Synthesis and enhanced humidity detection response of nanoscale Au-particle-decorated ZnS spheres
© Liang and Liu; licensee Springer. 2014
Received: 5 November 2014
Accepted: 25 November 2014
Published: 30 November 2014
We successfully prepared Au-nanoparticle-decorated ZnS (ZnS-Au) spheres by sputtering Au ultrathin films on surfaces of hydrothermally synthesized ZnS spheres and subsequently postannealed the samples in a high-vacuum atmosphere. The Au nanoparticles were distributed on ZnS surfaces without substantial aggregation. The Au nanoparticle diameter range was 5 to 10 nm. Structural information showed that the surface of the annealed ZnS-Au spheres became more irregular and rough. A humidity sensor constructed using the Au-nanoparticle-decorated ZnS spheres demonstrated a substantially improved response to the cyclic change in humidity from 11% relative humidity (RH) to 33% to 95% RH at room temperature. The improved response was associated with the enhanced efficiency of water molecule adsorption onto the surfaces of the ZnS because of the surface modification of the ZnS spheres through noble-metal nanoparticle decoration.
Semiconducting compounds have been proposed as potential materials for use in sensing devices for gas detection and humidity measurement [1–4]. In particular, because of their high surface-to-volume ratio, nanostructured semiconductors exhibit physical and chemical behaviors that are superior to their bulk counterparts [5–7]. Among various sensors, humidity sensors have crucial applications in semiconductor electronics and food-processing industries. Various semiconducting materials have been used in humidity-sensing devices [8, 9]. The ZnS-based humidity sensors have been realized through complex processes or a high-temperature process [10, 11]. Humidity sensors based on ZnS with a facile synthesis methodology are limited in number. ZnS is one of the most crucial II to VI semiconductor compounds . ZnS has shown promise for use in novel diverse applications including light-emitting diodes, sensors, infrared windows, electroluminescent materials, and flat-panel displays [3, 12–15]. ZnS nanostructures can be synthesized by various physical and chemical methodologies [3, 16]. Although thermal evaporation has been widely used for synthesizing nanoscale ZnS, both the extremely high process temperature and complex process control prevent the realization of high-performance ZnS-based sensors . From a morphological perspective, enhancing the sensing performance of nanostructures continues to be challenging, despite their sensing properties being superior to those of thin films and bulk materials. Recently, surface functionalization with noble metals or through noble-metal doping has been achieved, and it enhances the sensitivity and stability of nanostructure-based sensors [17–19]. However, most of these studies have focused on gas-sensing behaviors, and there are few reports on the humidity-sensing behavior. Recently, Pd2+-doped ZnO nanotetrapods were prepared and the humidity detection capability of ZnO was improved through noble-metal doping . Room-temperature humidity-sensing properties of boron nitride nanotubes have been enhanced through surface decoration with Au particles . In this study, a combination of a chemical solution process and the sputtering technique was used to prepare Au-nanoparticle-decorated ZnS spheres. The effects of the surface modification of ZnS-based humidity sensors through Au-nanoparticle decoration were investigated in this study. The ZnS-based humidity sensor performance was observed to be correlated with microstructural changes.
The zinc nitrate (Zn(NO3)2 · 6H2O) and thioacetamide (TAA) were used as source materials to prepare hydrothermally synthesized ZnS spheres in this work . The polyvinylpyrrolidone (PVP) was used as a surfactant to control the ZnS sphere size. The 200-nm-thick SiO2/Si (100) substrates were used as templates for deposition of ZnS spheres. The reaction solution contains equimolar of zinc nitrate (0.05 M) and TAA (0.05 M). The PVP was subsequently added to the above solution. The reaction solution was stirred at room temperature for 30 min. Subsequently, the reaction solutions and the substrates were transferred into a Teflon-lined stainless steel autoclave. The hydrothermal reaction temperature was fixed at 130°C and the duration is 6 h. At the end of the growth period, the substrates were removed from the solution, then immediately rinsed with deionized water to remove any residual salt from the surface, and dried in air. For synthesis of Au-nanoparticle-decorated ZnS spheres, Au ultrathin film was deposited onto the surfaces of the hydrothermally synthesized ZnS spheres using a home-built DC sputtering system. During deposition, substrate temperature was maintained at room temperature, and the deposition gas pressure was fixed at 20 mTorr with a pure Ar ambient. The sputtering time and power for the Au are 40 s and 20 W, respectively. The samples were further annealed in a high vacuum chamber (base pressure approximately 3 × 10−6 Torr) at 300°C for 30 min to induce ultra-thin Au film to form Au nanoparticles on the ZnS surfaces (ZnS-Au). The 200-nm-thick SiO2 layer herein was used as an insulator layer. The ZnS and ZnS-Au spheres were dispersed onto the SiO2 layer. Subsequently, the silver glue was used to fabricate two metal electrodes onto the ZnS/SiO2 for electric measurements.
Crystal structures of the samples were investigated by X-ray diffraction (XRD; Panalytical X’Pert Pro MPD) using Cu Kα radiation. Morphologies of the as-synthesized samples were characterized by scanning electron microscopy (SEM; Hitachi S-4800), and high-resolution transmittance electron microscopy (HRTEM; Philips Tecnai F20 G2) was used to investigate the coverage and morphology of Au nanoparticles on the surfaces of the ZnS spheres. The energy-dispersive X-ray spectroscopy (EDS) attached to TEM was used to evaluate the composition of the samples. Room temperature-dependent photoluminescence (PL; JOBIN-YVON T64000 Micro-PL Spectroscopy) spectra were obtained using the 325-nm line of a He-Cd laser. The electrical characteristics of the ZnS-based sensors were tested as a function of relative humidity (RH) with a fixed applied voltage of 5 V in a home-built testing chamber at room temperature. A computer was used to collect the signals from the sensor in the testing circuit. The RH levels for the humidity sensor test herein were controlled to be approximately 11%, 33%, 55%, 75%, 85%, and 95% and a hygrometer was used to monitor RH levels in the test chamber. The experimental setup has been described elsewhere . The different RH levels were generated by referencing various saturated salt solutions in closed chamber at room temperature . The humidity sensitivity test of the samples was performed with the sample initially stored in the dry ambient (11% RH); subsequently, the sensor was upon exposure to one of the higher selected RH levels (33% to 95%). Finally, the sensor was restored in the 11% RH environment again to finish a test run.
Results and discussion
Highly crystalline ZnS spheres were decorated with Au particles by combining the sputtering technique and high-vacuum thermal annealing. Detailed TEM images revealed that the as-synthesized Au particles had nanoscale sizes and that they were efficiently distributed on the surface of the ZnS spheres. PL spectra revealed that the nanoparticle surface modification changed the PL spectra intensity and intensity ratio of ZnS emission bands. Au nanoparticles decorating the surface of the ZnS spheres significantly affected the sensor’s humidity response. The ZnS-Au sensor exhibited considerably enhanced sensitivity compared with a pure ZnS sphere sensor at various percent RH levels operated at room temperature.
YCL is a professor of the Institute of Materials Engineering at National Taiwan Ocean University (Taiwan). SLL is a graduate student of the Institute of Materials Engineering at National Taiwan Ocean University (Taiwan).
This work is supported by the Ministry of Science and Technology of Taiwan (Grant No. NSC 102-2221-E-019-006-MY3).
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