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Triton-X mediated interconnected nanowalls network of cadmium sulfide thin films via chemical bath deposition and their photoelectrochemical performance
Solid State Sciences 36 (2014) 41e46
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Triton-X mediated interconnected nanowalls network of cadmium sulfide thin films via chemical bath deposition and their photoelectrochemical performance S.A. Vanalakar a, c, S.S. Mali b, E.A. Jo a, J.Y. Kim c, J.H. Kim a, **, P.S. Patil b, * a b c
Department of Materials Science and Engineering, Chonnam National University, Gwangju 500 757, South Korea Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, M.S. 416 204, India Department of Electronics and Computer Engineering, Chonnam National University, Gwangju 500 757, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2013 Received in revised form 13 July 2014 Accepted 15 July 2014 Available online 23 July 2014
Thin films of cadmium sulfide (CdS) have been wet chemically deposited onto fluorine-doped tin oxide (FTO) coated conducting glass substrates by using non-ionic surfactant; Triton-X 100. An aqueous solution contains cadmium sulphate as a cadmium and thiourea as sulphur precursor. Ammonia used as a complexing agent. The results of measurements of the x-ray diffraction, Raman spectroscopy, optical spectroscopy, energy dispersive spectroscopy, scanning electron microscopy, Brunauer Emmett Teller (BET) surface areas and atomic force microscopy were used for the characterization of the films. These results revealed that the films are polycrystalline, consisting of CdS cubic phase. The films show a direct band gap with energy 2.39 eV. The films show interconnected nanowalls like morphology with welldefined surface area. Finally, the photoelectrochemical (PEC) performance of Triton-X mediated CdS thin film samples were studied. The sample shows photoelectrochemical (PEC) performance with maximum short circuit current density (Jsc) 1.71 mA/cm2 for larger area (1 cm2) solar cells. © 2014 Elsevier Masson SAS. All rights reserved.
Keywords: Cadmium sulfide Surfactant Interconnected nanowalls morphology PEC performance
1. Introduction Nanostructured thin films of cadmium sulfide (CdS) have been intensively studied due to their applications in optoelectronic devices, photoelectrochemical solar cells, semiconductor sensitized solar cells, the window layer in a photovoltaic cell, and transparent conducting semiconductors [1e5]. However, the optoelectronic properties of CdS nanomaterials are strongly influenced by their morphologies and structures. Therefore, it is very important to explore the simple method to achieve the surface morphology by controlled synthesis. Surface morphology is a vital aspect of the science of nanomaterials and any research carried out in this direction will be of great interest, especially due to the several application possibilities. The synthesis of varieties of surface morphologies from ordering structures to complex functional architectures, offer great
* Corresponding author. Tel.: þ82 62 530 1709; fax: þ82 62 530 1699. ** Corresponding author. Tel.: þ91 231 2609230; fax: þ91 231 2691533. E-mail addresses: [email protected] (J.H. Kim), [email protected] (P.S. Patil). http://dx.doi.org/10.1016/j.solidstatesciences.2014.07.006 1293-2558/© 2014 Elsevier Masson SAS. All rights reserved.
opportunities to explore their novel properties for fabrication of various nano devices. Recently, research, development across the globe concentrating on invention of various nanostructures. Chemical bath deposition methods have proved to be effective and versatile and have been employed widely to synthesize a variety of nanomaterials. The amphiphilic nature of organic surfactants helps in modifying the surface morphology of the deposit owing to their concentration-dependent specific activity during chemical bath deposition. The specific activity of the surfactants is generally understood in terms of adsorption during deposition and depends on the critical micelle concentration of the surfactant molecules which forms the bilayers or multilayers [6]. Surfactants or micelles as the regulating structural agents or templates are readily employed in the fabrication of low dimensional nanostructures [7]. Surfactants influence on the complex of chemical properties of substrate in the water solution owing to the solubilization. Effect of organic surfactants on an oxide including ZnO [8], NiO [9] WO3 [10], SnO2 [11], TiO2 [12] as well as on metal chalcogenides like CdS [13,14], PbS [15], CuS [16] have been investigated extensively during the past decade. Surfactants have been widely used to promote stable dispersion of solids in different media. The distinct structural feature of the
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S.A. Vanalakar et al. / Solid State Sciences 36 (2014) 41e46
surfactant originates from its duality; the hydrophilic head-polar group and the hydrophobic tail group that usually consist of fewer hydrocarbon chains. Two important features that characterize surfactants are; the absorption at the interface and self-accumulation into micelle structures [17]. There are different types of surfactants mainly categories in anionic, cationic and non-ionic [18]. In view of this, a systematic study has been undertaken in the synthesis and characterization of interconnected nanowalls of cadmium sulfide using non-ionic surfactants; Triton X-100 (Octylphenol Ethoxylate). Triton X-100 is an excellent detergent, wetting agent and gives an effective performance across a broad temperature range. It acts as an emulsifier for oil-in-water system. Triton X-100 is soluble in water and it is chemically stable in most acidic and alkaline solutions. It acts as metal corrosion resistant as well. Recently, XeH Yang et al. synthesize CdS nanotubes using Triton X-l00 [19]. Jung et al. studied effect of Triton X-100 in water-added electrolytes on the performance of dye-sensitized solar cells [20]. In the present paper, attempts were made to engineer the morphology of the CdS thin films by using non-ionic surfactant; Triton X-100. Effects of Triton-X 100 on the growth of CdS have been studied by conducting experiments with and without surfactant. Thin films of CdS with interconnected nanowalls were prepared at the 90 C by using the CBD method. Further, the CdS thin films synthesized with and without Triton-X were characterized for their structural, morphological, and optical studies through the range of techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical absorption spectroscopy. Finally, the photoelectrochemical (PEC) performance, such as J-V characteristics in dark and under illumination, the ideality factor of prepared films were studied. 2. Experimental details 2.1. Materials and preparation of surfactant mediated CdS thin film All chemicals except Triton X 100 were purchased from s.d. finechemicals and used without any further purification. Triton X-100 was obtained from Aldrich. Soda lime glass (SLG) and fluorinedoped SnO2 conducting (FTO, 10Usq1) glass were used as substrates. The size of each substrate is 1 cm2. Cleaning of the substrates for thin film depositions is the most important factor. Cleanness is the process of breaking the bonds between substrates and contaminants without damaging the substrates. The substrates were first cleaned with a neutral cleaner and washed with double distilled water. Then, the substrate was cleaned ultrasonically and sequentially in acetone, ethanol and double distilled water for 10 min, and dried by blowing dry nitrogen gas. The cadmium sulfate (CdSO4$H2O) was used as cadmium (Cd) source and thiourea (H2N$CS$NH2) as a sulphur (S) source. Triton-X 100 and liquor ammonia (NH3) were used as organic surfactant and complexing agent respectively. The preparative parameters, including precursor concentration, deposition time and temperature, were varied to yield good quality CdS thin films. The CdS films have been synthesized using two different wet chemical routes: (i) without Triton-X 100 and (ii) with Triton-X 100. Finally, a standard recipe was chosen as follows: The matrix solution was prepared by adding an aqueous solution (1 weight %) of organic surfactants (Triton-X 100) to 1.2 M CdSO4. Aqueous ammonia (NH4OH) was added to maintain 11 pH of the solution. The initial turbid solution turns into a transparent, by adding excess ammonia. Then, 1 M thiourea was added into the above solution with constant stirring. The CdS films were deposited by vertically dipping the soda lime glass into the above solution at the 90 C for 10 min. The deposited films were rinsed in distilled water and dried at room temperature overnight. The color of deposited CdS thin films are pale yellow.
2.2. Characterizations The structural properties of the CdS thin films were studied using an X-ray diffractometer (Philips, PW 3710, Almelo, Holland) operated at 25 kV, 20 mA with CuKa radiation (1.5407 Å). FT-Raman spectra of the films were recorded in the spectral range between 250 and 1000 cm1 using FT-Raman spectrometer (Bruker MultiRAM, Germany) that employs Nd:YAG laser source with an excitation wavelength 1064 nm and resolution 4 cm1. Optical absorbance was measured using a UVevis spectrophotometer (UV1800, Shimadzu, Japan). The surface morphology of the films was examined by scanning electron microscope (SEM) (Model JEOLJSM-6360, Japan), operated at 20 kV. Field emission SEM (JSM6701F model) was employed to further examine the CdS morphology. The energy dispersive spectroscopic (EDS) analysis was investigated by FESEM coupled with an energy dispersive spectroscopy unit. The surface morphology and roughness of the films were observed using atomic force microscopy (AFM, Digital Instrument, nanoscope III) operated at room temperature. The thickness of the resulting CdS films was measured using surface profiler (Ambios XP-1). The BET surface area of CdS samples was determined by Micrometrics ASAP 2020 analyzer using nitrogen adsorption and desorption isotherms at 77 K. For the PEC characterization of the with and without Triton-X mediated CdS thin film samples, the measurements were performed in an electrolyte of 1 M polysulfide (Na2SeNaOHeS) in a two-electrode arrangement of following configuration: Glass/FTO/CdS/Na2SeNaOHeS/G In the above cell configuration, CdS thin film deposited on FTO acts as a working electrode (active area ~1.0 cm2), and G is graphite, which acts as a counter electrode. The J-V characteristics were measured using Semiconductor Characterization System SCS-4200 Keithley, Germany uses two electrode configurations in the dark and under illumination at 30 mW/cm2.
3. Results and discussion Chemical bath deposition (CBD) is a method in which controlled release of metal ions and chalcogenide ions take place that results in having control over grain size of thin films. The precipitation of metal chalcogenides in CBD occurs only when the ionic product exceeds the solubility product of metal chalcogenides [21]. Combinations of ions form nuclei on the substrate as well as in the solution results in precipitation. The film growth takes place via ion-by-ion condensation of the materials or by adsorption of colloidal particles from the solution onto the substrate. The complexing agents like NH3 help to control the reaction rate. The TritonX 100 approach to synthesis CdS by CBD process is closely related to understand the growth mechanism. The controllable growth of CdS interconnected nanowalls in the composite film was probably due to two factors. Firstly, Cd2þ and S2 were gradually released in the Triton X matrix-colloid. Triton X-100 molecule contains a plane benzene ring, which connects to both the hydrophobic and
Fig. 1. The structure of the benzene ring, which connects to both the hydrophobic and hydrophilic group of Triton X-100.
S.A. Vanalakar et al. / Solid State Sciences 36 (2014) 41e46
hydrophilic group at the meta position (Fig. 1). It was contingent that, the Triton X-100 cumulates in between the CdS crystallites during CBD crystallization and on washing the material; the improved structures of CdS were formed. CdSO4 at 11 pH dissociates as:
CdSO4 /Cd2þ þ SO2 4
(1)
Addition of thiourea leads to the release of sulfur ions:
CSðNH2 Þ2 þ 2OH /S2 þ CH2 N2 þ 2H2 O
(2)
CdS thin film is formed by virtue of Eqs. (1) and (2) reactions:
Cd2þ þ S2 /CdS
(3)
Secondly, the colloid cumulates turned into clear interconnected nanowalls shaped possibly due to steric hindrance provided by Triton X 100 and ammonia molecules. X-ray diffraction patterns were obtained for structural characterization. Fig. 2 shows the XRD pattern of with and without TritonX 100 mediated CdS interconnected nanowalls network on the soda lime glass substrate. The films deposited did not show peaks related to elemental cadmium or sulfur or carbon. CdS peaks in both the XRD pattern are (111), (200), (220), (311), (222), (400) and (331) appears at 2q ¼ 26.45, 31.71, 44.13, 51.87, 54.97, 62.67 and 70.23 respectively. The comparison of the observed XRD pattern with the standard JCPDS data (80-0019) confirms the formation of CdS phase with a cubic crystal structure. The lattice parameter ‘a’ is calculated using the following Eq. (4).
1 h2 þ k2 þ l2 ¼ 2 d a2
(4)
The mean value of a ¼ 5.810 Å is in good agreement with the reported value a ¼ 5.811 Å. Further, using the breadth of (111) peak, the average crystallite size is estimated using Scherrer's formula given below Eq. (5)
D¼
kl b cos q
(5)
where l ¼ 1.5406Å, k is the dimensionless constant (0.95), b is the corrected broadening of the diffraction line measured at half of its
Fig. 2. X ray diffraction patterns of with and without Triton-X 100 mediated CdS thin films.
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maximum intensity (taken in radians by multiplying a factor of p/ 360), D the diameter of crystallite and q is diffraction angle. The calculated crystallite size of Triton X mediated CdS is found to be 20 nm for (111) plane. The broadened peaks indicate the nanocrystalline nature of the films. The phase identification of the sample has been carried out using FT-Raman spectroscopy. The Raman spectrum of the Triton-X 100 mediated CdS film is shown in Fig. 3. The Raman spectrum of the CdS film exhibits a well-resolved band at 301 cm1; corresponding to the first order scattering of the longitudinal optical (LO) phonon mode and second-order band around 600 cm1. CdS can have both hexagonal wurtzite and cubic zinc blended structures, and it is reported that for both structures, the zone-center longitudinal-optical A1 (LO) phonon frequency is nearly 305 cm1 [22]. The full width half maxima (FWHM) of the 1 LO peak is 16.13 cm1. Though, this large width indicates poor crystallinity (lack of long-range order) in the films, but the well-defined peak indicates the crystalline nature of the material. Hence, the large FWHM in the present case can be attributed to a polycrystalline effect in the as-deposited film. Due to the nanocrystalline thin films, UVeVis spectroscopy has become an effective tool in determining the size and optical properties. Fig. 4 shows the room temperature optical absorption spectrum of the with and without triton-X mediated CdS thin film recorded in the range of 450e750 nm without taking into account scattering and reflection losses. It is clearly observed that the onset of the optical absorption at about 520 nm, which is the absorption edge of the CdS films. At a lower wavelength than 500 nm the optical absorption saturates at a value of 4. Inset of Fig. 4 shows a plot of (ahʋ)2 against the photon energy (hʋ). The band gap of Triton-X mediated CdS films is found to be 2.39 eV and 2.42 eV for without Triton-X CdS films by extrapolating the straight line of the square of absorption coefficient to the intercept of the X-axis of photon energy. The present UVevisible spectrum reveals that CdS interconnected nanowalls network have high absorbance of light in the visible region, indicating applicability as a captivating material in solar cell applications. Moreover, a significant shift in the spectral photoresponse is observed for Triton-X mediated CdS sample. The synthesized Triton-X mediated CdS thin films are examined by SEM (Fig. 5). The low magnification SEM image Fig. 5(a) reveals the film surface looks porous and the formation of CdS
Fig. 3. The Raman spectrum of Triton-X 100 mediated CdS thin film.
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Fig. 4. Optical absorption spectra of with and without Triton-X 100 mediated CdS thin film. The inset figures are the plots of (ahn)2 against hn for CdS thin films.
interconnected nanowalls over the entire substrate. Also, there is not any overgrowth on the substrate without any void, pinholes or cracks and they cover the substrate well. The SEM image Fig. 5(b) shows CdS nanowalls connected with each other forming
nanoconduits. Such walls having ~70 nm thickness are seen in the SEM and FESEM image in Fig. 5(c). The morphologies and structures of the as-synthesized with and without Triton-X mediated CdS thin films are also examined by FE-SEM. The representative FE-SEM image shows [Fig. 6(a)] the as synthesized samples without Triton-X. From Fig. 6(a) it clear that, as prepared sample without Triton-X have no regular shape, the nanowalls are covered with nanonet like structure. With Triton X, the morphologies of the samples are gradually improved, the nanowalls become regular without overlapping nanonet like structure [Fig. 6(b)]. Moreover, Fig. 6(b) confirms interconnected nanowalls like structure of CdS thin films. The surface area is an important physical property that determines the quality and utility of functional materials. The Brunauer, Emmett and Teller (BET) technique is the most common method for determining the surface area of porous materials. The BET specific surface area of without and with Triton-X mediated CdS thin films are 14.28 m2/g and 23.11 m2/g respectively (see inset of Fig. 6). The Triton-X mediated CdS thin film sample provides high surface area compared to without Triton-X CdS thin films, for enhanced surface activities like efficient permeability of electrolytes into the inner structure, and light absorption. It also scatters the light multiply into photoanode and hence improves the effective absorption. The light absorption path length of photons can be increased as it is trapped in the nanochannels. These mechanisms boost the PEC performance of CdS electrode. Atomic force microscopy (AFM) was employed to characterize the 2-D and 3-D surface morphology of the CdS thin films. Fig. 5(d) show the AFM image of
Fig. 5. (a) Low magnification SEM image of Triton-X 100 mediated CdS thin films sample (b) SEM image show the formation of interconnected nanowall network of Triton-X 100 mediated CdS over the substrate. The inset shows the EDS spectrum for Triton-X 100 mediated CdS thin films. (c)High magnification SEM image of Triton-X 100 mediated CdS with nanoconduits formed by numerous nanowalls for sample. The inset FESEM image shows the thickness of nanowall. (d) AFM image of Triton-X 100 mediated CdS with nanoconduits formed by numerous nanowalls.
S.A. Vanalakar et al. / Solid State Sciences 36 (2014) 41e46
45
Fig. 6. (a) FE-SEM image of CdS without Trinton-X thin film sample. Inset shows the Nitrogen adsorptionedesorption isotherms of CdS without Trinton-X thin film. (b) FE-SEM image of CdS with Trinton-X thin film sample. Inset shows the Nitrogen adsorptionedesorption isotherms of CdS with Trinton-X thin film.
Triton-X mediated CdS thin film sample. The AFM image clearly shows that the nanowall network is interconnected. An energy dispersive spectroscopic (EDS) analysis of the film (inset Fig. 5(b)) confirms the presence of S and Cd, and the molar ratio of Cd is slightly greater than S, which matches with the stoichiometry of CdS. Fig. 7(a and b) shows JeV characteristics of without and with Triton-X 100 CdS thin film samples. The JeV characteristic in the dark resembles diode-like characteristics of the PEC cells fabricated with all the samples. Under illumination, shifting of the JeV curve in the fourth quadrant of the graph suggests that the electrons are the majority carriers, confirming the n-type conductivity of CdS thin films. CdS without mediated Trinton-X 100 cell exhibits the power conversion efficiency (h) of 0.28% with (JSC) 0.81 mA/cm2, open-circuit voltage (VOC) ¼ 360 mV and fill factor (FF) ¼ 0.30. Where as, CdS with Trinton-X 100 cell exhibits the power conversion efficiency (h) of 0.87% with (JSC) ¼ 1.71 mA/cm2, open-circuit voltage (VOC) ¼ 461 mV and fill factor (FF) ¼ 0.33. The ideality factor is a fitting parameter that describes how closely the diode's behavior matches the behavior predicted by theory. The ideality factor is determined under forward bias and is normally found to be in between 1 and 2 depending upon the relation between diffusion current and recombination current. The ideality factor becomes 1 when the p-n junction of the diode is an infinite plane and no recombination occurs within the spacecharge region. When recombination current is more than diffusion current then ideality factor becomes 2. The ideality factor 'nd' of prepared CdS films is determined from following diode equation as.
I ¼ Io eqV=nd kT 1
(6)
where, Io is the reverse saturation current, V is forward bias voltage, k is Boltzmann's constant, T is ambient temperature in Kelvin and nd is an ideality factor. The ideality factor was found to be 2.06 and 2.16 (inset Fig. 7) for the sample under the light illumination for CdS without and with Trinton-X 100. The ideality factor for the samples are found to be greater than 1 indicates recombination current is more. The higher value of nd is indicative of the series resistance effect, surface states and the charge carrier recombination at the semiconductoreelectrolyte interface. These factors reduce the ideality of devices. 4. Conclusion In summary, CdS composed of uniform interconnected nanowalls network have been successfully prepared via a non-ionic
Fig. 7. The JeV characteristics of interconnected nanowall network of CdS thin film sample. (a): The JeV characteristics of CdS without Trinton-X thin film and; (b): The JeV characteristics of CdS with Trinton-X thin film. Inset figures show respective ideality factor graphs of CdS thin films.
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surfactant-assisted wet chemical reaction system at low temperature and ambient atmosphere. This method is simple and suitable for the preparation of large surface area thin film in a one step process using Triton-X 100. The as deposited CdS film showed a cubical crystal structure. The presence of characteristic bonds of CdS is observed from Raman spectroscopy. The well covered porous structure with the interconnected nanowalls network morphology leading to high surface area is observed by SEM studies. The PEC performance of Trinton-X 100 mediated CdS is higher than pure CdS thin films. This structure is a good prospective way for PEC application. Acknowledgment This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No.: 20124010203180). One of the authors SAV is grateful to DST-SERB, New Delhi for financial support (SR/ FTP/PS-083/2012). References [1] K. Senthil, D. Mangalaraj, S.K. Narayandass, Appl. Surf. Sci. 476 (2001) 169e170. [2] J.J. Loferski, Material for solar energy conversion, in: G.G. Libowitz (Ed.), Materials Science Series, Academic Press, New York, 1979 (Chapter 4).
[3] S.A. Vanalakar, R.C. Pawar, M.P. Suryawanshi, S.S. Mali, D.S. Dalavi, A.V. Moholkar, K.U. Sim, Y.B. Kown, J.H. Kim, P.S. Patil, Mater. Lett. 65 (2011) 548e551. [4] J. Britt, C. Ferekides, Appl. Phys. Lett. 62 (1993) 2851. [5] J. Jie, W. Zhang, I. Bello, C.S. Lee, S.T. Lee, Nanotoday 5 (4) (2010) 313e336. [6] U. Retter, M. Tchachnikova, J. Electroanal. Chem. 550 (2003) 201e208. [7] H. Zhang, D. Yang, X. Ma, Y. Ji, S. Li, D. Que, Mater. Chem. Phys. 93 (2005) 65. [8] A.I. Inamdar, S.H. Mujawar, V. Ganesan, P.S. Patil, Nanotechnology 19 (2008) 325706. [9] Y. Tan, S. Srinivasan, K.-S. Choi, J. Am. Chem. Soc. 127 (2005) 3596e3604. [10] M. Deepa, A.K. Srivastava, S.N. Sharma, Govind, S.M. Shiraprasad, Appl. Surf. Sci. 254 (8) (2008) 2342e2352. [11] K.K. Khun, A. Mahajan, R.K. Bedi, Appl. Surf. Sci. 257 (7) (2011) 2929e2934. [12] S.S. Mali, C.A. Betty, P.N. Bhosale, R.S. Devan, Y.R. Ma, S.S. Kolekar, P.S. Patil, CrystEngComm 14 (2012) 1920e1924. [13] C. O'Dwyer, V. Lavayen, N. Mirabal, M.A. Santa Ana, E. Benavente, lez, Z. Lopez, O. Scho € ps, U. Woggon, C.M. Sotomayor S. Ormazabal, G. Gonza Torres, Phot. Nano. Fund. Appl. 5 (2007) 45e52. [14] C.L. Perkins, F.S. Hasoon, J. Vac. Sci. Technol. A 243 (2006) 497e504. [15] L. Dong, Y. Chu, Y. Liu, M. Li, F. Yang, L. Li, J. Colloid Interface Sci. 301 (2) (2006) 503e510. [16] L. Chen, Y. Shang, H. Liu, Y. Hu, Mater. Des. 31 (4) (2010) 1661e1665. [17] L. Vaisman, H.D. Wanger, G. Marom, Adv. Colloid Interface Sci. 37 (46) (2006) 128e130. [18] C.G. Salzmann, B.T.T. Chu, G. Tobias, S.A. Llewellyn, M.L.H. Green, Carbon 45 (2007) 907e912. [19] X.-H. Yang, Q.-S. Wu, L. Lia, Y.-P. Ding, G.-X. Zhang, Col. Surf. A: Physicochem. Eng. Asp. 264 (2005) 172e178. [20] Y.S. Jung, B. Yoo, M.K. Lim, S.Y. Lee, K.J. Kim, Electrochim. Acta 54 (2009) 6286e6291. [21] K.L. Chopra, R.C. Kainthla, D.K. Pandya, A.P. Thakoor, Phys. Thin Films 12 (1982) 167. [22] G.Z. Shen, C. Lee, J. Growth Des. 5 (2005) 1085.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Triton-X mediated interconnected nanowalls network of cadmium sulfide thin films via chemical bath deposition and their photoelectrochemical performance S.A. Vanalakar a, c, S.S. Mali b, E.A. Jo a, J.Y. Kim c, J.H. Kim a, **, P.S. Patil b, * a b c
Department of Materials Science and Engineering, Chonnam National University, Gwangju 500 757, South Korea Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, M.S. 416 204, India Department of Electronics and Computer Engineering, Chonnam National University, Gwangju 500 757, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2013 Received in revised form 13 July 2014 Accepted 15 July 2014 Available online 23 July 2014
Thin films of cadmium sulfide (CdS) have been wet chemically deposited onto fluorine-doped tin oxide (FTO) coated conducting glass substrates by using non-ionic surfactant; Triton-X 100. An aqueous solution contains cadmium sulphate as a cadmium and thiourea as sulphur precursor. Ammonia used as a complexing agent. The results of measurements of the x-ray diffraction, Raman spectroscopy, optical spectroscopy, energy dispersive spectroscopy, scanning electron microscopy, Brunauer Emmett Teller (BET) surface areas and atomic force microscopy were used for the characterization of the films. These results revealed that the films are polycrystalline, consisting of CdS cubic phase. The films show a direct band gap with energy 2.39 eV. The films show interconnected nanowalls like morphology with welldefined surface area. Finally, the photoelectrochemical (PEC) performance of Triton-X mediated CdS thin film samples were studied. The sample shows photoelectrochemical (PEC) performance with maximum short circuit current density (Jsc) 1.71 mA/cm2 for larger area (1 cm2) solar cells. © 2014 Elsevier Masson SAS. All rights reserved.
Keywords: Cadmium sulfide Surfactant Interconnected nanowalls morphology PEC performance
1. Introduction Nanostructured thin films of cadmium sulfide (CdS) have been intensively studied due to their applications in optoelectronic devices, photoelectrochemical solar cells, semiconductor sensitized solar cells, the window layer in a photovoltaic cell, and transparent conducting semiconductors [1e5]. However, the optoelectronic properties of CdS nanomaterials are strongly influenced by their morphologies and structures. Therefore, it is very important to explore the simple method to achieve the surface morphology by controlled synthesis. Surface morphology is a vital aspect of the science of nanomaterials and any research carried out in this direction will be of great interest, especially due to the several application possibilities. The synthesis of varieties of surface morphologies from ordering structures to complex functional architectures, offer great
* Corresponding author. Tel.: þ82 62 530 1709; fax: þ82 62 530 1699. ** Corresponding author. Tel.: þ91 231 2609230; fax: þ91 231 2691533. E-mail addresses: [email protected] (J.H. Kim), [email protected] (P.S. Patil). http://dx.doi.org/10.1016/j.solidstatesciences.2014.07.006 1293-2558/© 2014 Elsevier Masson SAS. All rights reserved.
opportunities to explore their novel properties for fabrication of various nano devices. Recently, research, development across the globe concentrating on invention of various nanostructures. Chemical bath deposition methods have proved to be effective and versatile and have been employed widely to synthesize a variety of nanomaterials. The amphiphilic nature of organic surfactants helps in modifying the surface morphology of the deposit owing to their concentration-dependent specific activity during chemical bath deposition. The specific activity of the surfactants is generally understood in terms of adsorption during deposition and depends on the critical micelle concentration of the surfactant molecules which forms the bilayers or multilayers [6]. Surfactants or micelles as the regulating structural agents or templates are readily employed in the fabrication of low dimensional nanostructures [7]. Surfactants influence on the complex of chemical properties of substrate in the water solution owing to the solubilization. Effect of organic surfactants on an oxide including ZnO [8], NiO [9] WO3 [10], SnO2 [11], TiO2 [12] as well as on metal chalcogenides like CdS [13,14], PbS [15], CuS [16] have been investigated extensively during the past decade. Surfactants have been widely used to promote stable dispersion of solids in different media. The distinct structural feature of the
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surfactant originates from its duality; the hydrophilic head-polar group and the hydrophobic tail group that usually consist of fewer hydrocarbon chains. Two important features that characterize surfactants are; the absorption at the interface and self-accumulation into micelle structures [17]. There are different types of surfactants mainly categories in anionic, cationic and non-ionic [18]. In view of this, a systematic study has been undertaken in the synthesis and characterization of interconnected nanowalls of cadmium sulfide using non-ionic surfactants; Triton X-100 (Octylphenol Ethoxylate). Triton X-100 is an excellent detergent, wetting agent and gives an effective performance across a broad temperature range. It acts as an emulsifier for oil-in-water system. Triton X-100 is soluble in water and it is chemically stable in most acidic and alkaline solutions. It acts as metal corrosion resistant as well. Recently, XeH Yang et al. synthesize CdS nanotubes using Triton X-l00 [19]. Jung et al. studied effect of Triton X-100 in water-added electrolytes on the performance of dye-sensitized solar cells [20]. In the present paper, attempts were made to engineer the morphology of the CdS thin films by using non-ionic surfactant; Triton X-100. Effects of Triton-X 100 on the growth of CdS have been studied by conducting experiments with and without surfactant. Thin films of CdS with interconnected nanowalls were prepared at the 90 C by using the CBD method. Further, the CdS thin films synthesized with and without Triton-X were characterized for their structural, morphological, and optical studies through the range of techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical absorption spectroscopy. Finally, the photoelectrochemical (PEC) performance, such as J-V characteristics in dark and under illumination, the ideality factor of prepared films were studied. 2. Experimental details 2.1. Materials and preparation of surfactant mediated CdS thin film All chemicals except Triton X 100 were purchased from s.d. finechemicals and used without any further purification. Triton X-100 was obtained from Aldrich. Soda lime glass (SLG) and fluorinedoped SnO2 conducting (FTO, 10Usq1) glass were used as substrates. The size of each substrate is 1 cm2. Cleaning of the substrates for thin film depositions is the most important factor. Cleanness is the process of breaking the bonds between substrates and contaminants without damaging the substrates. The substrates were first cleaned with a neutral cleaner and washed with double distilled water. Then, the substrate was cleaned ultrasonically and sequentially in acetone, ethanol and double distilled water for 10 min, and dried by blowing dry nitrogen gas. The cadmium sulfate (CdSO4$H2O) was used as cadmium (Cd) source and thiourea (H2N$CS$NH2) as a sulphur (S) source. Triton-X 100 and liquor ammonia (NH3) were used as organic surfactant and complexing agent respectively. The preparative parameters, including precursor concentration, deposition time and temperature, were varied to yield good quality CdS thin films. The CdS films have been synthesized using two different wet chemical routes: (i) without Triton-X 100 and (ii) with Triton-X 100. Finally, a standard recipe was chosen as follows: The matrix solution was prepared by adding an aqueous solution (1 weight %) of organic surfactants (Triton-X 100) to 1.2 M CdSO4. Aqueous ammonia (NH4OH) was added to maintain 11 pH of the solution. The initial turbid solution turns into a transparent, by adding excess ammonia. Then, 1 M thiourea was added into the above solution with constant stirring. The CdS films were deposited by vertically dipping the soda lime glass into the above solution at the 90 C for 10 min. The deposited films were rinsed in distilled water and dried at room temperature overnight. The color of deposited CdS thin films are pale yellow.
2.2. Characterizations The structural properties of the CdS thin films were studied using an X-ray diffractometer (Philips, PW 3710, Almelo, Holland) operated at 25 kV, 20 mA with CuKa radiation (1.5407 Å). FT-Raman spectra of the films were recorded in the spectral range between 250 and 1000 cm1 using FT-Raman spectrometer (Bruker MultiRAM, Germany) that employs Nd:YAG laser source with an excitation wavelength 1064 nm and resolution 4 cm1. Optical absorbance was measured using a UVevis spectrophotometer (UV1800, Shimadzu, Japan). The surface morphology of the films was examined by scanning electron microscope (SEM) (Model JEOLJSM-6360, Japan), operated at 20 kV. Field emission SEM (JSM6701F model) was employed to further examine the CdS morphology. The energy dispersive spectroscopic (EDS) analysis was investigated by FESEM coupled with an energy dispersive spectroscopy unit. The surface morphology and roughness of the films were observed using atomic force microscopy (AFM, Digital Instrument, nanoscope III) operated at room temperature. The thickness of the resulting CdS films was measured using surface profiler (Ambios XP-1). The BET surface area of CdS samples was determined by Micrometrics ASAP 2020 analyzer using nitrogen adsorption and desorption isotherms at 77 K. For the PEC characterization of the with and without Triton-X mediated CdS thin film samples, the measurements were performed in an electrolyte of 1 M polysulfide (Na2SeNaOHeS) in a two-electrode arrangement of following configuration: Glass/FTO/CdS/Na2SeNaOHeS/G In the above cell configuration, CdS thin film deposited on FTO acts as a working electrode (active area ~1.0 cm2), and G is graphite, which acts as a counter electrode. The J-V characteristics were measured using Semiconductor Characterization System SCS-4200 Keithley, Germany uses two electrode configurations in the dark and under illumination at 30 mW/cm2.
3. Results and discussion Chemical bath deposition (CBD) is a method in which controlled release of metal ions and chalcogenide ions take place that results in having control over grain size of thin films. The precipitation of metal chalcogenides in CBD occurs only when the ionic product exceeds the solubility product of metal chalcogenides [21]. Combinations of ions form nuclei on the substrate as well as in the solution results in precipitation. The film growth takes place via ion-by-ion condensation of the materials or by adsorption of colloidal particles from the solution onto the substrate. The complexing agents like NH3 help to control the reaction rate. The TritonX 100 approach to synthesis CdS by CBD process is closely related to understand the growth mechanism. The controllable growth of CdS interconnected nanowalls in the composite film was probably due to two factors. Firstly, Cd2þ and S2 were gradually released in the Triton X matrix-colloid. Triton X-100 molecule contains a plane benzene ring, which connects to both the hydrophobic and
Fig. 1. The structure of the benzene ring, which connects to both the hydrophobic and hydrophilic group of Triton X-100.
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hydrophilic group at the meta position (Fig. 1). It was contingent that, the Triton X-100 cumulates in between the CdS crystallites during CBD crystallization and on washing the material; the improved structures of CdS were formed. CdSO4 at 11 pH dissociates as:
CdSO4 /Cd2þ þ SO2 4
(1)
Addition of thiourea leads to the release of sulfur ions:
CSðNH2 Þ2 þ 2OH /S2 þ CH2 N2 þ 2H2 O
(2)
CdS thin film is formed by virtue of Eqs. (1) and (2) reactions:
Cd2þ þ S2 /CdS
(3)
Secondly, the colloid cumulates turned into clear interconnected nanowalls shaped possibly due to steric hindrance provided by Triton X 100 and ammonia molecules. X-ray diffraction patterns were obtained for structural characterization. Fig. 2 shows the XRD pattern of with and without TritonX 100 mediated CdS interconnected nanowalls network on the soda lime glass substrate. The films deposited did not show peaks related to elemental cadmium or sulfur or carbon. CdS peaks in both the XRD pattern are (111), (200), (220), (311), (222), (400) and (331) appears at 2q ¼ 26.45, 31.71, 44.13, 51.87, 54.97, 62.67 and 70.23 respectively. The comparison of the observed XRD pattern with the standard JCPDS data (80-0019) confirms the formation of CdS phase with a cubic crystal structure. The lattice parameter ‘a’ is calculated using the following Eq. (4).
1 h2 þ k2 þ l2 ¼ 2 d a2
(4)
The mean value of a ¼ 5.810 Å is in good agreement with the reported value a ¼ 5.811 Å. Further, using the breadth of (111) peak, the average crystallite size is estimated using Scherrer's formula given below Eq. (5)
D¼
kl b cos q
(5)
where l ¼ 1.5406Å, k is the dimensionless constant (0.95), b is the corrected broadening of the diffraction line measured at half of its
Fig. 2. X ray diffraction patterns of with and without Triton-X 100 mediated CdS thin films.
43
maximum intensity (taken in radians by multiplying a factor of p/ 360), D the diameter of crystallite and q is diffraction angle. The calculated crystallite size of Triton X mediated CdS is found to be 20 nm for (111) plane. The broadened peaks indicate the nanocrystalline nature of the films. The phase identification of the sample has been carried out using FT-Raman spectroscopy. The Raman spectrum of the Triton-X 100 mediated CdS film is shown in Fig. 3. The Raman spectrum of the CdS film exhibits a well-resolved band at 301 cm1; corresponding to the first order scattering of the longitudinal optical (LO) phonon mode and second-order band around 600 cm1. CdS can have both hexagonal wurtzite and cubic zinc blended structures, and it is reported that for both structures, the zone-center longitudinal-optical A1 (LO) phonon frequency is nearly 305 cm1 [22]. The full width half maxima (FWHM) of the 1 LO peak is 16.13 cm1. Though, this large width indicates poor crystallinity (lack of long-range order) in the films, but the well-defined peak indicates the crystalline nature of the material. Hence, the large FWHM in the present case can be attributed to a polycrystalline effect in the as-deposited film. Due to the nanocrystalline thin films, UVeVis spectroscopy has become an effective tool in determining the size and optical properties. Fig. 4 shows the room temperature optical absorption spectrum of the with and without triton-X mediated CdS thin film recorded in the range of 450e750 nm without taking into account scattering and reflection losses. It is clearly observed that the onset of the optical absorption at about 520 nm, which is the absorption edge of the CdS films. At a lower wavelength than 500 nm the optical absorption saturates at a value of 4. Inset of Fig. 4 shows a plot of (ahʋ)2 against the photon energy (hʋ). The band gap of Triton-X mediated CdS films is found to be 2.39 eV and 2.42 eV for without Triton-X CdS films by extrapolating the straight line of the square of absorption coefficient to the intercept of the X-axis of photon energy. The present UVevisible spectrum reveals that CdS interconnected nanowalls network have high absorbance of light in the visible region, indicating applicability as a captivating material in solar cell applications. Moreover, a significant shift in the spectral photoresponse is observed for Triton-X mediated CdS sample. The synthesized Triton-X mediated CdS thin films are examined by SEM (Fig. 5). The low magnification SEM image Fig. 5(a) reveals the film surface looks porous and the formation of CdS
Fig. 3. The Raman spectrum of Triton-X 100 mediated CdS thin film.
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Fig. 4. Optical absorption spectra of with and without Triton-X 100 mediated CdS thin film. The inset figures are the plots of (ahn)2 against hn for CdS thin films.
interconnected nanowalls over the entire substrate. Also, there is not any overgrowth on the substrate without any void, pinholes or cracks and they cover the substrate well. The SEM image Fig. 5(b) shows CdS nanowalls connected with each other forming
nanoconduits. Such walls having ~70 nm thickness are seen in the SEM and FESEM image in Fig. 5(c). The morphologies and structures of the as-synthesized with and without Triton-X mediated CdS thin films are also examined by FE-SEM. The representative FE-SEM image shows [Fig. 6(a)] the as synthesized samples without Triton-X. From Fig. 6(a) it clear that, as prepared sample without Triton-X have no regular shape, the nanowalls are covered with nanonet like structure. With Triton X, the morphologies of the samples are gradually improved, the nanowalls become regular without overlapping nanonet like structure [Fig. 6(b)]. Moreover, Fig. 6(b) confirms interconnected nanowalls like structure of CdS thin films. The surface area is an important physical property that determines the quality and utility of functional materials. The Brunauer, Emmett and Teller (BET) technique is the most common method for determining the surface area of porous materials. The BET specific surface area of without and with Triton-X mediated CdS thin films are 14.28 m2/g and 23.11 m2/g respectively (see inset of Fig. 6). The Triton-X mediated CdS thin film sample provides high surface area compared to without Triton-X CdS thin films, for enhanced surface activities like efficient permeability of electrolytes into the inner structure, and light absorption. It also scatters the light multiply into photoanode and hence improves the effective absorption. The light absorption path length of photons can be increased as it is trapped in the nanochannels. These mechanisms boost the PEC performance of CdS electrode. Atomic force microscopy (AFM) was employed to characterize the 2-D and 3-D surface morphology of the CdS thin films. Fig. 5(d) show the AFM image of
Fig. 5. (a) Low magnification SEM image of Triton-X 100 mediated CdS thin films sample (b) SEM image show the formation of interconnected nanowall network of Triton-X 100 mediated CdS over the substrate. The inset shows the EDS spectrum for Triton-X 100 mediated CdS thin films. (c)High magnification SEM image of Triton-X 100 mediated CdS with nanoconduits formed by numerous nanowalls for sample. The inset FESEM image shows the thickness of nanowall. (d) AFM image of Triton-X 100 mediated CdS with nanoconduits formed by numerous nanowalls.
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Fig. 6. (a) FE-SEM image of CdS without Trinton-X thin film sample. Inset shows the Nitrogen adsorptionedesorption isotherms of CdS without Trinton-X thin film. (b) FE-SEM image of CdS with Trinton-X thin film sample. Inset shows the Nitrogen adsorptionedesorption isotherms of CdS with Trinton-X thin film.
Triton-X mediated CdS thin film sample. The AFM image clearly shows that the nanowall network is interconnected. An energy dispersive spectroscopic (EDS) analysis of the film (inset Fig. 5(b)) confirms the presence of S and Cd, and the molar ratio of Cd is slightly greater than S, which matches with the stoichiometry of CdS. Fig. 7(a and b) shows JeV characteristics of without and with Triton-X 100 CdS thin film samples. The JeV characteristic in the dark resembles diode-like characteristics of the PEC cells fabricated with all the samples. Under illumination, shifting of the JeV curve in the fourth quadrant of the graph suggests that the electrons are the majority carriers, confirming the n-type conductivity of CdS thin films. CdS without mediated Trinton-X 100 cell exhibits the power conversion efficiency (h) of 0.28% with (JSC) 0.81 mA/cm2, open-circuit voltage (VOC) ¼ 360 mV and fill factor (FF) ¼ 0.30. Where as, CdS with Trinton-X 100 cell exhibits the power conversion efficiency (h) of 0.87% with (JSC) ¼ 1.71 mA/cm2, open-circuit voltage (VOC) ¼ 461 mV and fill factor (FF) ¼ 0.33. The ideality factor is a fitting parameter that describes how closely the diode's behavior matches the behavior predicted by theory. The ideality factor is determined under forward bias and is normally found to be in between 1 and 2 depending upon the relation between diffusion current and recombination current. The ideality factor becomes 1 when the p-n junction of the diode is an infinite plane and no recombination occurs within the spacecharge region. When recombination current is more than diffusion current then ideality factor becomes 2. The ideality factor 'nd' of prepared CdS films is determined from following diode equation as.
I ¼ Io eqV=nd kT 1
(6)
where, Io is the reverse saturation current, V is forward bias voltage, k is Boltzmann's constant, T is ambient temperature in Kelvin and nd is an ideality factor. The ideality factor was found to be 2.06 and 2.16 (inset Fig. 7) for the sample under the light illumination for CdS without and with Trinton-X 100. The ideality factor for the samples are found to be greater than 1 indicates recombination current is more. The higher value of nd is indicative of the series resistance effect, surface states and the charge carrier recombination at the semiconductoreelectrolyte interface. These factors reduce the ideality of devices. 4. Conclusion In summary, CdS composed of uniform interconnected nanowalls network have been successfully prepared via a non-ionic
Fig. 7. The JeV characteristics of interconnected nanowall network of CdS thin film sample. (a): The JeV characteristics of CdS without Trinton-X thin film and; (b): The JeV characteristics of CdS with Trinton-X thin film. Inset figures show respective ideality factor graphs of CdS thin films.
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surfactant-assisted wet chemical reaction system at low temperature and ambient atmosphere. This method is simple and suitable for the preparation of large surface area thin film in a one step process using Triton-X 100. The as deposited CdS film showed a cubical crystal structure. The presence of characteristic bonds of CdS is observed from Raman spectroscopy. The well covered porous structure with the interconnected nanowalls network morphology leading to high surface area is observed by SEM studies. The PEC performance of Trinton-X 100 mediated CdS is higher than pure CdS thin films. This structure is a good prospective way for PEC application. Acknowledgment This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No.: 20124010203180). One of the authors SAV is grateful to DST-SERB, New Delhi for financial support (SR/ FTP/PS-083/2012). References [1] K. Senthil, D. Mangalaraj, S.K. Narayandass, Appl. Surf. Sci. 476 (2001) 169e170. [2] J.J. Loferski, Material for solar energy conversion, in: G.G. Libowitz (Ed.), Materials Science Series, Academic Press, New York, 1979 (Chapter 4).
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