Realizing the Control of Electronic Energy Level Structure and Gas

Subscriber access provided by TULANE UNIVERSITY

Surfaces, Interfaces, and Applications

Realizing the Control of Electronic Energy Level Structure and Gas-Sensing Selectivity over HeteroatomDoped In2O3 Spheres with Inverse Opal Microstructure Tianshuang Wang, Bin Jiang, Qi Yu, Xueying Kou, Peng Sun, Fangmeng Liu, Huiying Lu, Xu Yan, and Geyu Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21543 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Realizing the Control of Electronic Energy Level Structure and Gas-Sensing Selectivity over Heteroatom-Doped In2O3 Spheres with Inverse Opal Microstructure Tianshuang Wang, Bin Jiang, Qi Yu, Xueying Kou, Peng Sun,* Fangmeng Liu, Huiying Lu, Xu Yan and Geyu Lu* State Key Laboratory on Integrated Optoelectronics, Key Laboratory of Gas Sensors, Jilin Province, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China. E-mail Address: [email protected] (Peng Sun) E-mail Address: [email protected] (Geyu Lu)

KEYWORDS: gas sensor, Fermi level, inverse opal spheres, selectivity control, formaldehyde

ABSTRACT: Understanding the effect of substitutional doping on gas-sensing performances is essential for designing high activity sensing nanomaterials. Herein, formaldehyde sensors based on gallium-doped In2O3 inverse opal (IO-(GaxIn1-x)2O3) microspheres were purposefully prepared by simple ultrasonic spray pyrolysis method combined with self-assembly sulfonated polystyrene spheres template. The well-aligned inverse opal structure, with three different-sized pores, plays dual roles of accelerating the

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

diffusion of gas molecules and providing more active sites. The Ga substitutional doing can alter the electronic energy level structure of (GaxIn1-x)2O3, leading to the elevation of Fermi level and the modulation of band gap closed to a suitable value (3.90 eV), hence, effectively optimizing the oxidative catalytic activity for preferential CH2O oxidation and increasing the amount of absorbed oxygen. More importantly, the gas selectivity could be controlled by varying the energy level of adsorbed oxygen. Accordingly, the IO-(Ga0.2In0.8)2O3 microspheres sensor showed high response toward formaldehyde with fast response and recovery speeds, and ultralow detection limit (50 ppb). Our findings finally offer implications for designing Fermi level-tailorable semiconductor nanomaterials for the control of selectivity and monitoring indoor air pollutant. 1. INTRODUCTION Formaldehyde is a representative indoor air pollutant and has been classified as a primary carcinogen by the International Agency for Research on Cancer (IARC), and it is mainly found in decorative materials, furniture, plywood, paint, adhesives, chemical cleaner, and textile. Research manifested that the high concentration formaldehyde can not only cause irritation of the skin-mucous membrane, throat and eyes, but can also damage the nervous, hepar and immune system. Especially, being exposed to formaldehyde for a long time may induce nasopharyngeal, sinus cancer, lung cancer and leukemia.1-4 Note that, modern people spend an average of 70%-90% of their time in enclosed buildings, thus, being exposed to low concentration formaldehyde for a long time can still cause harmful effect on human health.5


Page 2 of 39

Page 3 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Accordingly, the Standardization Administration of the People’s Republic of China (SAC) has set the permissible release limits for indoor decoration pollutant chemicals and that for formaldehyde must be less than 0.1 ppm after closing door and window for 1 h.6 Considering protecting human beings from this carcinogenic gas, a precise and reliable monitoring of sub-ppm-level formaldehyde is essential to evaluate the indoor air quality. Semiconductor

metal

oxides

(SMO)

sensors

propose

a

cost-effective,

miniaturization and smart integration approach for rapid detection and identification of volatile organic compounds (VOCs) based on resistance changes caused by the reaction between adsorbed oxygen species and targeted gas molecules.7-9 However, members of SMO sensors generally suffer from weak sensitivity, slow response and recovery speeds, cross-response to other interfering gases, and can not reach ppb-level of detection limit.10-14 Thus, over the last decades, in order to get rid of these inherent defects of SMO sensors, several synthetic methods have been applied to upgrade simplex nanoparticles building blocks to complex porous structure.15-18 As one of the representative porous materials, inverse opal nanomaterials that prepared by removing of the initial SiO2 or polymer templates with aligned opal structure through calcination and chemical etching, has shown great potential in a variety of fields, such as catalysis, chemical sensor, and lithium-ion batteries (LIBs).19-21 Owing to their advantages of high specific surface area, well-aligned interconnected macroporous structure, delimited local voids, nanosized wall components and special optical characteristic (e.g., efficient utilization of light, etc.), 3D inverse opal nanomaterials



have became one of the hot spots in the modern research of microreactor.21-23 Uniform channel (aligned porous structure) not only promote analyte gas molecule transfer readily and quickly, but also provide effective discrete voids with more reactive sites for concentrating the reactants by selective interactions.20,22,24 Along with tremendous progress on basic growth mechanism of nanostructural materials, for the purpose of replacing traditional time-consuming and sophisticated synthesis methods (e.g., gravitational force method, centrifugal method, filtration method, vertical deposition method, etc.),19 we recently develop a new type of synthesis method with self-assembled and precisely controlled mechanisms, which represents a time-saving, economical and simple synthetic route to prepare 3D IO nanomaterials with well-defined structures. Especially, such 3D IO nanomaterials with sphere morphology encompasses trimodal pores (sizes ≈ 4, 80 and 160 nm) and the void space in an opal, as well as consists more rod-like skeletal walls. Additionally, the latest data reveal that the gas selectivity could be systematically controlled by varying the surface oxygen chemistry, because there is a kind of relationship between the energy level of surface adsorbed oxygen and the electronic energy level structure. For example, Zou et al.25,26 had pointed out that the heteroatom doping can be adopted to modulate semiconductor’s Fermi level and band gap, and thus helping to improve selectivity while optimizing gas sensitivity. In addition, the crystal structure (e.g., the mean diameter (dm) of semiconductor metal oxide grains, the thickness (L) of the pace charge layer, etc.) also can be tuned by the impregnation of foreign metal oxides.10,14,27-29 For instance, Kim et al.14 had obtained Sn-doped NiO


Page 4 of 39

Page 5 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


multiroom spheres sensor with high xylene sensitivity by ultrasonic spray pyrolysis, and observed that the crystallite sizes of Sn-doped NiO decreased with increasing Sn doping concentration, thus suggesting that controlling dm smaller than 2L is an important guideline for designing high-performance gas sensor.10 Hence, if we seek to combine the microreactor concept with the heteroatom doping technology for tuning the morphology of nanomaterials, varying the electronic energy level structure of semiconductors, optimizing the oxidative catalytic activity of surface adsorbed oxygen, and accelerating gas molecule transport, which will make a huge step towards the improvement of gas-sensing characteristics. With these considerations in mind, first of all, we prepare a kind of efficient microreactor of 3D inverse opal In2O3 spheres by a combination of sulfonated polystyrene (S-PS) spheres template and ultrasonic spray pyrolysis method. These monodisperse S-PS spheres possessed sulfonic acid (SO3−H+) functional group (also termed hydrophilic shells) will self assemble form 3D opal structure with triangular channels during spray pyrolysis process. Thus, the IO-In2O3 microspheres sensor showed better gas-sensing characteristics and higher reaction activity compared with solid-In2O3 microspheres sensor (Figure 1 and Figure S1), because there are several structural sections in spatial extent for facilitating gas penetration, such as, exterior surface, interior surface and aligned pore channel over the whole microsphere. And then, a series of IO-(GaxIn1-x)2O3 microspheres are prepared by substituting the In atoms in IO-In2O3 with Ga atoms via doping method, because gallium is a congener of indium in Group 13 and the ionic radius of Ga3+ (0.61 Å) is smaller than that of the



In3+ (0.80 Å).30 We found that the band gaps of (GaxIn1-x)2O3 can be enlarged via Ga doping, and the increased surface adsorbed oxygen on semiconductor induced by Ga doping was linked with the elevation of Fermi level, resulting in significantly enhanced sensing performances to formaldehyde. Furthermore, it was demonstrated that the change of gas selectivity from acetone to formaldehyde originates from the preferential oxidation of formaldehyde via tuning the oxidative catalytic activity of surface adsorbed oxygen. Accordingly, the IO-(Ga0.2In0.8)2O3 sensor showed high formaldehyde sensitivity with fast response and recovery speeds, ultralow detection limit (50 ppb) and remarkable selectivity against other interference gases. Our work aims at unraveling electronical energy level structural origin of controllable gas-sensing selectivity, and providing new route for monitoring indoor air pollutant.

Figure 1. Scheme diagram of inverse opal structure as microreactor. 2. EXPERIMENTAL PROCEDURES 2.1 Preparation of Gas-Sensing Materials Ga-doped inverse opal In2O3 microspheres with different concentrations of Ga atoms (referred to as IO-(GaxIn1-x)2O3, x = 0.1, 0.2 and 0.3) were prepared by a combination of S-PS spheres template and ultrasonic spray pyrolysis method, and followed by sequent heat treatment. Typically, for the synthesis of IO-(Ga0.2In0.8)2O3,


Page 6 of 39

Page 7 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In(NO3)3·4.5H2O (0.4606 mmol), Ga(NO3)3·xH2O (0.1535 mmol) and 0.2 g S-PS spheres powders were added to a mixed solution of hydrogen peroxide (0.3 mL), hydrochloric acid (0.16 mL, 0.2 M) and distilled water (15 mL), under continuous stirring for 2 h to obtain a homogeneous spray solution. The droplets of the solution were generated by an ultrasonic atomizer (frequency = 1.7 MHz), which were carried to a tube (temperature = 700 °C). Nitrogen (flow rate = 500 sccm) was used as the carrier gas. Three conical flasks containing distilled water were used to collect the as-prepared precursor powders. After spray pyrolysis, the as-prepared precursor powders were collected by three conical flasks containing distilled water, and dried at 80 °C for 12 h. Finally, the IO-(Ga0.2In0.8)2O3 microspheres with 3D inverse opal structure were prepared by annealing the precursor powders at 600 °C for 3 h in air. For comparison, the the IO-In2O3 and IO-Ga2O3 were prepared by similar approach, except that only one kind of metal salt (0.6141 mmol In(NO3)3·4.5H2O or Ga(NO3)3·xH2O) was added into to the spray solution. Furthermore, to investigate the effect of microreactor on gas-sensing characteristics, the solid-In2O3 microspheres were prepared by ultrasonic spray pyrolysis of the spray solution without S-PS spheres template. 2.2 Sensor Fabrication and Measurement The detailed procedures for fabricating the gas sensors were mentioned in our previous work (Figure S2).13 The gas-sensing performances were tested by a static measurement system, and the measurement procedures were shown as follows.31 The test gases with different gas volume fractions were prepared by a static volumetric



method. Firstly, we vacuumed the test chamber (1 liter) to a negative pressure and rinsed it several times with clean air. Then a given amount of test gas was injected into the test chamber by an injector (CTC GC PAL 700, Hamiton Co. Reno, Nevada, USA), and diluted with clean air (the relative humidity is maintained to be 30 ± 5%). Subsequently, the sensor was transferred from the clean air chamber to a test chamber. After its resistance reaching to a steady state, the sensor was transferred from the test chamber to the clean air chamber again, and its resistance gradually recovered to its original state. The gas responses (S = Ra/Rg, Ra: sensor resistance in air, Rg: sensor resistance in test gas) of the sensors to different reducing gases (such as toluene, ethanol, formaldehyde, etc.) were measured at 125-375 °C. The response (τ90%-response) and recovery (τ90%-recovery) times are defined as the time to reach 90% variation in the sensor resistance upon exposure to the analyte gas and air. 3. RESULTS AND DISCUSSION 3.1 Material Characterization Undoped and Ga-doped IO-In2O3 microspheres were prepared by using self-assembly S-PS spheres as template (as shown in Figure S3, the mean diameters of S-PS spheres were about 200 nm).24 Aqueous droplets contained indium nitrate and S-PS spheres were generated by ultrasonic transducer and carried into a tubular quartz pyrolysis reactor (Figure 2A(a-1), left). During spray pyrolysis (Figure 2A(a-2), middle), the detailed processes are as follows: (i) Solution in the droplets evaporated at the early stage of the reaction; (ii) S-PS spheres would self-assemble into opal microspheres structure, and the contact area between two adjacent S-PS spheres would increase


Page 8 of 39

Page 9 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


caused by softening the S-PS spheres template, resulting in the formation of additional via-holes;32 (iii) Indium source was then melted. Accordingly, the softened self-assembly S-PS spheres embedded in the inverse opal-structured molten indium source. After spray pyrolysis, IO-In2O3 microspheres contained small amounts of residual carbon species were caused by hardening the frame, the removal of S-PS spheres and the decomposition of the metal salts (Figure 2A(a-1), right).31 After thermal treatment at ~600 °C for 3h in air, the pure IO-In2O3 microspheres were obained by removing the residual carbon species. Similarly, the IO-(GaxIn1-x)2O3 microspheres were prepared by spray pyrolysis of the aqueous solution containing indium nitrate, gallium nitrate, and S-PS spheres and subsequent heat treatment (Figure 2A(a-2)). Thus, the formation of well-aligned inverse opal structure in the IO-(GaxIn1-x)2O3 samples caused high gas accessibility.22

Figure 2. (A) Schemes illustrating the preparation of (a-1) inverse opal-In2O3 (IO-In2O3) microspheres, (a-2) Ga-doped IO-In2O3 microspheres (IO-(GaxIn1-x)2O3);



(B and C) SEM and (D and E) TEM images of IO-(Ga0.2In0.8)2O3 microspheres. The interconnected and well-aligned 3D inverse opal skeletons were observed from all (GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) species (Figure 2B,C and Figure S4), and all the samples yield a long-range ordered hexagonal arrangement of the inverse opal microspheres structure. As shown in Figure 2B, the IO-(Ga0.2In0.8)2O3 microspheres were selected as representatives to investigate the morphology of the (GaxIn1-x)2O3, it could be found that the individual inverse opal microsphere was assembled by packed small nanoparticles, and the diameters of spherical pores (~160 nm) generated from S-PS spheres were slightly smaller than the S-PS spheres sizes (~200 nm), properly due to the slower spray pyrolysis rate of S-PS spheres templates than that of indium nitrate by the growth of In2O3 crystallites, and the softened of S-PS spheres template during spray pyrolysis.13,36 Accordingly, due to the closer distance between the adjacent S-PS spheres (increase in contact area), all IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) microspheres samples displayed additional via-holes among the side oxides walls (as indicated by the red arrow in Figure 2C). Besides, through further comparison and observation, it could be observed that the morphology of those IO-(GaxIn1-x)2O3 (x = 0.1, 0.2, 0.3) have changed delicately compared to primary IO-In2O3. With increasing Ga doped content, the nanoparticles composed the skeletons of inverse opal structure become obviously smaller, and the IO microspheres become more tightness and smooth.14,33 Moreover, the decrease of crystallite sizes on the skeletons, indicating that the Ga doping can effectively restrain the growth of In2O3 nanoparticles.18,36 SEM images (Figure S5(a-d)) showed that the


Page 10 of 39

Page 11 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


morphologies of all IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) samples were isolated uniform inverse opal microspheres. The diameters of IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) microspheres samples were affected by the change in grain sizes because Ga doping prevented the growth of crystals,28 and a histogram analyzing the diameter distributions of two hundreds IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) samples (Figure S5(a-1)-(d-1)), the average diameters of IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) were 783 ± 26, 740 ± 60, 609 ± 32, and 589 ± 12 nm, respectively, suggesting that the average diameters of IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) were decreasing with the increasing Ga doping.18,37 As shown in the XRD patterns of Figure 3a, the diffraction peaks of all (GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) samples were matched with the single phase of cubic In2O3 (JCPDS# 06-0416), without any detectable Ga2O3 phase. With increasing Ga doping concentration, the XRD peak positions of (GaxIn1-x)2O3 samples continuously shifted toward large angle sides (Figure 3b). Furthermore, according to the basis of Scherrer Formula, the mean crystallite sizes of In2O3, (Ga0.1In0.9)2O3, (Ga0.2In0.8)2O3 and (Ga0.3In0.7)2O3 were estimated to be about 14.55, 13.95, 10.2 and 9.5 nm, respectively (Figure 3c).33 The crystallite sizes gradually decrease with increasing Ga doping concentration, indicating that Ga3+ ions have successfully substituted In3+ ions in the In2O3 lattice and decrease the lattice distances, which is in agreement with the results of SEM images.28,34 This is feasible considering that the Ga3+ ions with an atomic radius of 0.62 Å can easily occupy the indium lattice sites (atomic radius of In3+ = 0.80 Å).30 It has been reported that the gas sensitivity



increases abruptly when the crystallite size becomes comparable or smaller than the Debye length.35

Figure 3. (a) XRD patterns, (b) partially enlarged XRD patterns between 25-40°, and (c) crystallite sizes of the IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2, 0.3); (d-g) HRTEM images and FFT patterns of IO-(Ga0.2In0.8)2O3 microspheres, and (h) EDS elemental mapping images of the IO-(Ga0.2In0.8)2O3 microspheres. Detailed information about the inner structures of the IO-(Ga0.2In0.8)2O3 was further confirmed by using transmission electron microscopy (TEM) (Figure 2D,E), an highly-aligned inverse opal structure was found in every IO-(Ga0.2In0.8)2O3


Page 12 of 39

Page 13 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


microsphere, and it could be found that the IO morphology remained after Ga doping. Besides, all spherical pores (mean pore size, ~160 nm) reflecting the shape of S-PS spheres were well-developed and uniform inside the microspheres, and oxides side formed stably between these pores walls, indicating that the inverse opal structure accumulated by many S-PS spheres could be remained even after decomposition of S-PS spheres templates. Moreover, an inverse opal structure was also found in every IO-In2O3 microsphere, and there were some voids among the indium oxides side walls, reflecting that the highly IO structure were arranged by loosely packed interconnected ultrasmall In2O3 particles.18 However, the IO-In2O3 microspheres seemed like largely destroyed without Ga doping (the inverse opal structure was not well-organized and the side walls was not clear). It could be concluded that the heteroatom Ga doping led to the improvement of IO structure and the formation of interconnected spherical pores (Figure S6a,b).33 Figure 3(d-g) exhibited a representative set of HRTEM images and corresponding FFT patterns of the IO-(Ga0.2In0.8)2O3 sample. Remarkably, with increasing Ga doping concentration, the Ga2O3 phase tended to separate from (GaxIn1-x)2O3, the existence of the Ga2O3 phase was confirmed by observing the (311) crystal plane with lattice spacing of 0.241 nm (Figure 3e).25,38 The inter-planar spacing 0.295 nm agreed well with the crystal plane (222) of cubic In2O3 (JCPDS# 06-0416) was observed in the HRTEM image (Figure 3(f-1,g-1)) and FFT pattern (Figure 3(f-2,g-2)).20 The corresponding selected-area electron diffraction (SAED) pattern of IO-(Ga0.2In0.8)2O3 was shown in Figure S7, and the SAED pattern of Ga2O3 (311) was relatively weak due to the low concentration of Ga2O3. The HRTEM



images and corresponding FFT patterns of the IO-In2O3 were exhibited in Figure S6c,d. The elemental distribution was further substantiated by EDS mapping of the elemental composition in the IO-(Ga0.2In0.8)2O3, revealing that the Ga element was uniformly distributed over the entire IO-(Ga0.2In0.8)2O3 microsphere (Figure 3h).

Figure 4. (a-d) N2 adsorption/desorption isotherms and (e) pore-size distributions and (f) corresponding BET surface areas of (a and f-1) IO-In2O3, (b and f-2) IO-(Ga0.1In0.9)2O3, (c and f-3) IO-(Ga0.2In0.8)2O3 and (d and f-4) IO-(Ga0.3In0.7)2O3 microspheres. The specific surface areas (SSA) and pore-size distributions were measured from nitrogen adsorption/desorption isotherms (Figure 4). The pore-size distribution of all IO-(GaxIn1-x)2O3 microspheres samples were exhibited in Figure 4e. Abundant mesopores were observed in IO-(GaxIn1-x)2O3 (x=0.1, 0.2 and 0.3) microspheres, whereas only a relatively low volume of mesopores with a diameter of ~4 nm was observed in the IO-In2O3 microspheres. This suggested that most of the mesopores were originated from the incorporation of Ga into In2O3, and a little was assigned to


Page 14 of 39

Page 15 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the outgassing produced by the decomposition of the S-PS sphere templates.13,14,18 Furthermore, pores with diameters of approximately 80 nm were observed in all IO-(GaxIn1-x)2O3 microspheres samples, which originated from the formation of area contact between two adjacent S-PS spheres by softening the template during pyrolysis.32 Note that the red arrows in Figure 2C more clearly pointed out the existence of the additional via-holes between the neighbouring frames. And, it also found that the amount of macropores (diameter: ~80 nm) increased with increasing Ga doping concentration. Besides, it was difficult to analyze the macropores (diameter: ~160 nm) via BET analysis, but combining with SEM/TEM analyses, it can be verified that both macropores (~80 nm and ~160 nm) and mesopores (~4 nm) are abundant in the IO-(GaxIn1-x)2O3 microspheres. In addition, the crystallite sizes of (GaxIn1-x)2O3 gradually decreased with increasing Ga doping concentration. Accordingly, the SSA of the IO-In2O3, (Ga0.1In0.9)2O3, (Ga0.2In0.8)2O3 and (Ga0.3In0.7)2O3 microspheres were 20.84, 20.3, 27.8 and 32.43 m2/g (Figure 4f), respectively, which showed a similar tendency as an increase in the amount of Ga ion. These results arise from the introduction of Ga3+ in the lattice of In2O3, resulting in a decrease of the crystallite sizes of In2O3 nanoparticles and an increase of the SSA.26,37 Based on above analysis, the coexistence of abundant macropores and mesopores, high-efficient gas accessibility of IO structures, and high SSA consist in the IO-(GaxIn1-x)2O3 microspheres, which can forebode the enhanced sensing properties.31 3.2 Gas-Sensing Characterization



Formaldehyde is a kind of representative indoor air pollutants, with irritant, pathopoiesia and mutagenesis properties.1,2 In this work, the Ga2O3 based sensor cannot be measured at operating temperatures below 400 °C because its base resistance in air is beyond the upper detection limit (1000 MΩ), thus, we fabricated gas sensors based on all (GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) samples to investigate their gas-sensing characteristics. The gas responses of IO-In2O3, IO-(Ga0.1In0.9)2O3, (Ga0.2In0.8)2O3 and (Ga0.3In0.7)2O3 sensors to 100 ppm formaldehyde, acetone, ethanol and methanol were measured in the range of 175-325 °C, 175-375 °C, 125-325 °C and 200-375 °C, respectively (Figure 5 and Figure S8). Note that the gas-sensing behaviors of n-type semiconductor metal oxides were observed over all sensors, the resistances in air of all sensors will decrease while being exposed into reducing gases.10 Furthermore, the gas responses of all sensors presented volcano-shaped curves in the operating temperature range, revealing that there will be a adsorption/desorption balances of gas molecules (adsorbed oxygen and formaldehyde) during surface gas-sensing reaction.39 As shown in Figure S8, the IO-In2O3 sensor showed high response and good selectivity toward acetone at 300 °C, and by combining with Figure S1, it could be concluded that the gas-sensing characteristics of the IO-In2O3 sensor was better than that of the solid-In2O3 sensor, because the inverse opal structure, with high catalytic activity and high utility factor, will be as a microreactor for improving gas-sensing performances.14,37 Besides, the IO-In2O3 sensor showed negligibly low response (Ra/Rg < 10) to 100 ppm formaldehyde at the range of 175-375 °C. In stark contrast, the Ga-doped IO-In2O3 sensors


Page 16 of 39

Page 17 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(IO-(GaxIn1-x)2O3, x = 0.1, 0.2 and 0.3) all showed remarkable selectivity toward formaldehyde, obviously lower responses to acetone, ethanol and methanol, at their respective optimal operating temperatures (Figure 5(a-c)), and the formaldehyde responses of IO-(GaxIn1-x)2O3 (x = 0.1, 0.2 and 0.3) sensors gradually increased with increasing Ga doping concentration. Note that the IO-(Ga0.2In0.8)2O3 sensor showed the highest formaldehyde response among these three Ga-doped In2O3 sensors, and reached the maximum (Ra/Rg = 48.8 to 100 ppm formaldehyde) at the optimal operating temperature of 200 °C. However, when more amount of Ga was doped into the In2O3 crystal, the formaldehyde response of the IO-(Ga0.3In0.7)2O3 sensor sharply declined in the range of 200-375 °C, which was probably attributed to the formation of tiny amounts of Ga2O3 clusters as a result of excess Ga doping, and was similar with the reported literature.14,24 These indicated that the formaldehyde sensing performance of the In2O3 sensor depends on the level of Ga doping. Moreover, we have also found that the selectivity of IO-(Ga0.1In0.9)2O3 and (Ga0.3In0.7)2O3 sensors has been changed with the increase of operating temperatures, it may be due to the different values of lowest unoccupied molecule orbit (LUMO) energy level for different test gases.40-42 The orange dotted boxes in Figure 5 highlighted the optimal sensing temperature ranges of the IO-(GaxIn1-x)2O3 (x = 0.1, 0.2 and 0.3) sensors for selective detection of formaldehyde. Compared with the IO-(Ga0.1In0.9)2O3 (225-275 °C) and (Ga0.3In0.7)2O3 (250 °C and 300 °C) sensors, the IO-(Ga0.2In0.8)2O3 sensor could selectively detect formaldehyde against other interfering gases at a wide operating temperature range (125-275 °C). It is certain that the optimal operating



temperature of semiconductor oxides reflects a combination of LUMO energy, heteroatom doping concentration, surface reaction activity and surface oxygen chemistry.28,34,41,43 Furthermore, in order to decrease the cross-responses to other indoor gases to a negligible level, such as ethanol, because most of the SMO gas sensors will violently react with ethanol.22 Thus, the selectivity to formaldehyde over ethanol interference (SF/SE) of the IO-(GaxIn1-x)2O3 (x = 0.1, 0.2 and 0.3) sensors were calculated (Figure 5d). Note that the SF/SE value of IO-(Ga0.2In0.8)2O3 sensor was significantly higher than that of the IO-(Ga0.1In0.9)2O3 and IO-(Ga0.3In0.7)2O3 sensors, confirming a significant effect of the Ga doping in enhancing the formaldehyde selectivity. Besides, the selectivity comparisons between Ga-doped In2O3 sensors and undoped In2O3 sensor implied that the Ga doping will obviously control the gas-sensing selectivity.


Page 18 of 39

Page 19 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Gas responses (S) of the (a) IO-(Ga0.1In0.9)2O3, (b) IO-(Ga0.2In0.8)2O3 and (c) IO-(Ga0.3In0.7)2O3 sensors to 100 ppm of various gases at at 175-375 °C, 125-325 °C and 200-375 °C, respectively; (d) Formaldehyde selectivity (Sformaldehyde/Sethanol, SF/SE) of the IO-(Ga0.1In0.9)2O3, IO-(Ga0.2In0.8)2O3 and IO-(Ga0.3In0.7)2O3 sensors at 225-275 °C, 125-275 °C and 250-300 °C, respectively; Formaldehyde-sensing characteristics of IO-(Ga0.2In0.8)2O3 sensor at 200 °C: (e and f) dynamic sensing transients to 0.05-100 ppm formaldehyde, and (g) response/recovery times to formaldehyde in concentration range of 0.05-100 ppm. The gas-sensing transients of the IO-(Ga0.2In0.8)2O3 sensor toward 0.05-100 ppm formaldehyde at 200 °C were shown in Figure 5e,f. Obviously, the IO-(Ga0.2In0.8)2O3 sensor possessed an excellent response-recovery kinetics characteristics in broad formaldehyde concentration range. And it can be observed that the IO-(Ga0.2In0.8)2O3 sensor still had a response of 1.53 when the formaldehyde concentration was as low as 50 ppb. For ensuring human healthy, the formaldehyde concentration in enclosed buildings is required to be lower than 100 ppb.3 Thus, the sub-ppm detection of formaldehyde of the IO-(Ga0.2In0.8)2O3 sensor device is necessary for monitoring indoor air quality. Figure S9 pointed out that the relationship between the response of the IO-(Ga0.2In0.8)2O3 sensor and formaldehyde concentration was a power function relationship, and the exponent in the power law was 0.39 corresponded to porous sensing layers.44 Furthermore, according to the power law equation in Figure S9, the detection limit of the IO-(Ga0.2In0.8)2O3 sensor was calculated to be 10 ppb when S (S= Ra/Rg) > 1.2 was used as the criterion for response.8 The corresponding response



Page 20 of 39

(τ90%-response values) and recovery (τ90%-recovery values) times of the IO-(Ga0.2In0.8)2O3 sensor in the formaldehyde concentration range of 0.05-100 ppm were calculated and depicted in Figure 5g. For the IO-(Ga0.2In0.8)2O3 sensor, the response time tended to decrease with increasing formaldehyde concentration, and the sensor showed long response time at low formaldehyde concentration. The unusual relationship between the response time and gas concentration could be explained by a formula proposed by I.-D. Kim, which was based on a non-linear diffusion reaction model.45 In this formula, the diffusion time (τ), that will influence response time, was represented as τ = kx02/(DC01-r) where k, x0, C0 and D denote the reaction rate constant, film thickness and gas concentration, and diffusion coefficient, respectively. And the constant r is in the range of 0.3-1.0. Therefore, higher concentration will cause faster diffusion kinetics, and result in shorter response time, meaning that the response time was determined by a non-linear adsorption isotherm. However, the recovery time tended to increase when the gas concentration and gas response increased, which emanated mainly from the sluggish surface kinetics of adsorption, dissociation and ionization of oxygen during the recovery.46 Besides, the IO-(Ga0.2In0.8)2O3 sensor showed ultra-fast response speed (τ90%-response = 1 s) to 100 ppm formaldehyde at 200 °C (Figure S10), which also can be explained by the rapid diffusion of gas molecules through the highly

gas-accessible

inverse

opal

structures

with

abundant

meso-

and

macropores.20,22 The repetitive sensing transients of IO-(Ga0.2In0.8)2O3 sensor to 100 ppm formaldehyde indicated the good reproducibility (Figure S11). Furthermore, for the IO-(Ga0.2In0.8)2O3 sensor, both the resistance in air and gas response to 100 ppm


Page 21 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formaldehyde were not apparently changed during long-term stability testing for 30 days (Figure S12). In addition, the formaldehyde sensing characteristics of IO-(Ga0.2In0.8)2O3 sensor were better than those of many previously reported formaldehyde sensors (Table S1).

Figure 6. (a) UV/visible diffuse reflection spectra and (b) corresponding band gaps of IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2, 0.3 and 1) samples; (c) The resistance in air of gas sensors based on IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2, 0.3 and 1) at different operating temperatures; (d-g) Work function area scan recorded for IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) samples via Kelvin Probe measurements; (h) The working principle of Kevin Prob measurements. 3.3 Gas-Sensing Mechanism According to the classical working principle of the semiconductor metal oxide-based gas sensors, the utility factor will take part in determining the gas-sensing



properties.10 By combining with the XRD and BET analysis results, note that compared with those of undoped IO-In2O3 microspheres, the IO-(Ga0.2In0.8)2O3 microspheres tend to be assembled from smaller nanoparticles and possess larger specific surface area. Therefore, the relatively higher formaldehyde response can be explained in part by the optimized structural regulation of IO-(Ga0.2In0.8)2O3 microspheres induced by Ga doping, the root cause is the increase of active sites participated in gas-sensing reaction and the accelerating gas diffusion.14,37 In addition, the specific surface area (SSA), pore structure and crystallite size of IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) microspheres specimens are listed in Table S2. It is worthwhile noting that the morphological variation of the undoped and Ga-doped IO-In2O3 microspheres are not evidently different from each other. Accordingly, it could be reasonably speculated that the obviously enhanced formaldehyde sensing performances mainly depended on the Ga incorporation. In the sections that follow, further experimental studies and theoretical will be carried out to explain the Ga substitutional doping how to affect the band gaps and the electronic energy level structure of Ga-doped In2O3, and contribute to improve formaldehyde sensing performances. Firstly, the UV-vis spectrophotometer were implemented to verify the variation in band gap upon the substitutional doping of Ga ion in the In2O3 crystal. The UV-vis adsorption spectra of (GaxIn1-x)2O3 (x = 0, 0.1, 0.2, 0.3 and 1) samples showed continuous blue shifts of adsorption edges with increasing Ga doping concentration (Figure 6a), suggesting a increase in the energy band gap. In addition, the band gaps


Page 22 of 39

Page 23 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of all above samples can be calculated from the transformation of UV-vis diffuse reflectance spectra by utilizing the Tauc plot equation (Figure S13 and Figure 6b).47,48 The calculated results further show that the band gaps of Ga-doped In2O3 tended to be larger values compared with that of undoped In2O3 (from 2.88 eV to 4.27 eV). Thus, the baseline resistances in air of IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) sensors gradually increased with increasing Ga doping (Figure 6c), which is in line with previously reported results.49 Furthermore, the work functions area map recorded for pure In2O3 and Ga-doped In2O3 (Figure 6(d-g)) are evaluated by employing the Kelvin Probe measurements to experimentally verify the elevation of Fermi energy level after Ga doping. Obviously, the Ga substitutional doping results in continuous reduction of work function values with increasing Ga doping concentration, and the measured results show the average work function values of Ga-doped In2O3 decreased from 4.97 eV to 3.91 eV. In addition, the work function (Φ) is the thermodynamic energy that needed to remove an electron from Fermi level to the vacuum energy (Figure 6h),50 thus, according to the measured results of the work function values of different IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) samples, it can be concluded that the Ga doping will cause the elevating of Fermi level in Ga-doped In2O3 samples. As well known that the SMO gas sensors detect reducing gases through a change in resistance caused by reactions between surface chemisorbed oxygen species and targeted gas molecules.10,50 Generally, As shown in Figure 7a, when gas sensors operate in the aerial atmosphere where the oxygen molecules will adsorb on the surface of SMO, thus leading to the electrons transfer from the conduction band to



surface, as well as the formation of chemisorbed oxygen species (O2-, O- and O2-) on the surface and the upward bend of energy band,51 until achieve at equilibrium meant that the lowest unoccupied molecular orbital (LUMO) energy level for adsorbed oxygen is same with the Fermi energy level.25,51 Accordingly, it could be inferred that more chemisorbed oxygen will be formed on the surface of IO-(Ga0.2In0.8)2O3 at the same condition while at equilibrium, and which would in turn increase their depletion layer, because the higher Fermi level will generate larger energy level difference between SMO and adsorbed oxygen.26 Thus, it can be observed that the baseline resistances in air (Ra) of all Ga-doped IO-In2O3 based sensors generally higher than that of undoped IO-In2O3 based sensor, while at the same conditions. Subsequently, when exposed to formaldehyde, more chemisorbed oxygen in IO-(Ga0.2In0.8)2O3 will take part in the formaldehyde sensing reaction and higher chemiresistive variations will be found in the IO-(Ga0.2In0.8)2O3 sensor, finally leading to higher response compared with IO-In2O3. Thus, the markedly increase in gas response through Ga substitutional doping in the present study can be mainly explained by the modulation of Fermi energy level. In order to prove that the increase amount of chemisorbed oxygen is due to the substitutional doping of Ga ion, and further investigate the incorporation of Ga in In2O3, all (GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) samples were analyzed by XPS spectra (Figure S14). The XPS analysis were adjusted for specimen charging via referencing the C 1 s peak at 284.6 eV. For pure Ga2O3 sample, the two peaks centered at ~1116.9 and ~1143.6 eV are correspond to the Ga 2p3/2 and Ga 2p1/2,


Page 24 of 39

Page 25 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


respectively (Figure S14a).52 The In 3d5/2 and In 3d3/2 peaks are observed at ~444.4 and ~452.0 eV, respectively, in the pure In2O3 (Figure S14b).24 Compared with the binding energy of Ga 2p in pure Ga2O3, the Ga 2p peaks of the (Ga0.3In0.7)2O3, (Ga0.2In0.8)2O3 and (Ga0.1In0.9)2O3 shift toward a lower binding energies, but the binding energies of In 3d in (Ga0.1In0.9)2O3, (Ga0.2In0.8)2O3 and (Ga0.3In0.7)2O3 shift toward higher values compared with those of pure In2O3 (Figure S14a,b). The shifts suggest that there will be charge redistribution between Ga and In because Ga has a higher electronegativity than In.50 In other words, when Ga is doped in the In2O3, the electrons will be easily transformed from In to Ga, leading to the increase of electrons screening effect for Ga with the decrease of that for In. Therefore, this verifies that the substitutional doping of Ga atom. The O 1s peaks of all (GaxIn1-x)2O3 (x = 0, 0.1, 0.2 and 0.3) samples can be separated into three parts: lattice oxygen (OI), oxygen-deficient regions (OII), and chemisorbed oxygen (OIII) (Figure S14c).8,51 Obviously, the contents of the OIII components gradually increase with increasing Ga doping concentration until the maximum content was obtained in (Ga0.2In0.8)2O3. This result is in agreement with the above proposed mechanism that the elevation of Fermi energy level participates in increasing adsorbed oxygen. Thus, this emphasizes the fact that the increased chemisorbed oxygen of the IO-(Ga0.2In0.8)2O3 sensor in the present study is a key parameter that promotes the formaldehyde sensing performance. Taking all above factors into account, the enhanced gas-sensing characteristics of the IO-(Ga0.2In0.8)2O3 sensor can be rationalized by the rapid gas diffusion generated



from interconnectivity between three different-sized pores, abundant reaction activity sites combined with high specific surface area, and increased contents of adsorbed oxygen induced by the elevation of Fermi level, all of which originate from the well-aligned inverse opal structure and substitutional doping of Ga ion. Finally, it is worth noting that the elevation of Fermi level changed the gas selectivity either (Figure 7b). For example, the IO-(Ga0.2In0.8)2O3 sensor showed a high selectivity to formaldehyde at 200 °C, whereas the IO-In2O3 sensor showed similar response to acetone, ethanol and formaldehyde. According to recent literature,25,26 the enhanced formaldehyde selectivity can be explained in part by the chemisorbed oxygen on the IO-(Ga0.2In0.8)2O3 surface maintains lower LUMO energy level (lower oxidizing ability), and the formaldehyde possesses relatively strong reducibility (higher LUMO energy of 0.219 eV) compared with other weak reducing gases (for example, the LUMO energy for acetone and ethanol are 0.205 eV and 0.126 eV, respectively), which is supported by an improvement of the gas selectivity. Therefore, another accomplishment of this work is the Fermi level-optimized regulation of IO-(Ga0.2In0.8)2O3 material for realizing superior formaldehyde selectivity without interference from one of the most common VOCs (such as ethanol, toluene, et al) in real life.


Page 26 of 39

Page 27 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) Schematic energy diagram illustrating the effect of Ga doping on formaldehyde sensing; (b) Gas responses of the IO-In2O3, IO-(Ga0.1In0.9)2O3, IO-(Ga0.2In0.8)2O3, and IO-(Ga0.3In0.7)2O3 sensors to various gases (concentration: 100 ppm of formaldehyde (F), acetone (A), ethanol (E) and methanol (M)) at 300, 250, 200 and 250 °C, respectively. 4. CONCLUSION Taken together, the results have demonstrated the electronic energy level structure controllability of the spray pyrolysis-doping synthesis of (GaxIn1-x)2O3 nanomaterials



Page 28 of 39

and its synergistic impact on the oxidative catalytic activity for preferential CH2O oxidation. Herein, we prepared 3D inverse opal Ga-doped In2O3 microspheres based sensor with ultrasensitive and highly selective for monitoring indoor air pollutant-formaldehyde, which adopted an effective method of heteroatom substitutional doping induced variation of the electronic energy level structure of (GaxIn1-x)2O3. The prepared sensor of 3D IO-(Ga0.2In0.8)2O3 microspheres achieved high response (Ra/Rg = 47.2 ± 5) toward 100 ppm formaldehyde with fast response and recovery speeds, and showed good selectivity and stability upon exposure to formaldehyde. In addition, it can successfully detect ppb-level formaldehyde. 3D inverse opal morphology with well-aligned interconnected porous structure significantly provided abundant gas diffusion channels and increased activity sites. More importantly, the Ga substitutional doping successfully elevated the Fermi level, and further enlarged the energy level difference between semiconductor and adsorbed oxygen, thereby bring more oxygen adsorption and improving sensing performances toward formaldehyde. Moreover, the Ga substitutional doping also played the role of decreasing the oxidative catalytic activity of absorbed oxygen to realize the selective detection

of

formaldehyde.

We

expect

this

strategy

of

fabricating

inverse-opal-structured doped semiconductor metal oxide will open new route to design indoor air quality monitoring gas sensor with higher sensitivity and selectivity.

ASSOCIATED CONTENT Supporting Information


Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The Supporting Information is available free of charge on the ACS Publications website. Schematic diagram of the gas sensor device; SEM images of the S-PS spheres, IO-In2O3, IO-(Ga0.1In0.9)2O3 and IO-(Ga0.3In0.7)2O3; TEM and HR-TEM images, and SAED patterns of IO-In2O3 and IO-In2O3 microspheres; UV/visible diffuse reflectance spectra of IO-(GaxIn1-x)2O3 (x = 0, 0.1, 0.2, 0.3 and 1); XPS spectra of different samples; Gas responses of the sensors; The comparison of gas-sensing properties and structural characterization.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]. E-mail: [email protected]. ORCID Tianshuang Wang: 0000-0003-3196-3005. Peng Sun: 0000-0002-9509-9431. Notes The authors declare no competing financial interest.

ACKNOWLEGEMENTS This work is supported by the National Key Research and Development Program (No. 2016YFC0207300). National Nature Science Foundation of China (Nos. 61722305,



61833006, 61520106003). Science and Technology Development Program of Jilin Province (No. 20170520162JH). China Postdoctoral Science Foundation funded project Nos. 2017T100208 and 2015M580247. Graduate Interdisciplinary Research Fund of Jilin University (No. 10183201833).


Page 30 of 39

Page 31 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) IARC Working Group on the Evaluation of Carcinogenic Risks to Humans, IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, 2006, 88, 1-478. (2) Logue, J. M.; Price, P. N.; Sherman, M. H.; Singer, B. C. A Method to Estimate the Chronic Health Impact of Air Pollutants in US Residences. Environ. Health Persp. 2012, 120, 216-222. (3) Michelot, N.; Marchand, C.; Ramalho, O.; Delmas, V.; Carrega, M. Monitoring Indoor Air Quality in French Schools and Day-Care Centers. Hvac&R Res. 2013, 8, 1083-1089. (4) Du, Z.; Mo, J.; Zhang, Y. Risk Assessment of Population Inhalation Exposure to Volatile Organic Compounds and Carbonyls in Urban China. Environ. Int. 2014, 73, 33-45. (5) Zhang, Y.; Mo, J.; Weschler, C. J. Reducing Health Risks from Indoor Exposures in Rapidly Developing Urban China. Environ. Health Persp. 2013, 121, 751-755. (6) http://www.sac.gov.cn/szhywb/ztzl/qzxgjbzdwtb/201706/t20170614_246608.htm, accessed May 29 2017. (7) Yamazoe, N.; Suematsu, K.; Shimanoe, K. Extension of Receptor Function Theory to Include Two Types of Adsorbed Oxygen for Oxide Semiconductor Gas Sensors. Sensor. Actuat. B-Chem. 2012, 163, 128-135. (8) Yamazoe, N.; Shimanoe, K. Proposal of Contact Potential Promoted Oxide Semiconductor Gas Sensor. Sensor. Actuat. B-Chem. 2013, 187, 162-167. (9) Choi, S.-J.; Ku, K.-H.; Kim, B. J.; Kim, I.-D. Novel Templating Route Using Pt



Infiltrated Block Copolymer Microparticles for Catalytic Pt Functionalized Macroporous WO3 Nanofibers and Its Application in Breath Pattern Recognition. ACS Sens. 2016, 1, 1124-1131. (10) Yamazoe, N.; Sakai, G.; Shimanoe, K. Oxide Semiconductor Gas Sensors. Catal. Surv. Asia 2003, 7, 63-75. (11) Postica, V.; Gröttrup, J.; Adelung, R.; Lupan, O.; Mishra, A. K.; Lee, N. H. D.; Ababii, N.; Carreira, J. F. C.; Rodrigues, J.; Sedrine, N. B.; Correia, M. R.; Monteiro, T.; Sontea, V.; Mishra, Y. K. Multifunctional Materials: A Case Study of the Effects of Metal Doping on ZnO Tetrapods with Bismuth and Tin Oxides. Adv. Funct. Mater. 2017, 27, 1604676. (12) Kim, H.-J.; Yoon, J.-W.; Choi, K.-I.; Jang, H. W.; Umar, A.; Lee, J.-H. Ultraselective and Sensitive Detection of Xylene and Toluene for Monitoring Indoor Air Pollution Using Cr-doped NiO Hierarchical Nanostructures. Nanoscale, 2013, 5, 7066-7073. (13) Wang, T.; Yu, Q.; Zhang, S.; Kou, X.; Sun, P.; Lu, G. Rational Design of 3D Inverse Opal Heterogeneous Composite Microspheres as Excellent Visible-Light Induced NO2 Sensors at Room Temperature. Nanoscale 2018, 10, 4841-4851. (14) Kim, B.-Y.; Yoon, J.-W.; Kim, J. K.; Kang, Y. C.; Lee, J.-H. Dual Role of Multiroom-Structured Sn-Doped NiO Microspheres for Ultrasensitive and Highly Selective Detection of Xylene. ACS Appl. Mater. Inter. 2018, 10, 16605-16612. (15) Zhu, W.; Chen, Z.; Pan, Y.; Dai, R.; Wu, Y.; Zhuang, Z.; Wang, D.; Peng, Q.; Chen, C.; Li, Y. Functionalization of Hollow Nanomaterials for Catalytic


Page 32 of 39

Page 33 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Applications: Nanoreactor Construction. Adv. Mater. 2018, e1800426, 1-30. (16) Sun, F.; Cai, W.; Li, Y.; Jia, L.; Lu, F. Direct Growth of Mono- and Multilayer Nanostructured Porous Films on Curved Surfaces and Their Application as Gas Sensors. Adv. Mater. 2005, 17, 2872-2877. (17) Koo, W.-T.; Choi, S.-J.; Kim, S.-J.; Jang, J.-S.; Tuller, H. L.; Kim, I.-D. Heterogeneous Sensitization of Metal-Organic Framework Driven Metal@Metal Oxide Complex Catalysts on an Oxide Nanofiber Scaffold Toward Superior Gas Sensors. J. Am. Chem. Soc. 2016, 138, 13431-13437. (18) Firooz, A. A.; Hyodo, T.; Mahjoub, A. R.; Khodadadi, A. A.; Shimizu, Y. Synthesis and Gas-Sensing Properties of Nano- and Meso-Porous MoO3-Doped SnO2. Sensor. Actuat. B-Chem. 2010, 147, 554-560. (19) Stein, A.; Wilson, B. E.; Rudisill, S. G. Design and Functionality of Colloidal-Crystal-Templated Materials-Chemical Applications of Inverse Opals. Chem. Soc. Rev. 2013, 42, 2763-2803. (20) Wang, T.; Can, I.; Zhang, S.; He, J.; Sun, P.; Liu, F.; Lu, G. Self-Assembly Template Driven 3D Inverse Opal Microspheres Functionalized with Catalyst Nanoparticles Enabling a Highly Efficient Chemical Sensing Platform. ACS Appl. Mater. Inter. 2018, 10, 5835-5844. (21) Xu, J.-J.; Wang, Z.-L.; Xu, D.; Meng, F.-Z.; Zhang, X.-B. 3D Ordered Macroporous LaFeO3 as Efficient Electrocatalyst for Li-O2 Batteries with Enhanced Rate Capability and Cyclic Performance. Energy Environ. Sci. 2014, 7, 2213-2219. (22) Yoon, J.-W.; Choi, S. H.; Kim, J.-S.; Jang, H. W.; Kang, Y. C.; Lee, J.-H.



Page 34 of 39

Trimodally Porous SnO2 Nanospheres with Three-Dimensional Interconnectivity and Size Tunability: A One-Pot Synthetic Route and Potential Application as An Extremely Sensitive Ethanol Detector. NPG Asia Mater. 2016, 8, e244. (23) Xie, Y.; Xing, R.; Li, Q.; Xu, L.; Song, H. Three-Dimensional Ordered ZnO-CuO Inverse Opals Toward Low Concentration Acetone Detection for Exhaled Breath Sensing. Sensor. Actuat. B-Chem. 2015, 211, 255-262. (24) Wang, T.; Zhang, S.; Yu, Q.; Kou, X.; Sun, P.; Liu, F.; Lu, H.; Yan, X.; Lu, G. 3D

Inverse

Opal

Nanostructured

Multilayer

Films

of

Two-Component

Heterostructure Composites: A New-Generation Synthetic Route and Potential Application as High-Performance Acetone Detector. Sensor. Actuat. B-Chem. 2018, 276, 262-270. (25) Chen, H.; Sun, L.; Li, G.-D.; Zou, X. Well-Tuned Surface Oxygen Chemistry of Cation Off-Stoichiometric Spinel Oxides for Highly Selective and Sensitive Formaldehyde Detection. Chem. Mater. 2018, 30, 2018-2027. (26) Chen, H.; Zhao, Y.; Shi, L.; Li, G.-D.; Sun, L.; Zou, X. Revealing the Relationship

between

Energy

Level

and

Gas

Sensing

Performance

in

Heteroatom-Doped Semiconducting Nanostructures. ACS Appl. Mater. Inter. 2018, 10, 29795-29804. (27) Kim, H.-J.; Lee, J.-H. Highly Sensitive and Selective Gas Sensors Using P-Type Oxide Semiconductors: Overview. Sensor. Actuat. B-Chem. 2014, 192, 607-627. (28) Wang, C.; Liu, J.; Yang, Q.; Sun, P.; Gao, Y.; Liu, F.; Zheng, J.; Lu, G. Ultrasensitive and Low Detection Limit of Acetone Gas Sensor Based on W-Doped


Page 35 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NiO Hierarchical Nanostructure. Sensor. Actuat. B-Chem. 2015, 220, 59-67. (29) Woo, H.-S.; Kwak, C.-H.; Chung, J.-H.; Lee, J.-H. Highly Selective and Sensitive Xylene Sensors Using Ni-Doped Branched ZnO Nanowire Networks. Sensor. Actuat. B-Chem. 2015, 216, 358-366. (30) http://abulafia.mt.ic.ac.uk/shannon/ptable.php. (31) Wang, T.; Zhang, S.; Yu, Q.; Wang, S.; Sun, P.; Lu, H.; Liu, F.; Yan, X.; Lu, G. Novel Self-Assembly Route Assisted Ultra-Fast Trace Volatile Organic Compounds Gas Sensing Based on Three-Dimensional Opal Microspheres Composites for Diabetes Diagnosis. ACS Appl. Mater. Inter. 2018, 10, 32913-32921. (32) Xing, R.; Li, Q.; Xia, L.; Song, J.; Xu, L.; Zhang, J.; Xie, Y.; Song, H. Au-Modified Three-Dimensional In2O3 Inverse Opals: Synthesis and Improved Performance for Acetone Sensing Toward Diagnosis of Diabetes. Nanoscale 2015, 7, 13051-13060. (33) Feng, C.; Wang, C.; Zhang, H.; Li, X.; Wang, C.; Cheng, P.; Ma, J.; Sun, P.; Gao, Y.; Zhang, H.; Sun, Y.; Zheng, J.; Lu, G. Enhanced Sensitive and Selective Xylene Sensors Using W-Doped NiO Nanotubes. Sensor. Actuat. B-Chem. 2015, 221, 1475-1482. (34) Gao, H.; Wei, D.; Lin, P.; Liu, C.; Sun, P.; Shimanoe, K.; Yamazoe, N.; Lu, G. The Design of Excellent Xylene Gas Sensor Using Sn-Doped NiO Hierarchical Nanostructure. Sensor. Actuat. B-Chem. 2017, 253, 1152-1162. (35) Xu, C.; Tamaki, J.; Miura, N.; Yamazoe, N. Promotion of Tin Oxide Gas Sensor by Aluminum Doping. Talanta 1991, 3, 147-155.



(36) Wang, C.; Cui, X.; Liu, J.; Zhou, X.; Cheng, X.; Sun, P.; Hu, X.; Li, X.; Zheng, J.; Lu, G. Design of Superior Ethanol Gas Sensor Based on Al-Doped NiO Nanorod-Flowers. ACS Sens. 2016, 1, 131-136. (37) Kwak, C.-H.; Kim, T.-H.; Jeong, S.-Y.; Yoon, J.-W.; Kim, J.-S.; Lee, J.-H. Humidity-Independent Oxide Semiconductor Chemiresistors Using Terbium-Doped SnO2 Yolk−Shell Spheres for Real-Time Breath Analysis. ACS Appl. Mater. Inter. 2018, 10, 18886-18894. (38) Arora, K.; Goel, N.; Kumar, M.; Kumar, M. Ultrahigh Performance of Self-Powered β-Ga2O3 Thin Film Solar-Blind Photodetector Grown on Cost-Effective Si Substrate Using High-Temperature Seed Layer. ACS Photonics 2018, 5, 2391-2401. (39) Sun, P.; Zhou, X.; Wang, C.; Wang, B.; Xu, X.; Lu, G. One-Step Synthesis and Gas Sensing Properties of Hierarchical Cd-Doped SnO2 Nanostructures. Sensor. Actuat. B-Chem. 2014, 190, 32-39. (40) Zen, W.; Liu, T. M. Hydrogen Sensing Characteristics and Mechanism of Nanosize TiO2 Dope with Metallic Ions. Phys. B-Condens. Mat. 2010, 405, 1345-1348. (41) Lee, A. P.; Reedy, B. J. Temperature Modulation in Semiconductor Gas Sensing. Sensor. Actuat. B-Chem. 1999, 60, 35-42. (42) Patzke, G. R.; Zhou, Y.; Kontic, R.; Conrad, F. Oxide Nanomaterials: Synthetic Developments, Mechanistic Studies, and Technological Innovations. Angew. Chem. Int. Ed. 2010, 49, 2-36.


Page 36 of 39

Page 37 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Gurlo, A. Nanosensors: Towards Morphological Control of Gas Sensing Activity. SnO2, In2O3, ZnO and WO3 Case Studies. Nanoscale 2011, 3, 154-165. (44) Barsan, N.; Weimar, U. Conduction Model of Metal Oxide Gas Sensors. J. Electroceram. 2001, 7, 143-167. (45) Choi, S.-J.; Chattopadhyay, S.; Kim, J. J.; Kim, S.-J.; Tuller, H. L.; Rutledge, G. C.; Kim, I.-D. Coaxial Electrospinning of WO3 Nanotubes Functionalized with Bio-Inspired Pd Catalysts and Their Superior Hydrogen Sensing Performance. Nanoscale 2016, 8, 9159-9166. (46) Jeong, S.-Y.; Yoon, J.-W.; Kim, T.-H.; Jeong, H.-M.; Lee, C.-S.; Kang, Y. C.; Lee, J.-H. Ultra-Selective Detection of Sub-ppm-Level Benzene Using Pd-SnO2 Yolk-Shell Micro-Reactors with a Catalytic Co3O4 Overlayer for Monitoring Air Quality. J. Mater. Chem. A 2017, 5, 1446-1454. (47) Pradhan, G. K.; Padhi, D. K.; Parida, K. M. Fabrication of Alpha-Fe2O3 Nanorod/RGO Composite: A Novel Hybrid Photocatalyst for Phenol Degradation. ACS Appl. Mater. Inter. 2013, 5, 9101-9110. (48) Pany, S.; Naik, B.; Martha, S.; Parida, K. Plasmon Induced Nano Au Particle Decorated over S,N-Modified TiO2 for Exceptional Photocatalytic Hydrogen Evolution under Visible Light. ACS Appl. Mater. Inter. 2014, 6, 839-846. (49) Chen, H.; Hu, J.; Li, G.-D.; Gao, Q.; Wei, C.; Zou, X. Porous Ga-In Bimetallic Oxide Nanofibers with Controllable Structures for Ultrasensitive and Selective Detection of Formaldehyde. ACS Appl. Mater. Inter. 2017, 9, 4692-4700. (50) Helander, M. G.; Greiner, M. T.; Wang, Z. B.; Lu, Z. H. Pitfalls in Measuring



Work Function Using Photoelectron Spectroscopy. Appl. Surf. Sci. 2010, 256, 2602-2605. (51) Korotcenkov, G. Metal Oxides for Solid-State Gas Sensors: What Determines Our Choice? Mater. Sci. Eng. B 2007, 139, 1-23. (52) Feng, Z.; Feng, Q.; Zhang, J.; Zhang, C.; Zhou, H.; Li, X.; Huang, L.; Xu, L.; Hu, Y.; Zhao, S.; Hao, Y. J. Band Alignments of SiO2 and HfO2 Dielectrics with (AlxGa1-x)2O3 Film (0≦x≦0.53) Grown on Ga2O3 Buffer Layer on Sapphire. Alloy. Comp. 2018, 745, 292-298.


Page 38 of 39

Page 39 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical TOC Entry


Realizing the Control of Electronic Energy Level Structure and Gas

Recommend Documents