Porous Ga-In bimetallic oxide nanofibers with controllable structures

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Porous Ga-In bimetallic oxide nanofibers with controllable structures for ultrasensitive and selective detection of formaldehyde Hui Chen, Jiabo Hu, Guo-Dong Li, Qian Gao, Cundi Wei, and Xiaoxin Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13520 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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ACS Applied Materials & Interfaces

Porous Ga-In bimetallic oxide nanofibers with controllable structures for ultrasensitive and selective detection of formaldehyde

Hui Chen,a,b Jiabo Hu,b Guo-Dong Li,b Qian Gao,a Cundi Wei,a,* Xiaoxin Zoub,* a

Key Laboratory of Automobile Materials, Ministry of Education, College of Materials Science and

Engineering, Jilin University, Changchun 130025, P. R. China b

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin

University, Changchun 130012, P. R. China

*Corresponding author. E-mail address: [email protected] (X. Zou); [email protected] (C. Wei)

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Abstract

The design of appropriate composite materials with unique surface structures is an important strategy to achieve ideal chemical gas sensing. In this paper, efficient and selective detection of formaldehyde vapor has been realized by a gas sensor based on porous GaxIn2-xO3 nanofibers assembled by small building blocks. By tuning the Ga/In atomic ratios in the materials, crystallite phase, nanostructure and band gap of as-obtained GaxIn2-xO3 nanofibers can be rationally altered. This further offers a good opportunity to optimize the gas sensing performances. In particular, the sensor based on porous Ga0.6In1.4O3 nanofibers assembled by small nanoparticles (≈ 4.6 nm) exhibits best sensing performances. Toward 100 ppm formaldehyde, its highest response (Ra/Rg = 52.4, at 150 oC) is ∼4 times higher than that of the pure In2O3 (Ra/Rg = 13.0, at 200 oC). Meanwhile, it has superior ability to selectively detect formaldehyde against other interfering volatile organic compound gases. The significantly improved sensing performance makes Ga0.6In1.4O3 sensor very promising for selective detection of formaldehyde.

Keywords. Gas sensor, formaldehyde, Ga2O3, In2O3, porous nanofibers


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Introduction Formaldehyde as a carcinogenic VOC (volatile organic compound) in indoor air is seriously threatening human health.1-3 As it is commonly used in building materials and could be released from household products, general population is exposed to formaldehyde frequently. Therefore, it is imperative to develop a simple and cost-effective method to detect formaldehyde. Currently, considerable attention has been paid to metal oxide semiconductor based gas sensors for the detection of various toxic and explosive gases owing to their low cost, high efficiency, portable size, and easy fabrication.4-7 Many oxide semiconductors, such as In2O3,8 Co3O4,9 ZnO,10 SnO211 and NiO12 have proven to be promising sensing materials for formaldehyde detection. The gas sensing performances of oxide semiconductor gas sensors can be enhanced by adopting strategies such as construction of heterostructures,13 decoration of noble or metal oxide catalysts,14-15 formation of hetero-metal oxides,1618

etc. Among these strategies, incorporation of hetero-metal oxides in the single metal oxide has been

demonstrated as a versatile tactic to alter grain size, phase structure, electrical conductivity, and band gap, thus synergistically enhancing the sensing properties.18 In this regard, the design and synthesis of novel mixed metal oxides are worthy of further investigation. As a starting point, we chose indium oxide (In2O3, Eg = 2.5~2.8 eV), one of the most important gas sensing materials, as formaldehyde detection materials. To composite with In2O3, wide band gap semiconductor beta-gallium oxide (β-Ga2O3, Eg = 4.4 - 4.9 eV) is a desirable candidate for the following reasons. Ga2O3 itself can serve as an active gas sensing material.19-21 In addition, it is possible for β-Ga2O3 to alloy with In2O3 due to the chemical relationship between gallium and indium. Gallium is a congener of indium in Group 13, and they are both in stable oxidation state (+3) in their corresponding oxides. Therefore, controllable substitution of In3+ in β-Ga2O3 and Ga3+ in cubic In2O3 can be easily realized by taking advantage of their similarity in chemistry.22 These render Ga2O3-In2O3 system more flexible and superior compared with their individual components; there are more opportunities to continuously tailor their phase, nanostructure, band gap and thereby gas sensing


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behavior. Inspired by these facts, we envisioned that GaxIn2-xO3 should be promising materials with the potential to be optimized for formaldehyde detection. On the other hand, according to the sensing mechanisms of semiconductor gas sensors, the adsorption/desorption behaviors of formaldehyde molecules mainly occur on the surface. Thus, besides composition, rational construction of nanostructures is crucial to the high performance of sensing materials.23-25 One of the ideal architectures of sensing materials should be porous nanofibers assembled of small nanoparticles where small size effect works, meeting the condition of D ≤ 2L where D is the grain size and L is the thickness of the space charge layer.26-28 In this case, electrons in nanoparticles of the sensing materials are almost fully depleted in air, so the sensitivity of the sensor under exposure to reducing gases will reach the maximum. Meanwhile, the one-dimensional porous nanostructures render such materials effective gas diffusion channels.29-30 However, due to the limitation of synthetic approaches, such an ideal architecture is challenging to achieve. Very recently, our group have developed a facile electrospinning technique to synthesize porous GaxIn2-xO3 nanofibers (1 ≤ x ≤ 1.8) with atomically thin pore walls,31 inspiring us to investigate the gas sensing performances of the Ga2O3-In2O3 system. In this work, we systematically study the phase formation, morphology evolution and band gap tuning of GaxIn2-xO3 nanofibers across the composition range (0 ≤ x ≤ 2), as well as their gas sensing behaviors. Among them, porous Ga0.6In1.4O3 nanofibers assembled by small nanoparticles (≈ 4.6 nm) and porous Ga1.4In0.6O3 nanofibers with atomically thin pore walls (0.85-2 nm), representing two typical nanostructures, exhibit advantageous gas properties compared with pure In2O3. Especially, Ga0.6In1.4O3 nanofibers show the best gas sensing performance with rapid response, excellent sensitivity and selectivity for formaldehyde detection at a low operating temperature (150 oC), indicating its potential application as a superior sensing material.

Experimental Section Material synthesis and characterization. GaxIn2-xO3 nanofibers, with different Ga/In atomic ratios, were synthesized by our previous procedures.31 Typically, for the synthesis of Ga0.6In1.4O3, gallium(III) ACS Paragon Plus Environment

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nitrate hydrate (0.077 g), indium(III) nitrate hydrate (0.163 g) and PVP (0.8 g) were mixed with ethanol (6.6 g) and DMF (2.2 g). The 3 : 1 weight ratio of ethanol to DMF was rationally chosen as solvent based on results of our previous electrospinning experiments,32 as it renders the solution with appropriate viscosity and conductivity for electrospinning. The electrospun solution was obtained by continuously stirring the mixture for 12 h at room temperature. Then, the solution was electrospun under 22 kV applied voltages with 20 cm tip-to-collector distance. The resulting metal/polymer nanofibers were collected and fully dried at 70 °C. Finally, the dried metal/polymer nanofibers were annealed at 600 °C for 3 h in air to obtain the Ga0.6In1.4O3 nanofibers. The X-ray diffraction (XRD) patterns were obtained with a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) measurement was carried out on an ESCALAB 250 X-ray photoelectron spectrometer with a monochromatic X-ray source (Al Kα hυ = 1486.6 eV). The scanning electron microscope (SEM) images were performed on a JEOL JSM 6700F electron microscope. The transmission electron microscope (TEM) images were recorded on a Philips-FEI Tecnai G2STwin microscope equipped with a field emission gun operating at 200 kV. The Brunauer-Emmett-Teller (BET) surface areas and N2 adsorption/desorption isotherms were obtained with a Micromeritics ASAP 2020M system. The UV/Vis diffuse reflectance spectra were performed on a Perkin-Elmer Lambda 20 UV/Vis spectrometer. Fabrication and measurement of gas sensor. The gas sensors (side-heated type) were fabricated as below: Viscous paste slurry was obtained in an agate mortar by mixing GaxIn2-xO3 samples with appropriate amount of ethanol. It was then evenly coated on the outer surface of ceramic tube of gas sensor (Figure S1). A Ni−Cr coil across the tube was used as the heater to control operating temperature. Finally, the as-obtained sensors were aged at 200 ◦C for 12 hours to improve their stability. Gas sensing tests of GaxIn2-xO3 samples were conducted in a commercial CGS-8 gas sensing measurement system (Beijing Elite Tech Company Limited). In this system, the upper detection limit of resistance was 500 MΩ. The electronic circuit of the gas sensing measurement system is shown in Figure S1. In the circuit, resistance variation of gas sensor can be monitored at 5V working voltage, and ACS Paragon Plus Environment

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operation temperature can be controlled by tuning heating voltage. We used ambient air with a relative humidity of 20 ± 5% (at room temperature, 20 ±3 oC) as both diluting gas and reference gas. To prepare the sample gas, a certain amount of target liquid was injected into the testing chamber (∼1 L) with a stopper by a microinjector. The mixtures were shaken well for about 10 min. When testing, the sensor was placed into the test chamber until resistance reached a steady value, then it was taken out and resistance began to recover to original value in fresh air. The response of the sensor is determined as the ratio, Ra/Rg, where Ra and Rg are the resistance values of the sensors in air and in the target gas, respectively. The response and recovery time represent the time required to reach 90% of the total resistance after the sensor is exposed to the target gas and air, respectively.

Results and Discussion Composition and microstructure analysis. XRD patterns (Figure 1) were obtained to identify crystalline phase and composition of all GaxIn2-xO3 (x = 0, 0.3, 0.6, 1, 1.4, 1.7, 2) nanofibers. For x = 0, the diffraction peaks are conclusively indexed as cubic In2O3 phase (JCPDS 06-0416). When small amounts of Ga3+ ions (x = 0.3) are doped into the cubic In2O3, a slight shift of all diffraction peaks is presented to high angle side. This indicates that Ga3+ ions incorporate into the In2O3 crystal lattice successfully and lower the crystal lattice constant, as Ga3+ (r Ga3+ = 0.062 nm) is smaller than In3+ (r In3+ = 0.080 nm). With the increase of x (0.6 ≤ x ≤1.7), the diffraction peaks of GaxIn2-xO3 gradually weaken and broaden, suggesting that their crystallite sizes decrease. Finally, the phase becomes monoclinic βGa2O3 (JCPDS 41-1103) with relatively high crystallinity. These facts confirm that In and Ga phases have high mutual solubility in the as-obtained Ga-In bimetallic oxide samples, and thereby mutually restrain the crystallite growth. The phase relationships in In2O3-Ga2O3 system have been recognized experimentally and theoretically in several works.33-36 The formation of the solid solutions is further confirmed by XPS spectra (Figure 2). The peaks centered at ~ 444.7 eV and ~ 452.0 eV in pure In2O3 are respectively assigned to 2p5/2 and 2p3/2 of In3+.37 The peaks located at ~ 1119.4 eV and ~ 1146.3 eV in pure β-Ga2O3 are respectively attributed to ACS Paragon Plus Environment

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2p3/2 and 2p1/2 of Ga3+.38 After alloying, the binding energies of In 3d in both Ga0.6In1.4O3 and Ga1.4In0.6O3 shift to higher values compared with those of pure In2O3, while the binding energies of Ga 2p in both Ga1.4In0.6O3 and Ga0.6In1.4O3 shift to lower values compared with those of pure β-Ga2O3. The obvious chemical shifts confirm that solid solution exists in the GaxIn2-xO3 system. Specifically, Ga has a higher electronegativity than In, so Ga is more powerful to attract electrons to itself in the system. When substitution of In3+ in β-Ga2O3 (i.e., Ga1.4In0.6O3) or Ga3+ in cubic In2O3 (i.e., Ga0.6In1.4O3) takes place, Ga attracts electrons from In, increasing the electrons screening effect for Ga while decreasing that for In. The charge redistribution results in shift of binding energy for In 3d and Ga 2p. GaxIn2-xO3 nanofibers (x = 0, 0.4, 0.6, 1, 1.4, 1.8, 1.9 and 2) were selected as representatives to investigate the inner structure changes of In2O3-Ga2O3 system. The TEM images in Figure 3A-C reveal that when x < 1, morphologies of GaxIn2-xO3 are nanofibers assembled by packed nanoparticles. With the introduction of Ga, the nanoparticles become smaller accordingly. Average grain sizes of In2O3, Ga0.4In1.6O3 and Ga0.6In1.4O3 are about 26.4 nm, 16.2 nm and 4.6 nm, respectively. The small nanoparticles shrink, interconnect and evolve into ultrathin walls. For 1 ≤ x ≤1.8 (Figure 3D, E and S2), GaxIn2-xO3 solid solutions preserve the porous structure with ultrathin pore walls. Once x ≥ 1.9 (Figure 3F), the ultrathin porous structure degenerates, and evolves into pea-like nanotube structure. As expected, pure β-Ga2O3 nanofibers also have a structure of pea-like nanotubes (formation mechanism is proposed in Figure S3). Therefore, the grain growth of β-Ga2O3 and In2O3 in the solid solutions is suppressed by In3+ and Ga3+, respectively. Notably, the small (or thin) crystallite sizes of GaxIn2-xO3 for 0.6 ≤ x ≤1.8 match the XRD results (Figure 1). The diameter of nanofibers is affected by the change in inner structure. SEM images (Figure 4A and Figure S4) show that the morphologies of GaxIn2-xO3 are isolated uniform nanofibers and the lengths of the nanofibers are up to several micrometers. The diameter distributions in the inset images exhibit the average diameters of GaxIn2-xO3 (x = 0, 0.3, 0.6, 1, 1.4, 1.7 and 2) nanofibers are 59 ± 12, 150 ± 27, 223 ± 32, 141 ± 15, 115 ± 17, 84 ± 10 and 87 ± 12 nm, respectively. The diameters of solid solutions nanofibers are generally larger than those of corresponding pure oxide nanofibers (Figure 4B). The ACS Paragon Plus Environment

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largest diameter of Ga0.6In1.4O3 nanofibers can be attributed to the highly porous structure assembled by loosely packed interconnected ultrasmall nanoparticles, the size of which was restrained during alloying. As typical representatives of two unique nanostructures, Ga0.6In1.4O3 and Ga1.4In0.6O3 nanofibers were further characterized. TEM images (Figure S5A and S5D) exhibit that they all have highly porous nanonetwork structures. However, Ga0.6In1.4O3 nanofibers consist of nanocrystals (about 4.6 nm in size) which are randomly but spatially interconnected with each other (Figure 4C), while Ga1.4In0.6O3 nanofibers are composed of ultrathin and three-dimensional interconnected pore walls of thickness 0.852.00 nm (Figure S5E-G). The lattice fringes of Ga0.6In1.4O3 (Figure 4D and S5B-C) with d-spacing of 0.292 nm and 0.413 nm agree well with the (222) and (211) planes of cubic In2O3, while the lattice fringes of Ga1.4In0.6O3 parallel to the pore walls (Figure S5E-G), with d-spacing of 0.282 nm, correspond well to the (002) planes of monoclinic β-Ga2O3. Mesopores may exist in Ga0.6In1.4O3 and Ga1.4In0.6O3, as indicated by their nitrogen adsorption-desorption isotherms (Figure S6), which are typical type IV with H3 hysteresis loops. BJH pore size distribution curves (Figure S7) show that pores in Ga0.6In1.4O3 are primarily centered at 10 nm, while there are mainly mesopores and macropores in

Ga1.4In0.6O3 nanofibers. These results match conclusions from their TEM analysis. The BET surface areas of Ga0.6In1.4O3 and Ga1.4In0.6O3 are 28.4 m2g−1 and 29.9 m2g−1, respectively. For semiconductor sensing materials, appropriate band gap is an essential premise of great gas sensing performance. Even if β-Ga2O3 materials can be used for reducing gases detection at around 6001000 oC, their wide band gap brings about high intrinsic resistance and low chemisorption at low temperatures, limiting their applicability.39-41 Alloying is a feasible approach to control the band gaps of β-Ga2O3. Figure 5A presents the UV/Vis diffuse reflectance spectra of GaxIn2-xO3 (x = 0, 0.3, 0.6, 1, 1.4, 1.7, 2) nanofibers. Pure β-Ga2O3 has a short absorption edge at about 275 nm. With the increase of In content, the absorption edges of GaxIn2-xO3 exhibit obvious and continuous red shifts, and the band gaps


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decrease from 4.5 eV to 2.6 eV (Figure 5B). Therefore, the precise control of band gaps of GaxIn2-xO3 nanofibers can be achieved by tuning the Ga/In ratio in electrospun solution. Gas sensing properties. We systematically investigated the formaldehyde sensing performances of sensors based on GaxIn2-xO3 (x = 0, 0.3, 0.6, 1, 1.4, 1.7, 2) materials after aging at 200 oC for 12 hours. Taking Ga0.6In1.4O3 nanofibers for example, their structures are maintained after the aging process (Figure S8). Responses of the sensors over a wide range of operating temperature of 115-400 oC to 100 ppm formaldehyde are displayed in Figure 5C. When x ≤ 1, the gas responses exhibit volcano shaped dependence on operating temperature. The response rises with the increases of temperature, reaches the pinnacle and then goes down. Before the optimum temperature, there are not enough activated formaldehyde molecules to react with the adsorbed oxygen. However, above the optimum temperature, the loss of reactant caused by larger amount of adsorbed formaldehyde molecules desorption leads to the decrease of sensing response.18 As a result, Ga0.6In1.4O3-based sensor presents the highest response (Ra/Rg = 52.5 at 150 oC) as compared to In2O3 (Ra/Rg = 13.0 at 200 oC) and GaInO3 (Ra/Rg = 41.2 at 150 o

C). When x > 1, sensors based on Ga1.4In0.6O3, Ga1.7In0.3O3 and pure β-Ga2O3 cannot be used for

formaldehyde detection until their operating temperatures reach 200 oC, 300 oC and over 400 oC, respectively, since their high resistances in air are beyond the upper detection limit (500 MΩ) at lower

temperatures (Figure 5D). Sensors based on Ga1.4In0.6O3 and Ga1.7In0.3O3, show high responses similar to GaInO3-based sensor above 200 oC, which is ascribed to their similar porous structure with ultrathin pore walls. As is known, low working temperature can offer huge advantages in practical applications, such as reducing energy consumption, improving sensor stability and reliability etc.42-43 In Ga2O3-In2O3 system, the changes of base resistance in air are determined by band gaps. Therefore, the band gaps tuning makes a crucial contribution to the low optimal operating temperature of GaxIn2-xO3 with x ≤ 1. As negative examples, Ga-rich GaxIn2-xO3 (i.e., Ga1.4In0.6O3, Ga1.7In0.3O3 and Ga2O3), they are single βGa2O3-type structure since solubility limit of In3+ in β-Ga2O3 is about 45 mol%.35 Such structure results ACS Paragon Plus Environment

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in sharply increasing band gap from 2.9 eV (GaInO3) to 3.4 eV (Ga1.4In0.6O3), which is unfavorable for gas sensing application at low operating temperature. Considering the advantages of high response and low optimal operating temperature, Ga0.6In1.4O3 is chosen as an optimal gas material for formaldehyde detection. Generally, selective detection of formaldehyde against other interference VOCs is a challenging but significant task to achieve for metal oxide-based gas sensors, especially when avoiding cross-responses to ethanol. Responses of the sensors based on In2O3 and Ga0.6In1.4O3 to 100 ppm ethanol at different operating temperatures are shown in Figure 6A. Ga0.6In1.4O3-based sensor exhibits significantly enhanced response to ethanol as compared to that of pure In2O3. However, different from formaldehyde detection, Ga0.6In1.4O3-based sensor shows the maximum response (Ra/Rg = 27.6) at 200 oC and a low response (Ra/Rg = 15.9) at 150 oC to 100 ppm ethanol. In other words, Ga0.6In1.4O3-based sensor achieves high selectivity to formaldehyde against ethanol at 150 °C, while the responses to them become indiscernible at 200 °C or higher. The result suggests that to different gases, responses of Ga0.6In1.4O3-based sensor are significantly influenced by operating temperature. Similar findings have been reported in other gas sensing materials.44-45 In addition, we investigated responses of the sensors based on Ga0.6In1.4O3 and In2O3 to a wide range of VOCs at 150 oC and 200 oC, respectively (Figure 6B). The responses of Ga0.6In1.4O3 sensor to formaldehyde are about 3.3, 7.7, 7.3, 32.8, 30.9 and 14.6 times than that to ethanol, methanol, acetone, benzene, toluene and xylene, respectively, while the responses of pure In2O3 sensor to formaldehyde are only 1.5, 2.6, 2.2, 9.3, 8.1 and 4.8 times than that to these interfering gases. These indicate that Ga0.6In1.4O3 sensor has enhanced ability to distinguish formaldehyde from other VOCs. Figure 6C presents responses of sensors based on Ga0.6In1.4O3 and In2O3 toward various concentrations of formaldehyde at 150 oC and 200 oC, respectively. They both have a wide formaldehyde response range of 0.2-500 ppm, and their response values increase accordingly with the increase of formaldehyde concentration. Dynamic sensing transients of the two sensors toward various ACS Paragon Plus Environment

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concentrations of formaldehyde at their optimal operating temperatures are shown in Figure 6D. Both sensors based on Ga0.6In1.4O3 and In2O3 display rapid response and recovery characteristics to formaldehyde in the concentration range 0.2-100 ppm. Ga0.6In1.4O3-based sensor exhibits superior sensitivity to formaldehyde compared with pure In2O3-based sensor. Moreover, the minimum detectable formaldehyde concentration (i.e., limit detection) of Ga0.6In1.4O3-based sensor is as low as 200 ppb with the response of 1.5, suggesting its active response to low concentration of formaldehyde. Further efforts (e.g. decoration of noble metal catalysts) are needed to meet lower detection limit (0.04 ppm) set by US Environmental Protection Agency.46 The concentration-dependent response time and recovery time of the Ga0.6In1.4O3-based sensor are presented in Figure 7A. The sensor shows short response time (1-14 s) and recovery time (25-70 s) to 0.2 ppm-100 ppm of formaldehyde at 150 oC. Response-recovery curves of Ga0.6In1.4O3 based sensor for continuous detection under 100 ppm formaldehyde at 150 oC are shown in Figure 7B. Obviously, the sensor maintains the original sensing characteristics after 10 cyclic tests, suggesting its outstanding reproducibility. Furthermore, we repeated the same test one month later. As shown in Figure S9, both base resistance in air and formaldehyde response remain almost unchanged, indicating its long-term stability. Table S1 lists the comparison of gas sensing responses, selectivity, limit detection and operating temperature of formaldehyde detection among Ga0.6In1.4O3 sensor and other nanostructured metal oxides based sensors in recent literatures. Ga0.6In1.4O3 in our study ranks the most outstanding formaldehyde sensing materials. Sensing mechanism. The sensing mechanism for formaldehyde detection was investigated in previous researches.47-48 When typical n-type In2O3 nanofibers are exposed to air, oxygen molecules are chemisorbed onto the surfaces of the sensing materials and subsequently capture electrons to form adsorbed oxygen (e.g., O2−, O−, and O2−). The decrease of carrier concentration on the surfaces of sensing materials produces an electron depletion layer, bringing about potential barrier called schottky barrier from surface to the bulk inside, and thereby increasing the overall sensor resistance. Thereafter, when In2O3 nanofibers are exposed to formaldehyde vapor, surface adsorbed oxygen will react with ACS Paragon Plus Environment

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formaldehyde (e.g., HCHO + 2O- → CO2 + H2O + 2e-)47 and will release electrons onto the sensing materials surfaces. This process results in the decrease in thickness of electron depletion layers, height of schottky barrier and resistance of the sensor materials. In this work, compared with undoped In2O3 nanofibers, Ga0.6In1.4O3 gas sensing material exhibits more than 4 times higher response to 100 ppm formaldehyde at 150 oC. Considering the mechanism above, the doping of Ga3+ in the system plays four main roles: (I) reducing the size of grain size; (II) increasing surface area and porosity; (III) increasing oxygen vacancy and surface chemisorbed oxygen; (IV) optimizing electrical conductivity. Substitution of Ga3+ in cubic In2O3 renders Ga0.6In1.4O3 porous nanostructure assembled by small nanoparticles (≈ 4.6 nm). A mature model explaining the dependence of gas sensing property on the size of grain has been well developed and frequently discussed in previous literatures.26-28 In the model, D represents the grain size and L represents the thickness of the electron depletion layers. For D >> 2L, (e.g. In2O3 nanofibers), the grain boundary barriers dominate the conductivity. The influence of surface gas reaction on main body is negligible. With the decrease of D, the weight of electron depletion layer in the grain increases, leading to the increase of sensing performance. For D ≤ 2L, (e.g. Ga0.6In1.4O3 nanofibers) electrons are fully depleted in the small nanoparticles. As the depletion areas fill in the entire material, the difference in energy state throughout the grain is eliminated, leading to a nearly flat energy bands. Therefore, precipitous decrease in resistance can be obtained with few electrons from surface reactions when exposed to formaldehyde. In other words, entire crystallites of Ga0.6In1.4O3 are responsible for the gas response, offering significant advantages over pure In2O3. The N2 sorption-desorption isotherms of Ga0.6In1.4O3 nanofibers and In2O3 nanofibers were shown in Figure S10. The BET surface areas of them were calculated to be 28.4 m2/g and 15.5 m2/g, respectively. Ga0.6In1.4O3 nanofibers with highly porous nanostructure possess larger specific surface area than pure In2O3 nanofibers, suggesting more surface adsorption sites and more efficient gas diffusion, which is helpful to the enhanced response. Oxygen vacancy (OV) and surface chemisorbed oxygen (OC) have major impact on the gas sensing performances of the sensing materials.17, 49-50 Material with higher content of OV has more surface active ACS Paragon Plus Environment

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sites where gas adsorption and reaction occur. Material containing more OC provides more reactants available for the surface redox reaction, leading to a larger variation of sensor resistance. In Figure 8, the XPS peaks of O 1s of In2O3 and Ga0.6In1.4O3 can be deconvoluted into three peaks including lattice oxygen (OL), OV and OC (The detailed information of each peak is listed in Table S2.) In Ga0.6In1.4O3, the relative percentages of OV (38.9%) and OC (33.0%) increase compared with that in the pure In2O3 (30.8% and 22.1%, respectively), leading to the enhanced sensing performances. The increased proportions of OV and OC can be attributed to the existence of structural defects in β-Ga2O3.22 Electrical conductivity, determined by the band gap, is an another important factor on sensing behaviors.29, 41 For n-type semiconductor sensing materials with low band gap, the low surface electron potential barrier is unfavorable to the resistance variation. For materials with band gap above the optimum, the diffusion of oxygen vacancy is sluggish and hinders the gas sensor behaviors at low temperature. As shown in Figure 5, band gaps tuning by Ga3+ doping (or In3+ doping) results in appropriate base resistance of Ga0.6In1.4O3 in air as well as the optimal gas sensing performance at low operating temperature. Besides, Ga0.6In1.4O3-based sensor achieves high selectivity to formaldehyde against other VOCs, especially ethanol at 150 °C, while the responses to formaldehyde and ethanol become indiscernible at 200 oC or higher. A possible mechanism for the excellent selectivity is proposed. Because Ga3+ (0.062 nm) has a smaller ion radius than In3+ (0.080 nm), the substitution of Ga3+ in In2O3 structure increases the overall ionic potential of the material (i.e., Ga0.6In1.4O3) and the material bind the adsorbed oxygen molecules (e.g., O2−, O−, and O2−) more tightly. Therefore, stronger Brønsted acid is required to carry away more adsorbed oxygen at equilibrium. Formaldehyde is more acidic than ethanol (pKa values for formaldehyde and ethanol are 13.27 and 15.5, respectively).51 When the sensing material is exposed to the target gas, the formaldehyde molecules are able to consume more adsorbed oxygen and thus result in higher response at relatively low temperature. However, the intrinsic acidity of formaldehyde is not dominant when Ga3+ ions are not doped or operating temperature is over 200 °C, because lower ionic


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potential of pure In2O3 renders lower energy barrier to interact with adsorbed oxygen, while higher temperature activates more molecules ready to interact with the adsorbed oxygen.

Conclusions We have successfully fabricated the GaxIn2-xO3 nanofibers (0 ≤ x ≤ 2) as gas sensing materials by an electrospinning method and investigated their gas sensing performances. The Ga3+ and In3+ in GaxIn2xO3

have high mutual solubility and restrain the grain growth. This renders the materials porous

structure assembled by ultra small (x = 0.6) or ultrathin (1 ≤ x ≤ 1.8) building blocks. The structures not only achieve electron depletion of the entire materials, but also provide rich surface adsorption sites and efficient gas diffusion channels. Meanwhile, the alterable crystallite phases and tunable band gap of the composites are in favor of tailoring their gas sensing behaviors. As a result, porous Ga0.6In1.4O3 assembled by small nanoparticles (≈ 4.6 nm) exhibits best gas sensing performances at a low operating temperature (150 oC) toward formaldehyde, with outstanding sensitivity (Ra/Rg = 52.5 to 100 ppm), excellent selectivity, rapid response speed (< 15 s) and low limit of detection (0.2 ppm).

Acknowledgements. This work was supported by the financial assistance of the NSFC (21371070 and 21401066), Science and Technology Research Program of Education Department of Jilin Province ([2016] No. 410), and Jilin Province Science and Technology Development Plan (20150520003JH).

Supporting Information Available. Circuit schematic diagram of sensor measurement. SEM images of GaxIn2-xO3 (x = 0, 0.3, 0.6, 1, 1.4, 1.7, 2.0) nanofibers. TEM images of GaxIn2-xO3 (x = 0.6, 1.4, 1.8, 2.0) nanofibers. HRTEM images of Ga0.6In1.4O3 and Ga1.4In0.6O3 nanofibers. Nitrogen adsorptiondesorption isotherms of In2O3, Ga0.6In1.4O3 and Ga1.4In0.6O3 nanofibers. Material characterization after aging process. Comparison of formaldehyde response, selectivity, limit detection and operating temperature with recent studies. Fitting Results of O 1s XPS Spectra. This material is available free of charge via the Internet at http://pubs.acs.org. ACS Paragon Plus Environment

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Figure captions Figure 1. XRD patterns of the GaxIn2-xO3 (x = 0, 0.3, 0.6, 1, 1.4, 1.7, and 2.0) nanofibers. Figure 2. High-resolution (A) In 3d XPS spectra of Ga1.4In0.6O3, Ga0.6In1.4O3, In2O3 and (B) Ga 2p XPS spectra of Ga0.6In1.4O3, Ga1.4In0.6O3, β-Ga2O3. Figure 3. TEM images of (A) In2O3 nanofibers, (B) Ga0.4In1.6O3 nanofibers, (C) Ga0.6In1.4O3 nanofibers, (D) GaInO3 nanofibers, (E) Ga1.4In0.6O3 nanofibers and (F) Ga1.9In0.1O3 nanofibers. Figure 4. (A) SEM image and corresponding nanofiber diameter distribution of Ga0.6In1.4O3 nanofibers. (B) Dependency of nanofiber diameter of GaxIn2-xO3 on composition. (C) TEM images and (D) HRTEM images of Ga0.6In1.4O3 nanofibers. Figure 5. (A) UV-visible diffuse reflection spectra and (B) corresponding band gaps of the GaxIn2-xO3 (x =0, 0.3, 0.6, 1, 1.4, 1.7 and 2.0) nanofibers. (C) Gas responses of the sensors based on GaxIn2-xO3 (x = 0, 0.3, 0.6, 1, 1.4, 1.7) materials at different operating temperatures to 100 ppm formaldehyde. (D) Base resistance in air of the sensors based on GaxIn2-xO3 (x = 0, 0.3, 0.6, 1, 1.4, 1.7) materials at different operating temperatures. Figure 6. (A) Gas response of Ga0.6In1.4O3 and pure In2O3-based sensors to 100 ppm formaldehyde and ethanol at different operating temperatures. (B) The selectivity tests of the two sensors toward 100 ppm of formaldehyde against ethanol, methanol, acetone, benzene, toluene and xylene at 150 oC and 200 oC, respectively. (C) Concentration dependent response curves of the two sensors at 150 oC and 200 oC, respectively. (D) Dynamic sensing transients of the two sensors to different formaldehyde concentrations at 150 oC and 200 oC, respectively. Figure 7. (A) Response time and recovery time of Ga0.6In1.4O3-based sensor to different formaldehyde concentrations at 150 °C. (B) The cycling response-recovery curve of the sensor to 100 ppm formaldehyde at 150 °C. Figure 8. O 1s XPS spectra of (A) pure In2O3 nanofibers and (B) Ga0.6In1.4O3 nanofibers. ACS Paragon Plus Environment

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Figure 1


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Table of Contents graphic


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Porous Ga-In bimetallic oxide nanofibers with controllable structures

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