CdS doped TiO2 nanotube composite was fabricated by chemical bath deposition, and was characterized by the structural, spectral and photoelectrochemical properties. The results of the structural and spectral properties showed that CdS particles were successfully deposited onto the surface of TiO2 nanotube. It is demonstrated that CdS doped TiO2 nanotube composite improved the light harvesting ability. Power conversion efficiency of about 0.32% was observed. This value is about 2.9 times higher than that of pure TiO2 nanotube. The CdS doped TiO2 nanotube composite possesses relatively higher photocatalytic activity and photodegradation efficiency than that of pure TiO2 nanotube under UV light irradiation, and the degradation efficiency of methyl orange was about 42% at UV intensity of 32 W.

INTRODUCTION

In recent decades, nanocomposites have attracted much attention due to their special and excellent properties in optics, electronics, magnetism and chemistry (Maurya & Chauhan 2011). TiO2 nanotube obtained by the anodization of Ti sheet attract more interest due to their large specific surface area and the precisely oriented nature of the nanotube arrays, which can improve the charge-collection efficiency by promoting the electron/hole transport and slower recombination (Zhu et al. 2007). However in photocatalysis, pure TiO2 nanotube demonstrates limited photoactivity since it absorbs only in the UV region which constitutes only about 4–5% of the solar spectrum (Wilson et al. 2012). To overcome this drawback, various methods have been studied including the substitutional non-metallic doping such as C, S, N, etc. (Ohno et al. 2004; Morawski et al. 2009; Zhang et al. 2010), and metallic doping such as Ag, Pt, Au, etc. (Stathatos et al. 2001; Chen et al. 2012; Tian et al. 2012). In contrast, the modifications of TiO2 nanotubes through coupling with a visible-light semiconducting materials can effectively improve the light-absorbing ability (Liu et al. 2013). CdS is normally considered as an important sensitizer since it has a narrow band gap (2.4 eV) with a higher conduction band level than that of TiO2 (Zhu et al. 2009). Therefore, CdS can induce an efficient charge separation by minimizing the electron–hole recombination in excited TiO2.

The application of CdS to TiO2 nanotube modification has been well investigated by some researchers. Kang et al. (2010) developed a ternary hybrid CdS/Pt-TiO2 nanotube photoelectrode by dipping and deposition as well as successive ionic layer adsorption and reaction (SILAR). Shao et al. (2012) deposited the CdS nanoparticles on the surface of TiO2 nanotube using the electrodeposition method. Li et al. (2011) reported that highly ordered TiO2 nanotube arrays modified by CdS, CdSe and ZnS quantum dots were successfully fabricated by chemical bath deposition (CBD) method and expanded the photoresponse range of TiO2 nanotube arrays from the ultraviolet region to visible region. Maurya & Chauhan (2011) revealed the advantage of CdS/TiO2 nanocomposite prepared by a simple co-precipitation method. Wilson et al. (2012) reported that CdS/TiO2 nanotube composite was prepared by a SILAR and photocatalytic degradation of methyl orange (MO) was about 35%.

In this study, CdS doped TiO2 nanotube composite was fabricated by the CBD method. Photocatalytic degradation of MO under UV irradiation was investigated. Also, the CdS doped TiO2 nanotube composite was characterized by the structural, spectral and photoelectrochemical properties.

MATERIALS AND METHODS

Preparation of TiO2 nanotube

The TiO2 nanotube was prepared on Ti foil using the potentiostatic anodization method. All the anodization experiments were carried out at room temperature using a commercially available Ti foil (99.5% purity, 0.5 mm thickness) as the working electrode, a standard Pt foil as the counter electrode. Before anodization, Ti foil was degreased by sonication in acetone, ethanol and deionized (DI) water for 30 minutes. Then Ti foil was pre-anodized in ethylene glycol (EG) electrolyte composed of 0.15 M NH4F and DI water at 60 V for 5 minutes. After anodization, TiO2 nanotube was rinsed with distilled water and dried in the air to wipe off the debris on the surface of TiO2 nanotube. Then TiO2 nanotube samples were annealed at 450 °C for 30 minutes in ambient air.

Fabrication of CdS doped TiO2 nanotube composite

The TiO2 nanotube was sensitized with CdS by the CBD method. The solutions, used for the deposition of CdS particles on the surface of TiO2 nanotube, were 0.5 M Cd(NO3)2 in ethanol and 0.5 M Na2S in methanol. The dipping solution temperature was maintained to be constant at 35 °C using a water bath. The TiO2 nanotube sample was first dipped into 0.5 M Cd(NO3)2 ethanol solution for 5 minutes, rinsed with ethanol and then dried in the air. The TiO2 nanotube sample has to be absolutely dry before it was dipped into the Na2S methanol solution to ensure that the CdS particles were only deposited on the TiO2 nanotube, not on the Ti foil surface. The dried TiO2 nanotube sample was dipped into 0.5 M Na2S methanol solution for 5 minutes, again rinsed with methanol and then dried in the air. The two-step dipping procedure is termed as one CBD cycle and the incorporated amount of CdS can be increased by repeating the assembly cycle (a total of four cycles).

Characterization

The structural characterization of the samples were carried out using field emission scanning microscopy (FE-SEM, Hitachi SU-70, Japan) equipped with an energy dispersive X-ray spectrometer (EDS). The crystal structure of pure TiO2 nanotube and CdS doped TiO2 nanotube was examined by X-ray diffraction (XRD, PANalytical X'pert PRO, Germany). The spectral characterization of the samples were analyzed by Fourier transform infra-red spectroscopy (FT-IR, Bruker Optics Vertex 80 & Hyperion 3000, USA) and photoluminescence (PL, Hitachi F-7000, Japan). The absorption spectra were recorded by UV–Vis spectroscopy (UV–Vis NIR spectrophotometer, Hitachi U-4100, Japan). The current density–voltage curves were performed under AM 1.5 G simulated sunlight illumination (100 mW/cm2) using an electrochemical workstation (604D, CH Instruments, USA).

Photocatalytic degradation of MO

The photocatalytic activities of pure TiO2 nanotube and CdS doped TiO2 nanotube were evaluated by degradation of MO as a target pollutant. All the experiments were performed in a 300 mL cylindrical glass reactor at a controlled temperature of 25 °C. UV sources were a 8 W × 4 high pressure mercury lamp of UV-A (352 nm), UV-B (306 nm) and UV-C (254 nm) lamp, respectively. The sheet of pure TiO2 nanotube and CdS doped TiO2 nanotube with an area of approximately 4 × 4 cm2 was soaked in 200 mL MO (concentration of 1 ppm) solution to achieve the degradation by UV irradiation. 0.1 M Na2S was added into the solution to suppress the photo-corrosion of CdS. Color and UV–Vis absorbance of MO samples were analyzed periodically using a DR-2010 (HACH) and spectrophotometer (Analytik Jena, SPECORD 50) by withdrawing small aliquots (25 mL) every 10 minutes. The relative absorbance of MO by UV–Vis spectrophotometer was determined at 465 nm. After each analysis, the sample solution was returned back to the photo-reactor. The samples were used several times to test the repeatability of their photocatalytic performance. All the data presented here have been analyzed at least twice.

RESULTS AND DISCUSSION

Structural characterization of pure TiO2 nanotube and CdS doped TiO2 nanotube

TiO2 nanotube was prepared by anodization using an electrolyte composed of EG, 0.15 M NH4F and DI water at 60 V for 5 minutes. Figures 1(a) and 1(b) show the SEM images of pure TiO2 nanotube. It is clearly seen that diameter and length of pure TiO2 nanotube were about 74.5 nm and 5.01 μm, respectively. In Figure 1(c), it was seen that CdS is successfully deposited on the surface of TiO2 nanotube. However, it can be seen that CdS deposits aggregate at the surface of TiO2 nanotube and nanopores was partially clogged by them. To analyze chemical composition of CdS doped TiO2 nanotube composite, EDS result is indicated in Figure 1(d). EDS of CdS doped TiO2 nanotube composite was composed of elements, 32.69 At% Ti, 57.62 At% O, 3.05 At% S and 3.64 At% Cd. The atomic ratio of S versus Cd is close to 1:1, which is consistent with that of the stoichiometric CdS compound, as expected for the format of CdS compound. The atomic ratio of Ti and O in the EDS was slightly close to 1:1.7, which did not substantiate the stoichiometric formula of TiO2.

Figure 1

SEM images of top (a) and cross-sectional (b) views of pure TiO2 nanotube. Top view of CdS doped TiO2 nanotube (c) and EDS of CdS doped TiO2 nanotube (d).

Figure 1

SEM images of top (a) and cross-sectional (b) views of pure TiO2 nanotube. Top view of CdS doped TiO2 nanotube (c) and EDS of CdS doped TiO2 nanotube (d).

XRD is conducted to characterize the phase structures of both pure TiO2 nanotube and CdS doped TiO2 nanotube. The XRD patterns of pure TiO2 nanotube and CdS doped TiO2 nanotube are shown in Figure 2. Peaks, marked ‘A’, ‘T’ and ‘C’ in Figure 2, correspond to anatase phase, Ti and CdS, respectively. The diffraction peaks located at 2θ = 25.31°, 37.03°, 37.93°, 38.39°, 47.9°, 54.00°, 55.00° and 62.94° were correspondingly attributed to (101), (103), (004), (112), (200), (105), (211) and (204) planes of anatase TiO2, respectively. CdS diffraction peaks at 2θ = 28.34° and 32.84° corresponded to (111) and (002) planes of the hexagonal CdS, respectively. The results of SEM, EDS and XRD patterns confirmed the formation of CdS doped TiO2 nanotube composite.

Figure 2

XRD patterns of pure TiO2 nanotube and CdS doped TiO2 nanotube.

Figure 2

XRD patterns of pure TiO2 nanotube and CdS doped TiO2 nanotube.

Spectral characterization of pure TiO2 nanotube and CdS doped TiO2 nanotube

FT-IR spectroscopy has been extensively used for identifying the various functional groups on the TiO2 nanotube itself, as well as for identifying the adsorbed species and reaction intermediates on the surface of TiO2 nanotube. FT-IR spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube are indicated in Figure 3. A FT-IR spectrum of pure TiO2 nanotube shows a vibrational peak at 2931 cm−1 and 1920 cm−1, respectively, which is attributed to bending vibration of surface hydroxyl groups. The band at 1023 cm−1 represents the stretching vibration of the Ti–O bond and the band at 815 cm−1 is due to the Ti–O–Ti vibrational band of TiO2 nanotube. In the CdS doped TiO2 nanotube, the hydroxyl group peaks appeared at 2711 cm−1 and 1714 cm−1, respectively. The band at 975 cm−1 and 814 cm−1 still exist in CdS doped TiO2 nanotube corresponding to the Ti–O bond and Ti–O–Ti vibrational band of pure TiO2 nanotube, respectively. The band corresponding to Cd–S vibration appeared at 624 cm−1. The band at 1413 cm−1 is attributed to the stretching vibration of C–OH bond.

Figure 3

FT-IR spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube.

Figure 3

FT-IR spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube.

UV–Vis diffuse reflectance spectra were performed to test the light-harvesting capability of pure TiO2 nanotube and CdS doped TiO2 nanotube. Figure 4 shows the UV–Vis diffuse reflectance spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube. It is found that pure TiO2 nanotube only exhibits the absorption band at UV region around 300–380 nm, corresponding to a band gap of 3.2 eV of anatase TiO2. After doping with CdS, stronger UV intensity in both ultraviolet and visible light was observed in comparison with the pure nanotube. The absorption band of CdS doped TiO2 nanotube composite is extended to the visible range, and wavelength is shifted around 400–490 nm, which facilitates the absorption of the abundant visible light. It seems that the spectrum of CdS doped TiO2 nanotube was somewhat shifted to the left. Until now, however, there has been no data to explain exactly why the vibrational peak was shifted. Only we imply that the bonding energy of Cd–hydroxyl complex is greater than that of Ti–hydroxyl complex (Zha et al. 2014). In addition, this shift of spectrum can be caused by the addition of sodium and nitrate used for the fabrication of CdS doped TiO2 nanotube.

Figure 4

UV–visible diffuse reflectance spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube.

Figure 4

UV–visible diffuse reflectance spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube.

The PL spectra are related to the transfer behavior of the photo-generated electrons and holes. A lower PL intensity of the emission peak means a lower electron–hole recombination rate, and hence a longer life of photo-generated carriers (Zhao et al. 2013). Figure 5 shows the PL spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube. The excitation wavelength of samples was 341 nm and the spectra were monitored in range of 350–500 nm. As shown in Figure 5, pure TiO2 nanotube and CdS doped TiO2 nanotube are similar to the same position of emission peak, whereas PL intensity of CdS doped TiO2 nanotube was lower than that of pure TiO2 nanotube. The weaker intensity of CdS doped TiO2 nanotube compared to the pure TiO2 nanotube shown in Figure 5 is due to the loading of the semiconductor CdS which allows more efficient light harvesting and results in an increase in the quantity of photo-generated charge carriers. This indicated that the recombination of photo-induced electron–hole pairs in CdS doped TiO2 nanotube composite was inhibited effectively.

Figure 5

PL spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube.

Figure 5

PL spectra of pure TiO2 nanotube and CdS doped TiO2 nanotube.

Photoelectrochemical characterization of pure TiO2 nanotube and CdS doped TiO2 nanotube

The current density–voltage curves of pure TiO2 nanotube and CdS doped TiO2 nanotube under the visible-light irradiation are shown in Figure 6, and their open circuit voltages (Voc), short circuit current density (Jsc), fill factors (FFs) and power conversion efficiency (PCE) are listed in Table 1. As shown in Figure 6 and Table 1, it is observed that CdS doped TiO2 nanotube exhibits much higher short circuit current density, and the Jsc is 2.2 times higher than that of pure TiO2 nanotube. Moreover, power conversion efficiencies of pure TiO2 nanotube and CdS doped TiO2 nanotube are about 0.11% and 0.32%, respectively, and CdS doped TiO2 nanotube is 2.9 times higher than that of pure TiO2 nanotube. The improved current density and efficiency show that the co-sensitization has influenced the photovoltaic performance significantly. The following reasons should be ascribed to the enhanced photoelectrical performance of CdS doped TiO2 nanotube composite. First, as confirmed by the UV–Vis spectrum (see Figure 4), more solar light can be harvested by the CdS doped TiO2 nanotube due to its broad absorption range. Secondly, more efficient light can be absorbed by increasing the quantity of photo-generated charge carriers due to the effective charge, and therefore a higher PCE is achieved (Li et al. 2010). To further improve the photovoltaic performance, it is necessary to pay attention to the study of some narrower band gap sensitizers such as CdSe, PbS and co-sensitizers which have been proved to be more effective in visible light harvesting.

Table 1

Photovoltaic performance parameters of CdS doped TiO2 nanotube composite

Photoanode Voc (V) Jsc (mA/cm2FF PCE (%) 
Pure TiO2 nanotube 0.64 0.51 0.36 0.11 
CdS doped TiO2 nanotube 0.70 1.11 0.40 0.32 
Photoanode Voc (V) Jsc (mA/cm2FF PCE (%) 
Pure TiO2 nanotube 0.64 0.51 0.36 0.11 
CdS doped TiO2 nanotube 0.70 1.11 0.40 0.32 

Voc, open circuit voltages; Jsc, short cirtuit current density; FF, fill factors; PCE, power conversion efficiency.

Figure 6

Current density–voltage curves of pure TiO2 nanotube and CdS doped TiO2 nanotube.

Figure 6

Current density–voltage curves of pure TiO2 nanotube and CdS doped TiO2 nanotube.

Photocatalytic activity of pure TiO2 nanotube and CdS doped TiO2 nanotube

The photocatalytic activities of pure TiO2 nanotube and CdS doped TiO2 nanotube were evaluated by photocatalytic degradation of MO solution under UV irradiation. Figure 7(a) shows the comparison of the photodegradation rate of pure TiO2 nanotube and CdS doped TiO2 nanotube prepared by different UV lamp types. The photodegradation rate of MO increased in order of UV lamp type C > B > A with respect to the pure TiO2 nanotube and CdS doped TiO2 nanotube. Under the same condition of UV-C 32 W, photodegradation efficiencies of pure TiO2 nanotube and CdS doped TiO2 nanotube were about 37% and 42% after UV irradiation of 100 minutes, respectively, and CdS doped TiO2 nanotube was 1.15 times higher than that of pure TiO2 nanotube due to the effective separation of the photo-generated electron–hole pairs by the doping of CdS (Zhang et al. 2013). When UV-C (254 nm) was applied, the relatively higher photodegradation rate can be caused by abundant reactive species formed at the surface of TiO2 catalyst (Wong & Chu 2003; Jeong et al. 2004). However, there is no explanation for the low photodegradation rate of CdS doped TiO2 nanotube at UV-A and UV-B.

Figure 7

(a) Photodegradation rate and (b) UV–Vis absorbance of MO under UV irradiation of 32 W at different UV lamp types.

Figure 7

(a) Photodegradation rate and (b) UV–Vis absorbance of MO under UV irradiation of 32 W at different UV lamp types.

Figure 7(b) shows the UV–Vis absorbance of MO degraded by the pure TiO2 nanotube and CdS doped TiO2 nanotube at different UV lamp types. It is observed that the absorption peak of MO was the wavelength of 465 nm. The lowest absorbance of MO photodegradation was indicated in CdS doped TiO2 nanotube composite after 100 minutes under the conditions of UV-C type and UV intensity of 32 W. When illuminated by UV, CdS and TiO2 nanotubes are excited and electron–hole pairs are produced (Equation (1)). Since the conductive band of CdS is more negative than that of TiO2 nanotube, the excited electrons from CdS can quickly transfer to the TiO2 nanotube (Equation (2)), whereas holes can accumulate in the valence band of CdS. Hydroxyl ions scavenge these holes to form strongly oxidizing hydroxyl radicals (Equation (3)). Thus, the interfacial electrons transfer from CdS to TiO2 nanotube can effectively separate the electrons and holes, inhibiting the recombination of electron–hole pairs. Electrons will react with oxygen adsorbed on the surface of TiO2 nanotube to form superoxide radicals (Equation (4)), which participate in a series of steps to form hydroxyl radicals (Equation (5)). It is the superoxide radicals and hydroxyl radicals, which are responsible for the degradation of organic compounds (Equation (6)). In addition, holes participate in the direct oxidation of organic compounds (Equation (7)). The degradation process can be expressed as follows (Li et al. 2009): 
formula
1
 
formula
2
 
formula
3
 
formula
4
 
formula
5
 
formula
6
 
formula
7

CONCLUSIONS

We have successfully fabricated a CdS doped TiO2 nanotube composite by CBD method of TiO2 nanotube and CdS precursors. The CdS doped TiO2 nanotube composite was extensively examined by the structural and spectral properties. These results showed that CdS was successfully deposited on the surface of TiO2 nanotube. It reveals that the CdS doped TiO2 nanotube composite can inhibit the recombination of electron–hole pairs and enhance the adsorption performance both in the UV light range and the visible-light range. The CdS doped TiO2 nanotube composite displayed excellent photoelectrochemical performance, with Jsc and PCE values of about 2.2 times and 2.9 times higher than that of pure TiO2 nanotube, respectively. The enhanced photoelectrical performance can harvest more solar light. The CdS doped TiO2 nanotube composite possesses relatively higher photocatalytic activity and photodegradation efficiency than that of pure TiO2 nanotube under UV light irradiation.

ACKNOWLEDGEMENTS

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2013R1A2A1A09007252).

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