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Synthesis, characterization and electrocatalytic activity of SnO2, Pt–SnO2 thin films for methanol oxidation
Accepted Manuscript Synthesis, characterization and electrocatalytic activity of SnO2, Pt-SnO2 thin films for methanol oxidation Sumanta Jana, Gopinath Mondal, Bibhas Chandra Mitra, Pulakesh Bera, Anup Mondal PII: DOI: Reference:
S0301-0104(14)00133-5 http://dx.doi.org/10.1016/j.chemphys.2014.05.003 CHEMPH 9095
To appear in:
Chemical Physics
Received Date: Accepted Date:
6 February 2014 6 May 2014
Please cite this article as: S. Jana, G. Mondal, B.C. Mitra, P. Bera, A. Mondal, Synthesis, characterization and electrocatalytic activity of SnO2, Pt-SnO2 thin films for methanol oxidation, Chemical Physics (2014), doi: http:// dx.doi.org/10.1016/j.chemphys.2014.05.003
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.
Synthesis, characterization and electrocatalytic activity of SnO2, PtSnO2 thin films for methanol oxidation
Sumanta Janaa, Gopinath Mondalb, Bibhas Chandra Mitrac, Pulakesh Berab, Anup Mondala* a
Department of Chemistry, Bengal Engineering and Science University, Botanic Garden, Howrah 711103, WB, India b
Department of Chemistry, Panskura Banamali College, Purba Medinipur, Panskura 721152, WB, India
c
Department of Physics, Bengal Engineering and Science University, Botanic Garden, Howrah 711103, 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 nanocrystalline SnO2 thin films on TCO (transparent conducting oxide) substrates. Un-doped and Pt-doped SnO2 thin films have been synthesized from the precursor solution of stannic chloride (SnCl4.5H2O) and chloroplatinic acid (H2PtCl6.H2O). X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive analysis of X rays (EDAX) and X ray photoelectron spectroscopy (XPS) techniques were used to characterize the thin films. Optical characterizations were carried out by Uv-vis and photoluminescenc (PL) spectroscopy. The present method provides a simple and cost-effective way to deposit highly stable SnO2 and Pt-SnO2 thin films. The synthesized films were used as electrode and its catalytic activity towards methanol oxidation was investigated. The study reveals that Pt-SnO2 electrode is more effective for methanol electrooxidation than SnO2. Keywords:
Thin
films;
nanocrystalline
electrooxidation.
2
material;
electrocatalytic
activity;
1. Introduction Recently, syntheses of nanostructure SnO2 thin films have attracted much attention because of its wide applications and diverse optical, electrical and electrochemical properties [1]. The material is used in several areas, such as electrode material in lithium ion batteries, photoelectrodes, sensors, heterogeneous catalysts [2-5]. Recently SnO2 materials are extensively used in dye sensitized solar cells [6-8]. Several synthetic methods were applied to prepare SnO2 thin films including chemical vapor deposition [9], sol-gel [10], spray pyrolysis [11], thermal evaporation [12], sputtering [13], electrodeposition etc [14]. But preparation of nanocrystalline SnO2 thin films on TCO substrate by galvanic method has not been noticed by us, which is being reported here. This technique is simpler than the conventional electrodeposition process of using a potentiostat/galvanostat. SnO2 is invariably anion deficient and oxygen vacancies are mainly responsible for making available free electrons for the conduction process [15]. Previous reports revealed that incorporating noble elements e.g., Pt, Pd etc. into SnO2 material enhance the conduction process and electrocatalytic activity [16]. Pt nanoparticles (NPs) usually possess large surface area which accounts for the number of available active sites towards electrochemical reaction and electron transfer [17]. Thus, the electrochemical reaction could greatly be promoted at the Pt NPs surfaces, leading to a more sensitive and rapid response. In this article, we have discussed the synthesis and detail characterization of un-doped and Pt-doped SnO2 thin films. The electrocatalytic activity of the materials was studied. It has been investigated that the doped material i.e; Pt-SnO2 shows an effective electrocatalytic activity towards methanol oxidation.
3
2. Experimental 2.1 Synthesis of SnO2 thin films A properly cleaned TCO glass substrate and a metallic Zn strip were clamped vertically and dipped into 0.1 M SnCl4 solution in a 100 ml reaction bath. The total volume of the working solution was maintained to 100 ml by adding distilled water. The pH of the solution was adjusted to 2.3 with dilute HCl solution. Zn strip served as an easily oxidisable anode and TCO acted as cathode. The deposition was carried out at 800C under stirred condition. When the electrochemical cell was externally short-circuited, Zn2+ ions were released from the Zn electrode and electrons within the short-circuited path moved to TCO, where they reduced Sn4+ ions to Sn0 (black colouration). Then, H2O2 (30%) was added dropwise onto this black surface and it turned white due to rapid oxidation of Sn to SnO2. 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)
H2O2 readily dissociates as H2O2 → H2O + [O]
(iv)
This nascent oxygen [O] readily reacts with elemental tin and finally converts to SnO2. Sn0 + 2 [O] → SnO2
(v)
A white transparent layer was developed on TCO within 40 minutes and it was airannealed at 4000C for one hour. 2.2 Synthesis of Pt doped SnO2 thin films
4
Dilute solutions of chloroplatinic acid (H2PtCl6.H2O) with different Pt percentages (5%, 10%, 15%, 20%) were used as the precursor solution for Pt doping. The synthesized SnO2 thin films were separately dipped into the precursor solution. Then, after 5 minutes the films were taken out form the bath, air dried and finally annealed at 300°C for 30 minutes. Thus, Pt-doped SnO2 thin films were synthesized with different Pt percentages. 3. Results and discussion 3.1 Characterization The structural and morphological analysis were carried out by X-ray diffractometer (XRD) (Seifert P300 Cu Kα radiations) and field emission scanning electron microscope (FESEM), associated with EDAX probe (Gemini Zeiss Supra 35VP,Germany). The optical properties were studied by UV–Vis (JASCO V-530) spectrophotometer and photoluminiscence (PL) spectra were recorded with Perkin–Elmer LS-55 Fluoremeter. Xray photoelectron spectroscopy (XPS) measurements were carried out on an ESCLAB KMII using Al as the exciting source. Electrocatalytic activity was measured in standard three electrode system (CH instruments 600 D series). 3.2 XRD analyses Figure 1a shows the XRD pattern of TCO and SnO2/TCO thin films. XRD patterns for TCO and SnO2 thin films are similar but their intensities are different. The similar 2θ value for each case is due to presence of same elements (SnO2) in all the films. The Conducting layer of commercially purchased TCO is FTO (fluorine doped tin oxide, SnO2:F) i.e F doped SnO2. For the as deposited SnO2 thin films, the thin layer of SnO2 was deposited on this conducting layer. Hence, all the two layers are basically tin oxide, so, their diffraction peaks were same. An excess of SnO2 due to the as deposited SnO2
5
thin films is responsible for the comparative high intensity than the TCO substrate. In Figure 1a the major diffractions were observed from (112), (006), (200), (130) planes whose 2θ value matches well with JCPDS # 78-1063 (orthorhombic). XRD pattern of PtSnO2 (10% Pt) shows intense extra peaks at 39.2°, 43° and 64.43° along with SnO2 peaks (Figure 1b). These extra peaks are due to platinum (Pt) incorporation into SnO2. No other diffraction peaks were observed in the XRD pattern, meaning that the deposited material is pure. 3.3 FESEM analyses Figure 2 represents the FESEM images of (a) TCO, (b) SnO2/TCO and (c) Pt-SnO2 (10% Pt) thin films respectively. SEM image of SnO2 (Figure b) shows a porous flake like nanostructure uniformly grown on TCO substrate. The grain growth is likely due to the assimilation of the smaller grains in a controlled manner to give a definite shape, with the consequent formation of pores in between the larger grains. For Pt-SnO2 thin films a drastic change in morphology is clearly observable from Figure c. The growth of Pt nanoparticles on SnO2 surface is discrete and results a porous morphology. The composition analysis was carried out by energy dispersive X-ray (EDX) spectroscopy (associated with SEM instrument) which confirms presence of Pt, Sn and O. EDAX of Pt-SnO2 (10%) shows 1.82% Pt incorporation in the SnO2 material (Figure (d)). The detail of compositional analysis was shown in Table 1. 3.4 Optical studies Figure 3a shows the UV-Vis absorption spectra of SnO2, a sharp increase in absorption was observed from ~ 400 nm. It is well-known that for a crystalline semi conductor the optical absorption near the band edge follows the equation (αhν)1/n = A(hν − Eg), where,
6
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 Eg from the plot of (αhν)2 vs. hν. The Eg value was calculated by plotting (αhν)2 versus hν and extrapolating the linear portion of the curve to the x-axis; α being the absorption coefficient and hν the photon energy [18]. From the plot Eg value was calculated to 3.48 eV (Figure 3a inset) comparable to bulk SnO2 (Eg = 3.6 eV) [19]. Figure 3b shows room-temperature photoluminescence (PL) spectra of SnO2 thin film. SnO2 thin films have an intense luminescence centered at 395 nm (3.14 eV). It should be noted that there are very few reports on strong blue luminescence from SnO2 nanostructures [20] although yellow red light emissions at ~605 nm were observed for SnO2 grown by laser-ablation, vapor-liquid-solid approach etc [21-22]. The blue luminescence of synthesized SnO2 can be attributed to oxygen-related defects that have been introduced during growth. 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 a strong PL signal at room temperature. 3.5 XPS Investigation XPS was carried out to investigate the surface composition and chemical state of the deposited thin films. As observed in Figure 4a, a common peak was detected at 486.3 eV i.e at the position of the main (j = 5/2) spin-orbit split component of the Sn3d peak, which is typical for Sn (IV) sites coordinated by oxygen atoms [23]. The Sn 3d region shows
7
regular doublet for Sn3d5/2 and Sn 3d3/2, with peaks at 486.3 and 496 eV, which is approximately the same value as that in the standard spectrum of SnO2 [24]. Whereas, the peak centered at 532.9 eV corresponds to the O 1s (Figure 4b), indicates a normal state of O2− in the compounds. These results are consistent with the previous reported values observed for SnO2 [23, 25]. Figure 4c represents the XPS spectrum of Pt4f. The presence of two peaks at 76.27 eV and 78.89 eV (j-j doublet separation of ca. 2.62 eV) is detected in the outer layers of the coatings, similar to the earlier reports of Moulder et al [27]. On the other hand, the Pt 4f7/2 peak at 72.39 eV and Pt 4f5/2 peak at 75.98 eV with a peak separation of 3.59 eV is ascribed to PtO (Pt II) [26]. Pt(IV) (BE Pt4f7/2 ≈ 75.9 eV, j-j doublet separation of ca. 3 eV) species were also detected with a peak separation of ca. 3 eV in the Pt-SnO2 Matrix [27]. The presence of PtO on the surface of Pt nanostructures is usual, since oxygen absorption can easily occur at annealing stage. In fact, presence of Pt (IV) was also detected from the XPS study. A different reduced platinum species (BE Pt4f7/2 ≈ 71.5 eV) appeared, very close in energy to the zero-valence state (Pt0). Hence, the Pt 4f spectrum resulted into several contribution with binding energies that are in agreement with the literatures values for Pt(0), PtO and PtO2. The XPS results for the Sn 3d and Pt 4f lines combined with the deconvoluted components of the O 1s suggest that Sn is present mainly as SnO2 while Pt is found in the zero valence and oxidised states. The electrochemical characterizations of the electrodes in supporting electrolyte were performed in 0.1M HClO4.
3.6 Resistivity studies The resistivity of the deposited thin films was measured by conventional four probe technique. SnO2 thin films have highest resistivity compare to TCO and Pt-SnO2
8
(lowest). The high resistivity of SnO2 might be due to presence of pores and its uneven distribution, where the free movement of electrons is restricted to large extent with agree to the statement by Coutts et al; [28]. Another reason is due to physisorbed oxygen, molecules receive electrons from the conduction band and change to O– ads or O2– ads species which form an electron depletion layer just below the surface of SnO2 and create a potential barrier [29]. Resistivity of Pt doped SnO2 is low due to increase in metallic character into the semiconductor matrix which accelerates the electrons movement. 3.7 Electrocatalytic activity Electrocatalytic measurements were carried out in three electrode system where SnO2 or Pt-SnO2 thin film was taken as working electrode, a platinum wire as counter and an Ag/AgCl electrode was used as reference electrodes respectively. A chemical bath (50 mL) of 0.1 M HClO4 in 1 M methanol was prepared and the electrodes were dipped into it. Before the experiment, the solution was degassed by purging with pure N2 for 20 minutes. Figure 4 represents the cyclic voltamograms of the TCO, SnO2 and Pt-SnO2 thin films in the working solution. From the CV study it was observed that the catalytic activity of Pt- SnO2 thin film was greater than SnO2. The anodic current increases sharply due to oxidation of methanol [30] and then decreases after peak potential (0.4V), due to loss of active sites (SnO2). Here, methanol oxidation starts at 0.37 V. The less reactivity of SnO2 thin films is due to its restricted movement of electrons. For Pt based system methanol oxidation involves generation of chemisorbed carbon monoxide (CO) on the Pt surface by the following reaction [31] Pt + CH3OH → Pt-CO + 4H+ + 4e
(i)
SnO2 + H2O → SnO2−OH + H+ + e¯
(ii)
9
The bond formed between Pt and CO may lead to deactivation. But coupling Pt with SnO2 increases the kinetic of methanol oxidation by decreasing oxidation potential. A continuous removal of adsorbed CO from the Pt active sites favours the oxidation. During the electrochemical process, reaction between chemisorbed CO and OH* occurs to produce CO2 that is removed from the electrode surface according to the following scheme: Pt-CO + SnO2-OH*→ Pt + SnO2 + CO2 + H+ + e¯
(iii)
It directly reacts with the methanol and results the oxidation. Figure 5a represents the cyclic voltamogram of Pt-SnO2 in the same working solution with gradual addition of 25 mM, 50 mM and 100 mM methanol. Steady increase in current occurs with increasing methanol concentration which indicates that the material is quite effective for the oxidation. Figure 5b shows current-time curves of SnO2 and Pt-SnO2 electrodes in same working solution at 0.37 V. There is a continuous current drop with polarization time, which is rapid at the beginning and follows a relatively slow decay after that. The rapid decay implies the poisoning phenomena on catalyst surface probably due to the formation of intermediate CO species [32] and for Pt-SnO2 species this decay is comparatively low and current density is high. The high current density of Pt-SnO2 electrode obviously indicates better stability and activity of the material. 4. Conclusions SnO2 thin films were successfully synthesized by simple galvanic technique. The films are porous and have small particle size. Comparative performances of SnO2 and Pt-SnO2 thin films were studied towards oxidation of methanol. Platinum is present as dopant in the SnO2 films with homogeneous in depth distribution. The electrical sensitivity (better
10
catalytic activity towards methanol oxidation) of Pt-SnO2 films is enhanced by Pt doping. It can be suggested that the increased porosity and the electronic sensitization due to the dopant have a synergistic effect in enhancing the electrical response of the films. The high current density of Pt-SnO2 electrode also indicates better stability and activity of the material. Acknowledgments The author S. Jana is thankful to UGC (India) for his research fellowship ((Ref. No. 2012/2009 (ii) EU-IV). The authors wish to acknowledge DST (India) and U.G.C.-S.A.P. (India) for providing instrumental facilities to the Department of Chemistry, IIEST, Shibpur, India.
11
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Md.T. Uddin, Y. Nicolas, C. Olivier, T. Toupance, M.M. Müller, H.J. Kleebe, K. Rachut, J. Ziegler, A. Klein, W. Jaegermann, J. Phys. Chem. C 117 (2013) 22098.
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14
Figure captions Figure 1: XRD patterns of (a) SnO2 (b) Pt-SnO2 thin film. Figure 2: FESEM images of (a) bare TCO (b) SnO2 thin film (c) Pt-SnO2 thin film (d) EDAX of Pt-SnO2 Figure 3: (a) UV-Vis spectra of bare TCO, and SnO2 thin film, inset Tauc plot (b) Room temperature PL spectrum of SnO2 thin film with an excitation wavelength at 250 nm. Figure 4: XPS spectra of Sn 3d (a), O1s (b) and Pt 4f (c) of SnO2 and Pt-SnO2 thin films. Figure 5: (a) Cyclic voltamogram (CV) of different electrodes in 1 M MeOH + 0.1 M HClO4 solution. Figure 6: (a) CV of Pt-SnO2 thin film with gradual addition of methanol (b) Current– time curves of SnO2, Pt- SnO2 thin films, measured at 0.37 V. Table1: Atomic percentages of EDAX analysis
15
Figures:
Figure 1
Figure 2
16
Figure 3
Figure 4
17
Figure 5
Figure 6
18
Table1
Set No.
Conc. of H2PtCl4
Atomic % of Sn:O:Pt
Reactivity (methanol oxidation)
1
5%
32·74:65·96:1.30
Low
2
10%
32·51:65·67:1.82
High
3
15%
32·63:64·81:2.56
Low
4
20%
33·61:63·17:3.22
Low
19
Graphical Abstract Nanostructure SnO2 thin films were synthesized by galvanic technique. Platinum (Pt) nanoparticles were doped onto SnO2 surface by dipping SnO2 films into dilute chloroplatinic acid solution. Comparative performances of SnO2 and Pt- SnO2 thin films were investigated for methanol oxidation and better reactivity was observed for the later case.
Pt –SnO2
20
Research highlights •
Galvanically, nanocrystalline SnO2 thin films were synthesized
•
Pt doped SnO2 thin films were synthesized from chloroplatinic acid
•
Electrocatalytic activity of these materials towards methanol oxidation was investigated
•
Pt- SnO2 thin films showed better reactivity than SnO2
21
S0301-0104(14)00133-5 http://dx.doi.org/10.1016/j.chemphys.2014.05.003 CHEMPH 9095
To appear in:
Chemical Physics
Received Date: Accepted Date:
6 February 2014 6 May 2014
Please cite this article as: S. Jana, G. Mondal, B.C. Mitra, P. Bera, A. Mondal, Synthesis, characterization and electrocatalytic activity of SnO2, Pt-SnO2 thin films for methanol oxidation, Chemical Physics (2014), doi: http:// dx.doi.org/10.1016/j.chemphys.2014.05.003
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.
Synthesis, characterization and electrocatalytic activity of SnO2, PtSnO2 thin films for methanol oxidation
Sumanta Janaa, Gopinath Mondalb, Bibhas Chandra Mitrac, Pulakesh Berab, Anup Mondala* a
Department of Chemistry, Bengal Engineering and Science University, Botanic Garden, Howrah 711103, WB, India b
Department of Chemistry, Panskura Banamali College, Purba Medinipur, Panskura 721152, WB, India
c
Department of Physics, Bengal Engineering and Science University, Botanic Garden, Howrah 711103, 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 nanocrystalline SnO2 thin films on TCO (transparent conducting oxide) substrates. Un-doped and Pt-doped SnO2 thin films have been synthesized from the precursor solution of stannic chloride (SnCl4.5H2O) and chloroplatinic acid (H2PtCl6.H2O). X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive analysis of X rays (EDAX) and X ray photoelectron spectroscopy (XPS) techniques were used to characterize the thin films. Optical characterizations were carried out by Uv-vis and photoluminescenc (PL) spectroscopy. The present method provides a simple and cost-effective way to deposit highly stable SnO2 and Pt-SnO2 thin films. The synthesized films were used as electrode and its catalytic activity towards methanol oxidation was investigated. The study reveals that Pt-SnO2 electrode is more effective for methanol electrooxidation than SnO2. Keywords:
Thin
films;
nanocrystalline
electrooxidation.
2
material;
electrocatalytic
activity;
1. Introduction Recently, syntheses of nanostructure SnO2 thin films have attracted much attention because of its wide applications and diverse optical, electrical and electrochemical properties [1]. The material is used in several areas, such as electrode material in lithium ion batteries, photoelectrodes, sensors, heterogeneous catalysts [2-5]. Recently SnO2 materials are extensively used in dye sensitized solar cells [6-8]. Several synthetic methods were applied to prepare SnO2 thin films including chemical vapor deposition [9], sol-gel [10], spray pyrolysis [11], thermal evaporation [12], sputtering [13], electrodeposition etc [14]. But preparation of nanocrystalline SnO2 thin films on TCO substrate by galvanic method has not been noticed by us, which is being reported here. This technique is simpler than the conventional electrodeposition process of using a potentiostat/galvanostat. SnO2 is invariably anion deficient and oxygen vacancies are mainly responsible for making available free electrons for the conduction process [15]. Previous reports revealed that incorporating noble elements e.g., Pt, Pd etc. into SnO2 material enhance the conduction process and electrocatalytic activity [16]. Pt nanoparticles (NPs) usually possess large surface area which accounts for the number of available active sites towards electrochemical reaction and electron transfer [17]. Thus, the electrochemical reaction could greatly be promoted at the Pt NPs surfaces, leading to a more sensitive and rapid response. In this article, we have discussed the synthesis and detail characterization of un-doped and Pt-doped SnO2 thin films. The electrocatalytic activity of the materials was studied. It has been investigated that the doped material i.e; Pt-SnO2 shows an effective electrocatalytic activity towards methanol oxidation.
3
2. Experimental 2.1 Synthesis of SnO2 thin films A properly cleaned TCO glass substrate and a metallic Zn strip were clamped vertically and dipped into 0.1 M SnCl4 solution in a 100 ml reaction bath. The total volume of the working solution was maintained to 100 ml by adding distilled water. The pH of the solution was adjusted to 2.3 with dilute HCl solution. Zn strip served as an easily oxidisable anode and TCO acted as cathode. The deposition was carried out at 800C under stirred condition. When the electrochemical cell was externally short-circuited, Zn2+ ions were released from the Zn electrode and electrons within the short-circuited path moved to TCO, where they reduced Sn4+ ions to Sn0 (black colouration). Then, H2O2 (30%) was added dropwise onto this black surface and it turned white due to rapid oxidation of Sn to SnO2. 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)
H2O2 readily dissociates as H2O2 → H2O + [O]
(iv)
This nascent oxygen [O] readily reacts with elemental tin and finally converts to SnO2. Sn0 + 2 [O] → SnO2
(v)
A white transparent layer was developed on TCO within 40 minutes and it was airannealed at 4000C for one hour. 2.2 Synthesis of Pt doped SnO2 thin films
4
Dilute solutions of chloroplatinic acid (H2PtCl6.H2O) with different Pt percentages (5%, 10%, 15%, 20%) were used as the precursor solution for Pt doping. The synthesized SnO2 thin films were separately dipped into the precursor solution. Then, after 5 minutes the films were taken out form the bath, air dried and finally annealed at 300°C for 30 minutes. Thus, Pt-doped SnO2 thin films were synthesized with different Pt percentages. 3. Results and discussion 3.1 Characterization The structural and morphological analysis were carried out by X-ray diffractometer (XRD) (Seifert P300 Cu Kα radiations) and field emission scanning electron microscope (FESEM), associated with EDAX probe (Gemini Zeiss Supra 35VP,Germany). The optical properties were studied by UV–Vis (JASCO V-530) spectrophotometer and photoluminiscence (PL) spectra were recorded with Perkin–Elmer LS-55 Fluoremeter. Xray photoelectron spectroscopy (XPS) measurements were carried out on an ESCLAB KMII using Al as the exciting source. Electrocatalytic activity was measured in standard three electrode system (CH instruments 600 D series). 3.2 XRD analyses Figure 1a shows the XRD pattern of TCO and SnO2/TCO thin films. XRD patterns for TCO and SnO2 thin films are similar but their intensities are different. The similar 2θ value for each case is due to presence of same elements (SnO2) in all the films. The Conducting layer of commercially purchased TCO is FTO (fluorine doped tin oxide, SnO2:F) i.e F doped SnO2. For the as deposited SnO2 thin films, the thin layer of SnO2 was deposited on this conducting layer. Hence, all the two layers are basically tin oxide, so, their diffraction peaks were same. An excess of SnO2 due to the as deposited SnO2
5
thin films is responsible for the comparative high intensity than the TCO substrate. In Figure 1a the major diffractions were observed from (112), (006), (200), (130) planes whose 2θ value matches well with JCPDS # 78-1063 (orthorhombic). XRD pattern of PtSnO2 (10% Pt) shows intense extra peaks at 39.2°, 43° and 64.43° along with SnO2 peaks (Figure 1b). These extra peaks are due to platinum (Pt) incorporation into SnO2. No other diffraction peaks were observed in the XRD pattern, meaning that the deposited material is pure. 3.3 FESEM analyses Figure 2 represents the FESEM images of (a) TCO, (b) SnO2/TCO and (c) Pt-SnO2 (10% Pt) thin films respectively. SEM image of SnO2 (Figure b) shows a porous flake like nanostructure uniformly grown on TCO substrate. The grain growth is likely due to the assimilation of the smaller grains in a controlled manner to give a definite shape, with the consequent formation of pores in between the larger grains. For Pt-SnO2 thin films a drastic change in morphology is clearly observable from Figure c. The growth of Pt nanoparticles on SnO2 surface is discrete and results a porous morphology. The composition analysis was carried out by energy dispersive X-ray (EDX) spectroscopy (associated with SEM instrument) which confirms presence of Pt, Sn and O. EDAX of Pt-SnO2 (10%) shows 1.82% Pt incorporation in the SnO2 material (Figure (d)). The detail of compositional analysis was shown in Table 1. 3.4 Optical studies Figure 3a shows the UV-Vis absorption spectra of SnO2, a sharp increase in absorption was observed from ~ 400 nm. It is well-known that for a crystalline semi conductor the optical absorption near the band edge follows the equation (αhν)1/n = A(hν − Eg), where,
6
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 Eg from the plot of (αhν)2 vs. hν. The Eg value was calculated by plotting (αhν)2 versus hν and extrapolating the linear portion of the curve to the x-axis; α being the absorption coefficient and hν the photon energy [18]. From the plot Eg value was calculated to 3.48 eV (Figure 3a inset) comparable to bulk SnO2 (Eg = 3.6 eV) [19]. Figure 3b shows room-temperature photoluminescence (PL) spectra of SnO2 thin film. SnO2 thin films have an intense luminescence centered at 395 nm (3.14 eV). It should be noted that there are very few reports on strong blue luminescence from SnO2 nanostructures [20] although yellow red light emissions at ~605 nm were observed for SnO2 grown by laser-ablation, vapor-liquid-solid approach etc [21-22]. The blue luminescence of synthesized SnO2 can be attributed to oxygen-related defects that have been introduced during growth. 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 a strong PL signal at room temperature. 3.5 XPS Investigation XPS was carried out to investigate the surface composition and chemical state of the deposited thin films. As observed in Figure 4a, a common peak was detected at 486.3 eV i.e at the position of the main (j = 5/2) spin-orbit split component of the Sn3d peak, which is typical for Sn (IV) sites coordinated by oxygen atoms [23]. The Sn 3d region shows
7
regular doublet for Sn3d5/2 and Sn 3d3/2, with peaks at 486.3 and 496 eV, which is approximately the same value as that in the standard spectrum of SnO2 [24]. Whereas, the peak centered at 532.9 eV corresponds to the O 1s (Figure 4b), indicates a normal state of O2− in the compounds. These results are consistent with the previous reported values observed for SnO2 [23, 25]. Figure 4c represents the XPS spectrum of Pt4f. The presence of two peaks at 76.27 eV and 78.89 eV (j-j doublet separation of ca. 2.62 eV) is detected in the outer layers of the coatings, similar to the earlier reports of Moulder et al [27]. On the other hand, the Pt 4f7/2 peak at 72.39 eV and Pt 4f5/2 peak at 75.98 eV with a peak separation of 3.59 eV is ascribed to PtO (Pt II) [26]. Pt(IV) (BE Pt4f7/2 ≈ 75.9 eV, j-j doublet separation of ca. 3 eV) species were also detected with a peak separation of ca. 3 eV in the Pt-SnO2 Matrix [27]. The presence of PtO on the surface of Pt nanostructures is usual, since oxygen absorption can easily occur at annealing stage. In fact, presence of Pt (IV) was also detected from the XPS study. A different reduced platinum species (BE Pt4f7/2 ≈ 71.5 eV) appeared, very close in energy to the zero-valence state (Pt0). Hence, the Pt 4f spectrum resulted into several contribution with binding energies that are in agreement with the literatures values for Pt(0), PtO and PtO2. The XPS results for the Sn 3d and Pt 4f lines combined with the deconvoluted components of the O 1s suggest that Sn is present mainly as SnO2 while Pt is found in the zero valence and oxidised states. The electrochemical characterizations of the electrodes in supporting electrolyte were performed in 0.1M HClO4.
3.6 Resistivity studies The resistivity of the deposited thin films was measured by conventional four probe technique. SnO2 thin films have highest resistivity compare to TCO and Pt-SnO2
8
(lowest). The high resistivity of SnO2 might be due to presence of pores and its uneven distribution, where the free movement of electrons is restricted to large extent with agree to the statement by Coutts et al; [28]. Another reason is due to physisorbed oxygen, molecules receive electrons from the conduction band and change to O– ads or O2– ads species which form an electron depletion layer just below the surface of SnO2 and create a potential barrier [29]. Resistivity of Pt doped SnO2 is low due to increase in metallic character into the semiconductor matrix which accelerates the electrons movement. 3.7 Electrocatalytic activity Electrocatalytic measurements were carried out in three electrode system where SnO2 or Pt-SnO2 thin film was taken as working electrode, a platinum wire as counter and an Ag/AgCl electrode was used as reference electrodes respectively. A chemical bath (50 mL) of 0.1 M HClO4 in 1 M methanol was prepared and the electrodes were dipped into it. Before the experiment, the solution was degassed by purging with pure N2 for 20 minutes. Figure 4 represents the cyclic voltamograms of the TCO, SnO2 and Pt-SnO2 thin films in the working solution. From the CV study it was observed that the catalytic activity of Pt- SnO2 thin film was greater than SnO2. The anodic current increases sharply due to oxidation of methanol [30] and then decreases after peak potential (0.4V), due to loss of active sites (SnO2). Here, methanol oxidation starts at 0.37 V. The less reactivity of SnO2 thin films is due to its restricted movement of electrons. For Pt based system methanol oxidation involves generation of chemisorbed carbon monoxide (CO) on the Pt surface by the following reaction [31] Pt + CH3OH → Pt-CO + 4H+ + 4e
(i)
SnO2 + H2O → SnO2−OH + H+ + e¯
(ii)
9
The bond formed between Pt and CO may lead to deactivation. But coupling Pt with SnO2 increases the kinetic of methanol oxidation by decreasing oxidation potential. A continuous removal of adsorbed CO from the Pt active sites favours the oxidation. During the electrochemical process, reaction between chemisorbed CO and OH* occurs to produce CO2 that is removed from the electrode surface according to the following scheme: Pt-CO + SnO2-OH*→ Pt + SnO2 + CO2 + H+ + e¯
(iii)
It directly reacts with the methanol and results the oxidation. Figure 5a represents the cyclic voltamogram of Pt-SnO2 in the same working solution with gradual addition of 25 mM, 50 mM and 100 mM methanol. Steady increase in current occurs with increasing methanol concentration which indicates that the material is quite effective for the oxidation. Figure 5b shows current-time curves of SnO2 and Pt-SnO2 electrodes in same working solution at 0.37 V. There is a continuous current drop with polarization time, which is rapid at the beginning and follows a relatively slow decay after that. The rapid decay implies the poisoning phenomena on catalyst surface probably due to the formation of intermediate CO species [32] and for Pt-SnO2 species this decay is comparatively low and current density is high. The high current density of Pt-SnO2 electrode obviously indicates better stability and activity of the material. 4. Conclusions SnO2 thin films were successfully synthesized by simple galvanic technique. The films are porous and have small particle size. Comparative performances of SnO2 and Pt-SnO2 thin films were studied towards oxidation of methanol. Platinum is present as dopant in the SnO2 films with homogeneous in depth distribution. The electrical sensitivity (better
10
catalytic activity towards methanol oxidation) of Pt-SnO2 films is enhanced by Pt doping. It can be suggested that the increased porosity and the electronic sensitization due to the dopant have a synergistic effect in enhancing the electrical response of the films. The high current density of Pt-SnO2 electrode also indicates better stability and activity of the material. Acknowledgments The author S. Jana is thankful to UGC (India) for his research fellowship ((Ref. No. 2012/2009 (ii) EU-IV). The authors wish to acknowledge DST (India) and U.G.C.-S.A.P. (India) for providing instrumental facilities to the Department of Chemistry, IIEST, Shibpur, India.
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Figure captions Figure 1: XRD patterns of (a) SnO2 (b) Pt-SnO2 thin film. Figure 2: FESEM images of (a) bare TCO (b) SnO2 thin film (c) Pt-SnO2 thin film (d) EDAX of Pt-SnO2 Figure 3: (a) UV-Vis spectra of bare TCO, and SnO2 thin film, inset Tauc plot (b) Room temperature PL spectrum of SnO2 thin film with an excitation wavelength at 250 nm. Figure 4: XPS spectra of Sn 3d (a), O1s (b) and Pt 4f (c) of SnO2 and Pt-SnO2 thin films. Figure 5: (a) Cyclic voltamogram (CV) of different electrodes in 1 M MeOH + 0.1 M HClO4 solution. Figure 6: (a) CV of Pt-SnO2 thin film with gradual addition of methanol (b) Current– time curves of SnO2, Pt- SnO2 thin films, measured at 0.37 V. Table1: Atomic percentages of EDAX analysis
15
Figures:
Figure 1
Figure 2
16
Figure 3
Figure 4
17
Figure 5
Figure 6
18
Table1
Set No.
Conc. of H2PtCl4
Atomic % of Sn:O:Pt
Reactivity (methanol oxidation)
1
5%
32·74:65·96:1.30
Low
2
10%
32·51:65·67:1.82
High
3
15%
32·63:64·81:2.56
Low
4
20%
33·61:63·17:3.22
Low
19
Graphical Abstract Nanostructure SnO2 thin films were synthesized by galvanic technique. Platinum (Pt) nanoparticles were doped onto SnO2 surface by dipping SnO2 films into dilute chloroplatinic acid solution. Comparative performances of SnO2 and Pt- SnO2 thin films were investigated for methanol oxidation and better reactivity was observed for the later case.
Pt –SnO2
20
Research highlights •
Galvanically, nanocrystalline SnO2 thin films were synthesized
•
Pt doped SnO2 thin films were synthesized from chloroplatinic acid
•
Electrocatalytic activity of these materials towards methanol oxidation was investigated
•
Pt- SnO2 thin films showed better reactivity than SnO2
21