Experimental section
Film deposition: gold NPs supported on tungsten oxide NNs were co-deposited at
375 ∘C via AACVD of tungsten hexaphenoxide (W(OPh)6) (Cross et al.,
2003) and ammonium tetrachloroaurate(III) hydrate
(NH4AuCl4.xH2O) (Alfa Aesar, 99.9 %) dissolved in acetone
(10 cm3, Sigma-Aldrich, ≥ 99.6%) and methanol (5 cm3,
Sigma-Aldrich, ≥ 99.6 %) (Table 1) using a
custom-built reactor.
Details of samples.
W(OPh)6 (g)
NH4 AuCl4 (g)
Solvent (acetone : methanol 2 : 1) (cm3 )
Au at. %
Temperature (∘C)
Sample 1
0.10
0.010
15
4.5
375
Sample 2
0.10
0.003
15
2.1
375
Sample 3
0.10
0
15
0
375
A Johnson Matthey Liquifog 2 operating at 1.6 MHz was used to generate an
aerosol from a solution containing both the tungsten and gold precursors,
with the aerosol droplets transported to the heated substrate by a nitrogen
(oxygen-free, BOC) gas flow. Annealing of the samples was carried out in air
at 500 ∘C for 2 h. Alumina (A493 Kyocera) or gas-sensor
platforms (as shown schematically in Fig. 1), on
which both the heater and sensor are printed on the same side of an alumina
tile, were used as substrates. Both were cleaned with acetone and isopropyl
alcohol prior to use.
Schematic fabrication processes of the micro-sensor based on Au NPs
supported on tungsten oxide NNs.
Film analysis: the crystalline structures of samples were determined via
X-ray diffraction (XRD) (Bruker, LinxEye D8-Discover, using Cu Kα
radiation operated at 40 kV and 40 mA) with glancing incident angle (tube at
1∘), 0.05∘ per step and 1 s per step, and 2θ from 10 to 66∘. The
microstructure of the films was examined with scanning electron microscopy
(SEM) (Jeol 6310F, 5 kV) and high-resolution transmission electron microscopy (HR-TEM) (Jeol-2100, 200 kV), which
was also used to obtain the information of lattice structure. The chemical
composition measurement was determined via energy dispersive X-ray spectroscopy
(EDX) (20 kV) (Jeol 6310F). Chemical and electronic states of the elements
in the thin films were examined by means of X-ray photoelectron spectroscopy (XPS)
(Thermo Scientific K-Alpha), using monochromatic Al Kα radiation
(0.6 eV) and charge compensation by means of dual-beam charge neutralisation with an
electron gun (1 eV) and argon-ion gun (≤ 10 eV), calibrated by the C 1 s
peak at 284.7 eV. UV–Vis spectroscopy was performed using a double-monochromated Perkin Elmer Lambda 950 UV–Vis–NIR spectrophotometer in the
300 to 1100 nm range.
Gas-sensing tests: gas sensors were exposed to various concentrations of
ethanol at 0.5, 1, 2, 3 and 4 ppm respectively, at an operating temperature
of 300 ∘C and relative humidity of 50 %. The sensor response is
defined as R=Ra/Rg, where Ra is the resistance of the
sensor in air and Rg is the resistance in ethanol. Sensors were exposed
to ethanol vapours for 4 min and afterwards to air for 4 min.
Results and discussion
Figure 2a shows pictures of Au-functionalised
and non-functionalised tungsten oxide films deposited on alumina substrates
before and after heat treatment in air at 500 ∘C. The pre-annealed
films were characterised by a dark colour, as expected in the presence of
partially reduced tungsten species, and after heating in air,
non-functionalised tungsten oxide turned bright yellow, as expected for
stoichiometric WO3, whilst Au-functionalised tungsten oxide was dark
purple in colour, indicative of the presence of gold nanoparticles.
UV–Vis analyses (Fig. 2b) were performed
collecting reflectance data (R) and then converting into absorbance (A)
using the following formula (Viscarra Rossel et al., 2006):
A=log10(1/R),
analogous to absorbance units, log(1/T), for transmission measurements.
(a) Pictures of films deposited onto alumina substrate, before and after
heating treatment at 500 ∘C. (b) UV–Vis analyses of Au-functionalised WO3 (in black, 4.5 at. %; red, 2.1 at. %) and
non-functionalised WO3 (in blue).
For the non-functionalised tungsten oxide film, a peak is observed at 400 nm,
corresponding to the WO3 band edge (Egap WO3 ∼ 2.7 eV).
The Au-functionalised WO3 films show an additional absorbance
at 585 nm, corresponding to the surface plasmon resonance (SPR) peak of gold
nanoparticles (El-Brolossy et al., 2008). Increasing the amount of gold precursor
led to an increase in the absorbance of the gold plasmon resonance peak,
suggesting a greater incorporation of gold particles.
XRD patterns of Au-functionalised WO3 (in black, 4.5 at. %; red
2.1 at. %) and non-functionalised WO3 (in blue). Monoclinic-phase (blue), Al2O3 trigonal (- - -) and Au FCC (• • •) structures are also presented.
XRD analysis (Fig. 3) of films deposited on
Kyocera alumina substrates, after annealing, revealed the presence of
monoclinic-phase WO3 (P21/n space group, a= 7.306 Å, b= 7.540 Å,
c= 792 Å, and β= 90.88∘; ICCD card
no. 72-0677; Loopstra and Rietveld, 1969). Alumina peaks, exhibited in all patterns and
depicted with dashed vertical lines, come from the alumina substrate. Both
the WO3 / Au and the WO3 patterns showed preferred orientation in
the [001] direction. In particular, the peak (002) at 23.11∘
2θ is very intense relative to the (020) peak. Gold FCC peaks
(dotted vertical lines) at 38.25∘ 2θ, 44.30∘
2θ and 64.55∘ 2θ were observed only in the
WO3 / Au pattern (zoomed patterns). This result confirms the
co-deposition of tungsten oxide and gold metal. The mean grain size of Au
nanoparticles was calculated to be about 26 nm based on the
Scherrer formula:
D=0.89λβcosθ,
where D is the mean size of the crystalline domains, λ is the Cu
Kα wavelength, β is the line broadening at half the maximum
intensity (FWHM) in radians and θ is half of the Bragg angle.
No shifts in the WO3 peaks position were observed, demonstrating that
the monoclinic crystal structure of tungsten oxide was not changed after the
addition of gold. Similar results for WO3 monoclinic phase were
obtained by Vallejos et al. (2013) when using HAuCl4 as the gold
precursor, but no Au peaks were observed in their XRD patterns, likely due to
the lower amount of gold in the samples (0.09 Au at. %).
Images from EDX analysis of the Au-functionalised and non-functionalised samples
are shown in Table 2. The measured Au atomic percentage
composition (Au at. % on Au–WO3) in the deposited films was 4.5 and
2.1 for sample 1 and sample 2 respectively, much lower than the theoretical
values based on the composition of the precursor solution, which were 10.4
and 3.4 at. % respectively. The incorporation efficiency was
44 % for sample 1 and 63 % for sample 2, much higher than the one
determined by Vallejos et al. (2013), which had 5–10 % efficiency.
SEM images are depicted in Fig. 4, clearly showing
the increase in the amount of decorating nanoparticles on increasing
the amount of gold precursor, with sample 1 (Fig. 4a) displaying a larger number of surface (gold) particles compared to
sample 2 (Fig. 4b). No decorating particles were
observed in the non-functionalised sample 3 (Fig. 4). The decorating particles were well dispersed all along the needles,
demonstrating the good efficiency of the synthesis in terms of composition
homogeneity. The thickness of the films, visible in a cross-section image in
Fig. 4d, was roughly 1.5 µm in all cases,
and the tungsten oxide “needles” (ca. 0.1–0.2 µm diameter and ca. 1–2 µm
length), although not perfectly vertically aligned, were
predominantly perpendicular to the substrate.
SEM images of (a) sample 1 (WO3 / Au 4.5 at. %), (b) sample 2
(WO3 / Au 2.1 at. %), (c) sample 3 (WO3) and (d) cross section of
sample 3.
Precursors theoretical and EDX experimental atomic percentage composition
for gold element.
Theoretical Au at. %
Experimental Au at. %
Sample 1
10.4
4.5
Sample 2
3.4
2.1
Sample 3
0
0
(a) and (b) TEM images of WO3 / Au sample. HR-TEM for (c) WO3 nanoneedle and
(d) gold nanoparticles.
Au 4f and W 4f scans fitting parameters: peak, FWHM and area.
Peak 1 (4f5/2 )
Peak 2 (4f7/2 )
Position
FWHM
Area
Position
FWHM
Area
Sample 1
Au 4.5 at. %
W 4f
37.84
1.098
51 429
35.65
1.098
68 586
Sample 2
Au 2.1 at. %
W 4f
37.79
1.115
52 690
35.60
1.115
70 267
Sample 3
Au 0 at. %
W 4f
37.84
1.165
86 442
35.65
1.165
115 284
Sample 1
Au 4.5 at. %
Au 4f
87.68
0.982
36 077
83.90
0.982
48 120
Sample 2
Au 2.1 at. %
Au 4f
87.68
0.997
37 438
83.90
0.997
49 935
HR-TEM was also used to characterise the crystalline habit of the
nanoneedles and decorating nanoparticles (Fig. 5). The crystal planes in
the long axis of the needles were separated by 0.378 nm
(Fig. 5c), corresponding to the (020) plane of
monoclinic WO3 monoclinic, and the decorating NP
(Fig. 5d) exhibited planes separated by 0.23 nm,
consistent with (111) atomic planes of FCC gold. TEM examination showed that the
gold nanoparticles were polydispersed both in shape (triangle, trapezoid,
rod, sphere) and size (ca. 18 and 62 nm diameter), with an average size
of 30 nm. This result matches with the gold nanoparticles' grain size, 26 nm,
observed by using the Scherrer formula on the XRD pattern.
This is different to that observed previously using HAuCl4 as a gold
precursor (Vallejos et al., 2013). In that case the gold NP displayed a spherical
shape with a size between 4 and 11 nm. This may be due to the different
nature of the precursor or the different gold loading.
XPS high-resolution spectra for WO3 and WO3 / Au, after annealing,
are shown in Fig. 6 and the relative fitting
parameters are listed in Table 3.
XPS spectra for sample 1 (WO3 / Au 4.5 at. %), sample 2 (WO3 / Au
2.1 at. %) and sample 3 (WO3) films on alumina. (a) W 4f peaks and
(b) Au 4f, fitted with parameters in Table 3.
The W 4f spectra in Fig. 6a show a single
tungsten environment for all the samples, with the 4f7/2 peak centred
at 35.6 eV, which corresponds to the W6+ oxidation state (Fleisch and Mains, 1982).
The Au 4f peaks for the Au-functionalised WO3 samples were fitted using
single Gaussian–Lorentzian functions and the Au 4f7/2 binding energy was
found to be 83.9 eV (Fig. 6b), corresponding to
metallic gold (Dückers and Bonzel, 1989). Therefore W(VI) and Au(0) were the only
oxidation states found for these elements.
XPS analysis of annealed samples indicated a W / O ratio of 2.8 for samples 2
and 3 and a ratio of 2.7 for sample 1 (Table 4). This value is much higher
compared to the 2.4 ratio obtained by Vallejos et al. (2013) in both W and
Au / W samples. The gold atomic percentage (Au at. % on Au–WO3) obtained
via XPS (Table 4) is much higher than the one
obtained via EDX (Table 2). This can be explained by taking into account the
morphology of the WO3 / Au system and the area analysed; SEM images
(Fig. 4) show the Au nanoparticles are dispersed
on the surface of the tungsten oxide nanoneedle, and therefore, with XPS
being a surface analysis technique, the Au at. % appears to be greater
than EDX, which gives bulk information.
XPS Au atomic percentage composition. Oxygen : tungsten ratio.
Au at. % EDX
Au at. % XPS
O / W XPS
Sample 1
4.5
9.5
2.7
Sample 2
2.1
7.1
2.8
Sample 3
0
0
2.8
Response time and recovery time variation to the different gold loadings.
Response time (s)
Recovery time (s)
Sample 1
2.8
14.0
Sample 2
5.3
9.1
Sample 3
4.5
8.5
Gas-sensing response towards different amount of ethanol at 300 ∘C as a function of increasing gold loading.
Gas-sensing properties
Gas-sensing properties of gold-functionalised and non-functionalised
WO3 nanoneedles were tested towards ethanol vapours.
Figure 7 shows the sensor response towards
different amounts of analyte at an operating temperature of 300 ∘C
in the presence of 50 % of relative humidity. Gas sensors based on
non-functionalised WO3 (sample 3) showed a very low response towards
0.5 ppm of ethanol, with sensitivity increasing in the presence of higher
amounts of analyte as expected, up to a highest value of 1.2 when the
concentration of ethanol was 4 ppm.
Sample 2 (Au 2.1 at. %) and sample 1 (Au 4.5 at. %) showed
respectively 2-fold and 5-fold higher sensing response than
non-functionalised gas sensors (sample 3), with increased response even
towards low concentrations of analyte, close to 1.3 towards 0.5 ppm of
ethanol for sample 1 (4.5 % Au). These results demonstrate an enhancement
in gas-sensing response by increasing the amount of gold loading. The
highest loading amount used in this study was 4.5 %; however, as has been
seen in catalysis studies (Epling and Hoflund, 1999), increasing the loading percentage does
not necessarily lead to a linear improvement in catalytic performance.
Therefore, further experiments will be conducted in order to determine the
optimum loading of gold beyond which the activity is stable or decreases.
All sensors displayed an n-type response with decreasing resistance in the
presence of the reducing gas (Fig. 7). The
response and recovery times calculated for 3 ppm ethanol
(Table 5) exhibit approximately the same values for
sample 2 (2.1 % Au) and 3 (non-functionalised), a response time of 5 s and recovery time of 9 s. Sample 1 (4.5 % Au) had the
fastest response time, 2.8 s, but the slowest recovery time, 14 s.