Fabrication of CdS quantum dots sensitized TiO2 nanowires/nanotubes arrays and their photoelectrochemical properties
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Herein, CdS quantum dots (QDs) sensitized TiO2 nanowires/nanotubes arrays (CdS–TiO2 NWs/NTs) have been successfully fabricated through an optimized one-step anodization method, followed by successively ion layer adsorption and reaction strategy. The structures of the as-prepared samples were characterized by scanning electrons microscopy, X-ray diffraction and X-ray photoelectron microscopy. Results indicated that TiO2 NWs/NTs were comprised of well-ordered NTs arrays in the bottom and NWs at the mouth of the NTs. CdS QDs were uniformly adhered to the surface of the NWs, as well as inner walls of the NTs. In addition, optical and photoelectronchemical properties of the as-prepared samples were investigated. All results showed that CdS–TiO2 NWs/NTs exhibited excellent photoelectric and photocatalytic activities, a photocurrent density of 1.04 mA cm−2, a photocatalytic efficiency of degradation methyl blue of 97.8% after 120 min, and the rate of repeated degradation maintained at 95.8% after 5 cycles. A possible mechanism was described to lay the foundation for future applications.
KeywordsTiO2 NWs/NTs arrays CdS Photoelectric performance Photocatalytic activity
Over the past few decades, Titanium dioxide (TiO2) has been received great attentions and considered as an ideal photocatalyst for solar cells and environmental purification due to its outstanding photoelectrochemical properties [1, 2, 3, 4]. Compared with pulverous TiO2, highly ordered nano-structures like nanowires (NWs), nanobelts (NBs), nanorods (NRs), nanoflakes (NFs) and nanotubes (TNs) have been as more effective architectures to improve the photocatalytic efficiency [5, 6, 7, 8]. Among these, TiO2 TNs have been regard as the most promising nanostructures for future application with remarkable light-harvesting properties, recyclability and high mass transport efficiency, in which photogenerated carriers can be easily transported along its ordered structures [9, 10].
Actually, enhancing its photocatalytic efficiency for future applications, it requires further enlarging the specific surface area to increase reactive sites and adsorption capacity of the reactants [11, 12]. Usually, the way of increasing the specific surface area is to extend the length of TiO2 TNs, Nevertheless, which would increase the recombination of electron–hole pairs during long distance transmission. Therefore, the challenge of how to further improve the specific surface area of TiO2 NTs with low recombination still remains. Moreover, TiO2 is a wide band gap semiconductor, it can only be excited by UV light, hence, only 2–3% solar energy can be used. Thus, numerous attempts were carried out to narrow this band gap and improve the light absorption capacity in the visible region, such as using non-metallic element doping (N, C, S, I, F) [13, 14, 15], organic dyes or metal chalcogenide quantum dots (QDs) sensitizing [16, 17], depositing noble metal nanoparticles (Au, Ag, Pt) [18, 19, 20], etc. Among these methods, QDs sensitized TiO2 was considered the best strategies to improve spectral absorptivity and electron transfer ability, with a vast amount of studies being reported, these include CdSe, CdS, CdTe, PbS, ZnS, PbSe and InP [21, 22, 23, 24, 25]. In the case of CdS, which has a narrow band gap (2.2 eV), it is a promising material for decorating TiO2, leading to a red shift of light absorption to visible light [26, 27]. By directly loading QDs onto the surface of TiO2 NTs, however, an accumulation at the NTs mouth would occur, which consequently seriously affects the efficiency of photoelectric conversion.
To overcome these problems, this work discovered a facile and effective synthetic strategy for the fabrication of a novel composite structure of TiO2 NWs/NTs with CdS QDs, resulting in excellent photocatalysis and photoelectronic properties. In TiO2 NWs/NTs structures, highly ordered NTs arrays were prepared on a Ti film, NWs were generated through splitting of the top of the NTs, which meaned that the NWs and the NTs was an organic whole. This unique structure is not only beneficial to utilizing the advantages of TiO2 NTs in efficiently transporting the photogenerated carriers, but also to increasing the specific surface area of TiO2 for loading additional QDs. The SILAR method was then used to synthesize CdS QDs to adhere to the TiO2 composite structure. The results showed that the compound possesses excellent photoelectrical performance and photocatalytic efficiency as well as high reusability rate.
2 Materials and methods
The TiO2 NWs/NTs arrays were fabricated using the anodic oxidation method. In a typical experiment, Tailored Ti foil (99.6% pure, 1 cm × 1 cm × 0.02 cm) was successively ultrasonic cleaned by acetone, isopropanol, ethanol and distilled water. Then the cleaned Ti foil was polished in a mixed solution of hydrofluoric acid and nitric acid for 30 s. The anodic oxidation process was conducted in ethylene glycol solution (include 0.3 wt% NH4F and 2 vol% H2O) by 55 V DC power for 5 h. The as-anodized Ti foil was cleaned by distilled water several times and then annealed in a muffle furnace at 500 °C in atmosphere for 3 h with a heating rate of 5 °C/min.
In SILAR process, the prepared TiO2 NWs/NTs arrays were successively dipped into ethanol + 0.1 M CdCl2 and methanol + 0.1 M Na2S for 5 min each time, respectively. After one immersion procedure, the sample was washed with ethanol and methanol to remove excess reagent, and then dried before entering the next dipping cycle. After this process, the prepared sample was annealed at 350 °C for 1 h in vacuum to obtain a more stable heterojunction. The prepared samples were respectively called CTWT0, CTWT1, CTWT2 and so on, according to the number of SILAR deposition cycles.
Crystal structure of the samples were characterized by X-ray diffraction (Shimadzu, XRD-6100) with Cu Kα irradiation (λ = 0.15418 nm) at a scanning rate of 4° min−1 in the range of 20° ≤ 2θ ≤ 80°. The surface morphology and the elemental concentration and distribution were analyzed by an energy dispersive spectrometer (EDS) attached to the field emission scanning electron microscopy (JSM-7800 FESEM). The chemical composition and electronic structure were determined through X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI) using Al Kα (hν = 1486.6 eV) irradiation with the electron energy analyzer operating at constant transmission energy (30 eV) and the measurement was performed under ultrahigh vacuum conditions 10−10 Torr. The UV–Vis absorption spectra were measured by a UV–visible spectrophotometer (Hitachi, U-3310). Photoluminescence measurements were performed on a fluorescence spectrophotometer (Hitachi, F-7000) using Xe lamp as excitation source.
The photoelectric behaviors were characterized using CHI 760E electrochemical workstation, a 300 W Xe lamp was used as the simulated solar light source, 0.5 mol L−1 Na2SO4 solution was used as electrolyte, whereas Pt foil and saturated Ag/AgCl were used as counter and reference electrodes, respectively. The current–voltage characteristics was tested from open circuit voltage to 1 V at a scan rate of 0.05 V s−1, and the transient current–time characteristics was measured at a bias voltage of 0.6 V for 380 s with an interval of 20 s between dark and light. To detect the photocatalytic efficiency of the prepared samples, 30 mL MB solution (5 mg L−1) was used as the contaminant in the photocatalytic test process, and this process was conducted under the same Xe lamp light source at room temperature.
3 Results and discussion
3.1 Morphological and elemental analysis
3.2 Optical and photoelectrical properties
Figure 5b shows the transient current–time characteristics of the as-prepared samples. A maximum 10% drop down is observed in the whole light process, indicating excellent charge transmission and strong photo stability. At 0.6 V applied potential, the stable photocurrent density of the four samples is 0.375, 0.598, 1.040 and 0.755 mA cm−2, respectively, which shows that those sensitized with CdS QDs are higher than pure TiO2 NWs/NTS arrays. The highest obtained value is ca. 1.040 mA cm−2, which is ca. 2.8 fold greater than the bare sample (0.375 mA cm−2). Moreover, when the number of SILAR cycles is increased by 5 times, the corresponding photocurrent density decrease to 0.755 mA cm−2, which is as a result of over surplus dense of the CdS QDs. From the insert imagine of Fig. 1d, QDs were completely wrapped around the TiO2 NWs/NTS structures and plied arsy-varsy on their surface, which would increase the interface between QDs and hinder charge transfer, hence a decline in photoelectric properties is observed .
Summarily, its excellent optoelectronic performance and photocatalytic activity of the prepared sample may be attributed to their excellent structure: CdS QDs possesses a narrow band gap, allowing the absorption spectrum of sensitized TiO2 to be extended into the visible light region; Nanowires will not only improve the surface area for increasing more reactive sites but also accommodate more QDs, compared with the usual TiO2 NTs arrays; The QDs deposited on the wall of the tube enable the light to form effective refraction and improve the utilization rate of light; NWs are grown in situ at the mouth of the NTs and closely connect between NWs and NTs, the excited electrons can be rapidly transported along NWs to vertical NTs, reducing the probability of recombination of electron–hole pairs.
A new nanostructure of CdS QDs deposited TiO2 NWs/NTs arrays were successfully fabricated by the anodic oxidation and SILAR cycle process. In this structure, NWs integrated with NTs can enlarge the surface areas for depositing more quantum dots and quickly transport electron to vertical NTs, enhancing the efficiency of photoelectric conversion. Therefore, the as-prepared samples showed an enhancement absorption of visible light, exhibiting an excellent photoelectric and photocatalytic properties. Meanwhile, this structure could be modified by other sensibilizers to further improve their performance and we will also carry out research on this issue. Thus, we believe this strategy of synthetically utilizing the advantages of TiO2 may provide a prospect for practical application in solar energy and environmental protection.
This study was funded by the Fundamental Research Funds for the Central Universities Key Project (XDJK2017B062) and the National Science Foundation for Young Scientists of China (Grant No. 51605392).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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