Microwave-Assisted Solvothermal Synthesis of Chalcogenide Composite Photocatalyst and Its Photocatalytic CO 2 Reduction Activity under Simulated Solar Light

: A novel heterostructure consisting of Ru and Cu co-doped ZnS nanopowders (RCZS) into a MoS 2 -graphene hybrid (MSG) is successfully prepared by the microwave-assisted solvothermal approach. RCZS nanopowders are fabricated on the surface of MSG, which produces a nanoscale interfacial between RCZS and MSG. As the photo-excited electrons of RCZS can easily migrate to MoS 2 through graphene by hindering the electron and hole (e – and h + ) recombination, the photocatalytic activity could be improved by e ﬀ ective charge transfer. As RCZS are anchored onto the MSG, the photoluminescence intensity of the chalcogenide composite photocatalyst obviously decreases. In addition, a quaternary ruthenium and copper-based chalcogenide RCZS / MSG is able to improve the harvest and utilization of light. With the increase in the concentrations of Ru until 4 mol%, the band gap signiﬁcantly decreases from 3.52 to 2.73 eV. At the same time, moderate modiﬁcation by ruthenium can decrease the PL intensity compared to the pristine CZS / MSG sample, which indicates the enhancement of e – and h + separation by Ru addition. The photocatalytic activity of as-synthesized chalcogenide composite photocatalysts is evaluated by the photocatalytic carbon dioxide reduction. Optimized operation conditions for carbon dioxide reduction have been performed, including the concentration of NaOH solution, the amount of RCZS / MSG photocatalyst, and the content of co-doped ruthenium. The doping of ruthenium would e ﬃ ciently improve the performance of the photocatalytic activity for carbon dioxide reduction. The optimal conditions, such as the concentration of 2 M NaOH and the 0.5RCZS / MSG dosage of 0.05 g L –1 , provide the maximum methane gas yield of 58.6 µ mol h − 1 g –1 .


Introduction
As solar energy is a kind of environmentally friendly energy source, it is a great idea to convert solar energy into solar fuels via the photocatalytic reaction, i.e., the photocatalytic carbon dioxide reduction [1,2]. It is also a sustainable carbon circle approach to solve both the energy crisis and greenhouse effect issues via photocatalytic hydrogen evolution or photocatalytic CO 2 reduction [3][4][5]. There are two sorts of solar fuels: (1) Hydrogen (H 2 )-and (2) carbon-based fuels, including carbon monoxide (CO), methane (CH 4 ), methanol (CH 3 OH), ethanol (C 2 H 5 OH), formic acid (HCOOH), and formaldehyde (HCHO) [1,2,4]. The production of carbon-based fuels by the photocatalytic CO 2 reduction is a useful strategy to reduce CO 2 in the atmosphere due to the capturing and recycling of CO 2 . However, CO 2 is a very stable molecule with extreme difficulty to chemically activate [2,6]. The single-electron reduction of CO 2 to form the radical CO 2¯i nvolves multiple electron processes, leading to the higher consumption of solar energy by the photocatalysis. Therefore, the best strategy to enhance the light harvesting is to create such a photocatalyst, which has not only a narrow band gap, but also the efficient separation of eand h + pairs in the photocatalytic system. In addition, a quaternary copper-based chalcogenide performed efficient capture and utilization of solar energy [7][8][9]. Meanwhile, heterostructured photocatalysts possess a synergic effect to augment the photocatalytic activity [10,11]. MoS 2 /graphene has been reported to have unique properties, such as large surface area, superior electron mobility, and excellent catalytic ability [7]. According to the references, many scientists have synthesized new heterostructured photocatalysts for exploring pronounced enhancements in photocatalytic performance [7][8][9][10][11][12].
Based on our previous studies, we know that copper-modified zinc sulfide could effectively enhance the photocatalytic activity efficiency [13,14]. Additionally, ruthenium addition could also create the acceptor energy levels to capture efrom the valence band (VB), leading to a light harvesting efficiency from the ultraviolet light region to the visible light region [15]. On top of that, the active ruthenium and CO 2 possess a sensitive interaction, resulting in a higher selectivity to reduce CO 2 [16,17]. Ruthenium anchored onto graphene oxide (Ru-phen-GO) could produce methanol at 82.9 µmol h -1 g -1 [18]. Therefore, in this study, we have synthesized novel Ru and Cu co-doped ZnS nanopowders (RCZS)/MoS 2 /graphene (MSG) heterostructured photocatalysts via the microwave-assisted solvothermal method to explore their photocatalytic performances in carbon dioxide reduction via simulated solar irradiation.

Chalcogenide Composite Photocatalysts
MoS 2 -graphene has been reported to not only provide the reactive sites, but also enhance the eand h + transfer [7,19]. Therefore, in this study, we attempted to add different amounts of the MSG hybrid to modify the ZnS photocatalyst. Figure 1a reveals the bare ZnS nanosphere with an average diameter of 113.7 nm. However, according to Figure 1b-d, the size of the ZnS particles decreased as the amount of MSG hybrid was increased from 0.1 wt.% to 1.0 wt.%. This clearly reveals that uniform structures of nanoparticles are well dispersed on exfoliated graphene nanosheets by the microwave-assisted solvothermal method, as shown in Figure 1(c1,d1). Moreover, MSG could increase the specific surface area of the samples from 72.91 to 104.60 m 2 /g by adding 0.5 wt.% MSG hybrid. Meanwhile, the charge transfer resistance was found to decrease from 6.19 to 5.07 Ω by adding 0.5 wt.% MSG hybrid. The photocurrent density was determined to slightly increase from 8.98 to 9.06 mA/cm 2 by adding 0.5 wt.% MSG hybrid due to the formation of heterostructured interfaces. However, excess interfaces, such as 1 wt.% MSG, would result in more grain boundaries for lowering the electron transportation. On the basis of the above results, we added a 0.5 wt.% MSG hybrid to conduct the next process. According to our previous study, we know that the doping of copper into ZnS would improve its photocatalytic activity more efficiently [20]. In addition, it shows the highest photocatalytic activity by 4 mol% copper addition. Therefore, 4 mol% copper doped into ZS/MSG was adopted in this study. Subsequently, we prepared the heterostructured photocatalysts, RCZS/MSG, via the doping of various amounts of ruthenium. The major peaks at 28.7°, 48.2°, and 57.0° pertain to (111), (220), and (311) planes of the cubic lattice of the ZnS phase (JCPDS card No. 05-0566), as shown in Figure 2. The average crystallite size of the as-synthesized ZnS nanoparticles, D111, is 2.48±0.032 nm. D50 of RCZS/MSG nanopowder is slightly decreased by doping ruthenium. The characteristics of the ruthenium-doped CZS/MSG nanoparticles are revealed in Table 1. The surface area decreases with the increase in ruthenium doping content from 46.23 to 35.90 m 2 /g due to the particle agglomeration in spite of the further slight increase in BET by doping more ruthenium up to 4%. In addition, the photocurrent density is 7.43 mA/cm 2 by doping 0.5 mol% ruthenium, which also has the highest percentage increase in photocurrent density (563.4%) than the other samples. A higher photocurrent density indicates that the photons absorbed by the photocatalyst can generate the photoexcited carriers efficiently. Therefore, the 0.5RCZS/MSG sample has outstanding photocatalytic activity. The band gap of pristine ZnS is 3.74 eV (data not shown). The band gap energy of RCZS/MSG nanopowder was calculated to redshift from 3.74 to 2.73 eV (Figure 3a), which can be assigned to ruthenium doping. Figure 3b indicates the conduction band (CB) and valence band (VB) edge positions of RCZS/MSG nanopowder. RCZS/MSG heterostructured photocatalysts possess a more negative potential than the reduction potential of the carbon dioxide level (E = −0.24 eV, CH4/CO2), leading to conversion from CO2 to CH4 [2]. Figure 4 reveals peaks at 360, 451, 469, 483, and 493 nm, and the intensity of the peaks declines by doping ruthenium due to the S vacancies, where surface defects can act as the adsorption sites to transfer eto the adsorbed species [21,22]. The emission band at 468-493 nm is due to the recombination of electrons in the sulfur vacancy level with the holes in the zinc vacancy (VZn) [21]. Therefore, the S vacancy could inhibit the eand h + pair recombination and enhance the blue-indigo-violet emission by the modification of ruthenium, as shown in Figure  4b. In addition, moderate modification by ruthenium can decrease the PL intensity compared to that of the CZS/MSG sample, leading to the effective eand h + pair separation. On the basis of the aforementioned discussion, we know that the new RCZS/MSG heterostructured photocatalysts could suitably improve the optical performance and enhance photocatalytic activity. According to our previous study, we know that the doping of copper into ZnS would improve its photocatalytic activity more efficiently [20]. In addition, it shows the highest photocatalytic activity by 4 mol% copper addition. Therefore, 4 mol% copper doped into ZS/MSG was adopted in this study. Subsequently, we prepared the heterostructured photocatalysts, RCZS/MSG, via the doping of various amounts of ruthenium. The major peaks at 28.7 • , 48.2 • , and 57.0 • pertain to (111), (220), and (311) planes of the cubic lattice of the ZnS phase (JCPDS card No. 05-0566), as shown in Figure 2. The average crystallite size of the as-synthesized ZnS nanoparticles, D 111 , is 2.48±0.032 nm. D 50 of RCZS/MSG nanopowder is slightly decreased by doping ruthenium. The characteristics of the ruthenium-doped CZS/MSG nanoparticles are revealed in Table 1. The surface area decreases with the increase in ruthenium doping content from 46.23 to 35.90 m 2 /g due to the particle agglomeration in spite of the further slight increase in BET by doping more ruthenium up to 4%. In addition, the photocurrent density is 7.43 mA/cm 2 by doping 0.5 mol% ruthenium, which also has the highest percentage increase in photocurrent density (563.4%) than the other samples. A higher photocurrent density indicates that the photons absorbed by the photocatalyst can generate the photoexcited carriers efficiently. Therefore, the 0.5RCZS/MSG sample has outstanding photocatalytic activity. The band gap of pristine ZnS is 3.74 eV (data not shown). The band gap energy of RCZS/MSG nanopowder was calculated to redshift from 3.74 to 2.73 eV (Figure 3a), which can be assigned to ruthenium doping. Figure 3b indicates the conduction band (CB) and valence band (VB) edge positions of RCZS/MSG nanopowder. RCZS/MSG heterostructured photocatalysts possess a more negative potential than the reduction potential of the carbon dioxide level (E = −0.24 eV, CH 4 /CO 2 ), leading to conversion from CO 2 to CH 4 [2]. Figure 4 reveals peaks at 360, 451, 469, 483, and 493 nm, and the intensity of the peaks declines by doping ruthenium due to the S vacancies, where surface defects can act as the adsorption sites to transfer eto the adsorbed species [21,22]. The emission band at 468-493 nm is due to the recombination of electrons in the sulfur vacancy level with the holes in the zinc vacancy (V Zn ) [21]. Therefore, the S vacancy could inhibit the eand h + pair recombination and enhance the blue-indigo-violet emission by the modification of ruthenium, as shown in Figure 4b. In addition, moderate modification by ruthenium can decrease the PL intensity compared to that of the CZS/MSG sample, leading to the effective eand h + pair separation. On the basis of the aforementioned discussion, we know that the new RCZS/MSG heterostructured photocatalysts could suitably improve the optical performance and enhance photocatalytic activity.

Photocatalytic Carbon Dioxide Reduction
The photocatalytic carbon dioxide reduction activities of RCZS/MSG heterostructured photocatalysts were performed via simulated solar light. The experimental parameters, such as the different types of RCZS/MSG heterostructured photocatalyst, the concentration of reductant, and the dosages of 0.5RCZS/MSG, affecting the solar fuel evolution efficiency are evaluated below.

The Different Types of RCZS/MSG Heterostructured Photocatalyst
The formation of a heterojunction structure could improve the eand h + pair separation and photocatalytic reaction efficiency [7,23]. In addition, our previous research indicated that the CZS composite photocatalyst exhibited a lower methane generation rate (2.9 μmole h -1 g -1 ) [20]. Therefore, we prepared the ruthenium and MSG-modified CZS to form the heterostructured photocatalyst, resulting in improved photocatalytic activity. First, the photolysis experiment was conducted and the result gave zero gas evolution, which means the mechanism by the direct photolysis can be negligible. Then, the addition of 0.5 wt.% MSG hybrid to the CZS composite photocatalyst slightly enhanced the methane generation rate from 2.9 to 12.4 μmole h -1 g -1 . It may be that the presence of the MSG hybrid cocatalyst can not only increase the eand h + pair separation efficiency, but also provide more catalytically active sites from MoS2 [7]. Meanwhile, Figure 5a shows the photocatalytic reduction activity of carbon dioxide in NaOH solution over CZS/MSG by various Ru 3+ ion amounts under 12 h illumination of simulated solar light. It can be seen that the co-doped 0.5 mol% ruthenium has the highest activity for solar fuel generation, such as methane (23.4 μmole h -1 g -1 ) and hydrogen (34.8 μmole h -1 g -1 ). The active ruthenium and CO2 possess a sensitive interaction, resulting in the high selectivity to reduce CO2 [16,17]. Furthermore, the high surface area of the photocatalyst favors better photocatalytic activity [24]. Therefore, we used the 0.5RCZS/MSG heterostructured photocatalyst to adjust the factors of photocatalytic activity in the next experimental procedure.

Photocatalytic Carbon Dioxide Reduction
The photocatalytic carbon dioxide reduction activities of RCZS/MSG heterostructured photocatalysts were performed via simulated solar light. The experimental parameters, such as the different types of RCZS/MSG heterostructured photocatalyst, the concentration of reductant, and the dosages of 0.5RCZS/MSG, affecting the solar fuel evolution efficiency are evaluated below.

The Different Types of RCZS/MSG Heterostructured Photocatalyst
The formation of a heterojunction structure could improve the eand h + pair separation and photocatalytic reaction efficiency [7,23]. In addition, our previous research indicated that the CZS composite photocatalyst exhibited a lower methane generation rate (2.9 µmole h -1 g -1 ) [20]. Therefore, we prepared the ruthenium and MSG-modified CZS to form the heterostructured photocatalyst, resulting in improved photocatalytic activity. First, the photolysis experiment was conducted and the result gave zero gas evolution, which means the mechanism by the direct photolysis can be negligible. Then, the addition of 0.5 wt.% MSG hybrid to the CZS composite photocatalyst slightly enhanced the methane generation rate from 2.9 to 12.4 µmole h -1 g -1 . It may be that the presence of the MSG hybrid cocatalyst can not only increase the eand h + pair separation efficiency, but also provide more catalytically active sites from MoS 2 [7]. Meanwhile, Figure 5a shows the photocatalytic reduction activity of carbon dioxide in NaOH solution over CZS/MSG by various Ru 3+ ion amounts under 12 h illumination of simulated solar light. It can be seen that the co-doped 0.5 mol% ruthenium has the highest activity for solar fuel generation, such as methane (23.4 µmole h -1 g -1 ) and hydrogen (34.8 µmole h -1 g -1 ). The active ruthenium and CO 2 possess a sensitive interaction, resulting in the high selectivity to reduce CO 2 [16,17]. Furthermore, the high surface area of the photocatalyst favors better photocatalytic activity [24]. Therefore, we used the 0.5RCZS/MSG heterostructured photocatalyst to adjust the factors of photocatalytic activity in the next experimental procedure.

The Concentration of Reductant
Caustic solution, NaOH, would increase the CO2 solubility into water [25,26]. In addition, the hydroxyl ion (OH -) can serve as a hole scavenger to enhance the eand h + pair separation efficiency [25]. Therefore, we prepared three NaOH concentrations to test the photocatalytic activity, as shown in Figure 5b. From the results, we know that the appropriate NaOH concentration at 2 M could perform with excellent photocatalytic activity. Figure 5c reveals the dosage influence (0.005-0.25 g L -1 ) on photocatalytic CO2 reduction for 0.5RCZS/MSG in aqueous NaOH solution for 12 h irradiation. It has maximum solar fuel yields as high as 56.8 μmole h -1 g -1 methane and 39.2 μmole h -1 g -1 hydrogen at 0.05 g L -1 photocatalyst dosage. While the photocatalyst amount is below 0.05 g L -1 , the available active sites of the system decrease, resulting in the decrease in photocatalytic activity. On the contrary, as the photocatalyst amount is more than 0.05 g L -1 , the excess suspended catalysts could block or scatter the simulated solar light, leading to low photocatalytic efficiency. Moreover, the excess suspended catalysts could enhance the CO2 adsorption by the catalyst, resulting in a decrease in free carbon dioxide in the solution, giving lower methane production than hydrogen production. Therefore, the solar fuel production efficiency reduces above and below 0.05 g L -1 .

The Dosages of 0.5RCZS/MSG
Based on the above results, Figure 6 shows the photocatalytic CO2 reduction mechanism as proposed. The reduction half-reaction involves two pathways for the formation of methane and

The Concentration of Reductant
Caustic solution, NaOH, would increase the CO 2 solubility into water [25,26]. In addition, the hydroxyl ion (OH -) can serve as a hole scavenger to enhance the eand h + pair separation efficiency [25]. Therefore, we prepared three NaOH concentrations to test the photocatalytic activity, as shown in Figure 5b. From the results, we know that the appropriate NaOH concentration at 2 M could perform with excellent photocatalytic activity. Figure 5c reveals the dosage influence (0.005-0.25 g L -1 ) on photocatalytic CO 2 reduction for 0.5RCZS/MSG in aqueous NaOH solution for 12 h irradiation. It has maximum solar fuel yields as high as 56.8 µmole h -1 g -1 methane and 39.2 µmole h -1 g -1 hydrogen at 0.05 g L -1 photocatalyst dosage. While the photocatalyst amount is below 0.05 g L -1 , the available active sites of the system decrease, resulting in the decrease in photocatalytic activity. On the contrary, as the photocatalyst amount is more than 0.05 g L -1 , the excess suspended catalysts could block or scatter the simulated solar light, leading to low photocatalytic efficiency. Moreover, the excess suspended catalysts could enhance the CO 2 adsorption by the catalyst, resulting in a decrease in free carbon dioxide in the solution, giving lower methane production than hydrogen production. Therefore, the solar fuel production efficiency reduces above and below 0.05 g L -1 .

The Dosages of 0.5RCZS/MSG
Based on the above results, Figure 6 shows the photocatalytic CO 2 reduction mechanism as proposed. The reduction half-reaction involves two pathways for the formation of methane and hydrogen via photocatalytic CO 2 reduction and water splitting reaction, as shown in Equations (1) and (2) [27]. Therefore, the photocatalytic system as reported herein allows the reduction of CO 2 at significant rates and the concomitant production of H 2 from water. According to the references, we know that dopants, ruthenium and copper, could increase the eand h + pair separation and migration, resulting in the enhanced photocatalytic efficiency [14,15]. The activated electrons can transfer to copper trap energy levels, allowing the activated electrons to transfer to the surface [13,14]. Additionally, the addition of ruthenium will create the acceptor energy levels to capture efrom the VB, leading to better light harvesting efficiency extending from the ultraviolet light region to the visible light region [15]. In addition, the heterojunction structure can improve the emigration efficiency from ZnS to MoS 2 /graphene (belonging to interfacial charge transfer), reducing the eand h + pairs recombination [23]. Thus, the reaction rate could be improved by the effective charge transfer. The half reactions of the photocatalytic CO 2 reduction are indicated in Equations (1) and (3).
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 11 hydrogen via photocatalytic CO2 reduction and water splitting reaction, as shown in Equations (1) and (2) [27]. Therefore, the photocatalytic system as reported herein allows the reduction of CO2 at significant rates and the concomitant production of H2 from water. According to the references, we know that dopants, ruthenium and copper, could increase the eand h + pair separation and migration, resulting in the enhanced photocatalytic efficiency [14,15]. The activated electrons can transfer to copper trap energy levels, allowing the activated electrons to transfer to the surface [13,14].
Additionally, the addition of ruthenium will create the acceptor energy levels to capture efrom the VB, leading to better light harvesting efficiency extending from the ultraviolet light region to the visible light region [15]. In addition, the heterojunction structure can improve the emigration efficiency from ZnS to MoS2/graphene (belonging to interfacial charge transfer), reducing the eand h + pairs recombination [23]. Thus, the reaction rate could be improved by the effective charge transfer. The half reactions of the photocatalytic CO2 reduction are indicated in Equations (1) and (3).   Figure 6. The photocatalytic CO 2 reduction mechanism over 0.5RCZS/MSG heterostructured photocatalysts.
The flat-band potential [14], charge transfer resistance, and photocurrent density were measured by the potentiostat/galvanostat PGSTAT302N, Metrohm Autolab (Herisau, Switzerland). The measurement of photocurrent density was conducted by adding 0.15 g as-synthesized catalyst into 0.5 mL Nafion solution (Type: DE-520) by ultrasound for 10 min, where 0.1 mL of the mixture solution was withdrawn and attached onto the 20 × 10 × 0.3 mm of indium tin oxide conductive glass (ITO). Then, the ITO conductive glass was dried overnight at room temperature under atmospheric conditions and employed as a working electrode. A Pt wire and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. The photocurrent density measurement was performed in the 0.1 M K 4 Fe(CN) 6 (99.9%, Sigma-Aldrich, St. Louis, MO, USA) electrolyte via an electrochemical workstation. In addition, a Pen-Ray lamp (8 W, λ = 365 nm; Model 11SC-1L; Series 90-0019-01; UVP, Cole-Parmer, Vernon Hills, IL, USA) was used as the light source.

Photocatalytic Hydrogen Evolution and Carbon Dioxide Reduction
The photocatalytic hydrogen evolution and carbon dioxide reduction were conducted by using 100 mL solution containing 1-4 M NaOH and 0-0.25 g L -1 of ZnS composite photocatalysts in a single reactor. Subsequently, CO 2(g) was purged to dissolve in the solution for 10 min. The reactor was irradiated by a 350 W xenon light (KIT-XENON-ADJ350W) for 12 h, and the productions of hydrogen and methane gases were determined by GC-TCD (GC-2014, SHIMADZU (Tokyo, Japan), and China Chromatograph 8700T (Beijing, China)).

Conclusions
It is demonstrated that ruthenium co-doped CZS/MSG heterostructured photocatalysts act for the excellent photocatalysis of carbon dioxide reduction. The appropriate doping concentration of ruthenium could reach the maximum methane yield, where 0.5RCZ/MSG possesses outstanding physical and chemical properties, leading to the enhancement of the photocatalytic efficiency. In addition, ruthenium is a superior methanation cocatalyst. Therefore, the appropriate conditions, such as the concentration of 2 M NaOH and the photocatalyst dose of 0.05 g L -1 , could perform with better photocatalytic activity.