Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Enhanced Visible-Light Photocatalytic H2 Evolution in Cu2O/Cu2Se Multilayer Heterostructure Nanowires Having {111} Facets and Physical Mechanism Bin Liu,† Lichao Ning,‡ Congjie Zhang,‡ Hairong Zheng,*,§ Shengzhong Frank Liu,# and Heqing Yang*,†
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Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, Key Laboratory of Macromolecular Science of Shaanxi Province, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, 710119, China ‡ Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, 710119, China § School of Physics and Information Technology, Shaanxi Normal University, Xi’an, 710119, China # Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, 710119, China S Supporting Information *
ABSTRACT: It is rather challenging to develop photocatalysts based on narrowband-gap semiconductors for water splitting under solar irradiation. Herein, we synthesized the Cu2O/Cu2Se multilayer heterostructure nanowires exposing {111} crystal facets by a hydrothermal reaction of Se with Cu and KBH4 in ethanol amine aqueous solution and subsequent annealing in air. The photocatalytic H2 production activity of Cu2O/Cu2Se multilayer heterostructure nanowires is dramatically improved, with an increase on the texture coefficient of Cu2O(111) and Cu2Se(111) planes, and thus the exposed {111} facets may be the active surfaces for photocatalytic H2 production. On the basis of the polar structure of Cu2O {111} and Cu2Se {111} surfaces, we presented a model of charge separation between the Cu−Cu2Se(111) and O−Cu2O(1̅ 1̅ 1̅) polar surfaces. An internal electric field is created between Cu− Cu2Se(111) and O−Cu2O(1̅ 1̅ 1̅) polar surfaces, because of spontaneous polarization. As a result, this internal electric field drives the photocreated charge separation. The oxidation and reduction reactions selectively occur at the negative O−Cu2O(1̅ 1̅ 1̅) and the positive Cu−Cu2Se(111) surfaces. The polar surface-engineering may be a general strategy for enhancing the photocatalytic H2-production activity of semiconductor photocatalysts. The charge separation mechanism not only can deepen the understanding of photocatalytic H2 production mechanism but also provides a novel insight into the design of advanced photocatalysts, other photoelectric devices, and solar cells.
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INTRODUCTION
benefit the development of 1D photoelectric nanodevices and open a new chapter in nanoscale research.12,13 However, the multilayer heterostructure nanowires have never been reported to function as a highly active photocatalyst. It is a constantly challenging issue in photocatalysis to develop highly active photocatalysts. To date, a large number of photocatalysts for hydrogen production from water splitting have developed.14−20 However, most of the developed photocatalysts are wide-band-gap semiconductors that can only utilize ultraviolet irradiation.15 Regarding optical absorption, narrow-band-gap semiconductor has a band gap matching the preferred range of the solar irradiation
Recently, one-dimensional (1D) nano/heterostructure compounds of two important functional materials have drawn continuous attention, because of their unique physical and chemical properties and potential applications in nanoelectronic and optoelectronic devices.1−9 Si/SiGe,4 GaAs/ GaP,1 InAs/InP,5 CdS/Ag2S,6 In−O/In-doped ZnO,7 ZnS/ ZnO,8 Ag2S/CdS/ZnS,9 AgInS2/Ag2S/AgInS2,9 Cu1.94S-ZnS,10 Cu1.94S-ZnS-Cu1.94S,10 Cu1.94S-ZnS-Cu1.94S-ZnS-Cu1.94S,10 and Cu1.94S-CuS11 superlattice/heterostructure nanowires have been successfully synthesized. The GaAs/GaP superlattice nanowires are displayed to be a suitable candidate to fabricate nanophotonic devices.1 The InAs/InP heterostructure nanowires are effective in high-electron-mobility devices.5 The achievement in the heterostructure nanowires will markedly © XXXX American Chemical Society
Received: May 1, 2018
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DOI: 10.1021/acs.inorgchem.8b01197 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry spectrum.21,22 Thus, it is highly desirable but challenging to develop the highly reactive photocatalysts based on narrowband-gap semiconductors for H2 production.23 Cu2Se and Cu2O are two very important p-type semiconductors. Cu2Se has both a direct band gap of 2.0−2.3 eV24 and an indirect band gap of 1.1−1.5 eV.24 Cu2O has a direct band gap of 2.2 eV.25 Both semiconductors are regarded as promising materials with potential application in solar energy conversion,26 lithium ion batteries,27 sensing,28,29 supercapacitors,30 thermoelectric devices,31,32 and photocatalysis.21,33,34 Cu2Se and Cu2O should be excellent solar light harvesters, because of their band gaps matching the preferred range of the solar irradiation spectrum. However, to our knowledge, to date, the fabrication and photocatalytic properties of Cu2O/Cu2Se multilayer heterostructure nanowires have never been reported in the literature. Recently, crystal face engineering has been demonstrated to be a promising and efficient strategy to enhancing photocatalytic hydrogen production performance,35−41 although the enhancement mechanism is yet unknown to date. Herein, we first report on the synthesis of Cu2O/Cu2Se multilayer heterostructure nanowires with different texture coefficients of the Cu2O(111) and Cu2Se(111) planes and texture coefficients of the Cu2O(111) and Cu2Se(111) dependent visible-light photocatalytic H2 generation activities. The enhancement in photocatalytic H2 evolution activity is ascribed to the exposed {111} surfaces. On the basis of polar structures of the Cu2O and Cu2Se {111} crystal faces, we presented a model of charge separation between polar Cu−Cu2Se(111) and O−Cu2O(1̅ 1̅ 1̅) surfaces to explain the enhancement in photocatalytic H2 production activities.
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Measurement of Photocatalytic H2 Production Activity. The photocatalytic reaction was conducted in a Pyrex reaction cell that was linked with a closed gas circulation and evacuation system. A quantity of 50.0 mg of the as-synthesized Cu2O/Cu2Se multilayer heterostructure nanowires, Cu2O commercial powders, or Cu2Se nanoplates were dispersed in a CH3OH aqueous solution (90 mL H2O and 10 mL CH3OH). The suspension was degassed thoroughly and then irradiated by a 300 W Xe lamp equipped with an optical filter (λ ≥ 420 nm) under constant stirring. The amount of generated H2 was determined using a GC-9560 gas chromatography (Huaai, TCD, Ar carrier). The average rate of H2 evolution in the initial 4 h was recorded for comparison, and the experimental error is
