ZnSnO3 Nanoparticle-Based Piezocatalysts for Ultrasound-Assisted

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ZnSnO3 Nanoparticle-Based Piezocatalysts for Ultrasound-Assisted Degradation of Organic Pollutants Aritra Biswas, Subhajit Saha, and Nikhil R. Jana ACS Appl. Nano Mater., Just Accepted Manuscript • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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ACS Applied Nano Materials

ZnSnO3 Nanoparticle-Based Piezocatalysts for Ultrasound-Assisted Degradation of Organic Pollutants Aritra Biswas, Subhajit Saha* and Nikhil R. Jana* Centre for Advanced Materials and School of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India *Address for correspondence to [email protected] and [email protected]

Abstract. Piezocatalysis is a promising alternative of photocatalysis where strained state of a piezoelectric material is exploited for electrochemical surface reactions under dark condition. Among various piezoelectric materials, ZnSnO3 has immense potential as piezocatalyst owing to the non-centrosymmetric structure and strong ferroelectric polarization. Herein, we report orthorhombic ZnSnO3 nanoparticles as efficient piezocatalyst under ultrasound treatment. Small particle size of 4-5 nm, phase purity, room temperature ferroelectric behavior and colloidal form of ZnSnO3 are responsible for efficient piezocatalysis. It is shown that hydroxyl radicals are generated during piezocatalysis that is responsible for degradation activity. Presented work demonstrates the bright prospect of ZnSnO3 nanoparticles for ultrasound assisted piezocatalytic decontamination of polluted water and other applications.

Keywords: nanoparticle, piezocatalysis, ferroelectric polarization, reactive oxygen species, water pollution

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Introduction In recent times piezoelectric materials have garnered considerable researchers’ attention owing to their capability of utilizing mechanical energy in wide spread applications ranging from energy harvesting nanogenerators to catalytic degradation of hazardous organic pollutants.1-4 Among these, piezocatalysis is a promising approach for next generation water decontamination technology.5-7 In piezocatalysis, the strained state of a piezoelectric crystal is exploited for the enhancement of electrochemical surface reactions. In particular, when the piezocatalyst crystals are subjected to an external force, they exhibit development of surface polarization caused by the separation of positive and negative charges to the opposite polar surfaces. These polar surfaces promote efficient charge transfer between catalyst material and organic chemical leading to their swift degradation phenomenon.5-7 While the current generation decontamination technologies such as photocatalysis,8-12 electrocatalysis13 and fenton reactions14 are already plugged by some major drawbacks like slow reaction rate, high cost, and conditional usage; piezocatalysis offers some striking advantages. Especially, operability under dark condition as well as availability of abundant low-frequency vibration energy as a source of mechanical energy resource establishes piezocatalysis as a highly propitious technology over its peers.15-17 Moreover, the built in electric field in the piezocatalysts are highly beneficial for inhibiting the electron-hole recombination which further ensures high degradation activity.18 Hence, it is imperative to design efficient piezocatalysts that can be effectively used in waste water management applications. Among the plethora of inorganic piezoelectric materials, lead free perovskite oxides have attracted substantial research heed by virtue of their excellent piezoelectric response, lower toxicity, high thermal and structural stability.19 In this context, ZnSnO3 is of immense interest as it possesses some unique inherent features suitable for piezocatalytic applications. First, owing to the non2 ACS Paragon Plus Environment

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centrosymmetric structure, ZnSnO3 generally exhibits large displacement of Zn atoms leading to strong ferroelectric polarization at room temperature -- greater than other commonly known lead free piezoelectrics such as KNbO3, BaTiO3 and ZnO.20 Second, unlike other lead free piezoelectric materials; ZnSnO3 is a semiconducting oxide with lower degree of electrical resistivity (410-3 ohm cm) which is a prerequisite for efficient charge transport in the piezocatalyst.21 In addition, presence of edge sharing SnO6-ZnO6 octahedral framework in ZnSnO3 ensures dispersed conduction band mediated enhanced mobility of charge carriers which in turn, can trigger faster degradation activity.22 However, despite its great promise, the full potential use of the ZnSnO3 nanocrystals for piezocatalytic applications remains largely unexplored. This is primarily due to unavailability of phase pure ZnSnO3 nanocrystals.23 In this context, solution based colloidal synthesis approach could be an ideal choice as it offers excellent control over shape and size of the nanocrystals.24 However, to the best of our knowledge, no prior report in the literature describes synthetic method for colloidal ZnSnO3 nanocrystals that could be used for piezocatalytic applications. Herein we report water dispersible colloidal form of ZnSnO3 nanoparticles for first time that can be used for ultrasound assisted piezocatalytic application. ZnSnO3 nanoparticles are synthesized via colloid chemical approach. The synthesized nanoparticles exhibit narrow size distribution of 4-5 nm with room temperature ferroelectric behavior that is suitable for piezocatalytic reactions. Colloidal form of nanoparticles show ultrasound assisted degradation of rhodamine B and it is shown that hydroxyl radical is involved in this catalytic degradation. Although various materials are used earlier as piezocatalyst, this is the first report of ZnSnO3 nanoparticles as piezocatalyst in their dispersible form. In particular we have used this dispersible/colloidal form for efficient piezocatalysis and demonstrated better performance as compared to reported piezocatalysts. 3 ACS Paragon Plus Environment


Presented ZnSnO3 nanoparticles can be used for piezocatalytic decontamination of polluted water and other applications.

Experimental Section Materials. Oleic acid, oleylamine, 1-octadecene, tin(IV) chloride pentahydrate, zinc acetate dihydrate, tetramethylammonium hydroxide solution (25 wt. % in methanol), p-benzoqinone, ethylenediamine tetraacetic acid disodium salt dihydrate and terephthalic acid were purchased from Sigma Aldrich. Rhodamine B was purchased from Loba Chemie Pvt. Ltd. Instrumentation. Wide-angle X-ray diffraction (XRD) was measured at room temperature with a Bruker D8 advance powder diffractometer equipped with Cu Kα (λ = 1.5406 Å) as X-ray source. JEOL, JEM 2100 F Transmission electron microscope was used to probe the size and shape of the nanoparticles. Nature of crystallinity and compositional analysis was investigated by the same instrument using selected area electron diffraction (SAED) pattern and energy dispersive X-ray spectra (EDS), respectively. Omicron X-ray photoelectron spectrometer (serial number: 0571) with Al Kα X-ray source and hemispherical analyzer was employed for investagating the surface chemical composition and corresponding high resolution core-level spectra were deconvoluted using CASA XPS software. Polarization vs electric field measurement is carried out by ferroelectric loop tracer (PE loop tracer) instrument by Radiant Technologies. Raman spectra were recorded by J-Y Horiba Confocal Triple Raman Spectrometer (Model: T64000) equipped with a 532 nm Nd:YAG laser (Spectra Physics). Elemental analysis was performed with a CHNS/O analyzer (2400 Series II CHNS/O PerkinElmer). ζ potentials and hydrodynamic sizes were measured by dynamic light scattering (DLS) technique using NanoZS (Malvern) instrument. UV−visible absorption spectra of samples were collected using Shimadzu UV-2550 UV−visible spectrophotometer. Ultrasonication for investigating the piezocatalytic behaviour was performed 4 ACS Paragon Plus Environment

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using a 120 W digital ultrasonicator by PCI Analytics Pvt. Ltd. (India) with operating frequency 33±3 kHz. Photoluminescence emission spectra were recorded by a Fluorescence Spectrometer LS 45 (Perkin Elmer). Fourier transform infrared (FTIR) spectra were taken on Perkin Elmer Spectrum 100 FTIR in transmittace mode using KBr pellets. BET surface area of the synthesized nanoparticles were investigated using N2 adsorption-desorption isotherm at 77 K with a Quantachrome Autosorb-1C. The pore-size distributions were analyzed by using nonlocal density functional theory (NLDFT). Piezoelectrochemical measurement was performed with a CHI633D Electrochemical Analyzer. ICP analysis was carried out using Optima 2100 DV (PerkinElmer) inductively coupled plasma optical emission spectroscopy (ICP-OES). Synthesis of ZnSnO3 nanoparticles. At first, metal oleate complexes were prepared by reacting zinc acetate dihydrate or tin (IV) chloride pentahydrate with oleic acid in the presence of tetramethylammonium hydroxide. Briefly, for preparation of tin (IV) oleate, 6.7 mL tetramethylammonium hydroxide solution was mixed with 250 mL methanolic solution of oleic acid (5.0 mL) under 50 °C. Then, 1.4 g tin (IV) chloride pentahydrate was dissolved in 15 mL of methanol and added dropwise to the above solution followed by stirring for 2 h. Solid precipitate of tin (IV) oleate complex was separated and washed several times with methanol and dried to obtain white powder. For preparation of zinc oleate, 6.7 mL tetramethylammonium hydroxide solution was mixed with 200 mL methanolic solution of oleic acid (5.0 mL) under 50 °C. In a separate vial 1.7 g zinc acetate dihydrate was dissolved in 20 mL of methanol and dropwise added to the above solution followed by stirring for 1 h. The resulting solid precipitate was separated and washed several times with methanol, and dried to obtain white powder. Next, 62.8 mg (0.1 mmole) zinc oleate was dssolved in 8 mL oleic acid-octadecene (1:7) mixture and loaded in a three neck round bottomed flask. The reaction mixture was degassed at 70 °C with 5 ACS Paragon Plus Environment


continous purging of argon for 15 min. Then, temperature was increased to 280 °C and solution was again degassed for 5-10 min and kept under inert condition during the experiment. After that, 124.4 mg (0.1 mmole) tin (IV) oleate in 2 mL oleylamine was quickly injected under stirring condition and temperature was maintained at 280 °C for 15-60 min. The clear solution gradually turned colorless to orangish within 10-15 min. After 60 min heating, the reaction was stopped. Nanoparticles were then purified from the solution by adding acetone to the reaction mixture and then isolated precipitates were dissolved in chloroform. Further, second round of purification of nanoparticles was performed by adding ethanol to the chloroform solution of nanoparticles followed by precipitation. Finally, purified nanoparticles can be redispersed in wide range of non polar solvents such as chloroform/toluene/cyclohexane for further use. Preparation of water dispersible ZnSnO3 nanoparticle. As synthesized ZnSnO3 nanoparticles were separated from free surfactants by adding acetone/ethanol based precipitation and purified by a standard precipitation-redispersion method as described earlier. In a typical purification process, 0.25 mL of toluene was mixed with 0.25 mL of nanoparticle solution and then precipitated by adding 1.5 mL acetone. The mixture was centrifuged at 6,000 rpm for 2 min and the supernatant was discarded. The precipitate obtained was then dissolved in 0.5 mL of toluene and further precipitated by adding 1.5 mL of ethanol. The mixture was then shaken vigorously for 1 min and centrifuged at 6,000 rpm for 2 min. This precipitation-redispersion procedure was repeated twice, and the particles obtained were dissolved in 4 mL of toluene prior to ligand exchange. Finally, 4 mL toluene solution of nanoparticle was mixed with 2 mL of a methanol solution of tetramethylammonium hydroxide (0.01 M). The mixture was stirred by a magnetic stirrer and heated to 60-70 °C for 30 min until complete precipitation. Next, the supernatant was discarded, and the particles obtained was washed repeatedly with toluene and/or ethanol and then dried in


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vaccume. These nanoparticles are then dispersed in water via mild sonication and used for piezocatalytic experiments. Piezocatalytic experiment. 25 mg ZnSnO3 nanoparticles were dispersed in 50 mL aqueous rhodamine B solution (4.7×10-6 M). The suspension was stirred in the dark for 60 min to establish adsorption−desorption equilibrium and then treated with ultrasound using a 120 W digital ultrasonic generator with operating frequency 33 ± 3 kHz. Next, 1 mL of aliquot was withdrawn at different time intervals, nanoparticles were separated by high speed centrifuge (14000 rpm, 2 min) and the aqueous supernatent was used to verify the degradation of rhodamine B via UVvisible spectroscopy. The absorbance value at 550 nm was monitored for detecting the degree of rhodamine B degradation. In order to understand the mechanism involved in the piezocatalytic process, control piezocatalysis experiments were performed in presence of various scavengers. They include tert-butyl alcohol (TBA), benzoquinone (BQ), and ethylenediamine tetraacetate dehydrate (EDTA) which are known to scavenge hydroxyl radicals (OH), superoxide radiclas (O2-) and holes (h+), respectively. Typically, aqueous nanoparticle dispersion was mixed with rhodamine B solution, stirred in the dark for 60 min and then 5 mL TBA or BQ (final concentration 0.5 mM) or EDTA (final concentration 0.5 mM) was added to the reaction mixture. Next, the reaction mixture was treated with ultrasound as described earlier. As hydroxyl radical is found to be the dominant species during piezocatalysis, it is estimated quantitatively using terephthalic acid. Typically, aqueous nanoparticle dispersion was mixed with terephthalic acid (5 × 10−4 M) and the pH of the solution was adjusted to pH 10.0 using NaOH solution. Then, the solution was stirred at room temperature for 1 h to achieve complete homogeneity and further treated with ultrasound source. Next, 1 mL of aliquots were collected at 7 ACS Paragon Plus Environment


different time intervals and nanoparticles were separated by centrifuge. Finally, the fluorescence spectra of the corresponding supernatants were investigated under 315 nm UV excitation. Piezoelectrochemical activity study. A glassy carbon electrode (GCE) of 3 mm diameter (surface area of 0.07 cm2) was polished carefully with 1, 0.3, and 0.05 μm alumina powder successively until a mirror finish was obtained. Then, the electrode was cleaned with ethanol/acetone followed by deionized water and dried in the air at room temperature. Then, the electrode was voltammetrically scanned in 0.5 M H2SO4 from 0.1 to 1.2 V (vs Ag/AgCl) at a rate of 100 mV/s. Finally, for piezoelectrochemical measurement, the dispersed nanoparticles mixed with 0.5 wt % of nafion solution were dropcoated on GCE surface and allowed to dry in air for 2 h. Piezoelectrochemical activity was investigated in a conventional three-electrode system having platinum wire as an auxiliary electrode, Ag/AgCl saturated KCl (1 M) as a reference electrode and modified glassy carbon as a working electrode. For generating piezopotential the three-electrode system was kept in an ultrasonic transducer (power 120 W and freequency 33 ± 3 kHz). The differential pulse voltammetry (DPV) signal was recorded with a scan rate of 10 mV/s at room temperature and freshly prepared aqueous solution of Na2SO4 (0.01 M) was used as an electrolyte for measurements.

Results and Discussion Structural and compositional analysis of ZnSnO3 nanoparticle. Synthesis of ZnSnO3 nanoparticles involves nucleation-growth at 280 °C from equimolar mixture of zinc and tin precursors in octadecene-oleic acid-oleylamine medium. The details of synthesis conditions are schematically shown in Scheme 1. Under this condition fcc-ZnSnO3 particles are formed within 10-15 min which eventually converted to orthorhombic ZnSnO3 with ferroelectric phase, as the reaction progresses for next 60 min. This is evident from XRD pattern of particles collected at 8 ACS Paragon Plus Environment

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different reaction time. (Supporting Information, Figure S1) The orthorhombic structure of ZnSnO3 nanoparticle is characterized using high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction analysis and Raman spectroscopy. (Figure 1) XRD pattern of ZnSnO3 nanoparticles exhibit broad diffraction peaks at 26.5°, 33.7° and 51.5° which are correlated to the (hkl) indices of (012), (110) and (116) planes, respectively. (Figure 1a) All XRD peaks are in well agreement with the JCPDS card no. 28-1486 corresponding to orthorhombicZnSnO3.25 The structure of such orthorhombic-ZnSnO3 can be visualized from three dimensional network of edge sharing ZnO6 and SnO6 octahedra shown in Figure 1b. The XRD peaks appear broad due to extremely small crystallite size of the synthesized materials. Low resolution TEM image (Figure 1d) of ZnSnO3 nanoparticles shows 4-5 nm particle size and HRTEM image shows very good crystallinity with the lattice spacing of 0.33 nm which is assigned to the (012) plane corresponding to the single-phase orthorhombic perovskite structure. (Figure 1e) Polycrystalline particles are also observed as evident from different lattice orientation in different grain. Selected area electron diffraction (SAED) pattern of nanoparticles, that collect signals from many particle, demonstrate ring like pattern of polycrystallinity. (Figure 1f) Although we understand that single crystal particles can have better piezocatalytic performance, synthesis of single crystalline ZnSnO3 nanoparticles (4-5 nm) is challenging and our presented method can produce polycrystalline particles only. We have used various experimental conditions but single crystal nanoparticles have not been obtained. Highly disordered structural aspects of these ZnSnO3 nanoparticle is also prominent from the broad Raman spectra of the samples.26 (Figure 1c) The broad Raman pattern can be deconvoluted into eight components located at: 407, 479, 547, 597, 636, 703, 737 and 828 cm-1. Two Raman peaks at 547 and 597 cm-1 can be assigned due to stretching vibrations of ZnO6/SnO6 octahedrons26,27 which are the main building block of ZnSnO3 perovskite. The 479 cm-



1

Raman peak can be attributed to the internal vibration of oxygen while the peak appearing at 636

cm-1 is due to stretching vibration of short Zn-O or Sn-O bonds.28 Other Raman peaks at 407, 703, 737 and 828 cm-1 are difficult to assign. In order to achieve orthorhombic ZnSnO3 nanoparticle with ferroelectric phase, we have varied the experimental conditions. Under the similar experimental condition, if only zinc precursor is used the ZnO nanoparticle is formed and if only tin precursor is used, the SnO2 nanoparticle is formed. (Supporting Information, Figure S2) However, if ZnO nanoparticles are prepared first and then used them as a substrate for preparing ZnSnO3 nanoparticles, only a physical mixture of both ZnO and SnO2 are obtained. (Supporting Information, Figure S3) We have also found that use of > 65 volume % oleyl amine (with respect to the oleic acid-oleylamine mixture) in the oleic acidoleylamine-octadecene solvent is essential for nucleation-growth of ZnSnO3 nanoparticles. Elemental composition of synthesized ZnSnO3 nanoparticles has been probed by EDS elemental mapping which reveals the presence of zinc, tin, and oxygen as major elemental constituents, homogenously distributed all over the nanoparticles (Supporting Information, Figure S4). Surface chemical composition and valence state identification of each constituent elements have been carried out by using X-ray photoelectron spectroscopy (XPS). All the spectra are charge corrected by using C 1s line at 284.6 eV which appears due to the presence carbon on the sample surface. Survey spectra of ZnSnO3 nanocrystals not only reveal the presence of Zn, Sn and O in the sample but also clearly discard the possibility of containing impurity contents. (Figure 2a) High resolution XPS scans were also recorded in order to probe the corresponding valence sates of each component elements. High resolution Zn 2p core-level spectra exhibit two strong peaks at 1044.1 eV and 1021 eV corresponding to spin orbit split components of Zn 2p1/2 and Zn 2p3/2, respectively. (Figure 2b) The binding energy difference of these two components is 23.1 eV which readily confirms the 10 ACS Paragon Plus Environment

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presence of Zn2+ in synthesized ZnSnO3 nanocrystals.11,29 Similarly, high resolution Sn 3d core level spectra is shown in Figure 2c which also demonstrate strong spin orbit split components of Sn 3d3/2 and Sn 3d5/2 at 494.3 eV and 485.9 eV, respectively. The separation of these doublets are 8.4 eV which indicates the existence of Sn4+ ions in the sample.30 The O 1s core-level peak can be deconvoluted in two components at 529.8 eV and 531.9 eV. The lower binding energy peak can be attributed to the lattice oxygen present in ZnSnO3 crystal31 while the higher binding energy part can be assigned to the surface hydroxyl or carboxylic group32 appearing from the capping agents used for the stabilization of ZnSnO3 nanocrystals during synthesis. Moreover, surface chemical composition of the synthesized nanocrystals is also determined by considering the area of corresponding core level peaks and their relative sensitivity factors. The obtained nanocrystals exhibit Zn:Sn ratio 1:1.05 which clearly reveals the formation of ZnSnO3 phase. ICP analysis show Zn:Sn ratio as 1:1.38 which also indicates the zinc deficiency in the synthesized ZnSnO 3 piezocatalyst.33,30 Characterization of ferroelectric property. In order to confirm the ferroelectric phase formation in ZnSnO3 nanocrystals the polarization vs electric field (P-E) measurements have been carried out. For the measurement of P-E loop, a lateral electrode configuration has been employed where colloidal ZnSnO3 nanoparticles were deposited within a 150 μm wide etched ITO channel. (Figure 3a) The ferroelectric hysteresis loop of the samples is shown in Figure 3b which clearly indicate ferroelectric behaviour with lossy structure. The energy dissipation through the nanocrystals gives rise to a phase separation between charge and voltage signals leading to a loop shape of the curve with a definite area under the curve.34 Actually, being a well-known semiconductor, ZnSnO3 often exhibits low electrical resistivity which in turn hinders the polarization saturation formalism in hysteresis measurement and thereby leads to a lossy structure of the P-E loop.35-37 Moreover,



ferroelectricity mediated charge storage ability of the material is also reflected from the pronounced loop shape of the curve. It is to be noted that ferroelectric materials are a special class of piezoelectric material. Although we have synthesized ZnSnO3 nanoparticles at different conditions and studied their piezocatalytic performances, only some selected conditions can make piezocatalytic ZnSnO3 nanoparticle. Water dispersible ZnSnO3 nanoparticles from hydrophobic ZnSnO3 nanoparticles. For piezocatalytic application hydrophobic ZnSnO3 nanoparticles are transformed to water soluble nanoparticle by removing the capping ligands with hydroxide groups.38 This is particularly because many real applications, biomedical and catalysis application in particular, require water dispersible nanoparticle in their colloidal form.38 Typically hydrophobic nanoparticles are dissolved in toluene and then mixed with methanol solution of tetramethylammonium hydoxide and heated to 60-70 °C. Under this condition, hydrophobic fatty acid/amine ligands at the nanoparticle surface are replaced by hydroxide groups that lead to the precipitation of particles. Next, the particles are isolated and dispersed in water. Exchange of fatty acid/amine by hydroxide groups is confirmed from FTIR spectroscoy that shows appearance of broad characteristics of O-H stretching at 3400 cm-1 and disappearance of -CH2 asymmetric and symmetric stretching of oleic acid at 2922 and 2852 cm-1.(Figure 4a) Elemental analysis shows drastic decrease in carbon content after this exchange process, as a result of removing oleic acid/oleylamine capping agents. (Supporting Information, Table S1) The water dispersible nanoparticles show ζ potential value of -29.8 mV, indicating that they are negatively charged due to the presence of hydroxyl groups. TEM study shows that nanoparticles do not change their size after ligand exchange. (Figure 4 b,c) However, the hydrodynamic size of water dispersible nanoparticles becomes larger (in the ranges between 150-350 nm), due to lower colloidal stability. (Figure 4d)


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Ultrasound assisted piezocatalysis by ZnSnO3 nanoparticle. Ultrasound assisted piezocatalytic property of ZnSnO3 nanoparticles is investigated using degradation of rhodamine B. Typically, colloidal ZnSnO3 nanoparticles are mixed with rhodamine B and then treated with ultrasound. Time dependent UV-visible absorption spectra show the gradual disappearance of rhodamine B color, which can also be observed under naked eye. (Figure 5a) Control experiments show that simultaneous presence of ultrasound and orthorhombic-ZnSnO3 nanoparticle is essential for degradation of rhodamine B. If ZnSnO3 nanoparticle is replaced by ZnO or fcc-ZnSnO3 nanoparticle, no such degradation is observed (Figure 5b). Moreover ZnSnO3 nanoparticle can be used multiple times without losing the catalytic efficiency. (Supporting Information, Figure S5) These results clearly indicate the piezocatalytic property of ZnSnO3 nanoparticle under the ultrasound treatment. In order to investigate the role of rhodamine B adsorption in this degradation processes; the relative adsorption capacity of the nanoparticles is measured using the nitrogen (N2) adsorption/desorption isotherms (Supporting Information, Figure S6). Isotherms can be classified as type IV with a H3 type hysteresis loop39 having surface area of 272 m2/g with the pore diameter in the 1.5-4.5 nm. Self-aggregation of the nanoparticles is responsible for this type mesoporous structure and narrow pore distribution suggests uniform assembly of crystallites throughout the material.40 It is interesting to note that, without ultrasound treatment 10-15 % adsorption of the rhodamine B is observed, even after 24 h of stirring. In contrast the complete degradation of rhodamine B is achieved within one hour of ultrasound treatment (Supporting Information, Figure S6 c,d). On this basis we conclude that adsorption of rhodamine B does not contribute this degradation processes, rather piezocatalysis is responsible for this degradation. Since adsorption processes is low during the piezocatalysis experiment, the kinetics of the degradation can be represented as a linear dependence of ln(C0/C) against ultrasound exposure time (t), as shown in



Figure 6b. As reactive oxygen species (ROS) are actually involved in the degradation of organic chemicals, we have investigated the role of different types of ROS in the piezocatalysis. We have used different free radical scavengers during the piezocatalytic degradation of rhodamine B. Scavengers include TBA,41 benzoquinone BQ42 and EDTA43 that are known to scavenge hydroxyl radicals (OH), superoxide radiclas (O2-) and holes (h+), respectively. Results show that TBA completely stops rhodamine B degradation, EDTA partially inhibits rhodamine B degradation and BQ has insignificant effect. (Figure 6a) Calculated rate constant is 4.5×10-2 min-1 in absence of any scavenger and values are 2.0×10-2 min-1, 4.6×10-3 min-1 and 4.7×10-5 min-1 in presence of BQ, EDTA and TBA, respectively. (Figure 6c) The reported rate constant for ZnSnO3 nanoparticle is higher than other lead free oxide piezocatalysts reported in the literature. (Supporting Information, Table S2) It is also revealed that overall piezocatalytic performance of ZnSnO3 nanoparticles in the current work is closely comparable with the other lead free oxide piezocatalysts reported earlier. Obtained results also indicate that rate constant is slowest in presence of hydroxyl radical scavenger where practically no degradation takes place. As hydroxyl radical is the major ROS component in the piezocatalytic degradation process, quantitative detection of it is critical. In the present investigation, quantitative estimation of hydroxyl radicals is carried out by fluorometric detection protocol using terephthalic acid as a OH trapper.44 Typically, mixture of ZnSnO3 nanoparticles and terephthalic acid are treated with ultrasound and under this condition intermediate hydroxyl radicals react with terephthalic acid with the formation of stable fluorescent product (2-hydroxy terephthalic acid)45 that increases with reaction time. (Figure 6d) This result clearly proves that hydroxyl radicals are generated during piezocatalysis that is responsible for degradation of rhodamine B.


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Based on these observations we have proposed a tentative mechanism of piezocatalysis by ZnSnO3 nanoparticle. (Scheme 2) ZnSnO3 is a wide band gap semiconductor material with band gap ~3.9 eV.46 (Supporting Information, Figure S7) In absence of external stress, free charges present on the crystal surface quickly exhaust before reacting with the dye solution. As a result, no significant degradation activity is noticed without external stimulation by ultrasound. Moreover, for any catalytic reaction a proper alignment of potential energy between the piezocatalyst and the aqueous solution is crucial for redox reactions.47 With the application of ultrasound, strong piezoelectric polarization is developed inside ZnSnO3 nanocrystals. As a result of this polarization induced piezopotential, band energy lowers at the positive piezopotential side and raises at the negative piezopotential side.30 Following this variation in energy, both valence band and conduction band bends across the nanocrystal which in turn provides the necessary slope to enforce the free electrons and holes to move to the opposite direction of the crystal surfaces.48 Confirmation of this piezopotential induced band bending and corresponding charge separation phenomenon is further verified from the current density vs potential (J-V) curve measured in presence and absence of ultrasound. (Figure 7) Ultrasound treated ZnSnO3 nanoparticles are found to exhibit greater current density, indicating efficient carrier separation under mechanical stress.49 Moreover, the ultrasound induced band bending results decreases the difference of potential energy between band edge and redox potential of H2O/OH and O2/O2- which further promotes electrons and holes to react easily with dissolved oxygen and water to form ROS.50 However, the free charge carriers that are consumed in the redox processes can be further regenerated from the dissipated heat produced by the absorption of ultrasound radiation.

Conclusion



In summary, colloidal ZnSnO3 nanoparticles have been synthesized using molecular precursors in a colloidal synthesis approach. The approach produces phase pure orthorhombic-ZnSnO3 nanoparticles of 4-5 nm size and exhibits stable ferroelectric behavior at room temperature. Under ultrasound exposure, the colloidal form of nanoparticles produce reactive oxygen species (mainly hydroxyl radical) that is able to degrade rhodamine B. The current work strongly highlights the merits of ZnSnO3 nanoparticle as an efficient piezocatalyst for decontamination of organic pollutants in water. ASSOCIATED CONTENT Supporting Information Elemental analysis of hydrophobic/hydrophilic ZnSnO3 nanoparticles, summary of ultrasoundassisted piezocatalyst reported earlier, additional control experimental data on nanoparticle synthesis/characterization. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interests. Acknowledgement. The authors acknowledge DST Nano Mission (Grant number SR/NM/NB/ 1009/2016) and CSIR (Grant number 02(0249)15/EMR-II) Government of India for financial assistance. AB acknowledges CSIR, India for providing research fellowship. References 1. Wang, X. Piezoelectric Nanogenerators-Harvesting Ambient Mechanical Energy at the Nanometer Scale. Nano Energy 2012, 1, 13-24.


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Liang,

Z.;

Yan,

C.-F.;

Rtimi,

S.;

Bandara,

J.

Piezoelectric

Materials

for

Catalytic/Photocatalytic Removal of Pollutants: Recent Advances and Outlook. Appl. Catal. B Environ. 2019, 241, 256-269. 18. Feng, Y.; Li, H.; Ling, L.; Yan, S.; Pan, D.; Ge, H.; Li, H.; Bian, Z. Enhanced Photocatalytic Degradation Performance by Fluid-Induced Piezoelectric Field. Environ. Sci. Technol. 2018, 52, 7842-7848.


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27. Para, A. T.; Reshi, A. H.; Shelke, V. Synthesis of ZnSnO3 Nanostructure by Sol Gel Method. AIP Conf. Proc. 2016, 1731, 050002-1-050002-3. 28. Mayedwa, N., Mongwaketsi, N., Khamlich, S.,Kaviyarasu, K., Matinise, N., Maaza, M. Green Synthesis of Zin Tin Oxide (ZnSnO3) Nanoparticles Using Aspalathus Linearis Natural Extracts: Structural, Morphological, Optical and Electrochemistry Study. Appl. Surf. Sci. 2018, 446, 250257. 29.Qin, Y.; Zhang, F.; Du, X.; Huang, G.; Liu, Y.; Wang, L. Controllable Synthesis of Cube-Like ZnSnO3@TiO2 Nanostructures as Lithium Ion Battery Anodes. J. Mater. Chem. A 2015, 3, 29852990. 30. Wang, Y.-T.; Chang, K.-S. Piezopotential-Induced Schottky Behavior of Zn1−xSnO3 Nanowire Arrays and PiezophotocatalyticApplications. J. Am. Ceram. Soc. 2016, 99, 2593-2600. 31. Wang, Y.; Gao, P.; Sha, L.; Chi, Q.; Yang, L.; Zhang, J.; Chen, Y.; Zhang, M. Spatial Separation of Electrons and Holes for Enhancing the Gas-sensing Property of a Semiconductor: ZnO/ZnSnO3 Nanorod Arrays Prepared by a Hetero-epitaxial Growth. Nanotechnology 2018, 29, 175501. 32.Li, Z. D.; Zhou, Y.; Zhang, J. Y.; Tu, W. G.; Liu, Q.; Yu, T.; Zou, Z. G.Hexagonal NanoplateTextured Micro-Octahedron Zn2SnO4: Combined Effects toward Enhanced Efficiencies of DyeSensitized Solar Cell and Photoreduction of CO2 into Hydrocarbon Fuels. Cryst. Growth Des. 2012, 12, 1476-1481. 33. Lo, M. K.; Lee, S. Y.; Chang, K. S. Study of ZnSnO3 Nanowire Piezophotocatalyst using Twostep Hydrothermal Synthesis. J. Phys. Chem. C 2015, 119, 5218-5224. 34. Barsoum, M.W. Fundamentals of Ceramics, Taylor & Francis: Boca Raton, Florida 2002.


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Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ar gas

Ar gas

Ar gas

Ar gas Sn-precursor in oleylamine

Zn-precursor in oleic acid

cooled to room temperature

280 C

Colloidal ZnSnO3 nanoparticle

ODE

Scheme 1. Synthetic approach for ZnSnO3 nanoparticle. In the first step oleic acid solution of Zn precursor is added to octadecene medium. Next, oleylamine solution of Sn precursor is injected at 280 ℃ for nucleation-growth of ZnSnO3 nanoparticle.



c)

b)

(110)

(116)

(012)

ZnO6

SnO6 Zn

20

30

d)

40 50 60 2 (degree)

70

Sn

Intensity (Counts)

a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

552

597 635

477

703 737

407

827

O

80

400

e)

500 600 700 -1 800 Raman shift (cm )

f) 5 nm 0.33 nm

(012) (110) (116)

5 1/nm

20 nm

Figure 1. (a) X-ray diffraction pattern of orthorhombic ZnSnO3 nanoparticles with respect to bulk, (b) Structural models for orthorhombic ZnSnO3 (size of atoms are based on arbitrary scale), (c) Raman spectrum of ZnSnO3 nanoparticles showing non-centrosymmetric phase, (d, e) TEM image of ZnSnO3 nanoparticle in low and high magnification showing the lattice spacing of 0.33 nm, corresponding to the (012) plane of ZnSnO3 and (f) Selected-area electron diffraction pattern of nanoparticles taken from Figure 1d showing the polycrystalline nature of the ZnSnO3 nanoparticles.


Page 25 of 32

a)

15 Zn:Sn = 1:1

b)

Sn 3d

6.6

3

Counts/sec (10 )

O 1s

3

Counts/sec (10 )

12 Sn 3p

9

Zn 2p 6 Sn 4d C 1s

3 0 1200 1000 800

600

400

200

Zn 2p3/2

6.0 5.4

Zn 2p1/2 E = 23.19 eV

4.8 4.2 1048

0

Binding energy (eV)

c)

d)

40 Sn 3d5/2 30

3

Counts/sec (10 )

3

Counts/sec (10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sn 3d3/2 20 E = 8.4 eV

10 0

496

492

488

1016

15

Experimental curve Fitted curve Lattice Oxygen 12 C=O/OH 9 6 3

484

1040 1032 1024 Binding energy (eV)

534

Binding energy (eV)

532

530

528

526

Binding energy (eV)

Figure 2. (a) Survey scan X-ray photoelectron spectroscopy (XPS) reveal the presence of elemental Sn, Zn and O as main elements in ZnSnO3 nanoparticles. b-d) XPS high resolution scans of Zn 2p (b), Sn 3d (c) and O 1s (d) regions with oxidation state (+2) for Zn and (+4) for Sn in ZnSnO3 nanoparticles.



b)

a) ITO

2

Polarization (C/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Glass

Page 26 of 32

0.003 0.002

ZnSnO3

0.001 0.000 -0.001 -0.002 -0.003 -4

V

-2

0

2

4

Electric Field (kV/cm)

Figure 3. a) Schematic representation for polarization vs electric field measurement protocol by PE loop tracer. A portion of the ITO coated glass is etched (with HCl and Zn powder) and then deposited with ZnSnO3 nanoparticles by dropcasting. b) Polarization vs electric field curve of ZnSnO3 nanoparticles measured at 1 kHz, exhibiting hysteresis loop with ferroelectric behavior.


Page 27 of 32

a)

b)

Transmittance (%) 4000

Hydrophobic Ligand exchanged 3200 2400 1600 800 -1 Wavenumber (cm )

20 nm d)

c)

Correlation Coefficient

18 15

Number (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12 9 6

0.8 0.6 0.4 0.2 0.0 0.1

103 105 10 Time (µs)

3 0

50 nm

150

300

450 600 Size (nm)

750

900

Figure 4. (a) FTIR spectra of hydrophobic and hydrophilic ZnSnO3 nanoparticles. Ligand exchange with hydroxyl group leads to the appearance of broad O-H stretching at ~3400 cm-1 and disappearance of -CH2 asymmetric and symmetric stretching of oleic acid at 2922 and 2852 cm-1, (b) TEM image of hydrophobic ZnSnO3 nanoparticles with colloidal dispersion in chloroform at inset, (c) TEM image of hydrophilic ZnSnO3 nanoparticles with colloidal dispersion in water at inset and (d) Hydrodynamic size of hydrophilic ZnSnO3 nanoparticle in aqueous phosphate buffer of pH 7.4 along with correlation coefficient at inset.



b)

0.7 0.0

0.5

0.6 0.5

1.0

1.5

2.0

1.0

2.5

0.8

Time (h)

0h 0.5 h 1h 1.5 h 2h 2.5 h

0.4 0.3 0.2 0.1 0.0 400

500 600 700 Wavelength (nm)

0.6

C/C0

a)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

control 1 control 2 control 3 control 4 ZnSnO3

0.4 0.2 0.0

800

0

30

60 90 Time (min)

120 150

Figure 5. a) Ultrasound assisted piezocatalytic degradation of rhodamine B in presence of colloidal ZnSnO3 nanoparticle, as observed by gradual decrease of rhodamine B absorption. Inset shows the optical images representing gradual discolouration of rhodamine B. b) Ultrasound assisted rate of degradation of rhodamine B (C/C0) by colloidal ZnSnO3 nanoparticle, as compared to no nanoparticle (control 1), ZnO nanoparticle (control 2) , orthorhombic-ZnSnO3 nanoparticle without any ultrasound (control 3) and fcc-ZnSnO3 nanoparticle with ultrasound (control 4).


Page 29 of 32

a)

b)

>h

+ > •O − 2

ln(C0/C)

•OH

8 7 6 5 4 3 2 1 0

no scavenger BQ EDTA TBA

0

c)

30

d)

60 90 120 Time (min)

150

2.5 h 2h 1.5 h 1h 0.5 h

800

Intensity (I/I0)

TBA (.OH scavenger)

EDTA (h+ scavenger)

BQ (-.O2 scavenger)

1000

no scavenger

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600 400 200 0 360

420 480 540 Wavelength (nm)

600

Figure 6. a) Ultrasound assisted degradation of rhodamine B (C/C0) by colloidal ZnSnO3 nanoparticle in presence of different free radical scavengers, showing that hydroxyl radical is the main reactive oxygen species for piezocatalysis. b) Plots of ln(C0/C) vs ultrasonic treatment time for the degradation of rhodamine B in presence of different scavengers. Linear fitting is carried out from the points obtained by averaging the data taken from five independent set of experiments. c) Kinetic rate constant for the ultrasound assisted rhodamine B degradation performed with different scavengers. d) Demonstration of hydroxyl radical formation during ultrasound treatment as evidenced by increase in fluorescence intensity of 2-hydroxy terephthalic acid.



-0.10 without ultrasound ultrasound

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-0.08 -0.06 -0.04 -0.02 0.00

0.2

0.4 0.6 0.8 1.0 Potential (V vs Ag/AgCl)

1.2

Figure 7. Ultrasound assisted current density v/s potential (J-V) curve obtained with a glassy carbon electrode modified with orthorhombic ZnSnO3 nanoparticles and measured in 0.01 M Na2SO4 at a scan rate of 10 mV/s.


Page 30 of 32

Page 31 of 32

Conduction band

Conduction band e-

e- e - e- e- e- e-

h+

h+

.OH

h+

h+

Valence band

e-

h+

Free charges

-

Polarization charges

h+

PUS

h+ h+

h+

e-

eO2

+ + + +

h+

H2O

e- e-

Thermal activation

h+

Stress

-

+

h+

O2/.O2-

e-

Thermal activation

H2O/.OH

stress free state

Thermal activation

O2/.O2-

Thermal activation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


.O 2

H2O/.OH

h+

Valence band

Scheme 2. Schematic representation demonstrating mechanism involved in ultrasound assisted reactive oxygen species (ROS) generation from colloidal ZnSnO3 nanoparticle. Free electrons and holes within the nanoparticle are attracted towards opposite direction upon ultrasound induced piezoelectric potential. This in turn shifts the conduction and valence band, resulting the easier reaction with dissolved oxygen and water to form ROS.



Page 32 of 32

TOC

rhodamine B ZnO6 ROS

stress

degradation

stress

SnO6

stress

power timer

nanoparticles-rhodamine B Zn

Sn

O


ZnSnO3 Nanoparticle-Based Piezocatalysts for Ultrasound-Assisted

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