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Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films
RINP 464
No. of Pages 5, Model 5G
3 December 2016 Results in Physics xxx (2016) xxx–xxx 1
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.journals.elsevier.com/results-in-physics 5 6
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Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films
7
K.P. Ganesan a,b, N. Anandhan a,⇑, V. Dharuman d, P. Sami c, R. Panneerselvam a, T. Marimuthu a
3
8 9 10 11 12 13 1 2 5 6 16 17 18 19 20 21 22 23 24 25
a
Advanced Materials and Thin Film Lab, Department of Physics, Alagappa University, Karaikudi 630 003, India Department of Physics, Saiva Bhanu Kshatriya College, Aruppukottai 626101, India c Department of Chemistry, V.H.N.S.N College, Virudhunagar 626 001, India d Molecular Electronics Laboratory, Department of Bioelectronics and Biosensors, Alagappa University, Karaikudi 630 003, India b
a r t i c l e
i n f o
Article history: Received 2 November 2016 Received in revised form 29 November 2016 Accepted 29 November 2016 Available online xxxx Keywords: Cuprous oxide Crystal orientation Electrodeposition and cubic structure
a b s t r a c t Cuprous oxide (Cu2O) thin films with different crystal orientations were electrochemically deposited in the presence of various molar concentrations of cetyl trimethyl ammonium bromide (CTAB) on fluorine doped tin oxide (FTO) glass substrate using standard three electrodes system. X-ray diffraction (XRD) studies reveal cubic structure of Cu2O with (1 1 1) plane orientation, after addition of CTAB in deposition solution, the orientation of crystal changes from (1 1 1) into (2 0 0) plane. Scanning electron microscope (SEM) images explored significant variation on morphology of Cu2O thin films deposited with addition of CTAB compared to without addition of CTAB. Photoluminescence (PL) spectra illustrate that the emission peak around at 650 nm is attributed to near band edge emission, and the film prepared at the 3 mM of CTAB exhibits much higher intensity than that of the all other films. UV–Visible spectra show optical absorption in the range of 480–610 nm and the highest transparency of Cu2O film prepared at the concentration of 3 mM CTAB. The optical band gap is increased in the range between 2.16 and 2.45 eV with increasing the CTAB concentrations. Ó 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
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42 43
Introduction
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Cuprous oxide (Cu2O) is an excellent p-type metal oxide semiconductor and is used in solar energy conversion devices due to their direct optical band gap (Eg = 1.9–2.2 eV), it has high absorption coefficient, hence it absorbs visible light up to 650 nm [1,2]. It has attracted much current interest because of its potential applications in the field of solar cells, lithium ion batteries, biological sensors, gas sensors, magnetic storage, micro devices, and negative electrodes [3–8]. Different physical and chemical methods have been employed to synthesize the Cu2O micro and nanostructures using sputtering [9], spray pyrolysis [10], electrodeposition [11] and Chemical vapor deposition [12]. Among the various deposition methods, electrochemical deposition is an attractive method because of it’s to control the crystallization, morphology and thickness of Cu2O thin films with respect to various applications point of view, the electrochemical deposition is used to prepare different Cu2O morphologies like corners of the cube (three sided pyramids), triangular prism, truncated cubes [13] and granular spherical morphology [14].
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⇑ Corresponding author. E-mail address: [email protected] (N. Anandhan).
The properties of the Cu2O are directly related to their welldefined shape and structure. These structure and shape the Cu2O films can be modified by using KCl as an additive [15]. Apart from additive, the use of surfactants is possible to change the properties of the films. Sodium dodecyl sulfate (SDS) is used as a surfactant to control both the morphology and crystal size of Cu2O thin films during the electrodeposition [16]. Cetyl trimethyl ammonium bromide (CTAB) is another important cationic surfactant to obtain the nanorod structured Cu2O films using (CBD) [17]. Since in the present work, electrochemical deposition was carried out to deposit Cu2O thin films at various concentrations of CTAB on FTO glass. The crystal structure, surface morphology and optical properties of Cu2O films were investigated and their results are discussed.
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Experimental method
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The electrochemical deposition of Cu2O films was carried out on FTO glass substrate in a bath solution consisting of 0.4 M copper sulfate pentahydrate (CuSO45H2O) and 3 M lactic acid. The pH of the bath solution was adjusted about 10 using sodium hydroxide (NaOH) [18]. To study the effect of surfactant on the properties of Cu2O films, different molar ratio of the CTAB was added in the range 0–3 mM. During the electrodeposition, the temperature of
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http://dx.doi.org/10.1016/j.rinp.2016.11.064 2211-3797/Ó 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Ganesan KP et al. Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films. Results Phys (2016), http://dx.doi.org/10.1016/j.rinp.2016.11.064
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bath solution kept at 60 °C. FTO glass served working electrode, and platinum sheet was used as a counter electrode. Ag/AgCl was used as a reference electrode. Prior to electrodeposition, FTO glass substrate was well cleaned using acetone, isopropanol and deionized water, respectively. The electrodeposition was potentiostatically performed at a constant potential of 0.3 V for 30 min using standard three electrode system of Model 362, EC&G Princeton Applied Research instrument, USA. Finally the deposited Cu2O films were rinsed with deionized water and dried at 100 °C for 1 h prior to subject the characterization. The crystal structure of Cu2O thin films were studied using PAN analytic X-ray diffractometer employing Cu-Ka radiation k = 1.504 Å. The surface morphology of Cu2O microstructure was scanned by scanning electron microscopy (SEM) using ZEISS instrument. The UV–visible absorption and transmittance spectra were recorded by a UV–Vis spectrometer from UV-1800, SHIMADZU instrument. The luminescence properties of Cu2O films were investigated by photoluminescence spectroscopy using RF6000, SHIMADZU instrument. Thickness of the films was measured by styles profilometer.
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Results and discussion
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Structure analysis
105
The XRD patterns of Cu2O films grown at different molar concentrations of CTAB on FTO substrate are shown Fig. 1. The unassigned diffraction peaks are related to FTO glass substrate. The remaining diffraction peaks are well indexed to cubic structure of Cu2O and good in accord with ICCD card No.:75-1531. The diffraction peaks appeared at 2 theta values of 36.5 and 42.35° correspond to (1 1 1) and (2 0 0) planes, respectively. None of the other characteristic peaks are appeared for impuries. It can be seen that the pure Cu2O films on FTO glass, the (1 1 1) peak is appeared as a prominent compared with (2 0 0) plane; it indicates crystalline growth along their preferential orientation. When the concentration of CTAB is increased from 0 to 1 mM, the intensity of diffraction peak of Cu2O film is notably decreased compared with pure Cu2O film. Further increasing the CTAB concentrations to 2 mM, the crystalline growth orientation is changed from (1 1 1) to (2 0 0) plane. As concentration of CTAB is about 3 mM, the (2 0 0) plane intensity appears as high which revealed that, the crystals are grown along the (2 0 0) plane orientation. In this XRD patterns, it can be observed that the concentration of CTAB plays an important role in changing crystal growth orientation. The crystalline size of the prepared thin films is calculated using Scherrer formula 1 [19].
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0:9k D¼ b cos h
ð1Þ
129
where D is the crystalline size, k is the wavelength of the X-ray, b is the full width and half maximum of the diffraction peak in radians and h is the diffraction angle. The crystalline size of the films deposited at the different concentrations of CTAB 0 mM, 1 mM, 2 mM and 3 mM is found to be 32.15 nm, 31.55 nm, 34.45 nm and 34.45 nm respectively. It could be observed the slight variation of the crystalline size of the Cu2O films prepared in the absence and presence of CTAB. The films deposited in the presence of 2 and 3 mM of CTAB exhibits the good stability with higher crystalline size.
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Surface morphology analysis
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Fig. 2 shows SEM images of the films that were electrodeposited from the solution containing different concentrations of CTAB. The faceted crystalline grains on the surface of the pure Cu2O films were observed as the corners of the cubes (three-side pyramids) as shown in Fig. 2(a). The pyramid structure of Cu2O thin films can be assigned to preferred crystal orientation plane at (1 1 1) [13]. The insert Fig. 2(a) displays uniform distribution of pyramid like morphology over the surface of the substrate. The size of the grains is about 2.98 lm. The morphology of the Cu2O film was slightly changed as the film deposited in the presence of 1 mM CTAB as shown Fig. 2(b). The insert Fig. 2(b) illustrate that, the density of the pyramid is increased with decreasing void between pyramids. The size of the grains is about 1.72 lm. When the film is electrodeposited at a concentration of 2 mM CTAB, the granular spherical grains are evenly planted on surface of the substrate as shown in Fig. 2(c). The insert of Fig. 2(c) shows that there is no pin hole appeared on the film. The size of the granular spherical grains is about 0.71 lm. Fig. 2(d) displays the densely packed granular grains on whole, the surface of the substrate as CTAB is added about 3 mM. The granular grains are of about 0.84 lm. It can observe from SEM images that the morphology of the Cu2O is notably changed from pyramid to granular by increasing the concentration of the CTAB.
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Photoluminescence spectra analysis
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Photoluminescence (PL) spectrum is a powerful tool which gives the information about the band structure and crystalline quality [20]. PL spectra of Cu2O films grown on FTO glass substrate were performed using excitation wavelength of 325 nm. In this Fig. 3, the visible emission peak centered at 650 nm is observed for all Cu2O films. The peak observed in visible region 650 nm is associated with near band emission [18]. The emission peak of
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Fig. 1. XRD patterns of Cu2O thin films deposited at different concentrations of CTAB (a) 0 mM, (b) 1 mM, (c) 2 mM, and (d) 3 mM.
Please cite this article in press as: Ganesan KP et al. Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films. Results Phys (2016), http://dx.doi.org/10.1016/j.rinp.2016.11.064
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Fig. 2. SEM images of Cu2O thin films deposited at different concentrations of CTAB (a) 0 mM, (b) 1 mM, (c) 2 mM and (d) 3 mM.
Fig. 3. Photoluminescence spectra of films deposited at different concentrations of CTAB of (a) 0 mM, (b) 1 mM, (c) 2 mM and (d) 3 mM.
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Cu2O film prepared at a concentration of 2 mM is blue shifted compared to pure Cu2O films. It may be argued that blue shift is caused by quantum confinement of excitation photo generated inside the micro particles [18]. The films prepared at the various concentrations of CTAB, the film deposited using 2 mM of CTAB exhibits high
intense emission peak. It reveals that more number electrons transfer from valence band to conduction band. From these PL spectra, one can conclude that the film deposited using 2 mM has better emission properties with higher crystalline quality than other films. Hence it can be used to fabricate solar cells.
Please cite this article in press as: Ganesan KP et al. Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films. Results Phys (2016), http://dx.doi.org/10.1016/j.rinp.2016.11.064
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UV–Visible spectra studies
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The optical absorption and transmittance spectra were recorded in order to determine the absorption coefficient and electron transition (direct and indirect band gap) in the deposited semiconducting thin films [20]. UV–Visible absorption spectra for Cu2O thin films deposited without and with addition of CTAB surfactant are shown in Fig. 4 in the range of 450–900 nm. It can be seen from absorption spectra that there is only one peak appeared at 614 nm due to presence of the photo carriers in the visible range of prepared thin films [21]. Generally, the absorbance depends on the crystal structure, morphology, surface roughness and film thickness [22]. The film deposited in the presence of 3 mM CTAB shows higher absorbance than that of the films prepared other concentrations. It may be due to availability of high density of granular grains thin films, which lead to multi light scattering at grains boundary for absorbing the more numbers photons. It can be seen that the absorbance edge of the film is slightly blue shifted with increasing the CTAB concentrations 0 mM, 1 mM, 2 mM and 3 mM for 614 nm, 607 nm, 565 and 538 nm, respectively. The shifting (blue shift) towards lower wavelength is attributed to distortion of the O–Cu–O bond in Cu2O thin films [23]. Fig. 5 shows transmittance spectra of films prepared using various concentrations of CTAB. The film deposited without addition of CTAB shows less transmittance compared to films prepared other concentrations; it may be higher thickness of the film about 4.61 lm. The thickness of the films prepared at the various concentrations of CTAB 0 mM, 1 mM, 2 mM and 3 mM is about 4.61 lm, 3.41 lm, 3.05 lm and 2.30 lm, respectively. The transmittance of the film is increased up to 2 mM of CTAB, after that transmittance is significantly decreased as the concentration of CTAB is about 3 mM. This might be morphology evolution (densely packed granular grains). The optical absorption is used to calculate the absorbance coefficient and optical band gap (Eg) of a semiconductor. The band gap is important parameter to determine the optical properties. Therefore the band gap of the Cu2O thin film is calculated using Tauc’s formula (24).
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218 220
1 A a ¼ ln t T
ð2Þ
Fig. 5. UV–Visible transmittance spectrum of Cu2O thin films deposited at various concentrations of (a) 0 mM, (b) 1 mM, (c) 2 mM, and (d) 3 mM.
where a is the optical absorption coefficient, A is the absorbance, T is the transmittance and t is the thickness of the thin films. The band gap of the Cu2O is estimated using the Eq. (3).
aht ¼ Aðht Eg Þ
n
ð3Þ
where a is the optical absorption coefficient, hm is the incident photon energy, Eg is the optical band gap of the thin films. The band gap of the Cu2O prepared at various concentrations of the CTAB 0, 1, 2 and 3 mM are found to be 2.16, 2.29, 2.39 and 2.45 eV, respectively. The band gap of the Cu2O increases with increasing in the CTAB concentrations, it is owing to anisotropic growth orientation of Cu2O [23].
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Conclusion
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Cu2O thin films were successfully grown on FTO glass substrates using electrodeposition technique at different concentrations of CTAB. It is found that surfactant CTAB played an important role in the formation microstructure orientation and morphology. The crystal growth orientation on Cu2O films was changed from (1 1 1) to (2 0 0) plane with increasing the CTAB concentrations. Cu2O film prepared in the absence of CTAB exhibited cubic with three side pyramid structure. The film prepared in the presence of 3 mM CTAB displayed densely packed sphere shape microstructure. The film prepared at a concentration of 2 mM CTAB exhibited better luminescence with higher crystalline quality than that of other films. The film deposited at a concentration of 3 mM CTAB show the best absorbance with moderate transmittance.
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Uncited reference
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Ref. [24]. References
Fig. 4. UV–Visible absorbance spectra of Cu2O thin films deposited at different concentrations of (a) 0 mM, (b) 1 mM, (c) 2 mM, and (d) 3 mM.
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[1] Gu YE, Su X, Du YL, Wang CM. Preparation of flower like Cu2O nanoparticles by pulse electrodeposition and their electrocatalytic application. Appl Surf Sci 2010;256:5862–6. [2] Tang YW, Chen ZG, Zhang LS, Jia ZY, Zhang X. Preparation and characterization of nanocrystalline Cu2O/TiO2 hetrojunction film electrodes. J Inorg Mater 2006;21:453–8. [3] Hiroki N, Tatsuya S, Hiroki H, Chihiro M, Lchiro T, Tohru H, Mits. Chemical fabrication of p-type Cu2O transparent thin films using molecular precursor method. Mater Chem Phys 2012;137:252–7.
Please cite this article in press as: Ganesan KP et al. Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films. Results Phys (2016), http://dx.doi.org/10.1016/j.rinp.2016.11.064
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[4] Ho JY, Huang MH. Synthesis of submicrometer-sized Cu2O crystals with morphological evolution from cubic to hexapod structures and their comparative photocatalytic activity. J Phys Chem 2009;113:14159–64. [5] Park JC, Kim J, Kwon H, Song H. Gram-scale synthesis of Cu2O nanocubes and Subsequent oxidation to CuO hollow nanostructures for lithium-ion battery anode materials. Adv Mater 2009;21:803–7. [6] Sharma P, Sharma SK. Microscopic investigations of Cu2O nanostructures. J Alloy Compd 2013;557:152–9. [7] Miyake M, Chen YC, Braun PV, Wilzuius P. Fabrication of three-dimensional photonic crystals using multi beam interference lithography and electrodeposition. Adv Mater 2009;21:3012–5. [8] Yu Y, Zhang LY, Wang J, Yang Z, Long MC, Hu NT, et al. Preparation of hollow porous Cu2O microspheres and photocatalytic activity under visible light irradiation. Nannoscale Res Lett 2012;7:347–52. [9] Antony Jiji, Qiang You, Faheen Muhammad, Meyer Daniel, McCready David E, Engelhard Mark H. Ferromagnetic semiconductor Co doped Cu2O. Appl Phys Lett 2005;90:013106. [10] Upadhyay Sumant, Sharma Dipika, Satsangi Vibha R, Shrivastav Rohit, Waghmare Umesh V, Dass Saha. Spray pyrolytically deposited Fe-doped Cu2O thin films for solar hydrogen generation: experiments & first-principles analysis. Mater Chem Phys 2015;160:32–9. [11] Jiang Xishun, Zhang Miao, Shi Shiwei, He Gang, Song Xueping, Sun Zhaoqi. Microstructure and optical optical properties of nanocrystalline Cu2O thin films prepared by electrodeposition. Nanoscale Res Lett 2014;9:219. [12] Hesjedal T. Continuous roll–to roll growth of grapheme films by chemical vapor deposition. Appl Phys Lett 2011;98:13310. [13] Amano Fumiaki, Ebina Toshihiro, Ohtaani Bunsho. Enhancement of photocathodic stability of p-type copper (I) oxide electrodes by surface etching treatment. Thin films 2014;550:340–6. [14] Jiang Xishun, Zhang Miao, Shiweishi, He Gang, Song Xueping, Sun Zhaoqi. Microstructue and optical properties of nanocrystalline Cu2O thin films prepared by electrodeposition. Nanoscale Res Lett 2014;9:219.
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[15] Yu Xiao-jiao, Li Xin-Ming, Qian Yang. Effect of KCl on crystal morphology of electrodeposited Cu2O thin films. Adv Mater Res 2011;239–242:2827–30. [16] Yang Ying, Han Juan, Ning Xiaohuli, CaO Wei, Wei Xu, Guo Liejin. Controllable morphology and conductivity of electrodeposited Cu2O thin film. Appl Mater Interfaces 2014;6:22534–43. [17] Aref AA, Xiong Liangbin, Yan Nannan, Abdul karem AM, Yu Ying. Cu2O nanorod thin films prepared by CBD method with CTAB substrate effect deposition mechanism and photo electrochemical properties. Mater Chem Phys 2011;127:433–9. [18] Liu YL, Liu YC, Mu R, Yang H, Shao CL, Zhang JY, Lu YM, Shen DZ, Fan XW. The structural and optical properties of Cu2O films electrodeposited and different substrate. Semicond. Sci. Technol. 2005;20:44–9. [19] Giilen Y, Bayansal F, Sahin B, Cetinkara HA, Giider HS. Fabrication and characterization of Mn-doped CuO thin films by the SILLAR method. Ceram Int 2013;39:6475–80. [20] Laidoudi S, Bioud AY, ZiZi AA, Schmerber G, Bartringer J, Bare S, Dinia A. Growth and characterization of electrodeposited Cu2O. Semicond Sci Technol 2013;28:0115005. [21] Kiryung, Seunghwan, Lee Donyoon, Seo Hyungtak. Physico-chemical interface effect in Cu2O-ZnO hetro junction on photocurrent spectrum. RSC Adv 2015;5:103803. [22] Halin DSC, Talib IA, Daud AR, Hamid MAA. Chracterizations of cuprous oxide thin films prepared by sol-gel spin coating technique with different additives for the photo electro chemical solar cell. Int J Photo Energy 2014:1–6. [23] Zhao Min, Yaoyao, Lv Jianguo, Sun Yue, Cao Li, He Gang, et al. Microstructure, morphology and sunlight response of cuprous oxide thin films. J Mater Sci: Mater Electron 2015;27:1799–804. [24] Messaoudi O, Assaker IB, Gannouni M, Souissi A, Makhlout H, Bardaoui A, et al. Structural morphological and electrical characteristics of electrodeposited Cu2O: effect of deposition time. Appl Surf Sci 2016;366:383–8.
Please cite this article in press as: Ganesan KP et al. Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films. Results Phys (2016), http://dx.doi.org/10.1016/j.rinp.2016.11.064
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No. of Pages 5, Model 5G
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Contents lists available at ScienceDirect
Results in Physics journal homepage: www.journals.elsevier.com/results-in-physics 5 6
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Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films
7
K.P. Ganesan a,b, N. Anandhan a,⇑, V. Dharuman d, P. Sami c, R. Panneerselvam a, T. Marimuthu a
3
8 9 10 11 12 13 1 2 5 6 16 17 18 19 20 21 22 23 24 25
a
Advanced Materials and Thin Film Lab, Department of Physics, Alagappa University, Karaikudi 630 003, India Department of Physics, Saiva Bhanu Kshatriya College, Aruppukottai 626101, India c Department of Chemistry, V.H.N.S.N College, Virudhunagar 626 001, India d Molecular Electronics Laboratory, Department of Bioelectronics and Biosensors, Alagappa University, Karaikudi 630 003, India b
a r t i c l e
i n f o
Article history: Received 2 November 2016 Received in revised form 29 November 2016 Accepted 29 November 2016 Available online xxxx Keywords: Cuprous oxide Crystal orientation Electrodeposition and cubic structure
a b s t r a c t Cuprous oxide (Cu2O) thin films with different crystal orientations were electrochemically deposited in the presence of various molar concentrations of cetyl trimethyl ammonium bromide (CTAB) on fluorine doped tin oxide (FTO) glass substrate using standard three electrodes system. X-ray diffraction (XRD) studies reveal cubic structure of Cu2O with (1 1 1) plane orientation, after addition of CTAB in deposition solution, the orientation of crystal changes from (1 1 1) into (2 0 0) plane. Scanning electron microscope (SEM) images explored significant variation on morphology of Cu2O thin films deposited with addition of CTAB compared to without addition of CTAB. Photoluminescence (PL) spectra illustrate that the emission peak around at 650 nm is attributed to near band edge emission, and the film prepared at the 3 mM of CTAB exhibits much higher intensity than that of the all other films. UV–Visible spectra show optical absorption in the range of 480–610 nm and the highest transparency of Cu2O film prepared at the concentration of 3 mM CTAB. The optical band gap is increased in the range between 2.16 and 2.45 eV with increasing the CTAB concentrations. Ó 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
42 43
Introduction
44
Cuprous oxide (Cu2O) is an excellent p-type metal oxide semiconductor and is used in solar energy conversion devices due to their direct optical band gap (Eg = 1.9–2.2 eV), it has high absorption coefficient, hence it absorbs visible light up to 650 nm [1,2]. It has attracted much current interest because of its potential applications in the field of solar cells, lithium ion batteries, biological sensors, gas sensors, magnetic storage, micro devices, and negative electrodes [3–8]. Different physical and chemical methods have been employed to synthesize the Cu2O micro and nanostructures using sputtering [9], spray pyrolysis [10], electrodeposition [11] and Chemical vapor deposition [12]. Among the various deposition methods, electrochemical deposition is an attractive method because of it’s to control the crystallization, morphology and thickness of Cu2O thin films with respect to various applications point of view, the electrochemical deposition is used to prepare different Cu2O morphologies like corners of the cube (three sided pyramids), triangular prism, truncated cubes [13] and granular spherical morphology [14].
45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
⇑ Corresponding author. E-mail address: [email protected] (N. Anandhan).
The properties of the Cu2O are directly related to their welldefined shape and structure. These structure and shape the Cu2O films can be modified by using KCl as an additive [15]. Apart from additive, the use of surfactants is possible to change the properties of the films. Sodium dodecyl sulfate (SDS) is used as a surfactant to control both the morphology and crystal size of Cu2O thin films during the electrodeposition [16]. Cetyl trimethyl ammonium bromide (CTAB) is another important cationic surfactant to obtain the nanorod structured Cu2O films using (CBD) [17]. Since in the present work, electrochemical deposition was carried out to deposit Cu2O thin films at various concentrations of CTAB on FTO glass. The crystal structure, surface morphology and optical properties of Cu2O films were investigated and their results are discussed.
62
Experimental method
75
The electrochemical deposition of Cu2O films was carried out on FTO glass substrate in a bath solution consisting of 0.4 M copper sulfate pentahydrate (CuSO45H2O) and 3 M lactic acid. The pH of the bath solution was adjusted about 10 using sodium hydroxide (NaOH) [18]. To study the effect of surfactant on the properties of Cu2O films, different molar ratio of the CTAB was added in the range 0–3 mM. During the electrodeposition, the temperature of
76
http://dx.doi.org/10.1016/j.rinp.2016.11.064 2211-3797/Ó 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Ganesan KP et al. Electrochemically modified crystal orientation, surface morphology and optical properties using CTAB on Cu2O thin films. Results Phys (2016), http://dx.doi.org/10.1016/j.rinp.2016.11.064
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bath solution kept at 60 °C. FTO glass served working electrode, and platinum sheet was used as a counter electrode. Ag/AgCl was used as a reference electrode. Prior to electrodeposition, FTO glass substrate was well cleaned using acetone, isopropanol and deionized water, respectively. The electrodeposition was potentiostatically performed at a constant potential of 0.3 V for 30 min using standard three electrode system of Model 362, EC&G Princeton Applied Research instrument, USA. Finally the deposited Cu2O films were rinsed with deionized water and dried at 100 °C for 1 h prior to subject the characterization. The crystal structure of Cu2O thin films were studied using PAN analytic X-ray diffractometer employing Cu-Ka radiation k = 1.504 Å. The surface morphology of Cu2O microstructure was scanned by scanning electron microscopy (SEM) using ZEISS instrument. The UV–visible absorption and transmittance spectra were recorded by a UV–Vis spectrometer from UV-1800, SHIMADZU instrument. The luminescence properties of Cu2O films were investigated by photoluminescence spectroscopy using RF6000, SHIMADZU instrument. Thickness of the films was measured by styles profilometer.
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Results and discussion
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Structure analysis
105
The XRD patterns of Cu2O films grown at different molar concentrations of CTAB on FTO substrate are shown Fig. 1. The unassigned diffraction peaks are related to FTO glass substrate. The remaining diffraction peaks are well indexed to cubic structure of Cu2O and good in accord with ICCD card No.:75-1531. The diffraction peaks appeared at 2 theta values of 36.5 and 42.35° correspond to (1 1 1) and (2 0 0) planes, respectively. None of the other characteristic peaks are appeared for impuries. It can be seen that the pure Cu2O films on FTO glass, the (1 1 1) peak is appeared as a prominent compared with (2 0 0) plane; it indicates crystalline growth along their preferential orientation. When the concentration of CTAB is increased from 0 to 1 mM, the intensity of diffraction peak of Cu2O film is notably decreased compared with pure Cu2O film. Further increasing the CTAB concentrations to 2 mM, the crystalline growth orientation is changed from (1 1 1) to (2 0 0) plane. As concentration of CTAB is about 3 mM, the (2 0 0) plane intensity appears as high which revealed that, the crystals are grown along the (2 0 0) plane orientation. In this XRD patterns, it can be observed that the concentration of CTAB plays an important role in changing crystal growth orientation. The crystalline size of the prepared thin films is calculated using Scherrer formula 1 [19].
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0:9k D¼ b cos h
ð1Þ
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where D is the crystalline size, k is the wavelength of the X-ray, b is the full width and half maximum of the diffraction peak in radians and h is the diffraction angle. The crystalline size of the films deposited at the different concentrations of CTAB 0 mM, 1 mM, 2 mM and 3 mM is found to be 32.15 nm, 31.55 nm, 34.45 nm and 34.45 nm respectively. It could be observed the slight variation of the crystalline size of the Cu2O films prepared in the absence and presence of CTAB. The films deposited in the presence of 2 and 3 mM of CTAB exhibits the good stability with higher crystalline size.
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Surface morphology analysis
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Fig. 2 shows SEM images of the films that were electrodeposited from the solution containing different concentrations of CTAB. The faceted crystalline grains on the surface of the pure Cu2O films were observed as the corners of the cubes (three-side pyramids) as shown in Fig. 2(a). The pyramid structure of Cu2O thin films can be assigned to preferred crystal orientation plane at (1 1 1) [13]. The insert Fig. 2(a) displays uniform distribution of pyramid like morphology over the surface of the substrate. The size of the grains is about 2.98 lm. The morphology of the Cu2O film was slightly changed as the film deposited in the presence of 1 mM CTAB as shown Fig. 2(b). The insert Fig. 2(b) illustrate that, the density of the pyramid is increased with decreasing void between pyramids. The size of the grains is about 1.72 lm. When the film is electrodeposited at a concentration of 2 mM CTAB, the granular spherical grains are evenly planted on surface of the substrate as shown in Fig. 2(c). The insert of Fig. 2(c) shows that there is no pin hole appeared on the film. The size of the granular spherical grains is about 0.71 lm. Fig. 2(d) displays the densely packed granular grains on whole, the surface of the substrate as CTAB is added about 3 mM. The granular grains are of about 0.84 lm. It can observe from SEM images that the morphology of the Cu2O is notably changed from pyramid to granular by increasing the concentration of the CTAB.
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Photoluminescence spectra analysis
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Photoluminescence (PL) spectrum is a powerful tool which gives the information about the band structure and crystalline quality [20]. PL spectra of Cu2O films grown on FTO glass substrate were performed using excitation wavelength of 325 nm. In this Fig. 3, the visible emission peak centered at 650 nm is observed for all Cu2O films. The peak observed in visible region 650 nm is associated with near band emission [18]. The emission peak of
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Fig. 1. XRD patterns of Cu2O thin films deposited at different concentrations of CTAB (a) 0 mM, (b) 1 mM, (c) 2 mM, and (d) 3 mM.
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Fig. 2. SEM images of Cu2O thin films deposited at different concentrations of CTAB (a) 0 mM, (b) 1 mM, (c) 2 mM and (d) 3 mM.
Fig. 3. Photoluminescence spectra of films deposited at different concentrations of CTAB of (a) 0 mM, (b) 1 mM, (c) 2 mM and (d) 3 mM.
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Cu2O film prepared at a concentration of 2 mM is blue shifted compared to pure Cu2O films. It may be argued that blue shift is caused by quantum confinement of excitation photo generated inside the micro particles [18]. The films prepared at the various concentrations of CTAB, the film deposited using 2 mM of CTAB exhibits high
intense emission peak. It reveals that more number electrons transfer from valence band to conduction band. From these PL spectra, one can conclude that the film deposited using 2 mM has better emission properties with higher crystalline quality than other films. Hence it can be used to fabricate solar cells.
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UV–Visible spectra studies
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The optical absorption and transmittance spectra were recorded in order to determine the absorption coefficient and electron transition (direct and indirect band gap) in the deposited semiconducting thin films [20]. UV–Visible absorption spectra for Cu2O thin films deposited without and with addition of CTAB surfactant are shown in Fig. 4 in the range of 450–900 nm. It can be seen from absorption spectra that there is only one peak appeared at 614 nm due to presence of the photo carriers in the visible range of prepared thin films [21]. Generally, the absorbance depends on the crystal structure, morphology, surface roughness and film thickness [22]. The film deposited in the presence of 3 mM CTAB shows higher absorbance than that of the films prepared other concentrations. It may be due to availability of high density of granular grains thin films, which lead to multi light scattering at grains boundary for absorbing the more numbers photons. It can be seen that the absorbance edge of the film is slightly blue shifted with increasing the CTAB concentrations 0 mM, 1 mM, 2 mM and 3 mM for 614 nm, 607 nm, 565 and 538 nm, respectively. The shifting (blue shift) towards lower wavelength is attributed to distortion of the O–Cu–O bond in Cu2O thin films [23]. Fig. 5 shows transmittance spectra of films prepared using various concentrations of CTAB. The film deposited without addition of CTAB shows less transmittance compared to films prepared other concentrations; it may be higher thickness of the film about 4.61 lm. The thickness of the films prepared at the various concentrations of CTAB 0 mM, 1 mM, 2 mM and 3 mM is about 4.61 lm, 3.41 lm, 3.05 lm and 2.30 lm, respectively. The transmittance of the film is increased up to 2 mM of CTAB, after that transmittance is significantly decreased as the concentration of CTAB is about 3 mM. This might be morphology evolution (densely packed granular grains). The optical absorption is used to calculate the absorbance coefficient and optical band gap (Eg) of a semiconductor. The band gap is important parameter to determine the optical properties. Therefore the band gap of the Cu2O thin film is calculated using Tauc’s formula (24).
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1 A a ¼ ln t T
ð2Þ
Fig. 5. UV–Visible transmittance spectrum of Cu2O thin films deposited at various concentrations of (a) 0 mM, (b) 1 mM, (c) 2 mM, and (d) 3 mM.
where a is the optical absorption coefficient, A is the absorbance, T is the transmittance and t is the thickness of the thin films. The band gap of the Cu2O is estimated using the Eq. (3).
aht ¼ Aðht Eg Þ
n
ð3Þ
where a is the optical absorption coefficient, hm is the incident photon energy, Eg is the optical band gap of the thin films. The band gap of the Cu2O prepared at various concentrations of the CTAB 0, 1, 2 and 3 mM are found to be 2.16, 2.29, 2.39 and 2.45 eV, respectively. The band gap of the Cu2O increases with increasing in the CTAB concentrations, it is owing to anisotropic growth orientation of Cu2O [23].
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Conclusion
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Cu2O thin films were successfully grown on FTO glass substrates using electrodeposition technique at different concentrations of CTAB. It is found that surfactant CTAB played an important role in the formation microstructure orientation and morphology. The crystal growth orientation on Cu2O films was changed from (1 1 1) to (2 0 0) plane with increasing the CTAB concentrations. Cu2O film prepared in the absence of CTAB exhibited cubic with three side pyramid structure. The film prepared in the presence of 3 mM CTAB displayed densely packed sphere shape microstructure. The film prepared at a concentration of 2 mM CTAB exhibited better luminescence with higher crystalline quality than that of other films. The film deposited at a concentration of 3 mM CTAB show the best absorbance with moderate transmittance.
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Uncited reference
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Ref. [24]. References
Fig. 4. UV–Visible absorbance spectra of Cu2O thin films deposited at different concentrations of (a) 0 mM, (b) 1 mM, (c) 2 mM, and (d) 3 mM.
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