CHEMISTRY AN ASIAN JOURNAL Accepted Article Title: Surfactant-Free Synthesis of Plasmonic Tungsten Oxide Nanowires with Visible-Light-Enhanced Hydrogen Generation from Ammonia Borane
Authors: Can Xue; Zaizhu Lou; Quan Gu; Lin Xu; Yusen Liao
This manuscript has been accepted after peer review and the authors have elected to post their Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201500319 Link to VoR: http://dx.doi.org/10.1002/asia.201500319
A sister journal of Angewandte Chemie and Chemistry – A European Journal
Supported by
Federation of Asian Chemical Societies
Chemistry - An Asian Journal
10.1002/asia.201500319
COMMUNICATION Surfactant-Free Synthesis of Plasmonic Tungsten Oxide Nanowires with Visible-Light-Enhanced Hydrogen Generation from Ammonia Borane Zaizhu Lou,[a] Quan Gu,[a] Lin Xu,[a] Yusen Liao,[a] and Can Xue*[a] Dedication ((optional))
Abstract: WO3-x nanowires were successfully synthesized through a simple surfactant-free solvothermal method. These nanowires exhibit strong plasmonic absorption in visible and near-infrared region due to the abundant oxygen vacancies. The plasmon excitation of these WO3-x nanowires provide five times enhancement on the hydrogen generation from ammonia borane.
The localized surface plasmon resonance (LSPR) of noble metal (e.g. Ag and Au) nanoparticles, arising from coherent oscillation of surface conduction electrons, leads to strong absorption in the visible and near-infrared (NIR) region.[1,2] Researchers have demonstrated that the integration of plasmonic metal particles with semiconductors allows for enhanced visible-light-driven photocatalytic activities.[3-6] However, the high-cost of noble metals restricts large-scale practical applications. Recently, the LSPR phenomena have been observed in some nonmental nanostructures of heavily-doped semiconductors, such as WO3[7] MoO3-x,[8,9] Cu2-xS,[10] and Sn-doped In2O3.[11] The incident x, light can induce collective oscillations of the free charges on the surface of doped nonmetal nanoparticles, giving opportunities to explore low-cost noble-metal-free plasmonic photocatalysts. Nevertheless, the LSPR absorption of those nonmetal nanoparticles usually locates in the NIR region due to the low density of free charges. Due to the easy variation of valance state, the color of tungsten oxide could be changed between white and blue under light irradiation or electrical stimulation, allowing for the application in chromic devices.[12,13] More recently, some researchers have found that plasmonic tungsten oxide nanostructures exhibit intense LSPR absorption in both visible and NIR region with plasmon-enhanced photocatalytic activities.[14-16] For instance, W18O49 nanowires were prepared by WCl6 in ethanol via a hydrothermal process, and capable for photo-reduction of CO2 under irradiation.14 By using oleylamine as shape-control surfactants, W18O49 nanowires with aspect ratio dependent photocatalytic activity were synthesized and investigated.[15] Tungsten oxide nanosheets made by alcohothermal exfoliation showed enhanced performance for photocurrent response and photocatalytic water oxidization due
[a]
Dr. Z. Z. Lou, Dr. Q. Gu, Dr. L. Xu, Y. S. Liao and Prof. C. Xue School of Material Science & Engineering Nanyang Technological University Nanyang Avernue, Singapore 639798 E-mail: [email protected]
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
to their LSPR.[16] It has been suggested that the oxygen vacancies on tungsten oxide surfaces are responsible to the plasmon-driven activities under irradiation of visible and NIR light.[17-19] Some reports have demonstrated that LSPR can enhance the photocatalytic acitivity of plasmonic tungsten oxides in the full spectum (UV-Vis-NIR) region.[14-16] Nevertheless, the applications of plasmonic tungsten oxides through their LSPR excitation still need further exploration. Herein, in this work, we present a simple one-step solvothermal method using W(CO)6 without any surfactants to synthesize blue WO 3-x nanowires that exhibited strong plasmonic absorption in the visible and NIR region due to the abundant oxygen vacancies on the nanowire surfaces. We also demonstrated firstly that catalytic activities of WO3-x nanowires for hydrogen generation from ammonia borane could be greatly enhanced upon LSPR excitation of the WO3-x nanowires.
Figure 1. a) UV-vis absorption spectra of the product; b) XRD patterns of the product and standard tungsten oxide (W 18O49, JCPDS no. 71-2450); c) TEM image of the sample (WO3-x-160); d) HRTEM image of the sample (WO3-x-160).
The WO3-x nanowires were synthesized by using W(CO) 6 as the precursor dissolved in ethanol with solvothermal treatment at 160 oC for 12 hours. The product appeared as blue solution with uniform dispersion of tungsten oxide (Figure 1). The blue color can be attributed to the abundant oxygen vacancies on the tungsten oxide surfaces, which induces intense LSPR absorption starting from ~500 nm. Upon treatment with H2O2, these oxygen vacancies could be passivated, and as a result, the visible-light absorption disappears as shown in Figure 1a.
Chemistry - An Asian Journal
10.1002/asia.201500319
COMMUNICATION The X-ray diffraction (XRD) pattern (Figure 1b) of obtained samples can be indexed as monoclinic W18O49 (JCPDS No. 712450). The crystal structures of W 18O49 is shown in the inset of Fig. 1b with a=18.334, b=3.786, c=14.044, and β=115.2. The strong (010) peak indicates the preferential growth of tungsten oxide along direction. Although the obtained nanowires have the same crystal structures with W 18O49, some oxygen vacancies will be passivated by air during washing and drying processes. Furthermore, the amount of oxygen vacancies is not directly correlated with the crystal structure of the obtained nanowires. As shown in figure S1, the sample treatment by H2O2 completely quenches the LSPR absorption, indicating the significantly reduced oxygen vacancies, but it does not induce observable changes in the XRD pattern. Similar results were also reported by other researchers.[7] As such, the obtained tungsten oxide nanowires can be labeled WO3-x, and the x value can be measured by XPS as discussed in blow. The transmission electron microscopy (TEM) image (Figure 1c) reveals that the obtained tungsten oxide samples are nanowires with ~10 nm diameter and lengths of several hundred nanometers to a few micrometers. The high-resolution TEM (HRTEM) analysis shows clear lattice fringes with interplanar dspacing of 0.376 nm (Figure 1d and S2), which further confirms the nanowire growth along direction.[20]
Figure 2. Formation scheme of WO3-x nanowires
The growth process of WO3-x nanowires can be illustrated in Figure 2. The dissolution of W(CO)6 in ethanol leads to W(OCH2CH3) precursors that are converted to tungsten oxide seeds upon solvothermal treatment. The continuous growth of the seeds along direction results in tungsten oxides nanowires.[15] Note that the tungsten oxide nanowires at high concentration intend to form bundle-like structures (S3 and S4 of supporting information) due to the surfactant-free surface and strong van der waals attraction between the nanowires. The effect of reaction temperature was also explored. When the solvothermal temperature is decreased from 160 to 120 oC, we obtained less quantity of tungsten oxide product which still appeared as nanowires with ~10 nm diameters (Figure 3a-c, S5 and S6). However, Figure 3d indicates that the LSPR absorption of the sample prepared at 120 oC and 140 oC appeared significantly weaker than that of the sample prepared at 160 oC, suggesting that the reaction temperature plays a critical role on the amount of oxygen vacancies on nanowires.
Figure 3. TEM images of tungsten oxides with different temperature: a) 120 oC, b) 140 oC and c) 160 oC; d) UV-vis light spectra of above samples.
Figure 4. W 4f XPS spectrums of commercial WO3 (a) and WO3-x-160 (b).
To clarify the chemical composition of the WO3-x nanowires and values of x, we have characterized the sample by X-ray photoelectron spectroscopy (XPS). For the commercial WO3 sample, Figure 4a indicates two binding energy peaks of W4f at 37.9 eV and 35.9 eV, corresponding to W 6+ 4f5/2 and 4f7/2, respectively. In comparison, the W4f XPS spectra of WO 3-x nanowires (Figure 4b) can be deconvoluted into two pairs of peaks. The two peaks at 37.9 eV and 35.9 eV are contributed by 4f5/2 and 4f7/2 of W 6+, and the other two peaks at 37.2 eV and 35.0 eV are attributed to 4f5/2 and 4f7/2 of W 5+. The quantitative analyses based on the XPS spectra suggest that the W 5+ cation accounts for 26% of total W states in the WO 3-x nanowires, W(0.26)5+W(0.74)6+O3-x, and the average oxidization state of W is about 5.74. Based on charge balance, the chemical composition of the prepared WO3-x nanowires can be estimated as WO2.87 and the x value is about 0.13. This result further confirms the existing of massive oxygen vacancies that enhance the density
Chemistry - An Asian Journal
10.1002/asia.201500319
COMMUNICATION of free electrons and lead to strong plasmon absorption in the visible and NIR region.
Figure 5. H2 evolution rate from ammonia borane over different catalysts: comparison between no catalyst, commercial WO3 and WO3-x -160 in dark (a) and under light irradiation (b); comparison between WO3-x -160, WO3-x-140 and WO3-x-120 in dark (c) and under light irradiation (d).
The plasmonic energy has been proved as an efficient strategy for catalytic hydrogen evolution from ammonia borane (BH3NH3),[9, 21] which is an important hydrogen storage material. As such, we have evaluated the capability of the plasmon absorption of WO3-x nanowires for enhancing hydrogen generation from BH3NH3. As shown in Figure 5a, without tungsten oxide catalyst, H2 evolution from BH3NH3 was hardly observed in dark. While the presence of WO3-x nanowires and commercial WO3 led to H2 evolution of 15.8 μmol and 9.5 μmol, respectively, in 60 minutes. The higher efficiency of WO3-x nanowires in dark might be attributed to the more surface active sites as compared to commercial WO3. Under visible light (>420 nm) irradiation, the commercial WO3 only showed 1.6 times enhancement on H2 evolution (14.8 μmol in 60 min) as compared to that in dark. In contrast, surprisingly, we observed 5 times enhanced H2 evolution rate (79.5 μmol in 60 min) over WO3-x nanowires under visible light irradiation versus dark, as shown in Figure 5b. Such a larger enhancement is attributed to the strong LSPRs absorption of the WO3-x nanowires in the visible and NIR region. Moreover, the preparation temperature of WO3-x nanowires influences their catalytic activities. Higher reaction temperature would induce more oxygen vacancies, which leads to more crystal defects serving as active sites for reactions on the surface of WO3-x nanowires. This is evidenced by Figure 5c, showing that the H2 evolution rates over the WO3-x nanowires prepared at lower temperature are in order of WO 3-x-120 < WO3x-140 < WO3-x-160 under dark. Under visible light irradiation, the H2 evolution rates also followed the same trend (Figure 5d) due
to the less oxygen vacancies on the WO3-x nanowires at lower preparation temperature and thereby weaker LSPR absorption. Nevertheless, all above results have demonstrated that the catalytic efficiency for hydrogen evolution on plasmonic WO 3-x nanowires can be greatly enhanced by LSPR excitation. To further verify the LSPR effect on hydrogen generation, we also tested the activity of the oxidized WO3-x nanowires (after H2O2 treatment) that do not show plasmon absorption because the surface oxygen vacancies have been passivated by H 2O2. As shown in figure S7 of supporting information, the H2 generation rate over the oxidized WO3-x nanowires is much lower than that over plasmonic WO3-x nanowires under visible light irradiation, which confirms the critical role of LSPR excitation to the enhanced H2 generation.
Figure 6. Mechanism of LSPRs enhanced catalytic H2 evolution from BH3NH3 against WO3-x nanowires.
The possible principle of the plasmon-enhanced H2 evolution over WO3-x nanowires is illustrated in Figure 6. On the surface of WO3-x nanowires, massive oxygen vacancies are created during the solvothermal process, leading to heavily self-doped tungsten oxides. These vacancies provide free electrons that can be oscillating upon excitation of incident light. Similar with LSPR of noble metal nanoparticles, the coherent oscillation of these free electrons leads to intensive light absorption. The intensity and width of the plasmon absorption are dependent on the density of free electrons and morphologies of nonmetal particles. Due to the large length, the prepared WO3-x nanowires exhibit wide LSPRs absorption from 500 to 1500 nm. The irradiation of visible and NIR light can induce two processes on the surface of WO3-x nanowires. First, charge carriers can be generated by “hot” plasmonic electrons of WO3-x nanowires,[9] and enhance the charge transfer between the reagents for hydrogen evolution on the WO3-x nanowire surface. Second, photothermal effect arising from the non-radiative decay of electron oscillations would supply addition localized heating on the WO3-x nanowire surface,[22,23] and thereby increase reaction rate for more efficient H2 evolution. In addition, the surface active sites generated by the solvothermal process under high temperature and pressure also facilitate the reaction of ammonia borane and water. Therefore, the overall enhancement on H 2 evolution over the WO3-x nanowires is attributed to strong plasmon absorption and active sites for surface reaction. In summary, plasmonic WO3-x nanowires were successfully prepared by a surfactant-free solvothermal process which
Chemistry - An Asian Journal
10.1002/asia.201500319
COMMUNICATION generates massive oxygen vacancies on WO3-x nanowires. As a result, the obtained WO3-x nanowires exhibit intensive LSPR absorption in the visible and NIR region. We have also demonstrated that the plasmon excitation of WO3-x nanowires provides five times enhancement on the hydrogen evolution from ammonia borane.
MOE AcRF-Tier2 (MOE2012-T2-2-041, ARC 5/13), and CRP (NRF-CRP5-2009-04) from NRF Singapore. Keywords: photocatalysis • tungsten oxides • surface plasmon resonance • hydrogen generation • ammonia borane [1]
Experimental Section
[2]
All reagents were analytical grade. Hexacarbonyltungsten W(CO)6, ethanol, ammonia borane were purchased from Sigma-Aldrich and used without any further purification.
[3]
Synthesis of WO3-x nanowires: In a typical procedure, 30 mg hexacarbonyltungsten was added into 40 ml absolute ethanol with constant stirring. When the solid was dissolved completely, yellow transparent solution was obtained as the precursor solution. Then, the solution was transferred to an 80 mL Teflon-lined stainless steel autoclave, heated up to 160 oC and kept for 12 hours. The produced samples were separated from solution by centrifugation, washed with ethanol for three times, and dried in a vacuum oven. The preparation of samples at different temperature followed the same procedures. Hydrogen evolution reaction: The hydrogen gas was generated through degradation of ammonia borane by using WO3-x nanowires as catalysts. Typically, 10 mg WO3-x nanowires were dispersed into 10 ml deionized water, and the suspension was placed in a photocatalytic reactor (80 mL) under constant stirring. The reactor was then sealed and degassed by nitrogen for 10 min. Subsequently, 2 ml DI water containing 2 mg ammonia borane was added into the solution. The generated hydrogen was periodically analyzed by Agilent 7890A gas chromatograph (GC) with TCD detector. For the H2 evolution under illumination, the reactor was exposed under a 300-W Xe lamp (MAX-302, Asahi Spectra Co. Ltd.) coupled with a UV cut-off filter ( > 420 nm, output light density of 200 mW/cm2). Characterization: The X-ray diffraction (XRD) patterns were obtained on Bruker D8-advanced X-ray powder diffractometer with Cu Kα radiation λ=1.5418 Å. The scanning electron microscopy (SEM) measurements were carried out on a JEOL JSM-7600F microscope. Transmission Electron Microscopy (TEM) measurements were carried out on a JEOL JEM-2010 microscope. UV-Vis-NIR absorption spectra were recorded on a Cary 5000 UV-Vis-NIR Spectrometer (Agilent Technologies). X-ray photoelectron spectroscopy was performed on the VG ESCALAB 220IXL XPS system (Thermo VG Scientific Ltd., UK).
[4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15] [16] [17]
[18] [19] [20]
[21]
Acknowledgements The authors acknowledge the financial support from NTU seed funding for Solar Fuels Laboratory, MOE AcRF-Tier1 (RG 44/11),
[22] [23]
X. Zhang, Y. L. Chen, R. S. Liu and D. P. Tsai, Rep. Prog. Phys. 2013, 76, 046401. M. M. Shahjamali, M. Salvador, M. Bosman, D. S. Ginger and C. Xue, J. Phys. Chem. C. 2014, 118, 12459. P. Wang, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai, J. Y. Wei and M. H. Whangbo, Angew. Chem. Int. Ed. 2008, 47, 7931. Y. P. Yuan, L. W. Ruan, J. Barber, S. C. J. Loo and C. Xue, Energy Environ. Sci. 2014, 7, 3934. Z. Y, Zhang, A. Li, S. W. Cao, M. Bosman, S. Z. Li and C. Xue, Nanoscale 2014, 6, 5217. H. Zhu, X. Ke, X. Yang, S. Sarina and H. Liu, Angew. Chem. Int. Ed. 2010, 49, 9657. K. Manthiram and A. P. Alivisatos, J. Am. Chem. Soc. 2012, 134, 3995. Q. Huang, S. Hu, J. Zhuang and X. Wang, Chem. Eur. J. 2012, 18, 15283. H. Cheng, T. Kamegawa, K. Mori and H. Yamashita, Angew. Chem. Int. Ed. 2014, 53, 2910. I. Kriegel, C. Jiang, J. Rodriguez-Fernandez, R. D. Schaller, D. V. Talapin, E. Como and J. Feldmann, J. Am. Chem. Soc. 2012, 134, 1583. M. Kanehara, H. Koike, T. Yoshinaga and T. Teranishi, J. Am. Chem. Soc. 2009, 131, 17736. Y. P. He , Z. Y. Wu , L. M. Fu , C. R. Li , Y. M. Miao , L. Cao, H. M. Fan and B. S. Zou, Chem. Mater. 2003, 15, 4039–4045. J. W. Liu, J. Zheng, J. L. Wang, J. Xu, H. H. Li and S. H. Yu, Nano Lett. 2013, 13, 3589. G. C. Xi, S. X. Ouyang, P. Li, J. H. Ye, Q. Ma, N. Su, H. Bai, and C. Wang, Angew. Chem. Int. Ed. 2012, 51, 2395. J. C. Liu, O. Margeat, W. Dachraoui, X. J. Liu, M. Fahlman and J. Ackermann, Adv. Funct. Mater. 2014, 24, 6029. J. Q. Tan, T. Wang, G. J. Wu, W. L. Dai, N. J. Guan, L. D. Li and J. L. Gong, Adv. Mater. 2015, 27, 1580. M. S. Bazarjani, M. Hojamberdiev, K. Morita, G. Q. Zhu, G. Cherkashinin, C. Fasel, T. Herrmann, H. Breitzke, A. Gurlo and R. Riedel, J. Am. Chem. Soc. 2013, 135, 4467. Y. M. Hsu, C. Y. Wang, P. Chang, T. R. Yew, Nanoscale, 2015, 21, 901-907. J. He, H. L. Liu, B. Xu and X. Wang, Small, 2015, 11, 1144. Z. G. Chen, Q. Wang, H. L. Wang, L. S. Zhang, G. S. Song, L. L. Song, J. Q. Hu, H. Z. Wang, J. S. Liu, M. F. Zhu and D. Y. Zhao, Adv. Mater. 2013, 25, 2095. K. Fuku, R. Hayashi, S. Takakura, T. Kamegawa, K. Mori, H. Yamashita, Angew. Chem. Int. Ed. 2013, 52, 7446. J. H. Liu, J. G. Han, Z. C. Kang, R. Golamaully, N. N. Xu, H. P. Li and X. L. Han, Nanoscale, 2014, 6, 5770. B. Li, Y. X. Zhang, R. J. Zou, Q. Wang, B. J. Zhang, L. An, F. Yin, Y. Q. Hu and J. Q. Hu, Dalton Trans., 2014, 43, 6244.
Chemistry - An Asian Journal
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COMMUNICATION Entry for the Table of Contents
COMMUNICATION Plasmonic WO3-x nanowires were successfully synthesized through a surfactant-free solvothermal method, and showed five times enhancement on hydrogen generation from ammonia borane under visible light irradiation.
Zaizhu Lou, Quan Gu, Lin Xu, Yusen Liao and Can Xue* Page No. – Page No. Surfactant-Free Synthesis of Plasmonic Tungsten Oxide Nanowires with Visible-LightEnhanced Hydrogen Generation from Ammonia Borane
Authors: Can Xue; Zaizhu Lou; Quan Gu; Lin Xu; Yusen Liao
This manuscript has been accepted after peer review and the authors have elected to post their Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201500319 Link to VoR: http://dx.doi.org/10.1002/asia.201500319
A sister journal of Angewandte Chemie and Chemistry – A European Journal
Supported by
Federation of Asian Chemical Societies
Chemistry - An Asian Journal
10.1002/asia.201500319
COMMUNICATION Surfactant-Free Synthesis of Plasmonic Tungsten Oxide Nanowires with Visible-Light-Enhanced Hydrogen Generation from Ammonia Borane Zaizhu Lou,[a] Quan Gu,[a] Lin Xu,[a] Yusen Liao,[a] and Can Xue*[a] Dedication ((optional))
Abstract: WO3-x nanowires were successfully synthesized through a simple surfactant-free solvothermal method. These nanowires exhibit strong plasmonic absorption in visible and near-infrared region due to the abundant oxygen vacancies. The plasmon excitation of these WO3-x nanowires provide five times enhancement on the hydrogen generation from ammonia borane.
The localized surface plasmon resonance (LSPR) of noble metal (e.g. Ag and Au) nanoparticles, arising from coherent oscillation of surface conduction electrons, leads to strong absorption in the visible and near-infrared (NIR) region.[1,2] Researchers have demonstrated that the integration of plasmonic metal particles with semiconductors allows for enhanced visible-light-driven photocatalytic activities.[3-6] However, the high-cost of noble metals restricts large-scale practical applications. Recently, the LSPR phenomena have been observed in some nonmental nanostructures of heavily-doped semiconductors, such as WO3[7] MoO3-x,[8,9] Cu2-xS,[10] and Sn-doped In2O3.[11] The incident x, light can induce collective oscillations of the free charges on the surface of doped nonmetal nanoparticles, giving opportunities to explore low-cost noble-metal-free plasmonic photocatalysts. Nevertheless, the LSPR absorption of those nonmetal nanoparticles usually locates in the NIR region due to the low density of free charges. Due to the easy variation of valance state, the color of tungsten oxide could be changed between white and blue under light irradiation or electrical stimulation, allowing for the application in chromic devices.[12,13] More recently, some researchers have found that plasmonic tungsten oxide nanostructures exhibit intense LSPR absorption in both visible and NIR region with plasmon-enhanced photocatalytic activities.[14-16] For instance, W18O49 nanowires were prepared by WCl6 in ethanol via a hydrothermal process, and capable for photo-reduction of CO2 under irradiation.14 By using oleylamine as shape-control surfactants, W18O49 nanowires with aspect ratio dependent photocatalytic activity were synthesized and investigated.[15] Tungsten oxide nanosheets made by alcohothermal exfoliation showed enhanced performance for photocurrent response and photocatalytic water oxidization due
[a]
Dr. Z. Z. Lou, Dr. Q. Gu, Dr. L. Xu, Y. S. Liao and Prof. C. Xue School of Material Science & Engineering Nanyang Technological University Nanyang Avernue, Singapore 639798 E-mail: [email protected]
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
to their LSPR.[16] It has been suggested that the oxygen vacancies on tungsten oxide surfaces are responsible to the plasmon-driven activities under irradiation of visible and NIR light.[17-19] Some reports have demonstrated that LSPR can enhance the photocatalytic acitivity of plasmonic tungsten oxides in the full spectum (UV-Vis-NIR) region.[14-16] Nevertheless, the applications of plasmonic tungsten oxides through their LSPR excitation still need further exploration. Herein, in this work, we present a simple one-step solvothermal method using W(CO)6 without any surfactants to synthesize blue WO 3-x nanowires that exhibited strong plasmonic absorption in the visible and NIR region due to the abundant oxygen vacancies on the nanowire surfaces. We also demonstrated firstly that catalytic activities of WO3-x nanowires for hydrogen generation from ammonia borane could be greatly enhanced upon LSPR excitation of the WO3-x nanowires.
Figure 1. a) UV-vis absorption spectra of the product; b) XRD patterns of the product and standard tungsten oxide (W 18O49, JCPDS no. 71-2450); c) TEM image of the sample (WO3-x-160); d) HRTEM image of the sample (WO3-x-160).
The WO3-x nanowires were synthesized by using W(CO) 6 as the precursor dissolved in ethanol with solvothermal treatment at 160 oC for 12 hours. The product appeared as blue solution with uniform dispersion of tungsten oxide (Figure 1). The blue color can be attributed to the abundant oxygen vacancies on the tungsten oxide surfaces, which induces intense LSPR absorption starting from ~500 nm. Upon treatment with H2O2, these oxygen vacancies could be passivated, and as a result, the visible-light absorption disappears as shown in Figure 1a.
Chemistry - An Asian Journal
10.1002/asia.201500319
COMMUNICATION The X-ray diffraction (XRD) pattern (Figure 1b) of obtained samples can be indexed as monoclinic W18O49 (JCPDS No. 712450). The crystal structures of W 18O49 is shown in the inset of Fig. 1b with a=18.334, b=3.786, c=14.044, and β=115.2. The strong (010) peak indicates the preferential growth of tungsten oxide along direction. Although the obtained nanowires have the same crystal structures with W 18O49, some oxygen vacancies will be passivated by air during washing and drying processes. Furthermore, the amount of oxygen vacancies is not directly correlated with the crystal structure of the obtained nanowires. As shown in figure S1, the sample treatment by H2O2 completely quenches the LSPR absorption, indicating the significantly reduced oxygen vacancies, but it does not induce observable changes in the XRD pattern. Similar results were also reported by other researchers.[7] As such, the obtained tungsten oxide nanowires can be labeled WO3-x, and the x value can be measured by XPS as discussed in blow. The transmission electron microscopy (TEM) image (Figure 1c) reveals that the obtained tungsten oxide samples are nanowires with ~10 nm diameter and lengths of several hundred nanometers to a few micrometers. The high-resolution TEM (HRTEM) analysis shows clear lattice fringes with interplanar dspacing of 0.376 nm (Figure 1d and S2), which further confirms the nanowire growth along direction.[20]
Figure 2. Formation scheme of WO3-x nanowires
The growth process of WO3-x nanowires can be illustrated in Figure 2. The dissolution of W(CO)6 in ethanol leads to W(OCH2CH3) precursors that are converted to tungsten oxide seeds upon solvothermal treatment. The continuous growth of the seeds along direction results in tungsten oxides nanowires.[15] Note that the tungsten oxide nanowires at high concentration intend to form bundle-like structures (S3 and S4 of supporting information) due to the surfactant-free surface and strong van der waals attraction between the nanowires. The effect of reaction temperature was also explored. When the solvothermal temperature is decreased from 160 to 120 oC, we obtained less quantity of tungsten oxide product which still appeared as nanowires with ~10 nm diameters (Figure 3a-c, S5 and S6). However, Figure 3d indicates that the LSPR absorption of the sample prepared at 120 oC and 140 oC appeared significantly weaker than that of the sample prepared at 160 oC, suggesting that the reaction temperature plays a critical role on the amount of oxygen vacancies on nanowires.
Figure 3. TEM images of tungsten oxides with different temperature: a) 120 oC, b) 140 oC and c) 160 oC; d) UV-vis light spectra of above samples.
Figure 4. W 4f XPS spectrums of commercial WO3 (a) and WO3-x-160 (b).
To clarify the chemical composition of the WO3-x nanowires and values of x, we have characterized the sample by X-ray photoelectron spectroscopy (XPS). For the commercial WO3 sample, Figure 4a indicates two binding energy peaks of W4f at 37.9 eV and 35.9 eV, corresponding to W 6+ 4f5/2 and 4f7/2, respectively. In comparison, the W4f XPS spectra of WO 3-x nanowires (Figure 4b) can be deconvoluted into two pairs of peaks. The two peaks at 37.9 eV and 35.9 eV are contributed by 4f5/2 and 4f7/2 of W 6+, and the other two peaks at 37.2 eV and 35.0 eV are attributed to 4f5/2 and 4f7/2 of W 5+. The quantitative analyses based on the XPS spectra suggest that the W 5+ cation accounts for 26% of total W states in the WO 3-x nanowires, W(0.26)5+W(0.74)6+O3-x, and the average oxidization state of W is about 5.74. Based on charge balance, the chemical composition of the prepared WO3-x nanowires can be estimated as WO2.87 and the x value is about 0.13. This result further confirms the existing of massive oxygen vacancies that enhance the density
Chemistry - An Asian Journal
10.1002/asia.201500319
COMMUNICATION of free electrons and lead to strong plasmon absorption in the visible and NIR region.
Figure 5. H2 evolution rate from ammonia borane over different catalysts: comparison between no catalyst, commercial WO3 and WO3-x -160 in dark (a) and under light irradiation (b); comparison between WO3-x -160, WO3-x-140 and WO3-x-120 in dark (c) and under light irradiation (d).
The plasmonic energy has been proved as an efficient strategy for catalytic hydrogen evolution from ammonia borane (BH3NH3),[9, 21] which is an important hydrogen storage material. As such, we have evaluated the capability of the plasmon absorption of WO3-x nanowires for enhancing hydrogen generation from BH3NH3. As shown in Figure 5a, without tungsten oxide catalyst, H2 evolution from BH3NH3 was hardly observed in dark. While the presence of WO3-x nanowires and commercial WO3 led to H2 evolution of 15.8 μmol and 9.5 μmol, respectively, in 60 minutes. The higher efficiency of WO3-x nanowires in dark might be attributed to the more surface active sites as compared to commercial WO3. Under visible light (>420 nm) irradiation, the commercial WO3 only showed 1.6 times enhancement on H2 evolution (14.8 μmol in 60 min) as compared to that in dark. In contrast, surprisingly, we observed 5 times enhanced H2 evolution rate (79.5 μmol in 60 min) over WO3-x nanowires under visible light irradiation versus dark, as shown in Figure 5b. Such a larger enhancement is attributed to the strong LSPRs absorption of the WO3-x nanowires in the visible and NIR region. Moreover, the preparation temperature of WO3-x nanowires influences their catalytic activities. Higher reaction temperature would induce more oxygen vacancies, which leads to more crystal defects serving as active sites for reactions on the surface of WO3-x nanowires. This is evidenced by Figure 5c, showing that the H2 evolution rates over the WO3-x nanowires prepared at lower temperature are in order of WO 3-x-120 < WO3x-140 < WO3-x-160 under dark. Under visible light irradiation, the H2 evolution rates also followed the same trend (Figure 5d) due
to the less oxygen vacancies on the WO3-x nanowires at lower preparation temperature and thereby weaker LSPR absorption. Nevertheless, all above results have demonstrated that the catalytic efficiency for hydrogen evolution on plasmonic WO 3-x nanowires can be greatly enhanced by LSPR excitation. To further verify the LSPR effect on hydrogen generation, we also tested the activity of the oxidized WO3-x nanowires (after H2O2 treatment) that do not show plasmon absorption because the surface oxygen vacancies have been passivated by H 2O2. As shown in figure S7 of supporting information, the H2 generation rate over the oxidized WO3-x nanowires is much lower than that over plasmonic WO3-x nanowires under visible light irradiation, which confirms the critical role of LSPR excitation to the enhanced H2 generation.
Figure 6. Mechanism of LSPRs enhanced catalytic H2 evolution from BH3NH3 against WO3-x nanowires.
The possible principle of the plasmon-enhanced H2 evolution over WO3-x nanowires is illustrated in Figure 6. On the surface of WO3-x nanowires, massive oxygen vacancies are created during the solvothermal process, leading to heavily self-doped tungsten oxides. These vacancies provide free electrons that can be oscillating upon excitation of incident light. Similar with LSPR of noble metal nanoparticles, the coherent oscillation of these free electrons leads to intensive light absorption. The intensity and width of the plasmon absorption are dependent on the density of free electrons and morphologies of nonmetal particles. Due to the large length, the prepared WO3-x nanowires exhibit wide LSPRs absorption from 500 to 1500 nm. The irradiation of visible and NIR light can induce two processes on the surface of WO3-x nanowires. First, charge carriers can be generated by “hot” plasmonic electrons of WO3-x nanowires,[9] and enhance the charge transfer between the reagents for hydrogen evolution on the WO3-x nanowire surface. Second, photothermal effect arising from the non-radiative decay of electron oscillations would supply addition localized heating on the WO3-x nanowire surface,[22,23] and thereby increase reaction rate for more efficient H2 evolution. In addition, the surface active sites generated by the solvothermal process under high temperature and pressure also facilitate the reaction of ammonia borane and water. Therefore, the overall enhancement on H 2 evolution over the WO3-x nanowires is attributed to strong plasmon absorption and active sites for surface reaction. In summary, plasmonic WO3-x nanowires were successfully prepared by a surfactant-free solvothermal process which
Chemistry - An Asian Journal
10.1002/asia.201500319
COMMUNICATION generates massive oxygen vacancies on WO3-x nanowires. As a result, the obtained WO3-x nanowires exhibit intensive LSPR absorption in the visible and NIR region. We have also demonstrated that the plasmon excitation of WO3-x nanowires provides five times enhancement on the hydrogen evolution from ammonia borane.
MOE AcRF-Tier2 (MOE2012-T2-2-041, ARC 5/13), and CRP (NRF-CRP5-2009-04) from NRF Singapore. Keywords: photocatalysis • tungsten oxides • surface plasmon resonance • hydrogen generation • ammonia borane [1]
Experimental Section
[2]
All reagents were analytical grade. Hexacarbonyltungsten W(CO)6, ethanol, ammonia borane were purchased from Sigma-Aldrich and used without any further purification.
[3]
Synthesis of WO3-x nanowires: In a typical procedure, 30 mg hexacarbonyltungsten was added into 40 ml absolute ethanol with constant stirring. When the solid was dissolved completely, yellow transparent solution was obtained as the precursor solution. Then, the solution was transferred to an 80 mL Teflon-lined stainless steel autoclave, heated up to 160 oC and kept for 12 hours. The produced samples were separated from solution by centrifugation, washed with ethanol for three times, and dried in a vacuum oven. The preparation of samples at different temperature followed the same procedures. Hydrogen evolution reaction: The hydrogen gas was generated through degradation of ammonia borane by using WO3-x nanowires as catalysts. Typically, 10 mg WO3-x nanowires were dispersed into 10 ml deionized water, and the suspension was placed in a photocatalytic reactor (80 mL) under constant stirring. The reactor was then sealed and degassed by nitrogen for 10 min. Subsequently, 2 ml DI water containing 2 mg ammonia borane was added into the solution. The generated hydrogen was periodically analyzed by Agilent 7890A gas chromatograph (GC) with TCD detector. For the H2 evolution under illumination, the reactor was exposed under a 300-W Xe lamp (MAX-302, Asahi Spectra Co. Ltd.) coupled with a UV cut-off filter ( > 420 nm, output light density of 200 mW/cm2). Characterization: The X-ray diffraction (XRD) patterns were obtained on Bruker D8-advanced X-ray powder diffractometer with Cu Kα radiation λ=1.5418 Å. The scanning electron microscopy (SEM) measurements were carried out on a JEOL JSM-7600F microscope. Transmission Electron Microscopy (TEM) measurements were carried out on a JEOL JEM-2010 microscope. UV-Vis-NIR absorption spectra were recorded on a Cary 5000 UV-Vis-NIR Spectrometer (Agilent Technologies). X-ray photoelectron spectroscopy was performed on the VG ESCALAB 220IXL XPS system (Thermo VG Scientific Ltd., UK).
[4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15] [16] [17]
[18] [19] [20]
[21]
Acknowledgements The authors acknowledge the financial support from NTU seed funding for Solar Fuels Laboratory, MOE AcRF-Tier1 (RG 44/11),
[22] [23]
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Chemistry - An Asian Journal
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COMMUNICATION Entry for the Table of Contents
COMMUNICATION Plasmonic WO3-x nanowires were successfully synthesized through a surfactant-free solvothermal method, and showed five times enhancement on hydrogen generation from ammonia borane under visible light irradiation.
Zaizhu Lou, Quan Gu, Lin Xu, Yusen Liao and Can Xue* Page No. – Page No. Surfactant-Free Synthesis of Plasmonic Tungsten Oxide Nanowires with Visible-LightEnhanced Hydrogen Generation from Ammonia Borane
