Abstract

In this paper, the TiO₂/HS-CH₂-COOH/Cu₃Se₂ composite film photoanodes were fabricated on conducting glass plates. Cu₃Se₂ nanoparticles were used as the sensitizer and the bi-functional modifier HS-CH₂-COOH was used at the interface between Cu₃Se₂ and TiO₂ films to improve the properties of film photoanode. The characterization results show that the sol-sel prepared anatase TiO₂ film has a compact and uniform surface while the tetragonal Cu₃Se₂ film has a coarse surface which is made up of uniform elongated particles. The photoelectrochemical experimental results indicate that the TiO₂/HS-CH₂-COOH/Cu₃Se₂ composite film photoanodes has a good photovoltaic property.

Keywords: TiO₂ film; Sensitizer; Cu₃Se₂; Photoelectrochemical; Solar cell

1. Introduction

The increasing demand for energy motivates the search for new and clean energy resources. As a renewable energy, solar radiation is a most promising energy source. Therefore, solar cells for harvesting solar energy have received great attention and considerable progress has been made in this field. In 1991, Grätzel and co-workers made an important discovery in the design of solar cell by using nanocrystalline TiO₂ to replace conventional crystalline silicon [1]. However, since TiO₂ is a wide band gap semiconductor (band gap around 3.2 eV),only the UV light in solar radiation can be adsorbed and used. To increase the photoactivity of TiO₂ in the visible range,a complex dye, cis-di (thiocyanato)-N,N-bis (2,2’-bipyridyl di-carboxylate) Ru(II), has been added as sensitizer in solar cells. Since then, other complex dyes, such as bis (tetrabutylammonium) cis-di (thiocyanato) bis (2,2’- bipyridine-4,4’carb-oxylic acid) Ru(II) and tri (cyanato)-2,2',2''-terpyridyl-4,4',4''- tricarboxylate) Ru(II) were synthesized and used as sensitizers in TiO₂ solar cells [2-3]. Recently, some inorganic sensitizers were used to replace the complex dye in TiO₂ solar cells. Among inorganic sensitizers, the semiconductors attracted much attention and various semiconductors, such as CdS [4], PbS [5-6], Bi₂S₃ [7], CdSe [8-9], InP [10], CuInSe₂ and In₂Se₃[11], have been used to sensitize TiO₂.

As a type of special semiconductors, copper selenides have diverse phases and structural forms [12-17], and have extensively been studied due to their potential applications in solar cells [18-20]. Our group have synthesized CuSe, Cu₂Se, Cu₃Se₂ and Cu_2-xSe, and studied their photoelectric properties, such as electrochemical property and electrogenerated chemiluminescence [21-22]. In this paper, as-synthesized Cu₃Se₂ nanoparticles were used as the sensitizer for a TiO₂ film photoanode. In addition, thioglyclic acid (HS-CH₂-COOH) was used as a ligand (L) at the interface between Cu₃Se₂ and TiO₂ thin films for improving the properties of film photoanode. The photoelectrochemical experimental results show that the as-fabricated TiO₂/L/Cu₃Se₂ photoanode has a good photovoltaic property.

2. Experimental Section

Preparation of TiO₂ Film The reagents are of analytical grade and were used in experiments without further purification. The preparation for TiO₂ film was typically carried out as follows. First, the solution A was prepared by adding 1 ml glacial acetic into the mixture solution containing 4 ml Ti(OC₄H₉)₄, 12 ml absolute ethanol and 0.05 g polyvinylpyrrolidone. The solution B (pH 2.3) was prepared by mixing 6 ml absolute ethanol with 2 ml distilled water and 0.5 ml 10 mol/L HCl. Then, the reaction solution was prepared by dropwise adding the solution B into the solution A under stirring. The TiO₂ sol was formed after gelling this reaction solution for 12 h at room temperature. Finally, the TiO₂ film based on the conducting glass plate (CGP, a tin oxide-coated glass plate, Bengbu Co. China), was prepared as follows. First, a piece of the CGP was dipped in the TiO₂ sol for 1 min and lifted up at a rate of 5.0 cm/min. Then, after dried at 100 ℃, the CGP covered with TiO₂ layer was calcined in a furnace at 450 ℃ for 30 min.

Preparation of Cu₃Se₂ Nanoparticles  First, the 24 ml CuSO₄ solution containing 5 mmol Cu²⁺ and 0.353 g tri-sodium citrate, was mixed with 8 ml Na₂SeSO₃ solution containing 5 mmol Se^2-. A dark brown precipitate was formed after keeping the mixture solution at 60 ℃ for 30 min. Then, the Cu₃Se₂ nanoparticles, for further use as sensitizer for TiO₂ were obtained by separating and washing the precipitate.

Preparation of TiO₂/L/Cu₃Se₂ Composite Film First, the CGP covered with TiO₂ film as-obtained was immerged in an acetonitrile solution containing HS-CH₂-COOH (L) at 80 ℃ for 12 h. Then, the CGP covered with TiO₂ film that adsorbed L was taken out, washed with acetonitrile and immerged in the toluene solution containing Cu₃Se₂ nanoparticles for 12 h. Finally, the TiO₂/L/Cu₃Se₂ composite film based on the surface of CGP, was obtained by washing and drying (at room temperature) processes. Meanwhile, the TiO₂/Cu₃Se₂ composite film was prepared with the same way, except for the introduction of L. The preparation course of the TiO₂/L/Cu₃Se₂ composite film is illustrated in scheme 1.

Scheme 1. Preparation course of the Cu₃Se₂/L/TiO₂ composite film

Characterization. The as-prepared films were characterized by scanning electron microscopy (SEM, X650, Hitachi, 10 kV) and X-ray diffractometry (XRD, Rigaku D/max-RA, graphite monochromatized CuKα1 radiation, λ = 0.15406 nm). The UV-Vis absorption and emission spectra of the products were recorded on a UV spectrometer (UV-3600, Japan) and a spectrofluorimeter (PL, F-2500, Japan), respectively. The thickness of the films was determined by the surface profile meter (Ambios XP-1).

Photoelectrochemical Measurement The measurements were performed on an electrochemistry work station (LK2005, Tian’jing, China). The electrochemical cell was made up of a KI (0.1 mol/L) electrolyte solution and a three-electrode system: a platinum sheet as counter electrode, a saturated calomel electrode (SCE) as reference electrode against which all potentials were recorded, and a CGP covered with given film as working electrode. A Xe lamp (250 W) was used as light resource.

3. Results and discussion

Figure 1. Characterization results: SEM images of TiO₂ layer (a) and Cu₃Se₂ layer (b); XRD patterns of TiO₂ layer (c) and Cu₃Se₂ layer (d).

The SEM images and XRD patterns of the as-prepared films are shown in Figure 1. From Figure 1a, it can be seen that the TiO₂ film has a compact and uniformity surface. As shown in Figure 1b, the Cu₃Se₂ film has a coarse surface which is made up of uniform grain-like particles about 40 nm in diameter and 150 nm in length. The thickness measurements show that the TiO₂ and Cu₃Se₂ films are about 110 and  40 nm thick, respectively.

The phase structure of the nanoparticles that constituted films was examined by XRD and the results are shown in Figure 1c and 1d. Figure 1c indicates that the TiO₂ layer has a pure anatase structure, since all the peaks in XRD pattern can be attributed to the anatase phase (JCPDS 83-2243), and no peak for other types was observed [23]. Moreover, the TiO₂ layer is made up of small size particles with good crystallinity, which is confirmed by the strong and wide features of the XRD peaks. Figure 1d shows that the Cu₃Se₂ layer has a tetragonal structure, as reported [21], since the peaks in the XRD pattern correspond well with the standard XRD pattern of Cu₃Se₂ in JCPDS 47-1745 file[24].

The UV–vis absorption spectra of the films are shown in Figure 2a. As reported [25], the UV region absorption of the films can be assigned to the absorption caused by the excitation of electrons from the band-to-band or band-defect transitions. From curve 1 in Figure 2a, it can be seen that the pure TiO₂ film shows a weak absorption in UV-visible region, and an absorption onset 380 nm from which the band energy gap (Eg) for TiO₂ film was calculated to be 3.26 eV. By coupling with the Cu₃Se₂ nanoparticles, the TiO₂/Cu₃Se₂ composite film as-formed shows an intense absorption in UV-visible region and has a large red-shift for absorbance onset comparing with pure TiO₂ film, which is clearly shown in Figure 2a. From curve 2 in Figure 2a, the Eg of the TiO₂/Cu₃Se₂ composite film is 2.30 eV. In order to study the effect of Cu₃Se₂ on the absorption of the TiO₂/Cu₃Se₂ composite film, the absorption and emission spectra of Cu₃Se₂ nanoparticles themselves were determined and the results are shown in Figure 2b. From the spectra, it can be seen that the strong absorption is located in visible region and the emission peak is at 530 nm from which the Eg of the Cu₃Se₂ nanoparticles was calculated to be 2.34 eV. Clearly, the Cu₃Se₂ is a semiconductor with narrow Eg as reported [13], and the presence of Cu₃Se₂ helps to improve the optical performance of TiO₂/Cu₃Se₂ composite film.

For the TiO₂/L/Cu₃Se₂ composite film, as shown in Figure 2a (curve 3), the absorption and the red-shift of the absorbance onset were further enhanced due to the introduction of interface modifier L. It had been proved that the thiol group can bind CdSe [26], and the carboxylate group can connect the TiO₂ [27]. Recently, this binding model was used in binding CdSe or CdS to TiO₂ [28-29]. Here, as a bi-functional linker molecule bearing carboxylate and thiol functional groups, the L could bind Cu₃Se₂ nanoparticles with TiO₂, as illustrated in scheme 1. Therefore, the optical property of the TiO₂/L/Cu₃Se₂ composite film was improved, in virtue of the introduction of L.

Figure 2. (a) UV–vis spectra of the films: (1) TiO₂ film; (2) TiO₂/Cu₃Se₂ composite film; (3) TiO₂/L/Cu₃Se₂ composite film; (b) absorbance and emission spectra of Cu₃Se₂ nanoparticles in toluene (Emission spectrum was recorded using 440 nm excitation).

As above, the photoresponse of TiO₂ in visible region can be enhanced by binding with narrow Eg Cu₃Se₂. Here, in order to study the photo-electron transfer between Cu₃Se₂ and TiO₂ under illumination, the photoelectrochemical behavior of the films as-obtained was determined and the results are shown in Figure 3. It has been demonstrated that the semiconductors such as CdS and CdSe with narrow Eg, are capable of injecting electrons into TiO₂ film [29-30]. For our TiO₂/Cu₃Se₂ (or TiO₂/L/Cu₃Se₂) composite film, the photo-excited electrons in Cu₃Se₂ film could be injected into TiO₂ film under illumination with visible light. This charge transfer enhanced the separation of photo-induced electron-hole pairs in system, as a result, a larger short circuit current was generated comparing pure TiO₂ film (shown in Figure 3a). The process of charge transfer could be expressed as follows: 

2 Cu₃Se₂ + hν →Cu₃Se₂ (e^-) + Cu₃Se₂ (h⁺)    (1)

Cu₃Se₂ (e^-) + TiO₂ →Cu₃Se₂ + TiO₂ (e^-)        (2)

Cu₃Se₂ (h⁺) + I^- →Cu₃Se₂ + 1/2 I₂                      (3)

Based on the equation (2), the electrons shifted into TiO₂ film, accumulated on the surface of the CGP and formed current in circuit.

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Figure 3. Photocurrent as a function of (a) time and (b) voltage. Curves 1, 2, 3 correspond to the TiO₂ film, TiO₂/Cu₃Se₂ composite film, and TiO₂/L/Cu₃Se₂ composite film, respectively.

It has been confirmed that the electron transfer could be facilitated by applying a positive bias to the semiconductor film electrode [29]. Here, the photocurrents generated at different potentials applied to the TiO₂, TiO₂/Cu₃Se₂ and TiO₂/L/Cu₃Se₂ films respectively, are shown in Figure 3b. From the curves in Figure 3b, it can be seen that when the potential was swept toward positive direction, the generated photocurrents all increased for three films. As known, the zero potential at which no current is produced, reflects the maximum attainable open-circuit voltage and is a measure of the flat band potential as well. From Figure 3b, it can be observed that the zero potentials are -0.58, -0.88 and -0.97 V for TiO₂, TiO₂/Cu₃Se₂ and TiO₂/L/Cu₃Se₂ films, respectively. Comparing with TiO₂ film, clearly, the open-circuit voltage for the TiO₂/L/Cu₃Se₂ film increases by 0.39 V. The phenomenon may result from the improvement in energetics, and the rapid and efficient separation of photogenerated charges.

4. Conclusion

In the interest of improving the photoelectric property of TiO₂ film, the sensitizer Cu₃Se₂ nanoparticles and the bi-functional modifier L (HS-CH₂-COOH) were introduced to TiO₂ film by dip-coating and calcining technique. The morphology and crystallization of various films as-obtained were characterized by SEM and XRD, and the photoelectric property was studied by a series of photoelectrochemical experiments. The spectral analysis results indicated that the TiO₂/L/Cu₃Se₂ composite film had a good optical performance in visible and ultraviolet region. Importantly, due to its efficient charge separation for the photo-induced electron-hole pairs, the TiO₂/L/Cu₃Se₂ composite film photoanode can generate larger short-circuit current and open-circuit voltage, comparing pure TiO₂ film, which is significant in solar energy conversion and optoelectronics devices.