Electron Paramagnetic Resonance Investigation of Charge Transfer in

Article pubs.acs.org/JPCC

Electron Paramagnetic Resonance Investigation of Charge Transfer in TiO2(B)/Anatase and N−TiO2(B)/Anatase Mixed-Phase Nanowires: The Relative Valence and Conduction Band Edges in the Two Phases Hsin-Hsi Lo,† Neeruganti O. Gopal,†,§ Shiann-Cherng Sheu,‡ and Shyue-Chu Ke*,† †

Physics Department, National Dong Hwa University, Hualien, Taiwan 97401 Department of Occupational Safety and Health, Chang Jung Christian University, Tainan, Taiwan 711

‡

S Supporting Information *

ABSTRACT: Regarding how photogenerated charge carriers are transferred in TiO2(B)/ anatase mixed-phase nanowires, no unified conclusion has been reached. Electron paramagnetic resonance (EPR) spectroscopy is employed to investigate the vectorial charge transfer in this material. When the material is subjected to UV irradiation, we show that holes stimulated in anatase are transferred to TiO2(B) by comparing EPR-detected amounts of trapped hole O− accumulated on TiO2(B) with X-ray diffraction (XRD)-determined TiO2(B) bulk phase compositions. Under visible-light irradiation which only activates the TiO2(B) phase, we unambiguously show that electron transfer occurs from TiO2(B) to anatase. Without intervention of other charge carriers generated by bandgap excitation, we monitor exclusively the fate of conducting electrons generated by specific excitation of N− midgap states of TiO2(B) with holes localized on the N atom in N-doped TiO2(B)/anatase. The result again clearly demonstrates that electrons migrate from TiO2(B) to anatase. Time-dependent decay of the N•-hole EPR signal shows that it is difficult for the transferred electron to return to TiO2(B). Both higher conduction band and valence band edge potentials in TiO2(B) than the corresponding ones of anatase are implicated. This study helps to point the way toward future development of TiO2(B) nanowire based material for photovoltaic and photocatalytic applications.

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INTRODUCTION Anatase titanium dioxide (TiO2) is the most commonly used material for photocatalytic and photovoltaic applications because of its excellent chemical stability, biological nontoxicity, and low cost.1−5 Two salient issues regarding the modification of TiO2, design of heterostructures for promoting spatial separation of charge carriers and extension of its absorption edge from the UV into the visible range for maximal utilization of solar energy, have been extensively investigated.6−10 UV excitation of anatase TiO2 promotes creation of conduction band (CB) electrons and valence band (VB) holes. These charge carriers either recombine or diffuse to the surface and act as initiators for the subsequent redox chemical reactions. Rapid electron−hole recombination is detrimental. Research efforts are directed toward understanding the mechanistic basis for improving charge separation in the design of new TiO2 nanostructures. Anatase and rutile mixed-phase TiO2, such as Degussa P25, is an excellent example of efficient spatial separation of charge carriers in which rutile electrons generated by visible-light irradiation are transferred to anatase, producing long-lived charge-separated states.11 Anatase− brookite and rutile−brookite TiO2 nanocomposites for enhancing photocatalytic activity have also been reported.12,13 Doping TiO2 with nonmetal elements like a nitrogen atom or boron atom is widely used to extend the optical absorption to the visible-light region.10,14−16 Among those doped materials, nitrogen-doped TiO2, the origin of photoactivity under visible light has clearly been elucidated.16 It was shown © 2014 American Chemical Society

that nitrogen doping in the anatase TiO2 system introduces a localized N− state in the midgap of TiO2 which can absorb photons with energies much less than the bandgap energies, and that generates a hole localized on a N atom (N•-hole) and a conducting electron. Recently, TiO2(B) has attracted much attention as a promising material for use in lithium ion batteries, dyesensitized solar cells, humidity sensors, and environmental photocatalysts.17−20 Monoclinic TiO2(B) was first synthesized by Marchand et al. in 1980 and was found in natural minerals by Banfield in 1991.21,22 Recently, various morphologies of TiO2(B) like nanowires and nanotubes were synthesized exhibiting high surface area and potential photocatalytic efficiency.23−26 TiO2(B)/anatase mixed-phase composites have also been prepared and exhibited enhanced activity under UV compared to the pure phase alone.18,19,27,28 Contradicting models18,19 regarding photogenerated charge transfer in TiO2(B)/anatase composites were proposed to account for the enhanced activity. One model favors electron migration from TiO2(B) to anatase and hole migration in the opposite direction. The other suggests that more holes than electrons migrate to TiO 2 (B), and that reduces the recombination of photogenerated charge carriers in anatase. No experimental evidence for electron migration to either Received: November 28, 2013 Revised: January 8, 2014 Published: January 14, 2014 2877

dx.doi.org/10.1021/jp411723m | J. Phys. Chem. C 2014, 118, 2877−2884

The Journal of Physical Chemistry C

Article

Figure 1. FE-SEM images of (a) HT-NW, (b) HT400, (c) HT500, and (d) HT550.

several times. The deposited powders were collected by centrifugation and dried at 70 °C for 8 h. The obtained HTNW was further subjected to heat treatment in air at various temperatures for 4 h. For nitrogen doping, the HT-NW was additionally stirred in ammonium hydroxide solution for 24 h, prior to dehydration and heat treatments. The heat-treated powders are designed as HTX for undoped and NHTX for Ndoped samples, where X represents the heating temperature in °C. Sample morphology was taken by field emission scanning electron microscopy (FE-SEM, JEOL JSM-7000F). Structures were characterized by an X-ray diffractometer (Rigaku D/Max2500) operating at 30 kV and 50 mA with Cu Kα (λ = 0.154 nm) radiation. Diffuse reflectance spectra were recorded using a Shimatzu UV-2550 spectrophotometer equipped with an integrating sphere. X-band EPR spectra were recorded by a Bruker EMX spectrometer with a TE102 cavity. The microwave frequency was measured with a Hewlett-Packard 5246L electronic counter. Measurement temperature at 12 K was maintained by an Advanced Research System Helitran continuous flow cryostat or at 77 K by immersion of the EPR sample tube in a liquid nitrogen containing finger Dewar. The irradiation light source was a Newport 1000 W xenon lamp with IR removed and transmitted into the EPR cavity through an optical fiber. For visible irradiation, an Andover (400FH9025) long pass filter was used. For N− site specific excitation, a monochromator (Newport, 66921) in conjunction with the xenon lamp was used. All samples were irradiated in situ for 10 min. EasySpin 3.1.7 was employed to simulate the spectra.30

direction was given in both models. No unified conclusion has been reached concerning the directional movements of charge carriers which circle around the relative position of CB and VB edges in the two phases. The purpose of this study is to employ electron paramagnetic resonance (EPR) techniques to provide direct spectroscopic evidence for the directional movement of charge carriers in the TiO2(B)/anatase mixed-phase nanowire. In addition, we have also prepared N-doped−TiO2(B) and in mixed phase with anatase (N−TiO2(B)/anatase), which offers the opportunity to monitor solely the fate of a photogenerated electron from a N− state by a N− site specific excitation, without intervention of other charge carriers generated by bandgap excitation.16 A model describing the relative CB and VB edges in the two phases is proposed.

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EXPERIMENTAL METHODS Hydrogen titanate nanowires (HT-NWs) were synthesized by hydrothermal treatment of precursors in an alkaline environment and further subjected to ion-exchange treatment following reported procedures.27,29 An amount of 2.05 g of commercial TiO2 was dispersed in 50 mL of aqueous solution of 10 M NaOH under vigorous and continuous stirring for 30 min at room temperature. The suspension was transferred into a Teflon-lined stainless steel autoclave. The autoclave was heated to 180 °C and maintained for 24 h in a box furnace and then slowly cooled to room temperature. The precipitates were washed with 0.5 M HCl aqueous solution and deionized water 2878



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RESULTS AND DISCUSSION Material Characteristics of HTX. The HT-NW exhibits wire-like morphology (Figure 1a). Heating of the HT-NW does not cause an obvious change in the nanowire morphology (Figure 1b−d). The average diameter is about 40−130 nm, and the length is several micrometers. The XRD pattern of the HT-NW (Figure 2a) is consistent with the crystal structure data of H2Ti3O7 (JCPDS 41-0192).

Figure 3. UV−visible diffuse reflectance spectra of (a) HT400 (black) and (b) HT800 (red). Inset: Tauc plots37 showing the bandgap energies calculated by the transformed Kubelka−Munk function.

and thus can be activated by λ > 400 nm photons. The HT800 (Figure 3b) has an absorption edge at 387 nm (∼3.2 eV), a common value of anatase nanoparticles, and thus can only be activated by UV. Origin of HTX EPR Spectra. Figure 4 shows the EPR spectra of HTX samples under in situ UV irradiation at 12 K. Figure 2. XRD patterns of (a) HT-NW, (b) HT400, (c) HT500, (d) HT550, (e) HT600, (f) HT700, and (g) HT800.

HT-NW nanowires are transformed mainly into the TiO2(B) phase (JCPDS 46-1238) when heated at 400 °C (Figure 2b).31−34 Between 500 and 700 °C (Figure 2c−f), mixtures of TiO2(B) and anatase (JCPDS 21-1272) are observed. The TiO2(B) contents vanished below the detection limit for HT800 (Figure 2g). Quantification of the phase compositions determined by the reference intensity ratio method is given in Table 1. HT400 consisting of 94% TiO2(B) is termed TiO2(B)Table 1. Ratio of XRD Phase Compositions and EPR Trapped Holes in TiO2(B) and Anatase TiO2 HT XRD (±15%) EPR (±10%)

400

500

550

600

700

800

Phase composition ratio (TiO2(B)/anatase TiO2) 94:6 80:20 37:63 20:80 14:86 0:100 Ratio of trapped holes (TiO2(B)/anatase TiO2) 93:7 86:14 79:21 26:74 10:90 0:100

type HT400. Analogous phase transformation phenomena from the HT-NW to TiO2(B) to anatase have been reported previously.32,33,35,36 In particular, the massive TiO2(B) to anatase phase transformation occurring at heating temperature between 500 °C (20% anatase) and 550 °C (63% anatase) (Table 1) is consistent with an earlier report32 showing an exothermic peak at 530 °C in thermogravimetry-differential thermal analysis which was attributed to phase transformation from TiO2(B) to anatase. For another example, an increase of anatase fraction from 24% (500 °C) to 74% (600 °C) was reported during the TiO2(B) nanoribbon phase transformation.36 UV−vis data show that HT400 (Figure 3a) dominated by TiO2(B) has its absorption edge extended into the visible range at ∼405 nm corresponding to a bandgap energy of ∼3.06 eV

Figure 4. Simulation and deconvolution of 12 K EPR spectra (UV minus dark) of (a) HT400, (b) HT500, (c) HT550, (d) HT600, (e) HT700, and (f) HT800. g > 1.995 features are decomposed into three components B, C, and D. A weighted sum of the three components gives rise to the simulated spectrum (blue) overlaid on the experimental spectrum (black). Red lines are to guide the eye. Settings: microwave frequency, 9.541 GHz; microwave power, 20 mW; modulation amplitude, 4 G at 100 kHz. Deconvoultions of each spectra are given in Supporting Information Figure S1.

At low temperature, there is not enough energy for photogenerated charges to diffuse from trapping sites for recombination, and that maximizes the signal intensities for EPR spectra deconvolution. The EPR spectrum of anatase HT800 (Figure 4f) is dominated by two sets of signals. The 2879



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first set with g⊥ = 1.991 and g// = 1.962 (signal ″A″) is assigned to photogenerated electrons trapped at coordinatively unsaturated cation sites located in the bulk of anatase to form Ti3+. These inner trapped electrons cannot react with surfaceadsorbed O2. The second set at g = 2.026, 2.016, 2.003 (signal ″C″) is assigned to trapped holes O− on the anatase surface. Two types of photogenerated holes have been commonly observed in TiO2: holes trapped at the lattice oxygen atoms located in the subsurface layer with a structure of [Ti4+O−•Ti4+OH−] and holes trapped on the surface bridging oxygen atoms with a structure of [Ti4+O2−Ti4+O−•]. Here, the superscript ″•″ denotes the location of the unpaired spin. By heating at a temperature above 400 °C for 4 h, most of the surface hydroxyl groups are removed, thus the structure of the trapped hole signal observed most likely corresponds to [Ti4+O2−Ti4+O−•]. The measured rhombic g tensors for the anatase trapped hole O− are in good agreement with literature values (Table 2).38−44 However, we note that there are also reported g-tensors with axial symmetry and different values for O− in anatase.45,46

are given in Table 3. The simulated spectra are in excellent agreement with the experimental ones. Table 3 shows that Table 3. Simulated Percentage of O− Contribution

HT400 HT500 HT550 HT600 HT700 HT800

2.026 2.016 2.027 2.019 2.030 2.018 2.028 2.016 2.026 2.015 2.025 2.016 2.026 2.017 2.027 2.015 g values for O−, 2.023 2.0210

2.003 2.007 2.004 2.002 2.005 2.003 2.002 2.003 in TiO2(B)

2.013 2.0135

assignment

reference

Ti4+O2−Ti4+O−• Ti4+O2−Ti4+O−• Ti4+O2−Ti4+O−• Ti4+O2−Ti4+O−• Ti4+O2−Ti4+O−• holes at anatase surface Ti4+O−• holes at anatase surface assignment

this work 38 39 40 41 42 43 44 reference

2.004 2.0042

O− O−

O− in anatase TiO2 (signal C)

O2•− (signal D)

66 63 64 21 8 0

5 11 17 62 74 83

29 26 19 17 18 17

upon increasing heating temperatures the contribution from scavenged electrons O2− to the EPR spectra decreases due to removal of surface-adsorbed O2, and the contribution from trapped holes O− in anatase (signal C) increases due to phase transformation from TiO2(B) to anatase while the contribution from O− in TiO2(B) (signal B) decreases. The ratio of trapped holes O− in TiO2(B) and anatase is then evaluated by taking the percentage of O− contributions of one phase divided by the total percentage of O− contributions from both phases. The data are compared with XRD determined bulk phase compositions in Table 1. Hole Transfer from Anatase to TiO2(B). Overall, the EPR spectra simulation determined ratios of trapped holes between TiO2(B) and anatase are consistent with the XRD determined phase compositions (Table 1), except that an apparent inconsistency exists for HT550. A plausible explanation is that whereas XRD detects the relative bulk phase compositions EPR measures the actual amount of trapped holes accumulated in different phases including the holes transferred through the anatase/TiO2(B) interface. Therefore, the comparison given in Table 1 demonstrates that a faction of photogenerated holes have migrated from anatase to TiO2(B). As a result, the EPRdetermined number of trapped holes in TiO2(B) is much higher than the XRD-determined bulk phase fraction of TiO2(B). This is likely to occur if the valence band edge of TiO2(B) is higher than that of anatase. Electron Transfer from TiO2(B) to Anatase. To investigate how electrons are transferred between TiO2(B) and anatase, we compare the EPR spectra of HT800, HT400, HT500, and HT550 under λ > 400 nm visible-light irradiation (Figure 5). The EPR measurement temperature was raised from 12 to 77 K to facilitate charge transfer. The visible-light photon does not have sufficient amount of energy to clear the 3.2 eV bandgap of anatase HT800, as evidenced by the flat line in Figure 5a. In contrast, both trapped electron and hole signals are observed in the other three TiO2(B)-containing samples (Figure 5b,c,d). These mean that the EPR signals originate from the electron−hole pairs created exclusively in TiO2(B). For HT400, the broad signal (peak to trough of ∼50 G) at g = 1.937 (signal ″E″, Figure 5), similar to that observed in titania nanotubes by Vijayan et al.,42 is analogously attributed to photogenerated electrons localized at surface trapping sites in the form of paramagnetic Ti3+ on the TiO2(B) surface. The surface strain might have led to a distribution of g tensors causing considerable line broadening. Signal ″F″ (g⊥ = 1.978, g// = 1.968), characteristic of electrons trapped in tetrahedral Ti4+ sites,48 is irrelevant in the present study. Very clearly, a sharp and prominent signal at g = 1.991 with a much weaker component at g = 1.962, which was not observed

Table 2. Comparison of EPR Trapped Hole g Values in Anatase and in TiO2(B) g values for O−, in anatase

O− in TiO2(B) (signal B)

this work 47

The EPR spectrum of TiO2(B)-type HT400 (Figure 4a) is dominated by signal ″B″ at g = 2.023, 2.013, and 2.003, assigned to trapped holes O− on the TiO2(B) surface. The g values and line shape are consistent with the data reported by Diwald et al. for trapped hole O− in nanotubes,47 which, to the best of our knowledge, is the only literature report that gave definite assignment of g values for comparison (Table 2). Additionally, signal ″D″ at g = 2.032, 2.009, and 2.000 attributed to adsorbed O2 scavenging photogenerated electrons forming O2− is also identified, which is probable because HT samples were prepared in air. Overall, Figure 4 reveals the changes of the trapped hole O− EPR pattern upon going from HT400 to HT800; that is, the decrease of the TiO2(B) trapped hole signal ″B″ is attended by the increases of anatase trapped hole signal ″C″. As guided by the red lines (Figure 4), the major EPR spectrum changes occur at 600 °C. EPR Spectra Simulation. Computer simulations and deconvolutions of the EPR trapped hole signals were performed to quantify the ratio of trapped holes in TiO2(B) and anatase phases for each HTX sample. The simulated EPR spectra of signals B, C, and D shown in Figure 4 are taken as standard spectra for the corresponding species and used for deconvolution of the experimental spectra into three components (Figure S1, Supporting Information). The percentages of each species were determined by least-squares fitting of their weighted sum to the experimental spectra and 2880



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Figure 6. UV−visible diffuse reflectance spectra of NHT400, NHT500, NHT550, and NHT600. Inset: Tauc plots37 showing the bandgap energies of NHT400 and NHT600 calculated by transformed Kubelka−Munk function.

Figure 5. 77 K visible-light-induced EPR spectra of (a) HT800, (b) HT400, (c) HT500, and (d) HT550.

nitrogen is absent in NHT600. Monochromatic light at λ = 450 ± 40 nm, avoiding bandgap excitation, is chosen to excite the N− state. Origin of NHTX EPR Spectra. The EPR spectrum of TiO2(B)-type NHT400 (Figure 7a) under λ = 450 ± 40 nm

in TIO2(B)-type HT400, emerged in HT500 and HT550. This set of signals is identical in both line shape and positions to signal ″A″ observed when HT800 was activated by UV (Figure 4f) and is ascribed to an inner trapped electron in the form of Ti3+ in anatase. As stated earlier, anatase is not activated by λ > 400 nm excitation, and thus observation of anatase signal ″A″ in HT500 and HT550 mixed-phase samples demonstrates unequivocally that electrons originally generated on TiO2(B) are transferred to anatase. This suggests that the CB edge of TiO2(B) lies higher than that of anatase. The height of signal ″A″ in HT550 is approximately 3 times higher than that in HT500; that is consistent with the ratio of anatase fractions between the two samples (Table 1). On the other hand, the intensity of signal ″E″ decreased on going from HT400 to HT550. That might suggest that the anatase CB edge lies even lower than that of TiO2(B) surface trapping sites; therefore, electrons are preferentially transferred to anatase rather than being trapped by TiO2(B) surface trapping sites. However, only a limited portion of anatase-conducting electrons are trapped by the available anatase lattice trapping sites and detected by EPR; others remain in the conduction band as EPR silent species. Material Characteristics of NHTX. Nitrogen doping in the anatase TiO2 system, making it a visible-light active material by introducing a localized state in the midgap, has been thoroughly investigated. Such a material offers the opportunity for site specific excitation with a much narrower bandwidth.16 That allows us to monitor exclusively the fate of the photogenerated electron without intervention of other charge carriers generated by bandgap excitation. N−TiO2(B)/anatase (NHTX) exhibits nanowire morphology (Figure S2, Supporting Information). XRD analyses (Figure S3, Supporting Information) show that NHT400 is mainly of the TiO2(B) phase, and mixtures of TiO2(B) and anatase are observed for NHT500 and NHT550. NHT600 is dominated by the anatase phase. UV−vis data (Figure 6) show that phase transformation from TiO2(B) to anatase causes a blue shift of the absorption edge from ∼405 nm (NHT400) to ∼390 nm (NHT600). The broad absorption shoulder at ∼450 nm is a hallmark spectral feature of nitrogen dopants in TiO2.16 The absorbance of this peak decreased rapidly with increasing heat-treatment temperatures and vanished at 600 °C; i.e.,

Figure 7. 77 K EPR spectra of (a) NHT400, (b) NHT500, and (c) NHT550 under in situ blue light (λ = 450 ± 40 nm) illumination. The intensities of the central component of the N•-hole signal (signal ″G″) are normalized to one for all three samples. Experimental spectra (blue); simulation (black) parameters for the N•-hole in TiO2(B): g = (2.006, 2.005, 2.004) and A = (1.71, 4.56, 33.0) G.

excitation exhibits a prominent triplet hyperfine splitting centered at g ≈ 2 (signal ″G″) and signal ″E″ at g = 1.937 attributed to photogenerated electrons localized at TiO2(B) surface trapping sites as aforementioned. The TiO2(B) EPR spectra are thus ascribed to site specific excitation that promotes electrons from the N− midgap state of TiO2(B) to the CB, with holes remaining localized on the nitrogen atom itself in TiO2(B) giving rise to the triplet N•-hole EPR signal (signal ″G″), in accord with the literature report on N-doped anatase TiO2.16 In addition, while a portion of the photogenerated electrons remain in the TiO2(B) CB as delocalized and EPR-silent electrons, a portion of them are localized on 2881



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static magnetic field corresponding to the central component of the triplet signals in Figure 7. The rise patterns of the three samples are essentially indistinguishable, but their decay patterns differ notably. The decays feature a fast decay after hν off, followed by a much slower decay. They are well described by a double-exponential decay plus an offset, y(t) = y0 + C1e‑k1t + C2e−k2t, where offset y0 is the baseline; k1 and k2 are the first-order decay rate constants; and C1 and C2 are the preexponential amplitudes giving the fraction of recombined N•holes through k1 and k2 processes, respectively. The fitted k1 and k2 values are approximately the same for the three samples (Table 4) suggesting that they follow similar electron−hole

TiO2(B) surface trapping sites, giving rise to the broad signal ″E″. We note that the dominant anisotropic hyperfine component A3 (Figure 6) of 33.0 G in TiO2(B) is larger than the reported value of 32.2 G for N-doped anatase. An explanation for the slightly decreased 14N hyperfine coupling in anatase may arise from the fact that the average Ti−O bond distance of anatase is slightly shorter49,50 compared to that of TiO2(B). As such, the crystal field of the surrounding Ti ions increases the admixtures of d-states into the 2p N ground state, and as a result, the 2p electron spreads out more and decreases its interaction with the N nucleus. Electron Transfer from N−TiO2(B) to Anatase. Given that the λ = 450 ± 40 nm photon (∼2.7 eV) excites only the N− midgap state of TiO2(B), the observations of electrons trapped in the anatase lattice (signal ″A″) in NHT500 and NHT550 (Figure 7b, c) demonstrate that a portion of TiO2(B) CB electrons have migrated to anatase CB and are further trapped by lattice trapping sites forming Ti3+. The approximated number of electrons transferred and trapped by anatase lattice trapping sites is about 5% of the number of generated N•-holes for NHT500 and increased to about 9% for NHT550 (Figure 7), qualitatively consistent with the increased anatase abundance for NHT550. Together, Figures 5 and 7 provide unambiguous precedence to demonstrate that electron transfer occurs from TiO2(B) to anatase. These observations do not support the previously proposed model19 which suggested that electrons in TiO2(B) cannot migrate to anatase on the basis of an assumed higher anatase CB edge. In contrast, our observations suggest that the CB edge of TiO2(B) lies higher than that of anatase; otherwise, it is thermodynamically unfavorable for electrons to accumulate in anatase to EPR detectable amounts at 77 K. Assume a 0.14 eV potential barrier between the two CB edges and that the energy distribution of TiO2(B) CB electrons follows Boltzmann distribution, then only less than 10−7% of the electrons could have enough energy to clear the barrier at 77 K to reach the anatase CB, if the CB edge of TiO2(B) was lower than that of anatase. Decay of the TiO2(B) N•-Hole EPR Signal. Figure 8 compares the N•-hole EPR signal intensities of NHT400, NHT500, and NHT550 during light on and off periods at a

Table 4. Fitting Parameters for Time-Dependent Decay of TiO2(B) N•-Hole EPR Signal Intensities NHT400 NHT500 NHT550

y0

C1

k1 (10−3 1/s)

C2

k2 (10−3 1/s)

0.27 0.45 0.51

0.26 0.21 0.22

18 18 16

0.38 0.28 0.21

1.6 1.6 1.6

recombination pathways. Origins of the faster (k1) and slower (k2) components are attributed to annihilation of N•-holes (signal ″A″) by recombination with the TiO2(B) CB electrons (EPR silent) and with the surface-trapped electrons (signal ″B″), respectively. Extrapolation of the decay curves to infinite (y0) yield signal depletion to 27% for NHT400, 45% for NHT500, and 51% for NHT550 of the maximum intensities achieved during hν on. The offset y0 is a measure of the amount of unrecombined N•-hole holes. Thus, compared to NHT400, the additional amount of unrecombined N•-holes in NHT500 (18%) and NHT550 (24%) suggests that it is difficult for electrons transferred to anatase to return to TiO2(B). Otherwise, we would expect to observe similar offset (y0) values for the three samples. A model (Figure 9) with both higher CB and VB edge potentials in TiO2(B) than the corresponding ones of anatase is proposed to account for the charge transfers observed in the TiO2(B)/anatase mixed-phase HT500 and HT550 and in the N−TiO2(B)/anatase mixed-phase NHT500 and NHT550. Figure 9(a) shows that the TiO2(B) bandgap excitation generates electrons and holes initially on TiO2(B). Accumulation of photogenerated electrons would shift the Fermi level within TiO2(B) upward, while the Fermi level within anatase remains close to the dark. The resulting upward shift of TiO2(B) CB would drive electrons to pass through its CB into the CB of anatase. Thermodynamically favored electron transfer moves the CB electrons of anatase to the available anatase trapping sites, consistent with the observations shown in Figure 5 where electrons originated from TiO2(B) migrate to the anatase. The visible-light generated holes remain on TiO2(B) because its VB edge lies higher than that of anatase. However, when HT550 is exposed to UV, both phases are simultaneously activated, and their relative band positions remain about the same as in the dark. Migration of anatase holes to TiO2(B) which has a higher VB potential is expected to occur and is as demonstrated by comparison of EPRdetected amount of trapped holes in TiO2(B) to XRDdetermined bulk phase compositions (Table 1). For N-doped TiO2(B)/anatase, Figure 9(b), λ = 450 ± 40 nm excitation generates CB electrons and N•-holes on TiO2(B). Accumulation of photogenerated electrons on TiO2(B) would shift its CB further upward and drive electron

Figure 8. Time evolution of TiO2(B) N•-hole signal intensities during λ = 450 ± 40 nm light on and off under Ar atmosphere at 300 K for (a) NHT400, (b) NHT500, and (c) NHT550. Experimental data in black and double-exponential decay fittings in blue colors. The plots were scaled to one at the hν off position. 2882



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ASSOCIATED CONTENT

S Supporting Information *

Supporting figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Department of Physics, Vikrama Simhapuri University PG Centre, Kavali-524201, SPSR Nellore District, Andhra Pradesh, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Science Council of Taiwan (Grant NSC-1012627-M-259) supported this research. Figure 9. Schematics showing an electron transfer pathway in (a) TiO2(B)/anatase and in (b) N−TiO2(B)/anatase mixed-phase nanowires. Dark (left), (a) under λ > 400 nm irradiation (right), (b) under λ = 450 ± 40 nm irradiation (right). For UV excitation, both phases are simultaneously activated, and their relative band positions would remain about the same as in the dark (left).

REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. (3) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J.; Sakamoto, M.; Wang, F. Highly Efficient Dye-Sensitized Solar Cells with a Titania Thin-Film Electrode Composed of a Network Structure of Single-Crystal-like TiO2 Nanowires Made by the ″Oriented Attachment″ Mechanism. J. Am. Chem. Soc. 2004, 126, 14943−14949. (4) Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655− 2661. (5) Hernández-Alonso, M. D.; Fresno, F.; Suárez, S.; Coronado, J. M. Development of alternative photocatalysts to TiO2: Challenges and opportunities. Energy Environ. Sci. 2009, 2, 1231−1257. (6) Zhou, W.; Liu, H.; Wang, J.; Liu, D.; Du, G.; Cui, J. Ag2O/TiO2 Nanobelts Heterostructure with Enhanced Ultraviolet and Visible Photocatalytic Activity. ACS Appl. Mater. Interfaces 2010, 2, 2385− 2392. (7) Bubenhofer, S. B.; Schumacher, C. M.; Koehler, F. M.; Luechinger, N. A.; Grass, R. N.; Stark, W. J. Large-Scale Synthesis of PbS−TiO2 Heterojunction Nanoparticles in a Single Step for Solar Cell Application. J. Phys. Chem. C 2012, 116, 16264−16270. (8) Xu, Q. C.; Wellia, D. V.; Ng, Y. H.; Amal, R.; Tan, T. T. Y. Synthesis of Porous and Visible-Light Absorbing Bi2WO6/TiO2 Heterojunction Films with Improved Photoelectrochemical and Photocatalytic Performances. J. Phys. Chem. C 2011, 115, 7419−7428. (9) Zhu, G.; Pan, L. K.; Xu, T.; Sun, Z. CdS/CdSe-Cosensitized TiO2 Photoanode for Quantum-Dot-Sensitized Solar Cells by a MicrowaveAssisted Chemical Bath Deposition Method. ACS. Appl. Mater. Interfaces 2011, 3, 3146−3151. (10) Zhao, W.; Ma, W. H.; Chen, C. C.; Zhao, J. C.; Shuai, Z. G. Efficient Degradation of Toxic Organic Pollutants with Ni2O3/ TiO2‑xBx under Visible Irradiation. J. Am. Chem. Soc. 2004, 126, 4782−4783. (11) Hurum, D. C.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. Explaining the Enhanced Photocatalytic Activity of Degussa P25 Mixed-Phase TiO2 Using EPR. J. Phys. Chem. B 2003, 107, 4545− 4549. (12) Vargeese, A. A.; Muralidharan, K. Anatase-Brookite Mixed Phase Nano TiO2 Catalyzed Homolytic Decomposition of Ammonium Nitrate. J. Hazard. Mater. 2011, 192, 1314−1320. (13) Boppella, R.; Basak, P.; Manorama, S. V. Viable Method for the Synthesis of Biphasic TiO2 Nanocrystals with Tunable Phase Composition and Enabled Visible-Light Photocatalytic Performance. ACS. Appl. Mater. Interfaces 2012, 4, 1239−1246.

transfer from TiO2(B) to anatase, while holes remain localized on the N atom itself, consistent with the observations in Figure 7. When the light is off, the intrinsic higher TiO2(B) CB edge in the dark would hamper the electrons to return to TiO2(B) once they have been transferred to anatase, consistent with the N•-hole decay curves (Figure 8). This modeling is thus fully consistent with the available experimental data reported herein and is also supported by the theoretical band structure calculations proposed by Li et al.18

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CONCLUSION TiO2(B)/anatase and N−TiO2(B)/anatase mixed-phase nanowires with various phase compositions are prepared, characterized, and subjected to EPR investigation under both UV and visible illumination. The data clearly resolve the apparent conflicts by providing experimental precedence for both electron and hole migration pathways in these nanowires. By comparing EPR-detected TiO2(B) trapped hole O− abundance with XRD-determined phase compositions, we show that holes stimulated in anatase by UV are transferred to TiO2(B). By comparing the EPR spectra of TiO2(B)-type and TiO2(B)/ anatase mixed-phase samples under visible irradiation, we show that electron transfer occurs from TiO2(B) to anatase. Without intervention of other charge carriers generated by bandgap excitation, EPR experiments with N− site specific excitation demonstrated that electron transfer also occurs from NTiO2(B) to anatase, and the transferred electrons face a potential barrier to return to TiO2(B). Higher CB and VB edge potentials in TiO2(B) than the corresponding ones of anatase are concluded. The anatase and TiO2(B) phases acting as electron and hole sinks, respectively, provide spatial charge separation in TiO2(B)/anatase mixed-phase nanowires which is useful for photovoltaic and photocatalytic applications. 2883



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(14) In, S.; Orlov, A.; Berg, R.; Garcia, F.; Jimenez, S. P.; Tikhov, M. S.; Wright, D. S.; Lambert, R. M. Effective Visible Light-Activated BDoped and B,N-Codoped TiO2 Photocatalysts. J. Am. Chem. Soc. 2007, 129, 13790−13791. (15) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269−271. (16) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Valentin, C. Di.; Pacchioni, G. Origin of Photoactivity of Nitrogen-Doped Titanium Dioxide under Visible Light. J. Am. Chem. Soc. 2006, 128, 15666−15671. (17) Armstrong, A. R.; Armstrong, G.; Canales, J.; Bruce, P. G. TiO2B Nanowires. Angew. Chem., Int. Ed. 2004, 43, 2286−2288. (18) Li, W.; Liu, C.; Zhou, Y. X.; Bai, Y.; Feng, X.; Yang, Z. H.; Lu, L. H.; Lu, X. H.; Chan, K. Y. Enhanced Photocatalytic Activity in Anatase/TiO2(B) Core−Shell Nanofiber. J. Phys. Chem. C 2008, 112, 20539−20545. (19) Yang, D. J.; Liu, H. W.; Zheng, Z. F.; Yuan, Y.; Zhao, J. C.; Waclawik, E. R.; Ke, X. B.; Zhu, H. Y. An Efficient Photocatalyst Structure: TiO2(B) Nanofibers with a Shell of Anatase Nanocrystals. J. Am. Chem. Soc. 2009, 131, 17885−17893. (20) Qi, L. H.; Li, C. Y.; Chen, Y. J. Dye-Sensitized Solar Cells Based on Nitrogen-Doped TiO2−B Nanowire/TiO2 Nanoparticle Composite Photoelectrode. Chem. Phys. Lett. 2012, 539−540, 128−132. (21) Marchand, R.; Brohan, L.; Tournoux, M. TiO2(B) a New Form of Titanium Dioxide and the Potassium Octatitanate K2Ti8O17. Mater. Res. Bull. 1980, 15, 1129−1133. (22) Banfield, J. F.; Veblen, D. R.; Smith, D. J. The Identification of Naturally Occurring TiO2(B) by Structure Determination Using HighResolution Electron Microscopy, Image Simulation, and DistanceLeast-Squares Refinement. Am. Mineral. 1991, 76, 343−353. (23) Wang, Q.; Wen, Z. H.; Li, J. H. Solvent-Controlled Synthesis and Electrochemical Lithium Storage of One-Dimensional TiO2 Nanostructures. Inorg. Chem. 2006, 45, 6944−6949. (24) Li, Q. J.; Zhang, J. W.; Liu, B. B.; Li, M.; Liu, R.; Li, X. L.; Ma, H. L.; Yu, S. D.; Wang, L.; Zou, Y. G.; et al. Synthesis of High-Density Nanocavities inside TiO2−B Nanoribbons and Their Enhanced Electrochemical Lithium Storage Properties. Inorg. Chem. 2008, 47, 9870−9873. (25) Dong, W.; Cogbill, A.; Zhang, T.; Ghosh, S.; Tian, Z. R. Multifunctional, Catalytic Nanowire Membranes and the MembraneBased 3D Devices. J. Phys. Chem. B 2006, 110, 16819−16822. (26) Wang, P.; Xie, T. F.; Wang, D. J.; Dong, S. Facile Synthesis of TiO2(B) Crystallites/Nanopores Structure: A Highly Efficient Photocatalyst. J. Colloid Interface Sci. 2010, 350, 417−420. (27) Wang, G.; Wang, Q.; Lu, W.; Li, J. H. Photoelectrochemical Study on Charge Transfer Properties of TiO2−B Nanowires with an Application as Humidity Sensors. J. Phys. Chem. B 2006, 110, 22029− 22034. (28) Liu, B.; Khare, A.; Aydil, E. S. TiO2−B/Anatase Core−Shell Heterojunction Nanowires for Photocatalysis. ACS Appl. Mater. Interfaces 2011, 3, 4444−4450. (29) Wang, Y.; Wu, M.; Zhang, W. F. Preparation and Electrochemical Characterization of TiO2 Nanowires as an Electrode Material for Lithium-Ion Batteries. Electrochim. Acta 2008, 53, 7863−7868. (30) Stoll, S.; Schweiger, A. EasySpin, A Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 42−55. (31) Feist, T. P.; Davies, P. K. The Soft Chemical Synthesis of TiO2(B) from Layered Titanates. J. Solid State Chem. 1992, 101, 275− 295. (32) Yoshida, R.; Suzuki, Y.; Yoshikawa, S. Synthesis of TiO2(B) Nanowires and TiO2 Anatase Nanowires by Hydrothermal and PostHeat Treatments. J. Solid State Chem. 2005, 178, 2179−2185. (33) Pavasupree, S.; Suzuki, Y.; Yoshikawa, S.; Kawahata, R. Natural Rutile-Derived Titanate Nanofibers Prepared by Direct Hydrothermal Processing. J. Solid State Chem. 2005, 178, 3110−3116. (34) Kolen’ko, Y. V.; Kovnir, K. A.; Gavrilov, A. I.; Garshev, A. V.; Frantti, J.; Lebedev, O. I.; Churagulov, B. R.; Tendeloo, G. V.;

Yoshimura, M. Hydrothermal Synthesis and Characterization of Nanorods of Various Titanates and Titanium Dioxide. J. Phys. Chem. B 2006, 110, 4030−4038. (35) Lin, C. H.; Chao, J. H.; Liu, C. H.; Chang, J. C.; Wang, F. C. Effect of Calcination Temperature on the Structure of a Pt/TiO2 (B) Nanofiber and Its Photocatalytic Activity in Generating H2. Langmuir 2008, 24, 9907−9915. (36) Kiatkittipong, K.; Scott, J.; Amal, R. Hydrothermally Synthesized Titanate Nanostructures: Impact of Heat Treatment on Particle Characteristics and Photocatalytic Properties. ACS Appl. Mater. Interfaces 2011, 3, 3988−3996. (37) Tauc, J.; Grigorovici, R.; Vancu, A. Optical Properties and Electronic Structure of Amorphous Germanium. Phys. Status Solidi 1966, 15, 627−637. (38) Micic, O. I.; Zhang, Y.; Cromack, K. R.; Trifunac, A. D.; Thurnauer, M. C. Trapped Holes on Titania Colloids Studied by Electron Paramagnetic Resonans. J. Phys. Chem. 1993, 97, 7277−7283. (39) Nakaoka, Y.; Nosaka, Y. ESR Investigation into the Effects of Heat Treatment and Crystal Structure on Radicals Produced over Irradiated TiO2 Powder. J. Photochem. Photobiol. A 1997, 110, 299− 307. (40) Coronado, J. M.; Maira, A. J.; Conesa, J. C.; Yeung, K. L.; Augugliaro, V.; Soria, J. EPR Study of the Surface Characteristics of Nanostructured TiO2 under UV Irradiation. Langmuir 2001, 17, 5368−5374. (41) Kumar, C. P.; Gopal, N. O.; Wang, T. C.; Wong, M. S.; Ke, S. C. EPR Investigation of TiO2 Nanoparticles with Temperature-Dependent Properties. J. Phys. Chem. B 2006, 110, 5223−5229. (42) Vijayan, B. K.; Dimitrijevic, N. M.; Wu, J. S.; Gray, K. A. The Effects of Pt Doping on the Structure and Visible Light Photoactivity of Titania Nanotubes. J. Phys. Chem. C 2010, 114, 21262−21269. (43) Dimitrijevic, N. M.; Vijayan, B. K.; Poluektov, O. G.; Rajh, T.; Gray, K. A.; He, H.; Zapol, P. Role of Water and Carbonates in Photocatalytic Transformation of CO2 to CH4 on Titania. J. Am. Chem. Soc. 2011, 133, 3964−3971. (44) Chiesa, M.; Paganini, M. C.; Livraghi, S.; Giamello, E. Charge Trapping in TiO2 Polymorphs as Seen by Electron Paramagnetic Resonance Spectroscopy. Phys. Chem. Chem. Phys. 2013, 15, 9435− 9447. (45) Berger, T.; Sterrer, M.; Diwald, O.; Knözinger, E.; Panayotov, D.; Thompson, T. L.; Yates, J. T. Light-Induced Charge Separation in Anatase TiO2 Particles. J. Phys. Chem. B 2005, 109, 6061−6068. (46) Scotti, R.; D’Arienzo, M.; Testino, A.; Morazzoni, F. Photocatalytic Mineralization of Phenol Catalyzed by Pure and Mixed Phase Hydrothermal Titanium Dioxide. Appl. Catal., B 2009, 88, 497−504. (47) Riss, A.; Elser, M. J.; Bernardi, J.; Diwald, O. Stability and Photoelectronic Properties of Layered Titanate Nanostructures. J. Am. Chem. Soc. 2009, 131, 6198−6206. (48) Li, G.; Dimitrijevic, N. M.; Chen, L.; Nichols, J. M.; Rajh, T.; Gray, K. A. The Important Role of Tetrahedral Ti4+ Sites in the Phase Transformation and Photocatalytic Activity of TiO2 Nanocomposites. J. Am. Chem. Soc. 2008, 130, 5402−5403. (49) Yang, K.; Dai, Y.; Huang, B. Study of the Nitrogen Concentration Influence on N-Doped TiO2 Anatase from FirstPrinciples Calculations. J. Phys. Chem. C 2007, 111, 12086−12090. (50) Vittadini, A.; Casarin, M.; Selloni, A. Structure and Stability of TiO2-B Surfaces: A Density Functional Study. J. Phys. Chem. C 2009, 113, 18973−18977.

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