Application of g-C₃N₄/Sn₃O₄ Heterostructure for Photoelectrochemical H₂ Generation

The hydrogen gas (H₂) has attracted attention due to its environmentally friendly burn with no emission of greenhouse gases [1]. Among the nanostructured materials studied for this purpose, the n-type Sn₃O₄ has been recently applied for H₂ generation. To avoid rapid rate of charge carrier recombination and improve photoelectrochemical performance in Sn₃O₄, the construction of heterojunctions can be generally employed to overcome charge recombination at the interfaces of two semiconductors.<br/>Herein, Sn₃O₄ and g-C₃N₄/Sn₃O₄heterostructures were synthesized via the microwave-assisted hydrothermal method, while g-C₃N₄ was obtained by the heating of melamine. The g-C₃N₄, Sn₃O₄ and g-C₃N₄/Sn₃O₄ films for the working electrodes were prepared by spin coating. The photoelectrochemical tests for H₂ generation was performed using a solar simulator (300 W Xenon lamp) and a sealed photoelectrochemical reactor containing two compartments and equipped with a quartz window (electrolyte: 0.1 mol L⁻¹ Na₂SO₄). The H₂ gas was identified and quantified using gas chromatography with a <i>thermal conductivity detector</i>.<br/>The characterization of g-C₃N₄by X-ray diffraction revealed peaks at 2θ = 13.1° and 27.4° corresponding to (100) and (002) hexagonal structure planes (JCPDS 87-1526), respectively; for the Sn₃O₄and the g-C₃N₄/Sn₃O₄materials, the diffraction patterns matched the triclinic structure (JCPDS 16-0737). The FTIR and Raman spectrum of the nanocomposites confirmed the obtainment of g-C₃N₄/Sn₃O₄ heterostructure. SEM and TEM images showed that g-C₃N₄ presented a two-dimensional structure consisting of micrometer-long wrinkled sheets (2D material), while the Sn₃O₄3D nanomaterial is formed by petals-like morphology with nanometric thickness. In comparison to pure materials, the nanocomposite presented Sn₃O₄ nanopetals distributed over the g-C₃N₄ sheets, suggesting the effective formation of the g-C₃N₄/Sn₃O₄ heterostructure. The Sn₃O₄presented a E_gap= 2.9 eV, while both g-C₃N₄and g-C₃N₄/Sn₃O₄ possessed E_gap= 2.7 eV, indicating that all the photoelectrocatalysts showed promising application in the visible light region.<br/>During photoelectrochemical tests the g-C₃N₄/Sn₃O₄ composite presented the highest photocurrent density of (1.2 mA.cm⁻²). Transient photocurrent curves showed that the g-C₃N₄/Sn₃O₄ nanocomposite showed a quick response in the presence of light and back to zero when the light was turned off, indicating that the nanomaterial is sensitive to the UV-vis irradiation and presents faster charge separation [2]. Constant potential electrolysis experiments were performed (at 0.8 V vs Ag/AgCl, under UV-vis irradiation) for all samples and indicated their high stability. Nyquist plots showed that g-C₃N₄/Sn₃O₄ has lower electron transport resistance than the g-C₃N₄ and Sn₃O₄, indicating that its charge transfer is faster. The cumulative H₂ formation using all electrodes showed a stable production of H₂ for pure g-C₃N₄ and Sn₃O₄ materials within the investigated time period of 3 h, while for the g-C₃N₄/Sn₃O₄ a crescent generation of H₂ was observed until reach the maximum of 0.487 mmol L^-1. The H₂ production rate was nearly 4.4 and 4.8 times higher than the H₂ production using of g-C₃N₄ and Sn₃O₄photoanode, respectively. This observation can be explained by the synergistic effect between Sn₃O₄nanostructures and g-C₃N₄ nanosheets, in which the heterostructure exhibited efficient separation of photoexcited electron–hole pairs, which decreased the recombination rate and therefore promoted higher H₂ generation. Therefore, this study offers a promising strategy to synthesize effective and low cost photocatalyst for energy conversion applications, especially for H₂ generation.