Jing
Wang
*a,
Mingying
Yu
a,
Xian
Li
b and
Yi
Xia
c
aThe Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China. E-mail: jingwang@jiangnan.edu.cn
bAgricultural Information Institute, Chinese Academy of Agricultural Sciences, Key Laboratory of Agricultural Information Service Technology of Ministry of Agriculture, Beijing 100081, China
cResearch Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming 650093, China
First published on 15th November 2018
Polystyrene sulfonate (PSS) functionalized ZnO nanowires were hydrothermally synthesized and assembled into a UV-activated optoelectronic NO2 sensor for the first time. In situ PSS functionalization during hydrothermal synthesis not only facilitated the formation of high-quality ZnO nanowires with small diameters and high aspect ratios, but also significantly improved the UV photoconductance of ZnO nanowires through photo-induced electron injection, leading to the enhanced optoelectronic properties of ZnO nanowires. The optoelectronic NO2 sensors based on PSS-functionalized ZnO nanowires showed much higher responses and faster response rates under UV illumination at room temperature (25 °C). The responses ranged from 60% to 690% toward NO2 concentrations of 50 ppb to 2 ppm. Moreover, the optoelectronic sensors exhibited excellent reversibility, good stability and a low detection limit (5 ppb), making the efficient, optically controlled sensing of ppb-level NO2 gas possible.
In order to further satisfy the practical applications of room-temperature NO2 sensors, many efforts have been devoted to improving the optoelectronic sensing performance of ZnO nanostructures. To date, mainly two effective strategies including morphology modulation and surface functionalization have been developed. On the one hand, successful attempts have been reported to improve the morphology-dependent optoelectronic properties by modulating ZnO nanostructures with tailored size, dimensions or microstructures.9–12 Particularly, one-dimensional ZnO nanostructures have been demonstrated as promising candidates for multifunctional optoelectronic devices owing to their large surface-to-volume ratios, high electron mobility and excellent device interconnecting ability.11,12 On the other hand, improved optoelectronic NO2 sensing properties have been achieved by surface functionalization of ZnO nanowires with various light-active materials, such as other semiconducting materials,13–15 noble metal nanoparticles16,17 or light-absorbing dyes and polymers.18,19 These surface modifiers significantly increase the free electrons in ZnO under light illumination through photo-induced charge separation and transfer processes; thus the number of electrons that participate in the surface reaction with NO2 molecules greatly increase, leading to the enhanced optoelectronic sensing performances.
In this work, polystyrene sulfonate (PSS) functionalized ZnO nanowires were hydrothermally synthesized and fabricated into a UV-activated optoelectronic NO2 sensor for the first time. The in situ PSS functionalization during hydrothermal synthesis facilitated the formation of high-quality ZnO nanowires with small diameters and high aspect ratios, serving as ideal sensing materials for optoelectronic devices. More importantly, the surface adsorbed PSS significantly improve the UV photoconductance of ZnO nanowires through photo-induced electron injection, leading to the enhanced optoelectronic properties of ZnO nanowires. Owing to the dual roles of PSS functionalization, the room-temperature NO2 sensors based on PSS-functionalized ZnO nanowires exhibited a highly sensitive, fast, reversible and stable optoelectronic response under UV illumination, making the efficient, optically controlled sensing of ppb-level NO2 gas possible.
The sensing tests were performed in a homemade chamber with dry air as the carrier gas at 25 °C unless otherwise specified. The input concentration of NO2 was 2 ppb to 2 ppm and the flow rate of the mixed gas was 200 mL min−1. The resistance of the films was measured in the dark or under UV illumination and recorded by using a Keithley 2700 multimeter/data acquisition system. The response (R) of the sensor is defined as , where Ra is the initial resistance in dry air, and Rg is the resistance of the device after being exposed to a NO2 atmosphere. The response time is defined as the time required until 90% of the whole response is reached while the recovery time denotes the time needed until 90% of the signal is recovered.
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Fig. 1 SEM images of ZnO–blank (a) and ZnO–PSS (b); (c) statistical diameter distribution of the nanowires; and (d) typical XRD pattern of ZnO–PSS; insets: HRTEM image and SAED pattern. |
The anisotropic growth of inorganic nanostructures under hydrothermal conditions was usually considered as a kinetically controlled process.20 In our experiments, it was observed that the growth rate of ZnO in the presence of PSS was significantly lowered, implying that the growth of ZnO was hindered by PSS. Such growth suppression was widely observed and attributed to the adsorption of the organic ligands at the crystal–solution interface.21 Moreover, these organic ligands could exert a highly selective effect, acting on certain crystallographic faces and thus directing the anisotropic growth of nanostructures.21 In the current case, we suggest that the PSS molecules may selectively adsorb on the (100) prism planes rather than on the (001) polar planes of ZnO, inhibiting their radial growth and thereby promoting the anisotropic growth of nanowires along the c-axis. The adsorption of PSS onto ZnO surfaces was examined by FT-IR analyses, as shown in Fig. 2. Compared with the IR curve of uncapped ZnO, some new peaks occurred in ZnO synthesized in the presence of PSS. The peaks at 2852 cm−1 and 2922 cm−1 were attributed the stretching vibrations of –CH2–. The peak at 1633 cm−1 attributable to the stretching of CC in the aromatic rings of PSS was also observed for the ZnO–PSS sample. The two peaks at 1173 cm−1 and 1095 cm−1 could be assigned to the two stretching vibration modes of S
O, νas(SO3) and νs(SO3), respectively, which shifted to the lower wavelength compared to those in free sulfonate groups (1180 and 1124 cm−1),22 implying the strong interactions of sulfonate groups in PSS with ZnO surfaces.
XPS spectra were further employed to study the surface structures of PSS-functionalized ZnO to identify the adsorption mechanisms, as shown in Fig. 3. Additional S 2p and Na 1s peaks were detected in the survey spectra (Fig. 3a). The high evolution S 2p peak of ZnO–PSS could be divided into two peaks centered at 168.9 and 167.7 eV (Fig. 3b), corresponding to the S 2p1/2 and S 2p3/2 transitions, respectively, which were quite identical to those in ZnSO3,23 suggesting the interaction between the –SO3 groups in PSS and the zinc ions on ZnO surfaces. For zinc-related species, Zn L3M45M45 Auger electron spectra (AES) were recorded since they were quite sensitive to the structural variations (Fig. 3c). The peaks at 497.7 and 494.3 eV could be assigned to the lattice Zn2+ and the zinc interstitials,24 respectively, while the additional peak centered at 496.3 eV should have originated from the surface zinc ions interacting with the PSS molecules. The above FT-IR and XPS analyses implied that the PSS molecules may serve as a surface ligand, with their sulfonate groups coordinating with the surface Zn2+ ions forming Zn–SO3 complexes. It was noticed that the anionic polymer functionalized ZnO nanostructures were generally prepared by electrostatic layer-by-layer assembly, with the assistance of positively charged polymers as intermediate layers, by considering the negatively charged surface of ZnO in aqueous solution.25 In contrast, herein, in situ PSS functionalization was carried out during the hydrothermal synthesis based on a surface ligand adsorption mechanism. Such an in situ functionalization method not only avoids the multistep assembly procedures and the use of other additives, but also renders a robust contact between ZnO and PSS, which is beneficial for the charge transfer in optoelectronic processes.
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Fig. 3 XPS spectra of ZnO–PSS: (a) survey curve; (b) high-resolution S 2p peak and (c) Zn L3M45M45 Auger peak. |
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Fig. 4 (a) The UV-Vis spectra and (b) room-temperature photoluminescence spectra of ZnO–blank and ZnO–PSS. |
The as-prepared ZnO nanowires were constructed into optoelectronic devices to investigate their electric transport properties at 25 °C under UV illumination. Fig. 5a and b present the current–voltage (I–V) characteristics of the devices based on ZnO-blank and ZnO–PSS, respectively, under dark and UV illumination conditions (λ ∼ 254 nm, P = 5 × 10−4 W cm−2). Both samples exhibited linear I–V characteristics, indicating good ohmic contacts between the nanowire films and electrodes.29 At an applied voltage of 5.0 V, the dark currents across ZnO-blank and ZnO–PSS were 2.49 μA and 0.87 μA, respectively. The suppressed dark current of ZnO–PSS could be attributed to the narrowed conducting channel induced by the thinner diameter, and the barrier effects of surface-absorbed PPS (considering that the PSS polymer itself is almost nonconductive). Under UV illumination, the current across the ZnO–PSS sample drastically increased to 212 μA, which was much higher than that of the ZnO-blank sample (14 μA), indicating that the PSS functionalization greatly improved the photoconductance of the ZnO nanowires. Due to the suppressed dark current and enhanced photoconductance, the PSS-functionalized ZnO nanowires exhibited a high UV-to-dark signal ratio of ∼240, being over 40 times larger than that of the ZnO-blank sample (IUV/Idark = 5.6). The enhanced UV photoconductance of the PSS-functionalized ZnO nanowires could be attributed to the charge transfer between PSS and ZnO under UV illumination, as illustrated schematically in Fig. 5c. Since the absorption peak of PSS is very close to the 254 nm UV source used in this study, it is expected that under UV illumination, electrons in PSS are excited from the highest occupied molecular orbital (HOMO) to its lowest unoccupied molecular orbital (LUMO), and then they are injected into the conduction band of ZnO. It is also possible that the electron in the valence band of ZnO is likely to be excited to the HOMO of PSS. Such a “hopping” process may enhance the transition probability of the valence electrons in ZnO to its conduction band, thus largely increasing the number of photogenerated charge carriers in ZnO.25,30 In addition, since the surface defects of ZnO were reduced by PSS functionalization (as revealed by PL analyses), the defect mediated recombination of electrons and holes could be greatly inhibited, which may also contribute to the enhanced optoelectronic performance of PSS-functionalized ZnO nanowires.
Based on the above experimental results, the roles of PPS functionalization in enhancing the optoelectronic performances of ZnO nanowires could be summarized by the following two aspects. On the one hand, the PSS molecules may adsorb preferentially onto the (100) prism planes of ZnO nanowires through sulfonate groups coordinating with surface zinc ions, thus hindering the radial growth and thereby leading to the formation of nanowires with thinner diameters and high aspect ratios. On the other hand, the energy levels introduced by PSS could be coupled with ZnO, serving as “hopping” states that promote the injection of electrons into the conduction band of ZnO under UV illumination. Therefore, a significant enhancement of the optoelectronic performance of ZnO nanowires was achieved. Although both the roles of polymers in directing the ZnO nanowire growth and improving the optoelectronic properties of ZnO–polymer hybrids have been previously reported, our current work reports for the first time that the UV-absorbing PSS polymer could play dual roles in enhancing the optoelectronic properties of ZnO nanowires.
The optoelectronic NO2 sensing performance of the PSS-functionalized ZnO nanowires was further evaluated at 8 different concentrations of NO2 ranging from 50 ppb to 2 ppm under UV light illumination (bias = 5.0 V, 5 × 10−4 W cm−2), as shown in Fig. 7a. One can see that the response increases quickly upon NO2 exposure and the response gradually decreases to the base line when NO2 is switched off, demonstrating excellent reversible response and recovery properties upon continuous NO2 exposure/release cycles. The ZnO–PSS based optoelectronic gas sensor shows responses ranging from 60% to 690% toward NO2 concentrations ranging from 50 ppb to 2 ppm. Fig. 7b shows the responses of the sensor as a function of NO2 concentrations. The data can be fitted well with the Langmuir adsorption isotherm, R = a/(1 + b/C), where C represents the concentration of NO2, which was further confirmed by the linear fitting of R−1versus C−1 (inset of Fig. 7b). These results can be understood as the surface coverage of NO2 molecules follows the Langmuir adsorption isotherm. At lower concentrations, the sensor exhibits an approximately linear relationship between the sensor response and the gas concentration because the surface adsorption sites are not saturated. At higher concentrations, the surface coverage tends to saturate and hence leads to the saturation response.32,33 The dynamic response curves measured at very low concentrations of NO2 revealed a clear response of ca. 4.5% to 5 ppb of NO2, which was considered as the detection limit of NO2. In comparison, the ZnO-blank sample did not show any obvious response to 20 ppb of NO2. Such a low detection limit of PSS-functionalized ZnO nanowires is beneficial for their practical application in NO2 sensors. In addition, Fig. 7d shows the responses of the sensor to 2 ppm of NO2 and other gases including NO, SO2, Cl2, HCHO, H2S, NH3 and CO, which are typical oxidative or reductive air pollutants. Obviously, the ZnO–PSS optoelectronic sensor exhibited very high selectivity to NO2.
In order to confirm the repeatability of the optoelectronic NO2 sensor, the time dependent photoresponse of the sensor in air or in 2 ppm of NO2 was tested by periodically turning on and off the UV light, as shown in Fig. 8a. It was observed that the sensor could work once light was illuminated and it was repeatable. The excellent optoelectronic properties of ZnO–PSS were favorable for the instant, optically controlled sensing of NO2 since the photoresponse was fast and the photocurrent was quite stable and repeatable. The sensor was also exposed to 2 ppm of NO2 for 6 successive exposure/release cycles (Fig. 8b). Stable responses of 670–700% were observed in 6 cycles with little appreciable baseline drift, demonstrating good reproducibility of the sensor. Furthermore, we checked the long-time stability of the sensor in air to find more practical application capability. The dynamic response curves of the sensor after 30 and 90 days’ storage are presented in Fig. 8c. After 90 days in air, the sensor could still accomplish effective detection with fast response and almost full recovery, and a response of 590% to 2 ppm of NO2 was achieved, indicating that ca. 85% of the initial response could be retained. The effect of the relative humidity (RH) on the sensor response of the ZnO–PSS sample was also studied, as shown in Fig. 8d, which is very important for a room-temperature gas sensor. It was found that the sensor response decreased with increasing RH, which should be attributed to the adsorption of water molecules that inhibited the adsorption of O2 and NO2. However, at a high RH of 80%, a sensor response of ca. 410% could still be achieved. The result indicated that although the sensing performance of the optoelectronic sensor showed a degradation at high RH, it could still meet the demand of NO2 detection. To sum up, owing to the dual roles of PSS functionalization, the room-temperature NO2 sensors based on PSS-functionalized ZnO nanowires exhibited a highly sensitive, fast, reversible and stable optoelectronic response under UV illumination, making the efficient, optically controlled sensing of ppb-level NO2 gas possible. The presently developed in situ functionalization approach also provides an alternative strategy for the design and construction of metal oxide/polymer nanocomposites, which opens a facile pathway to the fabrication of high-performance room-temperature NO2 sensors.
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