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Gold clusters on WO3 nanoneedles grown via AACVD: XPS and TEM studies
Materials Chemistry and Physics 134 (2012) 809e813
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Gold clusters on WO3 nanoneedles grown via AACVD: XPS and TEM studies Cristina Navío a, Stella Vallejos b, c, Toni Stoycheva b, Eduard Llobet b, Xavier Correig b, Rony Snyders a, Christopher Blackman c, Polona Umek d, Xiaoxing Ke e, Gustaaf Van Tendeloo e, Carla Bittencourt e, * a
Laboratory of Interaction Chemistry on Plasma Surfaces, University of Mons, Belgium MINOS, EMaS, Departament d’Enginyeria Electronica, Universitat Rovira i Virgili, Tarragona, Spain c Department of Chemistry, University College London, London, WC1H 0AJ, UK d Solid State Physcis Department Jozef Stefan Institute, Jamov cesta 39, 1000 Ljubljana, Slovenia e Electron Microscopy for Material Science, University of Antwerp, Antwerp, Belgium b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2011 Received in revised form 16 February 2012 Accepted 13 March 2012
We have prepared tungsten oxide films decorated with gold particles on Si substrates by aerosol assisted chemical vapor deposition (AACVD) and characterized them using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). SEM shows that the films are composed of needle-like structures and TEM shows that both the needles and the gold particles are crystalline. XPS indicates the presence of oxygen vacancies, i.e. the films are WO3x, and hence the deposited material is composed of semiconducting nanostructures and that the interaction between the gold particles and the WO3 needles surface is weak. The synthesis of semiconducting tungsten oxide nanostructures decorated with metal particles represents an important step towards the development of sensing devices with optimal properties. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: AACVD Oxide nanostructures Gas sensing Gold clusters
1. Introduction Transition metal oxides have electronic and chemical properties that can be fine tuned during their synthesis; this flexibility makes them outstanding candidates for a wide range of applications [1e6]. In this context, tungsten oxide films have already proved their applicability as active material in diverse applications including electrochromic devices and gas sensing, and much effort has been directed at optimizing the performance of tungsten oxide films in sensing devices [6e16]. Compared to the traditional 3D, thin films tungsten oxide low-dimensional structures exhibit higher surface to volume ratio and particular phase structures that attract great interest in gas sensing applications [17e21]. Additionally, low-dimensional metal oxides are often single crystalline nanostructures with well-defined catalytic properties, which helps in gaining insight into the gas sensing mechanisms of such nanomaterials. Consequently their synthesis and characterization has been the focus of much research in recent years. Several synthesis methods of tungsten oxide nanostructured films have been reported [22e26], among them aerosol assisted chemical vapor
* Corresponding author. Tel.: þ32 32653695; fax: þ32 32653318. E-mail address: [email protected] (C. Bittencourt). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.073
deposition (AACVD) has been shown to produce films formed by needle-like nanostructures with remarkable sensing properties significantly exceeding those of commercially available metal oxide sensors [27]. Although, pure metal oxide active layer sensors can have high sensitivity, they have typically poor selectivity. To overcome this drawback noble metals that act as catalysts can be added to the metal oxide active layer [15,28e30]. Several methods have been used to add catalytic active dopants to the sensing active material, in some of them the dopants are inserted within the bulk of the sensing active material and in others, often used, the dopants are deposited on the top of the sensing layer [1,7]. In this context, gold has been used regularly as an electronic or chemical sensitizer in tungsten oxide films for gas sensing. Yamazoe and co-workers showed that loading tungsten oxide films at 0.8% wt. with Au resulted in a highly enhanced sensitivity to ammonia vapors [31]. Gold loading has also been reported beneficial for enhancing sensitivity to nitrogen oxides [32] or hydrogen sulphide [28]. Haruta and co-workers reported that in the case of AueWO3 composite films, the combination of Au made such films sensitive mainly at the plasmon absorption region around visible wavelengths, which enhanced the optical detection of H2 [33]. Recently, we reported the co-growth using AACVD of preformed gold nanoparticles and gold precursors into dense-like MOX film [34,35]. These methods lead to
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the growth, in a single step, of metal oxide nanoneedles having their surface decorated with gold nanoparticles. Herein, we report on the characterization of Au-decorated WO3 needle like nanostructures deposited using AACVD. The WO3 films and the nature of the Au/WO3 interface were characterized using Xray photoelectron spectroscopy (XPS), transmission (TEM) and scanning electron (SEM) microscopy. The nature of the Au/WO3 interface strongly impacts in the electrical transparence of the Au/ WO3. 2. Experimental WO3 and Au/WO3 nanoneedles were grown on Si substrates by Aerosol Assisted Chemical Vapor Deposition (AACVD) at 500 C and 450 C respectively, details have been reported elsewhere [27]. Briefly, tungsten hexaphenoxide [W(OPh)6] and hydrogen tetrachloroaurate (III) [HAuCl4], were used as precursor materials. Precursors, in the form of an aerosol, were carried into the CVD reactor by a nitrogen flow. The chemical composition of the samples was investigated using X-ray photoelectron spectroscopy (XPS), VERSAPROBE PHI 5000 from Physical Electronics, equipped with a Monochromatic Al Ka X-Ray. The energy resolution was 0.6 eV. For the compensation of built up charge on the sample surface during the measurements, a dual beam charge neutralization composed of an electron gun (w1 eV) and the Argon Ion gun (10 eV) was used. All binding energies were calibrated to the C 1s peak at 284.3 eV. To support the interpretation of the XPS spectra, three standard samples were analyzed from Alfa Aesar: Tungsten (IV) oxide 99.9%, Tungsten (VI) oxide 99.8% and Au foil. To observe the morphology of the WO3 nanoneedles a FEI Nova 200 Nanolab Dual Beam SEM/FIB system was used while the microstructure of the WO3 nanoneedles was studied using a JEOL JEM-2100 transmission electron microscope (TEM). 3. Results and discussion Fig. 1 shows SEM images of the WO3 film grown on the Si substrate. A high density of non-aligned needles was observed both in the WO3 and Au/WO3 samples with similar morphological features. Further information about the structure of the nanoneedles was obtained by TEM (Fig. 2). The diameter of WO3
Fig. 1. SEM image of WO3 nanoneedles deposited via AACVD on Silicon substrate.
nanoneedles was found in the range of 15e60 nm. The nanoneedles are monophasic; clear lattice fringes can be observed in two insets in Fig. 2 with interplanar spacing is 0.35e0.37 nm consistent with an internal order of the WO3 nanoneedles in the [001] (0.5d ¼ 0.3650 nm) or [020] (0.5d ¼ 0.3770 nm) directions. The upper inset of Fig. 2, shows a typical high resolution image of Au nanoparticles, it can be observed a family of planes with lattice spacing of 2.4 Å, corresponding to (111) planes. In the inset of Fig. 2, we can observe that the atomic planes under the particles are not affected by the presence of the particle suggesting a weak interaction between particle and the surface of the WO3. The nanoneedle surface is homogeneously covered with well-dispersed quasi-spherical gold nanoparticles with diameters between 9 and 25 nm (Fig. 3). To study the nature of the chemical bonding at the interface Au/ WO3 we used XPS. The analysis of both core and valence (bonding electrons) electronic states gives information on the chemical nature of the samples. The fundamental phenomenon underlying the XPS is the photoelectric effect that describes the ejection of electrons from a surface when photons impinge upon it. The kinetic energy (or binding energy) of the ejected electron is characteristic of each element and its chemical state. The W 4f and W 5p3/2 core level spectra were recorded using XPS on a standard WO3 powder sample (Fig. 4a), on WO3 nanoneedles grown on Si (Fig. 4b) and the co-deposited Au nanoparticles and WO3 nanoneedles grown on Si (Fig. 4c). According to the reported procedure for fitting the spinorbit doublet of W 4f [36e39], to reproduce the nanoneedles spectra, 3 doublets were used to fit the W 4f core level and one Gaussian for the W 5p3/2. The FWHM of the W 4f peaks recorded on the WO3 needles and Au/WO3 (Fig. 4b and c respectively) were larger than the ones recorded on the reference powder. The main doublet (W 4f7/2) is centered at 35.4 eV, it is generated by photoelectrons emitted from W atoms with oxidation state þ6 (WO3). The second doublet (W 4f7/2) centered at 34.4 eV can be attributed to photoelectrons emitted from tungsten atoms near oxygen vacancies, where the oxidation state of W is lower than þ6 (substoichiometric WO3-x). This component has a weak contribution in the spectrum recorded on the WO3 standard (w9%, Fig. 4a), conversely on the spectrum recorded on the WO3 nanoneedles (Fig. 4b and c) the area under this component correspond to 27% of the total W 4f peak suggesting a higher concentration of oxygen vacancies. The third doublet used to reproduce the W 4f spectrum is centered (W 4f7/2) at 36.2 eV e the origin of this component was reported to be associated to local variations in the vacuum-level energy (Evl) caused by surface defects [40]. Photoelectrons emitted from the same chemical environment will experience different values of Evl in traversing the surface, and will arrive at the analyzer with a different kinetic energy. The contribution of surface defects can be expected to be quantitatively important in an XPS spectrum recorded using energy of excitation in the order of the used one (1486.4 eV). The third doublet has the same contribution to the total intensity of the W 4f spectrum recorded on the WO3 nanoneedles and Au/WO3 nanoneedles sample suggesting a weak interaction of the Au clusters with the WO3 needles. It has been shown that, in some oxides doped with metal atoms, under certain conditions the metal atoms can diffuse and localize preferentially at defect sites such as oxygen vacancies [40]. If this is the case, it is expected that Au particles nucleate on these sites, thus the photoelectrons emitted from W atoms beneath the gold particles would experience inelastic losses when passing through the metal particle, and thus no longer contribute to the W 4f peak. Therefore, the absence of a measurable reduction in the relative intensity of the component at 36.2 eV suggests that the gold particle distribution is not mediated by the presence of defects at the nanoneedles surface. Further studies will be performed aiming at determining
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Fig. 2. TEM image of a WO3. (a) The inset shows the crystalline structure of both gold particles and WO3 needles. (b) The atomic planes under the particle are not affected by its presence.
the mechanism that causes the intrinsic spatial particle distribution at the WO3 needle surface in the AACVD synthesis. The Au 4f core level spectrum recorded on the Au/WO3 at 84 eV (Fig. 5) is similar to the one reported for the Au crystal indicating that the codeposited Au particles are in the metallic state [41]. The absence of new structures in the Au 4f spectrum testifies our assertion of weak interaction at the gold/WO3 interface. In Fig. 6 the O 1s peaks are shown. Besides the small chemical shift from the powder reference sample to the pristine WO3 nanoneedles (also observed in the W 4f peak), a second component of oxygen centered at 533 eV is present in the case where Au nanoparticles are co-deposited. Due to the large area of that component (1/3 of the main peak) and the lack of additional structures on the W 4f core level that could suggest another oxidation state for the tungsten, the origin of this peak can be assigned to contamination on the sample. In fact, the carbon concentration in the Au/WO3 samples was found to be higher than on the WO3 samples suggesting the presence of organic fragments probably due to the lower temperature used during the synthesis.
Fig. 3. TEM image of WO3 nanoneedles decorated with gold particles (dark dots).
The valence electronic states of the samples can be seen in Fig. 7. The WO3 reference sample (blue line) and WO3 nanoneedles (black line) spectra show similar features. Covalent bonding interactions transform the O 2p and the metal d-orbitals into bonding and antibonding orbitals of mixed atomic character. Reported, calculations of the valence band density of states (VB-DOS) for monoclinic WO3 assigned the peak located at about 4.1 eV to an O 2p-derived band while the peak located at about 6 eV corresponds to a hybridized ‘W 5deO 2p’ band [42,43]. The rapid onset of the VB spectrum at about 3.0 eV can be assigned to the top of the valence band, and the well-defined structure near the Fermi level (EB ¼ 0) is associated to W 5d in-gap states, whose filling is due to the
Fig. 4. XPS core levels for pristine WO3 powder (a), WO3 nanoneedles (b) and Au on WO3 nanoneedles (c). Light gray: W 4f level for stoichiometric WO3. Red: W 4f level for sub-stoichiometric WO3x. Blue: W 4f level of W atoms near surface defects. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Au 4f core level of the gold particles at the WO3 nanoneedle.
presence of oxygen vacancies [37]. In stoichiometric WO3 samples, the 5d-derived states near the gap are empty and no states are observable at the Fermi level [37,40,42,43]. However, if there are oxygen vacancies or lower valence WOx (x 2) states as well as W (0) not all the ‘5d-electrons’ can be transferred and the 5d-derived states near the gap are populated. The presence of a structure near the Fermi level (not expected for an insulator) suggests that there are sub-oxides or oxygen vacancies, which support our assertion of the component at binding energy equal to 36.2 eV to the presence of defects (black component in Fig. 6). Comparing the spectra of the valence band recorded on the standard samples and on the pristine WO3 needles, it can be seen that the structure at the Fermi level is more pronounced in the needles spectrum confirming our assertion that the needles have more oxygen vacancies. The loss of oxygen atoms leaves behind extra electrons resulting in a material behaving as n-type semiconductor. The presence of tungsten oxide sub-oxides and surface defects (i.e. oxygen vacancies) is of interest for tungsten oxide based chemical sensors. The baseline conductance of semiconducting WO3 nanoneedles will depend on the oxygen species adsorbed at their surface defects [10e12]. The
Fig. 7. Valence band for pristine WO3 nanoneedles (black), WO3 powder (blue), Au on WO3 nanoneedles (red) and pure Au (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
presence of oxidizing or reducing species that alter the equilibrium concentration of oxygen adsorbates can be monitored via the change in electrical conductance experienced by the nanoneedles [27]. The valence band spectrum recorded on WO3 needles decorated with gold particles presents a new structure peaking at 6.0 eV and, another at 1.4 eV. The gold valence band is formed by a 5d band, characterized by the 5d doublet with weighted centers at 6.35 eV and 3.55 eV and a wide “sep” band formed mainly by 6s and 6p electrons (partially hybridized with 5d-orbitals), localized above the 5d states and below the Fermi level [41]. Thus the new observed features can be associated with photoelectrons emitted from gold atoms forming the clusters. The 5d spin orbit splitting reflects the number of Au nearest neighbors surrounding a gold atom and consequently the strength of the AueAu interactions. Thus, the observed 5d spineorbit splitting supports our assertion that the clusters are in the metallic state. 4. Conclusions In summary, using aerosol assisted chemical vapor deposition WO3 nanoneedles and Au-decorated WO3 nanoneedles can be synthesized with high yields. Both, WO3 and Au-decorated WO3 nanoneedles are single crystalline and show oxygen vacancies indicating a semiconducting behavior. Well-dispersed Au nanoparticles homogenously cover the WO3 surface. The absence of additional structures in the Au 4f peak suggests a weak interaction at the Au/WO3 interface. The well-dispersed Au nanoparticles can play an important role for sensitizing the tungsten oxide nanoneedles via a spill-over effect. The obtained nanomaterial represents an excellent model for both experimentally and theoretically studying the interaction between metal oxides nanostructures and gas molecules in view of gaining more insight in the mechanisms of gas sensing. Acknowledgments
Fig. 6. O 1s core levels for pristine WO3 nanoneedles (black), Au on WO3 nanoneedles (red) and WO3 powder (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
This work is partially supported by the Belgian Program on Interuniversity Attraction Pole (PAI 6/08) and ARC-UMONS. T.S., E.L and X.C. are supported by the Spanish Ministry of Science and Innovation and the Catalan Agency for Research under grants no.
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TEC2009-07107 and 2009 SGR 789, respectively. The support of the COST Action MP0901 “NanoTP” and the European Commission under contract ERC 246791 (COUNTATOMS) are gratefully acknowledged. References [1] B. Rajesh, V. Karthik, S. Karthikeyan, K.R. Thampi, J.M. Bonard, B. Viswanathan, Pt-WO3 supported on carbon nanotubes as possible anodes for direct methanol fuel cells, Fuel 81 (2002) 2177e2190. [2] W. Zhang, T. Yang, C.X. Yin, G.C. Li, K. Jiao, An enhanced strategy using poly(maminobenzene sulfonic acid) nanofibres for rapid detection of DNA oxidative damage induced by hollow TiO2 nanocubes, Electrochemistry Communications 11 (2009) 783e786. [3] H. Belatel, H. Al-Kandari, F. Al-Kharafi, F. Garin, A. Katrib, Catalytic reactions of methylcyclohexane (MCH), on partially reduced tungsten oxide(s), Applied Catalysis A-General 318 (2007) 227e233. [4] A. Enesca, A. Duta, J. Schoonman, Study of photoactivity of tungsten trioxide (WO3) for water splitting, Thin Solid Films 515 (2007) 6371e6374. [5] A.C. Dillon, A.H. Mahan, R. Deshpande, R. Parilla, K.M. Jones, S.H. Lee, Metal oxide nano-particles for improved electrochromic and lithium-ion battery technologies, Thin Solid Films 516 (2008) 794e797. [6] S.H. Lee, R. Deshpande, P.A. Parilla, K.M. Jones, B. To, A.H. Mahan, A.C. Dillon, Crystalline WO3 nanoparticles for highly improved electrochromic applications, Advanced Materials 18 (2006) 763. [7] C. Bittencourt, M.P. Felicissimo, A. Felten, L.A.O. Nunes, P. Ivanov, E. Llobet, J.J. Pireaux, L. Houssiau, Characterization of WO3: Ag films: ToF-SIMS studies of ammonia adsorption, Applied Surface Science 250 (2005) 21e28. [8] C. Bittencourt, A. Felten, E.H. Espinosa, R. Ionescu, E. Llobet, X. Corteig, J.J. Pireaux, WO3 films modified with functionalised multi-wall carbon nanotubes: morphological, compositional and gas response studies, Sensors and Actuators B-Chemical 115 (2006) 33e41. [9] R. Ionescu, E.H. Espinosa, R. Leghrib, A. Felten, J.J. Pireaux, R. Erni, G. Van Tendeloo, C. Bittencourt, N. Canellas, E. Llobet, Novel hybrid materials for gas sensing applications made of metal-decorated MWCNTs dispersed on nanoparticle metal oxides, Sensors and Actuators B-Chemical 131 (2008) 174e182. [10] R. Calavia, A. Mozalev, R. Vazquez, I. Gracia, C. Cane, R. Ionescu, E. Llobet, Fabrication of WO3 nanodot-based microsensors highly sensitive to hydrogen, Sensors and Actuators B-Chemical 149 (2010) 352e361. [11] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sensors and Actuators B-Chemical 5 (1991) 7e19. [12] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, Journal of Electroceramics 7 (2001) 143e167. [13] A. Rothschild, Y. Komem, The effect of grain size on the sensitivity of nanocrystalline metal-oxide gas sensors, Journal of Applied Physics 95 (2004) 6374e6380. [14] V. Khatko, S. Vallejos, J. Calderer, I. Gracia, C. Cane, E. Llobet, X. Correig, Micromachined WO3-based sensors with improved characteristics, Sensors and Actuators B-Chemical 140 (2009) 356e362. [15] M.E. Franke, T.J. Koplin, U. Simon, Metal and metal oxide nanoparticles in chemiresistors: does the nanoscale matter? Small 2 (2006) 36e50. [16] J. Tamaki, Z. Zhang, K. Fujimori, M. Akiyama, T. Harada, N. Miura, N. Yamazoe, Grain-size effects in tungsten oxide-based sensor for nitrogen-oxides, Journal of the Electrochemical Society 141 (1994) 2207e2210. [17] A. Kolmakov, M. Moskovits, Chemical sensing and catalysis by onedimensional metal-oxide nanostructures, Annual Review of Materials Research 34 (2004) 151e180. [18] B. Deb, S. Desai, G.U. Sumanasekera, M.K. Sunkara, Gas sensing behaviour of matlike networked tungsten oxide nanowire thin films, Nanotechnology 18 (2007). [19] C. Po-Chiang, S. Guozhen, Z. Chongwu, Chemical sensors and electronic noses based on 1-D metal oxide nanostructures, IEEE Transactions on Nanotechnology (2008) 668e682. [20] J. Huang, Q. Wan, Gas sensors based on semiconducting metal oxide onedimensional nanostructures, Sensors 9 (2009) 9903e9924. [21] A. Vomiero, A. Ponzoni, E. Comini, M. Ferroni, G. Faglia, G. Sberveglieri, Direct integration of metal oxide nanowires into an effective gas sensing device, Nanotechnology 21 (2010) 145502e145508.
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Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Gold clusters on WO3 nanoneedles grown via AACVD: XPS and TEM studies Cristina Navío a, Stella Vallejos b, c, Toni Stoycheva b, Eduard Llobet b, Xavier Correig b, Rony Snyders a, Christopher Blackman c, Polona Umek d, Xiaoxing Ke e, Gustaaf Van Tendeloo e, Carla Bittencourt e, * a
Laboratory of Interaction Chemistry on Plasma Surfaces, University of Mons, Belgium MINOS, EMaS, Departament d’Enginyeria Electronica, Universitat Rovira i Virgili, Tarragona, Spain c Department of Chemistry, University College London, London, WC1H 0AJ, UK d Solid State Physcis Department Jozef Stefan Institute, Jamov cesta 39, 1000 Ljubljana, Slovenia e Electron Microscopy for Material Science, University of Antwerp, Antwerp, Belgium b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2011 Received in revised form 16 February 2012 Accepted 13 March 2012
We have prepared tungsten oxide films decorated with gold particles on Si substrates by aerosol assisted chemical vapor deposition (AACVD) and characterized them using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). SEM shows that the films are composed of needle-like structures and TEM shows that both the needles and the gold particles are crystalline. XPS indicates the presence of oxygen vacancies, i.e. the films are WO3x, and hence the deposited material is composed of semiconducting nanostructures and that the interaction between the gold particles and the WO3 needles surface is weak. The synthesis of semiconducting tungsten oxide nanostructures decorated with metal particles represents an important step towards the development of sensing devices with optimal properties. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: AACVD Oxide nanostructures Gas sensing Gold clusters
1. Introduction Transition metal oxides have electronic and chemical properties that can be fine tuned during their synthesis; this flexibility makes them outstanding candidates for a wide range of applications [1e6]. In this context, tungsten oxide films have already proved their applicability as active material in diverse applications including electrochromic devices and gas sensing, and much effort has been directed at optimizing the performance of tungsten oxide films in sensing devices [6e16]. Compared to the traditional 3D, thin films tungsten oxide low-dimensional structures exhibit higher surface to volume ratio and particular phase structures that attract great interest in gas sensing applications [17e21]. Additionally, low-dimensional metal oxides are often single crystalline nanostructures with well-defined catalytic properties, which helps in gaining insight into the gas sensing mechanisms of such nanomaterials. Consequently their synthesis and characterization has been the focus of much research in recent years. Several synthesis methods of tungsten oxide nanostructured films have been reported [22e26], among them aerosol assisted chemical vapor
* Corresponding author. Tel.: þ32 32653695; fax: þ32 32653318. E-mail address: [email protected] (C. Bittencourt). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.073
deposition (AACVD) has been shown to produce films formed by needle-like nanostructures with remarkable sensing properties significantly exceeding those of commercially available metal oxide sensors [27]. Although, pure metal oxide active layer sensors can have high sensitivity, they have typically poor selectivity. To overcome this drawback noble metals that act as catalysts can be added to the metal oxide active layer [15,28e30]. Several methods have been used to add catalytic active dopants to the sensing active material, in some of them the dopants are inserted within the bulk of the sensing active material and in others, often used, the dopants are deposited on the top of the sensing layer [1,7]. In this context, gold has been used regularly as an electronic or chemical sensitizer in tungsten oxide films for gas sensing. Yamazoe and co-workers showed that loading tungsten oxide films at 0.8% wt. with Au resulted in a highly enhanced sensitivity to ammonia vapors [31]. Gold loading has also been reported beneficial for enhancing sensitivity to nitrogen oxides [32] or hydrogen sulphide [28]. Haruta and co-workers reported that in the case of AueWO3 composite films, the combination of Au made such films sensitive mainly at the plasmon absorption region around visible wavelengths, which enhanced the optical detection of H2 [33]. Recently, we reported the co-growth using AACVD of preformed gold nanoparticles and gold precursors into dense-like MOX film [34,35]. These methods lead to
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the growth, in a single step, of metal oxide nanoneedles having their surface decorated with gold nanoparticles. Herein, we report on the characterization of Au-decorated WO3 needle like nanostructures deposited using AACVD. The WO3 films and the nature of the Au/WO3 interface were characterized using Xray photoelectron spectroscopy (XPS), transmission (TEM) and scanning electron (SEM) microscopy. The nature of the Au/WO3 interface strongly impacts in the electrical transparence of the Au/ WO3. 2. Experimental WO3 and Au/WO3 nanoneedles were grown on Si substrates by Aerosol Assisted Chemical Vapor Deposition (AACVD) at 500 C and 450 C respectively, details have been reported elsewhere [27]. Briefly, tungsten hexaphenoxide [W(OPh)6] and hydrogen tetrachloroaurate (III) [HAuCl4], were used as precursor materials. Precursors, in the form of an aerosol, were carried into the CVD reactor by a nitrogen flow. The chemical composition of the samples was investigated using X-ray photoelectron spectroscopy (XPS), VERSAPROBE PHI 5000 from Physical Electronics, equipped with a Monochromatic Al Ka X-Ray. The energy resolution was 0.6 eV. For the compensation of built up charge on the sample surface during the measurements, a dual beam charge neutralization composed of an electron gun (w1 eV) and the Argon Ion gun (10 eV) was used. All binding energies were calibrated to the C 1s peak at 284.3 eV. To support the interpretation of the XPS spectra, three standard samples were analyzed from Alfa Aesar: Tungsten (IV) oxide 99.9%, Tungsten (VI) oxide 99.8% and Au foil. To observe the morphology of the WO3 nanoneedles a FEI Nova 200 Nanolab Dual Beam SEM/FIB system was used while the microstructure of the WO3 nanoneedles was studied using a JEOL JEM-2100 transmission electron microscope (TEM). 3. Results and discussion Fig. 1 shows SEM images of the WO3 film grown on the Si substrate. A high density of non-aligned needles was observed both in the WO3 and Au/WO3 samples with similar morphological features. Further information about the structure of the nanoneedles was obtained by TEM (Fig. 2). The diameter of WO3
Fig. 1. SEM image of WO3 nanoneedles deposited via AACVD on Silicon substrate.
nanoneedles was found in the range of 15e60 nm. The nanoneedles are monophasic; clear lattice fringes can be observed in two insets in Fig. 2 with interplanar spacing is 0.35e0.37 nm consistent with an internal order of the WO3 nanoneedles in the [001] (0.5d ¼ 0.3650 nm) or [020] (0.5d ¼ 0.3770 nm) directions. The upper inset of Fig. 2, shows a typical high resolution image of Au nanoparticles, it can be observed a family of planes with lattice spacing of 2.4 Å, corresponding to (111) planes. In the inset of Fig. 2, we can observe that the atomic planes under the particles are not affected by the presence of the particle suggesting a weak interaction between particle and the surface of the WO3. The nanoneedle surface is homogeneously covered with well-dispersed quasi-spherical gold nanoparticles with diameters between 9 and 25 nm (Fig. 3). To study the nature of the chemical bonding at the interface Au/ WO3 we used XPS. The analysis of both core and valence (bonding electrons) electronic states gives information on the chemical nature of the samples. The fundamental phenomenon underlying the XPS is the photoelectric effect that describes the ejection of electrons from a surface when photons impinge upon it. The kinetic energy (or binding energy) of the ejected electron is characteristic of each element and its chemical state. The W 4f and W 5p3/2 core level spectra were recorded using XPS on a standard WO3 powder sample (Fig. 4a), on WO3 nanoneedles grown on Si (Fig. 4b) and the co-deposited Au nanoparticles and WO3 nanoneedles grown on Si (Fig. 4c). According to the reported procedure for fitting the spinorbit doublet of W 4f [36e39], to reproduce the nanoneedles spectra, 3 doublets were used to fit the W 4f core level and one Gaussian for the W 5p3/2. The FWHM of the W 4f peaks recorded on the WO3 needles and Au/WO3 (Fig. 4b and c respectively) were larger than the ones recorded on the reference powder. The main doublet (W 4f7/2) is centered at 35.4 eV, it is generated by photoelectrons emitted from W atoms with oxidation state þ6 (WO3). The second doublet (W 4f7/2) centered at 34.4 eV can be attributed to photoelectrons emitted from tungsten atoms near oxygen vacancies, where the oxidation state of W is lower than þ6 (substoichiometric WO3-x). This component has a weak contribution in the spectrum recorded on the WO3 standard (w9%, Fig. 4a), conversely on the spectrum recorded on the WO3 nanoneedles (Fig. 4b and c) the area under this component correspond to 27% of the total W 4f peak suggesting a higher concentration of oxygen vacancies. The third doublet used to reproduce the W 4f spectrum is centered (W 4f7/2) at 36.2 eV e the origin of this component was reported to be associated to local variations in the vacuum-level energy (Evl) caused by surface defects [40]. Photoelectrons emitted from the same chemical environment will experience different values of Evl in traversing the surface, and will arrive at the analyzer with a different kinetic energy. The contribution of surface defects can be expected to be quantitatively important in an XPS spectrum recorded using energy of excitation in the order of the used one (1486.4 eV). The third doublet has the same contribution to the total intensity of the W 4f spectrum recorded on the WO3 nanoneedles and Au/WO3 nanoneedles sample suggesting a weak interaction of the Au clusters with the WO3 needles. It has been shown that, in some oxides doped with metal atoms, under certain conditions the metal atoms can diffuse and localize preferentially at defect sites such as oxygen vacancies [40]. If this is the case, it is expected that Au particles nucleate on these sites, thus the photoelectrons emitted from W atoms beneath the gold particles would experience inelastic losses when passing through the metal particle, and thus no longer contribute to the W 4f peak. Therefore, the absence of a measurable reduction in the relative intensity of the component at 36.2 eV suggests that the gold particle distribution is not mediated by the presence of defects at the nanoneedles surface. Further studies will be performed aiming at determining
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Fig. 2. TEM image of a WO3. (a) The inset shows the crystalline structure of both gold particles and WO3 needles. (b) The atomic planes under the particle are not affected by its presence.
the mechanism that causes the intrinsic spatial particle distribution at the WO3 needle surface in the AACVD synthesis. The Au 4f core level spectrum recorded on the Au/WO3 at 84 eV (Fig. 5) is similar to the one reported for the Au crystal indicating that the codeposited Au particles are in the metallic state [41]. The absence of new structures in the Au 4f spectrum testifies our assertion of weak interaction at the gold/WO3 interface. In Fig. 6 the O 1s peaks are shown. Besides the small chemical shift from the powder reference sample to the pristine WO3 nanoneedles (also observed in the W 4f peak), a second component of oxygen centered at 533 eV is present in the case where Au nanoparticles are co-deposited. Due to the large area of that component (1/3 of the main peak) and the lack of additional structures on the W 4f core level that could suggest another oxidation state for the tungsten, the origin of this peak can be assigned to contamination on the sample. In fact, the carbon concentration in the Au/WO3 samples was found to be higher than on the WO3 samples suggesting the presence of organic fragments probably due to the lower temperature used during the synthesis.
Fig. 3. TEM image of WO3 nanoneedles decorated with gold particles (dark dots).
The valence electronic states of the samples can be seen in Fig. 7. The WO3 reference sample (blue line) and WO3 nanoneedles (black line) spectra show similar features. Covalent bonding interactions transform the O 2p and the metal d-orbitals into bonding and antibonding orbitals of mixed atomic character. Reported, calculations of the valence band density of states (VB-DOS) for monoclinic WO3 assigned the peak located at about 4.1 eV to an O 2p-derived band while the peak located at about 6 eV corresponds to a hybridized ‘W 5deO 2p’ band [42,43]. The rapid onset of the VB spectrum at about 3.0 eV can be assigned to the top of the valence band, and the well-defined structure near the Fermi level (EB ¼ 0) is associated to W 5d in-gap states, whose filling is due to the
Fig. 4. XPS core levels for pristine WO3 powder (a), WO3 nanoneedles (b) and Au on WO3 nanoneedles (c). Light gray: W 4f level for stoichiometric WO3. Red: W 4f level for sub-stoichiometric WO3x. Blue: W 4f level of W atoms near surface defects. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Au 4f core level of the gold particles at the WO3 nanoneedle.
presence of oxygen vacancies [37]. In stoichiometric WO3 samples, the 5d-derived states near the gap are empty and no states are observable at the Fermi level [37,40,42,43]. However, if there are oxygen vacancies or lower valence WOx (x 2) states as well as W (0) not all the ‘5d-electrons’ can be transferred and the 5d-derived states near the gap are populated. The presence of a structure near the Fermi level (not expected for an insulator) suggests that there are sub-oxides or oxygen vacancies, which support our assertion of the component at binding energy equal to 36.2 eV to the presence of defects (black component in Fig. 6). Comparing the spectra of the valence band recorded on the standard samples and on the pristine WO3 needles, it can be seen that the structure at the Fermi level is more pronounced in the needles spectrum confirming our assertion that the needles have more oxygen vacancies. The loss of oxygen atoms leaves behind extra electrons resulting in a material behaving as n-type semiconductor. The presence of tungsten oxide sub-oxides and surface defects (i.e. oxygen vacancies) is of interest for tungsten oxide based chemical sensors. The baseline conductance of semiconducting WO3 nanoneedles will depend on the oxygen species adsorbed at their surface defects [10e12]. The
Fig. 7. Valence band for pristine WO3 nanoneedles (black), WO3 powder (blue), Au on WO3 nanoneedles (red) and pure Au (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
presence of oxidizing or reducing species that alter the equilibrium concentration of oxygen adsorbates can be monitored via the change in electrical conductance experienced by the nanoneedles [27]. The valence band spectrum recorded on WO3 needles decorated with gold particles presents a new structure peaking at 6.0 eV and, another at 1.4 eV. The gold valence band is formed by a 5d band, characterized by the 5d doublet with weighted centers at 6.35 eV and 3.55 eV and a wide “sep” band formed mainly by 6s and 6p electrons (partially hybridized with 5d-orbitals), localized above the 5d states and below the Fermi level [41]. Thus the new observed features can be associated with photoelectrons emitted from gold atoms forming the clusters. The 5d spin orbit splitting reflects the number of Au nearest neighbors surrounding a gold atom and consequently the strength of the AueAu interactions. Thus, the observed 5d spineorbit splitting supports our assertion that the clusters are in the metallic state. 4. Conclusions In summary, using aerosol assisted chemical vapor deposition WO3 nanoneedles and Au-decorated WO3 nanoneedles can be synthesized with high yields. Both, WO3 and Au-decorated WO3 nanoneedles are single crystalline and show oxygen vacancies indicating a semiconducting behavior. Well-dispersed Au nanoparticles homogenously cover the WO3 surface. The absence of additional structures in the Au 4f peak suggests a weak interaction at the Au/WO3 interface. The well-dispersed Au nanoparticles can play an important role for sensitizing the tungsten oxide nanoneedles via a spill-over effect. The obtained nanomaterial represents an excellent model for both experimentally and theoretically studying the interaction between metal oxides nanostructures and gas molecules in view of gaining more insight in the mechanisms of gas sensing. Acknowledgments
Fig. 6. O 1s core levels for pristine WO3 nanoneedles (black), Au on WO3 nanoneedles (red) and WO3 powder (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
This work is partially supported by the Belgian Program on Interuniversity Attraction Pole (PAI 6/08) and ARC-UMONS. T.S., E.L and X.C. are supported by the Spanish Ministry of Science and Innovation and the Catalan Agency for Research under grants no.
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