| As energy crisis and environmental pollution have become increasingly prominent, development of clean and sustainable new energy is indispensable. Photocatalytic H2production from water splitting using solar energy has attracted considerable attention in this regard. Traditionally, photocatalysts have high band gap energy (Eg>3.0eV) such that they are only active under ultraviolet light, which occupies only5%of the solar spectrum. To utilize the visible light of the solar spectrum (~44%), development of visible-light-responsive photocatalysts with high photocatalytic activity has been a research focus. In this study, quaternary sulfide photocatalysts with high visible-light absorption and photocatalytic activity were prepared base on the energy-band theory. A photocatalytic reaction system with an electron donor was established. Meanwhile, the photocatalytic conditions and mechanism of H2production from water splitting were investigated.In this study, a series of (CuAg)xIn2xZn2(1-2x)S2photocatalysts were successfully synthesized via the precipitation and calcination method. The band gap and conduction band edge of (CuAg)xIn2xZn2(1-2x)S2were manipulated by adjusting the composition ratio (x) based on the energy-band theory. In principle, the structure and photocatalytic activity are varied by the elemental composition. Based on the photocatalytic experiments,(CuAg)0.15In0.3Zn1.4S2（x=0.15）was found to exhibit the optimal visible-light absorption and photocatalytic activity.To obtain visible-light-responsive photocatalysts with high photocatalytic activity, the preparation conditions of (CuAg)0.15In0.3Zn1.4S2were optimized. The optimal process conditions were determined as: precipitation time15h, H2S flow rate50mL·min1, ultrasonic condition125W for30min, calcination temperature600℃, and calcination time5h. The photocatalysts were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N2physical adsorption, UV-Vis absorption spectroscopy, inductively coupled plasma mass spectrometry, et. al. The results show that the photocatalysts had high crystallinity, small particle size (3050nm), and strong visible-light absorption capacity (up to645nm). To sufficiently utilize visible light for high photocatalytic hydrogen production, a reactor for producing H2was designed and a photocatalytic water splitting process was set up. The radial dimensions of the reactor were reasonably settled for the gas-closed stainless steel reactor, which can ensure the parallel visible light through the filter and collimating lens to pass from the top to the bottom of the reaction suspension. During photocatalysis, I, S2/SO32, S2, SO32, Fe2+, and Ce3+were selected as electron donors. I was eventually selected as the electron donor in this study because of its greatest potential to increase the photocatalytic activity. In the reaction system containing the (CuAg)0.15In0.3Zn1.4S2photocatalyst, Ru cocatalyst and KI electron donor, the photocatalytic reaction conditions were optimized, which included:300W Xe lamp (400690nm); Photocatalyst concentration1.0g·L1; Ru cocatalyst0.7wt%; Reaction solution300mL; KI concentration0.2mol·L1; Reaction temperature25℃,and Reaction time24h. The (CuAg)0.15In0.3Zn1.4S2photocatalyst had the highest photocatalytic activity (the H2production rate of1750μmol·g1·h1and the quantum yield of12.8%at420±5nm). At present, the quantum yields of most of visible-light-responsive photocatalysts were less than20%and they only absorbed part of visible light. Therefore, the (CuAg)0.15In0.3Zn1.4S2photocatalyst is an excellent photocatalytic material with high photocatalytic activity and visible-light absorption.Except for the high photocatalytic activity, the stability and lifetime of the (CuAg)0.15In0.3Zn1.4S2photocatalyst are the important indicators to evaluate the photocatalyst. Without photocatalyst regeneration, the photocatalyst lost19%of its activity in photocatalytic H2production after seven reaction cycles (168h). In addition, the photocatalytic activity decreased obviously in the first three cycles and the activity decreased slowly in the back four cycles. After the photocatalyst was regenerated through calcining procedures, the photocatalytic activity recovered to the original level. The results indicate that the photocatalyst had a good photostability under visible light and was applicable in practical H2production.The mechanism and path of the photocatalytic reaction with KI as the electron donor was analyzed at different pH values. At low pHs, the oxidation product of I was mainly I3. When the solution pH increased, the end product of I oxidation gradually shifted from I3to IO3, highlighting the strong pH dependence of the overall H2evolution reaction pathways. According to the reaction mechanism, H2production terminated largely owing to the consumption of KI, pH changes and the buildup of the H2pressure in the suspension. For a dynamic reactor, the H+and KI can be added into the reactor, and the pH of the reaction solution is detected when the reaction is occurring. In addition, the H2in the upper of the reactor can be harvested anytime.In this paper, the (CuAg)xIn2xZn2(1-2x)S2photocatalysts were prepared through tuning the elemental composition to provide a new class of visible-light-responsive photocatalysts. In the reaction system containing the (CuAg)0.15In0.3Zn1.4S2photocatalyst, Ru cocatalyst and KI electron donor, the (CuAg)0.15In0.3Zn1.4S2photocatalyst can effectively absorb visible light to produce H2through optimizing the synthesis and reaction conditions. The overall goal of the study is to facilitate the search for highly efficient and stable water-splitting photocatalysts and to enhance our understanding of the photocatalytic H2production process under visible light irradiation. |
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