- Email: [email protected]
Hydrothermal synthesis of Ga-doped In2O3 nanostructure and its structural, optical and photocatalytic properties
Materials Science in Semiconductor Processing 71 (2017) 357–365
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Hydrothermal synthesis of Ga-doped In2O3 nanostructure and its structural, optical and photocatalytic properties
MARK
⁎
B. Shanmuga Priyaa, M. Shanthib, , C. Manoharana, M. Bououdinac a b c
Department of Physics, Annamalai University, Annamalai Nagar, Chidambaram 608002, Tamilnadu, India Department of Physics (FEAT), Annamalai University, Annamalai Nagar, Chidambaram 608002, Tamilnadu, India Department of Physics, College of Science, University of Bahrain, PO Box 32038, Kingdom of Bahrain
A R T I C L E I N F O
A B S T R A C T
Keywords: Ga-doped In2O3 Nanostructures TEM Photoluminescence Photocatalytic activity
In this work, pure and Ga-doped In2O3 nanostructures have been synthesized by facile template-free hydrothermal method. The structural, morphological and optical properties are characterized by using XRD, FT-IR, HR-SEM and TEM, EDS, XPS, UV-DRS, and PL techniques. X-ray diffraction analysis indicates a pure cubic phase while crystallite size decreases with Ga doping. HR-SEM and TEM observations reveal irregular-shaped and spindle-like nanostructures with enhanced crystallinity and reduction in particle size with Ga doping. XPS spectra reveal the oxidation state of Ga is +3. The energy band gap estimated by UV–vis DRS spectroscopy is found to increase slightly from 3.40 to 3.45 eV with Ga doping. Photoluminescence spectra display violet, blue and green emission peaks are observed during Ga doping concentrations. Photodegradation of Methylene blue dye under ultra violet light radiation is found to double with Ga doping; i.e. 48% for Ga- In2O3 compared to 22% for pure In2O3.
1. Introduction The fundamental components in optoelectronic devices are transparent conductive oxides (TCO), which are fully transparent with a wide range of wavelengths. TCOs are mostly used in a variety of applications such as solar energy conversion devices, flat panel display, photovoltaic and OLED screens. Ga-doped In2O3 is an effective transparent conductive material [1], which shows high optical transparency and inferior electrical conductivity in the visible region. Ga-doped In2O3 is used in a phase change random access memory devices [2,3], gas sensors [4] as well as a semiconductor for thin film transistor [5]. Nowadays, the synthesis of a semiconducting materials with enhanced electronic properties, by tuning the reaction conditions, becomes more challenging and of great importance. In addition, semiconductors can also be used as an effective photocatalysts due to feasible combination of their electronic structure, light absorbing characteristics, charge transport features, their relatively stable excited electronic states and their half-life period observed for some oxides. In2O3 is an efficient photocatalyst compared with ZnO and TiO2 towards the degradation of MB [6]. Indium oxide (In2O3) is one of the important n-type semiconductor materials with a band gap of 3.0–3.75 eV [7]. In recent years, Ga-doped In2O3 nanostructures have been synthesized by various methods
⁎
Corresponding author. E-mail address: [email protected] (M. Shanthi).
http://dx.doi.org/10.1016/j.mssp.2017.08.025 Received 31 May 2017; Received in revised form 7 August 2017; Accepted 18 August 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
[1–3,8–10], and the reported results show the effect of synthesis route in addition to Ga doping content on phase stability, morphological and optical modifications. Among the various available methods, hydrothermal method is facile and eco friendly one to synthesize Ga-doped In2O3. The nucleation rate, uniformity and different morphological features could be achieved in hydrothermal process under controlled and optimized conditions. Additionally, the hydrothermal process appears to be industrially more economical for large scale production of good quality nanomaterials. Doping semiconductors with appropriate elements (known as impurities) has a great influence on various properties including optical, electrical, magnetic, catalytic activity and gas sensing, etc. Moreover, the dopant is considered as critical factor in controlling the particle size and morphology, which have a direct effect on nanomaterials properties. The doping of metal ions in semiconductors restrict the recombination of electron – hole pairs and hence photodegradation efficiency is enhanced. In this present work, Ga doped In2O3 nanostructure is synthesized by hydrothermal method. The effect of Ga3+ dopant on the structural, optical photocatalytic activity of In2O3 is studied. Dopant Ga3+ into In2O3 act as a electron scavenger (i.e) inhibit the electron – hole recombination in the photocatalytic degradation process and so more number of hydroxyl radicals OH. are formed. The increased number of OH. radicals in an aqueous solution is the reason for
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
increased efficiency of degradation. Generally, the activity of photocatalyst depends on factors like morphology, crystallite size and surface area. Semiconducting photocatalyst with different nanostructure plays an important role for the photodegradation of organic pollutants in water [11,12]. Previously, Zn2+ doped In2O3 with different nanostructures were prepared using hydrothermal method. The structural, optical and photocatalytic performance was studied based on the effect of In/Zn ratio [13]. The high surface area with porous nature of the photocatalyst provides more adsorption sites for the dye molecules and improves the photo reaction [14]. 2. Materials and methods 2.1. Chemicals and reagents Fig. 1. XRD patterns of pure and Ga-doped In2O3 nanostructures.
The chemicals and reagents purchased from Sigma-Aldrich, viz. Indium nitrate pentahydrate [In (NO3)2·5H2O], Gallium nitrate [Ga (NO3)3·2H2O], urea [NH2 CO NH2] are of analytical grade and used as received without any further purification. Deionized (DI) water was used throughout the whole experimental procedures. 2.2. Synthesis of pure and doped In2O3 nanostructures In a typical synthesis, 20 ml of an aqueous solution of Indium nitrate pentahydrate [In(NO3)2·5H2O] (0.1 M) was taken and then mixed together with an aqueous solution of 20 ml of urea (0.3 M). The resulting solution was allowed to mix thoroughly using a magnetic stirrer for about 15 min. The as-obtained solution was transferred into a closed container and put inside the stainless steel autoclave at room temperature, which was then heated at 120 °C for over 6 h. After being cooled to room temperature naturally, the white precipitate was collected and washed with DI water and ethanol for several times and dried at 100 °C for 12 h in hot air oven. Then the samples were calcined for 2 h at 600 °C. Gallium nitrate [Ga(NO3)3·2H2O] with different concentrations (0.001, 0.003, 0.005, 0.007 M) were added with the aqueous solution of Indium nitrate pentahydrate for the preparation of Ga-doped In2O3 nanostructures, by following the same procedure adopted to prepare pure In2O3 compound.
Fig. 2. FT-IR spectra of pure and Ga-doped In2O3 nanostructures.
200 and Jeol/JEM 2100 high resolution transmission electron microscopy (TEM) with an accelerating voltage 200 kV. X-ray photoelectron spectra, arising during photoemission of electrons from sample surface, were obtained under vacuum 1.3 × 10−7 Pa at room temperature with electrostatic spectrometer HP 5950 A Hewlette – Packard firm using monochromotized AlKα1,2(hυ = 1486.6 eV) X-ray excitation and the gun of low-energy electrons for compensation of electrostatic charging of samples. UV–vis diffuse reflectance spectra were recorded using VARIAN Carry 5000 UV–vis spectrometer. PL study was carried out at room temperature using a VARIAN spectrophotometer equipped with a 450 W xenon lamp as the excitation source.
2.3. Photodegradation measurements Photocatalytic efficiency of as-synthesized pure and Ga-doped In2O3 nanostructures were investigated by the photochemical dye degradation of Methylene blue (MB). The reaction was carried out at ambient temperature using a photoreactor equipped with 365 nm UV lamp. A 0.05 g of photocatalyst was added to 40 ml of stirred methylene blue dye solution (1 × 10−4 M), then taken in a reaction tube. The solution in the reaction tube was stirred and catalyst was kept under constant motion by bubbling air using micro air-pump. At a given interval of irradiation times, the solution was sampled out from the reaction tube and UV spectra were recorded to study the degradation of MB dye. Heber multilamp photo-reactor model HML-MP88 was used for the photodegradation of MB dye. It consisted of eight number of 8 W medium pressure mercury lamps set in parallel and emitting 365 nm wavelength. UV spectra for Photocatalytic activity were recorded on a JASCO V-670 spectrometer at room temperature.
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the evolution of X–ray diffraction patterns of pure and Ga-doped In2O3 with different M% of Ga. A strong preferred orientation along (222) reflection is noticed with the presence of low intensity peaks at 21.6°, 35.6°, 45.6°, 51.2° and 60.8° corresponding to (211), (400), (431), (440) and (622) reflections, respectively, which is in good agreement with JCPDS card no. 71-2195 of bulk In2O3 cubic phase. A slight decrease of (222) reflection intensity with increasing Ga concentration, can be noticed. No additional peaks related to Ga-phases can be detected, thereby confirming the formation of single phase and the dissolution of Ga into In2O3 host lattice. The observed peaks’ broadening can be associated with defects produced due to Ga doping; may be due to the difference in ionic radii of Ga3+ and In3+; i.e. 0.62 and 0.80 Å, respectively. The substitution of In3+ by Ga3+, results in lattice mismatch, thereby enhancing strain and reduction in lattice parameter
2.4. Characterization techniques Powder XRD patterns of as-synthesized nanostructures were recorded using XPERT PRO X-ray diffractometer equipped with Cu-Kα radiation source (λ = 1.5406 Å). The representative FT-IR spectra of nanostructures were recorded using RX I Perkin Elmer FT-IR spectrometer. The morphological features and chemical composition were analyzed by using high resolution scanning electron microscopy (HRSEM) and energy dispersive spectroscopy (EDS) using FEI Quanta FEG 358
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Fig. 3. HR-SEM images of a) pure In2O3, b,c) Gadoped In2O3 nanostructures.
Fig. 4. EDS spectrum of a) pure In2O3, b) Ga-doped In2O3 nanostructures.
[15]. The crystallite size is found to decrease from 39 nm to 14 nm by Ga doping. The contraction in lattice parameter is due to the incorporation of Ga3+ with smaller ionic radius into In3+ sites within In2O3 crystal lattice [2,16], leading to the deterioration of crystal lattice, which in turn decreases the crystallite size [17]. The appearance of intense sharp peaks indicates the good crystallinity.
due to the asymmetric and symmetric stretching vibrations of In–O bond. It can be noticed that the bond position has been slightly shifted to a higher wavenumber as the concentration of Ga dopant increased, which once again confirms that Ga3+ dissolves into In2O3 lattice and occupy In3+ sites, as both elements have different ionic radii (0.62 and 0.8 Å).
3.2. Fourier transform infrared (FTIR) spectroscopy
3.3. High resolution scanning electron microscopy (HR-SEM) observations
The appearance of two main intense peak at 604 and 565 cm−1 in addition to weak peaks at 495, 450 and 436 cm−1 are attributed to the In-O phonon vibration mode of cubic In2O3 phase (Fig. 2). Panneerdoss et al. [18] stated that the observed peaks at 650, 580 and 490 cm−1 are
The surface morphology of pure and 7 M % Ga-doped In2O3 nanostructures are shown in Fig. 3. HR-SEM images reveal the effective morphological changes of In2O3 nanoparticles upon Ga doping. In2O3 nanoparticles are found to have irregular entities along with nanoflakes 359
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Fig. 5. TEM images of a) pure In2O3, b–d) low and high magnification of Ga-doped In2O3, SAED pattern of e) pure, f) Ga-doped In2O3 nanostructures.
Fig. 6. Histogram of particle size distribution of a) pure In2O3, b) Ga-In2O3 nanostructures.
(Fig. 4(a)) of pure In2O3 consists only In and O elements, with a chemical compositions 38.48 and 61.2 at%, respectively, which is very close to stoichiometry ratio of In2O3 compound. The EDS spectrum shown in Fig. 4(b) reveals the presence of Ga in addition to In and O, with a chemical composition of 36.92, 57.31 and 5.78 at%, for of In, O and Ga respectively.
like structure (Fig. 3(a)), whereas 7 M% Ga-doped In2O3 show remarkable new feature; from irregular shape into defined spindle–like morphology (Fig. 3(b, c)) composed of nanoparticles with a mean diameter in the range of 9–15 nm. The growth of Ga-doped In2O3 nanostructures is obtained by self assembly and oriented attachment mechanism (OAM) [19]. The spindle–like nanostructure consists of nanoparticles which are self-assembled into ordered chain, aligned parallel along the spindle axis. The spindle–like morphology is with an equatorial diameter of 100–200 nm and length of 750–1000 nm.
3.5. Transmission electron microscopy (TEM) observations Detailed morphological and structural analyses of pure and Gadoped In2O3 (7 M %) nanostructures are carried out using TEM and selected-area electron diffraction (SAED). Fig. 5(a) shows an irregular arrangement of nanocrystals that are assembled into irregular branched or network structure. Fig. 5(b, c) reveals porous spindle-like
3.4. Energy dispersive spectroscopy (EDS) analysis The constituent elements and the corresponding chemical composition analyzed by EDS are shown in Fig. 4. The EDS spectrum 360
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Fig. 7. XPS survey spectra of Ga-doped In2O3 nanostructure.
Fig. 9. UV-DRS spectra of pure and Ga-doped In2O3 nanostructures.
morphology formed by the agglomeration of all primary uniformlyshaped nanoparticles. It can be noticed that the particle size reduces with Ga doping; 9 nm. The network nanostructure observed in pure In2O3 changes into porous spindle-like nanostructure with the agglomeration of nanoparticles into chains that are parallel along the spindle axis with Ga doping. The observed d-spacing value of 0.298 nm from the diffraction pattern (Fig. 5(d)) is in close agreement with the reported values for the plane (222) of In2O3 [20]. The SAED patterns of
individual pure and Ga-doped In2O3 nanoparticles shown in Fig. 5(e, f), reflect the good crystalline nature of the as-prepared nanostructures, in agreement with XRD analysis. The SAED pattern Fig. 5(e) has spot-like appearance with fewer number of crystallite aggregates. Meanwhile, the slightly elongated diffraction spots (Fig. 5(f)) are due to the missorientation of planes in multiple nano-domains. The histograms of particle size distribution obtained from TEM images shown in the Fig. 6(a, b), indicate a mean diameter of 18 and
Fig. 8. High resolution XPS spectra of (a) In 3d, (b) O 1s, (c) Ga 3d.
361
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Fig. 10. Bandgap energy of pure and Ga-doped In2O3 nanostructures.
and 451 eV, which are in agreement with the binding energies of In3+d5/2 and In3+d3/2, [21]. The observed intense peak of O 1s at 530 eV can be ascribed to the lattice oxygen in In–O bonds as reported in Lopez et al. [22]. The binding energy of Ga 3d in the spectrum is about 20.7 eV, which demonstrates the presence of trivalent in the products as reported by Shih et al. [23]. Ga3+ indicates the oxidation state of Ga is +3 in Ga-doped In2O3.
9 nm, for pure and 7 M % of Ga doped In2O3 nanostructures respectively, which is consistent with XRD analysis. 3.6. XPS spectrum An XPS spectrum was recorded to ascertain the valency of gallium incorporated into the lattice position of In3+. XPS survey spectrum (Fig. 7) of Ga (7 M %)-doped In2O3, which identified significant signals of In, O, Ga and carbon. Fig. 8(a–c) shows the high resolution XPS spectra of In, O and Ga elements. In XPS, the calibration of binding energy is made by considering the C 1s peak (284 eV) as charge reference. In 3d spectrum exhibits two strong peaks centered at 443 eV
3.7. UV–vis reflectance spectroscopy The diffuse reflectance spectra (DRS) recorded for pure and Gadoped In2O3 nanostructures shown in Fig. 9, shows clear reflectance 362
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Scheme 1. Illustration of photodegradation mechanism of MB over Ga3+ doped In2O3.
Fig. 11. PL spectra of pure and Ga-doped In2O3 nanostructures.
edge at 339 nm, which slightly shifts towards visible region for Gadoped In2O3. As reported by Caroline Knapp et al. [24], pure and Gadoped In2O3 nanostructures have high reflectance in the mid-UV and visible regions. Ga-doped In2O3 nanostructures have slight decrease in the percentage of reflectance, which can be attributed to quantum confinement effect [25]. The band gap energy of pure and Ga-doped In2O3 nanostructures is determined from the plot of (αhν)2 versus photon energy (eV), as shown in Fig. 10; 3.4 eV for pure In2O3, which slightly increases to 3.45 eV for 7 M% Ga doping. The incorporation of Ga3+ with much smaller radius into In3+ sites is not sufficient in breaking the symmetry of disallowed valence band to conduction band transitions. Hence, the optical properties are not hugely altered. Regoutz et al. [16] reported that the fundamental optical band gap increased when Ga is effectively doped into In2O3 lattice.
Fig. 13. Plot of (C/C0) versus time for the photodegradation of MB dye by pure In2O3 and Ga-doped In2O3 nanostructures under UV irradiation.
crystal lattice [27]. The as-obtained emission values are in agreement with some previous reports [15,28]. No significant changes in the spectra can be noticed, apart from a slight decrease in emissions’ intensity with increasing Ga concentration. As reported in the literatures, PL emissions of nanocrystalline In2O3 are possibly due to the effect of oxygen vacancies [29,30]. The oxygen vacancies in the crystal lattice of pure In2O3 decreases with the occupation of Ga3+ in vacant sites created by oxygen. The decrease in oxygen vacancy reduces the possible recombination of photo-excited hole-electron pairs, which can be considered as the main reason for the decrease in photoemission intensity.
3.8. Photoluminescence (PL) The PL emission spectra of the pure and Ga-doped In2O3 nanostructures are shown in Fig. 11. It is well known that bulk In2O3 cannot emit light at room temperature, but its nanostructures can emit UV and visible lights [26]. However, the as-synthesized pure and Ga-doped In2O3 nanostructures reveal three peaks centered at 432, 485 and 536 nm associated with violet, blue and green emissions, respectively and due to the presence of defects and oxygen vacancies within In2O3
Fig. 12. Absorption spectra of MB solution containing a) pure In2O3, b) Ga-doped In2O3. nanostructures under exposure to UV light for various durations.
363
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
specific surface area (reduction of crystallite size from 39 to 14 nm as obtained by XRD analysis), thereby favoring a better reaction (faster kinetics) with the dye molecules [28]. Therefore, an enhancement of photocatalytic activity is noticed for Ga-doped In2O3. The photocatalytic efficiencies of as-synthesized pure and Ga-doped In2O3 are compared and presented in Fig. 13; During 100 mins of visible light irradiation the percentage of MB degradation of Ga-doped In2O3 is found to be twice higher than that of pure In2O3, 48% and 22%, respectively.
3.9. Photocatalytic activity In aqueous solution the photocatalytic degradation of MB is initiated by the UV-light with photon energies, which are greater than or equal to their band gaps. The electron in their valence band (VB) can be excited to their conduction band (CB) of Ga- In2O3 catalyst by leaving behind an equal amount of electron vacancies or holes in their valence band at the same time. The part of photogenerated electrons and holes on the surface of the catalyst involve in the oxidation and reduction reactions. The highly oxidative holes on VB could react with hydroxyl ions (OH-) and H2O adsorbed on the surface of the Ga- In2O3 catalyst to generate highly reactive hydroxyl radicals (OH•) and react with dye molecules directly in an aqueous solution. In addition, e- on the surface of the Ga- In2O3 catalyst could react with dissolved oxygen molecules to yield superoxide radicals O2•-. The superoxide radicals O2•- react with water to produce hydroperoxide (H2O2). Hydroperoxy is the key intermediate for the degradation, which can easily breakdown and also provide conditions for subsequent production of OH• radicals. These hydroxyl (OH•) and superoxide (O2•-) radicals then react with surrounding organic dye molecules, leading to easily evaporative degradation by-products or entire mineralization into CO2, H2O and mineral acids. The photocatalytic activity of pure and Ga-doped In2O3 are examined against the photodegradation of MB aqueous solutions under visible light irradiation. An aqueous MB dye solution with appropriate concentration is prepared and then allowed to irradiate under UV-light for different intervals of time with the presence of catalysts (pure and Ga-doped In2O3). The resulting solution is evaluated by UV–vis spectrometer and the concentration of MB is monitored by measuring its absorbance around 660 nm and by recording the UV–vis spectra. From Fig. 12(a, b), it is obvious that the concentration of MB slightly decreases with the increase in irradiation time of visible light in the presence of all catalysts; Ga3+ doped In2O3 nanostructures show better photocatalytic activity than pure In2O3. The possible mechanism is given by the following equations. PC + hν → h+ h+ VB
VB+
eˉ
+ OHˉ → OH•
CB
Pure and Ga-doped In2O3 nanostructures were synthesized using simple hydrothermal method. XRD analysis reveals the formation of pure single cubic crystal structure with preferred orientation along (222) reflection, while the crystallite size decreases with increasing Ga doping concentration (1–7 M%). The surface morphology changes from irregular into defined porous spindle-like nanostructure, with particle size distribution of 18 and 9 nm for pure and Ga-doped nanostructure, respectively, meanwhile SAED patterns confirm the good crystallinity. XPS results confirmed that the doped gallium ions are in trivalence. UVDRS spectra of the pure In2O3 shows a reflectance edge at 339 nm with a bandgap of 3.40 eV, which slightly shifts towards higher energy level and a bandgap of 3.45 eV with Ga doping. Violet, blue and green emissions are observed in the photoluminescence spectra. It is found that Ga-doped In2O3 nanostructure exhibited better photocatalytic activitmatrix absorb photoexcited electrons and changed y than pure In2O3 for the degradation of methylene blue dye, with an efficiency of 48% compared to only 22%. References [1] Diego Leon Sanchez, Jesus Alberto Ramos Ramon, Manuel Herrera Zaldívar, Umapada Pal, Efrain Rubio Rosas, Adv. Nano Res. 3 (2015) 81–96. [2] M. Ramzan, T. Kaewmaraya, R. Ahuja, Appl. Phys. Lett. 103 (2013) 072113–072114. [3] Bo Jin, Taekyung Lim, Sanghyun Ju, Marat I. Latypov, Dong-Hai Pi, Hyoung Seop, Kim M. Meyyappan, Jeong-Soo Lee, Appl. Phys. Lett. 104 (2014) 103510–103514. [4] Minoo Bagheria, Abbas Ali Khodadadi, Ali Reza Mahjoub, Yadollah Mortazavi, Sens. Actuators.: B Chem. 220 (2015) 590–599. [5] Sunho Jeong, Ji-Yoon Lee, Sun Sook Lee, Youngmin Choi, Beyong-Hwan Ryu, J. Phys. Chem. C 115 (2011) 11773–11780. [6] N. Talebian, M.R. Nilforoushan, Thin Solid Films 518 (2010) 2210–2215. [7] G. Mohan Kumar, A. Madhan Kumar, P. Ilanchezhiyan, T.W. Kang, Nanoscale 6 (2014) 11226–11231. [8] Yong Liu, Wei Xu, Da-Bo Liu, Meijuan Yu, Yuan-Hua Lin, Ce-Wen Nan, Phys. Chem. Chem. Phys. 17 (2015) 11229–11233. [9] Jing Lin, Yang Huang, Yoshio Bando, Chengchun Tang, Chun Li, Dmitri Golberg, ACS Nano 4 (2010) 2452–2458. [10] Lili Wu, Quan Li, Xitian Zhang, Tianyou Zhai, Yoshio Bando, Dmitri Golberg, J. Phys. Chem. C 115 (2011) 24564–24568. [11] Zhenmin Li, Pengyi Zhang, Tian Shao, Jinlong Wang, Ling Jin, Xiaoyun Li, J. Hazard. Mater. 260 (2013) 40–46. [12] R. Camposeco, S. Castillo, J. Navarrete, R. Gomez, Catal. Today 266 (2015) 90–101. [13] B. Shanmuga Priya, M. Shanthi, C. Manoharan, S. Dhanapandian, 〈http://doi.org/ 10.1007/s10854-017-7106-0〉. [14] S. Bharathi, D. Nataraj, D. Mangalaraj, Y. Masuda, K. Senthil, K. Yong, J. Phys. D: Appl. Phys. 43 (015501) (2010) 9. [15] Lizhu Liu, Yiqing Chen, Linliang Guo, Taibo Guo, Yunqing Zhu, Yong Su, Chong Jia, Meiqin Wei, Yinfen Cheng, Appl. Surf. Sci. 258 (2011) 923–927. [16] A. Regoutz, R.G. Egdell, D.J. Morgan, R.G. Palgrave, H. Tellez, S.J. Skinner, D.J. Payne, G.W. Watson, D.O. Scanlon, Appl. Surf. Sci. 349 (2015) 970–982. [17] Yogeshwar Kumar, Md. Ahamad Mohiddon, Alok Srivastava, K.L. Yadav, Indian J. Eng. Mater. Sci. 16 (2009) 390–394. [18] I. Joseph Panneerdoss, S. Johnson Jeyakumar, S. Ramalingam, M. Jothibas, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 147 (2015) 1–13. [19] Dewei Chu, Yu-Ping Zeng, Dongliang Jiang, Yoshitake Masuda, Sens. Actuators B 137 (2009) 630–636. [20] S.N. Anshu Singhal, J. Achary, O.D. Manjanna, R.M. Jayakumar, A.K. Kadam, J. Tyagi, Phys. Chem. C 113 (2009) 3600–3606. [21] Gunho Jo, Woong-Ki Hong, Jongsun Maeng, Tae-Wook Kim, Gunuk Wang, Ahnsook Yoon, Soon-Shin Kwon, Sunghoon Song, Takhee Lee, Colloids Surfaces A: Physicochem. Eng. Asp. 313–314 (2008) 308–311. [22] Iñaki Lopez, Antonio D. Utrilla, Emilio Nogales, Bianchi Mendez, J. Javier Piqueras, Phys. Chem. C 116 (2012) 3935–3943. [23] Huan-Yu Shih, Fu-Chuan Chu, Atanu Das, Chia-Yu Lee, Ming-Jang Chen, Ray-
(1)
CB
(2)
OH• + MB → degrade products eˉ
4. Conclusion
ˉ
+ O2 → O2•
(3) (4)
ˉ
O2• + 2H2O → H2O2+ O2 +2OHˉ
(5)
H2O2 + eˉ
CB
→ OH• + OHˉ
(6)
Dye + h+
VB
→ Final species
(7)
Dye + OH• → Final species
(8) 3+
It is well known that Ga doped into In2O3 matrix absorb photoexcited electrons and changed into Ga2+ ions. These Ga2+ ions react with O2 molecule and produce superoxide radical O2•- and Ga3+ ions (Scheme 1). Wider band gap of the Ga3+ doped In2O3 favor the efficiency of photodegradation. Ga3+ doped In2O3 has higher band gap than undoped In2O3 because of their smaller particle size, leads to extend the electron-hole stability and favor photodegradation efficiency. In2O3 + hυ → h+
VB
+ e-
CB
Ga3+ + e- → Ga2+ Ga
2+
+ O2 → O2• + Ga -
(9) (10)
3+
(11)
By this way, photogenerated electrons are continuously trapped and Ga3+ slow downs the electron – hole recombination. This means that Ga3+ on the surface of the In2O3 may act as electron scavenger. On the other hand, the reduction in particle size with Ga doping results in the appearance of porous nanostructure, in addition to the increase of the 364
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Optoelectron. Adv. Mater. 10 (2008) 161–165. [28] Heqing Yang, Ruyu Shi, Jie Yu, Ruini Liu, Ruigang Zhang, Hua Zhao, Lihui Zhang, Hairong Zheng, J. Phys. Chem. C 113 (2009) 21548–21554. [29] Liang Xu, Yong Su, Sen Li, Yiqing Chen, Qingtao Zhou, Song Yin, Yi Feng, J. Phys. Chem. B 111 (2007) 760–766. [30] Soumitra Kar, Subhadra Chaudhuri, Chem. Phys. Lett. 422 (2006) 424–428.
Ming Lin, Nanoscale Res. Lett. 11 (235) (2016) 9. [24] Caroline E. Knapp, Geoffrey Hyett, Ivan P. Parkin, Claire J. Carmalt, Chem. Mater. 23 (2011) 1719–1726. [25] H. Lin, C.P. Huang, W. Li, C. Ni, S. Ismat Shah, Yao-Hsuan Tseng, Appl. Catal. B: Environ. 68 (2006) 1–11. [26] E.C.C. Souza, J.F.Q. Rey, E.N.S. Muccillo, Appl. Surf. Sci. 255 (2009) 3779–3783. [27] S. Maensiri, P. Laokul, J. Klinkaewnarong, S. Phokha, V. Promarak, S. Seraphin, J.
365
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Hydrothermal synthesis of Ga-doped In2O3 nanostructure and its structural, optical and photocatalytic properties
MARK
⁎
B. Shanmuga Priyaa, M. Shanthib, , C. Manoharana, M. Bououdinac a b c
Department of Physics, Annamalai University, Annamalai Nagar, Chidambaram 608002, Tamilnadu, India Department of Physics (FEAT), Annamalai University, Annamalai Nagar, Chidambaram 608002, Tamilnadu, India Department of Physics, College of Science, University of Bahrain, PO Box 32038, Kingdom of Bahrain
A R T I C L E I N F O
A B S T R A C T
Keywords: Ga-doped In2O3 Nanostructures TEM Photoluminescence Photocatalytic activity
In this work, pure and Ga-doped In2O3 nanostructures have been synthesized by facile template-free hydrothermal method. The structural, morphological and optical properties are characterized by using XRD, FT-IR, HR-SEM and TEM, EDS, XPS, UV-DRS, and PL techniques. X-ray diffraction analysis indicates a pure cubic phase while crystallite size decreases with Ga doping. HR-SEM and TEM observations reveal irregular-shaped and spindle-like nanostructures with enhanced crystallinity and reduction in particle size with Ga doping. XPS spectra reveal the oxidation state of Ga is +3. The energy band gap estimated by UV–vis DRS spectroscopy is found to increase slightly from 3.40 to 3.45 eV with Ga doping. Photoluminescence spectra display violet, blue and green emission peaks are observed during Ga doping concentrations. Photodegradation of Methylene blue dye under ultra violet light radiation is found to double with Ga doping; i.e. 48% for Ga- In2O3 compared to 22% for pure In2O3.
1. Introduction The fundamental components in optoelectronic devices are transparent conductive oxides (TCO), which are fully transparent with a wide range of wavelengths. TCOs are mostly used in a variety of applications such as solar energy conversion devices, flat panel display, photovoltaic and OLED screens. Ga-doped In2O3 is an effective transparent conductive material [1], which shows high optical transparency and inferior electrical conductivity in the visible region. Ga-doped In2O3 is used in a phase change random access memory devices [2,3], gas sensors [4] as well as a semiconductor for thin film transistor [5]. Nowadays, the synthesis of a semiconducting materials with enhanced electronic properties, by tuning the reaction conditions, becomes more challenging and of great importance. In addition, semiconductors can also be used as an effective photocatalysts due to feasible combination of their electronic structure, light absorbing characteristics, charge transport features, their relatively stable excited electronic states and their half-life period observed for some oxides. In2O3 is an efficient photocatalyst compared with ZnO and TiO2 towards the degradation of MB [6]. Indium oxide (In2O3) is one of the important n-type semiconductor materials with a band gap of 3.0–3.75 eV [7]. In recent years, Ga-doped In2O3 nanostructures have been synthesized by various methods
⁎
Corresponding author. E-mail address: [email protected] (M. Shanthi).
http://dx.doi.org/10.1016/j.mssp.2017.08.025 Received 31 May 2017; Received in revised form 7 August 2017; Accepted 18 August 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
[1–3,8–10], and the reported results show the effect of synthesis route in addition to Ga doping content on phase stability, morphological and optical modifications. Among the various available methods, hydrothermal method is facile and eco friendly one to synthesize Ga-doped In2O3. The nucleation rate, uniformity and different morphological features could be achieved in hydrothermal process under controlled and optimized conditions. Additionally, the hydrothermal process appears to be industrially more economical for large scale production of good quality nanomaterials. Doping semiconductors with appropriate elements (known as impurities) has a great influence on various properties including optical, electrical, magnetic, catalytic activity and gas sensing, etc. Moreover, the dopant is considered as critical factor in controlling the particle size and morphology, which have a direct effect on nanomaterials properties. The doping of metal ions in semiconductors restrict the recombination of electron – hole pairs and hence photodegradation efficiency is enhanced. In this present work, Ga doped In2O3 nanostructure is synthesized by hydrothermal method. The effect of Ga3+ dopant on the structural, optical photocatalytic activity of In2O3 is studied. Dopant Ga3+ into In2O3 act as a electron scavenger (i.e) inhibit the electron – hole recombination in the photocatalytic degradation process and so more number of hydroxyl radicals OH. are formed. The increased number of OH. radicals in an aqueous solution is the reason for
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
increased efficiency of degradation. Generally, the activity of photocatalyst depends on factors like morphology, crystallite size and surface area. Semiconducting photocatalyst with different nanostructure plays an important role for the photodegradation of organic pollutants in water [11,12]. Previously, Zn2+ doped In2O3 with different nanostructures were prepared using hydrothermal method. The structural, optical and photocatalytic performance was studied based on the effect of In/Zn ratio [13]. The high surface area with porous nature of the photocatalyst provides more adsorption sites for the dye molecules and improves the photo reaction [14]. 2. Materials and methods 2.1. Chemicals and reagents Fig. 1. XRD patterns of pure and Ga-doped In2O3 nanostructures.
The chemicals and reagents purchased from Sigma-Aldrich, viz. Indium nitrate pentahydrate [In (NO3)2·5H2O], Gallium nitrate [Ga (NO3)3·2H2O], urea [NH2 CO NH2] are of analytical grade and used as received without any further purification. Deionized (DI) water was used throughout the whole experimental procedures. 2.2. Synthesis of pure and doped In2O3 nanostructures In a typical synthesis, 20 ml of an aqueous solution of Indium nitrate pentahydrate [In(NO3)2·5H2O] (0.1 M) was taken and then mixed together with an aqueous solution of 20 ml of urea (0.3 M). The resulting solution was allowed to mix thoroughly using a magnetic stirrer for about 15 min. The as-obtained solution was transferred into a closed container and put inside the stainless steel autoclave at room temperature, which was then heated at 120 °C for over 6 h. After being cooled to room temperature naturally, the white precipitate was collected and washed with DI water and ethanol for several times and dried at 100 °C for 12 h in hot air oven. Then the samples were calcined for 2 h at 600 °C. Gallium nitrate [Ga(NO3)3·2H2O] with different concentrations (0.001, 0.003, 0.005, 0.007 M) were added with the aqueous solution of Indium nitrate pentahydrate for the preparation of Ga-doped In2O3 nanostructures, by following the same procedure adopted to prepare pure In2O3 compound.
Fig. 2. FT-IR spectra of pure and Ga-doped In2O3 nanostructures.
200 and Jeol/JEM 2100 high resolution transmission electron microscopy (TEM) with an accelerating voltage 200 kV. X-ray photoelectron spectra, arising during photoemission of electrons from sample surface, were obtained under vacuum 1.3 × 10−7 Pa at room temperature with electrostatic spectrometer HP 5950 A Hewlette – Packard firm using monochromotized AlKα1,2(hυ = 1486.6 eV) X-ray excitation and the gun of low-energy electrons for compensation of electrostatic charging of samples. UV–vis diffuse reflectance spectra were recorded using VARIAN Carry 5000 UV–vis spectrometer. PL study was carried out at room temperature using a VARIAN spectrophotometer equipped with a 450 W xenon lamp as the excitation source.
2.3. Photodegradation measurements Photocatalytic efficiency of as-synthesized pure and Ga-doped In2O3 nanostructures were investigated by the photochemical dye degradation of Methylene blue (MB). The reaction was carried out at ambient temperature using a photoreactor equipped with 365 nm UV lamp. A 0.05 g of photocatalyst was added to 40 ml of stirred methylene blue dye solution (1 × 10−4 M), then taken in a reaction tube. The solution in the reaction tube was stirred and catalyst was kept under constant motion by bubbling air using micro air-pump. At a given interval of irradiation times, the solution was sampled out from the reaction tube and UV spectra were recorded to study the degradation of MB dye. Heber multilamp photo-reactor model HML-MP88 was used for the photodegradation of MB dye. It consisted of eight number of 8 W medium pressure mercury lamps set in parallel and emitting 365 nm wavelength. UV spectra for Photocatalytic activity were recorded on a JASCO V-670 spectrometer at room temperature.
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the evolution of X–ray diffraction patterns of pure and Ga-doped In2O3 with different M% of Ga. A strong preferred orientation along (222) reflection is noticed with the presence of low intensity peaks at 21.6°, 35.6°, 45.6°, 51.2° and 60.8° corresponding to (211), (400), (431), (440) and (622) reflections, respectively, which is in good agreement with JCPDS card no. 71-2195 of bulk In2O3 cubic phase. A slight decrease of (222) reflection intensity with increasing Ga concentration, can be noticed. No additional peaks related to Ga-phases can be detected, thereby confirming the formation of single phase and the dissolution of Ga into In2O3 host lattice. The observed peaks’ broadening can be associated with defects produced due to Ga doping; may be due to the difference in ionic radii of Ga3+ and In3+; i.e. 0.62 and 0.80 Å, respectively. The substitution of In3+ by Ga3+, results in lattice mismatch, thereby enhancing strain and reduction in lattice parameter
2.4. Characterization techniques Powder XRD patterns of as-synthesized nanostructures were recorded using XPERT PRO X-ray diffractometer equipped with Cu-Kα radiation source (λ = 1.5406 Å). The representative FT-IR spectra of nanostructures were recorded using RX I Perkin Elmer FT-IR spectrometer. The morphological features and chemical composition were analyzed by using high resolution scanning electron microscopy (HRSEM) and energy dispersive spectroscopy (EDS) using FEI Quanta FEG 358
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Fig. 3. HR-SEM images of a) pure In2O3, b,c) Gadoped In2O3 nanostructures.
Fig. 4. EDS spectrum of a) pure In2O3, b) Ga-doped In2O3 nanostructures.
[15]. The crystallite size is found to decrease from 39 nm to 14 nm by Ga doping. The contraction in lattice parameter is due to the incorporation of Ga3+ with smaller ionic radius into In3+ sites within In2O3 crystal lattice [2,16], leading to the deterioration of crystal lattice, which in turn decreases the crystallite size [17]. The appearance of intense sharp peaks indicates the good crystallinity.
due to the asymmetric and symmetric stretching vibrations of In–O bond. It can be noticed that the bond position has been slightly shifted to a higher wavenumber as the concentration of Ga dopant increased, which once again confirms that Ga3+ dissolves into In2O3 lattice and occupy In3+ sites, as both elements have different ionic radii (0.62 and 0.8 Å).
3.2. Fourier transform infrared (FTIR) spectroscopy
3.3. High resolution scanning electron microscopy (HR-SEM) observations
The appearance of two main intense peak at 604 and 565 cm−1 in addition to weak peaks at 495, 450 and 436 cm−1 are attributed to the In-O phonon vibration mode of cubic In2O3 phase (Fig. 2). Panneerdoss et al. [18] stated that the observed peaks at 650, 580 and 490 cm−1 are
The surface morphology of pure and 7 M % Ga-doped In2O3 nanostructures are shown in Fig. 3. HR-SEM images reveal the effective morphological changes of In2O3 nanoparticles upon Ga doping. In2O3 nanoparticles are found to have irregular entities along with nanoflakes 359
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Fig. 5. TEM images of a) pure In2O3, b–d) low and high magnification of Ga-doped In2O3, SAED pattern of e) pure, f) Ga-doped In2O3 nanostructures.
Fig. 6. Histogram of particle size distribution of a) pure In2O3, b) Ga-In2O3 nanostructures.
(Fig. 4(a)) of pure In2O3 consists only In and O elements, with a chemical compositions 38.48 and 61.2 at%, respectively, which is very close to stoichiometry ratio of In2O3 compound. The EDS spectrum shown in Fig. 4(b) reveals the presence of Ga in addition to In and O, with a chemical composition of 36.92, 57.31 and 5.78 at%, for of In, O and Ga respectively.
like structure (Fig. 3(a)), whereas 7 M% Ga-doped In2O3 show remarkable new feature; from irregular shape into defined spindle–like morphology (Fig. 3(b, c)) composed of nanoparticles with a mean diameter in the range of 9–15 nm. The growth of Ga-doped In2O3 nanostructures is obtained by self assembly and oriented attachment mechanism (OAM) [19]. The spindle–like nanostructure consists of nanoparticles which are self-assembled into ordered chain, aligned parallel along the spindle axis. The spindle–like morphology is with an equatorial diameter of 100–200 nm and length of 750–1000 nm.
3.5. Transmission electron microscopy (TEM) observations Detailed morphological and structural analyses of pure and Gadoped In2O3 (7 M %) nanostructures are carried out using TEM and selected-area electron diffraction (SAED). Fig. 5(a) shows an irregular arrangement of nanocrystals that are assembled into irregular branched or network structure. Fig. 5(b, c) reveals porous spindle-like
3.4. Energy dispersive spectroscopy (EDS) analysis The constituent elements and the corresponding chemical composition analyzed by EDS are shown in Fig. 4. The EDS spectrum 360
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Fig. 7. XPS survey spectra of Ga-doped In2O3 nanostructure.
Fig. 9. UV-DRS spectra of pure and Ga-doped In2O3 nanostructures.
morphology formed by the agglomeration of all primary uniformlyshaped nanoparticles. It can be noticed that the particle size reduces with Ga doping; 9 nm. The network nanostructure observed in pure In2O3 changes into porous spindle-like nanostructure with the agglomeration of nanoparticles into chains that are parallel along the spindle axis with Ga doping. The observed d-spacing value of 0.298 nm from the diffraction pattern (Fig. 5(d)) is in close agreement with the reported values for the plane (222) of In2O3 [20]. The SAED patterns of
individual pure and Ga-doped In2O3 nanoparticles shown in Fig. 5(e, f), reflect the good crystalline nature of the as-prepared nanostructures, in agreement with XRD analysis. The SAED pattern Fig. 5(e) has spot-like appearance with fewer number of crystallite aggregates. Meanwhile, the slightly elongated diffraction spots (Fig. 5(f)) are due to the missorientation of planes in multiple nano-domains. The histograms of particle size distribution obtained from TEM images shown in the Fig. 6(a, b), indicate a mean diameter of 18 and
Fig. 8. High resolution XPS spectra of (a) In 3d, (b) O 1s, (c) Ga 3d.
361
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Fig. 10. Bandgap energy of pure and Ga-doped In2O3 nanostructures.
and 451 eV, which are in agreement with the binding energies of In3+d5/2 and In3+d3/2, [21]. The observed intense peak of O 1s at 530 eV can be ascribed to the lattice oxygen in In–O bonds as reported in Lopez et al. [22]. The binding energy of Ga 3d in the spectrum is about 20.7 eV, which demonstrates the presence of trivalent in the products as reported by Shih et al. [23]. Ga3+ indicates the oxidation state of Ga is +3 in Ga-doped In2O3.
9 nm, for pure and 7 M % of Ga doped In2O3 nanostructures respectively, which is consistent with XRD analysis. 3.6. XPS spectrum An XPS spectrum was recorded to ascertain the valency of gallium incorporated into the lattice position of In3+. XPS survey spectrum (Fig. 7) of Ga (7 M %)-doped In2O3, which identified significant signals of In, O, Ga and carbon. Fig. 8(a–c) shows the high resolution XPS spectra of In, O and Ga elements. In XPS, the calibration of binding energy is made by considering the C 1s peak (284 eV) as charge reference. In 3d spectrum exhibits two strong peaks centered at 443 eV
3.7. UV–vis reflectance spectroscopy The diffuse reflectance spectra (DRS) recorded for pure and Gadoped In2O3 nanostructures shown in Fig. 9, shows clear reflectance 362
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Scheme 1. Illustration of photodegradation mechanism of MB over Ga3+ doped In2O3.
Fig. 11. PL spectra of pure and Ga-doped In2O3 nanostructures.
edge at 339 nm, which slightly shifts towards visible region for Gadoped In2O3. As reported by Caroline Knapp et al. [24], pure and Gadoped In2O3 nanostructures have high reflectance in the mid-UV and visible regions. Ga-doped In2O3 nanostructures have slight decrease in the percentage of reflectance, which can be attributed to quantum confinement effect [25]. The band gap energy of pure and Ga-doped In2O3 nanostructures is determined from the plot of (αhν)2 versus photon energy (eV), as shown in Fig. 10; 3.4 eV for pure In2O3, which slightly increases to 3.45 eV for 7 M% Ga doping. The incorporation of Ga3+ with much smaller radius into In3+ sites is not sufficient in breaking the symmetry of disallowed valence band to conduction band transitions. Hence, the optical properties are not hugely altered. Regoutz et al. [16] reported that the fundamental optical band gap increased when Ga is effectively doped into In2O3 lattice.
Fig. 13. Plot of (C/C0) versus time for the photodegradation of MB dye by pure In2O3 and Ga-doped In2O3 nanostructures under UV irradiation.
crystal lattice [27]. The as-obtained emission values are in agreement with some previous reports [15,28]. No significant changes in the spectra can be noticed, apart from a slight decrease in emissions’ intensity with increasing Ga concentration. As reported in the literatures, PL emissions of nanocrystalline In2O3 are possibly due to the effect of oxygen vacancies [29,30]. The oxygen vacancies in the crystal lattice of pure In2O3 decreases with the occupation of Ga3+ in vacant sites created by oxygen. The decrease in oxygen vacancy reduces the possible recombination of photo-excited hole-electron pairs, which can be considered as the main reason for the decrease in photoemission intensity.
3.8. Photoluminescence (PL) The PL emission spectra of the pure and Ga-doped In2O3 nanostructures are shown in Fig. 11. It is well known that bulk In2O3 cannot emit light at room temperature, but its nanostructures can emit UV and visible lights [26]. However, the as-synthesized pure and Ga-doped In2O3 nanostructures reveal three peaks centered at 432, 485 and 536 nm associated with violet, blue and green emissions, respectively and due to the presence of defects and oxygen vacancies within In2O3
Fig. 12. Absorption spectra of MB solution containing a) pure In2O3, b) Ga-doped In2O3. nanostructures under exposure to UV light for various durations.
363
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
specific surface area (reduction of crystallite size from 39 to 14 nm as obtained by XRD analysis), thereby favoring a better reaction (faster kinetics) with the dye molecules [28]. Therefore, an enhancement of photocatalytic activity is noticed for Ga-doped In2O3. The photocatalytic efficiencies of as-synthesized pure and Ga-doped In2O3 are compared and presented in Fig. 13; During 100 mins of visible light irradiation the percentage of MB degradation of Ga-doped In2O3 is found to be twice higher than that of pure In2O3, 48% and 22%, respectively.
3.9. Photocatalytic activity In aqueous solution the photocatalytic degradation of MB is initiated by the UV-light with photon energies, which are greater than or equal to their band gaps. The electron in their valence band (VB) can be excited to their conduction band (CB) of Ga- In2O3 catalyst by leaving behind an equal amount of electron vacancies or holes in their valence band at the same time. The part of photogenerated electrons and holes on the surface of the catalyst involve in the oxidation and reduction reactions. The highly oxidative holes on VB could react with hydroxyl ions (OH-) and H2O adsorbed on the surface of the Ga- In2O3 catalyst to generate highly reactive hydroxyl radicals (OH•) and react with dye molecules directly in an aqueous solution. In addition, e- on the surface of the Ga- In2O3 catalyst could react with dissolved oxygen molecules to yield superoxide radicals O2•-. The superoxide radicals O2•- react with water to produce hydroperoxide (H2O2). Hydroperoxy is the key intermediate for the degradation, which can easily breakdown and also provide conditions for subsequent production of OH• radicals. These hydroxyl (OH•) and superoxide (O2•-) radicals then react with surrounding organic dye molecules, leading to easily evaporative degradation by-products or entire mineralization into CO2, H2O and mineral acids. The photocatalytic activity of pure and Ga-doped In2O3 are examined against the photodegradation of MB aqueous solutions under visible light irradiation. An aqueous MB dye solution with appropriate concentration is prepared and then allowed to irradiate under UV-light for different intervals of time with the presence of catalysts (pure and Ga-doped In2O3). The resulting solution is evaluated by UV–vis spectrometer and the concentration of MB is monitored by measuring its absorbance around 660 nm and by recording the UV–vis spectra. From Fig. 12(a, b), it is obvious that the concentration of MB slightly decreases with the increase in irradiation time of visible light in the presence of all catalysts; Ga3+ doped In2O3 nanostructures show better photocatalytic activity than pure In2O3. The possible mechanism is given by the following equations. PC + hν → h+ h+ VB
VB+
eˉ
+ OHˉ → OH•
CB
Pure and Ga-doped In2O3 nanostructures were synthesized using simple hydrothermal method. XRD analysis reveals the formation of pure single cubic crystal structure with preferred orientation along (222) reflection, while the crystallite size decreases with increasing Ga doping concentration (1–7 M%). The surface morphology changes from irregular into defined porous spindle-like nanostructure, with particle size distribution of 18 and 9 nm for pure and Ga-doped nanostructure, respectively, meanwhile SAED patterns confirm the good crystallinity. XPS results confirmed that the doped gallium ions are in trivalence. UVDRS spectra of the pure In2O3 shows a reflectance edge at 339 nm with a bandgap of 3.40 eV, which slightly shifts towards higher energy level and a bandgap of 3.45 eV with Ga doping. Violet, blue and green emissions are observed in the photoluminescence spectra. It is found that Ga-doped In2O3 nanostructure exhibited better photocatalytic activitmatrix absorb photoexcited electrons and changed y than pure In2O3 for the degradation of methylene blue dye, with an efficiency of 48% compared to only 22%. References [1] Diego Leon Sanchez, Jesus Alberto Ramos Ramon, Manuel Herrera Zaldívar, Umapada Pal, Efrain Rubio Rosas, Adv. Nano Res. 3 (2015) 81–96. [2] M. Ramzan, T. Kaewmaraya, R. Ahuja, Appl. Phys. Lett. 103 (2013) 072113–072114. [3] Bo Jin, Taekyung Lim, Sanghyun Ju, Marat I. Latypov, Dong-Hai Pi, Hyoung Seop, Kim M. Meyyappan, Jeong-Soo Lee, Appl. Phys. Lett. 104 (2014) 103510–103514. [4] Minoo Bagheria, Abbas Ali Khodadadi, Ali Reza Mahjoub, Yadollah Mortazavi, Sens. Actuators.: B Chem. 220 (2015) 590–599. [5] Sunho Jeong, Ji-Yoon Lee, Sun Sook Lee, Youngmin Choi, Beyong-Hwan Ryu, J. Phys. Chem. C 115 (2011) 11773–11780. [6] N. Talebian, M.R. Nilforoushan, Thin Solid Films 518 (2010) 2210–2215. [7] G. Mohan Kumar, A. Madhan Kumar, P. Ilanchezhiyan, T.W. Kang, Nanoscale 6 (2014) 11226–11231. [8] Yong Liu, Wei Xu, Da-Bo Liu, Meijuan Yu, Yuan-Hua Lin, Ce-Wen Nan, Phys. Chem. Chem. Phys. 17 (2015) 11229–11233. [9] Jing Lin, Yang Huang, Yoshio Bando, Chengchun Tang, Chun Li, Dmitri Golberg, ACS Nano 4 (2010) 2452–2458. [10] Lili Wu, Quan Li, Xitian Zhang, Tianyou Zhai, Yoshio Bando, Dmitri Golberg, J. Phys. Chem. C 115 (2011) 24564–24568. [11] Zhenmin Li, Pengyi Zhang, Tian Shao, Jinlong Wang, Ling Jin, Xiaoyun Li, J. Hazard. Mater. 260 (2013) 40–46. [12] R. Camposeco, S. Castillo, J. Navarrete, R. Gomez, Catal. Today 266 (2015) 90–101. [13] B. Shanmuga Priya, M. Shanthi, C. Manoharan, S. Dhanapandian, 〈http://doi.org/ 10.1007/s10854-017-7106-0〉. [14] S. Bharathi, D. Nataraj, D. Mangalaraj, Y. Masuda, K. Senthil, K. Yong, J. Phys. D: Appl. Phys. 43 (015501) (2010) 9. [15] Lizhu Liu, Yiqing Chen, Linliang Guo, Taibo Guo, Yunqing Zhu, Yong Su, Chong Jia, Meiqin Wei, Yinfen Cheng, Appl. Surf. Sci. 258 (2011) 923–927. [16] A. Regoutz, R.G. Egdell, D.J. Morgan, R.G. Palgrave, H. Tellez, S.J. Skinner, D.J. Payne, G.W. Watson, D.O. Scanlon, Appl. Surf. Sci. 349 (2015) 970–982. [17] Yogeshwar Kumar, Md. Ahamad Mohiddon, Alok Srivastava, K.L. Yadav, Indian J. Eng. Mater. Sci. 16 (2009) 390–394. [18] I. Joseph Panneerdoss, S. Johnson Jeyakumar, S. Ramalingam, M. Jothibas, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 147 (2015) 1–13. [19] Dewei Chu, Yu-Ping Zeng, Dongliang Jiang, Yoshitake Masuda, Sens. Actuators B 137 (2009) 630–636. [20] S.N. Anshu Singhal, J. Achary, O.D. Manjanna, R.M. Jayakumar, A.K. Kadam, J. Tyagi, Phys. Chem. C 113 (2009) 3600–3606. [21] Gunho Jo, Woong-Ki Hong, Jongsun Maeng, Tae-Wook Kim, Gunuk Wang, Ahnsook Yoon, Soon-Shin Kwon, Sunghoon Song, Takhee Lee, Colloids Surfaces A: Physicochem. Eng. Asp. 313–314 (2008) 308–311. [22] Iñaki Lopez, Antonio D. Utrilla, Emilio Nogales, Bianchi Mendez, J. Javier Piqueras, Phys. Chem. C 116 (2012) 3935–3943. [23] Huan-Yu Shih, Fu-Chuan Chu, Atanu Das, Chia-Yu Lee, Ming-Jang Chen, Ray-
(1)
CB
(2)
OH• + MB → degrade products eˉ
4. Conclusion
ˉ
+ O2 → O2•
(3) (4)
ˉ
O2• + 2H2O → H2O2+ O2 +2OHˉ
(5)
H2O2 + eˉ
CB
→ OH• + OHˉ
(6)
Dye + h+
VB
→ Final species
(7)
Dye + OH• → Final species
(8) 3+
It is well known that Ga doped into In2O3 matrix absorb photoexcited electrons and changed into Ga2+ ions. These Ga2+ ions react with O2 molecule and produce superoxide radical O2•- and Ga3+ ions (Scheme 1). Wider band gap of the Ga3+ doped In2O3 favor the efficiency of photodegradation. Ga3+ doped In2O3 has higher band gap than undoped In2O3 because of their smaller particle size, leads to extend the electron-hole stability and favor photodegradation efficiency. In2O3 + hυ → h+
VB
+ e-
CB
Ga3+ + e- → Ga2+ Ga
2+
+ O2 → O2• + Ga -
(9) (10)
3+
(11)
By this way, photogenerated electrons are continuously trapped and Ga3+ slow downs the electron – hole recombination. This means that Ga3+ on the surface of the In2O3 may act as electron scavenger. On the other hand, the reduction in particle size with Ga doping results in the appearance of porous nanostructure, in addition to the increase of the 364
Materials Science in Semiconductor Processing 71 (2017) 357–365
B. Shanmuga Priya et al.
Optoelectron. Adv. Mater. 10 (2008) 161–165. [28] Heqing Yang, Ruyu Shi, Jie Yu, Ruini Liu, Ruigang Zhang, Hua Zhao, Lihui Zhang, Hairong Zheng, J. Phys. Chem. C 113 (2009) 21548–21554. [29] Liang Xu, Yong Su, Sen Li, Yiqing Chen, Qingtao Zhou, Song Yin, Yi Feng, J. Phys. Chem. B 111 (2007) 760–766. [30] Soumitra Kar, Subhadra Chaudhuri, Chem. Phys. Lett. 422 (2006) 424–428.
Ming Lin, Nanoscale Res. Lett. 11 (235) (2016) 9. [24] Caroline E. Knapp, Geoffrey Hyett, Ivan P. Parkin, Claire J. Carmalt, Chem. Mater. 23 (2011) 1719–1726. [25] H. Lin, C.P. Huang, W. Li, C. Ni, S. Ismat Shah, Yao-Hsuan Tseng, Appl. Catal. B: Environ. 68 (2006) 1–11. [26] E.C.C. Souza, J.F.Q. Rey, E.N.S. Muccillo, Appl. Surf. Sci. 255 (2009) 3779–3783. [27] S. Maensiri, P. Laokul, J. Klinkaewnarong, S. Phokha, V. Promarak, S. Seraphin, J.
365