g-C3N4 Microspheres with Enhanced Visible-Light

Oct 3, 2017 - The TiO2 surface (64.4 m2 g–1) was mostly covered by a photoactive g-C3N4 layer, while the interconnected porous network featured a la...
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Mesoporous TiO2/g‑C3N4 Microspheres with Enhanced Visible-Light Photocatalytic Activity Hao Wei,† William A. McMaster,† Jeannie Z. Y. Tan,†,‡ Lu Cao,† Dehong Chen,*,†,§ and Rachel A. Caruso*,†,‡,∥ †

Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia CSIRO, Manufacturing, Clayton South, Victoria 3169, Australia

‡

S Supporting Information *

ABSTRACT: Designing a heterojunction semiconductor is an efficient strategy to extend the light response of a photocatalyst to the visible range and thus improve photocatalytic activity. Starting with mesoporous anatase TiO2 microspheres, mesoporous TiO2/g-C3N4 microspheres were prepared via a facile nanocoating procedure, with the porous TiO2 as the active supporting scaffold and g-C3N4 (3 wt %) as the visible light sensitizer. Heterojunctions formed at the TiO2/g-C3N4 interfaces separated photogenerated charges. The TiO2 surface (64.4 m2 g−1) was mostly covered by a photoactive g-C3N4 layer, while the interconnected porous network featured a large pore volume (0.30 cm3 g−1) for mass diffusion. The g-C3N4 precursor, cyanamide, a nitrogen-rich molecule, also acted as a nitrogen source to form TiO2−xNx. Substitution of N in the TiO2 lattice triggered a visible light response due to an additional N level above the TiO2 valence band that resulted in band gap narrowing to 1.5 eV. Compared with mesoporous g-C3N4, the composite microspheres were 8.5 times more active in degrading phenol under visible light irradiation. A mechanism was proposed for the TiO2/g-C3N4 heterojunction incorporated within the mesoporous structure that enhanced the photocatalytic properties.

1. INTRODUCTION The semiconductor titanium dioxide (TiO2) has attracted substantial attention due to its strong oxidizing ability for the photodecomposition of organic pollutants and its uses in solar cells and ultraviolet (UV) light shielding.1−3 However, the wide band gap of TiO2 (3.0−3.2 eV) only allows a photoresponse (excitation of electrons from the valence band, VB, to the conduction band, CB) under UV light irradiation, which corresponds to just 5% of the sunlight that reaches the Earth’s surface.4,5 To extend the photoresponse of TiO2 into the visible light region for practical applications, efforts have focused on introducing energy states between the VB and CB (i.e., band gap narrowing) by means of hydrogenation,6 metal,7 and nonmetal doping.4,8,9 Although a visible light response has been achieved via doping, the dopant usually acts as a recombination center for electron−hole pairs leading to lower kinetic performance.10,11 One alternative approach to achieve a visible light response is to form a heterojunction (i.e., an interface between two dissimilar semiconductors of different band gaps) with more effective electron−hole pair separation.12 An optimized heterojunction should possess a well-aligned interface between the semiconductor sensitizer and the TiO2 substrate, with the positions of the respective VBs and CBs facilitating the separation of photogenerated charge carriers.13,14 Recently, titanium carbides and phosphorene cocatalyst metal sulfides were developed as efficient photocatalysts that function under visible irradiation and demonstrated improved performance in hydrogen production.15,16 As an emerging photocatalyst, graphitic carbon nitride (gC3N4) has been widely investigated as a visible light sensitizer © XXXX American Chemical Society

due to its band gap of 2.69 eV and its high chemical stability in water splitting and pollutant degradation under visible light.17−21 Moreover, the negative potential (−1.12 eV vs NHE) of the lowest unoccupied molecular orbital indicates that g-C3N4 can favorably form heterojunctions with other semiconductors, such as graphene oxide,22 TiO2,23,24 ZnO,25 Bi,26 and BiPO4,27 to achieve superior performance under visible light irradiation. For example, a core−shell photocatalyst of BiPO4 coated by g-C3N4 displayed enhanced activity in degrading methylene blue under UV and visible light.27 In addition, heterojunctions were obtained by grafting g-C3N4 nanosheets onto TiO2(B) nanofibers via a solid reaction at high temperature. Benefiting from the heterojunctions produced by aligned g-C3N4 (224̅0) and TiO2(B) (001) planes, the composite exhibited higher activity in degrading sulforhodamine B than either pristine TiO2(B) or g-C3N4 under visible light.24 To obtain heterojunction semiconductors, physical mixing and solid-state reactions are widely utilized; however, improvements in photocatalytic activity are limited because contact between the semiconductors is not optimized. Apart from forming heterojunctions, various structures, including porous materials, have been employed to optimize mass transfer while simultaneously supplying abundant active sites through high surface areas and large pore volumes.28,29 For instance, using the ordered mesoporous silica SBA-15 as a hard template, mesoporous g-C3N4 with a large surface area (505 m2 Received: July 2, 2017 Revised: September 22, 2017

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The Journal of Physical Chemistry C g−1) and pore volume (0.55 cm3 g−1) can be prepared.29 Compared to nonporous bulk g-C3N4, mesoporous g-C3N4 exhibited higher photoactivity to catalyze H2 production. Consequently, there are good prospects for a heterojunction semiconductor that combines mesoporous g-C3N4 and TiO2 to obtain enhanced photocatalytic properties in the visible light range. Hence, mesoporous TiO2/g-C3N4 microspheres were prepared via a facile nanocoating strategy. To form a homogeneous heterojunction, cyanamide, as the g-C3N 4 precursor, was infiltrated into mesoporous anatase TiO2 microspheres then polymerized to a thin layer by calcination, thus coating the anatase nanocrystals within the mesoporous structure. The resulting mesoporous TiO2/g-C3N4 microspheres exhibited a large pore volume and high surface area. Benefiting from heterojunctions formed at the TiO2/g-C3N4 interfaces, the photocatalytic activity (assessed by phenol degradation) was dramatically enhanced under visible light irradiation, being 8.5 times higher than the mesoporous gC3N4. Based on characterization of the surface composition, morphology, porosity, and optical and electronic properties, a mechanism was proposed to illustrate the superior photocatalytic performance of the purpose-built heterojunctions within the mesoporous TiO2/g-C3N4 microspheres under visible light irradiation.

were prepared via a nanocoating procedure. TO powder (0.8 g) was dispersed in a variable volume of cyanamide aqueous solution (0.8 g mL−1, x mL) and stirred at room temperature until the solvent evaporated completely. After calcination at 550 °C in nitrogen for 4 h, the final products were denoted as TOCN-x, where x was the volume of cyanamide solution. 2.2.3. Synthesis of the Mesoporous g-C3N4 (CN). Ordered mesoporous silica SBA-15 was introduced as a hard template, with CN fabricated as a replica via a nanocasting procedure.28 Typically, premade SBA-15 (0.5 g) was dispersed in cyanamide aqueous solution (0.8 g mL−1, 1 mL) and stirred at room temperature until the solvent evaporated completely. Next, a SBA-15/g-C3N4 composite was obtained after calcination at 550 °C in nitrogen for 4 h. The template was removed by dissolution in NH4HF2/HCl aqueous solution (1 M for NH4HF2, 1:2 in molar ratio) for 8 h. The as-prepared CN was dried at 80 °C after washing in a water/ethanol mixture (1:1 v/v) three times. 2.3. Material Characterization. Scanning electron microscopy (SEM) was performed to study the sample morphology on an FEI Quanta 200F environmental scanning electron microscope at an accelerating voltage of 15 kV and without Au coating. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) were conducted on an FEI Tecnai F20 transmission electron microscope operating at 200 kV. Powder X-ray diffraction (XRD) patterns were acquired on a Bruker D8 Advance diffractometer using Cu Kα radiation. The diffractometer was set at a 40 kV working voltage and 40 mA working current, with samples scanned from 5° to 80° in 2θ. Thermogravimetric analysis (TGA) was conducted on a Mettler Toledo TGA/SDTA851e thermogravimetric analyzer heating from 25 to 900 °C with a ramp rate of 10 °C min−1 under oxygen or nitrogen flow (30 mL min−1). Nitrogen sorption isotherms were measured at −196 °C using a Micromeritics TriStar 3000 Surface Area and Porosity Analyzer. Prior to measurement, calcined samples were degassed at 150 °C for at least 8 h on a vacuum line. The specific surface areas were calculated by a standard multipoint Brunauer−Emmett− Teller (BET) method using adsorption values in the range P/P0 = 0.05−0.20. The Barrett−Joyner−Halenda (BJH) model was applied to the adsorption branch of the isotherm to determine pore size distributions, and pore volume was estimated from the adsorbed amount at P/P0 = 0.983. UV−visible diffuse reflectance spectra were collected on a PerkinElmer Lambda 1050 UV/vis/NIR spectrophotometer with a 150 mm integrating sphere accessory from 300 to 800 nm. The diffuse reflectance versus wavelength data were used to estimate the bandgap by converting reflectance to absorption data according to the Kubelka−Munk equation, F(R) = (1 − R)2/(2R) = K/S, where R is the reflectance and K and S are the effective absorption and scattering coefficients, respectively. Fourier transform infrared (FT-IR) spectra were obtained using a Varian 7000 FT-IR spectrometer with an attenuated total reflectance accessory, collecting in the range 400−4000 cm−1. X-ray photoelectron spectroscopy (XPS) data were recorded on a VG ESCALAB 220i-XL spectrometer equipped with a twin crystal monochromated Al Kα X-ray source, which emitted a photon energy of 1486.6 eV at 10 kV and 22 mA. The C 1s peak at 285.0 eV was used as a reference for the calibration of the binding energy scale. Photoluminescence (PL) spectra (λ = 350−600 nm) were measured on a Varian Cary-Eclipse at room temperature using an excitation wavelength of 325 nm. Electron paramagnetic resonance (EPR)

2. EXPERIMENTAL SECTION 2.1. Chemicals. Titanium(IV) isopropoxide (TIP, 97%), cyanamide (H2NCN, AR), hexadecylamine (HDA, 90%), Pluronic P123 triblock copolymer (EO20PO70EO20), tetraethyl orthosilicate (TEOS, 98%), tert-butyl alcohol (t-BuOH, >99.0%), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, AR), and ammonium bifluoride (NH4HF2, AR) were purchased from Sigma-Aldrich. Phenol (AR), 1-butanol (99.8%), and ethanol (>99.5%) were purchased from ChemSupply. Ammonium chloride (NH4Cl, AR) was from BDH. Hydrochloric acid (HCl, 32%) and ammonia solution (NH3· H2O, 25 wt %) were purchased from Merck. All reagents were used as received. Milli-Q water was collected from a Millipore academic purification system with resistivity higher than 18.2 MΩ cm. 2.2. Material Synthesis. 2.2.1. Synthesis of the Mesoporous Anatase TiO2 Microspheres (TO). Amorphous TiO2 microspheres were prepared following the procedure reported previously.30 Typically, HDA (11.92 g) was dissolved in 1butanol (600 mL) with aqueous NH4Cl solution (0.1 M, 4.8 mL). Under intense stirring at room temperature, TIP (13.57 mL) was added quickly and stirred vigorously for 1 min. After keeping static for 18 h, the white precipitate was collected and washed with ethanol three times. The final product was dried at room temperature. Solvothermal treatment was conducted to prepare TO. Typically, amorphous TiO2 microspheres (3.2 g) were dispersed in a mixture of ethanol (40 mL), water (20 mL), and ammonia solution (2 mL) and stirred for 30 min at room temperature. The mixture was heated in a Teflon-lined autoclave (100 mL) at 160 °C for 16 h. The solvothermally treated product was recovered by centrifugation (Beckman Coulter Allegra 25R centrifuge) at 5000 rpm for 5 min and washed with a water/ethanol mixture (1:1 v/v), with TO obtained after calcination (Bel Tetlow K2 chamber furnace with Shimaden FP93 controller) at 550 °C for 4 h in air. 2.2.2. Synthesis of the Mesoporous TiO2/g-C3N4 Microspheres (TOCN-x). Mesoporous TiO2/g-C3N4 microspheres B

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The Journal of Physical Chemistry C spectra were recorded on a Bruker Elexsys E500 spectrometer with a Bruker ER 4122SHQ cavity by applying an X-band microwave (9.43 GHz, 1 mW) with a sweeping magnetic field (600 G). Samples were cooled to 77 K in liquid nitrogen, and the g factor was calibrated by reference to DPPH (1,1-diphenyl2-picrylhydrazyl, g = 2.000). 2.4. Photocatalysis Reaction. All experiments were conducted in a fume cupboard inside a dark room to avoid light exposure. The photocatalytic performance was evaluated by analyzing the degradation of aqueous phenol (10 ppm, 160 mL) with photocatalyst (80 mg) under visible light irradiation in a jacketed beaker. Prior to irradiation, the suspension was equilibrated by intense stirring in the dark for 1 h. The beaker was kept at 20 ± 1 °C using cooling water, and air was continuously bubbled (50 mL min−1, BOC air gas 052) into the solution during equilibration and degradation. A 500 W Hg (Xe) globe (Oriel) with a Schott filter (cutoff λ < 420 nm) was used as the visible light source (light intensity: 8.5 mW cm−2). Phenol degradation was monitored by taking 3 mL aliquots after 0, 15, 30, 45, and 60 min irradiation, collecting the supernatant after centrifugation (Hermle Z 233) at 15 000 rpm for 15 min, and then determining the concentration of phenol from the characteristic absorption peak at λ = 270 nm from a spectrum (λ = 200−500 nm) obtained using a 1 cm quartz cuvette on a PerkinElmer Lambda 1050 UV/vis/NIR spectrophotometer. To evaluate the stability of the photocatalysts, powder samples after photocatalytic reaction were carefully collected by centrifugation (Beckman Coulter Allegra 25R centrifuge) at 8000 rpm for 10 min and washed with a water/ethanol mixture (1:1 v/v). The active species generated during photocatalysis were detected through trapping tests with EDTA-2Na and tBuOH.25 The trapping tests were conducted under the same conditions as the photocatalysis experiments, where EDTA2Na (2.92 g, 10 mmol) or t-BuOH (1 mL, 10 mmol) was added to the phenol aqueous solution containing the photocatalyst under ultrasonication for 15 min. All photocatalysis experiments were performed five times for reproducibility. Graphical results are presented as means and standard deviations after calculation.

Scheme 1. Schematic of the Nanocoating Strategy for the Preparation of TOCN-xa

a

Cyanamide solution was infiltrated into the mesoporous TiO2 bead and then calcined under N2 to form g-C3N4 within the porous structure.

of cyanamide was added (Figure S1b), whereas the composite microspheres were light yellow (Figure S1c), yellow (Figure S1d), and dark yellow (Figure S1e), as a result of the g-C3N4 layer coated on the TiO2 substrate. This yellow coloring matched that of mesoporous g-C3N4 (CN) (Figure S1f), prepared using cyanamide solution to nanocast mesoporous silica, SBA-15.28 The morphologies of the TO (Figure S2) and TOCN-x (Figure 1a−d) were observed by SEM. The TO

3. RESULTS AND DISCUSSION Amorphous TiO2 microspheres were first prepared by sol−gel chemistry in the presence of a structure directing agent, from which mesoporous anatase TiO2 microspheres (TO) were obtained via solvothermal treatment followed by calcination. Next, cyanamide aqueous solution was introduced as the gC3N4 precursor, which infiltrated the TO and coated it with a thin layer. Calcination at 550 °C under an inert nitrogen atmosphere yielded mesoporous composite TiO2/g-C3N4 microspheres labeled as TOCN-x, where x correlates to the 0.1, 0.5, 1, or 2 mL of cyanamide solution used during synthesis (see Scheme 1). Due to capillary action arising from the abundant mesopores within the TO host, the cyanamide solution easily infiltrated the mesoporous structure, resulting in widespread TiO2/g-C3N4 interfaces forming during calcination (discussed below). Moreover, this infiltration method improves the interfacial contact beyond what could be achieved by standard ball-milling or sol−gel chemistry approaches. By adding different amounts of cyanamide solution to the white TO powder (Figure S1a, Supporting Information) different colored samples were obtained. TOCN-0.1 was gray, implying that g-C3N4 was not formed when only a low quantity

Figure 1. SEM images of (a, b) TOCN-1 and (c, d) TOCN-2. (e) TEM image of a TiO2/g-C3N4 interface in TOCN-1 and detailed HRTEM images of (f) anatase TiO2 and (g) g-C3N4. C

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Figure 2. (a) TGA curves of TO, TOCN-0.1, TOCN-0.5, TOCN-1, and TOCN-2 obtained under oxygen flow. (b) XRD patterns with patterns offset up the intensity axis for clarity, (c) nitrogen sorption isotherms, and (d) pore size distributions determined by BJH analysis for TO, TOCN0.5, TOCN-1, TOCN-2, and CN.

microspheres had a uniform diameter of 2 μm. After infiltrating with cyanamide aqueous solution and thermally converting to g-C3N4, the morphology of TOCN-1 remained the same as the raw microspheres (Figure S2) without any obvious g-C3N4 aggregation on the outer surface (Figure 1a). From the SEM image of TOCN-1, the open porous structure remained visible after the coating procedure (Figure 1b). The particle size did not change when a higher amount of g-C3N4 precursor was loaded, i.e., TOCN-2 (Figure 1c). However, aggregated nanoparticles, derived from excess growth of g-C3N4, were observed on the surface (Figure 1d). TEM of TOCN-1 (Figure 1e) demonstrated that the crystal structures of both anatase TiO2 and g-C3N4 were present in the composite microspheres and in close proximity. By using HRTEM, lattice fringes of 3.5 and 2.1 Å were indexed to the (101) plane of anatase TiO2 (Figure 1f) and the (110) plane of g-C3N4 (Figure 1g), respectively. From the HRTEM image of the TiO2/g-C3N4 interface, the crystal lattices of TiO2 and g-C3N4 connected to form a heterojunction. Based on the electron microscopy studies, the aqueous cyanamide solution was thermally polymerized on the TiO2 surface, resulting in a thin g-C3N4 layer.20 The TGA curves of the fabricated TOCN-x conducted in an oxygen atmosphere are shown in Figure 2a. A small mass loss occurred in all samples below 150 °C, which was attributed to the loss of adsorbed water from the pristine TiO2 materials (TO). An additional mass loss was observed in TOCN-0.5, TOCN-1, and TOCN-2 from 150 to 200 °C, which was ascribed to the decomposition of the s-triazine moiety in gC3N4. Furthermore, the greater mass loss in TOCN-2 between 300 and 400 °C was attributed to the decomposition of g-C3N4

with its higher thermal stability. Calculated from the mass loss difference between TO and TOCN-x after full combustion at 400 °C, the organic contents of TOCN-0.5, TOCN-1, and TOCN-2 were about 1, 3, and 5 wt %, respectively. Unlike the other TOCN-x materials, TOCN-0.1, which was prepared with a small addition of cyanamide, had negligible organic content and the mass loss curve followed that of the pristine TiO2 material (TO) very closely. Figure 3b shows the XRD patterns of the different TiO2-based samples and CN. TO and TOCN-x exhibited well-defined peaks at 25.3°, 37.8°, 47.9°, 53.8°, 55.1°, 62.7°, 68.7°, 70.3°, and 75.0° in 2θ that were respectively indexed to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO2 (JCPDS card no. 21-1272). Moreover, the crystal size calculated from the (101) peak of TOCN-x by the Scherrer equation (approximately 7.3 nm) was almost the same as TO (approximately 7.8 nm), implying no severe crystal growth during the nanocoating process. By contrast, g-C3N4 exhibited two well-resolved peaks at 13.1° and 27.4° in 2θ, corresponding to the (100) crystal plane of tri s-triazine units, and the (002) crystal plane, the latter attributed to interlayer stacking of aromatic segments and unique to the g-C3N4 phase. After nanocoating with g-C3N4, the XRD patterns of TOCN-x showed no detectable peak that could be indexed to g-C3N4 due to its low content and high dispersion in the mesoporous microspheres. Nitrogen sorption isotherms (Figure 2c) of TO and TOCNx exhibited type IV adsorption branches with hysteresis loops between the H1 and H2 types, which were related to irregular mesopores. As pristine TiO2 microspheres, TO showed a broad distribution of pore diameters centered at 28.3 nm (Figure 2d and Table 1). After coating with a thin g-C3N4 layer, the pore D

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Figure 3. (a) UV−visible absorbance, (b) the relationship between the transformed Kubelka−Munk functions and the photon energies, and (c) PL spectra for TO, TOCN-0.1, TOCN-0.5, TOCN-1, TOCN-2, and CN. (d) EPR spectra of TO, TOCN-0.1, and TOCN-1.

CN showed a strong absorption peak at 807 cm−1 associated with the breathing mode of s-triazine units belonging to gC3N4, as well as multiple peaks between 1240 and 1600 cm−1 that were assigned to aromatic C−N stretching vibrational modes.31 A broad peak around 3300 cm−1 was attributed to uncondensed amino functional groups and adsorbed water in TOCN-0.5 and TOCN-1, which was not observable in CN. Owing to its higher organic content, the stronger peaks of the aromatic C−N vibrations were observed in TOCN-2, but the striazine peak at 807 cm−1 could not be identified due to its low intensity. The light absorption properties of TO, TOCN-x, and CN were characterized by UV−visible diffuse reflectance spectroscopy (Figure 3a). The light absorption of TO was limited to wavelengths lower than 390 nm; however, the infiltration of gC3N4 significantly altered this. Compared to TOCN-0.1 and TOCN-0.5, TOCN-1 and TOCN-2 exhibited light absorption that extended through the entire visible range and even had a higher absorbance than CN due to widespread TiO2/g-C3N4 heterojunctions. Furthermore, TOCN-2 exhibited a shoulder with a red shift to 600 nm in the visible range.28,29 The band gap energies of the semiconductors were estimated by the Kubelka−Munk transformation, ahv = A(hv − Eg)2, where a represents the absorption coefficient, h is Planck’s constant, ν is the light frequency, Eg is the band gap energy, and A is a constant. The relationship between the transformed Kubelka− Munk functions and the photon energies of the photocatalysts is depicted in Figure 3b. Due to heterojunction formation, the band gap energies of TO, TOCN-1, TOCN-2, and CN were determined as 3.2, 1.5, 1.7, and 2.7 eV, respectively. In order to further investigate the separation of photogenerated charge

Table 1. Physical Properties of TO, TOCN-0.5, TOCN-1, TOCN-2, and CN sample

pore sizea (nm)

surface areab (m2 g−1)

pore volumec (cm3 g−1)

TO TOCN-0.5 TOCN-1 TOCN-2 CN

28.3 26.3 25.1 21.0 7.1

64.8 64.5 64.4 64.7 210.9

0.31 0.30 0.30 0.28 0.37

a

Peak of the pore size distribution. bSpecific surface area based on the BET method. cTotal pore volume at P/P0 = 0.983 based on the BJH adsorption branch model.

sizes of TOCN-0.5, TOCN-1, and TOCN-2 decreased slightly to 26.3, 25.1, and 21.0 nm, respectively (Table 1), which coincided with g-C3N4 layers between 1 and 4 nm thick coated on the TiO2 surface. Moreover, the pore volume decreased from 0.31 cm3 g−1 for TO to 0.28 cm3 g−1 for TOCN-2 (Table 1). The surface areas of TO and TOCN-x were almost the same at approximately 64 m2 g−1 (Table 1). Different to TOCN-x, CN showed a type IV isotherm with an H2 hysteresis loop (Figure 2c), which was attributed to cylindrical mesopores and in keeping with CN templated by SBA-15.26 The narrow pore distribution was centered at 7.1 nm, with a pore volume of 0.37 cm3 g−1 and a surface area of 210.9 m2 g−1 (Figure 2d and Table 1). The FT-IR spectra of TO, TOCN-x, and CN indicated the presence of different species (Figure S3). As pristine TiO2, TO exhibited a wide absorption band at 500−900 cm−1 that was attributed to Ti−O stretching, while TOCN-0.1 showed a very similar absorption band without the unique peaks of g-C3N4. E

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negligible photoactivity, unlike the composites (TOCN-x) where g-C3N4 content dramatically influenced the photocatalytic activities compared to CN, especially of TOCN-0.5, TOCN-1, and TOCN-2. TOCN-1 exhibited optimal activity in degrading phenol with a reaction rate constant of 6.8 × 10−3 min−1, approximately 8.5 times higher than CN. In contrast, TOCN-0.1, without g-C3N4, exhibited photocatalytic activity even lower than the TOCN-0.5, due to the absence of the TiO 2/g-C3 N4 heterojunction. To evaluate the intrinsic adsorption of phenol and the useful material lifetime of TOCN-1, a recycling test was conducted under visible light irradiation. As shown in Figure 4b, there was negligible adsorption of phenol in the dark, and only a slight decrease of activity resulted after repeating four times. The morphological stability of TOCN-1 was investigated by SEM and little change was observed after the five cycles, which demonstrated the excellent mechanical durability of the mesoporous composite microspheres (Figure S5). The enhanced photoactivity of TOCN-1 and TOCN-2 over CN could be ascribed to the achieved lower band gap energy and faster mass transfer of phenol and the degradation products through the mesoporous structures due to the larger diameter pores. In addition, the heterojunctions formed at the TiO2/gC3N4 interface provided efficient channels for charge carrier separation. To detect the main oxidation species in the photocatalytic process, trapping experiments were conducted on TOCN-x to scavenge hydroxyl radicals generated from the photoexcited holes and oxygen radicals from the photoexcited electrons. t-BuOH was used as a radical scavenger, while EDTA-2Na was a hole scavenger.25 As shown in Figure 5a, the

carriers (photoinduced electron−hole pairs formed under irradiation) PL spectra of all of the samples were obtained (Figure 3c). The PL spectrum of TO in the range of 350−420 nm was ascribed to band−band emission and excitonic fluorescence resulting from surface oxygen vacancies and defects (Figure S4).31 TOCN-0.1 and TOCN-0.5 showed smaller peaks in a similar range, which could be associated with the same phenomena as TO. In contrast, CN showed a very strong, broad peak centered at 460 nm (Figure 3c), implying a high recombination rate of the photoinduced electron−hole pairs. Significantly, only negligible peaks were observed in all composites, which suggested a fast and efficient separation of the electron−hole pairs generated by TOCN-x.32 To evaluate the importance of the TiO2/g-C3N4 heterojunction in TOCNx, EPR was conducted, and the spectra are shown in Figure 3d.33 Pristine TiO2 microspheres (TO) showed an EPR signal indexed to surface defects, while a weaker signal was observed for TOCN-0.1 due to the high recombination of electron-hole pairs in the presence of the N dopant. Remarkably, stronger paramagnetic signals were detected for TOCN-1 implying efficient electron generation and separation through the TiO2/ g-C3N4 interface. The photocatalytic activity of TO, TOCN-x, and CN for degrading phenol under visible light irradiation is shown in Figure 4a; CN was considered as a reference material for the CN-containing composites. The photocatalytic degradation process was fitted to pseudo-first-order kinetics, and the value of the apparent rate constant, k, was determined from the slope of the fitted line. In Figure 4a, anatase TiO2 (TO) showed

Figure 4. (a) Photocatalytic decomposition of phenol over TO, TOCN-0.1, TOCN-0.5, TOCN-1, TOCN-2, and CN under visible light irradiation. (b) Recycling test of the TOCN-1 photocatalyst. The system was kept in the dark prior to testing.

Figure 5. Comparison between photocatalytic degradation of TOCN0.5, TOCN-1, and TOCN-2 with and without (a) the radical scavenger t-BuOH or (b) the hole scavenger EDTA-2Na. F

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Figure 6. XPS of TOCN-1 and TOCN-2: (a) Ti 2p, (b) C 1s, and (c) N 1s spectra. (d) Valence band XPS of TO, TOCN-1, and TOCN-2. The intensities of TOCN-1 and TOCN-2 were shifted up the intensity axis to distinguish their band edges.

photocatalytic activity in TOCN-x was greatly suppressed with t-BuOH addition, implying that a considerable amount of photogenerated electrons were involved in the photocatalytic reaction under visible light irradiation. Owing to the intrinsic potential, the photoexcited holes in the VB of g-C3N4 could not generate hydroxyl radicals. In contrast, the addition of EDTA2Na caused a small decrease in the photocatalytic activity of TOCN-x, indicating that photoexcited holes contributed less to the radicals required in this visible light photocatalytic reaction (Figure 5b). XPS was conducted to investigate the chemical composition and bonding states of the samples (Figure 6). The full survey spectrum (Figure S6a) of TOCN-1 indicated the presence of Ti, O, C, and N, while Ti, O, and C (trace amounts) were present in TO. Owing to the low content of g-C3N4 in TOCN0.5, signals corresponding to g-C3N4 were not conclusively identified in the XPS results and have been excluded. The titanium spectra of TOCN-x illustrated in Figure 6a showed two main Ti 2p peaks centered at binding energies of 458.7 and 464.6 eV, which can be assigned to Ti 2p 3/2 and Ti 2p 1/2 of anatase TiO2. The very weak peaks centered at 457.4 and 460.6 eV were ascribed to Ti3+ bonded with N.30 The peak area corresponding to the Ti−N bond was greater for TOCN-1 than TOCN-2, indicating that more Ti3+ at the interface had bonded with N in TOCN-1 (Table 2). In addition, the titanium spectrum of TOCN-0.1 showed a weak peak located at 457.3 eV (Figure S6b) that was also indexed to the Ti−N bond. The carbon spectra of the TOCN-x samples were fitted to two major peaks, with binding energies located at 285.0 and 288.4 eV (Figure 6b). The former was assigned to sp2 C atoms in the graphitic structure and the latter to N−CN coordination in the triazine aromatic ring attached to NH2 groups.17,29 A third

Table 2. Peak Area Percentage Derived from Chemical Bonds in the XPS Spectra sample

Ti−O (% in Ti 2p)

Ti−N (% in Ti 2p)

C−N (% in C 1s)

N−Ti (% in N 1s)

TO TOCN-1 TOCN-2

100 89.8 93.3

0 10.2 6.7

N/A 15.9 9.8

N/A 6.1 4.4

peak observed at 286.5 eV was assigned to C−N coordination between the aromatic rings and the TiO2 surface.23,31 The nitrogen spectra showed three divided peaks centered at 396.9, 398.6, and 400.1 eV (Figure 6c). The second peak was assigned to the CN−C coordination in the aromatic ring, the third peak was related to amino groups, while the broad first peak was assigned to N−Ti bonds formed at the interface. Similar to the Ti spectra, peak areas derived from N−Ti bonding showed higher contributions in the N and C spectra of TOCN-1 than in TOCN-2 (Table 2). Unlike the N spectrum of g-C3N4 presented by Cao et al.,17 no peak around 399.6 eV for a trigonally bonded N atom could be identified for TOCN-1 in Figure 6c, while TOCN-2 revealed only a weak peak at this binding energy. The valence bands of TO, TOCN-1, and TOCN-2 were determined from Figure 6d, with band edges of 2.8, 1.8, and 1.6 eV, respectively. Thus, the presence of g-C3N4 in the composites shifted the main absorption onset to a lower binding energy. A proposed mechanism for charge transfer through the TiO2/g-C3N4 heterojunction during phenol degradation under visible light irradiation is shown in Scheme 2. The heterojunction was formed inside the anatase TiO2 structure, after infiltration of the cyanamide and calcination under nitrogen. Different from the layer structure observed in bulk G

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spheres. XPS of the composites indicated that amino groups of the organic precursor and intermediates had reacted with the TiO2 surface to induce N doping, thus forming an extra energy level between the CB and VB of TiO2 and resulting in a reduced band gap (1.5 eV) due to N doping. In turn, this led to improved visible light absorption and photocatalytic properties, including the fast and efficient separation of photoinduced electron−hole pairs. The surface area (64.4 m2 g−1) and pore volume (0.30 cm3 g−1) of the pristine TiO2 microspheres were maintained in the mesoporous TiO2/g-C3N4 microspheres, with only a slight decrease in pore diameter (25.1 nm). The mesoporous TiO2/g-C3N4 microspheres were photoactive in the visible range, showing potential for decomposing organic pollutants in aqueous environmental remediation.

Scheme 2. Proposed Mechanism for Charge Transfer through the Heterojunction at the TiO2/g-C3N4 Interface under Visible Light Irradiation

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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.7b06493. Optical photos, FT-IR, and detailed (350−410 nm) PL spectra of mesoporous TiO2, TiO2/g-C3N4 microspheres, and g-C3N4; SEM image of mesoporous TiO2 microspheres without g-C3N4 and TiO2/g-C3N4 microspheres (TOCN-1) after repeating the photocatalysis reaction four times; XPS survey spectra of mesoporous TiO2 and TiO2/g-C3N4 microspheres, and mesoporous g-C 3 N 4 , and Ti 2P spectra of TOCN-0.1 and summarized photocatalytic performance of TiO2/gC3N4 composites reported previously. (PDF)

g-C3N4,17,18,20 the growth of the g-C3N4 molecular structure was hindered by limited cyanamide infiltration within the TiO2 mesopores, and only a thin layer was obtained on the TiO2 surface. In addition, the amino groups of the cyanamide precursor and intermediate products (e.g., s-triazine and melem) could react with TiO2 to substitute N in the lattice, forming TiO2−xNx during calcination. This was demonstrated in the XPS results (Figure 6d), with the creation of an extra N 2p level above the VB maximum of pristine TiO2, effectively narrowing the band gap (Scheme 2).4,34,35 Consequently, with widespread TiO2/g-C3N4 heterojunctions throughout the interconnected mesoporous structure, both the g-C3N4 and TiO2−xNx species were capable of generating electron−hole pairs under visible light irradiation (Scheme 2). Electrons in the CB of g-C3N4 could transfer to the CB of TiO2−xNx through the interface, while holes in the substitutional N level could transfer to the VB of g-C3N4, resulting in separation of the charge carriers. Furthermore, excited electrons located in the CBs of TiO2−xNx and g-C3N4 can convert O2 to oxygen radicals for further phenol oxidation, while phenol molecules were also degraded by hydroxyl radicals derived from holes in the VB of TiO2−xNx. Together, the heterojunction and N level in TiO2−xNx endowed the final TOCN-x composites, especially TOCN-1, with an excellent response to visible light and enhanced photocatalytic activity compared with previous reports (Table S1). Moreover, the unique mesoporous structure with spherical morphology provided an abundant photocatalytic interface for charge separation and a fast pathway for mass transfer.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +61 2 6125 3536. *E-mail: [email protected]. Tel: +61 3 9925 2146. ORCID

Rachel A. Caruso: 0000-0003-4922-2256 Present Addresses §

Research School of Chemistry, The Australian National University, Canberra ACT 2601, Australia. ∥ RMIT University, Melbourne, Victoria 3000, Australia. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS Mesoporous TiO2/g-C3N4 microspheres were fabricated through the infiltration and thermal conversion of a cyanamide solution to g-C3N4 within a porous TiO2 material. The composites exhibited TiO2/g-C3N4 heterojunctions throughout the interconnected structure while maintaining mesoporosity. Benefiting from the heterojunctions formed at the interface between the anatase TiO2 and g-C3N4, photocatalytic activity for the degradation of phenol under visible light was 8.5 times higher than that of mesoporous g-C3N4, with the activity maintained across five cycles. This enhancement resulted from a 3 wt % g-C3N4 loading in the mesoporous TiO2 micro-

ACKNOWLEDGMENTS The Melbourne Advanced Microscopy Facility at The University of Melbourne provided access to electron microscopes. H.W. acknowledges a Melbourne International Research Scholarship (MIRS), and R.A.C. was supported by an Australian Research Council Future Fellowship (FT0990583). The schematics were prepared with assistance from Xiaobo Zheng. Drs. David Parris and Yasmina Dkhissi provided valuable comments in the preparation of this manuscript. H

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