Gas Sensing Properties of CaO-BaTiO - ACS Publications

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Highly Selective CO2 Gas Sensing Properties of CaO-BaTiO3 Heterostructures Effectuated Through Discreetly Created n-n Nanointerfaces Shravanti Joshi, Frank Antolasic, Manorama V. Sunkara, Suresh K. Bhargava, and Samuel J Ippolito ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04453 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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ACS Sustainable Chemistry & Engineering

Highly Selective CO2 Gas Sensing Properties of CaO-BaTiO3 Heterostructures Effectuated Through Discreetly Created n-n Nanointerfaces Shravanti Joshi, a, b* Frank Antolasic, a Manorama V. Sunkara, b * Suresh K. Bhargava, a and Samuel J. Ippolito a, c *

a

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

Science, Engineering & Health, RMIT University, 124 La Trobe Street, Melbourne, Victoria 3001, Australia. b

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

Chemical Technology, Uppal Road, IICT Colony, Tarnaka, Hyderabad, Telagana 500007, India. E-mail: [email protected], [email protected], Tel.: +91 40 27193225. c

School of Engineering, College of Science, Engineering & Health, RMIT University, 124 La Trobe

Street, Melbourne, Victoria 3001, Australia. E-mail: [email protected], Tel: +61 3 99252673.

*To whom all Correspondences should be addressed

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Abstract Globally recognized for its role as an occupational hazard, carbon dioxide (CO2) detection and monitoring is essential in agriculture, chemical manufacturing and healthcare/clinical oriented applications. Although, optical and chemical gas sensors are available commercially, current gas sensing technologies involving selective monitoring of CO2 at lower detection limits specifically for industrial conditions still remains a formidable challenge. Herein, we present a simple strategy for highly selective CO2 detection using an inexpensive transducer platform based on reversible chemisorbed carbonation between CO2 and CaO-BaTiO3 heterostructures. Microsensor showed an optimum sensitivity of 65% towards 1000 ppm CO2 gas and superior selectivity when operated at 160oC. Such a remarkable sensing performance originates from the discreetly created n-n nanointerfaces and conveniently actualized staggered energy band positions that promote favorable charge transfer upon exposure to CO2 gas molecules even at parts per million levels. Reversible sensing phenomenon is demonstrated using operando timeresolved diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and correlated with energy band alignment determined from the ultra-violet diffuse reflectance (UV-DRS) spectra to propose the sensing mechanism.

Keywords: spheroids, n-n heterostructures, DRIFTS, CO2, selectivity, operando, carbonation.

Introduction Carbon dioxide (CO2) is a versatile gas owing to its significant use in many industrial and biological processes, each taking benefit of its one or more inherent attributes such as thermodynamic stability, chemical inertness and high density. However, this odorless and colorless gas can be particularly perilous to humans at higher concentrations (>5000 ppm),


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causing damage to respiratory function, even to an extent of making the central nervous system dysfunctional. National Institute for Occupational Safety and Health (NIOSH), Occupational Safety and Health Administration (OSHA), and the American Conference of Governmental Industrial Hygienists (ACGIH) recognizes CO2 as a toxic contaminant and strong asphyxiant.1-2 Further, this gas plays an essential role in chemical-physical phenomenon of the atmosphere resulting in ocean acidification, global warming etc.3-4 Interestingly, CO2 is extensively used in the production of carbonated beverages, in fire extinguishers and as a compressed gas for many portable pneumatic systems.5 Furthermore, CO2 is an important gasotransmitter with potential application in non-invasive early diagnosis of stomach cancer.6 Hence, in the aforementioned broader context, it is imperative to quantify CO2 gas in the presence of common cross-interfering gases. Thus, the development of highly sensitive, accurately selective sensors detecting lower CO2 gas levels that can operate in a wide range of surroundings, is a significant step in the environmental monitoring and controlling its concentrations in many applications. Previous researchers have reported on fluorescent and/or colorimetric,7 surface plasmon resonance (SPR),8 surface acoustic wave (SAW),9 micro cantilever,10 based CO2 gas sensors to name a few. Although, high sensitivity is an attractive feature of these CO2 sensors, but there are still many concerns which restricts the commercial availability of these kinds of sensors. Briefly, due to the small dimension for light confinement, fluorescent and/or colorimetric sensors are highly sensitive to small variations in refractive index. However, major limitations includes nonreliability, contamination issues, poor selectivity and many a times the device will not provide a reading if it is clogged or is damaged.7 Simplistic features of SPR mediated devices allow its use as a versatile and highly sensitive optical sensor. But several challenges such as limit of detection, selectivity and incorporation of SPR into multiplexed platforms still restrict their wide



scale research.8 SAW and micro-cantilever based CO2 sensor exhibited exceptional limit of detection, other parameters such as selectivity, stability and response/recovery times were rarely reported which are important from sensor feasibility point of view.9-10 Apart from the aforementioned sensors, current CO2 monitoring technologies are principally based on non dispersive infrared (NDIR), electrochemical and chemo-resistive methods.11-12 NDIR spectroscopic methods are commercially utilized within various industrial processes. This technique depends on unique optical fingerprint of CO2 gas resulting in its high suitability for detecting lower concentration range.12 Nevertheless, there are some major disadvantages that are associated with this technology, that is, they have the tendency to suffer from cross-sensitivity issues when interfering compounds (such as SO2, NO2 and CO) are present in the environment or sample. Additionally, the instruments employed are expensive, complex and typically requires skilled operators to supervise their operation. Instead, electrochemical method has an advantage of being high selectivity and show robust long-term repeatability towards the target gas.12 Further, the electrochemical method predominantly relies on simplistic oxidation-reduction of CO2 gas, but at the cost of complex configuration of sensor element and high operation temperatures (~400-700oC) consequently, resulting in high operation expenses that limit their application mostly to petrochemicals, refineries, steel-making and automotive industries.1 In these scenarios, chemo-resistive sensing is cost-effective, easy to miniaturized and facile technique which leans on reversible carbonation reaction actualized through a charge transfer between metal oxides and physical/chemisorbed CO2 gas species.11-13 Interestingly, pristine or functionalized metal oxides show high sensitivity and stability, however in the majority of cases at the helm of poor selectivity in the presence of various interfering gases.13 Thus, considering the significant trade-offs between various sensing parameters and ease of fabrication, there still


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exists an ever progressing challenge for developing suitable sensing materials which exhibits selectivity and the pursuit for ideal yet economic transducing platform for CO2 gas sensing. Recently, we demonstrated BaTiO3 spheroids functionalized with CuO leaves as suitable candidate.14 Nanostructured CuO/BaTiO3 active layer demonstrated excellent sensitivity and long-term repeatability with ultra-fast response/recovery kinetics towards CO2 gas, although the selectivity was met with limited success. Calcium oxide (CaO) is widely investigated as CO2 capturing agent, 15 however it has been rarely reported as a sensitive layer material. Given that the CO2 sensing mechanism is primarily based on a reversible chemisorbed reaction between acid oxide (CO2) and basic oxide (for example, CuO) to form salt (for example, CuCO3),14 it is anticipated that CaO as an additive will be effective in increasing the selectivity of the sensitive layer owing to its high CO2 adsorption capacity. With regards to CO2 sensing, among the additives reported such as MgO, NiO, CuO, CeO2, PbO to name a few,16 CaO has shown high affinity towards CO2 and with a wide gap of 7.7 eV forms in theory a straddling heterojunction (type I) with BaTiO3 which has a bandgap of 3.2 eV. Recently, Sun and co-workers17 investigated NO2 and CO sensing properties of n-CaO decorated n-ZnO heterostructures and attributed its selective performance to the larger modulation of potential barrier at the n-n interface. Nonetheless, a comprehensive yet fresh perceptive on CaO-BaTiO3 based n-n heterostructures and its effect on the gas sensing performance still needs to be investigated. In this contribution, we present a convenient approach for selective and reversible CO2 monitoring based on n-n heterostructures of CaO-BaTiO3, integrated onto a cost effective sensing platform. To demonstrate this novel concept, a detailed selectivity study is presented for the various sensitive materials including its repeatability and long-term performance in presence of common cross-interfering gases (ethanol (C2H6O), acetone (C3H6O), hexane (C6H14), methanol



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(CH3OH), hydrogen (H2), nitrogen dioxide (NO2), nitric oxide (NO), sulfur dioxide (SO2) and carbon monoxide (CO)). Further, the focus of this work was also to understand the low temperature charge transfer phenomenon taking place between the surface of sensitive layer and adsorbed CO2 gas species that is evidenced by operando time-resolved DRIFTS technique. Additionally, a correlation between the chemisorbed based carbonation reaction and the band alignment at the nanointerface involved in the selective performance is outlined.

Experimental Materials Barium

hydroxide

octahydrate

((BaOH)2.8H2O),

titanium

dioxide

(TiO2, particle

size ~ 20 nm), Tween-80 (C64H124O26) and calcium hydroxide (Ca(OH)2) were procured from Sigma-Aldrich Chemicals Co. All the chemicals were of analytical reagent (AR) grade and were used without any further purification. Ethanol (undenatured 70% w/w LR grade) was procured from Chem-Supply Pty Ltd. Throughout the chemical synthesis, Sartorius Stedim Biotech S.A (Model-Arium 61316) deionized water (18.2 MΩ.cm) was used.

Synthesis of CaO-Decorated BaTiO3 spheroids BaTiO3 spheroids were synthesized by following synthesis route reported in our earlier work.14 To decorate BaTiO3 spheroids with 25 wt.% of CaO, a convenient strategy was employed. Briefly, ca. 67 mg of as-synthesized BaTiO3 was added to 40 ml of ethanol in a beaker and stirred for 45 min. To this solution, 33 mg of calcium hydroxide was added to prepare 100 mg/40 mL Ca-Ba-Ti-O complex mixture solution. The calculated amount of calcium hydroxide varied based on weight percentages. After stirring, the resultant mixture was allowed


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to evaporate at 70oC using hot plate. Subsequently, it was weighed and calcined at 600oC for 1 h in muffle furnace under air atmosphere. Heat treatment ensured complete conversion of Ca(OH)2 to CaO particles with increased adhesion of the CaO to the BaTiO3 spheroids.

Materials Characterization Transmission electron micrographs (TEM) and selected area electron diffraction (SAED) pattern were taken at 100 kV on JEOL 1010. Additionally, high-resolution transmission electron micrographs (HR-TEM) were captured at 200 kV on a JEOL 2010. TEM sample were prepared by dispersing sample powder in iso-propanol using an ultra-sonicator followed by drop-casting onto a strong carbon copper grid. Energy dispersive X-ray analysis (EDS) was performed on FEI Quanta 200 ESEM equipped with Oxford X-MaxN 20 EDXS detector without sputtering at an accelerating voltage of 25 kV. Sample preparation consisted of drop casting powdered solution (5 µL) prepared using iso-propanol on a gold (Au) coated silicon wafer. Powder X-ray diffractograms (XRD) were recorded on a Bruker AXS diffractometer (D8 ADVANCE) with Cu anode (Kα radiation, λ = 1.5406 Å) and data was collected in the 2ϴ range, 20 to 70° in continuous scanning mode with 0.01° sampling pitch and 5o min-1 scan rate. UV-Vis-NIR spectrophotometer (Varian Cary 5000) was used to record ultra-violet diffuse reflectance spectroscopy (UV-DRS) of the powdered samples in the wavelength range of 200-800 nm. Energy band gap energy of BaTiO3 and CaO powder samples was deduced from their diffuse reflectance spectra according to Kubelka-Munk (K-H) theory. Whereas, for the CaO decorated BaTiO3 composites in various weight percentages, band gap energy was calculated by extrapolating the linear region of the absorption spectra to the abscissa. X-ray photoelectron spectroscopy (XPS) was employed to analyze surface chemistry of the as-synthesized



nanomaterials using a Thermo Scientific K-Alpha instrument (monochromatic Al Kα radiation, Ephoton=1486.6 eV). Flood gun was switched off while running valence band maxima (VBM) analyses for the samples prepared by drop casting a thin layer of nanomaterial on Au substrate and then analyzing it. The binding energy (B.E) in each case, that is, core levels and valence band maxima was corrected with an internal reference peak, i.e. C 1s peak centered at 284.8 eV. The spectral components were background corrected using the Shirley logarithm and deconvoluted using a Gaussian-Lorentzian function taking least-squares fitting into account. CO2 sensing mechanism was substantiated using an operando technique following the protocol reported in our previous work.14 Conventional and time-resolved diffuse reflectance infrared Fourier transform (DRIFT) spectra were collected using a Perkin Elmer Fourier Transform Infrared (FT-IR) spectrometer fitted with a Harrick Praying Mantis DRIFTS unit. The sensitive material was mixed and grounded with KBr (ca. ratio 1:100) and 200 mg of this mixture was then placed into a custom-built sensor chamber fitted with a KBr window. The sensor chamber was heated to desired temperature using the heater placed at its bottom. Test gases were pumped into the chamber at a flow rate of 200 sccm using a homemade mixing setup. The absorbance spectra was collected for 1024 scans with a resolution interval of 32 cm-1, were corrected for baseline and normalized before comparative evaluation post each run. Care was taken to avoid contamination by using freshly prepared mixture of sensitive material and KBr powder during every DRIFTS run.


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Chemoresistive Sensor Set-up, Fabrication and Measurements Custom built gas sensor set-up, element fabrication and sensing test patterns are described extensively in our previous works.12,14 Briefly, ca.10 mg of as-synthesized nanomaterial was dispersed in 10 mL of 1:1 mixture of deionized water and ethanol via ultra-sonication at room temperature. In each case, the sensor element was fabricated by dip-coating the cylindrical alumina tube (length (L) - 15 mm and diameter (D) - 5 mm) provided with two platinum (Pt) electrodes (Sigma-Aldrich Chemicals Co.CAS No. 7440-06-4) for the electrical contacts into the nanomaterial suspended slurry. The platinum wire procured (D - 0.5 mm) showed an electrical resistivity of 10.6×10-6 Ω.cm at 20oC. Nikrothal 80 wire (Sandvik Asia Pvt. Ltd) was used as heater with coil inserted inside the alumina tube to maintain the high temperature during the sensing experiments. Prior to each sensing experiment, the sensor element was preheated at 200oC for 2-3 h in synthetic dry air to stabilize the sensor surface. During this process, care was taken to remove any adsorbed hydroxyl group and/or residual organic species formed over a period of time. All the gas sensing experiments were carried out in a dry synthetic air conditions. The sensors were tested at various operating temperatures, ranging from 100 to 200°C with an increment of 20oC. The temperature was controlled by a ceramic heater coupled to a power supply utilizing K-type thermocouple placed inside the sensor chamber as a feedback for proportional-integral-derivative (PID) controller. Industrial purity grade CO2 gas (>99.999%) was diluted using air to obtain concentrations in the range of 50-1000 ppm at a gas flow rate of 200 sccm. Sensor response (SR) is defined as,

‫= ܀܁‬

‫܉܀ ି܏܀‬

(1)

‫܉܀‬

While the sensitivity (S) of the sensor is defined as, S = ‫ × ܀܁‬100 (%)

(2)



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where ܴ௚ is the resistance of sensor in presence of CO2 gas and ܴ௔ is the resistance of sensor under dry air.14 Response and recovery times are defined as the time period required to attain 90% of the resistance change upon exposure to CO2 gas and air, respectively.17

Results and Discussion Physico-chemical Characterization of CaO-BaTiO3 Heterostructures The BaTiO3 powder was impregnated with Ca(OH)2 in 1, 3, 5, 10, 25 and 50 wt.% by adding ca. 1.3, 4, 6.60, 13.20, 33 and 66 mg, respectively to make a total of 100 mg Ca(OH)2-BaTiO3 mixture followed by calcination at 600oC for 1 h to achieve CaO-BaTiO3 heterostructures. Here after,

the

resulting

n-n

heterostructures

will

be

referred

to

as

“R-CaO-B”

(R = Ca(OH)2 concentration in wt.%), that is, 1CaO-B, 3CaO-B, 5CaO-B, 10CaO-B, 25CaO-B and 50CaO-B, respectively. This strategy conveniently resulted in creating discrete n-n nanointerfaces heterostructures, as illustrated in Figure S1 (Electronic Supporting Information (ESI)). TEM images of 25CaO-B composite are presented in Figure 1. Distribution of n-BaTiO3 spheroids over n-CaO sheets is intermittent and non-uniform, with clearly visible nanointerfaces in small numbers (Figure 1a-b). High-resolution TEM images of the nanocomposite in Figure 1c-d, 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].14 Further, the interplanar lattice spacing of ~0.24 nm corresponds to (200) plane of cubic CaO (Figure 1e-f).17 The BaTiO3 nanostructures exhibited highly polydispersed spheroids with diameter in the range ~15 to 75 nm (Figure S2a), which is a sufficient dimension choice to obtain stable and reproducible gas sensing performance.17 High resolution micrograph revealed sets of Moire fringes indicating relative orientation and lattice mismatch among the spheroids


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(Figure S2b). SAED pattern for BaTiO3 spheroids illustrated the Laue spots along with rings, indicative of polycrystalline nature of the as-synthesized nanomaterial (Figure S2c). CaO powder displayed an irregular sheet like morphology (Figure S3a), whereas the Laue spots observed from SAED pattern (Figure S3b), further confirmed its polycrystalline nature. Upon decoration, the stand-alone metal oxides showed high tendency in retaining its original morphology (Figure S4), thus verifying the feasibility of the synthesis route. In an attempt to carry out more precise chemical composition analyses, EDS area scanning and elemental mapping of 25CaO-B sample was performed over a true area of 10×10 µm (Figure S5 and S6). The results clearly indicated uniform distribution of elements, namely Ba, Ti, Ca, and O throughout the heterostructured composite sample. X-ray diffraction patterns for control samples and CaO-BaTiO3 heterostructures in various weight percentages post calcination at 600oC for 1 h were examined for phase purity and crystallinity (Figure 2). Sharp intensity peaks evident from each diffraction pattern confirmed crystallinity of as-synthesized nanoarchitectures. Highly crystalline Bragg’s diffraction pattern for pure BaTiO3 could be assigned to tetragonal crystal structure (JCPDS file no. 89-1428, a = 0.4006 nm and c = 0.4017 nm) with P4mm (99) space group (Figure 2a).14 CaO pattern exhibited peaks that could be readily indexed to (101), (111), (021), (200), (220), (041), (311) and (222) reflections from face centered cubic rock salt structured CaO (JCPDS file no. 82-1691, a = 0.4796 nm) (Figure 2b).17 Interestingly, calcium is one of the most highly reactive alkaline earth metals and hence it is always found as a compound.15 Usually, calcium compounds such as CaO or Ca(OH)2 readily reacts with atmospheric CO2 to form carbonates and bicarbonates. This was observed even in the present study, wherein the two low intensity peaks at 2ϴ = 28.41o and 28.83o indicates the presence of CaCO3 as an impurity in minor but detectible quantities



(Figure 2b). X-ray diffractograms corresponding to different weight percentages of CaO decorated BaTiO3 heterostructures confirmed that no significant modification of the crystal structure for the detached singularities took place (Figure 2c-h). For the heterostructures with CaO content ≤3 wt.%, peaks concerning only BaTiO3 in the XRD pattern were visible (Figure 2c-d). This could be a result of small sized CaO nanoparticles with relatively low crystallinity intermittently distributed without significant agglomeration.18 Above 3 wt.%, presence of CaO peaks became distinguishable (Figure 2e). The XRD pattern of BaTiO3 decorated with CaO in 10 and 25 wt.% that is, 10CaO-B and 25CaO-B (Figure 2f-g), showed peaks corresponding to tetragonal BaTiO3 and fcc-CaO, respectively suggesting their coexistence in the composite as independent phases. Further, till 25 wt.% loading, no obvious peak shifts were observed indicating that Ca ions were not incorporated into the BaTiO3 lattice even after heat treatment at 600oC for 1 h.18 At 50 wt.%, CaO peaks became dominant, with considerable shifting and splitting of peaks (Figure 2h). XPS was performed to probe into the surface composition and oxidation states of the metal ions in the as-synthesized nanomaterial. Wide scan survey spectra of pure BaTiO3 spheroids and 25CaO-B composite displayed photoemissions only of Ba 4d, Ba 4p, Ba 3d, Ti 2p, Ti 2s, C 1s, O 1s, O KLL, Ca 2s, Ca 3s and Ca 2p (Figure S7). Narrow elemental scan of Ba 3d spectrum showed doublet at 778.9 and 794.3 eV corresponding to 3d5/2 and 3d3/2, respectively and separated by a spin-orbital splitting energy of 15.4 eV (Figure S8a), indicating existence of Ba2+ in the composite.14 High resolution core level Ti 2p spectrum displayed binding energies of 458.2 and 464.1 eV, assigned to 2p3/2 and 2p1/2 respectively, confirming existence of Ti4+ in the perovskite phase of BaTiO3 (Figure S8b). The spin-orbital splitting energy of 5.9 eV for Ti 2p is in good agreement with our previous report.14 Core level Ca 2p spectrum for 25CaO-B


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heterostructured sample displayed two major peaks at binding energies of 346.9 and 350.5 eV, assigned to 2p3/2 and 2p1/2, respectively separated by a spin-orbital energy of 3.6 eV (Figure S9a).19 Further, the Ca 2p spectrum exhibited two characteristics satellite loss features, confirming presence of Ca2+ in the sample.19 Additionally, UV-DRS was employed to determine energy band gap of CaO, pure BaTiO3 and CaO-BaTiO3 heterostructures, which was deemed as an important information for the determination of band bending at the n-n nanointerfaces (Figure S9b). Pristine BaTiO3 spheroids showed high intensity absorption band edge at 391 nm compared to the low intensity band edge in the range 401-422 nm demonstrated by the CaO-BaTiO3 heterostructures. The absorption band edges were observed to be 391 nm (3.17 eV), 250 nm (4.96 eV), 414 nm (3.01 eV), 407 nm (3.05 eV), 401 nm (3.09 eV), 405 nm (3.06 eV), 407 nm (3.05 eV) and 422 nm (2.94 eV) for pure BaTiO3, CaO, 1CaO-B, 3CaO-B, 5CaO-B, 10CaO-B, 25CaO-B and 50CaO-B, respectively. To gain deeper insights into the change in energy band structure and subsequent n-n nano interface formation, a series of XPS measurements were carried on various CaO-BaTiO3 composites and displayed in Figure 3. Representative stacked core level of barium (Ba 3d) for pure BaTiO3 showed emission lines at 779.3 and 794.6 eV for 3d5/2 and 3d3/2 binding energies, respectively (Figure 3a). However, with an increase in CaO weight percentage, Ba 3d showed shifting of emission lines towards higher binding energies. This shift could be attributed to the formation of space charge layer at the n-n nanointerfaces in the heterostructured composite.20,21 For the 50CaO-B composite, Ba 3d5/2 and Ba 3d3/2 peak were positioned at 780.2 and 795.7 eV, respectively. The corresponding shift to a higher binding energy of 1±0.05 eV could be attributed to the shift of the Fermi level (EF) possibly induced by band alignment in the BaTiO3 spheroids. The core level photoemissions are also susceptible to the changes impelled by the adsorption of



molecules on the surface to form covalent and/or ionic bonds resulting in chemical shifted.21,22 Equivalent trend was observed in case of Ti 2p (Figure 3b). Similarly, increase in Ca 2p peak intensity and shift towards higher binding energy was observed with increase in CaO percentage (Figure S10). This spectrum exhibited asymmetric multiplet for 1CaO-B sample at 347.1 and 350.8 eV, which is in good agreement with the existence of the calcium cation as Ca2+ for CaO.19 Interestingly, for the heterostructured samples with lower CaO loading (< 10 wt.%), signature singlet Ca 2s remained undeveloped, indicating the formation of CaO decorated BaTiO3 spheroids in small quantities (Figure S10). However, above 25 wt.% CaO loading, a fully developed Ca 2s core level was clearly visible. The O 1s XPS spectrum was observed to be wide and displayed asymmetric multiplet (Figure 3c). However with an increase in CaO weight percentage, the spectra changed gradually forming a fully symmetric shape and in each case photoemission shifted towards lower binding energy, indicating strong presence of diverse forms of atomic oxygen in binding states.21,22 Additionally, the valence band maximum (VBM) was determined by linearly extrapolating the binding energy edge in the XPS scans to the base line (Figure 3d). In case of pure BaTiO3 (Figure 3d), peak at 5.4 eV is due to first valence band spectrum attributed primarily to O 2Π bonding orbital.23 In addition, peaks for Ba 5p, O 2s and Ba 5s are observed at 13.7, 21.3 and 28.6 eV, respectively.23 With 1 wt.% CaO loading, shift and split in Ba 5p peak was observed, thus confirming presence of CaO. Above 10 wt.% CaO, three new peaks emerged at 7.2, 24.9 and 28.3 eV, attribute to O 2p, O 2s oxygen levels and Ca 3p core level of Ca in CaO, respectively.24 Ca 2p showed small but noticeable energy dispersion. Further, a stark decrease in major peaks belonging to pure BaTiO3 VBM, namely, Ba 5p, O 2s and Ba 5s was observed with an increase in the CaO loading. Moreover, from Figure 3d, the evolution of the VB spectra of heterostructured samples revealed, that the VBM for 50CaO-B


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was shifted towards a higher binding energy with increasing CaO weight percentage. The valence band was shifted by binding energy of 0.9±0.05 eV, which was consistent with the Ba 3d core level shift. XPS spectra depicting evolution of core level peaks of Ba 3d, Ti 2p, O 1s, Ca 2p and Ca 2s (Figure 3a-c and Figure S10) as a function of CaO weight percentage displayed minimal change in the spectral lines, except for slight broadening of all peaks and binding energies shift. This further verifies our argument, which is in agreement with the X-ray diffractograms (Figure 2), that no solid solution formation or irreversible chemical reaction occurred between BaTiO3 and CaO. Interestingly, crystal ionic radius of Ba2+ and Ca2+ at a coordination number (CN) of 6 is 149 and 114 pm, respectively. Hence, the substitution of Ca2+ in Ba2+ of BaTiO3 matrix to form a solid state solution would have been impossible due to the considerable differences in the ionic radius, even though the heat treatment temperature was as high as of 600oC.18

Gas Sensing Characteristics Initial CO2 sensing studies involved determination of optimal operating temperature and appropriate amount of CaO decoration in weight percentage at which highest sensor response magnitude could be obtained. The sensor responses as a function of operating temperature for control samples (only BaTiO3 spheroids and CaO sheets) and CaO-BaTiO3 heterostructures are presented in Figure 4a. Pristine BaTiO3 spheroids showed negligible response towards 1000 ppm CO2 gas balanced with synthetic dry air at all temperatures, however CaO sheets exhibited minor response at 160oC (Figure 4a). Upon n-n heterocontact formation between these two metal oxides, the response magnitude increased substantially and a maximum sensor response of 65% at 160°C for 25CaO-B composite was achieved. Based on this result, 160oC was found to be the



optimal operating temperature. Further, the response magnitudes decreased rapidly beyond 160°C for each of the heterostructures tested towards CO2 gas. Additionally, deterioration in the sensor response for the nanocomposite with CaO loading of 50 wt.%, (50CaO-B) was observed, thus confirming that the 25CaO-Bcomposite is the most advantageous composition among the range of samples tested in this study. However, the sensor response curve for 25CaO-B as a function of CO2 gas concentrations at 160oC showed a steady fall in the response magnitude with decrease in CO2 gas concentration (Figure 4b). Figure 4c shows the dynamic transients for 25CaO-B composite at 160oC. The gas sensing transients for pure BaTiO3 and CaO sensitive layers are not shown in the Figure 4c, as neither of the sensitive layers showed significant response magnitude characteristics as observed from the temperature profile (Figure 4a). Sensing test pattern corresponding to different CO2 concentrations were recorded successively ranging from 50 to 1000 ppm. The microsensor exhibited typical n-type behavior of metal oxide semiconductor towards an oxidizing gas, that is, increase in resistance of sensitive layer upon exposure to target gas.14,17 Interestingly, it was observed that the response magnitude of the sensitive layer kept increasing even after the exposure period had finished for 100 to 500 ppm pulses, but displayed a very stable response for a significant amount of time for the 700 and 1000ppm pulses. This could be attributed to the unsaturation of CaO-BaTiO3 composite in presence of lower CO2 concentration owing to which response magnitude kept on increasing compared to its performance in presence of higher amounts. Response time (t90) is highly dependent on the gas concentration and for sake of simplicity, it was estimated as the difference between the time gas was pumped in (that is, start of the yellow period in Figure 4c) and the time gas was flushed out (that is, end of yellow period in Figure 4c). Sensor responses for 25CaO-B based microsensor showed a superior response and recovery times toward the tested CO2


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concentrations (Figure 5a) compared to the control samples (Figure S11). The response and recovery times for 25CaO-B composite for different gas concentrations were in the range 7-15 and 6-35 s, respectively. Calibration curves showed a linear dependence of sensor response on CO2 gas concentration in the 50-1000 ppm range for various sensors tested in the current study (Figure 5b). Long-term repeatability of the n-n heterostructured gas sensors was tested by exposing the control samples and 25CaO-B composite towards CO2 gas balanced with dry air over a period of 40 days continuously at 160°C (Figure 6a). Among the three sensitive layers, 25CaO-B heterostructured composite exhibited exceptional repeatability with a nearly constant response magnitude towards 1000 ppm gas concentration. However, response magnitude of CaO sheets decreased steadily and at the end of 30 days, it showed no response towards the target gas. Rapid fall in the sensor response observed here could be attributed to the high absorption capacity of CaO towards CO2, which could have led to irreversible formation of CaCO3.15 The capability to selectively distinguish a target gas in the presence of multiple interfering gases commonly found in simulated industrial ambience for example, agriculture, petrochemical, steel sectors etc for any sensor is essential from smart monitoring and accurate early diagnostics applications viewpoint. Thus, it was imperative to study CO2 selectivity in presence of various reducing, oxidizing and corrosive gases namely acetone (500 ppm), ethanol (500 ppm), hexane (1000 ppm), methanol (500 ppm), H2S (100 ppm), H2 (50 ppm), CO (50 ppm), NO2 (1000 ppm), NO (100 ppm) and SO2 (50 ppm) at 160oC taken in various industrially meaningful concentrations. These gases are well-known to play a decisive role in the chemistry of the earth’s atmosphere in producing acid rain, smog etc that have a damaging impact on the environment and living beings.13-14,17 Selectivity of 25CaO-B sample was confirmed with reference to control samples (pure BaTiO3



spheroids and CaO sheets) and is presented in Figure 6b. Pure BaTiO3 showed significantly lower response to CO2 and the specific analyte gases, however CaO sheets did exhibit a small but consistent response magnitude to all pulses which contained CO2 (Figure 6b). Further, the results revealed that 25CaO-B composite exhibited dominant CO2 selective sensing characteristics compared to the CaO towards various reducing and oxidizing gases balanced in synthetic dry air. Dominant selectivity shown by 25CaO-B composite was confirmed again by comparing the gas responses to CO2 containing mixed gases (1000 ppm CO2 + interference gas 1). The results clearly indicated that intermittently formed n-n nanointerfaces in an optimum amount played a pivotal role in achieving highly selective CO2 gas sensing response (Figure 6b). Selected gas concentrations were well below the immediately dangerous levels to life as defined by NIOSH.12,12

The exposure limit of CO2 defined by NIOSH is 5000 ppm.12 Considering this gas

concentration, the CO2 selectivity of heterostructured sensor was found to be ~65% at 1000 ppm, which is an encouraging result. Significantly stronger response to CO2 gas even in presence of interfering gases at low activation temperature exhibited by 25CaO-B sample could be attributed to the larger modulation of the potential barrier height at the CaO-BaTiO3 nanointerface,17 discreetly actualized through a facile strategy using Ba and Ca precursors available in abundant amount worldwide. However, the dominant selectivity observed here could be attributed to high CO2 adsorption ability shown by CaO compared to other common interfering gases.

Dominant and Reversible CO2 Sensing Mechanism Sensor response towards CO2 gas observed from 25 wt.% CaO-BaTiO3 heterostructured sensitive layer was corroborated by employing DRIFTS technique in absorbance mode at 160oC. Figure 7 shows the evolution of DRIFTS spectra for 25CaO-B composite obtained by following the experimental procedure described extensively in our earlier report.14 Briefly, the spectrum


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was taken at room temperature on freshly synthesized sample in dry synthetic air condition thus, allowing the determination of organic residues with an absolute certainty on the surface of the composite (Figure 7a). Thereon, under dry air environment the composite was maintained at stable temperature of 160oC for 10 mins and then the spectrum was collected (Figure 7b). Subsequently, at a stable temperature of 160oC, CO2 gas was introduced into the chamber at a flow rate of nearly 100 sccm measured using a rotameter (manufactured by Key Instruments) and the corresponding spectrum was recorded (Figure 7c). This was followed by collecting spectrum after flushing out of CO2 gas by dry air from the custom-built chamber to verify the reversibility of the sensing phenomenon (Figure 7d). The spectrum of the heterostructured composite achieved by exposing it to dry air at room temperature shows a weak peak at 416 cm-1 and broad peak in the range 450-750 cm-1, attributed to vibrations of R–O (where R = Ba and/or Ca) and Ti–O stretching modes of BaTiO3 in CaO-BaTiO3 composite (Figure 7a).14 Infrared (IR) bands at 862 and 1072 cm-1 are characteristic peaks due to physisorbed CO2.25 These peaks emerged in the spectrum primarily due to the high affinity of BaO and/or CaO with CO2 to form respective carbonates, that is, BaCO3 or CaCO3. The higher wavenumber band at 1418 cm-1 is attributed to the splitting of doubly degenerate stretching mode of –CO3 in the adsorbed condition.26-27 The –C–H stretching vibration at 2514 cm-1 are attributed to the presence of adsorbed organic species.28 A sharp peak due to superficial –OH group and adsorbed molecular water is visible at 3640 cm-1.11 Upon heating the sample to 160oC, IR band due adsorbed hydroxyl group increased in intensity whereas the band due to carboxylate group decreased (Figure 7b). Further, the intensity of IR bands at 862 and 1072 cm-1 decreased. Introduction of CO2 gas into the chamber, resulted in several new peaks that subsequently disappeared on evacuation, thereby confirming that the CO2 gas adsorption/desorption processes are completely



reversible (Figure 7c and 7d). The peak at 2334 cm-1 could be attributed to the atmospheric CO2 interference, which was not corrected while recording the spectrum emerged upon flushing CO2 gas into the chamber. For ease of interpretation, this peak was excluded from the studies. Appearance of peak at 666 cm-1 could be attributed to the fundamental mode of carbonate ion 29-30 group (−COଶି hints at the formation of carbonate species (CaCO3 ଷ ) due to in-plane bending,

and/or BaCO3) on the surface in presence of CO2 gas, which could be the reason for enhanced sensor response exhibited by CaO-BaTiO3 heterostructures. A similar protocol was followed and operando time-resolved DRIFTS study of 25CaO-B sensor exposed to 100 ppm of CO2 gas at 160oC was performed (Figure 8). The experiment consisted of stabilizing the sensor in N2 atmosphere (10 min) followed by alternating exposure to CO2 gas and recovery in N2.31 Timeresolved DRIFT spectrum (2D plot) revealed presence of continuously growing characteristic IR bands (660, 2340 and 3640 cm-1) only in presence of CO2 gas, clearly indicates that there are carbonate and hydroxyl groups on/near CaO-BaTiO3 surface involved in the sensing phenomenon. This study is in perfect agreement with repeatability and reproducibility test pattern demonstrated in presence of air (Figure 4c and 6a). Response to CO2 and recovery in N2 was further rationalized from Gram-Schmidt orthogonalized interferogram (Figure S12) and appropriately supported by camera captured images of the control computer evincing the various DRIFT spectra collected throughout the experiment (Figure S13) using custom built set-up (Figure S14). Interestingly, DRIFTS scan followed a similar trend for CaO and pure BaTiO3 nanoarchitectures (Figure S15 and S16). Operando technique employed opportunely establishes a correlation between the spectroscopic measurements and the chemo-resistive response magnitude, thus confirming reversible carbonation of either or both BaO and CaO results in the dominant sensing phenomenon.14 This exceptional reversibility is rarely observed in


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chemisorptive based metal oxide semiconductors CO2 sensors and usually the response/recovery times of such micro-sensors can be up to several magnitude longer owing slow kinetic processes.13 Further, based on the reversible dynamic transients, sensing characteristics exhibited by 25CaO-B composite are excellent and are nearly comparable with cost-effective optical and highly selective electrochemical sensors matured commercially over the years.12 Furthermore, the proposed sensitive layer of CaO-BaTiO3 heterostructured composite shows improved sensor characteristics and compares well with the best chemisorptive based metal oxide semiconductors sensors reported in the literature that have rarely been selective towards CO2 gas.32-37 Based on the theoretical estimate, CaO-BaTiO3 exhibits type I (straddled gap) n-n heterojunction (Figure 9a).17 However, from the band-gap values determined using UV-DRS data for CaO sheets and BaTiO3 spheroids, a type II (staggered gap) heterointerface was realized (Figure 9b). A comparatively narrow band-gap of ~4.96 eV was observed here for CaO sheets than its usual wide band-gap of 7.72 eV exhibited by bulk counterpart. Figure 9c-e illustrates the CO2 gas sensing mechanism of the CaO decorated BaTiO3 heterostructures. Under vacuum (Figure 9c), electrons in such system have tendency to transfer from CaO to BaTiO3, since the work function of BaTiO3 (4 eV) is larger than that of CaO (1.69 eV).12,17 This electron transfer continues till an electronic equilibrium is reached between CaO and BaTiO3. In vacuum, due to the charge confinement by highly reactive surface species, a thin depletion layer and accumulation layers are formed along the n-n interfacial region in the composite. Owing to n-n heterojunction formation, a cumulative depletion layer develops at the interface even under vacuum as a result of electrons and holes recombination in BaTiO3 and CaO respectively, leading to band alignment and modulation of potential barrier height.17,38-39 In air (Figure 9d), oxygen molecules are adsorbed on the surface of BaTiO3 and CaO that are subsequently ionized



by capturing free electrons from the electron-accumulation layer. This further leads to formation of accumulation and depletion layer around the CaO and BaTiO3, thus larger band alignment compared to that in vacuum is observed. Upon supplying the sensor chamber with CO2 gas (Figure 9e), CO2 molecules get chemisorbed on CaO and/or BaTiO3 surface to react reversibly with preadsorbed oxygen species resulting in the formation of CaCO3 and/or BaCO3 (Figure S15 and S16).11,16,29 This reversible surface carbonation of CaO and/or BaO results in formation of thick depletion layer and thinning of conduction channel. This modulation of potential barrier at CaO-BaTiO3 heterojunction is large compared to CaO-CaO and BaTiO3-BaTiO3 n-n based homojunctions,17 which eventually contributes to the enhanced sensor response magnitude as depicted by the 25CaO-B composite.

Conclusions In summary, we reported an important progress for selective detection of CO2 gas based on a novel staggered CaO-BaTiO3 heterostructures. The heterostructures comprising of CaO sheets decorated with BaTiO3 spheroids were synthesized by solution impregnation technique followed by calcination. The optimum CO2 gas sensing performance was shown by 25 wt.% CaO functionalized BaTiO3 composite at the optimum temperature of 160oC, when exposed to 1000 ppm CO2 gas concentration. The sensor showed an impressive response magnitude (~65%) with high repeatability (40 days), accuracy (89%) and faster recovery. Remarkable CO2 selectivity was demonstrated in presence of interfering gases commonly found in the industrial applications, which is rarely studied so extensively for metal oxide semiconductor based chemoresistive sensors. Energy band alignment proposed from UV-DRS measurements revealed type II heterojunction between n-CaO and n-BaTiO3 for the composite, in stark contrast to type I


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heterojunction calculated theoretical. The dominant sensing phenomenon was theoretically stated and was experimentally proven by employing DRIFTS technique that indicated a series of reversible chemisorbed carbonation reactions occurring at n-n nanointerfaces requiring very low thermal activation. Simple strategy and detail insights on the band bending provided in the present work is anticipated to provide a feasible approach towards realization of highly selectivity CO2 sensors for a wide range of applications in industrial and environmental monitoring.

Associated Content Supporting Information CaO-BaTiO3 synthesis scheme, additional TEM and SAED patterns for BaTiO3 and CaO, EDS and elemental mapping of CaO decorated BaTiO3 spheroids, stacked X-ray photoelectron survey spectra of pure BaTiO3 and 25CaO-B sample, core level spectra of Ba 3d, Ti 2p, Ca 2p and Ca 2s, Gram-Schmidt orthogonalized interferogram of 25CaO-B sensor, camera captured image of control computer integrated with FT-IR spectrometer depicting various DRIFT spectra, camera captured image of the time-resolved DRIFTS set-up and operando DRIFTS measurements for CaO and pure BaTiO3.

Authors Information Corresponding Author *E-mail: [email protected] (S. J) *E-mail: [email protected] (M. V. S), Tel.: +91 40 27193225. *E-mail: [email protected] (S. J. I), Tel: +61 3 99252673.



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ORCID Shravanti Joshi – 0000-0002-9242-3574 Manorama V. Sunkara – 0000-0001-6650-1834 Suresh K. Bhargava – 0000-0002-9298-5112 Samuel J. Ippolito – 0000-0002-6844-7963

Acknowledgements SJ acknowledges RMIT University, College of Science, Engineering and Health (SEH), Australia for financial assistance in the form of postgraduate scholarship (CLP-0092) and through the awardment of Higher Degree by Research Publication Grant (HDRPG). SJ appreciates generous bursary by ATA Scientific Instruments Australia in the form of young scientist

encouragement

award

for

the

year

2017

(https://www.atascientific.com.au/awards/encouragement-award-november-2017/). SJ and MVS duly acknowledges the strong support of the CSIR XII FYP Projects M2D (CSC-0134) and NanoSHE (BSC0112) for the grants received. All authors greatly acknowledge the facilities, scientific insights and the technical expertise provided by the Australian Microscopy & Microanalysis Research Facility (RMMF) at the RMIT University.

Conflict of Interest The authors declare ne conflict of interest.

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Figure 1 Morphological and structural characterization of CaO-BaTiO3 heterostructures. TEM micrograph illustrates (a) 25CaO-B composite, (b) n-n interfaces formed between BaTiO3 spheroids and CaO sheets. High resolution TEM micrographs revealing lattice fringes for (c-d) tetragonal structured BaTiO3 spheroids and (e-f) cubic structured CaO sheets.



Figure 2 X-ray diffraction patterns, where (a) pure BaTiO3, (b) CaO, (c) 1CaO-B, (d) 3CaO-B, (e) 5CaO-B, (f) 10CaO-B, (g) 25CaO-B and (h) 50CaO-B.


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Figure 3 Evolution of the X-ray photoemission, (a-c) core levels and (d) valence band maxima (VBM) spectra.



Figure 4 (a) Temperature profile for pure BaTiO3, CaO and CaO decorated BaTiO3 spheroids in various weight percentages, (b) temperature profile as a function of CO2 gas concentration for 25CaO-B sample and (c) gas sensing transients of 25CaO-B measured at 160oC in dry conditions. Error bars are fitted with standard deviation (±σ) measured for 5 consecutive test cycles.


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Figure 5 (a) Response/recovery times for 25CaO-B composite and (b) linear fit forming the calibration curves measured at 160oC fitted with standard deviation (±σ) measured for 5 consecutive test cycles.



Figure 6 (a) Long-term repeatability under continuous CO2 exposure and (b) dominant selective performance in presence of acetone (500 ppm), ethanol (500 ppm), hexane (1000 ppm), methanol (500 ppm), H2S (100 ppm), H2 (50 ppm), CO (50 ppm), NO2 (1000 ppm), NO (100 ppm) and SO2 (50 ppm). Error bars are fitted with standard deviation (±σ) measured for 10 consecutive test cycles. All studies were carried out at 160oC in presence of 1000 ppm CO2 gas concentration balanced with dry synthetic air.


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Figure 7 Diffuse reflectance infrared Fourier transform spectra of 25CaO-B, where the spectrum was collected in each case for time period of 10 mins (a) room temperature (b) in absence of CO2 gas at 160oC, (c) response curve in the presence of CO2 gas at 160oC and (d) room temperature in the absence of CO2 gas.



Figure 8 Time-resolved DRIFT spectra of the 25CaO-B sensor under alternating exposure to 100 ppm of CO2 and recovery in N2 at 160°C for 40 min. The scale is in the unit of absorbance and the spectra are shown taking the state of the sensor after stabilization in N2 (600 s) before the CO2 exposure as the background.


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Figure 9 Energy band diagrams of CaO-BaTiO3 heterostructures under vacuum before contact, where (a) theoretical estimate and (b) calculated based on band gap values determined from UV diffuse reflectance spectra. Band bending depicting dominant sensing mechanism exhibited by CaO-BaTiO3 heterostructure after formation of type II heterojunction in (c) vacuum, (d) air and (e) presence of CO2. Here, Eg – band gap of metal oxide semiconductor, E0 – vacuum level, Ec – conduction band level, Ev – valence band level, qχ – electron affinity, qΦ – work function, EAL – electron accumulation layer and EDL – electron depletion layer.



For Table of Contents Use Only Facile synthesis of CaO-BaTiO3 for selective detection of CO2 gas using an inexpensive transducer platform based on reversible chemisorbed carbonation.


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Gas Sensing Properties of CaO-BaTiO - ACS Publications

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