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Boron oxynitride nanoclusters on tungsten trioxide as a metal-free cocatalyst for photocatalytic oxygen evolution from water splitting

Ying Peng Xie, Gang Liu, Gao Qing (Max) Lu, Hui-Ming Cheng
2012 Nanoscale  
Here we show that B 2 O 3Àx N x nanoclusters can be formed on the surface of WO 3 particles by a combination of thermal oxidation of tungsten boride (WB) in air and the subsequent nitriding process in gaseous ammonia. The resultant nanoclusters are found to play an apparent role in improving the photocatalytic oxygen evolution of WO 3 by promoting the surface separation of photoexcited chargecarriers. Photocatalytic hydrogen and/or oxygen evolution from water splitting by using semiconductor
more » ... ng semiconductor photocatalysts is a highly desirable but very challenging process to convert solar energy to chemical energy. [1] [2] [3] [4] [5] [6] The practical applications of this attractive process are hindered by the low solar light conversion efficiency, which is controlled by the narrow light absorption range and/or the high recombination probability of photoinduced electron-hole pairs in most stable photocatalysts. 7,8 To increase solar light absorption, several strategies such as doping, 3,9 sensitization, 10 coupling with small bandgap quantum dots, 11 and surface disordering 12 have been intensively investigated in the past few decades. The increased visible light absorption, however, does not always result in an improved visible light photocatalytic activity. An additional key issue of concern is the effective separation and transfer of the excited electrons and holes on photocatalyst surfaces. It is established that the development of a well-matched cocatalyst on the photocatalyst surface can play a pivotal role in substantially promoting photocatalytic activity. So far, besides the well known cocatalysts (i.e. Pt, Rh, RuO 2 , NiO, IrO 2 ), 1,2 several impressive cocatalysts such as MoS 2 /WS 2 /PdS for CdS, 13 Cr-Rh oxide/Mn 3 O 4 for GaN : ZnO solid solution 4,14 have been explored and they exhibit a strong ability to improve the photocatalytic water splitting activity. With the assistance of these special cocatalysts, hydrogen evolution from visible light photocatalytic water splitting with and without sacrificial agents has been continuously increased. However, searching suitable cocatalysts for different photocatalysts is definitely important in achieving a high photocatalytic activity. Tungsten trioxide (WO 3 ) with a band gap of 2.7 eV and deep valence band maximum is a well-known O 2 evolution photocatalyst with visible light response, 15 and it is widely used to construct Zscheme systems for photocatalytic overall water splitting. 16 Typically, Pt nanoparticles as a cocatalyst are loaded on WO 3 photocatalyst to improve the surface separation probability of photoexcited electrons and holes. 16 However, Pt nanoparticles also act as a favorable catalyst for the undesired back reaction of H 2 with O 2 in the overall water splitting reaction thus limiting the photocatalysis efficiency. 17 There is an increasing demand for developing other alternative cocatalysts for WO 3 photocatalysts. It has been reported that B 2 O 3 can play an important role in improving the photocatalytic activity of B 2 O 3 -TiO 2 composites by potentially forming some favorable local structures and surface acid sites as active sites in photocatalytic reactions. 18 Therefore, it is expected that boron oxide nanoclusters might act as an effective cocatalyst to promote the activity of WO 3 . Furthermore, compared to a metal based cocatalyst, metal-free boron oxide has the merit of being easily modified by introducing heteroatoms, for example, the popular nitrogen dopant. With this in mind, we in situ prepared a B 2 O 3 loaded WO 3 photocatalyst first and subsequently modified B 2 O 3 to boron oxynitride (B 2 O 3Àx N x ). It is found that B 2 O 3Àx N x shows a more than 3 times stronger capability of promoting the oxygen evolution rate of WO 3 than B 2 O 3 . To the best of our knowledge, this is the first case of demonstration of B 2 O 3Àx N x nanoclusters as a metal-free cocatalyst in improving the activity of a photocatalyst, and we envisage that the results obtained may shed some light on designing other novel cocatalysts to maximize photocatalytic activity. B 2 O 3 nanoclusters were in situ introduced on the surface of WO 3 particles through a direct thermal oxidation of tungsten boride (WB) powder in air (the resultant sample is denoted as B 2 O 3 @WO 3 ). In this route, WB acts as the precursor of both WO 3 and B 2 O 3 nanoclusters. A subsequent nitriding process was conducted to produce B 2 O 3Àx N x nanoclusters on WO 3 particles (denoted as B 2 O 3Àx N x @WO 3 ) by heating the resultant B 2 O 3 @WO 3 in a gaseous ammonia atmosphere. The scanning electron microscopy (SEM) image in Fig. 1a shows that B 2 O 3 @WO 3 has a quasicuboid morphology with an average size of ca. 400 nm. Both the
doi:10.1039/c2nr11846g pmid:22241455 fatcat:f7c3e4h6rrg2ff6x4edwzh6bpa