Atomic-scale marriage of light-harvesting and charge-storing components for efficient photoenergy storage catalysis
Graphical abstract
Incorporation of the charge-storing {Mo(VI)Ox} component into TiO2 matrix to construct an atomic-scale heterojunction for photoenergy storage catalysis.
Introduction
With the accelerating depletion of fossil energy resources, solar energy is regarded as one of the most prospective alternative energy sources due to its excellent sustainability and zero-emission of pollutants [1]. Semiconductor photocatalysis, capable of converting sunlight into electrical and/or chemical energy, has attracted world-wide research interests as a promising method for solar energy utilization [2], [3]. However, most of the developed stable semiconductors such as TiO2 and BiVO4 can only work under the direct illumination of sunlight [4], [5]. Such indispensible dependence on the real-time solar energy supply could not satisfy the actual energy demands in many cases especially in night. Recently, photoenergy storage (PES) system, through partially storing the photoexcited charge carriers (e−/h+) under illumination and spontaneously releasing them as electrical/chemical energy, has been developed as one of the most promising solutions to compensate the fluctuating availability of solar energy [6], [7].
To date, the reported PES system can be classified into two classes: one is the single moiety photo-rechargeable material, such as MoO3, WoO3 [6], [8], [9] and the other is the heterojuncted composite containing a light-harvesting component and a charge-storing one, such as TiO2-MoO3 [7], [10], [11], [12]. Due to a high-recombination probability of the photoexcited e−/h+, the solar energy utilization and storage efficiency of a single moiety photo-rechargeable material is quite limited if without any noble metal modification [8]. The recombination probability of photoexcited e−/h+ could be essentially reduced in a heterojunction photo-rechargeable system, based on the formation of an intrinsic electrical potential to oppositely direct the photoexcited electrons and holes [13]. However, in most nano-scale heterojunction PES systems, the inevitable defects at the interface originated from the lattice mismatch between two moieties would capture/quench the photoexcited e−/h+ pairs and hinder the charge transferring process [14], [15]. Moreover, these excited e−/h+ pairs at the interface may suffer from a premature recombination therein due to the relatively short free path length compared with those in the separated components [16], and hence result in the unsatisfactory efficiency for solar energy storage in the developed nano-scale heterojunction materials. Consequently, designing a novel PES system with both light-harvesting and charge-storing moieties but no defined interface is of great significance for efficient utilization of solar energy.
Considering the impressive energy storage capability of molybdenum oxides [17] and suitable reduction potential of Mo(VI) to accept the photoexcited electrons from TiO2 [18], in addition to the smaller effective ionic radius of Mo(VI) (0.41 Å) than that of Ti(IV) (0.6 Å) in lattice octahedral site [19], it is possible to incorporate {Mo(VI)Ox} component into TiO2 matrix to construct atomic-scale heterojunction between the light-harvesting unit TiO2 and the charge-storing component {Mo(VI)Ox} without any defective interface, enabling an efficient photoenergy storage performance. However, uncontrolled separate nucleation and growth of TiO2 and MoO3 will take place independently rather than forming the atomic-scale heterojunction between {Mo(VI)Ox} and {TiO6} by common hydrothermal or sol-gel methods (Fig. S1), which probably result from the great difference in the hydrolysis and condensation rates between titanic and molybdate chemical compounds in aqueous solution [20].
In this work, through an accurate control over the kinetics of hydrolysis and condensation of precursors titanium (IV) butoxide (TBOT) and ammonium heptamolybdate ((NH4)6Mo7O24), we present a novel hydrothermal strategy to in situ integrate the heterogeneous {Mo(VI)Ox} component into TiO2 matrix (denoted as Mo-TiO2), constructing an artificial atomic-scale heterojunction between light-harvesting unit TiO2 and the charge-storing component {Mo(VI)Ox} in a single-phase (Fig. 1a). Under illumination, photoexcited electrons could effectively transfer from TiO2 to {Mo(VI)Ox} nearby and stored in the form of {Mo(V)Ox} [11]. Then the photoinduced {Mo(V)Ox} species could spontaneously activate oxygen to the highly active superoxide radical (•O2−) for further degradation of organic pollutant after light turning-off [21], enabling dark-continued catalytic activity (Fig. 1b). More importantly, benefiting from the photosensitization effects of {MoOx} component [22] and the constructed atomic-scale heterojunction in a single-phase to effectively eliminate the inevitable charge pair quenching at defective interfaces, Mo-TiO2 exhibits excellent solar energy storage catalytic performances. To our best knowledge, the photoenergy storage system based on a single-phase material with atomic-scale heterojunction is still not available to date in literatures.
Section snippets
Materials
Titanium(IV) butoxide (TBOT, 97%) and ammonium heptamolybdate ((NH4)6Mo7O24·4H2O, 99%) were purchased from Sigma-Aldrich. Hydrofluoric acid (HF, 47 wt%), hydrochloric acid (HCl, 37 wt%), Rhodamine B (RhB, 99%) were obtained from Sinopharm Chemical Reagent Co., Ltd, China. Formaldehyde solution (10 mg ml−1) was purchased from Aladdin Reagent Co., Ltd, Shanghai. Deionized water used throughout the experiments was prepared using ELGA water purification system (PURELAB Classic).
Preparation of samples
Mo-TiO2 compound was
Results and discussion
In our one-pot hydrothermal strategy of preparing Mo-TiO2, Titanium (IV) butoxide (TBOT) and ammonium heptamolybdate ((NH4)6Mo7O24) were used as the Ti and Mo sources, respectively. However, due to their great differences in hydrolysis and condensation rates in aqueous solutions, the uncontrolled separate nucleations and growths of TiO2 and MoO3 phases will take place independently instead of forming a single phase structure with the atomic-scale heterojunction between {Mo(VI)Ox} and {TiO6} via
Conclusion
In summary, through a physicochemical regulation on the corresponding hydrolysis and condensation kinetics, the charge-storing component {MoOx} has been successfully integrated into the light-harvesting TiO2 matrix to construct a novel photoenergy storage system with atomic-scale heterojunction. Due to the photosensitization effects of {MoOx} component, the band-gap of Mo-TiO2 is significantly reduced with remarkably enhanced visible light absorption. Moreover, the construction of atomic-scale
Conflict of interest
The authors declare no competing financial interest.
Acknowledgments
This work was financially supported by the National Key Basic Research Program of China (2013CB933200), National 863 Plans Projects (2012AA062703), National Natural Science Foundation of China (21177137), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Youth Innovation Promotion Association of CAS (2012200). The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.
Yingfeng Xu received her B.S. (2012) of material science and engineering from Xi'an Jiaotong University. Now she is a Ph.D. candidate in Shanghai Institute of Ceramics, Chinese Academy of Sciences, under the supervision of Prof. Jianlin Shi. Her current research concentrates on the development of novel photocatalysts for water splitting and pollutants degradation.
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doped TiO2 could extend the photo-response range to the visible region, but the introduction of deep impurity levels was found to accelerate photogenerated carrier recombination. Fortunately, the rational doping of Mo was found to mitigate this negative effect. Accordingly, Mo, C co-doped TiO2 was synthesized via a facile low-temperature calcination process, resulting in unique band structure that enhanced the photocatalytic oxidation of NH3. Notably, in-situ DRIFTS and DFT results demonstrated that the gradual reaction of NH3 adsorbed on Mo atoms with oxygen species through the N2H4 mechanism, promoting the formation of N2. These findings offer insights into the design of efficient photocatalysts for NH3 removal under visible light, and present a promising technology for indoor air pollution control.
Yingfeng Xu received her B.S. (2012) of material science and engineering from Xi'an Jiaotong University. Now she is a Ph.D. candidate in Shanghai Institute of Ceramics, Chinese Academy of Sciences, under the supervision of Prof. Jianlin Shi. Her current research concentrates on the development of novel photocatalysts for water splitting and pollutants degradation.
Ping Lu received her Ph.D. of material physics and chemistry from Shanghai Institute of Ceramics, Chinese Academy of Sciences in 2015. Currently her research focuses on the microstructure analysis and in-situ study of phase transition by using different techniques of TEM including SAED, HRTEM and EELS, etc.
Lisong Chen received his Ph.D. degree from the Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS) in 2011, under the supervision of Prof. Jianlin Shi. He then joined East China Normal University as a lecture. Now his research interest focuses on the design, synthesis of inorganic nanomaterials for energy conversion related applications.
Chen Zhang received his B.S. of composite science and engineering from Jiangsu University in 2012. Now he is a Ph.D. candidate in Shanghai Institute of Ceramics, Chinese Academy of Sciences, under the supervision of Prof. Jianlin Shi. Now his current research interest focuses on designing and synthesizing novel functional materials.
Lingxia Zhang received her Ph.D. degree from Shanghai Institute of Ceramics, Chinese Academy of Sciences in 2003. She has been working at the institute after graduation. Currently her research mainly includes mesoporous and low-dimensional materials applied in artificial photosynthesis and environmental purification.
Xiaohua Zhang received his Bachelor degree from Beijing University of Aeronautics and Astronautics in 2013. Now he is a Ph.D. candidate under the supervision of Professor Jianlin Shi at Shanghai Institute of Ceramics, Chinese Academy of Sciences. His current main research focuses on the synthesis of heterogeneous catalysts with high activity and the probe of its active site, especially on non-precious metal-based ORR electro-catalysts.
Jianlin Shi received his B.S. from Nanjing University of Technology in 1983. He obtained his Ph.D. degree in 1989 at Shanghai Institute of Ceramics, Chinese Academy of Sciences. Presently his main research interest includes the structural design and synthesis of mesoporous materials and mesostructured nanocomposites, and the catalytic and biomedical performances of the materials for applications in environmental protection and nanomedicine. He has published more than 400 scientific papers which received more than 19,000 citations from other researches.