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Boron oxynitride nanoclusters on tungsten trioxide as a metal-free cocatalyst for photocatalytic oxygen evolution from water splitting†
Downloaded by Nova Scotia Agriculture College on 24 October 2012 Published on 04 January 2012 on http://pubs.rsc.org | doi:10.1039/C2NR11846G
Ying Peng Xie,a Gang Liu,*a Gao Qing (Max) Lub and Hui-Ming Chenga Received 24th November 2011, Accepted 22nd December 2011 DOI: 10.1039/c2nr11846g
Here we show that B2O3 xNx nanoclusters can be formed on the surface of WO3 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 WO3 by promoting the surface separation of photoexcited chargecarriers. Photocatalytic hydrogen and/or oxygen evolution from water splitting by using semiconductor photocatalysts is a highly desirable but very challenging process to convert solar energy to chemical energy.1–6 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 disordering12 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, RuO2, NiO, IrO2),1,2 several impressive cocatalysts such as MoS2/WS2/PdS for CdS,13 Cr–Rh oxide/Mn3O4 for GaN : ZnO solid solution4,14 have been explored and they exhibit a strong ability to improve the photocatalytic water splitting activity. With the assistance of these special cocatalysts, a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua RD, Shenyang, 110016, China. E-mail: [email protected] b ARC Centre of Excellence for Functional Nanomaterials, The University of Queensland, Qld, 4072, Australia † Electronic supplementary information (ESI) available: (1) Experimental section. (2) XRD patterns, FT-IR and Raman spectra of B2O3@WO3 and B2O3 xNx@WO3. (3) Time course of O2 evolution from water splitting using B2O3@WO3 and B2O3 xNx@WO3. (4) XRD pattern and SEM image of pure WO3, UV-visible absorption spectra of pure WO3 and N–WO3. (5) UV-visible absorption spectra of bulk B2O3 and schematic of band edges of WO3, bulk B2O3, and B2O3 xNx nanocluster. See DOI: 10.1039/c2nr11846g
This journal is ª The Royal Society of Chemistry 2012
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 (WO3) with a band gap of 2.7 eV and deep valence band maximum is a well-known O2 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 WO3 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 H2 with O2 in the overall water splitting reaction thus limiting the photocatalysis efficiency.17 There is an increasing demand for developing other alternative cocatalysts for WO3 photocatalysts. It has been reported that B2O3 can play an important role in improving the photocatalytic activity of B2O3–TiO2 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 WO3. 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 B2O3 loaded WO3 photocatalyst first and subsequently modified B2O3 to boron oxynitride (B2O3 xNx). It is found that B2O3 xNx shows a more than 3 times stronger capability of promoting the oxygen evolution rate of WO3 than B2O3. To the best of our knowledge, this is the first case of demonstration of B2O3 xNx 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. B2O3 nanoclusters were in situ introduced on the surface of WO3 particles through a direct thermal oxidation of tungsten boride (WB) powder in air (the resultant sample is denoted as B2O3@WO3). In this route, WB acts as the precursor of both WO3 and B2O3 nanoclusters. A subsequent nitriding process was conducted to produce B2O3 xNx nanoclusters on WO3 particles (denoted as B2O3 xNx@WO3) by heating the resultant B2O3@WO3 in a gaseous ammonia atmosphere. The scanning electron microscopy (SEM) image in Fig. 1a shows that B2O3@WO3 has a quasicuboid morphology with an average size of ca. 400 nm. Both the Nanoscale, 2012, 4, 1267–1270 | 1267
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Fig. 1 Morphology and atomic structure of WO3 particles. (a) SEM image of WO3 particles with a coating of B2O3 nanoclusters; (b) SEM image of WO3 particles with a coating of B2O3 xNx nanoclusters; (c and d) High resolution TEM images of the edges of a WO3 particle with a coating of B2O3 and B2O3 xNx nanoclusters, respectively.
Fig. 2 XPS spectra of (A) B 1s, (B) O 1s, (C) N 1s and (D) W 4f in B2O3@WO3 (a) and B2O3 xNx@WO3 (b).
shape and particle size of B2O3@WO3 particles are retained after the nitriding process, as shown in Fig. 1b. X-Ray diffraction (XRD) patterns of B2O3@WO3 and B2O3 xNx@WO3 prepared (Fig. S1†) confirm the monoclinic g-WO3 phase (space group P21/ n; JCPDS card no. 43-1035), and no B2O3 or BN phase is detected. Although only a WO3 phase can be detected by XRD, an apparent thin layer with a thickness of less than 2 nm on some areas on the surface of the particle can be clearly observed in the high resolution TEM image of the B2O3@WO3 particle in Fig. 1c. The nitriding process results in the cracking of the layer and further agglomeration into some isolated nanoclusters with a size of 2–5 nm, as indicated by blue arrows in Fig. 1d. The lattice fringes in Fig. 1c and d with a distance of 0.365 nm can be assigned to (200) of WO3. In addition, it should be pointed out that most B2O3 byproducts from the oxidation of WB had automatically separated from the prepared WO3 particles by leaking into the ceramic container bottom due to the high fluidness of B2O3 at a high temperature as a result of which only a small amount of B2O3 could be coated on the WO3 particle surface as shown in Fig. 1c. The chemical compositions and their states in the surface layers of B2O3@WO3 and B2O3 xNx@WO3 were investigated by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 2, tungsten, oxygen, boron and nitrogen exist in both samples. Two states of boron with their B 1s level binding energies at 193.1 eV and 190.5 eV in B2O3@WO3 can be identified to be the predominant B–O bonds and minor N–B bonds in B2O3 nanoclusters.19 The amount of boron in the surface layer of B2O3@WO3 is determined to be as high as 23.7 atom%, where the ratio of B–N to B–O bonds is 0.029. The O 1s XPS spectrum shows a strong peak at 532.8 eV from B–O bonds, and only a very weak peak at 398.4 eV, which is from B–N bonds,19 appears in the N 1s XPS spectrum. This is consistent with the analysis of the B 1s XPS spectrum. The much lower percentage of B–N bonds than B–O bonds can be reasonably explained as being a result of the thermodynamically less favorable nitriding process of B species by N2 than its oxidation process by O2 during calcinations of WB.
After the subsequent nitriding treatment in ammonia atmosphere, the ratio of B–N bonds to B–O bonds has been substantially increased by nearly one order from 0.029 to 0.238 as indicated in Fig. 2A-b. Correspondingly, the signal intensity of the N 1s XPS peak at 398.4 eV is also improved in Fig. 2C-b (the peak at 401.7 eV originates from chemisorbed N2 molecules); the signal intensity of the O 1s XPS peak from O–B bonds is apparently weakened in Fig. 2B-b. Note that the binding energies of both B 1s and O 1s from O–B bonds are shifted to low energy levels from pristine 193.1 eV and 532.8 eV to 192.3 eV and 532.2 eV, respectively, with the increase of incorporated B–N bonds in B2O3 xNx nanoclusters. The plausible reason for this shift is the changed interface interaction between WO3 and B2O3 with the agglomeration of a uniform B2O3 layer into isolated B2O3 xNx nanoclusters, as shown in Fig. 1c and d. The changed interaction can be supported by the substantial shift of the W 4f XPS peak by 0.5 eV in Fig. 2D. The formation of B2O3 xNx nanoclusters on the surface of WO3 particles can be further confirmed by Fourier transform infrared (FTIR) spectroscopy. Fig. S2A† shows FT-IR spectra in the range of 400–4000 cm 1 of B2O3@WO3 and B2O3 xNx@WO3. Three important features can be identified: (i) the retained fingerprint region below 1000 cm 1 assigned to the stretching modes n(O–W–O),20 suggesting that the incorporation of more B–N bonds in B2O3 nanoclusters exerts no or negligible influence on the lattice framework of O–W–O. The unchanged Raman active modes in Fig. S2B† also support this result; (ii) the much weakened bands at 1200–1500 cm 1 attributed to the B–O stretching vibrations of trigonal BO3 units20,21 by the incorporation of B–N bonds in B2O3 nanoclusters. Moreover, the peak centered at 1450 cm 1 is shifted to 1400 cm 1; (iii) a new sharp peak centered at 1400 cm 1, which is assigned to the B–N stretching vibration in the hexagonal BN crystal,22 was observed in B2O3 xNx@WO3. All these features are consistent with XPS data analysis. In addition, the peak at 3200 cm 1 corresponding to the stretching mode of water molecules adsorbed on B2O3 xNx@WO3 is much weakened upon nitrogen incorporation. This indicates that the
1268 | Nanoscale, 2012, 4, 1267–1270
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stability of B2O3 in water can be improved by the incorporation of nitrogen, to some extent. The improved stability of the B2O3 xNx nanocluster is expected to play an important role in improving the photocatalytic activity of B2O3 xNx@WO3. We now estimate the photocatalytic activities of B2O3@WO3 and B2O3 xNx@WO3 samples by monitoring the water splitting oxygen evolution from an aqueous solution containing AgNO3 sacrificial agent under visible light. Table 1 shows the oxygen evolution rate of prepared photocatalysts. The B2O3 xNx@WO3 photocatalyst shows a 3.8 times higher oxygen evolution rate than B2O3@WO3 (also see Fig. S3 in the ESI†), suggesting that B2O3 xNx is more effective than B2O3 in promoting oxygen evolution from water splitting. Unfortunately, no comparable bare WO3 reference can be obtained from the preparation route with WB precursor so that it is hard to judge whether B2O3 plays a substantial role in promoting oxygen evolution of WO3 or not. To clarify this issue, we loaded 5 wt% B2O3 on WO3 by a chemical impregnation method (see Fig. S4 in the ESI†). It is found that the oxygen evolution rate of WO3 can be improved from pristine 7.8 to 24.8 mmol h 1 after the loading of B2O3, indicating that B2O3 indeed acts as an effective cocatalyst in promoting the activity of WO3. Consequently, we can further conclude that B2O3 xNx is a better cocatalyst than B2O3 based on the above results. In addition, it is noted that the pH value of the reaction solution remarkably decreases as a result of the increased concentration of H+ with oxygen evolution from water splitting (see Table 1). We also confirmed that the initial difference in the pH value will not exert an obvious influence on oxygen evolution in this case. To understand this improved photocatalytic oxygen evolution, we first examined the possible influence of introducing B–N bonds in B2O3 on electronic band structures of WO3 by UV-visible light absorption spectroscopy. As shown in Fig. 3a, B2O3@WO3 and B2O3 xNx@WO3 have an exactly overlapped intrinsic absorption edge with the threshold wavelength of 459 nm, suggesting the unchanged bandgap of WO3 by the incorporated B–N bonds in B2O3 nanoclusters. In contrast to B2O3@WO3, an additional absorption band beyond 500 nm up to the infrared range appears in B2O3 xNx@WO3. This absorption band is typically attributed to low-energy photon and/or thermal excitations of trapped electrons in localized states of defects (i.e. oxygen deficiency) just below the conduction band minimum.23 In this case, some additional oxygen vacancies would be created in the surface/subsurface of WO3 when B2O3@WO3 was treated in the gaseous ammonia atmosphere. To verify the above explanation, we treated bare WO3 reference in the gaseous ammonia. As shown in Fig. S5†, a similar absorption band beyond 500 nm was also formed in the absorption spectrum of the treated WO3 due to the partial loss of oxygen. Further photocatalytic Table 1 Oxygen evolution rate from water splitting with different photocatalystsa and pH value of the reaction solution before and after reaction
Samples
O2 evolutionb/ mmol h 1
pH value before/ after
Reference WO3 B2O3@WO3 B2O3 xNx@WO3
7.8 1 19.8 1 74.5 1
5.4/3.4 5.8/2.8 6.8/2.5
a Reaction conditions: 100 mg of photocatalyst was dispersed in 270 mL aqueous solution containing 0.85 g AgNO3; irradiation wavelength, l > 400 nm. b The average O2 evolution rate for 3 h.
This journal is ª The Royal Society of Chemistry 2012
Fig. 3 (a) UV-visible absorption spectra of B2O3@WO3 (black line) and B2O3 xNx@WO3 (red line); (b) photoluminescence spectra of B2O3@WO3 (black line) and B2O3 xNx@WO3 (red line).
activity measurements show that the introduced oxygen vacancies only cause a marginal oxygen evolution rate increase from pristine 7.8 to 11.3 mmol h 1. All these results indicate that the much superior oxygen evolution of B2O3 xNx@WO3 to B2O3@WO3 (74.5 vs. 19.8 mmol h 1) is mainly related to the effective role of B2O3 xNx nanoclusters in promoting surface separation of electrons and holes as discussed below. We used photoluminescence (PL) emission spectroscopy to investigate photoexcited charge-carrier trapping and recombination. As shown in Fig. 3b, both B2O3@WO3 and B2O3 xNx@WO3 exhibit an emission band with its centre at ca. 480 nm, which originates from the trapping state emission of oxygen-vacancy related states in the WO3 crystal.24 It is generally accepted that the emission intensity of PL emission increases with the increase of oxygen vacancies.25 Despite some additional oxygen vacancies introduced into WO3 crystal in B2O3 xNx@WO3 as mentioned above, the oxygen vacancy-related PL signal intensity of B2O3 xNx@WO3 is conversely decreased compared to that of B2O3@WO3. It has been reported that the defect-related PL emission in TiO2 can be weakened by surfaceloaded Pt nanoparticles due to a charge transfer effect from TiO2 to Pt.26 Therefore, similarly, we propose that this weakened PL emission intensity of B2O3 xNx@WO3 is caused by the more effective chargecarrier transfer from the WO3 to the B2O3 xNx nanoclusters, which will contribute to the improved photocatalytic activity of B2O3 xNx@WO3. Furthermore, TEM images of the B2O3 xNx@WO3 particles after photocatalytic reaction in Fig. 4a show that Ag nanoparticles from the reduction of sacrificial Ag+ by the photoexcited electrons predominately locate on the bare WO3 surface, indicating that the B2O3 xNx nanoclusters act as active sites for collecting photoexcited holes. Although the above results demonstrate the important role of B2O3 xNx nanoclusters in promoting oxygen evolution from water splitting, the mechanism of B2O3 xNx nanoclusters acting as a cocatalyst should be quite different from that of conventional cocatalysts such as Pt, Au with a lower Fermi energy level than photocatalysts.2,7a B2O3 and its nitride are typically insulating (widebandgap) so that the charge carrier transfer from WO3 to B2O3 xNx nanocluster would not occur in viewpoint of energy as indicated by the relative position of their band edges in Fig. S6†. On the other hand, it has been demonstrated that the electron tunnelling can indeed occur from narrow-bandgap Si to wide-bandgap TiO2 in the atomic layer-deposited oxide stabilized silicon photoanodes for water oxidation.27 The key to realize the tunnelling is that the thickness of the TiO2 layer must be thin enough. In our case, the size of the Nanoscale, 2012, 4, 1267–1270 | 1269
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Sciences for financial support. GL thanks the IMR SYNL-T.S. K^e Research Fellowship.
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Notes and references
Fig. 4 (a) TEM images of Ag-loaded B2O3 xNx@WO3 particles obtained after photocatalytic oxygen evolution reactions in the presence of AgNO3 as a sacrificial agent; (b) schematic of the bulk diffusion, surface separation and transfer processes of photoexcited electrons and holes in a B2O3 xNx@WO3 particle.
B2O3 xNx nanocluster ranges from 2 to 5 nm, which should be thin enough to allow carrier tunnelling between WO3 and the cluster for water oxidation as illustrated in Fig. 4b.
Conclusion We developed an easy route to in situ prepare B2O3 nanocluster loaded WO3 particles through the thermal oxidation of WB in air. It has been demonstrated that B2O3 nanoclusters show a capability of apparently improving photocatalytic oxygen evolution from water splitting on WO3 under the irradiation of visible light. Furthermore, the B2O3 xNx nanoclusters derived from B2O3 nanoclusters by thermal treating in ammonia atmosphere can further increase the oxygen evolution rate of WO3 by a factor of 3.8 due to their effectiveness in promoting electron–hole separation and transfer. This work not only demonstrates a unique metal-free cocatalyst but also might open up a door to develop other new cocatalysts.
Acknowledgements The authors thank the Major Basic Research Program, Ministry of Science and Technology of China (no. 2009CB220001), NSFC (no. 50921004, 51002160, 21090343, 51172243), Solar Energy Initiative and the funding (KJCX2-YW-H21-01) of the Chinese Academy of
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Cite this: Nanoscale, 2012, 4, 1267
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Boron oxynitride nanoclusters on tungsten trioxide as a metal-free cocatalyst for photocatalytic oxygen evolution from water splitting†
Downloaded by Nova Scotia Agriculture College on 24 October 2012 Published on 04 January 2012 on http://pubs.rsc.org | doi:10.1039/C2NR11846G
Ying Peng Xie,a Gang Liu,*a Gao Qing (Max) Lub and Hui-Ming Chenga Received 24th November 2011, Accepted 22nd December 2011 DOI: 10.1039/c2nr11846g
Here we show that B2O3 xNx nanoclusters can be formed on the surface of WO3 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 WO3 by promoting the surface separation of photoexcited chargecarriers. Photocatalytic hydrogen and/or oxygen evolution from water splitting by using semiconductor photocatalysts is a highly desirable but very challenging process to convert solar energy to chemical energy.1–6 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 disordering12 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, RuO2, NiO, IrO2),1,2 several impressive cocatalysts such as MoS2/WS2/PdS for CdS,13 Cr–Rh oxide/Mn3O4 for GaN : ZnO solid solution4,14 have been explored and they exhibit a strong ability to improve the photocatalytic water splitting activity. With the assistance of these special cocatalysts, a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua RD, Shenyang, 110016, China. E-mail: [email protected] b ARC Centre of Excellence for Functional Nanomaterials, The University of Queensland, Qld, 4072, Australia † Electronic supplementary information (ESI) available: (1) Experimental section. (2) XRD patterns, FT-IR and Raman spectra of B2O3@WO3 and B2O3 xNx@WO3. (3) Time course of O2 evolution from water splitting using B2O3@WO3 and B2O3 xNx@WO3. (4) XRD pattern and SEM image of pure WO3, UV-visible absorption spectra of pure WO3 and N–WO3. (5) UV-visible absorption spectra of bulk B2O3 and schematic of band edges of WO3, bulk B2O3, and B2O3 xNx nanocluster. See DOI: 10.1039/c2nr11846g
This journal is ª The Royal Society of Chemistry 2012
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 (WO3) with a band gap of 2.7 eV and deep valence band maximum is a well-known O2 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 WO3 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 H2 with O2 in the overall water splitting reaction thus limiting the photocatalysis efficiency.17 There is an increasing demand for developing other alternative cocatalysts for WO3 photocatalysts. It has been reported that B2O3 can play an important role in improving the photocatalytic activity of B2O3–TiO2 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 WO3. 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 B2O3 loaded WO3 photocatalyst first and subsequently modified B2O3 to boron oxynitride (B2O3 xNx). It is found that B2O3 xNx shows a more than 3 times stronger capability of promoting the oxygen evolution rate of WO3 than B2O3. To the best of our knowledge, this is the first case of demonstration of B2O3 xNx 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. B2O3 nanoclusters were in situ introduced on the surface of WO3 particles through a direct thermal oxidation of tungsten boride (WB) powder in air (the resultant sample is denoted as B2O3@WO3). In this route, WB acts as the precursor of both WO3 and B2O3 nanoclusters. A subsequent nitriding process was conducted to produce B2O3 xNx nanoclusters on WO3 particles (denoted as B2O3 xNx@WO3) by heating the resultant B2O3@WO3 in a gaseous ammonia atmosphere. The scanning electron microscopy (SEM) image in Fig. 1a shows that B2O3@WO3 has a quasicuboid morphology with an average size of ca. 400 nm. Both the Nanoscale, 2012, 4, 1267–1270 | 1267
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Fig. 1 Morphology and atomic structure of WO3 particles. (a) SEM image of WO3 particles with a coating of B2O3 nanoclusters; (b) SEM image of WO3 particles with a coating of B2O3 xNx nanoclusters; (c and d) High resolution TEM images of the edges of a WO3 particle with a coating of B2O3 and B2O3 xNx nanoclusters, respectively.
Fig. 2 XPS spectra of (A) B 1s, (B) O 1s, (C) N 1s and (D) W 4f in B2O3@WO3 (a) and B2O3 xNx@WO3 (b).
shape and particle size of B2O3@WO3 particles are retained after the nitriding process, as shown in Fig. 1b. X-Ray diffraction (XRD) patterns of B2O3@WO3 and B2O3 xNx@WO3 prepared (Fig. S1†) confirm the monoclinic g-WO3 phase (space group P21/ n; JCPDS card no. 43-1035), and no B2O3 or BN phase is detected. Although only a WO3 phase can be detected by XRD, an apparent thin layer with a thickness of less than 2 nm on some areas on the surface of the particle can be clearly observed in the high resolution TEM image of the B2O3@WO3 particle in Fig. 1c. The nitriding process results in the cracking of the layer and further agglomeration into some isolated nanoclusters with a size of 2–5 nm, as indicated by blue arrows in Fig. 1d. The lattice fringes in Fig. 1c and d with a distance of 0.365 nm can be assigned to (200) of WO3. In addition, it should be pointed out that most B2O3 byproducts from the oxidation of WB had automatically separated from the prepared WO3 particles by leaking into the ceramic container bottom due to the high fluidness of B2O3 at a high temperature as a result of which only a small amount of B2O3 could be coated on the WO3 particle surface as shown in Fig. 1c. The chemical compositions and their states in the surface layers of B2O3@WO3 and B2O3 xNx@WO3 were investigated by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 2, tungsten, oxygen, boron and nitrogen exist in both samples. Two states of boron with their B 1s level binding energies at 193.1 eV and 190.5 eV in B2O3@WO3 can be identified to be the predominant B–O bonds and minor N–B bonds in B2O3 nanoclusters.19 The amount of boron in the surface layer of B2O3@WO3 is determined to be as high as 23.7 atom%, where the ratio of B–N to B–O bonds is 0.029. The O 1s XPS spectrum shows a strong peak at 532.8 eV from B–O bonds, and only a very weak peak at 398.4 eV, which is from B–N bonds,19 appears in the N 1s XPS spectrum. This is consistent with the analysis of the B 1s XPS spectrum. The much lower percentage of B–N bonds than B–O bonds can be reasonably explained as being a result of the thermodynamically less favorable nitriding process of B species by N2 than its oxidation process by O2 during calcinations of WB.
After the subsequent nitriding treatment in ammonia atmosphere, the ratio of B–N bonds to B–O bonds has been substantially increased by nearly one order from 0.029 to 0.238 as indicated in Fig. 2A-b. Correspondingly, the signal intensity of the N 1s XPS peak at 398.4 eV is also improved in Fig. 2C-b (the peak at 401.7 eV originates from chemisorbed N2 molecules); the signal intensity of the O 1s XPS peak from O–B bonds is apparently weakened in Fig. 2B-b. Note that the binding energies of both B 1s and O 1s from O–B bonds are shifted to low energy levels from pristine 193.1 eV and 532.8 eV to 192.3 eV and 532.2 eV, respectively, with the increase of incorporated B–N bonds in B2O3 xNx nanoclusters. The plausible reason for this shift is the changed interface interaction between WO3 and B2O3 with the agglomeration of a uniform B2O3 layer into isolated B2O3 xNx nanoclusters, as shown in Fig. 1c and d. The changed interaction can be supported by the substantial shift of the W 4f XPS peak by 0.5 eV in Fig. 2D. The formation of B2O3 xNx nanoclusters on the surface of WO3 particles can be further confirmed by Fourier transform infrared (FTIR) spectroscopy. Fig. S2A† shows FT-IR spectra in the range of 400–4000 cm 1 of B2O3@WO3 and B2O3 xNx@WO3. Three important features can be identified: (i) the retained fingerprint region below 1000 cm 1 assigned to the stretching modes n(O–W–O),20 suggesting that the incorporation of more B–N bonds in B2O3 nanoclusters exerts no or negligible influence on the lattice framework of O–W–O. The unchanged Raman active modes in Fig. S2B† also support this result; (ii) the much weakened bands at 1200–1500 cm 1 attributed to the B–O stretching vibrations of trigonal BO3 units20,21 by the incorporation of B–N bonds in B2O3 nanoclusters. Moreover, the peak centered at 1450 cm 1 is shifted to 1400 cm 1; (iii) a new sharp peak centered at 1400 cm 1, which is assigned to the B–N stretching vibration in the hexagonal BN crystal,22 was observed in B2O3 xNx@WO3. All these features are consistent with XPS data analysis. In addition, the peak at 3200 cm 1 corresponding to the stretching mode of water molecules adsorbed on B2O3 xNx@WO3 is much weakened upon nitrogen incorporation. This indicates that the
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stability of B2O3 in water can be improved by the incorporation of nitrogen, to some extent. The improved stability of the B2O3 xNx nanocluster is expected to play an important role in improving the photocatalytic activity of B2O3 xNx@WO3. We now estimate the photocatalytic activities of B2O3@WO3 and B2O3 xNx@WO3 samples by monitoring the water splitting oxygen evolution from an aqueous solution containing AgNO3 sacrificial agent under visible light. Table 1 shows the oxygen evolution rate of prepared photocatalysts. The B2O3 xNx@WO3 photocatalyst shows a 3.8 times higher oxygen evolution rate than B2O3@WO3 (also see Fig. S3 in the ESI†), suggesting that B2O3 xNx is more effective than B2O3 in promoting oxygen evolution from water splitting. Unfortunately, no comparable bare WO3 reference can be obtained from the preparation route with WB precursor so that it is hard to judge whether B2O3 plays a substantial role in promoting oxygen evolution of WO3 or not. To clarify this issue, we loaded 5 wt% B2O3 on WO3 by a chemical impregnation method (see Fig. S4 in the ESI†). It is found that the oxygen evolution rate of WO3 can be improved from pristine 7.8 to 24.8 mmol h 1 after the loading of B2O3, indicating that B2O3 indeed acts as an effective cocatalyst in promoting the activity of WO3. Consequently, we can further conclude that B2O3 xNx is a better cocatalyst than B2O3 based on the above results. In addition, it is noted that the pH value of the reaction solution remarkably decreases as a result of the increased concentration of H+ with oxygen evolution from water splitting (see Table 1). We also confirmed that the initial difference in the pH value will not exert an obvious influence on oxygen evolution in this case. To understand this improved photocatalytic oxygen evolution, we first examined the possible influence of introducing B–N bonds in B2O3 on electronic band structures of WO3 by UV-visible light absorption spectroscopy. As shown in Fig. 3a, B2O3@WO3 and B2O3 xNx@WO3 have an exactly overlapped intrinsic absorption edge with the threshold wavelength of 459 nm, suggesting the unchanged bandgap of WO3 by the incorporated B–N bonds in B2O3 nanoclusters. In contrast to B2O3@WO3, an additional absorption band beyond 500 nm up to the infrared range appears in B2O3 xNx@WO3. This absorption band is typically attributed to low-energy photon and/or thermal excitations of trapped electrons in localized states of defects (i.e. oxygen deficiency) just below the conduction band minimum.23 In this case, some additional oxygen vacancies would be created in the surface/subsurface of WO3 when B2O3@WO3 was treated in the gaseous ammonia atmosphere. To verify the above explanation, we treated bare WO3 reference in the gaseous ammonia. As shown in Fig. S5†, a similar absorption band beyond 500 nm was also formed in the absorption spectrum of the treated WO3 due to the partial loss of oxygen. Further photocatalytic Table 1 Oxygen evolution rate from water splitting with different photocatalystsa and pH value of the reaction solution before and after reaction
Samples
O2 evolutionb/ mmol h 1
pH value before/ after
Reference WO3 B2O3@WO3 B2O3 xNx@WO3
7.8 1 19.8 1 74.5 1
5.4/3.4 5.8/2.8 6.8/2.5
a Reaction conditions: 100 mg of photocatalyst was dispersed in 270 mL aqueous solution containing 0.85 g AgNO3; irradiation wavelength, l > 400 nm. b The average O2 evolution rate for 3 h.
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Fig. 3 (a) UV-visible absorption spectra of B2O3@WO3 (black line) and B2O3 xNx@WO3 (red line); (b) photoluminescence spectra of B2O3@WO3 (black line) and B2O3 xNx@WO3 (red line).
activity measurements show that the introduced oxygen vacancies only cause a marginal oxygen evolution rate increase from pristine 7.8 to 11.3 mmol h 1. All these results indicate that the much superior oxygen evolution of B2O3 xNx@WO3 to B2O3@WO3 (74.5 vs. 19.8 mmol h 1) is mainly related to the effective role of B2O3 xNx nanoclusters in promoting surface separation of electrons and holes as discussed below. We used photoluminescence (PL) emission spectroscopy to investigate photoexcited charge-carrier trapping and recombination. As shown in Fig. 3b, both B2O3@WO3 and B2O3 xNx@WO3 exhibit an emission band with its centre at ca. 480 nm, which originates from the trapping state emission of oxygen-vacancy related states in the WO3 crystal.24 It is generally accepted that the emission intensity of PL emission increases with the increase of oxygen vacancies.25 Despite some additional oxygen vacancies introduced into WO3 crystal in B2O3 xNx@WO3 as mentioned above, the oxygen vacancy-related PL signal intensity of B2O3 xNx@WO3 is conversely decreased compared to that of B2O3@WO3. It has been reported that the defect-related PL emission in TiO2 can be weakened by surfaceloaded Pt nanoparticles due to a charge transfer effect from TiO2 to Pt.26 Therefore, similarly, we propose that this weakened PL emission intensity of B2O3 xNx@WO3 is caused by the more effective chargecarrier transfer from the WO3 to the B2O3 xNx nanoclusters, which will contribute to the improved photocatalytic activity of B2O3 xNx@WO3. Furthermore, TEM images of the B2O3 xNx@WO3 particles after photocatalytic reaction in Fig. 4a show that Ag nanoparticles from the reduction of sacrificial Ag+ by the photoexcited electrons predominately locate on the bare WO3 surface, indicating that the B2O3 xNx nanoclusters act as active sites for collecting photoexcited holes. Although the above results demonstrate the important role of B2O3 xNx nanoclusters in promoting oxygen evolution from water splitting, the mechanism of B2O3 xNx nanoclusters acting as a cocatalyst should be quite different from that of conventional cocatalysts such as Pt, Au with a lower Fermi energy level than photocatalysts.2,7a B2O3 and its nitride are typically insulating (widebandgap) so that the charge carrier transfer from WO3 to B2O3 xNx nanocluster would not occur in viewpoint of energy as indicated by the relative position of their band edges in Fig. S6†. On the other hand, it has been demonstrated that the electron tunnelling can indeed occur from narrow-bandgap Si to wide-bandgap TiO2 in the atomic layer-deposited oxide stabilized silicon photoanodes for water oxidation.27 The key to realize the tunnelling is that the thickness of the TiO2 layer must be thin enough. In our case, the size of the Nanoscale, 2012, 4, 1267–1270 | 1269
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Sciences for financial support. GL thanks the IMR SYNL-T.S. K^e Research Fellowship.
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Notes and references
Fig. 4 (a) TEM images of Ag-loaded B2O3 xNx@WO3 particles obtained after photocatalytic oxygen evolution reactions in the presence of AgNO3 as a sacrificial agent; (b) schematic of the bulk diffusion, surface separation and transfer processes of photoexcited electrons and holes in a B2O3 xNx@WO3 particle.
B2O3 xNx nanocluster ranges from 2 to 5 nm, which should be thin enough to allow carrier tunnelling between WO3 and the cluster for water oxidation as illustrated in Fig. 4b.
Conclusion We developed an easy route to in situ prepare B2O3 nanocluster loaded WO3 particles through the thermal oxidation of WB in air. It has been demonstrated that B2O3 nanoclusters show a capability of apparently improving photocatalytic oxygen evolution from water splitting on WO3 under the irradiation of visible light. Furthermore, the B2O3 xNx nanoclusters derived from B2O3 nanoclusters by thermal treating in ammonia atmosphere can further increase the oxygen evolution rate of WO3 by a factor of 3.8 due to their effectiveness in promoting electron–hole separation and transfer. This work not only demonstrates a unique metal-free cocatalyst but also might open up a door to develop other new cocatalysts.
Acknowledgements The authors thank the Major Basic Research Program, Ministry of Science and Technology of China (no. 2009CB220001), NSFC (no. 50921004, 51002160, 21090343, 51172243), Solar Energy Initiative and the funding (KJCX2-YW-H21-01) of the Chinese Academy of
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