Visible light responsive photocatalysts developed by substitution with metal cations aiming at artificial photosynthesis

Abstract

To solve resource, energy, and environmental issues, development of sustainable clean energy system is strongly required. In recent years, hydrogen has been paid much attention to as a clean energy. Solar hydrogen production by water splitting using a photocatalyst as artificial photosynthesis is a promising method to solve these issues. Efficient utilization of visible light comprised of solar light is essential for practical use. Three strategies, i.e., doping, control of valence band, and formation of solid solution are often utilized as the useful methods to develop visible light responsive photocatalysts. This mini-review introduces the recent work on visible-light-driven photocatalysts developed by substitution with metal cations of those strategies.

Graphical abstract

Keywords

visible light responsive photocatalyst / water splitting / artificial photosynthesis: metal ion substitution

1 Introduction

From the viewpoint of the recent resource, energy and environmental issues, development of the sustainable technology is strongly required. Solar water splitting using a photocatalyst is the candidate to solve these issues. When a semiconductor photocatalyst is irradiated with light at an energy more than a band gap, electrons are excited from a valence band to a conduction band of a host material as shown in Fig. 1. Photocatalytic water splitting proceeds when the levels of the conduction band and the valence band are more negative and positive than the redox potentials of H⁺/H₂ and O₂/H₂O, respectively. For practical application of solar hydrogen production using a photocatalyst, it is strongly required to develop visible light responsive photocatalysts which split water efficiently under solar light irradiation. The three strategies (doping, control of valence band, and formation of solid solution) described in Fig. 2 are mainly utilized as the useful methods for the development of visible light responsive photocatalyst [1,2].

Doping has often been employed to prepare visible light responsive photocatalysts. Doping means the substitution of a foreign element at the lattice point of host materials, which forms the impurity level in a forbidden band of a semiconductor, bringing response to visible light. Although a dopant contributes to sensitization of a photocatalyst to visible light, it also works as a recombination center which causes the decrease of the photocatalytic activity. Therefore, the optimization of a doping amount is greatly important.

In the case of metal oxide semiconductor photocatalysts, the valence band is usually formed by O 2p orbitals and locates at a largely more positive level than an oxidation potential of water. Therefore, the band gap of metal oxide widens to satisfy the sufficient reduction potential of water. Formation of a new valence band is an effective way to solve the issue. Bi6s² [3], Pb6s² [4], Sn5s² [5], Ag4d¹⁰ [6], and Cu3d¹⁰ electron-filled orbitals [7] of metal cations form such new valence bands at a more negative level than that of O 2p orbitals, resulting in the development of the photocatalysts having narrow band gaps. Additionally, because N 2p, S 3p, and Se 4p orbitals of non-metal anions composing (oxy) nitride, (oxy) sulfide, and selenide also form valence bands at a more negative level than O 2p orbitals, TaON, Ta₃N₅, graphitic carbon nitride (g-C₃N₄), Sm₂Ti₂O₅S₂, Y₂Ti₂O₅S₂, and CuGaSe₂ are the photocatalysts responding to visible light [8–15]. However, the non-atmospheric condition in the preparation of many valence band-controlled materials is required because they are easily oxidized in air.

Formation of solid solution by combination of several semiconductors having different band gaps is also employed to develop visible light responsive photocatalysts. This strategy is often applied to a chalcogenide photocatalyst. It has a great advantage because the band structure and band gap can easily be tuned by changing the ratio of solid solution [16]. Selection of the materials with a similar crystal structure is required to prepare solid solution photocatalysts.

To use dyes is also one of the methods to sensitize a photocatalyst to visible light [17]. When a dye is irradiated with visible light, an electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the dye and the excited electron transfers to a conduction band of the wide band gap photocatalyst, resulting in the fact that H₂ evolves on the photocatalyst [18,19].

This mini-review introduces doped-, valence band-controlled, and solid solution photocatalysts responding to visible light, developed with metal ion substitution. Inorganic semiconductor photocatalyst materials are especially focused on for water splitting and sacrificial hydrogen and oxygen evolutions aimed at artificial photosynthesis.

2 Doped photocatalysts

Various visible light responsive metal oxide photocatalysts have been developed by doping with transition metal ions such as Ni [20], Cr [21], Rh [22,23], and so on. Rh, Ir, and Ru-doped SrTiO₃ photocatalysts are focused on in the present mini-review, because they possess unique properties.

2.1 Rh-doped photocatalyst

A Rh-doped SrTiO₃ photocatalyst (SrTiO₃:Rh) with 2.3 eV of an energy gap shows a high activity for photocatalytic hydrogen evolution from an aqueous solution containing a sacrificial reagent under visible light irradiation. The Rh⁴⁺ ions doped in SrTiO₃ changes to Rh³⁺ during the photocatalytic reaction. The sacrificial hydrogen evolution proceeds by transition from the impurity levels formed by Rh³⁺ to the conduction band of SrTiO₃. In contrast, the SrTiO₃:Rh photocatalyst does not show the activity for sacrificial oxygen evolution. Therefore, the SrTiO₃:Rh photocatalyst does not singly show the activity for water splitting under visible light irradiation. It can be applied to a Z-schematic water splitting system as a hydrogen-evolving photocatalyst [2,24–26].

The SrTiO₃:Rh possesses a unique property as a p-type oxide semiconductor giving cathodic photocurrent under visible light irradiation [27]. Photoelectrochemical water splitting proceeds under visible light irradiation without electrical external bias when the SrTiO₃:Rh photocathode is combined with a BiVO₄ photoanodes as demonstrated in Fig. 3 [28].

The antipathogens performance using a photocatalyst is also well studied. A TiO₂ photocatalyst, which is a representative photocatalyst, easily inactivates bacteria than bacteriophage under UV light irradiation. In contrast to TiO₂, the SrTiO₃:Rh photocatalyst milled by a ball-milling device easily inactivates bacteriophage even in the presence of bacteria under visible light irradiation [29]. The high antiphage performance of the milled SrTiO₃:Rh photocatalyst is due to the presence of Rh⁴⁺ ions induced by visible light irradiation and the large surface area by ball-milling treatment. It is notable that the SrTiO₃:Rh photocatalyst also has a unique property, showing a selective antiphage performance.

In contrast to the single Rh-doped SrTiO₃, a Rh and Sb-codoped SrTiO₃ photocatalyst (SrTiO₃:Rh,Sb) shows photocatalytic activities for both sacrificial hydrogen and oxygen evolutions under visible light irradiation [30] and can split water under visible light irradiation in the presence of an IrO_x cocatalyst [31]. The change in the oxidation number of the Rh ion caused by codoping of Sb plays an important role. Both Rh³⁺ and Rh⁴⁺ ions exist in SrTiO₃:Rh as prepared. On the other hand, when the Rh and Sb ions are codoped in SrTiO₃, the Sb ion is doped as Sb⁵⁺ at a Ti⁴⁺ site in a SrTiO₃ host. Therefore, the oxidation number of Rh⁴⁺ is controlled to Rh³⁺ due to the charge compensation. Additionally, it is confirmed that the IrO_x cocatalyst enhances the activities for sacrificial hydrogen and oxygen evolutions of SrTiO₃:Rh,Sb, which indicates that it works as active sites for both hydrogen and oxygen evolutions on water splitting over an IrO_x/SrTiO₃:Rh,Sb photocatalyst. The Rh³⁺ ion and an IrO_x cocatalyst are the key factors for photocatalytic water splitting of SrTiO₃:Rh,Sb. IrO_x/SrTiO₃:Rh,Sb photocatalyst splits water into H₂ and O₂ stoichiometrically under visible light irradiation of up to 500 nm as depicted in Fig. 4 and shows the activity for solar water splitting [31]. Al-doped SrTiO₃, AgTaO₃, and Na_0.5Bi_0.5TiO₃ photocatalysts show the high activity for solar water splitting [32–35]. However, those photocatalysts do not respond to visible light. Therefore, it is notable that the IrO_x/SrTiO₃:Rh,Sb photocatalyst is the visible light responsive oxide photocatalyst, showing the activity for solar water splitting.

2.2 Ir-doped photocatalyst

SrTiO₃:Rh and SrTiO₃:Rh,Sb photocatalysts respond to a visible light of up to 540 and 500 nm, respectively. However, it is required to develop the photocatalyst being responsive to longer light wavelength for efficient uses of sunlight. An Ir ion is an effective dopant for response to long wavelength of visible light because the Ir³⁺ ion forms a shallower impurity level in a band gap than the Rh³⁺ ion. Recently, Ir-doped SrTiO₃ photocatalyst loaded with Ir cocatalyst (Ir/SrTiO₃:Ir) has been developed. The Ir/SrTiO₃:Ir photocatalyst treated with H₂ reduction at 673 K shows the activity for sacrificial hydrogen evolution under a visible light of up to approximately 800 nm as exhibited in Fig. 5 [36], indicating that Ir/SrTiO₃:Ir can utilize the whole range of visible light. The Ir ion is mainly doped as Ir⁴⁺ at a Ti⁴⁺ site in a SrTiO₃ host as prepared. After H₂ reduction and sacrificial hydrogen evolution, it was confirmed by diffuse reflectance spectra that the oxidation number of the Ir⁴⁺ ion changed to Ir³⁺. The sacrificial hydrogen evolution proceeds by transition from the impurity levels formed by Ir³⁺ to the conduction band of SrTiO₃. In addition, an Ir cocatalyst plays an important role in the activity for hydrogen evolution over Ir/SrTiO₃Ir. H₂ reduction contributes to the formation of metallic Ir and a good contact between the loaded Ir and SrTiO₃:Ir host. By these synergistic effects, an Ir cocatalyst works as an efficient site for hydrogen evolution.

NaNbO₃ and BaTa₂O₆ codoped with Ir ions and alkali earth metal ions or lanthanum ions have also been reported as visible light responsive photocatalysts [37,38]. The Ir ion is doped as Ir³⁺ by codoping with Ca²⁺, Sr²⁺, Ba²⁺, and La³⁺ due to charge compensation. NaNbO₃ codoped with Ir and Sr ions shows the activity for both sacrificial hydrogen and oxygen evolutions under visible light irradiation and responds to a visible light of up to 700 nm for sacrificial hydrogen evolution. In addition, NaTaO₃ and BaTa₂O₆ codoped with Ir and La ions shows the photocatalytic activity for sacrificial hydrogen evolution under a visible light of up to 600 and 640 nm, respectively. It suggests that the Ir ion is a suitable dopant for sensitization of a metal oxide photocatalyst to a long wavelength of visible light.

2.3 Ru-doped photocatalyst

Previously, a Ru-doped SrTiO₃ (SrTiO₃:Ru) photocatalyst showing the activity for both hydrogen and oxygen evolutions from aqueous solutions containing sacrificial reagents under visible light irradiation is reported [22]. This photocatalyst is a unique material which is active for both sacrificial hydrogen and oxygen evolutions by doping of a single metal ion. However, the photocatalytic properties have not been clarified so far. Thus, effects of co-doping and H₂ reduction to SrTiO₃:Ru on photocatalytic properties are expected. It is confirmed by electron spin resonance spectra and diffuse reflectance spectra that the oxidation number of a Ru dopant is controlled to trivalent by co-doping of a Sb⁵⁺ ion and H₂ reduction [39]. While the activities for sacrificial hydrogen evolution over SrTiO₃:Ru, Sb and H₂-red. SrTiO₃:Ru are lower than that over pristine SrTiO₃:Ru, the activities for sacrificial oxygen evolution are higher than that over pristine SrTiO₃:Ru. In particular, the activity over SrTiO₃:Ru treated with H₂ at 673 K is the highest among those photocatalysts, which shows a higher activity of about four times than pristine SrTiO₃:Ru. It is notable that H₂-red. SrTiO₃:Ru shows the activity for sacrificial oxygen evolution under a visible light of up to 750 nm.

3 Valence band-controlled photocatalysts developed by metal ion exchange for alkali ions in various metal oxides using molten salts treatment

BiVO₄ and SnNb₂O₆ are the representative valence band-controlled photocatalysts [3,5,40–42]. On the other hand, the novel visible light responsive photocatalyst can be developed by metal ion exchange of the alkali ion in metal oxides. The Ag(I) and Cu(I) ions contribute to forming a shallow valence band in oxide materials [6,43,44]. However, it is difficult to prepare the materials containing Ag(I) and Cu(I) ions by conventional solid state reaction because of the formation of Ag(0) and Cu(II). In such a background, the visible light responsive photocatalysts containing the Ag(I) and Cu(I) ions have been successfully developed by treatments of layered oxide materials with molten AgNO₃ and CuCl. Herein, various visible light responsive photocatalysts developed by Ag(I) and Cu(I) ions exchange of the alkali ions in various metal oxides by molten salts treatment are described.

3.1 Ag(I) ion-exchanged metal oxide photocatalysts with a layered structure

Photocatalytic activity for sacrificial oxygen evolution over various Ag(I) ion-exchanged layered oxide materials is listed in Table 1.

Na₂W₄O₁₃ consists of layered structure of [W₄O₁₃]²⁻ slabs and Na⁺ ions in the interlayer as displayed in Fig. 6. Na₂W₄O₁₃ cannot absorb visible light because its band gap is 3.12 eV. When Na₂W₄O₁₃ is treated with molten AgNO₃, Na⁺ ions are exchanged with Ag⁺ ions, keeping the layered structure, which results in the formation of a new valence band consisting of Ag4d orbitals, leading to the photocatalytic activity for the sacrificial oxygen evolution under visible light irradiation [45]. The Ag(I) ion-exchanged Na₂W₄O₁₃ photocatalyst responds to a visible light of up to 440 nm at which non-ion-exchanged Na₂W₄O₁₃ cannot show the activity. Additionally, Z-schematic water splitting using the Ag(I) ion-exchanged Na₂W₄O₁₃ as an oxygen-evolving photocatalyst with a SrTiO₃:Rh photocatalyst of a hydrogen-evolving photocatalyst proceeds under visible light irradiation.

Many metal oxide photocatalysts with Ruddlesden-Popper-type and Dion-Jacobson-type layered perovskite structures synthesized by molten AgNO₃ treatment such as Ag(I)-A₂SrTa₂O₇ (A= Li, K), Ag(I)-K₂SrNb_0.2Ta_1.8O₇, Ag(I)-K₂CaNaNb₃O₁₀, and Ag(I)-KLaNb₂O₇ are active for sacrificial oxygen evolution under light irradiation [46]. The visible light response is attributed to photoexcitation of electrons from a valence band consisting of Ag4d and O2p orbitals to a conduction band of layered oxides of host materials. In general, Ag(I) ions in interlayer to metallic Ag are usually reduced by photoexcited electrons, leading to the deactivation of the photocatalyst. However, the XRD pattern of Ag(I)-K₂SrTa₂O₇ of a representative Ag(I) ion-exchanged material hardly changes even after the photocatalytic sacrificial oxygen evolution for 5 h. This indicates that the Ag(I) ions in the interlayers of Ag(I)-K₂SrTa₂O₇ are relatively stable against the reduction.

3.2 Cu(I) ion-exchanged metal oxide photocatalysts with a layered and tunneling structure

Photocatalytic activity for sacrificial hydrogen evolution over various Cu(I) ion-exchanged layered materials is summarized in Table 2.

Cu(I) ion-exchanged Li₂TiO₃ (CuLi_1/3Ti_2/3O₂) with a delafossite structure has trigonal and hexagonal phases. Although it was reported that the hexagonal CuLi_1/3Ti_2/3O₂ photocatalyst showing the sacrificial hydrogen evolution under visible light irradiation was successfully prepared by a flux method [47], the single phase of trigonal had not been prepared so far. Cubic Li₂TiO₃ of a low temperature phase and monoclinic Li₂TiO₃ of a high temperature phase have a bulky structure and layered crystal structures, respectively. Hexagonal CuLi_1/3Ti_2/3O₂ and trigonal CuLi_1/3Ti_2/3O₂ can be selectively prepared in a single phase by a molten CuCl treatment of cubic and monoclinic Li₂TiO₃ [48]. The band gaps of hexagonal and trigonal CuLi_1/3Ti_2/3O₂ are 2.1 eV and they show the activity for photocatalytic hydrogen evolution from an aqueous solution containing sacrificial reagent under a visible light of up to about 600 nm. In addition, Z-schematic water splitting proceeds under solar light irradiation using those Pt-loaded CuLi_1/3Ti_2/3O₂ as hydrogen-evolving photocatalysts, a TiO₂ as an oxygen-evolving photocatalyst, and a reduced graphene oxide (RGO) as a solid-state electron mediator as shown in Fig. 7.

Various Cu(I) ion-exchanged Ruddlesden-Popper(RP)-type and Dion-Jacobson(DJ)-type layered perovskite metal oxides have also been developed [46,49], among which, RP-type metal oxides consisting of Ti(IV) or Ta(V) in the perovskite slabs, and K(I) in the interlayer are suitable host materials to obtain visible light responsive photocatalysts by Cu(I) ion exchange. In particular, Cu(I)-K₂SrTa₂O₇ photocatalyst with an RP structure shows the highest activity for sacrificial hydrogen evolution under a visible light irradiation of up to 600 nm. Although Cu(I)-KLaTa₂O₇ with a DJ structure has a similar slab to Cu(I)-K₂SrTa₂O₇, the photocatalytic activity of the Cu(I)-KLaTa₂O₇ is much lower than that of Cu(I)-K₂SrTa₂O₇. The difference in the photocatalytic activity is due to the density of the Cu(I) ion and the interaction between the Cu(I) ion at the interlayer in the photocatalyst. The high density of the Cu(I) ions is favorable for the migration of photogenerated holes to the edges of layered structure of photocatalyst particles because Cu(I) forms the valence band. On the other hand, even if Cu(I) ion-exchanged M₂La₂Ti₃O₁₀ (M= K, Na) photocatalysts possess the same layered structure as each other, Cu(I)-K₂La₂Ti₃O₁₀ shows a much higher activity than Cu(I)-Na₂La₂Ti₃O₁₀. It is confirmed that Cu(I)-Na₂La₂Ti₃O₁₀ contains Cu(II) impurities that work as a recombination center between photogenerated e^– and h⁺. These studies reveal that not only the intrinsic property of the host material and the rate of ion exchange but also the density of the Cu(I) ions exchanged in the interlayers and the formation of unfavorable Cu(II) species affect the photocatalytic activity.

Cu(I)-Li₂MTi₆O₁₄ (M= Na₂, Sr, Ba, Pb) with a tunneling structure is also successfully developed by Cu(I) ion exchange for the alkali ions in the tunnel [46], of which, Cu(I)-Li₂SrTi₆O₁₄ shows the highest activity for sacrificial hydrogen evolution responding to a visible light of up to 560 nm. This study is of great significance in terms of the successful development of visible-light-driven photocatalyst from a wide-band-gap photocatalyst with a tunneling structure by Cu(I) ion exchange.

4 Solid solution photocatalysts

Solid solution has often been applied to develop visible light responsive photocatalysts based on a calcogenide-type photocatalyst. Although photocorrosion of a metal sulfide photocatalyst easily occurs in general, it is suppressed by using a suitable reducing reagent of sacrificial electron donor such as S²⁻ or {\mathrm{SO}}_{\mathrm{3}}^{\mathrm{2}-}. Many metal sulfide photocatalysts containing the Cu(I) ion show a p-type semiconductor character and can be employed as a photocathode in a photoelectrochemical water splitting system. Construction of the system responsive to visible light by employing a sulfide photocatalyst as a photocathode and an n-type semiconductor photocatalyst as a photoanode is a significant research topic.

CuGaS₂-AgGaS₂ [50], ZnS-CuGaS₂ [51], and ZnS-CuGaS₂-CuInS₂ [52] solid solution photocatalysts have been developed. The optimal ratio of these solid solutions on photocathodic property are Cu_0.8Ag_0.2GaS₂ (BG: 2.2 eV), (CuGa)_0.5ZnS₂ (BG: 1.9 eV), and Cu_0.8Ga_0.4In_0.4Zn_0.4S₂ (BG: 2.35 eV), respectively. By employing a Cu_0.8Ga_0.4In_0.4Zn_0.4S₂-based photocathode and a BiVO₄-based photoanode as shown in Fig. 2, solar water splitting proceeds without any external bias.

Cu₃MS₄ (M= V, Nb, Ta) with a sulvanite structure is also the promising photocatalyst which shows the activity for sacrificial hydrogen evolution under visible light irradiation [53,54]. The band gaps of Cu₃VS₄, Cu₃NbS₄, and Cu₃TaS₄ are 1.6, 2.5, and 2.8 eV, respectively. Solid solutions using those photocatalysts, such as Cu₃Nb_1-xV_xS₄ and Cu₃Ta_1–xV_xS₄, show higher activities than the single component Cu₃MS₄ (M= V, Nb, Ta). The band gaps of those solid solutions are 1.6–1.7 eV, indicating the absorption of a wide range of visible light. In particular, the Cu₃Nb_0.9V_0.1S₄ solid solution shows the highest photocatalytic activity among those solid solutions. Additionally, the Cu₃Nb_1–xV_xS₄ and Cu₃Ta_1–xV_xS₄ give cathodic photocurrents under visible light irradiation, indicating that those solid solutions show a p-type semiconductor character, among which, the Cu₃Nb_0.9V_0.1S₄ shows the best photoelectrochemical performance. When the system consisting of Ru-loaded Cu₃Nb_0.9V_0.1S₄ as a photocathode and BiVO₄ loaded with CoOx cocatalyst as a photoanode is constructed as shown in Fig. 3, the photoelectrochemical water splitting proceeds under simulated sunlight irradiation. This result indicates that the formation of solid solutions is effective for Cu₃MS₄ (M= V, Nb, Ta)-based photocatalysts and photoelectrodes.

5 Conclusions

Various visible light responsive photocatalysts were developed by metal ion exchange. For doped-photocatalysts, a SrTiO₃:Rh showed a high activity for sacrificial hydrogen evolution under visible light irradiation and functioned as a photocathode in a photoelectrochemical system. IrO_x/SrTiO₃:Rh,Sb of a single particulate photocatalyst was also a promising photocatalyst which showed the activity for water splitting under visible light irradiation. Moreover, Ir and Ru were also an excellent dopant for sensitization of a photocatalyst to long wavelength of visible light. In particular, a Ir/SrTiO₃:Ir treated with H₂ showed the activity for sacrificial hydrogen evolution responding up to the whole range of a visible light of up to 800 nm. For valence band-controlled photocatalysts, various visible light responsive metal oxides with layered and tunneling structures were developed by exchange of the alkali ions in the metal oxides with Ag(I) and Cu(I) ions by molten salts treatment, among which, a CuLi_1/3Ti_1/3O₂ photocatalyst with a delafossite-like structure and a Cu(I)-K₂SrTa₂O₇ photocatalyst with a Ruddlesden-Popper-type layered structure showed a relatively high activity for sacrificial hydrogen evolution under visible light irradiation. For solid solution photocatalysts, CuGaS₂-AgGaS_2, ZnS-CuGaS₂, ZnS-CuGaS₂-CuInS₂, and Cu₃MS₄(M= V, Nb, Ta) photocatalysts which showed the activity for sacrificial hydrogen evolution under visible light were developed. In particular, Cu₃Nb_0.9V_0.1S₄ photocatalyst and photocathode showed a higher activity for sacrificial hydrogen evolution and a larger photocurrent than the single component Cu₃VS₄ and Cu₃NbS₄. This indicates that the formation of a solid solution is also an effective way to improve photocatalytic and photoelectrochemical performances.

For practical use of photocatalytic water splitting for hydrogen production, it is strongly required to develop the photocatalyst responding to the long wavelength of visible light and having a high quantum yield. Although the number of reported photocatalysts responding to a long wavelength of light is increasing, that of photocatalysts with a high quantum yield is still limited. The development of highly activated photocatalysts responsive to long wavelength visible lights can be achieved by further improving the preparation method. If the strategy of the design of a photocatalyst with a high quantum efficiency is clarified, great progresses will be made in this research area. It is expected that the photocatalytic water splitting technology is applicable for practical use by development of novel visible-light-driven photocatalysts.

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