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Photocatalytic activity of galvanically synthesized nanostructure SnO2 thin films
Accepted Manuscript Photocatalytic activity of galvanically synthesized nanostructure SnO2 thin films Sumanta Jana, Bibhas Chandra Mitra, Pulakesh Bera, Mousumi Sikdar, Anup Mondal PII: DOI: Reference:
S0925-8388(14)00552-0 http://dx.doi.org/10.1016/j.jallcom.2014.02.182 JALCOM 30767
To appear in: Received Date: Revised Date: Accepted Date:
25 October 2013 26 February 2014 27 February 2014
Please cite this article as: S. Jana, B.C. Mitra, P. Bera, M. Sikdar, A. Mondal, Photocatalytic activity of galvanically synthesized nanostructure SnO2 thin films, (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.02.182
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Photocatalytic activity of galvanically synthesized nanostructure SnO2 thin films Sumanta Janaa, Bibhas Chandra Mitrab, Pulakesh Berac, Mousumi Sikdard, Anup Mondala* a,d
Department of Chemistry, Bengal Engineering and Science University, Botanic Garden,
b
Howrah 711103, WB, India
Department of Physics, Bengal Engineering and Science University, Botanic Garden, Howrah 711103, WB, India
c
Department of Chemistry, Panskura Banamali College, Purba Medinipur, Panskura 721152, WB, India *Corresponding author email [email protected] (AM), [email protected] (SJ), fax: 91-33-2668-2916
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Abstract The study demonstrates an approach to synthesize nanostructure SnO2 thin films on TCO (transparent conducting oxide) coated glass substrates by galvanic technique. Aqueous solution of hydrated stannic chloride (SnCl4·5H2O) in potassium nitrate (KNO3) solution was used as the working solution. The process involves no sophisticated reactor or toxic chemicals, and proceeds continuously under ambient condition; it provides an economic way of synthesizing nanostructure SnO2 semiconductor thin films. The influence of sintering temperature on crystalline structure, morphology, electrical and dielectric properties has been studied. A detail analysis of I−V, C−V and dielectrics for annealed SnO2 thin films have been carried out. The morphological advantage i.e nanoporous flake like structure allows more efficient transport of reactant molecules to the active interfaces and results a strong photocatalytic activity for degrading methyl orange (MO) dye. Keywords: Thin films; Semiconductors; Nanostructure materials; Surfaces and interfaces; Electronic properties; Photocatalytic activity
Introduction
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Metal oxide semiconductors have received much attention because of its potential applications and surface active chemical and electronic properties. Several simple metal oxide semiconductors e.g. NiO [1], WO3 [2], ZnO [3], TiO2 [4], SnO2 [5], etc have sufficient band gap energy to catalyze photochemical reactions for environmental interest [6]. Among these metal oxide semiconductors, SnO2 have been investigated largely due to its stability and tunable optical, electrical and optoelectronic properties [7], which make it an excellent candidate for versatile applications [8−9]. The material is used extensively in several areas, such as electrode material in Li-ion battery and transparent conductive electrodes [10−12], solar cell and dye-sensitized solar cells [13−14], solidstate gas sensors [15], photo-electrochemical material [16] photo catalysis [17] etc. Nanoporous SnO2 has potential application in photocatalytic activity due to high surface area and stability, which provides more efficient transport of reactant molecules to the active surfaces [18−19]. All these applications are dependent on the size, morphology, phase, and crystallinity of the particles. Hence, size controlled synthesis of SnO2 thin films with high surface area has become an important area in nanoscience and nanotechnology. Recent studies show that the deposition procedure plays an important role with respect to a particular application, e.g., Karunakaran et al. has shown that hydrothermally grown SnO2 nanoparticles are more effective photo-catalytic material than that of SnO2 grown by sono-chemical synthesis [20]. There are different methods to prepare SnO2 thin films e.g sol–gel [21−22], thermal evaporation [23], microwave [24], electrodeposition [25], sonochemical [26], hydrothermal synthesis [27] etc. However, most of the reported methods have some limitations such as use of (i) costly instruments for preparing SnO2 thin films (ii) complex compounds as starting materials, which are
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difficult to synthesize and expensive (iii) sensitive reagents those are short lived and difficult to handle (iv) critical experimental conditions etc. Moreover, formation of mixed or undesired crystalline phases often occurs during the preparation of thin films [28]. Therefore, selecting a simple deposition path having minimum dispute is a challenging task till now. In order to overcome the limitations of the existing methods, we have used a technically simple and cost effective method to deposit high quality SnO2 thin films, called the galvanic technique. This same method was used earlier by our group for the deposition of ZnO, PbTe [29−30] and several other semiconductor thin films. The technique is simpler than the conventional electrodeposition process of using a potentiostat/galvanostat. Here, a Zn rod has been used as an oxidisable electrode (anode) in the electrolytic solution and a TCO glass is used as the cathode; the two being shortcircuited externally through a copper wire. The potential difference between the two electrodes leads to the deposition of SnO2 layer on the TCO substrate; hence, no external bias is required. Detail analyses of electrical and dielectric properties of synthesized SnO2 thin films were carried out and interesting features were observed. The surface morphology of the SnO2 layer is also an important parameter for application point of view. In this case, the porous morphological grain growth of the SnO2 thin film, having high surface area has resulted in higher photocatalytic activity for the photodegradation of methyl orange (MO) dye.
2. Experimental
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To start with, the TCO glass substrates (Thickness: 3.2 mm, Conducting layer: FTO (fluorine tin oxide, SnO2: F), Resistivity: 8~10 ohm/square) were cleaned with detergent, dipped into chromic acid solution and then washed thoroughly with cold distilled water, to remove any adhering impurities. They were then boiled in methanol and dried. A properly cleaned TCO glass substrate and a Zn strip were clamped vertically and dipped into 0.1 M SnCl4 solution containing 0.01 M KNO3 as a supporting electrolyte. The total volume of the working solution was maintained at 100 ml with distilled water and the pH of the working solution was adjusted to 2.3 with dilute nitric acid. The Zn strip and the TCO substrate were then short-circuited externally through a copper wire. The Zn strip served as an oxidisable anode, while the TCO substrate acted as the cathode. The deposition was carried out at 800C under stirred condition. When the electrochemical cell was set up and short-circuited, Zn2+ ions were released from the Zn electrode to the solution. The electrons that were generated due to this oxidation took the short-circuited path to move to the cathode (TCO), where Sn4+ gets reduced to elemental tin (Sn0). Anode reaction: Zn → Zn2+ + 2e
E0 = 0.7618 V
(i)
Cathode reaction: Sn4+ + 4e → Sn0
E0 = 0.00675 V
(ii)
Total reaction 2 Zn + Sn4+ → 2 Zn2+ + Sn0
(iii)
The NO3ˉ ions easily get converted to NO2ˉand generate nascent oxygen. NO3ˉ → NO2ˉ + [O]
E0 = 0.01 V
(iv)
This nascent oxygen [O] readily reacts with elemental tin (Sn0) (deposited on TCO) and converts it to SnO2. So, the overall reaction is Sn0 + 2 [O] → SnO2
(v)
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A white semi-transparent layer of SnO2 was deposited on TCO within one hour (this is as-deposited SnO2). The as synthesized material was then air annealed at 4500C for an hour (this is annealed SnO2). Fig.1 shows the photograph of bare TCO substrate and SnO2 thin film on TCO. The synthesized thin films were uniform and pinhole free. The thicknesses of the deposited films were measured by Surface Profilometer (Bruker Contour GT). The structural properties were analyzed through X-ray diffraction (XRD) with a Seifert P3000 Diffractometer having Cu Kα (λ=1.54A°) radiation. The surface microstructure and morphology were analyzed with a Gemini Zeiss Supra 38VP (Carl Zeiss Micro imaging GmbH, Berlin, Germany) field emission scanning electron microscope (FESEM). The optical properties were studied with UV−Vis (JASCO V−530), FTIR (JASCO FTIR−460) and Photoluminescence (PL) (Perkin–Elmer LS−55) Spectrophotometers. A detail analysis of electrical and dielectric properties was carried out using an LCR meter (Agilent 4284 A Precision LCR meter). 3. Results and discussion 3.1 Thickness measurement The thickness of the deposited films were measured with a Surface profilometer (Bruker Contour GT non-contacting mode) and was found to be ~400 nm for as deposited SnO2 and ~300 nm for annealed SnO2 thin films. The possible reasons for this decrease in thickness are: (i) Sn may evaporate from the surface due to annealing and (ii) since, asdeposited SnO2 has different surface morphology than annealed SnO2 thin film (the grains of as-deposited SnO2 are almost spherical in shape while the annealed films have
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flat, flake like morphology and bigger grain size). The change in shape and size may lead to decrease in the thickness of annealed SnO2 thin films. 3.2. X-ray diffraction analysis Fig. 2 indicates the formation of polycrystalline SnO2. The major diffractions were observed from (112), (006), (200), (130) planes whose 2θ values matches well with the standard JCPDS values (Joint Committee on Powder Diffraction Standards) for the orthorhombic structure (JCPDS # 78−1063). As the deposited particles have regular shape and size, Scherrer equation (D = 0.9λ /βcosθ) [where, λ = 1.540598 Å, β is the full width at half maxima value for the most intense peak] was applied to find out the average crystallite size (D), which was around 15 nm and the microstrain (ε=(βcosθ)/4) was calculated to be 16×10 −2 for as deposited sample. For the annealed (at 4500C in air for one hour) SnO2 thin films the peaks become sharper and the intensity increases to almost three times. Sharpness of the peaks indicates increase in crystallinity of SnO2; the average crystallite size (D) increases to around 55 nm. As the particle size increases the microstain reduces to 9.5 ×10 −2. The details of lattice parameters, average crystallite size, I/Imax ratio etc. have been provided in Table 1. 3.3. FESEM analysis of SnO2 thin films Fig. 3 represents the FESEM images of bare TCO (Fig. 3a), as-deposited SnO2 thin film on TCO (Fig. 3b) and air annealed SnO2 thin film on TCO (Fig. 3c), respectively. FESEM image (Fig. 3b) of the as-deposited SnO2 thin film shows a uniform, compact, nearly spherical nanostructure. However, on air-annealing the same film at 4500C for an hour, a significant grain growth takes place and a porous; flake like structure generates (Fig. 3c). The grain growth is likely due to the assimilation of the smaller grains in a
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controlled manner to give a definite shape, with the consequent formation of pores in between the larger grains. Thus, a drastic change in morphology occurs after annealing the thin film. 3.4. Optical Studies of SnO2 thin films Fig. 4(a) shows the UV−Vis absorption spectra of both as-deposited and air annealed SnO2 thin films. The as-deposited film shows a gradual rise in the absorption over a broad range of the wavelength from 500 to 400 nm, suggesting less crystallinity; however, for the air annealed SnO2 thin film a sharp increase in absorption was observed at about 400 nm. The band gap energy (Eg) of SnO2 thin films was calculated using the Tauc’s relation: (αhν)1/n = A(hν - Eg) where, hν is the incident photon energy, ‘A’ is a constant and ‘n’ is the exponent, the value of which is determined by the type of electronic transition causing the absorption and can take the values 1/2 or 2 depending upon whether the transition is direct or indirect, respectively. Since, SnO2 is well established as a direct band gap semiconductor, we can evaluate the value of E g from the plot of (αhν)2 vs. hν. The inset of Fig. 4 (a) shows the (αhν)2 vs. hν plot of the absorption spectrum of air annealed SnO2 thin films, from which the value of Eg was found to be 3.67 eV, which is comparable to bulk SnO2 (E g = 3.6 eV) [31−32]. Fig. 4(b) is the transmittance spectra of SnO2 thin films. For annealed SnO2 thin films, percentage (%) of transmittance is higher than that of as-deposited SnO2. This may be due to the decrease in thickness (~400 nm for as deposited and ~300 nm for annealed SnO2 thin films) and increased crystallinity. The transmittance of the films might also be influenced by surface roughness as shown in FESEM.
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The FTIR spectrum (Fig. 5(a)) of the as-deposited SnO2 thin film shows only one band at 592 cm−1, confirming the stretching frequency of Sn-O. Presence of any other bands was not observed. Air annealed spectrum shows a sharper band with a small shift to 652 cm−1 for Sn−O stretching. This agrees with the similar observation by Chang et al. [33]. Fig. 5(b) shows room-temperature PL spectra of the as deposited and annealed SnO2 thin films. It can be seen that as-deposited SnO2 thin films have comparatively low luminescence at room temperature, while an intense luminescence centered at 395 nm (3.14 eV) was observed for annealed SnO2. It should be noted that there are very few reports on strong blue luminescence for SnO2 nanostructures [34], though yellow red light emissions at ~605 nm were observed in SnO2 ribbons grown by a laser-ablation technique [35−36]. The interactions between oxygen vacancies and interfacial tin vacancies would lead to the formation of a significant number of trapped states, which form a series of metastable energy levels within the band gap, and results in a strong PL signal at room temperature. 3.4.1 Electrical properties (AC) of SnO2 thin films The electrical measurements of the annealed SnO2 thin films (porous) were carried out at room temperature with an LCR meter. Fig. 6 (a) and 6 (c) represents AC current- voltage (I−V) and capacitance-voltage (C−V) plots of SnO2 thin films, under two different fields of 50Hz and 1 KHz. From the I-V data under 50 Hz field, (Fig. 6(a)) a stepwise increase in current was observed between 0 and 760 mV, showing nonlinear behavior, but with increasing external bias from 800 mV onwards, the I−V plot becomes linear under high potential values. The nonlinear nature of the I−V plot might be due to tunneling effect [37−38]. Since these SnO2 thin films are porous and have small particle size, this material
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can be regarded as an assembly of many nanocapacitors. Thus, there is a great chance of electron entrapment inside these pores and particle boundaries. These create electron trapping zones of different energy levels. At a certain potential, the electrons that were previously occupied in the trapping levels get released, which is reflected in the smooth rise in current. The presence of different trapping zones with different energy levels can be assumed from the multistep I−V characteristics under 50 Hz field. This nonlinear phenomenon was again observed during C-V measurement for the annealed SnO2 thin films at lower potential values (up to 750 mV) under 50 Hz field (Fig. 6(c)), due to the intermittent release of trapped charges from different trapping zones. The bias levels at which the trapped charges are released, a fall in capacitance value was observed. On the other hand, on increasing the frequency to 1 KHz, the I−V (Fig. 6(b)) and C−V plots (Fig. 6(d)) become smooth, indicating that the trapped zones were no longer capable of holding on to the trapped charges at higher frequency and all the trapped charges were released simultaneously even at low potential value. The high resistivities of the porous samples are responsible for the low current observed in the I–V measurements. It is easy to note that the value of current shown by this SnO2 thin film at 500 mV bias under 50 Hz is about 0.014 μA which increases approximately 10 times to 0.12 μA under 1 kHz. 3.4.2 Dielectric studies Capacitance (Cp) of the films was measured with varying frequency (ω) between 1 to 10 KHz, under a constant bias of 1.0 V. The Dielectric constant (D.C.) of the deposited films was measured using the conventional equation: ε = (C × d) / (ε0 ×A)
(iii)
10
where C is the measured capacitance, d, the thickness of the film, A, the area of contact (0.4 cm2), and ε0 is the D.C. of vacuum (8.854 ×10-12 F/m). From the plot of D.C vs frequency (Fig.7 (a)), it is clear that these porous SnO2 thin films have a very low dielectric constant at any particular point of frequency. This might be due to the low number of nanoparticles of SnO2 present per unit volume, which could act as nanodipoles, for the porous materials. Due to the porous nature of SnO2 thin films; there occurs a significant amount of void space, which in turn lowers the number of SnO2 units per unit volume [38]. The dielectric tangent loss factor (tan δ) was calculated using the relation: tanδ = 1/(2πfRpCp), where δ is the loss angle, f is the frequency, Rp is the equivalent parallel resistance and Cp is the equivalent parallel capacitance. The dielectric tangent loss factor (tan δ) as a function of applied frequency () for SnO2 thin film samples has been shown in Fig. 7 (b). From the figure it is evident that the tanδ values decrease rapidly with the applied frequencies () and then reaches a constant value. 3.4.3 Resistivity measurements The resistivity values were determined using the conventional four probe measurement which suggests that the resistivity of annealed SnO2 is comparatively higher than as deposited SnO2 and TCO. The resistivity of the annealed thin films is comparatively high due to presence of pores and its uneven distribution, where the free movement of electrons is restricted to a large extent [39]. Another reason for higher resistivity might be due to air exposed physisorbed oxygen molecules which receive electrons from the conduction band of the film and change to O– ads or O2– ads species. These adsorbed molecules form an electron depletion layer just below the surface of SnO 2 particles and create a potential barrier between particles; consequently the film becomes resistive
11
[40−41]. In the conduction process, the electrical resistivity is known to be affected by grain growth, grain size, inter-grain distribution, pore distribution (porosity), in addition to other blocking effects. The pore distribution is an important parameter and plays a vital role in deciding the resistivity behavior. Due to the porous nature of SnO2 thin films; there occurs a significant amount of void space which generate a pore resistance [42]. Due to this pore resistance and particle boundary resistance, the resistivity of annealed SnO2 thin film is high. 3.4.4 Photocatalytic activity To demonstrate the potential applicability of the present nanoporous SnO2 thin films, we investigated its relative activity to that of commercial photocatalyst (Degussa P25 titania). The photocatalytic degradation of methyl orange (MO) dye was carried out in an aqueous solution at ambient temperature. Five annealed SnO2 thin films (surface area 2 ×1 cm2) were vertically immersed into a 200 ml aqueous solution of MO (20 mg /L). The light irradiation was carried out using a 200 W tungsten lamp which was placed vertically over the reaction vessel at a distance of 10 cm. Commercial TiO2 (Degussa-P25) was taken as the reference to compare the photo-catalytic activity under the same experimental conditions. Before the start of the experiment, the solution was purged with oxygen for a period of five minutes. At specific time intervals (20 minutes), 5 ml of the aliquot solution was withdrawn from the solution mixture and centrifuged and the changes in MO concentration were observed. From Fig. 8 (a) it is clear that maximum absorption occurs at ~450 nm which is the λmax of MO. The absorption decreases gradually with irradiation time and almost completely disappears after 120 min. Fig. 8 (b) represents the change in absorbance with irradiation
12
time. Comparative experiments were carried out to investigate and evaluate catalytic activity with commercial Degussa-P25. The aqueous solution of MO (20 mg/L) was subjected to a series of experimental conditions: (i) without catalyst in dark (ii) without catalyst in light (iii) TiO2 (Degussa-P25, 10 mg) in light (iv) SnO2 thin films in light (Fig. 9 (a)). There was no MO degradation in dark. When MO solution without nanoporous SnO2 was irradiated, a very slight degradation was observed. But in presence of SnO2 a significant degradation was observed after irradiation, this activity is even greater than P25 (specific surface area 50 m2 g/l) under this condition. From Fig. 9 (a) the degradation efficiency was calculated to ~83 %. The ln (C0/Ct) vs. time curve (Fig. 9 (b)) shows a linear relationship with the irradiation time, which indicates that the photodegradation proceeds through pseudo first-order kinetics, i.e. ln (C0/Ct)= kt, where, Ct = concentration of dye at time t, C0 =concentration of dye at time t = 0 and k = photodegradation rate constant. From Fig. 9(b), the rate constant (k) for dye degradation with SnO2 thin film was calculated to be 14 ×10−3 min−1. The synthesized porous SnO2 can absorb the visible light and utilize to photodegrade MO. Two possible reasons for the photo-degradation of MO by the synthesized porous SnO2 thin films are: (i) porous SnO2 has an absorption edge around 400 nm (band gap 3.67 eV), and thus can absorb and utilize a portion of the visible light and (ii) the morphological advantage i.e porous structure allows more efficient transport of reactant molecules (dye) to the active sites which results in high photo-catalytic activity of this material compared to that of Degussa P25 [43]. A schematic representation for photocatalytic degradation is illustrated in Fig. 10. The essential requirement is separation of electrons (e−) and holes (h+) by absorbing light. The photo generated
13
electrons and holes react with adsorbed surface substances, like O2, OH− and form reactive species O2−, OH* (hydroxyl radicals). These are the major oxidative species for the decomposition of organic pollutants. Oxidative species degrade the organic dye (MO) into small molecules like CO2, H2O, NO3¯, NH4+ etc. In the absence of electron acceptors (O2), there is a great chance of electron hole recombination. Presence of oxygen prevents this recombination by trapping electrons through the formation of superoxide radical anions (O2−). Now OH* radicals generated are able to react with organic molecules (MO) or to diffuse away from the SnO2 surface and then react with MO in the solution [44]. The morphological advantage (upon annealing, nanopores are grown) and proper stoichiometric ratio of Sn:O (enhanced oxygen incorporation after annealing) might play active role for this photocatalytic degradation. The same experiment was repeated for as deposited SnO2 thin films but no catalytic activity was observed. The compact surface and low oxygen content makes it inappropriate for this dye degradation. 4. Conclusions SnO2 thin films have been successfully synthesized by the galvanic technique. A drastic change in morphology was observed after annealing the as deposited SnO2 films. The annealed films were porous and have high surface area and display interesting optical, electrical, and dielectric properties at room temperature. Electrical studies show that the SnO2 thin films consist of an assembly of nanocapacitors. These can trap the charges inside the pores and particle boundaries. At a certain potential or energy the electrons that were previously occupied in the trapping levels, get released and separation of electrons and holes are then possible. Photodegradation study of MO dye with annealed SnO2 films
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showed that the typical morphological advantages e.g. small particle size, high specific surface area, nanopores with large oxygen content lead to an effective material for dye degradation. Acknowledgments One of the authors, S. Jana is thankful to UGC (India) for his fellowship (Ref. No. 2012/2009 (ii) EU-IV). M. Sikdar is grateful to DST-SERI (India) (Project No DST/TM/SERI/2K10/60) for project fellowship. The authors also acknowledge AICTE and U.G.C.-S.A.P. (India) for providing instrumental facilities to the Department of Chemistry, BESU, India.
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Figure captions Fig. 1: Photograph of bare TCO and SnO2 thin film on TCO Fig. 2: XRD patterns of bare TCO, as-deposited SnO2 thin film and annealed SnO2 thin film. Fig. 3: FESEM images of (a) bare TCO (b) as deposited SnO2 thin film (c) air annealed SnO2 thin film. Fig. 4: (a) UV-Vis spectra of bare TCO, as deposited and annealed SnO2 thin films, inset Tauc plot of annealed SnO2 thin film (b) Transmittance spectra of SnO2 thin films. Fig. 5: (a) FTIR spectra of as deposited SnO2 and annealed SnO2 thin films (b) PL spectra of SnO2 thin films. Fig. 6: (a) I vs V plot at 50 Hz (b) I vs V plot at 1 kHz (c) Capacitance (Cp) vs Voltage (V) at 50 Hz (d) Capacitance (Cp) vs Voltage (V) at 1 kHz. Fig. 7: (a) Dielectric constant (D.C) vs Frequency plot (b) Dielctric loss vs frequency plot. Fig. 8: (a) Time dependent spectral changes of MO solution by SnO2 thin films. (b) Change in absorbance with irradiation time. Fig. 9: (a) Photodegradation of MO under different conditions (i) without catalyst in dark (ii) without catalyst in light (iii) commercial degussa P25 (TiO2) in light (iv) SnO2 thin film in light. (b) The logarithmic change in concentration of MO as a function of irradiation time. Fig. 10: Schematic representation of photocatalytic degradation. Table 1: X− ray diffraction data for annealed SnO2 thin film
20
Figure
Figures
Fig. 1
Fig. 2
1
Fig. 3
2
Fig. 4
Fig. 5
3
Fig. 6
Fig. 7
4
Fig. 7
Fig. 8
Fig. 9
5
hγ (E > Eg)
e-
CB
TCO
Photo reduction
O2− + 2 H2O + e−
Eg
SnO2 VB
O2−
e− + O2
h+
Photo oxidation
h+ + H2O
2 OH* + 2OH− OH* + H+
Fig. 10
Planes
d std (A°)
d obs (A°)
I/Imax std
I/Imax obs
(112)
3.312
3.296
79.1
100
(006)
2.644
2.607
31.4
49.42
(200)
2.368
2.376
18.6
44
(130)
1.764
1.732
44.6
48.41
(133)
1.674
1.643
28
21.7
(226)
1.499
1.486
46
18.37
Avg. size 55 nm, [a = 4.737 A°, b = 5.708 A°, c = 15.865 A° (Std)] [a = 4.76 A°, b = 5.58 A°, c = 15.63 A° (Obs)]
Table 1
6
Graphical Abstract (for review)
Graphical Abstract Photocatalytic activity of galvanically synthesized nanostructure SnO2 thin films Nanostructured porous tin dioxide (SnO2) thin films have been synthesized by simple and cost effective galvanic technique. The synthesized porous SnO2 thin films show excellent photocatalytic activity for degrading methyl orange (MO) dye under light irradiation. The porous morphological grain growth due to annealing is likely to play an active role for this degradation.
4500C Annealed
as deposited SnO2 thin film
MO
annealed SnO2 thin film Photo degradation
*Highlights (for review)
Research highlights
SnO2 thin films have been successfully synthesized by galvanic technique.
A drastic morphological change occurs after annealing as deposited SnO2 thin films.
Morphological advantage results enhanced photodegradation of dye.
S0925-8388(14)00552-0 http://dx.doi.org/10.1016/j.jallcom.2014.02.182 JALCOM 30767
To appear in: Received Date: Revised Date: Accepted Date:
25 October 2013 26 February 2014 27 February 2014
Please cite this article as: S. Jana, B.C. Mitra, P. Bera, M. Sikdar, A. Mondal, Photocatalytic activity of galvanically synthesized nanostructure SnO2 thin films, (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.02.182
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photocatalytic activity of galvanically synthesized nanostructure SnO2 thin films Sumanta Janaa, Bibhas Chandra Mitrab, Pulakesh Berac, Mousumi Sikdard, Anup Mondala* a,d
Department of Chemistry, Bengal Engineering and Science University, Botanic Garden,
b
Howrah 711103, WB, India
Department of Physics, Bengal Engineering and Science University, Botanic Garden, Howrah 711103, WB, India
c
Department of Chemistry, Panskura Banamali College, Purba Medinipur, Panskura 721152, WB, India *Corresponding author email [email protected] (AM), [email protected] (SJ), fax: 91-33-2668-2916
1
Abstract The study demonstrates an approach to synthesize nanostructure SnO2 thin films on TCO (transparent conducting oxide) coated glass substrates by galvanic technique. Aqueous solution of hydrated stannic chloride (SnCl4·5H2O) in potassium nitrate (KNO3) solution was used as the working solution. The process involves no sophisticated reactor or toxic chemicals, and proceeds continuously under ambient condition; it provides an economic way of synthesizing nanostructure SnO2 semiconductor thin films. The influence of sintering temperature on crystalline structure, morphology, electrical and dielectric properties has been studied. A detail analysis of I−V, C−V and dielectrics for annealed SnO2 thin films have been carried out. The morphological advantage i.e nanoporous flake like structure allows more efficient transport of reactant molecules to the active interfaces and results a strong photocatalytic activity for degrading methyl orange (MO) dye. Keywords: Thin films; Semiconductors; Nanostructure materials; Surfaces and interfaces; Electronic properties; Photocatalytic activity
Introduction
2
Metal oxide semiconductors have received much attention because of its potential applications and surface active chemical and electronic properties. Several simple metal oxide semiconductors e.g. NiO [1], WO3 [2], ZnO [3], TiO2 [4], SnO2 [5], etc have sufficient band gap energy to catalyze photochemical reactions for environmental interest [6]. Among these metal oxide semiconductors, SnO2 have been investigated largely due to its stability and tunable optical, electrical and optoelectronic properties [7], which make it an excellent candidate for versatile applications [8−9]. The material is used extensively in several areas, such as electrode material in Li-ion battery and transparent conductive electrodes [10−12], solar cell and dye-sensitized solar cells [13−14], solidstate gas sensors [15], photo-electrochemical material [16] photo catalysis [17] etc. Nanoporous SnO2 has potential application in photocatalytic activity due to high surface area and stability, which provides more efficient transport of reactant molecules to the active surfaces [18−19]. All these applications are dependent on the size, morphology, phase, and crystallinity of the particles. Hence, size controlled synthesis of SnO2 thin films with high surface area has become an important area in nanoscience and nanotechnology. Recent studies show that the deposition procedure plays an important role with respect to a particular application, e.g., Karunakaran et al. has shown that hydrothermally grown SnO2 nanoparticles are more effective photo-catalytic material than that of SnO2 grown by sono-chemical synthesis [20]. There are different methods to prepare SnO2 thin films e.g sol–gel [21−22], thermal evaporation [23], microwave [24], electrodeposition [25], sonochemical [26], hydrothermal synthesis [27] etc. However, most of the reported methods have some limitations such as use of (i) costly instruments for preparing SnO2 thin films (ii) complex compounds as starting materials, which are
3
difficult to synthesize and expensive (iii) sensitive reagents those are short lived and difficult to handle (iv) critical experimental conditions etc. Moreover, formation of mixed or undesired crystalline phases often occurs during the preparation of thin films [28]. Therefore, selecting a simple deposition path having minimum dispute is a challenging task till now. In order to overcome the limitations of the existing methods, we have used a technically simple and cost effective method to deposit high quality SnO2 thin films, called the galvanic technique. This same method was used earlier by our group for the deposition of ZnO, PbTe [29−30] and several other semiconductor thin films. The technique is simpler than the conventional electrodeposition process of using a potentiostat/galvanostat. Here, a Zn rod has been used as an oxidisable electrode (anode) in the electrolytic solution and a TCO glass is used as the cathode; the two being shortcircuited externally through a copper wire. The potential difference between the two electrodes leads to the deposition of SnO2 layer on the TCO substrate; hence, no external bias is required. Detail analyses of electrical and dielectric properties of synthesized SnO2 thin films were carried out and interesting features were observed. The surface morphology of the SnO2 layer is also an important parameter for application point of view. In this case, the porous morphological grain growth of the SnO2 thin film, having high surface area has resulted in higher photocatalytic activity for the photodegradation of methyl orange (MO) dye.
2. Experimental
4
To start with, the TCO glass substrates (Thickness: 3.2 mm, Conducting layer: FTO (fluorine tin oxide, SnO2: F), Resistivity: 8~10 ohm/square) were cleaned with detergent, dipped into chromic acid solution and then washed thoroughly with cold distilled water, to remove any adhering impurities. They were then boiled in methanol and dried. A properly cleaned TCO glass substrate and a Zn strip were clamped vertically and dipped into 0.1 M SnCl4 solution containing 0.01 M KNO3 as a supporting electrolyte. The total volume of the working solution was maintained at 100 ml with distilled water and the pH of the working solution was adjusted to 2.3 with dilute nitric acid. The Zn strip and the TCO substrate were then short-circuited externally through a copper wire. The Zn strip served as an oxidisable anode, while the TCO substrate acted as the cathode. The deposition was carried out at 800C under stirred condition. When the electrochemical cell was set up and short-circuited, Zn2+ ions were released from the Zn electrode to the solution. The electrons that were generated due to this oxidation took the short-circuited path to move to the cathode (TCO), where Sn4+ gets reduced to elemental tin (Sn0). Anode reaction: Zn → Zn2+ + 2e
E0 = 0.7618 V
(i)
Cathode reaction: Sn4+ + 4e → Sn0
E0 = 0.00675 V
(ii)
Total reaction 2 Zn + Sn4+ → 2 Zn2+ + Sn0
(iii)
The NO3ˉ ions easily get converted to NO2ˉand generate nascent oxygen. NO3ˉ → NO2ˉ + [O]
E0 = 0.01 V
(iv)
This nascent oxygen [O] readily reacts with elemental tin (Sn0) (deposited on TCO) and converts it to SnO2. So, the overall reaction is Sn0 + 2 [O] → SnO2
(v)
5
A white semi-transparent layer of SnO2 was deposited on TCO within one hour (this is as-deposited SnO2). The as synthesized material was then air annealed at 4500C for an hour (this is annealed SnO2). Fig.1 shows the photograph of bare TCO substrate and SnO2 thin film on TCO. The synthesized thin films were uniform and pinhole free. The thicknesses of the deposited films were measured by Surface Profilometer (Bruker Contour GT). The structural properties were analyzed through X-ray diffraction (XRD) with a Seifert P3000 Diffractometer having Cu Kα (λ=1.54A°) radiation. The surface microstructure and morphology were analyzed with a Gemini Zeiss Supra 38VP (Carl Zeiss Micro imaging GmbH, Berlin, Germany) field emission scanning electron microscope (FESEM). The optical properties were studied with UV−Vis (JASCO V−530), FTIR (JASCO FTIR−460) and Photoluminescence (PL) (Perkin–Elmer LS−55) Spectrophotometers. A detail analysis of electrical and dielectric properties was carried out using an LCR meter (Agilent 4284 A Precision LCR meter). 3. Results and discussion 3.1 Thickness measurement The thickness of the deposited films were measured with a Surface profilometer (Bruker Contour GT non-contacting mode) and was found to be ~400 nm for as deposited SnO2 and ~300 nm for annealed SnO2 thin films. The possible reasons for this decrease in thickness are: (i) Sn may evaporate from the surface due to annealing and (ii) since, asdeposited SnO2 has different surface morphology than annealed SnO2 thin film (the grains of as-deposited SnO2 are almost spherical in shape while the annealed films have
6
flat, flake like morphology and bigger grain size). The change in shape and size may lead to decrease in the thickness of annealed SnO2 thin films. 3.2. X-ray diffraction analysis Fig. 2 indicates the formation of polycrystalline SnO2. The major diffractions were observed from (112), (006), (200), (130) planes whose 2θ values matches well with the standard JCPDS values (Joint Committee on Powder Diffraction Standards) for the orthorhombic structure (JCPDS # 78−1063). As the deposited particles have regular shape and size, Scherrer equation (D = 0.9λ /βcosθ) [where, λ = 1.540598 Å, β is the full width at half maxima value for the most intense peak] was applied to find out the average crystallite size (D), which was around 15 nm and the microstrain (ε=(βcosθ)/4) was calculated to be 16×10 −2 for as deposited sample. For the annealed (at 4500C in air for one hour) SnO2 thin films the peaks become sharper and the intensity increases to almost three times. Sharpness of the peaks indicates increase in crystallinity of SnO2; the average crystallite size (D) increases to around 55 nm. As the particle size increases the microstain reduces to 9.5 ×10 −2. The details of lattice parameters, average crystallite size, I/Imax ratio etc. have been provided in Table 1. 3.3. FESEM analysis of SnO2 thin films Fig. 3 represents the FESEM images of bare TCO (Fig. 3a), as-deposited SnO2 thin film on TCO (Fig. 3b) and air annealed SnO2 thin film on TCO (Fig. 3c), respectively. FESEM image (Fig. 3b) of the as-deposited SnO2 thin film shows a uniform, compact, nearly spherical nanostructure. However, on air-annealing the same film at 4500C for an hour, a significant grain growth takes place and a porous; flake like structure generates (Fig. 3c). The grain growth is likely due to the assimilation of the smaller grains in a
7
controlled manner to give a definite shape, with the consequent formation of pores in between the larger grains. Thus, a drastic change in morphology occurs after annealing the thin film. 3.4. Optical Studies of SnO2 thin films Fig. 4(a) shows the UV−Vis absorption spectra of both as-deposited and air annealed SnO2 thin films. The as-deposited film shows a gradual rise in the absorption over a broad range of the wavelength from 500 to 400 nm, suggesting less crystallinity; however, for the air annealed SnO2 thin film a sharp increase in absorption was observed at about 400 nm. The band gap energy (Eg) of SnO2 thin films was calculated using the Tauc’s relation: (αhν)1/n = A(hν - Eg) where, hν is the incident photon energy, ‘A’ is a constant and ‘n’ is the exponent, the value of which is determined by the type of electronic transition causing the absorption and can take the values 1/2 or 2 depending upon whether the transition is direct or indirect, respectively. Since, SnO2 is well established as a direct band gap semiconductor, we can evaluate the value of E g from the plot of (αhν)2 vs. hν. The inset of Fig. 4 (a) shows the (αhν)2 vs. hν plot of the absorption spectrum of air annealed SnO2 thin films, from which the value of Eg was found to be 3.67 eV, which is comparable to bulk SnO2 (E g = 3.6 eV) [31−32]. Fig. 4(b) is the transmittance spectra of SnO2 thin films. For annealed SnO2 thin films, percentage (%) of transmittance is higher than that of as-deposited SnO2. This may be due to the decrease in thickness (~400 nm for as deposited and ~300 nm for annealed SnO2 thin films) and increased crystallinity. The transmittance of the films might also be influenced by surface roughness as shown in FESEM.
8
The FTIR spectrum (Fig. 5(a)) of the as-deposited SnO2 thin film shows only one band at 592 cm−1, confirming the stretching frequency of Sn-O. Presence of any other bands was not observed. Air annealed spectrum shows a sharper band with a small shift to 652 cm−1 for Sn−O stretching. This agrees with the similar observation by Chang et al. [33]. Fig. 5(b) shows room-temperature PL spectra of the as deposited and annealed SnO2 thin films. It can be seen that as-deposited SnO2 thin films have comparatively low luminescence at room temperature, while an intense luminescence centered at 395 nm (3.14 eV) was observed for annealed SnO2. It should be noted that there are very few reports on strong blue luminescence for SnO2 nanostructures [34], though yellow red light emissions at ~605 nm were observed in SnO2 ribbons grown by a laser-ablation technique [35−36]. The interactions between oxygen vacancies and interfacial tin vacancies would lead to the formation of a significant number of trapped states, which form a series of metastable energy levels within the band gap, and results in a strong PL signal at room temperature. 3.4.1 Electrical properties (AC) of SnO2 thin films The electrical measurements of the annealed SnO2 thin films (porous) were carried out at room temperature with an LCR meter. Fig. 6 (a) and 6 (c) represents AC current- voltage (I−V) and capacitance-voltage (C−V) plots of SnO2 thin films, under two different fields of 50Hz and 1 KHz. From the I-V data under 50 Hz field, (Fig. 6(a)) a stepwise increase in current was observed between 0 and 760 mV, showing nonlinear behavior, but with increasing external bias from 800 mV onwards, the I−V plot becomes linear under high potential values. The nonlinear nature of the I−V plot might be due to tunneling effect [37−38]. Since these SnO2 thin films are porous and have small particle size, this material
9
can be regarded as an assembly of many nanocapacitors. Thus, there is a great chance of electron entrapment inside these pores and particle boundaries. These create electron trapping zones of different energy levels. At a certain potential, the electrons that were previously occupied in the trapping levels get released, which is reflected in the smooth rise in current. The presence of different trapping zones with different energy levels can be assumed from the multistep I−V characteristics under 50 Hz field. This nonlinear phenomenon was again observed during C-V measurement for the annealed SnO2 thin films at lower potential values (up to 750 mV) under 50 Hz field (Fig. 6(c)), due to the intermittent release of trapped charges from different trapping zones. The bias levels at which the trapped charges are released, a fall in capacitance value was observed. On the other hand, on increasing the frequency to 1 KHz, the I−V (Fig. 6(b)) and C−V plots (Fig. 6(d)) become smooth, indicating that the trapped zones were no longer capable of holding on to the trapped charges at higher frequency and all the trapped charges were released simultaneously even at low potential value. The high resistivities of the porous samples are responsible for the low current observed in the I–V measurements. It is easy to note that the value of current shown by this SnO2 thin film at 500 mV bias under 50 Hz is about 0.014 μA which increases approximately 10 times to 0.12 μA under 1 kHz. 3.4.2 Dielectric studies Capacitance (Cp) of the films was measured with varying frequency (ω) between 1 to 10 KHz, under a constant bias of 1.0 V. The Dielectric constant (D.C.) of the deposited films was measured using the conventional equation: ε = (C × d) / (ε0 ×A)
(iii)
10
where C is the measured capacitance, d, the thickness of the film, A, the area of contact (0.4 cm2), and ε0 is the D.C. of vacuum (8.854 ×10-12 F/m). From the plot of D.C vs frequency (Fig.7 (a)), it is clear that these porous SnO2 thin films have a very low dielectric constant at any particular point of frequency. This might be due to the low number of nanoparticles of SnO2 present per unit volume, which could act as nanodipoles, for the porous materials. Due to the porous nature of SnO2 thin films; there occurs a significant amount of void space, which in turn lowers the number of SnO2 units per unit volume [38]. The dielectric tangent loss factor (tan δ) was calculated using the relation: tanδ = 1/(2πfRpCp), where δ is the loss angle, f is the frequency, Rp is the equivalent parallel resistance and Cp is the equivalent parallel capacitance. The dielectric tangent loss factor (tan δ) as a function of applied frequency () for SnO2 thin film samples has been shown in Fig. 7 (b). From the figure it is evident that the tanδ values decrease rapidly with the applied frequencies () and then reaches a constant value. 3.4.3 Resistivity measurements The resistivity values were determined using the conventional four probe measurement which suggests that the resistivity of annealed SnO2 is comparatively higher than as deposited SnO2 and TCO. The resistivity of the annealed thin films is comparatively high due to presence of pores and its uneven distribution, where the free movement of electrons is restricted to a large extent [39]. Another reason for higher resistivity might be due to air exposed physisorbed oxygen molecules which receive electrons from the conduction band of the film and change to O– ads or O2– ads species. These adsorbed molecules form an electron depletion layer just below the surface of SnO 2 particles and create a potential barrier between particles; consequently the film becomes resistive
11
[40−41]. In the conduction process, the electrical resistivity is known to be affected by grain growth, grain size, inter-grain distribution, pore distribution (porosity), in addition to other blocking effects. The pore distribution is an important parameter and plays a vital role in deciding the resistivity behavior. Due to the porous nature of SnO2 thin films; there occurs a significant amount of void space which generate a pore resistance [42]. Due to this pore resistance and particle boundary resistance, the resistivity of annealed SnO2 thin film is high. 3.4.4 Photocatalytic activity To demonstrate the potential applicability of the present nanoporous SnO2 thin films, we investigated its relative activity to that of commercial photocatalyst (Degussa P25 titania). The photocatalytic degradation of methyl orange (MO) dye was carried out in an aqueous solution at ambient temperature. Five annealed SnO2 thin films (surface area 2 ×1 cm2) were vertically immersed into a 200 ml aqueous solution of MO (20 mg /L). The light irradiation was carried out using a 200 W tungsten lamp which was placed vertically over the reaction vessel at a distance of 10 cm. Commercial TiO2 (Degussa-P25) was taken as the reference to compare the photo-catalytic activity under the same experimental conditions. Before the start of the experiment, the solution was purged with oxygen for a period of five minutes. At specific time intervals (20 minutes), 5 ml of the aliquot solution was withdrawn from the solution mixture and centrifuged and the changes in MO concentration were observed. From Fig. 8 (a) it is clear that maximum absorption occurs at ~450 nm which is the λmax of MO. The absorption decreases gradually with irradiation time and almost completely disappears after 120 min. Fig. 8 (b) represents the change in absorbance with irradiation
12
time. Comparative experiments were carried out to investigate and evaluate catalytic activity with commercial Degussa-P25. The aqueous solution of MO (20 mg/L) was subjected to a series of experimental conditions: (i) without catalyst in dark (ii) without catalyst in light (iii) TiO2 (Degussa-P25, 10 mg) in light (iv) SnO2 thin films in light (Fig. 9 (a)). There was no MO degradation in dark. When MO solution without nanoporous SnO2 was irradiated, a very slight degradation was observed. But in presence of SnO2 a significant degradation was observed after irradiation, this activity is even greater than P25 (specific surface area 50 m2 g/l) under this condition. From Fig. 9 (a) the degradation efficiency was calculated to ~83 %. The ln (C0/Ct) vs. time curve (Fig. 9 (b)) shows a linear relationship with the irradiation time, which indicates that the photodegradation proceeds through pseudo first-order kinetics, i.e. ln (C0/Ct)= kt, where, Ct = concentration of dye at time t, C0 =concentration of dye at time t = 0 and k = photodegradation rate constant. From Fig. 9(b), the rate constant (k) for dye degradation with SnO2 thin film was calculated to be 14 ×10−3 min−1. The synthesized porous SnO2 can absorb the visible light and utilize to photodegrade MO. Two possible reasons for the photo-degradation of MO by the synthesized porous SnO2 thin films are: (i) porous SnO2 has an absorption edge around 400 nm (band gap 3.67 eV), and thus can absorb and utilize a portion of the visible light and (ii) the morphological advantage i.e porous structure allows more efficient transport of reactant molecules (dye) to the active sites which results in high photo-catalytic activity of this material compared to that of Degussa P25 [43]. A schematic representation for photocatalytic degradation is illustrated in Fig. 10. The essential requirement is separation of electrons (e−) and holes (h+) by absorbing light. The photo generated
13
electrons and holes react with adsorbed surface substances, like O2, OH− and form reactive species O2−, OH* (hydroxyl radicals). These are the major oxidative species for the decomposition of organic pollutants. Oxidative species degrade the organic dye (MO) into small molecules like CO2, H2O, NO3¯, NH4+ etc. In the absence of electron acceptors (O2), there is a great chance of electron hole recombination. Presence of oxygen prevents this recombination by trapping electrons through the formation of superoxide radical anions (O2−). Now OH* radicals generated are able to react with organic molecules (MO) or to diffuse away from the SnO2 surface and then react with MO in the solution [44]. The morphological advantage (upon annealing, nanopores are grown) and proper stoichiometric ratio of Sn:O (enhanced oxygen incorporation after annealing) might play active role for this photocatalytic degradation. The same experiment was repeated for as deposited SnO2 thin films but no catalytic activity was observed. The compact surface and low oxygen content makes it inappropriate for this dye degradation. 4. Conclusions SnO2 thin films have been successfully synthesized by the galvanic technique. A drastic change in morphology was observed after annealing the as deposited SnO2 films. The annealed films were porous and have high surface area and display interesting optical, electrical, and dielectric properties at room temperature. Electrical studies show that the SnO2 thin films consist of an assembly of nanocapacitors. These can trap the charges inside the pores and particle boundaries. At a certain potential or energy the electrons that were previously occupied in the trapping levels, get released and separation of electrons and holes are then possible. Photodegradation study of MO dye with annealed SnO2 films
14
showed that the typical morphological advantages e.g. small particle size, high specific surface area, nanopores with large oxygen content lead to an effective material for dye degradation. Acknowledgments One of the authors, S. Jana is thankful to UGC (India) for his fellowship (Ref. No. 2012/2009 (ii) EU-IV). M. Sikdar is grateful to DST-SERI (India) (Project No DST/TM/SERI/2K10/60) for project fellowship. The authors also acknowledge AICTE and U.G.C.-S.A.P. (India) for providing instrumental facilities to the Department of Chemistry, BESU, India.
15
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Figure captions Fig. 1: Photograph of bare TCO and SnO2 thin film on TCO Fig. 2: XRD patterns of bare TCO, as-deposited SnO2 thin film and annealed SnO2 thin film. Fig. 3: FESEM images of (a) bare TCO (b) as deposited SnO2 thin film (c) air annealed SnO2 thin film. Fig. 4: (a) UV-Vis spectra of bare TCO, as deposited and annealed SnO2 thin films, inset Tauc plot of annealed SnO2 thin film (b) Transmittance spectra of SnO2 thin films. Fig. 5: (a) FTIR spectra of as deposited SnO2 and annealed SnO2 thin films (b) PL spectra of SnO2 thin films. Fig. 6: (a) I vs V plot at 50 Hz (b) I vs V plot at 1 kHz (c) Capacitance (Cp) vs Voltage (V) at 50 Hz (d) Capacitance (Cp) vs Voltage (V) at 1 kHz. Fig. 7: (a) Dielectric constant (D.C) vs Frequency plot (b) Dielctric loss vs frequency plot. Fig. 8: (a) Time dependent spectral changes of MO solution by SnO2 thin films. (b) Change in absorbance with irradiation time. Fig. 9: (a) Photodegradation of MO under different conditions (i) without catalyst in dark (ii) without catalyst in light (iii) commercial degussa P25 (TiO2) in light (iv) SnO2 thin film in light. (b) The logarithmic change in concentration of MO as a function of irradiation time. Fig. 10: Schematic representation of photocatalytic degradation. Table 1: X− ray diffraction data for annealed SnO2 thin film
20
Figure
Figures
Fig. 1
Fig. 2
1
Fig. 3
2
Fig. 4
Fig. 5
3
Fig. 6
Fig. 7
4
Fig. 7
Fig. 8
Fig. 9
5
hγ (E > Eg)
e-
CB
TCO
Photo reduction
O2− + 2 H2O + e−
Eg
SnO2 VB
O2−
e− + O2
h+
Photo oxidation
h+ + H2O
2 OH* + 2OH− OH* + H+
Fig. 10
Planes
d std (A°)
d obs (A°)
I/Imax std
I/Imax obs
(112)
3.312
3.296
79.1
100
(006)
2.644
2.607
31.4
49.42
(200)
2.368
2.376
18.6
44
(130)
1.764
1.732
44.6
48.41
(133)
1.674
1.643
28
21.7
(226)
1.499
1.486
46
18.37
Avg. size 55 nm, [a = 4.737 A°, b = 5.708 A°, c = 15.865 A° (Std)] [a = 4.76 A°, b = 5.58 A°, c = 15.63 A° (Obs)]
Table 1
6
Graphical Abstract (for review)
Graphical Abstract Photocatalytic activity of galvanically synthesized nanostructure SnO2 thin films Nanostructured porous tin dioxide (SnO2) thin films have been synthesized by simple and cost effective galvanic technique. The synthesized porous SnO2 thin films show excellent photocatalytic activity for degrading methyl orange (MO) dye under light irradiation. The porous morphological grain growth due to annealing is likely to play an active role for this degradation.
4500C Annealed
as deposited SnO2 thin film
MO
annealed SnO2 thin film Photo degradation
*Highlights (for review)
Research highlights
SnO2 thin films have been successfully synthesized by galvanic technique.
A drastic morphological change occurs after annealing as deposited SnO2 thin films.
Morphological advantage results enhanced photodegradation of dye.