Strategies and reaction systems for solar-driven CO₂ reduction by water

Abstract

Solar driven CO₂ conversion into high-value-added chemicals and energy-rich fuels is one of the promising strategies to tackle global warming and to address the energy-supply crisis. Even though enormous effort has been devoted to exploring all sorts of homogeneous and heterogeneous photocatalysts, the current efficiency and more importantly selectivity to valuable chemicals are still rather moderate, thus it is desired to develop high-efficiency photocatalytic system toward CO₂ reduction with excellent selectivity. In this review, fundamental aspects of photocatalytic CO₂ reduction by pure water, the reaction systems and the reliable method for detection of the products are firstly described. Thereafter the recent advances of the main strategy for improving the photocatalytic CO₂ reduction from the perspective of promoting the CO₂ adsorption and activation, accelerating the kinetics of water oxidation, and modulating charge separation are overviewed. The prospects and challenges on precise designing heterogeneous catalysts for CO₂ photoreduction are proposed at the end, indicating the significance for the further development of photocatalytic systems with high CO₂ conversion efficiency and product selectivity.

1 Introduction

With the fast development of industrialization and rapid population growth, there is an ever-growing demand for energy consumption worldwide. Nowadays, 80 ~ 90% global energy consumption depends on the combustion of fossil fuels, which causes excessive carbon dioxide (CO₂) emission to the atmosphere and gives rise to global warming [1]. The latest data shows that global warming has increased dramatically during the past two decades, and the current global average temperature is 1.1 °C higher than that in the end of the nineteenth century [2]. That means human beings will face grim environmental and anthropological issues if anthropogenic CO₂ emission could not be effectively controlled and minimized. Thankfully, the global warming has aroused the public concerns and it is imperative to drastically reduce CO₂ emission. Numerous efforts have been devoted to replacing fossil fuels with renewable energy (solar, wave, wind, and biomass, etc.) and developing effective technologies for sustainable energy production [3, 4]. Representative water splitting and CO₂ reduction driven by inexhaustible solar energy are attractive approaches to energy conversion. As for photocatalytic water splitting, it is a relatively simple reaction with water as the only reactant. In particular overall water splitting has been considered as a cost-effective technology to scale up solar hydrogen production with the merits of ready synthesis of the reasonably priced photocatalyst along with the simple reactor and equipment designs.

Turning to CO₂ reduction, various technologies have been developed to convert CO₂ into carbonaceous fuels, such as electrocatalysis, thermocatalysis, and photocatalysis, etc. [5,6,7]. In particular, the transformation of CO₂ into chemical fuels with photovoltaic-powered electrochemical reduction and photoreduction are appealing in varieties of feasible strategies, since these two avenues could be achieved in normal pressure and temperature, and the required energy can be directly or indirectly provided by solar energy to truly realize carbon recycling [8, 9]. Photovoltaic-powered electrochemical reduction of CO₂ is driven by photovoltaic cells to generate sufficient photovoltage and then supplied to the cathode for CO₂ reduction and the anode for water oxidation. In this case, the design of photovoltaic and the employed electrocatalysts are flexible, which could be optimized individually and then assembled together to access the best overall performance. While the products selectivity and poor stability of electrocatalyst are still the obstacles to the development of electrocatalytic CO₂ reduction. In contrast, the wireless configuration of photocatalytic CO₂ reduction endows the device design much more facile and compact. Moreover, it could utilize the enormous solar energy directly, which is a competitive approach with the merits of nonpollution, inexhaustibility, and nonenergy inputs for energy conversion. In addition, the products selectivity could be ingeniously modulated from the perspective of energy band structures of photocatalysts, adsorption/activation of reactants, surface active sites as well as the adsorption/desorption of intermediates, and the desired hydrocarbons could be accessed based on the above considerations. This is more competitive than the electrochemical reduction pathway since most CO₂ reduction electrocatalysts yield CO or formate as the primary reduction products.

Since the first case that Inoue et al. realized the CO₂ photoreduction in the aqueous system [10], the photocatalytic CO₂ reduction field has witnessed a dramatic expansion in research directed at photocatalytic systems. There have been explored varieties of photocatalysts for CO₂ reduction in recent decades, including homogeneous catalysts as well as heterogeneous ones (Table 1). Of course, the activity and the selectivity of photocatalysts are closely related with the reaction media. An ideal photocatalytic CO₂ conversion system is similar to the natural photosynthesis of green plants which can exploit the energy of incident photons to convert CO₂ and H₂O into carbohydrates and O₂. That is to say, the photocatalysts are excited by photons and generate pairs of electrons and holes to reduce CO₂ with protons and oxidize H₂O, respectively [11]. However, many reported photocatalysts could not simultaneously accomplish the CO₂ reduction and H₂O oxidation, and thus the organic hole scavengers, for example, triethanolamine (TEOA), trimethylamine (TEA), and ethylenediaminetetraacetic acid (EDTA), were used in CO₂ photoreduction as electron/proton donors [12, 13], especially for some metal-organic framework (MOF) materials and metal sulfide semiconductors [14,15,16,17,18]. Such photocatalytic CO₂ reduction systems sacrifices expensive electron/proton donors instead of H₂O oxidation half-reaction which is unsustainable and uneconomical. In addition, the photocatalytic CO₂ reduction reaction is a complex multistep process, and the stoichiometric CO₂ reduction and H₂O oxidation is of great significance to deeply understand the reaction mechanism and tune product selectivity. While the H₂ evolution reaction (HER) usually becomes an observable competitive process in the presence of water, which possibly leads to low efficiency of CO₂ photoreduction [19]. Therefore, it is a grand challenge on smart design and fabrication of efficient photocatalytic systems for CO₂ reduction coupled with pure water oxidation.

Table 1 Representative summary of photocatalytic CO₂ reduction systems

From a thermodynamic point of view, the photocatalytic system must satisfy the reduction potentials of CO₂, and simultaneously maintain a high redox potential for the oxidation of H₂O to O₂. Besides, more attention should be paid to the CO₂ adsorption and activation, which is extremely important for inhibiting HER reactions and determining the CO₂ reduction reaction pathways. From a dynamic perspective, the water oxidation by holes proceeds at < s timescale [43], which is unambiguous to be the rate-determining step in overall CO₂ photoreduction process. Accordingly, the modulation and acceleration of photogenerated hole to initiate the surface oxidation reaction are critical to the conversion efficiency in water without the usage of any sacrificial agent. In addition, it should be noted that the recombination of photogenerated electron-hole pairs from the excited semiconductors occurs at < μs timescale, which is much faster than the water oxidation by holes [20, 29, 31, 44,45,46]. Therefore, strategies should be developed to enhance the charge separation, especially for the spatial charge separation, with efforts to achieving efficient CO₂ photoreduction.

Up to now, there have been many reviews focusing on the design and fabrication of various semiconductor photocatalysts, as well as the complex process of photocatalytic CO₂ reduction reaction [9, 47]. To differentiate from these review articles, we try to make an interesting review from the perspective of coupling CO₂ reduction with pure water oxidation. Firstly, the basic principle and procedure of photocatalytic CO₂ reduction coupled H₂O oxidation are discussed in detail. Then, the current widely-used reaction systems and products detection are comprehensively summarized. Special emphases are given on the strategies for improving photocatalytic CO₂ reduction performance, including co-catalysts engineering for CO₂ adsorption and activation, accelerating the kinetics of water oxidation, and promoting charge separation by constructing Z-scheme heterojunctions. Finally, the conclusions, challenges and perspectives concerning on efficient heterogeneous photocatalytic systems for CO₂ reduction with H₂O are presented.

2 Basic principle and procedure of photocatalytic CO₂ reduction

The basic principle of semiconductor photocatalysis is mainly based on energy band theory of solid-state physics. The band structure of a semiconductor is not successive, which consists of multiple energy bands, including the valence band (VB), the conduction band (CB), and the forbidden band. In general, the VB of a semiconductor is fully filled with valence electrons at low temperatures, while there are little electrons in the bottom of CB, in which most of the energy levels are unoccupied. There is a space between the top of the VB and the bottom of CB that electrons are unable to exist, which is denoted as the forbidden band. The energy difference between the CB and the VB is called bandgap. When a semiconductor photocatalyst absorbs photons with the energy equal to or greater than its bandgap, the electrons in the VB will be excited to the CB, while leaving the holes in the VB. The generated electrons and holes would further migrate to the surface of a semiconductor, and initiate the redox reactions with the adsorbed reactants on the surface. Meanwhile, a proportion of the electrons and holes would recombine in the bulk or on the surface of semiconductors through the electrostatic force [48]. Based on the fundamentals of photocatalysis, the procedure of CO₂ photoreduction by pure water is mainly involved with the following procedures:

(i) CO₂ adsorption. The CO₂ adsorption and activation play a crucial role in tuning the activity and selectivity of photocatalytic CO₂ conversion [49]. On one hand, if the photocatalyst surface is favorable for CO₂ adsorption, the photogenerated electrons would be more easily captured by CO₂ that adsorbed on the catalyst surface after being transferred from the bulk to the catalyst surface, which can reduce the recombination of photogenerated charge carriers to a certain extent, and improve the utilization efficiency of photoelectrons [50]. On the other hand, CO₂ is a thermodynamically stable nonpolar molecular with linear configuration, and the bond energy of C=O (750 kJ mol^− 1) is much higher than that of C − C (336 kJ mol^− 1), C − O (327 kJ mol^− 1) and C − H (411 kJ mol^− 1) [11]. As a result, the photoreduction of CO₂ needs to overcome a higher activation energy to break the C=O bonds. It is understood that the adsorbed CO₂ could become a partially charged CO₂^δ•- species via the interactions with surface atoms [51]. The adsorbate no longer has the initial linear symmetry of the free CO₂ molecule and thus lower the energy barriers for accepting an electron since the lowest unoccupied molecular orbital (LUMO) level of CO₂ decreases as the molecule bends [52]. Figure 1 shows the possible structures of adsorbed CO₂^δ•- species on catalysts [53]. Moreover, the main adsorbed structure of CO₂ on the surface of a photocatalyst is determined by its chemical properties, which has a significant influence on the subsequent reaction pathways and selectivity.

Fig. 1

The possible structures of adsorbed CO₂^δ•- species on catalysts [53] Copyright 2020, Elsevier

(ii) Excitation and migration of photogenerated charge carriers. The electron-hole pairs generates when a photocatalysts absorb the incident photons with energy equal to or greater than the bandgap, and then the photogenerated electrons and holes migrate independently to the surface of a photocatalyst (Fig. 2). Since the recombination of charge carriers (~ 10^− 9 s) is considerably faster than the surface redox processes (10^− 3 to 10^− 8 s) [54, 55], it is always accompanied by the undesirable electron-hole recombination in bulk or on the surface of photocatalysts. As a result, the rapid migration of charge carrier from bulk to the surface could effectively restrain the electron-hole recombination, which enables more photogenerated charge carriers to participate in the subsequent redox reactions [56]. The quantity of the initial photogenerated charge carriers of semiconductor photocatalysts depends on the inherent bandgap, while the redox capacity of the charge carriers is determined by the energy band alignments. In general, the bottom of the CB of a photocatalyst has to be more negative than the CO₂ reduction potential and the corresponding reduction products, while the top of the VB of a photocatalyst has to be more positive than the potential of water oxidation.

Fig. 2

Schematic illustration of photocatalytic CO₂ reduction by water over a photocatalyst at pH = 7

(iii) Surface catalytic reactions with CO₂ and H₂O. The photocatalytic CO₂ reduction by water consist of two half reactions, namely, the electron-initiated CO₂ reduction reaction and the hole-induced water oxidation reaction. In general, one-electron transfer to form a CO₂^•- intermediate [Eq. (1)] is believed to be the first step to initiate a series of subsequent reactions. Whereas, the fairly negative thermodynamic potential of − 1.9 V versus NHE at pH=7 makes the candidate semiconductors impossible to directly drive such reactions by single-electron transfer²³. Proton-assisted multiple-electron transfer turns to be an alternative and feasible route to reduce CO₂ via passing by the formation of CO₂^•- [Eqs. (2), (3), (4), (5), (6), (7), (8), (9) and (10)] [32, 57]. The activation barriers of CO₂ conversion decrease gradually with the merits of several electrons and protons simultaneously transferred in pairs to CO₂. According to the number of the involved electrons and protons transferred, CO₂ can be converted to C₁ compounds such as CO, CH₄, CHOH, HCOOH and CH₃OH as well as high-value-added hydrocarbons.

Of note, most studies on CO₂ photoreduction are focused on the reduction side, yet less attention has paid to the oxidation half-reaction. In principle, if the VB energy level of a semiconductor is positive than the redox potential of O₂/H₂O (0.81 V vs NHE), the photogenerated holes would react with the adsorbed H₂O or OH⁻_ads on the surface of semiconductors to produce O₂ and H⁺ [Eqs. (11)]. However, the formation of O₂ is relatively difficult due to the need for four holes per oxygen molecule and the challenge of its desorption. An alternative to the generation of oxygen is the formation of hydroxyl radicals with strong oxidizing [Eqs. (12)]. It is worth noting that the produced H⁺ could participate in the CO₂ reduction reaction, as well as the competitive reaction of water reduction to H₂ [Eqs. (13)]. The oxidative environment complicates the CO₂ reduction pathway since some intermediate species would be more favorable to oxidize than reduce, as a result interrupting the process before the multiple-electron reduction products are obtained.

The related potentials for CO₂ conversion at pH = 7, vs NHE

$$ {CO}_2(g)+{e}^{-}\to C{O}_2^{\cdotp -}\kern1em {E}^{\theta }=-1.90\kern0.5em V $$

(1)

$$ {CO}_2(g)+2{H}^{+}+2{e}^{-}\to CO(g)+{H}_20(l)\kern1em {E}^{\theta }=-0.53\;V $$

(2)

$$ {CO}_2(g)+8{H}^{+}+8{e}^{-}\to {CH}_4(g)+2{H}_20(l)\kern1em {E}^{\theta }=-0.24\;V $$

(3)

$$ {CO}_2(g)+4{H}^{+}+4{e}^{-}\to HCHO(g)+2{H}_20(l)\kern1em {E}^{\theta }=-0.48\;V $$

(4)

$$ {CO}_2(g)+2{H}^{+}+2{e}^{+}\to HCOOH(l)\kern1em {E}^{\theta }=-0.61\;V $$

(5)

$$ {CO}_2(g)+6{H}^{+}+6{e}^{-}\to {CH}_3 OH(l)\kern1em {E}^{\theta }=-0.38\;V $$

(6)

$$ 2{CO}_2(g)+8{H}^{+}+8{e}^{-}\to {CH}_3 COOH(l)+2{H}_20(l)\kern1em {E}^{\theta }=-0.30\;V $$

(7)

$$ 2{CO}_2(g)+12{H}^{+}+12{e}^{-}\to {CH}_3{CH}_2 OH(l)+3{H}_20(l)\kern1em {E}^{\theta }=-0.32\;V $$

(8)

$$ 2{CO}_2(g)+12{H}^{+}+12{e}^{-}\to {C}_2{H}_4(g)+4{H}_20(l)\kern1em {E}^{\theta }=-0.33\;V $$

(9)

$$ 2{CO}_2(g)+14{H}^{+}+14{e}^{-}\to {C}_2{H}_6(g)+4{H}_20(l)\kern1em {E}^{\theta }=-0.27\;V $$

(10)

$$ 2{H}_20(l)+4{h}^{+}\to {0}_2(g)+4{H}^{+}\kern1em {E}^{\theta }=+0.81\;V $$

(11)

$$ 2{H}_20(l)+{h}^{+}\to 0H+{H}^{+}\kern1em {E}^{\theta }=+2.31\;V $$

(12)

$$ 2{H}^{+}+2{e}^{-}\to {H}_2(g)\kern1em {E}^{\theta }=-0.42\;V $$

(13)

(iv) Desorption of the products. The catalytic active site plays a crucial role in the final activity and selectivity of CO₂ reduction. If the products cannot be desorbed from the surface of a photocatalyst in time, the active site would be coated or poisoned that result in decreasing the reaction rate and ceasing the subsequent chemical reaction [9]. In the course of photocatalytic CO₂ reduction, varieties of intermediates could be obtained, and the interactions between intermediates and photocatalysts have great influence on the yield and selectivity of CO₂ photoreduction. If the interaction was strong, the intermediates would be difficult to release from the surface of photocatalysts, and the subsequent hydrogenation reaction would be more likely occurred. On the contrary, if the binding energy of the intermediates on the surface of a photocatalyst is weak, the products desorption from the photocatalysts surface would be more likely performed, and the specific products would be preferentially produced.

3 Reaction systems and products detection for CO₂ reduction

3.1 Reaction systems

The photocatalytic CO₂ reduction requires suitable reactant, which could react with h⁺ and simultaneously provide protons for CO₂ reduction. The categories and dosage of reactant have a significant impact on the activity and the selectivity of CO₂ reduction. Abundant water with the merits of low-cost, pro-environment, etc. has been appealing electron/proton donors to replace those sacrificial regents to achieve sustainable CO₂ reduction.

To date, there are two kinds of preoccupant photocatalytic CO₂ reduction systems with water as reducing agent, namely, the solid-liquid reaction system and the gas-solid reaction system. The schematic diagram of the devices for these two reaction systems are shown in Fig. 3. The former one contains the catalyst powder and water in a sealed reactor, and the reactor is evacuated or bubbled with N₂ or Ar, etc. inert gas to remove air dissolved in liquid, and then filled with saturated CO₂ by means of bubbling or injection before formal photoreduction reactions [58, 59]. Some studies also directly disperse the catalyst into NaHCO₃ or KHCO₃ solutions with a certain concentration for photocatalytic CO₂ reduction [60, 61]. Turning to the gas-solid reaction system, in which both CO₂ and water in the form of gas to participate in the reaction, while the catalyst is coated or directly dispersed in the reactor [20, 29, 31, 44,45,46, 54, 55]. In this case, CO₂ and H₂O vapor are produced by the reaction of NaHCO₃/KHCO₃ with acid solution, and their content are controlled by adjusting the reaction temperature and the concentration of reactant [62, 63]. It is worth noting that the reductive products are diverse in these two reaction systems. CH₃OH, CH₃CH₂OH, HCOOH and some other liquid products are dominant reductive products in the solid-liquid suspension system [6, 7, 20, 29, 31, 36, 44,45,46], while CO, CH₄, C₂H₄, C₂H₆ and other hydrocarbons with small chains are primary products in the solid-gas reaction system [26, 37, 38, 64,65,66]. Of note, the competitive H₂ evolution reaction from direct photocatalytic water reduction is an observable process, which seriously reduces the CO₂ conversion efficiency and hinder the selectivity of products [67]. Whereas, the impact of the undesirable H₂ evolution reaction on the efficiency and selectivity of CO₂ reduction is much weaker in the gas-solid reaction system due to the lower H₂O vapor content.

Fig. 3

Schematic diagram of the devices of the solid-liquid reaction system (a) and the gas-solid reaction system (b), respectively

3.2 Products detection

3.2.1 Analysis of gaseous products

It is well acknowledged that the CO₂ reduction process involves multiple protons coupled electron reaction to produce a wide variety of products, such as H₂, CO, CH₄, CH₃OH, HCOOH, as well as even higher hydrocarbons. The analytical approaches are different in terms of the physical state of CO₂ reduction products. In general, the quantitative detection of gaseous products like CO, CH₄, HCHO and H₂ are analyzed by gas chromatograph equipped with a thermal conductivity detector (TCD) and/or flame ionization detector (FID). Thereinto, H₂ is detected by a TCD equipped with suitable capillary columns, for instance, TDX-01 columns, Molsieve 5 Å columns and so forth [68, 69]. While all organic substance including CH₄, HCHO and inorganic CO could be further detected by FID (Note: CO detection requires a methanation reactor which contains a Ni catalyst). The role of methanation reactor is to convert CO to CH₄ and then analyzed by the FID [20, 29, 31, 44,45,46]. In particular, in the water contained CO₂ reduction reaction, O₂ has been regarded as the major oxidative products. The detection of O₂ is significant for the confirmation of an overall CO₂ reduction reaction. The analytical method of O₂ is the same as H₂, which quantified by gas chromatograph equipped with a TCD. Definitely, prior to quantitative detection of the reaction products, a calibration using standard gas mixture with different concertation of the gas to be tested is indispensable [70].

3.2.2 Analysis of liquid products

In addition to gaseous products, the confirmation and detection of the potential liquid products in the course of photocatalytic CO₂ reduction are essential. HCOOH and CH₃OH are the most common liquid products. For instance, CH₃OH etc. small chain liquid products are usually taken from the reactor by syringe, and detected by gas chromatograph equipped with FID detector [26, 37, 38, 65, 71]. Other possible liquid products, such as HCOOH, is generally analyzed by high-performance liquid chromatography [72].

4 Strategies for improving photocatalytic performance of CO₂ reduction by water

4.1 Co-catalysts engineering for CO₂ adsorption and activation

Great efforts have been devoted to boost the photocatalytic activity of CO₂ reduction over the last decades, yet the efficiency is still moderate due to the thermodynamically stable nature of CO₂. Typically, the initial step for CO₂ reduction is the adsorption process, and the linear configuration of CO₂ would become bent for the subsequent activation. Modification of cocatalysts on the photocatalysts could effectively lower the potential barrier for CO₂ activation, enhance the charge separation and simultaneously modulate the selectivity of the products, which has been a widely explored route to promote the photocatalytic activity of CO₂ reduction [73,74,75]. Developing low-cost, robust and sustainable cocatalysts to couple with photocatalysts as catalytic active sites, is prospected to greatly improve the photocatalytic activity.

Yu et al. reported a 2D/2D heterojunction of ultrathin Ti₃C₂/Bi₂WO₆ nanosheets for efficient CO₂ reduction, in which the Ti₃C₂ with abundant exposed metal sites served as an economical and stable cocatalyst [21]. The coupled Ti₃C₂ with several atomic layers remarkably promotes the CO₂ adsorption, and meanwhile the charge transport pathway in the Ti₃C₂/Bi₂WO₆ hybrid is greatly shortened, which is quite favorable for the transferred electrons from Bi₂WO₆ accumulated on the surface of Ti₃C₂ (Fig. 4a and b). Accordingly, the total yield of CH₄ and CH₃OH obtained on the optimal one is 4.6 times than that of pristine Bi₂WO₆ nanosheets. Very recently, Ng and Tang et al. developed a surfactant-free method to fabricate Cu₂O@Cu₃(BTC)₂ composite photocatalyst for selective CO₂ conversion (Fig. 4c) [22]. The coated Cu₃(BTC)₂ enlarges the surface area and increases CO₂ uptake, which offers a dense CO₂ atmosphere near the active catalytic sites. Importantly, the uncoordinated -COOH in Cu₃-(BTC)₂ framework that nearby the catalytic sites could form H-bonds with the intermediates of CO₂ reduction (*CO, *CHO, *CH₂O, and *OCH₃). Such interactions could prevent the desorption of the intermediates and lower the energy barrier for the CH₄ production, thus contributes to the selective evolution of CH₄ for Cu₂O@Cu₃(BTC)₂ composite.

Fig. 4

(a) Energy level structure diagram of Bi₂WO₆ and Ti₃C₂ (b) Photo-induced electron transfer process at the interface of the hybrids [21] Copyright 2018, Wiley-VCH (c) Illustration of the synthetic route towards Cu₂O@Cu₃(BTC)₂ core–shell nanowires through a surfactant-free topotactic conversion strategy for selective photocatalytic carbon dioxide reduction to methane [22] Copyright 2021, Wiley-VCH

Metal nanoparticle has been considered as an effective cocatalyst for photocatalytic reactions. The function of extracting photogenerated electrons and providing additional active sites makes it a promising candidate for CO₂ photoreduction. Despite there have some progress achieved on metal particle/semiconductor, it still confronts challenges of the efficiency and mechanism elucidation. The concept of single atom catalysis has triggered enormous attention and interest in recent years with the merits of maximum atom utilization efficiency, impressive catalytic activity, and unique selectivity. The isolated single atoms differ from traditional nanoparticles, because the former could fully expose the atomically dispersed metals and generated electrons and holes are easier to move to the surface of the catalyst. In this regard, scaling down the metal nanoparticles to single metal atoms affords great opportunity to maximize metal atom utilization efficiency and to acquire uniform catalytic active sites [76]. Zhang et al. developed a simple strategy to dispersed isolated Ni single atoms on defect ZrO₂ for selective photocatalytic CO₂ conversion [23]. The crucial role of the atomically dispersed Ni sites has been systematically evidenced to lower the energy barrier for CO₂ to CO conversion via an adsorbed COOH intermediate (Fig. 5a and b). Meanwhile, it also suppressed H₂ desorption in the competitive water reduction reaction. Inspired by the single atom catalysis, the metal phthalocyanine (MPc) with the well-defined single atom metal sites that coordinated by four N atoms, has also been a potential single atom catalytic site for CO₂ conversion. Interestingly, the inherent M-N₄ sites of MPc with homogeneous configuration makes it more facile to be a uniform and stable isolate catalytic center than single atom sites, which needs to be precisely coordinated by N/C matrix (M-C/N unit). Jing’s group designed an ultrathin CuPc/g-C₃N₄ heterojunction via a surface hydroxyl induced assembly process for CO₂ conversion [24]. It is worth noting that the coated CuPc served as a high-level-energy electron transfer platform, greatly prolonged the electron lifetime of g-C₃N₄, and the subsequent charge transfer from the ligand of CuPc to the center Cu-N₄ sites dramatically promotes the CO₂ adsorption and activation (Fig. 5c). In addition, this work emphasized that the controllable assembly of CuPc with high dispersion is crucial to expose abundant isolated catalytic active site for CO₂ concentration and conversion.

Fig. 5

(a) Energy profile for the elementary steps of CO₂ reduction to CO over (010) facets of ZrO₂, Ni-SA/Zr, and Ni-SA/O (b) Differential charge density diagrams and intermediates during CO₂ reduction to CO over Ni-SA/O model [23] Copyright 2020, Wiley-VCH (c) Schematic of HLEEs transfer process in the fabricated MPc/CN heterojunction [24] Copyright 2020, Elsevier

Actually, except the well acknowledged noble metals, transition metal-based materials, for instance, ions, oxides, sulfides and phosphides, etc. also exhibit appealing potentials for catalysis as cocatalysts. To date, it was found that most reported cocatalysts are in solid state. Very recently, a smart ionized cocatalyst has come into sight. Yu et al. proposed core-triple shell Mn, C-codoped ZnO hollow spheres as efficient photocatalysts for CO₂ conversion [25]. It was authenticated that the ionized Mn species could restore their primal oxidation state by means of capturing the photogenerated electrons from the CB of ZnO, which function as the active sites for CO₂ adsorption and activation. The embedded Mn ions with switchable valence states served as an “ionized cocatalyst” to provide electrons for CO₂ conversion. Interestingly, this process could be continuously operated due to the light-switchable valence state of Mn ions. Moreover, it turns out that CO is the predominant reduction products, and H₂ could be hardly detected, which might be correlated with the competitive H₂ evolution from water reduction is inhibited without noble metal cocatalysts.

4.2 Accelerated water oxidation kinetics

In an ideal photocatalytic CO₂ reduction reaction, the excited pairs of photogenerated electrons and holes would transfer to the surface of a semiconductor to reduce CO₂, and simultaneously to oxidize water to produce O₂. Even though continuous efforts have been devoted to the optimization of cocatalysts for CO₂ activation, it is often neglected that the water oxidation reaction which proceeds at seconds timescale is much slower than the undesirable electron-hole recombination (occurs at < μs time scale). Therefore, it is much meaningful to develop preferable semiconductors with long-lived charge carriers along with economical cocatalysts to extract photogenerated holes, so as to accelerate the water oxidation half reaction instead of charge recombination. Tang et al. ingeniously designed a unique hole-accepting carbon-dots decorated carbon nitride (^mCD/CN) for selective CO₂ reduction nearly 100% to methanol by pure water [20]. The ^mCD modification tackles the difficult water oxidation by greatly prolonging the lifetime of charge carriers to allow for electrons accumulation. In addition, it is evidenced that the ^mCD could selectively transfer holes towards H₂O rather than methanol. Significantly, the unique ^mCD as a hole acceptor in the ^mCD/CN has been clearly distinguished by transient absorption spectroscopy (TAS), which is a powerful technique to directly reveal the charge carrier dynamics of photocatalysts. The half-life time of the signal observed at 700 nm of CN increased from 25 μs to 160 μs after decorating with ^mCD, indicating suppressed electron-hole recombination owing to the charge separation across the ^mCD/CN junction (Fig. 6). Accordingly, it offers longer time for the 6-electron process to reduce CO₂ to methanol.

Fig. 6

Diffuse reflectance TAS spectra for samples with (a–c) and without (d–f) 10 mM AgNO₃ (g) μs-TAS decay kinetics of CN, ^mCD/CN and ^sCD/CN in water monitored at 700 nm and excited by pulsed 355 nm excitation (460 μJ/cm²) and (h) fitted α parameters indicated in the same color as the associated trace [20] Copyright 2020, Springer Nature

Based on the discovery that carbon-dots could act as a hole acceptor of CN, Tang and Godin et al. further modulated the fine structure of CN from the perspective of band positions, bandgaps and hydrophilicity to stepwise optimize the photocatalytic activity of CN, and followed by modification of CD with the function of extracting holes [26]. A carbon nitride-like polymer (FAT) was designed by altering the terminal and linker groups in carbon nitride, and the replacement of some N atoms in CN to O atoms in FAT leading to a higher intensity of trapped holes with photoexcitation, which is different from the common CN with the nature of trapping electrons. As expected, the CD decorated FAT manifested impressive activity for selective CO₂ reduction to methanol by water. It was evidenced by TAS that the fast hole trapping in FAT is the key because of the CD can subsequently capture holes on the sub-microsecond timescale. This work proposed a significant thought to improve the activity from the perspective of retaining the reactivity of holes and increase the number of effective electrons, so as to accelerate the six-electron reduction of CO₂ conversion to methanol.

As stated before, modulating the photogenerated holes to facilitate water oxidation is a crucial aspect which needs to be considered in the course of CO₂ photoreduction. It is naturally expected that regulating the photogenerated electrons on the foundation of hole trapping is bound to build a fancy photocatalytic system with high performance for CO₂ reduction. Jing et al. developed a facile and green approach to anchor subnano Ni and Mn-oxo clusters on chitosan oligomer (COS)-functionalized ultrathin g-C₃N₄ (UCN) nanosheet for efficient CO₂ conversion [27]. The decorated Ni-oxo and Mn-oxo clusters with an average diameter of ca. 0.8 nm were supported on 5COS₂₀–UCN uniformly (Fig. 7a-c). Atmosphere-controlled SPS spectra uncovered the character of Ni-oxo clusters for capturing electrons, since the SPS response of 1.5Ni–5COS₂₀–UCN in O₂ is stronger compared with that in N₂ with the generally acknowledged O₂ capture electrons. While Mn-oxo clusters decorated 5COS₂₀-UCN delivered an opposite result, and the SPS signals of 1Mn–5COS₂₀–UCN in O₂ are weaker than that in N₂ atmosphere, which is indictive of the nature of Mn-oxo clusters for capturing holes (Fig. 7d). These results are further confirmed by TS-SPV spectra in N₂ atmosphere. Interestingly, the response of the optimal 1Ni0.75Mn–5COS₂₀–UCN shows a less-positive response than 1.5Ni–5COS₂₀–UCN, which strongly evidenced the dual modulation of photogenerated electrons and holes by Ni and Mn-oxo clusters (Fig. 7e). Moreover, collaborated with the results of electrochemical measurements, the synergetic effect between Ni- and Mn-oxo species, is capable of photogenerated electron trapping along with CO₂ activation and photogenerated hole trapping along with water activation are clarified (Fig. 7f and g). Matching capacity of Ni- and Mo-oxo species for the equalized modulation of electrons and holes is highlighted for the first time.

Fig. 7

(a) TEM image, (b) STEM image and the corresponding EDX mapping images of C, N, O, Ni, and Mn elements, and (c) HAADF-STEM image of 1Ni0.75Mn–COS₂₀–UCN (d) SPS responses of 1.5Ni–COS₂₀–UCN (left) and 1Mn–COS20–UCN (right) in different atmospheres (e) TS-SPV responses of UCN, 1Mn–COS₂₀–UCN, 1.5Ni–COS₂₀–UCN, and 1Ni0.75Mn–5COS₂₀ UCN in the N₂ atmosphere (f) EC reduction curves of UCN, 5COS₂₀–UCN and 1.5Ni–COS₂₀–UCN in the CO₂-bubbled system (g) EC oxidation curves of UCN, 5COS₂₀–UCN and 1Mn–COS₂₀–UCN samples in the N₂-bubbled system [27] Copyright 2021, Wiley-VCH

Likewise, they fabricated MnO_x and Au-TiO₂ co-modified porous g-C₃N₄ as efficient visible light responsive photocatalysts for CO₂ reduction. The lifetime of photogenerated charge carriers of g-C₃N₄ is greatly prolonged with the decoration of sheet-like nanostructured MnO_x, which is disclosed by transient-state surface photovoltage (TS-SPV) response, an advanced technique to investigate the dynamic properties of charge carriers. It is deduced that the modified MnO_x could capture holes and afford catalytic function for H₂O oxidation for producing hydroxyl and then evolving oxygen, while the coupled Au on TiO₂ could trap electrons and provide catalytic active sites for CO₂ reduction. Accordingly, the optimal one delivers impressive CO₂ conversion activity and quantum efficiency of ≈4.92% at 420 nm wavelength [28].

4.3 Promoting charge separation by constructing Z-scheme heterojunctions

As stated above, modification of cocatalyst on a single photocatalyst whose CB and VB could meet the thermodynamic potentials for both CO₂ reduction and water oxidation, is an effective strategy to accelerate the surface reaction kinetics, and simultaneously enhance the charge separation. However, it often requires a semiconductor endowed with a wide bandgap. In contrast, narrow bandgap semiconductors are helpful due to the wide visible light absorption range. Whereas, in most cases, it suffers from the insufficient reduction capacity of electrons or inadequate oxidation capacity of holes. Unfortunately, these two aspects are contradictory and, in consequence, the concept of heterojunction emerges on the scene.

Several types of heterojunctions have been explored in recent decades, in which the traditional type II heterojunction has been widely investigated and received considerable attention [15, 17, 77]. In fact, the type II heterojunction could suppress the recombination of charge carriers, while the charge separation is promoted by sacrificing the strong redox capacities of those photogenerated charge carriers, which is detrimental to CO₂ conversion with high thermodynamic requirements. Hence, developing a heterojunction with rapid charge separation and meanwhile maintaining the sufficient thermodynamic energy of photoelectrons and holes, is anticipated to be a more rational option to improve the photocatalytic activities for CO₂ reduction.

Enlightened by nature photosynthesis, constructing artificial Z(S)-scheme heterojunction via linking two semiconductors with staggered band structures has emerged as a fancy pathway to convert CO₂ with water into solar fuels. On one hand, the charge separation would be greatly facilitated with the help of recombination of the charge carriers with weak redox capacities. On the other hand, what makes it distinct from type II heterojunctions lie in the energetic electrons of the reductive semiconductor and the positive holes of the oxidative semiconductor could be preserved, which provides sufficient energy to drive the overall CO₂ reduction reaction. In this regard, it is feasible to construct a Z(S)-scheme heterojunction by combining a CO₂ reduction photocatalyst and a water oxidation photocatalyst for efficient CO₂ conversion.

Zhou et al. fabricated a high-performance BiVO₄{010}–Au–Cu₂O Z-scheme photocatalyst for CO₂ reduction from the view of charge separation [18]. This research pointed out that the hot electrons that accumulate on the {010} surface of BiVO₄ could easily overcome the Schottky barrier to facilitate the migration from BiVO₄ to Au, and then recombine with the photogenerated holes in Cu₂O. The long-live holes and electrons in the VB of BiVO₄ and CB of Cu₂O due to the deposition of crystal-facet-dependent electron shuttle could initiate water oxidation to produce O₂ and CO₂ reduction to CO and CH₄ with high-efficiency, respectively (Fig. 8a). This study integrated surface-induced charge directional transfer and the optimal contact interface, and provided a new strategy for designing efficient Z-scheme heterojunction for CO₂ reduction. Similarly, a three-dimensional BiVO₄/Carbon-Coated Cu₂O nanowire arrays were synthesized for CO₂ conversion to CO and CH₄ (Fig. 8b) [78]. The charge separation is significantly enhanced due to the construction of Z-scheme heterojunction, and as a result, the CO formation rate over BVO/C/Cu₂O exhibited ca. 9- and 5-fold improvements than those of Cu₂O mesh and Cu₂O NWAs, respectively (Fig. 8c). The accelerated Z-scheme charge flow between BVO and Cu₂O with the introduced carbon layer plays a crucial role in improving the photocatalytic activities. In addition, the BVO/C/Cu₂O heterojunction delivered outstanding stability in the course of CO₂ photoreduction with the help of incorporation of a carbon protective layer.

Fig. 8

(a) Scheme to describe the carrier migration behavior from BiVO₄ to Cu₂O with and without Au particles deposited on the different facets of BiVO4 [18]. Copyright 2018, Wiley-VCH (b) Schematic of the synthesized BVO/C/Cu₂O nanowire (c) Photocatalytic performance for CO₂ reduction of various catalysts [78]. Copyright 2018, American Chemical Society

It is worth noting that the current Z-scheme systems for CO₂ reduction still remains a great challenge on account of the inefficient interfacial charge transfer along with the competitive side and back reactions, namely the proton reduction and the oxidation of the reduction products from CO₂ conversion. In this condition, it is difficult to control the selectively of the CO₂ reduction products, in particular associated with the use of redox mediators. Demon and Reisner et al. innovatively proposed a photocatalyst sheet that converts CO₂ and H₂O into formate and oxygen as a potentially scalable technology for CO₂ utilization [29]. La- and Rh-doped SrTiO₃ (SrTiO₃:La,Rh) and Mo-doped BiVO₄ (BiVO₄:Mo) were integrated on to a gold layer as the CO₂ reduction and water oxidation photocatalysts, respectively. Meanwhile, phosphonated Co(II) bis(terpyridine) and RuO₂ were loaded on the reductive photocatalyst and the oxidative counterpart. The Z-scheme charge which transfers between BiVO₄:Mo and SrTiO₃:La,Rh is greatly facilitated with the modification of Au layer, and the modified CotpyP and RuO₂ were evidenced as highly efficient cocatalysts for CO₂ reduction and oxygen evolution, respectively. The enhanced charge separation and the accelerating surface reaction kinetics contribute to the excellent CO₂ reduction performance. The well-designed monolithic device delivered a superior solar-to-formate conversion efficiency of 0.08 ± 0.01% with a selectivity for formate of 97 ± 3%. The apparent quantum yield (AQY) at 420 ± 15 nm was achieved to 2.6%. Moreover, the produced HCOO⁻ with an STF of 0.06% was obtained on a sheet with an active irradiated area of ca. 20 cm². This impressive wireless device combines formate production and water oxidation with light as the sole energy source, which has seldom been reported in the field of wireless CO₂ reduction.

Modifying cocatalysts on Z-scheme heterojunctions has been evidenced as a feasible strategy to lower the barrier of CO₂ activation and improve the selectivity, yet the capacity of cocatalysts for the manipulation of Z-scheme charge separation still limited. While the rapid charge separation is the prerequisite for the subsequent CO₂ concentration and surface catalytic reactions. To this end, to further modulate the Z-scheme charge transfer and separation are crucial to boost the photocatalytic activities of CO₂ reduction. On one hand, the charge transfer and migration of both reductive and oxidative semiconductor along with the successful interfacial transfer influence the Z-scheme performance. On the other hand, the competitive type II charge transfer pathway (electrons transfer from the reductive seminconductor to the oxidative one, while the holes transfer in the opposite direction) due to the staggered band structures of the reductive photocatalyst and the oxidative photocatalyst would severely impair the Z-scheme charge transfer and separation. Accordingly, tackling the issues of interfacial charge transfer and modulating the desirable Z-scheme charge transfer pathway are of great significance for the architecture of highly efficient Z-scheme heterojunction.

Very recently, Jing and Tang et al. proposed a universal strategy to construct a cascade Z-scheme system with dimension-matched (001)TiO₂-g-C₃N₄/BiVO₄ nanosheet (T-CN/BVNS) heterojunction as a model [30]. The coupled (001)TiO₂ serving as an effective energy platform could direct Z-scheme charge transfer and separation, blocking the unexpected type-II charge transfer pathway (Fig. 9a). The optimal T-CN/BVNS heterojunction exhibits ca.19-fold photoactivity improvement for CO₂ reduction to CO in the absence of cocatalysts and costly sacrificial agents under visible-light irradiation, compared with pristine BiVO₄ nanosheet, which is also superior to other reported Z-Scheme systems even with noble metals as mediators. Notably, the introduced (001)TiO₂ could prolong the lifetimes of spatially separated electrons and holes and does not compromise their reduction and oxidation potentials, which was comprehensively validated by experimental results and DFT calculations from the view of ultrafast timescale. Importantly, this strategy is also applicable to facilitate the charge transfer in other Z-scheme heterojunctions (eg.C₃N₄/WO₃ and C₃N₄/Fe₂O₃), and other wide-band gap semiconductors, such as SnO₂, can also be used as an alternative electron-energy platform. Overall, this study highlights photocatalysts with advanced charge separation property is vital to achieve CO₂ photoreduction activity improvement, and enrich the methods for the design of delicate Z-scheme heterojunction with cascade charge transfer.

Fig. 9

(a) Schematic representation of the proposed cascade Z-Scheme mechanism of photogenerated charge transfer under visible light for efficient photocatalysis. T refers to (001)TiO₂, which can feasibly be replaced by other wide bandgap semiconductors like SnO₂ [30] Copyright 2021, Wiley-VCH (b) The electron transfer processes in Cu₂O–Pt/SiC/IrO_x under light illumination (c) The proposed mechanism of the separated system for the efficient CO₂ reduction and O₂ evolution [31] Copyright 2020, Springer Nature

In another study, Li et al. reported a Cu₂O–Pt/SiC/IrO_x hybrid photocatalyst by loading the IrO_x and Pt-Cu₂O on SiC surface through a step-by-step photodeposition method, and integrated with Pt/WO₃ to construct an artificial photosynthetic system [31]. Interestingly, a spatially separated reaction system with two reaction chambers was established, in which one chamber is loaded with the Cu₂O–Pt/SiC/IrO_x hybrid and Fe²⁺ for CO₂ reduction, and the other one with Pt/WO₃ and Fe³⁺ for water oxidation (Fig. 9b and c). Benefitting from the elaborately design, the artificial photosynthetic system exhibits superior photocatalytic activities for CO₂ reduction to HCOOH and H₂O oxidation to O₂ under visible light irradiation. Importantly, the yields of HCOOH and O₂ with stoichiometric ratio achieved 896.7 and 440.7 μmol g^− 1 h^− 1, respectively. It is revealed that the impressive efficiencies of CO₂ conversion and water oxidation are ascribed to the unique configuration of direct Z-scheme electronic structure of Cu₂O-Pt/SiC/IrO_x and the indirect Z-scheme spatially separated reduction and oxidation units, which enables photogenerated electrons and holes live longer and hold back the backward reaction of products.

Enormous studies about photocatalyst have been conducted by integrating the water oxidation with a CO₂ reduction to construct Z-scheme heterojunctions for CO₂ reduction by pure water [32, 54, 55, 57]. In addition to the traditional Z-scheme heterojunction with typical inorganic semiconductors as reductive constituents, some organic materials are emerged as a new frontier on account of the potentials for CO₂ adsorption, diffusion and activation. Recently, crystalline covalent organic frameworks (COFs) have attracted great attention and been continually developed. In particular, the rich porous structure and the tunable energy band alignments make it a desirable reductive photocatalyst for the construction of a novel Z-scheme heterojunction.

Lan et al. developed a versatile strategy to fabricate series of COF-semiconductor Z-scheme heterojunctions for CO₂ conversion, in which the water oxidation photocatalysts (TiO₂, Bi₂WO₆, Fe₂O₃) were elaborately integrated with COFs (COF-316/318) by covalent connection [32]. The optimal COF-318-TiO₂ Z-scheme heterojunction delivered the highest CO production rate of 69.67 μmol/g/h in the absence of additional photosensitizers and sacrificial agents, 5.2-fold and 6-time higher than the pure COF-318 and TiO₂, respectively (Fig. 10a and b). Meanwhile, the produced O₂ has also been confirmed by the isotope labeling experiment. Both experimental results and DFT calculations revealed the Z-scheme charge transfer pathway in the COF-semiconductor heterojunctions, resulting in the spatial separated electrons accumulated in COF for CO₂ reduction and the photogenerated holes leave in semiconductors for water oxidation. It also pointed out that the energy barrier for CO₂ activation is reduced due to the coexistence of pyridine group and cyano sites (Fig. 10c and d). The inherent CO₂ uptake capacity of COFs makes it different from traditional reductive photocatalyst. This study is the first case of COF-semiconductor Z-scheme heterojunction in the application of CO₂ reduction by pure water, and it can be seen that the efficient charge transfer and separation are of significance as well as in the organic-inorganic Z-scheme heterojunction towards CO₂ reduction.

Fig. 10

(a) Schematic representation of the preparation of COF-318-SCs via the condensation of COF-318 and semiconductor (b) Photocatalytic CO₂ to CO performances of COF-318-TiO₂ for gas-solid CO₂ reduction compared with COF-318, TiO₂ and COF-318/TiO₂ (c) Photocatalytic performances of various COF-318-SCs and bulk COF-318, TiO₂, Bi₂WO₆ and α-Fe₂O₃ (d) Schematic illustration of the charge transfer process under light irradiation with Z-scheme model (e) CO₂ RR process on COF-318-TiO₂ based on DFT calculations [32] Copyright 2020, Wiley-VCH

In the latest decade, the concept of isolated active site has been a hot topic in fields of photocatalysis and electrocatalysis. The isolated active site could maximize metal atom utilization efficiency and provide abundant active centers for the targeting reactions. Integrating the merits of Z-scheme dominant rapid charge separation and isolated active sites for CO₂ activation would undoubtedly improve the photocatalytic activities for CO₂ reduction. Jing and co-workers innovatively developed a zinc phthalocyanine/BiVO₄ nanosheet (ZnPc/BVNS) ultrathin nanocomposites for CO₂ photoreduction via a hydroxyl induced assembly strategy (Fig. 11a) [33]. The optimal one delivers ca. 16-fold enhancement in the quantum efficiency compared with the reported BiVO₄ nanoparticles at the excitation of 520 nm with an assistance of 660 nm photons. In addition, a certain amount of oxidation product O₂ with stoichiometric ratio is also produced. The impressive activities are primarily attributed to the cascade Z-scheme charge transfer between ZnPc and BVNS thanks to the dimension-matched ultrathin heterojunction nanostructure, which is comprehensively validated by experimental results and theoretical calculations (Fig. 11b-d). Importantly, the spatial separated electrons could be further transferred from the ligand of ZnPc to the isolated Zn-N₄ catalytic sites, leading to the accelerated CO₂ adsorption and activation (Fig. 11e). This work proposed a delicate reductive photocatalyst ZnPc which is distinguished from traditional ones, featuring isolated catalytic active sites and wide visible light absorption properties. This novel Z-scheme heterojunction opens up a new route for the design of highly efficient photocatalysts for CO₂ reduction from the perspective of both charge separation and catalytic active sites introduction.

Fig. 11

(a) Schematic illustration of the preparation of ultrathin ZnPc/BVNS heterojunction. SPS responses of 1ZnPc/BVNS and BVNS in N₂ atmosphere assisted with a (b) 660 nm and (c) 520 nm monochromatic beam (d) The time-dependent survival probability (TDSP) curves of the excited electrons transfer between partly hydroxylated BVNS and ZnPc during the injection process (Red: from ZnPc to BVNS; Blue: from BVNS to ZnPc) (e) Proposed cascade Z-scheme mechanism of photogenerated charge transfer on MPc-coupled BVNS. M represents a transition metal [33] Copyright 2019, Wiley-VCH

Since the pioneering work of the new step-scheme (S-scheme) heterojunction with WO₃/g-C₃N₄ as the first model was reported in 2019, plenty of related research is undergoing rapid development [79]. The emergence of S-scheme heterojunctions is based on the direct Z-scheme heterojunctions, while it aims to reveal the photocatalytic mechanism clearly and vividly. The charge transfer route in the S-scheme heterojunction is similar to the Z-scheme one, that is, the photogenerated electrons in the CB of oxidation photocatalyst would recombine with the photogenerated holes in the VB of reduction photocatalyst, and the spatial separated electrons and holes with high reductive and oxidative capacities could be preserved to initiate redox reactions. While the recombination step of the pointless electrons and holes in S-scheme heterojunctions is emphasized with the driving force, namely, the internal electric field, band bending, and Coulombic attraction. In this condition, the S-scheme heterojunction dominant charge separation would be more powerful than Z-scheme ones.

Yu et al. fabricated a unique 2D van der Waals heterostructure of Ti₃C₂ MXene quantum dots decorated TiO₂/C₃N₄ nanosheets for CO₂ photoreduction into hydrocarbon fuels [34]. The constructed 2D/2D/0D TiO₂/C₃N₄/Ti₃C₂ heterojunction exhibits much higher photocatalytic CO₂ reduction activity compared with that of TiO₂, C₃N₄, TiO₂/C₃N₄ and C₃N₄/Ti₃C₂. The S-scheme heterojunction at the TiO₂/C₃N₄ interface plays a critical role in enhancing the charge separation, and the formation of Schottky heterojunction at the C₃N₄/Ti₃C₂ quantum dots interface further facilitate the electrons transfer from C₃N₄ to Ti₃C₂ quantum dots. This charge modulation strategy is similar with the above mentioned one (T-CN/BVNS), in which TiO₂ as the energy platform to direct charge transfer in the cascade Z-scheme heterojunction [30]. It can be concluded that to maximized the charge transfer and separation is vital to the improvement for the photocatalytic activities for CO₂ reduction.

5 Conclusions and perspectives

At present, some significant advances in photocatalytic CO₂ reduction have been achieved, especially on the strategies for improving the activities of CO₂ photoreduction which concentrated on the CO₂ adsorption and activation, water oxidation kinetics acceleration, and highly efficient charge transfer and separation. Even though some successes have been made in designing CO₂ reduction systems, and reveal the reaction mechanism and pathway, the desirable activity and the selectivity of the target products are still in their infancy stage. There are still several perspectives for improvement in further architecture of photocatalysts toward solar fuels production with great challenges ahead.

First of all, comprehensive photocatalytic CO₂ reduction systems with strong light absorption capacity, rapid charge separation property, and abundant catalytic active sites need to be developed. It is well acknowledged that the overall efficiency of CO₂ reduction depends on these three core parameters, while the selectivity is determined by both the thermodynamic potential and kinetic energy barriers in terms of the CO₂ reduction pathway. Heterojunction photocatalysts, especially for Z(S)-scheme ones could tackle the irreconcilable aspects of visible light harvesting and the large bandgap as far as the sufficient redox potentials. Hence, rational design oxidative and reductive constituents with emerging materials, such as COFs, MOFs, and functional metal phthalocyanines, etc. that endowed with broad visible light response and catalytic active sites to construct integrative Z(S)-scheme heterojunctions, are promising routes to access the desirable photocatalytic CO₂ performance. Importantly, the tunable energy band alignments, enriched porous structure, the well-defined coordination environment and the isolated active sites provide plenty of opportunities for rationalizing the configuration of Z(S)-scheme heterojunctions, as well as improving the selectivity for the target reduction products.

An additional challenge is the standardized evaluation of the CO₂ photoreduction performance. Most current studies quantified the photocatalytic activities of CO₂ reduction via the formation rate of a certain product, and the final performance is presented in terms of μmol h^− 1, or μmol h^− 1 g^− 1 by normalizing the mass of the used photocatalyst. However, it is not quite rational for the comparison between different works in this way since the varieties of reaction systems, light intensities, and the mass of the used photocatalyst, etc. Even though to quantify the photocatalytic activities of CO₂ reduction by AQY could improve the accuracy of the performance evaluation, it is still somewhat not accurate as one hole scavenger can dramatically affect AQY. Therefore, a specific and unified regulation is essential to be established, including light source (wavelength and intensity), irradiation areas, photocatalyst dosage, and the volume of the used water, etc. Alternatively, the solar-to-chemical conversion efficiency which reflects the energy conversion ability of a photocatalyst could also be determined as a key criterion to evaluate the performance of CO₂ photoreduction.

A critical issue that should be taken into account in CO₂ photoreduction is the modulation of water oxidation half-reaction. It is accepted that the water oxidation half-reaction is a more challenging and rate-determining step in the overall CO₂ reduction reactions. Whereas, very few studies are focused on the oxidative aspect. The typical modulation strategies for facilitate O₂-evolution in photocatalytic water splitting are also applicable in designing the oxidative constituents of heterojunctions towards efficient CO₂ reduction and water oxidation.

Adjusting the selectivity of product is of great significance to improve the yield of high-value-added chemicals or even liquid fuels. Generally, water is more preferentially adsorbed onto the surface of photocatalysts than CO₂. Undoubtedly, the fierce competing HER is inevasible in CO₂ photoreduction by water. On one hand, it is feasible to introduce precise active sites such as metal single atoms with highly selective adsorption ability toward CO₂ molecules in order to guarantee the CO₂ instead of water to be reduced first. On the other hand, to achieve C=O bond cleavage and C-H bond formation simultaneously is imperative. Construction of ingenious dual sites with different functions of C=O bonds activation and hydrogenation is a viable strategy to access CO₂ conversion to CH₄. The d-band centers and the geometric structures for transition-metal atoms would be easily influenced when combining with another metal, and the electronic structure change could favor the sustainable multi-electron attracted CO₂ reduction process to hydrocarbon. Another crucial direction for improving the selectivity of CO₂ photoreduction lies in the rational design of photocatalysts. Considering the inherent characters of the employed photocatalyst for CO₂ adsorption, porous materials such as zeolites and metal-organic frameworks (MOFs), etc. with high surface areas, tunable porous structures, pore volumes, and dimensionalities are promising candidates to capture CO₂ molecules in their cavities for selective CO₂ conversion. The confinement of pores would play vital role in restricting reaction intermediates to achieve hydrogenation for the CH₄ production. Beyond that, the surface modification of photocatalysts would governing the CO₂ adsorption, activation, and further the selectivity. For instance, the engineering of surface defects includes oxygen vacancies, sulfur vacancies, the introduction of functional Lewis basic sites, as well as loading targeted noble-metal cocatalysts. Moreover, it is challenging convert CO₂ to multicarbon fuels with high energy density. The key step for the transformation of CO₂ to C₂₊ products is the sluggish kinetics process of C-C coupling. To stabilize and enrich the related intermediates on the active sites of a photocatalyst would allow the intermediates to be further protonated as well as increase the C-C coupling possibility, resulting in the evolution of C₂₊ or liquid products.

Another vital aspects are the identification of the active sites in CO₂ conversion, revelation of the mechanism of reaction process, and reaction pathway, which are favorable for the understanding of product selectivity of CO₂ reduction. In general, the identification of active sites could be achieved only under reacting conditions. Therefore, to precisely and comprehensively confirm the active sites with advanced characterization techniques is always pursued, especially in-situ and time resolved ones. For instance, in-situ Raman spectroscopy could deliver detailed information about chemical structures, and investigate the interactions between active sites of the photocatalyst and the adsorbed reactant. In situ nearly ambient pressure XPS (NAP-XPS) under working conditions is a powerful technique to reveal the intermediates species on the catalyst during the process of photocatalysis. Combining with the information of the detected change of surface groups and intermediates on a photocatalyst by in-situ Fourier transform infrared spectroscopy (FT-IR), a convincible and comprehensive process of CO₂ conversion on active sites could be deduced. Meanwhile, the acquired information about reaction process kinetics and possible intermediate radicals could also reveal the selectivity of products. In addition, in situ electron paramagnetic resonance (EPR) spectroscopy could detect the unpaired electrons or radicals, which normally formed on the active sites. In this regard, the involved charge transfer associated with active sites in CO₂ conversion could be directly unveiled by in-situ EPR measurement. Moreover, TAS is a powerful tool to uncover the electron-hole dynamics of semiconductors at a wide timescale. Although the applications of TAS in the field of CO₂ reduction are still limited, the development of in-situ and operando TAS with specific conditions (different atmosphere along with the saturated aqueous solution) would deeply elucidate the complicated reaction process of CO₂ reduction by pure water from the perspective of charge carrier dynamics.