Abstract
Two types of photocatalysts, 1%Pt/Cd1−xZnxS/g-C3N4 (x = 0.2–0.3) and Cd1−xZnxS/1%Pt/g-C3N4 (x = 0.2–0.3), were synthesized by varying the deposition order of platinum, and a solid solution of cadmium and zinc sulfides onto the surface of g-C3N4. The characterization of photocatalysts showed that, for 1%Pt/Cd1−xZnxS/g-C3N4, small platinum particles were deposited onto a solid solution of cadmium and zinc sulfides; in the case of Cd1−xZnxS/1%Pt/g-C3N4, enlarged platinum clusters were located on the surface of graphitic carbon nitride. Based on the structure of the photocatalysts, we assumed that, in the first case, type II heterojunctions and, in the latter case, S-scheme heterojunctions were realized. The activity of the synthesized samples was tested in hydrogen evolution from triethanolamine (TEOA) basic solution under visible light (λ = 450 nm). A remarkable increase in hydrogen evolution rate compared to single-phase platinized 1%Pt/Cd1−xZnxS photocatalysts was observed only in the case of ternary photocatalysts with platinum located on the g-C3N4 surface, Cd1−xZnxS/1%Pt/g-C3N4. Thus, we proved using kinetic experiments and characterization techniques that, for composite photocatalysts based on Cd1−xZnxS and g-C3N4, the formation of the S-scheme mechanism is more favorable than that for type II heterojunction. The highest activity, 2.5 mmol H2 g−1 h−1, with an apparent quantum efficiency equal to 6.0% at a wavelength of 450 nm was achieved by sample 20% Cd0.8Zn0.2S/1% Pt/g-C3N4.
Highlights
The main trend in the reduction in readily available high-quality carbon-containing fossil fuels is the urgent need for the development of available alternative-energy sources, renewable energy
The characterization of photocatalysts showed that, for 1%Pt/Cd1−xZnxS/g-C3N4, small platinum particles were deposited onto a solid solution of cadmium and zinc sulfides; in the case of Cd1−xZnxS/1%Pt/g-C3N4, enlarged platinum clusters were located on the surface of graphitic carbon nitride
Two groups of ternary photocatalysts based on platinized 10–50 wt. % Cd1−xZnxS/g-C3N4 (x = 0.2–0.3) were synthesized. It was shown for the first time that, by changing the order of platinum deposition, one can obtain two types of photocatalysts, Pt/Cd1−xZnxS/g-C3N4 and Cd1−xZnxS/Pt/g-C3N4, with a different structure: in the first case, small platinum particles are deposited onto the surface of the solid solution of cadmium and zinc sulfides; in the latter case, enlarged platinum clusters are located over the surface of graphitic carbon nitride
Summary
The main trend in the reduction in readily available high-quality carbon-containing fossil fuels is the urgent need for the development of available alternative-energy sources, renewable energy. A new impetus to the development of methods for the synthesis of materials for photocatalytic water splitting and hydrogen production was given by the discovery of a previously unknown photocatalyst: polymer graphitic carbon nitride g-C3N4 [3] This material possesses the properties of a semiconductor with a band gap of 2.7 eV (λ = 460 nm), a position of valence band (VB) levels of +1.6 V vs NHE, and conduction bands (CB) levels of −1.1 V vs NHE [4]. G-C3N4 is stable in both acidic and alkaline media and at thermal treatments up to 700 ◦C and may be synthesized from available nitrogen-containing organics such as cyanamides, melamine, and urea These properties make g-C3N4 an ideal semiconductor for photocatalytic hydrogen production, CO2 reduction, and solar-cell applications [5,6,7]. Quantum efficiency in the photocatalytic hydrogen production over pristine g-C3N4 is quite low [8]
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