Development of Ag/ZnO nanorods and nanoplates at low hydrothermal temperature and time for acetone sensing application: an insight into spillover mechanism
- 93 Downloads
Nanostructure synthesis at low temperature with high purity and yield has a great potential in the present competitive research and development of nanomaterials for varieties of application. Here, we demonstrate the facile hydrothermal synthesis of ZnO nanocomposites at low hydrothermal temperature and time. With barely 80 °C hydrothermal temperature and 2 h of reaction time, ZnO nanorods were successfully synthesized, while by slightly increasing the reaction time (6 h-and-on), nanoplates of the same material were developed. Both the obtained nanostructures were analyzed using physio-chemical characterizing tools and examined for practical relevance as gas sensing material. The optimized sample was further treated to Ag loading (1–5 mol%) for lowering the operating temperature of the sensor. ZnO sample with 3 mol% Ag loading showed excellent sensitivity of 92.7% for 1000 ppm of acetone concentration with a drop in the operating temperature from 325 °C (Pristine ZnO) to 250 °C (Ag-loaded ZnO). The morphological correlation with reaction time and thereby sensitivity and spillover mechanism are discussed.
KeywordsComposite materials Hydrothermal process Ag/ZnO Sensors Acetone
Since the discovery of ZnO and its usefulness in varieties of applications, it has attracted increasing attention as promising semiconducting material [1, 2, 3, 4, 5]. Apart from gas sensing, ZnO has attracted high technological applications, like photodetectors, photodiodes, surface acoustic wave filters, solar cells, photonic crystals, LEDs, optical waveguides, due to its wide direct band gap (3.3 eV) and large excitation binding energy (60 meV) [6, 7, 8, 9, 10, 11, 12]. In the state of the art, doping of noble metals like gold (Au), platinum (Pt), ruthenium (Ru), nickel (Ni), palladium (Pd), gallium (Ga), silver (Ag) and their combinations showed significant improvement in ZnO-based gas sensor [13, 14, 15, 16, 17, 18, 19, 20]. The loading of these noble metals into parent metal oxide amplifies the charge transfer mechanism. Amongst the noble metals, silver is one of the best candidates for loading into ZnO matrix, due to its least orbital energy, better solubility and larger ionic size. In comparison with other noble metals, Ag boosts the catalytic activities and thereby surface reactions.
Summary of hydrothermally developed ZnO nanostructures at various reaction temperature and time
2 Experimental procedure and characterizations
3 Results and discussion
Diffusion of gases to the active region
Adsorption of gases on to active region
Desorption of reaction products from active region
Diffusion of reaction products away from active region.
The following are the possible reactions for acetone, ammonia and ethanol, respectively.
In case of having the larger surface area of the sensor, a more number of active sites will be available for adsorption of the target gas. This will, in contrast, release a more number of electrons into the conduction band of ZnO and thereby decrease in the sensor . Remarkable performance distinction was observed amongst developed ZnOs at different hydrothermal treatments (Fig. 5a). Sample H6 showed the highest response (90.5% at 325 °C for 1000 ppm), due to the formation of precise phase (validated in XRD) as well as high surface area in comparison with nanorods (Fig. 4). The BET surface area for sample H4 and sample H6 is obtained as 10.78 m2/g and 28.31 m2/g, respectively.
In contrast, higher reaction time (> 6 h) resulted into blocked channels, which trapped the target gas. Therefore, due to unavailability of oxygen vacancy, the gas sensitivity got decreased. Hence, being the optimized pristine sample, Ag loading (1–5 mol%) was carried out in sample H6. By the addition of Ag in the pristine ZnO matrix, gas sensing performance got improved, which is highlighted in Fig. 5b. As expected, though marginal improvement in sensitivity (from 90.5% to 92.7%) was observed, a dramatic fall in the operating temperature (from 325 to 250 °C) was realized. Thereby, one of the objectives of the present work: to lower down the operating temperature by Ag incorporation was achieved. Amongst the different Ag loadings, sample A3 has the highest response of 92.7% for 1000 ppm. The dots in Fig. 5c describe the maximum value of sensitivity obtained for all the developed samples. The nature of the graph is attributed to the spillover mechanism, due to Ag loading, which is discussed later in the end of this section . With an increase in concentration of Ag (1–3 mol%), more electrons made available (spillover) which helped to have a better control of resistance. However, after threshold level of Ag dosing, the spillover effect got ceased due to agglomeration of nanoparticles . Therefore, Ag loading with 3 mol% in the pristine ZnO was treated as best doping level.
One of the important factors, selectivity, is shown in Fig. 5d. The optimized sample, A3 for gas sensing proficiency towards aforementioned reducing gases, was deduced. From all the target gases, Ag/ZnO demonstrated the best selectivity towards acetone. At an operating temperature of 250 °C, sample showed 92% sensitivity for 1000 ppm acetone concentration over the other target gases. This is due to the lighter nature of acetone which enables faster reaction steps (adsorption, diffusion and desorption) that all are involved in gas sensing mechanism, mentioned above.
The related gas sensing evaluation of all the developed sensors and their stability is further highlighted in the supplementary information section (Fig. S7–S10). The gas sensing measurements of H6, A1, A3 and A5 sensors were repeatedly measured on the regular intervals in order to confirm the stability and reproducibility of the sensors. The acetone performance of H6 sensor was measured at an operating temperature of 325 °C towards 1000 ppm concentration of acetone, whereas samples A1–A5 were measured at an operating temperature of 250 °C towards same acetone concentration (1000 ppm). The sensor performance of Ag-loaded ZnO sensors was carried out two times after the initial measurement with the interval of 10 days. It was observed that the sensor responses after 2 months of performance were found to be 90% (H6), 83% (A1), 90% (A3) and 89% (A5), respectively. Thus, the stability and reproducibility were found to be more than 90% in all the samples after 2 months of shelf life. Therefore, confirm the reproducibility and stability of the developed sensors.
3.1 Spillover mechanism in Ag/ZnO
An improvement in the sensor response can be greatly achieved by the surface modification of metal oxide (ZnO in the present case) either with -(i) doping, -(ii) multicomponent/binary compounds, or (iii) metals/noble metals such as Pd, Au and Ag as a catalyst (Spillover). These methods are highly exploited to enhance the gas sensing performance of metal oxide gas sensors [34, 35, 36]. The prime purpose of using these additives is to modify the catalytic activity of the base oxide, favouring the generation of active sites/phases and thereby enhancing the electron exchange rate and as well lowering the operating temperature. By modifying/altering the metal oxide surface through different surface atoms, a new chemical reactivity can be foreseen which enables the sensor to operate at lower temperature. This is the one of the prime motivations of using Ag in the present work that should trigger the spillover process and result into higher response at lower operating temperature.
Herein, emphasis is given to incorporate Ag in ZnO surface, where the support is ZnO whose electrical properties are made to change by active oxidation reactions on its surface. Spillover is a mechanism where the oxidation of reducing agent can be accelerated on the semiconductor surface at much lower temperatures by the presence of dispersed metallic catalyst (Ag in the present case). Ag acts as surface sites for adsorbates and promoters to trigger the surface catalysis. They create additional adsorption sites and surface electronic states. This in turn enhances the gas sensitivity, selectivity and rate of response. 1 mol% doping of Ag led the ZnO to operate at 275 °C from its pristine value of 325 °C. With further increase in the doping (3 mol%), the operating temperature got reduced to 250 °C with an improvement in the response of 92.7% towards acetone. However, the excessive amount of Ag (5 mol%) led to decrease in the response to 91.2% with the same operating temperature of 250 °C. This is due to the change in the functions (catalysis), turning into either shunting layer or active membrane filters, which obstructs the penetration of detecting gas in the surface of ZnO matrix.
Step 1 Oxygen molecules are adsorbed on the Ag surface
Step 2 and spilled over to ZnO, where dissociated oxygen species are ionized with electrons from ZnO.
- Step 3 In the presence of target gas, the ionosorbed oxygen reacts with target gas forming a by-product and injecting the electrons back into the conduction band of ZnO. As the concentration of ionosorbed oxygen species is larger on the Ag/ZnO, a larger number of electrons are released to the conduction band under target gas exposure causing a higher sensor signal. The proposed mechanism is illustrated in the following scheme (Fig. 6).
Comparative chart for acetone response of ZnO-based material over varying acetone concentration, operating temperature and synthesis route
Acetone concentration (ppm)
Hydrothermal, 200 °C
RF reactive sputtering
1000 to 15 ppm
92% @ 500 ppm
Hydrothermal, 200 °C
Hydrothermal, 150 °C
3.58 μAcm − 2 mM − 1
Hydrothermal, 180 °C
Hydrothermal, 120 °C
Hydrothermal, 110 °C
In conclusion, we demonstrated the facile hydrothermal synthesis of ZnO nanocomposites at lower hydrothermal temperature. The obtained nanorods (at 80 °C, 2–4 h) and nanoplates (at 80 °C, > 4 h) were analyzed through XRD, SEM/TEM, SAED and EDS analyses. As the material holds two distinct morphologies, gas sensing competence was found different for different hydrothermal reaction time. ZnO sample with 3 mol% Ag loading showed excellent sensitivity of 92.7% for 1000 ppm of acetone concentration with a drop in the operating temperature from 325 to 250 °C. Further, the spillover mechanism of Ag/ZnO is explained using schematic illustration by taking acetone as an example of target gas. The reported approach of fabricating gas sensor is easily reproducible at relatively lower cost and thus offers great promise for future industrial application of gas sensors.
The authors greatly acknowledge to CSIR, India, for the financial support of this work (03(1389)/16/EMR-II). Dr. Nadargi acknowledges CSIR, India, for awarding Research Associate under the same scheme.
Compliance with ethical standards
Conflict of interest
The authors declare that there is no competing interest whatsoever.