rGO decorated W doped BiVO4 novel material for sensing detection of trimethylamine

Accepted Manuscript Title: rGO decorated W doped BiVO4 novel material for sensing detection of trimethylamine Authors: Lixia Sun, Jianhua Sun, Ning Han, Dankui Liao, Shouli Bai, Xiaojun Yang, Ruixian Luo, Dianqing Li, Aifan Chen PII: DOI: Article Number:

S0925-4005(19)30950-5 https://doi.org/10.1016/j.snb.2019.126749 126749

Reference:

SNB 126749

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

28 February 2019 26 June 2019 27 June 2019

Please cite this article as: Sun L, Sun J, Han N, Liao D, Bai S, Yang X, Luo R, Li D, Chen A, rGO decorated W doped BiVO4 novel material for sensing detection of trimethylamine, Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.126749 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.

rGO decorated W doped BiVO4 novel material for sensing detection of triethylamine Lixia Suna, Jianhua Suna,*, Ning Hanb,*, Dankui Liaoa, Shouli Baic, Xiaojun Yangc, Ruixian Luoc, Dianqing Lic, Aifan Chenc,* Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification

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a.

Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning

b.

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530004, China

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering,

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of

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c.

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Chinese Academy of Sciences, Beijing 100190, China

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Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology,

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Beijing 100029, China

Corresponding author Tel. +86 010 64199706

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E-mails: [email protected]; [email protected]; [email protected]

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Graphical abstract

The novel gas sensing material of rGO decorated W-doped bismuth vanadate was

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synthesized for the first time by metal organic decomposition followed by

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hydrothermal treatment. The sensing properties of the 6BiVO4/rGO5 composite to triethylamine exhibit significant enhancement compared with pristine BiVO4, which

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is attributed to the formation of p-n heterojunction and the performance of rGO.

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The rGO decorated 6W:BiVO4 gas sensing material was synthesized for the first time. The structure, morphology and gas sensing properties of material were investigated The optimum composite exhibits excellent sensing properties towards trimethylamine.

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The enhancement is attributed to formation of heterojunction and rGO performance.

Abstract: The development of new materials forever is research front for various application fields, otherwise becomes making bricks without straw. A novel sensing material of rGO decorated W-doped BiVO4 was synthesized for the first time by 2

metal organic decomposition combined with hydrothermal methods for detection of trimethylamine (TEA) vapor. The structure and morphology of material were characterized by spectroscopy techniques. The sensing properties of sensor to TEA were measured. The results showed that the 6WBiVO4/rGO5 composite exhibits response of 12.8 and response time of 16 s, which is 5.12 times higher and 2.75 times lower than that of BiVO4, respectively. Moreover, the sensor shows excellent

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selectivity and stability to 20 ppm TEA at 135°C. The enhancement is ascribed to the increase of electron density, enhancement of specific surface and accelerating of

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electron transfer for BiVO4 due to W doping, rGO decoration and formation of heterojunctions.

Keywords: Semiconductor heterojunction; W-doping BiVO4; Reduced graphene

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oxide; Triethylamine sensor

1. Introduction Trimethylamine is a hazardous volatile organic compound that originates from spoiled fishes and sea creatures and has negative effects on air quality and human 3

health

[1]

. The gas sensors based on metal oxide semiconductors have attracted

considerable attention due to the advantages on high sensitivity, low cost synthesis route and compatible with micro-electronic manufacturing technology [2,3]. Nowadays, the wide band gap of semiconductor metal oxides as gas sensing materials, such as

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SnO2, ZnO, WO3 have obtained great progress and wide applications[4-6], but the development of new materials is still a long-term and urgent demand. The bismuth

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vanadate (BiVO4) with band gap of 2.4 eV as photoanode material for solar photoelectrochemical (PEC)water splitting has been widely investigated by various

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modifications to enhance its PEC performance due to its nontoxicity, stability, low

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cost synthesis and excellent compatibility to its modification. But BiVO4 as gas

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sensing material rarely was reported to date and still in the early investigation stages.

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BiVO4 as gas sensing material has to be modified by various strategies

[7,8]

, such as

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element doping, combining two semiconductors to create heterostructures or using carbon material to decorate BiVO4. Among them, doping is an effective and facile

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method to improve gas sensing properties of semiconductor metal oxide. Herein, high

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valence W ions as charge donors were doped into BiVO4 to increase electronic density and reduce band gap of BiVO4. Recently, graphene has been widely studied as an additive material of semiconducting metal oxide due to its large specific surface

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and high carrier mobility, thereby potentially providing superior sensitivity and rapid response time. So far, many graphene/metal oxide based gas sensors have been reported to effectively detect pollutant gases, combustible gases and volatile organic compounds [9-11].To further enhance the gas sensing properties of BiVO4, the rGO 4

nanosheets were decorated on surface of doped-BiVO4, it not only can form p-n heterojunction with doped BiVO4 but also as an effective electron shuttling mediator increases specific surface and accelerates electron transport of doped BiVO4. Therefore, the triadic composite of W-BiVO4/rGO is a promising novel sensing For comparison, the sensing performance of

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material for detection of TEA vapor.

BiVO4 based sensors reported in literature and this work were listed in Table 1.The

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enhanced sensing mechanism was also discussed in detail in the work.

2. Experimental section

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2.1 Preparation of graphene oxide

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All the chemical reagents we used were analytically pure without further

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purification. The graphene oxide (GO) was prepared from natural graphite powder by

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a two-step oxidation reaction of pre-oxidation followed by modified Hummer’s

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oxidation to reach complete oxidation of graphite. In the first step, 20 g of graphite powder, 10 g of K2S2O8, and 10 g of P2O5 were successively added to a 250 ml

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three-necked flask. 30 mL of concentrated H2SO4 was slowly poured into the solution.

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After 6 hours of vigorous reaction under 80°C of water bath temperature, the obtained solution with dark blue precipitate was thermally separated and gradually cooled to room temperature, and then dropwise added deionized water to dilute the above

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solution. The solution was filtered and washed with deionized water to pH 7, and then the obtained product was dried overnight at 60°C under vacuum. The obtained pre-oxidized graphene was further oxidized. 0.5 g of pre-oxidized graphite powder was mixed with 34 mL of H2SO4 (98%), 0.74 g of NaNO3, and 5 g of KMnO4 in an 5

ice water bath under vigorous stirring and maintained temperature at 20°C, then the solution up to 35°C and stirring for 3 h. Finally, 250 mL of deionized water and 4 mL of H2O2 (30 wt%) were slowly poured into the above solution, the obtained golden yellow suspension was washed several times with HCl and water (1:10 v/v). Finally,

2.2 Synthesis of BiVO4 and W-doped BiVO4 nanoparticles

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the obtained GO solution was dried overnight at 50°C[16].

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Pure and W-doped BiVO4 nanoparticles were synthesized by the modified metal

organic decomposition method. A typical procedure was given as follows: bismuth

(C10H14O5V,

0.03M)

and

ammonium

paratungstate

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acetylacetonate

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nitrate pentahydrate (Bi(NO3)3·5H2O) in acetic acid (CH3COOH, 0.2M), vanadyl

ratio of Bi : (V + W). The obtained precursor was sonicated 20 min then

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atom

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(NH4)6W7O24·6H2O, 0.01 M) in dimethyl sulfoxide (C2H6OS) were mixed with 1 : 1

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dried followed by calcined at 450°C for 2 h with a heating rate of 2oC/min. The samples with different atom ratios (W/Bi) of 0%, 4%, 6%, and 8% were obtained and

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named as BiVO4, 4W-BiVO4, 6W-BiVO4 and 8W-BiVO4, respectively.

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2.3 Synthesis of W-BiVO4/rGO nanocomposite

W-BiVO4/rGO triadic nanocomposites were synthesized by a high temperature

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thermal reduction process to remove oxygen-containing functional groups from GO. Weighed as-prepared 6W-BiVO4 nanoparticles were dispersed in deionized water by ultrasonication for 2 h. The different amount of GO powder was dispersed in water by ultrasonication to exfoliate the GO and obtain the uniform aqueous suspension of GO, followed by the exfoliated suspension was added into above W doped BiVO4 6

dispersion and magnetically stirred for 12 h at room temperature. Subsequently, the whole dispersion system was transferred to a Teflon-sealed autoclave and maintained at 180°C for 12 h. The final product was alternately washed by deionized water and ethanol for three times. The W-BiVO4/rGO composites with different weight

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percentages of rGO ( 0%, 3%, 5%, 10% and 100%) were prepared, they were marked as W-BiVO4, W-BiVO4/rGO3, W-BiVO4/rGO5,W-BiVO4/rGO10 and pure rGO,

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respectively. The schematic diagram of preparation process was shown in Fig. 1. 2.4 Characterization of material

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The crystal phase and structure were analyzed at Shimadzu XRD-6000

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diffractometer with Cu Kα radiation (λ= 0.15418 nm) operating at 40 kV and 30 mA

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in 10 ∼ 80° at a scanning rate of 10°/min. Scanning electron microscope (SEM)

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images and energy dispersive X-ray spectroscopy (EDX) were performed on a Hitachi

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field emission microscope equipped with EDX spectrometer (Zeiss Supra 55) operated at 20.0 kV. High-resolution transmission electron microscopy (HRTEM)

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images are obtained on JEOL JEM-2010 microscopy with an accelerating voltage of

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200 kV. The solid-state UV-Vis absorption spectra were measured at room temperature by using a spectrometer equipped with an integrating sphere attachment

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(Shimadzu UV-3000). A X-ray photoelectron spectrum (Thermo VG ESCALAB250) with Al Ka X-ray as the excitation source (1486.6 eV) was used to record and analyse the chemical composition and element valence state in composite. 2.5 fabrication of sensor

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The prepared sample was mixed with an appropriate amount of ethanol and coated on a small magnetic tube with a gold electrode. A Ni-Cr alloy wire for heating was put into the central tunnel of the magnetic tube to control the operating temperature of the sensor. The prepared sensing element was aged at certain

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temperature for a few days. The gas sensing tests were performed in a static state system (JF02E, Guiyan Jinfeng Technology Co Kunming, China) to measure

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responses of the sample in different concentration of TEA vapor. The response of

n-type semiconductor based sensor to reduce gas is defined as Ra/Rg and Rg/Ra for

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oxidizing gases, where Ra is the resistance of the gas sensor in air and Rg stands for

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resistance of the sensor in the target gas.

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3.1 Morphology and structure

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3. Results and discussion

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The morphologies of samples were observed as shown in Fig. 2(a) and (b), the uniform distribution of nanoparticles with 100–300 nm size were observed for the

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pure BiVO4 and W-doped BiVO4 samples. Fig.2 (c) indicates that the W-doped

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BiVO4 particles are wrapped by the extremely thin rGO nanosheets and the rGO was presented with a crumpled structure, which contributes to its high specific surface area. The elemental composition and distribution of the obtained W-BiVO4/rGO

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sample were quantitatively recorded on EDX spectroscopy (Fig. 2j). The mapping (Fig. 2e-i) indicates the existence of O, Bi, V, W, and C elements and the atomic ration of 0.056 between W and Bi, which suggests that the W has been doped successfully into BiVO4 and is corresponding to nominal 6% added in prepared 8

process. In addition, the content of C elements is roughly consistent with added amounts in the synthesized process. The XRD analysis of BiVO4, GO, rGO, 6W-BiVO4 and W-BiVO4/rGO samples were performed and shown in Fig. 3(a), In the XRD patterns of pure BiVO4 and

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W-doped BiVO4 nanocrystals, all the sharp diffraction peaks are good agreement with a monoclinic phase of BiVO4 (JCPDS 14-0688). There is no characteristic diffraction

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peak of GO (001) appeared at about 11° in W-BiVO4/rGO sample, implying that GO

has been completely reduced to rGO by the thermal reduction[17]. The enlarged view

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in Fig. 3(b) shows a slight shift of the diffraction peak of (-121) due to the W doped

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into the V sites of the lattice in BiVO4. The crystallite sizes of BiVO4 and 6W-BiVO4

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were estimated by Scherrer’s formula to be 25.58 nm and 29.02 nm, respectively,

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replaced V atom in BiVO4 lattice.

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which may be the lattice distortion caused by atomic radius of W slightly larger

The ultraviolet-visible absorption spectrum (UV-vis) is usually applied to

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evaluate the band gap of the semiconductors. From Fig. 3(c), the main absorption

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edge of BiVO4 appears at 520 nm, while the band absorption edge of 6W-BiVO4 shifts to longer wavelength of 530 nm. The relationship between the band gap and optical absorption obeys the following equation (1) for a crystalline semiconductor

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[18]

.

E



n

photon

 K E photon  Eg 

(1)

Where, α, Ephoton, K and Eg represent the absorption coefficient, the discrete photo energy, a constant and the band gap energy, respectively. The monoclinic BiVO4 has 9

a direct band gap and thus n value is 1 in the equation (1). The band gap energies of BiVO4 and doped BiVO4 were estimated to be 2.43 eV and 2.49, respectively by above equation (1) as shown in Fig. 3(d). The results showed that the band gap of doped BiVO4 sample was decreased due to the formation of impurity energy level,

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which is conductive to the electron transfer and the improvement of sensing properties for semiconductor metal oxides.

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The chemical compositions and valence states were analyzed by surface XPS

technique for the optimum composite as shown in Fig. 4. Two strong peaks

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corresponding to Bi 4f5/2 and Bi 4f7/2 in Bi 4f spectra appeared around 164.9 eV and

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159.6 eV of Fig. 4(b), which confirms the presence of Bi3+ ions. The split peaks of V

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2p observed at 524.9 and 516.4 eV are assigned to V 2p1/2 and V 2p3/2 orbits,

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respectively as shown in Fig. 4(c). The O 1s spectrum of the W-BiVO4/rGO sample

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can be decomposed into three peaks as shown in Fig. 4(d). The peak located at 530.4 eV, 531.6 eV, and 533.3 eV are the characteristic peaks of the O2- lattice (designated

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as Oβ), surface adsorbed oxygen (designated as Oα) and adsorbed molecular water

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(designated as Oγ) in the W-BiVO4/rGO sample, respectively. As shown in Fig. 4(e), the appearance of W 4f5/2 and W 4f7/2 peaks located at 37.8 and 35.7 eV also confirmed the presence of hexavalent tungsten species (W6+). In the spectrum of C 1s

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(Fig. 4f), carbons formed by sp2 bonds (284.8 eV) are in a leading position, and oxygen-containing functional groups such as 288.8 eV (C=O) and 285.8 eV (C−O) can be observed, this demonstrates that high-level oxidation occurs during the graphene exfoliation. 10

The Raman spectra of the GO, W-BVO4, and W-BVO4/rGO samples were shown in Fig. 3(b). It can be seen that Raman peaks located at 116, 200, 336, and 810 cm−1 are the typical vibrational peaks of BVO4[19]that basically consistent with the peak positions of BiVO4 reported in the literature. The Raman peaks located at 1371 cm−1

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for the D band and located at 1599 cm−1 for the G band correspond to the interruption of the symmetric hexagonal graphite lattice and the in-plane stretching motion of the

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symmetric sp2-bonded carbon, respectively [20]. The D band and G band peaks of rGO in W-BVO4/rGO composite exhibit a slight shift from their pristine positions

[21]

. From Fig.5, the increase in the ID/IG intensity ratio from 1.00

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domains inversely

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compared with GO. The ID/IG ratio can be used to evaluate the average size of the sp2

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(GO) to 1.16 (W-BVO/rGO) indicates that the reduction of GO decreased the average

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size of sp2 domain in the graphene structure and increased defect[22], that is, the

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high-intensity ratio of ID/IG indicates lower in planar stretching alignment of sp2 C-C bond and greater sp3 defects, so the rGO possess a large number of defects and lower

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in planar stretching alignment of sp2 C-C bond compared with GO. Fig.5 also

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confirms that the GO nanosheets have successfully been decorated on theW-BVO4 and itself reduced into rGO sheets. To characterize the specific surface area and porosity of the 6W-BVO4/rGO5

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composite,

the

nitrogen

adsorption–desorption

isotherms

was

carried

out

(Micromeritics Surface Area 2390 system). The BET specific surface area was calculated to be 38.35 m2 g−1 through adsorption of N2 quantity as shown in Fig. 6(a). The high specific surface results from decoration of rGO, which is beneficial to the 11

adsorption of gases and surface reaction between adsorbed oxygen species and TEA, thus resulting in enhancement of sensing response. The pore size distribution curve was shown in Fig. 6(b), the entire pore size is in mesoporous range of 3–15 nm. To roughly calculate the carrier density of materials, the M-S plots were [23]

. As shown in Fig.7(a), both plots showed

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measured by electrochemical method

positive slopes, which means the single and modified BiVO4 are n-type

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semiconductors. From M-S plots, the 6W-BiVO4 sample has the electron density of

3.22 × 1020 cm-3 that almost is 2.5 times higher than pristine BiVO4 (1.30 × 1020 cm-3).

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The enhancement of electron density for doped sample is important reason enhancing

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gas sensing properties of the composite, which has been confirmed by the gas sensing

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3.2 Gas-sensing properties

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experiments.

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The sensing properties of the materials towards TEA were measured by the resistance change in the air and in the TEA gas. Fig.8(a) showed that the operating

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temperature significantly affects response of a semiconductor sensor, since it greatly

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affects the gas adsorption and the sensing reaction.[24] The heating is to overcome the activation energy of chemisorption and surface reaction, but when the temperature higher than the optimum operating temperature of 135oC, the response decreases

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because the gas desorption becomes dominant. The temperature at which the response exhibits a maximum value is called the optimum operation temperature. However, the operating temperature of the gas sensor is related to the orbital energy of gas molecule, adsorption mode and amount of gas adsorption, and so on. So to clarify the 12

relationship will need to research further more. Fig. 8(b) showed the transient responses for the different composites at 135°C. It can be seen that the 6W-BiVO4/rGO5 composite showed the highest response of 12.8 that is 2.06 and 5.12 times higher than that of the 6W-BiVO4 (6.2) and pristine BiVO4 (2.5), respectively.

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As shown in Fig. 8(c), the response time of 6W-BiVO4/rGO5 composite was measured to be 16 s that is 1.12 and 2.75 times smaller than that of the 6W-BiVO4 (18

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s) and pristine BiVO4 (44 s), respectively. The transient resistances of BiVO4 and 6W-BiVO4/rGO5 composite to different concentrations of TEA (1-30 ppm) at 135°C

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were shown in Fig. 8(d).

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Fig. 9(a) showed the effect of TEA concentrations on transient response for the

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pristine BiVO4, 6W-BiVO4 and 6W-BVO4/rGO5 samples at 135°C. It indicated that

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the sensor response increases dramatically with the increase of TEA concentration.

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Transient responses (S(t)) were simulated according to the Langmuir–Hinshelwood equation[25]:

S(t) = Smax·(1 - e-k·Ca·t)

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(2)

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where Smax, k, Ca represent the maximum response, the surface rate constant and the initial gas concentration, respectively. Fig. 9(b) showed that the experimental response curves can fit well with the Langmuir–Hinshelwood model for the

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6W-BVO4/rGO5 composite, indicating the gas adsorption on material surface is single molecular uniform adsorption and the gas sensing response conforms to the gas-solid phase reaction model of Langmuir–Hinshelwood.

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The linear tendency was observed for pure BiVO4 and 6W-BVO4/rGO5 composite in different concentrations of TEA as shown in Fig. 10 (a). The linear tendency of composite is better than that of pure BiVO4. The detection limit (DL) is one of the important parameters representing the sensing properties, a high sensitivity

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usually has a lower detection limit. The detection limit (DL) is defined as the lowest test concentration in which the response is significantly different from the noise signal

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(generally 3 times than the standard deviation of noise). The sensor noise can be calculated by the variation in the relative response of sensor in the baseline. The 10

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consecutive data taken before exposure to TEA were averaged and a standard

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deviation (S) was calculated to be 0.35 using the root-mean-square deviation (RMSD)

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provided by Dua et al[26, 27]: 𝑆2

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𝑅𝑀𝑆𝑛𝑜𝑖𝑠𝑒 = √ 𝑁

(3)

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Where N is the number of data points, The RMSnoise can be calculated according to the above Eq. (3) to be 0.11. According to the definition of detection limit, the slopes

RMSnoise Slope

=3*0.11/0.15 = 2.2 ppm (for BVO4)

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DL = 3

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are 0.15 and 0.52 for BiVO4 and 6W-BVO4/rGO5 composite from Fig. 10(b), so that,

DL = 3

RMSnoise Slope

=3*0.11/0.52= 0.63 ppm (for 6W-BVO4/rGO5)

(4) (5)

The calculated detection limits of the sensors fit well with double logarithm liner

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fitting as shown in Fig. 10(b) (DL = 0.65 ppm for BVO4 and DL = 0.43 ppm for 6W-BVO4/rGO5 composite). The result evidently confirmed that the composite with high sensitivity also necessarily has low detection limit to same test gas.

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One of the challenges for semiconductor metal oxide gas sensors is to achieve high selectivity, which makes the sensor enables to exclusively test certain target gas. Therefore, the responses of 6W-BVO4/rGO5 composite to 20 ppm of formaldehyde, acetone, ammonia, butyl alcohol, methanol, and toluene were measured at same

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temperature of 135°C. Fig. 11(a) exhibited the selectivity of the optimum composite based sensor to 20 ppm TEA in other interference gases. The gas-sensing properties of

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semiconductor metal oxides are dominantly controlled by the surface oxygen adsorption and the reaction activity between chemisorbed oxygen ions and target gas

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molecules. The gas adsorption leads to the surface energy and the conductance change,

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which arises from electron affinity that is affected by the orbital energy of the gas

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molecule, that is, the electron affinity between gas and material results in different

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resistance of the sensor after gas adsorption. While the reaction activity depends on the

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lowest unoccupied molecule orbit (LUMO) energy of gas molecule at different operating temperature. If the LUMO energy is lower, the energy needed for the gas

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response will reduce and the sensing response can be enhanced. Moreover, the sensor

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can detect the target gas at lower operating temperature. So, the reducibility difference for reducing gases results from their LUMO energy. For example, from quantum chemistry calculation the value of LUMO energy for ethanol, methanol, acetone and

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formaldehyde are 0.12572, 0.19728, 0.20525 and 0.21965 eV, respectively. Based on the analysis above, the reducibility reduces in order of ethanol, methanol, acetone and formaldehyde, which is consistent with the difference of bond energies between gas molecules. However, to clarify the mechanism of selectivity, it will need to research 15

further more

[28, 29]

. Stability is one of the key parameters in the development of gas

sensors, the 6W-BVO4/rGO5 composite based sensor was exposed to 20 ppm TEA for 7 cycles in 30 days to investigate the stability of the sensor. The response of sensor maintains at same level around 12.8 as shown in Fig. 11(b), indicating the excellent

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stability of the sensor 3.3 Mechanism of enhancing gas sensing

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Although rGO is not semiconductor, it exhibits the p-type semiconductor behaviors because it exhibits the increase of resistance in reducing gas, while in [30, 31]

. Herein, the response of the

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oxidizing gas exhibits the decrease of resistance

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synthesized composite would occur along continuous W doped BiVO4 nanoparticles

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due to the concentration of rGO was lower compared to BiVO4. Accordingly, the

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sensing mechanism of the composite to TEA is based on the resistance change

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induced by interface electron deplete, which results from oxygen chemisorption and the sensing reactions between the adsorbed oxygen species and the target gas. When

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the composite exposed to air at temperature of 130°C, oxygen molecules in air were

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adsorbed on BiVO4 surface to form the oxygen species (O2–, O- or O2-) by the capture of electrons from the conduction band (CB) of BiVO4 and create a depletion layer close to the particle surface, which results in the resistance increase of composite in

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air. Once the composite was exposed to a reducing gas of TEA, TEA will react with the absorbed oxygen species (O-(ads)), removing these oxygen species and releasing the electrons back to CB of BiVO4, leading to a decrease in the depletion layer along with a resistance decrease. The reaction equation can be represented as following: 16

2N (C2H5)3 + 43 O− 15 H2O+ 2NO2 + 12 CO2 + 43 e−

(6)

Where, the main adsorbed oxygen species are O-(ads) at operating temperature of 135oC because the type of forming oxygen species is dependent on the operating temperature of sensor. In addition, when rGO and BiVO4 were contact each other, electrons were

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transferred from rGO to BiVO4 until their Fermi level equality and formation of rGO/BiVO4 heterojunction due to the work function (4.7 eV) of rGO lower than that

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of BiVO4 (5.4eV) as shown in Fig.11, resulting in a 0.7 eV band-bending and enhancement of interface potential barrier, which leads to the additional increase of

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composite resistance in air and the additional decrease compared in TEA compared

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with undecorated rGO sample of W-BVO4. As a result, the response of the composite

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was further enhanced according to the define (Ra/Rg) of response due to the formation

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of heterojunction. Moreover, rGO as an efficient electron shuttling mediator decorated

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on 6W-doped BiVO could more effectively enhance the TEA adsorption on surface of composite and accelerate electron transport from TEA to BiVO4 due to large specific

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surface, high surface defect and rapid electron transport of rGO. So, the 5% rGO

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decorated 6W-doped BiVO4 is a novel and promising sensing material for detection of TEA[32].

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4. Conclusions

Novel rGO decorated W doped BiVO4 gas sensing material was successfully

synthesized for the first time by a facile metal organic decomposition and hydrothermal methods. The 6W-BiVO4 sample exhibits the highest response of 6.2 and the response time of 18 s to 20 ppm TEA at 135°C, which is 2.48 times higher 17

than BiVO4 and 2.44 times smaller than BiVO4, respectively. The response is further enhanced to 12.8 and the response time is further decreased to 16 s by rGO decoration, which is 5.12 times higher and 2.75 times lower than that of BiVO4, respectively. Moreover, the sensor shows excellent selectivity and stability. The new kind of

Acknowledgments work

was

supported

by

Guangxi

Natural

Science

Foundation

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This

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sensing material may bring a new hope in gas sensor field.

(2017GXNSFAA198289，2018GXNSFAA294001), National Key R&D Program of

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China (2016YFC0207100) and National Natural Science Foundation of China (Grant

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No.51772015) and Chongzuo Science Foundation (FA2017004 、 FA2017003 、

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FA2018002)

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IP T SC R U N A

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CC E

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ED

M

Fig. 1 Schematic diagram of composite preparation process.

23

IP T SC R U N

A

Fig. 2 (a-c) SEM images of BiVO4, 6W-BiVO4 and 6W-BiVO4/rGO5 samples; (d)-(i)

A

CC E

PT

ED

M

Corresponding mappings and (j) EDS spectrum of 6W-BiVO4/rGO5 sample.

24

Fig. 3 (a, b) XRD patterns of BiVO4, GO, rGO, 6W-BiVO4 and 6W-BiVO4/rGO samples; (c) UV-Vis absorption spectra of BiVO4 and 6W-BiVO4 samples; (d) Band

A

N

U

SC R

IP T

gap energies of BiVO4 and 6W-BiVO4 samples.

M

Fig. 4 (a) XPS full survey spectrum of 6W-BiVO4/rGO5 composite, (b) Bi 4f, (c) V

A

CC E

PT

ED

2p, (d) O 1s, (e) W 4f, and (f) C 1s.

25

IP T SC R U N A

CC E

PT

ED

M

Fig. 5 Raman spectra of GO, 6W-BiVO4 and 6W-BVO4/rGO5 samples.

A

Fig. 6 (a) and (b) Nitrogen absorption–desorption isotherms and pore size distribution images for 6W-BVO4/rGO5 sample.

26

IP T SC R

A

CC E

PT

ED

M

A

N

U

Fig. 7 Mott-Schottky plots of BiVO4 and 6W-BVO4/rGO5 samples.

Fig. 8 (a) Effect of operating temperature on response for different samples; (b) transient responses to 20 ppm TEA for different samples at 135°C; (c) transient responses and response time of different samples to 20 ppm TEA at 135°C; (d) 27

transient resistances of BiVO4 and 6W-BVO4/rGO5 composite to different

SC R

IP T

concentrations of TEA at 135°C.

Fig. 9 (a) Transient responses in different concentration of TEA for pure and

N

U

composites at 135°C; (b) Experimental and fitted transient responses for optimum

CC E

PT

ED

M

A

composite in different concentrations of TEA at 135°C.

Fig. 10 (a) Linear relationship between response and gas concentration for pure BiVO4 and optimum composite; (b) Double logarithm fitting between the response

A

and gas concentration for pure BiVO4 and composite and corresponding detection limit.

28

IP T SC R

Fig. 11 (a) Response of different gases at 135°C for BVO4, 6W-BVO4, 6W-BVO4/rGO5 based sensors; (b) Stability of 6W-BVO4/rGO5 based sensor to 20

A

CC E

PT

ED

M

A

N

U

ppm TEA at 135°C.

Fig. 12 Schematic diagram of sensing mechanism.

29

Table 1 Sensing performance comparison of BiVO4 based sensors reported in literatures and this work.

Synthesized

Response

Operating

method

Refs

temperature(°C) 35.2 (100 ppm n-butanol)

BiVO4-polyoxometalate nanocomposite

hydrothermal

31.2% (50 ppm NO2)

La-Doped BiVO4

Chemical precipitation

3.5 (700 ppm ethanol)

Fe2O3/BiVO4 composites

Solvothermal

7.8 (2ppm NO2)

W doped BiVO4/rGO hybrids

hydrothermal

5.9 (10ppm triethylamine)

N

A M ED PT CC E A

30

260

[12]

Room temperature

[13]

450

[14]

110

[15]

180

this work

SC R

hydrothermal

U

monoclinic BiVO4 nanoplates

IP T

Materials