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Hierarchical ZnO nanosheet-nanorod architectures for fabrication of poly(3-hexylthiophene)/ZnO hybrid NO2 sensor Jing Wang, Xian Li, Yi Xia, Sridhar Komarneni, Haoyuan Chen, Jianlong Xu, Lan Xiang, and Dan Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12553 • Publication Date (Web): 15 Mar 2016 Downloaded from http://pubs.acs.org on March 19, 2016

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ACS Applied Materials & Interfaces

Hierarchical ZnO nanosheet-nanorod architectures for fabrication of poly(3-hexylthiophene)/ZnO hybrid NO2 sensor Jing Wanga, b†, Xian Lic†, Yi Xiaa, Sridhar Komarnenib*, Haoyuan Chena, Jianlong Xuc, Lan Xianga* and Dan Xiec* a

Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China

b

Materials Research Institute, Materials Research Laboratory, The Pennsylvania State University, University

Park, PA 16802, USA c

Institute of Microelectronics, Tsinghua University, Beijing 100084, China

Abstract A facile one-step solution method has been developed here to fabricate hierarchical ZnO nanosheet-nanorod architectures for compositing with poly(3-hexylthiophene) (P3HT) for fabricating a hybrid NO2 sensor. The hierarchical ZnO nanosheet-nanorod architectures were controllably synthesized by aging the solutions containing 0.05 mol·L-1 Zn2+ and 0.33 mol·L-1 OH- at 60 oC through a metastable phase-directed mechanism. Concentration of OH- played a huge role on the morphology evolution. When the [OH-] concentration was decreased from 0.5 to 0.3 mol·L-1, the morphology of the ZnO nanostructures changed gradually from mono-dispersed nanorods (NR) to nanorod-assemblies (NRA), and then to nanosheet-nanorod architectures (NS-NR) and nanosheet-assemblies (NSA), depending on the formation of various metastable, intermediate phases. The formation of NS-NR included the initial formation of ZnO nanosheets/γ-Zn(OH)2 mixed intermediates, followed by the dissolution of Zn(OH)2, which served as soluble zinc source. Soluble Zn(OH)2 facilitated the dislocation-driven secondary growth of ZnO nanorod-arrays on the primary defect-rich nanosheet substrates. Hybrid sensors based on composite films composed of P3HT and the as-prepared ZnO nanostructures were fabricated for the detection of NO2 at room temperature. The P3HT/ZnO NS-NR bilayer film exhibited not only the highest sensitivity but also good reproducibility and selectivity to NO2 at room temperature. The enhanced sensing performance was attributed to the formation of P3HT/ZnO heterojunction in addition to the enhanced adsorption of NO2 by NS-NR ZnO rich in oxygen-vacancy defects. † These two authors contributed equally to this work. * Corresponding authors. Fax: +86-10-62772051; Tel: +86-10-62788984 Emails: [email protected] (S. Komarneni); [email protected] (L. Xiang); [email protected] (D. Xie)

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Keywords: ZnO, nanosheet-nanorod architectures, metastable phase-directed synthesis, P3HT/ZnO heterojunction film, room-temperature NO2 sensor 1. Introduction Three-dimensional hierarchical metal oxide semiconductors assembled by one-dimensional (1D) or two-dimensional (2D) nanoscale building blocks (nanorods, nanowires, nanosheets, etc.) have attracted much attention owing to their unique architectures and wide applications in the fields of catalysis, sensors, batteries, solar cells, etc.1-4 Recently, with the aim of combining the features of 1D and 2D nanostructures, many efforts have been devoted to the solution synthesis of hierarchical metal oxides assembled by 1D and 2D building blocks simultaneously.5-9 For example, Chen et al., (2011) reported the solution synthesis of flowerlike SnO2 hierarchical structures which were assembled by nanosheets and ultrathin nanorods and exhibited enhanced gas sensing performance when compared with the nanosheet assemblies.5 Wu et al., (2014) developed a solution-based strategy for the fabrication of TiO2 nanowire-nanosheet-nanorod hyperbranched arrays with high performance in dye-sensitized solar cells owing to the enhanced capabilities for light trapping and scattering.9 These 1D/2D building blocks of co-assembled architectures may lead to potential applications in functional devices. Hierarchical ZnO structures co-assembled by 1D/2D building blocks have also been synthesized via solution-based approaches and they showed enhanced performance in luminescent devices, solar cells, photo-catalysts, gas sensors, etc.10-15 Such hierarchical structures were previously fabricated by time-consuming (2.0-24.0 h) multi-step strategies, which involved the stepwise fabrication of secondary nanostructures on the primary structures. For example, Qiu et al., (2011) synthesized hierarchical ZnO nanosheet-nanorod

structures

via

a

two-step

electrochemical

deposition

route

involving

the

electrode-deposition of ZnO nanosheets at 70 oC for 1.0 h and the subsequent electrochemical growth of the ZnO nanorods on the as-prepared nanosheets for another 1.0 h.13 A two-step hydrothermal method was developed to grow ZnO nanorod-arrays on the ZnO nanosheets by refreshing the solution after aging at 75 oC for 3.0 h to synthesize nanosheets, followed by aging at 90 oC for 5.0 h for the nanorod growth.10 Up to now little or no work was reported about the formation of the 1D/2D building blocks of co-assembled ZnO architectures via rapid one-step solution route.


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Nitrogen oxides (NOx) are amongst the most toxic gaseous pollutants, which are resulted from combustion or automotive emissions.16 Though NO is the major component of flue gas, it can be easily oxidized in the atmosphere to NO2;17 NO2 is also released into the atmosphere from industrial sources, and has become one of the most common air pollutants.18 Exposure to >3 ppm NO2 directly affect human health.19 Therefore, the monitoring of NO2 of ppm level has been an important environmental issue. ZnO nanostructures have been widely used for the detection of NO2 at elevated temperatures (>200 oC) owing to their high sensitivity and selectivity.20-23 To decrease the energy consumption and the risk of gas explosion, it is desirable to monitor NO2 at room temperature with high sensitivity. Recent studies showed that the detection of NO2 at room-temperature could be achieved by hybrid sensors based on composite materials such as ZnO/Au nanoparticles,24 ZnO/silicon,25 graphene/ZnO,26,27 and conducting polymer (CP)/ZnO,28-31 etc. Among them, CP/ZnO composites have advantages of easy processing and low cost. Hybrid sensors based on CP/ZnO composite films such as poly(3-hexylthiophene)/ZnO nanowires31 and polypyrrole/ZnO nanoparticles29 were fabricated, which showed sensitivities of 15-32% to 4-10 ppm NO2 at room temperature. However, up to now, to the best of our knowledge, little or no work was reported about the application of the hierarchical ZnO structures in CP/ZnO hybrid sensors. In addition, the structural effects of ZnO on the performance of CP/ZnO hybrid sensors have not been reported yet. The present work developed a rapid one-step solution method for the synthesis of ZnO nanosheet-nanorod architectures. The influence of the metastable intermediate phases on the self-assembly of the hierarchical architectures was investigated. The effects of the P3HT/ZnO heterojunction and the oxygen vacancy defects of ZnO nanostructures on the sensing performances of NO2 at room temperature were also discussed. Such ZnO nanosheet-nanorod architectures may also have potential applications in solar cells and catalysis, and the novel metastable phase-directed method may also be helpful for the synthesis of other inorganic hierarchically branched structures. 2. Experimental 2.1 Sample preparation Commercial chemicals of analytical grade and deionized water with a resistivity > 18 MΩ·cm-1 were used in all the experiments. ZnO hierarchical structures were synthesized from the supersaturated zinc-bearing alkaline



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solutions, using ZnO and NaOH as the starting chemicals. At first, 4.07 g of ZnO was dissolved into 100.0 mL NaOH with varying concentrations (3.0, 3.3, 4.0 and 5.0 mol·L-1). 10.0 mL of the solutions were then mixed quickly with 90.0 mL of water to form the diluted solutions containing 0.05 mol·L-1 Zn2+ and varying OH(0.30, 0.33, 0.4 and 0.50 mol·L-1). The diluted solutions were then heated to 60 oC and kept isothermally for 0-3.0 h, the precipitates were filtered, washed with deionized water and dried in a vacuum oven at 25 oC for 24.0 h. 2.2 Analysis Powder X-ray diffraction (XRD) was used for phase identification using an X-ray diffractometer (D8 Advance, Bruker, Germany) with CuKα (λ=0.154178 nm) radiation. The morphology and microstructures of the samples were examined with a field emission scanning electron microscope (FESEM, JSM 7401F, JEOL, Japan) and a high-resolution transmission electron microscope (HRTEM, JEM-2010, JEOL, Japan), respectively. The room temperature photoluminescence spectra of the samples were measured on a Hitachi F-7000 luminescence spectrometer using a Xe lamp with an excitation wavelength of 325 nm. The surface composition of the samples was characterized by X-ray photoelectron spectrometer (XPS, PHI-5300, PHI, USA). To determine the soluble zinc amount, the reaction suspensions were centrifuged (5000 rpm) and the supernatants were neutralized with 1:1 (v) hydrochloric acid, then buffered to pH=10 with NH4OH-NH4Cl buffer solution and titrated by EDTA method. 2.3 Fabrication and characterization of hybrid gas sensors N-type (100) oriented silicon wafers were used as the substrates. A 300 nm thick silicon dioxide (SiO2) dielectric-layer was thermally-grown on the silicon wafer. The inter-digital electrodes with finger width of 50 µm and gap width of 50 µm were patterned by standard photolithography and the titanium/gold (Ti/Au, 20 nm/100 nm) electrodes were deposited by RF sputtering and lift-off process. P3HT was dissolved in chloroform to achieve a concentration of 3.0 g·L-1. ZnO was dispersed in the mixture of deionized water and ethanol (1:1) to achieve a concentration of 3.0 g·L-1. For ZnO (up)/P3HT (down) bilayer film, P3HT solution was firstly spin-coated onto the interdigital electrodes at 1000 rpm for 6 s and 3000 rpm for 30 s, respectively, then ZnO suspension was spin-coated onto the P3HT film under the same condition. For ZnO (down)/P3HT (up) bilayer film, the coating procedures was reversed under the same condition. For ZnO/P3HT mixed film,


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the P3HT/ZnO (m/m, 1:1) were firstly mixed in chloroform to achieve a total concentration of 3.0 g·L-1, followed by spin-coating onto the interdigital electrodes. The fabricated sensors were placed in a homemade chamber with air inlet and outlet. The input concentration of NO2 was 1.0 - 100 ppm and the flow rate of the mixed gas was 200 mL·min-1. To make the sensing data more reliable, the gas feeding time of each dose was fixed at 15 min for all tests, and dry air was used as the carrier gas during comparative long intervals (fixed at 45 min) to allow the recovery as fully as possible.32 The resistances of the films was measured by Keithley 2700 multimeter/Data Acquisition System. All tests were done at 25 oC. The sensitivity (S) of the sensor is defined as (Ra-Rg)/Ra×100 (%), where Ra is the initial resistance in dry air, and Rg is the resistance of the composite film after exposing to NO2 atmosphere. 3. Results and discussion Figure 1 shows the morphology of the products formed after aging the solutions containing 0.30-0.50 mol·L-1 [OH-] and 0.05 mol·L-1 [Zn2+] at 60 oC for 3.0 h. The decrease of [OH-] from 0.50 mol·L-1 to 0.30 mol·L-1 led to the change of the building block from nanorods to nanosheets. Mono-dispersed nanorods (NR) and flower-like nanorod assemblies (NRA) were formed at [OH-]=0.50 mol·L-1 and 0.40 mol·L-1 (Figure 1a and b), respectively. Hierarchical nanosheet-nanorod architectures (NS-NR) co-assembled by 1D and 2D ZnO building blocks were achieved at [OH-]=0.33 mol·L-1 (Figure 1c), while ZnO nanosheet-assemblies (NSA) were obtained at [OH-]=0.30 mol·L-1 (Figure 1d). The magnified SEM image inset in Figure 1c indicated the dense growth of the arrayed nanorods with diameters of 50-80 nm and lengths of about 500 nm on the nanosheets. The XRD results (Figure S1) indicated that all the samples were indexed to würtzite ZnO (JCPDS card 36-1451). The intensity ratios of (100) to (002) diffraction peaks (I(100)/I(002)) of NR, NRA, NS-NR and NSA were 2.61, 1.45, 0.71 and 0.96, respectively, implying the increased proportion of (0001) facets in NS-NR, which should be attributed to the nanorod-arrays with top (0001) planes.



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Figure 1. SEM images of the products formed at different [OH-]. [OH-] (mol·L-1): (a) 0.50, (b) 0.40, (c) 0.33, (d) 0.30; [Zn2+]=0.05 mol·L-1; aging time: 3.0 h.


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Figure 2. SEM images (a-d) and corresponding XRD patterns (e) of the intermediated products formed at 10 min, [OH-] (mol·L-1): (a) 0.50, (b) 0.40, (c) 0.33, (d) 0.30; variation of the residual soluble zinc concentration in the solutions with aging time at different [OH-] (f); schematic illustration of the relationship among initial hydrolysis rate of soluble zinc, intermediate phase and product morphology (g). The formation of different ZnO nanostructures depended on the formation of intermediates with varying morphology and composition. The SEM images (Figure 2a-d) and XRD patterns (Figure 2e) of the intermediates formed in different solutions at 10 min indicated that the intermediates formed at [OH-]=0.50, 0.40, 0.33 and 0.30 mol·L-1 were ε-Zn(OH)2, ε-Zn(OH)2/ZnO nanorod assemblies (minor), ZnO nanosheets/γ-Zn(OH)2, and ZnO nanosheet-assemblies, respectively. The initial competitive precipitation of ε-Zn(OH)2, γ-Zn(OH)2 and ZnO from the supersaturated solutions was connected with the hydrolysis rates of the zinc-bearing alkaline solutions. Figure 2f shows the variation of the residual soluble zinc with reaction time at different [OH-]. The hydrolysis rates of the zinc-bearing alkaline solutions, especially those within 10 min, increased sharply with the decrease of [OH-], leading to the formation of various metastable intermediate phases and the subsequent conversion to different ZnO nanostructures. In the case of [OH-]=0.50 mol·L-1 and 0.40 mol·L-1, the slow hydrolysis of the zinc-bearing alkaline solutions led to the initial formation of ε-Zn(OH)2 as the major intermediates (Figure 2a and b) which would convert to ZnO NR or NRA via dissolution-precipitation or in-situ crystallization route.33,34 In the case of [OH-]=0.33 mol·L-1, the comparatively faster hydrolysis of the zinc-bearing alkaline solution led to the initial co-precipitation of ZnO nanosheets and rod-like γ-Zn(OH)2 (Figure 2c)35 and the subsequent formation of NS-NR by secondary growth of ZnO nanorod-arrays. At [OH-]=0.30 mol·L-1, solely ZnO NSA were precipitated directly due to the fast hydrolysis rate (Figure 2d), and the secondary growth could not occur in the absence of Zn(OH)2 intermediates. The correlations among the hydrolysis rates of the zinc-bearing alkaline solutions, the metastable intermediate phases and the morphology of ZnO products are schematically illustrated in Figure 2g.



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Figure 3. Morphology evolution of NS-NR in the products obtained at [OH-]=0.33 mol·L-1 (a-c), aging time (min): (a) 20, (b) 30, (c) 40; corresponding phase evolution of the products (d); TEM and HRTEM images of the products obtained at 20 min (e, f), inset in Figure 3e is the schematic illustration of the possible formation of screw dislocation at the attachment interface of two adjacent nanorods; schematic illustration for the metastable phase-directed formation mechanism of NS-NR (g). The above results indicated that the co-existence of ZnO nanosheets and γ-Zn(OH)2 intermediate was critical for the construction of NS-NR. Time-resolved growth process of NS-NR was investigated to identify the roles of ZnO nanosheets and γ-Zn(OH)2 intermediate in the one-step synthesis of NS-NR. The magnified SEM images of the NS-NR in the products formed at different growth stages (Figure 3a-c) demonstrated the morphology evolution of the hierarchical architectures, i.e., the initial formation of the primary bare nanosheets at 10 min (inset in Figure 2c), followed by the subsequent heterogeneous nucleation and growth of the nanorod-arrays with a length of 100-150 nm at 20 min (Figure 3a), then the elongation of the nanorod-arrays to 300-400 nm at 30 min (Figure 3b) and to 500 nm at 40 min (Figure 3c), while the diameter of the nanorods-arrays increased slightly from 20-30 nm to 50-80 nm. The low-magnification SEM images (Figure S2) and the corresponding XRD patterns during 20-40 min presented in Figure 3d indicated the gradual conversion of γ-Zn(OH)2 and ε-Zn(OH)2 intermediates to ZnO. These results revealed that NS-NR was actually constructed by a stepwise process during the one-pot synthesis, by the initial formation of ZnO nanosheets followed by the subsequent Zn(OH)2 intermediates induced secondary growth of ZnO


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nanorod-arrays. In the secondary growth stage, the initially formed ZnO nanosheets may serve as the defect-rich micro-substrate for the heterogeneous nucleation and growth of the nanorod-arrays. TEM observations revealed that the initially formed ZnO nanosheets with saw-teeth like heads and rough surfaces were assembled by side-by-side attachment of the parallel (001)-oriented nanorods, remaining at the crystal boundary of the interface (Figure S3), as has been suggested previously.36,37 The heterogeneous nucleation and growth of the nanorods occurred on the nanosheets at 20 min (Figure 3e and f), which may be driven by the dislocations originated from the imperfect oriented attachment of the parallel nanorods.38,39 The lattice fringes (0.24 nm) and the angle (28o) between the lattice fringes and the longitudinal axis of the nanorod (Figure 3f) indicated the preferential growth of the nanorod along c axis. The screw dislocation step edges on the tip of the as-grown nanorods in Figure 3f indicated the possible dislocation-driven spiral growth mechanism, in other words, the spiral growth may become dominant once the dislocations with screw character formed at the attachment interface of the adjacent nanorods, as illustrated in inset in Figure 3e.39 In addition, the soluble zinc concentration was maintained in the range of 0.15-0.18 mol·L-1 during 10-40 min (Figure 2f) owing to the continuous dissolution and re-precipitation of γ-Zn(OH)2 and ε-Zn(OH)2 intermediates. Such a comparatively low and stable soluble zinc concentration was a favorable environment for the dislocations-driven growth of 1D ZnO nanostructures.40 The detailed metastable phase-directed formation process of NS-NR architectures was schematically illustrated in Figure 3g. The fast hydrolysis of the zinc-bearing alkaline solution led to the initial formation of ZnO nanosheet-assemblies, along with the rapid precipitation of the metastable γ-Zn(OH)2, which gradually disappeared as more stable ε-Zn(OH)2 was crystallized. Meanwhile, heterogeneous nucleation and secondary growth of the nanorod-arrays occurred on the primary defect-rich ZnO nanosheets; the soluble zinc concentration supersaturated with respect to ZnO was maintained by the continuous dissolution of γ-Zn(OH)2 and ε-Zn(OH)2 intermediates, leading to the oriented elongation of the ZnO nanorod-arrays. In the case of [OH-]=0.30 mol·L-1, the soluble zinc dropped quickly to a low level due to the rapid hydrolysis and the absence of Zn(OH)2 intermediates (Figure 2f), which inhibited the secondary growth of nanorod-arrays. Compared with the previously reported synthesis of ZnO nanosheet-nanorod architectures, in which the secondary growth of the nanorods was usually promoted by replenishing the growth solution, in the present case, successive construction of ZnO NS-NR was achieved in a rapid one-step synthesis with the help of



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metastable Zn(OH)2 intermediates acting as “zinc reservoir” and initially formed ZnO nanosheets as “microsubstrates”. It is well-known that metal oxides usually exhibit poor sensing performance at room temperature. However, in this work, we fabricated P3HT/ZnO hybrid sensors for room-temperature NO2 detection. A p-type P3HT thin film layer was firstly coated onto the electrode followed by a ZnO layer to form a bilayer hybrid film, as illustrated in Figure 4a. Figure 4b shows the dynamic response curves of the sensor based on pure ZnO NS-NR film, pure P3HT film, and P3HT/ZnO NS-NR bilayer hybrid film using 4.0 ppm NO2 at room temperature. The sensors based on pure ZnO NS-NR exhibited a weak n-type response (i.e., increased film resistance in NO2) with a low sensitivity of about 6%, which is consistent with previously reported work.28,29 The low sensitivity was generally attributed to the lack of highly active surface oxygen species (O- and O2-) at room temperature.41 As a p-type organic material, P3HT adsorbed NO2 by providing the electrons, which led to an increase of the hole carriers and a decrease of the resistance,31 but the sensitivity of pure P3HT film was comparatively low (~11%). The P3HT/ZnO bilayer hybrid film displayed a p-type response to NO2 (i.e., a decrease of the film resistance in NO2) with a sensitivity (S) up to 59% at room temperature (Figure. 4b). Figure 4c shows the sensing performance of the P3HT/ZnO NS-NR composite films in the case of 4 ppm NO2 at room temperature for successive 6 cycles of NO2 gas in and off. It reveals that the hybrid sensor showed rather stable sensitivities around 60 % in 6 cycles, in spite of a small fluctuation and baseline drift. These results indicated the good reproducibility of the hybrid sensors, and also confirmed that the obtained sensing data were reliable. The enhancement of the sensing performance of the hybrid film was attributed to the p-P3HT/n-ZnO heterojunction effect. In the case of bare P3HT film, the surface resistance was 12.3 MΩ. After the coating of ZnO layer on P3HT film, the resistance was about 18.1 MΩ. This increase in resistance was due to the formation of a heterojunction with a hole–electron depletion layer formed at the interface of ZnO and P3HT (Figure 4d). The tunneling current across the heterojunction was exponentially related to the height of the junction barrier, which depended on the depletion width. Therefore, the conductivity change in the heterojunction was highly sensitive to the changes in the depletion layer.10 The gas effects a change of the depletion region and thus modulates the conductivity of the heterojunction. Since the NO2 molecule is electrophilic, charges are expected to transfer from ZnO to NO2. Therefore, exposure to NO2 would cause a


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decrease of the ZnO carrier concentration, leading to a reduction of the channel electric field of the junction. The above led to the reduction of the width of the depletion layer and the height of the junction barrier, thus increasing the conductivity of the heterojunction significantly.42

Figure 4. Schematic illustration for the gas sensor based on the P3HT/ZnO NS-NR bilayer hybrid film (a); dynamic response curves of the sensors based on pure ZnO NS-NR, pure P3HT film, and P3HT/ZnO NS-NR bilayer hybrid film to 4 ppm NO2 at room temperature (b); reproducibility of the hybrid sensors based on the P3HT/ZnO NS-NR composite films exposed to 4 ppm NO2 at room temperature (for successive 6 cycles) (c); energy-band of p-P3HT/n-ZnO heterojunction, showing the width of the depletion region and the junction barriers of the heterojunction in air and NO2 (d). Experimental results also revealed that the coating of ZnO as an upper layer [ZnO(up)] on P3HT film to form a ZnO(up)/P3HT(down) bilayer hybrid film was an optimized structure for the enhancement of sensing performance. For comparison, we also fabricated ZnO(down)/P3HT(up) bilayer film and ZnO/P3HT mixed hybrid film and tested their dynamic response to 4.0 ppm NO2 at room temperature, the results are shown in Figure S4. The ZnO(down)/P3HT(up) bilayer film and ZnO/P3HT mixed hybrid film exhibited sensitivities of 16% and 28% to 4.0 ppm NO2, respectively, which are significantly lower than the ZnO(up)/P3HT(down) bilayer hybrid film. These results indicated that the exposure of ZnO nanostructure to NO2 is critical. Therefore, we propose that the use of a hierarchical porous ZnO film as the upper layer in the hybrid film as it may promote the NO2 adsorption and improve the sensing performance.



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We also investigated the effects of ZnO nanostructures on the performance of the hybrid sensors. Different hybrid sensors were fabricated based on P3HT film and as-synthesized ZnO nanostructures shown in Figure 1. Figure 5a and b show the real-time dynamic response curves and the variations of the sensitivities of the hybrid sensors to 1.0-100 ppm NO2 at room temperature. It is evident that the P3HT/ZnO NS-NR hybrid film based sensor exhibited the highest sensitivity over the entire range (1.0-100 ppm) of NO2 concentrations, followed by P3HT/ZnO NSA, P3HT/ZnO NRA and P3HT/ZnO NR based devices in sequence. Moreover, at NO2 concentration above 50 ppm, the sensitivity of the hybrid sensors based on P3HT/NR and P3HT/NRA showed evidence for saturation, while the hybrid sensors based on P3HT/NSA and P3HT/NS-NR were still not saturated even at 100 ppm. This could be explained by a competition between the available adsorption sites on the surface of the ZnO nanostructure versus NO2 concentration. At low NO2 concentration (NSA>NRA>NR. Figure 6c summarizes the comparison of the sensitivity to 4 ppm NO2, relative intensity of VO emission (IVo) in PL intensities (at λ=550 nm) and peak area ratios of O2 to O1 peak in XPS spectra (RO2/O1) of the four samples. The resulting similar variation trends indicated that the higher NO2 sensing performance of ZnO NS-NR in the hybrid sensor correlated with their higher concentration of surface oxygen-vacancies compared to other nanostructures. Though the enhancement of NO2 sensing performance by oxygen-vacancies have been previously reported in ZnO nanostructure-based sensors at high working temperatures, the oxygen-vacancy effects have not been reported yet for CP/metal oxide hybrid sensors. Here, we attribute the improvement of NO2 sensing performance to the oxygen-vacancy-enhanced NO2 adsorption. NO2 could be adsorbed on different active sites of ZnO nanostructures with different energies. For example, it was previously reported that the adsorption energy of NO2 on the oxygen-vacancy site of ZnO surface (-0.98 eV) is lower than on the perfect site (-0.30 eV).46 Therefore, the surface oxygen-vacancy site may enhance the adsorption and electron transfer of NO2 to the ZnO surface despite of the absence of O- and O2- at room temperature: ܸை + ܱଶ ሺ݃ሻ ↔ ܱଶି ሺܽ݀‫ݏ‬ሻ + ܸைା 2ܱܰଶ ሺܽ݀‫ݏ‬ሻ + ܱଶି ሺܽ݀‫ݏ‬ሻ + ܸைା ↔ 2ܱܰଷି ሺܽ݀‫ݏ‬ሻ + ܸைାା


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According to the above equations, NS-NR with more oxygen vacancies promoted the adsorption and electron transfer of NO2, which led to the greater reduction of the width of the depletion layer and thus increasing the sensitivity to NO2 at room temperature. Moreover, the saturation of the hybrid sensors based on P3HT/NS-NR taking place at higher concentration level should also be attributed to the enhanced NO2 adsorption capacity mediated by rich oxygen-vacancies.

Figure 6. High-resolution XPS O1s spectra (a), room temperature PL spectra (b); comparison of PL intensities (at λ=550 nm), peak area ratios of O2 to O1 peak (RO2/O1) and sensitivities of the hybrid films to 4 ppm NO2 in the case of different ZnO nanostructures (c). A comparison of NO2 sensing performance at room temperature between the present P3HT/ZnO NS-NR based hybrid sensor and the former metal oxide-based and CP-based hybrid sensors operating at room temperature is summarized in Table 1. These reported metal oxide-based and CP-based composite films, which were fabricated by hybridization of ZnO nanostructure or other metal oxides with conducing polymers or carbon nanomaterials, exhibited sensitivities of 1.5-38% with 4-100 ppm NO2, while the P3HT/ZnO NS-NR hybrid film in the present work demonstrated high sensitivities up to 59 and 180% with 4 and 50 ppm NO2, respectively. Furthermore, the present hybrid sensors also showed high selectivity to NO2. Figure S5 shows the sensitivities of the P3HT film and P3HT/ZnO NS-NR hybrid film to 4 ppm NO2 and other gases including NO, SO2, H2S, CH4 and HCHO, which were all common air pollutants. Obviously, the hybrid film exhibited highly-improved selectivity to NO2 compared to bare P3HT film. Therefore, the present work provides a novel alternate route to prepare low-cost room-temperature NO2 sensors.



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Table 1. Comparison of room-temperature NO2 sensing performance of the metal oxide-based and CP-based hybrid gas sensors in the literatures and in this study. CP-based hybrid gas sensor

NO2 conc. (ppm)

S (%)

4

59

50

180

ZnO nanoparticles/PPy

100

38

28

Camphor sulfonic acid doped ZnO/PPy

10

15

29

ZnO nanoparticles/polyaniline

100

2.6

30

ZnO nanowires/P3HT

4

32

31

ZnO nanosheets/Au

5

Negligible without UV

24

ZnO nanorods/porous silicon

50

35.1

25

ZnO nanoparticles/graphene

50

14

26

3D ZnO/graphene aerogel

50

8

27

TiO2-PEDOT nanocables

20

1.5

47

WO3 nanoparticles/PPy

10

17

48

Reduced graphene oxide/P3HT

4

Hierarchical ZnO Nanosheet-Nanorod ... - ACS Publications

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