Synthesis and optical properties of Au decorated colloidal tungsten oxide nanoparticles

Accepted Manuscript Title: Synthesis and optical properties of Au decorated colloidal tungsten oxide nanoparticles Author: Nemat Tahmasebi Seyed Mohammad Mahdavi PII: DOI: Reference:

S0169-4332(15)01748-1 http://dx.doi.org/doi:10.1016/j.apsusc.2015.07.181 APSUSC 30908

To appear in:

APSUSC

Received date: Revised date: Accepted date:

27-3-2015 28-6-2015 25-7-2015

Please cite this article as: N. Tahmasebi, S.M. Mahdavi, Synthesis and optical properties of Au decorated colloidal tungsten oxide nanoparticles, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.07.181 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

> Tungsten oxide nanoparticles were prepared by Pulsed Laser Ablation (PLA). > A very fine metallic Au particles or coating are decorated on the surface of tungsten oxide nanoparticles.

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> UV-Vis spectroscopy shows an absorption peak at ~530 nm which is due to SPR effect of gold.

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> After exposing to hydrogen gas, Au/WO3 colloidal nanoparticles show excellent gasochromic coloring.

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*Manuscript Click here to view linked References

Synthesis and optical properties of Au decorated colloidal tungsten oxide

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b,c,2

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nanoparticles Nemat Tahmasebi , Seyed Mohammad Mahdavi Department of Basic Science, Jundi-Shapur University of Technology, Dezful, Iran b Department of Physics, Sharif University of Technology, Azadi Ave, Tehran, Iran c Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Azadi Ave, Tehran, Iran

1) Corresponding author

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Tel: +98 61 4242 8000; Fax: +98 61 4242 6666

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E-mail: [email protected] (N. Tahmasebi)

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2) Corresponding author

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Tel: +98 21 661 645 17; Fax: +98 21 660 227 11

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E-mail: [email protected] (S.M. Mahdavi)

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Abstract: In this study, colloidal tungsten oxide nanoparticles were fabricated by pulsed laser ablation of tungsten target using the first harmonic of a Nd:YAG laser (1064 nm) in deionized water. After ablation, a 0.33 g/lit HAuCl4 3+

aqueous solution was added into as-prepared colloidal nanoparticles. In this process, Au ions were reduced to 0

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decorate gold metallic state (Au ) onto colloidal tungsten oxide nanoparticles surface. The morphology and chemical composition of the synthesized nanoparticles were studied by AFM, XRD, TEM and XPS techniques. UV-

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Vis analysis reveals a distinct absorption peak at ~ 530 nm. This peak can be attributed to the surface plasmon resonance (SPR) of Au and confirms formation of gold state. Moreover, X-ray photoelectron spectroscopy reveals

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that Au ions’ reduction happens after adding HAuCl4 solution into as-prepared colloidal tungsten oxide nanoparticles. Transmission electron microscope shows that an Au shell has been decorated onto colloidal WO3

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nanoparticles. Noble metal decorated tungsten oxide nanostructure could be an excellent candidate for photocatalysis, gas sensing and gasochromic applications. Finally, the gasochromic behaviour of the synthesized

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samples were investigated by H2 and O2 gases bubbling into the produced colloidal Au/WO3 nanoparticles.

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Synthesized colloidal nanoparticles show excellent coloration contrast (~ 80%) through NIR spectra.

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Key words: Pulsed laser ablation; Optical properties; Colloidal WO3 nanoparticles; Au

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1.

Introduction

Oxides [1] generally and nanostructured oxides [2] especially are of utmost interest in both industrial and academic sections. Tungsten oxide is a very interesting wide band gap n-type semiconductor which has been

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studied in either bulk or films due to unique physical and chemical properties [3-5]. In recent years, tungsten oxide films have been used in wide applications such as photocatalyst, gas sensors, electrochromic and gasochromic

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devices [5-10]. On the other hand, nanostructure materials tend to reveal extremely distinct properties in comparison with bulk type because of higher surface area and lower dimensionality [11,12]. Nowadays, various

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WO3 nanostructured including nanowires [13], nanoplates [14], nanorods [15] and nanoparticles [16] have been synthesized by physical or chemical techniques. Pulsed-laser techniques are bringing the rare opportunity to

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investigate very interesting phenomena which occur in very short time scale [17]. They also help with great flexibility and simplicity for synthesis of nanostructure materials [18, 19]. Among which pulsed laser ablation is an

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effective, clean and safe method for synthesis of nanoparticles [17]. By this method, a number of factors such as energy per pulse, wavelength, repetition rate, and pulse width affect the synthesis process of nanoparticles;

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furthermore, different nanoparticles such as conductor, semiconductor and insulator can be produced.

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Pure tungsten oxide based sensors have very high working temperature and low sensitivity for gas sensing application [20]. To overcome these drawbacks noble metals can be added to the tungsten oxide nanostructure. It

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is well accepted that the catalytic activity of the noble metals such as (Pt, Pd) [20-22] is essential for gas response of the metal oxides. Noble metal can be inserted within the bulk of the materials or deposited on the top of the sensing film [23]. Gold in bulk state is chemically inert and shows low activity as a catalyst [24]. However, Au nanoparticles with diameters below 10 nm are surprisingly active for many reactions, such as H 2 dissociation and CO oxidation [24, 25]. In recent years, Au based catalysts have attracted more interest for gas sensing [26, 27]. In this context, Ando and co-workers reported the gas sensing properties of WO3 films activated by Au, Pd and Pt catalysts for sensing hydrogen gas. It was observed that combination of Pd and Au led to improvement of optical H2 sensitivity. However, no observable change in the absorbance of the Pt/WO 3 and pure WO3 sensor was observed [28]. Ahmad and co-workers reported that the dynamic performance including response and recovery time as well as stability of Au/WO3 sensor are very good compared to Pd/WO3 and Pt/WO3 sensors [28].

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So far, different physical and chemical techniques have been used to activate tungsten oxide nanostructure using noble metals. These techniques include electrochemical [29], electroless [30] and solution reduction using reducing chemicals [31-34]. Recently, we have introduced a facile method to synthesized colloidal Pd/WO3

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nanoparticles [35, 36]. In this method, tungsten oxide nanoparticles were fabricated by pulsed laser ablation inside DI water and were activated against hydrogen gas by addition of small amount of palladium salt (PdCl2). The

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prepared Pd/WO3 colloidal nanoparticles show a gasochromic response that have transparent and blue colour in the presence of oxygen and hydrogen gas, respectively.

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In this study, colloidal tungsten oxide nanoparticles are synthesized by pulsed laser ablation technique. Since the presence of the catalyst (noble metal) is necessary to have reasonable gasochromic properties. Therefore, by

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adding a HAuCl4 aqueous solution into colloidal tungsten oxide nanoparticles, Au

3+

ions are reduced to form a

metallic coating on the surface of tungsten oxide particles. The metallic Au act as catalyst for dissociation of

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hydrogen molecules in coloration process. The Colloidal Au/WO3 nanoparticles were found to have gasochromic switching capability and were characterized in the current study by atomic force microscopy (AFM), X-ray

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2. Experimental

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spectroscopy techniques.

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diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-Vis

Tungsten oxide nanoparticles were fabricated by pulsed laser ablation of a tungsten target (99.9%) in deionized (DI) water using the first harmonic of a Nd:YAG laser (λ=1064 nm) operating at 500 mJ/pulse, 5 ns pulse width and 10 Hz repetition rate. In these experiments, the target was submerged in a liquid cell that was filled with 20 ml of DI water as ablation environments. The laser beam was focused normal to the surface of the target by a set of optical components. The ablation time was set to be 20 minutes. HAuCl4 (1 mM) aqueous solution was produced by dissolving of 0.033 g hydrogen tetrachloroaurate (III) hydrate (99.9%, metals basis, Au 49% min., Alfa Aesar) in 99.9 ml DI water and 0.1 ml HCl ultrasonically. Immediately after laser irradiation, various amount of this aqueous solution including 0, 0.01, 0.1, 0.4, 0.6, 1.1 and 1.8 ml were added into 3 ml of as-prepared bluish colour tungsten oxide colloid. In case of no Au addition, a colour change was observed from bluish to a colourless state (a

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few minutes after irradiation). When HAuCL 4 aqueous solution was added drop by drop into as- prepared colloidal nanoparticles the mixed turned to a red-wine colour, while the aging process does not affect on it. The size distribution and particle morphology of nanoparticles were investigated by atomic force microscopy

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(AFM) (Veeco Cp-Research model) in air with a silicon tip of 10 nm radius, and by transmission electron microscope (TEM) (Philips, 100 KV, EM 208). The crystal structure was characterized by an X-ray diffraction (XRD). This analysis

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carried out by a Philips Xpert instrument operating with Cu Kα radiation (λ = 1.54 Å) at 40 kV/40 mA. For the X-ray photoelectron spectroscopy (XPS) experiment, an Al anode X-ray source was employed with a concentric

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hemispherical analyzer (Specs Company, model EA10 plus) to analyse the energy of the emitted photoelectron from the surface. The chamber pressure during the XPS experiment was 7.5× 10-10 Torr. The energy scale was

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calibrated by adjusting the carbon peak C(1s) at 284.8 eV. All the peaks were deconvoluted using SDP software (version 4.1). XPS and AFM measurements were carried out on dried amount of colloidal nanoparticles that were

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deposited on Si substrates. Optical transmission of colloidal solution was measured by Perkin Elmer Lambda 25 model spectrophotometer in the wavelength range from 200 to 1100 nm. A water filled quartz cell was also used

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as the reference. The absorption coefficient was determined according to Eq.1: (1)

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Here d is cell thickness and T is transmittance.

3. Results

AFM analysis can be applied as a technique to measure size distribution of pulsed laser ablated nanoparticles in liquid. Fig. 1 (a-c) shows the AFM image and analayzed data of ablated nanoparticles. Line scan of AFM images has been used to estimate the size distribution of nanoparticles (Fig 1(b)). The histogram in Fig. 1(c) shows the size distribution of nanoparticles ranges from 40 to 120 nm. The average size of nanoparticles is roughly 85 nm. “It should be noted that the measured radius of the nanoparticles in an AFM image can be overestimated by tip convolution effect, when the tip radius is comparable to the dimension of nanoparticles [37,38]. In this paper, because the average size of WO3 nanoparticles is higher than the tip radius, this effect has not been considered. Therefore, we reported the size as measured.

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Fig. 2 represents the XRD pattern of as-prepared colloidal tungsten oxide nanoparticles dried in air at 40 °C for 20 min. The diffraction peaks in Fig. 2 can be indexed to the monoclinic hydrated tungstate (WO 3.2H2O) with lattice constant of a=7.500 Å, b=6.930 Å and c= 3.700 Å, which are consistent with the values in the standard card (18-

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1420) [39]. Optical transmission of the colloidal solution was measured by UV-Vis spectroscopy. The absorption coefficient (α)

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of colloidal nanoparticles is calculated according to Eq.1. The optical absorption spectra of HAuCl4 solution (1 mM), colloidal tungsten oxide nanoparticles (5 min aged after irradiation) and after adding 0.6 ml HAuCl4 solution into 3

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ml as-prepared colloidal tungsten oxide nanoparticles are shown in Fig. 3. One can observe both the HAuCl4 solution and colloidal tungsten oxide nanoparticles are highly transparent in the visible and near-infrared spectra.

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However, the addition of HAuCl4 solution into the tungsten oxide colloid can be an effective method to grow catalyst (Au) on WO3 nanoparticles’ surface. Fig. 3(c) displays the absorption spectrum of colloidal tungsten oxide

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nanoparticles after adding HAuCl4 solution. The comparison between Fig. (3-c) and Fig. (3-a) clearly indicate lower absorption for the colloidal tungsten oxide nanoparticles. In addition, a peak has been appeared in the absorption

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spectra of Au/WO3 nanoparticles. This peak represents the characteristic of the plasmon absorption around ~ 550

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nm. In other words, Au nanosystems show a strong surface plasmon resonance (SPR) absorption between 520 and 560 nm. Therefore, formation of Au metallic state may be confirmed through the absorption spectrum which

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reveals a surface plasmon resonance band at ~ 550 nm. The inset of Fig. 3 shows (left to right) the photograph images of the colloidal tungsten oxide nanoparticles (5 min after irradiation), HAuCl 4 (1 mM) aqueous solution and after adding 0.6 ml HAuCl4 into as-prepared colloidal tungsten oxide nanoparticles. After adding HAuCl4 aqueous solution the colour of colloidal sample was changed to red wine. This colour changing was not observed for colloidal tungsten oxide nanoparticles that were allowed to age at ambient condition for more than 30 min. Depending on its average size, a pure Au nanoparticles colloid may show a reddish colour. Therefore, this process can be attributed to Au ions reduce to metal Au state due to charge transfer between ions and WO 3 nanoparticles surface. In laser ablation process, because of high pick power of the incident laser pulse, it makes possible to form high temperature plasma. This plasma might activate the tungsten oxide nanoparticles’ surface with free electrons and/or dangling bonds which will have high chemical activity at the initial stages of ablation. The passivation of nanoparticles surface gradually occurs through increasing particle size, surface oxidation and particle agglomeration

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[35, 36]. The presence of electron on WO3 naoparticles surface, at the initial stages of ablation, provides negative charges which require for reduction of positive ions. Fig. 4 (A) reveals absorption spectra of colloidal Au/WO3 nanoparticles with various HAuCl4 concentrations.

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Different quantities of HAuCl4 were added into 3 ml of as-prepared tungsten oxide colloidal nanoparticles. The inset of Fig. 4 (A) illustrates the photographic images of the colloidal nanoparticles after adding various quantities

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of HAuCl4 into as-prepared colloidal tungsten oxide nanoparticles. These photographs clearly depicts that colour intensity has diminished for samples with 1 ml and 1.8 ml HAuCl4 in comparison to that with 0.6 ml HAuCl4

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solution. Furthermore, absorption at the wavelength of maximum absorption (λmax) enhances dramatically with increasing HAuCl4 concentration up to 0.6 ml. Further rise in the HAuCl4 concentration leads to reduction in the

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absorption peak intensity (Fig. 4(B)). It might be concluded that there are not sufficient active species to reduce all 3+

3+

Au ions at higher HAuCl4 contents (> 0.6 ml). Moreover, a number of them remain in unreduced form (Au ) and

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as a result, Au (and also WO3) concentration declines within synthesized colloidal solution. Therefore a SPR height reduction would occur. In addition, the absorption spectra show a small red shift in the wavelength of SPR as

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HAuCl4 content increases which could be attributed to formation of larger Au particles (Fig. 4(b)) [40].

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The band gap energy of the sample have been measured by using their linear plots of (αhν)1/µ versus 1/ɳ [35], where, α is the optical adsorption coefficient and hν is incident photon energy. Details of calculation method has been

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explained in our previous work [35]. By examining the various values of ɳ, as a result of line fitting, ɳ=2 was determined for the both pure WO3 and Au/WO3, showing the indirect transitions in the samples. The Eg value is determined by extrapolating the linear region of (αhν) 1/2 plot to hν=0. Fig. 5 shows the optical band gap as a function of HAuCl4 concentration. This figure reveals a reduction in band gap energy for Au/WO3 nanoparticles in comparison to bare WO3 nanoparticles. The energy gap of colloidal WO3 nanoparticles is 3.45 eV. The measured band gap energy for Au/WO3 nanoparticles is found to vary from 3.30 eV (for 0.1 HAuCl4 containing solution) to 3 eV (for 1.8 HAuCl4 containing solution) which disclose a slight red shift. Au catalyst’s introduction to metal oxide surface initiates formation of additional inter-band-gap electronic states and consequently diminishes the optical band gap [28]. XPS analysis was carried out to determine the surface chemical composition of the particles. Survey scan spectra of an accumulated HAuCl4 solution layer on the substrate by drop drying of a 0.33 g/l HAuCl 4 solution and two

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Au/WO3 nanoparticles with 0.6 ml and 1.8 ml HAuCl4 containing solution on silicon substrate confirmed presence of Au, W, Cl, oxygen, and carbon species. The carbon and oxygen peaks are partially due to exposure to atmosphere. The XPS spectra of W4f and O1s revealed that valence state of W is solely +6 (not shown here) [35].

3+

+

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High resolution XPS spectrum and peak fitting of Au4f region are displayed in Fig. 6. The peak area (%) of the 0

different Au valance state (Au , Au and Au ) were summarized in table 1. The Au4f spectrum of the dried HAuCl 4

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solution (Fig 6(a)), before adding to colloidal WO3 nanoparticles, includes two Au related contributions. The first +

doublet in concentration 62% is at 84.7 eV (Au4f7/2) and 88.4 eV (Au4f5/2) binding energies is related to Au ions 3+

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[41]. The second doublet (38 %) with binding energies of 86.2 eV (Au4f7/2) and 89.7 eV (Au4f5/2) are assigned to Au

[41]. Fig. 6 (b) demonstrates the peak fitting for the Au4f spectrum of Au/WO3 nanoparticles with 0.6 ml HAuCl4

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solution. The Au4f peak is better fitted with two maxima at binding energies values of 84.0 eV and 87.5 eV that are consistent with the Au metallic state which confirms the Au formation [41,42]. Therefore, these results reveal that +

3+

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Au atoms are in Au and Au chemical state in the HAuCl4 solution whilst majority of reduced sample are in the 0

metallic (Au ) state. On the other hand, Fig. 6(c) and table 1 reveals that solutions with higher added HAuCl4 +

3+

(30% and 21%, respectively). Therefore, in

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concentration (1.8 ml), contain substantial amounts of Au and Au

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comparison to the prepared sample with 0.6 ml HAuCl4 solution, it can be concluded that there are not sufficient active species to reduce all Au ions. So, high amount of unreduced ions remain in the colloidal solution. These

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results are in agreement with the aforementioned UV-Vis data. On the other hand, the chloride anion can have a negative effect on catalyst activity of Au nanoparticles. Cl ions might poison the active sites of the Au catalyst [44]. For more evidence, the Cl2p region in the XPS spectra of HAuCl4 solution and colloidal Au/WO3 nanoparticles with different HAuCl4 concentrations (0.6 ml and 1.8 ml) are shown in Fig. 6 (a-c). For HAuCl4 solution and colloidal Au/WO3 nanoparticles with 1.8 ml HAuCl4 concentration the peak positions of Cl2p3/2 and Cl2p1/2, centered at 199.17 and 201 eV, respectively, are related to the ionic chloride of HAuCl4 composition. No Cl2p signal within the bonding energies ranging from 194 eV to 206 eV was detected for Au/WO3 sample with 0.6 ml HAuCl4 concentration. This suggests that HAuCl4 is completely reduced to Au. Therefore, XPS analysis reveals that Au ions’ reduction happens after introduction of HAuCl4 solution into asprepared colloidal tungsten oxide nanoparticles.

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The morphology and shape of Au/WO 3 nanoparticles was investigated using transmission electron microscopy (TEM). TEM images of two Au/WO3 nanoparticles with 0.4 ml and 1 ml HAuCl4 containing solution are shown in Fig. 7a, b, respectively. For the sample with 0.4 ml HAuCl 4 content, if we consider the clusters (shown by black arrows)

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in Fig. 2a to be Au, a thin layer of Au has been decorated on the WO 3 nanoparticles surface. Fig. 2b shows that the thickness of this shell has increased with increasing HAuCl4 content and an Au shell with thickness of

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approximately 15 nm was formed on tungsten oxide nanoparticles surface. Furthermore, for sample with higher HAuCl4 content (1 ml), there are chains of small particles (shown by white arrows) that have been attached to the

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Au/WO3 nanoparticles surface (Fig. 7(b)). Probably, these particles can be attributed to individual metallic Au nanoparticles which were produced spontaneously and small Au/WO3 nanoparticles.

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The gasochromic switching behaviour of Au/WO3 colloidal solution was examined by the bubbling of hydrogen or oxygen gases into the liquid containing cell. Fig. 8 (a-c) represents photographic images corresponding to Au/WO3

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nanoparticles with various content of HAuCl4 solution (0.01, 0.1 and 0.4 ml) before and after hydrogen exposure. From this figure, one can see the depth of gasochromic coloring effect for sample with 0.1 ml HAuCl 4 is

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considerably better that other samples. For other HAuCl4 content (0.6, 1 and 1.8 ml), when they are exposed to H 2

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gas (not shown here) even for more than 2 hours, no significant change in their colour is observed which might be attributed to catalyst activity of Au. For noble metal nanoparticles (Pd, Pt or Au) decorated on WO3 support,

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gasochromic phenomena occur via spillover mechanism [28,45,46]. In this process, when Au/WO3 nanoparticles are exposed to H2, Au will dissociate H2 molecules into H atoms, and these hydrogen atoms may be transferred onto the WO3 surface by spillover mechanism, and then injected into the body of WO 3 nanoparticles. This process can lead to coloration of WO3 nanoparticles. The effectiveness of spillover mechanism is largely dependent on size and distribution of Au catalyst [28,45,46]. According to UV-Vis and TEM results, we were expecting at higher HAuCl4 concentration more and larger Au particles are decorated on tungsten oxide nanoparticles’ surface. Therefore, the enhancing of Au content can reduce the catalyst activity of Au and also can have a negative effect on spillover process of hydrogen atom on WO3 surface [46]. The coloration dynamic was determined by the change in optical density function (∆OD), which is defined as ∆OD=ln(Ic/Ib); Ic and Ib are optical transmission intensity of coloured and fully bleached state at λ=632.8 nm, respectively. Fig. 8(d) shows time variation of optical density of 0.1 ml HAuCl 4 concentration during H2/O2 exposure. One can

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observe, when the colloidal sample is exposed to hydrogen gas at room temperature, its optical density gradually is switched from a transparent state to an absorbing blue state, and the optical variation is reversible, i.e. when hydrogen gas is removed and oxygen gas is flushed into the quartz cell, ΔOD decreases quickly to initial value.

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On the other hand, the visual appearance is essential for a gasochromic devices and smart windows. These devices should have a high transmittance in non-hydrogenated state and show a high coloration contrast after hydrogen

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exposure. Fig. 9 shows the optical transmission spectra in the wavelength range from 400 to 1100 nm. In the nonhydrogenated state, the sample shows high transmission in Vis and NIR regions (with a red wine appearance).

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After exposure to hydrogen, the overall transmission at 500-1100 nm decreases and at the same time, the sample’s colour gradually changes to blue colour. The blue appearance is caused by reduction of sample

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transmittance in the red wavelength region. To analyse the behaviour of optical modulation in Vis and NIR regions under hydrogenation, the optical transmission of exposed to hydrogen (T c) was subtracted from that of hydrogen

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free samples (Tb). The transmission modulation (∆T= Tb-Tc) of colloidal tungsten oxide nanoparticles is around 80% at 630 nm and 85% at 1000 nm. That shows excellent coloration change through visible and NIR spectra compared

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to reported values for electrochromic WO 3.2H2O films (∆T~ 60%) [39], gasochromic Au/WO3 thin films [28] and

4. Conclusion

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gasochromic Pd/WO3 colloidal nanoparticles [36, 47].

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In the present study, colloidal tungsten oxide nanoparticles were prepared by pulsed laser ablation in DI water and Au as catalyst was deposited on them using a HAuCl4 aqueous solution. The AFM results reveal the average size of nanoparticles to be roughly 85 nm. UV-Vis spectroscopy shows a SPR absorption peak at ~550 nm which is due to 0

the reduction of Au ions and formation of metallic gold state (Au ). Moreover, the band gap energy of colloidal Au/WO3 nanoparticles decreased on increasing Au concentration. The TEM and also XPS results indicate when 3+

HAuCl4 solution is added into as-prepared colloidal nanoparticles, Au ions are reduced and metallic Au coating are decorated on the surface of tungsten oxide nanoparticles. The gasochromic response for colloidal sample shows the reversibility of the coloration process from red wine colour to blue in the presence of oxygen and hydrogen gas, respectively. The Au/WO3 colloidal sample with 0.1 ml HAuCl4 showed good gasochromic coloring. This sample exhibit a high transmission (90%) in bleached state, while low transmission values (<20%) are observed at colored

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state through NIR region. In addition, the mentioned method to synthesis Au decorated tungsten oxide

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nanostructure may find potential application in photocatalysis, gas sensing and gasochromic devices.

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Table captions: Table 1. The peak area (%) of the Au4f spectrum for HAuCl4 aqueous solution and after adding different quantities

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of HAuCl4 into 3 ml as-prepared colloidal tungsten oxide nanoparticles

Figure captions:

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nanoparticles prepared by laser ablation of tungsten target in DI water.

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Figure 1: Two dimensions AFM micrograph (a), a typical line scan (b) and associated histogram (c) of colloidal

Figure 2. XRD patterns of powder extracted from drop-drying of colloidal tungsten oxide nanoparticles

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Figure 3. Optical absorption spectra of (a) colloidal tungsten oxide nanoparticles, (b) HAuCl4 (1 mM) aqueous solution and (c) colloidal Au/WO3 nanoparticles. The inset displays the corresponding photograph of the (a-c)

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samples

Figure 4. (A) Absorption spectra of Au/WO 3 sample with various content of HAuCl4 solution in 3 ml WO3 colloidal

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nanoparticles. The content of HAuCl4 solution are (a) 0.1 ml (b) 0.4 ml (c) 0.6 ml (d) 1 ml and (e) 1.8 ml. The inset displays the corresponding photograph of the (a-e) samples. (B) The wavelength of maximum absorption (λmax) and

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absorption at λmax as a function of HAuCl4 content

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Figure 5. Optical band gap of Au/WO3 colloidal nanoparticles as a function of HAuCl4 content.

Figure 6. High resolution XPS spectra of Au4f and Cl2p for (a) HAuCl 4 aqueous solution (1 mM), colloidal tungsten oxide nanoparticles after adding (b) 0.6 ml, and (c) 1.8 ml HAuCl4 aqueous solution

Figure 7. High magnification TEM image of an Au decorated tungsten oxide nanoparticles (Au/WO 3) with (a) 0.4 ml, and (b) 1 ml HAuCl4 aqueous solution content

Figure 8. Corresponding photographic images of Au/WO3 colloidal nanoparticles with various content of HAuCl4 solution (a) 0.01 ml, (b) 0.1 ml, (c) 0.4 ml HAuCl4 before and after gasochromic coloring by hydrogen exposure, (d) Coloration dynamic response of (b) in the presence of hydrogen and oxygen gases.

Figure 9. Optical transmission spectra of Au/WO3 colloidal nanoparticles (for 0.1 ml HAuCl4 solution) in the presence of (a) oxygen and (b) hydrogen gas.

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(b)

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(a)

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Figure 1 Click here to download Figure: Figure 1.docx

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Particle Number

(c)

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Diameter (nm)

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(-201) (220)

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(020) (101) (011)

(200) (001)

Intensity (arb. units)

(010)

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2 Theta (degree)

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(b)

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Wavelength (nm)

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Figure 4 Click here to download Figure: figure 4.docx

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(c)

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Absorption coefficient (cm-1)

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Wavelength (nm)

λmax (nm)

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Abs at λmax (cm-1)

(B)

HAuCl4 content (ml)

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Ebg (eV)

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HAuCl4 content (ml)

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Figure 6 Click here to download Figure: figure 6.docx

Au4f

Cl2p (c)

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(c) Au0 Au+ 3+

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Intensity (arb. units)

Au0

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Au

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Au3+

Au+

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Binding Energy (eV)

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Figure 7 Click here to download Figure: figure 7.docx

(b)

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Au

Au Au

WO3

WO3

50 nm

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Figure 8 Click here to download Figure: figure 8.docx

(a)

(b)

H2

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O2

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Transmission (%)

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Table 1

Table 1.

The peak area (%) of Au4f spectrum

1

Au3+ (%)

HAuCl4

Au+ (%)

38

62

Au0 (%) 0

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Samples

0.6 ml HAuCl4 +3 ml WO3

0

0

100

3

1.8 ml HAuCl4 +3 ml WO3

21

30

49

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2

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