Ag2S/CdS/TiO2 Nanotube Array Films with High Photocurrent Density by Spotting Sample Method
© Sun et al. 2015
Received: 18 May 2015
Accepted: 22 September 2015
Published: 1 October 2015
Ag2S/CdS/TiO2 hybrid nanotube array films (Ag2S/CdS/TNTs) were prepared by selectively depositing a narrow-gap semiconductor—Ag2S (0.9 eV) quantum dots (QDs)—in the local domain of the CdS/TiO2 nanotube array films by spotting sample method (SSM). The improvement of sunlight absorption ability and photocurrent density of titanium dioxide (TiO2) nanotube array films (TNTs) which were obtained by anodic oxidation method was realized because of modifying semiconductor QDs. The CdS/TNTs, Ag2S/TNTs, and Ag2S/CdS/TNTs fabricated by uniformly depositing the QDs into the TNTs via the successive ionic layer adsorption and reaction (SILAR) method were synthesized, respectively. The X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectrum (XPS) results demonstrated that the Ag2S/CdS/TNTs prepared by SSM and other films were successfully prepared. In comparison with the four films of TNTs, CdS/TNTs, Ag2S/TNTs, and Ag2S/CdS/TNTs by SILAR, the Ag2S/CdS/TNTs prepared by SSM showed much better absorption capability and the highest photocurrent density in UV-vis range (320~800 nm). The cycles of local deposition have great influence on their photoelectric properties. The photocurrent density of Ag2S/CdS/TNTs by SSM with optimum deposition cycles of 6 was about 37 times that of TNTs without modification, demonstrating their great prospective applications in solar energy utilization fields.
As useful wide-bandgap semiconductor materials, titanium dioxide (TiO2) has been extensively used in wastewater treatment , photocatalysis [2–4], gas sensors [5, 6], photochemical water splitting [7, 8], solar cells [9, 10], etc. Because of the broad applications of TiO2 and development of nanotechnology, various TiO2 structured materials have been synthesized in recent years, such as nanoparticles [11, 12], mesoporous materials , nanofilms , nanowires [2, 14], nanobelts , nanorods , and nanotubes [17, 18]. With large surface area and high aspect ratio, nanotubes are nontoxic and environmental friendly, which have attracted much attention in many fields. In 1999, Zwilling et al. reported the preparation of the TiO2 nanotube arrays (TNTs) using electrochemical anodization . Since then, many studies have focused on TNTs with advanced structures and improved properties [10, 20–23]. Unlike general TiO2 nanotubes with random arrays , light comes readily inside TNTs and electrons transfer freely due to the vertical structure and close packing inside the nanotube arrays.
With a bandgap of 3.2 eV, TiO2 can absorb only ultraviolet light with a wavelength less than 380 nm, which leads to a very low efficiency of sunlight utilization. In order to solve this problem, the TNTs are usually modified using various methods. Dye sensitization  and doping noble metals  or semiconductor materials  are typical methods in the preparation of hybrid TNT materials, which can widen spectral response range to visible light and exhibit better photoelectric properties. Among various photosensitizers, some large bandgap inorganic semiconductor quantum dots (QDs), such as CdS [26–28], CdSe , and PbS , are usually used as dopants. These QDs have different valence band and conduction band energy from TiO2. In addition to widen the range of visible light response, the QDs/TiO2 hybrids can improve charge separation capability and minimize charge carrier recombination probabilities. However, little has been focused on the use of small bandgap semiconductors in the field of sensitizers because photoelectrons and holes recombine readily, though they may absorb a wide range of sunlight.
To our best knowledge, nearly all sensitizers are uniformly distributed on TiO2. In this study, we prepared the hybrid nanotube array films of Ag2S/CdS/TNTs by spotting sample method (SSM). Firstly, nanoporous TNTs were prepared using electrochemical anodization under controlled reaction conditions, which have better photoelectric properties ; secondly, CdS was deposited inside TNTs via the successive ionic layer adsorption and reaction (SILAR) method, which can widen visible light response spectrum; thirdly, Ag2S was deposited in the local domain of the CdS/TNTs using spotting sample method. As a narrow-bandgap (0.9 eV) semiconductor material, Ag2S strongly absorbs visible light. However, too much Ag2S deposition can lead to clog-up of nanotube inlet, which could decrease the sunlight absorption capabilities of TiO2 nanotubes. In order to overcome this problem, a narrow-gap semiconductor material, Ag2S, was deposited in the local domain of CdS/TNTs by spotting sample method, which may result in a lower coverage of Ag2S on TNTs, and the presence of these three semiconductor materials can decrease the recombination possibilities of photoelectrons and holes. Five different films, Ag2S/CdS/TNTs by SSM, TNTs, CdS/TNTs, Ag2S/TNTs, and Ag2S/CdS/TNTs by SILAR, were prepared and analyzed to compare their photoelectro-chemical properties.
Chemicals and Materials
All the chemicals used are analytical grade, and the titanium foil (0.3 mm thick, purity > 99.9 %) is obtained from Sumitomo. Milli-Q water is from a three-stage Milli-Q plus 185 purification system with a resistivity larger than 18.2 MΩ · cm.
Preparation of TiO2 Nanotube Array Films
TNTs were prepared by anodic oxidation. Titanium foils were tailored into small pieces (2.5 × 3.0 mm). In order to be dust-free and oil-free on the titanium foils, titanium pieces were ultrasonic cleaned for 30 min in deionized water and acetone, respectively. After that, titanium foils were polished in mixed solution of HF (40 %):HNO3 (65 %):H2O = 1:4:5 (volume) for 3 min, then were rinsed in Milli-Q water and dried in nitrogen gas. Finally, anodic oxidation was carried out in electrolyte solution (0.8 g NH4F + 150 g HOCH2CH2OH + 0.5 mL 40 % HF) at 10 °C for 30 min. The crystalline phase anatase-TiO2 was obtained using thermal treatment at 450 °C for a period of 2 h. In this experiment, the constant voltage was 60 V, current is 0.07~0.10 A/cm2, the titanium foils were used as anode, and a graphite rod served as a counter electrode.
Preparation of Ag2S/CdS/TiO2 Hybrid Nanotube Array Films
Surface morphology of nanotube array films and direct cross section of TNTs thickness measurements were carried out using a JEOL JSM-6700F field emission scanning electron microscopy (FE-SEM). The morphology and microstructure of TNTs coupled with CdS and Ag2S.
QDs were characterized using a JEOL JEM-1400 transmission electron microscope (TEM) and a JEOL JEM-2100F high-resolution transmission electron microscope (HR-TEM). Energy dispersive X-ray spectroscopy (EDX) was also carried out in the TEM. The X-ray powder diffraction (XRD) patterns were collected on a Bruker D8 advance X-ray diffractometer with Cu Kα (λ = 0.15418 nm) radiation for structure analysis. The surface composition of nanotube array films and binding energy were characterized using ESCALAB 250 X-ray photoelectron spectrum (XPS). The optical absorption of TNTs and hybrid QDs/TNTs was characterized using TU-1901 UV-vis diffuse reflectance absorption spectra (DRS).
Photocurrent measurements were carried out with a model 263A potentiostat/galvanostat and a three-electrode test cell without applying any bias. The TNTs, CdS/TNTs, Ag2S/TNTs, Ag2S/CdS/TNTs by SSM, and Ag2S/CdS/TNTs by SILAR method were subsequently used as the working electrode. A platinum wire was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The measurements were carried out with a 300-W xenon lamp (PLS-SXE300/300UV) as light irradiation source. 0.20 mol · L−1 Na2S and 0.20 mol · L−1 Na2SO3 aqueous solutions were used as electrolyte with sacrificial agent. The incident photon to current conversion efficiency (IPCE) measurements was performed employing a 150-W Xe lamp coupled with a computer-controlled monochromator. Electrochemical impedance spectroscopy (EIS) was performed under illumination with an AC amplitude of 5 mV and frequency range between 100 kHz and 0.1 Hz.
Results and Discussion
Ag2S/CdS/TNTs by SSM are fabricated via spotting sample method. SEM, TEM, XRD, and XPS results show that most of the CdS and Ag2S can be deposited into the TNTs and some Ag2S particles are deposited on the surface of the array films. The Ag2S nanoparticles deposited in the local range of the CdS/TNTs and Ag2S/CdS/TNTs by SSM may widen the absorption spectra of TNTs significantly to the visible light region and even enhance the absorption of the UV light (320–800 nm). Unlike TNTs, CdS/TNTs, Ag2S/TNTs, and Ag2S/CdS/TNTs via SILAR, Ag2S/CdS/TNTs by SSM have a much larger photocurrent under UV-vis light irradiation. The photocurrent density of Ag2S/CdS/TNTs by SSM with an optimal number of deposition cycles of 6 was about 37 times than that of TNTs under light (320–800 nm) irradiation of Xe lamp. With better photoelectric properties and stability of Ag2S/CdS/TNTs by SSM, they have good prospective application in solar energy utilization fields.
This work was financially supported by the NSFC (Grant Nos. 21420102006 and 21273134).
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