ARTICLE pubs.acs.org/JPCC
Photoelectrochemical and Photocatalytic Behaviors of Hematite-Decorated Titania Nanotube Arrays: Energy Level Mismatch versus Surface Specific Reactivity Tae Hwa Jeon,† Wonyong Choi,§ and Hyunwoong Park†,‡,* †
Department of Energy Science and ‡School of Energy Engineering, Kyungpook National University, Daegu 702-701, Korea School of Environmental Science and Engineering, POSTECH, Pohang 790-784, Korea
§
bS Supporting Information ABSTRACT: Nanocrystalline hematite particles (R-Fe2O3) were electrodeposited on the TiO2 nanotube (TiNT) arrays that were fabricated via anodization of Ti foils. The short precontact time (1 h) of aqueous ferric ions (Fe3þ) on TiNT resulted in formation of hematite particles selectively on the mouth surface of TiNT (hematite@1 h/TiNT), whereas the long precontact time (24 h) resulted in complete filling of the TiNT inside and an even full-covering of the TiNT top surface with the hematite particles (hematite@24 h/ TiNT). For comparison, hematite particles were also electrodeposited on TiO2-nanoparticulate films obtained via oxidative annealing of Ti foil resulting in hematite fully covered TiO2 nanoparticles (hematite/TiNP). Photoelectrochemical (PEC) study with AM 1.5 light (UV þ Vis) indicated that the PEC activity of TiNT decreased by ca. 40% and almost completely vanished when hematite covered the full surface of TiNT (hematite@ 24 h/TiNT) and loaded on the mouth surface of TiNT (hematite@1 h/TiNT), respectively. The relatively higher PEC activity of hematite@24 h/TiNT was further observed under varying visible light conditions (400 nm < λ < 500 nm). Hematite/TiNP also has ca. 40%-reduced PEC activity as compared to TiNP under AM 1.5 light, the tendency of which is similar to hematite@24 h/TiNT. Photocatalytic (PC) activities of TiNT and hematite/TiNT for degradation of aqueous phenol under AM 1.5-light were also compared, which indicates that the PC activity of TiNT vanishes almost completely with hematite@1 h/TiNT, whereas it is recovered at a moderate level with hematite@24 h/TiNT. All of these PEC and PC behaviors of TiNT and hematite/ TiNT were discussed in terms of hematite-induced charge recombination due to an energy level mismatch between TiO2 and hematite, as well as surface-specific photoactivity of TiNT (i.e., mouth surface vs interwall and/or underlying base layer). Various surface analysis techniques (XRD, XPS, TEM, UVvis diffuse reflectance) were employed to understand the surface states of TiNT and hematite/ TiNT. Finally, more detailed charge transfer mechanism was proposed.
’ INTRODUCTION TiO2 nanotubes (TiNT) arrays have received growing attention1,2 from various research areas such as sensors,3 dyesensitized solar cells,46 photoelectrochemical fuel generation,710 Li-ion battery,11 environmental remediation,1216 biomedical applications,17 and so forth due to their unique photophysicochemical properties. Particularly, the TiNT grown on Ti foil via anodization has vertically aligned tubular architectures with a large surface area and porosity. Such geometric configuration, which is not found in TiO2 nanoparticles (TiNP), allows electrolyte to be filled in the pores and thereby increases semiconductor liquid contact. Additionally, the internal scattering of the transmitted photons inside of the tube improves photon capture more effectively leading to greater quantum yields than those of TiNP.1 Various attempts have been made to decorate TiNT with guest materials such as electrocatalysts and narrow bandgap semiconductors to further increase the efficiency and widen its photoelectrochemical activity window to the visible light region, respectively. The former includes, for example, Pt for cathodic r 2011 American Chemical Society
hydrogen evolution from water,8 NiO for redox capacitance,18 and SbSnO2 for anodic treatment of water pollutants.19 However, the latter includes CdS,20,21 WO3,22 SrTiO3,23 Fe2O3,24,25 and so forth mainly for improving the photoelectrochemical performances and fuel generation. Among them, iron oxides play very unique roles when hybridized with TiNT. First, they provide TiNT with better light-harvesting properties such that the theoretical maximum efficiency of solar-to-hydrogen is 15% with hematite (R-Fe2O3). In comparison, the maximum efficiency of TiO2 (anatase) is only 1%.26 Second, the magnetic property of iron oxides (e.g., feroxyhyte, magnetite, and maghematite) can be combined with the biomedical functions of TiNT. One can imagine that TiNT delivers drugs to targeted locations in vivo; after TiO2-mediated phototherapy, drugs are released from TiNT, which are finally recovered from the location by an external magnetic field. Received: February 7, 2011 Revised: March 7, 2011 Published: March 22, 2011 7134
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Figure 1. SEM images of (a) TiNP, (b) hematite/TiNP, (c, d) TiNT, (e) hematite@1 h/TiNT, and (f) hematite@24 h/TiNT. 1 h and 24 h refer to precontact times of Fe(III) to TiNT substrates for 1 h and 24 h, respectively, before electrodeposition.
Despite such great application potentials, however, the iron oxideTiNT hybrids may suffer from low photoelectrochemical and photocatalytic performances primarily resulting from the following. 1) Iron oxides have intrinsically low conductivity and high recombination rates of charge carriers. 2) The band positions are mismatched in that the conduction band (CB) level of iron oxides is lower (more positive), whereas its valence band (VB) level is higher (more negative) as compared to those of TiNT. Such band positions may be beneficial for utilization of holes (e.g., water oxidation), whereas detrimental for utilization of electrons (e.g., water reduction) under UV irradiation. 3) Iron oxides can block active surface sites of TiNT. TiNT has largely two different surfaces, mouth surface versus internal surface (interwall þ bottom layer). The former might directly absorb
photons and the consequently created electrons are transported to the bulk tube, whereas the latter may have multiscattered photons and the charge carriers will be created in the deeper region. Because the latter should have a much larger surface area, its contribution to photoefficiency also might be greater. This work has been intended for synthesis of TiNT and hematite (R-Fe2O3) hybrid electrodes and study on their photoelectrochemical (PEC) and photocatalytic (PC) activities under various experimental conditions. In particular, the PEC and PC activities of bare and hematite-decorated TiNT samples were compared with those of TiNP analogues to obtain insight on how the energy level mismatch affects the collection efficiency of photogenerated charge carriers and such mismatch-induced deactivation is related to kind of TiNT surfaces. Detailed surface 7135
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Figure 2. TEM images (side views) of (a, b) TiNT (c, d) hematite@1 h/TiNT and (e, f) hematite@24 h/TiNT.
analyses and various PEC and PC experimental results were provided to propose the charge transfer mechanism occurring at hematite and TiNT interfaces.
’ EXPERIMENTAL SECTION Preparation of TiO2 Nanotubes and Hematite-Decorated TiO2 Nanotubes. TiO2 nanotubes (TiNT) arrays were prepared
by following a typical sonochemical anodization method of titanium foils in a two-electrode system.27 A titanium sheet (0.127 mm thick, 99.7%-pure, Aldrich) was cut into small pieces with equal size of 1.5 cm 3 cm, which were ultrasonically cleaned in ethanol for 10 min in a sonication bath (WiseClean, 40 kHz, 100 W) and rinsed with distilled water. A couple of a Ti foil
piece and a graphite rod (Alfa Aesar, 99.9995%) that was immersed in a mixed aqueous solution (0.2 L) of H3PO4 (0.5 M) and NaF (0.14 M) in a beaker was placed in the sonication bath and a dc voltage of þ20 V was applied to the Ti foil (anode) against the graphite rod (cathode) for 45 min along with ultrasonication. Then the Ti foil was quickly rinsed with distilled water, dried in air, and annealed at 500 °C for 6 h in the presence of air. For comparison, a Ti foil piece was annealed at the same condition without the anodization step to obtain TiO2 particulate films on Ti foil (hereafter TiNP, Figure S1 of the Supporting Information). For growing hematite particles on TiNT (or TiNP), a typical electrodeposition method was employed.28 In brief, as-prepared TiNT foils (or TiNP) were immersed in a mixed aqueous solution of NaF (5 mM), NaCl (0.1 M), H2O2 7136
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Figure 3. XRD patterns of TiNP, TiNT, hematite/TiNP, and hematite@1 h/TiNT. TiNT and TiNP were annealed at 500 °C for 6 h; hematite/TiNP and hematite/TiNT were annealed at 500 °C for 0.5 h, subsequently. A: anatase, R: rutile, H: hematite. Unmarked peaks originate from underlying Ti substrates.
(1 M), and FeCl3 3 6H2O (5 mM) for different times (1 h vs 24 h), and then 50 potential cycles were applied to them at a sweep rate of 0.1 V/s from 0.52 V to þ0.41 V vs saturated calomel electrode (SCE) with a cyclic voltammetry (Versastat 3400). Finally, the hematite/TiNT (or hematite/TiNP) was annealed at 500 °C for 30 min in the presence of air. Surface Characterizations. A field emission scanning electron microscope (FE-SEM, Hitachi S-4800) and a scanning transmission electron microscope (STEM, Hitachi HD-2300) were used to analyze the morphologies of tube structures and the side views of TiNT, respectively. X-ray diffractometer (XRD, Rigaku D/Max-2500) and X-ray photoelectron spectroscopy (XPS, VG scientific, ESCA LAB 220i XL, MgKR source) were employed to analyze crystalline patterns of TiO2 and hematite and binding energies of their elements, respectively. Diffuse reflectance spectra were obtained by using a UVvis spectrometer (PerkinElmer, Lambda 950). A BaSO4 pellet was used as a reference. Photoelectrochemical and Photocatalytic Activity Measurements. The photoelectrochemical performance of hematite/TiNT was studied in a three-electrode system with SCE and Pt mesh as a reference and a counter electrode, respectively, in 0.1 M Na2SO4 as a supporting electrolyte. A 150 W Xenon arc lamp (Ushio 150-MO) equipped with AM 1.5G air mass filter was used as a light source. If necessary, long-wave pass filters were placed between the light source and the cell to investigate the effect of incident wavelengths on the photoelectrochemical behaviors. For photocatalytic activity, the samples were immersed in aqueous phenol of 0.1 mM (18 mL) and the AM 1.5-light was irradiated to the front side of samples through the solution. Phenol and its intermediates were analyzed with a high performance liquid chromatography (HPLC, YL9100). The mixed eluent with distilled water (with 0.1 wt % phosphoric acid) and acetonitrile (55/45 v/v) was flowed through a C18inverse column (4.6 mm 150 mm, Waters) at 1 mL/min.
’ RESULTS AND DISCUSSION Surface Characterization of Hematite/TiO2 Nanotube Arrays. Figure 1 shows the SEM images (top views) of TiNP and
TiNT without and with hematite nanoparticles. It is obvious that TiNP, which was obtained via oxidative annealing of Ti foil at 500 °C for 6 h without the electrochemical anodization process, has a porous structure created by protruding particles of ca.
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30 nm with distinct grain boundary (part a of Figure 1 and Figure S1 of the Supporting Information). XRD analysis indicates that this TiNP has the crystalline structure of rutile (2θ = 27.4° (110), 36.1° (101); Figure 3). Such crystallinity formation is not surprising because the annealing process induces the oxidation of surface Ti metal to TiO2 in the presence of air (i.e., oxygen).29 Iron oxide/TiNP hybrid obtained via electrodeposition of Fe(III) onto TiNP shows a similar porous structure yet with more packed particles of ca. 50 nm (part b of Figure 1). XRD result shows that the hybrid has a peak at 2θ = 35.6° (110), indicative of hematite structure (R-Fe2O3) (Figure 3). However, the anodization and postheat treatment of Ti foil created a number of densely packed self-standing nanotubes with pore sizes of ca. 120 nm and wall thickness of ca. 10 nm (parts c and d of Figure 1). According to TEM analysis, the tube has a node-like, bamboo-type skeleton with full lengths of around 500 nm (parts a and b of Figure 2). As-prepared TiNT has been found to have anatase/rutile-mixed crystallinity at the same annealing temperature of 500 °C (Figure 3) suggesting that the anatase-to-rutile phase transition is inhibited in TiNT as compared to TiNP. The anatase structure is likely to originate from nanotube walls while the rutile structure may result from the barrier layer created on the underlying Ti substrate.1 Iron oxide particles deposited on TiNT were also found to have the hematite structure (2θ = 33.2° (014), 35.6° (110), 40.9° (113), 49.5° (024), and 54.1° (116); Figure 3), which indicates that the physicochemical property of substrate (TiNP vs TiNT) does not significantly affect the nucleation and growth of hematite. It is of note that different contacting times of aqueous Fe(III) with TiNT prior to electrodeposition step results in different morphologies. Electrodeposition after one-hour precontact (i.e., adsorption) shows that hematite particles of 2030 nm with distinct grain boundary are selectively formed only on the mouth surface of the tube via interparticle connection (part e of Figure 1 and Figure S2 of the Supporting Information). In addition, such donut-like hematite configuration with similar inner and outer diameters with the tube arrays further suggests that the mouth surface works as an active site for formation and growth of hematite. Unfortunately, TEM analysis for side view of hematite/ TiNT did not give clear images for hematite particle (parts c and d of Figure 2), giving no obvious information about the thickness or number of the hematite ring layers. One might speculate that the donut-like hematite rings result from insufficient precontact time for Fe(III) to penetrate to the inside of tubes; hence if a longer precontact time is given, hematite might grow from the inside or interwall of the tubes and thus fill them. To verify this conjecture, TiNT was immersed in aqueous Fe(III) solution for 24 h and the electrodeposition was applied. As shown in part f of Figure 1, many hematite particles of ca. 25 nm formed and almost cover the entire mouth surface of the tube. It has been found through TEM analysis that the hematite particles completely fill the tube inside as well (parts e and f of Figure 2 and Figure S3 of the Supporting Information), suggesting that penetration of aqueous Fe(III) into the tube is allowed and the TiNT interwall may have similar surface species (e.g., >TiOH) with the mouth of TiNT array. The TiNT surfaces can be divided into mouth surface, internal surface (interwall and bottom), and outer void surface (outer-wall and bottom). The former is formed mainly via chemical dissolution of pre-existed metal oxide/metal interface (reactions 1, 2), whereas the other two are created by an applied electric field (reaction 3) along with chemical dissolution. Full knowledge on the physicochemical properties of the three 7137
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Figure 4. XPS spectra of hematite/TiNP and hematite@1 h/TiNT for (a) Ti2p and (b) Fe2p. Deconvoluted O1s spectra of (c) hematite/TiNP and (d) hematite@1 h/TiNT.
surfaces has not been obtained yet but it has been considered that they have similar photoelectrochemical and photocatalytic properties. Ti þ H2 Oðor O2 Þ f TiO2 þ 2Hþ þ 4e
ð1Þ
TiO2 ðor TiðOHÞ4 Þ þ 6F þ 4Hþ f TiF6 2 þ 2H2 O ð2Þ Ti4þ þ 6F f TiF6 2
ð3Þ
XPS analyses of hematite/TiNT hybrids have been attempted as well. As shown in part a of Figure 4, Ti2p3/2 and Ti2p1/2 bands of TiNT were found at 458.3 eV and ca. 464 eV respectively, which are identical to those of TiO2 particles. Also, no Ti2p band of Ti metal located at 454 eV was observed. Ti2p bands of TiNP, however, are shifted to lower binding energy regions by 0.3 eV likely due to less complete change of Ti metal into TiO2 during the oxidative annealing process. In the case of hematite (part b of Figure 4), both hybrids have Fe2p bands identically located at 710.9 eV originating from Fe2O3 and Fe2þ shakeup satellite peaks at ca. 715 eV were not found indicating that iron oxide films are composed primarily with hematite. In addition, no chemical shift of Fe2p band suggests that hematite particles simply use TiO2 as a platform or support without chemical bonding with it. Finally, O1s spectra were also studied (parts c and d of Figure 4). Hematite/TiNP hybrid has an asymmetric O1s band centered at 529.5 eV, whereas the O1s band of hematite/TiNT was shifted to higher binding energy direction by 0.3 eV. For more detailed comparison, the asymmetric O1s spectra were deconvoluted, which produces three hidden bands: OTi centered at 529.8 eV originating from TiO2, OFe centered at 529.6 eV from Fe2O3, and OFe(OH) centered at 531.2 eV originating from FeOOH. The existence of FeOOH is interesting since such iron oxide hydroxide was not found at XRD analysis likely due to minor
Figure 5. UVvis diffuse reflectance spectra (DRS) of TiNP, TiNT, hematite/TiNP, and hematite@1 h/TiNT. The insert shows the magnified DRS spectra of TiNP (26) and TiNT in the UV region. The absorbance was expressed as a KubelkaMunk unit.
content. The contribution degrees of three O1s bands to overall O1s spectra were estimated by calculating the band areas, which are 40.5% (OTi), 39.3% (OFe), and 20.1% (OFe(OH)) for hematite/TiNT while 17.6% (OTi), 52.0% (OFe), and 30.5% (OFe(OH)) for hematite/TiNP. The larger % content of OTi in TiNT indicates that oxygen atoms coordinated to Ti become more abundant with anodization/annealing and further suggests that Ti oxidation is more favorable with anodization/ annealing than that with the simple oxidative annealing. Finally, UVvis diffuse reflectance spectra of TiNT and hematite/TiNT samples were obtained and compared with TiNP analogues (Figure 5). The absorbance (expressed as a KubelkaMunk unit) of TiNT linearly grows with decreasing wavelength from at λ ∼ 580 nm which is ca. 180200 nm red-shifted with respect to typical TiO2 particles with bandgap energy of ca. 3.0 eV (rutile) ∼ 3.2 eV (anatase).29 Such optical property of 7138
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Figure 6. (a) IV patterns (cyclic voltammograms with a scan rate 0.1 V s1) of samples in 0.1 M Na2SO4 under AM 1.5-light. The insert shows the magnification of black-lined box. (b) Comparison of photocurrent generations at 1.0 VSCE in 0.1 M Na2SO4 under AM 1.5-light. n.c. and d refer to not calcined and dark, respectively.
TiNT to absorb visible light likely results from the underlying metal layer30 or a photon confinement effect within the tubes. Meanwhile, the absorbance of TiNP unexpectedly decreases with increasing wavelength and this optical behavior has been repeatedly observed for other TiNP samples. However, the magnified spectrum of TiNP shows that it absorbs UV light and has an absorption band centered at ca. 350 nm (insert of Figure 5). Loading of hematite particles on TiNP results in significantly larger light absorption from around 610 nm (corresponding to bandgap of ca. 2.03 eV) due to hematite particles and such optical property is similarly found in hematite/TiNT. Photoelectrochemical and Photocatalytic Behaviors of Hematite/TiO2 Nanotube Arrays. As-prepared hematite/TiNT samples were further investigated on their photoelectrochemical (PEC) behaviors under various conditions and compared with respect to hematite/TiNP. As shown in part a of Figure 6, TiNP (i.e., annealed Ti foil) has a current onset potential of ca. 0.1 VSCE and generates photocurrent of ca. 1.1 mA/cm2 at a biased potential of 1.0 VSCE in 0.1 M Na2SO4 electrolyte under AM 1.5light. Meanwhile nonannealed Ti foil generates only a minor photocurrent of ca. 50 μA/cm2 under the same condition (insert in part a of Figure 6 and part b of Figure 6). This obviously indicates that simple annealing of Ti foil has a certain effect on enhancing photocurrent generation due to creation of TiO2 particles (rutile phase) as confirmed by XRD study (Figure 3). Similarly to TiNP, TiNT without the annealing step (TiNT (n.c.)) has also an insignificant photocurrent generation similar to Ti foil (∼ 35 μA/cm2 at 1.0 VSCE), whereas with the annealing step (TiNT) has over 10 times greater photocurrent of
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Figure 7. (a) Comparison of photocurrent generations among samples under visible light (λ > 420 nm) and (b) effect of irradiation conditions on photocurrent generations of samples. Different long-wave pass filters were placed in front of AM 1.5-light source. E = 1.0 VSCE in 0.1 M Na2SO4.
ca. 1.75 mA/cm2. Hence, the oxidative annealing should improve the PEC activities of TiNT (TiNP as well) and the pure effect of TiO2 tubular configuration might be obtained by extracting the photocurrent of TiNP from that of TiNT (1.75 1.1 = 0.65 mA/cm2 at 1.0 VSCE, part b of Figure 6). Such tubular structureinduced PEC activity enhancement is likely to be ascribed to facilitated electron transfer from the tubular TiO2 to Ti bulk along the tube frame and/or improved TiO2 crystallinity. Loading of hematite particles, however, results in remarkably different PEC behaviors. When loaded on TiNP, hematite particles reduce the PEC activity of TiNP by around 33% from 1.1 mA/cm2 to 0.72 mA/cm2 at 1.0 VSCE (part b of Figure 6) despite similar current onset potentials (part a of Figure 6). Such decrease in photocurrent becomes far greater in TiNT samples (i.e., ∼97% decrease from 1.75 mA/cm2 at TiNT to 60 μA/cm2 at hematite@1 h/TiNT). It is important to note that pure (untreated) Ti foil has the similar photocurrent with hematite/TiNT (insert in part b of Figure 6) suggesting that TiNT almost completely loses its PEC activity by loading hematite particles only on its mouth surface. However, the TiNT internally filled and fully covered with hematite (i.e., hematite@24 h/TiNT) has 17-fold enhanced photocurrent of ca. 1 mA/cm2 as compared to hematite@1 h/TiNT despite much smaller TiO2 surface area exposed to AM 1.5-light. For more systematic comparison of PEC performances, irradiation condition was varied by placing different long-wave pass filters between the AM 1.5-light and the PEC cell. As shown in part a of Figure 7, irradiation of λ > 420 nm results in different PEC behaviors from the AM 1.5 condition (also Figure S4 of the Supporting Information). In the absence of hematite particles, TiNP and TiNT generate around 4 and 5.6 μA/cm2 at 1.0 VSCE. Despite the marginal difference, however, the higher 7139
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Scheme 1. Proposed Charge Transfer Mechanism of TiNT, Hematite@1h/TiNT, and Hematite@24h/TiNT under UV and Visible (vis) Lighta
Figure 8. Comparison of photocatalytic activities for phenol among samples under AM 1.5-light. [Phenol]0 = 0.1 mM.
a
M: mouth, W: inter-wall plus bottom layer.
photocurrent of TiNT is consistently observed even under the different irradiation conditions of λ > 455 and 495 nm (part b of Figure 7), indicating that the current differences are not due to experimental deviation. The visible light (λ > 420 nm) activity of TiNP likely results from the created rutile TiO2 with bandgap of ca. 3.0 eV capable of absorbing λ ∼ 413 nm. The similar optical property of TiNT to absorb the light of λ < ∼ 570 nm (Figure 5) might be also a primary reason for the photocurrent generation under visible light. In addition, high anodic potential bias (1.0 VSCE) can pump out, if generated, all the conduction band (CB) electrons, contributing to the photocurrent generation of TiNP and TiNT. Interestingly, loading of hematite particles on TiNP does not increase the photocurrent and rather slightly decreases it. Since hematite/TiNP could absorb light of λ < ca. 600 nm (Figure 5), no contribution of hematite to photocurrent generation suggests that the most CB electrons generated at hematite by irradiation of λ > 420 nm might recombine with valence band (VB) holes of TiNP (if generated) and/or that synthesized hematite particles have less efficient charge separation likely due to defect sites or low conductivity. In contrast to the case of TiNP, hematite@1 h/TiNT generates a greater photocurrent (ca. 14 μA/cm2) than TiNT (5.6 μA/cm2) and even hematite/ TiNP (3.5 μA/cm2), which might be due to facilitated longitudinal electron transfer from hematite to Ti bulk along the TiO2 wall at the potential bias of 1.0 VSCE. Such effect becomes more pronounced in hematite@24 h/TiNT since hematite particles fully cover TiNT surface and thus more CB electrons are generated even under the conditions of λ > 455 and 495 nm (part b of Figure 7). In fact, the hybridization of hematite and TiO2 is not a desirable strategy in terms of photoenergy conversion, since their energetics are not matched at all. For example, CB level of hematite is more positively located than TiO2 CB level whereas VB level of hematite is more negatively positioned than TiO2 VB level (Scheme 1). Such mismatch should result in ready recombination of electronhole pairs at the surface of hematite. However, the recombination can be minimized by applying positive bias potentials to the hybrid electrode and the selective excitation of the semiconductors (hematite vs TiO2) is possible just by employing different long-wave filters (e.g., UV vs visible light) due to different optical property between the two. Moreover, since hematite particles are located at specific surfaces of
TiNT (i.e., top mouth vs interwall/bottom layer) and work as a UV filter, TiNT can be excited selectively. We have further checked if hematite-loading reduces the photocatalytic (PC) activity of TiNT and performed photocatalytic degradation of phenol with the samples used in the PEC tests. As shown in Figure 8, phenol is rapidly degraded with TiNT to ca. 50% of the initial amount in 5 h whereas only 7% of the initial amount is degraded with TiNP. Kinetic comparison also results in that the pseudofirst order rate constant (k) for phenol degradation with TiNT is ca. 7.4 times greater than that with TiNP (kTiNT = 0.119 h1 vs kTiNP = 0.016 h1). The far higher activity of TiNT is not surprising and can be attributed to the different crystalline structures between TiNT and TiNP. As XRD patterns indicate (Figure 3), the TiNT has anatase and rutile-mixed crystalline structure which is favorable for efficient charge separation due to facile electron transfer from anatase CB to rutile CB and adopted in Degussa P25, a benchmark TiO2.31 However, TiNP has only the rutile structure which is known to be less effective than anatase and anatase/rutile mixture in degradation of phenol.32 When hematite particles are loaded on the mouth surface of TiNT (hematite@1 h/TiNT), however, the PC activity of TiNT for phenol under AM 1.5 condition vanishes almost completely (khematite@1 h/TiNT = 0.008 h1) and such phenomenon, despite being less pronounced, is also observed with TiNP (khematite/TiNP = 0.009 h1). The reduced PC activity by hematite loading is attributed to rapid hole transfer from TiO2 VB to hematite VB through the formed potential gradient; despite capability of oxidizing phenol, the hematite VB hole has lower potential than that of TiO2. However, hematite@24 h/TiNT has a moderate but enhanced PC activity (khematite@24 h/TiNT = 0.026 h1) when compared to hematite@1 h/TiNT. Photogenerated Charge Transfer Mechanism. Scheme 1 illustrates charge separation/transfer mechanism of TiNT and hematite/TiNT under the conditions of AM 1.5 (UV þ vis) and λ > 420 nm (vis). In principle, UV fraction of AM 1.5-light absorbed by TiNT excites not only the mouth surface TiO2 but also the interwall TiO2. Despite electrical contact between the two surfaces; however, the excitation/charge separation of each surface is separately illustrated in Scheme 1 because they have not necessarily the identical surface property; for example, the tube wall has anatase crystallinity, whereas the barrier layer has rutile crystallinity. In addition, as reported by Ong et al.,33 the propagation of light in TiNT is different: when they hit the mouth surface, most of the incident photons are absorbed by the mouth surface 7140
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The Journal of Physical Chemistry C with negligible portion of reflection whereas a significant portion of light that traveled into the tube is reflected back from the barrier layer with greatly reduced intensity. Hence, when UV is irradiated, both surfaces are excited, and generate electronhole pairs, and contribute to photocurrent generation (case I). Hematite particles loaded on the mouth surface of TiNT, however, absorb the most portion of incident light (UV þ vis), minimizing the effect of the mouth surface TiO2 on contributing to the photocurrent generation (case II). Thus, in this case, only the mouth surface hematite and the interwall TiO2 are active in generating photocurrent and the CB electrons of both semiconductors are likely to be transported away by applied bias potentials (pathway 1 of case II). However, this argument does not seem reasonable since the photocurrent is significantly reduced by a factor of 29 (i.e., from ca. 1.75 mA/cm2 to 60 μA/cm2 at 1.0 VSCE; part b of Figure 6) when hematite particles are selectively loaded on the mouth surface. Because the geometrical surface area ratio (rs) of the tube inside (interwall þ barrier layer) and the mouth is estimated to be around 24 (i.e., rs = (120 000 nm2 (interwall) þ 5020 nm2 (barrier layer))/5200 nm2 (mouth)) and the UV-excited hematite may produce higher photocurrent than the visible lightinduced (ca. 8 μA/cm2 at 1.0 VSCE, part a of Figure 7), the photocurrent of hematite@1 h/TiNT at 1.0 VSCE should be larger than min. 1.68 mA/cm2 (1.75 23/24 mA/cm2 at the tube inside þ0.008 mA/cm2 at the mouth hematite), not 60 μA/cm2. The reduced PC activity of hematite@1 h/TiNT for the degradation of phenol also can be revisited in a similar way and inclusion of rs suggests that the rate constant of hematite@1 h/TiNT should be 0.114 h1, not 0.008 h1. It should be noted that the PEC results are based on the electron transfer whereas the PC results are based on the hole transfer. The substantially reduced PEC and PC performances of the hematite@1 h/TiNT hybrid might be ascribed to rapid charge transfer of CB electrons and VB holes of TiO2 to CB and VB of hematite, respectively, and the charge recombination therein (pathway 2 of case II). This conjecture, therefore, disregards the effect of the applied potential bias on both separating the electronhole pairs and driving the CB electrons to move along hematite CB f TiO2 CB f counter electrode (pathway 1 of case II). Meanwhile, one can argue that such potential bias is sufficient to drive a vectorial charge transfer and inhibit the recombination since the photocurrents gradually level off from at around 1.0 VSCE (part a of Figure 6). In such a case, hematite may not induce the recombination so significantly and, if so, the reduced PEC and PC performances of the hybrid are likely to result from no activity of the interwall TiO2 (as well as barrier layer TiO2). According to our best knowledge, there is no direct evidence for the photoactivity of the interwall TiO2 in literature; it can be only inferred from the effect of tube length on the photoactivity since the tube length is directly correlated to the surface area of the interwall (under the condition of identical mouth wall thickness). Literature reports that the photo(electro)catalytic activity of TiNT increases with increasing the tube length due to reduced recombination of electronhole pairs.5,34,35 A few other studies, on the contrary, report that longer TiNT has lower photo(electro)catalytic activity likely due to increased resistance of TiNT, reduced intensity of the electric field within TiNT, and increased probability of the charge recombination.13,36 Thus a certain range of optimal length should exist in obtaining the highest photocatalytic effect,16 which further indicates that the interwall has some photocatalytic activity although its relative activity compared
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to the mouth surface is not known. In this study, the primary reason for the significantly reduced activity is not fully understood and might be ascribed to the ultrafast charge recombination occurring at hematite and TiO2 interface even at sufficiently high potential bias. Yet we could not fully exclude the possibility of the inactive interwall TiO2, which needs further detailed study. Finally, the photoelectrochemical charge generation/transfer of the hematite@24 h/TiNT was illustrated in Scheme 1 as well (case III). In this case, all the TiNT surfaces are covered with hematite particles and the most irradiated UV photons are absorbed by the hematite with no activation of TiNT. Thus the effect of the mouth surface hematite is same; yet the filled hematite, instead of the interwall TiO2, contributes to the overall photocurrent. It is noteworthy that the photocurrent of hematite@24 h/TiNT is ca. 17-fold higher than that of hematite@1 h/ TiNT (case II), the reasons for which may be the following. First, the enhancement is due to large amount of surface hematite (Figures 1 and 2) and thereby more photons are absorbed by hematite, contributing to the photocurrent. Second, if the interwall TiO2 has much lower PEC and PC activity, the relatively inactive interwall surface becomes active by the filled hematite. Finally, if the interwall TiO2 is as active as the mouth surface TiO2, the recombination occurring at hematite and TiO2 interface (hematite/TiO2 contact) is minimized or vanishes due to replacement of the interwall TiO2 with the hematite (hematite/ hematite contact).
’ CONCLUSIONS Hematite/TiNT hybrid electrodes were successfully fabricated via electrodeposition of hematite particles on TiNT and studied on their PEC and PC behaviors under various experimental conditions. TiNT was very active in both generating photocurrents at biased potentials and degrading aqueous phenol under AM 1.5-light but its PEC and PC activities almost completely vanished when hematite was selectively located only on the mouth surface of TiNT whereas the activities are recovered to a moderate level when hematite filled the TiNT inside and covered the TiNT surface. The photoactivity enhancement of the latter was still observed under visible light by which hematite was excited. These behaviors were interpreted in two different mechanisms: hematite-induced recombination versus surface-specific reactivity. The former is commonly considered in terms of energetics (energy level mismatch) and has not received further detailed attention in literature. Instead of hematite and TiO2 hybrids, therefore, Ti-doped hematite37 and TiO2/hematite alloy oxides38 have been studied to overcome such energetics mismatch and enhance the PEC activity of hematite. Meanwhile, the latter mechanism is likely to raise controversy because the mouth surface and the interwall surface of TiNT have been speculated to have equal or similar PEC and PC activities. However, neither definite evidence on such speculation has been presented nor the photophysicochemical property and reactivity of the TiNT interior have been disclosed in detail. This suggests that detailed and systematic studies on the surface-specific photoactivity of TiNT are necessary. Site-selective masking of three different kinds of TiNT surface (mouth, interwall, and base layer) by photoinactive materials will be an approach to addressing such an issue. 7141
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The Journal of Physical Chemistry C
’ ASSOCIATED CONTENT
bS
Supporting Information. SEM and TEM images and cyclic voltammograms of samples in this study. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected], phone: þ82-53-950-7371.
’ ACKNOWLEDGMENT This research was supported by Basic Science Research Programs (No. 2009-0089904, No. 2009-0071350, No. 20100002674) and by the Korea Center for Artificial Photosynthesis (NRF-2009-C1AAA001-2009-0093879) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Korea. ’ REFERENCES (1) Grimes, C. A.; Mor, G. K. TiO2 Nanotube Arrays: Synthesis, Properties, and Applications; Springer: New York, 2009. (2) Macak, J. M.; Tsuchiya, H.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 2100–2102. (3) Mor, G. K.; Varghese, O. K.; Paulose, M.; Grimes, C. A. Sens. Lett. 2003, 1, 42–46. (4) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. D: Appl. Phys. 2006, 39, 2498–2503. (5) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69–74. (6) Kang, T.-S.; Smith, A. P.; Taylor, B. E.; Durstock, M. F. Nano Lett. 2009, 9, 601–606. (7) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2006, 6, 24–28. (8) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. J. Phys. Chem. C 2007, 111, 8677–8685. (9) Beranek, R.; Tsuchiya, H.; Sugishima, T.; Macak, J. M.; Taveira, L.; Fujimoto, S.; Kisch, H.; Schmuki, P. Appl. Phys. Lett. 2005, 87. (10) Baker, D. R.; Kamat, P. V. J. Phys. Chem. C 2009, 113, 17967–17972. (11) Meekins, B. H.; Kamat, P. V. ACS Nano 2009, 3, 3437–3446. (12) Quan, X.; Yang, S. G.; Ruan, X. L.; Zhao, H. M. Environ. Sci. Technol. 2005, 39, 3770–3775. (13) Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. J. Phys. Chem. C 2008, 112, 253–259. (14) Naito, K.; Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Am. Chem. Soc. 2009, 131, 934–936. (15) Lai, Y. K.; Sun, L.; Chen, Y. C.; Zhuang, H. F.; Lin, C. J.; Chin, J. W. J. Electrochem. Soc. 2006, 153, D123–D127. (16) Zhuang, H. F.; Lin, C. J.; Lai, Y. K.; Sun, L.; Li, J. Environ. Sci. Technol. 2007, 41, 4735–4740. (17) Song, Y. Y.; Schmidt-Stein, F.; Bauer, S.; Schmuki, P. J. Am. Chem. Soc. 2009, 131, 4230. (18) Wang, Y. G.; Zhang, X. G. J. Electrochem. Soc. 2005, 152, A671–A676. (19) Cui, X.; Zhao, G. H.; Lei, Y. Z.; Li, H. X.; Li, P. Q.; Liu, M. C. Mater. Chem. Phys. 2009, 113, 314–321. (20) Bai, J.; Li, J. H.; Liu, Y. B.; Zhou, B. X.; Cai, W. M. Appl. Catal., B 2010, 95, 408–413. (21) Yin, Y. X.; Jin, Z. G.; Hou, F. Nanotechnology 2007, 18. (22) Park, J. H.; Park, O. O.; Kim, S. Appl. Phys. Lett. 2006, 89. (23) Zhang, J.; Bang, J. H.; Tang, C. C.; Kamat, P. V. ACS Nano 2010, 4, 387–395. (24) Kuang, S. Y.; Yang, L. X.; Luo, S. L.; Cai, Q. Y. Appl. Surf. Sci. 2009, 255, 7385–7388.
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