Synthesis and Photoinduced Charge-Transfer Properties of a

Feb 18, 2011 - standing of the photovoltaic properties, SPV, PL, and TPV techniques were employed to investigate the CT dynamics in the electrode of ...
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Synthesis and Photoinduced Charge-Transfer Properties of a ZnFe2O4-Sensitized TiO2 Nanotube Array Electrode Xinyong Li,*,†,‡ Yang Hou,*,† Qidong Zhao,† and Guohua Chen‡ †

Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education (MOE), and State Key Laboratory of Fine Chemical, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ABSTRACT: TiO2 nanotube arrays sensitized with ZnFe2O4 nanocrystals were successfully fabricated by a two-step process of anodization and a vacuum-assistant impregnation method followed by annealing. The sample was studied by an environmental scanning electron microscope, a transmission electron microscope, energydispersive X-ray analysis, and X-ray diffraction to characterize its morphology and chemical composition. Ultraviolet-visible (UVvis) absorption spectra and a photoelectrochemical measurement approved that the ZnFe2O4 sensitization enhanced the probability of photoinduced charge separation and extended the range of the photoresponse of TiO2 nanotube arrays from the UV to visible region. In addition, the behaviors of photoinduced charge transfer in a TiO2 nanotube array electrode before and after sensitization by ZnFe2O4 nanocrystals were comparatively studied. The photoluminescence of the TiO2 nanotube array electrode became suppressed, and the surface photovoltage responses on the spectrum were significantly enhanced after the introduction of ZnFe2O4 nanocrystals. The transfer dynamics of the photoinduced charges were observed directly by a transient photovoltage measurement, which revealed a fast charge separation at the interface between ZnFe2O4 nanocrystals and TiO2 nanotubes upon light excitation.

’ INTRODUCTION In recent years, photoinduced charge-transfer (CT) dynamics at the interface of a narrow band gap semiconductor/TiO2 has been intensively studied,1,2 because understanding the interfacial CT is crucial to the development of numerous nanomaterialbased devices, such as molecular electronics,3,4 solar cells,5,6 and fluorescence-based sensors.7,8 In most of these devices and applications, CT behavior plays an important role. In fact, the effective observation of CT behavior is a great challenge because of the complexity of influencing factors, which are related to the individual properties of nanoparticles, their electrical interconnects, and the local geometry.9 Nevertheless, understanding the CT behavior that governs the device operation and the performance of nanomaterials in various applications is essential for further improvement and development.10 A large amount of research work has been devoted to the study of CT, separation, and recombination phenomena. Peng et al. developed novel Fe2O3/TiO2 heterogeneous photocatalysts for degradation of Orange II. It was proven to have great advantage over the individual materials, owing to its enhanced chargeseparation ability, as demonstrated by surface photovoltage (SPV) and transient photovoltage (TPV) analysis.11 Jing et al. reported the deposition of WS2 onto the channel surface of crystalline mesoporous TiO2. It was postulated that under visible r 2011 American Chemical Society

light only electrons transferred from nanosized WS2 to the TiO2. This resulted in the effective charge separation of electrons and holes photogenerated in the WS2 and, thus, the efficient photocatalytic activity for hydrogen production on the WS2/TiO2 system.12 Gross et al. provided strong indirect evidence for efficient charge separation at the CdTe/CdSe interface of typeII layer-by-layer deposited structures by a photoluminescence (PL) quenching study.13 More recently, a transient spectroscopic measurement was used to probe the CT transitions in CdSe quantum dots-sensitized TiO2 nanoparticles, which revealed the localized nature of the CT state. The authors proved that the recombination and regeneration processes related with quantum dot sensitizers were not the main limiting factors leading to the low power conversion efficiency.14 For all kinds of the above photo-aided applications, there are four fundamental aspects determining the CT feature, including photoinduced charge generation,15 electron and hole separation,16 charge transport,17 and charge recombination,18 upon light excitation. To date, welldefined methods have been developed to effectively disclose these mechanisms in diverse functional systems, such as transient absorption,19 time-resolved photovoltage,20 transient photoconductivity,21 time-resolved PL,22 etc. Received: June 8, 2010 Published: February 18, 2011 3113

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Langmuir Recently, vertically oriented TiO2 nanotube arrays (TiO2NTAs) obtained by anodization of titanium have attracted significant interest because of their suitable photoelectric property and excellent stability.23,24 In comparison to TiO2 powders, the nanotube array structure can facilitate separation of the photoexcited charges, leading to higher charge collection efficiency.25 However, a critical drawback of TiO2 is its wide band gap of 3.2 eV, which can only be excited by ultraviolet (UV) light with a wavelength below 387 nm. This factor strongly limits the absorption of sunlight to the UV region of the solar spectrum.26 To solve this problem, much attention has been paid to extend the optical absorption edge of TiO2 into the visible region.27 A promising solution is to combine TiO2 with a semiconductor that has a narrow band gap and an energetically highlying conduction band. ZnFe2O4, with a band gap of 1.86 eV, is considered to be an important sensitizer used for large band gap semiconductors, and its conduction band level is slightly higher than that of TiO2.28,29 Yin et al. found that ZnFe2O4/TiO2 double-layered films have enhanced photoelectrochemical activity, because ZnFe2O4 and TiO2 can form a type-II band structure offset, which could efficiently separate photoinduced electronhole pairs in each semiconductor material and reduce their recombination.30 In a recent work, we have reported a ZnFe2O4/ TiO2 composite nanotube array prepared using a novel electrodeposition technique and proved that it has improved photoconversion efficiency.28 However, the photoinduced CT behaviors of the ZnFe2O4/TiO2 system, for example, the photoinduced electron transfer at the surface and interface of the material, have not been adequately addressed.11,31 In this work, we report the synthesis of a ZnFe2O4-sensitized TiO2-NTA electrode by a strategy combining electrochemical processes of anodization and vacuum-assistant impregnation method followed by annealing. To provide an in-depth understanding of the photovoltaic properties, SPV, PL, and TPV techniques were employed to investigate the CT dynamics in the electrode of TiO2-NTA modified by ZnFe2O4 nanoparticles. The transfer dynamics of the photoinduced charges were obtained directly and analyzed in detail based on a systematic characterization.

’ EXPERIMENTAL SECTION Preparation of the TiO2-NTA Electrode. A self-organized TiO2-NTA electrode was prepared in a two-electrode electrochemical cell. More details of anodic oxidation had been described previously.32 After oxidation, the amorphous TiO2-NTA was converted into anatase phase by annealing in air at 773 K for 1 h with heating rates of 2 C min-1. Preparation of the ZnFe2O4/TiO2 Composite Nanotube Array (ZnFe2O4/TiO2-CNTA) Electrode. The precursor of ZnFe2O4 was prepared using ethylene glycol as the solvent and citric acid as the coordinate agent. The stoichiometric amounts of zinc nitrate [Zn(NO3)2 3 6H2O] and ferric nitrate [Fe(NO3)3 3 9H2O] were dissolved in a volume of mixture solution (50 mL) containing citric acid and ethylene glycol (1:4, molar ratio). The molar amount of citric acid was equal to a total molar amount of metal nitrates in the solution. Ammonia solutions were slowly dropped in appropriate proportions, and a chemical reaction happened. During this procedure, the solution was kept at a temperature of 80 C and continuously stirred. Thus, a transparent and homogeneous precursor was obtained. Subsequently, a TiO2-NTA electrode was vertically placed into a buffer bottle under a vacuum of 1  10-3 Pa at room temperature for 120 s (vacuum assistant). Then, the obtained TiO2-NTA electrode was immediately

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immersed in a ZnFe2O4 precursor for 10 min. After that, it was withdrawn and dried at 100 C for 10 min. This procedure was repeated 5 times. Finally, the electrode was rinsed with deionized water and annealed at different temperatures (from 673 to 973 K) for 1 h at a heating rate of 2 C min-1 in air to obtain the ZnFe2O4/TiO2-CNTA electrode. For comparison, the ZnFe2O4 nanoparticles were loaded on a Ti foil (ZnFe2O4/Ti) by the vacuum-assistant impregnation method followed by annealing at 773 K for 1 h under the same conditions. Characterization. The morphology of the ZnFe2O4/TiO2CNTA electrode was characterized using an environmental scanning electron microscope (ESEM; Quanta 200 FEG) with an accelerating voltage of 30 kV. Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained with a Hitachi 800 system at 200 kV and a JEOL-2010 system also at 200 kV, respectively. The dopant concentration was verified by energy dispersive X-ray analysis (EDX; Horiba 7593H), and EDX was performed to determine the elemental concentration distribution on the catalyst using Link Isis Series 300 software. The crystallinity of the prepared samples was determined from X-ray diffraction (XRD) using a diffractometer with Cu KR radiation (Philips PW1825). The accelerating voltage and applied current were 40 kV and 30 mA, respectively. Light absorption properties were measured using UV-vis DRS (JASCO, UV-550) with a wavelength range of 200-800 nm. The PL spectra were measured at room temperature using an RPM2000 spectrophotometer (Accent Semiconductor Technologies Ltd., U.K.), with a He-Cd laser as the light source. Kelvin Probe (KP)-based SPV measurements were carried out on a scanning KP system (KP Technology, Ltd., U.K.). The SPV response was measured by tracking the contact potential difference (CPD) between the sample and the probe. The SPV value is equal to -ΔCPD. A 500 W xenon lamp and a double-prism monochromator provided the monochromatic light, which was less than 80 μW cm-2 at the maximum. Constant light intensity at each wavelength was not applied in the KP or alternating current (AC) SPV measurements. The SPV spectra were obtained by scanning the wavelength of monochromatic light from the visible to UV spectral region with the rate at about 30 nm min-1. The TPV device has been described in detail in a previous report.33 The TPV was excited with a laser radiation pulse (whose wavelength could be 355 or 532 nm, with a pulse width of 5 ns and a power of about 50 μJ) from a third harmonic Nd:YAG laser (Polaris II, New Wave Research, Inc.). The energy of the laser beam was measured by a joulemeter (Molectron Detector, Inc., EM500). The sample holder screened in a metal chamber is like a parallel-plate capacitor, which consisted of the sample electrode, a piece of 10 μm thick mica, and a counter electrode made of platinum wire gauze (from bottom to top). TPV signals were registered by a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix). All of the measurements were performed at room temperature and under ambient pressure. Photoelectrochemical Measurements. Photocurrent density was measured using a CHI electrochemical analyzer (CH instruments 760C, Shanghai, Chenhua, China) in a standard three-electrode configuration, with the ZnFe2O4/TiO2-CNTA electrode (an effective area of 6 cm2), a platinum foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Na2SO4 (0.01 M) purged with N2 was used as the electrolyte. A 500 W high-pressure xenon short arc lamp (Phillips) with a filter to remove light of wavelength below 420 nm was used as the visible light source to provide a light intensity of 100 mW cm-2 (Figure 1).

’ RESULTS AND DISCUSSION Figure 2 shows the XRD patterns of ZnFe2O4/TiO2-CNTA calcined at different temperatures. For comparison, an XRD pattern from TiO2-NTA is given in curve a. Qualitative analysis 3114

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Figure 1. Emission spectral intensity of the atlas fitted with a Xe lamp. Figure 3. UV-vis absorption spectra of TiO2-NTA, ZnFe2O4/Ti, and ZnFe2O4/TiO2-CNTA (calcined at 773 K).

Figure 2. XRD patterns of (a) TiO2-NTA calcined at 773 K for 1 h and ZnFe2O4/TiO2-CNTA calcined at (b) 673, (c) 773, (d) 873, and (e) 973 K for 1 h.

of this pattern shows that all peaks in the pattern can be indexed using the anatase TiO2-NTA [Joint Committee on Powder Diffraction Standards (JCPDS) 21-1272] and the Ti substrate (JCPDS 44-1294). In curves b-e, ZnFe2O4/TiO2-CNTA exhibits not only characteristic TiO2 (anatase and rutile) and Ti substrate diffraction peaks but also other new diffraction peaks at 30.1, 35.3, and 57.2, corresponding to Æ220æ, Æ311æ, and Æ511æ facets of ZnFe2O4, respectively. Clearly, the diffraction peak at 35.3 appears overlapping, which is from the two diffraction peaks of 2θ (ZnFe2O4) = 35.3 and 2θ (Ti) = 35.1. Peaks of ZnFe2O4 are well-matched to cubic spinel ZnFe2O4 (JCPDS 22-1012). The broad peaks associated with the spinel ZnFe2O4 reflect the small grain size of ZnFe2O4 nanocrystals on TiO2-NTA. In comparison of XRD patterns of curves a and c, the addition of ZnFe2O4 by the vacuum-assistant impregnation and calcination method promotes the phase transformation of TiO2 from anatase to rutile and results in the decrease of the phase transformation temperature. Additionally, further observation indicates that the diffraction peaks from the Ti substrate obviously decrease with an increasing calcination temperature, which might be ascribed to the fact that Ti is directly oxidized and transformed into rutile TiO2 at high temperatures.34 Shown in Figure 3 are UV-vis absorption spectra obtained from TiO2-NTA, ZnFe2O4/Ti, and ZnFe2O4/TiO2-CNTA (calcined at 773 K). TiO2-NTA mainly absorbs the UV light with wavelengths below 400 nm, while the ZnFe2O4/Ti sample

Figure 4. (a) Top surface ESEM image of TiO2-NTA. The inset is the corresponding high-magnification image. (b) Cross-sectional ESEM image of TiO2-NTA. The inset is the corresponding high-magnification image. (c and d) Top surface ESEM images of ZnFe2O4/TiO2CNTA (calcined at 773 K).

exhibits an obvious absorption in a wide wavelength range from UV to visible light. Moreover, a significant shift of the spectral response up to approximately 620 nm is observed in ZnFe2O4/ TiO2-CNTA. This indicates that the loading of ZnFe2O4 significantly improved the visible light absorption of TiO2NTA. From the top view (Figure 4a) and the cross-sectional (Figure 4b) examination, it can be observed that the diameter of the nanotubes is about 80-90 nm, the wall thickness is around 15 nm, and the average tube length is found to be approximately 700 nm. After vacuum-assistant impregnation and calcinations, the nanotube surface areas become decorated with aggregates of fine ZnFe2O4 nanoparticles (Figure 4c). ZnFe2O4 nanoparticles are not only visible on the top but also inside the tubes. No obvious blocks of ZnFe2O4 are observed at the entrances of TiO2-NTA (Figure 4d). Moreover, the structural integrity and morphology of TiO2-NTA remain unaltered after ZnFe2O4 loading. 3115

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Figure 5. (a) TEM image of TiO2-NTA. The inset is the corresponding high-magnification image. (b and c) TEM images of ZnFe2O4/ TiO2-CNTA (calcined at 773 K). The inset is the corresponding EDX pattern of ZnFe2O4/TiO2-CNTA. (d) HRTEM image of ZnFe2O4/ TiO2-CNTA (calcined at 773 K).

The microscopic structure of ZnFe2O4-sensitized TiO2NTA was further investigated by TEM. Figure 5a is a TEM image of the TiO2 nanotubes, showing that the sample has an ordered array tubular structure. The TEM image of ZnFe2O4/ TiO2-CNTA clearly displays that ZnFe2O4 nanoparticles (marked with brown arrows and yellow circles) have been deposited on the region near the top of the nanotubes and into the pore of the TiO2 nanotubes. The size of the ZnFe2O4 nanoparticles is around 30 nm (panels b and c of Figure 5). The corresponding EDX spectrum of ZnFe2O4/TiO2-CNTA also demonstrates the presence of Ti, O, Zn, Fe, and Cu (inset of Figure 5c). The quantitative analysis reveals that the atomic ratio of Zn and Fe is close to 1:2. The clearly resolved lattice fringes of 0.351 nm in the HRTEM image (Figure 5d) correspond to the Æ101æ plane of the anatase TiO2, which is in good agreement with the Æ101æ peak in the above XRD measurement. The observed lattice fringes of 0.254 and 0.172 nm in the right part of the image correspond to the Æ311æ and Æ422æ planes,35,36 respectively, of the cubic spinel phase of ZnFe2O4. These results confirm that ZnFe2O4 nanoparticles have been successfully assembled into TiO2-NTA. Figure 6a shows the photocurrent density variations of TiO2NTA, ZnFe2O4/Ti, and ZnFe2O4/TiO2-CNTA (calcined at 773 K) electrodes with measured potential in 0.01 M Na2SO4 solution. The dark current densities in all cases can be negligible. While upon irradiation, a significant increase in the photocurrent density is observed throughout the potential window at the ZnFe2O4/TiO2-CNTA electrode. The saturated photocurrent density of the ZnFe2O4/TiO2-CNTA electrode is about 0.61 mA cm-2, higher than those over the ZnFe2O4/Ti (0.29 mA cm-2) and TiO2-NTA (0.21 mA cm-2) electrodes by a factor of 2.1 and 2.9 at 1.2 V (versus SCE). Photoconversion efficiency, η, which is the light energy to chemical energy conversion efficiency, is calculated as37

Figure 6. (a) Variation of the photocurrent density versus bias potential (versus SCE). (b) Photoconversion efficiency as a function of the applied potential (versus SCE). (c) Short-circuit photocurrent density versus time plotted (0 V versus SCE) for TiO2-NTA, ZnFe2O4/Ti, and ZnFe2O4/ TiO2-CNTA (calcined at 773 K) electrodes in 0.01 M Na2SO4 solution under visible light irradiation (I0 = 100 mW cm-2; λ > 420 nm).

η ð%Þ ¼ ½ðtotal power output - electrical power inputÞ= light power input  100 ¼ jp ½Erev  - jEapp j  100=ðI0 Þ ð1Þ 3116

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Figure 7. PL spectra of TiO2-NTA and ZnFe2O4/TiO2-CNTA (calcined at 773 K).

where jp is the photocurrent density (mA cm-2), jpErev is the total power output, jp|Eapp| is the electrical power input, and I0 is the power density of incident light (mW cm-2). Erev is the standard reversible potential of 1.23 V [versus the normal hydrogen electrode (NHE)]. The applied potential is Eapp = Emeas - Eaoc, where Emeas is the electrode potential (versus SCE) of the working electrode at which the photocurrent is measured under irradiation and Eaoc is the electrode potential (versus SCE) of the same working electrode under open-circuit conditions under the same irradiation and in the same electrolyte. Figure 6b shows the corresponding photoconversion efficiency of the TiO2-NTA, ZnFe2O4/Ti, and ZnFe2O4/TiO2CNTA electrodes. A maximum photoconversion efficiency of the ZnFe2O4/TiO2-CNTA electrode increases to 0.57%, which is 2.7 and 4.1 times as much as that of the ZnFe2O4/Ti electrode (0.21%) and the TiO2-NTA electrode (0.14%), respectively. This result confirms that the ZnFe2O4/TiO2-CNTA electrode possesses enhanced photoconversion capability compared to the ZnFe2O4/Ti and aligned TiO2-NTA electrodes. Moreover, Figure 6c shows that the ZnFe2O4/TiO2-CNTA electrode has a strong instant photoresponse to the visible light irradiation. The short-circuit photocurrent density of the ZnFe2O4/TiO2CNTA electrode is as great as 4.8 times that of the TiO2-NTA electrode. This demonstrates that the separation rate of photogenerated holes and electrons increases because of the loading of ZnFe2O4. More photogenerated electrons collected from the ZnFe2O4/TiO2-CNTA electrode suggest that more photogenerated holes survive from recombination or from the longer lifetime that the holes had. Figure 7 shows the PL spectra of TiO2-NTA and ZnFe2O4/ TiO2-CNTA (calcined at 773 K) with an excitation wavelength of 325 nm. The PL spectra of anatase TiO2 materials were mainly attributed to three kinds of physical origins: self-trapped excitons, oxygen vacancies, and surface states.38,39 Clearly, the peak 1 at 394 nm should be attributed to self-trapped excitons located on the TiO6 octahedral. This is an intrinsic property of the TiO2 nanocrystal structure. Peaks 2 (455 nm) and 3 (528 nm) are also detected, and it seems that they are associated with the oxygen vacancies forming at the surface of TiO2-NTA.40 The weaker PL intensity of ZnFe2O4/TiO2-CNTA represents the fact that the sensitization of ZnFe2O4 decreases the density of the luminescence center/recombination center (surface states). Meanwhile, the lower PL intensity of ZnFe2O4/TiO2-CNTA than that of TiO2-NTA indicates that the charge separation

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Figure 8. SPV spectra of TiO2-NTA (curve a) and ZnFe2O4 /TiO2CNTA (curve b, calcined at 773 K) with irradiation from the top, taken on the KP. (Inset) Schematic setup of the KP-based SPV measurement.

efficiency may have been improved for the composite. This result is consistent with the charge transport properties of the above electrodes. SPV techniques based on a lock-in amplifier were employed to characterize the behavior of photogenerated charge carriers in the system.41 Figure 8 shows the SPV spectra of TiO2-NTA and ZnFe2O4/TiO2-CNTA (calcined at 773 K) taken on a KP. The KP-based SPV response of TiO2-NTA was apparently enhanced after the loading of ZnFe2O4 for the spectral region of 300-600 nm, which is attributed to the higher charge separation efficiency related to the sensitization of ZnFe2O4. In addition, the SPV onset of curve b (∼588 nm) is dramatically red-shifted compared to that of curve a (∼387 nm). In the spectral region of 387-600 nm, TiO2 cannot generate a SPV response because its band gap is about 3.2 eV42 and only the ZnFe2O4 nanoparticles can be excited. Because the conduction band of TiO2 lies more positive than that of the ZnFe2O4 conduction band,30 electron injection is expected from the photoexcited ZnFe2O4 into the TiO2 conduction band under visible light region irradiation (λ > 420 nm), whereas holes can accumulate in the valence band ZnFe2O4 to form a hole center. Thus, the exciton pairs are wellseparated in ZnFe2O4/TiO2-CNTA, producing a much broader and stronger SPV response than TiO2-NTA. The TPV technique, known as time-resolved photovoltage, is a very promising method for the investigation of dynamic properties of the photoinduced charge carriers in semiconductor materials.43-47 We can directly obtain information about the charge dynamics, including generation, separation, and recombination of the photoinduced charges via this technique. Under the laser pulse of 355 nm, the observed positive TPV polarity indicates that photogenerated electrons are transferred toward the bottom electrode (Figure 9a). For TiO2-NTA, an abrupt rise of photovoltage response peaked at the time scale of about 10-7 s corresponds to the maximum separation of photogenerated charge carriers. This feature should be attributed to the fast separation of photogenerated electron-hole pairs and surface charge accumulation. In contrast to bulk crystals, the electric field of the surface space charge region can be negligible in the TiO2 nanotubes. The slow TPV decay is limited by the interparticle diffusion in the TiO2 nanotubes and delayed charge recombination.44 As a result, after the quick photovoltage transient, the charge concentration at the surface of TiO2 nanotubes decreases gradually with time. For ZnFe2O4/TiO2-CNTA (calcined at 3117

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Scheme 1. Possible Pathway of the Photoelectron Transfer Excited by Visible Light Irradiation for ZnFe2O4/ TiO2-CNTA

Figure 9. TPV curves of TiO2-NTA and ZnFe2O4/TiO2-CNTA (calcined at 773 K) excited by a laser at (a) 355 nm and (b) 532 nm, respectively, with 5 ns of pulse duration.

773 K), upon excitation, large numbers of charges could also be generated at the surface of TiO2 and ZnFe2O4 nanoparticles. The surface-accumulated holes could then diffuse and recombine slowly with the excess electrons. The TPV response curves for both samples have peaks at the level of 10-7 s, which reflects the same nature of the fast charge separation and accumulation of the photogenerated holes. The TPV curves under the excitation by laser pulse of 532 nm show two main features (Figure 9b). First, positive photovoltage transients are observed for both samples. A positive TPV signal means positive charges transferring toward the top electrode or negative charges transferring to the bottom electrode. Second, the maximum TPV response increases after the sensitization of ZnFe2O4 nanoparticles. The TPV response is relatively low (about 10 μV) for TiO2-NTA, while it is much higher (about 60 μV) for TiO2-NTA sensitized by ZnFe2O4 nanoparticles. The stronger photovoltage intensity of ZnFe2O4/TiO2-CNTA indicates the higher charge separation efficiency. This feature also means that the photovoltage response has been successfully extended to the visible region. For the TPV pattern of TiO2-NTA in Figure 9b, there are weak peaks of photovoltage responses at four time scales of about 2.2  10-7, 4.4  10-7, 5.3  10-6, and 2.2  10-5 s, respectively, which could be attributed to trapping and detrapping of the electrons by the TiO2 nanoparticles composing the nanotubes. Because the incident photon energy (2.33 eV) was much lower than the band gap of TiO2, the photoinduced charge separation could only originate from the sub-band gap transition between surface states and energy bands of TiO2 nanoparticles. Then, the slow transport process of charge carriers was due to the

interaction between the small amount of photoinduced electrons and the surface states of TiO2 nanoparticles. While for ZnFe2O4/TiO2-CNTA under the excitation by a laser pulse of 532 nm, a peak value appears at 2.2  10-7 s. Because the ZnFe2O4 nanoparticles can be effectively excited by the laser at this wavelength, the fast positive TPV response peaked at 2.6  10-7 s indicates that plenty of photogenerated electrons have been injected into TiO2 from photoexcited ZnFe2O4. Thereafter, the slow decay of the TPV response is corresponding to the subsequent relaxation of the maximum charge separation state with time. According to the previous report,48 the injection from ZnFe2O4 to TiO2 through their interface could occur on a sub-nanosecond time scale, so that the first peak of the TPV response at 2.6  10-7 s should be attributed to the fast charge accumulation at the surface of ZnFe2O4 nanoparticles on the TiO2 nanotubes. Therefore, the dynamics of photogenerated charge carriers in the composite should undergo the processes of injection from ZnFe2O4 to TiO2, diffusion, and transfer across the TiO2 nanoparticles slowly toward the Ti substrate. The first step of interfacial CT is key and effective in reducing the recombination of electron-hole pairs generated in ZnFe2O4, although it could not be detected effectively by the TPV characterization becuase of the ultra-fast interfacial process within a time scale below 1 ps.49 According to the above results, it is further deduced that the coupling between ZnFe2O4 and TiO2 is responsible for the efficient charge separation and enhanced response under visible light excitation (Scheme 1). ZnFe2O4 with narrow-band gap energy could be easily excited by visible light (λ < 666 nm) and induce the generation of excess electrons and holes. For the case of TiO2, it could not be effectively excited by the photons with energy less than 2.95 eV (λ > 420 nm) because of its wide band gap of about 3.2 eV. With photons of larger than 2.95 eV (λ < 420 nm), electrons in the valence band of ZnFe2O4 could be excited up to a higher potential edge. The reformed conduction band edge potential of ZnFe2O4 is more active than that of TiO2. Hence, photoinduced electrons on the surface of ZnFe2O4 would easily transfer to TiO2, leaving the holes in the valence band of ZnFe2O4. Then, the TiO2 nanoparticles of the nanotube should allow for further transport of the excess electrons to the back contact electrode (Ti substrate). In such a way, the photoinduced electron-hole pairs could be more effectively separated. 3118

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Langmuir With the recombination of the minimized charge carrier and the enhanced photoelectric activity, the ZnFe2O4/TiO2-CNTA electrode should provide a valuable scaffold for many potential applications, such as photocatalysis, solar cell, water splitting, and sensors.

’ CONCLUSIONS In summary, the surface sensitization of a highly ordered TiO2-NTA by ZnFe2O4 nanoparticles was achieved by a vacuum-assistant impregnation method followed by annealing. The introduction of ZnFe2O4 nanoparticles significantly extended the spectral response toward the visible region. Under the influences of ZnFe2O4, higher photoinduced charge separation efficiency in the system was observed. Our results indicated that the separation of the charge carriers could undergo the processes of ultra-fast (about the level of 10-7 s) injection from ZnFe2O4 to TiO2 and then diffusion in the TiO2 nanotubes. The major rate-controlling step for charge decay lies in the interparticle diffusion between the TiO2 nanoparticles in the nanotubes. The information provided here may provide useful support for the further understanding of the dynamic properties of the CT in the composite system, especially for the synthesis of highly efficient visible light catalysts and fabricating photoelectric devices with higher performance. ’ AUTHOR INFORMATION Corresponding Author

*Key Laboratory of Industrial Ecology and Environmental Engineering, MOE, and State Key Laboratory of Fine Chemical, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China. Telephone: þ86-411-8470-7733. Fax: þ86-411-84708083. E-mail: [email protected].

’ ACKNOWLEDGMENT This research was supported financially by the National Nature Science Foundation of China (20837001, 20877013, and NSFC-RGC 21061160495), the National High Technology Research and Development Program of China (863 Program) (2007AA061402), and the Major State Basic Research Development Program of China (973 Program) (2007CB613306). We appreciate Dr. Yu Zhang and Prof. Dejun Wang at Jilin University for the warm support of photovoltage measurements. ’ REFERENCES (1) Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. Chem. Rev. 2010, 110, 6503–6570. (2) Robel, I.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2007, 129, 4136–4137. (3) Astruc, D.; Boisselier, E.; Ornelas, C. Chem. Rev. 2010, 110, 1857–1959. (4) Eckermann, A. L.; Shaw, J. A.; Meade, T. J. Langmuir 2010, 26, 2904–2913. (5) Kim, J. Y.; Choi, S. B.; Noh, J. H.; Yoon, S. H.; Lee, S.; Noh, T. H.; Frank, A. J.; Hong, K. S. Langmuir 2009, 25, 5348–5351. (6) Chun, K. Y.; Park, B. W.; Sung, Y. M.; Kwak, D. J.; Hyun, Y. T.; Park, M. W. Thin Solid Films 2009, 517, 4196–4198. (7) Takashi, T.; Tetsuro, M. Chem. Soc. Rev. 2010, 39, 4802–4819. (8) Yang, L. X.; Chen, B. B.; Luo, S. L.; Li, J. X.; Liu, R. H.; Cai, Q. Y. Environ. Sci. Technol. 2010, 44, 7884–7889. (9) Ofir, A.; Dittrich, T.; Tirosh, S.; Grinis, L.; Zaban, A. J. Appl. Phycol. 2006, 100, No. 074317.

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