CuBi2O4 Prepared by the Polymerized Complex Method for Gas

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Functional Inorganic Materials and Devices

CuBi2O4 Prepared by Polymerized Complex Method for Gas Sensing Applications Yun-Hyuk Choi, Dai-Hong Kim, and Seong-Hyeon Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02439 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Applied Materials & Interfaces

CuBi2O4 Prepared by Polymerized Complex Method for Gas Sensing Applications Yun-Hyuk Choi,†,* Dai-Hong Kim,‡ and Seong-Hyeon Hong‡ †

Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United

States ‡

Department of Materials Science and Engineering and Research Institute of Advanced Materials

(RIAM), Seoul National University, Seoul 151-744, Republic of Korea

* Corresponding author E-mail: [email protected] (Y.-H. Choi)

Keywords: CuBi2O4; Polymerized complex method; P-type; Chemical defect; Gas sensor

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Abstract The multi-component oxides can be extensively explored as alternative gas sensing materials to binary oxides with their structural and compositional versatility. In this work, the gas sensing properties of CuBi2O4 have been investigated toward various reducing gases (C2H5OH, NH3, H2, CO, and H2S) and oxidizing gas (NO2) for the first time. For this, the powder synthesis has been developed using the polymerized complex method (Pechini method) to obtain a single phase polycrystalline CuBi2O4. The defect, optical, and electronic properties in the prepared CuBi2O4 powder were modulated by varying the calcination temperature from 500 to 700°C. Noticeably, high concentration of Cu+−oxygen vacancy (VO•) defect complexes and isolated Cu2+ ion clusters was found in the 500°C-calcined CuBi2O4, where they were removed through aircalcination at higher temperatures (up to 700°C) while making the compound more stoichiometric. The change in intrinsic defect concentration with calcination temperature led to the variation of electronic band gap energy and hole concentration in CuBi2O4 with the polaronic hopping conduction (activation energy = 0.43 eV). The CuBi2O4 sensor with 500°C-calcined powder showed the highest gas responses (specifically, 10.4 toward 1000 ppm C2H5OH at the operating temperature of 400°C), with the highest defect concentration. As a result, the gas sensing characteristics of CuBi2O4 are found to be dominantly affected by the intrinsic defect concentration, which is controlled by calcination temperature. Towards reducing H2S and oxidizing NO2 gases, the multiple reactions arising simultaneously on the surface of CuBi2O4 sensor govern its response behavior, depending on gas concentration and operating temperature. We believe that this work can be a cornerstone for understanding the effect of chemical defect on the gas sensing characteristics in multi-component oxides.


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1. Introduction

Chemiresistive solid-state gas sensors represent the most attractive solution as a facile, lowcost means of environmental monitoring and medical diagnostics.1-4 Such sensors have been made from various oxide semiconductor materials allowing for high sensitivity and selectivity, long-term stability, and fast response and recovery.5-7 A lot of work has been carried out to enhance the gas sensing properties, primarily in binary oxides such as SnO2, In2O3, NiO, and CuO.5-7 However, the sensor performance and material functionality are still required to be further improved for practical gas sensing applications. The exploration of novel composition in sensor materials can be an alternative solution. For instance, the gas sensing properties of multicomponent oxides have been rarely investigated in previous studies despite their structural and compositional versatility. The urgent need for such a composition study is particularly recognized in p-type semiconductor materials, which show theoretically lower gas response characteristics than n-type counterparts to the same gas at similar morphological configurations.7-9 Several reports on the gas sensing properties of multi-component oxide compounds were found in the literature. For example, the dependence of point defect concentration and electrical conductivity on the oxygen partial pressure in perovskite oxides with an ABO3 formula unit (e.g. SrTiO3, CaTiO3, BaTiO3, and BaSnO3) allowed for their use in high-temperature oxygen (O2) sensors.10,11 In particular, p-type perovskite SrTi1-xFexO3-δ showed the effective responses toward hydrocarbons such as butane, methane, propene, and propane.12-14 P-type perovskite LaFeO3 and LaCoO3, wherein the concentration of defect and charge carrier was controlled by elemental doping, exhibited the remarkable responses toward carbon monoxide (CO), ethanol (C2H5OH), methanol (CH3OH), acetone (CH3COCH3), and nitrogen oxide (NO2), together with their good catalytic properties.10,15,16 P-type perovskite hexagonal YMnO3 revealed a high response toward 3 ACS Paragon Plus Environment


hydrogen sulfide (H2S).17 The gas response characteristics of p-type lanthanoid (Ln) perovskite oxides (LnFeO3 and LnCrO3) toward propylene (C3H6) and nitrogen oxide (NO2/NOx) were systematically investigated.18 Meanwhile, remarkable gas sensing properties toward ozone (O3) or oxygen (O2) were reported in p-type delafossite transparent conducting oxides (TCO) such as CuAlO2, CuCrO2, and CuFeO2.19-21 Recently, Šutka et al. showed that spinel ferrite oxides with an AB2O4 formula unit, (ZnFe2O4, CdFe2O4, MgFe2O4, and NiFe2O4) were very promising gas sensor materials in a series of studies.22-27 Orthorhombic CaFe2O4 and spinel CuFe2O4 were also proposed as promising p-type gas sensor materials with high sensitivity toward C2H5OH.28,29 Their gas sensing mechanism was based on change in the oxidation state of divalent cation in octahedral sites of the spinel structure by oxygen chemisorption, thereby varying charge carrier concentration, hopping-type electrical conductivity, and width of space charge layer. As a result, the gas sensing properties of multi-component oxides are considered to be strongly affected by their stoichiometry and defect chemistry, modulating the electronic structure. Here, we explore p-type semiconductor CuBi2O4 as a new candidate material for gas sensing applications. CuBi2O4 possesses a tetragonal crystal structure with a space group of P4/ncc.30,31 It is constructed from isolated CuO4 plaquettes (square-planar CuO46- complex clusters) forming staggered, colinear chains along c axis, which are connected by BiO4 units, with a twist angle of 33.3° between adjacent plaquettes.30-33 It represents a three-dimensional S = 1/2 Heisenberg antiferromagnetic ordering below 42−43 K (Neél temperature, TN).30,32,34 The electrical properties of CuBi2O4 are known to be governed by the multi-phonon hopping of charge carriers with a weak electron-phonon coupling.35-37 Since the 2000s, CuBi2O4 has been sought for use in photocatalyst and photoelectrochemical photocathode for the generation of hydrogen from water because it is a visible-light-sensitive p-type semiconductor with a narrow band gap energy of 1.40−1.80 eV. Furthermore, its conduction band minimum is located on a 4 ACS Paragon Plus Environment

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more negative position than the water reduction potential, allowing for spontaneous hydrogen generation with the minimum overpotentials under visible-light irradation.38-43 Such superior catalytic and defect-dependent electrical properties of CuBi2O4 lead to a high expectation that it can work as a promising gas sensing material. In this work, the polymerized complex method (or Pechini process)44 is exploited for powder synthesis of CuBi2O4 with variation of calcination temperature. This method is based on a series of processes, i.e. the intensive blending of positive ions in a solution, the controlled transformation of the solution into a polymer gel, the removal of the polymer matrix, and the development of an oxide precursor with a high degree of homogeneity.45 When compared to other conventional methods, the Pechini method secures better compositional homogeneity, better stoichiometry, lower toxicity, lower cost, and lower processing temperature. Thus, it is considered to be suitable for powder synthesis of multi-component oxide compounds.45 Subsequently, we demonstrate the gas sensing properties mediated by chemical defect in CuBi2O4 for the first time. The response characteristics are investigated toward various gases with varying gas concentration and operating temperature, in comparison with those of conventional CuO. Finally, the phase formation and defect, optical, electronic, and electrical transport properties of CuBi2O4 are correlated with its characteristic gas sensing properties to address the structure-function relationships.

2. Experimental

2.1. Powder synthesis and preparation of sintered bodies

CuBi2O4 powders were synthesized by polymerized complex method, also known as the Pechini process.44 First, 1 mol of citric acid (anhydrous, 99.5% purity, Junsei) was dissolved in 4 5 ACS Paragon Plus Environment


mol of ethylene glycol (anhydrous, 99.8% purity, Sigma-Aldrich) with constant stirring at room temperature for 24 h to obtain a transparent solution. Subsequently, 0.0333 mol of copper(II) chloride dihydrate (CuCl2·2H2O, 99.9% purity, High Purity Chemicals, Japan) and 0.0667 mol of bismuth(III) chloride (BiCl3, Kanto Chemical) were dissolved in the solution and stirred at 90°C for 5 h in order to form a metal−citric acid complex through a chelating process. The transparent green solution obtained was stirred at 140°C for another 16 h for polyesterification, and a yellowish sticky polymeric resin was finally obtained. This polymeric product was decomposed to black powder via pyrolysis at 400°C for 5 h in air, after which it was ground. The final calcination process was performed between 500 and 700°C for 6 h in air. The heating rate for high-temperature processing was 5°C/min and naturally cooled to room temperature in a furnace. The flow diagram for powder synthesis is represented in Figure S1 (Supporting Information). For the preparation of bulk CuBi2O4 pellets, the black powder was obtained via burning of the polyesterified polymer resin at 400°C for 5 h in air, followed by soft grinding. Subsequently, it was calcined at 500°C for 1 h in air, after which it was ground again. The as-prepared powder (10 g) was uniaxially pressed at 3 MPa into the pellet of 12 mm diameter and then cold isostatically pressed at 200 MPa. Sintering was carried out in air in a box furnace at 700°C for 12 h, where the pressed pellet was found to be dissociated during sintering at 800°C. For sensor comparison, CuO powder was separately prepared with the same process except for using 0.1 mol of copper(II) chloride dihydrate as a metal-organic precursor. The CuO powder was obtained through pyrolysis of the polymeric product at 400°C for 5 h in air without the additional calcination process.

2.2. Characterization


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The thermal properties of powders were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and differential thermal analysis (DTA) using a simultaneous TGA/DSC analyzer (SDT-Q600, TA Instruments, USA) under air or N2 atmosphere with a heating rate of 5°C/min from room temperature to 1000°C. The phase and crystal structure of powders and sintered pellets were characterized by the X-ray diffraction (XRD) method using a Bruker D8-Advance instrument equipped with a Cu Kα source (λ = 1.5406 Å). The lattice parameters were determined by the Rietveld refinement method using a TOPAS program. The morphology of powders, sintered pellets, and sensor films was observed by a field emission scanning electron microscope (FE-SEM, JSM-6500F, JEOL). Nitrogen adsorption/desorption isotherms, Brunauer-Emmett-Teller (BET) surface area analysis, and Barrett-Joyner-Halenda (BJH) pore size and volume analysis were performed for analyzing the specific surface area, pore size, and pore volume of powders. The chemical state of powders was investigated by X-ray photoelectron spectroscopy (XPS, AXIS-His, Kratos) with a microfocused Mg Kα radiation (1253.6 eV). The core-level XPS spectra for Cu2p, Bi4f, and O1s were measured, and energy calibration was achieved by setting the hydrocarbon C1s line equal to 284.5 eV. Photoluminescence (PL, PerkinElmer LS 55) spectra of powders were acquired with 259 nm wavelength excitation. Electron spin resonance (ESR) signals of powder samples were acquired using a JES-TE200 ESR spectrometer (JEOL, Tokyo, Japan) with an X-band microwave frequency (9.19 GHz) at room temperature. To identify the peaks, the signal components were analyzed by the ES-IPRITS data system with version 3.01 analysis software installed in the ESR instrument. The following ESR parameters were used: a frequency of 9.19 GHz, center field of 400 mT, modulation frequency of 100 kHz, time constant of 0.3 s, and power of 1 mW. The UVVis-NIR diffuse reflectance spectra were obtained using a UV-Vis-NIR spectrometer (Cary 5000, Agilent) with an integrating sphere attachment in order to measure the optical absorbance and 7 ACS Paragon Plus Environment


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band gap energy of powers. The band structure calculations, based on plane wave density functional theory (DFT+U), were investigated for CuBi2O4 using the CASTEP package program. The applied convergence criteria were 10-6 eV for the total energy change per atom. Geometric optimization was performed using a cut-off energy of 600 eV and 2 × 2 × 2 k-point sampling. Generalized gradient approximation (GGA) using Perdewe–Burkee–Ernzerhof (PBE) functionals and ultra-soft pseudopotentials were calculated, and then basis set corrections were conducted. The sintered pellet was cut into a parallelepiped measuring ca. 2.5 mm × 2.5 mm × 10 mm, and the electrical conductivity and absolute thermopower were measured by a D. C. 4-probe and steady-state method against oxygen activity in the range of -3.00 ≤ log10 aO < 0 in the 2

equilibrium state at 550, 600, and 650°C. The experimental details of measurement were described in ref. (46) and (47).

2.3. Preparation and measurements of gas sensors CuBi2O4 films were prepared on clean SiO2 (2 µm)/Si ⟨100⟩ substrates from homemade pastes including the synthesized powders via a doctor-blade method. The coated films were annealed at 500°C for 1 h under the ambient atmosphere. For sensor comparison, the CuO film with the same thickness was separately prepared on the substrate under the same annealing condition. With regard to the gas-sensor measurements, a pair of comb-like Pt electrodes was deposited on the films formed on the substrates, by sputtering through a mask with the gap between Pt electrodes of 0.2 mm and the width of 8 mm. This was followed by firing for a short time at 500°C without a change in the morphology or phase. Thereafter, Au wires were attached to the electrodes using Ag paste, and the samples were dried at 80°C in a conventional oven. The sensors were placed in a quartz tube located inside an electrical tube furnace with a gas 8 ACS Paragon Plus Environment

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inlet/outlet system. The sensor responses were obtained by measuring the changes in the electrical resistance between gas flow (with varying concentrations) balanced with air and pure dry-air flow at 300−500°C using a multimeter (Keithley 2002).

3. Results and discussion 3.1. Phase formation, crystal structure, and microstructure

To investigate the phase behavior of this compound, thermal analysis of the powders prepared through calcination at 400°C was performed by TGA, DSC, and DTA under air or N2 atmosphere (Figure 1). In the TG profiles, a slight weight loss, observed at elevated temperature up to 550°C, is considered to be due to vaporization of water and organic residue from the precursor powder, regardless of atmosphere. The weight gain was observed during further heating from 550 to 850°C in air, associated with the formation of CuBi2O4 compound by oxygen uptake from the atmospheric air (Figure 1a), while it did not take place in N2 (Figure 1b). Furthermore, in air, the catastrophic TG weight loss involving enormous DSC endothermic and sharp DTA peaks was observed at ca. 850°C, which resulted from the decomposition of powder into liquid phase, further corroborated by a contaminated coagulum left in the pans after heating cycle in the thermal analysis. The decomposition temperature observed in air was almost the same as that of the reported phase diagram (845°C).48 In contrast, the powder in N2 was decomposed at ca. 810°C, which was lower than in air. The decomposition temperature of CuBi2O4 into liquid phase is known to be reduced with decreasing the oxygen partial pressure in the phase diagram.48 The XRD patterns of the powders obtained at different calcination temperature are shown in Figure 2. The XRD pattern of black precursor powder, obtained via burning of the 9 ACS Paragon Plus Environment


polyesterified polymer resin at 400°C for 5 h in air, indicated that it includes CuO and Bi2O3 intermediate phases along with CuBi2O4 (see the indexed XRD pattern with Joint Committee on Powder Diffraction Standards (JCPDS) in Figure S2). When the black precursor powder was further calcined for 6 h in air at 500°C or higher, pure polycrystalline CuBi2O4 powder without any second phase was eventually obtained. The lattice parameters with a tetragonal symmetry (P4/ncc), determined by the Rietveld refinement, were a = 8.479 Å and c = 5.806 Å for the 500°C-calcined CuBi2O4 powder, a = 8.486 Å and c = 5.815 Å for 600°C-calcined CuBi2O4 powder, and a = 8.490 Å and c = 5.817 Å for 700°C-calcined CuBi2O4 powder, showing that the lattice parameters slightly increased with calcination temperature. Furthermore, it was experimentally found that both black precursor powder and 700°C-sintered CuBi2O4 pellet melted when heated to 800°C and then kept during a few hours. Thus, the synthesis of CuBi2O4 confirms the phase diagram of Bi2O3−CuOx where this stoichiometric compound is decomposed into liquid phase and CuO at 845°C in air,48 and thereafter the heat treatment temperature for CuBi2O4 preparation was decided in the range between 500 and 700°C. Figure 3 shows FE-SEM micrographs of the CuBi2O4 powders prepared through calcination of the precursor powder at 500, 600, and 700°C in air. The powders consisted of submicron/micro-sized particles which were ca. 500 nm, 1 µm, and 2 µm in diameter for the powders calcined at 500, 600, and 700°C, respectively, suggesting that the calcination at higher temperature leads to an increase in particle size. Furthermore, the interconnections among particles by necking were apparently observed in the powder calcined at 700°C. These indicate that the increase of calcination temperature decreases the specific surface area of CuBi2O4 powder. Through further quantitative nitrogen adsorption/desorption isotherms (Figure S3), BET, and BJH analysis, it was indeed revealed that the BET specific surface area of the powder decreased from 1.28 to 0.58 to 0.12 m2/g, the total pore volume decreased from 0.0038 to 0.0018 10 ACS Paragon Plus Environment

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to 0.0007 cm3/g, and the average pore diameter increased from 11.96 to 12.31 to 25.40 nm, as the calcination temperature increased from 500 to 600 to 700°C. The type of the measured physisorption isotherms (Figure S3) shows the normal form of isotherm with a non-porous or macroporous adsorbent, representing the unrestricted monolayer-multilayer adsorption.49 The powder synthesis of CuBi2O4 was sensitive to molar ratio between Cu(II) and Bi(III) precursors and thus stoichiometric mixture of the precursors was critical to obtain a pure CuBi2O4 phase, since it is formed in a very narrow intermetallic composition range as shown in the phase diagram.48 For example, when the precursors reacted with a mole ratio of 1:1 (0.0333 mol CuCl2·2H2O and 0.0667 mol BiCl3), CuBi2O4 was formed along with a small amount of CuO (see the XRD patterns in Figure S4). On the other hand, the synthesis seemed to be generous about the precursor type because a pure CuBi2O4 powder was similarly obtained even though copper(II) formate tetrahydrate (Cu(HCOO)2·4H2O, 97% purity, Sigma-Aldrich) was used instead of copper(II) chloride dihydrate (see FE-SEM image and XRD pattern of the synthesized CuBi2O4 powder in Figure S5).

3.2. Defect, optical, and electronic properties

The chemical states of the CuBi2O4 powders calcined at different temperatures have been characterized by XPS, and their Cu2p, Bi4f, and O1s core level spectra are compared in Figure 4. As shown in Figure 4a, the Cu2p spectra consist of the characteristic spin-orbit split 2p1/2 (953 eV) and 2p3/2 (933 eV) peaks (dash lines in Figure 4a) with their shake-up satellite peaks on the left sides (higher binding energies), which are indicative of the typical Cu(II) oxide.38,50-53 Notably, the Cu2p1/2 and 2p3/2 peaks of the 500°C-calcined CuBi2O4 powder revealed their rightside shoulders at 951.65 and 932.25 eV, respectively. The shoulders disappeared under 11 ACS Paragon Plus Environment


calcination at higher temperatures over 600°C. In contrast, their shake-up satellite peaks remained unchanged. This indicates that the Cu(I) component is highly contained in the 500°Ccalcined CuBi2O4 and is reduced with increasing the calcination temperature.38,50-53 For more detailed comparison of the samples, the Cu2p3/2 spectra are deconvoluted into two sub-peaks arising from the Cu(II) and Cu(I) components in Figure S6. It is indeed revealed that the 500°Ccalcined CuBi2O4 possesses the largest amount of Cu(I) component (17.6 at%). In Figure 4b, the Bi4f spectra are shown with the characteristic spin-orbit split 4f5/2 (163.77 eV) and 4f7/2 (158.47 eV) peaks which resulted from the Bi(III) component of CuBi2O4.38,54 The binding energies of Bi4f spectra are maintained at the constant values against the calcination temperature although the peak widths slightly decrease, which suggests that the bismuth valence and related defect state are rather stable against the calcination temperature. The Bi4f spectra of the CuBi2O4 samples are compared with that of a commercial Bi2O3 powder (99.999% purity, STREM Chemicals) in Figure S7. Compared with the Bi4f spectra of Bi2O3, those of CuBi2O4 are observed to shift to lower binding energy by roughly 0.4 eV. It indicates that the CuBi2O4 samples possess a slightly larger amount of Bi(II) component compared with Bi2O3. However, the Bi4f spectra acquired for the CuBi2O4 samples with 163.77 eV for Bi4f5/2 and 158.47 eV for Bi4f7/2 are still located on higher binding energies, compared with the spectra for pure Bi(II) component with around 163.3 eV for Bi4f5/2 and around 157.9 eV for Bi4f7/2.38,54 Thus, it is concluded that the prepared CuBi2O4 samples consist primarily of Bi(III) component. Furthermore, since no difference in binding energy of Bi4f spectra among the CuBi2O4 samples is observed, the bismuth valence and related defect state are regarded to be constant across all the CuBi2O4 samples although the slight amount of Bi(II) component is commonly contained in the samples. In the O1s spectra of Figure 4c, the main peaks are observed at 529.20 eV along with weak shoulders at 530.88 eV, which originated from the lattice oxygen species bound to Cu(II) 12 ACS Paragon Plus Environment

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and Bi(III) in CuBi2O4, respectively.38,54 This lattice oxygen peaks are constant against the calcination temperature. On the other hand, the shoulder peak at 531.87 eV, originating from oxygen vacancy,38,54 is gradually reduced with increasing the calcination temperature up to 700°C. As a result, it was found that the 500°C-calcined CuBi2O4 contained high concentration of Cu(I) component and oxygen vacancy, all of which were removed through calcination at higher temperatures. Such a defect healing with increasing the calcination temperature can be reconfirmed by the aforementioned DSC result showing the uptake of oxygen species from air during air−heating from 550 to 850°C (Figure 1a). Here, the Cu(I) component and oxygen vacancy detected by XPS demonstrate the creation of Cu+−oxygen vacancy (VO•) defect complexes in CuBi2O4 as the charge compensation, which is associated with the creation of oxygen vacancies by thermal calcination, causes the valence of the Cu ions to be reduced from 2+ to 1+.55,56 Indeed, the chemical defects in CuBi2O4 are apt to be generated within strongly ionic Cu(II)−O bonding surroundings rather than strongly covalent Bi(III)−O counterparts.30-32 The oxygen nonstoichiometry in CuBi2O4 was also reported.34 The XPS results show that they can be removed through air-calcination at higher temperatures in CuBi2O4. The defect states in CuBi2O4 have been further investigated by PL and ESR spectroscopy. Figure 5a shows the room-temperature PL spectra of the CuBi2O4 powders prepared with different calcination temperatures. All the powders revealed the broad peaks at 404 nm (3.07 eV), 480 nm (2.58 eV), and 528 nm (2.35 eV) which are larger energies than a band gap energy (1.40−1.80 eV) of CuBi2O4. Such broad large-energy emission peaks stem from the defect levels created by oxygen vacancies in CuBi2O4.39,50,57,58 In particular, the emission peaks were observed to be slightly decreased with the decrease of the defect concentration as the calcination temperature increased.



The room-temperature ESR spectra of the CuBi2O4 prepared with different calcination temperatures were collected at an X-band frequency and shown in Figure 5b. The ESR signals provide the structural information about the surroundings of only paramagnetic Cu(II) ions with unpaired 3d9 electrons (S = 1/2) in the CuBi2O4 compound since the diamagnetic Cu(I) and Bi(III) ions with the closed 3d10 and 6s2 shells, respectively, are ESR silent species. Furthermore, the CuBi2O4 possesses an ESR-active paramagnetic ordering at room temperature since it undergoes the antiferromagnetic-to-paramagnetic phase transition at TN = 42−43 K.30,32,34 The spectrum of 500°C-calcined CuBi2O4 powder exhibited partially resolved hyperfine splitting structure with the broadest resonance line. Particularly, quartet splitting features due to the hyperfine interaction between the unpaired electron and the nuclear spin of Cu2+ (I = 3/2) appeared on the parallel component in the low field region of the spectrum, which indicate the existence of the isolated Cu2+ ion clusters in the 500°C-calcined CuBi2O4.59-62 Moreover, the weak broad spectrum line indicates high concentration of oxygen vacancies in the 500°Ccalcined CuBi2O4.60,61 The isolated Cu2+ ion clusters are regarded to be formed along with the creation of oxygen vacancies.60 The hyperfine splitting structure disappeared in the spectrum and the peak intensity was enhanced with higher calcination temperature over 600°C. Furthermore, the effective g-value and the peak-to-peak line width gradually decreased from 2.125 to 2.088 to 2.038 and from 209.426 to 190.280 to 152.576 mT, respectively, as the calcination temperature increased from 500 to 600 to 700°C. Such results demonstrate that the Cu+−VO• defect complexes and isolated Cu2+ ion clusters are removed in CuBi2O4 by calcination at higher temperatures, which is consistent with the XPS and PL results.60,61,63,64 Consequently, it is concluded that an air−calcination process at higher temperatures produces more stoichiometric CuBi2O4 compound. The optical absorbance of the CuBi2O4 prepared with different calcination temperatures was measured using UV-vis-NIR diffuse reflectance spectroscopy. As shown in Figure 6a, it was 14 ACS Paragon Plus Environment

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found that the difference in calcination temperature gave rise to the deviation of absorbance intensity in CuBi2O4. Specifically, the 700°C-calcined CuBi2O4 powder absorbed the most amount of visible light among them and the absorbance was reduced with decreasing the calcination temperature. As the absorbed light range corresponds particularly to a red spectrum of wavelength ca. 700 nm, a brown color appears in the 500°C-calcined CuBi2O4 powder with the least absorbance (the largest reflectance) of red light. The color of CuBi2O4 powder was getting dark from brown to dark brown to black with increasing the calcination temperature from 500 to 600 to 700°C in accordance with a change in the light-absorbed capacity (see Figure 6c). Their direct band gap energies (m = 0.5) are compared with the Tauc plots in Figure 6b using the following equation (1):

αhν = A(hν − E g ) m

(1)

where α, ν, Eg, and m are the absorption coefficient, optical frequency, band gap energy, and the characteristic transition value, respectively. The band gap energies are noticeably obtained with two d-d and p-d transitions. Such a dual energy transitional behavior in CuBi2O4 can be explained in terms of charge transfers within the square-planar CuO46- complexes (intracenter p-d transitions) and between the neighboring complexes (intercenter d-d transitions) in its crystal structure.31 It can be further understood by an electronic band structure of CuBi2O4 consituted through a DFT simulation (Figure 7a). The valence and conduction band edges of CuBi2O4 arise from the combination of O2p and Cu3d orbitals, respectively, as reported.33,40 The p-d transition (Ep-d) from the valence band edge of O2p to the conduction band edge of Cu3d is calculated to be 1.750 eV (Figure 7b). Moreover, the d-d transition (Ed-d) from the additional d-orbital located on 0.207 eV above the valence band edge to the conduction band edge of Cu3d is confirmed to be 1.543 eV. The measured optical band gap energies (Ed-d = 1.54 eV and Ep-d = 1.79 eV for 500°Ccalcined CuBi2O4, Ed-d = 1.55 eV and Ep-d = 1.78 eV for 600°C-calcined CuBi2O4, and Ed-d = 1.49 15 ACS Paragon Plus Environment


eV and Ep-d = 1.52 eV for 700°C-calcined CuBi2O4) in Figure 6b conform to such an elecrtronic band structure of CuBi2O4. These band gap energies are in the range of the reported values, 1.40−1.80 eV.38-43 Noticeably, a decrease in the band gap energies with increasing the calcination temperature was observed, which was more dominantly for Ep-d. It is attributed to the reduction in the concentration of defects (i.e., Cu+−VO• complexes) with increasing the calcination temperature.

3.3. Electrical transport properties

The bulk CuBi2O4 pellet has been prepared via a sintering process in air at 700°C for 12 h to investigate its electrical transport properties. The apparent density of the CuBi2O4 pellet measured by the Archimedes method was 8.12 g/cm3, 94.3% of the theoretical density (8.61 g/cm3).32 The FE-SEM image of fractured surface and XRD pattern of the CuBi2O4 pellet are shown in Figure 8. The pores of roughly 500-nm diameter are sparsely observed among the dense grains of about 1.5-µm diameter (Figure 8a). The FE-SEM images acquired at various scales are further shown in Figure S8. The XRD pattern of bulk pellet revealed a pure crystalline CuBi2O4 phase (Figure 8b). The lattice parameters of the pellet determined by the Rietveld refinement were a = 8.502 Å and c = 5.820 Å, higher than those of non-sintered powders. The increase in lattice parameters with sintering relative to the non-sintered powders is attributed to the decrease in lattice strain by the annihilation of defect such as oxygen vacancies, as mentioned above. Figure 9a shows the equilibrium total conductivity isotherms σ of the CuBi2O4 pellet as a function of oxygen activity, measured at 550, 600, and 650°C. The σ increased with increasing the measurement temperature. In addition, the σ gradually increased with increasing oxygen activity in the given range of -3.00 ≤ log10 aO < 0, throughout the temperature range. Such 2


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behavior indicates the p-type semiconducting characteristics of CuBi2O4, especially under air condition ( log10 aO = -0.678) for gas sensing applications. Noticeably, at the lowest measurement 2

temperature 550°C, the σ was found to react most sensitively to the change of oxygen activity. In other words, the electrical conduction changes highly depending on the adsorption and desorption of oxygen molecules on the surface of CuBi2O4 at low temperatures, and thus it allows an operating temperature to be decided for an effective gas sensing. Figure 9b displays the absolute thermoelectric powers θ (Seebeck coefficient) of the CuBi2O4 pellet as a function of oxygen activity, acquired at 550, 600, and 650°C. The positive θ values also demonstrate the p-type semiconducting characteristics of CuBi2O4. The θ was observed to gradually increase with increasing oxygen activity. At log10 aO > -1.5, the θ value was inversely proportional to the 2

measurement temperature. Arrhenius plot of the electrical conductivity measured at 200−600°C for the CuBi2O4 pellet sintered at 700°C is shown in Figure 10. The activation energy (Ea) of electrical conduction was acquired from the Arrhenius equation:

σ = σ 0 exp(− Ea / kT )

(2)

Where, σ0 is the pre-exponential factor, Ea is the activation energy, and k is the Boltzmann’s constant. The Ea for electrical conduction was found to be 0.43 eV, which is close to the reported value 0.37 eV of the crystalline CuBi2O4 measured along the c-axis.35,36 The much lower Ea value than the band gap energy indicates that the electrical conduction of p-type semiconductor CuBi2O4 is governed by the polaronic hopping mechanism rather than the band conduction. Specifically, the electrical transport behavior of the crystalline CuBi2O4 has been interpreted in terms of the multi-phonon hopping of charge carriers with a weak electron-phonon coupling.35-37



Thus, it is considered that the chemical defect state strongly affects the gas sensing properties of CuBi2O4.

3.4. Gas sensing properties

To fabricate the gas sensor, CuBi2O4 films with the identical thickness of ca. 35 µm were prepared on clean SiO2 (2 µm)/Si ⟨100⟩ substrate using homemade paste with the synthesized CuBi2O4 powders via a doctor-blade method, followed by annealing at 500°C for 1 h under the ambient atmosphere. Their cross-sectional FE-SEM images are shown in Figure 11a−c. A series of steps toward the film preparation did not change the particle size; the particles of different sizes of ca. 500 nm, 1 µm, and 2 µm in diameter which correspond to those of the neat powders are observed for the films prepared with the calcined powders at 500, 600, and 700°C, respectively (Figure 11d−f). Furthermore, we have separately prepared the CuO film with the same thickness on the substrate to compare the gas sensing properties. The CuO powder was synthesized by the polymerized complex method which is identical to the synthesis of CuBi2O4. Figure S9a and b show the FE-SEM images and XRD pattern of the CuO powder obtained with the particle size of ca. 3 µm in diameter, respectively. The CuO film with the thickness of ca. 35 µm was also prepared on the substrate with the synthesized CuO powder (Figure S9c). A photograph of the final gas sensor fabricated through additional electroding and wiring processes on the prepared CuBi2O4 film (or CuO film) is displayed in Figure S10. The sensing properties of the fabricated CuBi2O4 and CuO sensors were investigated toward various reducing gases (C2H5OH, NH3, H2, CO, and H2S) and oxidizing gas (NO2) balanced with air at operating temperatures of 300, 400, and 500°C. As shown in Figure 12a, the stable and reversible response transients were obtained under the cross-input conditions of the air and the gas with the various 18 ACS Paragon Plus Environment

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concentrations of 50−1000 ppm, guaranteeing the stability and reproducibility of the CuBi2O4 sensors. The CuBi2O4 sensors exhibited the typical sensing behavior of p-type semiconductor with the increases of electrical resistance toward the reducing gas. The magnitude of gas response can be defined as the ratio (Rg/Ra) of the resistance in a target gas (Rg) to that in air (Ra), which is obtained from the measured response transient curves. As shown in the gas response versus gas concentration plot of Figure 12b, the response values of the CuBi2O4 sensors increased gradually with increasing gas concentration. The C2H5OH sensing mechanism for these p-type semiconductor CuBi2O4 and CuO sensors can be described as follows. The O2 molecules in air are adsorbed to form O− ions on the oxide surface at the operating temperature of 300°C or higher, with the surface reactions (3) and (4).9,50 (O2 ) gas ↔ 2(O) adsorption

(3)

(O) adsorption + (e − )lattice ↔ (O − ) adsorption

(4)

These reactions generate the hole-accumulation layer in the surface region of oxide grains in air, reducing the sensor resistance. When the sensor is exposed to reducing C2H5OH gas, the surface chemical reaction is as follows:

(C 2 H 5 OH ) gas + 6(O − ) adsorption → 2(CO2 ) gas + 3( H 2 O) gas + 6e −

(5)

The adsorption of the reducing C2H5OH gas onto the oxide-grain surface decreases the width of hole-accumulation layer in the surface region of oxide grains and therefore increases the sensor resistance. Eventually, a change in the resistance depending on the gas adsorption and desorption is served as a signal for gas response or sensitivity. The electrical resistance of all the sensors measured in air decreased gradually with increasing the operating temperature from 300 to 500°C (Figure 12c). In particular, as the calcination temperature of CuBi2O4 powder composing the CuBi2O4 sensor increased from 500 to 19 ACS Paragon Plus Environment


700°C, the resistance increased by nearly two orders of magnitude throughout the operating temperature (Figure 12c). Such an increase of resistance with calcination temperature is believed to be due to the reduction in the concentration of chemical defect leading to the decrease of charge carrier concentration with increasing the calcination temperature, as discussed in the section 3.2. It indicates a negative enthalpy of the defect generation reaction, similar to the behavior of CuO.50,65 Of course, the decrease of inter-particle contacts by coarsening of particles with calcination temperature can also increase the sensor resistance (see Figure 11d−f). However, previous studies demonstrated that the defect concentration and carrier action could dominate over the morphological factor for the resistance control in high-temperature synthesized semiconductor oxides with a high crystallinity.50,66,67 Therefore, the resistance change of CuBi2O4 is considered to be dominantly affected by the concentration of defect and electronic hole rather than crystallinity and morphological factors such as grain boundary, particle size, and surface area-to-volume ratio, similarly to those of CuO and ZnO.50,66,67 The response values of the CuBi2O4 sensors toward 1000 ppm C2H5OH gas are comparatively represented as a function of operating temperature in Figure 12d, together with those of CuO sensor (the detailed gas response values of CuO sensor are shown as a function of gas concentration in Figure S11). All the CuBi2O4 sensors showed the maximum responses toward C2H5OH gas at the operating temperature of 400°C, and they showed higher responses than CuO sensor at all the measured operating temperatures; the CuO sensor showed the maximum response value of 2 at the operating temperature of 350°C. In particular, the CuBi2O4 sensor with 500°C-calcined powder exhibited the highest responses toward 1000 ppm C2H5OH gas throughout the operating temperature, specifically with the highest response value of 10.4 at the operating temperature of 400°C. The response values of the CuBi2O4 sensor with 600°Ccalcined powder came second, followed by those of the CuBi2O4 sensor with 700°C-calcined 20 ACS Paragon Plus Environment

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powder. The relatively smallest particles and highest surface area-to-volume ratio in the CuBi2O4 sensor with 500°C-calcined powder can be advantages to gas sensing, but they are considered to be a relatively small contribution to the total gas response because the scale of particle size is still much larger than Debye length. Thus, it should be noticed that lower resistance and higher intrinsic defect concentration give rise to higher gas response characteristics in CuBi2O4. The increased hole concentration by the generation of intrinsic defect decreases the electrical resistance and the width of hole-accumulation layer of p-type oxide grains.9 In the sensing system with the marginal defect effect, when the hole concentration is high, the injection of equal amounts of electrons by the sensing reaction between reactive gas molecules and chemisorbed oxygen ions will lead to a lower variation in sensor resistance, and thus reduce the gas response (i.e., electronic sensitization mechanism).7,68,69 However, in this work, the CuBi2O4 with higher intrinsic defect concentration exhibited higher gas response in spite of its lower resistance and thus lower resistance variation in the sensing reaction. It noticeably demonstrates that the gas sensing properties of CuBi2O4 are dominantly dependent on the state of chemical defects including the concentration. Indeed, it was recently found that the high concentration of intrinsic defect and electronic hole in semiconductor oxides can increase their gas response, taking priority over crystallinity and morphological factors such as grain boundary, particle size, and surface area-to-volume ratio.50,70 The C2H5OH response characteristics of the CuBi2O4 sensor with 500°C-calcined powder have been particularly investigated at lower concentrations with the optimum operating temperature of 400°C. As shown in Figure S12a, the stable and reversible gas response transient is obtained up to 5 ppm, which is the detection limit of our measurement system. The gas response value of 2.3 is acquired toward 5 ppm C2H5OH. Figure S12b plots the gas response values of the CuBi2O4 sensor with 500°C-calcined powder as a function of gas concentration 21 ACS Paragon Plus Environment


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toward 5−1000 ppm C2H5OH gas. The gas response value is linearly increased with increasing gas concentration on a logarithmic scale (see the inset of Figure S12b). The gas responses to the other reducing gases, NH3, H2, and CO, have also been investigated. Figure 13 shows the comparison of gas response values which were measured at the operating temperature of 400°C toward 1000 ppm gases. To all the reducing gases investigated, the CuBi2O4 sensor with 500°C-calcined powder showed the most excellent gas response values. The most sensitive reducing gas was found to be C2H5OH in both CuBi2O4 and CuO sensors. Meanwhile, the CuBi2O4 sensors showed the second-highest response to NH3 gas, while the CuO sensor exhibited the second-highest response to H2. When the sensors are exposed to NH3, H2, and CO gases, the surface reactions are as follows:71

2( NH 3 ) gas + 3(O − ) adsorption → 3( H 2O) gas + ( N 2 ) gas + 3e −

(6)

( H 2 ) gas + (O − ) adsorption → ( H 2 O ) gas + e −

(7)

(CO ) gas + (O − ) adsorption → (CO2 ) gas + e −

(8)

The adsorption of these reducing gases onto the sensor surface decreases the width of holeaccumulation layer and thus increases the sensor resistance, similarly to the case of C2H5OH gas. Depending on the gas species, the difference in the molecular dissociation and charge transfer for gas adsorption and desorption on the sensor surface is believed to cause the difference in gas response value. In particular, the adsorption of C2H5OH contributes to a greater annihilation of the six holes in the reaction (5), in comparison with that of NH3 leading to the annihilation of the three holes (the reaction (6)) and those of H2 and CO leading to the annihilation of a single hole (the reactions (7) and (8)). This seems to be the reason for gas selectivity in the CuBi2O4 sensors. The response and recovery times can be defined as the time required for the response and the recovery to reach 90% of the final equilibrium signal, respectively. The average response 22 ACS Paragon Plus Environment

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times of the CuBi2O4 sensor with 500°C-calcined powder, were measured to be 57 s, 13 s, 10 s, 11 s toward C2H5OH, NH3, H2, and CO gases at the operating temperature of 400°C, respectively, while the CuO sensor represented relatively slower responses of 70 s, 94 s, 68 s, and 14 s. In all the sensors, the recovery times are measured to be an order of magnitude longer than the response times. The average recovery times of the CuBi2O4 sensor with 500°C-calcined powder were 294 s, 260 s, 176 s, and 175 s toward C2H5OH, NH3, H2, and CO gases at 400°C, respectively, while those of CuO sensor represented two-fold slower values of 420 s, 538 s, 314 s, and 623 s. As a result, the response and recovery times of CuBi2O4 sensor are found to be shorter than those of CuO sensor toward all the reducing gases investigated, which indicates the relatively fast operation of the CuBi2O4 sensor. The gas response values acquired for the CuBi2O4 sensor with 500°C-calcined powder are compared with the reported values for other multi-component oxides in Table S1. In particular, the responses toward C2H5OH are evaluated to be comparable to those of ZnFe2O4, while the responses toward NH3 and H2 represent higher levels than the reported values for the other materials despite relatively few reports. Toward another reducing gas, H2S, the sensing properties of the CuBi2O4 with 500°Ccalcined powder have been examined at the low concentrations between 1 and 5 ppm. As shown in Figure 14a, the sensor showed somewhat instable response transients to the H2S gas, although the response values can be obtained: 1.2 for 1 ppm, 3.8 for 2 ppm, and 5.3 for 5 ppm at the operating temperature of 400°C and 1.3 for 1 ppm, 1.6 for 2 ppm, and 1.9 for 5 ppm at 500°C. In particular, the response does not recover at the H2S concentrations more than 2 ppm at the operating temperature of 400°C. This non-recovery phenomenon can be demonstrated in terms of its H2S sensing mechanism. Two different sensing reactions between CuBi2O4 and H2S simultaneously happen, as found previously in the CuO nanostructured sensor.71-74 At a low H2S 23 ACS Paragon Plus Environment


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concentration (1 ppm), the adsorbed oxygen on the CuBi2O4 surface causes the oxidation of H2S, as follows: −

2( H 2 S ) gas + 3(O2 ) adsorption → 2( H 2 O ) gas + 2( SO2 ) gas + 3e −

(9)

The released electrons recombine with holes on the CuBi2O4 surface, resulting in the increase of the resistance. This process is easily recovered by removing H2S and exposing the CuBi2O4 surface to air. At a high H2S concentration (≥2 ppm), the H2S gas can cause the sulfidation of CuBi2O4. The final resistance and response are determined by the mutual compromise between the sulfidation and the reaction (9). The sulfidation process is somewhat reversible, but is much slower than the reaction (9). When an air is supplied, the sulfide is oxidized and converted back to the CuBi2O4, but the process is so slow that a baseline cannot recover in a reasonable period of time (minutes). However, because the process can be thermally activated or is hard to occur at high temperatures,72 such a non-recovery in the response is not observed at 500°C (Figure 14a). We have further investigated the sensing properties of the CuBi2O4 with 500°C-calcined powder toward an oxidizing NO2 gas at the low concentrations of 5−10 ppm. The response transients were acquired with the response values of 1.3 for 10 ppm and 1.2 for 5 ppm at 400°C, as shown in Figure 14b. Contrary to our expectations, the sensor resistance increases under exposure to the NO2 gas, which is an uncommon behavior because the increased formation of negatively charged oxygen upon exposure to the oxidizing NO2 gas increases the hole concentration, leading to the decrease of the p-type sensor resistance. A similar phenomenon was recently reported by Kim at al. for p-type CuO gas sensors for the first time, where two opposite NO2 sensing behavior depending on the NO2 concentration was found.75 In their result, the resistance of p-type CuO sensor decreased upon exposure to the NO2 gas with high


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concentrations (30−100 ppm), but increased at the low NO2 concentrations (≤5 ppm) at 400°C by the following reactions:

( NO2 ) gas + (O − ) adsorption → ( NO ) gas + (O2 ) gas + e − (low NO2 concentrations) ( NO2 ) gas + e − → ( NO ) gas + (O − ) adsorption (high NO2 concentrations)

(10) (11)

The electron generation by the reaction (10) at low NO2 concentrations and the electron consumption by the reaction (11) at high NO2 concentrations increased and decreased the resistance of p-type CuO sensor, respectively.75 In the present study, the increase in the resistance of p-type CuBi2O4 sensor, under exposure to the NO2 gas with low concentrations (≤10 ppm), can similarly be explained in terms of the electron generation by the reaction (10). Such a resistance increase of p-type gas sensors, upon the atmosphere that both oxidizing NO2 (low concentrations) and reducing CO gases coexist, can be used as an air quality sensor to control the air influx from outside of the automotive cabin, as noted by Kim and co-workers.75 Meanwhile, the CuO sensor prepared herein has not shown the responses toward the H2S and NO2 gases under given conditions, presumably because of its low surface-to-volume ratio. To test the reproducibility and stability of CuBi2O4 sensor, we have repeatedly obtained the reversible response transient curves toward each gas. As an example, the reversible response transient of the CuBi2O4 sensor with 500°C-calcined powder, acquired by repeated measurement for more than 5 h toward 10 ppm C2H5OH at operating temperature of 400°C, is displayed in Figure S13. Such repetitive measurements toward C2H5OH, NH3, H2, and CO gases corroborated the reproducible and stable response transients, guaranteeing the reproducibility and stability of CuBi2O4 sensor toward those gases. However, somewhat instable response transients were acquired for H2S and NO2 gases as mentioned above. The stability of CuBi2O4 sensor to the H2S and NO2 gases is believed to be improved in the well-defined forms of nanostructure or thin film. 25 ACS Paragon Plus Environment


In this work, the gas sensing properties of p-type semiconductor CuBi2O4 have been explored for the first time. We have found that the concentration of intrinsic defect (controlled by the change of calcination temperature) is the dominant factor governing the gas sensing characteristics in CuBi2O4. Such an intrinsic defect, considered Cu+−VO• defect complexes, is believed to be the primary reaction site for gas sensing on the surface of CuBi2O4. Finding the gas reaction sites in CuBi2O4 is suggested for future work. We believe that this work can become a cornerstone of exploring other multi-component compounds which remain to be discovered.

4. Conclusion

In the present study, the powder synthesis of p-type semiconductor CuBi2O4 has been realized by the polymerized complex method for the first time. A pure polycrystalline CuBi2O4 powder was obtained with a tetragonal crystal structure from a precursor powder through air calcination in the temperature range between 500 and 700°C. The increase of calcination temperature resulted in the increase of particle size and the decrease of specific surface area in CuBi2O4 powder. The defect, optical, and electronic properties of the CuBi2O4 powders prepared through calcination at 500, 600, and 700°C have been characterized comparatively by various spectroscopy methods such as X-ray photoelectron, photoluminescence, electron spin resonance, and UV-vis-NIR diffuse reflectance. As a result, high concentration of Cu+−oxygen vacancy (VO•) defect complexes and isolated Cu2+ ion clusters was found in the 500°C-calcined CuBi2O4, where they were removed through air-calcination at higher temperatures (up to 700°C) while making the compound more stoichiometric. The visible-light (particularly, a red spectrum) absorbance of CuBi2O4 was observed to increase with increasing the calcincation temperature. The band gap energy of CuBi2O4 was characterized by a dual energy transitional behavior: p-d 26 ACS Paragon Plus Environment

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transition (Ep-d) with 1.75 eV and d-d (Ed-d) transition with 1.54 eV. In paticular, the band gap energies were found to decrease with increasing the calcination temperature, which is more dominantly for Ep-d due to the reduction in defect cocentration. The electrical conductivity (σ) of CuBi2O4 investigated with the sintered pellet was measured to be proportional to both temperature and oxygen activity under the given conditions including ambient air. Furthermore, the measured thermoelectric powers (θ) revealed positive values. Such characteristics of σ and θ definitely indicated the p-type semiconducting nature of CuBi2O4. The activation energy value (Ea) measured to be 0.43 eV suggested that the polaronic hopping conduction is the primary mechanism for electrical transport in CuBi2O4. The gas sensing properties of CuBi2O4 have been investigated toward various reducing gases (C2H5OH, NH3, H2, CO, and H2S) and oxidizing gas (NO2) for the first time. The sensor resistance and gas response characteristics were found to be dominantly affected by the concentration of intrinsic defect in CuBi2O4. As a result, the CuBi2O4 sensor with 500°C-calcined powder showed the highest gas responses (specifically, 10.4 toward 1000 ppm C2H5OH at the operating temperature of 400°C), with the highest defect concentration. Towards reducing H2S and oxidizing NO2 gases, the multiple reactions arising simultaneously on the surface of CuBi2O4 sensor are found to govern its response behavior, depending on gas concentration and operating temperature.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Flow diagram for powder synthesis of CuBi2O4; XRD patterns, nitrogen adsorption/desorption isotherms, and XPS spectra of powders; FE-SEM images of powders and sintered pellet; XRD pattern and FE-SEM images of the prepared CuO; photograph of the fabricated gas sensor;



additional gas response values and transients of CuO and CuBi2O4 sensors; the reported gas response values for multi-component oxides.

Author Information Corresponding Author *E-mail: [email protected] (Y.-H. Choi)

ORCID Yun-Hyuk Choi: 0000-0002-3120-1556 Seong-Hyeon Hong: 0000-0001-8350-2724

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4009757).

References (1) Zhou, X.; Lee, S.; Xu, Z.; Yoon, J. Recent Progress on the Development of Chemosensors for Gases. Chem. Rev. 2015, 115, 7944-8000. (2) Potyrailo, R. A. Multivariable Sensors for Ubiquitous Monitoring of Gases in the Era of Internet of Things and Industrial Internet. Chem. Rev. 2016, 116, 11877-11923. (3) Righettoni, M.; Amann, A.; Pratsinis, S. E. Breath Analysis by Nanostructured Metal Oxides as Chemo-Resistive Gas Sensors. Mater. Today 2015, 18, 163-171. (4) Di Natale, C.; Paolesse, R.; Martinelli, E.; Capuano, R. Solid-State Gas Sensors for Breath Analysis: A Review. Anal. Chim. Acta 2014, 824, 1-17. (5) Tricoli, A.; Righettoni, M.; Teleki, A. Semiconductor Gas Sensors: Dry Synthesis and Application. Angew. Chem. Int. Ed. 2010, 49, 7632-7659. (6) Bai, J.; Zhou, B. Titanium Dioxide Nanomaterials for Sensor Applications. Chem. Rev. 2014, 114, 10131-10176. (7) Kim, H.-J.; Lee, J.-H. Highly Sensitive and Selective Gas Sensors Using p-Type Oxide Semiconductors: Overview. Sens. Actuators, B 2014, 192, 607-627. (8) Hübner, M.; Simion, C. E.; Tomescu-Stănoiu, A.; Pokhrel, S.; Bârsan, N.; Weimar, U. Influence of Humidity on CO Sensing with p-Type CuO Thick Film Gas Sensors. Sens. Actuators, B 2011, 153, 347-353. 28 ACS Paragon Plus Environment

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Figures

Figure 1. TGA, DSC, and DTA curves of the 400°C-calcined powder, acquired under (a) air and (b) N2 atmospheres with a heating rate of 5°C/min from room temperature to 1000°C.



Figure 2. (a) XRD pattern of the black precursor powder obtained via burning of the polyesterified polymer resin at 400°C for 5 h in air. XRD patterns of the CuBi2O4 powders prepared through further calcination at (b) 500, (c) 600, and (d) 700°C for 6 h in air. The bottom bars are JCPDS standard for tetragonal CuBi2O4 (no. 42-0334).


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Figure 3. FE-SEM micrographs of the CuBi2O4 powders prepared through calcination of the precursor powder at (a) 500, (b) 600, and (c) 700°C for 6 h in air.

Figure 4. The core-level XPS spectra of (a) Cu2p, (b) Bi4f, and (c) O1s acquired for the CuBi2O4 powders prepared through calcination of the precursor powder at 500, 600, and 700°C for 6 h in air.



Figure 5. (a) PL spectra (excited by 259 nm wavelength) and (b) ESR spectra (recorded at an Xband microwave frequency, 9.19 GHz) of the CuBi2O4 prepared with different calcination temperatures 500, 600, and 700°C. The measurements were performed at room temperature.

Figure 6. (a) The optical absorbance plotted as a function of wavelength, (b) Tauc plots, and (c) photographs of the CuBi2O4 powders prepared at different calcination temperatures. 36 ACS Paragon Plus Environment

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Figure 7. (a) Electronic band structure and (b) density of states (DOS) of CuBi2O4. The Fermi level (EF) is set at 0 eV.

Figure 8. (a) FE-SEM image of fractured surface and (b) XRD pattern of the bulk CuBi2O4 pellet sintered at 700°C for 12 h. In (b), the red bars are JCPDS standard for tetragonal CuBi2O4 (no. 42-0334).



Figure 9. (a) Total electrical conductivity versus oxygen activity and (b) thermoelectric power versus oxygen activity of the bulk CuBi2O4 pellet measured at 550, 600, and 650°C.

Figure 10. Arrhenius plot for the estimation of activation energy (Ea) in the CuBi2O4 pellet sintered at 700°C. The acquired Ea is determined to be 0.43 eV.


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Figure 11. Cross-sectional FE-SEM images of the CuBi2O4 films prepared on SiO2/Si substrates using the synthesized powders with different calcination temperatures; (a) 500, (b) 600, and (c) 700°C, and their high-magnification images; (d) 500, (e) 600, and (f) 700°C.



Figure 12. (a) Gas response transients of the CuBi2O4 sensor prepared using 500°C-calcined powder and (b) its gas response values shown as a function of gas concentration toward 50−1000 ppm C2H5OH gas. The gas response transients and response values were obtained with the operating temperatures of 300, 400, and 500°C. (c) The resistances and (d) 1000 ppm C2H5OH gas response values of the CuBi2O4 sensors prepared with the calcined powders at 500, 600, and 700°C plotted as a function of operating temperature, measured in air. The 1000 ppm gas response values of CuO sensor are shown together for comparison in (d).


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Figure 13. The comparision of gas response values of the CuBi2O4 sensors prepared with different calcination temperatures of 500 and 700°C and CuO sensor. The gas response values were obtained with the operating temperature of 400°C toward 1000 ppm C2H5OH, NH3, H2, and CO gases.



Figure 14. Response transients of the CuBi2O4 sensor prepared with 500°C-calcined powder toward (a) 1−5 ppm H2S and (b) 5−10 ppm NO2 gases. The response transients were obtained with the operating temperatures of 400 or 500°C.


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Table of Contents (TOC)


CuBi2O4 Prepared by the Polymerized Complex Method for Gas

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