Keywords

1 Introduction

Highly parallel systems such as neuronal networks combine processing and memory at the local synaptic level and are capable of completing complex tasks like pattern separation and completion with outstanding efficiency [1]. Neuromorphic engineering takes inspiration from neural networks, pursuing the aim of developing novel, neuron-inspired, efficient computing approaches. Memristive devices, due to their capability of unifying processing and data storage, have been studied broadly in the context of the development of novel computing paradigms and in the field of neuromorphic engineering [2]. Commonly, memristive devices are applied as an electrical representation of synapses and arranged in crossbar arrays, which enable interesting applications such as in-memory computing [3]. In the field of neuromorphic engineering however, new paradigms are necessary to capture the entirety of signal processing in neural networks [1, 4]. One important aspect in neural networks is their capability of dynamic on-demand reconfiguration of synaptic connections between the individual neurons. This is impressively shown by the onset of synaptic pruning, which describes the decrease of the number of connections between neurons in a fully developed brain. Accordingly, the development of biological neural networks happens on two different time scales: The fast synaptic plasticity (at the level of local synaptic connections between two neurons) and the slow blooming and pruning (at the global level throughout the neural network). While the first aspect, the synaptic plasticity, has attracted considerable research interest and memristive devices have been readily applied to mimic a variety of synaptic properties, the latter aspect of global plasticity is still under research and novel approaches are necessary to incorporate it into future bio-inspired hardware [4,5,6]. First approaches to replicate the global interactions of neuron assemblies are considering global connectivity through electrolyte gating, thus in a liquid medium [7]. In this chapter, metal line formation on a templated substrate from a liquid phase is discussed as an interesting approach with the potential to pave the way to achieve long-range, global plasticity.

The formation of a metallic lines on a template substrate requires a method that allows the formation of well-defined, localized metal structures. An effective approach to creating lines with high accuracy is the fabrication by localized reaction on a pre-structured substrate. A suitable compound is needed that can react in an aqueous medium to cause deposition of the metal atom in such a manner to have a localized growth of the metal on the surface of a template. TiO2 is a metal oxide which is known for its photoreduction ability and high photocatalytic property. In the presence of UV light, photoreduction of an electro positive metal ion will occur when TiO2 is present in the medium. For example, Dawson et al. showed that HAuCl4 can be reduced on surface of TiO2 nanoparticles to form gold capped nanocomposites with TiO2 core [8]. It was discussed that the concentration of the TiO2 core influenced the nanocomposite size and stability. Having a thin film of pre-structured TiO2 on a surface could serve as a base for gold nanoparticle growth when placed in HAuCl4 aqueous medium. This process is based on photocatalytic reduction of the gold precursor ions by TiO2 which would then result in the immobilization of the Au nanoparticles on areas patterned with TiO2 forming a structured metallic growth on the substrate. Thus, understanding the photocatalytic process involved is thus vital.

Semiconductor-based photocatalysis covers a wide range of cutting-edge applications in some important areas such as energy, environment, hygiene and disinfection [9, 10]. Principally, photocatalysis is described as the integration of photoexcitation and surface catalysis. The photoexcitation contains light absorption and charge output, while surface catalysis concerns the utilization of photoinduced charge carriers (electron (e−) and hole (h+)) for reduction or oxidation reactions [11]. This process is highly dependent on various parameters, including the photocatalytic activity of semiconductors, light wavelength, and intensity as well as reaction temperature [12]. Recently, semiconductor-based photocatalysts (TiO2, ZnO, WO3, etc.) have gained considerable attention due to their high potential for solving environmental and energy problems [13]. However, these photocatalytic materials can be also considered to easily synthesize metallic or metal oxide structures (forming hybrid structures) for different kinds of applications (hydrophobic and hydrophilic coatings, sensor devices, reconfigurable connections, etc.) rather than only using them for environmental and energy applications [14].

In the recent decade, titanium (IV) oxide (TiO2) is one of the most widely studied semiconductor photocatalysts due to its significant properties such as its strong photo-oxidizing power, non-toxicity, very good stability over a wide range of pH, chemical inertness, low-cost preparation as well as photocatalytic properties resulting from its bandgap energy and positions of the conduction band (CB) and valence band (VB) [15,16,17]. An overall mechanism for the photocatalytic reaction on TiO2 surface is categorized as two main processes such as (i) the reduction of oxygen molecules (O2) and (ii) oxidation of water molecules (H2O) by photoinduced charge carriers (e− and h+) [18]. Initially, the photocatalytic reaction is started when a photoelectron is excited from the filled VB of TiO2 to the empty CB as a result of irradiation. The absorbed photon must have the energy either equal or greater than the bandgap of TiO2 (3.0–3.2 eV) to excite the electron in the VB. Then, the excitation process leaves behind a hole in the VB [19]. Here, as a net result, electrons (e−) and holes (h+) pair are generated by the Eq. (1) below.

$${\text{Ti}}{{\text{O}}}_{2}+{\text{hv}} \left({\text{UV}}\right)\to {\text{Ti}}{{\text{O}}}_{2}({e}^{-}\left({\text{CB}}\right))+{\text{Ti}}{{\text{O}}}_{2}({{\text{h}}}^{+}\left({\text{VB}}\right))$$
(1)

Afterward, the photogenerated holes at the VB react with a water molecule to generate hydroxyl (OH*) radicals. These hydroxyl radicals are extremely powerful oxidizing agents, which attract adsorbed organic molecules to mineralize them depending upon their structure and stability level [19]. While the photogenerated holes react with water to generate the hydroxyl radical, at the same time, the electrons in the CB are taken up by oxygen (O2) molecules to produce anionic superoxide radical (O2−*). These superoxide radicals are formed into hydroperoxyl (HO2*) radicals and the subsequent hydrogen peroxide (H2O2), which further dissociates into highly reactive hydroxyl (OH*) radicals. All these oxidation and reduction processes commonly take place on the surface of the TiO2 photocatalyst [19]. Therefore, the surface area of the TiO2 is highly important to achieve the high photocatalytic activity for desired application.

Recent studies have been revealed that the particle size of TiO2 plays a crucial role in photocatalytic activity [20]. Decreasing the particle size to the nanoscale (increasing active surface area) leads to a higher photocatalytic activity. However, the conventional nano-powdered photocatalysts need post-treatment separation in a slurry system after the photocatalytic reaction. This problem can be overcome by immobilizing TiO2 particles as a robust and stable thin film on solid surfaces [21, 22]. However, thin films have a limited surface area in comparison to nanoparticle systems and they show limited photocatalytic activity, which has to be improved by further strategies such as (i) tailoring morphology and crystal structure, (ii) decorating/depositing noble metals (Au, Ag, Pt, etc.) and (iii) coupling metal oxide semiconductors (ZnO, CeO2, etc.) [23].

Noble metal nanoparticles (Au, Ag, Pt, etc.) deposited on semiconductor surfaces have been extensively studied due to their unusual (photo)catalytic and optoelectronic properties [24] as well as their potential to tailor the sensor properties of semiconducting metal oxide gas sensors 11. In particular, TiO2 decorated by noble metal nanoparticles like Au and Ag has received more and more attention because of its significant improvement in the photocatalytic activity of TiO2 for specific applications [22]. The size and distribution of these noble metal nanoparticles are the most effective parameters on the activity of photocatalyst [24]. Additionally, the photocatalytic performance of TiO2 modified by noble metals highly depends on some external factors such as strong contact with metal and support [25]. Noble metals can be deposited on TiO2 surface by various methods including sputtering, physical mixing, chemical reduction, electrodeposition, photocatalytic reduction, and so on. However, among the mentioned methods, the photocatalytic deposition (sometimes it is called photodeposition or photoreduction) is a facile, simple, and low-temperature process with efficient metal-support interaction especially for TiO2 thin film as support [26].

The photocatalytic deposition method is based on the photocatalytic properties (bandgap, activity, etc.) of semiconductor materials. Generally, several conditions are needed to allow the deposition of metals on the semiconductor surface. First, the photon energy of the light (solar or artificial light) should be larger than the energy bandgap of the semiconductor. Mostly, photocatalytic deposition of metal to TiO2 employs under artificial ultra-violet (UV) light illumination due to TiO2’s wide band gap, which is around 3.0 eV. When the light has proper energy, the light is absorbed by a semiconductor, and electrons in its VB are excited to its CB. These electrons are used for the reduction process. Second, the CB energy level of the semiconductor should be more negative than the reduction potential of the metal ion (M+). Third, the efficient charge (e− and h+) separation and migration, by electron donor, is necessary to continue the reaction [27]. Finally, the semiconductor acts as a template for metallic structures as shown in Fig. 1.

Fig. 1
figure 1

Schematic illustration for photodeposition of metal structures on semiconductor surface (M: Metal and D: Electron donor)

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The depositing of noble metals on semiconductor surface by photocatalytic deposition is possible, if all conditions, which are mentioned above, are available. However, various parameters are also needed to be considered to allow the well-controlled (precise shape, size, and distribution) photodeposition process to occur on semiconductor surfaces such as sacrificial reagent, pH, temperature, metal precursor, light exposure time and intensity, absence, or presence of oxygen in the media [14, 28, 29].

In this chapter, photocatalytic deposition is discussed as an approach to form metallic structures from a global liquid reservoir. In this context, the photocatalytic properties of TiO2 thin films are employed to reduce metallic species from the sur-rounding solution. This chapter will elucidate the fundamental process of photo-reduction with photocatalytic TiO2 thin films and will showcase the applicability towards the formation of metallic structures at the example of arrangements of locally grown metallic Au structures.

2 Reactive Sputtering of Photocatalytic TiO2 Thin Films

TiO2 is typically characterized as a n-type semiconductor and has three crystalline phases such as anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic) [30]. In general, anatase shows higher photocatalytic activity than rutile and brookite due to mainly its indirect bandgap structure (low electron–hole recombination rate). Therefore, most of the researchers focused on preparing of mainly anatase TiO2 structures for achieving high photocatalytic activity [31]. However, recent studies have shown that the mixture of anatase and rutile has a synergetic effect on decreasing the recombination rate of photogenerated electron–hole pairs, which boosts the overall photocatalytic performance [32].

As mentioned before, TiO2 is extensively employed in various applications due to its unique optical, electronic, and photocatalytic properties. In these applications, it is mainly used as powder form (nanoparticles around 25 nm size), which shows high photocatalytic activity results from the high surface area. However, the separation of these tiny particles from the reaction media after the end of the reaction is highly difficult with conventional filtration systems [33]. Recently, TiO2 thin films have gradually replaced conventional TiO2 powders due to separation and agglomeration problems, especially for their long-term use.

Various deposition methods have been utilized to prepare TiO2 thin films such as sol–gel dip coating, sol–gel spin coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), electron beam, spray pyrolysis method [34]. Among these deposition methods, PVD methods have been extensively used because they can easily control the composition and morphology of the prepared thin film. Various PVD methods are being utilized for the deposition of TiO2 thin film such as thermal evaporation, pulsed laser deposition, DC, and RF magnetron sputtering [35]. DC magnetron sputtering is one of the most preferred methods because of its good adhesion, high deposition rate, high quality, and uniform film [36]. In this deposition method, there are many vital parameters (sputtering power, distance between substrate and target, argon/oxygen ratio, working pressure, substrate temperature, and annealing treatment) to optimize the quality of the thin film. However, it is still a challenge to achieve highly photocatalytic active TiO2 thin films, which compete with TiO2 nanoparticles, due to their limited surface area [33]. To achieve a high surface area, nowadays an effective process is established, which is called as nano-crack network formation, within sputter-deposited TiO2 thin films. For example, Henkel et al. reported on thermally controlled nano-crack formation as a possible method to improve the photocatalytic performance of well adhering, reactively DC sputtered TiO2 thin film [37]. Networks of nanoscopic cracks were produced into tailored columnar TiO2 thin films by thermal annealing as shown in Fig. 2.

Fig. 2
figure 2

SEM images of sputtered TiO2 thin films, a before and b after thermal annealing (at 650 °C for 1 h), important morphological features are marked by red and blue lines. (Printed with permission from [37]. Copyright (2018) IOP)

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It can be easily seen from Fig. 2a, irregular and narrow columns (underlined by blue and red marks, respectively) dominate the formation of the film at the TiO2-substrate interface. According to the author’s observations, broader columnar structures start to grow at a distance of 200 nm from the substrate and become more dominant features at TiO2 thin film surface. However, after thermal annealing, the deep trenches separate into small bundles of TiO2 columns, which enhance the overall photocatalytically active surface area (Fig. 2b). Additionally, they also reported that the thermal annealing step (at 650 °C for 1 h) is highly crucial not only for the formation of the nano-crack network but also the transformation of amorph phase (as-deposited TiO2) to the mixture of anatase and rutile phase (anatase is more dominant), which is extremely beneficial for enhancing photocatalytic activity. Similarly, Zhan et al. observed that the correlation between annealing temperature and phase transformation of TiO2 from amorph phase to anatase (>300 °C), partially rutile (around 600–800 °C), and fully rutile phase (at 1000 °C) [34, 38].

Nano-crack network formation is a promising approach to achieve a high surface on the thin film surface. It can be one of the biggest features on competition between thin film and nanoparticles photocatalyst. Ghori et al. demonstrated that sputter-deposited TiO2 film with nano-crack networks shows extremely high photocatalytic activity in comparison to a reference TiO2 thin film prepared by immobilized TiO2 (Degussa P25) nanoparticles [21]. Here, the authors reported that sputtering of TiO2 at high oxygen partial pressure and low deposition rate leads to the formation of high aspect ratio structures with weak inter-columnar bonding, which can be transformed to a crack network by thermal annealing process (Fig. 3a).

Fig. 3
figure 3

SEM images of a sputter deposited TiO2 film after thermal annealing and b spin-coated reference TiO2 film. (Insets show cross-sectional SEM micrographs) c Normalized degradation (plot) of MB solution in the presence of sputter-deposited and reference TiO2 thin films under UV illumination. (Quartz substrate (given as blank) is used as the control substrate). (Printed with permission from [21]. Copyright (2018) Elsevier)

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As a comparison, they prepared the reference thin film by spin-coating. The prepared thin film has a porous structure composed of commercial spherical TiO2 nanoparticles, which are well distributed and maintained their original size (25–30 nm) and shape (Fig. 3b). The thickness of the sputter-deposited and the reference TiO2 thin films are around 400–450 nm for a reliable comparison. The photocatalytic activity of both TiO2 thin films was investigated by degradation of methylene blue (MB) aqueous solution under UV illumination. The results showed that the sputter deposited TiO2 film degrades 90% of MB in 4.95 min while the reference TiO2 thin film needs about 25 min (Fig. 3c).

The thermal annealing process is an easy and reliable process for phase transformation and formation of a nano-crack network for better photocatalytic performance as aforementioned. However, some additional process parameters can be also controlled for high photocatalytic performance. Generally, crystalline TiO2 thin film is deposited on the substrate at low operating temperatures. Here, the crystallinity of TiO2 thin film can be mainly tuned by changing total pressure and oxygen partial pressure [39]. The total gas pressure affects the kinetic energy of sputtered atoms, which increases the probability of collisions and acceleration of particles and subsequently the particle energy [40]. Therefore, the anatase phase forms at high total pressure when the rutile phase forms at low total pressure [40, 41].

During magnetron sputtering, argon (Ar) and oxygen (O2) are used as the plasma gas and the reactive gas, respectively. Oxygen plays a significant role in the deposition of TiO2 thin film via magnetron sputtering especially for reactive sputtering. Many researchers have pointed out that the oxygen concentration during the magnetron sputtering might affect the formation of oxygen vacancies in the deposited thin film [41]. Additionally, the oxygen partial pressure (the oxygen/argon ratio) has a huge effect on the discharge parameters, such as plasma potential, discharge voltage, and ion composition of discharge [42]. For example, Zhang et al. reported that the discharge voltage increases with increasing oxygen flow rate up to a certain threshold. Before the threshold is reached, a mixture of metallic Ti and TiO2 is deposited on the substrate. The uniform TiO2 thin film forms on the substrate only after the oxygen flow rate reaches this certain threshold [34, 43].

Although numerous attempts have been performed to investigate the relationship between the oxygen/argon ratio and photocatalytic activity of TiO2 thin film, no clear agreement has been reached so far. However, a lot of properties of TiO2 thin film are affected by oxygen/argon ratio such as deposition rate, grain size and surface roughness, surface chemical composition, and optical parameters, which might directly or indirectly influence the photocatalytic activity [44, 45]. For example, Vahl et al. reported the availability of oxygen during reactive sputtering is a vital parameter for tailoring of thin film morphology for high photocatalytic performance (Fig. 4) [46].

Fig. 4
figure 4

Top-view SEM images of TiO2 thin films for different oxygen/argon flow ratios, before (top row) and after (bottom row) thermal treatment (Printed with permission from [46]. Copyright (2019) IOP)

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The authors reported that the formation of nano-crack networks is more noticeable for TiO2 thin films deposited at higher oxygen/argon ratios. The reason for this might be the presence of crystalline anatase seeds for high oxygen partial flows. These seeds grow by thermal annealing process more effectively as a well-defined nano-crack network. Here, the annealed TiO2 thin films deposited different oxygen/argon ratios showed different activity on degradation of MB under UV illumination. After 7 h UV illumination, the TiO2 thin film deposited at 3% oxygen/argon ratio degraded almost 81.2% of MB when this value is 96.1% for TiO2 thin film deposited at 6% oxygen/argon ratio.

3 Deposition of Metal Structures from Photocatalytic Reduction

As discussed in the previous chapters, various strategies have been proposed to improve the volume to surface area ratio of TiO2 thin films for a high photocatalytic activity. Additionally, TiO2 thin film incorporated with noble metal nanoparticles can be considered another strategy to enhance the (photo)catalytic properties by means of tuning the Fermi level of TiO2 and acting as an electron sink [47]. The prepared noble metal/TiO2 hybrid structure has a huge potential on not only environmental applications but also different kinds of applications including hydrophobic and hydrophilic coatings, sensor devices, reconfigurable connections, etc. [13].

In the literature, there are numerous methods, which can be used to synthesize the noble metal/semiconductor nanocomposites such as hydrothermal, droplet, precipitation, electrochemical methods, and so on [27]. The photodeposition is one of the simplest methods to prepare the noble/semiconductor hybrid structure by using only the bandgap of the semiconductor [48]. This method needs only the irradiation of a light source (artificial or sunlight) rather than requiring additional reducing agents, high temperature, multi-step processing, etc. [49]. Furthermore, the photodeposition method is easily applicable to 2D substrates (such as thin film) by adjusting the concentration of the (noble)metal precursors, irradiation intensity, and duration [50]. For example, Mendoza-Diaz et al. demonstrated that Au nanoparticles were deposited on columnar TiO2 structure by photodeposition process under UV irradiation (365 nm, 100 W, 30 min) by using an Au precursor solution (Fig. 5) [51].

Fig. 5
figure 5

(Left) Schematic illustrations of TiO2 and TiO2/Au hybrid structures. (Middle) SEM images of the sample surface area. (Right) HAADF-STEM cross-sectional images: a TiO2 thin film; b Au-on-TiO2 (inset: Au NP size dispersion in yellow). Letter C corresponds to the top layer of protective sputtered carbon. (Printed with permission from [51]. Copyright (2020) American Chemical Society)

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It can be easily seen from the SEM images in Fig. 5a, the TiO2 thin film has a columnar morphology with grain sizes from ∼20 to 50 nm. This columnar morphology is generally observed in TiO2 films, which are deposited by chemical and physical vapor deposition techniques [52, 53]. For the Au-on-TiO2 sample, Au nanoparticles were deposited on top of the TiO2 thin film surface (Fig. 5b). The photodeposited Au nanoparticles have a spherical shape with a size range of 30−70 nm (inset in Fig. 5b). The authors reported that the size dispersity of the photodeposited nanoparticles is closely related to the geometrical restraints enforced by the roughness of the columnar TiO2 structure [51].

Similarly, Veziroglu et al. showed that not only the TiO2 morphology but also the composition of the precursor solution has a significant effect on the size and distribution of the photodeposited Au structure as demonstrated in Fig. 6 [28]. Figure 6a shows that only a few Au nanoclusters (surface coverage of 9.2%) were photodeposited on TiO2 surface water was used as a solvent. However, densely distributed Au nanoclusters (surface coverage of 51.6%) on TiO2 were obtained when a mixture of 1-hexanol−water (v/v: 20/ 80) was used (Fig. 6d).

Fig. 6
figure 6

Helium ion microscopy (HIM) images of photodeposited Au nanoclusters onto TiO2 thin film surface in a water, b acetone−water (v/v: 20/80), c isopropanol−water (v/v: 20/80), and d 1-hexanol−water (v/v: 20/80) mixtures. e Surface coverage (%) of photodeposited Au nanoclusters with different solvents. (Printed with permission from [28]. Copyright (2020) American Chemical Society…)

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Here, one can easily see differences in morphologies given in helium ion microscopy (HIM) images (Fig. 6a−d), the solvent type (or composition of the precursor solution) significantly affects the final morphology of the photodeposited Au nanoclusters. Because, during the photodeposition process, the TiO2 thin film contact with the Au precursor solution, which forms the solid−liquid interface. This interface plays a crucial role in the electron transferring from a photocatalyst surface to electron-acceptor species (e.g., Au3+ ions) in the solution. When the composition of the precursor solution changes, the distribution of the electrical potential also changes [54]. This phenomenon creates various pathways to reduce Au3+ ions on TiO2 as metallic Au nanoclusters, which form different shapes, sizes, and distribution profiles.

Similarly, the pH of the precursor solution also affects the charge distribution in the solid–liquid interface. Unfortunately, according to the author's best knowledge, there is not any systematic research related direct effect of the pH on photodeposited Au structure especially on TiO2 thin film surface, yet. However, similar studies can give us an overall understanding of the pH effect on the photodeposited particles. For example, Guo et al. reported that pH plays a significant role in the nucleation position of the photodeposited Au nanoparticles on to plate-like BiOBr [55]. It can be seen in Fig. 7, SEM images after the photodeposition process show facet-selective deposition of Au particles with pH-dependency. At pH 3, they observed an overall deposition of Au nanoparticles, favorably on the top ([001] facet) as shown in Fig. 7a. However, when increasing the pH of the precursor solution to 3, 5, and 9, the pattern of the photodeposited nanoparticles gradually changes (Fig. 7a–c). For example, Au nanoparticles are founded on every facet ([001] and [102], top and side, respectively) at pH 5 while only on the side ([102] facet] at pH 9.

Fig. 7
figure 7

SEM images of BiOBr showing pH-dependent geometrical deposition of Au nanoparticles. Au/BiOBr at a pH 3; b pH 5; c pH 9. The scale bar always corresponds to 200 nm. (Printed with permission from [55]. Copyright (2018) Royal Society of Chemistry)

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The authors reported that only at a low pH value, the charge density of the semi-conductor and precursor interface is weak. Therefore, [001] facets are weakly positively charged, which might significantly reduce the recombination of the photoinduced charge carriers (electron and holes). This phenomenon may enhance the electron/ hole mediated reaction on the semiconductor surface during the photodeposition process. However, at a high pH value, all facets are rather strongly negatively charged. Therefore, electrons are no longer attracted towards the [001] facet surface. They are no longer available for the reduction of Au ions on the surface. Hence, the facet-selectivity of the photodeposition reaction of Au ions to metallic Au nanoparticles reverses from the [001] facet to [102] facet at a high pH value [55].

As already mentioned, the composition and pH of the precursor solution have a huge effect on the size, distribution, and position of the photodeposited nanoparticles on the semiconductor surface due to various charge distributions in the solid–liquid interface during the photodeposition process. Additionally, the photocatalytic activity of the semiconductor affects the final morphology of the deposited particles on the surface because it limits how many electrons can be generated by the photocatalyst under light illumination. For instance, Veziroglu et al. demonstrated that hierarchical Au needle clusters (HAuNCs) were deposited on a highly active TiO2 thin film surface via UV illumination (Fig. 8a–b). Here, the size and the geometry of deposited HAuNCs were controlled by simply altering the photocatalytic activity of the TiO2 (depends on the crystal structure), UV light intensity, and irradiation time [14]. The basic growth mechanism of a HAuNC is semantically shown in Fig. 8c. First, the photoinduced charge carriers (electrons and holes) in TiO2 thin film are generated by UV illumination. These electrons reduce the Au3+ ions into a stable Au cluster. This first step is almost same with all conventional photodeposition process for different kind of metallic structures [56]. However, a high density of electrons can be generated by the prepared highly photocatalytic active TiO2 thin film. Therefore, the relative decrease of Au3+ ions on the surface, where the first embryonic Au cluster nucleate, occurs in a very short time. This promotes the diffusion of more Au3+ ions (from higher to lower concentration regions), which is followed by preferential piling of Au nanoclusters into needle-like structures (Fig. 8a–b). It seems that the high density of electrons triggers the directional growth of Au nanostructures (Growth). These dense electrons, which are generated by TiO2 thin film, can be trapped by former Au nanoclusters (nuclei) and they act as the further nucleation sites for incoming Au3+ ions from the precursor solution (Needle formation).

Fig. 8
figure 8

a SEM image of HAuNCs deposited on TiO2. b HIM image of a single HAuNC. c Schematic representation of HAuNC growth mechanism. SEM images of d periodic HAuNC arrays and e a single HAuNC. (Printed with permission from [14]. Copyright (2020) Wiley)

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Additionally, the authors reported that UV light intensity plays a major role in the final. size and sharpness of the deposited Au structures during the deposition process. Because the UV light intensity also decide the number of photogenerated electrons by TiO2, which leads to quick nucleation and a high growth rate. This is a piece of key information about controlling the nucleation point during the photodeposition process. For this purpose, they used the polymer mask between the light source and the sample. Here, patterned HAuNC structures on TiO2 thin film surface were obtained by selective light illumination as shown in Fig. 8d–e. These patterned HAuNC structures find some applications especially for catalysis, plasmonic, and biomedical technologies.

4 Metal Line Formation by Photocatalytic Reduction

As discussed in the previous section, localized irradiation of a continuous TiO2 thin film with the use of a shadow mask results in the formation of metallic nano- and microstructures from the precursor solution. In the following, a second approach to obtain localized formation of metallic structures will be discussed. In essence, this approach is based on the image reversal lithography process. The general process scheme is depicted in Fig. 9. The process is similar to the positive resist lithography except that in this case, a chemical modification step is included to ensure the cross linking of the resist. Also, the process involves a double exposure of the resist which causes the positive resist to act like a high-resolution negative resist.

Fig. 9
figure 9

An illustration of the major steps in the image reversal process necessary to obtain structures with high resolution

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The procedure involves: cleaning and priming of the glass wafer, spin coating of the resist, soft baking (110 °C for 50 s), exposure, hard bake (120 °C for 2 min), flood exposure, development, inspection, TiO2 deposition, lift off, final inspection.

The image reversal technique is preferred because the idea is to have the reversed structure of the mask transferred on to the glass substrate. The TiO2 which is then later deposited directly on to the glass surface takes the pattern of the mask while everywhere else is covered with resist. After the lift off (using acetone), only the structured TiO2 (which is unaffected by the acetone) remains on the substrate. With this approach, very thin lines of TiO2 are transferred to the glass substrate. It allows for fabrication of quite small structures on a substrate. For example, TiO2 lines of a few microns in width is structured on a glass substrate with sufficient accuracy.

The substrates with lithographically structured TiO2 thin films are consecutively used as substrates for photocatalytic deposition of metallic gold from precursor solution. For in-situ monitoring, the absorption and reflection of the photocatalytically grown gold on the titanium dioxide surface is taken as a measure. A standard transmission microscope (Leica DMi8) setup is used with an external UV-LED to stimulate the photocatalytic process as shown in Fig. 10.

Fig. 10
figure 10

Transmission microscope setup for in-situ monitoring of photocatalytic gold growth. Photocatalytic gold growth is excited by an external UV-LED. A standard microscope setup (Leica DmI8) with bright-field illumination is used to track the growth by recording images during the growth process

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A spectrograph extension of the setup is used to record the bright-field and UV-LED spectra. The UV-LED has its peak at approximately 365 nm, while the bright field source has no energy in the UV regime and is thus suitable for the in-situ monitoring. The intensity of the UV-light is controlled by the distance to the sample and was determined with a power meter (Newport 2936-C) for different distances. Samples are positioned in a beaker filled with HAuCl4 solution. Images are taken in regular intervals and are computed to obtain growth dynamic graphs of the process.

In a first experiment Au is grown using a 0.736 10−3 mol/l HAuCl4 solution on a sample covered partially with a titanium dioxide layer for different illumination intervals. Figure 11 shows three example transmission images of the sample. On the left side there is no TiO2, while the right side is covered with titanium dioxide. Due to photocatalytic Au growth on the TiO2 the transmission intensity is reduced. The total normalized transmission was 0.72 after completion of the total duration of the experiment.

Fig. 11
figure 11

Bright-field microscope transmission images at three different times during photocatalytic gold growth for a sample half-covered with TiO2. The computation areas are depicted in the left picture. In the red reference areas, there is no TiO2 and negligible growth occurs. The blue areas are on the TiO2 and become darker as the gold layer on the surface increases during growth. The normalized transmission on the titanium dioxide goes down to 0.72 after two hours

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Within the MATLAB environment different areas of the images are selected and evaluated over time. First the RGB channels as well as all pixels are summed up for these areas and are subsequently normalized to the first pictures, which are regarded as full normalized transmission for every measurement. The normalization accounts for different loss mechanisms such as thin-film interferences, absorption and scattering in the solution and the beaker. In the following, the three red reference areas and the three blue areas on the titanium dioxide are considered in more detail. The time evolution of the transmission for these six areas is depicted in Fig. 12. Time intervals of two minutes without illumination are followed by five minutes of UV illumination. Two illumination levels are employed–a lower excitation intensity of 1.8 mW/cm2 for the first two intervals and a higher intensity of 2.7 mW/cm2 for the third illumination phase.

Fig. 12
figure 12

Normalized transmission measurement of gold growth dynamics on titanium dioxide surface with different illumination powers. The image areas corresponding to the six timelines are indicated in Fig. 11. The blue lines show the transmission for areas on the titanium dioxide surface, the red lines are reference areas on the substrate. Different illumination intervals are depicted

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The results presented in Fig. 12 clearly show the suitability of the chosen method for monitoring the gold growth process. In the reference areas (red lines) almost no intensity change is observed indicating that no gold is deposited on the substrate. In the three areas on the titanium dioxide (blue lines) the change in transmission intensity is clearly visible. Furthermore, it is observed that the growth speed is controlled by the illumination intensity.

Figure 13 shows the growth results on a TiO2 line with 200 μm width. Three growth processes were conducted as depicted from stages (a–d). The first growth experiment (0.736 10−3 mol/l HAuCl4, 2.7 mW/cm2 UV intensity, 21 h) yielded only gold clusters and no full coverage. Repeating the experiment led to a full coverage of the surface, which was then destroyed by drying the sample with nitrogen as seen in Fig. 13c. To have full coverage the sample underwent the growth experiment a fourth time, showing fast and homogeneous growth over the titanium structure. As shown in the graph and Fig. 13d. We found that the growth process is faster and has a better coverage, when the growth is done repeatedly. This effect was also observable in other experiments within the same setup and is currently under investigation. A picture of the grown gold lines can be seen in Fig. 14. The linewidth varies from 10 to 500 µm.

Fig. 13
figure 13

Different stages of gold growth on a 200 µm TiO2 line. a shows the structure before growth, b, c and d after the first, second and third growth experiments. The graph depicts the normalized transmission measurement for the growth process between (c) and (d). The green lines are off structure reference spots, blue is on the connector dots (here to be seen just the left one) and red is on the line itself

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Fig. 14
figure 14

Reflection microscopy image of the grown Au lines with varying widths and lengths

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Figure 15 shows the results for grown metal lines with different line widths, ranging from 50 to 250 µm. The growth showed to be homogenous. Damages within the lines resulted from drying the sample.

Fig. 15
figure 15

Gold growth on titanium dioxide lines with different line widths: 50 µm (a, f), 100 µm (b, g), 150 µm (c, h), 200 µm (d, i), 250 µm (e, j)

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The samples were further characterized after the growth process with scanning electron microscopy (SEM) to get a better understanding of the layer composition and the coverage of the TiO2 with Au particles. After the first growth in HAuCl4 solution, sparsely distributed Au nanoclusters are observed with large spaces between the growing clusters (compare Fig. 16a). An explanation for this observation could be the rate of the electron transfer at the TiO2–solvent interface [28]. The rate of electron transfer is dependent on the potential gradient at the interface which could be inversely proportional to the dielectric constant of the liquid. For water, which has a high dielectric constant, a lower electron transfer rate is expected [28].

Fig. 16
figure 16

SEM-images with 8000 × times magnification of TiO2 with photocatalytic growth of Au clusters one time (a) and three times successively (b)

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After a second and third growth process on the same sample, the surface coverage of the Au nanoclusters increased drastically. It is possible that already deposited Au nanoparticles act as nucleation sites for aggregation, and the nanocluster will tend to grow in the more energetically favorable direction. The SEM image in Fig. 16b shows that large clusters have formed compared to the first growth, supporting the hypothesis.

It shows additionally that photocatalytic deposition of Au3+ still occurs on the surface without a nucleation source, leading to new formations of clusters. Their growth depends on how much of the TiO2–layer can be still excited by the UV-light despite the presence of Au particles. They reflect the light, which prohibits the generation of electrons in that area. The experiment shows that the surface coverage can be improved a lot by successive photocatalytic gold growth.

5 Conclusion

In this chapter the photodeposition of metallic nano- and microstructures is showcased at the example of locally grown metallic Au structures on photocatalytically active TiO2 thin films. Among the various deposition methods for TiO2 thin films, ranging from sol gel synthesis over evaporation to sputtering, in particular reactive DC magnetron sputter deposition has been found to offer the potential to tailor the morphology of the photocatalytic thin films. At the example of introducing nano-crack networks upon heat treatment it is shown how photocatalytic performance of thin films can be tailored.

Using photocatalytic TiO2 thin films as a substrate, metal structures such as Au micro- and nanoparticles can be deposited on the thin film surface via a photoreduction reaction. Upon the illumination of TiO2 thin films with UV light, the metal precursor is reduced via reaction with photogenerated electrons/holes, which results in the formation of solid metal structures. The morphology, size and coverage of the photodeposited metal structures can be greatly influenced by the choice of TiO2 substrate, illumination intensity and the composition of the precursor solution. With high photocatalytic performances of the TiO2 thin film it is possible to obtain high aspect ratio structures such as nanoneedles. The choice of thin film as well as the illumination intensity determines the morphology of the obtained structures. Lateral selectivity in the deposition of the metallic structures was showcased at two examples: On the one hand, selective illumination (e.g., by using shadow mask) results in localized gold growth. On the other hand, by structuring the TiO2 thin film (e.g., via lithography and lift-off) the formation of metal structures can be locally restricted. To obtain better insight and control, the formation of metal structures via UV-stimulated photodeposition can be monitored in-situ via changes in the transmission. The density of the grown nano- and microstructures can be varied upon performing consecutive growth steps from individual nanostructures towards a dense coverage. In addition, the surface coverage can be tailored further upon adding additives to the precursor solution. As such, light-stimulated photodeposition of metal structures is an interesting technique for the local on-demand formation of metallic aggregates and has the potential to contribute towards an inclusion of long-range connections and globally mediated aspects like homeoplasticity into the field of neuromorphic systems.