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Straddled Band Aligned CuO/BaTiO3 Heterostructures: Role of Energetics at Nanointerface in Improving Photocatalytic and CO2 Sensing Performance Shravanti Joshi, Ram Kumar Canjeevaram Balasubramanyam, Samuel J Ippolito, Ylias M. Sabri, Ahmad Esmaielzadeh Kandjani, Suresh K. Bhargava, and Manorama V. Sunkara ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00583 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Nano Materials

Straddled Band Aligned CuO/BaTiO3 Heterostructures: Role of Energetics at Nanointerface in Improving Photocatalytic and CO2 Sensing Performance Shravanti Joshi, a, b, c* Ram Kumar Canjeevaram Balasubramanyam, c, d Samuel J. Ippolito, a, c, d Ylias M. Sabri, a, c* Ahmad E. Kandjani, a* Suresh K. Bhargava, a, c 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, 124 La Trobe street, Melbourne, VIC 3001, Australia. E-mail: [email protected]; [email protected]; Tel.: +61 399252330. b

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

Chemical Technology, IICT Colony, Uppal Road, Hyderabad 500007, India. E-mail: [email protected]; [email protected]; Tel.: +91 40 27193225; Fax: +91 40 27160921. c

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

Uppal Road, Hyderabad - 500007, India. d

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

Trobe street, Melbourne, VIC 3001, Australia. *To whom all Correspondences should be addressed.

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Abstract This work details novel insights on the role of energetics that is, energy band bending and builtin potential at the nanointerface of CuO/BaTiO3 forming type I p/n heterostructures, evaluated by correlating XPS and UV-DRS studies. Cetyl trimethylammonium bromide (CTAB) assisted hydrothermal route was used to synthesize BaTiO3 cuboids with six active {100} facets and its CuO based heterostructures were tested for bifunctional applications in environmental nanoremediation. Straddled CuO/BaTiO3 heterostructures reported herein showcased exceptional flexibility as a ultra-violet (UV) active photocatalyst for methyl orange (MO) degradation and chemo-resistive CO2 gas sensor. CuO/BaTiO3 heterostructures in equimole ratio could degrade 99% MO in 50 min with rate constant (κ) of a first order reaction observed to be 10 and 100-fold greater in comparison with BaTiO3 and CuO samples, respectively. Subsequently in a parallel application, trials were carried out on CuO/BaTiO3 heterostructures for their sensitivity and stability towards CO2 gas below 5000 ppm. Upon Ag decoration, the sensor response improved compared to CuO/BaTiO3 heterostructures at 160oC, with enhanced response/recovery times (t90) of 300 and 320 s, respectively. Improved photoactivity was rationalized in terms of effective charge severance of photogenerated e-h pairs owing to favourable band alignment while, the optimum CO2 sensor response was attributed to efficient nanointerfaces configured in large numbers and Ag0/Ag+ acting as redox couple.

Keywords: p/n heterojunction, band offset, perovskite oxide, photocatalysis, CO2, straddled bandgap.


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1. Introduction Environmental pollution as a result of unmonitored hazardous by-products primarily arising due to rapid industrialization has become a global nuisance.1 In this context, owing to their high surface area and activity, nanomaterials are being utilized extensively in the environmental applications for detection and disposal of toxic gases or wastes.2 Interestingly, the performance capability

of

any

nanomaterial

is

highly

dependent

on

its

size,

shape,

functionalization/decoration with other (organic moieties, metal/metal oxides, etc) and other intrinsic properties (i.e. ferroelectricity, multiferroic, energy band structure in case of semiconductor materials).3 Over the past few decades, ferroelectric metal oxides have come into view as an important class of versatile materials thus, finding applications in chemical sensors, supercapacitors, catalysis and multilayered ceramic capacitors (MLCC) just to name a few.3 But more recently, stirred by their inherent atomic off-centre displacements, intense research efforts have been directed in tailoring the synthesis routes and correlating the ferroelectric nature with photocatalysis.6-7 Barium titanate (BaTiO3) is widely documented as a n-type dielectric ceramic, that exhibits cubic to tetragonal structural phase transition3 as a function of temperature and the crystallite size.7 Recently, Cui et al. studied the role of ferroelectricity exhibited by BaTiO3 on the solar light induced dye degradation, thus proposing that this inherent property aids in effective charge carrier and redox separation.8 Similarly, Kappadana et al. reported on photocatalytic application of cubic and tetragonal structured BaTiO3 nanostructures synthesized by polymeric complex method.9 Their investigation further revealed that tetragonal BaTiO3 showed enhanced ability to degrade the dye compared to cubic BaTiO3.9 Although, the studies reported significant discernment, photocatalytic studies involving functionalization of n-BaTiO3



with p-type metal oxides has rarely been performed. Formation of such multi-component heterojunction is widely documented as an effectual approach to tailor highly active photocatalyst, owing to their efficient charge separation upon band bending at the nanointerfaces.10 Among the p-type semiconductors, multiferroic cupric oxide (CuO) exhibiting constricted band gap (Eg = 1.2-1.7 eV) is highly favoured for photocatalysis applications.11-13 Although, there have been many reports correlating ferroelectricity with superior catalytic properties in stand-alone metal oxides and type II heterojunction systems, investigations concerning type I energy band alignment for example in heterostructures such as p-CuO/nBaTiO3, and rationalizing it with improved multifunctional properties still, largely remains unexplored. In our earlier work,14 we reported on CO2 sensing performance of Ag@CuO decorated BaTiO3 spheroids and attempted to deduce the sensing mechanism by in-situ DRIFTS technique. Despite this, a thorough understanding of p/n heterojunction type and its consequence on the CO2 sensing characteristics still requires to be explored. Inspired by these ideas, here we detail an account on the multifunctional properties of straddled energy band gap CuO/BaTiO3 heterostructures and their optimization for photocatalysis (MO degradation under UV irradiation) and gas sensing (CO2 monitoring) applications. BaTiO3 cuboids with six active {100} facets were synthesized using cetyl trimethylammonium bromide (CTAB) as cationic surfactant. Thereon, these cuboids were anchored with CuO nanospheres in controlled quantities to achieve a type I p/n heterojunction. The influence of CuO loading on the photocatalysis was investigated and the optimum amount was determined depending on the dye mineralization. In an analogous application as CO2 sensitive material, Ago/Ag+ redox couple mediated CuO carbonation capability was enhanced by decorating the heterostructures with silver (Ag) nanoparticles. Additionally, a rationale was


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outlined between the electronic properties and the mechanism involved in the multifunctional performance by investigating energy bandgap alignment at the p/n interface achieved from ultraviolet diffuse reflectance spectroscopy (UV-DRS) and X-ray photoelectron spectroscopy (XPS) studies.

2. Materials and Methods 2.1 Chemicals Analytical

reagent

((BaOH)2.8H2O),

(AR)

grade

chemicals

titanium

(IV)

dioxide

such

(TiO2),

as

barium

copper

(II)

hydroxide acetate

octahydrate monohydrate

(Cu(CO2CH3)2.H2O), cetyl trimethylammonium bromide ([(C16H33)N(CH3)3]Br), silver nitrate (AgNO3), sodium tetrahydridoborate (NaBH4),14 potassium bromide (KBr) and platinum (Pt) wire were obtained from Sigma-Aldrich Chemical Co. and were used in procured conditions. Formic acid (CH2O2) and methyl orange (C14H14N3NaO3S) were procured from Chem-Supply Pty Ltd. Anhydrous sodium sulphate (Na2SO4) was purchased from Ajax Finechem.

2.2 Preparation of BaTiO3 cuboids Firstly, ca. 1.577 g (5 mmoles) of barium hydroxide octahydrate and 0.4 g (5 mmoles) of titanium (IV) dioxide was added in beaker containing deionized water (30 ml). Thereon, 2.92 g (8 mmoles) of cetyl trimethylammonium bromide was added. Precursors were added under vigorous stirring conditions of 750 rpm and in the end left to stir for a period of 30 min. The solution was then sonicated for 15-20 min followed by transferring of contents to a 75 mL Teﬂon-lined hydrothermal autoclave.14 Autoclave was then kept in an oven that was held steady



at 200oC for 12 h. Afterwards, the resulting mixture was dispersed in deionized water and washed to achieve white coloured powder. A continual run consisting of centrifuging (6000 rpm for 20 min) and washing with deionized water followed by ethanol guaranteed total elimination of organic moities.14 This mixture was then dehydrated at 75°C for 12 h using an oven. It was observed from XRD data that BaCO3 as an impurity formed in minor percentages. Hence, the synthesized BaTiO3 powder was treated with 0.1 M formic acid at 30oC for 2-3 h, followed by washing with deionized water 5-6 times. Thereafter, the white residue was dehydrated at 75°C for 12 h. Effect of surfactant on cuboids morphology was ascertained by synthesizing samples in various concentrations of CTAB. BaTiO3 powders including 1, 2, 4, 6 and 10 mmoles were prepared by CTAB addition of 0.365, 0.73, 1.46, 2.2 and 3.65 g in 40 mL of solution, respectively.

2.3 Preparation of CuO/BaTiO3 Architectures CuO/BaTiO3 heterostructures were prepared by impregnation of BaTiO3 powder with copper (II) acetate monohydrate by modifying a previously reported methodology.15 Preparation route involved suspending BaTiO3 nanopowder (0.5 g) in ethanol (10 mL) with calculated of copper (II) acetate monohydrate (0.43 g).14 After being sonicated for 30 min at 30oC, resulting solids were achieved by drying the homogenous mixture at 75oC overnight. Thereafter, calcination at 300oC in air at 2 h yielded the CuO/BaTiO3 heterostructures. Functionalization of BaTiO3 with CuO in 16:1, 8:1, 4:1, 2:1, 1:1, 1:1.25 and 1:1.5 mole ratios was achieved by adding 0.03, 0.054, 0.107, 0.214, 0.43, 0.535 and 0.642 g of copper (II) acetate monohydrate, respectively. The resulting p/n heterostructures are hereafter named as BC-16:1, BC-8:1, BC-4:1, BC-2:1, BC-1:1, BC-1:1.25 and BC-1:1.5, respectively.


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2.4 Heterostructures Decoration with Silver (Ag) Wet solution impregnation route was employed to decorate CuO/BaTiO3 heterostructures with zerovalent silver (Ag) nanoparticles in 1 wt.%.14 Briefly, ca. 7.9 mg (0.04 mmoles or nearly 1 wt.%) of silver nitrate was added to p/n heterostructured powder (0.5 g) dispersed in ethanol (40 ml) and stirred continuously under magnetic agitation.14 Thereon, 3 mg (0.08 mmoles) of NaBH4 was added to the solution to reduce the silver ions. Consequently, the solution was subjected to centrifugation, sonication and washed 3-4 times with water followed by ethanol to achieve blackish powder that was dehydrated at 75°C for 12 h using an oven. Figure 1 illustrates synthesis routes employed to achieve Ag decorated CuO/BaTiO3 heterostructured system.

2.5 Characterization Techniques Electron micrographs in transmission mode (that is, TEM images) were captured using Gatan Orius SC600 CCD Camera interfaced with JEOL 1010 at 100 kV. As well, Gatan Orius SC600A CCD Camera integrated with JEOL 2010 was used to capture high resolution transmission electron microscopy, that is, HR-TEM images at 200 kV. For the TEM sample preparation, synthesized nanomaterials were dispersed in toulene and ultra-sonicated for 15 mins. Then this solution was drop casted onto a holey carbon copper grid before allowing it to dry. High-angle annular dark-ﬁeld scanning transmission electron microscopy (HAADF-STEM) images were collected at 200 kV using a Thermo Scientific Talos F200X scanning/transmission electron microscope (S/TEM) equipped with Bruker Nano ESPRIT 1.9 microanalysis software for elemental mapping. X-ray diffraction (XRD) patterns were collected using a Bruker AXS diffractometer (D8 ADVANCE) in the 2ϴ ranging from 10 to 70°.14 Energy dispersive X-ray spectroscopy (EDS) analyses were carried out on unsputtered samples using FEI Quanta 200



ESEM fitted with Oxford X-MaxN 20 EDXS Detector at 25 kV. Ultra-violet diffused reflectance spectra (UV-DRS) of nanostructures were collected in 200-800 nm range using Varian Cary 5000 spectrophotometer. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Thermo Fisher Scientific K-Alpha with monochromatic Al Kα radiation (Ephoton=1486.6 eV).14 The binding energy (B.E) in each synthesized sample was background corrected with C 1s peak 284.8 eV, arising from the adventitious carbon to ensure reliability of the data. Valence band maximum (VBM) analyses were done by first preparing sample, by coating a layer of assynthesized sample on gold (Au) deposited silicon (Si) wafer and then examining it, whilst keeping flood gun off. VBM-XPS data acquirement and investigation was carried out using Avantage v.488 software provided by Thermo Fisher Scientific. The energy band diagram of the CuO/BaTiO3 heterostructures was proposed by tallying up VBM-XPS data with UV-DRS studies using methodology reported in the literature.10

2.6 Photocatalysis Experiments Photocatalytic runs were assessed by following degradation rate of methyl orange (MO) at room temperature in an ambient air atmosphere. RMR 600 photochemical reactor (Rayonet, The Southern New England Ultraviolet Co, USA) equipped with 4 lamps placed at equal distance with an arc length of 3 inches each, was used to carry out the photocatalytic studies. In each run, 20 mg of nanomaterial was suspended through ultrasonication in MO solution (1 x 10-5 M, 20 mL) in a quartz tube (height (H) – 17 cm, outer diameter (D) – 2.5 cm and thickness (t) – 0.2 cm) to achieve a concentration of the catalyst in MO solution at 1 g/L. The solution was mixed continuously for 30 min under magnetic agitation in dark to equilibrate the physical deposition and removal process of MO on the nanomaterial surface. After mixing for 30 min, it was


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illuminated with 4 ultra-violet lamps (Rayonet RMR-3000, 300 nm, 4 W each with 6 inches in length), while stirring continuously to make certain that suspension is consistently exposed throughout the photocatalytic run. During the run, temperature was maintained pneumatically at ~30oC with the help of cooling fan placed at the bottom. After a particular time gap, aliquots (0.8 mL) were taken out from reactor and supernatant was collected post centrifuging to remove any nanomaterial. MO concentration was determined by observing the wavelength of maximum absorption at λmax= 464 nm of the centrifuged solutions by recording the spectra using spectrophotometer. Calibration plot following Beer-Lambert’s law was established by correlating the absorbance to the MO concentration. Reusability was probed by replicating the MO photodegradation runs for six successive times under comparable settings. Post each run, the nanomaterial was subjected to washing subsequently, overnight dehydration at 75oC using an oven.

2.7 Photoelectrochemical (PEC) Measurements 2.7.1 Photoelectrodes Fabrication Thin film electrodes comprising CuO/BaTiO3 (BC-1:1) heterostructures were prepared using electrophoretic deposition on a conducting fluorine doped tin oxide (FTO) coated glass with surface resistivity ~7Ω/sq (FTO glass TEC – 7, Sigma-Aldrich Chemicals Co.) by following a previously reported methodology.16 The electrophoretic deposition was carried out in an acetone solution (25 ml) containing heterostructured powder (20 mg) and iodine (7 mg), which was later dispersed by sonication for 5 min. The two FTO electrodes (1×4 cm) were immersed parallel (8 mm apart) in the solution, followed by applying 10 V bias across the electrodes for 10 min using a potentiostat (Ivium Stat Vertex). The coated area was controlled to be ~ 1.0×3 cm. Total



three electrodes were fabricated under identical protocols to evaluate the reliability and reproducibility of the results. The electrode was dried and then calcined at 300°C for 2 h.

2.7.2 Photoelectrochemical Analysis Photoelectrochemical analysis (PEC) was carried out on IVIUM stat vertex electrochemical work station in a custom-built photoelectrochemical chamber comprising of a quartz cuvette that could accommodate three electrodes and intensity tuneable to 370, 405, 470, and 525 nm light emitting diode (LED) sources. LEDs were calibrated using optical power meter (PM16-140 THOR Labs). The three electrode configuration consisted of CuO/BaTiO3 coated FTO substrate as the working electrode, Ag/AgCl as the reference electrode and platinum (Pt) wire as the counter electrode. An aqueous solution of 0.5 M Na2SO4 was used as the supporting electrolyte. The distance between the PEC cell and the light source was fixed at 1.5 cm. The transient photocurrent measurements were carried out using a custom-built pulse generator that could modulate the light source (10 s for OFF and 5 s for ON states, respectively). All the transient measurements were performed using chronoamperometry under open circuit conditions. Electrochemical impedance spectrum (EIS) was obtained in a frequency sweep from 50 mHz to 200 Hz (10 points/decade) under open circuit voltage (VOC) conditions with an AC amplitude of 10 mV. The EIS measurements were carried out in a Faraday cage setup to isolate the stray electrical noises.


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2.8 Gas Sensor Fabrication, Set-up and Test Patterns To fabricate sensor element, synthesized nanopowder (~5 mg) was made into a paste with ethanol (

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