Photoinduced Crystallization and Activation of Amorphous Titanium

13 May 2015 - Here we show that amorphous TiO2 prepared by the commonly used sol–gel method of tetrabutyl titanate hydrolysis is active in photocata...
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Photoinduced Crystallization and Activation of Amorphous Titanium Dioxide Galyna Krylova and Chongzheng Na* Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Titanium dioxide (TiO2) is one of the most common photosensitive materials used in photocatalysis, solar cells, self-cleaning coatings, and sunscreens. Although the crystalline TiO2 phases such as anatase and rutile are well-known to be photoactive, whether amorphous TiO2 is active in photocatalytic reactions is still controversial. Here we show that amorphous TiO2 prepared by the commonly used sol−gel method of tetrabutyl titanate hydrolysis is active in photocatalytic water reduction and methylene blue oxidation under the irradiation of a xenon lamp. The amorphous TiO2 gains photoactivity after an induction period of approximately an hour, suggesting that phase transition is involved. Using an extensive series of microscopic and spectroscopic analyses, we further show that the photoinduced crystallization by amorphous TiO2 forms a nanometer-thin layer of rutile nanocrystallites under the irradiation in the middle ultraviolet range. The resulting core−shell nanoparticles have a bandgap of 3.3 eV and are enriched with surface-active sites including reduced titanium and oxygen vacancies. The revelation of photoinduced crystallization raises the possibility of preparing photosensitive TiO2 using low-temperature radiation techniques that can not only save energy but also incorporate heat-sensitive components into manufacturing.

1. INTRODUCTION Titanium dioxide (titania; TiO2), with an annual global production of more than 14 million tons,1 is one of the most common photosensitive materials used in photocatalysis,2 solar cells,3 self-cleaning coatings,4 and sunscreens.5 Solid TiO2 can be either crystalline or amorphous. Polymorphs of crystalline TiO2 such as anatase and rutile are well-known for their photoactivity. In comparison, amorphous TiO2 is generally believed to be photochemically inactive, as measured in the photoreduction of alcohols6,7 and silver7 as well as the photooxidation of acetic acid and acetaldehyde.6 The lack of photoactivity is attributed to the presence of a large number of defects in the amorphous phase, which can lead to rapid recombination of photogenerated electrons and holes before they can be involved in reactions.6,7 Recently, a growing number of studies start to challenge the belief that amorphous TiO2 is not photoactive. Significant activity has been associated with amorphous TiO2 in the photodecomposition of stearic acid,8 the phototransformation of methylene blue,9,10 the photogeneration of hydrogen from water,11 and the photo-oxidation of dibenzothiophene.12 In addition, photoconductivity,13 photoluminescence,14 and lightexcited superhydrophilicity15 have been reported for using amorphous TiO2 as the starting material. A couple of mechanisms have been proposed including the increase in surface active site density (e.g., Ti-peroxide for oxidation9,12,16 and Ti3+ for reduction)11 as well as charge trapping by dopants to explain the observed photoactivity of amorphous TiO2.10 © XXXX American Chemical Society

An overlooked mechanism that may also contribute to the observed photoactivity of amorphous TiO2 is the photoinduced crystallization of the inactive amorphous phase to one or more active crystalline phases. Previously, photoinduced crystallization has been reported for selenium,17 metal chalcogenide glass,18 and sol−gel-derived TiO2 and other oxide thin films.19 For most of tested materials, the occurrence of photoinduced crystallization requires intense ultraviolet (UV) laser beams. Only thin films made of zinc oxide have been reported to crystallize under the irradiation of a mercury lamp with a narrow UV band.20 For amorphous TiO2, the question remains whether photoinduced crystallization can occur under conditions relevant to photocatalysis in the laboratory and the natural environment. In this study, we investigate the possibility of photoinduced crystallization as a potential mechanism for activating amorphous TiO2 in photocatalytic reactions. Using the photoreduction of water and the photooxidation of methylene blue as model reactions, we show that amorphous TiO2 is activated in ∼1 h by a xenon lamp. By comparing the lattice structures and band structures of TiO2 before and after being exposed to the irradiation, we show that a thin layer of rutile with a thickness of ∼10 nm is formed on the outside of the Received: March 2, 2015 Revised: May 7, 2015

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spectroscopy (DRS; Jasco V-670), thermogravimetric analysis, differential scanning calorimetry analysis (Mettler Toledo TGA/DSC-1 STAR), X-ray photoelectron spectroscopy (XPS; Physical Electronics PHI 5000 VersalProbe II), and photoluminescence spectroscopy (Horiba Fluorolog). XRD was collected using a monochromated Cu Kα radiation (λ = 1.54056 Å) at 40 kV and 40 mA. The Raman spectrum was recorded with a 532 nm laser source at magnification ×100. UV/vis DRS was recorded with an integrating sphere and a deuterium lamp using BaSO4 as the background. In XPS, the sample was excited by the monochromatic Al-Ka X-ray source (spot size 100 mm, voltage 15 kV, current 10 mA) under the activated charge-neutralization routine. Binding energy referencing was performed with the adventitious carbon peak position of 284.8 eV. Photoluminescence measurements were conducted on a Jobin Yvon single photon counting system with a 277 nm LED excitation source. The photoelectrochemical activity was measured using a three-electrode potentiostat equipped with a platinum mesh counter electrode and an Ag/AgCl reference electrode (Princeton Applied Research PARStat 2273). The working photoelectrode was fabricated by dip-coating on a piece of FTO glass (0.8 cm × 5.5 cm). The coating suspension was made by dispersing 20 mg TiO2 in a 20 mL ethanol solution of ethyl cellulose (0.2857 g L−1) and benzyl alcohol (8 mM) under sonication. The photoelectrode was annealed at 65 °C for 24 h before use. To perform the photoelectrochemical measurement, we immersed the electrode in an aqueous solution containing 1.0 M KCl and 10 volume percent of methanol (pH 6). Prior to the measurement, the electrolyte solution was degassed by bubbling nitrogen for 30 min. The electrode was illuminated from the backside of the FTO glass using a 300 W xenon lamp equipped with an AM 1.5G filter (Newport). The electrode was positioned to receive 100 W m−2 of incident power with an active area of 0.286 cm2. Linear sweep voltammetry was then performed from −1.0 to 0.8 V with respect to the reference electrode.

amorphous phase, rendering photoactivity to the otherwise inactive material.

2. EXPERIMENTAL METHODS Tetrabutyl titanate (99.0%), hydrochloric acid (36.5 to 38.0%), and ammonium hydroxide (29.7%) were purchased from Acros Organics, BHD Aristar, and Fisher Scientific, respectively. Fluorine-doped tin oxide (FTO) glass was obtained from Pilkington (TEC-7). All solvents used in this study were purchased from Fisher Scientific and used without further purification. Deionized (DI) water was produced using a Millipore Synergy Water Purification System. To synthesize amorphous TiO2, we mixed 100 μL of hydrochloric acid with 50 mL of 2-propanol, followed by the dropwise addition of 520 μL of tetrabutyl titanate. The resulting pH of the solution was ∼0.5, suppressing the hydrolysis of tetrabutyl titanate. The hydrolysis of tetrabutyl titanate was then initiated by the slow addition of the ammonia hydroxide solution until the pH reached 8.0. The suspension was stirred for 12 h. The resulting amorphous TiO2 precipitates were successively washed with 2-propanol five times, with DI water five times, and with ethanol five times. The precipitates were dried in air into a powder. Part of the amorphous powder was annealed in air at 450 °C for 2 h to obtain the anatase standard and at 750 °C for 2 h to obtain the rutile standard for X-ray diffraction (XRD) and Raman analyses (see later). The photocatalytic reduction of water was performed in a quartz cuvette. The cuvette was cooled by a water jacket connected to a thermostat that was in turn maintained at 25 °C. To perform the experiment, 1 mg amorphous TiO2 powder was ultrasonically dispersed in an aqueous solution. The solution contains 10% methanol (by volume) as the hole scavenger. The cuvette was then sealed with a Precision Seal septum rubber stopper. Nitrogen gas was bubbled through the suspension for 30 min to remove dissolved oxygen, creating an anaerobic condition for photocatalytic reduction. To initiate the reaction, we irradiated the cuvette with 1000 W xenon arc lamp (Newport). The lamp was equipped with a 10 cm water filter and a hot mirror to eliminate the infrared radiation that can cause rapid temperature change upon absorption. The cuvette was placed at the position with the light intensity of 1000 W m−2. To quantify hydrogen produced from the photocatalytic reduction of water, we periodically sampled 200 μL of gas by the airtight syringe through the stopper. The concentration of hydrogen was analyzed by a gas chromatograph equipped with a thermal conductivity detector (Thermo Scientific Trace 1300). The photooxidation of methylene blue was performed in the same cuvette without the elimination of dissolved oxygen. Instead, the reactive solution was constantly bubbled with air to supply oxygen. The photooxidation reaction was initiated in a suspension containing 10 μM methylene blue and 1 mg of amorphous TiO2 powder. Before the reaction was started, the suspension was stirred in the dark for 1 h to achieve the equilibrium of methylene blue adsorption. The concentration of methylene blue was monitored by the absorption peak at 663 nm.21 Before and after being exposed to the xenon lamp, the structure of amorphous TiO2 was examined using transmission electron microscopy (TEM; Titan FEI 80-300 at 300 kV), selected-area electron diffraction (SAED; JEOL 2100F at 200 kV), powder XRD (Bruker D8 Advance Davinci), Raman spectroscopy (Jasco NRS-5100), UV−visible diffuse reflectance

3. RESULTS We synthesized amorphous TiO2 using the commonly used sol−gel method of tetrabutyl titanate hydrolysis in isopropanol.22 The morphology and structure of amorphous TiO2 are examined using TEM, SAED, and powder XRD, as shown in Figure 1. The sample appears as a powder after drying, consisting of aggregates of ultrafine nanoparticles (Figure 1a). High-resolution TEM of the sample shows no discernible ordering typical of a crystalline material (Figure 1b), consistent with being an amorphous phase. The lack of long-range ordering is further confirmed by SAED (Figure 1c) and XRD (Figure 1d), neither of which reveals any discernible reflection pattern. The photocatalytic activity of amorphous TiO2 is examined at 25 °C using model reactions of water photoreduction and methylene blue photooxidation, as shown in Figure 2. In both reactions, a 1000 W xenon lamp equipped with infrared filters is used as the light source. By monitoring hydrogen evolution, we observe that water photoreduction in the presence of amorphous TiO2 follows a zeroth-order rate law after an induction period of ti: q = k0(t − ti), where k0 is the rate constant (Figure 2a). The zeroth order rate law is consistent with the fact that water as the reactant is in excess in this reaction. Least-squares regression gives ti = 42(±40) min (R2 = 0.99), suggesting that an induction period may exist. Because B

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0.99), consistent with the value estimated from water photoreduction. The slow decrease in C/C0 at t < ti is likely due to the slow adsorption of methylene blue by the amorphous TiO2 powder.23 Compared with am-TiO2, commercially available TiO2 P25 (78% anatase and 14% rutile) shows no induction in the same reactions but gives similar rate constants: k0 = 0.27(±0.02) mmol g−1 h−1 and k1 = 25(±1) h−1 (Figure 2c,d). Attempts to fit for induction give ti = −6(±3) min (R2 = 0.98) and ti = 0.2(±0.1) min (R2 = 0.99), respectively. The induction period revealed by both photoreduction and photooxidation reactions suggests that changes have occurred with amorphous TiO2 in the first hour of the irradiation by the xenon lamp. The similarity of measured reaction rates to those of P25 suggests that the change of amorphous TiO2 during induction is likely related to the transformation to the crystalline phases. To test the hypothesis of photoinduced crystallization, we examined the morphology and structure of amorphous TiO2 after being irradiated at 25 °C for 1 h using TEM, SAED, Raman spectroscopy, and powder XRD, as shown in Figure 3. After irradiation, the loosely bound powder has disintegrated into coarser nanoparticles of ca. 20 nm in diameter (Figure 3a). More importantly, lattice fringes lacking by pristine amorphous TiO2 are now visible on the surface of the irradiated sample (Figure 3b), indicating that amorphous TiO2 has been partially crystallized. According to the high-resolution TEM (cf. Figure S1 in the SI), the photocrystallized TiO2 (pc-TiO2) nanoparticles consist of amorphous cores covered by nanocrystallites of ∼2 nm in size. We have ruled out the possibility that crystallinity is created by the bombardment of electrons because amorphous TiO2 does not show lattice fringes after extended exposure in TEM (Figure S2 in the SI). The photoinduced crystallization of amorphous TiO2 is also confirmed by SAED, showing clear reflection ring patterns (Figure 3c). Lattice spacings of pc-TiO2 computed from the reciprocals of ring radii are compared with the lattice spacings of bulk anatase, rutile, and brookite (Figure 3d).24,25 The nearperfect match between pc-TiO2 and rutile suggests that the nanocrystallites have a rutile structure. This is further supported by Raman spectroscopy, showing that pc-TiO2 has the same broadened and shifted Raman modes of A1g, Eg, and a secondorder effect (SOE) as bulk rutile (Figure 3e).26 The fortification of the SOE peak near 220 cm−1 is attributable to the effect of nanocrystallinity.26 XRD of pc-TiO2, however, shows no distinctive reflections from rutile lattices (Figure 3f), consistent with the peak broadening of nanocrystals.27 To further understand the photoinduced crystallization of amorphous TiO2, we performed differential scanning calorimetry and thermogravimetric analysis, as shown in Figure 4. As temperature increases, amorphous TiO2 transforms according to the well-known amorphous → anatase → rutile sequence.27 DSC reveals a broad endothermic feature below 200 °C and two distinctive exothermic features near 305 and 450 °C, respectively (solid curve in Figure 4a). The endothermic feature is accompanied by a 12.4% weight loss, as revealed by TGA (solid curve in Figure 4b) and thus is attributed to the desorption and evaporation of volatile solvents such as unbound water, isopropanol, and ethanol. The first exothermic peak is accompanied by a 1.9% weight loss, corresponding to the release of water and butanol from the condensation of H− O−Ti and C4H9−O−Ti.28 The presence of water and alcohols in amorphous TiO2 is confirmed by Raman spectroscopy (Figure S3 in the SI). The second exothermic peak has a

Figure 1. Morphology and structure of amorphous TiO2. (a,b) Transmission electron micrographs. (c) Selected area electron diffraction. (d) Powder X-ray diffraction. Scale bars: (a) 500 nm; (b) 5 nm; and (c) 2 nm−1.

Figure 2. Photocatalytic activity of amorphous TiO2 in (a) hydrogen (H2) generation from water photoreduction and (b) methylene blue (MB) photooxidation compared with P25 in (c) water photoreduction and (d) methylene blue photooxidation.

hydrogen has to be extracted from the experimental system for analysis, only a limited number of measurements can be made at t < ti, resulting in a high level of uncertainty associated with the estimation of ti. To confirm the existence of induction, we performed methylene blue photooxidation with amorphous TiO2, whose progress can be conveniently monitored. This reaction also exhibits an induction but conforms to a first-order rate law: C/C0 = ln[−k1(t − ti)], where C and C0 are residual and initial methylene blue concentrations and k1 is the rate constant (Figure 2b). Regression gives ti = 59(±1) min (R2 = C

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Figure 4. Mechanism of photoinduced crystallization. (a,b) Differential scanning calorimetry and thermogravimetric analysis of amorphous TiO2 (solid curves) compared with photocrystallized TiO2 (3 h of irradiation; dashed curves). (c) Crystallization percentage estimated from the areal reduction of the anatase formation peak around 450 °C in DSC. The curve in panel c is an exponential fit. Cartoons show the progression of crystallization of amorphous TiO2, forming a core−shell structure.

amorphous-rutile core−shell structure observed by TEM and surface-sensitive Raman spectroscopy (cf. Figure 3b,e). Further examination of samples prepared by varying the irradiation time shows that the thickness of rutile shell grows when irradiation is extended from 1 to 6 h and the amorphous phase still remains after 6 h of irradiation. By comparing the areas of the anatase peak between am-TiO2 and pc-TiO2, we estimate that the content of rutile increases from 0 in amorphous TiO2 to 13% after 1 h of irradiation, 24% after 3 h of irradiation, and 32% after 6 h of irradiation. The progression of phase transition with time is summarized in Figure 4c. To understand the photocatalytic properties of pc-TiO2, we construct its band structure using optical and photoelectric spectroscopies, as shown in Figures 5. Measurements of light absorption show a distinctive bandgap of ΔE = 3.3(±0.1) eV (Figure 5a),30 broader compared with the bandgap of 3.03 eV for bulk rutile.31 The conduction band (CB) potential is estimated from the onset of photocurrent (Figure 5b). The interpolation of dark and light current curves give a CB potential of −0.66 V versus normal hydrogen electrode (NHE) or −3.84 eV versus vacuum, much higher than ECB = −4.8 eV for bulk rutile.31 The bandgap broadening and the negative shift of CB potential are likely due to the quantum confinement effect.32 According to ΔE and ECB, the valence band (VB) potential for pc-TiO2 is thus EVB = ECB − ΔE = −7.135 eV. In comparison, the bandgap and the onset of photocurrent for amTiO2 are measured at 3.4(±0.1) eV (Figure 5c) and −0.30 V (Figure 5d), respectively, suggesting that for am-TiO2, ECB = −4.2 eV and EVB = −7.6 eV. The CB and VB potentials of pc-TiO2 and am-TiO2 are visualized in Figure 5e. The ECB value of pc-TiO2 is significantly higher than the potential required for H+/H2 reduction (−4.3 eV at pH 6), indicating that photogenerated CB electrons can provide sufficient electrochemical driving force to reduce H+ to

Figure 3. Morphology and structure of photocrystallized TiO2 (pcTiO2). (a,b) Transmission electron micrographs. (c) Selected area electron diffraction patterns. (d) Comparison of lattice spacings estimated from c with spacings of bulk rutile (squares; reflections from left to right: 110, 011, 111, 121, 002), anatase (circles; reflections: 011, 004, 020, 015/121, 024), and brookite (diamonds; reflections: 210/ 111, 211, 321, 421, 123). The diagonal line shows the perfect match of pc-TiO2 and standard spacings. (e) Comparison of Raman spectrum of pc-TiO2 with rutile and anatase. (f) Powder X-ray diffraction of pcTiO2. Scale bars: (a) 20 nm; (b) 2 nm; (c) 2 nm−1.

heat output of 285 J g−1 (estimated from the integral of the peak) but shows no weight loss. This event is attributed to the formation of anatase (Figure S4 in the SI).7 The phase transition from anatase to rutile, which requires a temperature above the temperature limit of our DSC instrument (max: 700 °C),29 is confirmed by annealing amorphous TiO2 at 750 °C for 2 h, as examined by TEM and XRD (Figure S5 in the SI). In comparison with pristine amorphous TiO2, pc-TiO2 made by irradiating the amorphous powder in air for 3 h behaves quite differently in DSC/TGA. First, pc-TiO2 does not have the peak corresponding to the exothermic release of water and butanol in DSC (dashed curve in Figure 4a). The lack of this peak suggests that photoinduced crystallization of amorphous TiO 2 is likely accompanied by hydroxyl and butanyl condensation, although hydroxyl and butanyl may also have reacted with oxygen or decomposed under irradiation. Second, the peak corresponding to the amorphous-anatase transition is smaller for pc-TiO2 than that for pristine amorphous TiO2, suggesting that part of the amorphous phase has already been crystallized after 3 h of irradiation. This is consistent with the D

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Figure 6. Comparison of (a) absorption spectrum of amorphous TiO2 and (b) emission spectrum of the xenon lamp. The radiation absorbed by amorphous TiO2 is estimated by integrating the shaded area: R = 12.3 W m−2.

the access of irradiation to the am-TiO2 core. Rutile has an absorption coefficient of 108 m−1 at 310 nm,34 blocking 90% of light at a thickness of 10 nm. To examine whether the photoinduced crystallization is related to the previously proposed mechanism for amorphous TiO2 activation related to surface active sites,9,11,12,16 we have performed X-ray photoelectron and photoluminescence measurements with pc-TiO 2 , as shown in Figure 7. The deconvolution of the XPS of Ti 2p3/2 spectrum reveals a significant amount of surface titanium sites with five-fold coordination (Ti5C3+, 14.8%; Figure 7a), which is known to promote interfacial electron transfer.11 A similar amount of surface adsorbed water is found by deconvoluting the XPS of O 1s spectrum (14.4%, Figure 7b), suggesting the association of oxygen vacancies (VO) with Ti5C3+.35 Oxygen vacancies can facilitate the dissociative adsorption of reactive molecules such as water and methanol in photocatalytic reactions36 and serve as rapid hole scavengers.37,38 In comparison, XPS reveals much lower Ti5C3+ and VO concentrations on the am-TiO2 surface (ca. 6%; Figure 7c,d). Detailed energy levels of the surface defects of pc-TiO2 are further revealed by photoluminescence spectroscopy performed in the aqueous solution of 10% methanol. The spectrum exhibits a broad band (Figure 7e), consisting of five excitonic recombinations from 3.7 to 3.1 eV and five recombinations involving intraband surface states from 3.0 to 2.5 eV.39−42 The surface states include four types of electron-trapping oxygen vacancies (VO)i (i = 1, 2, 3, or 4)39,43 and the hole-trapping defect Ti5C3+.35 Although the energy levels of (VO)i and Ti5C3+ can shift with crystal size,42 it is generally believed that Ti5C3+ is located below (VO)i under the conduction band in the energy diagram (Figure 7f).40 These results suggest that the photoinduced crystallization mechanism can be reconciled with the active-site mechanism for explaining the photoactivity of amorphous TiO2.

Figure 5. Band structures of photocrystallized (pc) and amorphous (am) TiO2. (a) Tauc plot of light absorption revealing the bandgap of pc-TiO2. (b) Photocurrent measurement revealing the conduction band (CB) potential of pc-TiO2. (c) Tauc plot of light absorption revealing the bandgap of am-TiO2. (d) Photocurrent measurement revealing the CB potential of am-TiO2. (e) Band structures comparedwith the reduction potential of hydrogen generation and methylene blue (MB) oxidation. VB, valence band; NHE, normal hydrogen electrode. Symbols: α, the absorption coefficient; h, the Planck constant; ν, frequency.

H2 that is not normally possible for bulk rutile. The EVB value of pc-TiO2 is thus EVB = −7.135 eV, suggesting that the photogenerated holes can both oxidize methylene blue directly and through reactive oxygen species such as superoxide and hydroxyl radicals. Similarly, the CB and VB potentials of amTiO2 suggest that am-TiO2 should also be able to reduce water and oxidize methylene blue photochemically; therefore, the lack of activity of am-TiO2 is not a result of the lack of thermodynamic driving force. To investigate the mechanism of photoinduced crystallization, we have performed DRS for amorphous TiO2 and compared it with the irradiation spectrum of the xenon lamp, as shown in Figure 6. Significant light absorption only occurs with photons having wavelengths below 310 nm, corresponding to the shaded area of the irradiation from the xenon lamp. Previously, Davidson et al.33 and Ohtani et al.7 observed no activity with amorphous TiO2 in photocatalytic hydrogen generation using light sources when radiation with short wavelengths was cut off at 300 nm. On the contrary, Zhang and Maggard11 observed that amorphous TiO2 was photocatalytically active in hydrogen generation when the cutoff was reduced to 200 nm. Zhang and Maggard11 also measured a hydrogen production rate of 0.314 mmol h−1 for each gram of amorphous TiO2, similar to k0 = 0.30(±0.02) mmol g−1 h−1 measured in our experiments. Together, our results and these literature reports suggest that the photoinduced crystallization of amorphous TiO2 requires radiation in the middle UV range (i.e., 200−300 nm). This explains the rapid decrease in crystallization rate after 3 h of irradiation (cf. Figure 5c) because of the filtering effect of the rutile shell, which blocks

4. DISCUSSION We have observed crystallization and activation of amorphous TiO2 under the radiation of a xenon lamp. The radiation required for the amorphous-rutile transition can thus be estimated by integrating the shaded area in Figure 6b: R = 12.3 W m−2. R is comparable to the energy density allocated in the same wavelength region for the standard air mass 1.5 (AM1.5) solar spectrum (i.e., 11 W m−2). The similarity E

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photocatalytically less active than anatase because photogenerated electrons and holes are more likely to recombine in rutile.44,46 The intimate contact with amorphous TiO2 can reduce the frequency of charge recombination by allowing either electron or hole to migrate to am-TiO2 and thus being physically separated from its recombination partner. Charge separation due to bandgap alignment is well understood at the rutile−anatase interface, where electrons flow toward anatase while holes flow toward rutile due to differences in conduction and valence energy levels between the two crystalline polymorphs.31 As shown in Figure 5e, the CB potential of the amorphous core is lower than that of the rutile shell (−4.2 vs −3.84 eV), suggesting that the core has a greater affinity with electrons and thus electrons can migrate from the shell to the core. In contrast, the VB potential of the rutile shell is greater than that of the amorphous core (−7.135 vs −7.6 eV), suggesting that holes can migrate from the core to the shell. The presence of the amorphous core is also likely an important factor for the formation of rutile rather than anatase or brookite in photoinduced crystallization. Rutile is the thermodynamically stable phase for bulk TiO2; however, for TiO2 nanoparticles