Revealing the Relationship Between Energy Level and Gas Sensing

Subscriber access provided by Kaohsiung Medical University

Functional Inorganic Materials and Devices

Revealing the Relationship Between Energy Level and Gas Sensing Performance in Heteroatom-doped Semiconducting Nanostructures Hui Chen, Yanfang Zhao, Lei Shi, Guo-Dong Li, Lei Sun, and Xiaoxin Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10057 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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 25 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

Revealing the Relationship Between Energy Level and Gas Sensing Performance in Heteroatom-doped Semiconducting Nanostructures

Hui Chen, ‡a Yanfang Zhao, ‡a Lei Shi,a Guo-Dong Li,a Lei Sun,b Xiaoxin Zoua,* a

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

University, Changchun 130012, P. R. China b

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Dalian

116023 P. R. China

Hui Chen and Yanfang Zhao contributed equally to this work. *Corresponding author. E-mail address: [email protected]

ACS Paragon Plus Environment

1

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

Page 2 of 25

Abstract

The cation substitutional doping of metal-oxide semiconductors plays pivotal roles in improving the gas sensing performances, but the doping effect on surface sensing reaction is still not well understood. In this study, indium oxides doped with various heteroatoms are investigated to obtain in-depth understanding of how doping (or the resulting change in electronic structure) alters the surface absorbed oxygen chemistry and subsequent sensing process. The experimental results reveal that energy level of In2O3 can be modulated by introduction of these dopants, some of which (e.g., Al, Ga, Zr) lead to the elevation of Fermi level, while others (e.g., Ti, V, Cr, Mo, W, Sn) bring about relative drop in Fermi level. But only the former can improve the response to formaldehyde, indicating a strong link between Fermi level and sensing properties. Mechanistic study suggests that the elevation of fermi level increases energy level difference between oxide semiconductor and oxygen molecules, and facilitates the surface absorption of oxygen species, resulting in superior formaldehyde sensing activity. Especially Al-doped In2O3 exhibit remarkably enhanced sensing performances toward formaldehyde at low working temperature (150 oC) with high response, good selectivity, ultralow limit of detection (60 ppb) and short response time (2-23 s). Our findings not only promote the understanding of sensing reaction process and its correlation with semiconductor electronic structure, but also offer a general guideline for large-scale screening of promising oxide semiconductor-based sensing materials for gas detection.

Keywords: Gas sensor, In2O3, Sensing mechanism, Fermi level, Electronic structure


2

Page 3 of 25 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


Introduction Chemiresistive gas sensors hold great promise for monitoring air pollutants,1-3 specific biomarkers,4 gas leakage5-7 and food quality,8 owing to their advantages of real-time operation, simple structure, portability and ease of use. At the heart of the sensor are sensing materials mainly based on semiconducting metal oxides (MOS), which have the ability to identify target gases/vapors through a change in resistance induced by reversible interactions between target gases/vapor molecules and surface chemisorbed oxygens. Up to date, promising candidates of MOS sensing materials have been developed, including many transition-metal oxides and post-transition metal oxides.9,10 Among these oxides, SnO2,11-13 ZnO14-16 and In2O317-19 are the most prominent sensing materials used for the fabrication of gas sensors, since their high stability and high mobility of conduction electrons. Unfortunately, practical applications of these undoped MOS often suffer from low response and poor selectivity. To enhance their sensing performance, considerable efforts for optimizing surface oxygen properties and gas/vapor transport kinetics have thus been devoted. For oxide semiconductors, their physical and chemical properties, including adsorbed oxygen properties, are closely linked with their intrinsic electronic structure. Elemental doping is effective way to modulate the electronic structure of semiconductors, and thus influences their versatile functional properties.20-22 In this regard, establishing the relationship between elemental doping and semiconductor surface adsorbed oxygen, and thereby optimizing gas sensing performance from electronic structure is a promising option to explore high-performance sensing materials. Up to now, although some dopedsensing materials with improved sensing performances have been reported,23-29 the role of doping in affecting gas sensing reaction still remains controversial, owing to the complexity of the sensing mechanism and many parameters determining the surface reaction.30 Indeed, most experimental studies on single-phase MOS are currently focused on morphology (or porosity) of sensing materials due to the importance of gas molecule transport in sensing process.31-34 The sensing performances of MOS by various chemical doping are unexpected theoretically because of the lack of physical understanding of ACS Paragon Plus Environment

3


Page 4 of 25

how chemical doping influences the surface oxygen absorption and sensing reaction. The confusing issue has restrained the development of design principles for semiconductor sensing materials. In this work, we systematically study the doping effects by substituting the In atoms in indium oxide with different metals (denoted as M-doped In2O3, M = Al, Ti, Zr, V, Cr, Mo, W, Sn and Ga) via a facile electrospinning method. We found that the electronic structure In2O3 of can be tailored via these heteroatoms doping, resulting in significantly different sensing performances to formaldehyde. In particular, Al, Ga and Zr substitutions lead to both the elevation of fermi level and enhanced sensing activity to formaldehyde, while introduction of V, Cr, Mo, W, Sn and Ti dopants lower the fermi level and cause obvious inhibition to formaldehyde response. Furthermore, we demonstrate that the enhanced sensing performances by doping the former heteroatoms (especially Al) originate from the elevation of fermi level, which enlarges the energy level difference between oxide semiconductor and oxygen molecules, and brings more absorbed oxygen on semiconductor surface, promoting subsequent gas sensing reaction. As a consequence of optimal doping, Al-doped In2O3 exhibits superior response, rapid response/recovery speed and good stability for formaldehyde detection. Our work reveals the fundamental relationship between electronic structure and sensing property, and renders fermi level a suitable descriptor for fast screening of efficient sensing materials.

Experimental Section Material synthesis. Al-doped In2O3 nanofibers (denoted as AlxIn2-xO3, 0 < x ≤ 0.2), with different amounts of Al doping, were synthesized via a facile electrospinning method followed by calcination process. Typically, for the synthesis of Al0.15In1.85O3, Al(NO3)3 (0.046 mmol), In(NO3)3·(0.570 mmol) and PVP (0.8 g) were added to a mixed solution of ethanol (6.6 g) and DMF (2.2 g). The mixture was continuously stirred at room temperature for 12 h to form a homogeneous electrospun solution. Then, the electrospun solution was loaded into an injection syringe (5 mL) with needle, and a piece of aluminum foil (15 cm × 20 cm) was used as a collector. Electrospinning was initiated by applying a voltage of 22 kV between the needle tip and the collector with a distance of around 20 cm. After ACS Paragon Plus Environment

4

Page 5 of 25 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


electrospinning, the resulting product (Al- and In-embedded PVP nanofibers) was collected and fully dried at 70 °C. Finally, the product was annealed at 600 °C for 3 h in air, giving the Al0.15In1.85O3 material. For comparison, various heteroatoms (M = Ti, Zr, V, Cr, Mo, W, Sn and Ga) doped In2O3 nanofibers were also prepared. Their synthetic procedures were similar to that of Al0.15In1.85O3 material, except that we used Ti(OC4H9)4 (0.046 mmol), Zr(NO3)4 (0.046 mmol), NH4VO3 (0.046 mmol), Cr(NO3)3 (0.046 mmol), (NH4)6Mo7O24 (0.0066 mmol), (NH4)10H2(W2O7)6 (0.0038 mmol), SnCl4 (0.046 mmol) or Ga(NO3)3 (0.046 mmol) to replace Al(NO3)3 (0.046 mmol) in the electrospun solution. Gas Sensing Measurement. According to our previous work,35 the chemiresistive gas sensors (side-heated type) were fabricated by applying the similar method and procedure. The obtained materials were mixed with ethanol and ground to form viscous slurry. The slurry was coated on a designed ceramic tube equipped with a pair of Au electrodes and four Pt wires on both ends of the tube. An Ni-Cr alloy coil was inserted into the tube as a heater to tune operating temperature. The obtained sensors were kept at 200 ◦C for 24 hours before gas sensing measurement to improve stability. A commercial CGS-8 gas sensing measurement system (Beijing Elite Tech Co. Ltd., China) was applied for testing the sensing performances upon exposure to different tested gases. The tested gas was prepared by a stationary state gas distribution method, in which a given amount of target gases/vapours were injected into test chamber and diluted with fresh air (relative humidity of ∼20%). For measurement, the sensor was placed into the test chamber until resistance reached a constant value. Then the sensor was taken out from the test chamber, and resistance gradually recovers to original value in air. The sensor response was defined as S = Ra/Rg, where Ra and Rg were the sensor resistance in air and in tested gas, respectively.


5


Page 6 of 25

Results and Discussion

Figure 1. (A) XRD patterns, (B) partially enlarged XRD patterns between 28° and 32°, and (C) lattice constant a of the AlxIn2-xO3 (x = 0, 0.05, 0.1, 0.15, 0.2). Taking In2O3, one of the most promising candidates, as a model sensing material, our work herein presents that the sensing performances of In2O3 can be significantly affected by various heteroatoms doping. Especially Al doping was found to be the optimal promoter for enhancing formaldehyde sensing. Therefore, the crystal structure, morphology, energy structure and sensing property were investigated for the Al-doped In2O3 nanofibers and compared with undoped In2O3 nanofibers. Initially, the precursors (Al- and In-embedded PVP nanofibers, Figure S1, Supporting Information) were prepared by electrospinning ethanol and DMF mixed solution containing Al(NO3)3, In(NO3)3 and PVP. After thermal treatment at 600 oC in air, a series of Al-doped In2O3 nanofibers (dubbed AlxIn2-xO3) were obtained. XRD patterns (Figure 1A) show that the diffraction peaks of AlxIn2-xO3 (x = 0, 0.05, 0.1, 0.15, 0.2) samples are in good agreement with the single phase of cubic In2O3 (PDF No. 06-0416). With the Al content increased in AlxIn2-xO3 samples, the XRD peak positions continuously shift toward large angle sides (Figure 1B) and the lattice constants (a) gradually decrease (Figure 1C). These results suggest that Al3+ ions have successfully substituted In3+ ions in the cubic In2O3 lattice and decrease the lattice distances, because the ionic radius of Al3+ (0.54 Å) is smaller than that of the In3+ (0.80 Å). ACS Paragon Plus Environment

6

Page 7 of 25 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 2. (A) SEM image, (B) SEM elemental mapping images, (C) TEM image, (D) HRTEM image, and (E) size-distribution diagram of Al0.15In1.85O3. (F) Nitrogen adsorption-desorption isotherms of In2O3 and Al0.15In1.85O3; inset is the corresponding pore size distribution curves. SEM images (Figure 2A; Figure S2 in Supporting Information) reveal that the morphologies of AlxIn2-xO3 (x = 0, 0.05, 0.1, 0.15, 0.2) are homogeneous nanofibers with diameter of 50-150 nm and length up to several micrometers. Taking Al0.15In1.85O3 as a typical sample, we further conducted SEM elemental mapping and TEM to characterize the sample structure. SEM elemental mapping (Figure 2B) of Al0.15In1.85O3 shows the homogeneous distribution of Al, In and O elements throughout the entire nanofibers. TEM image (Figure 2C) identifies that Al0.15In1.85O3 nanofibers are porous structure assembled by small nanoparticles. And in the Al0.15In1.85O3 nanoparticle, In2O3 (222) crystal plane with lattice spacing of 0.292 nm is clearly observed by high-resolution TEM (Figure 2D). The nanoparticle in Al0.15In1.85O3 nanofibers are well distributed with an average diameter of 16.6 nm (Figure 2E). Compared with undoped In2O3 nanofibers (Figure S3, Supporting Information), Al0.15In1.85O3 nanofibers are assembled by smaller nanoparticles because Al ion doping prevents the growth of nanocrystals. ACS Paragon Plus Environment

7


Page 8 of 25

Furthermore, the nitrogen adsorption-desorption isotherms of undoped In2O3 and Al0.15In1.85O3 (Figure 2F) are of typical type IV, suggesting the existence of porous structures. The corresponding BJH pore size distribution curves (Figure 2F inset) reveal that pore sizes in the two samples are mainly below 30 nm. The BET surface area of Al0.15In1.85O3 and undoped In2O3 are 33 m2 g-1 and 14 m2 g-1, respectively. The larger BET surface areas of Al0.15In1.85O3 can be attributed to the formation of smaller aggregated nanoparticles.

Figure 3. (A) The responses of sensors based on AlxIn2-xO3 (x = 0, 0.05, 0.1, 0.15, 0.2) vs the operating temperatures to 100 ppm of formaldehyde. (B) Responses of sensors based on In2O3 and Al0.15In1.85O3 to 100 ppm of formaldehyde and interfering gases at 150 oC. (C) Dynamic response-recovery curve of Al0.15In1.85O3 based sensor to different concentrations of formaldehyde at 150 oC, and (D) the corresponding linear relationship between response and formaldehyde concentration. The error bars represent standard deviations based on three independently fabricated sensors. Formaldehyde is a volatile pollutant in indoor environments, with highly toxic and carcinogenic properties.36, 37 Therefore, early detection of formaldehyde is highly desired to ensure human health. ACS Paragon Plus Environment

8

Page 9 of 25 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


Herein, we fabricated gas sensors based on AlxIn2-xO3 (x = 0, 0.05, 0.1, 0.15, 0.2) samples to investigate their response toward formaldehyde. To identify the optimized sensing materials and their optimal operating temperature for best formaldehyde response, the AlxIn2-xO3 based sensors were tested in 100 ppm of formaldehyde at different temperatures (Figure 3A). The formaldehyde responses of all sensors present Volcano-shaped curves at the temperature range, suggesting that the optimal operating temperature exists. Before the optimal temperature is reached, surface reaction between adsorbed oxygen and formaldehyde are insufficiently activated, causing the increase of response with temperature. When operating temperature is higher than the optimum, the adsorption ability of gas molecules becomes weaker, and adsorbed formaldehyde molecules are easily desorbed before involving surface reaction, thus response decreases with temperature.38 The undoped In2O3 based sensor presents low responses (Ra/Rg < 15.0 to 100 ppm of formaldehyde) at the temperature range of 135-250 oC. With the increase of Al doping content (0 < x ≤ 0.15) in In2O3, the formaldehyde responses gradually enhance. It is worthy of noted that, Al0.15In1.85O3 based sensor reaches the maximum (Ra/Rg = 60.3±4.9 to 100 ppm of formaldehyde) at the optimal operating temperature of 150 oC. When more amount of Al was doped into the In2O3 crystal (for the Al0.2In1.8O3 sample), the response sharply declines in the low-temperature region (T ≤ 200 oC) but slightly increases in the hightemperature region (T > 200 oC). This is probably due to the formation of tiny amounts of Al2O3 clusters as a result of excess Al doping, just like the reported for Al-doped ZnO in the literature.39 It can be confirmed by Al 2p XPS spectrum (Figure S4 in Supporting Information), in which the peak with the binding energy at 74.6 eV is assigned to Al-O bond in Al2O3 structure.40 The Al2O3 composition (with wide band gap of 8.8 eV) 41 is nonconducting at relatively low temperature (T ≤ 200 oC), and it may occupy some sensing sites as inactive phase and hinders carrier transport, producing an adverse effect on sensing activity. However, the Al2O3 composition can be thermally activated at relatively high temperature (T > 200 oC), and combines with Al-doped In2O3 forming n-n heterostructure, which is


9


Page 10 of 25

believed to enhance response. Therefore, Al0.2In1.8O3 exhibits higher response than pristine or less Aldoped In2O3 samples at higher temperature. On basis of the results above, we determine that Al0.15In1.85O3 sample represents the optimized sensing material and 150 oC is its optimal operating temperature. In fact, the formaldehyde response of Al0.15In1.85O3 based sensor is much higher than that of many previously reported In2O3-based sensors (see Table S1 in Supporting Information). Subsequently, the responses of sensors based on In2O3 and Al0.15In1.85O3 to 100 ppm of various gases/vapors (formaldehyde and potential interfering gases/vapors) were tested at 150 oC (Figure 3B). These interfering gases/vapors include ethanol, acetone, xylene, toluene, benzene, H2, CO and CH4. Compared with In2O3 based sensor, the Al0.15In1.85O3 based sensor displays enhanced responses to each gas/vapor. The Al0.15In1.85O3 based sensor shows the highest response to formaldehyde, obviously lower responses to ethanol, acetone and xylene, and almost insensitive to toluene, benzene, H2, CO and CH4 at the same test condition. The selectivity comparisons between Al0.15In1.85O3 based sensor and In2O3 based sensor imply that the former possesses an improved selectivity toward formaldehyde against other interfering gases/vapors. The dynamic response-recovery curves of Al0.15In1.85O3 based sensor and In2O3 based sensor toward different concentrations of formaldehyde at 150 oC are shown in Figure 3C and Figure S5 (Supporting Information). The two sensors exhibit good response-recovery kinetics in wide formaldehyde concentration ranges (0.06-100 ppm for Al0.15In1.85O3 and 0.2-100 ppm for In2O3, respectively). With the increase of formaldehyde concentration, the responses of Al0.15In1.85O3 based sensor increase rapidly, presenting much higher responses and lower limit of detection (60 ppb) compared with that of In2O3-based sensor. Figure 3D shows the linear relationship between responses and formaldehyde concentrations of Al0.15In1.85O3 based sensor. The sensor responses exhibit a sharply increasing rate (β = 1.1100) at low formaldehyde concentrations of 60 ppb-20 ppm, while the increasing rate (β = 0.4655) slows down at the concentration range of 20-100 ppm. The phenomenon is attributed to the limited sites used for gas molecular adsorption and reaction on semiconductor surface. ACS Paragon Plus Environment

10

Page 11 of 25 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 4. Gas-sensing properties of sensors at 150 °C. (A) Dynamic response-recovery of Al0.15In1.85O3based sensor to 1 ppm of formaldehyde. (B) Response times and recovery times of Al0.15In1.85O3 and In2O3-based sensors to formaldehyde in concentration range of 1-100 ppm. (C) 10th cycles of dynamic response-recovery curves, and (D) long-term stability tests over 30 days of Al0.15In1.85O3-based sensor to 100 ppm of formaldehyde. The response and recovery time of gas sensors are very important for their practical applications. In this work, the response and recovery time are defined as the time required to reach 90% of the total changes in resistance in the case of response and recovery processes, respectively. As shown in Figure 4A, Al0.15In1.85O3 based sensor shows fast response time (23 s) and acceptable recovery time (103 s) toward 1 ppm formaldehyde at 150 oC. Furthermore, the response/recovery times of Al0.15In1.85O3 and In2O3-based sensors to formaldehyde in the concentration range of 1 to 100 ppm is given in Figure 4B. For both the two sensors, response times drastically decrease along with the increase of formaldehyde concentrations, while recovery times gradually increase. The observation of opposite correlations between response time and recovery time with increasing gas concentration has been reported ACS Paragon Plus Environment

11


Page 12 of 25

previously.42-43 Il-Doo Kim and co-workers have recently proposed a formula to explain the concentration-dependent response time based on a non-linear diffusion reaction model.44 According to the study, response time is determined by diffusion time (τ), which can be represented as τ = kx02/(DC01r

), where k, x0 and D represent the reaction rate constant, film thickness and diffusion coefficient,

respectively. C0 denotes gas concentration, and r is a constant between 0.3 and 1.0. Thus, higher concentration suggests the faster diffusion kinetics of gas molecules in the test chamber, resulting in shorter response time. But then, the higher concentration also renders sensing materials weaker ability to desorb the large quantity of gas species, leading to longer recovery time. In addition, the response/recovery time comparison between the two sensors illustrates that Al0.15In1.85O3-based sensors have faster response time but slower recovery time than In2O3-based sensors. The faster response time can be explained by that the more porous structure of Al0.15In1.85O3 facilitates effective and rapid diffusion of gas molecules,45 while the slower recovery time may be linked to the increased surface sensing sites which prolongs the desorption of gas species, re-adsorption and ionization of oxygen. The stability of Al0.15In1.85O3 based sensor was also carefully investigated. After 10 cycles of dynamic response-recovery tests toward 100 ppm formaldehyde (Figure 4C), the sensor response remains unchanged. Moreover, no apparent decrease trend in response is observed in a testing period of one month for detecting 100 ppm formaldehyde (Figure 4D). These results demonstrate the good repeatability and long-term stability of Al0.15In1.85O3 based sensor in practical application. Compared with undoped In2O3, the Al0.15In1.85O3 nanofibers are assembled by smaller nanoparticles and have larger BET surface areas. It is certain that the nanostructure of Al0.15In1.85O3 can bring about numerous active sites for surface sensing reaction, and partially accounts for superior sensing performances. In addition, it is worthy of noted that, the formaldehyde response of Al0.15In1.85O3-based sensor is about 5 times higher than that of In2O3-based sensors, while the BET surface area of Al0.15In1.85O3 is 2.4 times higher than that of In2O3. These results suggest that the increased surface area may be not the only reason behind the superior sensing performance exhibited by Al0.15In1.85O3. Thus,


12

Page 13 of 25 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


further theoretical and experimental studies were carried out to uncover how Al incorporation affected the electronic structure of cubic-In2O3 and helped to enhance formaldehyde sensing performances.

Figure 5. (A, C) DFT band structure calculations and (B, D) corresponding structural models of In2O3 and Al-doped In2O3 (In purple, Al yellow, O red). Note that the band gap values determined by DFT calculations are always less than those determined by experiments.46 (E) UV-visible diffuse reflectance spectra from 200 nm to 550 nm, and (F) base resistances in air at different operating temperatures of the AlxIn2-xO3 (x = 0, 0.05, 0.1, 0.15, 0.2) based sensors. Firstly, density functional theory (DFT) calculations were implemented (see theoretical details and Figure S6, Supporting Information) to reveal the variation in energy band structure (including band gap and Fermi energy) upon the substitution of In atom in the In2O3 structure by Al dopant atom. As shown in Figure 5A-D, the calculated band gap of the Al-doped In2O3 is about 0.99 eV, which is slightly larger than that of undoped In2O3 (0.94 eV), meaning an increased band gap upon the introduction of Al ion. This result was verified by the UV-visible diffuse reflectance spectra (Figure 5E), which exhibit continuous blue-shifts of absorption edges with the increasing of Al dopant contents, suggesting gradually widening band gap of AlxIn2-xO3. The band-gap tuning can strongly influence the resistivity of oxide semiconductors. Therefore, base resistances in air (Ra) of sensors based on AlxIn2-xO3 samples ACS Paragon Plus Environment

13


Page 14 of 25

increase progressively with Al content (Figure 5F), in agreement with theoretical calculation results shown above. Simultaneously, the effect of Al doping on the Fermi energy (EF) level was studied. Our calculated results show that compared with that of undoped In2O3, the calculated Fermi energy of Al-doped In2O3 increases from 5.17 eV to 5.24 eV. The elevation of fermi level by Al doping was also experimentally confirmed by employing scanning Kelvin probe method to evaluate the work functions (ϕ, Fermi energy relative to vacuum energy47) of undoped In2O3 and Al-doped In2O3. The measured work function of In2O3 is 4.99 eV (consistent with the previously reported value48), which is larger than that of Al0.15In1.85O3 (4.85 eV), demonstrating the higher Fermi level in Al-doped In2O3 sample. According to the well-known sensing mechanism49,50 and classical theory of semiconductor physics,51,52 a reasonable model was proposed to illustrate the effect of Fermi level change on the chemisorbed oxygen property and surface sensing reaction of oxide semiconductor. As shown in Figure 6A-C, the sensing reaction of In2O3 toward formaldehyde involves three processes: (1) In air, oxygen molecules adsorb on In2O3 surface and capture free electrons from conduction band (O2 (ad) + e− ↔ O2−, in the form of O2− at 150 °C),53 followed by upward bend of energy band. (2) Electronic equilibrium between In2O3 and adsorbed oxygen is established. In this situation, the LUMO level of adsorbed oxygen maintains same energy with the Fermi level of In2O3. (3) When exposed to formaldehyde, chemisorbed oxygen reacts with formaldehyde (HCHO (ad)+ O2− (ad) ↔ CO2 + H2O + e−) and returns electrons to conduction band, leading to diminishment of band bending. For Al-doped In2O3 with higher Fermi level (Figure 6D-F), its energy level difference between semiconductor and adsorbed oxygen is greater than undoped In2O3, endowing more chemisorbed oxygen on Al-doped In2O3 surface in air at equilibrium. Accordingly, when formaldehyde sensing occurs, the Al-doped In2O3 possesses more chemisorbed oxygen to react with formaldehyde and exhibits higher response compared with undoped In2O3.


14

Page 15 of 25 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


To demonstrate that the introduction of Al ions brings the increased chemisorbed oxygen, O 1s XPS analysis of AlxIn2-xO3 (x = 0, 0.05, 0.1, 0.15, 0.2) were carried out (Figure 6G-H and Figure S7A-C, Supporting Information). The XPS peaks of O 1s can be deconvoluted into three peaks including lattice oxygen (OL), Oxygen vacancy (OV) and absorbed oxygen species (OC). The results displayed that the contents of absorbed oxygen species were gradually increased with Al doping level until a maximum content was obtained at x= 0.15 (for the Al0.15In1.85O3 sample). The trend is in agreement with their corresponding sensor responses, confirming that the increase in absorbed oxygen species is crucial factor responsible for the greatly improved sensing performance of Al-doped In2O3. Furthermore, compared with undoped In2O3-based sensor, the enhanced selectivity of the Al0.15In1.85O3-based sensor toward formaldehyde is interpreted as follows. Our recent study has identified that selectivity of sensing material is strongly linked to the energy level of surface adsorbed oxygen which indicate the oxidizing ability to different reducing gases.35 As to Al0.15In1.85O3 material with higher Fermi level, its surface adsorbed oxygen maintains a lower oxidizing ability, and thereby can selectively oxidize strong reducing gas (e.g. formaldehyde) against others with relatively weak reducibility (e.g. methanol, acetone), leading to a better selectivity. In the light of the substantial improvement of formaldehyde sensing of In2O3 via Al atoms doping, the formaldehyde sensing performances of In2O3 doped with various heteroatoms (M = Ti, Zr, V, Cr, Mo, W, Sn and Ga) were also investigated. These dopant atoms have appropriate cation sizes for In atom substitution in cubic-In2O3 (Table S2, Supporting Information). XRD patterns of various M-doped In2O3 (Figure S8A, Supporting Information) all show the pure In2O3 phase, without any detectable impurity peaks. And diffraction peaks of these M-doped In2O3 materials (Figure S8B, Supporting Information) shift toward large angle sides with different degrees, indicating the successful doping of these dopant atoms in In2O3. The sensing performances of these M-doped In2O3 materials to formaldehyde are exhibited in Figure 7A. Compared with In2O3 (Ra/Rg = 11.2 at 200 oC), Ti, W, Sn, Cr, Mo and V doping decrease the response to 100 ppm formaldehyde. Their formaldehyde responses are not closely related to the operating temperature, and the highest responses in the temperature ranges are ACS Paragon Plus Environment

15


Page 16 of 25

quite low (below 10). On the contrary, Al, Ga and Zr-doped In2O3 presents enhanced responses to 100 ppm formaldehyde at their optimal operating temperature of 150 oC. These results reveal that the sensing performance of In2O3 can be regulated easily by variation of the dopant.

Figure 6. Schematic energy diagram of (A-C) In2O3 and (D-F) Al-doped In2O3: (A, D) before and (B, E) after equilibrium chemisorption of oxygen species in air; and (C, F) after exposure to formaldehyde. The dark circles and open circles represent free electrons and holes. In addition, ϕ, χo, d, CB, VB, Evac, and EF represent work function, electron affinity of O2 molecule, thickness of charge depletion layer, conduction band, valence band, vacuum energy, and Fermi energy, respectively. (G-H) O 1s XPS spectra of In2O3 and Al0.15In1.85O3. The change of crystallite size, meso-porosity, BET surface area, and band gap of In2O3 nanofibers doped with various heteroatoms (M = Ti, Zr, V, Cr, Mo, W, Sn and Ga) were also investigated (Table S3, Supporting Information). The results display that nanostructure and pore structure of the 8 samples are not significantly different from each other. In addition, no consistent correlation of sensing ACS Paragon Plus Environment

16

Page 17 of 25 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


performance exclusively with band-gap tuning is evident across the 8 samples. Therefore, it could be reasonably speculated that the modulation of fermi level is responsible for the above presented sensing properties. Scanning Kelvin probe method was further used to determine the fermi energy levels (reflected by their work functions) of these M-doped In2O3 materials (Figure 7B). As observed, Fermi level of M-doped In2O3 materials varies significantly with different doped metals. Specifically, Al, Ga and Zr doping significantly elevates the Fermi level of In2O3, while the Fermi level of Ti, W, Sn, Cr, Mo and V-doped In2O3 shifts negatively with different degrees. These results agree with the above proposed mechanism that the elevation in Fermi level of doped In2O3 material is beneficial for promoting oxygen molecule adsorption, enhancing response to formaldehyde.

Figure 7. (A) The responses of sensors based on M-doped In2O3 (M = Al, Ti, Zr, V, Cr, Mo, W, Sn, Ga) and undoped In2O3 vs the operating temperatures to 100 ppm of formaldehyde. The error bars represent standard deviations based on three independently fabricated sensors. (B) Fermi levels of above ten sensing materials. ACS Paragon Plus Environment

17


Page 18 of 25

Conclusions To conclude, we present that energy level engineering of oxide semiconductors is an alternative means for enhancing the gas sensing performances. Our results confirm that different heteroatom substitutions can modulate the fermi level of In2O3 and significantly influence the formaldehyde response. Especially Al doping leads to a higher Fermi level than other In2O3-based sensing materials, represents the most effective promoter among them. The Al-doped In2O3 presents much high response (Ra/Rg = 60.3 ± 4.9) toward 100 ppm of formaldehyde at low operating temperature (150 oC) with fast response speed and good selectivity. The great improvement benefits from the doping effect, which guarantees higher fermi level, enlarges the energy level difference between oxide semiconductor and oxygen molecules, and brings more absorbed oxygen species on the Al-doped In2O3 nanofibers, leading to superior sensing performance to formaldehyde. These results offer new insights on the design of doped semiconductor oxides for efficient gas detection from the angle of electron structure. Acknowledgements. X. Zou acknowledges financial support from the National Key R&D Program of China (Grant No. 2017YFA0207800), the National Natural Science Foundation of China (NSFC; 21771079), Jilin Province Science and Technology Development Plan (20170101141JC), Young Elite Scientist Sponsorship Program by CAST, Program for JLU Science and Technology Innovative Research Team (JLUSTIRT) and Fok Ying Tung Education Foundation (Grant No. 161011). H. Chen acknowledges financial support from Postdoctoral Innovative Talent Support Program (Grant No. BX20180120). Supporting Information Available. Additional experimental and theoretical section. Materials characterization results, including TG curve, SEM images, TEM images, XRD patterns, XPS Spectra, nanoparticle size, pore structure and band gap. Sensing property results, including dynamic responserecovery characteristic curve, relationship between concentration and formaldehyde response. This material is available free of charge via the Internet at http://pubs.acs.org. ACS Paragon Plus Environment

18

Page 19 of 25 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)

Zhang, J.; Tang, P.; Liu, T.; Feng, Y.; Blackman, C.; Li, D. Facile Synthesis of Mesoporous

Hierarchical Co3O4-TiO2 p-n Heterojunctions with Greatly Enhanced Gas Sensing Performance. J. Mater. Chem. A 2017, 5, 10387-10397. (2)

Wu, J.; Tao, K.; Guo, Y.; Li, Z.; Wang, X.; Luo, Z.; Feng, S.; Du, C.; Chen, D.; Miao, J.;

Norford Leslie, K. A 3D Chemically Modified Graphene Hydrogel for Fast, Highly Sensitive, and Selective Gas Sensor. Adv. Sci. 2016, 4, 1600319. (3)

Hao, J.; Zhang, D.; Sun, Q.; Zheng, S.; Sun, J.; Wang, Y. Hierarchical SnS2/SnO2

Nanoheterojunctions with Increased Active-Sites and Charge Transfer for Ultrasensitive NO2 Detection. Nanoscale 2018, 10, 7210-7217. (4)

Kim, S.-J.; Choi, S.-J.; Jang, J.-S.; Cho, H.-J.; Kim, I.-D. Innovative Nanosensor for Disease

Diagnosis. Acc. Chem. Res. 2017, 50, 1587-1596. (5)

Ma, J.; Ren, Y.; Zhou, X.; Liu, L.; Zhu, Y.; Cheng, X.; Xu, P.; Li, X.; Deng, Y.; Zhao, D. Pt

Nanoparticles Sensitized Ordered Mesoporous WO3 Semiconductor: Gas Sensing Performance and Mechanism Study. Adv. Funct. Mater. 2017, 28, 1705268. (6)

Wu, H.; Chen, Z.; Zhang, J.; Wu, F.; He, C.; Ren, Z.; Wu, Y. Manipulating Polyaniline Fibrous

Networks by Doping Tetra-β-carboxyphthalocyanine Cobalt(II) for Remarkably Enhanced Ammonia Sensing. Chem. Mater. 2017, 29, 9509-9517. (7)

Yao, M. S.; Lv, X. J.; Fu, Z. H.; Li, W. H.; Deng, W. H.; Wu, G. D.; Xu, G. Layer-by-Layer

Assembled Conductive Metal-Organic Framework Nanofilms for Room-Temperature Chemiresistive Sensing. Angew. Chem. Int. Ed. 2017, 56, 16510-16514. (8)

Zhu, Y.; Zhao, Y.; Ma, J.; Cheng, X.; Xie, J.; Xu, P.; Liu, H.; Liu, H.; Zhang, H.; Wu, M.;

Elzatahry, A. A.; Alghamdi, A.; Deng, Y.; Zhao, D. Mesoporous Tungsten Oxides with Crystalline Framework for Highly Sensitive and Selective Detection of Foodborne Pathogens. J. Am. Chem. Soc. 2017, 139, 10365-10373. ACS Paragon Plus Environment

19


(9)

Page 20 of 25

Zhou, X.; Lee, S.; Xu, Z.; Yoon, J. Recent Progress on the Development of Chemosensors for

Gases. Chem. Rev. 2015, 115, 7944-8000. (10)

Gurlo, A. Nanosensors: towards Morphological Control of Gas Sensing Activity. SnO2, In2O3,

ZnO and WO3 Case Studies. Nanoscale 2011, 3, 154-165. (11)

Degler, D.; Muller, S. A.; Doronkin, D. E.; Wang, D.; Grunwaldt, J.-D.; Weimar, U.; Barsan, N.

Platinum Loaded Tin Dioxide: a Model System for Unravelling the Interplay between Heterogeneous Catalysis and Gas Sensing. J. Mater. Chem. A 2018, 6, 2034-2046. (12)

Jang, J.-S.; Koo, W.-T.; Choi, S.-J.; Kim, I.-D. Metal Organic Framework-Templated

Chemiresistor: Sensing Type Transition from P-to-N Using Hollow Metal Oxide Polyhedron via Galvanic Replacement. J. Am. Chem. Soc. 2017, 139, 11868-11876. (13)

Jang, J. S.; Yu, S.; Choi, S. J.; Kim, S. J.; Koo, W. T.; Kim, I. D. Metal Chelation Assisted In

Situ Migration and Functionalization of Catalysts on Peapod-Like Hollow SnO2 toward a Superior Chemical Sensor. Small 2016, 12, 5989-5997. (14)

Yao, M. S.; Tang, W. X.; Wang, G. E.; Nath, B.; Xu, G. MOF Thin Film-Coated Metal Oxide

Nanowire Array: Significantly Improved Chemiresistor Sensor Performance. Adv. Mater. 2016, 28, 5229-5234. (15)

Woo, H.-S.; Kwak, C.-H.; Kim, I.-D.; Lee, J.-H. Selective, Sensitive, and Reversible Detection

of H2S Using Mo-Doped ZnO Nanowire Network Sensors. J. Mater. Chem. A 2014, 2, 6412-6418. (16)

Bai, S.; Guo, T.; Zhao, Y.; Luo, R.; Li, D.; Chen, A.; Liu, C. C. Mechanism Enhancing Gas

Sensing and First-Principle Calculations of Al-Doped ZnO Nanostructures. J. Mater. Chem. A 2013, 1, 11335-11342. (17)

Rai, P.; Yoon, J.-W.; Kwak, C.-H.; Lee, J.-H. Role of Pd Nanoparticles in Gas Sensing

Behaviour of Pd@In2O3 Yolk-Shell Nanoreactors. J. Mater. Chem. A 2016, 4, 264-269. (18)

Wang, S.; Xiao, B.; Yang, T.; Wang, P.; Xiao, C.; Li, Z.; Zhao, R.; Zhang, M. Enhanced HCHO

Gas Sensing Properties by Ag-Loaded Sunflower-Like In2O3 Hierarchical Nanostructures. J. Mater. Chem. A 2014, 2, 6598-6604. ACS Paragon Plus Environment

20

Page 21 of 25 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

(19)


Liu, W.; Xu, L.; Sheng, K.; Zhou, X.; Dong, B.; Lu, G.; Song, H. A Highly Sensitive and

Moisture-Resistant Gas Sensor for Diabetes Diagnosis with Pt@In2O3 Nanowires and a Molecular Sieve for Protection. NPG Asia Mater. 2018, 10, 293-308. (20)

Shi, Y.; Zhou, Y.; Yang, D.-R.; Xu, W.-X.; Wang, C.; Wang, F.-B.; Xu, J.-J.; Xia, X.-H.; Chen,

H.-Y. Energy Level Engineering of MoS2 by Transition-Metal Doping for Accelerating Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, 15479-15485. (21)

Lei, F.; Zhang, L.; Sun, Y.; Liang, L.; Liu, K.; Xu, J.; Zhang, Q.; Pan, B.; Luo, Y.; Xie, Y.

Atomic-Layer-Confined Doping for Atomic-Level Insights into Visible-Light Water Splitting. Angew. Chem. Int. Ed. 2015, 54, 9266-9270. (22)

Chen, H.; Yu, G.; Li, G. D.; Xie, T.; Sun, Y.; Liu, J.; Li, H.; Huang, X.; Wang, D.; Asefa, T.;

Chen, W.; Zou, X. Unique Electronic Structure in a Porous Ga-In Bimetallic Oxide Nano-Photocatalyst with Atomically Thin Pore Walls. Angew. Chem. Int. Ed. 2016, 55, 11442-11446. (23)

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. 2015, 1, 131-136. (24)

Hagedorn, K.; Li, W.; Liang, Q.; Dilger, S.; Noebels, M.; Wagner, M. R.; Reparaz, J. S.;

Dollinger, A.; Schmedt auf der Günne, J.; Dekorsy, T.; Schmidt-Mende, L.; Polarz, S. Catalytically Doped Semiconductors for Chemical Gas Sensing: Aerogel-Like Aluminum-Containing Zinc Oxide Materials Prepared in the Gas Phase. Adv. Funct. Mater. 2016, 26, 3424-3437. (25)

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. Sens. Actuators, B 2010, 147, 554560. (26)

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. Interfaces 2017, 9, 4692-4700.


21


(27)

Page 22 of 25

Phan, D.-T.; Chung, G.-S. Effects of Defects in Ga-Doped ZnO Nanorods Formed by a

Hydrothermal Method on CO Sensing Properties. Sens. Actuators, B 2013, 187, 191-197. (28)

Wang, Z.; Tian, Z.; Han, D.; Gu, F. Highly Sensitive and Selective Ethanol Sensor Fabricated

with In-Doped 3DOM ZnO. ACS Appl. Mater. Interfaces 2016, 8, 5466-5474. (29)

Inyawilert, K.; Wisitsoraat, A.; Sriprachaubwong, C.; Tuantranont, A.; Phanichphant, S.;

Liewhiran, C. Rapid Ethanol Sensor Based on Electrolytically-Exfoliated Graphene-Loaded FlameMade In-Doped SnO2 Composite Film. Sens. Actuators, B 2015, 209, 40-55. (30)

Govardhan, K.; Grace, A. N. Metal/Metal Oxide Doped Semiconductor Based Metal Oxide Gas

Sensors-A Review. Sens. Lett. 2016, 14, 741-750. (31)

Yoon, J.-W.; Choi, S. H.; Kim, J.-S.; Jang, H. W.; Kang, Y. C.; Lee, J.-H., 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. (32)

Wang, Z.; Zhu, Y.; Luo, W.; Ren, Y.; Cheng, X.; Xu, P.; Li, X.; Deng, Y.; Zhao, D. Controlled

Synthesis of Ordered Mesoporous Carbon-Cobalt Oxide Nanocomposites with Large Mesopores and Graphitic Walls. Chem. Mater. 2016, 28, 7773-7780. (33)

Wagner, T.; Haffer, S.; Weinberger, C.; Klaus, D.; Tiemann, M. Mesoporous Materials as Gas

Sensors. Chem. Soc. Rev. 2013, 42, 4036-4053. (34)

Li, Y.; Luo, W.; Qin, N.; Dong, J.; Wei, J.; Li, W.; Feng, S.; Chen, J.; Xu, J.; Elzatahry Ahmed,

A.; Es-Saheb Mahir, H.; Deng, Y.; Zhao, D. Highly Ordered Mesoporous Tungsten Oxides with a Large Pore Size and Crystalline Framework for H2S Sensing. Angew. Chem. Int. Ed. 2014, 53, 9035-9040. (35)

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. (36)

Salthammer, T. Formaldehyde in the Ambient Atmosphere: from an Indoor Pollutant to an

Outdoor Pollutant? Angew. Chem. Int. Ed. 2013, 52, 3320-3327. ACS Paragon Plus Environment

22

Page 23 of 25 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

(37)


Salthammer, T.; Mentese, S.; Marutzky, R. Formaldehyde in the Indoor Environment. Chem.

Rev. 2010, 110, 2536-2572. (38)

Bie, L.-J.; Yan, X.-N.; Yin, J.; Duan, Y.-Q.; Yuan, Z.-H. Nanopillar ZnO Gas Sensor for

Hydrogen and Ethanol. Sens. Actuators, B 2007, 126, 604-608. (39)

Jayanthi, K; Chawla, S. Electronic and Luminescent Characteristics of Aluminum Doped ZnO

Nanocrystals. Electron. Mater. Lett. 2015, 11, 55-59. (40)

Koushik, D.; Verhees, W. J. H.; Kuang, Y.; Veenstra, S.; Zhang, D.; Verheijen, M. A.; Creatore,

M.; Schropp, R. E. I. High-Efficiency Humidity-Stable Planar Perovskite Solar Cells Based on Atomic Layer Architecture. Energy Environ. Sci. 2017, 10, 91-100. (41)

Robertson, J. Band Offsets of Wide-Band-Gap Oxides and Implications for Future Electronic

Devices. J. Vac. Sci. Technol. B 2000, 18, 1785-1789. (42)

Ab Kadir, R.; Rani, R. A.; Alsaif, M. M. Y. A.; Ou, J. Z.; Wlodarski, W.; O’Mullane, A. P.;

Kalantar-Zadeh, K. Optical Gas Sensing Properties of Nanoporous Nb2O5 Films. ACS Appl. Mater. Interfaces 2015, 7, 4751-4758. (43)

Park, S.; Sun, G.-J.; Jin, C.; Kim, H. W.; Lee, S.; Lee, C. Synergistic Effects of a Combination

of Cr2O3-Functionalization and UV-Irradiation Techniques on the Ethanol Gas Sensing Performance of ZnO Nanorod Gas Sensors. ACS Appl. Mater. Interfaces 2016, 8, 2805-2811. (44)

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. (45)

Lee, J.-H. Gas Sensors Using Hierarchical and Hollow Oxide Nanostructures: Overview. Sens.

Actuators, B 2009, 140, 319-336. (46)

Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Ángyán, J. G. Screened Hybrid

Density Functionals Applied to Solids. J. Chem. Phy. 2006, 124, 154709.


23


(47)

Page 24 of 25

Greiner Mark, T.; Chai, L.; Helander Michael, G.; Tang, W. M.; Lu, Z. H. Transition Metal

Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies. Adv. Funct. Mater. 2012, 22, 4557-4568. (48)

Pan, C. A.; Ma, T. P. Work Function of In2O3 Film as Determined from Internal Photoemission.

Appl. Phys. Lett. 1980, 37, 714-716. (49)

Wang, C.; Yin, L.; Zhang, L.; Xiang, D.; Gao, R. Metal Oxide Gas Sensors: Sensitivity and

Influencing Factors. Sensors 2010, 10, 2088-2106. (50)

Franke, M. E.; Koplin, T. J.; Simon, U. Metal and Metal Oxide Nanoparticles in Chemiresistors:

Does the Nanoscale Matter? Small 2006, 2, 36-50. (51)

Zhang, Z.; Yates Jr, J. T. Band Bending in Semiconductors: Chemical and Physical

Consequences at Surfaces and Interfaces. Chem. Rev. 2012, 112, 5520-5551. (52)

Akbashev, A. R.; Zhang, L.; Mefford, J. T.; Park, J.; Butz, B.; Luftman, H.; Chueh, W. C.;

Vojvodic, A. Activation of Ultrathin SrTiO3 with Subsurface SrRuO3 for the Oxygen Evolution Reaction. Energy Environ. Sci. 2018, 11, 1762-1769. (53)

Barsan, N.; Weimar, U. Conduction Model of Metal Oxide Gas Sensors. J. Electroceram. 2001,

7, 143-167.


24

Page 25 of 25 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


Table of Contents graphic


25

Revealing the Relationship Between Energy Level and Gas Sensing

Recommend Documents