Indium-Containing Visible-Light-Driven (VLD) Photocatalysts for Solar Energy Conversion and Environment Remediation

Abstract

Indium-containing visible-light-driven (VLD) photocatalysts including indium-containing oxides, indium-containing sulfides, indium-containing hydroxides, and other categories have attracted more attention due to their high catalytic activities for oxidation and reduction ability under visible light irradiation. This chapter will therefore concentrate on indium-containing nano-structured materials that demonstrate useful activity under solar excitation in fields concerned with the elimination of pollutants, partial oxidation and the vaporization of chemical compounds, water splitting, and CO2 reduction processes. The indium-containing photocatalysts can extend the light absorption range and improve the photocatalytic activity by doping, heterogeneous structures, load promoter, and morphology regulation. A number of synthetic and modification techniques for adjusting the band structure to harvest visible light and improve the charge separation in photocatalysis are discussed. In this chapter, preparation, properties, and potential applications of indium-containing nano-structured materials used as photocatalysis will be systematically summarized, which is beneficial for understanding the mechanism and developing the potential applications.

1. Introduction

The current rapid industrial development causes the serious energy and environmental crises. Since Fujishima [1] reported the photocatalytic activity of TiO₂, the research enthusiasm has not diminished. Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. Photocatalysis is a light-driven chemical process over the surface of photocatalyst that can produce hydrogen from water, convert solar energy into electric energy, degrade organic pollutants, and reduce CO₂ into organic fuels [2]. Photocatalyts provide a potential strategy to solve these problems because these materials not only convert solar energy directly into usable or storable energy resources, but can also decompose organic pollutants under solar-light irradiation [3–9].

Over the past several years, researchers have made considerable effort to increase the visible-light-driven (VLD) photocatalytic activity of the photocatalysts [10, 11]. There are two strategies employed in the design of the VLD photocatalysts. One is the chemical modifications on a UV-active photocatalyst, including doping of foreign elements or coupling with a narrow band gap semiconductor. In order to extend the absorption of light into the visible region, three approaches have been widely used on semiconductors: (I) modification of the VB, (II) adjustment of the CB, and (III) continuous modulation of the VB and/or CB (see Figure 1) [12]. The other is to develop novel photocatalysts with VLD photocatalytic activity. The development of photocatalysts under visible light irradiation is one of the major goals for enhancing the efficient utilization of solar energy and realizing practical industrialization.

Figure 1.

Three strategies to narrow the band gap of semiconductor photocatalysts to match the solar spectrum [12].

Recently, photocatalysis driven by visible light has gained great attention, as the visible light occupies most part of the solar spectrum such as: bismuth compounds (Bi₂WO₆ [13], Bi₂MoO₆ [14], BiVO₄ [15], BiOBr [16]), silver compounds(AgAlO₂ [17], Ag₂CrO₄ [18], Ag₂CO₃ [19], AgVO₃ [20], Ag₃PO₄ [21]), and indium compounds(In₂O₃ [22], CaIn₂O₄ [23], InVO₄ [24], In₂S₃ [25]). Up to now, much attention has been given to a series of visible light active indium compound. The In 5s orbital in the valence band of the semiconductor may hybrid with O 2p or S 2p orbital to form a new energy level, which could narrow the band gap of the indium compound and enhance the photocatalytic activity with the visible light irradiations [26]. Many novel indium-containing VLD photocatalysts were reported. Herein we review the fundamental challenges and recent progress on indium-containing VLD photocatalysts.

Starting with a brief introduction, we will give an overview on the development of high-efficiency, indium-containing VLD photocatalysts. Section 2 covers indium-containing oxides including single-metal oxide (In₂O₃), double-metal containing indium oxides A_xB_yO_z (A site containing indium compounds InMO₄ (M=V, Nb, Ta), B site containing indium compounds AInO₂ (A=Ag, Na, Li), MIn₂O₄ (M = Ca, Sr, Ba). Section 3 describes systems involving indium based sulfides such as single-metal sulphide (In₂S₃), double-metal containing indium sulfides AIn_xS_y (A=Na, Cu, Ag, Cd, Zn), and containing indium solid solution ZnS-CuInS₂-AgInS₂, (CuIn)_xZn_2(1-x)S₂. Indium-containing hydroxides will be discussed in section 4. A number of synthetic and modification techniques for adjusting the band structure to harvest visible light and improve the charge separation in photocatalysis are discussed. A comparative analysis of the systems discussed and their future projection as environmentally friendly photocatalytic systems will conclude the review. Finally, some feasible ways to design and improve the visible-light responding photocatalysts are concluded, and the development of indium-containing oxides semiconductor photocatalysts is also proposed．

The application of indium-containing VLD photocatalysts for solar energy conversion and environment remediation as an important challenge will be listed. We aim to put together the research effort having been made so far, with a view of providing a good reference and inspiring new ideas for tackling this important challenge. In this chapter, preparation, properties, and potential applications of indium-containing nano-structured materials used as photocatalysts will be systematically summarized, which is beneficial for mechanism understanding and developing potential applications.

2. Indium-containing oxides

2.1. Indium oxide

Recently, the reported research has investigated In₂O₃ in an attempt to develop novel photocatalysts for water splitting. In₂O₃ fulfils some important requirements for the direct photo electrolysis of water such that the position of the conduction and valence band edges bracket the radix potentials of water, and In₂O₃ has an excellent conductivity and stability. In₂O₃ generally exists in two forms: cubic (C-In₂O₃) and hexagonal (H-In₂O₃). Currently, there are reports on the morphology of In₂O₃ mostly in the cubic structure of C-In₂O₃ and a few hexagonal structure of H-In₂O₃. So far, researchers have successfully synthesized the In₂O₃ with various morphology such as particles [27], fibers [28], porous particles [29], and cubes of nanostructures [30]. Figure 2 shows the SEM images of In₂O₃ nanostructures with different morphologies.

Figure 2.

SEM images of In₂O₃ nanostructures with different morphologies synthesized at 800℃ for 1 h: a, b-octahedrons; c, d-nanobelts and dentate nanowires; e, f-nanocrystal chains [31].

In₂O₃ is transparent to visible light because of its wide band gap (E_g=3.55-3.75 eV) [31], which decreases its potential efficiency for water splitting under solar illumination. In₂O₃ was modified by various methods. The results showed that the modified In₂O₃ have better response in the visible light region and have higher photocatalytic activity than In₂O₃. For example, Karla et al. [32] reported that N-doped In₂O₃ were prepared and found that the rate of decomposition of water under visible light has improved compared with pure In₂O₃. Compared to ion doping methods, constructing compound semiconductor heterojunction broaden the optical response range and effectively, the separation of electrons and holes. Li et al. [33] successfully synthesized heterojunction CuO/In₂O₃ composite photocatalysts by hydrothermal method. Under visible light, Rhodamine B as the target pollutants examined the catalytic properties of the composite photocatalysts and found its catalytic activity much higher than pure In₂O₃. The enhanced photocatalytic activity is due to the CuO and In₂O₃ forming heterogeneous structures, which can effectively improve the separation efficiency of the light-generated charge and extend the light absorption range. For solid solutions consisting of Ga₂O₃ and In₂O₃, Ga_1.14In_0.86O₃ showed the highest photocatalytic activity for H₂ evolution from aqueous methanol solutions and for O₂ evolution from aqueous silver nitrate solutions. In comparison, the solid solutions of Y₂O₃ and In₂O₃, Y_1.3In_0.7O₃, showed the highest photocatalytic activity for the overall water splitting when combined with RuO₂ as a promoter.

So far, the preparation methods of In₂O₃ mainly are thermal evaporation (TE) [34], chemical vapour deposition (CVD) [35], laser ablation (PLD) [36], metal organic chemical vapour deposition (MOCVD) [37], and a variety of wet chemical methods [38–40]. Among them, the characteristic of wet chemical method is the lowest preparation temperature, but their degree of crystallinity is poor; their morphology is mainly nanowire hexagonal structure, nanoparticles, and squares. In addition to the main method as described above, preparation cubic In₂O₃ by processing the precursor material is also suffering much attention [41–43].

2.2. Double-metal containing indium oxides

Double-metal containing indium oxides (A_xB_yO_z) due to the different site of element In position (A or B bits), can be divided into two categories: A site containing indium compounds InMO₄(M=V, Nb, Ta), B site containing indium compounds AInO₂ (A=Ag, K, Na, Li), MIn₂O₄ (M = Ca, Sr, Ba).

2.2.1. A site containing indium oxides

InMO₄ (M = V, Nb, Ta) compounds belong to ABO₄ compound, where In is in A bit. Their crystal structures were: InNbO₄ (InTaO₄) belong to monoclinic system with octahedral InO₆ and NbO₆ (TaO₆); InVO₄ belongs to orthorhombic system with octahedral InO₆ and tetrahedral VO₄. InTaO₄ was 5d compound (E_g = 2.6 eV), InNbO₄ was 4d compound (E_g = 2.5 eV), and InVO₄ was 3d compound (E_g = 2.0 eV), bandgap of the InMO₄ compound with M from 5d Ta to 4d Nb to 3d V reduced [44]. Song et al. [45] prepared one-dimensional InVO₄ nanofibers with width of 30-100 nm under visible light illuminated through 6 h, wherein the nitrobenzene degradation reached 69%. Zou [46] prepared InMO₄ (M = Nb⁵⁺, Ta⁵⁺) by high-temperature solid phase method. Under visible light (λ > 420 nm) irradiation, the hydrogen production rate of InMO₄ (M = Nb⁵⁺, Ta⁵⁺) is that of P25, 4.0 and 3.5 times, respectively. Meanwhile, the InMO₄ (M = V, Nb, Ta) photocatalysts were modified by doping heterogeneous structures method. The results showed that visible light absorption and photocatalytic activity of InVO₄ after modification had been enhanced [47, 48]. Zhang et al. [49] prepared graphene (Gr)/InNbO₄ composite photocatalysts by hydrothermal method from which the apparent rate constant of (0.0346 min^-1) degradation MB is higher than pure InNbO₄ (0.0185 min^-1) under visible light illumination.

InVO₄, due to suitable conduction band can be a promising photocatalyst for H₂ production under visible light irradiation. In addition, there are many reports indicating that the desired morphology and size of photocatalysts could regulate the position of the energy band to achieve higher radix ability. Yan et al. [50] reported that the nanosized InVO₄ nanoparticles with the size of 20 nm showed higher photocatalytic activity of H₂ production than InVO₄ microspheres. Hu et al. [51] synthesized g-C₃N₄/nano-InVO₄ heterojunction-type photocatalysts by in situ growth of InVO₄ nanoparticles onto the surface of g-C₃N₄ sheets via hydrothermal process. The formation of interfaces could promote the charge transfer and inhibit recombination of charge-hole pairs, which significantly improved the photocatalytic activity of H₂ evolution of 212 μmol/g⋅h from water-splitting. Figure 3 is a schematic illustration of
g-C₃N₄/InVO₄ composite under visible light irradiation.

Currently, these are the following methods for synthetic InVO₄: solid-phase synthesis [52], which is difficult to obtain a large surface area, pore volume, and a high mesoporous materials; mesoporous InVO₄ obtained by sol-gel method [53] are disorderly, has wide pore size distribution, pore walls were generally amorphous, and with poor thermal stability; surfactant templating method [54] can obtain larger surface area mesoporous InVO₄, but the manufacturing process requires high temperatures and the morphology is irregular.

Figure 3.

Schematic illustration of g-C₃N₄/InVO₄composite under visible light irradiation [51].

2.2.2. B site containing indium oxides

Photochemical dye degradation has been limited by the efficiency of the catalyst materials with respect to photon absorption. An ideal catalyst would be capable of using as much of the solar spectrum as possible, in particular the visible region. As we know, delafossites structure of materials have the potential to provide this photoactivity. These materials have the general formula ABO₂ and are based on the mineral CuFeO₂, also known as delafossite. AInO₂ (A = Ag, K, Na, Li) belong to ternary oxide ABO₂, where In is in B bit. Crystal structure of AgInO₂, LiInO₂ and NaInO₂ are delafossite, α-LiFeO₂ and α-NaFeO₂, respectively. The band gap values of AgInO₂, LiInO₂ and NaInO₂ are 2.0eV, 3.7eV and 3.9eV, respectively. These materials share the ability to alter the band structure by using chemical substitution. In particular, substitution on the B-site in these materials can be used to tune the physical properties of delafossites for specific applications. AgInO₂ is a narrow bandgap semiconductor material which responds in the visible light range. Wang et al. [55] reported 0.5 wt% Pt/NaInO₂ can completely degrade MB in 1h. Jonathan et al. [56] reported the effect of electronic structure changes in NaInO₂ and NaIn_0.9Fe_0.1O₂ on the photo reduction of Methylene Blue. Figure 4 shows the (A) crystal structure, (B) diffuse reflectance spectroscopy, and (C) energy level diagram of the NaInO₂ and NaIn_0.9Fe_0.1O₂, respectively. Diffuse reflectance spectroscopy was used to determine the band gap values of 3.9 eV and 2.8 eV for NaInO₂ and NaIn_0.9Fe_0.1O₂, respectively. Energy level diagram describing the flat band (EFB; dashed line), conduction band (ECB), and valence band (EVB) potentials of TiO₂, NaInO₂, and NaIn_0.9Fe_0.1O₂ in relation to some relevant electrochemical radix couples; potentials are in the reversible hydrogen electrode (RHE) scale (b). The spread in CB and VB potentials represents the experimental uncertainty in band edge determination.

Figure 4.

(A) Rhombohedral crystal structure,(B) Diffuse reflectance spectroscopy, and (C) Energy level diagram of the NaInO₂ and NaIn_0.9Fe_0.1O₂ [56].

MIn₂O₄ (M = Ca, Sr, Ba) belongs to the family of ternary oxide AB₂O₄, where In is in B bit. Their crystal structures are: CaIn₂O₄ and SrIn₂O₄ having the same octahedral InO₆ network structure, and BaIn₂O₄ having a more complex polyhedron InO_x structure. Sato and Tang team synthesized a series of photocatalysts MIn₂O₄ (M = Ca, Sr, Ba) and studied the crystal and electronic structure of the photocatalysts relationship with their visible light photocatalytic activity. Sato group [57] found that different crystal structures of MIn₂O₄ (M = Ca, Sr, Ba) have an impact on their photocatalytic activity. The crystal structures of CaIn₂O₄ and SrIn₂O₄ are orthorhombic and BaIn₂O₄ was monoclinic. The order of catalytic activity of water splitting in the xenon lamp irradiation were: CaIn₂O₄> SrIn₂O₄> BaIn₂O₄. Tang et al. [58] prepared MIn₂O₄ (M = Ca, Sr, Ba) by solid phase methods. They fall in the visible order solution MB catalytic activity were: CaIn₂O₄> SrIn₂O₄> BaIn₂O₄, wherein CaIn₂O₄ showed the highest activity (Figure 5). The reason of the order activity is that mesh structure of CaIn₂O₄ and SrIn₂O₄ helps photo-generated electron transfer. Inoue et al. [59] investigated the photocatalytic properties for water decomposition of alkali metal, alkaline earth metal, and lanthanum indates with an octahedrally coordinated In³⁺ d10 configuration ion. The photocatalytic activity for water decomposition under UV irradiation was considerably large for RuO₂-dispersed CaIn₂O₄, SrIn₂O₄, and Sr_0.93Ba_0.07In₂O₄ but very poor for RuO₂-dispersed AInO₂ (A = Li, Na) and LnInO₃ (Ln = La, Nd). The geometric structures of the InO₆ octahedral units for these indate were compared. As shown, the photo-catalytic active indates possessed distorted InO₆ octahedral with dipole moments. The internal fields that arose due to the dipole moment promoted the charge separation in the initial process of photo-excitation.

Figure 5.

MB original concentration, MB photolysis and its concentration variation after 120 min visible light irradiation (λ > 420 nm) on the different oxides [58].

3. Indium-containing sulfide

Indium based sulphides include single-metal sulfide (In₂S₃), double-metal containing indium sulfides AIn_xS_y (A = Na, Cu, Ag, Cd, Zn), and containing indium solid solution ZnS-CuInS₂-AgInS₂, (CuIn)_xZn_2(1-x)S₂. Compared with indium-containing oxides, Indium based sulphides has narrow band gap which can make good use of visible light and have been extensively studied. But its low stability, prone to light decay, and other shortcomings limit its further application.

3.1. Indium sulfide

In₂S₃ have three different forms of structural defects: α-In₂S₃(defect cube), β-In₂S₃(defect spinel, cubic, or tetragonal structure), and γ-In₂S₃ (layered hexagonal). The band gap of In₂S₃ is E_g=1.9-2.3 eV [60], and it belongs to the n-type narrow band gap semiconductor. Researchers mostly used solvothermal or hydrothermal methods to prepare excellent performance of visible light photocatalyst In₂S₃. Liu et al. [61] prepared tetragonal β-In₂S₃ nanotube by solvothermal method. The nanotube with diameter of about 10-20 nm, pipe wall thickness of 2 nm, tube length 1μm, and under simulated sunlight degraded Rhodamine with high rate. In addition, the researchers have further explored In₂S₃, such as the following: (1) the relationship between crystal or precious metal co-catalyst species and the catalytic activity and selectivity of light catalysis: Xing et al. [62] prepared a heterogeneous structure photocatalyst In₂S₃/g-C₃N₄ by hydrothermal method and they found 40 wt% In₂S₃/g-C₃N₄ degradation RhB rate in 30 min, can be 96% under visible light (λ > 420 nm) irradiation, far higher than the 50% of pure In₂S₃; (2) the relationship between crystal or precious metal co-catalyst species: Fu et al. [63] reported the preparation of the tetragonal and cubic phases In₂S₃ by hydrothermal method, both of their photocatalytic hydrogen production under visible light were investigated (Figure 6). The results showed that ordered tetragonal In₂S₃ has no hydrogen production activity, while disordered cubic structure In₂S₃ showed stable photocatalytic hydrogen production activity. At the same time, the authors also investigated the effect of precious metal as co-catalyst on the order photocatalytic activity of In₂S₃ and results are: Pd>Pt>Ru>Au; (3) the catalytic activity and selectivity of light catalysis: Xie et al. [64] used the microspheres In₂S₃ prepared by hydrothermal method, with selective degradation using 41.4% benzyl alcohol in 4 h invisible light.

Currently, researchers have explored a variety of ways for preparing In₂S₃ with different morphology. For example: Afzaal et al. [65] used high temperature vapour deposition to get In₂S₃ nanorods on a glass substrate. Liu et al. [66] used indium nitrate as an indium source, dodecyl mercaptan as a sulfur source to synthesis β-In₂S₃ nanotube structure with nanotube length of 1-10 μm, and width less than 15 nm by pyridine solvent thermal reaction method. Son et al. [67] reported the use of InCl₃⋅4H₂O and elemental sulfur as a precursor with a certain proportion of the oleylamine oil, then obtained hexagonal indium sulfide nanosheets with thickness of 0.76 nm. In addition, the researchers also used the hot water or solvent hot methods for preparing a variety of three-dimensional structures In₂S₃, such as flower microspheres [68], hollow microspheres [69], and dendrites [70].

Figure 6.

Schematic illustration of the In₂S₃/g-C₃N₄ photocatalytic reaction process under visible light irradiation [62].

3.2. Double-metal containing indium sulfide

The band gap of double-metal containing indium sulphides AIn_xS_y (A = Na, Cu, Ag, Zn) range from 1.87 to 2.5eV which have light response in the visible light. In recent years, the AIn_xS_y as visible light catalyst have been extensively studied with focus mainly on ZnIn₂S₄.

ZnIn₂S₄ is a II-III-VI family ternary metal sulfides with hexagonal and cubic spinel structure and have narrow band gap (E_g = 2.1-2.4 eV). ZnIn₂S₄ has strong light absorption in the visible region which is worth studying as a visible light catalyst. Li et al. [71] were first using ZnIn₂S₄ on visible light catalytic hydrogen. Hexagonal ZnIn₂S₄ (space group P3m1) is a typical layered compound and the band gap is about 2.4 eV (CB: –0.29 eV vs NHE; VB: 2.11 eV vs NHE) which have a strong and appropriate response on visible light. Li et al. [72] prepared cubic ZnIn₂S₄ nanoparticles and hexagonal phase ZnIn₂S₄ microspheres by changing the indium precursor and then investigated two crystal phases ZnIn₂S₄ degrade methyl orange in visible light. The results showed cubic phase ZnIn₂S₄ having activity just at the beginning, while the hexagonal phase has shown a high catalytic activity.

Researchers modified ZnIn₂S₄ to further improve its photocatalytic activity by means of ion doping and semiconductor composite. Wen-Hui Yuan group [73] first reported that doping N can improve visible light photocatalytic activity of ZnIn₂S₄ degradation of MB. Shen et al. [74] reported composite photocatalysts Cu-ZnIn₂S₄ have higher visible light catalytic activity than pure ZnIn₂S₄ (Figure 7).

So far, the preparation methods of ZnIn₂S₄ mainly are: chemical precipitation method [75], precursor route [76], hydrothermal method [77], microwave-hydrothermal method [78], solid phase method [79], the template [80], and so on. Among them, the hydrothermal method is the typical preparation method, which is more beneficial to controlling the different morphologies of ZnIn₂S₄.

Figure 7.

Hydrogen production under visible-light irradiation over Cu(X)-ZnIn₂S₄; the values of X were (a) 0.0 wt%, (b) 0.1 wt%,(c) 0.3 wt%, (d) 0.5 wt%, (e) 0.7 wt%, (f) 0.9 wt%, (g) 1.2 wt%, (h)1.6 wt%, (i) 2.0 wt%[74].

CdIn₂S₄ also is a kind of II-II-VI family ternary metal sulphides, its band gap E_g=2.12-2.29 eV [81]. Researchers prepared different morphologies CdIn₂S₄ by different methods and studied their visible light photocatalytic activity. Bhirud et al. [81] prepared different morphologies CdIn₂S₄. When not added, the active agent was flower shape; adding a surfactant polyvinylpyrrolidone was double cone; adding a surfactant CTAB was hollow spheres. Three different morphologies CdIn₂S₄ catalysed water splitting under visible light have shown different catalytic activity and double cone is the highest (3238 μmol. (g.h)^-1). Mu et al. [82] prepared spherical particles CdIn₂S₄ with average size of 236 nm from which degradation rate for methyl orange is 98% in the visible light illumination.

=2.12-2.29 eV [56AgIn_xS_y include AgInS₂ and AgIn₅S₈. AgInS₂ has chalcopyrite (t-AgInS₂) and orthogonal (o-AgInS₂) two crystal phases and the band gap values respectively were 1.87 and 1.98 eV [83, 84]; AgIn₅S₈ is cubic crystalline phase (c-AgIn₅S₈) and its the band gap value is 1.7-2.0 eV [85]. Both of the degrading organic substances under visible light irradiation exhibited good photocatalytic properties [86, 87]. NaInS₂ belongs to a narrow band gap semiconductor material (E_g = 2.3 eV) [88]. Researchers applied hydrogen production and degradation of organic pollutants under visible light [88, 89].

3.3. Solid solution containing indium sulfide

Solid solution containing indium adjust the content of the different components of the solid solution to achieve the band gap of a solid solution of regulation. For now, there are reports of solid solution system ZnS-CuInS₂, ZnS-AgInS₂, and ZnS-AgInS₂-CuInS₂. In this regard, Kudo teams have done a lot of research. Tsuji group [90, 91] used ZnS and narrow bandgap CuInS₂ or AgInS₂ by calculating to form a visible light catalyst (CuIn)_xZn_2(1-x)S₂, (AgIn)_xZn_2(1-x)S₂, and ZnS-CuInS₂-AgInS₂. Guo [92, 93] prepared (CuIn)_xZn_2(1-x)S₂ (x = 0.01–0.5) solid solution using CTAB as surfactant by hydrothermal method. Compared with the method used by Kudo group, their method has no calcination and the products have smaller particle size. Studies have shown that with the increase of x value, the absorption band edge of (CuIn)_xZn_2(1-x)S₂ became obvious red shift and the composition of the solid solution have great influence on hydrogen production performance (Figure 8).

Figure 8.

The photocatalytic hydrogen production activities of (CuIn)_xZn_2(1-x)S₂ solid solutions under visible-light irradiation [92].

In recent years, containing indium solid solution has made great progress in China. Guijun Ma [94] who belongs to CAS Dalian Institute of Physical Chemistry used solvothermal method to synthesize CuInS₂-ZnS solid solution. Compared with the results of Kudo, their products have smaller particle size (average particle size 30-50 nm) and exhibit stable photocatalytic hydrogen production. Chen et al. [95, 96] from Harbin Institute of Technology synthesized nano-porous solid solution photocatalysts ZnS-In₂S₃-Ag₂S and ZnS-In₂S₃-CuS by self-assembled solvothermal method. Under visible light, the photocatalysts exhibit excellent photocatalytic hydrogen performance and the apparent quantum yield is 19.8% and 22.6%, respectively.

Kudo et al. [97] synthesized (AgIn)_xZn_2(1-x)S₂(x = 0.17–0.5) solid solutions. The band gaps can be tuned from 2.40 to 1.95 eV, which lie between that of ZnS and AgInS₂ (Figure 9A). The intensive absorption bands with steep edges of the doped ZnS photocatalysts indicate that the visible light absorption was due to the band transition instead of the transition from impurity levels to the conduction band of ZnS. DFT calculations revealed that the valence band of it is mainly composed of hybrid orbitals of S 3p and Ag 4d, and the conduction band is composed of hybrid orbitals of In 5s5p +Zn 4s4p in (AgIn)_xZn_2(1-x)S₂ solid solution, which is located between those of ZnS and AgInS₂ (Figure 9B). The photocatalytic activity is greatly dependent on the composition and the Pt (3 wt%) loaded. (AgIn)_0.22Zn_1.56S₂ photocatalyst exhibited the highest activity for H₂ evolution in the presence of sacrificial reagent under visible light irradiation (λ > 420 nm); the quantum yield of the samples was as high as 20% at 420 nm.

Figure 9.

(A) Diffuse reflection spectra of (AgIn)_xZn_2(1-x)S₂ solid solutions; the values of x are (a) 0, (b) 0.17, (c) 0.22, (d) 0.29, (e) 0.33, (f) 0.40, (g) 0.5, and (h) 1. (B) Band structures of (AgIn)_xZn_2(1-x)S₂ solid solutions, ZnS and AgInS₂[97].

4. Indium-containing hydroxides

Indium-containing hydroxides include In(OH)₃ and InOOH. The band gap values were 5.15 eV and 3.70 eV respectively [98], and they belongs to the wide band gap semiconductor photocatalyst.

In(OH)₃ is a very important n-type photocatalyst and its crystal structure is: per In^{3 +} ions with six OH^- ions constituting the octahedral structure and In(OH)₃ belongs to wide-band gap light catalyst which does not respond in the visible light range. The researchers modified it to try to expand its range of light absorption by a variety of methods. For example, Lei et al. [99] synthesized photocatalyst In(OH)_yS_z by hydrothermal method. The result is that the absorption edge of In(OH)₃ followed S^2- substituted with OH^- moved from 240 nm to 570 nm (Figure 10).

Figure 10.

DRS of ZnS, In(OH)_yS_z and In(OH)_yS_z:Zn catalysts with different X value (X denotes the atomic ratio of Zn/In in the synthesis solution). The In(OH)_yS_z and In(OH)_yS_z:Zn were obtained with atomic ratio of S/In = 2.0 [99].

InOOH is orthorhombic crystal, compared with the crystal structure of In(OH)₃, InOOH can be seen as distorted octahedral InO₆ co-built by the edge of the way. Researchers, through the composite doping methods such as wide band gap photocatalyst InOOH modified to the scope and the absorption of visible light photocatalytic activity. Song et al. [100] prepared porous carbon spheres/InOOH composite photocatalyst by in-situ reaction. Compared with the pure InOOH, the composite photocatalyst of optical absorption edge had undergone a drastic red shift and improved the visible light photocatalytic activity. Ge et al. [101] reported that N, C doping InOOH has a response in the visible light range due to the doping, and narrowing of the band gap of InOOH (Figure 11).

Figure 11.

Comparison of UV-vis diffuse reflectance spectral of CN₂ and RhB (a) and the photo degradation efficiencies of RhB by 0.1 g of CN₂ under monochromatic light source (λ= 420 nm) (b). C₀ and C are the initial concentration after the adsorption equilibrium and temporal concentration of RhB at different times, respectively [101].

5. Conclusion and prospective

The use of semiconductor materials photocatalytic degradation organic pollutants and producing hydrogen has very important significance for environmental management, and energy depletion. However, the low visible light catalytic efficiency has hampered the development of photocatalytic technology. There is an urgent task in developing new and efficient visible light catalytic system.

In summary, the indium-based photocatalytic materials have good visible response and strong visible light photocatalytic activity and possess broad prospects on photocatalytic water environment, capacity, and other degradation of organic pollutants. The current findings indicate that photocatalytic activity of indium-based photocatalysts is affected by its size, morphology, and crystal structure; doping, heterogeneous structures, load promoter, and morphology regulation methods can extend the light absorption range and improve the photocatalytic activity. Indium-based photocatalysts also have the following problems which needs to do further exploration and research:

1. At the moment, researches of indium-based photocatalysts mainly focus on degradation of organic compounds and hydrogen production with little research for CO₂ reduction, which may be the weakness of reducing capacity. Using widely the photocatalysts on CO₂ reduction by various channels to improve their reducing ability is needed.

2. Indium-based photocatalyst of electron transfer mechanism, carrier generation and recombination, free radical generation and detection, and photocatalytic mechanism of research reports is little and this may require deeper and more systematic research and inspection.

3. So far, the reports on some narrow band gap and good photoelectric properties of indium-based photocatalysts application on the visible light catalysis have not appeared yet. For example, InN (E_g = 0.7 eV) [102].

4. Finally, the high cost for all indium-based compound limited its large-scale application. Therefore, how to significantly reduce its production cost is an issue to consider.

Acknowledgments

We gratefully acknowledge financial support from Chinese National Foundation of Natural Science (No. 51272032) and Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.