Titania Heterojunctions from

Aug 30, 2016 - A microwave-assisted hydrothermal method has been developed as an efficient approach to readily induce phase transition of titanate ass...
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Formation of Cu2O/Titanate/Titania Heterojunctions from Hydrothermally Induced Dual Phase Transitions Manchal Chaudhary,† Sue-min Chang,*,† Ruey-an Doong,†,‡ and Hsin-mu Tsai‡ †

Institute of Environmental Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu 30013, Taiwan

‡

S Supporting Information *

ABSTRACT: A microwave-assisted hydrothermal method has been developed as an efficient approach to readily induce phase transition of titanate assemblies in conjunction with decoration of Cu2O clusters on the surface. The influence of Cu2+ ions on the hydrothermally induced structural evolution was examined, and the roles of heterojunctions in the resulting composites in charge separation for improved photocatalytic activity were clarified. Hierarchical titanate assemblies with high adsorption capacity for Cu2+ ions (95.7 mg/g) were prepared from a low alkaline condition. Microwave-assisted hydrothermal treatment was then used to transform the adsorbents into Cu2O/titanate/ titania photocatalysts in 20 min via inducing titanate-to-titania and Cu2+-to-Cu2O dual transitions. While tubular architecture was maintained in the composites, the Cu2O clusters highly dispersed on the surface. Adsorbed Cu2+ ions have been found to retard the titanate-to-titania transformation locally, thus leading to Cu2O/titanate/titania heterojunctions. The multiheterojunctions enabled the composites to exhibit 1.7−5.1 times higher activity than the commercial product P25 (kobs, 0.06 min−1) for decomposition of bisphenol A due to charge separation. EPR results clearly reveal that the type II band alignment effectively drove electrons and holes to migrate toward the titania and the Cu2O moieties, respectively, and the titanate moiety positioning in between prevented back recombination. The optimal Cu2O loading to the highest activity (kobs, 0.306 min−1) was 3.7 wt %. Over the optimal amount, the lower reduction potential in the valence band of the Cu2O clusters compensated for the positive effect from charge separation, thus causing the activity to decline in turn.

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INTRODUCTION Titanate nanotubes (TNTs) have been a subject of significant interest in photocatalysis and adsorption because they contain a low-dimensional structure, large surface areas, and high density of hydroxyl groups, which benefits charge transport, adsorption, and ion exchange, respectively.1−5 TNTs are usually derived from alkaline treatment of titania compounds.6−8 It is believed that under a hydrothermal conditions reversible dissociation and condensation of TiO6 tetrahedrons generate titanate nanosheets, and the sheets then roll up to become tubes in order to reduce surface energy. Formation of different morphology of titanate species is a kinetically controlled process. Mao et al. prepared TNTs via a modified hydrothermal method involving redox reactions of a titanium source with H2O2 in the presence of NaOH.9 TNTs aggregated into hollow spherical architectures with a diameter of 100−200 nm. Interfacial tension and van der Waals attractive forces were considered to be responsible for the assembly. Different from conventional methods, Tang et al. successfully fabricated TNT-assembled hollow spheres under mild alkaline conditions.10 They used titania microspheres as the raw © 2016 American Chemical Society

materials as well as the template of the hierarchical structure and demonstrated that H2O2 played a crucial role in scrolling titanate nanosheets into tubes. The applications of TNTassembled hollow spheres have attracted more attention recently. The well-aligned TNTs in spherical architecture not only prevent random aggregation to preserve high surface areas but also improve light reflection to enhance photon utilization, thus exhibiting higher adsorption efficiency and photocatalytic activity than low-dimensional titanate species.11,12 Serving as photocatalysts, TNTs actually are not as active as titania due to the wide bandgap (ca. 3.6−3.8 eV) and fast charge recombination.1,13 To improve the activity, many studies transformed TNTs into anatase TiO2 crystals by dry or wet thermal treatment.14−17 Recently, titanate and titania heterostructures have been demonstrated to performed even higher photocatalytic activity than simple anatase TiO2 form, and their synthesis has attracted much attention.14,18,19 Cai et al. calcined Received: May 26, 2016 Revised: August 30, 2016 Published: August 30, 2016 21381

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The Journal of Physical Chemistry C titanate nanowires at 600 °C in H2 atmosphere and found that the photocatalytic activity was 20 times higher than the titanate nanowires.14 Formation of a titanate/titania heterojunction and a surface disordered shell were considered responsible for the high activity because such architecture synergistically assisted charge migration and charge separation. Jiang et al. fabricated a titania/titanate heterojunction by using a one-pot hydrothermal process at 150 °C for 24 h.20 Not only was the heterojunction introduced for charge separation, the wet process intercalated nanoparticles between the titanate species to inhibit aggregation, thus enhancing the activity for H2 evolution. Modifications of TNTs with ions, noble metals, or semiconductors are alternative ways to create charge trapping sites and heterojunctions.21−25 Zhao et al. prepared N-doped titanate and anatase TiO2 with a core−shell nanostructure and demonstrated the high photoactivity under visible light irradiation as the results of reduced bandgap, high electron− hole mobility and low electron−hole recombination.18 Loading TNTs with semiconductors, including In2O3, CuO, Fe2O3, WO3, or CdS crystals, have been demonstrated to improve photocatalytic activity both in the UV- and the visible-light regions.26−31 Unlike doping, heterojunction prevents the defect-mediated recombination within the lattice and thus is more beneficial to photocatalytic activity.32,33 The semiconductor composites can be prepared through hydrothermal processes, photodeposition, or adsorption of salts onto TNTs followed by calcination or sulfurization.26,29,34,35 Most of the conventional methods need multiple steps and are time and energy consuming. In this study, we prepared hierarchical titanate assemblies as an advanced adsorbent for Cu2+ ions and simply turned the titanate species into Cu2O/titanate/titania nanocomposites by post microwave-assisted hydrothermal treatment. Compared to conventional calcination which is processed at high temperatures for hours, the post microwave-assisted hydrothermal treatment only took 20 min to form the nanocomposites at 150 °C. In addition, the composites exhibited 1.7−5.1 times higher photocatalytic activity than the commercial product P25. The microstructures and surface properties of the titanate assemblies and the corresponding nanocomposites were explicitly characterized and discussed on the basis of a methodological design. Moreover, the roles of the heterojunctions in charge separation and the photocatalytic behavior were clarified.

solution was near neutral, and then the powders were dried at 60 °C for 10 h in oven. To load Cu2+ ions onto the TNTs, the TNTs (5 mg) were dispersed into the Cu2+ solutions (5 mL) at different concentrations (10, 20, 50, 70, 80, 90, and 100 mg/L) which were buffered with 10 mM MES (Aldrich) at pH = 5.0. After adsorption for 1 h, the suspension was added with 0.1 mL of 35 mM urea solution and then irradiated with microwave at 150 °C for 20 min. Characterizations. The morphology of as-prepared TNTs and Cu2O/titanate/titania hollow microspheres was characterized using a scanning electron microscope (SEM, JEOL, JSM-6700) and a scanning transmission electron microscope (STEM, JEOL, JEM-2010) operated at accelerating voltages of 5 and 200 kV, respectively. The Brunauer−Emmett−Teller (BET) specific surface area, pore-size distribution, and pore volumes were estimated based on N2 adsorption and desorption isotherms measured at 77 K using a surface area and porosimetry system (ASAP 2020, Micromeritics). The crystal structures were identified by using an X-ray diffractometer (Bruker NEW D8 ADVANCE, Germany) equipped with a Lynx eye high-speed strip detector and a Ni-filtered Cu Kα radiation (λ = 1.5406 Å) source operating at a voltage and an emission current of 40 kV and 40 mA, respectively. The Xray diffraction patterns were acquired at the 2θ angle ranging from 20° to 90° with a sampling step of 0.05° (step time = 0.5 s). The quantity of the Cu species in the TNTs and the titaniabased composites was determined by using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Horiba Jobin Yvon JY 2000). The amounts of adsorbed Cu2+ ions on the TNTs were derived from the remaining Cu concentration in the solution. For titania-based composites, the Cu species was dissolved into the solutions with acid digestion first. The Cu concentrations were then measured and based on to calculate the Cu2O loadings in the composites. The chemical states of elements were determined using an X-ray photoelectron spectrometer (XPS, ESCA PHI 1600) with an Al Kα X-ray source at 1486.6 ± 0.2 eV. To eliminate the chemical shifts resulting from the charge effects, the binding energies of the photoelectrons were calibrated by fixing the C 1s from the carbonaceous contaminants at 284.5 eV. The electronic structures of the TNTs-assembled microspheres and the Cu2O/titanate/titania composite were determined by using an UV−visible spectrometer (HITACHI U-3010) equipped with an integrated sphere. Diffused reflection spectra were recorded from 550 to 275 nm and were then converted into absorption spectra using the Kubelka−Munk equation. Photocatalytic Degradation of BPA by Cu2O/Titanate/ Titania Composites. Photocatalytic activity of the composites was examined in a BPA (15 mL, 10 mg/L) solution at a dosage of 0.83 g/L. The lamp was positioned within the inner part of the photoreactor and the temperature of the system was maintained at 25 °C by circulating water through the outer Pyrex jacket. Prior to the photocatalysis, the suspension was magnetically stirred for 30 min in darkness to reach adsorption equilibrium. The photodegradation of BPA molecules was measured by sampling 1 mL aliquot from the reactor at different time intervals. After centrifugation at 14,000 rpm for 5 min to remove the photocatalysts, the BPA concentrations in the supernatants were determined by a high-performance liquid chromatography (HPLC) equipped with a Phenomenex C-18 column and a photodiode array (Agilent Technol). The chromatography was conducted using a methanol/acetonitrile/water solution (50:30:20, v/v) as the eluent at a flow rate

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EXPERIMENTAL SECTION Fabrication of Cu2O/Titanate/Titania Heterojunctions. TNTs-assembled hollow microspheres, used as the raw material for the Cu2O/titanate/titania composites, were prepared with hydrothermal treatment of sol−gel-derived titania particles. Amorphous titania particles were obtained via hydrolysis of 0.8 mL of titanium isopropoxide (Acros, 98%) with 0.15 mL of 0.1 M HCl solution in 35 mL of ethanol. After aging for 24 h under an ambient condition, the particles were harvested by centrifugation at 14 000 rpm for 10 min. The titania particles (0.1 g) were suspended into 25 mL of 0.1 M NaOH solution, and then 0.03 mL of H2O2 solution (Riedel-de Haën, 30 wt %) was added into the suspension. Before being transferred to a 50 mL Teflon-lined vessel, the suspension was sonicated at 25 °C for 60 min. The hydrothermal process was carried out at different temperatures (120, 150, and 180 °C) for different times (10 and 24 h). The resulting TNTs were washed with 10 mM HEPES buffer solution (Aldrich) repeatedly until the 21382

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The Journal of Physical Chemistry C of 0.5 mL/min. The BPA concentration was determined based on the absorbance at 225 nm. To explore the photocatalytic mechanism, free radicals generated from the photocatalytic system were determined by using an electron paramagnetic resonance (EPR) spectrometer (Bruker, EMX-10, Germany) working at X-band frequency of 9.49−9.88 GHz with power of 8.02 mW. A 250 W Xe lamp (Ushio Inc.) was equipped to the sample cavity by a lined optical fiber. Oxygen-saturated suspensions were added with 5,5-dimethyl-1-pyrroline Noxide (DMPO, 4.4 mM, serving as a spin trap) to stabilize radicals during irradiation. The spectra were recorded at a center field of 3400−3510 G and a sweep width of 200 G.

Figure 2 shows XRD patterns and N2 adsorption−desorption isotherms of the microspheres. All the microspheres had

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RESULTS AND DISCUSSION TNT-Assembled Hollow Microspheres. Figure 1 shows SEM and TEM images of TNTs-assembled hollow micro-

Figure 2. (a) XRD patterns and (b) N2 gas adsorption−desorption isotherms and pore-size distribution of the hollow microspheres prepared at different hydrothermal temperatures.

diffraction peaks at 9.8°, 48.0°, and 61.9° 2θ positions, which can be assigned to (200), (020) and (002) orientations, respectively, of the layered structure in the titanate hydrate.34,39 The crystallite size (D200) was estimated using the Scherrer equation, and it increased from 4.7 to 5.9 nm when the temperature raised from 120 to 180 °C. It is noted that the sample treated at 180 °C for 10 h already exhibited similar crystallinity as the samples obtained at 120−150 °C for 24 h. The textures of the microspheres were characterized by gas adsorption. All the titanate assemblies exhibited type IV adsorption isotherm, indicating mesoporous structures. The sample prepared at 120 °C showed a type H4 hysteresis loop, and the loop stepwise transited to type H2 at elevated temperatures. This feature reveals that low-temperature treatment led to a slit-shaped structure, while a tubular structure with bottlenecks resulted from high temperature in order to reduce surface energy.6 The mesopores mainly appeared in the internal wall of the microspheres where the titanate sheets/ tubes grew up and interconnected with each other. The pore sizes were in the range of 3.9−5.3 nm and shrank with increasing temperature. Although the textures of the assemblies changed, a similar specific surface area of 101−104 m2/g was obtained at different temperatures. Compared to the starting material (the sol−gel-derived titania particles, 469 m2/g), a large dimension and aggregation of the titanate species reduced the surface area. The microstructural data are summarized in Table 1. According to the microstructural results, the sample, which prepared at 180 °C for 10 h, was selected as the raw material for further fabrication of Cu2O/titanate/titania heterojunctions because it contained high crystallinity and the high surface area after the short-term hydrothermal treatment. Characterization of Cu2O/Titanate/Titania Heterojunctions. Loading of Cu species was carried out in MES buffer solution with controlled pH at 5.0. Prior to microwaveassisted hydrothermal treatment, Cu2+ ions were adsorbed onto the TNTs-assembled microspheres for 1 h. Figure 3 shows the

Figure 1. SEM images of hollow microspheres prepared at different hydrothermal temperatures. (a) 120 °C for 24 h, (b) 150 °C for 24 h, (c) 180 °C for 24 h, (d) 180 °C for 10 h, 10k magnification, and (e) 180 °C for 10 h, 50k magnification (circled areas and the inset show the hollow interior of the microspheres). (f) TEM image of the hollow microspheres prepared at 180 °C for 10 h.

spheres synthesized from the hydrothermal treatment at 120− 180 °C for 10 or 24 h. After reaction at 120−150 °C for 24 h, folded nanosheets formed and assembled into marigold-flowerlike microspheres. The microspheres were hollow and had a diameter in the range of 500−700 nm. When the temperature increased to 180 °C, the nanosheets transformed into tubular structures but still aggregated. As shown in the TEM image, well-defined TNTs-assembled hollow microspheres were obtained after 10-h reaction at 180 °C. The diameter of the microspheres was ca. 800 nm, and the diameters and lengths of the tubes were in the range of 30−50 and 200−300 nm, respectively.10,36,37 With extending the reaction time to 24 h, the interior hollow space expanded and shape of the assembly became irregular as the result of Ostwald ripening.38 21383

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The corresponding number of the strong binding sites was 56.02 mg/g. When the Ce was higher than 0.8 mg/L, the fitting parameters were Kf = 57.85, n = 10.76, and R2 = 0.99, indicating weak electrostatic interaction on the remaining bind sites.40 The pHzpc of the TNTs was 2.65. Negative charges on the surface at pH = 5 enable the TNTs to be advanced in the adsorption for cations. After hydrothermal treatment, the adsorbed Cu2+ ions converted into oxides and deposited on the TNTs. The contents of the Cu moieties in the composites were 0.8, 1.6, 3.7, 4.5, 5.5, 6.0, and 6.9 wt % for the initial Cu2+concentration of 10, 20, 50, 70, 80, 90, and 100 mg/L, respectively. Figure 4 shows XRD patterns, TEM, and mapping images of the Cu2+-loaded TNTs samples after the microwave-assisted hydrothermal treatment. While the diffraction peaks of titanate structure diminished, typical diffraction signals of anatase titania crystals, which centered at 25.4°, 48.1°, and 62.8° 2θ positions, appeared in the sample, indicating titanate-to-titania transformation. There were no peaks for the Cu species observed in the XRD patterns, suggesting high dispersion and tiny sizes of the moieties. Calcination is often employed to convert TNTs into anatase titania and high temperature (450−500 °C) is inevitably needed to induce dehydration and elemental rearrangement for the phase transition.11,41 Recently, hydrothermal treatment has been demonstrated to readily transform TNTs into different structural analogues. In contrast to calcination, the wet chemical processes involve dissolution and reprecipitation of building blocks, thus allowing the structural transformation to occur at lower temperatures.42−44 We successfully transformed the TNTs into the anatase titania at 150 °C in 20 min. Microwave radiation and the MES buffer played the vital roles in promoting the phase transition. Microwave radiation intensified the interaction between the molecules by rotating polar molecules billion times per second.

Table 1. Crystalline Phase, BET Surface Area, Pore Volume, and Pore Size of the Hollow Microspheres conditions 120 150 180 180 a

°C, °C, °C, °C,

24 24 10 24

h h h h

crystalline phase

D200 (nm)

SAa (m2/g)

Dpb (nm)

titanate titanate titanate titanate

4.7 5.0 5.3 5.9

101 101 101 104

5.3 4.0 4.0 3.9

SA represents specific surface area. bDp means pore diameter.

Figure 3. (a) Adsorption isotherm and (b) Freundlich adsorption model of linear regression of Cu2+ ions onto TNTs-assembled hollow microspheres [Ce is the equilibrium concentration (mg/L) and qe is the adsorption amount (mg/g)].

adsorption isotherm and model fitting. The TNTs-assembled microspheres exhibited outstanding affinity for Cu2+ ions and the maximal capacity was 95.7 mg/g. The adsorption isotherm fitted the Freundlich model and exhibited two linear dependences between log qe and log Ce, where qe (mg/g) and Ce (mg/ L) are the adsorption amount and the equilibrium concentration, respectively. When the Ce was lower than 0.8 mg/L, the fitting parameters were Kf = 93.30, n = 0.55, and R2 = 0.99, indicating irreversible binding on the strong adsorption sites.

Figure 4. (a) XRD patterns of the Cu2+-loaded TNTs after microwave-assisted hydrothermal treatment at 150 °C for 20 min. (b) TEM and (c) HRTEM images of the 3.7 wt % Cu2O/titanate/titania composite. (d) STEM image of the composite and (e−g) mappings of Ti, O, and Cu elements. 21384

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extended the adsorption edge to 500 nm. In addition, higher density of states in the anatase TiO2 crystals enhanced the absorption in the UV range.45 The two absorption onsets refer to the bandgaps of 3.14 and 2.48 eV for the anatase TiO2 and the Cu2O nanocrystals, respectively. Bulk Cu2O is a p-type semiconductor with a bandgap of 1.9−2.2 eV.46,47 The higher bandgap of the Cu2O moiety (2.48 eV) not only reveals quantum confinement but also supports the tiny sizes in the composite.48,49 According to the measured bandgaps and the reduction potential in the bands, the electronic structure of the composite can be established. Anatase TiO2 crystals have reduction potential of 2.94 V in the valence band.50 The bandgap energy of anatase TiO2 is 3.14 eV, hence the corresponding reduction potential in the conduction band is −0.20 V. Compared to the anatase TiO2 crystals, the TNTs have the higher reduction potential of 3.02 V in the valence band and the lower reduction potential of −0.52 V in the conduction band.51 The reduction potential of the valence band of Cu2O crystals is 0.64 V.52 The bandgap energy of 2.48 eV reflects the reduction potential of −1.84 V in the conduction band. Photocatalytic Activity and Mechanism. The degradation curves of BPA molecules in the presence of Cu2O/ titanate/titania nanocomposites under 365 nm UV irradiation are shown in Figure 6. Prior to the photocatalysis, the suspensions had undergone adsorption in the dark for 1 h, and all the composites showed insignificant adsorption. TNTsassembled hollow microspheres performed low photoactivity and decomposed only 7% of BPA after 120 min irradiation. Their wide bandgap (3.54 eV) which limited available transitions under 365 nm UV irradiation was responsible for the low activity. After formation of Cu2O/titanate/titania heterojunctions, the photoactivity remarkably promoted. It reached 73% removal efficiency within 15 min when the composite contained 0.8 wt % Cu2O, and almost decomposed 90% of the target in 10 min when the content of the Cu2O moiety increased to 1.6−6.9 wt %. The composites exhibited 1.7−5.1 times higher photocatalytic activity than the commercial product P25, in which the 3.7 wt % Cu2O/ titanate/titania sample was the most active one with the observed pseudo-first-order rate constant (kobs) of 0.306 min−1. Over this optimal content, the activity declined with the increasing Cu2O loading. Photocatalysis involves adsorption and surface reactions. To clarify the influence of adsorption on the photocatalytic activity, adsorption isotherm of BPA molecules in the presence of the 3.7 wt % Cu2O/titanate/ titania composite was measured. The adsorption was well fitted with the Langmuir model which give the adsorption coefficient

In addition, the zwitterionic MES buffer absorbed the microwave radiation to create hot spots in terms of dielectric heating, thus synergistically facilitating the structural transformation. Anatase titania was favorable in this study. Formation of the metastable titania phase under the moderate acidic condition (pH = 5) is a kinetically controlled result because it has a similar edge-shared structure as titanate crystals.43 When the concentration of Cu2+ ions was higher than 50 mg/L, the titanate-to-titania transformation was slightly inhibited. This phenomenon indicates that the adsorbed Cu2+ ions hindered the elemental rearrangement of the titanate species. According to the TEM images, the sample still retained tubular structures but formed some extra protrusions and nanoparticles after the microwave-assisted hydrothermal treatment. Lattice fringes indicate that both the tubes and particles are mainly anatase titania species. In addition, the titania moieties were highly crystalline except for the outmost layer. Tiny Cu2O clusters formed against the titania crystals and dispersed evenly across the sample. This finding reveals that the surface Cu species retarded the titanate-to-titania conversion and resulted in the Cu2O/titanate/titania heterojunctions. Figure 5 shows XPS spectrum of Cu 2p states and UV−vis absorption spectra of the composite. The Cu 2p3/2 and 2p1/2

Figure 5. (a) XPS spectrum of the Cu 2p states in the Cu2O/titanate/ titania composite, and (b) UV−visible spectra of the TNTs and the Cu2O/titanate/titania composite.

photoelectron peaks centered at 932.7 and 952.5 eV, respectively, and appeared without shakeup lines along with, indicating Cu2O species on the surface. The reduced species generated due to the urea that acts as a reducing agent in the hydrothermal reaction. The electronic structure of the Cu2O/ titanate/titania composite was examined with UV−vis spectrometry. The TNT-assembled hollow microspheres showed an absorption edge at 350 nm. Correspondingly, their bandgap energy was 3.54 eV. Anatase TiO2 nanocrystals in the Cu2O/ titanate/titania composite red-shifted the onset of the steep absorption from 350 to 395 nm and the Cu2O moiety further

Figure 6. (a) Photocatalytic degradation of BPA by Cu2O/titanate/titania photocatalysts with different Cu2O loadings, and (b) the corresponding rate constants (kobs, min−1). 21385

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Figure 7. EPR spectra of DMPO spin adducts in the Cu2O/titanate/titania suspensions recorded in different irradiation times (a) 5 s, (b) 1 min, and (c) 3 min. (d) Proposed photocatalytic mechanism for the Cu2O/titanate/titania heterojunctions.

(kcd) and the adsorption capacity (Qmax) of 6.1 × 10−1 L/mg and 1.8 × 10−1 mg/g, respectively. The small Qmax and kcd values indicate weak attraction between the target and the photocatalyst (see Supporting Information, Figure S1). Photocatalytic degradation in different BPA concentrations was examined. According to a Langmuir−Hinshelwood model fitting, the adsorption coefficient (kcp) during the photocatalysis was 1.5 × 10−1 L/mg (see Supporting Information, Figure S2). The kcp represents the chemical interaction of BPA molecules with the photogenerated holes in the valence band, and the smaller kcp value than the kcd (6.1 × 10−1 L/mg) in the dark reveals that the decomposition of BPA molecules mainly results from radical attacks rather than direct oxidization by the holes. Under this circumstance, the compositions and configuration of the photocatalysts which govern the number of effective charge carriers is the key to the photocatalytic activity. The role of heterojunctions in photocatalytic activity was explored from detection of the photogenerated radicals by EPR. Figure 7 shows EPR spectra of DMPO spin adducts in the Cu2O/titanate/titania suspensions during irradiation. There were no radicals detectable in the darkness. After irradiation, a characteristic 1:2:2:1 quartet of the DMPO−•OH spin adduct and typical 1:2:1:2:1:2:1 hyperfine peaks of 5,5-dimethyl-2pyrrolidone-N-oxyl (DMPOX) were measured.53−55 The DMPO−•OH spin adduct generated immediately in the titanate/titania suspension. Whereas, this generation was suppressed when the composites contained Cu2O moiety. After irradiation for 1 min, the intensity of DMPO−•OH spin adduct almost diminished in the 1.6 wt % Cu2O/titanate/ titania suspension. Instead, DMPOX signals started to appear and their intensity increased with the increasing Cu2O loadings. After irradiation for 3 min, DMPOX tuned out to be dominant in all Cu2O/titanate/titania suspensions. It is reported that DMPOX species generates from decomposition of DMPO-•OOH adducts.56 In photocatalytic systems, the holes in the valence band react with H2O molecules to generate •OH radicals while O2 molecules trap electrons from the conduction band to generate •O2− radicals.

The •OOH radicals produced from protonation of the •O2− radicals with water molecules. According to the EPR results and the band structures, photocatalytic mechanism of the Cu2O/ titanate/titania composite is proposed and illustrated in Figure 7d. The band alignment in the titanate/titania heterojunction belongs to Type I. The energies of the conduction band and the valence band of the titania, which position within the bandgap of the titanate, drive both the electrons and holes to migrate from the tinanate to the titania. Although the band energies of the anatase crystals are suitable for producing both • OH and •O2− radicals, only the DMPO−•OH spin adduct was detected in the photocatalytic system because of the faster interaction between the spin trap and the •OH radicals.57 The Cu2O clusters have the higher reduction potential than the titania and the titanate species in both the conduction band (−1.84 V) and valence band (0.64 V).52 Band alignment in the Cu2O/titanate/titania heterojunctions belongs to type II and the built-in potential drives electrons and holes to move toward the titania and the Cu2O semiconductors, respectively. Such charge separation is supported by the detection of DMPOX in the Cu2O/titanate/titania suspensions. The holes in the valence band of the Cu2O clusters were incapable of generating •OH radicals because the band potential (E = 0.64 V) was lower than that for the •OH/−OH couple (E = 2.27 V).58 In the absence of •OH radicals, •O2− radicals and the holes in the Cu2O clusters were responsible for the decomposition of BPA in the Cu2O/titanate/titania composite-based system. The availability of the holes for direct oxidation of BPA molecules was verified by their visible-light-driven activity. When the composite was irradiated with visible light and activation of the Cu2O clusters was dominant, BPA molecules still decomposed and the degradation rate increased with the Cu2O loadings (see Supporting Information, Figure S3). Charge separation in the Cu2O/titanate/titania heterojunctions improved the photocatalytic activity. Although the Cu2O moiety separated the holes from the composite to suppress recombination, the lower reduction potential in the valence band decreased the oxidation rate. The consequence 21386

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compensates the advantage and leads to an optimal loading of the Cu2O moiety for the highest photocatalytic activity. The titanate/titania composite loaded with 3.7 wt % Cu2O clusters exhibited the highest photocatalytic activity. When the loading was lower than this value, the quantity of the Cu2O moiety was insufficient for effective charge separation. The detection of the DMP−•OH spin adducts in the 0.8−1.6 wt % Cu2O/titanate/ titania suspensions in the beginning of irradiation supports this phenomenon. When the loading was higher than the optimal amount, declined photocatalytic activity was attributed to slow charge transfer between the holes in the Cu2O clusters and the BPA molecules. In addition, higher fractions of the titanate species in the composite also reduced the photocatalytic activity. Although many approaches have been developed for preparation of similar composites, we provide an alternative way in this study to reach highly active photocatalysts more readily and efficiently. This approach is not only beneficial to microstructural adjustment of titanate species, but also promising for converting waste adsorbents into advanced materials.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology (MOST), Taiwan, for financial support under Grant No. MOST 104-2221-E009-020-MY3 and Grant No. MOST 1042628-E-009-002-MY2.

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CONCLUSIONS We have prepared hierarchical titanate assemblies and demonstrated their high adsorption capacity for Cu2+ ions. In addition, post microwave-assisted hydrothermal method was developed for phase transition to fabricate Cu2O/titanate/ titania nanocomposites at a low temperature within a short period of time. The Cu2O/titanate/titania heterojunctions formed because adsorbed Cu2+ ions inhibited titanate-to-titania transition. Without calcination, the composites maintained a tubular structure and high surface areas, which are beneficial to photocatalytic reactions. The composites exhibited substantially high photoactivity for BPA degradation. The heterojunctions effectively separated electrons and holes in the two ends of the heterojunctions, and the middle titanate species suppressed back recombination. The optimal Cu2O loading for the highest activity was 3.7 wt %. Overloading in turn declined the activity because low reduction potential in the valence band of the Cu2O moiety slow charge transfer between the holes and the BPA molecules. In summary, we not only extend the knowledge on phase transition of titanate assemblies under a hydrothermal condition but also provide an alternative green approach for turning waste adsorbents into advanced materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05301. Adsorption isotherm of BPA molecules in the presence of 3.7 wt % Cu2O/titanate/titania composite and linear fitting of the Langmuir model, dependence of initial reaction rates upon initial BPA concentrations and linear fitting of the Langmuir−Hinshelwood model, and photocatalytic degradation curve of BPA in the presence of different dosages of 3.7 wt % Cu2O/titanate/titania composites under 465 nm blue LED illumination and the corresponding pseudo-first-order rate constants (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +886-3-5712121 ext. 55506. 21387

DOI: 10.1021/acs.jpcc.6b05301 J. Phys. Chem. C 2016, 120, 21381−21389

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