Enhanced photocatalytic H₂-production activity of WO₃/TiO₂ step-scheme heterojunction by graphene modification

Sunlight-driven photocatalytic water-splitting for hydrogen (H₂) evolution is a desirable strategy to utilize solar energy. However, this strategy is restricted by insufficient light harvesting and high photogenerated electron–hole recombination rates of TiO₂-based photocatalysts. Here, a graphene-modified WO₃/TiO₂ step-scheme heterojunction (S-scheme heterojunction) composite photocatalyst was fabricated by a facile one-step hydrothermal method. In the ternary composite, TiO₂ and WO₃ nanoparticles adhered closely to reduced graphene oxide (rGO) and formed a novel S-scheme heterojunction. Moreover, rGO in the composite not only supplied abundant adsorption and catalytically active sites as an ideal support but also promoted electron separation and transfer from the conduction band of TiO₂ by forming a Schottky junction between TiO₂ and rGO. The positive cooperative effect of the S-scheme heterojunction formed between WO₃ and TiO₂ and the Schottky heterojunction formed between TiO₂ and graphene sheets suppressed the recombination of relatively useful electrons and holes. This effect also enhanced the light harvesting and promoted the reduction reaction at the active sites. Thus, the novel ternary WO₃/TiO₂/rGO composite demonstrated a remarkably enhanced photocatalytic H₂ evolution rate of 245.8 μmol g⁻¹h⁻¹, which was approximately 3.5-fold that of pure TiO₂. This work not only presents a low-cost graphene-based S-scheme heterojunction photocatalyst that was obtained via a feasible one-step hydrothermal approach to realize highly efficient H₂ generation without using noble metals, but also provides new insights into the design of novel heterojunction photocatalysts.
太陽光驅動的光催化分解水產氫是利用太陽能解決當前能源危機和環境問題的理想策略。二氧化鈦由於其穩定、環境友好和成本低等優點受到廣泛研究，在光催化領域具有不可或缺的作用。然而，純二氧化鈦光催化劑具有光生電子-空穴復合率高、太陽能利用率低等缺點，使其在光催化產氫領域的應用受到限制。迄今為止，人們探索了多種改性策略來提高二氧化鈦的光催化活性，如貴金屬負載、金屬或非金屬元素摻雜、構建異質結等。
通過復合兩個具有合適能帶排布的半導體來構建異質結可以大大提高光生載流子的分離，被認為是一種有效的解決方案。最近提出了一種新的S型異質結概念，以解釋不同半導體異質界面載流子轉移分離的問題。S型異質結是在傳統Ⅱ型和Z型（液相Z型、全固態Z型、間接Z型、直接Z型）基礎上提出的，但又揚長避短，優於傳統Ⅱ型和Z型。通常，S型異質結是由功函數較小、費米能級較高的還原型半導體光催化劑和功函數較大、費米能級較低的氧化型半導體光催化劑構建而成。三氧化鎢禁帶寬度較小（2.4–2.8eV），功函數較大，是典型的氧化型光催化劑，也是構建S型異質結的理想半導體光催化劑。根據S型電荷轉移機制，三氧化鎢/二氧化鈦復合物在光輻照下，三氧化鎢導帶上相對無用的電子與二氧化鈦價帶上相對無用的空穴復合，二氧化鈦導帶上還原能力較強的電子和三氧化鎢價帶上氧化能力較強的空穴得以保留，從而在異質界面上實現了氧化還原能力較強的光生電子-空穴對的分離。同時，石墨烯作為一種蜂窩狀碳原子二維材料，是理想的電子受體，在異質結光催化劑中能及時轉移電子。而且，石墨烯具有較好的導熱性和電子遷移率，光吸收強，比表面積大，可為光催化反應提供豐富的吸附和活性位點，已經被認為是一種重要催化劑載體和光電分解水產氫的有效共催化劑。
本文採用簡便的一步水熱法制備石墨烯修飾的三氧化鎢/二氧化鈦S型異質結光催化劑。光催化產氫性能測試表明，三氧化鎢/二氧化鈦/石墨烯復合材料的光催化產氫速率顯著提高(245.8 μmol g ⁻¹ h ⁻¹)，約為純TiO₂的3.5倍。高分辨透射電子顯微鏡、拉曼光譜和X射線光電子能譜結果證明了TiO₂和WO₃納米顆粒的緊密接觸，並成功負載在還原氧化石墨烯（rGO）上。X射線光電子能譜中Ti 2p結合能的增加證實TiO₂和WO₃之間強的相互作用和S型異質結的形成。此外，復合材料中的rGO大大拓展了復合物的光吸收範圍（紫外-可見漫反射光譜），增強了光熱轉換效應，而且rGO與TiO₂之間形成肖特基結，促進了TiO₂導帶電子的轉移和分離。總之，WO₃和TiO₂的S型異質結與TiO₂和rGO之間的肖特基異質結的協同效應抑制了相對有用的電子和空穴的復合，有利於氧化還原能力較強的載流子的分離和進一步轉移，加速了表面產氫動力學，於是增強了三元復合光催化劑的光催化產氫活性。Copyright © 2020 Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.