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Synthesis of Sb doping hierarchical WO3 microspheres and mechanism of enhancing sensing properties to NO2
Accepted Manuscript Synthesis of Sb doping hierarchical WO3 microspheres and mechanism of enhancing sensing properties to NO2 Jianhua Sun, Jun Guo, Jingyao Ye, Bangjun Song, Kewei Zhang, Shouli Bai, Ruixian Luo, Dianqing Li, Aifan Chen PII:
S0925-8388(16)32795-5
DOI:
10.1016/j.jallcom.2016.09.061
Reference:
JALCOM 38895
To appear in:
Journal of Alloys and Compounds
Received Date: 20 May 2016 Revised Date:
2 September 2016
Accepted Date: 5 September 2016
Please cite this article as: J. Sun, J. Guo, J. Ye, B. Song, K. Zhang, S. Bai, R. Luo, D. Li, A. Chen, Synthesis of Sb doping hierarchical WO3 microspheres and mechanism of enhancing sensing properties to NO2, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.061. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis of Sb doping hierarchical WO3 microspheres and mechanism of enhancing sensing properties to NO2 Jianhua Sun a,b, Jingyao Ye a, Meng Cui a, Zhangfa Tong b*, Shouli Bai a*, Ruixian Luo a, Dianqing
a
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Li a*, Aifan Chena
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Guangxi Key Laboratory of Petrochemical Resource Processing and Process
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b
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Chemical Technology, Beijing 100029, China.
Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi
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University, Nanning 530004 , China.
Sb-doped tungsten oxides have been synthesized successfully by hydrothermal method without any surfactant at 120oC for 24 h, which not only exhibists high response to NO2, good selectivity to ethanol, acetone, NH3, methanol and CO, but also exhibits good linear responses in the concentration range of 1ppm to 8ppm.
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Synthesis of Sb doping hierarchical WO3 microspheres and mechanism of enhancing sensing properties to NO2
a
a
a
a*
,
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Jianhua Sun a,b, Jun Guoa, Jingyao Ye , Bangjun Song, Kewei Zhang, Shouli Bai
Ruixian Luo a, Dianqing Li a*, Aifan Chena
State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of
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a
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Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing 100029, China. b
Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification
Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning
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530004 , China.
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Abstract Sb-doped tungsten oxides have been synthesized successfully by a hydrothermal method without any surfactant at 120oC for 24 h. The structure and morphology of product were examined by XRD and SEM. The sensing tests reveal that the
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enhancement to NO2 response by Sb doping is very significant compared to that reported in literatures. Especially, the highest response of 3.5 wt%-Sb doped WO3 exhibits 9 times higher than that of undoped sample. The transient responses of the sample to different concentrations of NO2 were also measured and modeled using L–
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H heterogeneous reaction mechanism. The mechanism of enhancing sensing performance is also discussed in detail, which attributed to the changes of material
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intrinsic defects and binding energies and has been confirmed by the room temperature PL and XPS spectra. The first-principle calculations indicate that the dopant as acceptor enter into tungsten oxide resulting in reduce in free electron concentration of system, which is in good agreement with the increase in resistance obtained from experiment measurements.
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calculation.
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Key words: WO3; Sb-doping; NO2 sensing; L–H reaction mechanism; First-principle
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1. Introduction: In recent years, air pollution has been becoming more and more serious with the development of industries and automobiles. Among the air pollutants, nitrogen dioxide (NO2) is not only toxic itself, but also a main precursor of acid rain and
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photochemical smog [1-3]. Many efforts have been devoted to develop NO2 sensors for accurately monitoring of the NO2 concentration in environment.
The semiconducting metal oxides have been widely investigated as sensing materials of NO2 sensors. Among them, tungsten trioxide (WO3), as a wide band-gap
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n-type semiconductor with intrinsic non-stoichiometry, has been proven to be a promising sensing material for detection of NO2 [4, 5]. Nowadays, various
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morphologies of WO3 with different dimensional nanostructures have been synthesized to improve the sensing performance of the WO3, such as nanorods [6], nanowires [7] and nanoplates [8], etc. Compared with low dimensional nanostructures, the hierarchical micro/nanometer microspheres[9] consisting of low-dimensional building blocks provide an opportunity to realize fast gas diffusion and surface
blocks[10-12].
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reactivity due to porous and spatial orientation arrangement of the building
In order to further enhance the gas sensing performance of semiconducting metal
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oxides, many attempts have been made such as loading of noble metal, doping and surface modification via element or oxide, and composite formed with another oxide,
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etc. Although doping of metal elements already has been recognized to be one of the facial and effective methods to enhance sensor sensitivity or reduce operating temperature of sensor [13-16], the dopants to enter crystalline structure of oxide is rather difficult, leading to the doping effect is not significant. For example, Xia et al. reported that 1.0 wt% Au-doped WO3 powder prepared by a colloidal chemical method for detection of NO2, although the product exhibited higher sensitivity and better selectivity to 10 ppm NO2 at optimum operating temperature of 150 oC, the sensitivity of doped sample only enhanced 1.5 times than that of undoped WO3 at 150oC [17], and the noble metal (Au) dopant is expensive. 3
ACCEPTED MANUSCRIPT In the work, we also made a lot of attempts to improve the sensing properties of tungsten trioxide to NO2 through doping different metal elements with the same mass fraction to WO3, such as Zn, Fe, Sn, Mo, La and Sb in the hydrothermal process. Among them, the 3.5 wt% Sb doped WO3 exhibits the best result of gas sensing. To
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comparison, we have searched about WO3 sensing materials with deferent doping elements and their sensitivity to NO2. The data have been listed in Table 1[18-22], from Table 1 it can be seen that the enhancement of sensitivity is very significant in
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the work compared with that reported in literatures.
2.1 Preparation of Sb-doped WO3
WO3
architectures
were
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All chemicals were analytical-grade reagents without further purification. Sb-doped prepared
by
following
hydrothermal
procedure:
Na2WO4·2H2O (1.0g) and citric acid(1.2 g) were dissolved in 60 mL of distilled water, proper amount of antimony chloride (Sb/WO3=2.0, 3.5 and 5.0 wt%)was added into the above mixed solutions under constant stirring. After ultrasonic 3 min and stirred
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30 min, 8 mL of 3 M HCl aqueous solution was added dropwise into the above solution. The solution was stirred for 10 min, then transferred to a Teflon-lined autoclave with 100 mL capacity and subsequently heated at 120oC for 24 h. After the reaction, the pale yellow products were harvested by centrifugation and thoroughly
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washing with deionized water. Finally, the product was dried at 80oC overnight and
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then annealed at 400oC, 600oC and 800oC, respectively for 4h in air.
2.2 Characterization
Powder X-ray diffraction (XRD) patterns of the products were recorded on a
Rigaku D/MAX-2500 X-ray diffractometer at 30 kV and 100 mA with copper Kα radiation (λ = 1.54Å). Scanning rate of 5o min−1 was applied to record the patterns in range of 10-70o(2θ).Field emission scanning electron microscopy (FESEM) images were obtained with a Hitachi S-4700 instrument operated at 20.0 kV. An elemental composition analysis was carried out by energy dispersive spectrometry (EDS) using an EDS spectrometer attached to the same microscope. X-ray photoelectron 4
ACCEPTED MANUSCRIPT spectroscopy (XPS) spectra were recorded on an X-ray photoelectron spectrometer (VG ESCALAB-MK) with aluminum Kα radiation. PL spectra were recorded from 350 to 550 nm at room temperature by a 325 nm excitation (RF-5301PC
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spectrometer)
2.3 First-principles calculations
First-principles calculations based on density-functional theory (DFT) were carried out using Materials Studio 5.5 (Accelrys Inc.). The exchange-correlation energy was
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represented by the local density approximation (LDA) of Ceperley and Alder (CA-PZ). A cutoffenergy of 400 eV was used as the plane-wave basis set and a 3x3x3
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Monkhorst k-point mesh was used for the Brillouinzone integrations. The lattice parameters of WO3 were obtained from the experiment and then fully optimized. The convergence threshold for self-consistent field (SCF) iteration was set at 10 5 eV.
3.Sensor fabrication and gas-sensing test
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The sensors based on Sb doped WO3 was fabricated[23] and different concentrations of NO2 (1 ppm, 2 ppm, 4 ppm and 8 ppm) were used as the target gas to examine the sensing performance of the sensor using a JF02E test system (Guiyan
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Jinfeng Technology Co., Kunming, China). The operating temperature of the sensor was adjusted by varying the heating voltage of system. The resistance of the sensor in air (Rair) and in the air–test gas mixture (Rgas) was recorded, respectively. The
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response for NO2 is defined as the ratio of RNO2/Rair. The response and recovery time were measured as the time for the sensor reaches 90% of its maximum response after introduction or removal of the test gas, respectively.
4. Results and discussion 4.1 Structure and morphology of WO3 X-ray powder diffraction analysis was carried out to investigate the crystal structures of pure and doped samples. Strong diffraction peaks in Fig. 1 indicate that the pure WO3 shows a high degree of crystallinity after calcination and the crystalline 5
ACCEPTED MANUSCRIPT phase can be indexed as a monoclinic structure due to all the diffraction peaks agree well with the values standard JCPDS#43-1035( space group: P21/n, lattice constants: a=7.297Å, b=7.539Å, c=7.688 Å). When the Sb doping amount is more than 3.5 wt%, an extra small diffraction peak emerges at 30o in addition to the diffraction peaks of
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pure tungsten oxide and the increase of diffraction peak intensity with the doping. The peak can be indexed as (222) peak of the cubic phase Sb6O13 compared with JCPDS # 33-0111.
Morphology and size of the as-synthesized WO3 and Sb-doped WO3 sample were
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observed by FE-SEM. Fig. 2(a) and (b) shows the FESEM image of pure WO3 at low and high magnification, respectively. The observation reveals that these microspheres
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with diameter of 2-3 µm are actually formed from 0D primary nanoparticles via self-assembled into 2D interlaced nanosheets, and finally into 3D hierarchical microspheres, as shown in Fig. 2(d). The WO42– would gel gradually from solution with dropping of dilute HCl solution, and finally small nanoparticles were obtained during the hydrothermal process. In the preparation process of precursors, the
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introduction of citric acid will force the tungstate ions to chelate with the dissociated carboxylic groups, forming a relatively stable tungstate complex, even in strong acidic aqueous solution. The growth of particles would be promoted or prevented if a
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chemical additive could effectively adsorb on the surface of particles. When large amount of citric acid was added, these coordinate nucleuses undergo further condensation polymerization under elevated temperature, and finally form sheet-like
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nanostructures. Under the tendency of thermodynamic stability, the sheet-like nanostructures tend to spontaneously rearrange themselves and become stacking to each other in an oriented fashion by dipole-dipole interactions. The steric barrier of carboxylic ligands provides relatively weak interaction between adjacent sheet-like structures, dominating their stacking and reconstruction, and the equilibrium between the steric barrier and condensation-polymerization dominates the morphological evolution of WO3. Fig. 2(c) indicates that the morphology of WO3 microspheres partially destroyed after doping. EDS was used to observe the existence of Sb component in doped sample as illustrated in Fig. 3, which reveals the presence of Sb, 6
ACCEPTED MANUSCRIPT W and O elements and the Sb concentration is about 1.92 wt%. 4.2 Sensing properties of pure and Sb-doped WO3 based sensors Fig. 4 presents that the response of a semiconductor sensor is greatly affected by the operating temperature, which results from the effect of temperature on
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adsorption/desorption and surface reaction of gas [23-25]. The response of pure WO3 annealed at 600oC reaches a maximum value of 17 at operating temperature of 125oC， so, 125oC is called to be the optimum operating temperature and is chosen to test the transient responses hereinafter. The hierarchical structures of WO3 promote the
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enhancement of response due to a fast diffusion of gas and surface reaction at low operating temperature. Fig. 4 shows the temperature dependence of response for
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sample annealed at different temperatures of (a) 400oC, (b) 600oC and (c) 800oC to 8 ppm NO2. The results suggest that the sensing response increases at first and then decreases with the change in operating temperature from 80 to 150 °C. Because sufficient thermal energy is essential to overcome the activation energy barrier of oxygen chemisorption and surface reaction at temperature range from 80 to 125 °C,
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however, once the operating temperature over 125 °C, the amount of desorbed gas will increase, leading to the reduce of response. The temperature (125 °C) that the chemisorbed amount of gas on the surface of material reaches maximum is called
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optimum operating temperature, and meanwhile the sensing response reaches maximum value. From Fig.4 it also can be seen that the annealing temperature of 600oC is found to be the appropriate annealing temperature for WO3 at which the
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sample not only has higher crystallinity but also has appropriate specific surface area [26].
To investigate the effect of doping amount on the gas sensing properties, the
responses of samples with different Sb doping amount to NO2 were examined. Fig. 5 presents responses of these samples to 1 ppm to 8 ppm NO2 at operating temperature of 125oC. It is found that the response increased with the increase of NO2 concentration for all samples and the responses of 3.5 wt% Sb-doped sample are obviously higher than pure WO3 and other doped samples. The 3.5 wt% Sb-doped WO3 sample exhibits the highest response of 195 to 8 ppm of NO2 gas, which is 9 7
ACCEPTED MANUSCRIPT times higher than pure tungsten trioxide. The response achieved 25.6 to 2 ppm of NO2. Fig. 6 shows the transient responses of pure and 3.5 wt% Sb-doped samples to different concentrations of NO2 at the optimum operating temperature of 125oC. From
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Fig.6 it can be seen that the response increases with increasing of gas concentration and has a good linear relationship between the response and the gas concentration in range of 1–8 ppm for 3.5 wt% Sb-doped sample, which is better than that of pure WO3 as shown in Fig.7.
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The cross sensing (selectivity) between NO2 and other reducing gases is still one of the major problems for practical NO2 sensor. Fig. 8 shows the selectivity of NO2 to
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these interference gases of ethanol, NH3, acetone, methanol and CO for 3.5 wt% Sb-doped WO3 based sensor. The experimental results indicate that the sensor not only exhibits high sensitivity and excellent selectivity but also has good linear response in NO2 concentration range of 1 ppm to 8 ppm at relatively low operating temperature of 125oC. So, the 3.5 wt% Sb-doped WO3 microspheres is a promising
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sensing material for NO2 detection.
4.3. Nonlinear fit of NO2 sensor based on 3.5 wt% Sb-doped WO3 sensors Fig. 9 indicates the time dependence of the sensitivity in measurement process. The response time and the recovery time were measured to be 38 s and 10 s, respectively,
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to 8 ppm NO2 at 125oC. The transient responses can be modeled by Langmuir– Hinshelwood reaction mechanism and the response transient (S(t)) can be expressed
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by Eq. (1)[27]
St = 1 − [/ ]
(1)
where τirrev= 1/(k × Cg) (k—rate constant; Cg—gas concentration) is referred as characteristic response time for the gas sensor. Respective response transients for NO2 concentrations in 2, 4 and 8 ppm were fitted nonlinearly according to Eq. (1), and the fitted results were shown in Fig. 9(a)–(c). Fig. 9(d) shows the characteristic response time, which was estimated from above results. The characteristic response time is reduced disproportionately with the increase of the NO2 concentration for the gas sensor. 8
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4.4 Factors influencing sensing performance of Sb-doped WO3 Two different sensing factors, structural and electronic sensitization, were provided herein. Sb doping induced the electronic sensitization of WO3 was investigated by an
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XPS spectra. Fig. 10 shows high-resolution XPS survey spectra (0–1350 eV) and spectra of W 4f, O 1s and Sb 3d for the pure and Sb-doped WO3 samples. From Fig. 10, only C, W, O and Sb related core levels are detectable in the spectra, indicating no impurities are introduced. Fig. 10(a) depicts the XPS spectrum of W 4f level for pure
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WO3. The peak positions corresponding to W6+ state located at 36.05 and 37.2 eV, respectively, while the W5+ state located at 34.8 and 36.5 eV, respectively. Therefore,
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the molecular formula of the WO3 should be precisely expressed as WO3−x, where x represents the number of oxygen vacancies. Furthermore, the binding energy for the 3.5 wt% Sb-doped WO3 sample increases 0.15 eV compared with the pure WO3 sample (from 37.1 eV to 37.25 eV for W 4f5/2 and from 35 eV to 35.15 eV for W 4f7/2) as shown in Fig. 10(b)[28]. The chemical shift in binding energies reflect the
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electronic interaction between the WO3 and the dopant[29], which is responsible for the enhancement of gas response due to the Sb doping. By estimating the area ratio of W6+ to W5+ from XPS spectrum, we can conclude that the x is 0.12 and 0.17 for the
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pure and Sb-doped WO3, respectively, that is, the molecular formula of WO3 should be written as WO2.88 and WO2.83, respectively. To further confirm the Sb element doped into the WO3 and identify the chemical state of doped Sb, the doped sample
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was analyzed by XPS spectrum as shown in Fig 10(c). We found that the Sb 3d peak appeared at 539.89 eV, and according to XRD analysis shown in Figure1, the diffraction peak of cubic phase Sb6O13 appears in the patterns. Accordingly, we assume that the doped element consist of two oxide states of Sb3+ and Sb5+ and the ratio between them is 1:2. On this basis, the Sb 3d peak can be divided into two peaks located at 539.31 eV and 540.02 eV and correspond well to the Sb3+ and Sb5+, respectively [30,31], which indicates that the Sb has been introduced in WO3 in the form of Sb6O13 (Sb2O3:Sb2O5 = 1:2). In addition, the chemical shifts of O and W toward high energy direction also show that antimony is doped into the lattice of 9
ACCEPTED MANUSCRIPT tungsten oxide [32]. In the case of structural sensitization, the defects such as oxygen vacancies are known to be the most common defects in semiconductor metal oxides. When W6+ ions in the WO3 lattice was substituted by Sb3+ and Sb5+, respectively, which changes
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the defect equilibrium of the WO3 crystals, therefore, negative 3 of valence and negative 1 of valence charges of the substituted W site have to be compensated in the form of oxygen vacancy to maintain electrical neutral of the structure, which can be described through the following equation:
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%%% !" 2$ + 3( + 3 )(••
% + !" 2 + 5( + )(••
(2)
(3)
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%%% It can be seen from Eq.(2) and (3) that $ represents the site of W6+ substituted % by Sb3+ with three negative charges, while the represents the site of W6+
substituted by Sb5+ with one negative charges. The . denotes a neutral oxygen atom on oxygen site and )(•• represents a vacancy with positive 2 valence charge
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with respect to the normal O2- site [33,34].
When the sample was exposed in air, the oxygen molecules in air capture electrons from surface of material,
(4)
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)(•• + 1/2 + 2 ⟶ (
when the sample was exposed in oxidizing gas of NO2, the NO2 molecules with high electron affinity capture electrons from oxide conductance band to form
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adsorbed NO2−,
)(•• + 12 + 3 ⟶ 134
(5)
The above reactions result in decrease in conductance for the samples (that is
resistance increase), which enhances the sensing response of the doped sample. So, from crystal structure analysis, the huge improvement in response of the sensor to NO2 can be attributed to the valence different substitution of Sb3+ and Sb5+ for W6+ ions in the WO3 lattice. It is in good agreement with the measurement of sensor response. 10
ACCEPTED MANUSCRIPT Fig. 11(a) shows room-temperature PL spectra of the pure WO3 and 3.5 wt% Sb-doped WO3 samples when excited at 325 nm. As observed in Fig. 11(a), the PL intensity of 3.5 wt% Sb-doped WO3 is lower than that of pure WO3. The lower PL intensity indicates a lower recombination rate of photo-generated electrons and holes.
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Because the PL emission originates from the recombination of excited electrons and holes, which can effectively promote the sensing response of Sb-doped WO3 sample [35, 36]. Emission in the ultraviolet region, also called near-band-edge (NBE) emission, originates from the free exciton emission. The emission in the visible region,
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also called deep level emission (DL), which can be attributed to the recombination of photogenerated electron and hole. The DL emission is usually related to the presence
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of intrinsic defects (such as oxygen vacancy), and its strength is positively correlated with the defect density. The area ratio of DL emission and NBE emission is generally considered to be defect characteristics (the higher the ratio, the more the defects). In this study, the photoluminescence spectrum can be divided into two wavelength regions as shown in Fig. 11(b) and (c): the NBE emission centered at 350–400 nm is
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abbreviated as INBE and the DL emission centered at 400–500 nm is abbreviated as IDL. By calculation, the PL ratio IDL/INBE of pure WO3 and 3.5 wt% Sb-doped WO3 was estimated to be about 8 and 10.8, respectively. Crystal defects are beneficial for the
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adsorption of oxygen and surface reaction, which means that the 3.5 wt% Sb-doped WO3 has a higher response than pure WO3. The PL analysis is in good agreement with
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the gas-sensing results.
5. First-principle calculations based on density functional theory
There are two kinds of mainly positions for metal elements doping, the replace
site and the interstitial site. The antimony ions replace tungsten ions resulting in the produce of holes, i.e., it reduces the concentration of free electron in doped system. If antimony ions enter into the lattice space as electron donors, it will increase the free electron concentration in system, leading to reduce in resistance. According to above Eq. (2), (3) and Fig.9 it can be seen that the Sb3+ replaces W6+ site resulting in produce of 3 holes, while two Sb5+ replaces W6+ resulting in produce of 2 holes, both all cause 11
ACCEPTED MANUSCRIPT the reduce in concentration of free electrons in system. If both of Sb3+ and Sb5+ all enter into lattice space, it will increase the concentration of free electron in system, leading to the reduce in resistance. But according to the experimental results the material resistance is increased after doping. Therefore, the Sb atoms doped into the tungsten oxide lattice actually all occupied the replace sites.
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Fig. 12 is the lattice structure of tungsten oxide before and after antimony atoms doping. Thus, it can be seen that doping results in strain of crystal cell and atomic rearrangement of the tungsten oxide. The results are shown in Table 2: the four O atoms in neighboring Sb move to the outside, (0.019 Å、0.086 Å、0.207 Å、0.146 Å);
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the other two neighboring O atoms move to the inner side(0.096 Å、0.155 Å), reducing the asymmetric distortion of octahedral SbO6.
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Fig. 13 shows the band structure of WO3 and Sb-doped WO3 obtained by first-principle calculation. As shown, the conduction band energy level and the valence band energy level become denser after doping. The energy level in conduction band and the energy level in valence band also become denser. And the appearance of Sb 5p impurity levels at 2 eV ~ 4 eV and the interaction between Sb 5p
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and W 5p are responsible for causing the G point removal. As shown in Fig. 14, the state density of tungsten oxide is obviously changed at 0 eV ~ 2 eV, which may be the reason of the conduction band shift after doping. These results are in accordance with
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the resistance change in gas sensing tests.
6. Conclusion
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In summary, Sb-doped WO3 architectures have been successfully synthesized by
a facile hydrothermal method at low temperature of 120oC. The sensitivity of sensor based on 3.5% Sb-doped WO3 is 9 times higher than that of pure WO3 to NO2 at operating temperature of 125oC. The characteristic response time of the sensor is reduced disproportionately with the increase of the NO2 concentration by using the L– H heterogeneous reaction mechanism to model the transient responses of the sensor. The key factor enhancing sensing performance is attributed to the intrinsic defect changes and electronic interaction between oxide and dopant. The first-principle calculations show that the substitution in valence different of Sb3+ and Sb5+ for W6+ 12
ACCEPTED MANUSCRIPT causes the reduce in concentration of free electron in doped system, leading to the increase in resistance of material, which is main factor of enhancing sensing response of material. This work provides the useful information for the synthesis of cheaper element doped metal oxides by using a cost-effective and facile synthesized method,
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which is pretty competitive.
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Acknowledgments
This work was supported by the National Natural Science Foundation of China
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(Grant No. 51372013), the Fundamental Research Funds for the Central Universities (YS1406), Beijing Engineering Center for Hierarchical Catalysts and the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University.
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[21] M. Epifani, N. Garcia-Castello, J.D. Prades, A. Cirera, T. Andreu, J. Arbiol, P. Siciliano, J.R. Morante, Suppression of the NO2 interference by chromium addition in WO3-based ammonia sensors. Investigation of the structural properties and of the related sensing pathways, Sens. Actuators, B, 187 (2013) 308-312. [22] X. Bai, H. Ji, P. Gao, Y. Zhang, X. Sun, Morphology, phase structure and acetone
(2014) 100-106.
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sensitive properties of copper-doped tungsten oxide sensors, Sens. Actuators, B, 193
[23] S. Bai, K. Zhang, R. Luo, D. Li, A. Chen, C.C. Liu, Low-temperature
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hydrothermal synthesis of WO3 nanorods and their sensing properties for NO2, Journal of Materials Chemistry, 22 (2012) 12643. [24] S. Bai, T. Guo, Y. Zhao, J. Sun, D. Li, A. Chen, C.C. Liu, Sensing performance
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and mechanism of Fe-doped ZnO microflowers, Sens. Actuators, B, 195 (2014) 657– 666.
[25] A.P. Lee, B.J. Reedy, Temperature modulation in semiconductor gas sensing, Sens. Actuators, B, 60 (1999) 35-42. [26] L. You, Y.F. Sun, J. Ma, Y. Guan, J.M. Sun, Y. Du, G.Y. Lu, Highly sensitive NO2 sensor based on square-like tungsten oxide prepared with hydrothermal treatment, Sens. Actuators, B, 157 (2011) 401-407. [27] C.Y. Lee, M.S. Strano, Understanding the dynamics of signal transduction for adsorption of
gases and vapors on carbon nanotube sensors, Langmuir 21(2005) 15
ACCEPTED MANUSCRIPT 5192–5196. [28] A.P. Shpak, A.M. Korduban, M.M. Medvedskij, V.O. Kandyba, XPS studies of active elements surface of gas sensors based on WO3-x nanoparticles, J. Electron Spectrosc. Relat. Phenom., 156-158 (2007) 172-175.
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[29] A. Samariya, R.K. Singhal, S. Kumar, Y.T. Xing, M. Alzamora, S.N. Dolia, U.P. Deshpande, T. Shripathi, E.B. Saitovitch, Defect-induced reversible ferromagnetism in Fe-doped ZnO semiconductor: An electronic structure and magnetization study, Mater. Chem. Phys. 123 (2010) 678–684.
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[30] B.L. Zhu, C.S. Xie, D.W. Zeng, W.L. Song, A.H. Wang, Investigation of gas sensitivity of Sb-doped ZnO nanoparticles, Mater. Chem. Phys., 89 (2005) 148-153.
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[31] D. Szczuko, J. Werner, S. Oswald, G. Behr, K. Wetzig, XPS investigations of surface segregation of doping elements in SnO2, Appl. Surf. Sci., 179 (2001) 301-306.
[32] J. Song, X. Ni, D. Zhang, H. Zheng, Fabrication and photoluminescence properties of hexagonal MoO3 rods, Solid State Sci., 8 (2006) 1164-1167.
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[33] B. Lyson-Sypien, A. Czapla, M. Lubecka, P. Gwizdz, K. Schneider, K. Zakrzewska, K. Michalow, T. Graule, A. Reszka, M. Rekas, A. Lacz, M. Radecka, Nanopowders of chromium doped TiO2 for gas sensors, Sens. Actuators, B, 175
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(2012) 163-172.
[34] X. Bai, H. Ji, P. Gao, Y. Zhang, X. Sun, Morphology, phase structure and acetone sensitive properties of copper-doped tungsten oxide sensors, Sens. Actuators, B, 193
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(2014) 100-106.
[35] I. Navas, R. Vinodkumar, V.P.M. Pillai, Self-assembly and photoluminescence of molybdenum oxide nanoparticles, Applied Physics A 103 (2011) 373–380. [36] X. Yang, C. Cao, L. Erickson, K. Hohn, R. Maghirang, K. Klabunde, Synthesis of visible-light-active TiO2-based photocatalysts by carbon and nitrogen doping, Journal of Catalysis 260 (2008) 128–133.
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ACCEPTED MANUSCRIPT Figure Captions Fig.1 XRD patterns of pure WO3 and Sb-doped WO3. Fig.2 (a) FESEM image of pure WO3 hierarchical microspheres , (b) magnificated FESEM image of WO3 hierarchical microspheres, (c)FESEM image of 3.5% Sb-doped WO3 (d) schematic illustration for the formation of WO3 hierarchical microspheres.
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Fig.3 EDS patterns of 3.5% Sb-doped WO3. Fig.4 Temperature dependence of response for sample annealed at different temperatures of (a) 400oC, (b) 600oC and (c) 800oC to 8 ppm NO2.
Fig.5 Sensitivity of (a)pure WO3, (b) 2.0 wt%Sb-doped WO3, (c) 3.5 wt% Sb-doped WO3 and (d)5.0 wt% Sb-doped WO3 annealed at 600oC and operated at 125oC.
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Fig.6 Transient response curves of sensors based on pure WO3 (green curves) and 3.5 wt% Sb-doped WO3 (purple curves) annealed at 600oC to 1–8 ppm NO2 at operating temperature of
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125oC.
Fig. 7 Linear relationship of the sensitivity and NO2 concentration for WO3 and 3.5 wt% Sb doped WO3 sensors at 125oC.
Fig. 8 Sensitivity of sensor based on 3.5 wt% Sb-doped WO3 to different test gases at operating temperatures of 125oC.
Fig. 9 Experimental (scattered point) and fitted (solid line) response transients of 3.5 wt%-Sb
response time.
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doped WO3 based sensor to NO2 gases (a) 2 ppm, (b) 4 ppm and (c) 8 ppm, (d) characteristic
Fig. 10 (a) and (b) W region for pure and 3.5 wt%-Sb doped WO3 samples respectively, (c) Sb region, and（d）O1s XPS spectra of the pure and Sb-doped WO3 sample.
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Fig. 11 (a) PL spectra of pure and 3.5 wt% Sb-doped WO3, (b)-(c)Gaussian deconvolutions of PL spectra of pure WO3 and 3.5 wt% Sb-doped WO3 annealed at 600oC. Fig. 12 The computational model of pure WO3 and Sb-doped WO3.
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Fig. 13 Band structure of (a) WO3 and (b) SbxW1-xO3. Fig. 14 The TDOS and PDOS of SbxW1-xO3. The inset is the magnification of the DOS in the conduction band between 2.5 eV and 4.5 eV.
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Table Legends Table 1. Sensitivity of doped WO3 to different gases reported in literatures. Table 2. The change of bond length of M-O before and after Sb-doping (unit: Å).
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Concentration
Test gas
Operating
Enhanced times
Reference
o
temperature( C) 10ppm
NO2
150
1.4
[18]
Ag-doped WO3
40ppm
NO
200
2.4
[19]
Sn-doped WO3
10ppm
CH3OH
250
1.5
[20]
Cr-doped WO3
50ppm
NH3
250
1.25
[21]
Cu-doped WO3
20ppm
CH3COCH3
300
1.5
[22]
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8ppm
NO2
125
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Au-doped WO3
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-x
+y
-y
+z
-z
WO3
1.919
1.852
2.009
1.799
2.074
1.776
SbxW1-xO3
1.938
1.938
1.913
2.006
1.919
1.922
0.019
0.086
-0.096
0.207
-0.155
0.146
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ACCEPTED MANUSCRIPT Highlights Sb-doped hierarchical WO3 was synthesized with the assistance of citric acid.
Sensitivity of WO3 to 8 ppm NO2 at 125°C is enhanced 9 times by doping 3.5 wt% Sb.
Response time is shortened disproportionately with increasing gas concentration.
The enhanced sensing is attributed to the change of defect and binding energy.
First principle calculations reveal the bandgap of WO3 is widened after Sb doping.
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S0925-8388(16)32795-5
DOI:
10.1016/j.jallcom.2016.09.061
Reference:
JALCOM 38895
To appear in:
Journal of Alloys and Compounds
Received Date: 20 May 2016 Revised Date:
2 September 2016
Accepted Date: 5 September 2016
Please cite this article as: J. Sun, J. Guo, J. Ye, B. Song, K. Zhang, S. Bai, R. Luo, D. Li, A. Chen, Synthesis of Sb doping hierarchical WO3 microspheres and mechanism of enhancing sensing properties to NO2, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.061. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis of Sb doping hierarchical WO3 microspheres and mechanism of enhancing sensing properties to NO2 Jianhua Sun a,b, Jingyao Ye a, Meng Cui a, Zhangfa Tong b*, Shouli Bai a*, Ruixian Luo a, Dianqing
a
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Li a*, Aifan Chena
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Guangxi Key Laboratory of Petrochemical Resource Processing and Process
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b
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Chemical Technology, Beijing 100029, China.
Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi
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University, Nanning 530004 , China.
Sb-doped tungsten oxides have been synthesized successfully by hydrothermal method without any surfactant at 120oC for 24 h, which not only exhibists high response to NO2, good selectivity to ethanol, acetone, NH3, methanol and CO, but also exhibits good linear responses in the concentration range of 1ppm to 8ppm.
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Synthesis of Sb doping hierarchical WO3 microspheres and mechanism of enhancing sensing properties to NO2
a
a
a
a*
,
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Jianhua Sun a,b, Jun Guoa, Jingyao Ye , Bangjun Song, Kewei Zhang, Shouli Bai
Ruixian Luo a, Dianqing Li a*, Aifan Chena
State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of
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a
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Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing 100029, China. b
Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification
Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning
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530004 , China.
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Abstract Sb-doped tungsten oxides have been synthesized successfully by a hydrothermal method without any surfactant at 120oC for 24 h. The structure and morphology of product were examined by XRD and SEM. The sensing tests reveal that the
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enhancement to NO2 response by Sb doping is very significant compared to that reported in literatures. Especially, the highest response of 3.5 wt%-Sb doped WO3 exhibits 9 times higher than that of undoped sample. The transient responses of the sample to different concentrations of NO2 were also measured and modeled using L–
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H heterogeneous reaction mechanism. The mechanism of enhancing sensing performance is also discussed in detail, which attributed to the changes of material
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intrinsic defects and binding energies and has been confirmed by the room temperature PL and XPS spectra. The first-principle calculations indicate that the dopant as acceptor enter into tungsten oxide resulting in reduce in free electron concentration of system, which is in good agreement with the increase in resistance obtained from experiment measurements.
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calculation.
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Key words: WO3; Sb-doping; NO2 sensing; L–H reaction mechanism; First-principle
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1. Introduction: In recent years, air pollution has been becoming more and more serious with the development of industries and automobiles. Among the air pollutants, nitrogen dioxide (NO2) is not only toxic itself, but also a main precursor of acid rain and
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photochemical smog [1-3]. Many efforts have been devoted to develop NO2 sensors for accurately monitoring of the NO2 concentration in environment.
The semiconducting metal oxides have been widely investigated as sensing materials of NO2 sensors. Among them, tungsten trioxide (WO3), as a wide band-gap
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n-type semiconductor with intrinsic non-stoichiometry, has been proven to be a promising sensing material for detection of NO2 [4, 5]. Nowadays, various
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morphologies of WO3 with different dimensional nanostructures have been synthesized to improve the sensing performance of the WO3, such as nanorods [6], nanowires [7] and nanoplates [8], etc. Compared with low dimensional nanostructures, the hierarchical micro/nanometer microspheres[9] consisting of low-dimensional building blocks provide an opportunity to realize fast gas diffusion and surface
blocks[10-12].
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reactivity due to porous and spatial orientation arrangement of the building
In order to further enhance the gas sensing performance of semiconducting metal
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oxides, many attempts have been made such as loading of noble metal, doping and surface modification via element or oxide, and composite formed with another oxide,
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etc. Although doping of metal elements already has been recognized to be one of the facial and effective methods to enhance sensor sensitivity or reduce operating temperature of sensor [13-16], the dopants to enter crystalline structure of oxide is rather difficult, leading to the doping effect is not significant. For example, Xia et al. reported that 1.0 wt% Au-doped WO3 powder prepared by a colloidal chemical method for detection of NO2, although the product exhibited higher sensitivity and better selectivity to 10 ppm NO2 at optimum operating temperature of 150 oC, the sensitivity of doped sample only enhanced 1.5 times than that of undoped WO3 at 150oC [17], and the noble metal (Au) dopant is expensive. 3
ACCEPTED MANUSCRIPT In the work, we also made a lot of attempts to improve the sensing properties of tungsten trioxide to NO2 through doping different metal elements with the same mass fraction to WO3, such as Zn, Fe, Sn, Mo, La and Sb in the hydrothermal process. Among them, the 3.5 wt% Sb doped WO3 exhibits the best result of gas sensing. To
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comparison, we have searched about WO3 sensing materials with deferent doping elements and their sensitivity to NO2. The data have been listed in Table 1[18-22], from Table 1 it can be seen that the enhancement of sensitivity is very significant in
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the work compared with that reported in literatures.
2.1 Preparation of Sb-doped WO3
WO3
architectures
were
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All chemicals were analytical-grade reagents without further purification. Sb-doped prepared
by
following
hydrothermal
procedure:
Na2WO4·2H2O (1.0g) and citric acid(1.2 g) were dissolved in 60 mL of distilled water, proper amount of antimony chloride (Sb/WO3=2.0, 3.5 and 5.0 wt%)was added into the above mixed solutions under constant stirring. After ultrasonic 3 min and stirred
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30 min, 8 mL of 3 M HCl aqueous solution was added dropwise into the above solution. The solution was stirred for 10 min, then transferred to a Teflon-lined autoclave with 100 mL capacity and subsequently heated at 120oC for 24 h. After the reaction, the pale yellow products were harvested by centrifugation and thoroughly
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washing with deionized water. Finally, the product was dried at 80oC overnight and
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then annealed at 400oC, 600oC and 800oC, respectively for 4h in air.
2.2 Characterization
Powder X-ray diffraction (XRD) patterns of the products were recorded on a
Rigaku D/MAX-2500 X-ray diffractometer at 30 kV and 100 mA with copper Kα radiation (λ = 1.54Å). Scanning rate of 5o min−1 was applied to record the patterns in range of 10-70o(2θ).Field emission scanning electron microscopy (FESEM) images were obtained with a Hitachi S-4700 instrument operated at 20.0 kV. An elemental composition analysis was carried out by energy dispersive spectrometry (EDS) using an EDS spectrometer attached to the same microscope. X-ray photoelectron 4
ACCEPTED MANUSCRIPT spectroscopy (XPS) spectra were recorded on an X-ray photoelectron spectrometer (VG ESCALAB-MK) with aluminum Kα radiation. PL spectra were recorded from 350 to 550 nm at room temperature by a 325 nm excitation (RF-5301PC
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spectrometer)
2.3 First-principles calculations
First-principles calculations based on density-functional theory (DFT) were carried out using Materials Studio 5.5 (Accelrys Inc.). The exchange-correlation energy was
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represented by the local density approximation (LDA) of Ceperley and Alder (CA-PZ). A cutoffenergy of 400 eV was used as the plane-wave basis set and a 3x3x3
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Monkhorst k-point mesh was used for the Brillouinzone integrations. The lattice parameters of WO3 were obtained from the experiment and then fully optimized. The convergence threshold for self-consistent field (SCF) iteration was set at 10 5 eV.
3.Sensor fabrication and gas-sensing test
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The sensors based on Sb doped WO3 was fabricated[23] and different concentrations of NO2 (1 ppm, 2 ppm, 4 ppm and 8 ppm) were used as the target gas to examine the sensing performance of the sensor using a JF02E test system (Guiyan
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Jinfeng Technology Co., Kunming, China). The operating temperature of the sensor was adjusted by varying the heating voltage of system. The resistance of the sensor in air (Rair) and in the air–test gas mixture (Rgas) was recorded, respectively. The
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response for NO2 is defined as the ratio of RNO2/Rair. The response and recovery time were measured as the time for the sensor reaches 90% of its maximum response after introduction or removal of the test gas, respectively.
4. Results and discussion 4.1 Structure and morphology of WO3 X-ray powder diffraction analysis was carried out to investigate the crystal structures of pure and doped samples. Strong diffraction peaks in Fig. 1 indicate that the pure WO3 shows a high degree of crystallinity after calcination and the crystalline 5
ACCEPTED MANUSCRIPT phase can be indexed as a monoclinic structure due to all the diffraction peaks agree well with the values standard JCPDS#43-1035( space group: P21/n, lattice constants: a=7.297Å, b=7.539Å, c=7.688 Å). When the Sb doping amount is more than 3.5 wt%, an extra small diffraction peak emerges at 30o in addition to the diffraction peaks of
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pure tungsten oxide and the increase of diffraction peak intensity with the doping. The peak can be indexed as (222) peak of the cubic phase Sb6O13 compared with JCPDS # 33-0111.
Morphology and size of the as-synthesized WO3 and Sb-doped WO3 sample were
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observed by FE-SEM. Fig. 2(a) and (b) shows the FESEM image of pure WO3 at low and high magnification, respectively. The observation reveals that these microspheres
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with diameter of 2-3 µm are actually formed from 0D primary nanoparticles via self-assembled into 2D interlaced nanosheets, and finally into 3D hierarchical microspheres, as shown in Fig. 2(d). The WO42– would gel gradually from solution with dropping of dilute HCl solution, and finally small nanoparticles were obtained during the hydrothermal process. In the preparation process of precursors, the
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introduction of citric acid will force the tungstate ions to chelate with the dissociated carboxylic groups, forming a relatively stable tungstate complex, even in strong acidic aqueous solution. The growth of particles would be promoted or prevented if a
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chemical additive could effectively adsorb on the surface of particles. When large amount of citric acid was added, these coordinate nucleuses undergo further condensation polymerization under elevated temperature, and finally form sheet-like
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nanostructures. Under the tendency of thermodynamic stability, the sheet-like nanostructures tend to spontaneously rearrange themselves and become stacking to each other in an oriented fashion by dipole-dipole interactions. The steric barrier of carboxylic ligands provides relatively weak interaction between adjacent sheet-like structures, dominating their stacking and reconstruction, and the equilibrium between the steric barrier and condensation-polymerization dominates the morphological evolution of WO3. Fig. 2(c) indicates that the morphology of WO3 microspheres partially destroyed after doping. EDS was used to observe the existence of Sb component in doped sample as illustrated in Fig. 3, which reveals the presence of Sb, 6
ACCEPTED MANUSCRIPT W and O elements and the Sb concentration is about 1.92 wt%. 4.2 Sensing properties of pure and Sb-doped WO3 based sensors Fig. 4 presents that the response of a semiconductor sensor is greatly affected by the operating temperature, which results from the effect of temperature on
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adsorption/desorption and surface reaction of gas [23-25]. The response of pure WO3 annealed at 600oC reaches a maximum value of 17 at operating temperature of 125oC， so, 125oC is called to be the optimum operating temperature and is chosen to test the transient responses hereinafter. The hierarchical structures of WO3 promote the
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enhancement of response due to a fast diffusion of gas and surface reaction at low operating temperature. Fig. 4 shows the temperature dependence of response for
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sample annealed at different temperatures of (a) 400oC, (b) 600oC and (c) 800oC to 8 ppm NO2. The results suggest that the sensing response increases at first and then decreases with the change in operating temperature from 80 to 150 °C. Because sufficient thermal energy is essential to overcome the activation energy barrier of oxygen chemisorption and surface reaction at temperature range from 80 to 125 °C,
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however, once the operating temperature over 125 °C, the amount of desorbed gas will increase, leading to the reduce of response. The temperature (125 °C) that the chemisorbed amount of gas on the surface of material reaches maximum is called
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optimum operating temperature, and meanwhile the sensing response reaches maximum value. From Fig.4 it also can be seen that the annealing temperature of 600oC is found to be the appropriate annealing temperature for WO3 at which the
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sample not only has higher crystallinity but also has appropriate specific surface area [26].
To investigate the effect of doping amount on the gas sensing properties, the
responses of samples with different Sb doping amount to NO2 were examined. Fig. 5 presents responses of these samples to 1 ppm to 8 ppm NO2 at operating temperature of 125oC. It is found that the response increased with the increase of NO2 concentration for all samples and the responses of 3.5 wt% Sb-doped sample are obviously higher than pure WO3 and other doped samples. The 3.5 wt% Sb-doped WO3 sample exhibits the highest response of 195 to 8 ppm of NO2 gas, which is 9 7
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Fig.6 it can be seen that the response increases with increasing of gas concentration and has a good linear relationship between the response and the gas concentration in range of 1–8 ppm for 3.5 wt% Sb-doped sample, which is better than that of pure WO3 as shown in Fig.7.
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The cross sensing (selectivity) between NO2 and other reducing gases is still one of the major problems for practical NO2 sensor. Fig. 8 shows the selectivity of NO2 to
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these interference gases of ethanol, NH3, acetone, methanol and CO for 3.5 wt% Sb-doped WO3 based sensor. The experimental results indicate that the sensor not only exhibits high sensitivity and excellent selectivity but also has good linear response in NO2 concentration range of 1 ppm to 8 ppm at relatively low operating temperature of 125oC. So, the 3.5 wt% Sb-doped WO3 microspheres is a promising
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4.3. Nonlinear fit of NO2 sensor based on 3.5 wt% Sb-doped WO3 sensors Fig. 9 indicates the time dependence of the sensitivity in measurement process. The response time and the recovery time were measured to be 38 s and 10 s, respectively,
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to 8 ppm NO2 at 125oC. The transient responses can be modeled by Langmuir– Hinshelwood reaction mechanism and the response transient (S(t)) can be expressed
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St = 1 − [/ ]
(1)
where τirrev= 1/(k × Cg) (k—rate constant; Cg—gas concentration) is referred as characteristic response time for the gas sensor. Respective response transients for NO2 concentrations in 2, 4 and 8 ppm were fitted nonlinearly according to Eq. (1), and the fitted results were shown in Fig. 9(a)–(c). Fig. 9(d) shows the characteristic response time, which was estimated from above results. The characteristic response time is reduced disproportionately with the increase of the NO2 concentration for the gas sensor. 8
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4.4 Factors influencing sensing performance of Sb-doped WO3 Two different sensing factors, structural and electronic sensitization, were provided herein. Sb doping induced the electronic sensitization of WO3 was investigated by an
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XPS spectra. Fig. 10 shows high-resolution XPS survey spectra (0–1350 eV) and spectra of W 4f, O 1s and Sb 3d for the pure and Sb-doped WO3 samples. From Fig. 10, only C, W, O and Sb related core levels are detectable in the spectra, indicating no impurities are introduced. Fig. 10(a) depicts the XPS spectrum of W 4f level for pure
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WO3. The peak positions corresponding to W6+ state located at 36.05 and 37.2 eV, respectively, while the W5+ state located at 34.8 and 36.5 eV, respectively. Therefore,
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the molecular formula of the WO3 should be precisely expressed as WO3−x, where x represents the number of oxygen vacancies. Furthermore, the binding energy for the 3.5 wt% Sb-doped WO3 sample increases 0.15 eV compared with the pure WO3 sample (from 37.1 eV to 37.25 eV for W 4f5/2 and from 35 eV to 35.15 eV for W 4f7/2) as shown in Fig. 10(b)[28]. The chemical shift in binding energies reflect the
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electronic interaction between the WO3 and the dopant[29], which is responsible for the enhancement of gas response due to the Sb doping. By estimating the area ratio of W6+ to W5+ from XPS spectrum, we can conclude that the x is 0.12 and 0.17 for the
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pure and Sb-doped WO3, respectively, that is, the molecular formula of WO3 should be written as WO2.88 and WO2.83, respectively. To further confirm the Sb element doped into the WO3 and identify the chemical state of doped Sb, the doped sample
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was analyzed by XPS spectrum as shown in Fig 10(c). We found that the Sb 3d peak appeared at 539.89 eV, and according to XRD analysis shown in Figure1, the diffraction peak of cubic phase Sb6O13 appears in the patterns. Accordingly, we assume that the doped element consist of two oxide states of Sb3+ and Sb5+ and the ratio between them is 1:2. On this basis, the Sb 3d peak can be divided into two peaks located at 539.31 eV and 540.02 eV and correspond well to the Sb3+ and Sb5+, respectively [30,31], which indicates that the Sb has been introduced in WO3 in the form of Sb6O13 (Sb2O3:Sb2O5 = 1:2). In addition, the chemical shifts of O and W toward high energy direction also show that antimony is doped into the lattice of 9
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the defect equilibrium of the WO3 crystals, therefore, negative 3 of valence and negative 1 of valence charges of the substituted W site have to be compensated in the form of oxygen vacancy to maintain electrical neutral of the structure, which can be described through the following equation:
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%%% !" 2$ + 3( + 3 )(••
% + !" 2 + 5( + )(••
(2)
(3)
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%%% It can be seen from Eq.(2) and (3) that $ represents the site of W6+ substituted % by Sb3+ with three negative charges, while the represents the site of W6+
substituted by Sb5+ with one negative charges. The . denotes a neutral oxygen atom on oxygen site and )(•• represents a vacancy with positive 2 valence charge
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with respect to the normal O2- site [33,34].
When the sample was exposed in air, the oxygen molecules in air capture electrons from surface of material,
(4)
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)(•• + 1/2 + 2 ⟶ (
when the sample was exposed in oxidizing gas of NO2, the NO2 molecules with high electron affinity capture electrons from oxide conductance band to form
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adsorbed NO2−,
)(•• + 12 + 3 ⟶ 134
(5)
The above reactions result in decrease in conductance for the samples (that is
resistance increase), which enhances the sensing response of the doped sample. So, from crystal structure analysis, the huge improvement in response of the sensor to NO2 can be attributed to the valence different substitution of Sb3+ and Sb5+ for W6+ ions in the WO3 lattice. It is in good agreement with the measurement of sensor response. 10
ACCEPTED MANUSCRIPT Fig. 11(a) shows room-temperature PL spectra of the pure WO3 and 3.5 wt% Sb-doped WO3 samples when excited at 325 nm. As observed in Fig. 11(a), the PL intensity of 3.5 wt% Sb-doped WO3 is lower than that of pure WO3. The lower PL intensity indicates a lower recombination rate of photo-generated electrons and holes.
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Because the PL emission originates from the recombination of excited electrons and holes, which can effectively promote the sensing response of Sb-doped WO3 sample [35, 36]. Emission in the ultraviolet region, also called near-band-edge (NBE) emission, originates from the free exciton emission. The emission in the visible region,
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also called deep level emission (DL), which can be attributed to the recombination of photogenerated electron and hole. The DL emission is usually related to the presence
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of intrinsic defects (such as oxygen vacancy), and its strength is positively correlated with the defect density. The area ratio of DL emission and NBE emission is generally considered to be defect characteristics (the higher the ratio, the more the defects). In this study, the photoluminescence spectrum can be divided into two wavelength regions as shown in Fig. 11(b) and (c): the NBE emission centered at 350–400 nm is
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abbreviated as INBE and the DL emission centered at 400–500 nm is abbreviated as IDL. By calculation, the PL ratio IDL/INBE of pure WO3 and 3.5 wt% Sb-doped WO3 was estimated to be about 8 and 10.8, respectively. Crystal defects are beneficial for the
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adsorption of oxygen and surface reaction, which means that the 3.5 wt% Sb-doped WO3 has a higher response than pure WO3. The PL analysis is in good agreement with
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the gas-sensing results.
5. First-principle calculations based on density functional theory
There are two kinds of mainly positions for metal elements doping, the replace
site and the interstitial site. The antimony ions replace tungsten ions resulting in the produce of holes, i.e., it reduces the concentration of free electron in doped system. If antimony ions enter into the lattice space as electron donors, it will increase the free electron concentration in system, leading to reduce in resistance. According to above Eq. (2), (3) and Fig.9 it can be seen that the Sb3+ replaces W6+ site resulting in produce of 3 holes, while two Sb5+ replaces W6+ resulting in produce of 2 holes, both all cause 11
ACCEPTED MANUSCRIPT the reduce in concentration of free electrons in system. If both of Sb3+ and Sb5+ all enter into lattice space, it will increase the concentration of free electron in system, leading to the reduce in resistance. But according to the experimental results the material resistance is increased after doping. Therefore, the Sb atoms doped into the tungsten oxide lattice actually all occupied the replace sites.
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Fig. 12 is the lattice structure of tungsten oxide before and after antimony atoms doping. Thus, it can be seen that doping results in strain of crystal cell and atomic rearrangement of the tungsten oxide. The results are shown in Table 2: the four O atoms in neighboring Sb move to the outside, (0.019 Å、0.086 Å、0.207 Å、0.146 Å);
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the other two neighboring O atoms move to the inner side(0.096 Å、0.155 Å), reducing the asymmetric distortion of octahedral SbO6.
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Fig. 13 shows the band structure of WO3 and Sb-doped WO3 obtained by first-principle calculation. As shown, the conduction band energy level and the valence band energy level become denser after doping. The energy level in conduction band and the energy level in valence band also become denser. And the appearance of Sb 5p impurity levels at 2 eV ~ 4 eV and the interaction between Sb 5p
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and W 5p are responsible for causing the G point removal. As shown in Fig. 14, the state density of tungsten oxide is obviously changed at 0 eV ~ 2 eV, which may be the reason of the conduction band shift after doping. These results are in accordance with
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the resistance change in gas sensing tests.
6. Conclusion
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In summary, Sb-doped WO3 architectures have been successfully synthesized by
a facile hydrothermal method at low temperature of 120oC. The sensitivity of sensor based on 3.5% Sb-doped WO3 is 9 times higher than that of pure WO3 to NO2 at operating temperature of 125oC. The characteristic response time of the sensor is reduced disproportionately with the increase of the NO2 concentration by using the L– H heterogeneous reaction mechanism to model the transient responses of the sensor. The key factor enhancing sensing performance is attributed to the intrinsic defect changes and electronic interaction between oxide and dopant. The first-principle calculations show that the substitution in valence different of Sb3+ and Sb5+ for W6+ 12
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which is pretty competitive.
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Acknowledgments
This work was supported by the National Natural Science Foundation of China
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(Grant No. 51372013), the Fundamental Research Funds for the Central Universities (YS1406), Beijing Engineering Center for Hierarchical Catalysts and the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University.
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ACCEPTED MANUSCRIPT Figure Captions Fig.1 XRD patterns of pure WO3 and Sb-doped WO3. Fig.2 (a) FESEM image of pure WO3 hierarchical microspheres , (b) magnificated FESEM image of WO3 hierarchical microspheres, (c)FESEM image of 3.5% Sb-doped WO3 (d) schematic illustration for the formation of WO3 hierarchical microspheres.
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Fig.3 EDS patterns of 3.5% Sb-doped WO3. Fig.4 Temperature dependence of response for sample annealed at different temperatures of (a) 400oC, (b) 600oC and (c) 800oC to 8 ppm NO2.
Fig.5 Sensitivity of (a)pure WO3, (b) 2.0 wt%Sb-doped WO3, (c) 3.5 wt% Sb-doped WO3 and (d)5.0 wt% Sb-doped WO3 annealed at 600oC and operated at 125oC.
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Fig.6 Transient response curves of sensors based on pure WO3 (green curves) and 3.5 wt% Sb-doped WO3 (purple curves) annealed at 600oC to 1–8 ppm NO2 at operating temperature of
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125oC.
Fig. 7 Linear relationship of the sensitivity and NO2 concentration for WO3 and 3.5 wt% Sb doped WO3 sensors at 125oC.
Fig. 8 Sensitivity of sensor based on 3.5 wt% Sb-doped WO3 to different test gases at operating temperatures of 125oC.
Fig. 9 Experimental (scattered point) and fitted (solid line) response transients of 3.5 wt%-Sb
response time.
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doped WO3 based sensor to NO2 gases (a) 2 ppm, (b) 4 ppm and (c) 8 ppm, (d) characteristic
Fig. 10 (a) and (b) W region for pure and 3.5 wt%-Sb doped WO3 samples respectively, (c) Sb region, and（d）O1s XPS spectra of the pure and Sb-doped WO3 sample.
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Fig. 11 (a) PL spectra of pure and 3.5 wt% Sb-doped WO3, (b)-(c)Gaussian deconvolutions of PL spectra of pure WO3 and 3.5 wt% Sb-doped WO3 annealed at 600oC. Fig. 12 The computational model of pure WO3 and Sb-doped WO3.
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Fig. 13 Band structure of (a) WO3 and (b) SbxW1-xO3. Fig. 14 The TDOS and PDOS of SbxW1-xO3. The inset is the magnification of the DOS in the conduction band between 2.5 eV and 4.5 eV.
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Table Legends Table 1. Sensitivity of doped WO3 to different gases reported in literatures. Table 2. The change of bond length of M-O before and after Sb-doping (unit: Å).
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Concentration
Test gas
Operating
Enhanced times
Reference
o
temperature( C) 10ppm
NO2
150
1.4
[18]
Ag-doped WO3
40ppm
NO
200
2.4
[19]
Sn-doped WO3
10ppm
CH3OH
250
1.5
[20]
Cr-doped WO3
50ppm
NH3
250
1.25
[21]
Cu-doped WO3
20ppm
CH3COCH3
300
1.5
[22]
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8ppm
NO2
125
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Au-doped WO3
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-x
+y
-y
+z
-z
WO3
1.919
1.852
2.009
1.799
2.074
1.776
SbxW1-xO3
1.938
1.938
1.913
2.006
1.919
1.922
0.019
0.086
-0.096
0.207
-0.155
0.146
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ACCEPTED MANUSCRIPT Highlights Sb-doped hierarchical WO3 was synthesized with the assistance of citric acid.
Sensitivity of WO3 to 8 ppm NO2 at 125°C is enhanced 9 times by doping 3.5 wt% Sb.
Response time is shortened disproportionately with increasing gas concentration.
The enhanced sensing is attributed to the change of defect and binding energy.
First principle calculations reveal the bandgap of WO3 is widened after Sb doping.
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