Controlled electrochemical functionalization of MOx nanostructures by Au NPs for gas sensing application
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. A sch heme of MO Ox-based resiistive gas sennsor device. MOx M and Au u@MOx, aft fter the anneealing, were dispersed iin acetonitriile solution to be drop-castted as sensin ng layers bettween the tw wo Au contaacts of the seensor devicee. The devicee was annealed d at 300°C fo or 2 h to staabilize the seensor materiial. The expeerimental seet up used foor gas sensing measuremen m nts is reporteed elsewheree.[11] The seensing expeeriments werre conductedd at a sensor teemperature of o 300°C. Th he sensor reesponse to a given gas cconcentration was definned as the perceentage relative resistance change, ΔR/Ri (%), where ΔR is the channge in resisstance
between the values of steady-state of the electrical resistance of the sensor upon a target gas and in air, Rf and Ri, respectively. The mean gas sensitivity, Sm (% ppm-1), is defined as weighted mean of the ratio between percentage relative resistance change (%) over gas concentration unit (ppm); it can be calculated by Eq. (1): ∆ =
1
(%
)
(1)
where cj is a defined gas concentration to which corresponds the [∆R/Ri]j response. RESULTS AND DISCUSSION Characterization of MOx and Au@MOx nanostructures The detailed chemical speciation of MOx and Au@MOx nanocomposites was obtained by XPS analysis and reported in Table 1. The chemical speciation revealed that the O-M/M atomic ratio (e.g. the percentage of oxygen-bound-to-metal divided by the total metal percentage) was stoichiometric, that means that it is about 2 in ZrO2 and 1 in ZnO. Moreover, also in the hybrid systems annealed at 550°C, the O-M/M ratio (%at.) was stoichiometric. N1s and Cl2p signals, corresponding to TOAC stabilizer of Au NPs, were below the limit of detection, moreover carbon percentage decreased, revealing that most of the surfactant shell was removed by the thermal annealing. Moreover, a significant amount of elemental gold (Au0), in the range of 1.5-2 at.%, was deposited on MOx by SAE process (Table 1). Table 1. XPS surface chemical composition of MOx and Au@MOx. The O-M percentage refers to the atomic percentage of oxygen-bound-to-metal. ZrO2
Au@ZrO2
ZnO
Au@ZnO
C%
19.8% ± 0.5%
14.2% ± 0.5%
19.2% ± 0.5%
12.0% ± 0.5%
O(Total)% O-M%
59.0% ± 0.5% 35.8% ± 0.5%
65.6% ± 0.5% 36.5% ± 0.5%
49.1% ± 0.5% 31.5% ± 0.5%
46.0% ± 0.5% 40.1% ± 0.5%
Zr%
21.2% ± 0.5%
18.0% ± 0.5%
-
-
Zn%
-
-
31.7% ± 0.5%
40.5% ± 0.5%
Au%
-
2.2% ± 0.2%
-
1.5% ± 0.2%
In Figure 2 A the high resolution region of Au4f XPS spectrum is reported; it is composed by a single doublet, attributed to Au in the elemental oxidation state. The position of the Au4f7/2 peak at 83.7 eV ± 0.2 eV, lower than bulk metallic Au, is a well-known effect, commonly attributed to initial state size-effects, already highlighted for very small gold particles.[8] Figures 2 B), 2 C), 2 D) and 2 E) show SEM and TEM images of Au@MOx. Both metal oxides are composed of nanostructures successfully decorated by Au NPs with a diameter of
about 12 2 nm. In paarticular, ZrO O2-based co omposites reeveal a polyycrystalline nanostructurre o
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