Polymer brushes assisted loading of high density CdS/CdSe quantum dots onto TiO₂ nanotubes and the resulting photoelectric performance

Abstract

Cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dots (QDs) are sequentially loaded onto anodized TiO₂ nanotubes to fabricate a CdS/CdSe co-sensitized photoelectrode for visible light response. These QDs are loaded on TiO₂ nanotubes by polymer brushes assisted in situ formation, and they are distributed uniformly around both inner and outer walls of TiO₂ nanotubes. The interpenetrating polymer can provide a superior nano-confined environment and will prevent the nanoparticles from growing further during nucleation. More importantly, the potential advantage of using polymer brushes is the capability of tuning the loading amount of QDs via multiple cation exchange. The photoresponse of CdS/CdSe-TiO₂ nanotubes has been efficiently extended into the visible-light region, indicating potential applications in solar cell devices.

---

1. Introduction

Titanium oxide (TiO₂) is one of the most important inorganic materials in fabricating photovoltaic devices such as dye-sensitized solar cells (DSSCs), quantum dots solar cells and TiO₂/polymer solar cells.^1–7 Among different types of titania nanomaterials, anodized TiO₂ nanotubes have attracted widespread interest through being a promising one-dimensional nanostructured material for constructing solar energy conversion assemblies,⁸ due to their high surface-to-volume ratio available for dye adsorption and their good electronic transmission efficiency.^9–11 However, the wider band gap (3.2 eV) of anatase TiO₂ limits its photoresponse to the ultraviolet (UV) region and thus hinders the efficient utilization of solar energy. In order to make TiO₂ responsive to visible light, many strategies have been proposed, mainly based on photosensitization of TiO₂ materials by visible-responsive sensitizers, including dye sensitizing,¹² making compounds that include quantum dots,¹³ and element doping.¹⁴

Till now, organic dyes have been regarded as one of the most efficient sensitizers for TiO₂ materials for solar cells, and the solar cell conversion efficiencies have achieved about 11%.^15,16 However, the poor stability and high cost of organic dyes limit their applications. In recent years, quantum dots (QDs) have emerged as a promising candidate that can trigger a wider light absorption range and have potential applications in photovoltaic cells,¹⁷ which have opened new approaches to design and fabricate QDs or QDs combinations for promising solar cells efficiencies and for improving the stability of the material. These semiconductor QDs with tunable energy gaps offer new opportunities for solar energy conversion, in which the size quantization effect makes the photoresponse occurr over a wider spectral region via tuning the size of QDs.¹⁸ In addition, the quantum confinement effect enables the possibility of generating multiple electron–hole pairs per photon through the impact ionization effect.¹⁹ Many methods have been developed to sensitize TiO₂ nanotubes (TiO₂ NTs) for visible light harvesting, such as chemical bath deposition,^20,21 cathodic reduction,²² close space sublimation technique,²³ spray pyrolysis deposition,²⁴ organic linker assisted deposition^25,26 and electrochemical atomic layer deposition.²⁷ Chemical bath deposition is very popular for generating CdS QDs on TiO₂ NTs, a photoelectrochemical cell efficiency of 4.15% has been attained.²⁰ Single CdS QDs can not overlap the entire visible solar spectrum, so CdS and CdSe QDs were both assembled onto nanoparticulated TiO₂ films for favoring the separation and transfer of photoinduced electrons and holes, and 4.22% efficiency was attained.²⁸

Surface initiated polymerization can graft polymer brushes onto a wide variety of substrates to prepare functional surfaces.^29–33 Polymer brushes can form a nano-confined environment for fabrication of quantum dots, so that their size will be well controlled with uniform distribution.^34,35 Directly grafting polymer brushes via surface-initiated polymerization on TiO₂ NTs with chemical bonding will provide a superior platform to introduce diverse QDs onto TiO₂ NTs, thus we can finely tune the photoresponse properities. Catechol and its derived compounds can self-assemble on various inorganic and organic materials with a good adhesion,^36,37 which is an effective approach to modify TiO₂ surfaces with polymer chains chemically tethered to an interface. Here, we fabricated a high density polymer brushes modified TiO₂ NTs via surface initiated polymerization using catecholic initiator.^38–40 Then it was followed by simple gas–solid and liquid–solid reactions to obtain narrow size distributed CdS and CdSe QDs with high coverage on the inner and outer walls of TiO₂ NTs, which can be ascribed to the grafted polymer chains that stabilized newly formed QDs and prevented their further collisions, which therefore prevented nucleation. The light absorption of the as-prepared materials was expanded to the visible light region, as shown by photoelectrochemical cell measurements, and these composite materials showed good photocurrent and photovoltage response properties.

2. Experimental

2.1. Reagent

Initiator 2-bromo-2-methyl-N-[2-(3,4-dihydroxy-phenyl)-ethyl]-propionamide was synthesized according to previous studies.⁴¹ Copper(I) bromide (CuBr) was purified by reflux in acetic acid. 3-hydroxytyramine hydrochloride was obtained from Aldrich. The ultrapure water used in all experiments was obtained from a NANOpure Infinity system from Barnstead/Thermolyne Corporation. Other reagents were used as received.

2.2. Synthesis of monomer cadmium dimethacrylate (Cd(MA)₂)

32.1 g CdO, 45 mL methacrylic acid (MA) and 60 mL distilled water were added to a 250 mL flask and the mixture was refluxed for 2 h at 60 °C. Then the product was filtrated and the filtrate was vacuum distilled to remove some water and the unreacted MA, the remainder was cooled and further collected by filtration and recrystallized from ethanol to give white flaky crystals, which are the cadmium dimethacrylate (Cd(MA)₂) target product.⁴² Finally, it was dried in a vacuum oven at room temperature for 24 h.

2.3. Substrate preparation, initiator modification and surface-initiated polymerization

Highly ordered TiO₂ NTs were prepared by a two-step potentiostatic anodization using commercial Ti foil as the anode, with a graphite slice as the counter electrode. The voltage was applied using a DC power supply (LW10J2, Shanghai). Before anodization, the titanium foils were washed with ethanol and acetone by ultrasonication to remove the oil layer, then washed with distilled water and dried in the air. The first anodization was carried out in ethylene glycol electrolyte containing 0.5 wt% NH₄F at 20 °C for about 2 h under a voltage of 60 V, then the TiO₂ nanotube films were easily peeled off in 1 M HCl under ultrasonication, resulting in bowl-like footprints on the Ti foil. The second anodization was carried out under the same conditions, with the addition of 0.5% HF in the electrolyte, for about 2 h to obtain highly ordered TiO₂ NTs. The as-prepared samples were annealed at 450 °C for 2 h in air to induce crystallinity to obtain anatase TiO₂ NTs.^43,44

The initiator modification process: anodized TiO₂ NTs (4.5 cm²) were immersed in 1.5 mg mL⁻¹ solution (ethanol : distilled water = 1 : 4) of initiator and stirred for 24 h in the dark under N₂ protection at room temperature. The initiator modified TiO₂ NTs were washed with distilled water and ethanol.

Surface-initiated polymerization on TiO₂ NTs: 2 g Cd(MA)₂ monomer and 20 mL distilled water were placed in a flask under N₂ flow and stirred for about 30 min to dissolve the monomer, then 60 mg bipyridyl and 30 mg CuBr were charged into the flask under N₂ flow, 30 min later the initiator modified TiO₂ NTs were added into the monomer solution, the polymerization was performed at 60 °C under N₂ protection for 4 h. The polymer-grafted TiO₂ NTs (denoted as TiO₂ NTs-PMAA-Cd) were taken out of the polymerization solution and washed with distilled water and ethanol, then dried under N₂ flow. The whole polymer grafting process on TiO₂ NTs is shown in Scheme 1(A).

Scheme 1 (A) Preparation of polymer brushes grafted TiO₂ NTs. (B) Successive loading of CdS/CdSe QDs onto TiO₂ NTs.

2.4. Preparation of CdS/CdSe QDs sensitized TiO₂ NTs

The polymer brushes grafted TiO₂ NTs were put into an evacuated vial and H₂S gas was injected into it. The gas–solid reaction was performed for 48 h at room temperature to convert the Cd ions into CdS nanoparticles. For reusing the polymer chains to prepare CdSe nanocrystals, CdS-PMAA-TiO₂ NTs were immersed in 30 mL 0.5 M NaOH (methanol as the solvent). After stirring for 12 h, the sample was purified using centrifugation with copious ultrapure water and ethanol, then the Cd ion exchange was carried out in 30 mL 0.5 M Cd(Ac)₂ (methanol as the solvent), then CdS-PMAA-Cd-TiO₂ NTs were put into Na₂SeSO₃ solution at 50 °C for 3 h to prepare CdSe/CdS-TiO₂ NTs. Sodium selenosulphate (Na₂SeSO₃) was prepared by refluxing Se (0.3 M) in an aqueous solution of Na₂SO₃ (0.6 M) at 70 °C for about 7 h.²⁸Scheme 1(B) depicts the fabrication process for CdS/CdSe QDs decorated TiO₂ NTs.

2.5. Photoelectrochemical tests

Photoelectrochemical measurements were carried out using a two electrode cell system: CdS/CdSe QDs sensitized TiO₂ NTs as the photoanode and Pt foil as the counter electrode. The measurements were carried out in 0.1 M Na₂S solution as the redox couple, a 150 W xenon lamp was used as the monochromatic light source (SM-25 Hyper Monolight, Japan) and a Keithley 2635A System SourceMeter was used to record the current values, the active area of the photoelectrochemical cell is 0.16 cm². The photocurrent and photovoltage response measurements were performed at room temperature using a CHI 660B electrochemical workstation (Shanghai, China), a conventional three-electrode cell was used, including a saturated Ag/AgCl electrode (SCE) as the reference electrode, a platinum counter electrode, and CdS and CdSe QDs sensitized TiO₂ NTs as the working electrode. The light intensity is 150 mW cm⁻² under simulated sunlight and the cell area is also 0.16 cm².

2.6. Characterization

Attenuated total reflection infrared (ATR-IR) spectra were measured using a Nicolet iS10 instrument (Thermo Nicolet Corporation). Chemical composition information about the samples was obtained by X-ray photoelectron spectroscopy (XPS), the measurement was carried out on a PHI-5702 multifunctional spectrometer using Al Kα radiation, and the binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The morphology of CdSe/CdS-TiO₂ NTs was determined by field emission scanning electron microcopy (JSM-6701F, JEOL Inc., Japan), and energy dispersive spectroscopy (EDS) was also carried out with a FESEM (JSM-6701F), which was used for the cross-sectional elemental analysis of the nanotubes. The phase identification of the samples was conducted with powder X-ray diffraction (XRD) experiments on a RigaKu D/max-RB diffractometer with Ni-filtered graphite-monochromatized Cu-Kα radiation (λ = 1.54056 Å). UV-vis diffuse-reflectance spectra were measured at room temperature using a Lambda 950 UV-vis spectrometer (Perkin Elmer).

3. Results and discussion

3.1. ATR-IR spectrum

The polymer grafting was first confirmed by ATR-IR spectra. The peak that appears at 1544.2 cm⁻¹ is assigned to the asymmetric vibration of the carboxylate groups in TiO₂ NTs-PMAA-Cd (Fig. 1A(c)). After reacting with H₂S gas, the carboxylate groups were almost converted to protonated carboxylic groups as indicated by the appearance of peak at 1704.6 cm⁻¹ attributed to the carboxylic stretching vibration (Fig. 1B(a)). These results indicate that the ionized carboxylate form of PMAA-Cd is fully converted to the protonated carboxylic form after reaction with H₂S gas, which suggests that approximately all the Cd(MA)₂ molecules have reacted with H₂S gas to generate CdS nanoparticles. TiO₂ NTs-PMAA-CdS-Cd was prepared by exchanging with Na⁺ and reloading Cd²⁺ ions into polymer chains, the IR absorbance of PMAA-CdS-Cd appears at 1542.0 cm⁻¹ due to the ionized carboxylate (Fig. 1B(c)) and it has similar absorption to that of the sodium ionized carboxylate form at 1543.7 cm⁻¹ (Fig. 1B(b)).

Fig. 1 ATR-IR spectra of A: (a) Anodized TiO₂ NTs, (b) TiO₂ NTs-initiator modified, (c) TiO₂ NTs-PMAA-Cd. B: (a) TiO₂ NTs-PMAA-CdS, (b) TiO₂ NTs-PMAA-Na-CdS, (c) TiO₂ NTs- PMAA-Cd-CdS.

3.2. FESEM images and elemental analysis

Fig. 2(A–F) represents the surface topography of TiO₂ NTs, CdS-TiO₂ NTs and CdS/CdSe-TiO₂ NTs and their cross-section morphology obtained by FESEM (Fig. 2D–F). Fig. 2(A) shows the FESEM view of anodized TiO₂ NTs, highly ordered alignment of nanotubes with well-defined tubular structures and smooth walls is clearly evident. The pore size ranges from 90 nm to 120 nm, and the average size is approximately 100 nm. Fig. 2(B) depicts that CdS nanoparticles are introduced onto TiO₂ NTs, some precipitates on the surface make TiO₂ NTs rather rougher than plain TiO₂ NTs. After further loading with CdSe nanoparticles, as seen from Fig. 2 (C), CdSe/CdS QDs decorated TiO₂ NTs have thicker tube walls than non-decorated ones due to the appearance of surface precipitates, and some nanoparticles have blocked the tube mouths, which could be used to confirm that CdSe/CdS QDs have been formed on the surfaces of TiO₂ NTs.

Fig. 2 FESEM images the surfaces of (A) Anodized TiO₂ NTs arrays; (B) CdS-TiO₂ NTs; (C) CdSe/CdS-TiO₂ NTs. Cross-section FESEM views of (D) plain TiO₂ NTs; (E) CdS-TiO₂ NTs; (F) CdSe/CdS-TiO₂ NTs. Energy dispersive spectroscopy (EDS) spectra on cross-section: (G) CdS-TiO₂ NTs and (H) CdSe/CdS-TiO₂ NTs.

Cross-section FESEM images were used to verify the QDs loading on the tube walls. Fig. 2(D) displays a cross-sectional FESEM image of the anodized TiO₂ NTs, having apparent tubular morphology with smooth tube walls. The nanotube length is about 20 μm, which is the optimum length for nanotubes in dye sensitized solar cells for realizing the highest efficiency.⁴⁵Fig. 2(E) is a FESEM image of TiO₂ NTs after CdS loading, as can be seen clearly that well dispersed nanoparticles are decorated on both the inner and outer walls, which makes the nanotubes rougher than the plain TiO₂ NTs. Fig. 2(F) is a cross-section FESEM image of CdS/CdSe-TiO₂ NTs, which shows that high density nanoparticles have covered both the inner and outer walls of the TiO₂ NTs. The direct contact between TiO₂ and the semiconducting QDs is helpful for maximizing the charge separation efficiency in solar cell devices. After the introduction of secondary QDs, the nanotube walls showed enhanced roughness after CdSe QDs loading.

Energy dispersive spectroscopy (EDS) was employed to further confirm the successful CdS/CdSe QDs loading on TiO₂ NTs. Fig. 2(G) shows the EDS analysis of a cross-section of the CdS-TiO₂ NTs. It indicates the as-prepared sample contains Cd and S elements, and the Ti and O signals originated from the TiO₂ and Ti substrate. EDS quantitative analysis of CdS-TiO₂ NTs also gives a precisely 1 : 1 stoichiometric ratio of Cd to S (Cd, 0.92%; S, 0.92%), which can confirm the formation of the CdS compound. In Fig. 2(H), the Se signal additionally appears besides the S and Cd peaks after the preparation of CdSe QDs, which provides direct proof that CdSe and CdS QDs co-exist on TiO₂ NTs. From the quantitative analysis of CdSe/CdS-TiO₂ NTs, the atom ratio of Cd (2.54%) is larger than the total amount of S and Se (1.52%), which is probably due to the incomplete conversion of Cd ions to CdSe/CdS QDs.

3.3. Chemical composition analysis

Successful initiator modification, polymer grafting, and CdS/CdSe QDs loading on TiO₂ NTs were verified by XPS. Fig. 3(A) shows the XPS survey spectra of TiO₂ NTs (a), TiO₂ NTs-initiator modified (b), TiO₂ NTs-PMMA-Cd (c) and TiO₂ NTs-PMMA-CdS (d). TiO₂ NTs are mainly composed of Ti and O as well as a small amount of C resulting from adventitious hydrocarbon contamination. Successful anchoring of initiator-modified TiO₂ NTs was confirmed by the presence of N signals not detected for the TiO₂ NTs. After surface initiated polymerization on TiO₂ NTs, polymer grafting was verified by high-resolution XPS spectral analysis in Fig. 3(C), which shows that the Cd 3d peaks appeared at binding energies of about 405.8 eV and 412.6 eV. After reaction with H₂S gas, successful loading of CdS nanoparticles onto TiO₂ NTs was primarily confirmed by the signals of the S (2p) peak at binding energies of 160.9 and 162.2 eV (Fig. 3(D)).

Fig. 3 XPS survey spectra of A: (a) TiO₂ NTs, (b) initiator modified TiO₂ NTs, (c) TiO₂ NTs-PMAA-Cd, (d) TiO₂ NTs-PMAA-CdS. B: (a) TiO₂ NTs-PMAA-CdS-Na, (b) TiO₂ NTs-PMAA-CdS-Cd, (c) TiO₂ NTs-PMAA-CdS-CdSe. (C): XPS survey spectra of Cd (3d) of TiO₂ NTs-PMAA-Cd. (D): S (2p) of TiO₂ NTs-PMAA-CdS. (E): Na (1s) of TiO₂ NTs-PMAA-CdS-Na. (F): Se (3d) of TiO₂ NTs-PMAA-CdS-CdSe.

In order to reuse the polymer brushes for the secondary QDs loading, an ion exchange strategy was performed to immobilize Cd ions into polymer brushes. Fig. 3(B) displays the XPS survey spectra of TiO₂ NTs-PMAA-CdS-Na, (b) TiO₂ NTs-PMAA-CdS and (c) TiO₂ NTs-PMAA-CdS-CdSe. TiO₂ NTs-PMAA-CdS was dipped in NaOH solution (methanol as solvent) to exchange Na⁺ for the production of TiO₂ NTs-PMAA-CdS-Na, as depicted in Fig. 3(E), the appearance of a Na (1s) signal at 1071.7 eV is the direct proof of the successful cation exchange. Also, the peaks of Na KLL in Fig. 3B(a) can be used to further confirm the Na⁺ exchange, and the clear Cd 3d peaks in Fig. 3B(b) prove the successful replacement of Na⁺ ions by Cd²⁺ cations. High-resolution XPS measurements were performed to analyze the Se species in CdSe quantum dots as shown in Fig. 3(F), where Se peaks appear at 53.5 and 54.1 eV (Se 3d).

3.4. Structure analysis

X-ray diffraction (XRD) experiments were used to determine the phase structure of these materials, all samples were annealed at 500 °C under N₂ protection before measurement. Fig. 4(a) is an experimental XRD profile of TiO₂ NTs that have been sintered at 450 °C under air, all the diffraction peaks corresponding to (101), (004), (200), (105), (211) and (204) can be indexed by the anatase phase of TiO₂ (JCPDS file No. 78-2486) and metallic Ti substrate (JCPDS file No. 1-1197), respectively.⁴³Fig. 4(b) is an experimental XRD profile of CdSe/CdS-TiO₂ NTs. It can be easily concluded from the standard card (JCPDS file No. 19-191) that the diffraction peaks corresponding to the (111), (220), (311) planes of CdSe after being sintered to induce crystallization can be readily indexed as the cubic CdSe phase, which is consistent with a previous report.⁴⁶Fig. 4(c) is an XRD profile of annealed CdS-TiO₂ NTs, showing the diffraction peaks of (100), (002), (101), (102), (110), (103) and (112) we can easily conclude that the CdS nanoparticles actually have hexagonal structure (JCPDS file No. 1-783).⁴⁷ Additionally, in Fig. 4(b), some peaks of CdS can also be seen, indicating the co-existence of CdS and CdSe QDs.

Fig. 4 Experimental XRD profiles taken from (a) bare TiO₂ NTs, (b) CdS/CdSe-TiO₂ NTs and (c) CdS-TiO₂ NTs.

3.5. Photoelectric properties

In solar cell devices, when CdS and CdSe are used to sensitize TiO₂ materials with a cascade structure, the conduction band and valence band of the three materials are in the following order: TiO₂ < CdS < CdSe as shown in Fig. 5(A), which was very efficient in accelerating charge separation and exciton injection, thus a greatly expanded spectral response of TiO₂ can be obtained.²⁸ In order to confirm the spectral absorption of conventional anodized TiO₂ NTs and CdS/CdSe QDs sensitized TiO₂ NTs, UV-vis diffuse-reflectance spectra are shown in Fig. 5(B), the plain TiO₂ NTs only have ultraviolet response. In order to further broaden the photoresponse of TiO₂ to the visible light region, CdS QDs were incorporated into TiO₂ NTs and thus the reflectance spectrum has been efficiently extended to approximately 550 nm, corresponding to the energy band gap of CdS (2.25 eV). For further broadening of the spectral absorption, CdSe QDs were fabricated onto TiO₂ NTs, due to its narrow energy band gap of 1.7 eV, the reflectance spectrum is extended to 700 nm, towards even wider visible light absorption.

Fig. 5 (A) The band edge structure for the efficient transport of the excited electrons and holes in CdS/CdSe co-sensitized TiO₂ NTs. (B) UV-vis diffuse-reflectance spectra of (a) Anodized TiO₂ NTs, (b) CdS-TiO₂ NTs, (c) CdSe/CdS-TiO₂ NTs. (C) The photocurrent dependence on the wavelength of illumination monochromatic light: (a) TiO₂ NTs; (b) CdS-TiO₂ NTs; (c) CdSe/CdS-TiO₂ NTs. (D) Incident photon to current conversion efficiencies (IPCEs) measured as a function of wavelength for QDs-sensitized TiO₂ NTs: (a) TiO₂ NTs; (b) CdS-TiO₂ NTs; (c) CdSe/CdS-TiO₂ NTs.

A comparison between the as-prepared TiO₂ NTs and CdSe/CdS-TiO₂ NTs was made using a two-electrode photoelectrochemical cell in 0.1 M Na₂S solution. Fig. 5(C) provides typical photocurrent responses of TiO₂ NTs before and after modification with CdS and CdSe/CdS QDs, the plain TiO₂ NTs shows very low photocurrent density under visible irradiation (Fig. 5(C)a), corresponding to the bandgap of anatase TiO₂ (3.2 eV) that only possess ultraviolet photoresponse.²⁸ After the loading of CdS QDs, the photocurrent increased from 400 nm to 550 nm as shown in Fig. 5(C)b, which matches with the bandgap of CdS (2.25 eV) that can extend the spectral response of TiO₂ to 550 nm. TiO₂/CdS is the superior combination to harvest solar energy due to the higher conduction band edge of CdS with respect to that of TiO₂, which has advantages to realize efficient electron transfer. For further expanding the photo absorption, CdSe QDs were prepared and showed enhanced photocurrent almost in the entire visible region (Fig. 5(C)c).

In general, the IPCE spectra of CdS-TiO₂ and CdSe/CdS-TiO₂ should be consistent with the corresponding UV-vis diffuse-reflectance spectra shown in Fig. 5(B). The incident photon to current conversion efficiency (IPCE) spectra are depicted in Fig. 5(D) and show a much wider light harvesting range after CdS/CdSe QDs loading on TiO₂. IPCE values increase in the spectrum of 400 nm to 550 nm achieved by the CdS QDs sensitization, the energy band gap matching of CdS and TiO₂ is the major driving force for the efficient charge transfer between CdS and TiO₂. When CdSe QDs are prepared on CdS QDs sensitized TiO₂ NTs, a higher IPCE is obtained in the short-wavelength region (<500 nm), where both CdS and CdSe QDs are photoactive so that the increase of IPCE mainly depends on the CdSe QDs. TiO₂/CdS/CdSe architecture is the perfect combination for visible light harvesting and higher charge separation and transfer efficiencies. In the long-wavelength visible light region (>500 nm), where only CdSe materials can be photoexcited, the IPCE is greatly enhanced and extended into the visible region.

To better understand the roles of CdS and CdSe QDs, reproducible photocurrent and photovoltage responses to on-off light cycles were tested and shown in Fig. 6(A) and (B). As depicted in Fig. 6(A), the plain TiO₂ NTs shows smaller photocurrent density than CdS-TiO₂ NTs, and after the introduction of CdSe QDs the photocurrent is greatly enhanced. When the illuminated light was regularly switched on and off, a series of almost identical electric signals appeared and it was very stable after several cycling measurements. Furthermore, the photovoltage response in Fig. 6(B) is also regular under light on-off tests and shows reversible switching properties. The CdSe/CdS-TiO₂ NTs electrode has the largest open circuit voltage (V_oc) and a more negative potential than CdS-TiO₂ NTs and TiO₂ NTs, ensuring the biggest photoresponse upon light illumination. As an n-type semiconductor, TiO₂, the electron was excited from the valance band to the conduction band under light illumination, and the open circuit potential of an n-type semiconductor is negative.⁴⁸

Fig. 6 Photocurrent (A) and photovoltage (B) responses of TiO₂ NTs, CdS-TiO₂ NTs and CdSe/CdS-TiO₂ NTs.

4. Conclusions

In summary, surface-initiated polymerization has been employed to graft polymer brushes onto the surfaces of TiO₂ nanotubes for CdS/CdSe QDs loading with the aim of being able to tune the response of TiO₂ to the visible region. These polymer brushes provide a nano-confined environment as the template for CdS and CdSe QDs formation with uniform dispersion on TiO₂ surfaces. The spectral absorption of these composite materials has been extended to the visible region and the composite materials show strong reversible photoelectric responses.

Acknowledgements

The authors gratefully acknowledge the support of NSFC (20973188, 21125316) and the “Hundred Talents Program” of Chinese Academy of Sciences.

References

1. R. Li, D. Liu, D. Zhou, Y. Shi, Y. Wang and P. Wang, Energy Environ. Sci., 2010, 3, 1765.
2. F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin and M. Grätzel, J. Am. Chem. Soc., 2008, 130, 10720.
3. S. Lim and S. Rhee, RSC Adv., 2011, 1, 518.
4. H. Lee, M. Wang, P. Chen, D. R. Gamelin, S. M. Zakeeruddin, M. Grätzel and M. K. Nazeeruddin, Nano Lett., 2009, 9, 4221.
5. G. K. Mor, S. Kim, M. Paulose, O. K. Varghese, K. Shankar, J. Basham and C. A. Grimes, Nano Lett., 2009, 9, 4250.
6. Q. Tai, X. Zhao and F. Yan, J. Mater. Chem., 2010, 20, 7366.
7. D. Wang, Q. Ye, B. Yu and F. Zhou, J. Mater. Chem., 2010, 20, 6910.
8. P. Roy, D. Kim, K. Lee, E. Spiecker and P. Schmuki, Nanoscale, 2010, 2, 45.
9. K. Shankar, J. Bandara, M. Paulose, H. Wietasch, O. K. Varghese, G. K. Mor, T. J. LaTempa, M. Thelakkat and C. A. Grimes, Nano Lett., 2008, 8, 1654.
10. I. Robel, V. Subramanian, M. Kuno and P. V. Kamat, J. Am. Chem. Soc., 2006, 128, 2385.
11. P. Roy, S. Berger and P. Schmuki, Angew. Chem., Int. Ed., 2011, 50, 2904.
12. J. Y. Li, C. Y. Chen, J. G. Chen, C. J. Tan, K. M. Lee, S. J. Wu, Y. L. Tung, H. H. Tsai, K. C. Ho and C. G. Wu, J. Mater. Chem., 2010, 20, 7158.
13. Y. Shengyuan, A. S. Nair, R. Jose and S. Ramakrishna, Energy Environ. Sci., 2010, 3, 2010.
14. S. Bingham and W. A. Daoud, J. Mater. Chem., 2011, 21, 2041.
15. M. K. Nazeeruddin, F. D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Grätzel, J. Am. Chem. Soc., 2005, 127, 16835.
16. A. Mishra, M. K. R. Fischer and P. Bäuerle, Angew. Chem., Int. Ed., 2009, 48, 2474.
17. S. Q. Fan, D. Kim, J. J. Kim, D. W. Jung, S. O. Kang and J. Ko, Electrochem. Commun., 2009, 11, 1337.
18. A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno and P. V. Kamat, J. Am. Chem. Soc., 2008, 130, 4007.
19. Y. L. Lee, B. M. Huang and H. T. Chien, Chem. Mater., 2008, 20, 6903.
20. W. T. Sun, Y. Yu, H. Y. Pan, X. F. Gao, Q. Chen and L. M. Peng, J. Am. Chem. Soc., 2008, 130, 1124.
21. D. R. Baker and P. V. Kamat, Adv. Funct. Mater., 2009, 19, 805.
22. S. Chen, M. Paulose, C. Ruan, G. K. Mor, O. K. Varghese, D. Kouzoudis and C. A. Grimes, J. Photochem. Photobiol., A, 2006, 177, 177.
23. X. F. Gao, W. T. Sun, Z. D. Hu, G. Ai, Y. L. Zhang, S. Feng, F. Li and L. M. Peng, J. Phys. Chem. C, 2009, 113, 20481.
24. K. Shin, S. i. S, S. H. Im and J. H. Park, Chem. Commun., 2010, 46, 2385.
25. C. Ratanatawanate, C. Xiong and K. J. B. Jr, ACS Nano, 2008, 2, 1682.
26. X. F. Gao, H. B. Li, W. T. Sun, Q. Chen, F. Q. Tang and L. M. Peng, J. Phys. Chem. C, 2009, 113, 7531.
27. W. Zhu, X. Liu, H. Liu, D. Tong, J. Yang and J. Peng, J. Am. Chem. Soc., 2010, 132, 12619.
28. Y. L. Lee and Y. S. Lo, Adv. Funct. Mater., 2009, 19, 604.
29. O. Azzaroni, S. E. Moya, A. A. Brown, Z. Zheng, E. Donath and W. T. S. Huck, Adv. Funct. Mater., 2006, 16, 1037.
30. X. Wang, H. Hu, Y. Shen, X. Zhou and Z. Zheng, Adv. Mater., 2011, 23, 3090.
31. Z. Liu, H. Hu, B. Yu, M. Chen, Z. Zheng and F. Zhou, Electrochem. Commun., 2009, 11, 492.
32. X. Liu, H. Chang, Y. Li, W. T. S. Huck and Z. Zheng, ACS Appl. Mater. Interfaces, 2010, 2, 529.
33. R. Barbey, L. Lavanant, D. Paripovic, N. Schüwer, C. Sugnaux, S. Tugulu and H. Klok, Chem. Rev., 2009, 109, 5437.
34. A. Calvo, M. C. Fuertes, B. Yameen, F. J. Williams, O. Azzaroni and G. J. A. A. Soler-Illia, Langmuir, 2010, 26, 5559.
35. L. Ionov, S. Sapra, A. Synytska, A. L. Rogach, M. Stamm and S. Diez, Adv. Mater., 2006, 18, 1453.
36. Q. Ye, F. Zhou and W. Liu, Chem. Soc. Rev., 2011, 40, 4244.
37. Q. Ye, X. Wang, S. Li and F. Zhou, Macromolecules, 2010, 43, 5554.
38. O. Azzaroni, Z. Zheng, Z. Yang and W. T. S. Huck, Langmuir, 2006, 22, 6730.
39. O. Azzaroni, A. A. Brown, N. Cheng, A. Wei, A. M. Jonas and W. T. S. Huck, J. Mater. Chem., 2007, 17, 3433.
40. Q. Ye, X. Wang, H. Hu, D. Wang, S. Li and F. Zhou, J. Phys. Chem. C, 2009, 113, 7677.
41. X. Fan, L. Lin, J. L. Dalsin and P. B. Messersmith, J. Am. Chem. Soc., 2005, 127, 15843.
42. T. Cui, J. Zhang, J. Wang, F. Cui, W. Chen, F. Xu, Z. Wang, K. Zhang and B. Yang, Adv. Funct. Mater., 2005, 15, 481.
43. D. Wang, B. Yu, C. Wang, F. Zhou and W. Liu, Adv. Mater., 2009, 21, 1964.
44. D. Wang, Y. Liu, B. Yu, F. Zhou and W. Liu, Chem. Mater., 2009, 21, 1198.
45. J. Yan and F. Zhou, J. Mater. Chem., 2011, 21, 9406.
46. J. H. Bang and P. V. Kamat, Adv. Funct. Mater., 2010, 20, 1970.
47. J. Cao, J. Sun, H. Li, J. Hong and M. Wang, J. Mater. Chem., 2004, 14, 1203.
48. C. J. Brabec, A. Cravino, D. Meissner, N. S. Sariciftci, T. Fromherz, M. T. Rispens, L. Sanchez and J. C. Hummelen, Adv. Funct. Mater., 2001, 11, 374.

---