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Hierarchical nanoheterostructure of tungsten disulfide nanoflowers doped with zinc oxide hollow spheres: Benzene gas sensing properties and first-principles study Dongzhi Zhang, Junfeng Wu, Peng Li, Yuhua Cao, and Zhimin Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07021 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 2, 2019

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ACS Applied Materials & Interfaces

Hierarchical nanoheterostructure of tungsten disulfide nanoflowers doped with zinc oxide hollow spheres: Benzene gas sensing properties and first-principles study

Dongzhi Zhanga, *, Junfeng Wua, Peng Lib, Yuhua Caoa, Zhimin Yanga

a

College of Information and Control Engineering, China University of Petroleum

(East China), Qingdao 266580, China b

State Key Laboratory of Precision Measurement Technology and Instruments,

Department of Precision Instruments, Tsinghua University, Beijing 100084, China

*Corresponding author: Dongzhi Zhang E-mail address: [email protected] Tel: +86-532-86982928 Fax: +86-532-86983326

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Abstract This paper reports an original fabrication of a benzene gas sensor based on tungsten disulfide nanoflowers (WS2 NFs)/zinc oxide hollow spheres (ZnO HMDs) hierarchical nanoheterostructure. The ZnO/WS2 hierarchical composite was characterized for the inspection of its nanostructure, elementary composition and surface morphology. The benzene-sensing properties of the ZnO/WS2 nanofilm sensor were exactly investigated. The results illustrate that the ZnO/WS2 sensor exhibits a remarkable sensing performance toward benzene gas, including good sensitivity, rapid detection, outstanding repeatability and stability. This is attributed to the ZnO/WS2 nanoheterostructure can dramatically enhance the benzene sensing performance. Furthermore, density functional theory (DFT) was employed to construct the benzene gas adsorption model for the ZnO/WS2 heterostructure, from which the determined parameters in geometry, energy and charge provided a powerful support for the mechanism explanation. This work suggests that ZnO/WS2 nanoheterostructure is competent to detect trace benzene gas at room temperature. Keywords:

ZnO

hollow

spheres;

WS2

nanoflowers;

benzene-sensing; density functional theory

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nanoheterostructure;

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1. Introduction Benzene (C6H6), one typical gas species of volatile organic compounds (VOCs), is a colourless, flammable, and poisonous vapor emitted from household items, construction materials, and vehicle exhaust emissions, which can be extremely detrimental to human health.1-4 Moreover, benzene is a major cause for diseases of leukemia and lymphomas.5 Hence, the real-time detection of benzene in low concentration is of great importance.6 To date, monitoring benzene in the everyday run of things has caused great concern in the domain of gas sensors, leading to that a large collection of benzene detection technologies, such as spectroscopy,7 cataluminescence,8 and metal oxide semiconductors (MOSs)-based sensors (i.e., Co3O4, SnO2, In2O3).

9-11

Among them, MOSs gas sensors could be a promising

approach by virtue of its low cost, flexibility, stability and superior gas sensing properties.12-15 However, the majority of MOs based gas sensors needs a high working temperature to improve its sensing-performance, which leads to some limitations for their practical appplications.16 In recent years, transition metal dichalcogenides (TMDs) have aroused intense interest in various fields of field-effect transistors (FETs), photodetectors, and notably gas sensors.17-19 As a typical kind of TMDs, tungsten disulfide (WS2) has layered structure and possesses tunable band gap of 1.35-2.1 eV.20, 21 Currently, WS2 based gas sensors have been reported for the detection of H2,22 NH3,23 NO2,24 and ethanol.25 Constructing a benzene sensor that can realize rapid detection and excellent performance still attracts attention. To date, the modification of WS2 with MOs is a 3/51



potential way to improve its gas sensing properties.

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Qin et al. decorated TiO2

quantum dot (QD) with 2D WS2 under different ratios via vacuum filtering. The sensor exhibited a higher gas-sensing performance toward ammonia gas compared to pristine TiO2 and WS2 sensor.26 Perrozzi et al. developed a WS2/WO3 composite based gas sensor upon exposure to H2, NH3, and NO2, and showed ppm-level detection limit at 150oC operating temperature.22 Zhang et al. reported a SnO2/WS2 based capacitance-type humidity sensor via self-assembly method.27 However, the synergetic mechanism between WS2 and MOs is still unclear. Further, there is no effort has been made on benzene sensor using WS2/MOs composite as sensitive film. In this work, WS2 nanoflower/ZnO hollow spheres hybrid was synthesized by hydrothermal route, and its synthesis were verified by characterization techniques. The sensing characteristics of the ZnO/WS2 film were investigated at room temperature. The results demonstrated that the ZnO/WS2 film sensor has excellent sensing performance toward benzene gas, including good selectivity, outstanding repeatability, excellent stability, fast response/recovery characteristics. Furthermore, density functional theory simulation model for ZnO/WS2 heterostructure was firstly established, and the sensing mechanism of the ZnO/WS2 heterostructure toward benzene is discovered. 2. Experimental 2.1 Materials Thioacetamide

(TAA),

[(NH4)6H2W12O40·xH2O],

oxalic

ammonium acid

(C2H2O4), 4/51


metatungstate zinc

acetate

dehydrate dehydrate

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[Zn(CH3COO)2·2H2O], ethanol and glucose were from Chinese Medicine Group (Shanghai, China). 2.2 Sample fabrication In our experiment, carbon microspheres were firstly synthesized and served as sacrificial templates to prepare ZnO hollow spheres. In the synthesis process of carbon microsphere, 3 g of glucose in 50 mL distilled water was stirred for 10 min. Subsequently, the obtained solution was hydrothermally treated in a 100 mL autoclave at 190ºC for 9 h, and followed by centrifugation, washing, and drying at 80ºC for 8 h to obtain the carbon microspheres. The preparation of ZnO hollow spheres is described as below: Zn(CH3COO)2·2H2O (0.438 g) in 16 mL ethanol was stirred for 30 min. After that, 2 mL 1.25 M NaOH and carbon microspheres were added and stirred for 30 min, followed by heating in a 50 mL autoclave at 120ºC for 24 h. The ZnO hollow spheres were annealed at 600ºC for 3 h at the velocity of 2ºC min-1. Subsequently,

ZnO

hollow

spheres/WS2

nanoflowers

nanohybrid

was

hydrothermally synthesized. Firstly, TAA (1.5 g), (NH4)6H2W12O40·xH2O (0.2 g), oxalic acid (1.2 g), and 0.016 g ZnO powders were added into 30 mL distilled water under the continuous stirring for 10 min. Then, the resulting ZnO/WS2 solution was hydrothermally treated at 200ºC for 24 h. The ZnO/WS2 film was screen-printed on an epoxy substrate with interdigital electrodes (IDEs). As a benchmarked sample, pure WS2 was prepared in the absence of ZnO powders. 2.3 Instrument and analysis 5/51



Figure 1 shows the schematic of benzene sensing experimental setup. The benzene-sensing experimental measurement was carried out at room temperature. The sensing signal is collected from the Cu/Ni IDEs with interdigital spacing of 200 μm. The dimension for the sensor is 1 cm×1 cm. The desired benzene gas concentration was obtained by dilution methods, and was injected into a sealed container using a syringe, and the sensor was placed inside the chamber. A computer-controlled data logger (Agilent 34970A) was used to measure the resistance of the sensors. In our experiment, the sensors were exposed to various benzene concentrations (100 ppb-100 ppm). The sensor response (R) was defined by R=|Ra–Rg|/Ra×100 %, where Ra and Rg were the resistance of the sensor in air and benzene, respectively.16 3. Results and discussion 3.1 Structure characterization The XRD analysis (X’Pert Pro MPD) of WS2, ZnO and ZnO/WS2 samples were carried out to examine the crystal structures, as illustrated in Figure 2 (a). In all resulting XRD patterns, no peaks of impurity crystal phases are observed, leading to a fact that the carbon based templates are thoroughly removed. All observed peaks can be well indexed by the standard cards of WS2 (JCPDS card no. 84-1398) and ZnO (JCPDS card no. 36-1451), respectively.28,

29

As for XRD spectrum of WS2, the

diffraction peaks located at 2θ of 14.37°, 28.92°, 32.68°, 33.42°, 39.57°, 43.96°, 49.85°, 58.37°and 59.87°, which correspond to (002), (004), (100), (101), (103), (006), (105), (110) and (112) planes, respectively. Meanwhile, the diffraction peaks of ZnO sample are observed at 31.55°, 34.30°, 36.06°, 47.34°, 56.36°, 62.62°, 66.59°, 6/51


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67.85°and 68.97°, which attributes to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes of ZnO. XPS characterization (Thermo Scientific Instrument) was performed for as-prepared ZnO/WS2 composite. And the survey spectrum is plotted in Figure 2 (b), which demonstrates that the main elements were W, S, Zn and O. Figure 2 (c) illustrates the W 4f spectrum of the ZnO/WS2 composite. Two peaks at 32.69 and 34.79 eV are ascribed to the doublet W 4f7/2 and W 4f5/2 of W4+ from WS2. Moreover, another two characteristic peaks observed at 35.76 and 37.91 eV assigned to W 4f7/2 and W 4f5/2 of W6+ in WS2, respectively.30 The spectra of S 2p shown in Figure 2 (d) exhibit two main peaks at 161.78 and 162.99 eV, which respectively correspond to S2− 2p3/2 and S2− 2p1/2.31 Figure 2 (e) displays two distinct peaks associated with Zn 2p1/2 and Zn 2p3/2 at 1046.16 and 1022.67 eV, which ascribed to the Zn2+ of ZnO.32 Figure 2 (f) displays the XPS pattern of O 1s with peaks at 529.72 and 531.53 eV which are indexed to the lattice oxygen in ZnO and adsorbed oxygen on ZnO/WS2 surface, respectively.32 SEM inspection (Hitachi S-4800) was conducted to identify the morphology of WS2, ZnO, and ZnO/WS2 composite, as shown in Figure 3. Figure 3 (a, d) depict the SEM characterization of ZnO with hollow nanostructure. Figure 3 (b, e) show the WS2 with nanoflower-shape nanostructure, which is constituted of curving sheets. Figure 3 (c, f) exhibit the ZnO/WS2 composite, in which hollow ZnO and WS2 flowers can be clearly observed. TEM (JEOL JEM-2100) is employed to analyze the nanostructure of ZnO/WS2 composite, and the results are plotted in Figure 4. Figure 4 7/51



(a) exhibits the TEM photograph of the ZnO hollow spheres with the size of approximately 1 μm, and the hollow structure is obvious. Figure 4 (b) and (c) display the nanostructure of ZnO/WS2 composite at different sizes, which consists of WS2 nanoflowers and ZnO hollow spheres. The HRTEM images of ZnO/WS2 nanostructure are scheduled in Figure 4 (d), (e) and (f). Figure 4 (d) and (e) demonstrate the crystal faces of the labeled ZnO and WS2.28, 33 The lattice spacing of 0.249 nm is ascribed to the (101) plane of the ZnO crystal, while the lattice spacing of 0.616 nm is related to the (002) plane of WS2. As shown in Figure 4 (f), the lattice fringe of (004) plane for WS2 and (100) plane for ZnO is 0.308 nm and 0.283 nm, respectively. Brunauer-Emmett-Teller (BET) was used to evaluate the surface area of the ZnO/WS2 sample by the Micromeritics ASAP 2020M analyzer. Figure 5 shows the nitrogen adsorption desorption isotherm and corresponding pore size distribution. The ZnO/WS2 sample combines ZnO hollow spheres with WS2 nanoflowers, which are interconnected to form a mesoporous structure and specific surface area. A significant hysteresis loop at P/P0 of 0.67-0.98 is observed, indicating the existence of mesopores and macropores.34 The specific surface area of the ZnO/WS2 sample was 103.99 m2/g. The pore size distribution of ZnO/WS2 sample is about 10 nm. 3.2 Benzene-sensing properties To highlight the superiority of ZnO/WS2 composite sensor, a comparison experiment was firstly conducted to compare the benzene-sensing performance of WS2, ZnO, and ZnO/WS2 nanohybrid. The three sensors were measured toward 20 8/51


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ppm benzene gas, and the measurement result is shown in Figure 6. It can be easily found that ZnO/WS2 film sensor exhibits the largest response among the three sensors, the pristine ZnO sensor is the second in order, and the last is the pristine WS2 sensor. The response/recovery times for the WS2, ZnO, and ZnO/WS2 sensors are 72 s/56 s, 41 s/79 s and 8 s/6 s, respectively. Therefore, ZnO/WS2 sensor shows a much more rapid response/recovery speed. Thus, by virtue of the incorporation modification, the ZnO/WS2 sensor achieved a significantly enhancement in benzene-sensing. Figure 7 (a) illustrates the response-recovery curves of ZnO/WS2 film sensor toward benzene gas (0.1-100 ppm). The resistance of ZnO/WS2 sensor was measured by alternately switching between differing concentrations of benzene and air with interval of 120 s. The resistance of ZnO/WS2 sensor is decreased with the increasing of benzene concentration, indicating that the ZnO/WS2 sensor possesses a typical property of n-type semiconductor. Figure 7 (b) plots the response values of the ZnO/WS2 sensor as a power function of benzene gas concentration, which can be represented as Y = 9.9807X0.3687 with R2 of 0.98849. Figure 7 (c) reveals the impulse response/recovery curves for benzene gas sensing. The measurement was achieved by switching the ZnO/WS2 film sensor toward

given

concentrations

of

benzene

and

air.

There

is

a

good

correspondence between the sensor response and benzene concentration. Figure 7 (d) illustrates the selectivity of the ZnO/WS2 sensor toward 1 ppm of benzene (C6H6), formaldehyde

(HCHO),

ethanol

(C2H5OH),

methanol

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(CH3OH),

acetone


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(CH3COCH3), methane (CH4), ammonia (NH3) and ethylene (C2H4) at room temperature, respectively. The ZnO/WS2 sensor exhibits the largest response toward benzene among the tested gas species. The measurement results suggest a good selectivity of ZnO/WS2 sensor for benzene detection. Figure 7 (e) shows the ZnO/WS2 film sensor maintains a good repeatability under the repeatable test. Figure 7 (f) exhibits the stability of the ZnO/WS2 film sensor upon exposure to 0.5, 20, and 60 ppm benzene, which was measured five days a time over 35 days.. Table 1 compares the benzene-gas sensing properties of the ZnO/WS2 film sensor with the other state-of-the-art benzene sensors.35-41 The items of these sensors, in terms of fabrication method, detection range, working temperature, and response value, are listed and compared in Table 1. The comparative results demonstrate that the ZnO/WS2 film sensor has a remarkable potential for room-temperature benzene detection. 3.3 Benzene-sensing mechanism and DFT simulation The resistance changes of the ZnO/WS2 sensor upon exposure to benzene mainly for the sake of adsorption-oxidation-desorption process.42 The increased BET surface area, hollow features of ZnO and nanoflower-shaped WS2 facilitate the diffusion, penetration and adsorption of benzene gas and bring more chemisorbed oxygen species. In air, the oxygen molecules adsorbed on the surface of ZnO/WS2 film are ionized to O2- (ads) through capturing free electrons from ZnO conduction band.

43

Hence, the decreasing electrons lead to a higher initial resistance of ZnO/WS2 sensor. The reducing gas such as benzene molecules react with O2- (ads) and release the 10/51


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captured electrons back into the conduction band, thereby the sensor resistance reduced in benzene gas. And the reaction equation is summarized as Eq. (1):44 C6H6(ads) + 15O2-(ads) → 12CO2(gas) + 6H2O(gas) + 15e-

(1)

An underlying mechanism for the enhancement of benzene-sensing properties of the ZnO/WS2 sensor in comparison to the pristine ZnO and WS2 sensors is contributed to the ZnO/WS2 heterojunctions. Figure 8 (a) and (b) demonstrates the energy band structure diagram of ZnO/WS2 heterojunction. P-type WS2 has band gap of 1.35 eV, and n-type ZnO has band gap of 3.37 eV.45 When ZnO and WS2 contact with each other, heterojunction is created at their contacting interface. 46-48 When ZnO is combined with WS2, the difference in Fermi level of ZnO and WS2 results in a depletion layer, composed of an electron depletion layer (EDL) at the side of ZnO and a hole depletion layer (HDL) at the side of WS2, formed at the interface of ZnO/WS2, leading to an increase of initial resistance contrasting with pristine ZnO and WS2.49 In this way, the resistance of the ZnO/WS2 sensor increases in air and decreases in benzene vapor on the ground of the width modulation of the depletion layer.50, 51 In order to deeply investigate the underlying sensing mechanism of ZnO/WS2 composite toward benzene, a DFT calculation was carried out using Dmol3 program (Supporting Information).52-54 In this work, the ZnO(10 1 0) surface is selected to be the study surface by virtue of its electrostatically stability and crucial role served as the main surface of ZnO,55 which have been investigated on the DFT research of gas adsorption on ZnO in several previous works.56-58 In order to investigate the benzene-sensing mechanism of ZnO/WS2 composite, two types of geometry models 11/51



of ZnO/WS2 heterostructures were established (see Figure S1, Supporting Information), including models constructed by respectively contacting WS2 surface with O atomic plane or Zn atomic plane. Table 2 lists the formation energy (Ef), the spin polarization, and Muliken charge distribution of the two kinds of models. Ef is given by Ef = EZnO/WS2 - EZnO - EWS2,59 where EZnO/WS2, EZnO, and EWS2 are the energies of ZnO/WS2 nanostructure, pristine ZnO, and pristine WS2, respectively. The negative value indicates that the combination reaction of ZnO and WS2 is an exothermic reaction. From the listed parameters, geometry model of ZnO/WS2 established by contacting WS2 with O atomic layer of ZnO has a smaller Ef than with Zn atomic layer, illustrating that the former is the more stable structure of ZnO/WS2 heterostructure for the use in the subsequent works. In this model, there are 1.212 e positive charges distributed in ZnO, versus 1.212 e negative charge distributed in WS2. It is equivalent to an electron flow of 1.212 e from ZnO to WS2, which conforms to the above mentioned generation process of ZnO/WS2 p-n junction. The ZnO/WS2 composite is a non-magnetic material after combination of nonmagnetic ZnO and WS2. For the sake of a further investigation of the variation of the electronic structure, the density of states (DOSs) of ZnO and WS2 before and after combination of the heterostructure has been calculated and shown in Figure 9. On the whole, both DOSs of ZnO and WS2 have an immense change after combination, and the changed DOSs exhibit a tremendous resonance. It suggests an obvious change of electronic structure of ZnO and WS2, and this change promotes a stable combination. For WS2, the whole DOS value has a great increase, and DOS in the range of -6.2 eV -2.4 eV 12/51


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has a downward energy shift about 1 eV after combination. As for ZnO, the whole DOS has a similar downward energy shift into the same energy level of WS2, and a newly formed energy state resonating with the corresponding DOS of WS2 at -14.6 eV to 12 eV also bear out a good combination. Consequently, all analysis results lead to a fact that ZnO and WS2 combine into a stable heterostructure. In the succeeding research of gas adsorption, an ideal benzene adsorption system (one benzene molecule adsorbed on an undefected adsorption surface) is initially investigated to explore the mechanism of adsorption between isolated benzene and the undefected adsorption surface. Firstly, three labeled adsorption sites on either ZnO or WS2 sheets are set (see Figure S2, Supporting Information). In a bid to acquire the most stable gas adsorption model, several initial adsorption configurations are considered as follows: benzene molecule parallel to the substrate at 1 site (P1), 2 site (P2) and 3 site (P3), and benzene molecule vertical to the substrate at 1 site (V1), 2 site (V2) and 3 site (V3). For the sake of a full-scale investigation, four adsorption substrates including ZnO/WS2 (ZnO served as the adsorption surface), WS2/ZnO (WS2 served as the adsorption surface), pristine ZnO, and pristine WS2 are investigated. Adsorption energy (Ead) of all these configurations is summarized in Table 3. Ead is calculated using the following equation: Ead = Egas/sub - Esub -Egas,60 where Egas/sub, Esub and Egas are the total energies of the whole adsorption system, isolated substrate, and isolated gas molecule, respectively. The negative value of Ead demonstrates an exothermic process, which means that the adsorption is energetically favorable. According to the calculated result, benzene molecule parallelly adsorbed on 13/51



substrate generally has a higher Ead than the vertically adsorbed on substrate, which means that configurations with a parallelly adsorbed benzene molecule has a greater thermodynamically stability and adsorption strength than with a vertically adsorbed benzene molecule under the same circumstance. Furthermore, the adsorption systems have the largest Ead, including P3 for ZnO/WS2, P2 for WS2/ZnO, P1 for ZnO, and P2 for WS2 (see Figure S3, Supporting Information). And the detailed parameters of these four configurations, are summarized in Table 4. It can be found that the bond length and geometry structure of benzene molecule has little variation after adsorption among these four configurations from the corresponding diagram and the parameter. It may be attributed to the stable chemical property of benzene. Thus, it leads to a merely physical adsorption without chemical reaction, so that the size of contact region between the adsorbed benzene and substrate surface plays an important role of adsorption strength, which is the reason why the adsorption strength of parallelly adsorbed benzene on the substrate is stronger than vertically adsorbed benzene. Furthermore, the adsorption system gained a large Ead value correspondingly has a shorter adsorption distance and charge transfer between benzene molecule and the substrate. It indicates that the strong adsorption strength will commonly diminish the distance between benzene molecule and substrate as well as promote the charge transfer. In conclusion, the benzene adsorption strengths for the studied adsorption surface are in orders: ZnO (Ead=-2.427 eV)>ZnO/WS2 (Ead=-1.752 eV)>WS2 (Ead=-1.075 eV)>WS2/ZnO (Ead=-1.064 eV). From the computed results, ZnO surface has a greater interaction with benzene molecule than WS2. It can be also seen that 14/51


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stronger adsorption ability is shown between benzene molecule and ZnO/WS2 than WS2/ZnO, illustrating that ZnO makes a bigger contribution than WS2 in the adsorption process of ZnO/WS2 heterostructure. Due to the weaker Ead of ZnO/WS2 than ZnO as well as WS2/ZnO than WS2, it tells that the adsorption strength of both ZnO and WS2 shows a decline after the generation of heterostructure. This phenomenon is by the reason that monolayer of isolated ZnO or WS2, especially ZnO, curves to make a larger contact area with benzene. Nevertheless, the transformation of electronic structure of heterostructure is not capable to break the stability of benzene molecule and alter the adsorption approach. By contrary, the stability of heterostructure restricts the curve of surface structure. As a result, just a single factor for heterostructure formation can’t fully account for the experimental results. The DOSs of benzene adsorption systems of ZnO, WS2, ZnO/WS2, and WS2/ZnO before and after adsorption are calculated and depicted in Figure 10. For DOS of benzene, four adsorption systems all have a change in shape and a downward energy shift in different degree, and the DOS in the range of -8.5 eV to 15.1 eV increases in the ZnO/WS2 and WS2/ZnO adsorption systems other than ZnO and WS2 adsorption systems. For DOS of ZnO, there is a bigger variation in ZnO adsorption system than in ZnO/WS2 adsorption system, and almost no variation occurs in WS2/ZnO system. For DOS of WS2, there is little variation that can be hardly observed in WS2 and WS2/ZnO adsorption system, and almost no variation occurs in ZnO/WS2 adsorption system. For the DOS of atom having biggest charge transfer, including O atom or S atom having shortest distance to benzene in the adsorption 15/51



system, there is a bigger variation of ZnO/WS2 than ZnO, WS2/ZnO and WS2. In the adsorption system of pristine ZnO and pristine WS2, DOS of adsorption surface changes more and DOS of O or S atom changes less than the adsorption system of ZnO/WS2 and WS2/ZnO. In conclusion, pristine material react with benzene through a more average contribution of all constituting atoms and have a stronger adsorption of benzene, due to the big curve of isolated materials making a large reaction area. Only a few atoms close to the benzene molecule play an important role in adsorption systems of heterostructure that have a weaker adsorption of benzene, indicating a few atoms close enough to the benzene molecule make a main contribution to the reaction. This phenomenon conforms that the contacting area is the main reason of benzene adsorption strength. Subsequently, the reduced (Red) model with the defect, including ZnO with an O defect and WS2 with an S defect, was constructed by taking an O or S atom out to further investigate the benzene adsorption property. There are six types of adsorption system considered in the subsequent work, including Red-ZnO, Red-WS2, Red-ZnO/WS2, Red-WS2/ZnO, Red-ZnO/Red-WS2, and Red-WS2/Red-ZnO. The most energetically favorable configurations of these six systems are obtained in this work (see Figure S4, Supporting Information). Table 5 lists the detailed parameters of these six configurations. From the calculation results, the bond length and geometry structure of benzene molecule still has little variation after adsorption among these six configurations, indicating that the atom defect is insufficient to change the adsorption condition owing to the chemical stability of benzene. In this section, the adsorption 16/51


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system having a large Ead value also correspondingly has a shorter adsorption distance and charge transfer between benzene molecule and substrate, except the distance of Red-WS2/ZnO, which may be ascribed to that the benzene has a great interaction with the region of S defect and reduce the spacing, but it doesn’t increase the whole reaction intensity. The benzene adsorption strengths for studied adsorption surface are in

orders:

Red-ZnO

(Ead=-1.721

eV)>Red-ZnO/WS2

(Ead=-1.660

eV)>Red-ZnO/Red-WS2 (Ead=-1.658 eV)>Red-WS2 (Ead=-1.079 eV)>Red-WS2/ZnO (Ead=-1.069 eV)>Red-WS2/Red-ZnO (Ead=-1.064 eV). Figure 11 plots the DOS of these six configurations to explore the fundamental adsorption mechanism. The degree of DOSs variation of adsorption systems based on reduced materials is smaller than that of impeccable materials, directly indicating that the atom defect leads to a weaker interaction between benzene molecule and reduced materials. All the six configurations have the similar change in the DOS shape, which means that the either reaction intensity toward benzene is approximately the same. The defects on the ZnO/WS2 material are detrimental to its response toward benzene molecules. As we described above, in the experimental situation, benzene molecules adsorbed on the sensing materials react with the oxygen adsorbed on the surface, and served as an active site. The optimized adsorption configurations that benzene interacts with the reduced material having an oxygen molecule adsorbed on the defect,

including

Red-ZnO@O2,

Red-WS2@O2,

Red-ZnO@O2/WS2,

and

Red-WS2@O2/ZnO (see Figure S5, Supporting Information). And the specific parameters of these four configurations, including the bond length of benzene 17/51



molecule, the shortest atomic distance between adsorbed benzene molecule and the substrate surface, the adsorption energy, and the charge transfer (CT) from benzene molecule to the substrate are summarized in Table 6. The bond length and geometry structure of benzene in the four configurations has some changes. And the adsorption surface of ZnO is still more sensitive toward benzene than WS2. The adsorption systems of Red-ZnO@O2/WS2 and Red-WS2@O2/ZnO have a larger Ead and CT than Red-ZnO@O2 and Red-WS2@O2. This consequence illustrates that the generation of heterostructure improves the sensitivity of benzene when oxygen adsorbed on material surface. The distance of Red-WS2@O2/ZnO is shorter than Red-WS2@O2, but the distance of Red-ZnO@O2/WS2 is longer than Red-ZnO@O2. The adsorption systems with an adsorbed oxygen molecules on the defect exhibits a stronger adsorption strength, which indicates that the adsorbed oxygen enhance the adsorption reaction. Figure 12 shows the DOSs of benzene adsorption systems of Red-ZnO@O2, Red-WS2@O2, Red-ZnO@O2/WS2, and Red-WS2@O2/ZnO. For DOS of benzene, variations of these four adsorption systems are all bigger than the corresponding adsorption system of reduced substrate, indicating a greater impact on the electronic structure of benzene through the additional adsorbed oxygen. In adsorption system of Red-ZnO@O2, there are some small peaks newly formed in the range of -7.3 eV to 16 eV, which resonates with the peaks of benzene. And the DOS of O atom has a relatively large change, indicating a promotion of benzene adsorption to a certain extent by the nearby atom. But the DOS of O atom makes no contribution to the 18/51


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newly formed peaks of DOS for ZnO. It means that the other atoms contribute less to the charge transfer for the newly formed peaks, indicating a strong adsorption influences the atom far from benzene molecule. Nevertheless, in adsorption system of Red-ZnO@O2/WS2, the DOS of ZnO has the largest change by an overall reduction among all these configuration investigated, whereas the DOS of O atom has a relatively small change. And the DOS of WS2 still has a fairly no change. It suggests that the whole ZnO surface have a considerably strong interaction with benzene molecule after the heterostructure formation and oxygen adsorption. However, for both adsorption systems of Red-ZnO@O2/WS2 and Red-ZnO@O2, the DOS of adsorbed oxygen only has a little change, which indicates that the adsorbed oxygen mainly play a role of bridge that only promote the charge transfer without alteration of its own electronic structure. When WS2 served as the adsorption surface, all DOSs, except for benzene, have a slight transformation just like the adsorption system of reduced substrate without adsorbed oxygen. But from the DOS and models before and after adsorption of oxygen, we can observe that the DOS of WS2 has a terrifically great change after adsorption of oxygen, leading to an immense variation of electronic structure of WS2 due to a strong reaction between oxygen and WS2. This change may be the reason for enhancing sensitivity toward benzene after the adsorption of oxygen and after the formation of heterostructure. In a conclusion, through the comprehensive DFT calculation, the benzene adsorption mechanism of impeccable material, defected material, and oxygen adsorbed defected material, are presented in depth discussion. Under the circumstance 19/51



Page 20 of 51

without oxygen, heterostructure has no positive, even a little negative effect on benzene adsorption compared to pristine material of ZnO and WS2. Under air atmosphere with the oxygen adsorbed on the defect, the ZnO/WS2 heterostructure exhibits enhanced benzene sensitivity compared to pristine material of ZnO and WS2, and the adsorption strength of ZnO is stronger than WS2, verifying the experimental results. In the reaction between ZnO/WS2 heterostructure and benzene, ZnO will play a more important role in directly gas adsorption, and WS2 contributes lot in reinforcing the reaction intensity and promoting the charge transfer. 4. Conclusions In summary, a benzene sensor based on ZnO/WS2 composite was fabricated in this paper by a facile hydrothermal route. The ZnO/WS2 composite were characterized by XRD, SEM, XPS, TEM, and BET. The benzene-sensing properties of the ZnO/WS2 sensor were systematically measured, which exhibited fast detection, good

selectivity,

outstanding

repeatability

and

stability.

The

underlying

benzene-sensing mechanism of the ZnO/WS2 heterostructure sensor was explored. Ultimately, benzene molecular adsorption model on the ZnO/WS2 heterostructure was established by DFT calculation based on first-principle. Through combining experimental investigation with DFT simulation, this work suggests that ZnO/WS2 nanoheterostructure is competent to detect trace benzene gas at room temperature. Supporting Information DFT model setup, geometry structures of ZnO/WS2 system, gas adsorption sites of ZnO and WS2, optimized benzene adsorption configurations, optimized benzene 20/51


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adsorption configurations, and optimized benzene adsorption configurations are supplied as Supporting Information. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China (51777215, 51775306).

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Table 1. Benzene-sensing properties in this work compared with previous works. Sensor materials

Fabrication method

Measuring range

Operating temp. (°C)

Response (%)

Ref.

ZnO nanorods

Dip-coating

1-200 ppm

370°C

20 (50 ppm)

[35]

ZnO microbeits

Chemical solution

1-100 ppm

300°C

7.5 (100 ppm)

[36]

FeTPP/SWCNT s

Drop-casting

1-25 ppm

RT

15 (20 ppm)

[37]

CdFe2O4

Spray pyrolysis

15-75ppm

RT

7 (30 ppm)

[38]

MWCNTs-g-PE G-b-PS

Atom transfer radical Polymerization

10-8000 ppm

RT

11 (6000 ppm)

[39]

PANI-TiO2

Drop-casting

10-150 ppm

RT

17 (40 ppm)

[40]

APHX

Drop-casting

10-48ppm

RT

22 (25ppm)

[41]

ZnO/WS2

Screen-printing

0.1-100 ppm

RT

30 (20 ppm)

This work

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Page 32 of 51

Table 2. The formation energy (Ef) and the Muliken charge distribution of ZnO/WS2 model established by contacting WS2 with O and Zn atomic layer of ZnO. Type of ZnO/WS2

Ef (eV)

Spin (μB)

O-WS2

-7.769

Zn-WS2

-5.262

heterostructure

Muliken charge distribution (e) ZnO

WS2

0

1.212

-1.212

0

-0.735

0.735

Note: O-WS2 represents the ZnO/WS2 model of O atomic plane of ZnO (10 1 0) surface on WS2, while Zn-WS2 denotes the ZnO/WS2 model of O atomic plane of ZnO (10 1 0) surface on WS2.

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Table 3. Adsorption energy for adsorption configurations of P1, P2, P3, V1, V2, V3 on ZnO/WS2, WS2/ZnO, ZnO, and WS2 substrate. Adsorption energy (eV) Substrate P1

P2

P3

V1

V2

V3

ZnO/WS2

-1.749

-1.747

-1.752

-1.303

-1.289

-1.364

WS2/ZnO

-1.002

-1.064

-1.011

-0.647

-0.718

-0.715

ZnO

-2.427

-2.226

-2.384

-1.626

-1.723

-1.808

WS2

-1.013

-1.075

-1.009

-0.674

-0.741

-0.733

Note: ZnO/WS2 represents the ZnO/WS2 model of O atomic plane of ZnO (10 1 0) surface on WS2, while Zn-WS2 denotes the ZnO/WS2 model of O atomic plane of ZnO (10 1 0) surface on WS2.

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Page 34 of 51

Table 4. The bond length of benzene molecule, the shortest atomic distance between adsorbed benzene molecule and the substrate surface, the adsorption energy, and the charge transfer (CT) from benzene molecule to the intact substrate. Adsorption

Bond length (Å) Distance (Å)

Ead (eV)

CT (e)

1.095/1.096

2.619

-1.752

0.571

1.418/1.419

1.096/1.098

3.452

-1.064

0.239

ZnO

1.416/1.417

1.094/1.096

2.309

-2.427

0.583

WS2

1.418

1.096

3.512

-1.075

0.244

C6H6

1.399

1.091

—

—

Systems

C—C

H—C

ZnO/WS2

1.416/1.418

WS2/ZnO

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Table 5. The bond length of benzene molecule, the shortest atomic distance between adsorbed benzene molecule and the substrate surface, the adsorption energy, and the charge transfer (CT) from benzene molecule to the reduced (Red)-substrate. Adsorption


Ead (eV)

CT (e)

1.095/1.096

2.501

-1.658

0.496

1.416/1.418

1.096

2.947

-1.064

0.253

Red-ZnO/WS2

1.417/1.420

1.095/1.097

2.493

-1.660

0.501

Red-WS2/ZnO

1.416/1.418

1.096/1.097

2.931

-1.069

0.254

Red-ZnO

1.416/1.417

1.094/1.096

2.401

-1.721

0.503

Red-WS2

1.417

1.096

3.378

-1.079

0.254

C6H6

1.399

1.091

—

—

systems

C—C

H—C

Red-ZnO/Red-WS2

1.417/1.420

Red-WS2/Red-ZnO

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Table 6. The bond length of benzene molecule, the shortest atomic distance between adsorbed benzene molecule and the substrate surface, the adsorption energy, and the charge transfer (CT) from benzene molecule to the O2 adsorbed reduced (Red)-substrate. Adsorption


Ead (eV)

CT (e)

1.095/1.096

2.631

-1.610

0.543

1.418/1.419

1.096/1.098

2.182

-1.277

0.325

Red-ZnO@O2

1.417/1.420

1.096/1.097

2.536

-1.573

0.524

Red-WS2@O2

1.417/1.420

1.096/1.097

2.483

-1.201

0.297

C6H6

1.399

1.091

—

—

systems

C—C

H—C

Red-ZnO@O2/WS2

1.416/1.418

Red-WS2@O2/ZnO

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Figure captions Figure 1. Schematic diagram of benzene sensing experimental setup. Figure 2. (a) XRD pattern of WS2, ZnO, and ZnO/WS2 samples. XPS spectra of ZnO/WS2 sample: (b) survey spectrum, (c) W 4f, (d) S 2p, (f) Zn 2p, and (e) O 1s. Figure 3. SEM characterization of (a, d) ZnO, (b, e) WS2, and (c, f) ZnO/WS2. Figure 4. TEM images of (a) ZnO hollow spheres, (b) and (c) ZnO/WS2 composite. HRTEM images of (d) ZnO hollow spheres, (e) WS2 nanoflowers, and (f) ZnO/WS2 composite. Figure 5. Nitrogen adsorption-desorption isotherms of ZnO/WS2 heterostructure. Inset: pore size distribution of ZnO/WS2 heterostructure. Figure 6. The responses and recovery characteristics of WS2, ZnO, and ZnO/WS2 sensors exposed to 20 ppm benzene. Figure 7. (a) Resistance measurement, (b) function fitting curve of the ZnO/WS2 sensor, (c) typical response-recovery curves for various concentrations of benzene, (d) selectivity for 1 ppm of interference gas species, (e) repeatability and (f) long-term stability of the ZnO/WS2 sensor. Figure 8. (a) The sensing mechanism of the ZnO/WS2 sensor upon exposure to air and benzene gas. (b) Energy band structure for the n-type ZnO/p-type WS2 heterostructure. (E0: vacuum-energy level; Eg: energy band gap; Ef: Fermi-energy level; Ec:

conduction band; Ev: valence band).

Figure 9. DOSs of WS2 and ZnO before and after combination of ZnO/WS2 heterostructure. 37/51



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Figure 10. DOSs of benzene adsorption systems of (a) ZnO, (b) WS2, (c) ZnO/WS2, and (d) WS2/ZnO before and after benzene adsorption. Figure 11. DOSs of benzene adsorption systems of (a) Red-ZnO, (b) Red-WS2, (c) Red-ZnO/WS2,

(d)

Red-WS2/ZnO,

(e)

Red-ZnO/Red-WS2,

and

(f)

Red-WS2/Red-ZnO before and after benzene adsorption. Figure 12. DOSs of benzene adsorption systems of (a) Red-ZnO@O2, (b) Red-WS2@O2, (c) Red-ZnO@O2/WS2, and (d) Red-WS2@O2/ZnO.

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Figure 1. Schematic diagram of benzene sensing experimental setup.

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Figure 2. (a) XRD pattern of WS2, ZnO, and ZnO/WS2 samples. XPS spectra of ZnO/WS2 sample: (b) survey spectrum, (c) W 4f, (d) S 2p, (f) Zn 2p, and (e) O 1s.

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Figure 3. SEM characterization of (a, d) ZnO, (b, e) WS2, and (c, f) ZnO/WS2.

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Figure 4. TEM images of (a) ZnO hollow spheres, (b) and (c) ZnO/WS2 composite. HRTEM images of (d) ZnO hollow spheres, (e) WS2 nanoflowers, and (f) ZnO/WS2 composite.

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Figure 5. Nitrogen adsorption-desorption isotherms of ZnO/WS2 heterostructure. Inset: pore size distribution of ZnO/WS2 heterostructure.

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Figure 6. The responses and recovery characteristics of WS2, ZnO, and ZnO/WS2 sensors exposed to 20 ppm benzene.

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Figure 7. (a) Resistance measurement, (b) function fitting curve of the ZnO/WS2 sensor, (c) typical response-recovery curves for various concentrations of benzene, (d) selectivity for 1 ppm of interference gas species, (e) repeatability and (f) long-term stability of the ZnO/WS2 sensor.

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Figure 8. (a) The sensing mechanism of the ZnO/WS2 sensor upon exposure to air and benzene gas. (b) Energy band structure for the n-type ZnO/p-type WS2 heterostructure. (E0: vacuum-energy level; Eg: energy band gap; Ef: Fermi-energy level; Ec:

conduction band; Ev: valence band).

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Figure 9. DOSs of WS2 and ZnO before and after combination of ZnO/WS2 heterostructure.

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Figure 10. DOSs of benzene adsorption systems of (a) ZnO, (b) WS2, (c) ZnO/WS2, and (d) WS2/ZnO before and after benzene adsorption.

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Figure 11. DOSs of benzene adsorption systems of (a) Red-ZnO, (b) Red-WS2, (c) Red-ZnO/WS2,

(d)

Red-WS2/ZnO,

(e)

Red-ZnO/Red-WS2,

Red-WS2/Red-ZnO before and after benzene adsorption.

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and

(f)


Figure 12. DOSs of benzene adsorption systems of (a) Red-ZnO@O2, (b) Red-WS2@O2, (c) Red-ZnO@O2/WS2, and (d) Red-WS2@O2/ZnO.

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TOC graphic

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Benzene Gas Sensing Pro - ACS Publications - American Chemical

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