Photocatalytic CO2 Reduction Using Water as an Electron Donor under Visible Light Irradiation by Z-Scheme and Photoelectrochemical Systems over (CuGa)0.5ZnS2 in the Presence of Basic Additives.

We demonstrated photocatalytic CO2 reduction using water as an electron donor under visible light irradiation by a Z-scheme photocatalyst and a photoelectrochemical cell using bare (CuGa)0.5ZnS2 prepared by a flux method as a CO2-reducing photocatalyst. The Z-scheme system employing the bare (CuGa)0.5ZnS2 photocatalyst and RGO-(CoOx/BiVO4) as an O2-evolving photocatalyst produced CO of a CO2 reduction product accompanied by H2 and O2 in a simple suspension system without any additives under visible light irradiation and 1 atm of CO2. When a basic salt (i.e., NaHCO3, NaOH, etc.) was added into the reactant solution (H2O + CO2), the CO formation rate and the CO selectivity increased. The same effect of the basic salt was observed for sacrificial CO2 reduction using SO32- as an electron donor over the bare (CuGa)0.5ZnS2 photocatalyst. The selectivity for the CO formation of the Z-schematic CO2 reduction reached 10-20% in the presence of the basic salt even in an aqueous solution and without loading any cocatalysts on the (CuGa)0.5ZnS2 metal sulfide photocatalyst. It is notable that CO was obtained accompanied by reasonable O2 evolution, indicating that water was an electron donor for the CO2 reduction. Moreover, the present Z-scheme system also showed activity for solar CO2 reduction using water as an electron donor. The bare (CuGa)0.5ZnS2 powder loaded on an FTO glass was also used as a photocathode for CO2 reduction under visible light irradiation. CO and H2 were obtained on the photocathode with 20% and 80% Faradaic efficiencies at 0.1 V vs RHE, respectively.

Introduction

Beneficial CO2 fixation technology especially utilizing renewable energy is strongly demanded to solve resources, energy, and environmental issues. Photocatalytic CO2 reduction using water as an electron donor has been paid much attention as the promising reaction to convert solar energy to chemicals such as CO, HCOOH, and CH4, which is referred to as artificial photosynthesis.[1,2] The photocatalytic CO2 reduction of an artificial photosynthesis has significant potential because chemical products such as CO are directly obtained from CO2 by utilizing sunlight and water, which is a chemically stable and abundant resource.[3]

The following points are key issues for photocatalytic CO2 reduction in artificial photosynthesis: high activity and selectivity, visible light response, using water as an electron donor, durability, and a simple aqueous suspension system. For achieving CO2 conversion in artificial photosynthesis, we must consider a Gibbs free energy change of the reaction (ΔG). Although photocatalytic CO2 reduction efficiently proceeds using sacrificial electron donors (i.e., triethanolamine and sulfite), the reaction is not artificial photosynthesis because of ΔG < 0, as shown in Figure (a). It is indispensable to reduce CO2 using water as an electron donor, namely, accompanied by O2 evolution by water oxidation, for achieving artificial photosynthesis with ΔG > 0. A Z-scheme photocatalyst and a photoelectrochemical cell are attractive systems to achieve CO2 reduction using water as an electron donor as shown in Figure (b) and (c).[4] Moreover, these systems can widely employ visible light responsive photocatalysts aiming at efficient sunlight utilization. In a UV light responsive system using wide band gap photocatalysts, highly efficient photocatalytic CO2 reduction using water as an electron donor has been achieved in the last 10 years since the discovery of the Ag/BaLa4Ti4O15 photocatalyst via one-photon excitation.[5] However, it cannot utilize visible and solar light because of its wide band gap. On the other hand, in a visible light responsive system, there are few suspension photocatalysts active for CO2 reduction using water as an electron donor accompanied by O2 evolution. So, construction of an efficient CO2 reduction system with a visible light response is demanded in the current stage. Ideally, the photocatalytic CO2 reduction system should be composed of powder-based photocatalyst materials for simplicity and low cost toward practical use.[6−9] From the background, we have focused on metal sulfide powdered photocatalysts as a CO2-reducing photocatalyst in a Z-scheme system and a photocathode in a photoelectrochemical system.

Figure 1

(a) CO2 reduction of a downhill reaction over a (CuGa)0.5ZnS2 photocatalyst using a sacrificial electron donor, (b) CO2 reduction of an uphill reaction by a Z-scheme photocatalyst system consisting of (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4), and (c) a photoelectrochemical system employing a (CuGa)0.5ZnS2 photocathode using water as an electron donor. An external bias lower than the redox potentials of CO2 and H2O should be applied for energy conversion in a photoelectrochemical cell.

Many metal sulfides are active for CO2 reduction under visible light irradiation in an aqueous solution containing a sacrificial reagent as an electron donor (Figure (a)).[10−13] However, they cannot oxidize water to form O2 because photocorrosion (i.e., self-photooxidation) occurs in an aqueous solution in the absence of strong electron donors. In other words, the metal sulfides cannot reduce CO2 using water as an electron donor as a single-particulate photocatalyst. In contrast, we have successfully constructed a Z-schematic CO2 reduction system using water as an electron donor employing bare CuGaS2 as a CO2-reducing photocatalyst combined with CoO/BiVO4 as an O2-evolving photocatalyst and reduced graphene oxide (RGO) as a solid-state electron mediator driven via interparticle electron transfer (Figure (b)).[12] The Z-scheme photocatalyst system can reduce CO2 to CO accompanied by O2 evolution under visible light irradiation even using a photocorrosive metal sulfide photocatalyst.[12] However, the CO evolution activity in the Z-schematic CO2 reduction was quite low and the CO selectivity was about 1%. Thus, improvement of the Z-schematic CO2 reduction system using metal sulfide and RGO-(CoO/BiVO4) photocatalysts is a key issue for efficient CO2 fixation by an artificial photosynthesis.

Usage of a photocatalyst system that shows a high activity for water splitting under visible light irradiation is one approach to achieve efficient photocatalytic CO2 reduction. Recently, we have successfully improved a Z-schematic water splitting system using RGO-(CoO/BiVO4) by combining it with Pt-loaded (CuGa)0.5ZnS2 prepared by a flux method as a H2-evolving photocatalyst instead of conventional Pt-loaded CuGaS2 prepared by a solid-state reaction (SSR).[14] The Z-scheme photocatalyst system consisting of Pt/(CuGa)0.5ZnS2 by a flux method and RGO-(CoO/BiVO4) shows the highest solar-to-hydrogen conversion efficiency among Z-schematic water splitting systems using a photocorrosive metal sulfide as a H2-evolving photocatalyst in a simple suspension system. It is expected that the (CuGa)0.5ZnS2 prepared by flux works for efficient Z-schematic CO2 reduction using water as an electron donor under visible light irradiation.

Photocatalytic CO2 reduction activity strongly depends on the condition of a reactant solution, especially pH and additives. For instance, the CO evolution rate and CO selectivity over cocatalyst-loaded wide band gap metal oxide photocatalysts are enhanced by adding a basic salt into a reactant solution because of pH adjustment and efficient supply of hydrated CO2 molecules of a reactant substrate.[15−17] However, at the current stage, there are no reports on the effects of salt addition on Z-schematic CO2 reduction using metal sulfides as a reducing photocatalyst driven via interparticle electron transfer through RGO combined with an O2-evolving photocatalyst, except for usage of a [Ru(dpbpy)] complex as a CO2-reducing catalyst and a [Co(tpy)2] complex as an electron mediator.[13] It is expected that the addition of a basic salt enhances the Z-schematic CO2 reduction employing metal sulfide and RGO-(CoO/BiVO4) photocatalysts using water as an electron donor under visible light irradiation.

In addition to the Z-schematic CO2 reduction in a simple suspension system, a photoelectrochemical system also has potential to utilize a metal sulfide photocatalyst material for the CO2 reduction (Figure (c)). Although there are some reports on photoelectrochemical CO2 reduction using photocathodes of metal sulfides,[18−21] it is still an important topic to develop a new photoelectrochemical CO2 reduction cell with a metal sulfide. (CuGa)0.5ZnS2 is a candidate as a new photocathode for the photoelectrochemical CO2 reduction because it has a p-type character and a suitable conduction band level for CO2 reduction.[13,14,22]

In the present study, we demonstrated Z-schematic CO2 reduction using bare (CuGa)0.5ZnS2 prepared by a flux method and RGO-(CoO/BiVO4) using water as an electron donor under visible light irradiation in the presence of various salts as shown in Figure (b) as an artificial photosynthesis. Sacrificial CO2 reduction over bare (CuGa)0.5ZnS2, which is a half-reaction of Z-schematic CO2 reduction, was also investigated to support the data of the Z-scheme system (Figure (a)). We also conducted bulk electrolysis of photoelectrochemical CO2 reduction employing a bare (CuGa)0.5ZnS2 photocathode under visible light irradiation, as shown in Figure (c).

Experimental Section

Preparation of Photocatalysts

Powdered (CuGa)0.5ZnS2 was synthesized at 723 K for 15 h under vacuum by a flux method using a LiCl–CsCl flux (LiCl:CsCl = 3:2, melting point 600 K at the molar ratio of 3:2), according to a previous report.[14] Starting materials of metal sulfides, Cu2S (Kojundo Chemical; 99%), Ga2S3 (Kojundo Chemical; 99.99%), and ZnS (Rare Metal Chemical; 99.99%), were mixed in an agate mortar with an atomic ratio of Cu:Ga:Zn = 1.0:1.2:2.4 for (CuGa)0.5ZnS2, containing 20 at. % of excess amounts of Ga and Zn. The obtained powders were washed with water to remove the flux reagent. Powdered (CuGa)0.5ZnS2 and CuGaS2 were also synthesized by an SSR method at 1073 and 873 K, respectively, for 10 h under vacuum according to previous reports as reference materials.[12,14]

BiVO4 was prepared by a liquid–solid-state reaction at room temperature, according to a previous report.[23] A CoO cocatalyst was loaded on the BiVO4 by impregnation with an aqueous Co(NO3)2 (Wako; 99.5%) solution. The BiVO4 powder (0.5 g) and an aqueous Co(NO3)2 solution (80 mmol L–1, 0.53 mL) were placed in a porcelain crucible and dried on a boiling water bath. The Co(NO3)2-impregnated powder was calcined in air at 673 K for 2 h to obtain CoO/BiVO4.[12] An RGO-(CoO/BiVO4) composite was prepared by photocatalytic reduction of a graphene oxide (NiSiNa materials; Rap TQ2-10) over CoO/BiVO4 under visible light irradiation.[12,24] CoO/BiVO4 (0.3 g) and graphene oxide (5 wt % to CoO/BiVO4) were dispersed in an aqueous methanol solution (50 vol %, 40 mL). The suspensions were stirred and bubbled with N2 gas under visible light irradiation for 3 h to prepare the RGO-(CoO/BiVO4) composite.

(CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) powders were characterized before and after photocatalytic CO2 reduction. The crystal phases were clarified by X-ray diffraction (XRD; Rigaku; MiniFlex). Diffuse reflectance spectra were obtained using a UV–vis–NIR spectrometer (JASCO; Ubest-570) equipped with an integrating sphere and were transferred from reflection to absorbance by the Kubelka–Munk method. The surface component and chemical state were analyzed by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) using an X-ray photoelectron spectrometer (Shimazu; ESCA-3400) with a Mg Kα anode. The samples were loaded on a carbon tape or an indium foil for the XPS measurement. The chemical shift was corrected using a C 1s peak or an In 3d peak. Electron spin resonance (ESR) was measured at liquid nitrogen temperature (77 K) using an ESR spectrometer (ADANI; SPINSCAN) with a microwave frequency of 9.4 GHz.

Photocatalytic Reactions

Sacrificial CO2 reduction (Figure (a)), which is a half-reaction of Z-schematic CO2 reduction, was conducted under 1 atm of CO2 gas flow (99.995%) at standard ambient temperature. Bare (CuGa)0.5ZnS2 photocatalyst (0.2 g) was dispersed in an aqueous solution containing 0.1 mol L–1 K2SO3 (Kanto Chemical; 95%) as a sacrificial reagent and 0–1.0 mol L–1 NaHCO3 (Wako Pure Chemical; 99.5%) in a top-irradiation reaction cell with a Pyrex window. Z-schematic water splitting and CO2 reduction using water as an electron donor (Figure (b)) were carried out at standard ambient temperature under 1 atm of Ar and CO2 gas flow, respectively. (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) (0.05–0.1 g each) were dispersed in water (120 mL) in a top-irradiation cell with a Pyrex window. Li2CO3 (Kojundo Chemical; 99%), NaOH (Kanto Chemical; 95%), NaHCO3, Na2CO3 (Kanto Chemical; 99.8%), KHCO3 (Kanto Chemical; 99.5%), CsHCO3 (Wako Pure Chemical; 99%), NH4HCO3 (Kanto Chemical; 96.0%), H3BO3 (Kanto Chemical; 99.5%), and NaCl (Kanto Chemical; 99.5%) were added into the reactant solution, if necessary. A 300 W Xe lamp (PerkinElmer; CERMAX PE300BF) with a long-pass filter (HOYA; L42) was employed as a light source. The light irradiation area was 33 cm2. The power of incident light at the center was adjusted to 25 mW cm–2 at 520 nm using a band-pass filter (Asahi Spectra) and a photodiode head (OPHIR; PD300-UV head and NOVA display). A solar simulator (Asahi spectra; HAL-320; AM-1.5 G; 100 mW cm–2) was employed as a light source for photocatalytic solar CO2 reduction. The light irradiation area was 16 cm2. Amounts of evolved H2, O2, and CO were determined using an online gas chromatograph (Shimadzu; GC-8A, MS-5A column, TCD, Ar carrier for H2 and O2; Shimadzu; GC-8A, MS-13X column, FID with a methanizer, N2 carrier for CO). Formic acid of an aqueous product was analyzed using an ion chromatograph (IC; TOSOH; IC-2010, TSKgel SuperIC-Anion HR). The CO selectivity, the ratio of reacted electrons to holes, and the solar to chemical energy conversion efficiency were calculated as follows:

CO selectivity % = 100 × (rate of CO formation [μmol h–1])/(sum of rates of H2 and CO formations [μmol h–1])

e–/h+ = (2 × sum of rates of H2 and CO formations [μmol h–1])/(4 × rate of O2 formation [μmol h–1])

solar to chemical energy conversion efficiency % = 100 × ((ΔG°298 associated with water splitting × rate of H2 formation + ΔG°298 associated with CO2 reduction to CO using water as an electron donor × rate of CO formation [J mol–1 × μmol h–1])/(irradiation time [h] × solar energy (AM-1.5 G) [W cm–2] × irradiation area [cm2]))

Photoelectrochemical Measurement

A (CuGa)0.5ZnS2 photocathode was prepared by a drop-cast method. Bare (CuGa)0.5ZnS2 powder (1 mg) was dispersed in 1 mL of ethanol (Kanto Chemical; 99.5%). The suspension was drop cast on a fluorine-doped tin oxide (FTO) transparent electrode. The (CuGa)0.5ZnS2-loaded FTO substrate was calcined at 573 K for 2 h under N2. Photoelectrochemical measurement was conducted using a three-electrode system with working, Ag/AgCl reference, and Pt counter electrodes connected to a potentiostat (Hokuto Denko; HSV-110) in an H-type glass cell with an optical window made of quartz. The glass cell was divided into cathode and anode parts using a Nafion membrane. An aqueous KHCO3 (Kanto Chemical; 99.5%) solution of an electrolyte was bubbled with CO2 gas at 15 mL min–1 during bulk electrolysis at −0.5 V vs Ag/AgCl at pH 6.9 (0.1 V vs RHE) (Figure (c)). A 300 W Xe lamp (PerkinElmer; CERMAX PE300BF) with a long-pass filter (HOYA; L42) was employed as a light source. The power of incident light at the center was adjusted to 25 mW cm–2 at 520 nm using a band-pass filter (Asahi Spectra) and a photodiode head (OPHIR; PD300-UV head and NOVA display). Amounts of evolved H2 and CO were determined using an online gas chromatograph (Shimadzu; GC-8A, MS-5A column, TCD, Ar carrier for H2; Shimadzu; GC-8A, MS-13X column, FID with a methanizer, N2 carrier for CO).

Results and Discussion

Z-Schematic CO2 Reduction under Visible Light Irradiation Using (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) in the Presence of Various Salts in a Suspension System

Table shows Z-schematic water splitting and CO2 reduction using bare (CuGa)0.5ZnS2 powder by a flux method as a reducing photocatalyst and RGO-(CoO/BiVO4) as an O2-evolving photocatalyst under visible light irradiation. In a previous report, a Pt cocatalyst was loaded on the (CuGa)0.5ZnS2 for Z-schematic water splitting.[14] However, the Pt cocatalyst does not work as an effective active site for CO2 reduction in Z-schematic CO2 reduction using a metal sulfide as a reducing photocatalyst,[11] because Pt is well known to enhance water reduction and be poisoned by CO. So, first, Z-schematic water splitting was carried out dispersing (CuGa)0.5ZnS2 without any cocatalysts and RGO-(CoO/BiVO4) in pure water under Ar and visible light irradiation (Table ; entry 1). H2 and O2 steadily evolved in a stoichiometric ratio for 8 h under visible light irradiation (Figure S1). CO was not obtained, indicating RGO could be neglected as an origin of carbon-containing product. Thus, it was confirmed that the (CuGa)0.5ZnS2 worked as a H2-evolving photocatalyst even without cocatalyst in the Z-scheme photocatalyst system combined with RGO-(CoO/BiVO4).

Table 1

Effect of Salt Addition on Z-Schematic CO2 Reduction under Visible Light Irradiation Using Bare (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) Photocatalystsa

				activity [μmol h–1]
entry	flow gas	additive [mmol L–1]	pH	H2	O2	CO	CO selectivity [%]	e–/h+
1	Ar	none	6.3	17.1	8.5	trace	0	1.0
2	CO2	none	4.1	1.1	0.5	0.1	11	1.23
3	Ar	H2SO4	4.0	1.4	0.7	trace	0	1.0
4	CO2	NaHCO3 (1)	5.2	3.5	1.9	0.4	11	1.04
5	CO2	NaHCO3 (10)	5.9	12.0	6.4	1.8	13	1.08
6	CO2	NaHCO3 (50)	6.7	8.3	3.8	2.4	23	1.20
7	CO2	NaHCO3 (100)	6.8	8.9	3.5	3.2	26	1.73
8	CO2	Li2CO3 (5)	5.9	8.1	4.4	2.3	22	1.17
9	CO2	NaOH (10)	5.8	10.2	5.2	1.4	12	1.07
10	CO2	Na2CO3 (10)	6.2	9.7	5.4	2.8	23	1.16
11	CO2	KHCO3 (1)	5.1	3.8	2.0	0.5	11	1.07
12	CO2	KHCO3 (10)	5.9	8.1	4.6	2.1	20	1.11
13	CO2	CsHCO3 (10)	5.9	8.3	4.3	2.4	23	1.24
14	CO2	NH4HCO3 (10)	5.9	9.4	5.0	2.4	20	1.17
15	CO2	H3BO3 (10)	4.1	1.2	trace	0.1	7
16	CO2	NaCl (10)	4.1	0.4	trace	trace	0

Photocatalyst: 0.05 or 0.1 g each, reactant solution: water (120 mL), flow gas: CO2 and Ar (1 atm), light source: 300 W Xe lamp (λ > 420 nm), light irradiation area: 33 cm2, cell: top-irradiation cell with a Pyrex window. CO selectivity [%] = 100 × (rate of CO formation)/(sum of rates of H2 and CO formations).

Z-schematic CO2 reduction was carried out suspending the (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) powders in CO2-dissolved water without any salts of additives under visible light irradiation. The pH of the reactant solution became around 4 because of dissolved CO2. CO of a CO2 reduction product was obtained in addition to H2 and O2 by water splitting (entry 2). We confirmed by IC analysis that HCOOH production was negligible in the Z-schematic CO2 reduction. We can rule out the possibility that obtained CO came from contaminants because CO was not obtained in a Z-schematic reaction under Ar even at pH 4 (entry 3). Thus, it was concluded that the Z-scheme photocatalyst system of bare (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) reduced CO2 to CO using water as an electron donor under visible light irradiation.

The ability of the Z-scheme photocatalyst should depend on the pH of the reactant solution. An acidic condition resulted in low activity of the Z-scheme photocatalyst (entries 1–3). Thus, tuning the pH and salt addition are key issues for enhancing the Z-schematic CO2 reduction. The Z-schematic CO2 reduction was investigated at different pHs from 4 to 7 adjusted with various concentrations of NaHCO3 of a basic additive (entries 2, 4–7). CO, H2, and O2 evolved at any pH under visible light irradiation, but negligible HCOOH was obtained (<0.1 μmol h–1). Adjusting the pH to 5–6 by addition of 1–10 mmol L–1 NaHCO3 (entries 4, 5) enhanced H2, O2, and CO formation compared with pH 4 without any additives (entry 2). The ratio of reacted electrons to holes (e–/h+) was estimated to be almost 1 in the Z-schematic CO2 reduction (entries 4, 5). However, when the concentration of NaHCO3 was equal to or higher than 50 mmol L–1, CO formation was more enhanced but O2 evolution was not. The increase in the selectivity for CO formation in the presence of a high concentration of NaHCO3 was due to an efficient supply of hydrated CO2 molecules of a reactant substrate. However, when 100 mmol L–1 of NaHCO3 was used, the e–/h+ was 1.73, being exceedingly beyond unity (entry 7). We confirmed that H2O2 of another candidate as a water oxidation product was not detected after the Z-schematic CO2 reduction. In the Z-schematic water splitting at around neutral pH (entry 1), stoichiometric amounts of H2 and O2 were obtained. In other words, e–/h+ being beyond unity at around neutral pH was observed in Z-schematic CO2 reduction but not Z-schematic water splitting. These results implied that photocorrosion of a metal sulfide photocatalyst might be accelerated in Z-schematic CO2 reduction in an aqueous solution containing 100 mmol L–1 NaHCO3.

Figure shows time courses of Z-schematic CO2 reduction using bare (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) in the absence and the presence of 10 or 100 mmol L–1 NaHCO3, corresponding to Table , entries 2, 5, and 7. H2, O2, and CO continuously evolved for 32 h under visible light irradiation in all cases. The CO2 reduction activity was quite low at pH 4 in the absence of NaHCO3 (Figure (a)). In contrast, addition of NaHCO3 much enhanced the CO2 reduction (Figure (b), (c)). The unfavorable Z-scheme photocatalyst ability at pH 4 would be due to damage of the metal sulfide material because the pH might be too low for the metal sulfide employed in the present study. When NaHCO3 with a high concentration of 100 mmol L–1 was used, obvious deactivation was observed even under neutral pH, though CO formation was significantly enhanced (Figure (c)). The deactivation was suppressed at the low concentration of 10 mmol L–1 NaHCO3 (Figure (b)). The Z-scheme photocatalyst in 10 mmol L–1 NaHCO3 maintained 80% of the highest activity at 6–22 h. The turnover number of the photocatalytic reaction was calculated to see if the CO2 reduction proceeded photocatalytically. The turnover number of a molar quantity of reacted electrons used for the CO and H2 formations to that of the employed (CuGa)0.5ZnS2 was calculated to be 1.9 at 32 h, as shown in Figure (b). No peeled RGO was observed in the reactant solution after the Z-schematic CO2 reduction. In addition, the products were not obtained under dark conditions. These results indicated the Z-schematic CO2 reduction proceeded photocatalytically. Thus, a low concentration of NaHCO3 (10 mmol L–1) was suitable to enhance and stabilize the Z-schematic CO2 reduction using water as an electron donor over bare (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) under visible light irradiation.

Figure 2

Z-schematic CO2 reduction under visible light irradiation using (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) photocatalysts (a) without any additives, (b) with 10 mmol L–1 NaHCO3, and (c) with 100 mmol L–1 NaHCO3. Photocatalyst: 0.1 g each, reactant solution: 120 mL, flow gas: CO2 (1 atm), light source: 300 W Xe lamp (λ > 420 nm), light irradiation area: 33 cm2, cell: top-irradiation cell with a Pyrex window.

The effects of addition of acidic, neutral, and other basic salts in a reactant solution on the Z-schematic CO2 reduction were also investigated. The addition of basic salts of Li2CO3, NaOH, Na2CO3, KHCO3, CsHCO3, and NH4HCO3 enhanced Z-schematic CO2 reduction using water as an electron donor (Table ; entries 8–14). HCOOH was not obtained with any basic additives, indicating CO was produced as a CO2 reduction product being independent of the kind of additives in this experimental condition. Entry 9 (with 10 mmol L–1 NaOH) is in the same line as the entry 5 (with 10 mmol L–1 NaHCO3) because the addition of alkaline compounds such as NaOH in the CO2 flowing system gives HCO3–, being similar to the condition of entry 5. Basic salts gave positive effects of “pH adjustment” and “efficient supply of hydrated CO2” for enhancement of the Z-schematic CO2 reduction. In contrast to the basic salts, the acidic salt H3BO3 and the neutral salt NaCl gave negative effects for not only the Z-schematic CO2 reduction but also the Z-schematic water splitting (entries 15, 16). H3BO3 and NaCl additions were not suitable for Z-schematic water splitting and CO2 reduction because the pH of the CO2-saturated aqueous solution was around 4. Moreover, adsorption of ions from H3BO3 and NaCl salts on a photocatalyst surface might change the surface and, therefore, suppress the contact between CO2-reducing and O2-evolving photocatalyst particles, resulting in a low-efficient interparticle electron transfer. We also confirmed K2SO4 salt addition gave a negative effect for Z-schematic water splitting even in the absence of CO2.

To compare the ability for photocatalytic CO2 reduction between Z-scheme systems developed in the present and previous studies, we also examined the Z-schematic CO2 reduction using our previous Z-scheme system consisting of bare CuGaS2 by an SSR and RGO-(CoO/BiVO4)[12] in the presence of 10 mmol L–1 NaHCO3, which was the optimized concentration in the present study as summarized in Figure . Z-schematic CO2 reduction continuously proceeded even using CuGaS2 (Figure S2(a)). The selectivity for CO formation in the Z-schematic CO2 reduction using CuGaS2 with 10 mmol L–1 NaHCO3 reached 5.3%, which was higher than the previously reported selectivity of about 1% without any additives,[12] indicating NaHCO3 addition was effective for Z-schematic CO2 reduction using not only (CuGa)0.5ZnS2 but also CuGaS2. Moreover, Z-schematic CO2 reduction using (CuGa)0.5ZnS2 prepared by an SSR not by a flux steadily proceeded in the presence of 10 mmol L–1 NaHCO3 under visible light irradiation (Figure S2(b)). The present Z-scheme system using bare (CuGa)0.5ZnS2 prepared by a flux as a reducing photocatalyst showed the highest CO formation rate and CO selectivity compared with CuGaS2 and (CuGa)0.5ZnS2 prepared by an SSR, as shown in Figure . Thus, we successfully improved Z-schematic CO2 reduction using water as an electron donor under visible light irradiation by applying (CuGa)0.5ZnS2 prepared by a flux method and adding a basic salt into the reactant solution. It is notable that the Z-scheme system can produce CO with a selectivity of 10–20% accompanied by almost stoichiometric O2 evolution under visible light irradiation in a simple suspension system in an aqueous medium even without special surface modification of a metal sulfide photocatalyst.

Figure 3

Z-schematic CO2 reduction under visible light irradiation using CuGaS2 or (CuGa)0.5ZnS2 as a reducing photocatalyst combined with an RGO-(CoO/BiVO4) photocatalyst in the presence of 10 mmol L–1 NaHCO3. Photocatalyst: 0.05–0.1 g each, reactant solution: 10 mmol L–1 NaHCO3 (120 mL), flow gas: CO2 (1 atm), light source: 300 W Xe lamp (λ > 420 nm), light irradiation area: 33 cm2, cell: top-irradiation cell with a Pyrex window. CuGaS2 was prepared by an SSR at 873 K for 10 h, and (CuGa)0.5ZnS2 was prepared by an SSR at 1073 K for 10 h or by a flux at 723 K for 15 h.

We also carried out Z-schematic solar CO2 reduction using the improved Z-scheme system consisting of (CuGa)0.5ZnS2 prepared by a flux method and RGO-(CoO/BiVO4). Z-schematic CO2 reduction proceeded under simulated sunlight with almost unity e–/h+ for 30 h, as shown in Figure . The solar to chemical energy conversion efficiency was 0.012%. Therefore, we successfully demonstrated artificial photosynthetic solar CO2 reduction in a simple powdered suspension system.

Figure 4

Z-schematic solar CO2 reduction using (CuGa)0.5ZnS2 prepared by a flux method and RGO-(CoO/BiVO4) photocatalysts in the presence of 10 mmol L–1 NaHCO3. Photocatalyst: 0.1 g each, reactant solution: 10 mmol L–1 NaHCO3 (120 mL), flow gas: CO2 (1 atm), light source: simulated sunlight (AM-1.5 G), light irradiation area: 16 cm2, cell: top-irradiation cell with a Pyrex window.

Sacrificial CO2 Reduction under Visible Light Irradiation over (CuGa)0.5ZnS2 Photocatalyst in the Presence of NaHCO3 in a Suspension System

CO2 reduction and H2 evolution over (CuGa)0.5ZnS2 prepared by a flux method from an aqueous solution containing K2SO3 as a sacrificial electron donor were investigated at various pHs as a half-reaction of Z-schematic CO2 reduction as shown in Table . When sacrificial CO2 reduction was carried out using an aqueous solution with K2SO3 in the absence of NaHCO3 (entry 1), (CuGa)0.5ZnS2 produced CO in addition to H2 under visible light irradiation. The formation of HCOOH was negligible. Twenty-three percent of the high CO selectivity was obtained, even though any cocatalysts including a metal complex catalyst[13,25] working as a CO2 reduction site were not loaded on the (CuGa)0.5ZnS2. As the concentration of NaHCO3 and pH became high, the CO evolution rate and CO selectivity became high (entries 1–4). The enhancement of CO selectivity was due to an efficient supply of hydrated CO2 molecules of a reactant species in the presence of NaHCO3 that worked as a CO2 buffer.[15,16] On the other hand, the enhancement of H2 and CO evolution, namely, an increase in the number of reacted electrons, was caused by an increase in the concentration of SO32– ions working as a sacrificial electron donor at high pH. The ratio of SO32– to HSO3– in an aqueous solution depends on the follow equation:The pKa of the equation is 6.91.[26] Therefore, we also evaluated the pH dependence of sacrificial H2 evolution over bare (CuGa)0.5ZnS2 under 1 atm of Ar flow (entries 5–8). The H2 evolution rates, namely, the number of reacted electrons, at pH 7.6 and 9.7 (entries 7, 8) were larger than those at pH 6.6–6.9 (entries 5, 6). Thus, when SO32– mainly existed at 6.9 < pH, a high H2 evolution rate was obtained. On the other hand, the H2 evolution rate decreased due to the decrease in SO32– of a sacrificial electron donor at pH ≤ 6.9. A similar pH dependence of sacrificial H2 evolution activity has been reported over a bare ZnS photocatalyst using SO32– as an electron donor.[26] In the sacrificial CO2 reduction over (CuGa)0.5ZnS2 under visible light irradiation, 1.0 mol L–1 NaHCO3 addition improved not only CO selectivity by efficient supply of hydrated CO2 molecules of a reactant species but also the number of reacted electrons because of the increase in SO32– as a sacrificial electron donor. Thus, NaHCO3 addition was also effective for enhancement of sacrificial CO2 reduction over bare (CuGa)0.5ZnS2 as well as Z-schematic CO2 reduction using (CuGa)0.5ZnS2 as a CO2-reducing photocatalyst. It is stressed that (CuGa)0.5ZnS2 has potential to reduce CO2 to CO with a selectivity of about 40% by only tuning a reactant solution even without any cocatalysts and special surface modification.

Table 2

Effect of pH on Sacrificial CO2 Reduction and H2 Evolution under Visible Light Irradiation over a Bare (CuGa)0.5ZnS2 Photocatalyst from an Aqueous Solution Containing K2SO3 as a Sacrificial Reagenta

				activity [μmol h–1]
entry	flow gas	additive [mol L–1]	pH	H2	CO	CO selectivity [%]
1	CO2	none	6.6	6.5	1.9	23
2	CO2	NaHCO3 (0.1)	6.9	6.3	2.5	28
3	CO2	NaHCO3 (0.5)	7.2	9.2	5.0	35
4	CO2	NaHCO3 (1.0)	7.4	15.9	11.7	42
5	Ar	H2SO4	6.6	16.5
6	Ar	H2SO4	6.9	16.2
7	Ar	H2SO4	7.6	32.4
8	Ar	none	9.7	35.8

Photocatalyst: 0.2 g, reactant solution: 0.1 mol L–1 K2SO3 aqueous solution (120 mL), flow gas: CO2 or Ar (1 atm), light source: 300 W Xe lamp (λ > 420 nm), light irradiation area: 33 cm2, cell: top-irradiation cell with a Pyrex window. CO selectivity [%] = (rate of CO formation)/(sum of rates of H2 and CO formation).

Figure shows the time courses of sacrificial CO2 reduction over (CuGa)0.5ZnS2 in the presence of 0.1 mol L–1 K2SO3 as a sacrificial electron donor with and without 1.0 mol L–1 NaHCO3 under visible light irradiation corresponding to Table , entries 1 and 4. CO was produced accompanied by H2 evolution. The turnover number of reacted electrons used for the CO and H2 formation to S atoms on the surface of (CuGa)0.5ZnS2 powders was estimated to be 15.5 at 8 h, by calculating the sum of S atoms on the surface calculated from the specific surface area[14] and the (111) lattice plane in Figure (b). Moreover, H2 and CO were not obtained under dark conditions. These results indicated the sacrificial CO2 reduction over (CuGa)0.5ZnS2 proceeded photocatalytically. However, the CO2 reduction activity decreased at the later period. Similar behavior was also previously reported for a bare CuGaS2 photocatalyst.[11] The deactivation was more significant with 1.0 mol L–1 NaHCO3 than without the additive. It is difficult to examine the sacrificial reaction with a concentration higher than 1 mol L–1 NaHCO3 because of the solubility. The deactivation in the sacrificial CO2 reduction (Figure ) was more significant than that in the Z-schematic CO2 reduction system in the absence of a sacrificial electron donor (Figure (b)). In sacrificial CO2 reduction, SO32– should be photooxidized to SO42–, which is an inert ion. If produced SO42– ions irreversibly adsorbed on a metal sulfide photocatalyst, it would suppress adsorption of SO32– as a reactant. The suppression of SO32– adsorption might lead to self-photooxidation of a metal sulfide photocatalyst. In addition, a localized pH gradient might damage the metal sulfide powder. In contrast to the sacrificial reaction, photogenerated holes on a metal sulfide photocatalyst are continuously consumed by photogenerated electrons on BiVO4 via interparticle electron transfer through RGO without negative effects of the ions, resulting in more steady CO2 reduction in a Z-scheme system.

Figure 5

Sacrificial CO2 reduction under visible light irradiation over a (CuGa)0.5ZnS2 photocatalyst from an aqueous solution containing 0.1 mol L–1 K2SO3 as a sacrificial reagent (a) without NaHCO3 and (b) with 1.0 mol L–1 NaHCO3. Photocatalyst: 0.2 g, reactant solution: 120 mL, flow gas: CO2 (1 atm), light source: 300 W Xe lamp (λ > 420 nm), light irradiation area: 33 cm2, cell: top-irradiation cell with a Pyrex window.

Characterization of Photocatalyst Powders after Z-Schematic and Sacrificial CO2 Reduction

Z-scheme photocatalysts after the CO2 reduction in the presence of 10 and 100 mmol L–1 NaHCO3 (Figure (b), (c)) and (CuGa)0.5ZnS2 after the sacrificial CO2 reduction with and without 1.0 mol L–1 NaHCO3 (Figure ) were characterized by XRD, DRS, AES, and ESR to reveal the cause of deactivation. There was no significant difference in XRD patterns of (CuGa)0.5ZnS2 before and after the sacrificial CO2 reduction (Figure S3(a), (f), (g)). Moreover, XRD patterns of a mixture of (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) of the Z-scheme photocatalysts did not change before and after Z-schematic CO2 reduction (Figure S3(c)–(e)). The above indicated that the crystal structure of (CuGa)0.5ZnS2 and BiVO4 did not change and no crystalline impurity phases existed even after the sacrificial CO2 reduction and the Z-schematic CO2 reduction.

We also measured diffuse reflectance spectroscopy (DRS) of samples before and after CO2 reduction as shown in Figure . First, we see (CuGa)0.5ZnS2 before and after the sacrificial CO2 reduction. (CuGa)0.5ZnS2 before CO2 reduction had an obvious band edge without a risen baseline (Figure (a)). In contrast, (CuGa)0.5ZnS2 after the sacrificial CO2 reduction obviously possessed a risen baseline (Figure (b), (c)), especially using NaHCO3. XPS analysis indicated that the surface composition of Ga, Zn, and S to Cu decreased after sacrificial CO2 reduction (Table S1), suggesting the Ga, Zn, and S might dissolve into the reactant solution during the sacrificial CO2 reduction. These DRS and XPS measurements implied that some defect levels and/or Cu-containing impurities existed on the metal sulfide surface after the sacrificial CO2 reduction, resulting in absorbing light up to the near-IR region. Moreover, the alternation after the sacrificial CO2 reduction using NaHCO3 was more significant than that without NaHCO3. The alternation of the surface of (CuGa)0.5ZnS2 might cause deactivation of sacrificial CO2 reduction, as shown in Figure . Next, DRS of a mixture of (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) of a Z-scheme photocatalyst was investigated. The Z-scheme photocatalyst before CO2 reduction had a risen baseline (Figure (d)) because of RGO (Figure (g)).[24] However, the different risen baseline from the RGO was also observed after the Z-schematic CO2 reduction (Figure (e), (f)), especially using 100 mmol L–1 NaHCO3. The shape of the risen baseline observed after Z-schematic CO2 reduction (Figure (e), (f)) was similar to that after sacrificial CO2 reduction (Figure (b), (c)). Moreover, the surface composition of Ga, Zn, and S to Cu also decreased after the Z-schematic CO2 reduction (Table S2). (CuGa)0.5ZnS2 also changed during Z-schematic CO2 reduction, especially using 100 mmol L–1 NaHCO3.

Figure 6

Diffuse reflectance spectra of (a) (CuGa)0.5ZnS2, (CuGa)0.5ZnS2 after sacrificial CO2 reduction (b) without NaHCO3 and (c) with 1.0 mol L–1 NaHCO3, a Z-scheme photocatalyst of a mixture of (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) (d) before CO2 reduction and after CO2 reduction (e) using 10 mmol L–1 NaHCO3 and (f) using 100 mmol L–1 NaHCO3, and (g) RGO-(CoO/BiVO4).

The chemical state of Cu should also affect the durability of photocatalytic CO2 reduction. An Auger peak of Cu L3M4,5M4,5 of (CuGa)0.5ZnS2 before and after sacrificial CO2 reduction was measured using an XPS instrument attached with a Mg target to reveal the chemical state of Cu on the surface of (CuGa)0.5ZnS2 as shown in Figure . ESR was also measured at 77 K to clarify the chemical state of Cu in the bulk in addition to the surface of (CuGa)0.5ZnS2, as shown in Figure , because Cu2+ is an ESR-active species but Cu+ is not. First, we determined whether the chemical state of Cu in (CuGa)0.5ZnS2 changes before and after the sacrificial CO2 reduction. The (CuGa)0.5ZnS2 before sacrificial CO2 reduction gave a sharp peak around 917 eV of kinetic energy (Figure (a)), agreeing with reported Cu+ at 916.7–917.3 eV for Cu2S.[27,28] This is reasonable because Cu+ in (CuGa)0.5ZnS2 should be the major species. On the other hand, the slight peak shift to a high kinetic energy was observed, and the shoulder peak around 917.8–918.0 eV corresponding to Cu2+ of CuS and CuO[28,29] appeared in the (CuGa)0.5ZnS2 after sacrificial CO2 reduction (Figure (b), (c)), in which (CuGa)0.5ZnS2 after sacrificial CO2 reduction using NaHCO3 showed a larger peak for the oxidized Cu than that without NaHCO3. An obvious signal of Cu0 around 918.6–918.8 eV was not observed. No ESR signals were observed for (CuGa)0.5ZnS2 before the CO2 reduction (Figure (a)), whereas an obvious signal appeared after the sacrificial CO2 reduction (Figure (b), (c)). The g value of 2.1 was close to the reported values (2.0–2.4) of Cu2+.[30,31] It was also confirmed using Cu(1 wt %)-loaded ZnS treated with H2 reduction that the ESR signal was not due to Cu0. These AES and ESR measurements revealed a part of Cu+ of (CuGa)0.5ZnS2 was oxidized to Cu2+ during the CO2 reduction even in the presence of a sacrificial electron donor. Thus, the deactivation observed in sacrificial CO2 reduction in Figure was also due to Cu2+ formation, which could work as a recombination site or kill an active site. The huge self-oxidation to form Cu2+ might be due to adsorption of some ions as discussed above. Next, AES and ESR for a Z-scheme photocatalyst are compared before and after CO2 reduction, as shown in Figure (b), (c). An ESR signal of RGO (Figure S4) was observed for an RGO-(CoO/BiVO4) composite. However, obvious ESR signals due to Cu2+ were not observed for the Z-scheme photocatalysts before and after Z-schematic CO2 reduction (Figure (d)–(f)), being different from that after sacrificial CO2 reduction (Figure (b), (c)). We also confirmed that the mixture of (CuGa)0.5ZnS2 after the sacrificial CO2 reduction and RGO-(CoO/BiVO4) gave the signal of Cu2+ as a blank test. In contrast to the ESR, peak shifts and obvious shoulder peaks of Cu2+ were observed in the AES for the Z-scheme photocatalyst after CO2 reduction (Figure (e), (f)), being similar to that after sacrificial CO2 reduction, although the signal of Cu2+ was not clear in that before Z-schematic CO2 reduction (Figure (d)). The shoulder peak assigned to CuS or CuO in the Z-scheme photocatalyst after CO2 reduction using 100 mmol L–1 NaHCO3 was larger than that using 10 mmol L–1 NaHCO3, while the peak of the Z-scheme photocatalyst after CO2 reduction using 10 mmol L–1 NaHCO3 also shifted to high kinetic energy. The difference in the intensity for the shoulder peak might suggest that the Z-scheme photocatalyst after CO2 reduction using 100 mmol L–1 NaHCO3 contained more Cu2+ than that using 10 mmol L–1 NaHCO3. Although a part of Cu+ on the surface of (CuGa)0.5ZnS2 was slightly photooxidized to Cu2+ during the Z-schematic CO2 reduction, Cu+ in the bulk was not, indicating Cu2+ was suppressed in Z-schematic CO2 reduction compared with that in sacrificial CO2 reduction. Therefore, Z-schematic CO2 reduction proceeded more stably than sacrificial CO2 reduction, as shown in Figures and 5.

Figure 7

Auger spectra for Cu L3M4,5M4,5 of (a) (CuGa)0.5ZnS2, (CuGa)0.5ZnS2 after sacrificial CO2 reduction (b) without NaHCO3 and (c) with 1.0 mol L–1 NaHCO3 , and a Z-scheme photocatalyst of a mixture of (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) (d) before CO2 reduction and after CO2 reduction (e) using 10 mmol L–1 NaHCO3 and (f) using 100 mmol L–1 NaHCO3. The kinetic energy was corrected using a C 1s peak for (a)–(c) and an In 3d peak for (d)–(f).

Figure 8

ESR at 77 K of (a) (CuGa)0.5ZnS2, (CuGa)0.5ZnS2 after sacrificial CO2 reduction (b) without NaHCO3 and (c) with 1.0 mol L–1 NaHCO3, and a Z-scheme photocatalyst of a mixture of (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) (d) before CO2 reduction and after CO2 reduction (e) using 10 mmol L–1 NaHCO3 and (f) using 100 mmol L–1 NaHCO3. The intensity of (b) and (c) is 1/5.

Photoelectrochemical CO2 Reduction Using a (CuGa)0.5ZnS2 Photocathode under Visible Light Irradiation

Photoelectrochemical measurement is beneficial to reduce CO2. Moreover, the photoelectrochemical property of the employed photocatalyst material is also important to consider in a Z-scheme photocatalyst system using RGO, because the electron flow mechanism is similar between the Z-scheme system with RGO and the photoelectrode system with an electric wire.[12,32] In other words, the condition to give a high photoelectrochemical performance should be preferable for an efficient Z-scheme system. Photoelectrochemical CO2 reduction was examined using a (CuGa)0.5ZnS2 photocathode under visible light irradiation. A (CuGa)0.5ZnS2 powder/FTO electrode gave a cathodic photocurrent by irradiating visible light under both Ar and CO2 (Figure S5). The photocurrent and the onset potential under CO2 were similar to those under Ar. We also evaluated the photoelectrochemical property under acidic and neutral pH in the presence of CO2 (Figure S6). The photocurrent density around pH 4 was much smaller than that around neutral pH, indicating the presence of a basic salt was also effective for the photoelectrochemical property for bare (CuGa)0.5ZnS2. In addition, efficient performance of the present Z-scheme system by tuning the pH higher than pH 4 is reasonable judging from the photoelectrochemical measurement. Since the similarity (Figure S5) does not mean CO2 reduction, bulk electrolysis was conducted to determine the Faradaic efficiency. Photoelectrochemical CO2 reduction over the (CuGa)0.5ZnS2 photocathode using an aqueous KHCO3 solution under visible light irradiation at −0.5 V vs Ag/AgCl at pH 6.9 (0.1 V vs RHE) was carried out as shown in Figure . The cathodic current was not observed under dark conditions, whereas (CuGa)0.5ZnS2 gave a steady cathodic current by irradiating visible light after a 5 h induction period. The reduction products of H2 and CO were obtained with 79% and 21% Faradaic efficiencies, respectively, giving almost 100% total Faradaic efficiency. The Faradaic efficiencies of H2 and CO formation were almost constant after the induction period. These results indicated that (CuGa)0.5ZnS2 could reduce CO2 to CO without obvious deactivation in a photoelectrochemical cell under visible light irradiation, being different from Z-schematic CO2 reduction in which slight deactivation was observed. The photoelectrochemical CO2 reduction with high stability was due to efficient hole transportation from the metal sulfide to the FTO substrate by applying a suitable potential. Thus, we successfully constructed a photoelectrochemical CO2 reduction system using a metal sulfide photocathode based on powdered materials working under visible light irradiation. The photoelectrochemical CO2 reduction system has an advantage from the viewpoint of collection of evolved syngas without separating it from evolved O2.

Figure 9

Photoelectrochemical CO2 reduction under visible light irradiation over a (CuGa)0.5ZnS2 photocathode in an aqueous solution containing 0.1 mol L–1 KHCO3 at a constant applied bias of 0.1 V vs RHE. Photoelectrode: drop cast, electrolyte: 0.1 mol L–1 KHCO3(aq) with dissolved CO2 at 1 atm, light source: 300 W Xe lamp (λ > 420 nm), applied bias: 0.1 V vs RHE (−0.5 V vs Ag/AgCl (pH 6.9)), reference electrode: Ag/AgCl, counter electrode: Pt.

Conclusions

The (CuGa)0.5ZnS2 photocatalyst reduced CO2 to CO using SO32– as a sacrificial electron donor under visible light irradiation. Adding NaHCO3 into a reactant solution of the sacrificial CO2 reduction enhanced the CO evolution and gave 42% of the selectivity for CO formation even without any cocatalysts. A Z-scheme photocatalyst consisting of the bare (CuGa)0.5ZnS2 and RGO-(CoO/BiVO4) reduced CO2 to CO using water as an electron donor without any salt additives under visible light irradiation. Moreover, adding a basic salt into the reactant solution improved CO evolution, reaching 10–20% of the CO selectivity accompanied by almost stoichiometric O2 evolution. The enhancement of Z-schematic CO2 reduction was due to the suitable pH condition and efficient supply of hydrated CO2 molecules of a reactant by adding a basic salt. The basic additive with suitable concentration also stabilized the Z-scheme photocatalyst using a photocorrosive metal sulfide material in CO2 reduction. Thus, reactant solution tuning contributed to both improvement and stabilization of the Z-schematic CO2 reduction. In addition to the reactant solution control, we successfully enhanced the Z-schematic CO2 reduction by employing an improved (CuGa)0.5ZnS2 photocatalyst. The present Z-scheme system was also active for CO2 reduction to CO using water as an electron donor under simulated sunlight. The Z-schematic CO2 reduction proceeded more stably than the sacrificial CO2 reduction, because self-photooxidation of Cu+ on (CuGa)0.5ZnS2 in the Z-schematic CO2 reduction was suppressed compared with that in the sacrificial CO2 reduction judging from the AES and ESR measurements. Photoelectrochemical CO2 reduction was also demonstrated using our original powdered (CuGa)0.5ZnS2 photocatalyst. Usually, PEC systems are composed of high-quality thin films prepared by a complex process and require surface modification with some thin-layer compounds and a cocatalyst. In contrast, the present PEC using (CuGa)0.5ZnS2 gave a reasonable efficiency even when employing simple powdered materials and without such a surface modification. The Faradaic efficiency for CO formation reached 21% at 0.1 V vs RHE with high stability because of efficient hole transportation to the FTO substrate. Thus, a PEC cell with a (CuGa)0.5ZnS2 photocathode was beneficial to demonstrate efficient and stable CO2 reduction. Moreover, it indicates that the (CuGa)0.5ZnS2 photocatalyst itself possesses an excellent electrocatalytic site for CO2 reduction on the surface. Thus, photocatalytic CO2 reduction with high activity, selectivity, and durability using water as an electron donor was achieved under visible light irradiation by employing a metal sulfide photocatalyst in a simple aqueous suspension and photoelectrochemical systems. Our finding about tuning the reactant solution will contribute to the construction of efficient Z-scheme and photoelectrochemical systems employing metal sulfide photocatalysts for CO2 reduction using water as an electron donor, namely, accompanied by O2 evolution, under visible light irradiation. We expect that highly selective CO2 reduction will further be achieved by introducing a suitable active site and surface modification of the metal sulfide photocatalyst for Z-schematic and photoelectrochemical CO2 reduction.