Roles and Properties of Cocatalysts in Semiconductor-Based Materials for Efficient CO₂ Photoreduction

Abstract

With the development of human society, excessive CO₂ emission has caused a series of environmental problems. Semiconductor-based CO₂ photocatalytic reduction (CO2PR) technology could not only replace the traditional fossil fuels into renewable solar energy but also address the above problematic CO₂ emission, which has attracted worldwide attentions. Generally, the CO₂PR reaction involved three key steps: light excitation of the semiconductor, photo-generated electron/hole pair separation and transfer, and the surface catalytic reactions. However, without modifications, bare semiconductors often show poor performance in both the activity and the selectivity in CO₂PR. Hopefully, with the assistance of suitable cocatalysts, the performance of the photocatalyst composite can be greatly enhanced. In this chapter, we summarized the most efficient and common cocatalysts in CO₂PR such as metal or alloy nanoparticles (NPs), graphene, carbon dots, metal organic frameworks (MOFs), semiconductors, etc. in recent years. Besides the synthesis methods and CO₂PR evaluation measurements, we focused on the discussion of the distinct roles and properties of the cocatalysts combined with the advanced characterization techniques. Lastly, a short perspective further points out the challenge and limitations in this field. We hope this review could bring inspiration to the development of highly efficient photocatalytic system in CO₂PR through the deep understanding of the roles of cocatalysts in the CO₂PR reaction.

12.1 Introduction

Since the industrial revolution, the rapid development of the economy resulting in the emission amount of CO₂ to the atmosphere increased year by year. As a greenhouse gas, CO₂ could adsorb the solar energy and raise the temperature itself. Consequently, the excessive CO₂ emission would cause global warming and lead to serious environmental problems such as ice mountain melting, sea level rise, ecological balance break, etc., which have gain great attention from the whole world [1]. Recently, the Intergovernmental Panel on Climate Change (IPCC) reports that currently global warming has raised the average temperature of 1.5 °C compared with the preindustrial level [2]; no wonder, it is urgent for us to find possible solutions to cope with the CO₂ excessive emission problem. Therefore, the efficient capture and storage of CO₂ to further transform it into desirable chemicals with renewable energy input could address the above problems ideally.

Since Fujishima and Honda observed the H₂ production by TiO₂ electrode under light irradiation [3], semiconductor-based photocatalysts and its applications in environment management and energy transformation have gained much attention [4]. Some reported semiconductors like TiO₂, C₃N₄, CdS, etc.. possessed suitable bandgaps and reduction potentials which can trigger the CO₂ photoreduction reaction (CO2PR) with H₂O. However, as a very stable molecule, CO₂ photoreduction with H₂O is difficult to occur, owing to this process involved with multiple steps like breaking the C=O bonds and forming the C-H bonds, which both are endothermic reactions and also required abundant electrons and corresponding H protons. In addition, as a major competing reaction, H₂O reduction to H₂ is more easily to occur in both thermodynamics and kinetics [5,6,7]. In this way, bare semiconductors without modification often show low CO₂ photoreduction activity and selectivity.

In the use of semiconductor as the photocatalyst to trigger CO₂ conversion with H₂O, the activity is mainly affected by three key factors: (i) light adsorption and the excitation of semiconductors, (ii) photo-generated electron-hole pairs’ separation and transfer, and (iii) surface catalytic reactions. Based on these principles, semiconductors modified with suitable cocatalysts can greatly boost the CO2PR activity and selectivity [4, 8]. In Table 12.1 we summarized the semiconductor-based photocatalysts with various cocatalysts loaded in CO2PR based on the researches in recent years. The content of Table 12.1 including the optimal products (such as CH₄, CO, CH₃OH, HCOOH, and H₂) yields brief introduction of photocatalysts’ preparation, CO2PR evaluation details, etc. Among these cocatalysts, for example, noble metal NPs like Pt [8,9,10,11,12,13], Au [14], Pd [15,16,17,18], Ag [19,20,21], etc., lower Fermi level is often considered as efficient cocatalysts for electron trapping and active site with suitable binding energy to some intermediates [5]; combined semiconductors with suitable band structures together can form p–n junction or Z-scheme and thus favored the light adsorption and charge separation [22,23,24,25]; the semiconductor–MOF or semiconductor–graphene composites often show enhanced CO₂ adsorption performance and effective CO₂ activation properties, etc.

Table 12.1 Semiconductor-based photocatalysts with cocatalysts in CO₂ photoreduction

In this chapter, we plan to present a short review to discuss about the cocatalysts including metal NPs, metal alloy NPs, graphene, carbon nanodots, MOFs, semiconductors, etc., synthesis methods, and basic roles in CO₂ photoreduction based on recent research progress. The advanced techniques such as time-resolved DRIFTS, time-resolved PL decay, EPR, and DFT calculations applied to clarify the charge transfer mechanism and surface catalytic reaction pathways are also discussed in detail.

12.2 Basic Principles of CO₂ Photoreduction

Generally, semiconductors can be excited by photon carriers with energy higher than its bandgap energy; after the excitation, the photo-generated electron-hole pairs would migrate to the surface for certain reduction/oxidation reactions or recombine together and release energy by means of heat. In order to catalyze the CO2PR reaction with H₂O, the photo-generated electron-hole pairs must possess suitable reduction and oxidation potential (Fig. 12.1). According to previous reports, different standard reduction potentials of CO₂ reduction with H protons to yield different products are shown in Eqs. 12.1, 12.2, 12.3, 12.4, and 12.5 [5, 26].

$$ {\mathrm{CO}}_2+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to \mathrm{HCOOH}\kern2em {{\mathrm{E}}_{\mathrm{redox}}}^0\kern0.5em =-0.61\mathrm{V} $$

(12.1)

$$ {\mathrm{CO}}_2+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to \mathrm{CO}\kern3em {{\mathrm{E}}_{\mathrm{redox}}}^0\kern0.5em =-0.53\mathrm{V} $$

(12.2)

$$ {\mathrm{CO}}_2+{6\mathrm{H}}^{+}+{6\mathrm{e}}^{-}\to {\mathrm{CH}}_3\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}\kern0.75em {{\mathrm{E}}_{\mathrm{redox}}}^0\kern0.5em =-0.38\mathrm{V} $$

(12.3)

$$ {\mathrm{CO}}_2+{8\mathrm{H}}^{+}+{8\mathrm{e}}^{-}\to {\mathrm{CH}}_4+{4\mathrm{H}}_2\mathrm{O}\kern1.25em {{\mathrm{E}}_{\mathrm{redox}}}^0\kern0.5em =-0.24\mathrm{V} $$

(12.4)

$$ {\mathrm{CO}}_2+{\mathrm{e}}^{-}\to {{\mathrm{CO}}_2}^{-}\kern6.25em {{\mathrm{E}}_{\mathrm{redox}}}^0\kern0.5em =-1.90\mathrm{V} $$

(12.5)

Fig. 12.1

Band structures of several common semiconductors and corresponding standard reduction potential involved with CO₂ and H₂O photocatalytic reduction at pH =7

In most semiconductor-based CO2PR reactions, H₂O is selected as the sole electron donor, in this case, the H₂O oxidation reaction to release H protons and O₂; hydrogen evolution reaction (HER) to generate H₂ has also two major steps: Eqs. 12.6 and 12.7:

$$ {2\mathrm{H}}_2\mathrm{O}+{4\mathrm{h}}^{+}\to {4\mathrm{H}}^{+}+{\mathrm{O}}_2\kern3em {{\mathrm{E}}_{\mathrm{redox}}}^0\kern0.5em =-0.82\mathrm{V} $$

(12.6)

$$ {2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_2\kern5em {{\mathrm{E}}_{\mathrm{redox}}}^0\kern0.5em =-0.41\mathrm{V} $$

(12.7)

Owing to the complexity of CO₂ reduction reaction that involved multi-electrons and protons’ participation and various intermediates’ adsorption/desorption processes, the deep understanding of the CO₂ reduction is still unclear both experimentally and theoretically. To date, researchers have proposed two classic CO₂ reduction pathways which are called formaldehyde pathway (also known as multiple proton-coupled electron transfer) and carbene pathway (also known as deoxygenation path), respectively [5, 27]. However, the carbene pathway is initially involved with one electron’s CO₂ reduction to generate CO₂ ⁻ (Eq. 12.5) which cannot be accomplished by many semiconductors owing to the limited reduction potential (Fig. 12.1) [5, 26]. Recently, Ji et al. [27] using TiO₂ (101) as the prototype proposed a new mechanism which involved CO₂ photoreduction at Ti site and oxygen vacancy site based on DFT calculation. Their result shows that the O vacancy (O_v) served as the active site to bind the intermediates like CH₂O or CH₃O and facilitate the CH₄ or CH₃OH generation; besides, the O_v also offers two electrons to protect the intermediates from reoxidation. Meanwhile, the intermediates adsorbed on Ti site could be oxidized by holes rapidly and result in low CO₂ photoreduction efficiency. Still, considering the alternative catalytic conditions among different photocatalysts, the study of the reaction mechanism of CO2PR remains challengeable.

12.3 Cocatalysts in Semiconductor-Based CO₂ Photoreduction

12.3.1 Preparations

12.3.1.1 Metal–Semiconductor Composites

There are numerous methods that are involved with the metal precursors’ reduction and further deposit metal NPs on the semiconductors’ surface, leading to the formation of metal–semiconductor composite. The common methods include photo-deposition method, alcohol reduction method, chemical reduction method (common reducing agents such as NaBH₄, ascorbic acid, glucose, trisodium citrate, hydrazine, etc.), deposition–precipitation (DP) method, atomic layer deposition (ALD) method, etc. (Table 12.1).

Xie et al. [10] synthesized Pt–TiO₂ composites with the use of three different methods: (1) photo-deposition of Pt NPs on titania with 300 W Xe lamp as light source and methanol as the sacrificial reagent, (2) impregnation of Pt precursor with titania and followed by calcination treatment in H₂ at 673 K, and (3) hydrazine reduction of H₂PtCl₆ an aqueous solution containing titania. The TEM results indicate that the photo-deposition method and hydrazine reduction method both result in smaller-sized Pt particles (mean size, 3.7 and 4.2 nm, respectively) and the impregnation method followed by H₂ calcination will result in bigger-sized Pt particles (6.8 nm). Wang et al. [9] adopted a unique tilted-target sputtering (TTS) method for the ultrafine Pt cluster deposition on 1D TiO₂ single-crystal film. The loading amount of Pt and cluster size (0.5–2 nm) were manipulated by adjusting the deposition time (5–60s). Song et al. [15] studied the shape-dependent Pd/C₃N₄ few-layer composites in CO2PR. During the synthesis process, the author used HCHO and Na₂C₂O₄ to promote the formation of Pd (111) facets, while the Br⁻ and I⁻ were introduced to stabilize the Pd (100) facet. As a result, Pd cube/C₃N₄ and Pd nanotetrahedron/C₃N₄ can be well obtained through a solution-phase solvothermal method. Despite their shape, these two Pd polyhedrons have comparable particle size (4–6 nm), which are the smallest Pd nanocrystals with specific facets obtained in aqueous phase by now.

Compared with single-unit metal–semiconductor composite, binary metal alloy NPs with diverse surface active sites and metal-support interfaces thus could possess more potential in photocatalysis. In the synthesis of alloy NPs involved with at least two metal precursors, Long et al. [18] synthesized a series of PdxCu1 fcc-phased NPs in situ growth on the TiO₂ nanosheet in the presence of ascorbic acid and PVP. Through varying the ratio of K₂PdCl₄ to CuCl₂, sphere-like NPs of CuPd1Cu1, Pd3Cu1, Pd5Cu1, Pd7Cu1, and Pd11Cu1 could be obtained. Neaţu et al. [14] adopted the stepwise deposition–precipitation method to prepare Au–Cu alloy/TiO₂ photocatalyst, using 0.2 M NaOH to tune the pH value of TiO₂-HAuCl₄ and Au/TiO₂-Cu(NO₃)₂ aqueous slurry to 8.5 and annealing in air and H₂ atmosphere, respectively. The Au–Cu alloy NPs were characterized by HR-TEM which constitute Au–Cu lattice fringes; meanwhile, the redshift of Au SPR adsorption peak in UV-vis DRS spectra also suggests the Au and Cu formed alloy status. It should be noted that, using this synthesis method, the unalloyed Au and Cu NPs also could be detected.

12.3.1.2 Semiconductor Heterojunctions and Z-Scheme Composites

In order to develop the economic noble metal-free photocatalysts with high efficiency in CO2PR, some semiconductor junction composites have been developed; the common methods include hydrothermal/solvothermal method, impregnation method, self-template method, ALD method, etc. (Table 12.1).

Jin et al. [38] reported a hierarchical-structured Z-scheme CdS-WO₃ photocatalyst and applied it in CO2PR. The hierarchical hollow WO₃ spheres were formed by immersing SrWO₄ in HNO₃ at first, and then the precipitate was washed and calcined at 500 °C in air. The as-prepared WO₃ spheres were negatively charged at pH = 7; therefore, stepwise adding Cd²⁺ and S⁻ source slowly will generate heterostructure CdS–WO₃ composite. Wang et al. [34] developed a porous ZnO@Co₃O₄ composite by using ZIF-8 and ZIF-67 as precursor templates (Fig. 12.2). First, the ZIF-8@ZIF-67 core–shell structure was synthesized through a solvothermal process and then followed by a N₂-400 °C 2-h calcination and air-400 °C 2 h calcination treatment. The two-step calcination process was determined by the TG/DTA analysis, while one-step calcination under air atmosphere will lead to nonporous ZnO NPs.

Fig. 12.2

Schematic illustration of the synthesis of polyhedral ZnO and ZnO@Co₃O₄ originated from ZIF8 and ZIF-8@ZIF-67, respectively. (Reprinted with permission from Ref. [34]. Copyright 2016, Royal Society of Chemistry)

In et al. [37] designed a novel CuO–TiO₂–_xN_x hybrid hollow nanocubes with the use of CuN₃ nanocubes as reactive templates (Fig. 12.3a). After slow hydrolysis of titanium-n-butoxide on the surface of CuN₃, the calcination treatment at 450 °C was carried out. During the calcination process, the CuN₃ reacts with the oxygen and form hollow CuO nanocube; meanwhile, the nitrogen diffuses outward and reacts with the crystalline TiO₂ to form TiO₂–_xN_x. Park and coworkers [57] once proposed a novel Cu_xO–TiO₂ p–n heterojunction (Fig. 12.3b). The Cu/Cu₂O nanoparticles were first synthesized through a thermal decomposition method, and then the TiCl₄ was mixed with the Cu/Cu₂O NPs in argon; after calcination in air, the TiCl₄ crystallized to TiO_2, and the Cu/Cu₂O NPs were oxidized to Cu_xO with organic ligands removed at the same time; at last, the mesoporous Cu_xO–TiO₂ composites were obtained.

Fig. 12.3

(a) Scheme illustration of multistep template strategy to convert Cu₃N nanocube into TiO₂@Cu₃N and hollow CuO–TiO₂–_xN_x nanocubes. Reprinted with permission from Ref. [37]. Copyright 2012 Jonh Wiley & Sons, Inc. (b) Scheme diagram of the step-by-step synthesis of mesoporous Cu_xO–TiO₂ composite. (Reprinted with permission from Ref. [57]. Copyright 2016, American Chemical Society)

12.3.1.3 Dual Cocatalysts

Considering the precise construct of the configuration of photocatalyst with synergistic dual cocatalysts will enhance the CO2PR efficiency greatly. Recently, more and more researches have focused on the dual cocatalysts deposition with advanced structures. Generally, compared with sole cocatalyst composites, the synthesis of dual cocatalysts is more complicated, which need involve with stepwise deposition of dual units. The spatial locations of dual cocatalysts should depend on the functions of these two species which are either separated or combined with each other.

The construction of spatial separated electron trapping agents and hole collectors could greatly promote the charge separation efficiency of the photocatalyst. Dong et al. [8] developed a 3D hierarchical structured TiO₂–SiO₂ with CoO_x and Pt growing inside and outside of the skeleton. Firstly, the Co(AC)₂.4H₂O and Ti–Si sol were mixed together and underwent a synchronizing self-assemble process; after the 500 °C calcination, the hydrolyzed Co(OH)_x transformed into CoO_x NPs embedded under the hierarchical TiO₂–SiO₂ skeleton homogenerously (denoted as the HCTSO). Subsequently, the Pt NPs were growing in situ on the outer surface of the HCTSO via alcohol reduction of H₂PtCl₆. In order to improve the CO₂ adsorption and the selectivity toward CH₄, Pan et al. [47] developed a 5-nm-thick carbon layer coated on In₂O₃ nanobelt coupled with Pt NP+ loadings. The glucose was used as the carbon source and the carbon layer was formed at 600 °C under the Ar atmosphere. Afterward, the Pt NPs were deposited on the surface of carbon layer through a photo-deposition method. The loading amount of carbon layer and Pt are 8% and 2%, respectively. Another classic dual cocatalysts structure of Pt@Cu_xO loaded on TiO₂ (p25) was proposed by Zhai and coworkers [41]. At first, Pt–TiO₂ was first prepared by photoreduction of H₂PtCl₆ in the TiO₂ suspension; afterward, the Cu species were deposited on the Pt surface under the illumination and using the CuSO₄ as the precursor. The Cu is easily oxidized into Cu¹ in air; therefore, the Cu existed as Cu_xO form. Moreover, prolonging the photo-deposition time of Cu species will increase the Cu_xO coverage on Pt NPs; the 5-h irradiation will form a complete Pt@Cu_xO core–shell structure.

The all-solid-state Z-scheme photocatalysts could take advantage of more negative reduction potential electrons and more positive oxidation potential holes from different semiconductors counterparts, thus attract more and more attention in CO2PR. Generally speaking, the photosystem II (oxidation part PSII) and photosystem I (reduction part PSI) are connected by a conductor. Li et al. [45] developed an elegant all-solid Z-scheme WO₃/Au/In₂S₃ nanowire photocatalyst; the WO₃ nanowire was first grown on the tungsten foil under Ar flow with WO₃ powder as precursor, then Au NPs were deposited on WO₃ nanowire by plasma sputtering method, and the In₂S₃ shell coated on Au surface was finally obtained through a chemical vapor deposition method (In₂S₃ powder and Au/WO₃ were placed in quartz furnace separately, the temperature of the furnace will be increased to 800 °C with certain Ar flow, and the deposition time is 10 min).

12.3.1.4 Carbon-Based Cocatalysts

Besides the metal nanoparticles and semiconductors cocatalysts, there are intensive research works focusing on the carbon-based cocatalysts in CO2PR. Graphene, carbon nanotube, carbon nanodots, carbon nanosheet layer, metal organic frameworks (MOFs), metal–ligand complex, etc. are common carbon-based cocatalysts in recent years (Table 12.1).

Graphene, owing to its flexible mechanical strength, remarkable electric conductivity, high surface area, etc., has been applied to many fields. In photocatalysis, graphene can transfer the photo-generated electrons, hence improving the electron-hole separation efficiency and prolonging the lifetime of charge carriers (Fig. 12.5). Tu et al. [49] fabricated a sandwich structure TiO₂–graphene nanosheet over a one-step hydrothermal method. The graphene oxide (GO) was prepared according to Hummers’ method which is a common method for many reported graphene–semiconductor composites. During the hydrothermal process, the GO, Ti precursor, and solvent (ethylenediamine abbreviated as En/H₂O) underwent an in situ simultaneous reduction–hydrolysis process, the GO was reduced by En, and the Ti precursor hydrolyzed to form TiO₂ NPs. Different weight ratios of TiO₂/graphene were obtained by varying the GO amount during the synthesis. Ong et al. [58] adopted the electrostatistic self-assembly strategy to prepare the reduced graphene oxide (rGO)/protonated C₃N₄ (pCN) composites. Owing to abundant CN motifs existing on the g-C₃N₄ surface, the surface protonation by HCl could be easily conducted. After the HCl treatment, the pCN was positively charged according to the Zeta potential test, which could spontaneously assemble on the negative-charged GO (prepared by Hummers’ method). Finally, the GO was reduced to rGO by NaBH₄ to form 2D/2D rGO/pCN composite. Unlike 2D graphene–semiconductor composites, Zhang proposed that encapsulation by graphene-like carbon sheet could enhance the confinement effect of the core nanoparticles compared with its naked counterparts. Therefore, Fe@C NPs were fabricated for the use of MIL-101 as self-sacrificing template and precursor. During the synthesis, two-step calcination method was utilized; first, MIL-101(Fe) was collapsed and formed Fe₃C and Fe₃O₄ in Ar-500 °C; meanwhile, the Fe species could avoid sintering into large NPs and , subsequently, the temperature raised to 700 °C to obtain the Fe@C NPs. It should be noted that rational regulate the calcination temperature and retention time is the key to control the particle size and graphite carbon layer’s thickness.

Metal organic frameworks (MOFs), as one class of porous nanocrystals, possess huge surface area, tunable surface functional groups, and alternative compositions which have been applied to multiple fields such as catalysis, gas capture and separation, drug delivery, molecule identification, etc. Due to strong CO₂ adsorption capability of UiO-66, cooperation with some narrow bandgap semiconductor could improve the CO2PR efficiency. Shi et al. [56] developed an electrostatic self-assembly strategy to fabricate the UiO-66/C₃N₄ composite. Firstly, the C₃N₄ nanosheet (CNNS) was prepared by liquid-state ultrasound exfoliation method; after the centrifuge to remove the large bulk C₃N₄, the CNNS was obtained. Secondly, the as-prepared UiO-66 and CNNS were mixed in water, because the CNNS is negatively charged in water with −35.91 mV Zeta potential and + 7.71 mV for UiO-66; that is the reason why electronic statistic self-assembly happened. Li et al. [53] developed a Cu₃(BTC)₂ (HKUST-1)@TiO₂ core–shell structured composite; the solid Cu precursor and the involvement of PVP are key to coat TiO₂ on the Cu₃(BTC)₂ nanocrystals uniformly. During the control experiments, using Cu(OH)₂ as Cu precursor and in the absence of PVP, the TiO₂ cannot be coated on the Cu₃(BTC)₂ uniformly; besides, using unsolid Cu(NO₃)₂ as precursor, the thermal stability of Cu₃(BTC)₂ is low; in this case, it will decompose at the 180 °C coating process and also cannot get desirable result.

Carbon nanodots, including carbon quantum dots, carbon dots, and graphene quantum dots, which are a new class of zero-dimensional (0D) carbon materials have attracted people’s attention over the past few decades; the unique properties of carbon dots such as superior up-conversion and size-dependent photoluminescence, high stability, low cytotoxicity, earth abundance, etc., thus made it a plausible candidate in many fields. The synthesis of carbon nanodots can be roughly classified into two approaches: bottom-up approach and top-down approach. Kang et al. [59] reported a facile electrochemical approach to synthesize large-scale high-quality carbon dots. The authors used two graphite rods as the counter electrode and ultrapure water as the electrolyte; statistic potential with 15–60 V was applied to the two electrodes; after 120-h electrolysis, a dark-yellow solution was formed, and the water-soluble carbon dots were obtained after filter and centrifuge. Ong et al. [55] adopted glucose as the carbon source using alkali-assisted ultrasonication method to prepare carbon nanodots (CND). Briefly, glucose and NaOH solution was mixed together and sonicated for 2 h and resulted in a dark-brown solution; after neutralization and filter, a brown carbon dot solution was obtained. Owing to the natural properties of same negative polarity of C₃N₄ and carbon dots, the coupled CND/C₃N₄ in this paper was obtained by protonation C₃N₄ in HCl solution in advance; after that, the C₃N₄ is positively charged thus can attract the CND by electrostatic force.

12.4 Roles and Properties of Different Cocatalysts

12.4.1 Promote the Charge Separation and Transfer

It is well-known that the noble metal NPs such as Pt, Pd, Au, Ag, Ru, etc. loaded onto the semiconductors could trap the photo-generated electrons and promote the separation of charge carriers. The reason could be attributed to the Fermi level of metal NPs which lies energetically below the conduction band level of its semiconductor counterpart; besides, the Schottky barrier formed at the metal–semiconductor interface thus could prevent the electrons from flowing backward. In this way, the surface sites of metal NPs become the active sites for the CO2PR reduction, and the performance of the metal–semiconductor composite is highly depending on the electron trapping ability of the supported metal NPs.

Xie et al. [10] compared the activity of five noble metals (Ag, Au, Rh, Pt, Pd) supported on TiO₂ in CO2PR. The yield of CH₄ and the rate of total electrons’ consumption in the CO2PR increase with the order of TiO₂ < Ag-TiO₂ < Rh-TiO₂ < Au-TiO₂ < Pd-TiO₂ < Pt–TiO₂, which equals with the same trend of the work function of these noble metals. This result reflects the fact that the electron trapping ability contributes to the reductive performance of supported metal catalyst and Pt–TiO₂ in this evaluation system is superior to the others. Since Pt is a very efficient cocatalyst in photocatalysis, the rational designation of Pt NPs with suitable shape (expose certain facets) and particle size (both geometric and electronic) is important. As we mentioned before, Wang et al. [9] synthesized a series of different-sized Pt NPs ranging from 0.5 to 1.5 nm loaded on the 1D TiO₂ single crystals through a TTS method, and the 1 nm Pt NPs show the highest CH₄ yield. The author claims that the ultrasmall Pt NPs (less than 1 nm) could prevent the electrons transferring from TiO₂ because of its higher energy band compared with the CB of TiO₂; on the contrary, bigger Pt NPs act as electron-hole recombination center which is also detrimental in the photocatalysis. Furthermore, the author adopted the femtosecond time-resolved TA spectroscopy to elucidate the charge transfer dynamics. After liner fitting, Pt–TiO₂ shows a greater slope compared with its TiO₂ counterpart which directly reflects the Pt NPs suppress the charge recombination process.

In order to replace the expensive noble metal cocatalysts into some earth-abundant materials, the development of noble metal-free cocatalysts with comparable performance is essential. Among them, carbon dots and graphene also play key role in promoting the charge separation in photocatalysis. Ong et al. [55] report carbon nanodots (CND) supported on protonated C₃N₄ composite. The obtained CND/pCN shows the CND with 4.4 nm diameter dispersed well on the pCN surface, and the CND did not affect the adsorption edge of C₃N₄ but act as conductive electron channel for charge separation (Fig. 12.4a, b). The author adopted steady-state PL spectroscopy and time-resolved transient PL decay to verify the charge separation kinetics. The pure p-C₃N₄ shows an intensive and broad PL emission peak which means a great extent of electron-hole recombination; the CND/pCN hybrids on the other hand show obvious decrease of peak intensity which suggests the recombination of charge carriers were suppressed (Fig. 12.4c). The emission lifetime of CND/pCN reduced compared with pCN which means the rapid interfacial electrons inject from pCN to CNDs and participate in the CO2PR reaction (Fig. 12.4d). Besides the electrons’ trapping ability, CNDs also can serve as the photosensitizer. Yu et al. [60] reported CDs/TiO₂ composite with enhanced visible light hydrogen production rate. The author claimed that π-conjugated CDs sensitize TiO₂ by forming C-O-Ti bond and donate electrons under visible light irradiation.

Fig. 12.4

(a) TEM image of CND/pCN-3 sample, the inset image shows the particle size distribution of CNDs deposited on the pCN nanosheet. (b) UV–vis DRS spectra of different samples; the digital photographs showing the colors of different samples are inset. (c) PL spectra of pCN and CND/pCN samples. (d) Time-resolved transient PL decay curves of pCN and CND/pCN samples excited at 405 nm. (Reprinted from Ref. [55]. Copyright 2017, Wee-Jun Ong et al. licensee Springer)

When graphene was introduced as the cocatalyst, Yu et al. [54] developed a metal-free CdS/rGO composite with enhanced CH₄ generation rate (2.51 μmol/g·h) which is ten times higher than pure CdS rods and overperforms the Pt/CdS. The enhanced performance was attributed to the π–π conjugate interaction between CO₂ and graphene and thus improves the CO₂ adsorption amount and destabilizes CO₂; besides, the rGO promotes the electron transfer, and storage was confirmed by conducting transient photocurrent and impedance analysis (Fig. 12.5b). Recently, Xu et al. [52] reported a CsPbBr₃ perovskite graphene composite which could efficiently convert CO₂ into CH₄ with 99.3% selectivity. The author also adopted steady-state PL and time-resolved PL decay to probe the electron transfer dynamic. Distinct PL intensity quenching of CsPbBr₃ QD and the PL decay time of CsPbBr₃ QD/GO composite are shorter compared with CsPbBr₃ QDs, which both reflect the introduction of GO benefits to the electron transfer and suppress the electron-hole recombination (Fig. 12.5d).

Fig. 12.5

(a) Schematic illustration of charge separation and transfer in G-TiO₂ system and photoreduction of CO₂ and H₂O. Reprinted with permission from Ref. [49]. Copyright 2013 Jonh Wiley & Sons, Inc. (b) Schematic illustration of charge separation and transfer in CdS-rGO composite. Reprinted with permission from Ref. [54]. Copyright 2014 Royal Society of Chemistry. (c) Schematic illustration of the charge transfer and separation in rGO/pCN nanocomposite for CO₂ photoreduction with H₂O to CH₄. Reproduced from Ref. [58] with permission of Elsevier. (d) Schematic diagram of CO₂ photoreduction over CsPbBr₃ QDs/rGO. (Reprinted with permission from Ref. [52]. Copyright 2017 American Chemical Society)

Construction of the semiconductors’ heterojunction is another strategy to improving the charge separation efficiency in CO2PR. Shi et al. [61] reported a visible light responsive g-C₃N₄/NaNbO₃ nanowire with higher CO2PR activity than either g-C₃N₄ or NaNbO₃. From Fig. 12.6 (a), the HR-TEM image shows the obvious intimate interface, which implies the existence of heterojunction between C₃N₄ and NaNbO₃. The band structures of g-C₃N₄ and NaNbO₃ were determined by UV–vis DRS and VB-XPS in Fig. 12.6 (b, c). The wavelength (λ) of adsorption edge of g-C₃N₄ and NaNbO₃ is 365 nm and 450 nm, respectively; therefore, the E_g (bandgap energy = 1240/λ) was calculated to be 3.4 eV and 2.7 eV, respectively. Meanwhile, the VB XPS result shows that the E_VB (valence band energy) of g-C₃N₄ and NaNbO₃ was located at ~1.57 eV and 2.7 eV, respectively; therefore, the E_CB (conduction band energy) of g-C₃N₄ and NaNbO₃ was calculated to be −1.13 eV and − 0.77 eV based on the equation E_CB = E_VB – E_g. Since the CB level of C₃N₄ is more negative than NaNbO₃, the photo-excited electrons from C₃N₄ could migrate to NaNbO₃ and suppress the electron-hole carriers’ recombination.

Fig. 12.6

(a) HR-TEM image of g-C₃N₄/NaNbO₃ heterojunction. (b) UV–vis DRS spectra of different samples. (c) VB-XPS spectra of g-C₃N₄ and NaNbO₃. (Reprinted with permission from Ref. [61]. Copyright 2014, American Chemical Society)

Although p–n heterojunctions greatly inhibit the recombination of photo-generated electrons and holes, however, after the photo-generated electrons migrate to the CB with lower reduction potential, the redox ability of the integral composite will be impaired [23]. Therefore, the all-solid-state Z-scheme system was developed to solve this problem. Generally, the all-solid-state Z-scheme photocatalytic system (PS) was divided into two components: PS-PS and three-component PS-conductor(C)-PS. Wang et al. [39] construct an indirect PS-PS Z-scheme BiOI/C₃N₄ composite with enhanced visible light CO2PR performance (Fig. 12.7a). In order to investigate the charge transfer modes, the author proposed two possible charge transfer routes: double-transfer mechanism and Z-scheme mechanism. The contrast experiment was adopted by using visible light as the light source; the result of no product yield proved that the electron transfer was not followed by the previous one. Furthermore, when the EDTA was added as the hole scavengers, the result shows an improved CO and H₂ yield but decreased O₂ yield which further reflects the enhanced electron-hole separation (Fig. 12.7b).

Fig. 12.7

(a) HR-TEM image of 7.4-BiOI/C₃N₄ composite. (b) Schematic illustrations of double-charge transfer mechanism (left) and Z-scheme charge transfer mechanism (right). (Reprinted with permission from Ref. [39]. Copyright 2016 American Chemical Society)

Compared with the PS–PS Z-scheme, the interface between two solids usually contains many defects, which may inhibit the charge transfer. Therefore, the PS-C-PS Z-scheme with a conductor insertion could reduce the electron transfer resistance and thus improve the CO2PR efficiency. He et al. [25] reported an Ag₃PO₄/C₃N₄ composite with enhanced CO2PR activity. Since the Ag₃PO₄ is not stable, therefore, Ag NPs were formed in situ within the composites under light irradiation and acted as the electron mediator. Taking into account of the CB level of Ag₃PO₄ (0.45 eV), if the composites followed the double-charge transfer mechanism, the introduction of Ag₃PO₄ cannot promote the CO2PR. So it is reasonable to believe that the charge transfer route followed Z-scheme mechanism. In this way, Ag accepts the photo-generated electrons from Ag₃PO₄ and recombines with holes from C₃N₄; subsequently, the photo-generated electrons with more negative reduction potential could be used into the CO2PR reaction. Wei et al. [44] developed a PS–C–PS Z-scheme structure photocatalyst that contains CdS (shell), Pt (core), and TiO₂ (support), which show enhanced CO₂ photoreduction activity and selectivity (36.8 μmol/g.h CH₄ yield and 98.1% CH₄ selectivity). The location of reduction sites and electron transfer route was confirmed by Ag photo-deposition method. The Ag NPs selectively deposited on the shell of CdS instead of TiO₂ surface which clearly demonstrates that the CdS acts as the reduction site and the electron transfer follows the TiO₂ → Pt → CdS route. In order to present consolidate proof to Z-scheme charge transfer behavior, Li et al. [45] first adopt Kelvin probe force microscopy to detect surface potential change of In₂S₃-Au-WO₃. Compared with WO₃/In₂S₃, the SPV image (reflect the concentration of photo-generated holes) of WO₃/Au/In₂S₃ shows significant change from 10 mV to 30 mV; this difference vividly reflects the efficient charge separation and the role of Au as the electron mediator.

Another way to improve the charge separation efficiency in photocatalysis is the construction of double cocatalysts (usually refers to the electron trapping agent and hole collect agent) with spatial separated configurations. Domen and coworkers [62] first developed Ta₃N₅ photocatalyst hollow shell with Pt and CoO_x deposited inside and outside of the shell, respectively. Followed by this pioneered work, similar strategies have been proposed such as the thin heterojunction Pt–TiO₂@In₂O₃@MnO_x hollow shell structure, porous TiO₂ tube, or hollow C₃N₄ shell with spatial separated Pt and CoO_x NPs [63,64,65], etc. Recently, our group developed a new strategy to construct spatial configuration by introducing Pt NPs and CoO_x NPs outside and inside of the skeleton of hierarchical TiO₂-SiO₂ (HTSO) [8], abbreviated as Pt/HCTSO. The HR-TEM image clearly indicates that the Pt NPs and CoO_x NPs separated by the HTSO skeleton (Fig. 12.8a); on the other hand, EDS-mapping image shows the Pt and Co species are well-dispersed throughout the framework of HTSO and no aggregation happened (Fig. 12.8b). The CO₂ photoreduction evaluation result revealed that the 0.8% Pt/HCTSO (0.8%) shows enhanced CH₄ yield and selectivity for CH₄ which are 1.9 and 4.4 times higher than 0.8% Pt/HTSO. To highlight the spatial locations of Pt and CoO_x in CO2PR, the Pt-CoO_x/HTSO was prepared by randomly loading Pt and CoO_x on the surface of HTSO; the CO2PR result shows even lower CO₂ reduction activity. The enhanced transient photocurrent response and decreased intensity of PL emission peaks (360 and 470 nm) all confirmed the spatial separated double cocatalysts promote the charge separation effectively (Fig. 12.8c, d). On the contrary, the random loading of Pt and CoO_x on the surface of HTSO results in many electron-hole recombination centers, which is detrimental to charge separation and thus shows poor performance in CO2PR.

Fig. 12.8

(a) TEM and selective HR-TEM images of Pt/HRTSO. (b) Elemental mapping image of Pt/HRTSO, the red dots denote as Pt element and the green dots denote as Co element. (c) Transient photocurrent spectra of different samples (300 W Xe lamp with AM 1.5 filter was used as the light source and 0.5 M Na₂SO₄ solution is used as the electrolyte). (d) Room temperature PL spectra of different samples (excitation wavelength at 315 nm). (Reprinted with permission from Ref. [8]. Copyright 2016, Royal Society of Chemistry)

12.4.2 Improve CO₂ Adsorption and Activation

The CO₂ adsorption and activation on the surface of photocatalyst are two important steps; however, without modification, common semiconductor-based photocatalysts often show low CO₂ uptake. Therefore, combining some unique cocatalysts with higher CO₂ adsorption amount should be a proper way to improve CO2PR efficiency. Xie et al. [66] in the use of MgO, a basic metal oxide, as the cocatalyst deposited on the TiO₂ surface, with the addition of Pt NPs, the Pt-MgO/TiO₂ composite shows an enhanced activity for CH₄ production. During the experiments, a linear relationship between different CO₂ chemisorption by different basic metal oxide-modified Pt–TiO₂ and CH₄ yields clearly demonstrates the important role of CO₂ adsorption; the MgO modification shows the highest CO₂ chemisorption compared with other basic metal oxides. Besides, the optimal MgO content is measured to be 1%; excess MgO adding will cause a thicker MgO layer and cover the Pt sites which are detrimental for CO₂ photoreduction. Li et al. [53] adopt MOF (Cu₃(BTC)₂) as the CO₂ adsorption cocatalyst and coat porous TiO₂ shell on the MOF crystals’ surface. This unique design hybrid shows enhanced CH₄ yield and selectivity compared with bare TiO₂ counterpart. The CO₂ adsorption results between bare MOF and MOF@TiO₂ suggest the CO₂ molecules can easily pass the TiO₂ shell. In order to investigate the charge transfer and working mechanism, the author adopts TA analysis and first-principle simulation. The result indicates the photo-excited electrons can transfer to the MOF core; subsequently, the CO₂ molecules adsorbed in MOF can be activated and convert into CH₄ effectively. Similarly, Shi et al. [56] reported a C₃N₄/UiO-66 composite, in this work zirconium-based MOF: UiO-66 acts as both CO₂ absorber and a semiconductor-like material to promote the electron-hole separation. ESR was used to verify the electron transfer route. Specifically, signal of g = 2.009 is attribute to O₂ ⁻ which was found in C₃N₄/UiO-66 under visible light irradiation but absent in pristine UiO-66. This indicates that the C₃N₄ was performed as a photosensitizer; the photo-generated electrons transferred to UiO-66 and thus suppress the electron-hole recombination and enhance the CO2PR performance. Pan et al. [47] reported a carbon-coated In₂O₃ photocatalyst with the use of glucose as the carbon source; the 5 nm carbon layer could enhance the CO₂ chemisorption and suppress the hydrogen generation (Fig. 12.9). Compared with the pure In₂O₃ nanobelt, C-In₂O₃ shows enhanced CO₂ adsorption capacity compared with pristine In₂O₃ nanobelt, and the maximum CO₂ adsorption was reached with the use of 0.8 g glucose (Fig. 12.9b). The selectivity of CH₄ was studied by the thermodynamic and kinetic behavior of H proton transfer route in the assistance of DFT calculation. The result indicates that the H proton transfer to adsorbed CO₂ in Pt₂/C-In₂O₃ is easier than H₂ formation (endothermic); on the contrary, H proton reduction to H₂ on Pt₂/P-In₂O₃ is exothermic, which is easier than Pt₂/C-In₂O_3. This result well-explained the high CH₄ yield and CO2PR selectivity of Pt/C-In₂O₃ compared with Pt/P-In₂O₃.

Fig. 12.9

(a) SEM and TEM (inset) images and EDX elemental mapping images of C-In₂O₃. (b) CO₂ adsorption capacities of In₂O₃-based samples. (c) H₂, CO, and CH₄ evolution rates from CO₂ photoreduction on Pt/C-In₂O₃ and Pt/P-In₂O₃. (Reprinted with permission from Ref. [47]. Copyright 2017, American Chemical Society)

12.4.3 Surface Active Sites in CO₂ Photoreduction

For better understanding the roles of cocatalysts in CO2PR, the identification of the active sites in cocatalysts and the study of reaction mechanism are very important. Only in this way can we develop photocatalysts with both high performance and selectivity in CO2PR. In order to investigate the active facet of Pd in CO₂ photoreduction, Bai et al. [15] synthesized Pd cube NPs (exposed mainly (100) facets) and Pd tetrahedron NPs (exposed mainly (111) facets) and deposit them on C₃N₄ layer separately. The size of Pd with different shapes is all around 4–6 nm; however, the activity was quite different, and the selectivity toward CO2PR of Pd nanotetrahedrons/C₃N₄ is obviously higher than Pd nanocubes/C₃N₄. Deep understanding of the shape-dependent selectivity of Pd was investigated by first-principle theory. Firstly, the adsorption energy for CO₂ and H₂O on Pd (111) is 0.23 eV and 0.37 eV and for Pd (100) is 0.064 and 0.554 eV, which indicates the CO₂ and H₂O tend to adsorb on Pd (111) and Pd (100), respectively. Secondly, when accepting two electrons, the Pd (111) shows a lower CO₂ activation energy barrier compared with Pd (100). This result reflects that the Pd (111) is the active site for CO₂ reduction and Pd (100) is more active for H₂O reduction. Generally, the active sites of supported metal catalysts rely on two factors: surface geometric structure and electronic structure, therefore, lattice engineering through alloy different metals could tuning the above two factors and further improve the activity and selectivity of the catalyst. Pd₇Cu₁ supported on TiO₂ nanoplates with isolation Cu in Pd lattice for CO2PR was reported by Long and coworkers [18]. In this research, when the Cu loading amount is below 12.5%, the XAFS results show that the oxidation of Cu (absent of Cu-O) could be inhibited effectively and Cu atoms were isolated in the Pd lattice (absent of Cu–Cu bonds). During the CO2PR evaluation, the Pd₇Cu₁/TiO₂ sample shows the optimal CO₂ reduction activity and CH₄ selectivity. In situ DRIFTS experiments show the enhanced signals of HCO₃ ⁻, CO₃ ⁼, and CO₂ ⁻ species over samples with isolation of Cu atoms; in addition, the first-principle theory also indicate the Pd–Cu pairs could enhance the CO₂ adsorption. Both experimental and theoretical results suggest the Pd-Cu pairs favor the CO₂ adsorption. The different d band centers of Cu in Pd₇Cu₁ and Pd₁Cu₁ revealed the Pd-surrounded environment could tune the electronic structure of Cu and improve the catalytic activity of Cu. Au–Cu alloy NPs supported on TiO₂ (p25) reported previously also show enhanced performance in CO2PR compared with Au/TiO₂ or Cu/TiO₂ (Fig. 12.10) [14].

Fig. 12.10

(a) HR-TEM image of Au–Cu alloy loaded on TiO₂; the lattice spacing distance is 0.222 nm, which is different from Au or Cu. (b) UV–vis DRS spectra of Au–Cu/TiO₂ (Au/Cu = 1:2) sample before and after reduction in 400 °C H₂ atmosphere calcination. (c and d) Time-resolved in situ FTIR spectra of intermediates that generate from adsorption and evolution during irradiation of CO₂ and H₂O bounded on Au–Cu/TiO₂ (Au/Cu = 1:2). (Reprinted with permission from Ref. [14]. Copyright 2014, American Chemical Society)

To gain deep understanding of the reaction mechanism and intermediates along the CO2PR, time-resolved in situ FTIR was applied. During the reaction, the generation of Cu-CO band (2126 cm⁻¹) indicates the Cu favors the CO₂ reduction instead of H₂O reduction; also the CO₂ ^.- (1589 cm⁻¹) shows a continuously decreasing trend during the irradiation, which is assumed as a reactive species generated from the surface Ti³⁺ (Fig. 12.10c, d). Further studies were carried out by using two light sources (visible light and UV light) to trigger CO2PR. Under the visible light irradiation, CH₄ and H₂ were the main products over the optimal Au-Cu/TiO₂, indicates the hot electrons generated from the surface plasma resonance of Au NPs and reacts with the activated CO₂ to generate CH₄. However, when using UV light as the light source, the Au–Cu alloy NPs act as the electron sink and promote the charge separation, which generate higher amount of H₂.

Low-coordinated sites (i.e., edge or corner sites) in metal NP-supported catalyst are often treated as active sites. Generally, these low-coordinated sites possess unique properties like strong binding energy toward certain reaction intermediates and low free energy which to some rate determines steps. Combining the experimental results with the DFT calculation, Mistry et al. [67] proposed that, in CO₂ electroreduction reaction, Au NPs show the size-dependent activity; the smaller-sized Au with more low-coordinated sites is more active in HER than CRR. Gao et al. [68] studied the Pd NPs with variable size in CO₂ electroreduction, and the result shows that low-coordinated sites of Pd are more suitable for COOH* generation but HER is insensitive to different surface sites. Zhu et al. [16] synthesized Pd nanosheet with similar thickness but different size (TiO₂-Pd NSs-s, small; TiO₂-Pd NSs-m, middle; TiO₂-Pd NSs-l, large) and proposed the edge sites of Pd nanosheet are the active site for CO2PR. Keeping the Pd loading amount as constant, with decrease of the size of Pd nanosheet, results in increased Pd edge density, and the CO and CH₄ yield increased as well. So the edge sites of Pd may act as the active site in CO2PR; to further confirm this edge-dependent activity, the Pd nanorings with even smaller size and higher density of edge sites were prepared and deposited on TiO₂ (denoted as TiO₂-Pd NRs-s). The obtained TiO₂-Pd NRs-s show a lower TiO₂-Pd interface to Pd volume but higher edge to volume ratio compared with TiO₂-Pd NSs-s. Consequently, the TiO₂-Pd NRs-s show a higher CO2PR activity but lower HER yield (lower electron transfer ability). As a result, the density of edge sites of Pd is highly related with the CO2PR performance which is reasonable to assume as the active sites. To verify the roles of metal active sites in CO2PR more specifically, with the assistance of DFT calculation, Gao et al. [69] report a step-by-step CO₂ photoreduction over single-atom Pt or Pd supported on g-C₃N₄. The calculated relative binding energy between Pd and Pt within C₃N₄ sixfold cavity proved the existence of charge transfer and strong interaction between the metal and support. Two possible product pathways, HCOOH and CH3OH, are studied for Pd/C₃N₄. The calculated desorption energy barrier for the key intermediate HCOOH* on Pd/C3N4 is 0.46 eV, which is much lower than the formation of HCHO*, suggesting that HCOOH is the more preferred product than CH₃OH. For Pt/C₃N₄, the strong interaction between Pt and HCOOH* (1.06 eV) and the favorable CH₂* and H₂O* generation instead of CH₂OH* hydrogenation made it the suitable candidate for CH₄ production.

12.5 Summary and Perspective

So far, the synthesis of photocatalyst with cocatalysts incorporation and the unique properties of various cocatalysts in CO2PR have been carefully summarized. The roles of these cocatalysts such as promote the charge separation efficiency, improve the adsorption of the CO₂ amount, expand the light harvesting range, provide active sites for the activation of CO₂ or other intermediates, etc. also have been briefly discussed. Besides, the important roles of spatial configurations of the photocatalyst composite and the deposition amount of the cocatalysts are also illustrated carefully: inappropriate incorporation of cocatalysts would lead to negative effect of the photocatalyst’s performance; on the contrary, rational structure design such as the Z-scheme model or cocatalysts with spatial separated configurations could enhance the performance of the photocatalyst. It should be noted that we mainly focus on the solid-state cocatalysts in this chapter; beside this, the molecular-state cocatalysts like metal complex and dyes also could act as the cocatalyst in the CO2PR; however, this type of photocatalytic system is often conducted in the liquid phase and in the presence of hole scavenger, which is quite different from the solid-phase cocatalysts, so these types of cocatalysts are not discussed in here.

Although numerous efforts have been done in the selection of suitable cocatalysts and the development of fine structures of photocatalyst in CO2PR, many problems still existed and need to be answered and improved:

• 1. The CO2PR evaluation method is alternative among different research groups; therefore, the product yields comparison of different photocatalysts which is problematic; other evaluation methods such quantum yield efficiency and turnover number (TON) are highly encouraged in the following studies.

• 2. The origination of the products should be verified carefully; the organic impurities or carbon-involved species also could be converted into the products and cause the illusion result; therefore, control experiment of CO₂ photocatalytic reduction reaction should be conducted with the use of isotope-labeled ¹³CO₂ as the reactant for comparison.

• 3. The reaction pathways and mechanism in CO2PR are still ambiguous; deep understanding of the CO2PR could bring inspiration to the researchers to design highly efficient and selective catalysts; in this case, the DFT calculation along with the in situ characterizations is highly advocated.

• 4. The stability of the cocatalysts in the long-term CO2PR reaction is another concern; many photocatalysts suffer from low stability due to the carbon-involved species accumulation and deactivate gradually; therefore, the development of highly efficient and stable photocatalyst and the study of the reason of catalysts’ deactivation are important.