CdTe Quantum Dots Encapsulated ZnO Nanorods for Highly Efficient

Dec 5, 2013 - (21, 22) Many attempts have been performed to increase the photocatalytic efficiency of semiconducting materials, such as doping TiO2 wi...
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CdTe Quantum Dots Encapsulated ZnO Nanorods for Highly Efficient Photoelectrochemical Degradation of Phenols Danqing Liu,†,§ Zhaozhu Zheng,† Chaoqun Wang,§ Yongqi Yin,‡ Shaoqin Liu,*,† Bin Yang,*,‡ and Zhaohua Jiang*,§ †

Key Laboratory of Microsystems and Microstructures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin 150080, China ‡ Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin 150080, China § School of Chemical Engineering, Harbin Institute of Technology, Harbin 150080, China S Supporting Information *

ABSTRACT: Vertically aligned CdTe−ZnO composite nanorods are constructed on the indium tin oxide substrates by layer-by-layer deposition of CdTe quantum dots on ZnO nanorod arrays. The CdTe shell forms an intact interface with the wurtzite ZnO nanorod, and its thickness can be accurately tuned by changing the deposition cycles. Photoluminescent measurements further disclose the band alignment between the CdTe shell and the ZnO core, which makes CdTe−ZnO composite nanorods exhibiting good photoelectron-chemical properties and being a prospective material for removal of phenol from wastewater under visible light irradiation. Impressively, about 75% degradation of 100 mg/L phenol solution and up to 53.2% removal of the total organic carbon are achieved within 150 min using the optimized CdTe−ZnO composite nanorods as photoelectrocatalysts under visible light.

1. INTRODUCTION Phenol and phenolic compounds are well-known for their acute toxicity and bio-recalcitrant nature. These compounds are mainly present in the wastes from pharmaceutical, petroleum manufacture, painting industries, and so on.1 Exposure to phenol is related to severe illnesses such as leukemia and some serious human organ malfunctions. Several technologies such as solvent extraction, adsorption, chemical oxidation, incineration, biological degradation, and photocatalysis have been employed for removal of phenol.2 However, removal of phenol by physical methods, chemical oxidation, and other nonbiological methods has serious drawbacks, such as high cost and formation of the hazardous byproducts. Biological degradation is generally favorable due to lower costs and possibility of complete mineralization; however, biodegradation of phenol in water or soil may be hindered or precluded by the presence of high concentrations of phenol or other chemicals or by other factors such as a lack of nutrients or microorganisms being capable of degrading phenol. Therefore, the development of clean and green processes for complete degradation of phenol from waste is imperative. Several outstanding advantages, such as ambient operating conditions, complete destruction of parents and their intermediate compounds, and relatively low operating cost, make photocatalytic degradation of phenol and its derivatives with semiconducting materials being one of the best solutions.3−9 Among the semiconductor photocatalysts tested, TiO2,10−12 Bi2WO6,13,14 Bi2MoO6,15 and ZnO16 have been © 2013 American Chemical Society

proven to be the highly active photocatalysts. So far, the application of the semiconducting material mediated photoreactions for water treatment is still suffering several big challenges.17 First, most reported photocatalysts possess wide band gap and require ultraviolet irradiation that just accounts for only 4−5% of the spectrum of solar energy. Thus, to obtain more efficient utilization of solar irradiation, there is a need for the photosensitive materials with a strong absorption in the visible spectrum region while maintaining the good stability.18,19 Additionally, the problem arising from employment of suspended semiconductor particles is their subsequent separation and recovery after water treatment,20 together with low quantum yield coming from high recombination rate of the photogenerated electron−hole pairs.21,22 Many attempts have been performed to increase the photocatalytic efficiency of semiconducting materials, such as doping TiO2 with transition metal cations,23,24 nonmetal anions,25 polymers,26 or metal− organic complexes,27 to enhance the visible light response and modifying TiO2 with noble metals28−30 or semiconductors31−34 to improve separation and transport of photocarriers during the photocatalysis process. Recently, electrochemically assisted photocatalysis with the supported films has received much attention as a useful remediation tool for water and wastewater because it can efficiently overcome the drawbacks of using Received: October 30, 2013 Revised: November 21, 2013 Published: December 5, 2013 26529

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containing 0.5 mg/mL PEI (pH 7.2) and the prepared CdTe QD solution, using an immersion time of 10 min. The introduction of [cobalt(o-phen)3]2+/3+, which has shown efficient hole carrier properties,41,42 was used for enhancing the capture and transport of photogenerated charges. After each layer deposition, the sample was thoroughly rinsed with water and dried under N2 flow. For each cycle, a bilayer of PEI[cobalt(o-phen)3]2+/3+/CdTe QDs was deposited on the surface of ZnO nanorods. The above cycle was repeated 20 times to obtain a stable photocurrent. Characterization. The morphologies and the crystallographic structures of the CdTe QD encapsulated ZnO nanorod arrays were studied using a field-emission scanning electron microscopy (FESEM, FEI Quanta 200F), high resolution transmission electron microscopy (HRTEM, FEI TECNAI), and X-ray diffraction (XRD, D/max-rb). The UV−vis absorption spectra were measured by using a U4100 UV− vis−NIR spectrometer (Hitachi, Japan), and the fluorescence spectra were recorded with a Fluoromax-4 fluorescence spectrophotometer (HORIBA Jobin Yvon, Japan). If not specially stated, the samples were excited at 325 nm, and the exciting slit and the emission slit were 2 and 5 nm, respectively. The optical properties of solutions and the multilayer films were measured using quartz cuvettes of 10 mm path length and a standard solid sample holder, respectively. Photoelectrochemical measurements were performed with a 660 series potentiostat (CH Instruments, Austin, TX). A photoelectrochemical cell was designed with the CdTe QD encapsulated ZnO/ITO as the working electrode, a platinum wire as counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and 0.3 M Na2S aqueous solution as the electrolyte. A 150 W xenon lamp (CROWNTECH) was used as the simulated solar light, the light intensity of which was ca. 190 mW/cm2. When applied a 400 nm optical filters, the light with the wavelength longer than 400 nm was introduced, the intensity of which was 130 mW/cm2. X-band electron spin resonance (ESR) spectra were recorded on an ER-300 ESR spectrometer (Burker Instruments Inc.) with a modulation frequency of 100 kHz. The center field was 3570 G, and the sweep width was 73 G. Photoelectrocatalytic Degradation of Phenol under Visible Light. The photoelectrocatalytic degradation of phenol in the aqueous solution with 0.07 M Na2SO4 as the electrolyte was carried out under the 150 W xenon lamp irradiation in a quartz reactor. The light intensity with 400 nm optical filter was 130 mW/cm2. The initial concentration of phenol and the solution volume were 100 mg/L and 50 mL, respectively. Typically, 20 bilayers of PEI-[cobalt(o-phen)3]2+/3+/CdTe QD encapsulated ZnO nanorod arrays/ITO electrode (4.15 cm2) were served as the photoanode, and the ZnO nanorod arrays/ ITO electrode was also used as the photoanode for comparison. Prior to irradiation, the resulting reaction system was stirred in the dark for 30 min to achieve an adsorption/desorption equilibrium for phenol between the solution and the electrode. The concentration of phenol was determined based on the Emerson’s method (more details regarding the experimental processes are displayed in the Supporting Information).43 Liquid samples were collected at regular intervals and analyzed by an ultrahigh performance liquid chromatography (UPLC, Waters Aquity) using a C18 column for separation. The wavelength of detector was set at 280 nm. The eluent was composed of 65% Milli-Q water (0.1 wt % formic acid) and 35% acetonitrile at a flow rate of 0.4 mL/min.

suspended semiconductor particles as well as reduce the recombination rate with holes by applying a positive bias to withdraw the electrons from the semiconductor particles to the electrode.35 In this work, we fabricate the large-scale CdTe quantum dot (QD) encapsulated ZnO nanorod arrays through accurate deposition of CdTe QDs on ZnO nanorod arrays by a layer-bylayer assembly technique. An intact interface is formed between single-crystal ZnO nanorods and the uniform CdTe QDs with tunable thickness. CdTe has a narrow band gap of 1.5 eV, matching most range of the solar radiation spectrum. Moreover, the electron−hole recombination inside CdTe QDs could be considerably inhibited by injecting electrons from CdTe into ZnO conduction band.36 Therefore, coupling CdTe with ZnO nanorod arrays gives rise to extension of the sensitivity of ZnO into the visible region and enhancement of the photocatalytic efficiency of ZnO. The resulting nanostructured photoelectrodes are proved to be a prospective material for removal of phenol from wastewater under visible light irradiation.

2. EXPERIMENTAL SECTION Materials. Cd(ClO4)2·6H2O, Al2Te3 powders, and mercaptopropionic acid (MPA) were purchased from Alfa Aesar. Poly(ethylenimine) (PEI, MW 25 000), N-methyl-N(trimethylsilyl)trifluoroacetamide (MSTFA), trimethyliodosilane (TMIS), and spin trapping reagent of 5,5-dimethyl-1pyrroline-N-oxide (DMPO) were purchased from SigmaAldrich. All other reagents were of analytical reagent grade. All solutions were prepared with ultrapure water from a Milli-Q water purification system (Billerica, MA). The indium-doped tin oxide (ITO, about 15 Ω/sq) coated glass obtained from Leaguer Film technology (Shenzhen) Co. Ltd. was used as the optically transparent electrodes. Tris(l,10-phenanthroline) complexes of cobalt(II), [Co(Phen)3]Cl2, were prepared by mixing CoCl2 and l,10phenanthroline (Phen) at a 1:3 molar ratio.37 The MPAcapped CdTe QDs were synthesized according to a previously published procedure from our laboratory.38 The absorbance shoulder of MPA-capped CdTe QDs was located at 545 nm, while the photoluminescence peak of QDs was situated at 595 nm (Figure S1 in Supporting Information). The size and concentration of CdTe QDs in solution estimated from the UV−vis spectrum were determined be around 3.24 nm and 6.99 × 10−5 M, respectively, according to Peng’s empirical equation.39 Preparation of CdTe QD Encapsulated ZnO Nanorod Arrays. The CdTe QD encapsulated ZnO nanorod arrays were deposited on the substrates by a two-step process. The substrates used in our experiments included silicon wafer, glass, quartz, and ITO. Initially, the film of ZnO nanorod arrays was grown on the substrates using the hydrothermal method. Deposition of ZnO nanorods was achieved by immersing the clean substrates in aqueous solution mixture of Zn(NO3)2 (0.1 M) and hexamethylenetetramine (0.1 M). The mixture was then refluxed at 90 °C for different duration time.40 Duration time was controlled at 2.5−15 h to obtain the nanorod arrays with the height of 1−8 μm. The ZnO nanorod samples were then thoroughly rinsed with deionized water to remove the surfactant and the residual salt and annealed at 300 °C for 0.5 h in air to increase the conductivity. In order to deposit CdTe QDs on the surface of ZnO nanorods, the film of ZnO nanorod arrays was alternately dipped in 1 mg/mL [cobalt(o-phen)3]2+ aqueous solution 26530

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Figure 1. (a, b) Top-view SEM images of ZnO nanorod arrays fabricated on ITO substrates by a hydrothermal method with pH 8.4 at 90 °C for 2.5 h. (c) XRD pattern of the as-grown sample. The inset shows the magnified XRD pattern.

Figure 2. (a, b) Top-view and (c) cross-sectional SEM images of 20 bilayers of [cobalt(o-phen)3]2+/3+-PEI/CdTe QD encapsulated ZnO nanorod arrays on ITO substrates. (d) XRD pattern of the sample after the LBL process. The inset shows the magnified XRD pattern.

To coat a uniform thin film of CdTe QDs onto the ZnO nanorod arrays, the film of ZnO nanorod arrays was alternately dipped in positively charged 1 mg/mL [cobalt(o-phen)3]2+ aqueous solution containing 0.5 mg/mL PEI (pH 7.2) and negatively charged CdTe QD solution. Figure S4 displays the UV−vis spectra of ([cobalt(o-phen)3]2+/3+-PEI/CdTe QDs)x multilayers deposited on the ZnO nanorod arrays, where the first layer is [cobalt(o-phen)3]2+/3+-PEI and the outermost layer is QDs. The absorbance shoulder at 550 nm (λ550 nm) is increased steadily with the number of [cobalt(o-phen)3]2+/3+PEI/CdTe QDs bilayers, x, which confirms irreversible adsorption of [cobalt(o-phen)3]2+/3+-PEI and QDs on the ZnO nanorod arrays. The inset shows a plot of λ550 nm versus the number of bilayers. Regular layer growth is revealed by a linear dependence of the absorbance determined at 550 nm versus x, and the absorbance increase for one bilayer of PEI[cobalt(o-phen)3]2+/3+/CdTe QDs at 550 nm is 0.021 ± 0.0003. The detailed structure of the CdTe QD encapsulated ZnO nanorod arrays was further investigated by both SEM and TEM. Figures 2a and 2b show the low- and high-magnification SEM images of 20 bilayers of [cobalt(o-phen)3]2+/3+-PEI/CdTe QD encapsulated ZnO nanorod arrays, respectively. After deposition, the ZnO nanorods are fully covered by dense CdTe nanoshells. The 20 bilayers of [cobalt(o-phen)3]2+/3+-PEI/ CdTe QDs encapsulated ZnO nanorod arrays have a uniform diameter around 310 ± 40 nm. The cross-sectional SEM image (Figure 2c) demonstrates that the CdTe shell is fairly homogeneous along the length of the ZnO nanorods. The energy dispersive X-ray (EDX) spectrum confirms that the composite is composed of Zn, O, Cd, Te, and S, and both ratios of Zn/O and Cd/Te are close to 1 (Figure S5a). The TEM image shows a clear core−shell contrast (Figure S5b). The core diameter is 130 nm, while the shell thickness is 80 nm. Figure

The reaction intermediates were analyzed by gas chromatography coupled with mass spectrometry (GC-MS, Agilent 7890A and 5975C, Agilent Tech.) using a 30 cm HP-5 column (Hewlett-Packard). Before GC-MS analysis, the liquid samples (15 mL) were freezing-dried and extracted by diethyl ether. The extracted product was then subjected to silylation with 50 μL of MSTFA and 0.5 μL of TMIS at 62 °C for 70 min to give TMS derivatives according to ref 44.

3. RESULTS AND DISCUSSION Preparation and Characterization of CdTe QD Encapsulated ZnO Nanorod Arrays. The CdTe QD encapsulated ZnO nanorod arrays were deposited on the ITO or quartz substrates by a two-step growth method. First, the films of ZnO nanorod arrays were grown on the substrates using the hydrothermal method. Figures 1a and 1b present the low- and high-magnified FESEM images of the ZnO nanorod arrays grown on the ITO substrate via a hydrothermal process with pH 8.4 at 90 °C for 2.5 h. The length of these nanorods is about 1.2 μm, and their average diameter is 160 nm with a narrow size distribution of ±20 nm. The nanorod length could be tuned by changing reaction time. When the reaction time increases to 5, 10, and 15 h, rods with 3.4, 5.2, and 8.1 μm in length, respectively, are obtained as revealed in the tilt view SEM image in Figure S2. Further observation from HRTEM (Figure S3) and the corresponding diffraction pattern reveal that these large-scale vertically aligned ZnO nanorods belong to the single-crystalline hexagonal structure of ZnO with the growth direction along [0001]. The peaks in the XRD pattern (Figure 1c) correspond to the characteristic (002), (100), (101), and (110) peaks of ZnO with the hexagonal wurtzite structure. The intensity of the (002) plane is obviously stronger than the others, which is consistent with the SEM image showing ZnO nanorods grown along one direction. 26531

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electrons accumulate in the conduction band of ZnO nanorods and the holes accumulate in the valence band of CdTe QDs. In this way, the charge separation is realized. Photoelectrochemical Properties of CdTe QD Encapsulated ZnO Nanorod Arrays. The absorption properties of the composite and the observed PL quenching suggest that the CdTe QD encapsulated ZnO nanorod arrays may show good photovoltaic performance. The photoelectrochemical response of the samples was analyzed under an illumination of 150 W xenon lamp with the measured density of 190 mW/cm2. Figure 4 indicates the current density versus potential (J−V) curves for

2d presents the typical XRD pattern of 20 bilayers of [cobalt(ophen)3]2+/3+-PEI/CdTe QD encapsulated ZnO nanorod arrays, where one can observe the (002), (100), (101), and (110) peaks corresponding to the hexagonal wurtzite structure of ZnO. The rather weak and broad peak centered at 23.96 can be indexed to the (111) planes of cubic CdTe (JCPDS file No. 65880) (the inset of Figure 2d). The much lower intensity of CdTe compared with ZnO is attributed to the relatively low crystallinity of the as-prepared CdTe QDs and the surrounding of polymer during the layer-by-layer preparation, which could be confirmed by the HRTEM imaging (Figure S5). Figure 3a shows typical absorption of the pure ZnO nanorod arrays and 20 bilayers of [cobalt(o-phen)3]2+/3+-PEI/CdTe QD

Figure 4. Current density (J) versus potential (V) curves for ZnO nanorods grown on ITO coated with 10 [cobalt(o-phen)3]2+/3+-PEI/ CdTe QD bilayers measured in the dark and under an illumination of 150 W xenon lamp. The inset shows the photocurrent response to ON−OFF cycles at a constant potential of 0 V vs SCE. A 0.3 M Na2S aqueous solution was used as electrolyte.

10 [cobalt(o-phen)3]2+/3+-PEI/CdTe QD coated 5.0 μm long ZnO nanorod arrays measured in the dark or under illumination. In the dark, the J−V curve shows typically rectifying behavior with a weak current density of 25 μA/cm2 at a potential of 0 V. Under illumination, the photocurrent density increased 10-fold compared to that in the dark. The corresponding photocurrent response to ON−OFF cycles is shown in the inset of Figure 4. Under illumination, the CdTe− ZnO composite modified electrode reaches over 95% of the steady-state current within 1 s. When the light source is turned off, the system immediately returns to its initial state. That is a steady and prompt photocurrent generation is obtained during on and off cycles of illumination. To study the effect of the ZnO nanorod length on the cell performance, six different lengths of ZnO nanorod arrays were prepared on ITO using a hydrothermal method by controlling the growth time of ZnO nanorods between 2.5 and 15 h at the same growth temperature. The average lengths of ZnO nanorod arrays are increased from 1.2 ± 0.3 μm to 3.6 ± 0.3, 4.9 ± 0.4, 5.2 ± 0.3, 6.8 ± 0.3, and 8.1 ± 0.4 μm for the growth time of 2.5, 5.0, 7.5, 10.0, 12.5, and 15 h, respectively (Figure S2). The lengths of the nanorods are linearly increased with the growth time at a growth rate of 0.511 μm/h. After coated with 10 [cobalt(o-phen)3]2+/3+-PEI/CdTe QD bilayers, J−V measurements are then performed on six different lengths of ZnO nanorod arrays (Figure 5). Photocurrent measurements show that the onset potential of the photocurrent (−0.7 V vs SCE) remains the same as the length of the nanorods increases, indicating that the onset potential of the photocurrent is mainly

Figure 3. (a) UV−vis spectra and (b) photoluminescence spectra of ZnO nanorods grown on the ITO electrode before (black curve) and after coated with 20 [cobalt(o-phen)3]2+/3+-PEI/CdTe QD bilayers (red curve).

encapsulated ZnO nanorod arrays on the ITO substrate. For the pure ZnO nanorod arrays, a steep absorption edge at 380 nm is consistent with the band gap of ZnO. After CdTe deposition, an absorption shoulder appears at 550 nm, corresponding well with the band gap of CdTe. Figure 3b shows the photoluminescence (PL) spectra of the pure ZnO nanorod arrays and 20 bilayers of [cobalt(o-phen)3]2+/3+-PEI/ CdTe QD encapsulated ZnO nanorod arrays on the ITO substrate. For the bare ZnO nanorod array, a strong emission can be clearly found at 380 nm. After the CdTe deposition, significant quenching of ZnO emission is observed. In addition, the emission of CdTe QDs is also largely depressed compared to that of [cobalt(o-phen)3]2+/3+-PEI/CdTe QD multilayers (Figure S6). The obvious PL quenching for both ZnO and CdTe QDs should be attributed to separation of photogenerated charge carriers driven by the proper band alignment between ZnO and CdTe QDs.45−47 The band diagram of the CdTe/ZnO composite is shown in Figure S7, and it is clear that both the valence band and conduction band of CdTe lie above those of ZnO. The photogenerated electrons from the conduction band of CdTe can easily transfer to the conduction band of ZnO, leading to the ZnO PL quenching. As a result, the 26532

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Figure 5. (a) J−V curves for ZnO nanorods with different length coated with 10 [cobalt(o-phen)3]2+/3+-PEI/CdTe QD bilayers. (b) Photocurrent response to ON−OFF cycles of ZnO nanorods with different length coated with 10 [cobalt(o-phen)3]2+/3+-PEI/CdTe QD bilayers. The applied potential was 0 V vs SCE upon irradiation with 150 W xenon lamp. (c) Dependence of photocurrent on the length of ZnO nanorod arrays.

determined by the properties of ZnO nanorods and CdTe QDs rather than the length of ZnO nanorods. The photocurrent is found to be increased with the length of ZnO nanorods when shorter than 5.2 μm, which is expected since longer nanorods could provide larger surface area for CdTe QD loading to enhance the light-harvesting efficiency. The maximum photocurrent (0.26 mA/cm2) is obtained for 5.2 μm nanorods under the same experimental conditions. However, further increasing the length of ZnO nanorods yields a decrease of the photoelectrical performance though the CdTe loading might be further increased. As the length of ZnO nanorods increases, two unfavorable factors cause the decrease in the photocurrent: (1) Light could not penetrate to the root of the CdTe-encapsulated ZnO nanorods, prevent production of the hole and electron pairs. (2) The photogenerated carriers at the top part of the nanorods must travel the entire length of the nanorods to reach the ITO substrate, thus giving rise to more pathways for hole and electron recombination. Therefore, the 5.2 μm ZnO nanorods are used in the following experiment. To examine the dependence of the photovoltaic performance on the CdTe shell thickness, a series of different [cobalt(ophen)3]2+/3+-PEI/CdTe QD bilayers are assembled on 5.2 μm long ZnO nanorods and their photocurrent responses are summarized in Figure 6a. With a thin CdTe shell (less than 20 bilayers), the photocurrent is increased gradually with the number increase of the [cobalt(o-phen)3]2+/3+-PEI/CdTe QD bilayer (Figure 6b). The photocurrent densities of 10 and 20 [cobalt(o-phen)3]2+/3+-PEI/CdTe QD bilayer coated ZnO nanorods are 0.26 and 0.29 mA cm−2, respectively, over 2 times larger than that of pure ZnO nanorods (0.13 mA cm−2). This reflects the higher separation efficiency of the photoinduced electrons and holes in the case of CdTe-ZnO composites. It is believed that the CdTe QD shell greatly enhances absorption in the visible light range, and furthermore the band alignment between ZnO and CdTe favors charge separation, which together result in the photocurrent enhancement. However, continuous increase of the CdTe shell thickness leads to a decrease in photocurrent (Figure 6b), most likely because the thicker CdTe shell can block charge transfer across the ZnO/electrolyte junction.48,49 Comparison of Photoelectrocatalytic and Photocatalytic Performance of CdTe QD Encapsulated ZnO Nanorod Arrays for Phenol Degradation. During the photoelectrocatalytic degradation of phenol using CdTe QD encapsulated ZnO nanorod arrays under visible light, bias potential applied on these electrodes is set to 1.0 V (vs SCE) and Na2SO4 or O2 are employed as the scavengers of

Figure 6. (a) Time−photocurrent response of ITO/ZnO/([cobalt(ophen)3]2+/3+-PEI/CdTe QD)x (x = 0, 10, 20, 30, 40, and 50) with light on and off. The applied potential was 0 V vs SCE upon irradiation with a xenon light source. (b) Dependence of photocurrent on the layer number of, x, [cobalt(o-phen)3]2+/3+-PEI/CdTe QD bilayers.

photogenerated electrons or hydroxy radicals, respectively. For comparison, we also examine the direct photolysis of phenol, electrolysis with bare ITO electrode, and photocatalysis with CdTe QD encapsulated ZnO nanorod arrays. The results are summarized in Figure 7a. The visible light irradiation does not cause any degradation of phenol because phenol is very stable under light irradiation. In the electrochemical process with the ITO electrode and in the photocatalytic process with CdTe QD encapsulated ZnO nanorod arrays under visible light, a slight electrolysis or photocatalysis of phenol can be observed. While in the photoelectrocatalytic process with CdTe QD encapsulated ZnO nanorod arrays under visible light, the degradation efficiency of phenol is significantly improved. The photoelectrocatalytic degradation efficiency of phenol with a bias potential (1.0 V vs SCE) is much higher than those in the photocatalytic cases because the applied bias potential makes more efficient separation of photogenerated electron−hole pairs. After being degraded for 150 min, the degradation efficiencies of phenol by the photocatalytic and electrochemical processes are 17.6 and 19.4%, respectively, whereas it is 75% via 26533

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Figure 7. (a) Phenol degradation in direct photolysis (■), electrolysis (●), photocatalysis (▲), and photoelectrocatalysis (▼) with CdTe QD encapsulated ZnO nanorod arrays. The photoelectrocatalysis was carried out under the bias of 1.0 V. Incident light: λ > 400 nm. (b) Phenol degradation in the photoelectrocatalytic process with ZnO nanorods (■) and CdTe QD encapsulated ZnO nanorod array (●). (c) Plot of ln(C/C0) versus visible light.

photoelectrocatalytic values clearly demonstrate that CdTe QDs/ZnO nanorod arrays exhibit much higher photoelectrocatalytic activity compared with that of pure ZnO nanorods. The enhancement of the photoelectrocatalytic activity of CdTe QD encapsulated ZnO nanorod arrays is most likely due to its stronger absorption of visible light that accounts for more photoinduced electron and holes. The degradation mechanism of phenol and its derivatives on TiO2 and ZnO under UV light and visible light has been described in the literature.50−53 Generally, the photogenerated electrons are scavenged by dissolved oxygen, and then superoxide (O2•−) would be obtained first, followed by formation of other reactive oxygen species including hydroperoxyl radical (HOO•), H2O2, or hydroxy radical (OH•). All these reactive oxygen species possess sufficient energy for oxidation of pollutants. To compare the ability of ZnO nanorod arrays and CdTe QD encapsulated ZnO nanorod arrays producing the reactive oxygen species, ESR spin-trap with 5,5-dimethyl-1-pyrroline-n-oxide (DMPO) was employed to monitor reactive oxygen species generated in the reaction system. The ESR spectra of the reaction solution using CdTe QD encapsulated ZnO nanorod arrays as the catalysts are shown in Figure 8a. When the reaction was conducted under

photoelectrocatalysis under the visible light. The results highlight that there is an obvious synergetic effect between the electrochemical process and the photocatalytic process. In order to investigate the mineralizing ability of CdTe QD encapsulated ZnO nanorod arrays, the total organic carbons (TOCs) during the photoelectrocatalytic process are measured. Notably, up to 53.2% of TOC is removed in 2.5 h, which is much higher than that of the photocatalytic process (the TOC removal during the photocatalytic process is ca. 1%). This result demonstrates that the CdTe QD encapsulated ZnO nanorod arrays possess good mineralizing ability. During the course of photoelectrochemical degradation of phenol with CdTe QD encapsulated ZnO nanorod arrays, the common intermediates from phenol oxidation such as catechol, benzoquinone, aromatic carboxylic acids, and fumaric acid could not be detected in this process, and only very trace amounts of intermediates occurred (Figure S8a). The gas chromatography/mass spectrometry (GC-MS) is further used to identify the intermediates of photoelectrochemical degradation of phenol through oxidative coupling intermediates with MSTFA and TMIS, which can derivatize organic substances with active hydrogen. Following this identification strategy, hydroxypropyl acid, glycerol, maleic acid, and hexanoic acid could be identified as the main intermediates (Figure S9). However, the peak value of hydroxypropyl acid, glycerol, maleic acid, and hexanoic acid in this process is low even at the end of reaction. Evidently, the low concentration of the organic intermediates again reveals that the mineralization rate via the photoelectrochemical treatment is very high. Additionally, one can noticed from Figure S8b that for the case of photocatalytic degradation of phenol using CdTe QD encapsulated ZnO nanorod arrays, catechol, and hydroquinone are main intermediates. Comparison on Photoelectrocatalytic Activity of Different Nanostructures. The control experiment is also done using pure ZnO nanorod arrays. As shown in Figure 7b, the photoelectrocatalytic activity of CdTe QD encapsulated ZnO nanorod arrays is also remarkably higher than that of ZnO nanorod arrays (the phenol conversion over ZnO nanorod arrays is ca. 40%). The photoelectrocatalytic degradation reaction of phenol on both ZnO nanorod arrays and CdTe QD encapsulated ZnO nanorod arrays follows the pseudo-firstorder reaction, which can be evidenced by the linear photodegradation process as a function of the irradiation time shown in Figure 7c. The reaction rate constant using the CdTe QD encapsulated ZnO nanorod arrays is 0.546 h−1, about 3 times as large as that using ZnO nanorod arrays. These

Figure 8. DMPO spin-trap ESR spectra of photoelectrochemical systems: (a) CdTe QD encapsulated ZnO nanorod arrays and (b) ZnO nanorod arrays under the same conditions.

visible light irradiation for 20 min, ESR signals with the characteristic intensity of 1:2:2:1 for the DMPO−OH• adducts appeared, and it was remarkably enhanced as increasing the reaction time, demonstrating that hydroxide radicals were indeed generated in the reaction. The control experiment was also done using pure ZnO nanorod arrays (Figure 8b). It was observed that the DMPO−OH• adducts were produced in proportion to the reaction time. It is noteworthy that the ESR signals of CdTe QD encapsulated ZnO nanorod arrays are 26534

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4. CONCLUSIONS The large-scale and vertically aligned CdTe−ZnO composite nanostructures were successfully grown on the ITO substrates by aqueous chemical growth of ZnO nanorods arrays followed by layer-by-layer deposition of CdTe QDs. The prepared CdTe−ZnO nanocomposites displayed good photovoltaic performance, which was assigned to the band alignment between CdTe and ZnO and the favorable absorption properties of CdTe. Furthermore, the photoelectrocatalytic ability of CdTe−ZnO nanocomposites to degrade phenol under visible light irradiation was investigated. Notably, compared to previously reported chemical, photocatalytic, or photoelectrochemical treatments, the photoelectrocatalytic degradation of phenol by using the aligned CdTe−ZnO composite nanostructures showed considerably high efficiency (see Table S1 in Supporting Information). The CdTe−ZnO nanocomposites were able to reduce 53.2% of TOC in the samples within 150 min. The mechanism study reveals that the photogenerated holes in CdTe nanoparticles can directly oxidize phenol and water, while the photogenerated electrons arrived at the counter electrode could be scavenged by dissolved oxygen to generate the hydroxyl radicals’ species via a chain reaction, resulting in high efficient phenol oxidization. Both reaction processes subsequently give rise to the complete mineralization of phenol and other intermediates. Such accurate structure control over the composite nanostructures will open the door toward development of next generation of photoelectrical devices or catalytic materials with high performances.

almost 2.5-fold as that of ZnO nanorod arrays, disclosing that CdTe QD/ZnO nanorod arrays have the stronger ability to produce hydroxyl radicals. Mechanism. On the basis of the intermediates detected in the experiment and Anderson’s model, a possible charge transfer diagram shown in Figure 9 is proposed to elucidate the

Figure 9. Schematic diagram of the routes of charge carrier transfer and phenol degradation with CdTe QD encapsulated ZnO nanorod arrays.

routes of charge carrier transfer and phenol degradation. The visible radiation generates electron−hole pairs in CdTe QDs. The electrons from the conduction band of CdTe QDs are quickly transferred to the conduction band of ZnO nanorods. Once the electrons diffuse into the conduction band of ZnO, the probability of its decay is small because there can be no free holes in ZnO under visible excitation. As a result, the electrons accumulate in the conduction band of ZnO and the holes accumulate in the valence band of CdTe. In this way, the holes in CdTe can interact with phenol and hydroxyl to return back to its ground state. However, in the photocatalytic process, the electrons would greatly accumulate in the conduction band of ZnO, which inhibits the electron transfer. As a result, the degradation efficiency of phenol in the photocatalytic process is low (Figure 7a). This could be avoided by applying a positive bias potential. On one hand, positive bias potential could improve the photogenerated charge separation ability and eliminate the barriers of electron transfer. On the other hand, under the positive bias, the electrons flowing to the counter electrode could be donated to an electron acceptor such as dissolved oxygen (leading to formation of O2•−), and then a sequence of reactions occur, including combination of O2•− with protons to yield HOO•, H2O2, or hydroxy radical (OH•), which possess sufficient energy for oxidation of pollutants. Generation of OH• has been confirmed by ESR (Figure 8a). Moreover, the control experiments using O2 as scavengers of the photogenerated electrons also confirm the contribution of hydroxy radicals in the photoelectrochemical reaction with CdTe QD encapsulated ZnO nanorod arrays. As shown in Figure S10, for O2-bubbled solution, the degradation efficiency of phenol is 75%. However, for N2-bubbled solution, the degradation efficiency of phenol is only 43% after 150 min, which is 32% less than that with O2-bubbled solution. These results indicate that both O2 and hydroxy radicals contribute the degradation of phenol, leading to the complete mineralization of phenol and other intermediates.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information for synthesis of CdTe QDs, phenol determination, identification of the reaction intermediates, Figures S1−S10, and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.L.). *E-mail: [email protected] (B.Y.). *E-mail: [email protected] (Z.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Natural Science Foundation of China (grants 91023007, 20773033, and 20975028), the National Key Basic Research of China (973 Program, 2013CB632900), New Century Excellent Talents in University, Outstanding Young Funding of Heilongjiang Province, and the Fundamental Research Funds for the Central Universities (HIT. ICRST. 2010 004).

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