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Efficient Heterostructures of Ag@CuO/BaTiO3 for LowTemperature CO2 Gas Detection: Assessing the Role of Nanointerfaces During Sensing by Operando DRIFTS Technique Shravanti Joshi, Samuel J Ippolito, Selvakannan R. Periasamy, Ylias M. Sabri, and Manorama V. Sunkara ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07051 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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ACS Applied Materials & Interfaces

Efficient Heterostructures of Ag@CuO/BaTiO3 for Low-Temperature CO2 Gas Detection: Assessing the Role of Nanointerfaces during Sensing by Operando DRIFTS Technique Shravanti Joshi, a, b, c Samuel J. Ippolito, a, b, d Selvakannan Periasamy, a, b Ylias M. Sabri, a, b and Manorama V. Sunkara b, c*

a

Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science, College

of Science, Engineering & Health, RMIT University, Melbourne, VIC 3001, Australia. b

RMIT-IICT Research Centre, CSIR-Indian Institute of Chemical Technology, Hyderabad -

500007, India. c

Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, CSIR-Indian Institute of

Chemical Technology, Hyderabad - 500007, India. E-mail: [email protected], Tel.: +91 40 27193225; Fax: +91 40 27160921. d

School of Engineering, College of Science, Engineering & Health, RMIT University,

Melbourne, VIC 3001, Australia.

*To whom all Correspondences should be addressed

ACS Paragon Plus Environment


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ABSTRACT Tetragonal BaTiO3 spheroids synthesized by a facile hydrothermal route using Tween 80 were observed to be polydispersed with a diameter in the range ~15 to 75 nm. Thereon, BaTiO3 spheroids were decorated with different percentages of Ag@CuO by wet impregnation and their affinity towards CO2 gas when employed as sensitive layers in a microsensor was investigated. The results revealed that the metal-nanocomposite based sensor had an exceptional selectivity and sensitivity toward CO2 gas (6-fold higher response) with appreciable response and recovery times (250oC).11-22 However, the downside to many of the reported cases is that they typically utilised complex synthesis routes involving sintering at elevated temperatures, that are well known to result in the final material having a reduced surface area and oxygen defects, both of which are crucial for gas sensing applications. Among the hybrid heterostructures reported, p-CuO/n-BaTiO3 was observed to show high sensitivity towards CO2 gas, owing to the following probable reasons. Being a piezoelectric ceramic, BaTiO3 has a large bandgap of 3.3 eV at 300 K with a high carrier concentration23 of up to ~7 × 1021 cm-3 and shows minimal lattice mismatch (99.999%) was introduced into the sensing chamber. The CO2 concentrations obtained using air as a diluent were in the range of 100-10,000 ppm. BaTiO3 spheroids decorated with CuO micro-leaves to form CuO/BaTiO3 nanocomposites in 1:8, 1:4, 1:2, 1:1, 1.25:1, 1.5:1 mole ratios, hereafter will be referred as CB-1:8, CB-1:4, CB-1:2, CB-1:1, CB-1.25:1 and CB-1.5:1. Thus, the sensors tested for CO2 gas were pure BaTiO3, pure CuO and CuO/BaTiO3 nanocomposites in various mole ratios. All gas sensing experiments were carried out at a gas flow rate of 200 cm3/min. Sensor response (SR) is defined as (( − )/ ), while the sensitivity (S) of the sensor is defined as SR × 100 (%), where is the resistance of sensor in presence of CO2 gas and is the resistance of sensor under dry air.11-22

3. RESULTS AND DISCUSSION 3.1 Crystal Structure and Morphological Evaluation of BaTiO3 Spheroids Details on the influence of reaction parameters and growth mechanism of BaTiO3 spheroids are given elsewhere (See Supporting Information (SI), Section 1 and Figure S1-S4). Raman spectroscopy is an excellent technique to probe the atomic vibrations in a crystalline material. In case of BaTiO3, it conveniently enables investigating the cubic-to-tetragonal phase transition,


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invoking the fact that the cubic phase inherently does not exhibit Raman active vibrational modes.27 Figure 1a shows the micro-Raman spectrum of as-synthesized BaTiO3 powder at optimized reaction parameters, that is, 200oC for 24 h with 10 mM of Tween 80. Raman active vibrational modes of BaTiO3 can be identified as A1 (LO) at 187 cm-1 and [B1, E (TO+LO)] at 306 cm-1 indicating local asymmetry within the TiO6 octahedra of BaTiO3, [A1 (TO), E (TO)] at 518 cm-1 indicative of coupling of TO modes associated only with tetragonal phase and [A1 (LO), E (LO)] at 715 cm-1, consistent with the reported values.28-31 Characteristic peaks at 306 and 715 cm-1 confirm the tetragonal phase of BaTiO3.29-30 Interestingly, the peak observed at 306 cm-1 is sharp and suggests that the tetragonal phase is dominant over pseudocubic phase.31 A weak peak at 249 cm-1 due to [A1 (TO), E (LO)] mode of BaTiO3, usually arises from the splitting of triply degenerate mode F1u (TO) in the cubic phase.32-33 The diffraction peaks observed from X-ray diffractogram demonstrated high crystallinity (Figure 1b). The wellresolved Bragg’s diffraction in this pattern could be labeled and indexed to tetragonal crystal structure (JCPDS file no. 89-1428, a = 0.4006 nm and c = 0.4017 nm) with P4mm (99) space group. The mean crystallite size estimated by applying Scherrer’s formula to the intense diffraction peaks is ~23 nm. The calculated lattice constants from Rietveld refinement a = 0.4013 nm and c = 0.40212 nm are in good agreement with the reported values.29-31 Tetragonality (defined as c/a ratio) of the final product as determined by Rietveld refinement was found to be 1.002, which is less than the standard value of tetragonal phase (c/a = 1.0027 from JCPDS card no. 89-1428). The tetragonal BaTiO3 phase from the XRD pattern can usually be identiﬁed by the asymmetric broadening at 2Ɵ of ~45o which can be attributed to (200)/(002) planes (Inset of Figure 1b), while the cubic BaTiO3 phase only has one single diffraction peak at 2Ɵ of ~45o. Absence of diffraction peaks corresponding to by-products such as BaCO3 and TiO2



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indicate phase purity of the sample. The BET surface area of BaTiO3 powder calculated from the linear fit of the multipoint plot was found to be ~13 m2.g-1, in good agreement with the theoretical estimate and earlier report (Figure S5).4 X-ray photoelectron spectroscopy was performed to confirm oxidation states of metal ions in the synthesized material. Figure 1c and 1d illustrates the core level XPS spectra of Ba and Ti. The Ba elemental spectrum shows doublet at 3d5/2 and 3d3/2 separated by a spin-orbital splitting energy of 15.3 eV. The typical Ba (3d5/2) peak has two components separated by ~1.47 eV, one at 779 and other at 777 eV, indicating existence of Ba2+ in two different oxidation states for the sample in the region selected.34 The peaks centered at 779 and 794 eV confirm +2 oxidation state of Ba atoms in perovskite phase of BaTiO3, while the peaks centered at 777 and 793 eV confirm +2 oxidation state of Ba atoms on the surface phase of BaTiO3.35 Peaks centered at 780 and 796 eV can be assigned to Ba atom in BaO or linked to surface hydroxyl group.31 These findings were further substantiated with C 1s and O 1s analysis (Figure S6).36-37 The Ti 2p peak upon deconvolution shows the presence of two peaks centered at 458 and 457 eV. The former is assigned to +4 oxidation state of Ti in BaTiO3 and the latter the latter attributed to +3 oxidation state in BaTiO3 due to surface oxygen deficiency.31-37 The spin-orbital splitting energy of 5.8 eV for Ti is in agreement with the reported values.38-41. The formation of oxygen and/or barium vacancies is usually accompanied by a change in the oxidation state of the nearest neighbor atom in order to maintain the local charge balance, in this case, its the Ti atom which shows change in oxidation state from +4 to +3.31 These inferences validates that few structural defects are present in BaTiO3 due to local oxygen vacancies attributed primarily to the synthesis routes and influence of reaction parameters. The ratio of Ba:Ti is ~1.54, calculated from the peak area of Ba 3d and Ti 2p supports the presence of dormant BaO in the sample.


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The typical TEM images of the formed BaTiO3 which appear as spheroids, when synthesized by the hydrothermal route at 200oC for 24 h are illustrated in Figure 2a-c. The spheroids were seen to amalgamate and append on all sides (Figure 2a). High magnification image reveals highly polydispersed spheroids with diameter in the range ~15 to 75 nm (Figure 2b-c). The selected-area electron diffraction pattern for the BaTiO3 spheroids presented in Figure 2d elucidates the Laue spots along with rings, indicating polycrystalline nature of synthesized material. The SAED pattern could be indexed to the tetragonal P4mm (99) space group using the unit cell parameters. The high-resolution TEM image of spheroid in Figure 2e, further reveals clear lattice fringes with measured interplanar crystal spacing of ~0.28 nm that corresponds to the (101) crystalline plane, confirming growth direction perpendicular to [110].31,42 These results are in line with the micro-Raman and XRD patterns, validating that the hydrothermally synthesized BaTiO3 spheroids show a tetragonal dominant structure with a possibility of pseudocubic phase present in minor percentage. Elemental quantification and mapping of equimole ratio of prolated BaTiO3 spheroids showed a uniform distribution of Ba, Ti, and O elements throughout the sample (Figure S7). The Ba:Ti ratio calculated from EDS is ~1.72 (Table S7), nearly agreeing with the Ba:Ti ratio calculated from XPS, thus supporting the presence of dormant BaO. Nevertheless, presence of this dormant BaO plays an important role in sensing mechanism and will be discussed in detail in succeeding section (See Section 3.4).

3.2 Physicochemical Characterization of Ag@CuO/BaTiO3 Heterostructures The powder X-ray diffractograms of the as-synthesized pure BaTiO3, CuO and CuO/BaTiO3 nanocomposites are presented in detail elsewhere (Figure S8-S9). Figure 3a shows the stacked in-situ high temperature X-ray diffractograms of CB-1:1 recorded from 40 to 320oC with step of



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40oC. The nanocomposite was observed to be stable at high temperatures. Peak splitting was clearly visible at every temperature step at 2Ɵ ~40o, 46o and 66o corresponding to (111), (200) and (220) planes. Impurities due to by-products such as BaCO3, TiO2 or any other complexes were not visible at high temperatures, thus confirming phase purity of as-synthesized CuO/BaTiO3 nanocomposite. The BET surface area of pure CuO and CB-1:1 sample calculated from the linear fit of the multipoint plot was found to be ~36 and 29 m2.g-1 (See ESI Figure S10S11).The chemical composition of the nanocomposite after CuO decoration and in-situ reduction of Ag was ascertained by XPS. Figure 3b is the stacked XPS survey spectrum for pure BaTiO3, CB-1:1 and the equimole mole ratio CuO/BaTiO3 nanocomposite impregnated with 1 wt.% Ag (1%Ag-CB-1:1). The existence of Cu and Ag species was indicated by the presence of Cu 3p, Cu LMM, Cu 2p and Ag 3d peaks respectively. Core level Cu 2p XPS spectrum represented by the characteristic 2p3/2 and 2p1/2 peaks centered at 934 and 953 eV respectively indicated the presence of Cu in +2 oxidation state (Figure 3c).43 Deconvolution of Cu 2p core-level spectrum of copper at 933 eV resulted in two peaks, where the peak at 933 and 934 eV confirmed the presence of Cu in +2 oxidation state.43 Further, the existence of the two shake-up satellite peaks, one in the range 938-946 eV, deconvoluted into two Gaussian peaks centered at 941 and 943 eV, and the other at 962 eV in the spectrum also reiterates the oxidation state of copper in the nanocomposite as Cu2+ (CuO). The Ag 3d core level XPS spectrum shows peaks centered at 368 and 374 eV which can be assigned to 3d5/2 and 3d3/2 spin-orbit components respectively, separated by 6 eV (Figure 3d), thus confirming the metallic nature of Ag.44-45 This inference supports the efficiency of solution impregnation technique in reducing the silver salt Ag (I) to metallic silver Ag (0), in-situ. Presence of two characteristic loss features further substantiates the existence of zero valent silver in the nanocomposite.


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The FE-SEM image clearly revealed that CuO has a leaf-like structure with thickness around 10-20 nm (Figure 4a). Particle size and morphology of CB-1:1 nanocomposite was examined by FE-SEM (Figure 4b), TEM (Figure 4c) and HR-TEM (Figure 4d). The representative FE-SEM and TEM images of the nanocomposite show spheroids with size in the range 20-80 nm in close contact with CuO micro leaves (Figure 4b-c). Morphology of CuO and BaTiO3 in the nanocomposite is retained, thereby implying that the CuO decoration does not modify the morphology of the spheroids. From HR-TEM image (Figure 4d), the lattice fringes belonging to two interplanar distances of 0.28 and 0.25 nm, corresponding to (101) and (111) crystalline planes of BaTiO3 and CuO respectively were observed, in perfect agreement with earlier reports.31 This image further confirmed the formation of p/n heterojunction at the interface between CuO and BaTiO3. In summary, HT-XRD, XPS and HR-TEM results imply that BaTiO3 spheroids form an intermittent heterocontact with CuO micro-leaves in the nanocomposite, with a strong presence of metallic Ag.

3.3 Sensor Response Characteristics The gas detection system and electrode, ceramic tube heater type sensor element in theoretical test circuit is illustrated in Figure 5a. Prior to each experiment, the sensing element was preheated at 400oC for 6 h in air to stabilize the sensor surface, by removing adsorbed water formed over a period of time and residual organic species (Figure S12). The gas sensing studies involved measuring the resistance of pure BaTiO3 spheroids, pure CuO micro leaves and the CuO-BaTiO3 nanocomposites towards CO2 gas in 100-10,000 ppm range balanced in air at various operating temperatures followed by cross-sensitivity and long term repeatability patterns. It is well-known, that in metal oxide semiconductor based sensors, surface reactions taking place upon interaction of target gas with the sensitive layer leads to sensing phenomenon, that is



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dependent on operating temperature and hence its optimization is important.6-10,26 Electrical resistance measured in air as a function of operating temperature for controls and composites used as sensor material is detailed elsewhere (Figure S13.a). The sensor response as a function of operating temperature for pure BaTiO3, pure CuO, and CuO micro-leaves decorated BaTiO3 spheroids in different mole ratios is illustrated in Figure 5b. Pristine BaTiO3 spheroids and CuO micro leaves showed negligible response towards 1,000 ppm CO2 gas till the operating temperature reached 200oC. Upon p/n heterocontact formation between these two metal oxides, the sensor response improved significantly and with an increase in CuO loading, a maximum sensor response of 52% at 140°C for equimole ratio was observed, thereby establishing the promotional effect of CuO decoration.11-12,14 This temperature profile is attributed to surface adsorption-desorption phenomenon that takes place in metal oxides based gas sensors.26 Beyond 140°C, sensor response decreased steadily for each of the nanocomposites tested in this study. Saturation in response was observed for nanocomposites with CuO loading in mole ratio 1.25:1 and 1.5:1, suggesting that 1:1 mole ratio is the most suitable composition. Earlier reports suggests that the formation of equal p/n junction units compare to unequal units, facilitates decreases in the energy barrier at the grain boundary of BaTiO3 and CuO upon interaction of CO2 gas effectively, that subsequently reduces the width of the depletion layer, resulting in response improvement upon exposure to CO2 gas.11-22 The sensor response profiles of CB-1:1 as a function of operating temperature for different gas concentrations is illustrated in Figure 5c. Steady increase in sensor response was observed with increase in CO2 gas concentration, with a maximum sensor response of 69.80% towards 10,000 ppm gas at 140oC, thereby establishing it as the optimal operating temperature for CB-1:1. Interestingly, increase in sensor response with increase in CO2 concentration and nonattainment of saturation at 100 ppm suggests the broad


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detection scope of the sensor under study. Figure 5d shows the sensor response of CB-1:1 as a function of operating temperature for different weight percentages (wt.%) of Ag towards 1,000 ppm CO2 gas. Sensor response increased with increase in Ag wt.% till 1, thereafter it decreased steadily. The maximum value of 58.64% towards 1000 ppm gas concentration at 120°C was observed for 1%Ag-CB-1:1 nanocomposite. The decrease in optimum operating temperature from 140°C to 120°C indicates influence of Ag in promoting CO2 adsorption by catalyzing the CuO carbonation.11-12,14 Figure 6a exemplifies the sensor response as a function of CO2 gas concentration. The calibration curve shows the linear dependence of sensor response on CO2 gas concentration in the 100-1,000 ppm range. Beyond 1,000 ppm, saturation in sensor response was observed. Figure 6b illustrates the dynamic transients for CB-1:1 and 1%Ag-CB-1:1 at an operating temperature of 120oC. The cycles corresponding to different concentrations of CO2 gas were recorded sequentially ranging from 100 to 1,000 ppm. Over the entire range of gas concentrations tested, the Ag-impregnated CuO/BaTiO3 nanocomposite showed enhanced response magnitude and response/recovery times compared to the nanocomposite without Ag (Figure 6c). It was observed that CuO/BaTiO3 based sensors showed a fast response (

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