Article
In-situ preparation of TiO2/N-doped graphene hollow sphere photocatalyst with enhanced photocatalytic CO2 reduction performance

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Abstract

Photocatalytic CO2 conversion efficiency is hampered by the rapid recombination of photogenerated charge carriers. It is effective to suppress the recombination by constructing cocatalysts on photocatalysts with high-quality interfacial contact. Herein, we develop a novel strategy to in-situ grow ultrathin N-doped graphene (NG) layer on TiO2 hollow spheres (HS) with large area and intimate interfacial contact via a chemical vapor deposition (CVD). The optimized TiO2/NG HS nanocomposite achieves total CO2 conversion rates (the sum yield of CO, CH3OH and CH4) of 18.11 μmol g−1 h−1, which is about 4.6 times higher than blank TiO2 HS. Experimental results demonstrate that intimate interfacial contact and abundant pyridinic N sites can effectively facilitate photogenerated charge carrier separation and transport, realizing enhanced photocatalytic CO2 reduction performance. In addition, this work provides an effective strategy for in-situ construction of graphene-based photocatalysts for highly efficient photocatalytic CO2 conversion.

Graphical Abstract

Intimate interfacial contact between N-doped graphene and TiO2 can effectively promote the photogenerated charge carriers transfer and enhance CO2 photoreduction performance.

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Introduction

Conversion of CO2 into chemical fuels by photocatalysis is considered to be promising to mitigating greenhouse effect and energy crisis [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. Therefore, building highly efficient photocatalysts is of paramount interest. And concerted efforts have been made on TiO2 owing to its excellent physicochemical stability, low cost, nontoxicity and environmental friendliness [15, 16, 17, 18, 19, 20, 21]. Furthermore, hollow spheres (HS) become promising candidates for photocatalytic CO2 reduction (PCR) due to manifold advantages including reduced charge carrier diffusion distance, improved light scattering and larger specific surface area [22]. Unfortunately, pristine TiO2 HS suffer from low efficiency owing to the rapid recombination of photogenerated electrons and holes (RPEH) [23, 24]. To tackle this challenge, we attempt to load cocatalyst on the TiO2 HS. Among various cocatalysts, noble metals have proved to be effective [23, 25, 26]. However, high cost and scarcity restrict their widespread application. Hence, other low-cost substitutes need to be devised.

Graphene, with excellent conductivity, larger work function and earth abundance, has attracted much attention [27, 28]. When graphene is grafted on n-type semiconductor photocatalysts, direct photogenerated electron migration from semiconductor to graphene is promoted and the RPEH is restricted [27, 28, 29]. When graphene is doped by N, the electron density of graphene skeleton is further improved, which facilitates electron transport due to lone-pair electrons of N atoms [30, 31, 32, 33, 34]. Moreover, various N, such as pyridinic, pyrrolic and graphitic N, serve as Lewis base sites for the CO2 adsorption and activation [28, 31, 34, 35, 36]. Unfortunately, the common practice for preparing semiconductor/N-doped graphene (NG) nanocomposite is growing semiconductor photocatalyst on NG surface [28, 34, 37]. The attained point-contact interface leads to limited interface area, inhibiting the rapid transfer and separation of photogenerated charge carriers [28, 37]. Therefore, it is necessary to detour the drawback of traditional method and design high-quality NG cocatalyst with abundant active sites and large area together with seamless interfacial contact.

Under this scenario, in-situ growth of NG on photocatalyst surface via chemical vapor deposition (CVD) is sensible, assuring intimate and large-area contact [31, 34, 38, 39]. Recently, our group has developed a recipe to directly grow few–layer graphene on ZnO surface via CVD using benzene as the graphene source [40]. With such strategy, ZnO surface is accessible to graphene precursors due to the excellent diffusion properties of organic gas molecules, thus achieving intimate contact between graphene and ZnO surface. Detailed characterization proved the effectiveness of this method in inhibiting the RPEH and enhancing CO2 photoreduction performance [40]. Therefore, after screening, pyridine, similar to benzene in structure, is chosen as the NG precursor, which can directly self–assemble into graphene skeleton inheriting abundant pyridinic N active sites [31, 34, 39, 41].

Herein, ultrathin NG layer (1–2 layers) was in-situ grown on the surface of TiO2 HS by CVD. In this case, each TiO2 nanoparticle in the TiO2 hollow shell is coated by ultrathin NG layer with intimate interfacial contact. We compare the PCR performance of in-situ fabricated TiO2/NG HS photocatalyst with that of blank TiO2 HS, TiO2/non-N doped graphene HS nanocomposite and ex-situ prepared NG-TiO2 HS nanocomposite. The advantages of this in-situ method are reflected in the excellent PCR performance. Besides, the PCR reaction mechanism was also thoroughly discussed.

Section snippets

Preparation

SiO2 spheres: SiO2 spheres with average sizes of about 350 nm were prepared [42]. Typically, 18 mL of ammonium hydroxide (AR, 28%), 32.5 mL of ethanol and 49.5 mL of deionized water were added to a 500 mL beaker and stirred at room temperature to prepare solution A. 9 mL of tetraethyl orthosilicate (AR, 99%) was added to 91 mL of ethanol (AR, 99.7%) and stirred to prepare solution B. Afterwards, solution B was quickly added to solution A with vigorous stirring. The mixture solution was kept

Physicochemical properties of samples

The preparation processes of TiO2/NG HS are schematically illustrated in Fig. 1(a). Initially, TiO2 nanoparticles are deposited on SiO2 spheres as templates (Fig. S1(a)), forming SiO2/TiO2 core-shell spheres (Fig. 1(b)). Subsequently, NG is loaded on SiO2/TiO2 surface at 700 °C under the protection of nitrogen, where pyridine serves as the NG precursor. The oxygen vacancies on SiO2/TiO2 surface act as NG nucleation sites (Fig. 2 and Fig. S2), and the active dehydrogenated pyridine ring radical

Conclusions

In summary, TiO2/NG HS photocatalyst was successfully constructed via CVD. The total photoreduction yield of TiO2/NG HS is obviously higher than those of blank TiO2, TiO2/G and NG-TiO2 HS. The significantly improved PCR performance originates from the ultrathin NG layer. NG, as an effective cocatalyst, accelerates the separation and transfer of photogenerated charge carriers via its intimate contact with TiO2. Besides, NG also produces a number of pyridinic N sites, which facilitate the

Jiaguo Yu received his BS and MS in chemistry from Central China Normal University and Xi’an Jiaotong University, respectively; his PhD in Materials Science from Wuhan University of Technology (WUT). In 2000, he became a Professor at WUT. His research interests include semiconductor photocatalysis, photocatalytic hydrogen production, CO2 reduction, dye-sensitized and perovskite Solar cells, indoor air purification and adsorption, supercapacitor, electrocatalysis and so on. He is Thomson Reuters

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    Jiaguo Yu received his BS and MS in chemistry from Central China Normal University and Xi’an Jiaotong University, respectively; his PhD in Materials Science from Wuhan University of Technology (WUT). In 2000, he became a Professor at WUT. His research interests include semiconductor photocatalysis, photocatalytic hydrogen production, CO2 reduction, dye-sensitized and perovskite Solar cells, indoor air purification and adsorption, supercapacitor, electrocatalysis and so on. He is Thomson Reuters “Hottest Researcher” of 2012. His name is also in the lists of 2014−2020 Highly Cited Researchers from Clarivate Analytics (Thomson Reuters) in Materials Science, Chemistry and Engineering. He is Foreign Member of Academia Europaea (The Academy of Europe) (2020), Foreign Fellow of the European Academy of Sciences (2020) and Fellow of the Royal Society of Chemistry (2015). He was appointed as the Associate Editor of Chin. J. Catal. in 2020.

    Available online 20 June 2021

    This work was supported by the National Natural Science Foundation of China (21905219, 51872220, 51932007, 51961135303, 21871217, U1905215, U1705251), the Fundamental Research Funds for the Central Universities (WUT: 2019IVB050) and the Innovative Research Funds of Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory (XHD2020-001).

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