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TiO2 nanocomposite by atmospheric microplasma electrochemistry – Its application as photoanode in dye-sensitized solar cell
Accepted Manuscript One-step synthesis of CuO/TiO2 nanocomposite by atmospheric microplasma electrochemistry – Its application as photoanode in dye-sensitized solar cell Zohreh Dehghani Mahmoudabadi, Esmaeil Eslami PII:
S0925-8388(19)31472-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.04.185
Reference:
JALCOM 50369
To appear in:
Journal of Alloys and Compounds
Received Date: 26 February 2019 Revised Date:
16 April 2019
Accepted Date: 19 April 2019
Please cite this article as: Z.D. Mahmoudabadi, E. Eslami, One-step synthesis of CuO/TiO2 nanocomposite by atmospheric microplasma electrochemistry – Its application as photoanode in dye-sensitized solar cell, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.04.185. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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One-Step Synthesis of CuO/TiO2 nanocomposite by atmospheric microplasma electrochemistry – its application as photoanode in Dye-Sensitized Solar Cell
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Zohreh Dehghani Mahmoudabadi1, Esmaeil Eslami Department of Physics, Iran University of Science and Technology, Narmak, Tehran, 16846- 13114, Iran
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Abstract
The necessity for improving performance of solar cell has been caused the development of
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methods for nanomaterial synthesis with tunable properties. Herein, CuO/TiO2 nanocomposite, for the first time, is prepared by microplasma-assisted electrochemical method in aqueous solution for using dye sensitized solar cells (DSSCs). The results of TEM and EDS indicate that the Cu nanoparticles dispersed in TiO2 lattice. Moreover, based on the current density–voltage (J–V), the power conversion efficiency, open-circuit voltage (Voc) and the short-circuit current density are 9.3%, 0.68 V and 18.8 mA cm-2 in CuO/TiO2, and 6.5%, 0.71 V and 12.94 mA cm-2
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in pure TiO2, respectively. These results reveal that the performance improvement could be due to the deduced band-gap energy, increasing life time of charge carriers and surface coverage of the sensitizing dye over CuO/TiO2 nanocomposite.
method.
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1. Introduction
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Keywords: CuO/TiO2 nanocomposite, dye sensitized solar cells, microplasma, electrochemical
In recent years, there have been several studies on possible alternatives to Dye-sensitized solar cells (DSSCs) due to their various excellent features such as low-cost, semi-transparency and high power conversion efficiency [1, 2]. DSSCs include a photoanode, a counter electrode, dye and an electrolyte. Among these elements, the semiconductors material used in photoanode directly determines the working process of 1
Corresponding author: E-mail address: [email protected] (z.dehghani)
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DSSCs because of photo-induced electron transport and organic dye adsorption [3-6]. Titanium dioxide, among different oxide materials available for making photoanode, is the attractive candidate with high photo catalytic activity and non-toxicity. However, the band gap measurement of TiO2 restricts its performance in solar cells [7]. There are a few light-excited
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dyes that could inject electrons into the conduction band of TiO2 due to inadequate electron injection driving force [8]. All of these materials are synthesized using various procedures to obtain the optimum states [9-14].
Today, TiO2 doped with some materials significantly improves the photocurrent and decreases
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the amount of electron recombination in solar cells [15-24]. These materials expand response ranges of TiO2 into the visible light range. Moreover, they play an important role in increase of
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specific surface area [8, 25-31]. To improve the efficiency of the solar cell, Cu as relatively cheap and abundant metal is one of the most promising materials among all [32, 33]. Although synthesis of the CuO/TiO2 nanocomposite has been reported in the many works, they all succeed to synthesize this combination with traditionally time - consuming steps [34-36]. For this reason, in addition to these methods, there are several researches which have been used to improve this problem, such as using a simple and fast method for nanocomposite synthesis [37, 38].
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Recently, the wide researches have been done on atmospheric pressure microplasma-liquid interactions in a wide range of applications such as synthesis of metal oxides and nanostructures [39-44]. By using this method, we have been able to synthesize nanocomposites consisting of metals and semiconductors, resulting in improved performance of the DSSC [38, 45]. In
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particular, the interaction between microplasma and solution suggests a strong technique for solution processing of nanocomposites due to the enriched chemical environment in the vicinity
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of the plasma/ liquid interface [46-51]. In our previous research, it is found that Ag/TiO2 nanocomposite is an appropriate combination to attain improvements in the efficiency of DSSCs. we indicated the synthesis of Ag/TiO2 nanocomposite by using microplasma-assisted electrochemical method and discussed the effect of Ag addition on the photoactivity of the solar cell. The Ag/TiO2 nanocomposite decreased the conduction band edge of TiO2 and expanded the response in the visible region, resulting in enhanced efficiency of DSSCs [38]. Herein, for the first time, CuO/TiO2 nanocomposite was synthesized using the microplasma electrochemical method for the application in DSSC. The properties of CuO/TiO2 nanocomposite have been characterized by scanning electron microscope (SEM), energy dispersive spectroscopy
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(EDS), and transmission electron microscopy (TEM), X-ray diffract meter (XRD). As a result, potential applications of the CuO/TiO2 nanocomposite have been discussed as a photoanode in the dye sensitized solar cell.
2.1. Preparation of CuO/TiO2 nanocomposite
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2. Materials & methods
The corresponding reaction cell is illustrated in Fig. 1. After polished with up to 1200 grit SiC
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paper and washed with deionized water, the copper rod (99.99%, 80 mm×2 mm×2 mm) as the anode was immersed in the solvent (50 ml aqueous solution of H2O and 25 gr TiO2 powder). The stainless steel tube (0.5 mm inside diameter, 0.7 mm external diameter and 9 cm length) as the
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cathode was set at 3 mm up the solvent and 2 cm away from the anode. During the experiment, the solution was stirred by a magnetic stirrer. The TiO2/CuO nanocomposite was rinsed several times with deionized water and ethanol at the end of process. Finally, the product was dried in an oven at 80º C for 1 hour. The solution temperature in reaction cell was kept constant (about 30°C) for the duration of the process. The Ar gas flow was coupled to the capillary tube and set to 200 sccm. The applied voltage to the cathode was 1.9 kV (reduced to 1.2 kV). The plasma was
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kept stable by a ballast resistor (RA=100 kΩ). The total voltage applied between the two electrodes and the microplasma current was measured by means of a high voltage probe
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(Tektronix P6015A) and resistor RC (R=100 Ω).
Fig. 1. The experimental apparatus diagram of microplasma electrochemical method.
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2.2. Preparation of Photoanode and counter electrode Two kinds of the pure TiO2 film and CuO/TiO2 nanocomposite film as photo anode was prepared by the doctor’s blade. Fluorine doped tin oxide (FTO) conducting glass before
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deposition was sequentially cleaned in an ultrasonic bath filled with a detergent solution, deionized water and ethanol for 15 min, respectively. A creamy paste was first prepared by mixing 0.20 g of prepared powders (pure TiO2 or CuO/TiO2 nanocomposite) with one drop of Triton X-100, 12 drops of glacial acetic acid, and about 2 ml of ethanol after sonication for 12 h at 1200 W cm-2. Then the two types of TiO2 thin film and CuO/TiO2 nanocomposite film were
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deposited on the FTO substrate (Hartford Glass, ~15 Ω cm-2, 80% transmittance in visible region). After that, the FTO substrate immersed into the 40 mM aqueous TiCl4 solution at 70 C
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for 45 min, and the sample was calcined at 500º C for 1 h. To load the dyes on the TiO2 electrode, they were soaked for 24 h at room temperature in an ethanol solution of 0.4 mM N719 (Ruthenizer 535-bisTBA) dye. A Pt-coated FTO glass as the counter electrode was used and placed over the photoanode. Finally, the cell openings were sealed with a sealing sheet (Surlyn) and a redox electrolyte (Iodolyte Z-100, Solaronix) solution was injected into the cell. An active
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area of 0.25 cm2 was used to measure the cell performance.
2.3. Material characteristic
The morphology of the CuO/TiO2 nanocomposite was studied by field emission scanning
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electron microscope (FESEM) (Model TE-SCAN-MIRA3) and transmission electron microscope (TEM) (Zeiss-EM10C-80 KV with holey carbon coated Cu grid with 300 meshes). To analyze
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the composition of the CuO/TiO2 nanocomposite, Energy-dispersive X-ray spectroscopy (EDS) was used. The XRD pattern was obtained using an X’Pert MPD (Philips) X-ray diffractometer with Co Kα radiation (λ=1.79 A°) to confirm the crystalline nature of the nanocomposite. The Raman spectra of samples were also measured by employing a Nicolet Almega dispersive Raman spectrometer (λex = 532 nm). To investigate the optical properties of CuO/TiO2 nanocomposite, UV/VIS absorption (UV-Vis, Lamda 25, Perkin Elmer) was used. The BET specific surface area (SBET) and porous structure were determined on ASAP micromeritics 2020 nitrogen adsorption apparatus. Moreover, the current density–voltage (J–V) performance of the corresponding DSSCs was performed with a solar simulator equipped with a Keithley 2400
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source meter. The power of the simulated light was calibrated to AM 1.5 (100 mW/cm2) by using a standard silicon photodiode (BPW21R). 3. Results and discussion
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We have recently used the microplasma electrochemical method for synthesis of Ag/TiO2 nanocomposite [38]. As explained in the previous work, regarding the role of the microplasma in the synthesis of nanocomposite, plasma could hasten the rate of nanocomposite synthesis and achieve the optimized nanoparticles with changing the plasma discharge parameters. As the
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result, the plasma reactive species initiate a waterfall of chemical reactions and the colour of the solution alters successively (with plasma irradiation). This is the direct result of the reduction of
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ions in solution and the synthesis of nanocomposite (Fig. 2).
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Fig. 2. Photographs of the solution, (a) without plasma irradiation and (b) after 25 min plasma irradiation.
In this method, some plasma electrons entered into the liquid could interact with species in solution to produce short-lived radicals. These species could either lead to dissociative electron
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attachment or pertain to the hydrated electrons that can produce OH and H. They can actually be used for the reducing of the copper ions in solution.
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3.1. Morphological characteristics of CuO/TiO2 nanocomposite The morphology and chemical compositions of CuO/TiO2 nanocomposites were characterized using SEM and EDX. The processing time was at 25 min. The Figures 3(a) and 3(b) show the SEM morphology of pure TiO2 and CuO/TiO2 nanocomposites. It has been seen that the crystalline sizes for pure TiO2 and CuO/TiO2 nanocomposite are in about 15–40 nm without any particular morphology. Figures 3(c) and 3(d) indicate EDX spectra and elemental map derived from the CuO/TiO2 nanocomposite. These analyses disclose the spatial uniform distribution of
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Ti, O, and Cu elements in CuO/TiO2 nanocomposite which confirms the absence of any other
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impurities in elemental mapping.
Fig 3: The morphologies imaged by SEM for (a) pure TiO2 and (b) CuO/TiO2 nanocomposite. (c) EDS spectrum and (d) Elemental mapping of the CuO/TiO2 nanocomposite after 25 min plasma irradiation.
TEM image of CuO/TiO2 nanocomposite is shown in Fig. 4. As seen, the synthesized CuO/TiO2
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nanocomposite was extremely aggregated with irregularly shaped nanoparticles (size distribution between 10 to 40 nm). Moreover, the dark spots on the TEM micrograph indicate the presence of
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Cu nanoparticles on the surface of titanium oxide, with values ranging between 3 and 7 nm in diameter.
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Fig 4: TEM micrograph for CuO/TiO2 nanocomposite after plasma irradiation (25 min).
The PL spectra of pure TiO2 and CuO/TiO2 nanocomposite were shown in Fig. 5. It can be used
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to illustrate electron/hole pair separation efficiency and charge carrier lifetimes. As seen, the PL intensity of CuO/TiO2 nanocomposite was significantly reduced in comparison with the pure TiO2. These results indicated that the electron/hole pair recombination is decreased in CuO/TiO2
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nanocomposite due to increasing life time of charge carriers.
Fig 5: Photoluminescence spectra of CuO/TiO2 nanocomposite and pure TiO2.
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N2 adsorption-desorption isotherms and pore size distribution profile CuO/TiO2 nanocomposite are shown in Fig. 6. It can be seen that the N2 adsorption-desorption isotherms displayed the presence of mesoporous materials with abroad pore size distribution (3–100 nm) and peak sizes of about 3–7 nm for CuO/TiO2 nanocomposite. The adsorption isotherm demonstrated type IV
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Langmuir characteristics. The obtained surface area of the pure TiO2 was 83.2 m2/g, while the surface areas of the CuO/TiO2 nanocomposite synthesized in 25 min processing time was 65 m2/g. This decrease was due to the insertion of small Cu nanoparticles into the pores of pure
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TiO2.
Fig 6: (a) Nitrogen adsorption-desorption isotherm ((a) pure TiO2 and (b) TiO2/CuO nanocomposite) and (b) Pore
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size distribution of synthesized TiO2/CuO nanocomposite.
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The X-ray diffraction spectra of the pure TiO2 and CuO/TiO2 nanocomposite were measured by X-ray diffraction in the about of 20-70o using Cu-kα radiation (Fig. 7). The diffraction peaks occur at 2θ values at 25.28 o, 27 o, 37.8 o, 48 o and 62.8 o for the pure TiO2, which have a good matching with the standard diffraction data (with JCPDS Card No.84-1286). After plasma irradiation, new small diffraction peaks are observed around 58.1 and 61.5 corresponding to CuO. It can be seen that the XRD pattern of CuO/TiO2 nanocomposite indicates a peak related to metallic Cu (JCPDS card 04-0836). These results confirm that the nanocomposite consists of TiO2 and copper (both CuO and Cu). Moreover, after 5 min, there is no peak related to CuO, indicating low Cu contents in sample. From XRD pattern it can also be seen that the peak
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intensity of CuO is enhanced with increase of time, demonstrating that the Cu effectively interact
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with species available in solution to produce both CuO and CuO/TiO2 nanocomposite.
Fig 7: XRD patterns of (a) pure TiO2 and synthesized CuO/TiO2 nanocomposite film after (b) 15 min and (c) 25 min plasma irradiation.
The band edge positions of photoanode material in DSSCs are extremely important electronic
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parameter because of effect on the driving force for electron injection in the conduction band of titanium oxide. Figure 8 indicates the optical properties of CuO/TiO2 nanocomposite and Tauc
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plot by using UV–Vis spectroscopy (during various irradiation times and current 7 mA). As seen, the absorption spectrum of pure TiO2 reveals an absorption threshold in the region of 300–400 nm, while the absorption edge of the absorption spectrum of CuO/TiO2 nanocomposite moves toward the visible light region of the spectrum with increase in processing time. It causes a powerful field around Cu NPs and raises their interaction with dye molecules. As a result, effective resonant energy transfers between excited state of dye and surface plasmons.
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Fig 8: The UV–Vis spectra for (a) pure TiO2 and CuO/TiO2 nanocomposite after (b) 15 min and (c) 25 min plasma irradiation.
The band gap of CuO/TiO2 nanocomposite were estimated by using a well-known Tauc’s plot
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procedure and the following equation
(1)
where a, h, t, E and A are the absorption coefficient, Planck’s constant, beam frequency, band
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gap and a constant, respectively[31, 52]. The reduced band gap energy in CuO/TiO2 nanocomposite confirms that reduced bottom of conduction band of TiO2 after addition of Cu
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has decreased the band gap of TiO2 which result in enhanced electron concentration in conduction band.
Figure 9 indicates the UV–Vis spectra of CuO/TiO2 nanocomposite synthesized by using microplasma for different current (4, 6 and 7 mA) after time processing 25 min. It has been seen that the intensity of the peaks of the CuO/TiO2 nanocomposite is increased by changing the current from 4 mA to 7 mA.
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Fig 9: The UV–Vis spectra for CuO/TiO2 nanocomposite nanoparticle after 25 min plasma irradiation in current (a)
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4 mA, (b) 6 mA and (c) 7 mA.
3.2. Photovoltaic performance of DSSCs based on pure TiO2 and CuO/TiO2 nanocomposite To consider the influence of CuO/TiO2 nanocomposite on the DSSCs efficiency, different
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photoanodes based on TiO2 and CuO/TiO2 nanocomposite were fabricated. Then the photovoltaic efficiency of the samples was measured under standard temperature condition. Figure 10 indicates the current density–voltage characteristics of samples Table 1 also shows
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solar cell parameters, such as short-circuit current density (Jsc), the open circuit voltage (Voc), the fill factor (FF) and the power conversion efficiency (PCE). For the DSSCs based on pure TiO2, the power conversion efficiency, the short-circuit current and the open-circuit voltage were 6.5 %, 12.94 mA/cm2 and 0.71 V, respectively. As seen, these parameters can alter by adding CuO to TiO2. For this reason, the short - circuit current increase and the open circuit voltage decrease after 25 min plasma irradiation. The power conversion efficiency also reaches up to 9.3 %.
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Fig 10: J-V curves of DSSCs for (a) pure TiO2 and CuO/TiO2 nanocomposite after (b) 15 min and (c) 25 min plasma
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irradiation.
The value of open-circuit voltage is depended essentially on the Fermi level (EF) related to Conduction Band-Edge Potential (Ecb) position, while the condition of Reduction potential (Ered) in the electrolyte does not alter by addition of Cu. The reduction of Voc in samples based
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on CuO/TiO2 nanocomposite is caused from movement of conduction band edge. Moreover, injection efficiency from the LUMO of the dye to the conduction band of TiO2 is also increased,
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caused the increase of Jsc.
Table 1: The performance of dye-sensitized solar cells obtained from the J–V curves indicated in Fig. 10.
DSSC photoanode
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
(a) TiO2
12.94
0.71
70
6.5
(b) CuO/TiO2
15.38
0.70
68
7.4
(c) CuO/TiO2
18.8
0.68
73
9.3
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Fig 11: J-V curves of DSSCs for CuO/TiO2 composite nanoparticle (a) 4 mA, (b) 6 mA and (c) 7 mA after 25 min plasma irradiation.
The current density–voltage characteristics of CuO/TiO2 nanocomposite and pure TiO2 based
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DSSCs for currents of 4 mA, 6 mA and 7 mA are shown in Fig 11. It can been seen that, with the increase of current values, the photoelectric conversion performances of the samples are improved, indicating that the values of Cu and CuO in DSSCs has increased. As a result, the plasma parameters also improve the formation mechanism of nanocomposite and DSSC
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performance with the enhancement of collection and transport of electrons.
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Figure 12 shows the incident photon-to-electron conversion efficiency (IPCE) spectra of the samples based on pure TiO2 and CuO/TiO2. In the present work, the IPCE values significantly were enhanced in sample based on CuO/TiO2 nanocomposite compared to those of sample based on the pure TiO2. The increase in IPCE may be the result of changing charge transfer efficiency and increasing dye loading with addition of Cu in TiO2 semiconductor.
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Fig 12: Normalized IPCE curves of DSSCs for (a) pure TiO2 and (b) CuO/TiO2 nanocomposite.
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The mechanisms of the formation of CuO/TiO2 nanocomposite will be now considered. Owing to the fact that there are several substances in the reaction system (Cu+, TiO2, H2O), multiple reaction mechanisms may be contributed to formation of the CuO/TiO2 nanocomposite. On the other hands, due to interactions of the plasma species with these substances, chemical reactions
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are at first occurred and produced radicals/compounds and hydrated electrons at the plasmaliquid interface. It can change drastically physical and chemical phenomena at the surface or
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within the solution. Regarding our previous work, we can discuss the mechanisms of the formation of CuO/TiO2 nanocomposite as follows: Copper ions are firstly separated from the anode by using anodic dissolution which happens because of the applied current (Eq. (2)), and afterwards combined with the species in solution to form a nanocomposite. The synthesis of Cu and CuO nanoparticles with species synthesized by plasma such as electron, H and H2O2, could be reasonable [9]. Among the plasma species, electrons can behave as reducing agent producing copper (Eq. (3)). On the other hand, the
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interaction of produced H2O2 (Eq. (4)) with copper atoms causes to the formation of CuO in the solution region closest to the plasma–solution interface. (2)
+ →
(3)
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→ +
+ → +
(4)
Moreover, this method could enhance the rate of CuO/TiO2 nanocomposite synthesis and
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achieve the optimized nanocomposite under different discharge conditions. By enhancing the current, the ratio of plasma species in solution especially electrons is increased. The CuO/TiO2
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nanocomposite could better improve the photoanode performance in DSSCs. Among the available methods for synthesis of nanocomposite, the presented method consists of a new technique for nanomaterial synthesis that there is possibility of plasma parameter adjustment to control the electrochemical reactions in the solution [38].
4. Conclusions
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In summary, we have been successfully indicated the synthesis of CuO/TiO2 nanocomposite by the microplasma-assisted electrochemical method. Then DSSCs based on the CuO/TiO2 nanocomposite and pure TiO2 films were synthesized by the doctor-blade procedure. The analysis of samples showed that the performance of DSSC could be improved by adding copper
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to TiO2. With increase of irradiation time and Cu in samples, the Voc deducted, while the Jsc and the efficiency were enhanced. Indeed, the samples based on CuO/TiO2 nanocomposite are
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capable of the more harvesting sunlight and improve the electron transport properties in TiO2. Moreover, the optimized characteristics of CuO/TiO2 nanocomposite are largely attained by plasma parameters such as irradiation time and discharge current. Finally, approaches based on microplasma could be utilized in the many applications as a simple route like that used for the synthesis of different types of nanomaterial. Compared with the other methods, this approach are usually faster and cheaper to synthesize nanocomposite due to the finite number of working steps involved in synthesis of nanomaterial.
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Highlights
CuO/TiO2 nanocomposite was prepared by plasma in aqueous solution.
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The performance of DSSC could be improved by adding copper to TiO2.
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This method is usually fast and cheap to synthesize nanocomposite.
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Nanocomposite characteristics are improved by plasma parameters.
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S0925-8388(19)31472-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.04.185
Reference:
JALCOM 50369
To appear in:
Journal of Alloys and Compounds
Received Date: 26 February 2019 Revised Date:
16 April 2019
Accepted Date: 19 April 2019
Please cite this article as: Z.D. Mahmoudabadi, E. Eslami, One-step synthesis of CuO/TiO2 nanocomposite by atmospheric microplasma electrochemistry – Its application as photoanode in dye-sensitized solar cell, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.04.185. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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One-Step Synthesis of CuO/TiO2 nanocomposite by atmospheric microplasma electrochemistry – its application as photoanode in Dye-Sensitized Solar Cell
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Zohreh Dehghani Mahmoudabadi1, Esmaeil Eslami Department of Physics, Iran University of Science and Technology, Narmak, Tehran, 16846- 13114, Iran
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Abstract
The necessity for improving performance of solar cell has been caused the development of
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methods for nanomaterial synthesis with tunable properties. Herein, CuO/TiO2 nanocomposite, for the first time, is prepared by microplasma-assisted electrochemical method in aqueous solution for using dye sensitized solar cells (DSSCs). The results of TEM and EDS indicate that the Cu nanoparticles dispersed in TiO2 lattice. Moreover, based on the current density–voltage (J–V), the power conversion efficiency, open-circuit voltage (Voc) and the short-circuit current density are 9.3%, 0.68 V and 18.8 mA cm-2 in CuO/TiO2, and 6.5%, 0.71 V and 12.94 mA cm-2
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in pure TiO2, respectively. These results reveal that the performance improvement could be due to the deduced band-gap energy, increasing life time of charge carriers and surface coverage of the sensitizing dye over CuO/TiO2 nanocomposite.
method.
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1. Introduction
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Keywords: CuO/TiO2 nanocomposite, dye sensitized solar cells, microplasma, electrochemical
In recent years, there have been several studies on possible alternatives to Dye-sensitized solar cells (DSSCs) due to their various excellent features such as low-cost, semi-transparency and high power conversion efficiency [1, 2]. DSSCs include a photoanode, a counter electrode, dye and an electrolyte. Among these elements, the semiconductors material used in photoanode directly determines the working process of 1
Corresponding author: E-mail address: [email protected] (z.dehghani)
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DSSCs because of photo-induced electron transport and organic dye adsorption [3-6]. Titanium dioxide, among different oxide materials available for making photoanode, is the attractive candidate with high photo catalytic activity and non-toxicity. However, the band gap measurement of TiO2 restricts its performance in solar cells [7]. There are a few light-excited
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dyes that could inject electrons into the conduction band of TiO2 due to inadequate electron injection driving force [8]. All of these materials are synthesized using various procedures to obtain the optimum states [9-14].
Today, TiO2 doped with some materials significantly improves the photocurrent and decreases
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the amount of electron recombination in solar cells [15-24]. These materials expand response ranges of TiO2 into the visible light range. Moreover, they play an important role in increase of
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specific surface area [8, 25-31]. To improve the efficiency of the solar cell, Cu as relatively cheap and abundant metal is one of the most promising materials among all [32, 33]. Although synthesis of the CuO/TiO2 nanocomposite has been reported in the many works, they all succeed to synthesize this combination with traditionally time - consuming steps [34-36]. For this reason, in addition to these methods, there are several researches which have been used to improve this problem, such as using a simple and fast method for nanocomposite synthesis [37, 38].
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Recently, the wide researches have been done on atmospheric pressure microplasma-liquid interactions in a wide range of applications such as synthesis of metal oxides and nanostructures [39-44]. By using this method, we have been able to synthesize nanocomposites consisting of metals and semiconductors, resulting in improved performance of the DSSC [38, 45]. In
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particular, the interaction between microplasma and solution suggests a strong technique for solution processing of nanocomposites due to the enriched chemical environment in the vicinity
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of the plasma/ liquid interface [46-51]. In our previous research, it is found that Ag/TiO2 nanocomposite is an appropriate combination to attain improvements in the efficiency of DSSCs. we indicated the synthesis of Ag/TiO2 nanocomposite by using microplasma-assisted electrochemical method and discussed the effect of Ag addition on the photoactivity of the solar cell. The Ag/TiO2 nanocomposite decreased the conduction band edge of TiO2 and expanded the response in the visible region, resulting in enhanced efficiency of DSSCs [38]. Herein, for the first time, CuO/TiO2 nanocomposite was synthesized using the microplasma electrochemical method for the application in DSSC. The properties of CuO/TiO2 nanocomposite have been characterized by scanning electron microscope (SEM), energy dispersive spectroscopy
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(EDS), and transmission electron microscopy (TEM), X-ray diffract meter (XRD). As a result, potential applications of the CuO/TiO2 nanocomposite have been discussed as a photoanode in the dye sensitized solar cell.
2.1. Preparation of CuO/TiO2 nanocomposite
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2. Materials & methods
The corresponding reaction cell is illustrated in Fig. 1. After polished with up to 1200 grit SiC
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paper and washed with deionized water, the copper rod (99.99%, 80 mm×2 mm×2 mm) as the anode was immersed in the solvent (50 ml aqueous solution of H2O and 25 gr TiO2 powder). The stainless steel tube (0.5 mm inside diameter, 0.7 mm external diameter and 9 cm length) as the
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cathode was set at 3 mm up the solvent and 2 cm away from the anode. During the experiment, the solution was stirred by a magnetic stirrer. The TiO2/CuO nanocomposite was rinsed several times with deionized water and ethanol at the end of process. Finally, the product was dried in an oven at 80º C for 1 hour. The solution temperature in reaction cell was kept constant (about 30°C) for the duration of the process. The Ar gas flow was coupled to the capillary tube and set to 200 sccm. The applied voltage to the cathode was 1.9 kV (reduced to 1.2 kV). The plasma was
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kept stable by a ballast resistor (RA=100 kΩ). The total voltage applied between the two electrodes and the microplasma current was measured by means of a high voltage probe
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(Tektronix P6015A) and resistor RC (R=100 Ω).
Fig. 1. The experimental apparatus diagram of microplasma electrochemical method.
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2.2. Preparation of Photoanode and counter electrode Two kinds of the pure TiO2 film and CuO/TiO2 nanocomposite film as photo anode was prepared by the doctor’s blade. Fluorine doped tin oxide (FTO) conducting glass before
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deposition was sequentially cleaned in an ultrasonic bath filled with a detergent solution, deionized water and ethanol for 15 min, respectively. A creamy paste was first prepared by mixing 0.20 g of prepared powders (pure TiO2 or CuO/TiO2 nanocomposite) with one drop of Triton X-100, 12 drops of glacial acetic acid, and about 2 ml of ethanol after sonication for 12 h at 1200 W cm-2. Then the two types of TiO2 thin film and CuO/TiO2 nanocomposite film were
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deposited on the FTO substrate (Hartford Glass, ~15 Ω cm-2, 80% transmittance in visible region). After that, the FTO substrate immersed into the 40 mM aqueous TiCl4 solution at 70 C
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for 45 min, and the sample was calcined at 500º C for 1 h. To load the dyes on the TiO2 electrode, they were soaked for 24 h at room temperature in an ethanol solution of 0.4 mM N719 (Ruthenizer 535-bisTBA) dye. A Pt-coated FTO glass as the counter electrode was used and placed over the photoanode. Finally, the cell openings were sealed with a sealing sheet (Surlyn) and a redox electrolyte (Iodolyte Z-100, Solaronix) solution was injected into the cell. An active
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area of 0.25 cm2 was used to measure the cell performance.
2.3. Material characteristic
The morphology of the CuO/TiO2 nanocomposite was studied by field emission scanning
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electron microscope (FESEM) (Model TE-SCAN-MIRA3) and transmission electron microscope (TEM) (Zeiss-EM10C-80 KV with holey carbon coated Cu grid with 300 meshes). To analyze
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the composition of the CuO/TiO2 nanocomposite, Energy-dispersive X-ray spectroscopy (EDS) was used. The XRD pattern was obtained using an X’Pert MPD (Philips) X-ray diffractometer with Co Kα radiation (λ=1.79 A°) to confirm the crystalline nature of the nanocomposite. The Raman spectra of samples were also measured by employing a Nicolet Almega dispersive Raman spectrometer (λex = 532 nm). To investigate the optical properties of CuO/TiO2 nanocomposite, UV/VIS absorption (UV-Vis, Lamda 25, Perkin Elmer) was used. The BET specific surface area (SBET) and porous structure were determined on ASAP micromeritics 2020 nitrogen adsorption apparatus. Moreover, the current density–voltage (J–V) performance of the corresponding DSSCs was performed with a solar simulator equipped with a Keithley 2400
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source meter. The power of the simulated light was calibrated to AM 1.5 (100 mW/cm2) by using a standard silicon photodiode (BPW21R). 3. Results and discussion
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We have recently used the microplasma electrochemical method for synthesis of Ag/TiO2 nanocomposite [38]. As explained in the previous work, regarding the role of the microplasma in the synthesis of nanocomposite, plasma could hasten the rate of nanocomposite synthesis and achieve the optimized nanoparticles with changing the plasma discharge parameters. As the
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result, the plasma reactive species initiate a waterfall of chemical reactions and the colour of the solution alters successively (with plasma irradiation). This is the direct result of the reduction of
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ions in solution and the synthesis of nanocomposite (Fig. 2).
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Fig. 2. Photographs of the solution, (a) without plasma irradiation and (b) after 25 min plasma irradiation.
In this method, some plasma electrons entered into the liquid could interact with species in solution to produce short-lived radicals. These species could either lead to dissociative electron
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attachment or pertain to the hydrated electrons that can produce OH and H. They can actually be used for the reducing of the copper ions in solution.
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3.1. Morphological characteristics of CuO/TiO2 nanocomposite The morphology and chemical compositions of CuO/TiO2 nanocomposites were characterized using SEM and EDX. The processing time was at 25 min. The Figures 3(a) and 3(b) show the SEM morphology of pure TiO2 and CuO/TiO2 nanocomposites. It has been seen that the crystalline sizes for pure TiO2 and CuO/TiO2 nanocomposite are in about 15–40 nm without any particular morphology. Figures 3(c) and 3(d) indicate EDX spectra and elemental map derived from the CuO/TiO2 nanocomposite. These analyses disclose the spatial uniform distribution of
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Ti, O, and Cu elements in CuO/TiO2 nanocomposite which confirms the absence of any other
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impurities in elemental mapping.
Fig 3: The morphologies imaged by SEM for (a) pure TiO2 and (b) CuO/TiO2 nanocomposite. (c) EDS spectrum and (d) Elemental mapping of the CuO/TiO2 nanocomposite after 25 min plasma irradiation.
TEM image of CuO/TiO2 nanocomposite is shown in Fig. 4. As seen, the synthesized CuO/TiO2
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nanocomposite was extremely aggregated with irregularly shaped nanoparticles (size distribution between 10 to 40 nm). Moreover, the dark spots on the TEM micrograph indicate the presence of
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Cu nanoparticles on the surface of titanium oxide, with values ranging between 3 and 7 nm in diameter.
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Fig 4: TEM micrograph for CuO/TiO2 nanocomposite after plasma irradiation (25 min).
The PL spectra of pure TiO2 and CuO/TiO2 nanocomposite were shown in Fig. 5. It can be used
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to illustrate electron/hole pair separation efficiency and charge carrier lifetimes. As seen, the PL intensity of CuO/TiO2 nanocomposite was significantly reduced in comparison with the pure TiO2. These results indicated that the electron/hole pair recombination is decreased in CuO/TiO2
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nanocomposite due to increasing life time of charge carriers.
Fig 5: Photoluminescence spectra of CuO/TiO2 nanocomposite and pure TiO2.
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N2 adsorption-desorption isotherms and pore size distribution profile CuO/TiO2 nanocomposite are shown in Fig. 6. It can be seen that the N2 adsorption-desorption isotherms displayed the presence of mesoporous materials with abroad pore size distribution (3–100 nm) and peak sizes of about 3–7 nm for CuO/TiO2 nanocomposite. The adsorption isotherm demonstrated type IV
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Langmuir characteristics. The obtained surface area of the pure TiO2 was 83.2 m2/g, while the surface areas of the CuO/TiO2 nanocomposite synthesized in 25 min processing time was 65 m2/g. This decrease was due to the insertion of small Cu nanoparticles into the pores of pure
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TiO2.
Fig 6: (a) Nitrogen adsorption-desorption isotherm ((a) pure TiO2 and (b) TiO2/CuO nanocomposite) and (b) Pore
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size distribution of synthesized TiO2/CuO nanocomposite.
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The X-ray diffraction spectra of the pure TiO2 and CuO/TiO2 nanocomposite were measured by X-ray diffraction in the about of 20-70o using Cu-kα radiation (Fig. 7). The diffraction peaks occur at 2θ values at 25.28 o, 27 o, 37.8 o, 48 o and 62.8 o for the pure TiO2, which have a good matching with the standard diffraction data (with JCPDS Card No.84-1286). After plasma irradiation, new small diffraction peaks are observed around 58.1 and 61.5 corresponding to CuO. It can be seen that the XRD pattern of CuO/TiO2 nanocomposite indicates a peak related to metallic Cu (JCPDS card 04-0836). These results confirm that the nanocomposite consists of TiO2 and copper (both CuO and Cu). Moreover, after 5 min, there is no peak related to CuO, indicating low Cu contents in sample. From XRD pattern it can also be seen that the peak
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intensity of CuO is enhanced with increase of time, demonstrating that the Cu effectively interact
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with species available in solution to produce both CuO and CuO/TiO2 nanocomposite.
Fig 7: XRD patterns of (a) pure TiO2 and synthesized CuO/TiO2 nanocomposite film after (b) 15 min and (c) 25 min plasma irradiation.
The band edge positions of photoanode material in DSSCs are extremely important electronic
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parameter because of effect on the driving force for electron injection in the conduction band of titanium oxide. Figure 8 indicates the optical properties of CuO/TiO2 nanocomposite and Tauc
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plot by using UV–Vis spectroscopy (during various irradiation times and current 7 mA). As seen, the absorption spectrum of pure TiO2 reveals an absorption threshold in the region of 300–400 nm, while the absorption edge of the absorption spectrum of CuO/TiO2 nanocomposite moves toward the visible light region of the spectrum with increase in processing time. It causes a powerful field around Cu NPs and raises their interaction with dye molecules. As a result, effective resonant energy transfers between excited state of dye and surface plasmons.
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Fig 8: The UV–Vis spectra for (a) pure TiO2 and CuO/TiO2 nanocomposite after (b) 15 min and (c) 25 min plasma irradiation.
The band gap of CuO/TiO2 nanocomposite were estimated by using a well-known Tauc’s plot
1
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procedure and the following equation
(1)
where a, h, t, E and A are the absorption coefficient, Planck’s constant, beam frequency, band
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gap and a constant, respectively[31, 52]. The reduced band gap energy in CuO/TiO2 nanocomposite confirms that reduced bottom of conduction band of TiO2 after addition of Cu
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has decreased the band gap of TiO2 which result in enhanced electron concentration in conduction band.
Figure 9 indicates the UV–Vis spectra of CuO/TiO2 nanocomposite synthesized by using microplasma for different current (4, 6 and 7 mA) after time processing 25 min. It has been seen that the intensity of the peaks of the CuO/TiO2 nanocomposite is increased by changing the current from 4 mA to 7 mA.
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Fig 9: The UV–Vis spectra for CuO/TiO2 nanocomposite nanoparticle after 25 min plasma irradiation in current (a)
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4 mA, (b) 6 mA and (c) 7 mA.
3.2. Photovoltaic performance of DSSCs based on pure TiO2 and CuO/TiO2 nanocomposite To consider the influence of CuO/TiO2 nanocomposite on the DSSCs efficiency, different
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photoanodes based on TiO2 and CuO/TiO2 nanocomposite were fabricated. Then the photovoltaic efficiency of the samples was measured under standard temperature condition. Figure 10 indicates the current density–voltage characteristics of samples Table 1 also shows
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solar cell parameters, such as short-circuit current density (Jsc), the open circuit voltage (Voc), the fill factor (FF) and the power conversion efficiency (PCE). For the DSSCs based on pure TiO2, the power conversion efficiency, the short-circuit current and the open-circuit voltage were 6.5 %, 12.94 mA/cm2 and 0.71 V, respectively. As seen, these parameters can alter by adding CuO to TiO2. For this reason, the short - circuit current increase and the open circuit voltage decrease after 25 min plasma irradiation. The power conversion efficiency also reaches up to 9.3 %.
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Fig 10: J-V curves of DSSCs for (a) pure TiO2 and CuO/TiO2 nanocomposite after (b) 15 min and (c) 25 min plasma
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irradiation.
The value of open-circuit voltage is depended essentially on the Fermi level (EF) related to Conduction Band-Edge Potential (Ecb) position, while the condition of Reduction potential (Ered) in the electrolyte does not alter by addition of Cu. The reduction of Voc in samples based
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on CuO/TiO2 nanocomposite is caused from movement of conduction band edge. Moreover, injection efficiency from the LUMO of the dye to the conduction band of TiO2 is also increased,
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caused the increase of Jsc.
Table 1: The performance of dye-sensitized solar cells obtained from the J–V curves indicated in Fig. 10.
DSSC photoanode
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
(a) TiO2
12.94
0.71
70
6.5
(b) CuO/TiO2
15.38
0.70
68
7.4
(c) CuO/TiO2
18.8
0.68
73
9.3
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Fig 11: J-V curves of DSSCs for CuO/TiO2 composite nanoparticle (a) 4 mA, (b) 6 mA and (c) 7 mA after 25 min plasma irradiation.
The current density–voltage characteristics of CuO/TiO2 nanocomposite and pure TiO2 based
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DSSCs for currents of 4 mA, 6 mA and 7 mA are shown in Fig 11. It can been seen that, with the increase of current values, the photoelectric conversion performances of the samples are improved, indicating that the values of Cu and CuO in DSSCs has increased. As a result, the plasma parameters also improve the formation mechanism of nanocomposite and DSSC
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performance with the enhancement of collection and transport of electrons.
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Figure 12 shows the incident photon-to-electron conversion efficiency (IPCE) spectra of the samples based on pure TiO2 and CuO/TiO2. In the present work, the IPCE values significantly were enhanced in sample based on CuO/TiO2 nanocomposite compared to those of sample based on the pure TiO2. The increase in IPCE may be the result of changing charge transfer efficiency and increasing dye loading with addition of Cu in TiO2 semiconductor.
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Fig 12: Normalized IPCE curves of DSSCs for (a) pure TiO2 and (b) CuO/TiO2 nanocomposite.
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The mechanisms of the formation of CuO/TiO2 nanocomposite will be now considered. Owing to the fact that there are several substances in the reaction system (Cu+, TiO2, H2O), multiple reaction mechanisms may be contributed to formation of the CuO/TiO2 nanocomposite. On the other hands, due to interactions of the plasma species with these substances, chemical reactions
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are at first occurred and produced radicals/compounds and hydrated electrons at the plasmaliquid interface. It can change drastically physical and chemical phenomena at the surface or
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within the solution. Regarding our previous work, we can discuss the mechanisms of the formation of CuO/TiO2 nanocomposite as follows: Copper ions are firstly separated from the anode by using anodic dissolution which happens because of the applied current (Eq. (2)), and afterwards combined with the species in solution to form a nanocomposite. The synthesis of Cu and CuO nanoparticles with species synthesized by plasma such as electron, H and H2O2, could be reasonable [9]. Among the plasma species, electrons can behave as reducing agent producing copper (Eq. (3)). On the other hand, the
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interaction of produced H2O2 (Eq. (4)) with copper atoms causes to the formation of CuO in the solution region closest to the plasma–solution interface. (2)
+ →
(3)
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→ +
+ → +
(4)
Moreover, this method could enhance the rate of CuO/TiO2 nanocomposite synthesis and
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achieve the optimized nanocomposite under different discharge conditions. By enhancing the current, the ratio of plasma species in solution especially electrons is increased. The CuO/TiO2
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nanocomposite could better improve the photoanode performance in DSSCs. Among the available methods for synthesis of nanocomposite, the presented method consists of a new technique for nanomaterial synthesis that there is possibility of plasma parameter adjustment to control the electrochemical reactions in the solution [38].
4. Conclusions
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In summary, we have been successfully indicated the synthesis of CuO/TiO2 nanocomposite by the microplasma-assisted electrochemical method. Then DSSCs based on the CuO/TiO2 nanocomposite and pure TiO2 films were synthesized by the doctor-blade procedure. The analysis of samples showed that the performance of DSSC could be improved by adding copper
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to TiO2. With increase of irradiation time and Cu in samples, the Voc deducted, while the Jsc and the efficiency were enhanced. Indeed, the samples based on CuO/TiO2 nanocomposite are
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capable of the more harvesting sunlight and improve the electron transport properties in TiO2. Moreover, the optimized characteristics of CuO/TiO2 nanocomposite are largely attained by plasma parameters such as irradiation time and discharge current. Finally, approaches based on microplasma could be utilized in the many applications as a simple route like that used for the synthesis of different types of nanomaterial. Compared with the other methods, this approach are usually faster and cheaper to synthesize nanocomposite due to the finite number of working steps involved in synthesis of nanomaterial.
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Highlights
CuO/TiO2 nanocomposite was prepared by plasma in aqueous solution.
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The performance of DSSC could be improved by adding copper to TiO2.
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This method is usually fast and cheap to synthesize nanocomposite.
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Nanocomposite characteristics are improved by plasma parameters.
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•