Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization

Abstract

Nanomaterials for highly selective and sensitive sensors toward specific gas molecules of volatile organic compounds (VOCs) are most important in developing new-generation of detector devices, for example, for biomarkers of diseases as well as for continuous air quality monitoring. Here, we present an innovative preparation approach for engineering sensors, which allow for full control of the dopant concentrations and the nanoparticles functionalization of columnar material surfaces. The main outcome of this powerful design concept lies in fine-tuning the reactivity of the sensor surfaces toward the VOCs of interest. First, nanocolumnar and well-distributed Ag-doped zinc oxide (ZnO:Ag) thin films are synthesized from chemical solution, and, at a second stage, noble nanoparticles of the required size are deposited using a gas aggregation source, ensuring that no percolating paths are formed between them. Typical samples that were investigated are Ag-doped and Ag nanoparticle-functionalized ZnO:Ag nanocolumnar films. The highest responses to VOCs, in particular to (CH3)2CHOH, were obtained at a low operating temperature (250 °C) for the samples synergistically enhanced with dopants and nanoparticles simultaneously. In addition, the response times, particularly the recovery times, are greatly reduced for the fully modified nanocolumnar thin films for a wide range of operating temperatures. The adsorption of propanol, acetone, methane, and hydrogen at various surface sites of the Ag-doped Ag8/ZnO­(0001) surface has been examined with the density functional theory (DFT) calculations to understand the preference for organic compounds and to confirm experimental results. The response of the synergistically enhanced sensors to gas molecules containing certain functional groups is in excellent agreement with density functional theory calculations performed in this work too. This new fabrication strategy can underpin the next generation of advanced materials for gas sensing applications and prevent VOC levels that are hazardous to human health and can cause environmental damages.

Surface functionalization of ZnO:Ag columnar thin films with AgAu and AgPt bimetallic alloy nanoparticles as an efficient pathway for highly sensitive gas discrimination and early hazard detection in batteries

“…Compared to the pristine ZnO:Ag thin lm, the gas response of AgAu/ZnO:Ag to ethanol, acetone, n-butanol, 2-propanol and methanol was increased by factor of $2.8, $6, $3.2, $3 and $4.5, respectively. The gas response values are also considerably higher compared to those obtained for ZnO:Ag columnar lms decorated with monometallic Ag NP-synthesized by the same method, 32 proving the higher efficiency of bimetallic NP decoration. The low gas response to hydrogen gas is characteristic for all samples, with the exception of the AgPt/ZnO:Ag columnar lms.…”

“…[28][29][30][31] The Ag doping content in the prepared samples was adjusted to 0.95 wt%, which was conrmed in an earlier investigation by energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). 32 All samples were thermally annealed at 650 C for 2 h in air aer deposition.…”

“…4b, which was used for charge referencing, indicates atmospheric surface contamination from organic compounds, such as carbohydrates. 32 The prominent signal of Zn and O is attributed to the ZnO base layer. The comparison of the O-1s high resolution spectra (Fig.…”

“…Despite modelling a dopant concentration almost threefold higher than the 0.95 wt% used for the experiments, previous works have shown excellent agreement between the two approaches. 32,49 The substitutional doping of the symmetrically inequivalent exposed Zn atoms was inspected, corresponding to the following solid-state reaction, Zn 48 O 48 (10 10) + Ag(fcc) ¼ Zn 47 AgO 48 (10 10) + Zn(hcp) (8) where the face-centred cubic (fcc) Ag and hexagonal close packed (hcp) Zn were modelled using the one and two atoms primitive unit cells, respectively.…”

“…The electronic properties of the doped surfaces were further interrogated by measuring the work function (F) in order to gauge their reactivity as reducing agents. 32,49 The work function is the energy needed to remove an electron from the Fermi level (E F ) of the surface into the electrostatic potential of the vacuum Table 3 shows large reductions of over 50% in the work function values of all the doped surfaces with respect to their pristine counterparts. The absolute value for this type of thermodynamic work is between 2.61 and 2.64 eV for the doped 4-fold position in both terminations.…”