anatase heterophase junction TiO2 thin film hydrogen sensors

Sensors & Actuators: B. Chemical 301 (2019) 127143

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Remarkably enhanced H2 response and detection range in Nb doped rutile/ anatase heterophase junction TiO2 thin film hydrogen sensors ⁎

Yuwen Baoa, Ping Weib, Xiaohong Xiaa, , Zhongbing Huanga, Kevin Homewooda, Yun Gaoa,

T

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a Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, School of Materials Science & Engineering and Faculty of Physics & Electronic Technology, Hubei University, Wuhan, 430062, China b State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrogen sensor Nb doped TiO2 Heterophase junction Charged oxygen ions Schottky barrier

Hydrogen sensors combining wide detection range and high response, that can operate at room temperature, are urgently demanded for monitoring different concentrations of hydrogen on a single device. Here, a novel onestep hydrothermal method is reported to construct Nb doped rutile/anatase TiO2 heterophase junctions for a highly sensitive H2 sensor. The Nb concentration in the solvent plays an important role for the heterophase juncton self-assemble, and the reaction time determines the surface morphology giving either a rutile TiO2 nanorod decorated anatase form, or a Nb doped rutile/anatase TiO2 bi-layer structure. An inverse dependence of the detectivity on H2 concentration is obtained as theoretically predicted but previously not experimentally verified for the high concentration region at low temperatures. The H2 concentration detection range is remarkably expanded, extending from 1 ppm to 12000 ppm for nanorod decorated film. The H2 response at 1 ppm is significantly enhanced to 22.5%, and reaches 98.9% in 8000 ppm H2 for the bilayer structure. The remarkablely extended detection range is the result of the massive accumulation of reactive pre-absorbed O−2 on the surface due to the Nb doping, and the significantly enhanced response at room temperature results from the joint contributions of pre-absorbed O−2 , film compactness and the heterophase junction. Moreover, Nb doped rutile/anatase TiO2 heterophase juncton films show high stability and humidity resistance for hydrogen sensing.

1. Introduction

heterojunction interface will affect the long term stability of the sensors. So far, there have been no reports on heterophase junction film sensors composed of anatase TiO2 and rutile TiO2, though the synergy between anatase TiO2 and rutile TiO2 was previously proven to help carrier separation in photocatalysts [13]. Transition metal doping, including Co, Ni, Cr, W, Ta, Al, and Nb, was found to be an important route for improving the gas sensing performance of TiO2 through band adjustment, surface potential modification, or charge carrier concentration regulation [14–22]. Among them, Nb doped TiO2 has attracted great interest due to its wide response to reducing gases such as H2 and CO [23–25], oxidizing gases such as NO2 and O2 [21,26], and even volatile organic compounds [27,28]. It was reported that Nb doping introduces new occupied states in the TiO2 band gap, arising from the Nb 4d valence electrons, changing the electronic structure of TiO2, enhancing charge transfer and O ionic adsorption [27,29,30]. However, crucially so far most Nb-doped sensors are made of TiO2 powder and only show sensing at high temperatures above 150 °C. Liu reported Nb-doped TiO2 nanotubes fabricated through anodization that worked at room temperature over a

Hydrogen is widely used in many industrial applications such as oil refining, metal smelting, electronics and aerospace [1,2]. Due to its combustible and explosive nature, it is extremely important to find H2 sensors working at room temperature with fast response, high response and wide detection range to satisfy a wide range of applications with differing detection requirements. Hydrogen sensors based on titanium dioxide have attracted extensive interest with the advantage of being nontoxic, biocompatible, low cost and photo-corrosion resistant. Great progress has been achieved in TiO2 based H2 sensors, especially for ordered TiO2 nanotubes and nanorods thin films which showed very high response to H2 at low concentrations [3–6]. Forming a heterostructure was found to dramatically improve the response, selectivity and response time [7]. The n-n isotypic heterostructure in αFe2O3@TiO2 [8], V2O5@TiO2 [9], TiO2@SnO2 [10], TiO2@WO3 [11], TiO2/ZnO [12] hydrogen sensors modulates the junction barrier height, forms an “accumulation layer”, promotes oxygen adsorption, and so enhances the response. However, elemental interdiffusion at the

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Corresponding authors. E-mail addresses: [email protected] (X. Xia), [email protected] (Y. Gao).

https://doi.org/10.1016/j.snb.2019.127143 Received 25 April 2019; Received in revised form 10 September 2019; Accepted 12 September 2019 Available online 13 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

Sensors & Actuators: B. Chemical 301 (2019) 127143

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Thermo Fisher), UV–vis absorption (UV-3600, Shimadzu) were adopted to characterize the microstructure, morphology, chemical component and optical properties of the samples.

wide detection range from 50 ppm to 20000 ppm. However, the corresponding response change was only in a narrow range from 7.7% to 41%, inadequate for precise measurements of H2 concentration [23]. Here, we simultaneously take advantage of the “accumulation layer” formed in a heterophase junction, the enhanced charge transfer, and augmented O ionic absorption in Nb doped TiO2. The Nb-doped rutile/anatase TiO2 heterophase junction thin film was fabricated by a one-step hydrothermal method via a self-adjusting Nb concentration during the hydrothermal processing. By controlling the reaction time, either rutile-decorated anatase film or rutile/anatase bi-layer film could be fabricated. A special lattice match is formed at the interface between the two phases, which reduces carrier scattering and enables fast charge transfer. We found that the Nb doped rutile/anatase hetrophase junction sensors show excellent H2 sensing properties working at room temperature with remarkably expanded detection range from 1 ppm to 12000 ppm, and significantly enhanced response up to 22.5% at 1 ppm. This work also provides the first experimental proof of the inverse dependence of the detectivity on H2 concentration theoretically predicted for the low temperature high concentration region [31].

2.4. Gas sensing test

2. Experimental

Platinum interdigitated electrodes about 500 nm thick were deposited onto the thin films by DC magnetron sputtering. The finger spacing of interdigitated electrodes was 1 mm. Hydrogen sensing measurements were carried out at room temperature (25 °C) at environmental humidity of about 40% in air. A Keithley 2400 multimeter was used to test the resistance variation of the sensors with an applied constant voltage of 1 V on the samples. Gas flow through the test chamber was precisely controlled via a computer-controlled mass flow controller. The targeted H2 concentration was maintained for about 8 min in a sealed chamber, and then the chamber was opened to release the H2. This process was repeated for several cycles to measure the hydrogen response at different concentrations, and the variation of resistance versus time was recorded once a second by a self-developed Labview program. Gas response S of the samples are calculated according to Eq. (1):

2.1. Seed Layer Synthesis

S=

Prior to synthesis, quartz substrates (2.5 cm*2.5 cm) were cleaned with acetone, absolute ethanol and deionized water in sequence. The substrates were then dried at 60 °C in air. A seed layer about 88 nm in thickness was deposited onto the cleaned quartz substrates by a RF magnetron sputtering system (ShenKeyi, China) using a Nb0.12Ti1.88O3 target at room temperature. The chamber base pressure was less than 3*10−4 Pa. High purity O2 (99.995%) and Ar (99.995%) were introduced as reacting and sputtering gas with fluxes of 0.4 sccm and 20 sccm respectively. Deposition was carried out at a magnetic sputtering power of 60 W at a working pressure of 0.1 Pa for 40 min. Subsequently the seed layer was annealed at 10−4 Pa, 400 °C for 60 min in a vacuum tube furnace.

in which Rair and Rgas denote the resistances of the sensor in air and H2 respectively. The response and recovery time are defined as the time taken for resistance variation to reach 90% of ΔR (ΔR = Rair − Rgas ).

Rair − Rgas Rair

*100%

(1)

3. Results and discussion 3.1. Microstructural characterization and heterophase junction growth mechanisms Fig. 1(a) presents the XRD spectra of samples with various Nb/Ti molar ratios, together with the reference diffraction patterns of anatase and rutile TiO2 (JCPDS No.21–1272 and No.21–1276). Only one weak peak corresponding to anatase TiO2 (101) is found in the seed layer, suggesting that the seed layer is composed of anatase TiO2. Two peaks R (101) and R (002) are observed in the un-doped TiO2_4H sample, indicating oriented growth of pristine rutile TiO2, similar to reported results [6,32]. In sample Nb1%_8H, the oriented growth of TiO2 thin films is disturbed; all the diffraction peaks of rutile TiO2 appear, together with a weak A(101) peak. The suppression of oriented growth is attributed to the doping of Nb into TiO2 lattice. The lattice parameters of TiO2 show a minor expansion after doping, due to the larger cation size of Nb5+ (0.70 Å) compared with Ti4+ (0.68 Å). In addition, the surface free energy could be modified by Nb dopants [33]. On further increasing the Nb concentration to 2%, both the intensities of R(110) and A(101) are reduced dramatically, signaling retardation of the growth for both rutile and anatase phases. Meanwhile, the higher relative intensity of A(101) over R(110) indicates preferential growth of anatase TiO2. No diffraction peaks assigned to Nb-based compounds were detected in Nb2%_8H, evidencing that Nb has been incorporated into the TiO2 lattice as a dopant. When the Nb concentration is increased to 4%, the R(110) peak disappears in sample Nb4%_8H, and the A(101) peak is very weak, similar to the seed layer, implying that there may not be any growth on the substrate. Fig. 1(b)–(e) display the surface morphologies of TiO2_4H, Nb1% _8H, Nb2%_8H and Nb4%_8H with their cross sectional images inserted. The TiO2_4H sample is composed of oriented nanorods growing perpendicular to the substrate with a thickness of 2.54 μm (Fig. 1(b)). With the addition of 1% Nb, the preferred growth orientation along the [002] direction was disturbed. The nanorods grow into larger bundles and no longer grow perpendicular to the substrate, consistent with the XRD results. The thickness of the film is only 1.77 μm for double the reaction time for pristine TiO2, giving an average growth rate only about 1/3 of

2.2. Thin Film Synthesis The pristine and Nb doped TiO2 thin films were fabricated using the hydrothermal method. The precursor solution was obtained by mixing 28 mL deionized water, 2 mL absolute ethanol, 25 mL concentrated hydrochloric acid and 1 mL tetrabutyl titanate (TBOT ≥ 99.0%) under magnetic stirring for about 15 min. Different doses of niobium ethoxide (Nb(OCH2CH3)5, ≥99.9%) were dissolved in 5 mL concentrated hydrochloric acid to form the Nb source solution. After 20 min of ultrasonic, Nb source solution was added to the precursor solution dropwise under stirring for about 20 min. The clear solution was then transferred to a 200 mL Teflon-lined steel autoclave with two quartz substrates leaning against the inner wall and the seed layer facing downward. The hydrothermal reaction was carried out at 150 °C for several hours. After synthesis, the autoclaves were cooled to room temperature in air. All samples were removed, rinsed with deionized water and then dried in air. Finally, the synthesized thin films were annealed at 400 °C for 20 min using rapid thermal processing under Ar atmosphere. The samples prepared with the Nb/Ti molar ratio of 0%, 1%, 2% and 4% are labeled as TiO2_4H, Nb1%_8H, Nb2%_8H, Nb4%_8H and Nb2%_12H, Nb2%_16H with reaction times of 4 h (4 H), 8 h (8 H), 12 h (12 H) and 16 h (16 H). 2.3. Characterization X-ray diffraction (D8-Advance, Bruker, Cu Kα line as the X-ray source), field emission scanning electron microscopy (Sigma 500, Zeiss), high resolution transmission electron microscope (Talos F200S, Thermo Fisher), X-ray photoelectron spectroscopy (ESCALAB 250Xi, 2

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Fig. 1. (a) XRD spectra of the thin films: the seed layer, pristine TiO2 (TiO2_4 H) and TiO2 (TiO2_8 H) with different Nb doping concentrations Nb/Ti= (1 at.%, 2 at.% and 4 at.%). (b–d) SEM images showing the surface and cross-sectional morphology of pristine TiO2 and Nb-doped TiO2 films: (b) TiO2_4H, (c) Nb1%_8H, (d) Nb2% _8H, (e) Nb4%_8H.

Fig. 2(a) presents XRD patterns of the 2 at.% Nb-doped TiO2 thin films grown at 150 °C for 8 h (Nb2%_8 H), 12 h (Nb2%_12 H) and 16 h (Nb2% _16 H), respectively. The A(101) peak exists in all the patterns with almost unchanged intensity, indicating that the anatase layer growth is complete after 8 h. However, the intensity of the R(110) peak increases gradually with increasing reaction time, indicating enhanced growth of the rutile phase. Crystallite sizes of the rutile phase were calculated from the R(110) peaks using the Debye–Scherrer formula. The mean grain sizes are 41.9 nm and 42.8 nm for the samples for 12H and 16H, suggesting that subgrains did not grow into large-size grains. The surface and crosssection SEM images of Nb2%_8H and Nb2%_12H samples are examined to clarify the growth process. Different from the rutile nanorods decorated on the surface of anatase of sample Nb2%_8H in Fig. 2(b), the surface of sample Nb2%_12H is composed of large bundles of rutile nanorods lying on top of the anatase layer to form a continuous compact layer as seen in Fig. 2(c). A clear three-layer structure, with thicknesses of 88 nm, 160 nm and 594 nm from bottom to top, is seen in cross-section, Fig. 2(d). The bottom layer has the same thickness as that

that using non-doped TBOT solvent. Increasing the Nb concentration to 2% results in a dramatic decrease of film thickness to 166 nm. The film is composed of a compact layer with small rods decorated on the surface. The change of surface morphology is due to transformation of the TiO2 crystal structure from rutile to anatase phase [34]. Given the A(101) peak is stronger than R(110) and that rutile TiO2 tends to grow into nanorods along the (001) direction in hydrochloric acid environment [32], we can deduce that the compact layer is Nb doped anatase TiO2 thin film, and the surface decorated nanorods are rutile TiO2. As the Nb concentration rises to 4%, a thin layer, about 88 nm thick, is seen in the SEM image very close to the seed layer, suggesting no film growth on the substrate. From this and the XRD results, we deduce that Nb ion participation in growth of TiO2, favours anatase phase formation, and retards the TiO2 growth rate. Excessive Nb doping, under the same hydrothermal conditions, results in no growth of either rutile or anatase TiO2. After successfully obtaining the Nb doped rutile TiO2 decorated anatase TiO2 junction, shown in Fig. 1(d), a longer reaction time was used to further promote growth of the surface-decorated rutile TiO2.

Fig. 2. (a) XRD spectra of 2 at.% Nb-doped TiO2 prepared at different growth time (8H, 12H and 16 H). SEM images of surface morphology of (b) Nb2%_8H and (c) Nb2%_12H, (d) cross-sectional morphology of Nb2%_12H. 3

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Fig. 3. High resolution TEM cross-sectional images with the inserted corresponding SEAD patterns, STEM elemental mapping, and the corresponding atomic Nb/Ti ratio of (a–c) Nb2%_8H and (d–f) Nb2%_12H.

Fig. 4. (a) HRTEM image of Nb2%_12H showing the anatase/rutile heterophase interface, and the corresponding fast Fourier transform (FFT) images taken within areas a1 and a2. (b) Interface atomic match between anatase (11ī) and rutile (10ī) plane simulated with Crystal Maker.

the anatase and rutile layers; located about 160 nm above the seed layer. Three layers with thicknesses of about 90 nm, 160 nm and 570 nm can be distinguished consistent with the SEM results. Similarly, the Nb/Ti atomic ratio gradually reduces from 12.1% to 6.3% in the anatase layer, and then from 6.3% to 3.0% in the subsequently grown rutile layer. The Nb/Ti ratio of the anatase layer in sample Nb2%_12H is similar to that in sample Nb2%_8H. The decreasing Nb atomic fraction implies consumption of Nb during the film growth. These results indicated that at least 6.3% Nb incorporation into the TiO2 lattice is required to maintain the stable anatase phase under high hydrochloric acid growth conditions. The self-adjustment of Nb concentration during the hydrothermal reaction is the key factor to grow anatase/rutile heterophase structure using the simple one-step hydrothermal method. High Resolution TEM image and corresponding SAED patterns of the Nb2%_12H sample at the interface between anatase and rutile layers are shown in Fig. 4(a). The SAED pattern of the area, a1, at the top layer corresponds to the {101}, {010} and {121} planes of rutile TiO2, perpendicular to the [10ī] direction. And the SAED from the area, a2, at the bottom layer corresponds to anatase TiO2 {101}, {011}, {112} planes, perpendicular to the [11ī] direction. The interphase lattice matching was simulated with Crystal Maker software and the results is shown in Fig. 4(b). The line mis-matching is less than 2.2%, which is favourable for rutile seeding at the edge position of the anatase (11ī) plane and growth along the [10ī] direction.

of the seed layer, the middle layer has similar thickness to the anatase layer in the Nb2%_8H sample, confirming the XRD results that the anatase layer growth stopped after 8 h reaction. After this rutile phase TiO2 starts to grow and the thickness of the rutile layer in the Nb2% _12H sample reached 594 nm. Both XRD and SEM results indicate that at 8 h the transition is from anatase to rutile layer growth. High Resolution TEM cross-sectional images, elemental mapping of each thin film, measured by energy dispersive X-ray spectroscopy (EDXs), and the corresponding calculated Nb/Ti atomic ratios of samples Nb2%_8H and Nb2%_12H are shown in Fig. 3(a)–(f), respectively. Nb distributed in both anatase and rutile TiO2 region in the two films as shown in the elemental mapping. The anatase layer forms a zigzag surface in sample Nb2%_8H. The corresponding Selected Area Electron Diffraction (SAED) patterns, inserted in Fig. 3(a), suggest that the anatase grains grow with exposed planes favoured along the [004] and [101] directions. The atomic ratio of Nb/Ti in the seed layer is about 6.8% consistent with the Nb/Ti ratio in the magnetic sputtering target material (Nb0.12Ti1.88O3). In the hydrothermal grown anatase TiO2 layer, the Nb/Ti ratio is greatest at the interface, and then gradually reduces from 12.0% to 6.5% along the growth direction. The upper layer SAED pattern of the sample Nb2%_12H, inserted in Fig. 3(d), indicates the upper layer is composed of rutile TiO2, the grains grow with exposed planes favoured along the [101] and [110] directions. The same zigzag boundary is seen at the interface between 4

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Fig. 5. Hydrogen sensing curves: resistance change at different H2 concentrations of (a) Nb2%_8H, and (b) Nb2%_12H, (c) response and (d) response and recovery time of sample Nb2%_8H and 12H at different H2 concentrations.

40%, showing excellent long-term stability. The difference in the three measurement cycles is less than 3%, indicating high repeatability. The same sample is also used for humidity test after ten months storage in ambient air condition, and the sensing curves are presented in Fig. 6(b). Most remarkably, it was found that the hydrogen time keeps almost unchanged with increasing the humidity from 40% to 60% in the high concentration region larger than 1200 ppm, but is slightly reduced in the low concentration region, e. g. for 1 ppm H2, the response changes from 21% to 17% when the humidity increases from 40% to 60%. Selectivity of the Nb2%_12H sample was tested in reductive and oxidative gas environments of carbon monoxide, ammonia, and nitrogen dioxide, respectively, with the gas concentration fixed at 1200 ppm. The resistance is reduced in reductive gas environment and the corresponding response are 88.6%, 18.9%, and 2.5% for H2, CO and NH3 respectively, as shown in Fig. 7. When oxidative gas NO2 is let in, the resistance is increased and the response is -12.8%. The response times exceed 10 min for all reductive gases other than H2. The ultrahigh response and fast response in H2 gas environment indicates excellent selectivity for H2 sensing. Plots of Ln(R H2 / Rair ) versus Ln(CH2) for the rutile/anatase heterophase junction sensors are presented in Fig. 8(a) to help understanding the sensing mechanism. The sensing curve could be divided into three regions, i.e., low concentration (LC), transitional concentration (TC) and high concentration (HC) regions. In the HC region (larger than 1200 ppm for Nb2%_8H, and larger than 200 ppm for Nb2%_12 H), both Nb2%_8H and Nb2%_12H sensors demonstrate a constant slope with a power law dependence close to -1. As the concentration is decreased into the TC region (100 ppm to 1200 ppm for Nb2%_8H, and 50 ppm to 200 ppm for Nb2%_12 H), the slope is no longer constant, deviating from -1 and varying with the hydrogen concentration. In the widely accepted redox reaction mechanism between hydrogen and adsorbed negatively charged oxygen ions for a metal oxide semiconductor H2 sensor, the resistance change is caused by charge transfer between H2 and the semiconductor surface due to reaction of H2 and the oxygen ions. Previous studies indicated that O2− dominates below 150 °C, while atomic O− (O 2 − ) ions are the main adsorbed charged species in the temperature range from 150 °C ∼ 397 °C (above

3.2. Hydrogen sensing and mechanisms Hydrogen sensing of Nb2%_8H and Nb2%_12H was characterized at room temperature. Typical resistance changes of the thin films at different hydrogen concentrations are shown in Fig. 5(a) and (b). Clearly both thin films give a substantial resistance change with introduction of hydrogen into the test chamber, and recover to their original resistance after hydrogen removal. The detectable H2 concentration, of the Nb2% _8H sensor, spans a wide range from 1 ppm to 12000 ppm, with the response changing from 9.3% to 95.2%. As the H2 concentration exceeds 12000 ppm, the resistance recovery reduces, which limits detection of higher concentrations. The highest detection concentration of the Nb2%_12H sensor is limited to 8000 ppm with response of 22.5% at 1 ppm and 98.9% at 8000 ppm. The high response, close to 100%, makes it difficult to distinguish further resistance changes with further H2 concentration increases. The response to H2 concentration for both samples is compared in Fig. 5(c). Clearly the response of sample Nb2%_12H at each concentration point is much larger compared to sample Nb2%_8H, while sample Nb2%_8H reached its saturation value more slowly than sample Nb2%_12H. Both the detection range and the response are greatly improved over previously reported TiO2 hydrogen sensors without Nb doping [35]. The response and recovery times of both the Nb2%_8H and Nb2% _12H sensors decreased sharply with increasing hydrogen concentration until 400 ppm and then gradually stabilized, as shown in Fig. 5(d). The response/recovery times are 288 s/324 s at 1 ppm and 127/119 s at 4000 ppm for the Nb2%_8H sensor and 231/236 s at 1 ppm and 43/ 107 s at 4000 ppm for the Nb2%_12H sensor. The Nb2%_12H sensor has faster response and recovery times compared with Nb2%_8H sensor over the whole test concentration range. Fig. 6(a) shows the repeatability and stability of Nb2%_12H measured at three different H2 concentrations for three cycles after two months storage at room temperature in ambient air conditions. The average sensitivities are 20.7% at 1 ppm, 45.8% at 100 ppm, and 86.2% at 1200 ppm H2 atmospheres, which are very close to the initial sensitivities (22.4%, 48.2% and 88.6%) measured at the same humidity of 5

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Fig. 6. (a) Repeatability and stability measurement of Nb2%_12H at three different H2 concentrations (1 ppm,100 ppm,1200 ppm) repeated for three cycles after 2 month storage at room temperature in ambient air conditions. (b) Hydrogen sensing curves at different humidities after 10 month storage.

the O2− dominated temperature region T < 150 °C, the reaction between H2 and the adsorbed oxygen is described in Eq. (3), and the corresponding n derived based on Eq. (3) is related to m by the following Eqs. (4) and (5),

H2 + O2− = H2 O +

1 O2 + e 2

(3)

1 ⎞ N n = −⎛1 − 2 for t ≪ 1 m + 1 ⎠ cPH2 ⎝ n = −cPH2 (1 −

1 N )for t ≫ 1 m2 + 1 cPH2

(4)

(5)

with m = Nt/(NdLD). Here, c and PH2 are the reaction constant and hydrogen pressure. m, the ratio of the depletion width to the Debye length, is a measure of the depletion strength. Nt and Nd represent the adsorbed surface charge density and the donor density, respectively. LD is the Debye length. According to Eq. (4), n tends to the value of -1 N when cP t ≪ 1 and m ≫ 1 are satisfied simultaneously. Although hy-

Fig. 7. Sensing performance of Nb2%_12H sample to H2, CO, NH3 and NO2 at a fixed concentration of 1200 ppm.

H2

drogen detection of semiconductor metal oxide (SMO) sensors at room temperature has been reported, no previous experimental results gave n equal to -1. The reason is either m was not large enough or the detection range did not reach a high enough concentration to satisfy the condition Nt ≪ 1. cP

397 °C) [36]. A power-law dependence with exponent, n, was usually used to describe the relationship between semiconductor resistance and the concentration of hydrogen, as calculated from Eq. (2) [37]:

n=

ln(R H2 / Rair ) lnPH2

H2

In this work, two sensors in the HC region reached the limit n∼-1.0 but at different H2 concentrations as shown in Fig. 8(a). As n reaches -1 N only when the condition cP t ≪ 1 is satisfied, reaching the inverse

(2)

Where, Rair and RH2 are the resistance of the sensor in air and in hydrogen, and PH2 is the partial pressure of hydrogen in the measured gas environment. Hua et al. presented a theoretical model to show that n should take values of -1, -1/2 and -1/4, respectively, corresponding to stoichiometric reactions of reducing gas with O2−, O−, and O 2 − [31]. In

H2

concentration dependence region at a higher hydrogen concentration for the Nb2%_8H sensor implies much greater pre-adsorbed O2− on its surface than on the Nb2%_12H sensor. The different adsorptions are based on the different film structures and Nb doping densities of the

Fig. 8. Plots of Ln(R H2/R air) versus Ln(CH2) : (a) The solid lines are fits to the high concentration region of H2 (from 1200 to 12000 ppm for Nb2%_8H, from 200 ppm to 8000 ppm for Nb2%_12 H). (b) The solid lines are fits to the low concentration region of H2 (from 1 ppm to100 ppm for Nb2%_8H, from 1 ppm to 50 ppm for Nb2% _12 H). 6

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Fig. 9. Energy band diagrams of (a) metal Pt and Nb-doped anatase TiO2 (Nb2%_8 H), (b) metal Pt and Nb-doped rutile TiO2 (Nb2%_12 H). (c) Nb-doped anatase/ rutile TiO2 heterophase junction.

fast response. The recover process in the LC region corresponds to migrating H atoms to the TiO2 surface, where they combine to form the H2 molecule, while it corresponds to readsorption O2− on the TiO2 surface in the HC region. According to the above analyses, it is expected that the recovery time is much faster in the HC region. Although the detecting range of Nb2%_8H is much larger than Nb2%_12H, the response of Nb2%_8H in the whole concentration range is lower than Nb2%_12H. To clarify this difference, the band structure of the rutile/anatase heterophase junction was determined by combining XPS, UPS, and UV–vis absorption spectra. The band gaps of the Nb doped rutile and anatase layers are 3.00 eV and 3.23 eV, determined from the optical absorption spectra (Fig. S1). UPS measurements show that the valence band edges are located at 2.58 eV, 2.76 eV, 3.08 eV and 2.81 eV below the EF for TiO2_4H, Nb1%_8H, Nb2%_8H and Nb2% _12H, respectively. (Fig. S2). The VBM energies move toward the direction of high binding energy, and the EF shifts toward to CBM with increase of Nb doping concentration at the surfaces due to stronger interactions between Nb 4d and O 2p states than that between Ti 3d and O 2p states, as well as the increased electron concentration in the conduction band because the pentavalent Nb cation replaces the tetravalent Ti. The work functions of polycrystalline platinum and the electron affinities of the anatase and rutile TiO2 are 5.65 eV, 5.1 eV and 4.8 eV, respectively [13,43]. Based on these results, the band structures of the Schottky barrier between Pt and Nb doped TiO2 as well as the Nbdoped rutile/anatase heterophase junction are shown in Fig. 9. From the band alignment in Fig. 9(a) and (b), we see that for the rutile decorated anatase structure (Nb2%_8 H), the Schottky barrier mainly exists between Pt and anatase TiO2 and the barrier height is 0.55 eV, while for the rutile/anatase bi-layer structure, the Schottky barrier of 0.85 eV mainly exists between Pt and the rutile TiO2 layer. The higher Schottky barrier in the bi-layer structure produces higher response, manifested by a larger n in the LC region as shown in Fig. 8 (b). As illustrated in Fig. 9(c), an electron accumulation layer and a depletion layer form at the interface of the heterophase junction between anatase and rutile TiO2. When the charge carriers (electrons) move from the surface into the heterophase junction region, the height of barrier is reduced, leading to a decrease in resistance across the junction and increasing the response. This mechanism provides a natural explanation for higher response in the Nb2%_12H sensor than in the Nb2%_8H sensor in the LC, TC and HC regions. Since the bi-layer film has a much larger area of heterophase junction than in the rutile decorated anatase film, the decrease of resistance is expected to be stronger for the former. It is also worth noting that the well-crystallized and compact structure greatly reduced the defect density at the surface, which could otherwise pin the Fermi level within the energy band and hinder changes in the barrier potential in the presence of H2. The relative percentages of [OH]/[O2−] decrease with increasing Nb doping on the TiO2 surfaces as illustrated by XPS spectra (Fig. S3), indicating that Nb doping can effectively reduce the lattice O vacancy defects. Moreover, the highly aligned and compact film as well as the lattice-matched interface will reduce carrier scattering, providing faster carrier transport

two sensors. The Nb2%_8H sensor has both anatase and rutile phase TiO2 exposed on the surface. The rutile nanorods have high energy surfaces (001) and (101) exposed, corresponding to higher adsorption of hydrogen and O2− [38]. The anatase layer has a relatively high concentration of Nb donors, leading to an increase of the surface negative charge density Nt of adsorbed O2−, which is proportional to the square route of the donor density. For the Nb2%_12H sensor there is a thick layer of rutile nanorods on the device surface, in which the exposed low energy rutile (110) surface has weaker adsorption for O2−. Moreover, the concentration of Nb donors in the rutile layer is smaller than the anatase layer. Consequently, Nt in the Nb2%_12H sensor is expected to be lower than in the Nb2%_8H sensor. Our results show that a sufficiently large Nt is effective in increasing the detection concentration range of the sensor. The sensing behavior in the TC region can be understood also based on O2− adsorption. Clearly, as the hydrogen concentration decreases into the TC region, where pre-adsorbed O2− still plays the dominant role, the N relatively lower hydrogen pressure leads to cP t ≫ 1, making n deH2

crease gradually with the decrease of hydrogen concentration, consistent with the theory deduced from Eq. (5). Upon further decreasing the hydrogen concentration into the LC region (less than 100 ppm for Nb2%_8H, and less than 50 ppm for Nb2%_12 H), the slope of the curve becomes constant again, with a much smaller value, about −0.022 and −0.051 for Nb2%_8H and Nb2%_12H samples, respectively, as shown in Fig. 8(b). The sudden change of slope, in combination with the sharp increase of response/ recovery time for low H2 concentration as shown in Fig. 5(d), indicates a further mechanism takes effect in the LC region. Previous investigations have suggested that noble metal/TiO2 Schottky contacts play a dominant role in the LC region [39,40]. Noble metal (Pt) interdigitated electrodes act as a catalyst to dissociate hydrogen molecules into hydrogen atoms, which then migrate into the TiO2. It has been shown theoretically and experimentally that doping of hydrogen atoms into TiO2 enhances its conductivity and decreases its work function, thus lower the Schottky barrier between Pt and TiO2, which is responsible for H2 sensing at LC. [41,42]. Based on the above analyses, we can conclude that reaction of H2 with O2− is the main cause for the sensing in the HC and TC regions, while H2 splitting at the Pt electrodes dominates the sensing in the LC region. The humidity-dependent hydrogen sensing and response/recovery time can be understood by the sensing mechanisms. In the LC region, the H2O molecules adsorbed on the Pt surface occupy part of the active sites for catalyzing H2 into H atoms, thus reducing the response. Moreover, the H2 splitting mainly occurs at Pt electrode instead on the TiO2 surface and TiO2/Pt interface, and it takes a longer time for H migrating into TiO2, resulting in a long response time in the LC region. In the HC region, large amount of pre-adsorbed O2− prevents H2O molecular being adsorbed on the TiO2 surface. As a result, the response dominated by reacting H2 with adsorbed O2− is hardly affected by the humidity. Meanwhile, the direct reaction of H2 with O2− on the TiO2 surface causes an immediate change of resistance, corresponding to the 7

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to enable the wide detection range and high response. In addition, the high concentration of O2− adsorbed at the surface effectively protects the surface from contamination, and the heterophase junction eliminates elements diffusing across the interface, so enabling high repeatability and long-term stability.

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4. Conclusion A rutile decorated anatase or rutile/anatase bi-layer heterophase junction was obtained by a simple one step hydrothermal method through self-adjusting Nb doping. The highly crystallized, aligned and compact film structure provided a low defect density and fast transport route, greatly improved sensor performance. The heterophase junction based hydrogen sensor demonstrated a remarkably wide detection range and high response. At high hydrogen concentration region, the sensing mechanism is mainly determined by H2 react with the surface adsorbed O2−, while at low hydrogen concentration, the Schottky barrier at the Pt and TiO2 interface and the area of the heterophase junction becomes important. The rutile decorated anatase film has higher Nb doping concentration at the surface, which increases the surface O2− absorption capacity, broadening the hydrogen detection range, maintaining a high stability and excellent humidity resistance. The rutile/ anatase bi-layer structure has a higher Schottky barrier and larger heterophase junction area, enhancing the response. The controllable growth of rutile/anatase TiO2 heterophase junction results from the self-adjusting Nb doping. Our heterophase junction H2 sensors demonstrate significantly enhanced performance over previous sensors in both the wide detection range and high response. Perhaps, of paramount significance for practical applications is that this is achieved at room temperature rather than the high temperatures, typically 350 °C usually required. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgements The authors are grateful to the financial support from National Science Foundation of China (No. 11374091, 11574076, 11674087, 11874144, and 51602094), the Overseas Expertise Introduction Center for Discipline Innovation (“111 center”, D18025), Wuhan Science and Technology Bureau (No. 2018010401011268) and from Science and Technology Department of Hubei Province (No. 2018CFA026). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.127143. References [1] J. Bai, B. Zhou, Titanium dioxide nanomaterials for sensor applications, Chem. Rev. 114 (2014) 10131–10176. [2] G. Korotcenkov, B.K. Cho, Metal oxide composites in conductometric gas sensors: achievements and challenges, Sens. Actuators B Chem. 244 (2017) 182–210. [3] V. Galstyan, E. Comini, G. Faglia, G. Sberveglieri, TiO2 nanotubes: recent advances in synthesis and gas sensing properties, Sensors 13 (2013) 14813–14838. [4] P. Maggie, K.V. Oomman, K.M. Gopal, A.G. Craig, G.O. Keat, Unprecedented ultrahigh hydrogen gas sensitivity in undoped titania nanotubes, Nanotechnology 17 (2006) 398–402. [5] O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, E.C. Dickey, C.A. Grimes, Extreme changes in the electrical resistance of titania nanotubes with hydrogen exposure, Adv. Mater. 15 (2003) 624–627. [6] X. Xia, W. Wu, Z. Wang, Y. Bao, Z. Huang, Y. Gao, A hydrogen sensor based on orientation aligned TiO2 thin films with low concentration detecting limit and short response time, Sens. Actuators B Chem. 234 (2016) 192–200. [7] D. Zappa, V. Galstyan, N. Kaur, H.M.M. Munasinghe Arachchige, O. Sisman, E. Comini, Metal oxide -based heterostructures for gas sensors- a review, Anal. Chim. Acta 1039 (2018) 1–23.

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Xiaohong Xia is a professor at Hubei University. She received her PhD degree from the Huazhong Normal University in 2007, when she joined the Hubei University as a lecturer. She worked at the University of Bolton as a visiting scientist and then worked as a postdoctoral research fellow in 2010–2011. After that she went back to Hubei University and been promoted to associate professor in 2012 and then to professor in 2017. Her current research interest is design and fabrication of semiconductor materials and devices. Zhongbing Huang is a Professor at Hubei University, Faculty of Physics and Elec-tronic Technology. He earned his PhD from the Chinese University of Hong Kong (CUHK) in 2002. He then worked as a postdoctoral research fellow in Wuerzburg Uni-versity, Germany. He joined the Hubei University as Professor of Physics in 2004. His current research interest is mainly on condensed matter physics and computational materials.

Yuwen Bao is PhD student at the School of Materials Science and Engineering of the Hubei University. He received his master degree from the Hubei University in 2009. He then worked as an engineer at Wuhan Analog Tek Technology Co., Ltd. and then joined the Hubei University as a lecturer in 2011. His current research is mainly focused on nanostructured metal oxide semiconductor materials and their application in gas sensors.

Kevin Homewood is a professor at Hubei University. He earned his PhD in Physics at University of Manchester in 1981 and thereupon worked as a research fellow at University of Manchester and University of Hull, until transferring to University of Surrey as a Professor in 1984. He worked at the Queen Mary University of London during 2016–2017 and then joined Hubei University as a full time professor of materials physics in 2017. His current interest covers semiconductor materials for mid-infrared photodectors.

Ping Wei is currently working at the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, as an associate professor. He received PhD in materials science at Wuhan University of Technology in 2012, then worked at University of Washington as a postdoctoral research fellow. In 2015, he joined Wuhan University of Technology as a faculty. His primary research focuses on the performance optimization and transmission electron microscopy studies of electronic functional materials.

Yun Gao is Deputy Dean of the School of Materials Science and Engineering of the Hubei University, China. She earned her PhD from the Chinese University of Hong Kong (CUHK) in 2002. She then worked as a postdoctoral research fellow and moved to the SAET of TDK afterwards as a chief Engineer. She joined the Hubei University as Professor of Materials Physics in 2004. Her current research interest is mainly on semiconductors materials and their various applications.

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