Advancements in black titanium dioxide nanomaterials for solar cells: a comprehensive review

Abstract

Over the past decade, black titanium dioxide (B-TiO₂) has garnered considerable attention within the scientific community due to its exceptional properties in optoelectronic and photovoltaic applications. This review offers a thorough examination of the synthesis, characteristics, and utilization of B-TiO₂ nanomaterials in solar cell technologies. It underscores the pivotal role played by B-TiO₂ in bolstering light absorption, facilitating charge separation, and optimizing charge transport mechanisms within solar cell architectures. Notable research endeavors are highlighted, showcasing the strides made in enhancing solar cell efficiency through the incorporation of B-TiO_2. Furthermore, this review delves into the challenges inherent in the utilization of B-TiO_2, including considerations of stability, scalability, and manufacturing methodologies, thereby stressing the imperative for refined synthesis techniques. Environmental concerns related to B-TiO₂ are also addressed, underscoring the significance of evaluating its ecological footprint and advocating for sustainable deployment practices. Moreover, the review elucidates the future prospects of B-TiO₂ in solar energy conversion, emphasizing innovations in device design and novel applications while championing environmentally conscious utilization practices. Serving as a comprehensive resource, this review is poised to empower researchers and practitioners alike, fostering deeper insights and facilitating advancements in the realm of B-TiO₂ nanomaterials within the domain of solar cells.

1 Introduction

Global energy consumption is rising significantly due to several variables, including industrial expansion and population increase. Due to this growth, carefully evaluating energy options that can both satisfy global energy demands and minimize this issue is imperative [1]. There are two types of energy sources: renewable and non-renewable. Reliance on non-renewable sources can exacerbate climate change, causing catastrophic natural disasters and ecological damage [2,3,4]. Furthermore, non-renewable energy sources cannot completely fulfill the world's energy needs due to their limited supply and exhaustibility. Prioritizing renewable energy sources over non-renewable alternatives, such as solar energy, is essential to ensuring a sustainable future [5, 6]. Due to its availability and affordability as a renewable energy source, solar energy stands out as a feasible option for the future. Solar energy has no limits and is infinitely available, in contrast to non-renewable sources. When compared to other energy sources, it provides greater production efficiency and stability. [7]. Solar energy may be harvested by a variety of technical techniques, including photovoltaic cells and panels, concentrated solar energy, and solar architecture, all of which can provide electricity [7]. Out of all these techniques, photovoltaic is the most popular and commonly applied technology for solar energy harvesting. Typically, photovoltaic arrays are made up of solar panels, each of which has several solar cells. Semiconductors in these cells absorb sunlight and emit electrons. These free electrons are guided by an electrical field, which results in a one-way electric current. This current is directed to an external system or device via metal contacts at the top and bottom of the solar cell [8,9,10]. Much research is now being done to create solar cells that, in terms of efficiency and production costs, can compete with first-generation solar cells. To create efficient and affordable solar cells, researchers are investigating a variety of materials, including thick crystalline silicon layers, organic photovoltaic materials, cadmium telluride, and copper indium gallium selenide [11,12,13] One substance that has drawn significant interest in solar cell applications in the pursuit of cutting-edge solar cell technology is titanium dioxide (TiO₂). This n-type semiconductor material exists in three different polymorphs, which are Anatase, Rutile, and Brookite, where rutile is the most thermodynamically stable, while anatase and brookite are metastable [14]. It also possesses properties such as high electron mobility, and excellent thermal stability which makes it a prospective material for solar cell applications, however, despite its advantages, white TiO₂ suffers from two major challenges that hinder its optimum potential. Firstly, it has a large bandgap energy (ranging from 3.0 to 3.4 eV, depending on the polymorph), limiting the wavelength range of solar radiation that TiO₂ can absorb. Secondly, the inherently high density of trap states in TiO₂ leads to fast charge carrier recombination, resulting in low photocatalytic efficiency. Although introducing dopants can narrow the bandgap, it often accelerates the recombination process, necessitating dopant-free approaches to address both limitations [15, 16]. One of the dopant-free materials that can potentially overcome these drawbacks is black TiO₂ (B-TiO₂). The synthesis of B-TiO₂ involves a two-step method, where the normal white TiO₂ is synthesized via different methods such as Sol–gel, Hydrothermal, Solvothermal, Direct oxidation method, and Chemical vapor deposition (CVP). The resultant white TiO₂ is then subjected to different conditions such as hydrogenation, metal, and chemical reductions, ultrasonication, etc. These strategies produce black TiO₂, which exhibits superior properties, including a decreased bandgap of approximately 1.54 eV. This expansion of the absorption spectrum into the visible range enhances the ability of black TiO₂ to harness solar energy, potentially improving the efficiency of PSCs [17]. Owing to its distinct characteristics, B-TiO₂ has garnered significant attention as a material under investigation for possible solar cell applications. It is a viable option for raising the performance and efficiency of solar cells. Black TiO₂ has more light-absorbing properties than conventional TiO₂, which is utilized in solar cells and can convert sunlight into energy more effectively. Its distinct nanostructured morphology and improved charge transfer capabilities have the power to completely transform the solar energy industry and greatly raise the efficiency of solar cells [18, 19]. Researchers seek to enhance the efficacy as well as the affordability of solar energy by concentrating on the production of novel materials like black titanium dioxide and developing solar cell technology. With further study, researchers aim to further alleviate the problem of global energy consumption by encouraging the broad use of solar energy as a clean, sustainable energy source. By compiling the most recent developments and research results on black TiO₂, this review seeks to close the current knowledge gap and illuminate both the material's special properties and its potential to completely transform the solar energy industry. This paper offers important insights into the potential paths for improving the performance and efficiency of solar cells by examining the synthesis techniques, clarifying the characteristics, and going over the useful applications of black TiO₂ nanoparticles. This review's methodical and analytical examination advances knowledge of black TiO₂ as a novel material and emphasizes its importance in the quest for efficient and sustainable solar energy conversion. This review offers distinctive contributions that set it apart. Firstly, it delves into the limited literature on black TiO₂'s application in Perovskite solar cells (PSCs), a novel exploration beyond its traditional use in dye-sensitized solar cells (DSSCs). By addressing the instability issues associated with DSSCs' liquid electrolytes and highlighting PSCs as a promising alternative, our review underscores the evolving landscape of solar cell technology and the potential role of black TiO₂ in enhancing efficiency and stability. Furthermore, our analysis provides an in-depth examination of synthesis methods, offering insights into fabrication techniques and their implications on material properties. By consolidating recent advancements and offering forward-looking perspectives, our review serves as a valuable resource for researchers and practitioners in the field of materials science and solar cell technology.

Table 1 presents a comprehensive synthesis and advancements in black TiO₂ research since its discovery, encompassing various synthesis methods and diverse applications beyond solar cell technologies. In 2018, Sanjay Gopal Ullattil provided an update on the latest developments in B-TiO₂ nanomaterials. In their review, they concentrated on the different ways that black TiO₂ nanomaterials can be synthesized, as well as their structure, morphological variations, electronic structure, and range of uses in environmental and technological domains, including dye-sensitized solar cells, photocatalytic water splitting, batteries, supercapacitors, and photothermal therapy. In another study, T.S. Rajaraman et al. (2020) reported on the characteristics and contradictory patterns associated with B-TiO₂. Researchers have devoted much attention to understanding its superior activities, most of which indicate that defect species like Ti³⁺ and oxygen vacancies are in charge of the increased photoactivity. On closer inspection, though, it becomes clear that defects alone do not always translate into better performance from black samples. Research indicates that an identical defective species may have adverse effects on its functionality. That is to say, a variety of parameters, including anatase/rutile ratio, synthesis technique, synthesis circumstances, valence band and conduction band levels, defect concentration, position, and so on play a crucial part in the processes that allow black TiO₂ samples to function. These findings have given rise to various contradictory theories on the role of defects, photo activity in the visible spectrum, the presence or coexistence of Ti³⁺ species and oxygen vacancies, the location of these species and their relationships, the relationship between band gap values and photocatalytic activity, etc. The synthesis of black- TiO₂ for photocatalytic H₂ generation was also reported by Soontorn Tuntithavorwat et al. (2024). Owing to its exceptional qualities, which include its extraordinary thermal-chemical resistance and strong capacity to absorb visible light, Defective black TiO₂ is being developed and used for photocatalytic H₂ generation. The synthesis of B- TiO₂ on a laboratory scale has advanced recently, and their study highlighted some of the methods that have been used. These methods include semiconductor coupling, conductive material, and metal and non-metal doping, as well as faulty self-doping. The connection of B-TiO₂ with semiconductors showed the greatest photocatalytic H₂ generation among the many techniques used. Considering that black TiO₂ has undergone substantial research since its discovery, including a range of synthesis techniques and a wide range of uses outside solar cell technologies, recent advancements in this material have made it a potential in lithium-oxygen batteries (LOBs) as reported in the research conducted by Juanjuan Ge et al. (2020). The black TiO₂ -based cathodes have strong electrochemical activity and better cycle stability for LOBs because the researchers produced colored TiO₂ and it had high electrical conductivity and good catalytic activity supported by oxygen vacancies and Ti³⁺ ions. According to Yufang Li et al. (2023), To increase visible light absorption, black TiO₂ is synthesized using the magnesium thermal reduction process. Then, to increase the photocatalytic activity, a hetero-junction is formed by chemically synthesizing the black TiO₂/SnO₂ composites. In their study, they looked at Rhodamine B's (RhB) shape, chemical makeup, phase structure, and photocatalytic degradation when exposed to visible light. The morphological characteristics showed that the surface of the TiO₂ nanophase generates a Ti³⁺ self-doped disorder layer, and that the black TiO₂ nanoparticles are well-decorated with SnO2 nanoparticles.

Table 1 A summary of recent advances and applications of black TiO₂

2 Synthesis and properties of black titanium dioxide

2.1 Different methods for synthesizing black titanium dioxide nanoparticles

Since 2011, many techniques have been developed to synthesize black titanium dioxide, increasing the range of alternatives available for its manufacture [20, 21]. Although there is still a lot of research being done on hydrogenation, there have been a lot of additional approaches identified recently. Two general categories may be utilized to categorize these techniques: partial reduction from TiO₂ and incomplete oxidation from low-valence states of titanium species. In general, the reaction equation below describes the reduction approach:

$${{\text{TiO}}}_{2}+\mathrm{A }\to {{\text{TiO}}}_{2-{\text{x}}}+ {{\text{AO}}}_{{\text{x}}}$$

(1)

where A represents the reductant. Studies have shown that reduction of white TiO₂ in the presence of imidazole, Al, Mg, and hydrogen can result in the production of black titania [8]. Apart from chemical reductants, other methods including electrochemical reduction and high-energy particle bombardment (such as electron beam, proton beam, and H₂ plasma) have also demonstrated potential in producing black titania with defects. Moreover, low valence-state Ti species have been used as precursors for oxidation procedures to produce black titania, such as elemental Ti, TiO, and Ti₂O₃ [22]. The production of B-TiO₂ nanoparticles has special qualities that draw interest for a range of uses, including solar cells. Improved solar energy conversion efficiency is made possible by these nanoparticles' superior light-absorbing qualities over traditional white TiO₂ [22]. Due to a decreased bandgap and enhanced absorption of visible light, the black coloring is the consequence of the change in the TiO₂ band structure. This characteristic encourages increased power production in solar cells and makes it possible to use sunlight more effectively. Additionally, B-TiO₂ nanoparticles have been shown to have longer photoexcited electron lifetimes and higher charge carrier mobility, which improves charge separation and lowers recombination rates [22]. Increased photocurrent generation and overall device performance in solar cells are facilitated by these features. To increase the efficiency and stability of black titanium dioxide nanoparticles, researchers are continuously investigating different synthesis methods and refining the particles' characteristics. Through enhanced synthesis techniques and comprehension of the underlying principles, researchers hope to fully realize black titanium dioxide's potential to transform solar cell technology and propel the area of renewable energy.

2.1.1 Synthesis of B-TiO₂ using hydrogenation method

A substance is treated with hydrogen or hydrogen plasma using the hydrogenation procedure, which calls for temperatures and times. The physical and chemical characteristics of the material are known to be significantly altered by this approach, which is known to introduce many reduction states [13]. The hydrogenation process has become favored as a successful strategy for the manufacture of black titanium dioxide. Through the process of hydrogenation, scientists have successfully produced B-TiO₂ nanoparticles, which possess distinct characteristics. With the help of this technique, hydrogen atoms may more easily be incorporated into the TiO₂ lattice, changing its structural makeup and electrical band structure [23, 24]. TiO₂ is reduced because of the hydrogenation process, which also creates oxygen vacancies and the states Ti³⁺ and Ti²⁺ in the material. These modifications lead to the production of B-TiO₂ with improved characteristics for light absorption and modified electrical behavior. The identification of B-TiO₂ and the accessibility of the hydrogenation process have given scientists a potent tool for producing B-TiO₂ nanomaterials. The capacity to manipulate the reduction states and alter the characteristics of TiO₂ by hydrogenation has created new avenues for enhancing the efficiency of several applications, such as photocatalysis, energy storage, and solar cells. B-TiO₂ nanomaterials can also be produced via alternative reduction methods, such as chemical reductants and high-energy particle bombardment, in addition to hydrogenation. These techniques provide several ways to cause reduction and change the material's characteristics in the ways that are intended. In the realm of nanomaterials, the hydrogenation method's production of B-TiO₂ is a noteworthy breakthrough. It offers a way to improve light absorption, charge separation, and photocatalytic activity and allows attributes to be tailored for specific applications. To fully realize the potential of hydrogenation process and its influence on the synthesis of B-TiO₂ nanomaterials for sustainable energy and environmental applications, research endeavors are still underway. To synthesize B-TiO₂, Chen et al. subjected virgin TiO₂ to a high pressure of 20 bar for five days at 200 °C [25]. Titanium tetraisopropoxide was hydrolyzed in a solution of ethanol, water, and hydrochloric acid to create the first white nano-TiO₂. After adding Pluronic F127, the mixture was air-calcined at 500 °C. The B-TiO₂ powder after preparation is shown in Fig. 1a [26]. As can be seen in Fig. 1b, Chen et al. found that the black hydrogenated TiO₂ nanocrystals' optical bandgap had been drastically lowered to around 1.54 eV. The process behind the bandgap narrowing of black TiO₂ (B-TiO₂) in the presence of different defects/disorders has been extensively investigated by various models over the past decade. These models have provided valuable insights into the underlying mechanisms driving bandgap modification in black TiO₂. Notably, preliminary results from these studies suggest that mid-gap states induced by disorder, rather than the presence of Ti³⁺ species, serve as the primary source of band narrowing in hydrogenated black TiO₂. The emergence of a conduction band (CB) tail and an upshift in the valence band (VB) edge have been observed as consequential effects of the disorder-induced mid-gap states. Moreover, researchers have observed distinct mechanisms for bandgap narrowing in oxygen-deficient TiO₂-x and hydrogenated titania (TiO_2-x H_x). In these cases, lattice disorder leads to the formation of VB and CB tails, resulting in a reduction of the bandgap length. These findings highlight the complex interplay between defects, disorders, and bandgap modification in black TiO₂, contributing to a deeper understanding of its unique electronic properties and its potential for addressing the limitations of conventional TiO₂ materials. In addition, as Fig. 1c shows, the optical absorption start was decreased to around 1.0 eV (1200 nm). As seen in Fig. 1d, the author ascribed these modifications in the optical characteristics to the existence of a surface lattice-disordered shell on the black TiO₂ nanoparticles.

Fig. 1

Copyright 2016. Advanced energy materials

(a) Pictures of white and B-TiO₂ nanocrystals. (b) Optical absorption spectra of (a) white and (b) B-TiO₂ nanoparticles. (c) Schematic illustration of the density. (d) HRTEM image of B-TiO₂ nanocrystal. Reprinted with permission from ref. [27].

Liu et al. carried out more study on the hydrogenation process and looked at how time affected the hydrogenated TiO₂ nanoparticles' color shift. They noticed that the color of the nanoparticles gradually changed as the hydrogen treatment time increased. Following a 3-day hydrogenation period, the white "P25" nanoparticles had a subtle yellow hue [26, 28, 29]. The color darkened with prolonged hydrogenation times and finally turned gray after 15 days of treatment. Hydrogenation must be done for over 15 days to produce B-TiO₂ nanoparticles [16]. Figures 2a and b show how these differences correspond to changes in optical and visual absorption qualities.

Fig. 2

Copyright 2018. Chemical engineering journal

(a) Photographic images and (b) optical absorption spectra of P25 treated under a high-pressure H₂ atmosphere at different time intervals. Reprinted with permission from ref. [30].

In another work, Wang et al. produced black hydrogenated TiO₂ nanowire arrays by ambient annealing pure H₂ at temperatures over 450 °C for thirty minutes. Rather than the yellowish-green TiO₂ nanowires shown in Fig. 3a [31], which were synthesized at lower temperatures, this synthesis technique yielded B-TiO₂ nanowires. It is important to consider that ambient H₂ has a far lower capacity for reduction than high-pressure H₂. When ambient H₂ is present, crystalline white TiO₂ nanoparticles typically cannot change into B-TiO₂. Therefore, using higher pressure settings or alternate hydrogenation processes that give more effective reduction capabilities is required to generate the desired B-TiO₂ material. Using a previously described hydrothermal technique, rutile TiO₂ nanowire arrays were synthesized on a glass substrate with fluorine-doped tin oxide (FTO). The white, uniform film produced on the FTO substrate is shown in a scanning electron microscopy (SEM) image to contain dense nanowire arrays that are vertically oriented (Fig. 3a). With a rectangular cross-section, these nanowires are uniform. The typical nanowire length is 23 m, and the diameters of the wires range from 100 to 200 nm. Every single nanowire that can be seen using SEM is made up of a bundle of smaller nanowires that have been combined using transmission electron microscopy (TEM) analysis. Clear lattice fringes with interplanar spacings of 0.32 and 0.29 nm are seen in a lattice-resolved TEM image acquired from the tiny nanowire, which are consistent with the d-spacings of the (110) and (001) planes of rutile TiO₂ (Fig. 3b). These results show that the TiO₂ nanowires develop in the (001) direction and further support the single-crystalline structure. The as-prepared TiO₂ nanowire arrays were first heated to 550 °C in the air for 3 h, then heated to varied temperatures between 200 °C and 550 °C in a hydrogen atmosphere for an additional 30 min. As seen in Fig. 3c, the hydrogen annealing temperature affects the colour of the H: TiO₂ nanowire films, which transition from white (untreated sample) to yellowish green (350 °C), then black (450 °C or above). Because of the hydrogen treatment, the TiO₂ appears to be dark, indicating that it exhibits visible light absorption. The pristine TiO₂ nanowires and H: TiO₂ nanowire arrays were synthesized at various annealing temperatures, and X-ray diffraction (XRD) spectra were obtained to investigate the crystal structure and potential phase changes during hydrogen annealing (Fig. 3d). Two diffraction peaks centred at 2 angles of 36.5 and 63.2 were seen in each sample after the peaks from FTO glass were subtracted. These two distinct peaks prove the nanowires are made of rutile TiO₂ and are indexed to the tetragonal rutile TiO₂ characteristic peaks (JCPDS No. 88–1175). Evidence that the TiO₂ nanowires on the FTO substrate are strongly orientated in the (001) direction can be found in the peak centred at 63.2, which is consistent with the observed development of the TiO₂ nanowires. XRD spectra were used to analyse the crystal structure and potential phase shifts during hydrogen annealing. Figure 3d demonstrates that although the TiO₂ peak intensity drops as the annealing temperature rises, there is no phase change after hydrogenation. The rise in defect density in the TiO₂ structure, which was also noted in previous research of hydrogenated TiO₂ nanoparticles, may be the cause of this. Additionally, when the annealing temperature was above 450 °C, the diffraction peaks of FTO glass gradually vanished and new peaks (depicted by arrows) corresponding to Sn metal arose. It suggests that by converting SnO₂ to Sn metal, the hydrogen treatment at high temperatures destroyed the FTO conducting layer [32, 33].

Fig. 3

Copyright 2011. Nano letters

(a) SEM image of vertically aligned TiO₂ nanowire arrays prepared on a FTO substrate. The scale bar is 4 μm. (b) Lattice-resolved TEM image of a single TiO₂ nanowire. Scale bar is 5 nm. (c) Digital pictures and (d) XRD spectra of pristine TiO₂ and H: TiO₂ nanowires annealed in hydrogen at various temperatures (300, 350, 400, 450, 500, and 550 °C). XRD spectrum of FTO substrate is added as reference. Arrows in (d) highlight the diffraction peaks corresponding to Sn metal. Reprinted with permission from ref. [34].

2.1.2 Synthesis of B-TiO₂ using the Metal reduction method

A more practical and affordable way to produce black titania is by metal reduction. Effective reductants in this process have been discovered to be active metals like magnesium, zinc, and aluminum. There are economic, safety, and simplicity benefits to metal reduction over hydrogen reduction [35]. When titanium dioxide is reacted with active metals, black titania is formed, and Ti species are reduced. This process is known as reductant usage. With this technique, black titania films or nanoparticles with modified characteristics can be produced alternately. Using metal reduction provides useful advantages because it does not require specialized hydrogenation equipment or high-pressure situations to be handled. In addition, this method is economically feasible for producing black titania on a big scale because of the accessibility and comparatively low cost of metals like magnesium, zinc, and aluminum.

Aluminum reduction

B-TiO₂ nanoparticles were produced by Wang et al. using aluminum reduction. By using molten aluminum, the procedure called for temperatures between 300 and 500 °C in an evacuated two-zone vacuum furnace [36]. The process of mixing aluminum with TiO₂ to create an Al-Ti alloy served as inspiration for the researchers. For the reduction process, aluminum and TiO₂ were placed independently in a two-zone tube furnace as part of the experimental setup. The samples were subsequently removed to a base pressure of less than 0.5 Pa. Aluminum was heated at 800 °C for 20 to 30 h, whereas the TiO₂ samples were heated between 300 and 600 °C. The aluminum was surrounded by a low partial pressure of oxygen because of the melting of TiO₂ and the subsequent release of oxygen. As shown in Fig. 4 [18], the reduced TiO₂ nanoparticles produced by this technique had a black hue and showed high absorption of visible and near-infrared light. Other black nano oxides with strong optical absorption, such as black brookite TiO₂, NbO₂O₅, and SrTiO₃, as well as TiO₂ nanotubes and gray TiO₂ nanowires, have also been produced using the aluminum reduction process. TiO₂@ TiO_2-x is the name of the crystalline-disordered core–shell structure seen in the black titania formed by aluminum reduction [27]. Aluminum reduction offers a novel approach for producing B-TiO₂ nanoparticles with distinctive optical characteristics. The resultant materials have potential uses in photocatalysis, optoelectronics, and energy conversion. In addition to helping to produce new materials, more investigation and study of the aluminum reduction process can improve our knowledge of the characteristics and uses of black titania.

Fig. 4

Copyright 2016. Advanced energy materials

(a) Schematic low-temperature reduction of TiO₂ in a vacuum two-zone furnace, Pictures of mass-produced black TiO₂ and P25 powder. (b) absorption spectra of TiO₂ samples reduced at different temperatures (300 °C, 400 °C, and 500 °C), the high-pressure hydrogenated black titania the H₂ -reduced titania (H₂ –TiO₂), and pristine titania (TiO₂ Reprinted with permission from ref. [27].

Wang et al. synthesized black TiO₂ at 400, 500, and 600 °C using Al as the reductant. At 500 °C, the black TiO₂ exhibited strong black color, absorbance, and photoactivity. When the reductant is the same, the gas used will affect how dark the color is. For instance, the sample treated at 350 °C showed maximum black color in the presence of Ar gas flow [37] when NaBH₄ was used as a reductant to produce black TiO₂ whereas strong black color was detected at a temperature of 500 °C for N₂ gas flow [38]. The so-formed defective TiO_2-x was treated for 3 h at a range of temperatures (300–700 °C) with a flow rate of 150 sccm N₂ gas (Fig. 5). The greatest blackness was observed at 500 °C as depicted in the image, and a color change took place. Additionally, anatase peak intensity increased with rising temperature [39, 40].

Fig. 5

Copyright 2018. Chemical engineering journal

Ti³⁺ self-doped TiO₂ samples prepared by post-annealing treatment in the 300 to 700 °C temperature range for 3 h under N₂ gas flow and b) the corresponding XRD pattern. Reprinted with permission from ref. [41].

Magnesium reduction

Black TiO₂ was synthesized in the study by Sinhamahapatra et al. using magnesium reduction. Commercial nano TiO₂ and magnesium powder were combined, and the combination was then annealed for five hours at 650 °C in a 5% H₂/Ar flowing environment. B-TiO₂ nanoparticles were produced using this technique. The scientists noticed that the production of a decreased black titanium oxide-based photocatalyst with high activity and stability was largely dependent on the interaction of H₂ and Mg. Nonetheless, they stressed that the main element affecting the synthesis of black titania was magnesium [42]. As seen in Fig. 6a, the black TiO₂ material showed much higher optical absorption in the visible and infrared range, indicating that it was very effective at capturing light energy. The remarkable stability of the B-TiO₂ nanoparticles produced by magnesium reduction over the whole solar spectrum is one of their unique properties. With rates of up to 43 mmol per hour per gram, these nanoparticles showed exceptional performance in terms of hydrogen production [42]. This characteristic has potential uses in photocatalysis and solar energy conversion. In addition to showing an optimal bandgap and band location, the B-TiO₂ material also displayed charge recombination centres, surface defects, and oxygen vacancies. Its increased optical absorption and photocatalytic capabilities were facilitated by these characteristics [43]. Black titania's band structure and characteristics may be customized by magnesium reduction, creating new possibilities for the creation of sophisticated photocatalysts and solar energy conversion devices. The research by Sinhamahapatra et al. shows how important magnesium reduction in producing B-TiO₂ nanoparticles with the right characteristics for effective light absorption and photocatalytic uses. Additional research in this field may yield new approaches to improve the production procedure and boost the functionality of materials based on black titania.

Fig. 6

Copyright 2017. Science bulletin

(a) the absorption spectra and (b) Mg reduction approach for 60,120,240, and 400 mg powder with their corresponding XRD spectra. Reprinted with permission from ref. [36].

In another study, TiO_2-x nanoparticles of various colours (white, gray, blue, and black) were successfully synthesized using a Mg reduction approach inspired by the classic alumina-thermic reduction reaction [43]. Commercial P25 TiO₂ nanoparticles (400 mg) were uniformly mixed with 60, 120, 240, and 400 mg of Mg powder before being purged with argon for 15 min and calcined at 600 °C in an Ar atmosphere for 4 h. After washing with excess HCl and distilled (DI) water, the TiO_x nanoparticles with varied coloration were produced as indicated by Fig. 6b [43].

Zinc reduction

Researchers have effectively produced self-doped TiO₂ nanoparticles by metallic zinc-assisted solvothermal methods, yielding two distinct TiO_2-x samples with increased sensitivity to visible light [44]. Zinc powder was added to the TiCl₄/absolute ethanol solution, causing Zheng et al. to detect a colour shift from light yellow to blue (Fig. 7a). The blue TiO_2-x nanoparticles' ability to capture visible light during the hydrothermal process was precisely controlled by the amount of zinc powder utilized (Fig. 7b) [43]. The solvothermal method of incorporating zinc into the TiO₂ structure introduced doping effects that affected the material's optical properties. Figure 7b illustrates this, displaying the optical absorption results of self-doped TiO_2-x samples that were obtained with varying ratios of Zn and TiCl₄ reactants. The self-doped TiO_2-x samples showed enhanced light sensitivity, which broadened their possible uses in solar energy conversion and photocatalysis [43]. Adjusting the quantity of zinc powder allows for the fine-tuning of the visible-light harvesting properties, which presents opportunities for maximizing the performance of TiO_2-x nanoparticles for uses. These investigations' results highlight the importance of zinc reduction as a successful strategy to produce self-doped TiO₂ nanoparticles with enhanced visible light sensitivity. The solvothermal process's capacity to regulate the doping level offers a way to modify the material's optical characteristics and improve its performance in light absorption and photocatalytic applications.

Fig. 7

Copyright 2016. Advanced energy materials

(a) Photographs of TiCl₄/absolute ethanol solution before and after adding Zn powder. (b) Optical absorption spectra of self-doped TiO_2-x samples obtained with different ratios of Zn and TiCl₄ reactants. Reprinted with permission from ref. [27].

2.1.3 High energy particle reduction

Hydrogen plasma

Black TiO₂ nanoparticles have been produced using hydrogen plasma in two separate ways: chemical vapor deposition assisted by hydrogen plasma and thermal plasma furnace. By heating P25 TiO₂ to 500°C for 4–8 h and using 200 W of input plasma power, Wang et al. produced hydrogenated B-TiO₂ nanoparticles using the thermal plasma furnace technique [45]. The resultant nanoparticles showed significant absorption of light, both visible and near-infrared, especially at wavelengths greater than 400 nm. The increased optical characteristics were ascribed by the researchers to H-doping and a crystalline-disordered core–shell structure, as seen by the TEM picture presented in Fig. 8A [46].

Fig. 8

Copyright 2016. Advanced energy materials

(a) High-resolution transmission electron microscopy (HRTEM) image of H-doped TiO₂ (TiO_2–x H_x) nanocrystal, with a crystalline-disordered core–shell structure (TiO₂ @TiO_{2− x} H_x). (b) The white TiO₂ sample prepared by alkali treatment and the black TiO₂ samples prepared by hydrogen plasma treatment at 350 and 500 °C, respectively. The last two are the samples contained chromium ions (Cr ³⁺). Reprinted with permission from ref. [27].

Teng and colleagues have successfully synthesized B-TiO₂ nanotubes using the hydrogen plasma assisted chemical vapor deposition technique. In a hot-film chemical vapor deposition system, hydrogen was used as the reaction gas during the hydrogenation process. The furnace was kept at a constant temperature of 200 °C while pure TiO₂ samples were heated for three hours at temperatures between 350 and 500 °C in a corundum boat placed beneath it. The resultant black hydrogenated TiO₂ nanotubes have unique optical characteristics [47]. These techniques show how well hydrogen plasma works to introduce hydrogen and change the optical characteristics of TiO₂ nanoparticles. These hydrogenated B-TiO₂ materials have prospective uses in photovoltaics, photocatalysis, and sensing, among other areas.

${{\varvec{N}}{\varvec{a}}{\varvec{B}}{\varvec{H}}}_{4}$ reduction

This is a chemical reduction method that makes use of the potent reducing ingredient sodium borohydride (NaBH₄). Because NaBH₄ can efficiently reduce a variety of compounds and give hydride ions (H-), it is used extensively in reduction processes. NaBH₄ was used as the reducing agent in the instance of producing reduced B-TiO₂ nanotubes by Kang et al. [48]. During the synthesis, a titanium (Ti) foil was anodized for 30 min at 500°C using a graphite cathode in an ethylene glycol solution containing 0.3 weight percent F and 2 volume percent X under 80 V. The first nanotube structure was then achieved by annealing the TiO₂ nanotubes for three hours at 450°C. Next, the reduction phase was performed in a 0.1 M NaBH₄ solution at room temperature for 10–60 min. The TiO₂ nanotubes underwent a significant change in their optical characteristics and a decrease in size because of the NaBH₄ reduction, showing a substantial absorption of light in the visible to near-infrared region [49]. The TiO₂ NTAs had an average length of 7 mm and an inner pore width of roughly 100 nm, according to the FE-SEM image in (Fig. 9a). Due to the one-dimensional characteristics of the tubes, the existence of well-aligned NTs that are vertically directed from the Ti foil substrate enables directional charge transmission. The decreased TiO₂ NTAs after the NaBH₄ treatment are depicted in (Fig. 9b) to still retain a porous structure, proving that the NaBH₄ treatment did not damage the nanostructure of the TiO₂ NTAs. When compared to the pure nanotubes illustrated in (Fig. 9a), the reduced nanotubes in (Fig. 9b) clearly split from one another, and the wall becomes thinner [50].

Fig. 9

SEM images of the TiO₂ NTAs before (a) and after (b) NaBH₄ treatment

Oxidation approach

Under this oxidation method, large oxygen vacancies or doping in the bulk Ti–O matrix led to the production of black titanium dioxide (TiO_2-x). Significant flaws and oxygen vacancies are present in the resultant TiO_2-x materials when the low-valence Ti species are not completely oxidized to TiO_2-x. These substances have a pronounced black colour and exhibit significant visible light absorption [51]. Elevated amounts of doping or oxygen vacancies cause the crystal structure of TiO₂ to be disrupted, resulting in the production of TiO_2-x, which has different optical and electrical characteristics. A wider spectrum of wavelengths, including visible light, are absorbed because of the insertion of defects and oxygen vacancies into the material's lattice structure. TiO_2-x materials' dark hue is a result of their greater absorption of light. Furthermore, doping TiO_2-x with foreign elements may potentially be responsible for its dark colour and increased light absorption. Dopants cause the electrical band structure to change, which makes it easier for light to be absorbed across a larger spectrum. The absorption spectra of the unreduced and reduced TiO₂ NTAs are shown in Fig. 10. While the photo-response of decreased TiO₂ NTAs is effectively extended into the visible and near-infrared ranges, the pristine TiO₂ NTAs are only active when exposed to UV radiation. According to a study, oxygen vacancy in TiO₂ has a favorable relationship with the absorption of visible and infrared light. According to the main absorption edge of the profile, the band gaps of the decreased and unaltered TiO₂ NTAs are approximately 2.99 and 3.09 eV, respectively, as shown in the inset. The idea that an electronic band is directly below the conduction band of pure TiO₂ and that the band gap has decreased supports this finding [52].

Fig. 10

UV–vis-NIR diffuse reflectance spectra of pristine and reduced TiO₂ NTAs

Chemical oxidation

Black titanium dioxide can also be produced via chemical oxidation. Black anatase TiO₂ nanoparticles with a crystalline/amorphous core/shell structure were created using a chemical oxidation process in the work carried out by Xin et al. [41]. TiH₂ and H₂O₂ were combined to create a yellowish gel, which was further diluted with ethanol in the procedure. Solvent-thermal treatment was applied to the combination for a duration of 24 h at 180°C. Following treatment, ethanol, water, and HCl were used to wash the resultant sample. Following a 12-h vacuum-dried process, bright blue TiO₂ nanocrystals were extracted from the precipitate. The last stage was a post-annealing treatment that was done for three hours under a nitrogen flow at temperatures between 300 and 700°C. To bring the mixture's pH down to 9.0, NaOH was added, and NaBH₄ was added as an antioxidant. The colour of the produced TiO₂ nanocrystals was shown to be significantly influenced by the annealing temperature [41]. As indicated by Fig. 11 below, the colour of the brookite changed after post-annealing treatment, going from blue to brown at 300 °C (T300), then to black at 500 °C (T500), and finally to light grey blue at 700 °C (T700). Following this pattern, the amount of light absorbed by each post-annealing treatment nanosheet showed improved absorption: T500 > T300 > T700 [53]. Precise control over the reaction conditions is possible through the chemical oxidation process, which yields black TiO₂ nanoparticles with distinct optical and structural characteristics. The improved light absorption properties of the crystalline/amorphous core/shell structure make these materials appropriate for a range of uses, including solar energy conversion, photocatalysis, and optoelectronic devices.

Fig. 11

Copyright 2017. International journal of photoenergy

UV–vis diffuse reflectance spectra of the as-prepared brookite TiO_2-x, T300, T500, and T700 (TiO_2-x was the sample without post-annealing). Reprinted with permission from ref. [25].

Ultrasonication

The production of amorphous B-TiO₂ nanoparticles with increased absorption of visible light may also be achieved by ultrasonication. First, a solution of titanium dioxide (20 mL, 4 mol/L) and titanium isopropoxide (12 mL, 8 wt%) was produced for the investigation. In a bath of freezing water, the combination was left to react for two hours. Then, using an ultrasonic apparatus with an output power density of 1,500 W/100 mL, ultrasonic treatment was carried out at 80 °C for 0.5 to 8 h. Through the application of high-frequency sound waves to the TiO₂ solution, intensive mixing and particle dispersion were produced because of cavitation and microstreaming effects. Amorphous B-TiO₂ nanoparticles with improved visible-light absorption capabilities were produced because of the ultrasonication method. Strong visible light absorption is exhibited by these nanoparticles, which opens prospective uses in photocatalysis, solar cells, and optoelectronics, among other areas. The study's findings, which illustrate the amorphous black TiO₂ nanoparticles produced by ultrasonication's visible-light absorption capabilities, are shown in Fig. 12 [54,55,56].

Fig. 12

Copyright 2017. International journal of photoenergy

UV–vis absorbance spectroscopy of original TiO₂ and ultrasonic treated TiO₂ for different hours. Reprinted with permission from ref. [25].

3 Role of synthesis route

The method of synthesis appears to be quite important for the performance of black TiO₂. Wang et al. compared the photocatalytic activities of B- TiO₂ prepared by both hydrogen thermal treatment at 600 ⁰ C and plasma treatment under an Ar (95%)/ H₂ (5%) atmosphere. The results showed that the hydrogen-treated samples had higher photocatalytic activity than the plasma-treated samples due to a higher concentration of oxygen vacancy defects and surface hydroxyl groups, which resulted in a thicker surface amorphous layer in the nanowire morphology and higher visible light absorption. [24] Surface and bulk flaws were present in the hydrogenated samples, while only the bulk defects were in the plasma-treated samples. The properties of the black TiO₂ sample prepared by aluminum reduction by Wang et al. were compared to those of the black TiO₂ prepared by Chen et al. at high pressure as well as those prepared under H₂ annealing at atmospheric pressure. The Al decreased sample had an improved solar absorption of up to 65%, according to the results, but black TiO₂ synthesized under high pressure only had a 30% solar absorption. The same team that used the hydrogen plasma approach to synthesize hydrogen-doped TiO₂ (TiO_2-x H_x) core/shell nanostructures reported an additionally improved absorption of solar energy of up to 83% [24, 54]. Compared to the oxygen-deficient TiO_2-x produced by hydrogenation, this resulted in much better light absorption, photocatalytic MO degradation, and hydrogen production performance under AM 1.5 irradiation. A vivid image of the impact of hydrogenation conditions on the photocatalytic characteristics and appearance of black TiO₂ was also provided by studies conducted by Leshuk et al. [57] in 2013. TiO₂ samples made by slightly different methods of synthesis had noticeably varied hydrogenation reactions. One of the samples went brown at 400 °C while the other remained white, showing that even little changes to the synthesis pathway have an impact on the reaction sites, which ultimately determine how hydrogenation will turn out. Additionally, both samples developed to a black color that was unrecognizable from one another at 500 °C. After hydrogenation, P25 particles remained unaltered as well. As white TiO₂ was synthesized, the irregular particles were more prone to hydrogenation than the regular P25 particles, which highlighted the influence of the precursor on the final hydrogenation product [57].

Table 2 below summarizes different synthesis methods, synthesis conditions, and applications of the synthesized black TiO₂.

Table 2 Synthesis routes, conditions, properties, and applications of TiO₂

In the research conducted by Danwen Yao et al. (2021), the PCE of QDSSCs was built utilizing a photoanode made of black TiO₂ that had been treated in both solution and air, as well as with pristine TiO₂. Their findings are shown in the Table below, which demonstrate that the QDSSCs assembled using the black TiO₂ by femtosecond filament treatment can successfully improve the photovoltaic performance of QDSSCs. The current densities of the QDSSCs for the two black TiO₂ cases are significantly higher than that of the pristine TiO₂ case. Additionally, the table's parameters demonstrate the existence of distinct relationships between the kinds of TiO₂ samples and the photovoltaic characteristics. Due to the increased size of TiO₂ nanoparticles (70-90 nm), the device built with pure anatase TiO₂ has poor efficiency (PCE = 1.76%). The QDSSCs based on black TiO₂ are found to have PCEs of 3.73% and 4.11%, respectively, for the solution and air environments when compared to pristine TiO₂ structure; this leads to enhancements of 233.5% and 212.3% [61] (Table 3).

Table 3 Comparison of parameters between White TiO₂ and Black TiO₂

In a separate study, Danwen Yao et al. (2023) further fabricated Black rutile TiO₂ nanoparticles in Deionized water (DI-Black TiO₂) and Absolute ethanol (AE-Black TiO₂) by a Ti–Sapphire. From Table 4 below, the three QDSSCs exhibit extremely different PCEs, which are 1.43%, 3.68%, and 9.11% for the pristine, DI-black, and AE-black TiO₂-NPs, respectively. The PCE of the DI-black TiO₂ rutile-based device is improved by about 2.6 times than the pristine one, which is like the case of anatase-based devices. Furthermore, the parameters outlined in the table suggest that Black-TiO₂ has superior properties and can perform better than normal white TiO₂ [61, 62]

Table 4 Further comparison of parameters between White TiO₂ and Black TiO₂

3.1 Properties of black titanium dioxide nanomaterials

Regular TiO₂ (white TiO₂) and black TiO₂ exhibit distinct characteristics that stem from differences in their structural, optical, and functional properties. In terms of color and light absorption, regular TiO₂ appears white and has limited visibility in the visible light spectrum, while black TiO₂ showcases a deep black color owing to its superior light-absorbing abilities across a broader electromagnetic spectrum, especially within the visible range. These differences in optical properties extend further, with regular TiO₂ primarily interacting with ultraviolet (UV) light due to its bandgap, whereas black TiO₂ possesses a narrower bandgap and a propensity for efficiently absorbing visible light, rendering it valuable for applications reliant on solar energy conversion. Moreover, the surface morphology and structure of the two materials diverge significantly. Regular TiO₂ typically boasts a smooth, crystalline structure, whereas black TiO₂ often features a complex nanostructured morphology that enhances its light-absorbing potential and photocatalytic activity. Consequently, black TiO₂ demonstrates notably heightened photocatalytic activity compared to regular TiO₂, a result of its enhanced light-absorption capabilities. These distinctions have profound implications for applications. Regular TiO₂ finds usage in products like sunscreens, cosmetics, and coatings, given its UV-blocking properties and white color. On the other hand, the unique attributes of black TiO₂ position it as a promising contender for applications such as solar cells and photocatalysis, where efficient light absorption and energy conversion are paramount. Notably, the energy conversion efficiency of black TiO₂ surpasses that of regular TiO₂, particularly in contexts where visible light is harnessed. This is why the unique characteristics that differentiate black titanium dioxide nanoparticles from their white equivalent have garnered a lot of interest. In this segment, we will delve into the noteworthy characteristics of B-TiO₂ nanoparticles and furnish supplementary information to augment comprehension of these intriguing substances.

3.1.1 Structural properties

The unique features of B-TiO₂ are mostly determined by its structural qualities. The change from white to black indicates a substantial adjustment in the optical characteristics, which can be ascribed to structural changes. Significant morphological and structural changes in B-TiO₂ are notably brought about by hydrogenation, which results in the creation of a crystalline/amorphous core/shell structure. Several investigations have demonstrated the existence of this core–shell structure using high-resolution transmission electron microscopy (HRTEM), where the crystalline core is covered by a thin amorphous layer [57]. Several studies have found that this core–shell configuration is associated with superior activities. B-TiO₂ nanoparticles have outstanding optical qualities in addition to their unique structural traits. They exhibit increased absorption of visible and near-infrared light as compared to white TiO₂ [57]. The material's structure contains flaws, oxygen vacancies, and surface imperfections, which are responsible for the enhanced absorption of light. As a result, B-TiO₂ has a wide absorption spectrum, which makes it ideal for use in photocatalysis and solar energy conversion applications. The structural changes of B-TiO₂ also have a substantial impact on its electrical characteristics. The electrical structure of the material is changed by the insertion of flaws and oxygen vacancies, which improves charge migration and separation. Thus, the efficiency of redox processes is increased, and charge carrier recombination is decreased [63]. Furthermore, the distinct surface characteristics of B-TiO₂ nanomaterials are influenced by the amorphous shell in their core–shell structure. The shell's existence increases the material's catalytic activity and fosters efficient interactions with surrounding molecules by offering a sizable surface area and plenty of active sites for chemical reactions [64]. B-TiO₂ nanoparticles exhibit superior activity due to their unique surface features, improved electrical properties, increased light absorption, and core–shell structure. They are the focus of much investigation and study due to their enhanced performance in a variety of applications.

A noteworthy finding in the investigation of B-TiO₂ nanoparticles is the identification of a disordered core–shell structure in the crystal structure. A hydrogenated B-TiO₂ nanoparticle is shown in Fig. 13a, where the outer layer clearly shows a structural departure from the usual crystalline anatase. In particular, the plane distance is no longer homogeneous, and the straight lattice lines are twisted near the border of the nanoparticle. A crystalline core surrounded by an amorphous shell is indicated by this core–shell configuration. Similarly, a similar crystalline core/amorphous shell configuration has been seen in B-TiO₂ nanoparticles generated by aluminum reduction. The thickness of the disordered layer grows as the reduction temperature rises in the 300–500 °C range, as seen in Fig. 13b. This phenomenon demonstrates how the reduction circumstances affect the structural properties. Moreover, unique characteristics in the diffraction patterns of B-TiO₂ nanoparticles are revealed by analyzing their surface disorder layer. The fast Fourier transform (FFT) from the high-resolution transmission electron microscopy (HRTEM) picture of the surface disorder layer may result in a fuzzy cloud-like pattern, in contrast to the Fourier transform of the crystalline phase, which shows distinct dot or ring diffraction patterns [65]. This differentiation provides more evidence for the existence of structural disorder in B-TiO₂ nanoparticles.

Fig. 13

Copyright 2016. Advanced energy materials

(a) image and line analysis of black hydrogenated TiO₂ nanocrystal prepared by high-pressure H₂ treatment., (b) HRTEM images of black TiO₂ nanocrystals obtained from Al reduction method at different temperatures. (c) Atomic-resolution high-angle annular dark-field (HAADF) image (left) and the schematic (right) of the surface of black rutile TiO₂ nanocrystal, which was derived from the amorphous TiO₂ nanoparticle annealed in Ar atmosphere. Reprinted with permission from ref. [27].

3.1.2 Electrical and electronic properties

Defect disorder is known to affect TiO₂'s charge carrier mobility and electrical conductivity. Numerous studies have demonstrated that because crystalline TiO₂ has a disordered structure, hydrogenation enhances charge transfer mechanisms. Hydrogen molecules are essential for changing the surface of TiO₂ during the hydrogenation process. Hydrogen is thought to function as a "scissor," severing the Ti–O connections and creating Ti-H and O–H bonds on the surface, according to Liu et al. This results in a highly confined mid-gap state above the valence band maximum (VBM) being formed. The substantial localization of the holes results in spatial separation from itinerant electrons, resulting in better photocatalytic efficiency. The material's electrical characteristics are changed by the hydrogen-induced alteration, which improves charge carrier mobility and conductivity. Following hydrogenation treatment, TiO₂ nanoparticles typically undergo modifications to their electrical structures, such as valence band shifting. A red shift of the valence band from 1.26 to 0.92 eV was observed in the hydrogenated TiO₂ nanoparticles treated at 200 °C for 5 days under 20 bar H₂ (Fig. 14a), according to Chen et al. According to Yu et al.'s findings (Fig. 14b), the magnesiothermic reduction of black TiO₂ nanoparticles also caused a shift in the valence band from 2.04 to 1.82 eV and a tail up to 1.06 eV. However, Wang et al. hypothesized that the hydrogen treatment had no effect on the valence band position of hydrogenated TiO₂ nanoparticles compared to pure TiO₂. Similar results were found when studying hydrogenated TiO₂ nanosheets [65, 66].

Fig. 14

Copyright 2017. International journal of photoenergy

(A) Valence-band XPS spectra of the white and black TiO₂ nanocrystals (Chen et al., 2011). (B) Valence-band XPS spectra of the commercially nano-anatase TiO₂ (CT) and H-TiO₂. Reprinted with permission from ref. [25].

Charge carriers may travel more easily in hydrogenated black TiO₂ nanoparticles because of their disordered structure, which offers more active and defect sites. Applications where effective electron or hole transport is essential, such photocatalysis, solar cells, and sensors, benefit from this improved charge transfer mechanism. Furthermore, the addition of hydrogen to the lattice can widen the absorption spectrum into the visible range by adding new energy levels to TiO₂'s bandgap. Because of their increased capacity to absorb light in the visible spectrum, B-TiO₂ nanoparticles may more efficiently use a wider spectrum of solar energy for photoconversion activities.

3.1.3 Electrochemical properties of Black-TiO₂

The enhanced performance of B-TiO₂ in photoelectrochemical cells (PEC) is a result of the interaction of several factors. Among these impacts are:

• (i) Increased light absorption: where a wider range of wavelengths, including the visible area, are better absorbed by B-TiO₂ nanoparticles. Defects and structural alterations are responsible for this increased capacity to absorb solar light, which enables more effective use of a broader spectrum.

• (ii) Improved charge separation: The separation of photogenerated electron–hole pairs is facilitated by the introduction of defects and structural changes in B-TiO₂. The effective operation of photocatalytic and photoelectrochemical processes depends on this enhanced charge separation.

• (iii) Decreased resistance to charge transport in the bulk and surface: B-TiO₂ nanoparticle defects and alterations lower the barrier to charge transport, allowing charge carriers to pass through the material more effectively. Improved charge transfer kinetics and increased electrical conductivity result from this.

• (iv) Improved charge transfer at the semiconductor/electrolyte interface: Defect site-containing B-TiO₂ surfaces facilitate effective charge transfer between the electrolyte solution and the semiconductor material. This leads to improved performance by facilitating the interchange of ions and electrons during photocatalytic and photoelectrochemical processes.

• (v) Suppressed recombination: Defects, like oxygen vacancies, can provide as locations for photogenerated electron–hole pairs to recombine. Black TiO₂ nanoparticles' structural alterations, on the other hand, aid in suppressing recombination, extending the lifespan of charge carriers, and boosting photocatalytic performance. One of the distinctive flaws in B-TiO₂ nanomaterials is thought to be oxygen vacancies. The activity and kinetics of surface reactions in metal oxides are strongly influenced by oxygen vacancies. Regarding their contribution to the photocatalytic activity of B-TiO₂, opinions are divided. According to some research, the effectiveness of photocatalysis may be decreased by the recombination sites that oxygen vacancies and associated states may serve. On the other hand, it has also been shown that these oxygen vacancies oversee visible light absorption and bandgap narrowing, which enhance photocatalytic activity. It has been found that hydrogenated B-TiO₂ nanoparticles include Ti-H and Ti–OH groups, the amount of which changes according on the hydrogenation procedure. These functional groups can affect the characteristics and functionality of the nanoparticles by altering their surface chemistry. Thus, it is essential to comprehend the function of oxygen vacancies and defects in B-TiO₂ nanomaterials to modify their characteristics and maximize their effectiveness in a range of applications, such as photocatalysis, solar energy conversion, and sensing. To fully use B-TiO₂ nanomaterials, more research is being done to clarify the exact processes and improve defect engineering techniques.

3.2 Factors affecting the properties of black TiO₂

3.2.1 Charge Separation and Transport in B-TiO₂

The superiority of B-TiO₂ over white TiO₂ in charge separation and transport has been extensively recognized. Photoluminescence (PL) emission spectra, which show the recombination of free carriers, are frequently used to assess the effectiveness of charge separation and transport in B-TiO₂ [67, 68]. Studies have shown that B-TiO₂ significantly lowers the intensity of photogenerated electron and hole emission when compared to white TiO₂, which may indicate that B-TiO₂ efficiently reduces the rate at which photogenerated electrons and holes recombine. This reduced electron–hole recombination is caused by several reasons, some of which are detailed below.

3.2.2 Effective electron and hole scavengers and traps for photo-generated electrons

B-TiO₂'s disordered structure, self-doping, H-doping, and oxygen vacancy cause a variety of mid-gap states to form, which can trap photogenerated electrons and holes and delay their recombination [69]. These localized mid-gap states, which are the result of doping and structural flaws, give the carriers efficient places to be trapped, extending their lifespan. Additionally, by serving as additional trap sites or carrier scavengers, surface adsorbates and their derivatives are essential to B-TiO₂. By interacting with photogenerated carriers, these adsorbates can increase their lifespan and decrease the chance of recombination. Additional charge transfer channels are made possible by the presence of surface adsorbates and their derivatives, which facilitates the movement of electrons and holes within the material and allows for effective charge separation. Moreover, the distinct electronic band structure of B-TiO₂, which results from its altered surface and defect states, encourages the spatial dispersion of charge carriers produced by photolysis. The electrical structure of the material has energy bands that facilitate the movement of electrons and holes in various directions, hence reducing the likelihood of their recombination and improving the efficiency of charge transport.

3.2.3 Facet effect

Lu et al.'s first-principal calculations have shed light on the facet-dependent characteristics of B-TiO₂ [70, 71]. They discovered that in hydrogenated B-TiO₂, the conduction band minimum (CBM) of the (001) surface layer shows a lower negative potential than that of the (101) surface layer. The differing distributions of bottom states in the conduction band (CB) between these two surface layers are the cause of this divergence in the CBM potentials (Fig. 15a-d). More specifically, the deeper layers of the material are the primary source of the bottom states of the CB in the (101) surface layer. This suggests that because these bottom states are localized in deeper layers, the electronic transitions involving them need more energy. On the other hand, the hydrogenated B-TiO₂ surface layer (001) exhibits a more uniform distribution of bottom states throughout all layers. Because of this, electronic transitions involving these states are made easier because they need less energy overall. According to these results, B-TiO₂'s electronic structure and, by extension, its charge separation and transport capabilities, may be greatly influenced by its surface orientation or facet. Charge carrier energy levels can impact their mobility and recombination dynamics because to differences in CBM potentials across various surface layers.

Fig. 15

Copyright 2016. Advanced energy materials

Three-dimensional isosurface contour plots of selected lowest unoccupied wave functions (LUWFs) on hydrogenated (101) (a, b), and (001) (c, d) with the eigenvalues in the range of 1.00–1.38 and 1.38–1.58 eV. Reprinted with permission from ref. [27].

3.2.4 Crystalline-disordered core–shell structure

The increased charge separation and transport capabilities of B-TiO₂ nanoparticles can be attributed to their distinct crystalline-disordered core–shell structure. Effective electron mobility is made possible in this arrangement that the wave function of itinerant conduction electrons extends to both the core and shell regions. On the other hand, the thin disordered shell encircling the crystalline core is where the valence band maximum (VBM) hole is heavily concentrated. Recombination kinetics are improved by the decreased spatial overlap between the localized holes and electrons caused by this dispersion of charge carriers inside the core–shell structure. Reduced overlap increases the likelihood of productive charge transfer processes by prolonging the recombination period for photoexcited electron–hole couples. Moreover, any photoexcited holes created in the anatase core of B-TiO₂ nanoparticles are forced to migrate toward the surface layers by the greater VBM energy of the disordered surface layer [72, 73]. The energy gradient, which causes the surface layers with greater VBM energy to function as sinks for the photoexcited holes, is what propels this migration. The core–shell structure helps electron–hole couples separate by localizing the holes at the surface, which delays their rapid recombination. B-TiO₂ nanoparticles' crystalline-disordered core–shell structure offers an environment that is conducive to effective charge separation and transport. The extended lifetime of photoexcited carriers and the inhibition of recombination events are caused by the dispersal of wandering electrons throughout the core and shell regions as well as the concentration of holes in the disordered shell and their migration to the surface layers. Because of these qualities, B-TiO₂ is a material that shows promise for several uses, such as photocatalysis, solar energy conversion, and optoelectronics.

3.2.5 Carrier (Electron) concentration

Because of the existence of electron donors such oxygen vacancies or H-doping, black TiO₂ is recognized for having a high electron concentration and increased electrical conductivity. By adding more electrons to the material, these electron donors raise the concentration of free electrons that are accessible for charge transfer. The deliberate insertion of hydrogen atoms into the TiO₂ lattice is referred to as "H-doping." Hydrogen may form more electron donor states in the TiO₂ bandgap, which raises the number of free electrons in the material. The improved electrical conductivity of B-TiO₂ is a result of these extra electrons. In a comparable manner, oxygen vacancies may likewise serve as electron donors. When there are gaps in the oxygen atoms inside the TiO₂ lattice, oxygen vacancies arise. The bandgap's localized states produced by these vacancies can capture and transfer electrons to the conduction band, raising the total electron concentration. Black TiO₂'s electrical conductivity is increased due to its larger concentration of free electrons [74, 75]. The performance of B-TiO₂ in diverse applications is significantly affected by its increasing electron concentration. It makes effective electron transport possible, enhancing charge separation and permitting quicker reaction kinetics in procedures like energy conversion and photocatalysis. Furthermore, applications requiring effective electron transport, such as photovoltaic devices or electronics based on B-TiO₂, benefit from the increased electrical conductivity.

4 Applications of black titanium dioxide nanomaterials in solar cells

4.1 The role of black titanium dioxide in solar cell applications

4.1.1 Crystalline silicon solar cells

The most popular solar cell technology in use today is crystalline silicon (c-Si). To improve the efficiency and light-absorbing capacity of c-Si solar cells, B-TiO₂ nanoparticles can be added. By decreasing light reflection and increasing the quantity of incident light that the solar cell absorbs, the B-TiO₂ layer serves as an antireflection coating. As a result, photon harvesting is increased, and total energy conversion efficiency is raised. Moreover, B-TiO₂ nanoparticles may help improve charge transport and separation inside the structure of the c-Si solar cell. The overall efficiency of the solar cell is improved by the increased electron concentration and improved electrical conductivity of B-TiO₂, which also reduces recombination losses and promotes effective charge carrier movement. The accomplishments and function of B-TiO₂ nanoparticles in improving the efficiency of crystalline silicon (c-Si) solar cells have been the subject of several recent research. B-TiO₂ nanoparticles were studied by Zhang et al. (2022) as an antireflection coating for c-Si solar cells. They showed better performance by decreasing light reflection and raising light absorption [76]. Wu et al. (2021) investigated the use of B-TiO₂ as an antireflection coating in further detail with an emphasis on improving c-Si solar cells' energy conversion efficiency [77]. The authors Li et al. (2021) reported that the addition of B-TiO₂ nanoparticles improved photon harvesting in c-Si solar cells [78]. The study conducted by Liu et al. (2020) examined how B-TiO₂ nanoparticles contribute to better charge separation and transport in c-Si solar cells, which ultimately results in improved performance [79]. The improved light absorption and charge carrier dynamics brought about by adding B-TiO₂ to c-Si solar cells were examined by Wang et al. (2020). Furthermore, B-TiO₂ nanotube arrays were investigated by Huang et al. (2019) as antireflection coatings, which increased the efficiency of c-Si solar cells [80]. All these studies show that B-TiO₂ nanoparticles significantly improve the absorption of light, charge separation, and overall performance of c-Si solar cells. As such, they provide a viable option for producing more sustainable and effective solar energy conversion.

4.1.2 Thin-film solar cells

The benefits of thin-film solar cells include reduced material prices and flexibility. Examples of these cells are amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium selenide (CIS), and copper indium gallium diselenide (CIGS). To improve the light absorption and charge carrier dynamics of thin-film solar cells, B-TiO₂ nanoparticles are used. In thin-film solar cells, adding B-TiO₂ can enhance the cells' capacity to capture light. Due to the special optical characteristics of the nanomaterial, incoming photons may be used more effectively because of their broadband light absorption and light scattering capabilities. Enhanced current production follows from greater photon absorption. Furthermore, in thin-film solar cells, B-TiO₂ nanoparticles can help with charge carrier separation and transport. B-TiO₂ contributes to the enhancement of thin-film solar cell devices' overall efficiency by mitigating recombination losses and enabling effective charge extraction. Recent studies have shown that the use of B-TiO₂ nanoparticles has significantly advanced the technology of thin-film solar cells. The application of B-TiO₂ nanoparticles to enhance charge carrier dynamics and light absorption in amorphous silicon (a-Si) solar cells was studied by Liu et al. (2022) [81]. According to their research, adding B-TiO₂ improved light trapping properties, which raised photon absorption and enhanced current production. Wang et al. (2021) investigated the use of B-TiO₂ nanoparticles in thin-film solar cells made of cadmium telluride (CdTe) in a similar manner. Their research demonstrated how B-TiO₂ may improve charge carrier separation and light absorption, which in turn can increase device efficiency. Additionally, B-TiO₂ nanoparticle integration in copper indium selenide (CIS) solar cells was studied by Chen et al. (2020) [82]. The results of the investigation showed that B-TiO₂ improved device performance by facilitating effective charge extraction and lowering recombination losses. Like this, Zhang et al. (2019) concentrated on using B-TiO₂ in thin-film solar cells made of copper indium gallium diselenide (CIGS) [83]. Their research highlighted how B-TiO₂ enhances charge carrier movement and separation, raising CIGS solar cells' total efficiency. Furthermore, Xu et al. (2018) looked at how adding B-TiO₂ nanoparticles to thin-film solar cells improved light absorption and charge carrier dynamics [84]. Their research demonstrated the wide range of applications for B-TiO₂ in a-Si, CdTe, and CIS thin-film solar cells. Additionally, B-TiO₂ nanoparticles were investigated for use in tandem solar cells by Lin et al. (2017), who showed improved light absorption and charge carrier extraction in a multilayered thin-film solar cell structure [85]. Taken as a whole, these investigations highlight the noteworthy advancements in thin-film solar cell technology made possible using B-TiO₂ nanoparticles. B-TiO₂ is used in thin-film solar cells to improve charge carrier dynamics, increase light absorption capacity, and boost overall device efficiency.

4.1.3 Next-generation solar cells

Due to their potential to reach high efficiencies and cost-effectiveness, next-generation solar cells, such as dye-sensitized solar cells (DSSCs) and perovskite solar cells, have attracted a lot of attention. Both the DSSCs and PSCs consist of a hole transport layer (HTL) which facilitates hole extraction and transportation while blocking electron flux. However, the HTL in DSSCs is a liquid electrolyte that has relatively poor physical and chemical stability, hence PSCs containing a solid electrolyte are preferred [86, 87]. B-TiO₂ nanoparticles have demonstrated potential for improving this cutting-edge solar cell technologies’ performance. Rather than using conventional white TiO₂, B-TiO₂ nanomaterials may be used as the photoanode material in DSSCs. Greater light absorption capabilities provided by the B-TiO₂ enable greater use of a wider range of light. In DSSCs, this results in higher photocurrent production and improved power conversion efficiency. In a similar vein, B-TiO₂ nanoparticles may be added to perovskite solar cells to improve charge carrier transport and light absorption. B-TiO₂'s special qualities reduce recombination losses and enable effective charge extraction, which improves perovskite solar cells' overall efficiency. The application of B-TiO₂ as the photoanode material in DSSCs was studied by Chen et al. (2022), and their results showed that adding B-TiO₂ nanomaterials improved light absorption and charge transfer, which in turn improved DSSC performance [86]. In a similar vein, Li et al.'s (2021) research on perovskite solar cells showed that adding B-TiO₂ nanoparticles increased charge transport and light absorption, which in turn improved overall performance [88]. To enhance the efficiency of these solar cells, Wu et al. (2021) modified the characteristics of B-TiO₂ for high-performance DSSCs. B-TiO₂ has also been studied for its potential to improve charge extraction, electron transport, and light harvesting in perovskite solar cells by Zhang et al. (2020), Song et al. (2020), and Lee et al. (2019) [87, 89]. These investigations show how B-TiO₂ nanoparticles have been used to improve the performance of perovskite solar cells and DSSCs, and they also present encouraging opportunities for further developments in solar cell technology.

4.2 Recent research studies and their findings

4.2.1 Perovskite solar cells

The strong optical absorption of perovskite materials has piqued the interest of academics in recent years, to the point where, following the first demonstration of the photovoltaic effect on perovskite materials in 2009, with an efficiency of 3.8%, researchers have risen to the occasion of improving the instability issues associated with the liquid electrolyte. This liquid electrolyte caused fast decomposition of perovskite materials in the cells. In 2012, solid-device configurations were disclosed, which alleviated the instability concerns. Since then, PCEs of around 10% have been recorded, along with increased operating stability. In 2021, a new world record of 25.6% for single-junction Perovskite solar cells (PSC) was revealed. Furthermore, because of their readily adjustable bandgap through components, Perovskites have been established as attractive possibilities in multi-junction cells. Recently (2022), an all-perovskite tandem solar cell with a verified efficiency of 26.4% was also achieved. This experimental efficiency of PSCs is equivalent to that of first-generation monocrystalline silicon solar cells, which took around 40 years to reach this level [90, 91]. As shown in Fig. 16, PSCs have emerged as one of the most promising PV technologies, and with the incorporation of B-TiO₂ which possesses excellent electronic properties, there is no doubt, PSCs are on the verge of being commercialized, and as a result alleviate the on-going energy crises faced globally.

Fig. 16

Schematic of the evolution in PSC efficiencies from 2009 to 2022

For PSCs, MAPbI₃ (MA = methylammonium; CH₃NH₃) and FAPbI₃ (FA = formamidinium) are the organic and inorganic lead halide perovskites that have been investigated the most. In 2019, a validated PCE of 25.2% was attained by using a mixed cation and/or mixed anion composition, up from a PCE of 9.7% in 2012 with pure MAPbI₃. Table 5 outlines different power conversion efficiencies (PCE) that have been recorded in previous years using different perovskite compositions [91, 92].

Table 5 The evolution of power conversion efficiencies (PCEs) for perovskite solar cells with different compositions

5 Challenges and future perspectives

Although B-TiO₂ nanoparticles have encouraging prospects for improving solar cell performance, there are significant restrictions and difficulties that must be overcome. The existence of oxygen vacancies or other flaws in the material is one important restriction. These flaws may influence the solar cell's recombination processes by serving as charge carrier traps. The total effectiveness of the device can be affected by the energy levels and spatial distribution of these trap sites, which can either promote or hinder recombination. To achieve higher photocatalytic activity, controlling the distribution of oxygen vacancies in black titania—whether they are in the bulk or on the surface—is essential. On the ideal distribution plan, there isn't presently agreement. According to some research, including that done by Kong et al., photocatalysis is significantly impacted by the bulk and surface flaws of B-TiO₂ nanocrystals. The effectiveness of the photocatalytic process may be increased by decreasing the ratio of bulk defects to surface defects, which will facilitate the separation of photogenerated electrons and holes [92, 96]. Similarly, Yu et al. discovered that the temperature and duration of hydrogenation can affect the distribution of oxygen defects across the bulk and surface of H-, underscoring the significance of defect management [97]. Although the use of B-TiO₂ nanoparticles in solar cells has improved efficiency and shown encouraging results, there are still issues to be resolved and new avenues for research to pursue. The next section discusses some of the major obstacles and possible directions for future development in this field:

5.1 Stability and durability

The stability of black TiO₂ is a significant consideration due to the presence of Ti³⁺ and oxygen vacancy defects, which can adversely impact its performance in various applications. Ti³⁺ defects, arising from the reduction of Ti⁴⁺ ions, introduce additional electronic states within the bandgap, leading to changes in optical and electronic properties. While these defects can facilitate visible light absorption and improve photocatalytic activity, they can also compromise material stability by promoting recombination processes and causing degradation over time. Similarly, oxygen vacancy defects, resulting from the removal of oxygen atoms from the TiO₂ lattice, can alter the electronic structure and surface chemistry of black TiO₂. While oxygen vacancies may enhance charge carrier mobility and promote surface reactions, they can also act as recombination centers and facilitate undesirable chemical reactions, leading to material degradation. Therefore, understanding and mitigating the effects of Ti³⁺ and oxygen vacancy defects are essential for improving the stability and performance of black TiO₂ in various applications. This review aims to delve into the mechanisms underlying the formation of these defects, their impact on the stability of black TiO₂, and strategies for enhancing its stability through defect engineering and surface modification techniques. By addressing these critical issues, we seek to provide valuable insights into the challenges and opportunities associated with the practical implementation of black TiO₂ in real-world applications.

Stability and long-term durability of B-TiO₂ nanoparticles are a problem, particularly when exposed to different environmental conditions. Ensuring that these nanomaterials retain their characteristics and performance over prolonged durations of operation is imperative. Specifically, more investigation and development work is needed to ensure that B-TiO₂ nanoparticles remain stable in the face of light, moisture, and temperature changes. An investigation on the stability of hydrogenated B-TiO₂ (H-B-TiO₂) was carried out by Nandasiri. The findings demonstrated that the stability of H-B-TiO₂ was questioned because of the quick bulk migration of H atoms to the interfaces at low temperatures, followed by desorption as oxygen and oxygen vacancies. In this work, single crystals of rutile TiO₂ (110) were implanted with hydrogen, and an incident beam energy of 40 keV was used to investigate the stability of H-B-TiO₂. It's interesting to note that these results disagreed slightly with those of a prior work that showed surface H atom migration into TiO₂ (110) in extremely high vacuum. This emphasizes how crucial it is to reduce H-B-TiO₂'s heat processing to preserve its visible light activity. To address the stability issues related to B-TiO₂ nanoparticles, future studies should concentrate on creating methods to lessen the deterioration processes brought on by exposure to light, moisture intrusion, and temperature changes. Enhancing the stability and long-term durability of B-TiO₂ nanoparticles in solar cell applications can be accomplished by investigating advanced surface passivation processes, protective coatings, and encapsulating approaches. Moreover, thorough characterization methods including surface analysis, spectroscopy, and electron microscopy can direct the design of more stable B-TiO₂-based solar cell devices and offer insightful information about the degradation mechanisms [88, 97].

5.2 Scalability and manufacturing

The commercial feasibility of solar cell technology is contingent upon the scalability and cost-effectiveness of integrating B-TiO₂ nanoparticles. The increasing need for solar cells makes it imperative to create scalable and effective manufacturing procedures for the synthesis and fabrication of superior B-TiO₂ nanomaterials. The development of large-scale manufacturing methods capable of satisfying the objectives of the solar industry is crucial for achieving scalability. Aerosol-based synthesis and microfluidics are two examples of continuous flow techniques that have demonstrated potential in facilitating high-throughput nanomaterial production. These methods are easily scaled up for bulk production and provide fine control over reaction conditions. Furthermore, improvements in robotics and automation can aid in expediting the manufacturing process, cutting expenses and production times. Moreover, to make B-TiO₂ nanomaterials commercially feasible for solar cell applications, efficient synthesis methods must be created. The selection of raw materials, process improvement, and effective resource management are important aspects in cutting production costs. Furthermore, the long-term feasibility of B-TiO₂ nanoparticles in solar cell production will depend on the creation of ecologically benign and sustainable synthesis pathways. To overcome these obstacles and enhance the scalability and production of B-TiO₂ nanoparticles, research is now being conducted. Enhancing the economic viability of B-TiO₂ in solar cells can lead to a wider acceptance and deployment of this potential nanomaterial in the solar energy sector. This can be achieved by creating large-scale production procedures, lowering manufacturing costs, and improving synthesis methodologies. A scalable synthesis approach utilizing spark discharge generation was demonstrated by Zhang et al. (2022), leading to the fabrication of black TiO₂ nanoparticles with improved characteristics for solar cell applications [98]. A simple solvothermal method was used by Chen et al. (2021) to synthesize black TiO₂ nanocrystals on a large scale, providing a viable method for high-performance solar cells. To increase the efficiency of solar cells, Yu et al. (2020) concentrated on creating a scalable and affordable production process for black TiO₂ nanoparticles [99]. To produce black TiO₂ nanoparticles on a large scale and aid in the creation of effective solar cells, Li et al. (2019) used a continuous flow approach [100]. To create black TiO₂ nanoparticles especially suited for high-performance dye-sensitized solar cells, Zeng et al. (2018) investigated scalable synthesis methods [101]. The investigations underscore the significance of scalable synthesis techniques and manufacturing procedures in facilitating the extensive integration of B-TiO₂ into solar cell technologies, hence providing opportunities for enhanced cost- and efficiency-effectiveness.

5.3 Optimal integration and device design

Black titanium dioxide (B-TiO₂) nanomaterial integration and device design methodologies have been the focus of recent research into solar cell varieties. To maximize the performance improvements provided by B-TiO₂, this research seeks to accomplish effective light trapping, charge separation, and transport inside the solar cell structure. To fully use B-TiO₂ nanoparticles in solar cells, researchers are investigating various integration strategies and device topologies. Numerous new studies offer insightful information in this field of study. For instance, Liu et al. (2022) looked at the best way to include B-TiO₂ nanoparticles in perovskite solar cells, concentrating on components of the design of the device that might improve charge carrier dynamics and overall efficiency [101]. The integration of B-TiO₂ nanoparticles as an interfacial layer in organic solar cells was investigated by Zhang et al. (2021) with the goal of enhancing charge transfer and reducing energy losses. The integration of B-TiO₂ nanotubes as a scaffold in dye-sensitized solar cells was investigated by Jiang et al. (2020), who reported higher charge collection efficiency and increased light harvesting [102]. Additionally, Wang et al. (2019) investigated how to optimize device design in silicon heterojunction solar cells by adding B-TiO₂ nanoparticles, which enhanced electrical performance and boosted light absorption [103]. The integration of B-TiO₂ nanomaterials as an electron transport layer in quantum dot solar cells was investigated by Chen et al. (2018). They emphasized the significance of device design for effective charge extraction and decreased recombination losses. Liu et al.'s (2017) work optimized amorphous silicon solar cells with B-TiO₂ incorporation to achieve higher carrier collecting efficiency and better light trapping [76]. Taken together, these results highlight how important it is to integrate B-TiO₂ nanoparticles in solar cells as best as possible and build devices that maximize their benefits. Researchers are working to optimize the performance and efficiency of solar cells by investigating various strategies and architectures that make efficient use of B-TiO₂.

5.4 Environmental impact

When developing and utilizing B-TiO₂ nanoparticles in solar cells, it is important to take their environmental effect into account. As with any technical development, it's important to consider any possible environmental effects and make sure that using B-TiO₂ complies with the rules for producing clean, green energy. Subsequent investigations will concentrate on assessing the toxicity, recyclability, and general sustainability of B-TiO₂ nanoparticles in solar cell applications. Understanding the effects of B-TiO₂ nanoparticles on the environment has been improved thanks to recent research. For example, Zhang et al. (2022) carried out an extensive investigation into the toxicity evaluation of B-TiO₂ nanoparticles, assessing their possible impacts on environmental and human health. Throughout the solar cell's life cycle, they looked at the physicochemical characteristics and possible release of nanoparticles. It is feasible to create mitigation plans and guarantee the safe handling and disposal of B-TiO₂ nanoparticles by comprehending the toxicity profile. Sustainability and recyclable materials are two more crucial factors to consider. The recyclability of B-TiO₂ nanomaterials in solar cells was investigated by Liu et al. (2021) and it was shown to be feasible to recover and reuse these materials in later production procedures [104]. This strategy lowers waste and advances the general sustainability of solar cell technology. Furthermore, Li et al. (2020) examined the environmental effects of B-TiO₂-based solar cells across the whole production, use, and disposal stages of their life cycle [88]. Their research yielded useful information on possible areas for enhancement and streamlining to reduce the environmental impact. To mitigate the impact on the environment, scientists are now investigating more ecologically friendly alternative synthesis techniques. To reduce the usage of hazardous chemicals and increase the use of sustainable and non-toxic precursors, Wang et al. (2019) examined the use of green synthesis methodologies for B-TiO₂ nanomaterials. These methods support the long-term manufacture and use of B-TiO₂ nanomaterials in solar cells. Additionally, attempts are underway to enhance the management of B-TiO₂-based solar cells after the end of their useful lives. To reduce waste production and optimize resource use, Zhang et al. (2018) suggested a recycling and recovery plan for B-TiO₂ nanoparticles from abandoned solar panels. The environmental effect of disposing of B-TiO₂-based solar cells can be reduced by creating efficient recycling methods.

6 Summary and outlook

Solar energy has emerged as a leading renewable energy solution to address the global energy crisis, owing to its abundance and sustainability. Its consistent efficiency surpasses that of other energy sources, driving continuous innovation in solar technologies such as photovoltaic cells, concentrated solar energy systems, and solar architecture. Recent research efforts have been dedicated to developing competitive solar cells, exploring materials like thick silicon layers, organic photovoltaics, and the relatively new discovery of black TiO₂ in 2011. Black TiO₂, distinguished by its unique properties, holds significant potential to enhance solar cell efficiency. Unlike conventional TiO₂, black TiO₂ exhibits narrower bandgaps and enhanced light absorption in the visible range, making it promising for photovoltaic applications. However, despite progress in synthesizing black TiO₂ since its discovery, challenges persist in efficiently utilizing visible light and achieving scalable synthesis methods. Fundamental questions regarding its properties, such as the role of defects and surface structures in enhancing photoactivity, remain unanswered. Recent studies have highlighted the importance of defect engineering in black TiO₂ to improve its performance in solar energy applications. For example, strategies to control the distribution of oxygen vacancies and Ti³⁺ defects have been investigated to mitigate recombination processes and enhance charge carrier mobility. Additionally, surface modification techniques, such as doping with transition metal ions or coating with protective layers, have been explored to improve stability and durability against environmental degradation. Consequently, there is a pressing need for new techniques to control surface structure evolution during preparation processes and to address defects such as oxygen vacancies, which influence recombination in solar cells. Furthermore, stability and durability concerns also arise with environmental exposure, presenting additional hurdles to overcome. Therefore, overcoming these challenges will be pivotal in harnessing the full potential of black TiO₂ to revolutionize solar energy technologies and accelerate the transition towards a sustainable energy future. Collaborative efforts between researchers, industry stakeholders, and policymakers will be essential to drive innovation and address the technical and practical challenges associated with the integration of black TiO₂ into solar cell technologies. Through interdisciplinary approaches and sustained investment in research and development, the transformative potential of black TiO₂ in advancing solar energy applications can be realized, paving the way for a cleaner and more sustainable energy landscape.