Directed charge transfer in all solid state heterojunction of Fe doped MoS2 and C–TiO2 nanosheet for enhanced nitrogen photofixation
Graphical abstract
Introduction
The Harbor-Bosch (HB) process has produced the largest amount of commercial ammonia and hence promoted the fast development of modern agriculture [[1], [2], [3]]. However, the high emission of carbon dioxides due to the HB process has also worsened the global climate because of the greenhouse effect [4,5]. Photocatalytic nitrogen reduction reaction (PNRR) that produces ammonia at ambient conditions is becoming a promising alternative to the HB process to produce ammonia and contributes to global decarbonization [6,7]. While its low performance is motivating researchers to develop feasible strategies including defect engineering, band gap regulation, plasmonic resonance and doping with anions and/or cations [[8], [9], [10], [11], [12], [13], [14]]. Also, the in situ observation by advanced techniques and systematical investigation including experimental design and theoretical calculations are still ongoing to explore the mechanisms, which is complicated by the competitive water splitting reaction, not to mention the identification of ammonia quantitatively and qualitatively. However, in any case, nitrogen photofixation is still worthy of intensive research for a sustainable and clean economy [[15], [16], [17]].
High performance PNRR requires the catalyst possessing enough active sites to facilitate the surface reaction kinetics [18,19]. For example, active sites can be increased by immobilizing noble metal nanoparticles, and/or creating surface defects on semiconducting supports. As such, the interfacial multiple electron transfer processes can be speed up, and the surface affinity to inert nitrogen molecules can also be increased for subsequential chemisorption and activation [20,21]. However, noble metals are expensive, and defects are also the recombination center to trap photocarriers. Transition metal based materials typically have lower cost and high catalytic activity [[22], [23], [24], [25], [26]], with the Fe and Mo based nanomaterials being particularly dazzling [27,28]. Recent studies have shown that Fe, Mo, and S are active to PNRR. The structural analysis to the biological nitrogenases has uncovered the existence of Fe–Mo–S cofactors [[29], [30], [31]]. The artificially developed Mo2Fe6S8–Sn2S6 biomimetic chalcogenides are also active to photocatalytically convert nitrogen to ammonia [16]. These results imply that Fe, Mo, S are necessary elements for the development of photocatalysts for efficient PNRR.
While high-performance PNRR also demands directed flow of charge carriers since energetic electrons have more driven force to overcome the high reduction potential of N2/NH3. All solid-state z-scheme photocatalysts are successful examples with directed charge transfer and have excelled in water splitting and CO2 photoreduction [32]. As one of z-scheme heterojunctions, the semiconductor-conductor-semiconductor heterojunction (the PS-C-PS junction) is more eye-catching since the first example of PS-C-PS system consisted of TiO2–Au–CdS with the metallic Au as the conductor [33]. In the system, the separation of the photoinduced charges is efficiently promoted, and more energetic electrons from the conduction band of CdS can be efficiently used to reduce water molecules while avoiding flowing to a lower conduction band of TiO2. Therefore, the PS-C-PS heterojunction could be particularly effective for PNRR [34,35].
TiO2 as a well-known n-type semiconductor and shows excellent performance in catalyzing nitrogen reduction reaction under light irradiation, especially the Ti3+ sites in titanium-based materials exhibiting excellent activity toward nitrogen reduction [36,37]. MoS2 is a well-known p-type two-dimensional material and has already been widely used for catalysis applications for clean fuels generation [38,39]. In this study, an all solid state z-scheme heterojunction is built by immobilizing Fe doped MoS2 nanobundles on porous TiO2 nanosheets with the in situ formed carbon as the conductor. The Fe doped MoS2@C–TiO2 system shows a higher ammonia yield of 205.7 μg gcat−1 h−1 than Fe doped MoS2@TiO2 and MoS2@C–TiO2. The enhanced performance is ascribed to the spatially separated electrons and holes in the PS-C-PS system. The present study has provided an effective strategy for the design of active heterojunctions for solar driven ammonia production.
Section snippets
Results and discussion
The all solid-state z-scheme heterojunction was constructed by immobilizing Fe doped MoS2 nanobundles on porous C–TiO2 nanosheets, as illustrated in Fig. 1a. Firstly, the Fe doping to MoS2 was realized by a solvothermal process. As shown in Fig. S1a, pure MoS2 without Fe doping shows three obvious diffraction peaks at about 14.1°, 32.9° and 58.8°, belonging to the (002), (100) and (110) plane of hexagonal phase of layered MoS2 (JCPDF Card No. 75–1539), respectively. Increasing the amount of FeCl
Conclusions
In summary, we designed and prepared an all solid-state z-scheme heterojunction of Fe–MoS2@C–TiO2 nanosheets. To realize the efficient doping of Fe to MoS2, a solvothermal process combined with a cryo-mediated exfoliation and fracturing method was employed to prepare Fe doped MoS2 nanobundles which was further immobilized on porous carbon coated TiO2 nanosheets for the construction of the z-scheme system. A systematical investigation has been conducted to demonstrate that the z-scheme system is
Author statement
Q. Song and C. C. Sun conducted the materials synthesis and characterization. K. M. Wu conducted the time-resolved photoluminescence spectroscopy measurements. Z. Wang and X. X. Bai designed the photoelectrochemical measurements and analyzed the data. Q. Li, H. Zhang, L. Zhou, H. L. Pang and Y. P. Liang discussed the data. Q. Song wrote the manuscript. Z. Wang, X. X. Bai, and Z. H. Zhao revised the manuscript. Z. H. Zhao designed and supervised the study.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors thank the financial support from the National Key Research and Development Program of China (2019YFE0121600), the National Natural Science Foundation of China (6217032156), Guangdong Basic and Applied Basic Research Foundation (2020A515110112), the Fundamental Research Funds for the Central Universities (JB211402, JB211411). The shared facilities of Analysis and Test Center of Xidian University are also appreciated.
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Authors contributed equally.