Suvarta D. Kharadea,
Nita B. Pawara,
Kishorkumar V. Khota,
Pallavi B. Patila,
Sawanta S. Malibc,
Chang K. Hongc,
Pramod S. Patilb and
Popatrao N. Bhosale*a
aMaterials Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur-416004, India. E-mail: p_n_bhosale@rediffmail.com; Fax: +91-231-2691533; Tel: +91-231-2692333
bThin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416004, India
cPolymer Energy Materials Laboratory, Department of Advanced Chemical Engineering, Chonnam National University, Gwangju, South Korea 500-757
First published on 18th February 2016
Low-dimensional nanostructures with reduced grain boundaries show superior charge transportation in a photoelectrochemical cell. Therefore, nanostructures of MoBiCuSe4 thin films deposited using different surfactants are expected to be favorable for providing a direct pathway for smooth transport of photogenerated charge carriers across a reduced number of grain boundaries within the photoelectrode. In the present investigation, we have studied the effect of different surfactants, such as polyethylene glycol (PEG), sodium dodecylsulfate (SDS) and trioctylphosphine oxide (TOPO), on the opto-structural, morphological and photoelectrochemical (PEC) properties of MoBiCuSe4 thin films. We have demonstrated a soft chemical route that facilitates the formation of a compact, homogeneous deposition with a large effective (photoactive) surface area, which could be suitable for PEC cells. The MoBiCuSe4 thin films have been deposited using the arrested precipitation technique (APT) and their formation confirmed by energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The nanocrystalline nature of the MoBiCuSe4 thin films and the mixed rhombohedral crystal structure with reduced number of grain boundaries were confirmed by the X-ray diffraction (XRD) pattern. The direct allowed type of transition in the material, with an average absorption coefficient above 104 cm−1, makes it suitable for PEC applications. The maximum light conversion efficiency achieved for MoBiCuSe4 thin films deposited with surfactant is 0.18%. PEC analysis verifies that the synthesized nanostructures of the surfactant-assisted MoBiCuSe4 photoelectrode material are suitable for PEC cells.
Earlier, we have reported the deposition of novel mixed metal chalcogenide thin films of MoBi2−xCuxSe4 by a facile chemical method for photoelectrochemical (PEC) applications and we have reported the maximum light conversion efficiency of 0.074% for MoBiCuSe4 thin films deposited without surfactant.20 Herein, we report the deposition of MoBiCuSe4 thin films by a simple surfactant-mediated solution route. The surface directing agents, such as PEG, SDS and TOPO, were used to obtain different nanostructures of MoBiCuSe4. The films were obtained just by keeping the deposition baths at room temperature for several hours without stirring. The structural, morphological, optical and PEC properties of the MoBiCuSe4 thin films were studied as a function of the different surfactants. We have demonstrated a soft chemical route that facilitated the formation of a compact, defect-free deposit with a large effective (photoactive) surface area to improve PEC cell performance.
The optimized experimental conditions for deposition of surfactant-mediated MoBiCuSe4 thin films were kept fixed as per our previous report.20 Briefly, a bath containing 1 ml 0.05 M Mo–TEA complex solution, 5 ml 0.05 M Bi–TEA complex solution and 10 ml 0.05 M Cu–TEA complex solution was taken, as a source of Mo4+, Bi3+ and Cu+ ions, and the pH of the solution was adjusted to 10 by adding aqueous ammonia solution. Subsequently, 10 ml 0.25 M Na2SeSO3 solution was added with continuous stirring as a source of Se2− ions. The total volume of the bath was made up to 40 ml by adding double-distilled water and designated as the 1st bath, without surfactant. Preparation of a 0.25 M Na2SeSO3 solution is the same as described in our earlier report.20 Furthermore, in order to deposit MoBiCuSe4 thin films with different nanostructures and tuned surface morphologies, we have used PEG, SDS and TOPO as surfactants. In a typical experiment, 1 ml 0.05% solutions of PEG, SDS and TOPO (in pure ethanol) were added separately into three (40 ml) baths prepared same as that of the bath 1st and they were designated as bath 2nd, 3rd and 4th, respectively. For deposition, well-cleaned FTO-coated glass substrates were immersed in the four baths and kept at room temperature (300 K) for 8 h. After the desired deposition time, the films were rinsed with double-distilled water and dried at room temperature. In order to remove surfactants and to improve the crystallinity, the as-deposited samples were annealed at 130 °C for 3 h. The MoBiCuSe4 thin film deposited without surfactant is denoted by MBCS and those deposited using different surfactant media, namely PEG, SDS and TOPO, are denoted by MBCS:PEG, MBCS:SDS and MBCS:TOPO, respectively, and used for further characterization.
The surface morphology and elemental analysis of all the samples were examined using a scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) analyzer (JEOL-JSM-6360A). Inferences about the oxidation states of molybdenum, bismuth, copper and selenium elements in the chemically deposited MoBiCuSe4 thin films were analyzed by X-ray photoelectron spectroscopy (XPS) (VG Multilab 2000-Thermo Scientific, USA, K-α) with a multi-channel detector, which can endure high photonic energies from 0.1 to 3.0 keV. The X-ray diffraction (XRD) patterns of the MoBiCuSe4 thin films were recorded using an X-ray diffractometer (Bruker AXS, D8 Model) using Cu Kα (λ = 1.5418 Å) radiation. A UV-vis-NIR spectrophotometer (model: Shimadzu UV-1800) was used to measure the optical absorption of the thin films in the wavelength range 400–1100 nm. PEC performance was recorded using a semiconductor characterization system (Keithely, SCS-4200 Germany) with a two electrode configuration in the dark and under illumination (30 mW cm−2). A glass-FTO/MoBiCuSe4/I−/I3−/G cell configuration was used to record J–V plots. The MoBiCuSe4 film and graphite rod (G) were employed as the working and counter electrodes, respectively. A 0.5 M aqueous solution of iodide–polyiodide (I−–I3−) was used as the redox electrolyte. The charge recombination properties and electron transport resistance of the MoBiCuSe4 thin films were investigated by electrochemical impedance measurements (model: AUTOLAB PGSTAT100 potentiostat) in the frequency range 0.1–105 Hz.
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Fig. 1 Chemical structures of (i) polyethylene glycol (PEG), (ii) sodium dodecyl sulfate (SDS) and (iii) trioctylphosphine oxide (TOPO). |
PEG is a polymer compound of ethylene oxide with a molecular weight of 20000 g mol−1. SDS is an organosulphate compound consisting of a 12-carbon tail attached to the sulphate functional group. TOPO is an organophosphorus compound with uncharged alkyl chains. It also has polarity resulting from the dipolar phosphorus–oxygen (P–O) bond. The chemical reaction kinetics for the formation of MoBiCuSe4 thin films has been briefly discussed in our previous report.20 The possible growth mechanism of surfactant-mediated MoBiCuSe4 thin film nanostructures using different surfactants is shown in Fig. 2.
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Fig. 2 Possible growth mechanism of surfactant-mediated MoBiCuSe4 thin film nanostructures using PEG, SDS and TOPO. |
PEG does not ionize in water and has good stability, which leads to a better tuning of surface morphology. The non-ionic surfactant PEG controls the growth rate of different faces by joining selectively to certain crystal faces.21 Initially, during the chemical growth, seeds are formed which act as nucleation centers for the growth of nanoparticles. The hydrophilic groups in the polymeric chain of PEG may get adsorbed on the surface of the nanoparticles in order to reduce their surface energy, preventing quick aggregation of the particles.23 As a result, anisotropic growth occurred, leading to nanostructured film formation via oriented attachment.8 PEG plays an important role as a template in optimizing the growth rate and orientation in the formation of crystallites.24 The different low-dimensional morphologies evident from the morphological analysis indicate that the involvement of surface active agents in the synthesis process is a simple and effective way to control the nucleation and growth process of MoBiCuSe4 nanostructures.
It is well known that SDS acts as a soft template.25–27 The surfactant molecules self-assemble into aggregates in aqueous solution above a critical concentration, called the micelle concentration, which is the concentration above which surfactant molecules arrange themselves in a spherical form in aqueous solution. In the process of chemical deposition, a group of freestanding crystallites with irregular sizes in the non-equilibrium state were formed, which ultimately dissolve in the solution followed by nucleation growth to form nanoparticles. Under prolonged reaction conditions, nanoparticles self-assemble and the process of dissolution–recrystallization takes place, leading to the formation of different nanostructures depending on the reaction conditions.28 The nanostructures formed through this approach grow mainly by the Ostwald ripening mechanism; nanostructures of larger size grow at the cost of smaller ones due to higher surface free energy.29 Particle formation is a complex process, involving nucleation, growth, coagulation and flocculation, which may be influenced significantly by SDS additive assemblies. Therefore, SDS plays an important role in the preparation of chalcogenide nanoclusters.
The cationic TOPO has polarity resulting from the dipolar P–O bond. Also, the structure of TOPO is responsible for its excellent complexing properties due to the presence of the phosphine group (PO). It acts as a surface active amphiphilic molecule containing a hydrophilic head and hydrophobic tail. It also contains a lone pair of electrons, which is used for the formation of complexes with metal ions. Thus, TOPO plays a dual role, as a complexing agent as well as a surfactant in the reaction bath. Due to this dual role performed by TOPO, the rate of reaction and deposition may lower, which results in ordered and nanostructured growth of thin films.30,31 Notable differences in the morphology of the MoBiCuSe4 were observed due to the different surfactants; hence, it can be predicted that the PEC performance could be enhanced significantly.
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Fig. 3 SEM images of MoBiCuSe4 thin films deposited without and with surfactant: (a and b) MBCS, (c and d) MBCS-PEG, (e and f) MBCS:SDS, and (g and h) MBCS:TOPO. |
From the XPS analysis data, the peaks of the XPS binding energies were deconvoluted with Gaussian peak shapes using the Origin software package. The typical XPS survey spectrum of the MoBiCuSe4 thin film (MBCS) shown in Fig. 4a reveals that all the peaks are assigned to C, O, Mo, Bi, Cu and Se elements. The binding energy of the C 1s transition was used as a reference at 284.60 eV to standardize the binding energy of the other elements. It should be mentioned that the presence of trace amounts of C and O on the surface of the thin films is infrequent, and could be due to the adsorption of CO2, O2 or H2O on the film surface.35,36 An overlapping of peaks (Bi 4f and Se 3p) is observed, but the intensities of the Bi 4f peaks are higher than that of the Se 3p transitions, which indicated the deficiency of selenium in the film. Moreover, it is known that Se atoms escape easily during the ablation process, thus resulting in Bi-rich MoBiCuSe4 thin films. The high resolution core spectrum for Mo 3d is shown in Fig. 4b. The two intense peaks of Mo 3d3/2 and Mo 3d5/2 at 230.82 and 228.23 eV binding energies indicate the +4 oxidation state of the Mo ions. The high resolution core spectrum for Bi 4f is presented in Fig. 4c and shows that peaks appear at 158.00 eV (Bi 4f7/2) and 163.27 eV (Bi 4f5/2) binding energies with a splitting of 5.27 eV, indicating the +3 oxidation state of the Bi ions, which is in good agreement with previous reports.36,37 The core level spectrum of Cu 2p is shown in Fig. 4d; the two narrow peaks at 931.34 eV (Cu 2p3/2) and 951.26 eV (Cu 2p1/2) binding energies with a peak separation of 19.92 eV indicate the +1 oxidation state of the copper ions. Moreover, the characteristic peak of Cu2+ (942 eV) cannot be observed, which implies that only the valence state Cu(I) exists in the film.38–41 The Se 3d spectrum (Fig. 4e) can be fitted perfectly using a single peak for 3d3/2 and 3d5/2 and a binding energy of about 53.31 eV. This means that the Se exists in the deposited films as the MoBiCuSe4 compound. No evidence of the contribution of elemental Se atoms at a binding energy of about 54.1 eV was observed.41 A distinctive peak at around 58 eV binding energy appeared in the core level spectrum of Se 3d due to the exposure of the sample to the air.42
The average crystallite size (D) was calculated from the broadening of the diffraction peak according to the well-known Scherrer equation;44
![]() | (1a) |
![]() | (1b) |
This value is appropriate for spherical crystals with cubic symmetry, where the peak width is defined using the FWHM. If one can use an integral breadth then the constant would be 0.89. Here, the peak width is defined using the FWHM, hence the value 0.94 is taken for K. In general, the constant K varies between 0.62 and 2.08. ‘β’ is the corrected broadening of the diffraction peak measured at half of its maximum (FWHM) intensity (taken in radians by multiplying a factor of π/180), ‘λ’ is the wavelength of X-rays used (1.5406 Å) and ‘θ’ is the diffraction angle. The crystallite size calculated for all MoBiCuSe4 thin film samples deposited without and with surfactant is summarized in Table 1. As can be seen from the diffraction pattern, improved crystallinity is observed for samples MBCS:SDS, MBCS:PEG and MBCS:TOPO, which can be attributed to the use of different surfactant media, which leads to ordered growth of crystallites and the subsequent Ostwald ripening process.
Sample code | Crystallite size (D) nm | Dislocation density (δ) × 10−3 lines per nm2 | Microstrain (ε) × 10−3 |
---|---|---|---|
MBCS | 89 | 0.13 | 1.84 |
MBCS:PEG | 98 | 0.10 | 1.76 |
MBCS:SDS | 91 | 0.12 | 1.81 |
MBCS:TOPO | 95 | 0.11 | 1.80 |
From the calculated values of crystallite size, the dislocation density (δ) (lines per m2) and microstrain (ε) of all the samples were determined by using eqn (2) and (3) and listed in Table 1.
![]() | (2) |
![]() | (3) |
αhν = A(hν − Eg)n | (4) |
![]() | (5) |
ν is the frequency of the incident beam (ω = 2πν), A is a constant, Eg is the optical band gap and n is an exponent, which can be assumed to have values of 1/2, 3/2, 2 and 3 depending on the nature of the electronic transition responsible for the absorption: n = 1/2 for allowed direct transition, n = 3/2 for forbidden direct transition, n = 2 for allowed indirect transition and n = 3 for forbidden indirect transition.47 The optical transitions in transition metal chalcogenides are found to be the direct allowed type.48,49 Hence, plots of (αhν)2 vs. hν shown in the insets of Fig. 6a and b are drown and the optical band gap energy has been estimated by extrapolating the straight line portion to cut the energy axis. A prominent feature that can be observed from the value of the absorption coefficient is found in the range of 104 cm−1, supporting the direct band gap nature of the semiconductor material,50,51 which is in good agreement with an earlier report.20
The MoBiCuSe4 thin film deposited without surfactant (MBCS) has a band gap energy of 1.26 eV, while the MBCS:PEG, MBCS:SDS and MBCS:TOPO samples exhibit energies of 1.47, 1.42 and 1.37 eV, respectively. A slight difference in the absorption edge observed for all the samples results in a small difference in their band gap. The difference in the absorption edge occurred due to strengthening of the quantum confinement in these nanostructures, where the Fermi level becomes discrete by eliminating its consecutive nature.50,52,53 The optical absorption study results reveal that the morphologically different nanostructures formed using different surfactants have suitable band gaps for solar cell applications.52
The output parameters of the PEC solar cell, i.e. the fill factor (FF) and conversion efficiency (η%), are calculated from the relations (6) and (7) respectively,57
![]() | (6) |
![]() | (7) |
![]() | (8) |
![]() | (9) |
The parameters of the PEC cell performance, namely Jsc, Voc, Jmax, Vmax, FF, η, Rs and Rsh, when using different MoBiCuSe4 photoanodes illuminated under light radiation are depicted in Table 2.
Sample code | Jsc (mA cm−2) | Voc (mV) | Jmax (mA cm−2) | Vmax (mV) | Rs (Ω) | Rsh (Ω) | FF | η (%) |
---|---|---|---|---|---|---|---|---|
MBCS | 0.145 | 356 | 0.103 | 216 | 917 | 2454 | 0.44 | 0.074 |
MBCS:PEG | 0.282 | 420 | 0.216 | 147 | 308 | 6666 | 0.45 | 0.178 |
MBCS:SDS | 0.175 | 318 | 0.130 | 180 | 622 | 6876 | 0.42 | 0.078 |
MBCS:TOPO | 0.197 | 330 | 0.145 | 192 | 714 | 3425 | 0.43 | 0.093 |
The light conversion efficiency of PEC cells using a semiconductor photoelectrode is effectively inhibited by the factor of recombination of photogenerated electrons and holes within the thin film photoelectrode. Thus, a better charge transport path is advantageous to achieve efficient electron transfer to minimize the recombination rate. As a low-dimensional morphology with reduced grain boundaries shows superior charge transportation,58 the nanostructured MoBiCuSe4 thin films deposited using different surfactants are expected to be favorable to provide an effortless pathway for smooth transport of the photogenerated charge carriers across a reduced number of grain boundaries within the photoelectrode. On the other hand, the observed improvement in the photoconversion efficiency of the surfactant-mediated samples due to the oriented nanostructured morphology is still not as significant as expected. This consequence may be rationalized by taking into account that the morphology is not the sole factor influencing the photoelectronic transport properties and some other factors, such as the crystallinity of the primary nanocrystals and the connection between the neighboring nanocrystals, may also have significant effects. Therefore, the photoconversion efficiency in the thin films is further increased by optimizing all of these factors.
From Table 2 it is observed that the surfactant-mediated MoBiCuSe4 thin films (MBCS:PEG, MBCS:SDS and MBCS:TOPO) show better performance as compared to the MoBiCuSe4 thin film without surfactant (MBCS). As observed from the structural and morphological studies, sample MBCS has smaller crystallite size, higher dislocation density and a petal-like surface morphology which offer a larger surface area to the photoelectrode for light absorption, however it has a recombination problem due to the existence of considerable grain boundaries in the film. Therefore, the electron trapping at the surface and intergrain boundaries results in lower light conversion efficiency. Samples MBCS:PEG, MBCS:SDS and MBCS:TOPO have improved crystallite size and lower dislocation density, which leads to reduced grain boundaries. This significantly reduces the recombination losses of photogenerated charge carriers due to the decrease in the grain boundary resistance. Also, they have nanofibrous, hollow-sphere-like vertically aligned nano-platelets and a marigold flower-like surface morphology, respectively, which is advantageous in providing a larger effective surface area for light absorption as well as providing a direct pathway for electron transport through the compactly interconnected nanofibers, nano-platelets and flowers, much faster than with loosely connected nanopetals. Further, the current–voltage characteristics are largely dependent on Rs and Rsh. As compared to the MoBiCuSe4 thin film without surfactant, the surfactant-mediated MoBiCuSe4 thin films give lower Rs and higher Rsh values. A lower Rs means that higher current will flow through the device and high Rsh corresponds to fewer shorts or leaks in the device. The ideal cell would have Rs approaching zero and Rsh approaching infinity. Thus, the nanostructured MBCS:PEG, MBCS:SDS and MBCS:TOPO thin films deposited using PEG, SDS and TOPO surfactants showed enhanced photoconversion efficiency.
From the J–V measurements it is observed that a higher magnitude of Jsc (0.282 mA cm−2), Voc (420 mV) and light conversion efficiency, η (0.18%) were obtained for the MoBiCuSe4 thin film deposited using the PEG surfactant. This is a considerable enhancement compared to the MoBiCuSe4 thin film (MBCS, η = 0.07%) deposited without surfactant. As observed from the morphological studies, sample MBCS:PEG has a nanofibrous hollow-sphere-like structure and it is believed that the multiple reflections of light within the interior cavity of the hollow microspheres are responsible for the enhanced photoconversion efficiency of the core shell structures. Also, the fibrous nature of the microspheres is responsible for the better interconnection between crystallites, as well as the involvement of surfactant during the deposition giving thin films with an improved degree of crystallinity, which provides a direct pathway for smooth transport of photogenerated charge carriers across a reduced number of grain boundaries and is indirectly responsible for the enhancement in PEC conversion efficiency. Hence, it is concluded that the increased light conversion efficiency of surfactant-mediated MoBiCuSe4 thin films (MBCS:PEG, MBCS:SDS and MBCS:TOPO) may be due to the increased effective surface area for light absorption and lower density of grain boundaries in the electron transport.
Though the photoconversion efficiency of the MoBiCuSe4 thin films is still very low compared to other chalcogenide PEC cells,59 study and development of an efficient p-type MoBiCuSe4 thin film photoelectrode can greatly help in the improvement of solar radiation conversion efficiency by forming p–n heterojunction PEC solar cells, as they offer more protection against photocorrosion because the p-type semiconductors are cathodically protected under illumination. Efforts are in progress to design a p–n heterojunction solar cell by considering MoBiCuSe4 as a p-type material in contact with another suitable n-type material, which will completely cover the interfaces across the MoBiCuSe4 nanostructures.
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Fig. 8 The Nyquist plots of EIS of MoBiCuSe4 thin films deposited without (MBCS) and with surfactants (MBCS:PEG, MBCS:SDS and MBCS:TOPO). |
Generally, the semicircle, i.e. the intermediate frequency region, represents the charge transfer resistance (Rct) (resulting from the interfacial redox reaction resistance) connected in parallel with an electrical double layer capacitance (Cdl) at the electrolyte–electrode interface and the sloping line with a 45° angle, i.e. the low frequency region, signifies the line of the Warburg diffusion element (Zw) which corresponds to the diffusion of charge carriers into the bulk of the electrode material and charging of the film. The first arc in the high frequency region was attributed to impedance related to the series resistance (Rs) of the system (i.e. electrolyte/substrate resistance). The magnitude of Rs, Rct, Cdl and Zw were calculated by data fitting into a Randles circuit model, presented in the inset of Fig. 8, and the values are listed in Table 3.
Sample code | Rs (Ω) | Rct (Ω) | Cdl (μF) | Zw (mMho) |
---|---|---|---|---|
MBCS | 22.2 | 560 | 8.27 | 2.76 |
MBCS:PEG | 24.1 | 303 | 7.36 | 2.15 |
MBCS:SDS | 21.3 | 533 | 8.56 | 3.51 |
MBCS:TOPO | 23.6 | 355 | 6.67 | 1.59 |
From Table 3 it is observed that the surfactant-mediated MoBiCuSe4 thin films (MBCS:PEG, MBCS:SDS and MBCS:TOPO) show smaller values of Rct as compared to the MoBiCuSe4 thin film without surfactant (MBCS), which is attributed to morphological modifications. Further, it can be seen that the MBCS:PEG thin film exhibits a much smaller semicircle in the high frequency region and lower slope in the low frequency region, compared to those of the MBCS:SDS and MBCS:TOPO thin films. According to former reports,60–62 a smaller semicircle means a lower charge-transfer resistance. Hence, it can be concluded that the MBCS:PEG thin film has a high charge-transfer rate. This information supports the PEC results of the MoBiCuSe4 thin films.
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