Enhancing Solar Light-Driven Photocatalytic Activity of Mesoporous

Nov 23, 2017 - Our novel hybrid nanostructure and its underlined synthesis strategy reflect a promising route to improve solar energy utilization in e...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 1310−1317

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Enhancing Solar Light-Driven Photocatalytic Activity of Mesoporous Carbon−TiO2 Hybrid Films via Upconversion Coupling Hannah Kwon,† Filipe Marques Mota,† Kyungwha Chung,† Yu Jin Jang,† Jerome K. Hyun,† Jiseok Lee,‡ and Dong Ha Kim*,† †

Department of Chemistry and Nano Science, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea ‡ School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, Eonyang-eup, Ulju-gun, Ulsan 689-798, Korea S Supporting Information *

ABSTRACT: Solar energy conversion has emerged as an attractive pathway in the decomposition of hazardous organic pollutants. Herein, tridoped β-NaYF4:Yb3+,Tm3+,Gd3+ upconversion (UC) nanorods were embedded in a carbon-doped mesostructured TiO2 hybrid film using triblock copolymer P123 acting as a mesoporous template and carbon source. The photoactivity of our novel material was reflected in the degradation of nitrobenzene, as a representative organic waste. The broad-band absorption of our rationally designed UC nanorod-embedded C-doped TiO2 in the UV to NIR range unveiled a remarkable increase in nitrobenzene degradation (83%) within 3 h compared with pristine TiO2 (50%) upon light irradiation. These results establish for the first time a synergetic bridge between the effects of a creative photon trapping TiO2 architecture, improved NIR light-harvesting efficiency upon UC nanorod incorporation, and a simultaneous decrease in the band gap energy and increased visible light absorption by C-doping of the oxide lattice. The resulting nanostructure was believed to favor efficient charge and energy transfer between the photocatalyst components and to reduce charge recombination. Our novel hybrid nanostructure and its underlined synthesis strategy reflect a promising route to improve solar energy utilization in environmental remediation and in a wide range of photocatalytic applications, e.g., water splitting, CO2 reutilization, and production of fuels. KEYWORDS: UV−vis−NIR photocatalysis, Solar energy, Upconversion nanoparticles, TiO2 mesostructure, Carbon−TiO2 hybrid, Nitrobenzene degradation

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INTRODUCTION

visible spectrum and, therefore, to extend the photoactivity of the resulting material. With carbon-doped TiO2, the conduction band remains above the redox potential as carbon moieties behave as sensitizers, enhancing a wide range of light absorption.20 Throughout the years, a wide number of approaches have showcased the incorporation of carbon moieties in metal oxides. Block copolymers (BCPs) have been shown to be simultaneously suitable as carbon precursors and as soft templates in the design of creative mesoporous architectures.21−25 Commercially available structure-directing agents, e.g., amphiphilic poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) triblock copolymer Pluronic P123, have been widely used in the synthesis of mesostructured TiO2 (mTiO2).26 Alternatively, following a simple one-step carbonization procedure, Pluronic P123 has been simulta-

In the context of environmental remediation as a critical issue in global society, the development of highly efficient photocatalysts has been actively pursued for the degradation of environmental organic pollutants.1−7 In this context, TiO2 has emerged as a representative material owing to its nontoxicity, thermal and photostability and cost effectiveness.8−11 A relatively large band gap (3.2 eV) with absorption in the ultraviolet (UV) region, which corresponds to ca. 5% of the total solar light spectrum, represents however a critical bottleneck in the enhancement of the photoactivity of TiO2. Strategies to utilize TiO2 under visible and near-infrared (NIR) illumination are in this sense of crucial interest.12 To induce TiO2 excitation by low-energy irradiation, the incorporation of dyes or quantum dots with low Eg, doping with metal and nonmetal elements, e.g., nitrogen, sulfur, and carbon, hybridization, and coupling with other semiconductors have been reported.13−19 Carbon doping reflects a facile strategy to reduce the band gap of TiO2 to coincide with the © 2017 American Chemical Society

Received: October 10, 2017 Revised: November 14, 2017 Published: November 23, 2017 1310

DOI: 10.1021/acssuschemeng.7b03658 ACS Sustainable Chem. Eng. 2018, 6, 1310−1317

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ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Diagram for the Fabrication of mTiO2, mC−TiO2, and UCN-Embedded mC−TiO2 Hybrid Films

process. NaOH solution (3 mL), ethanol (10 mL), and oleic acid (10 mL) were added to the Teflon reactor. Rare-earth chlorides, Gd3+, Y3+, Yb3+, and Tm3+ (0.2 M, 30:51.8:18:0.2), were dissolved in NH4F (2 M, 2 mL) and mixed with the above solutions under vigorous stirring for 30 min. The chamber was sealed and transferred to a stainless steel autoclave, following heat treatment at 200 °C for 4 h. After cooling to room temperature, UCNs were collected by centrifugation and sequentially washed with ethanol and water. Fabrication of Hybrid TiO2 Thin Films. The details of the preparation of TiO2 precursor solution herein outlined have been reported elsewhere.26 The TiO2 sol−gel precursor was prepared by mixing titanium isopropoxide (1.05 g) and hydrochloric acid (0.74 g) under vigorous stirring for 20 min. The sol−gel solution was dissolved into Pluronic P123 (0.2 g) in ethanol (3.0 g) and stirred for an additional 30 min period. In the fabrication of a mesoporous TiO2 thin film (mTiO2), the mixture solution of P123 and TiO2 precursor was spin coated at 2000 rpm for 2 min on the FTO substrate. The resulting material was treated at 450 °C for 4 h under air flow. Alternatively, a carbon-doped mesoporous TiO2 counterpart (mC− TiO2) was obtained at the same temperature for 4 h under an Ar atmosphere. Under these conditions, P123 was confirmed to be carbonized, leading to the presence of carbon moieties in the resulting architecture. In the synthesis of tridoped NaYF4:Yb3+,Tm3+,Gd3+ UCNs embedded in a carbon-doped mesostructured TiO2 hybrid film (mC−TiO2/UCN), the UCN powder was first dispersed in the mixture solution of P123 and TiO2 precursor. The resulting solution was spin coated at 800 rpm for 1 min on the FTO substrate, followed by heat treatment under Ar, as described above for the synthesis of mC−TiO2. Powder-based samples were additionally prepared for supplementary characterization (e.g., TEM). Photocatalytic Measurements. The photocatalytic degradation of NB over mTiO2, mC−TiO2, and mC−TiO2/UCN hybrid films was tested under UV−vis−NIR light irradiation (one-sun). The hybrid films were immersed in 30 mL of a 10 ppm of NB solution and magnetically stirred under dark conditions for 30 min prior to the reaction. Samples were illuminated using a Xe lamp (200−2500 nm range, Newport Co. Ltd., Model 66984) equipped with a 420 nm cutoff filter at 300 W as a visible light source. Alternatively, a 980 nm laser excitation beam was used as a NIR light source. The absorbance of the characteristic peak of NB at 270 nm of periodically collected samples was monitored in each case using UV−vis spectroscopy. The photocatalytic activity of each sample was determined according to the gradual decrease of the peak of NB. Instruments and Measurements. The surface morphologies of hybrid films were investigated with scanning electron microscopy (SEM; JEOL JSM6700-F) and atomic force microscope (AFM) in tapping mode (Digital Instruments Dimension 3100 scanning force microscope). Transmission electron microscopy (TEM) was carried out on a JEOL JSM2100-F microscope at 100 K, and elemental analysis was performed by energy-dispersive X-ray spectroscopy

neously reported by some of us in the synthesis of carbondoped mTiO2.27−32 In recent years, the doping of upconversion nanocrystals (UCNs) has emerged as a highly active research area toward feasible utilization of infrared (IR) and NIR light.33−42 In particular, lanthanides-doped UCNs have been shown to convert low-energy photons to higher-energy radiation in the visible and UV range via an anti-Stokes emission process. βNaYF4 has been underlined as the most efficient host material for luminescence,43,44 with selected lanthanide ion dopants acting as a sensitizer (Yb3+) or activator (Tm3+, Er3+, and Ho3+).45−47 Whereas the effect of carbon doping and the incorporation of UCN materials have separately opened new possibilities in TiO2-based photocatalytic applications, to date, no attempts to simultaneously consider both promising strategies have been made. Herein, previous literature reports inspired the synthesis of novel β-NaYF4:Yb3+,Tm3+,Gd3+ UCNs embedded in a carbon-doped mesostructured TiO2 film, in an attempt to harvest high-energy photons and maximize the photocatalytic performance of the resulting hybrid nanostructures under UV− vis−NIR light illumination. We chose the tridoped NaYF4:Yb3+,Tm3+,Gd3+ because of its efficient UC luminescence in the UV−vis range and well-established synthesis methods. Pluronic P123 was used as a structure-directing template and carbon source by direct carbonization of BCP. The strategic integration of a well-defined and interconnected mesoporous system was expected to lead to an improved charge carrier transfer efficiency and enhanced mass flow in chemical reaction.48 The monodispersed UC nanorods were embedded in the mesoporous carbon−TiO2 hybrid thin film (mC−TiO2/ UCN) by spin-coating and heat treatment at 450 °C under argon flow. The photocatalytic performance of our novel hybrid nanomaterials was investigated in the degradation of nitrobenzene (NB), as a representative hazardous organic waste, and used to shed additional light on the proposed reaction mechanism.

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EXPERIMENTAL SECTION

Chemicals. Titanium isopropoxide, Pluronic P123, nitrobenzene (NB), yttrium(III) chloride hexahydrate, thulium(III) chloride hexahydrate, gadolinium(III) chloride hexahydrate, ammonium fluoride, oleic acid, and sodium hydroxide were purchased from Sigma-Aldrich. Synthesis of β-NaYF4:Yb3+,Tm3+,Gd3+ Nanorods. β-NaYF4:Yb3+,Tm3+,Gd3+ nanorods were prepared by a hydrothermal 1311

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Scheme 2. (a) Energy Level Diagram of Yb3+, Tm3+, Gd3+, and TiO2, and the Corresponding Observed UC Emission Process under 980 nm Laser Irradiation and (b) Illustrative Diagram of Energy Transfer among Yb3+, Tm3+, Gd3+, and mC−TiO2

(EDX) at an accelerating voltage of 200 kV (JEOL JEM-3011). The crystalline structures of the mTiO2, mC−TiO2, and mC−TiO2/UCN were investigated by X-ray diffraction (XRD) Cu−Kα radiation (D/ max RA, Rigaku Co). Raman scattering spectra were recorded on a LabRam HREvo 800 (HORIABA Jobin Yvon, France). UV−vis absorbance was measured using a Varian Cary5000 UV−vis−NIR spectrophotometer.

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RESULTS AND DISCUSSION Characterization of the Hybrid Nanostructures. The synthesis procedure for the fabrication of mTiO2, mC−TiO2, and mC−TiO2/UCN has been conveniently summarized in Scheme 1. mTiO2 was prepared by spin coating, using a solution of both P123 and TiO2 precursors, followed by calcination at 450 °C under air atmosphere. mC−TiO2 was alternatively synthesized by carbonization at the same temperature under Ar flow. For the synthesis of mC−TiO2/UCN, UCN was mixed in advanced with a solution containing P123 and the TiO2 precursor, spin-coated, and carbonized as mentioned above. In Figure 1a, XRD patterns of the UC nanorods synthesized by a hydrothermal process were confirmed to agree with those

Upon 980 nm light irradiation, the population of the 2F5/2 level of Yb3+ has been proposed to lead to subsequent excitation of Tm3+ ions to populate the 3H5, 3F2, and 1G4 levels. The photoluminescence spectrum herein obtained is in accordance with previous reports for the UCN material applied in this work. Reported peaks yielded in the UV region at 290, 345, and 361 nm ascribed to Tm3+ transitions upon NIR light illumination could however not be observed as a result of our equipment limitation.52 These peaks have been ascribed to high-energy 1I6 → 3H6, 1I6 → 3F4, and 1D2 → 3H6 transitions, respectively. Similarly reported minor peaks in the UV region (270−281, 305, and 311 nm) accounting for Gd3+ transitions could not be detected. Gd3+ has been shown to be unable to directly absorb the incident 980 nm irradiation due to the large energy gap between 8S7/2 and 6PJ.52 Instead, Tm3+ acts as a sensitizer for Gd3+, with energy transfer from Tm3+ in the 3P2 level to Gd3+ to stimulate its excitation through energy transfer of 3P2 → 3H6. Overall comparison of the morphology of the synthesized hybrid nanostructures was assessed by representative SEM and TEM photographs and discussed below. SEM images of P123/ TiO2 precursor with UCN film spin-coated on the FTO revealed the presence of a relatively smooth surface film with fully dispersed UC nanorods (Figure S2). Figure 2a−c depicts the morphology of mTiO2, mC−TiO2, and UCN-embedded mC−TiO2 after corresponding calcination and carbonization treatments at 450 °C on FTO. A relatively rough and uneven surface is noticeable with the mC−TiO2/UCN hybrid film following the high-temperature heat treatment (Figure 2f). The result was corroborated in a series of height-constant AFM

Figure 1. Characterization of the prepared UC nanorods. (a) X-ray diffraction patterns, (b) SEM image, (c) photoluminescence spectrum, and (d) optical photograph under 980 nm laser excitation.

referenced for the pure hexagonal β-NaYF4 phase (JCPDS 160334).49,50 The incorporation of Gd ion dopants with increasing concentration has been shown to facilitate the transition from cubic to hexagonal phases.51 According to representative SEM images, well-dispersed UCN displayed uniform rod crystal morphology with an average size of 340 ± 33 nm in length and 43 nm in diameter (Figure 1b). The particle size distribution is provided in Figure S1. Synthesized UC nanorods were shown to effectively generate UC luminescence via a multiphoton process. Under 980 nm laser excitation, the bright blue light of the tridoped UCN system was noticeable to the naked eye (Figure 1d). The photoluminescence spectrum of UCN revealed two strong blue emission peaks centered at 450 and 474 nm, ascribed to 1D2 → 3 F4 and 1G4 → 3H6, respectively (Figure 1c). Observed peaks centered at 450, 474, 645, 695, and 802 nm of UCNs are attributed to Tm3+ transitions of 1D2 → 3F4, 1G4 → 3H6, 1G4 → 3 F4, 3F3 → 3H6, and 3H4 → 3H6 (Figure 1c and Scheme 2a). 1312

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sectional SEM images further confirmed a relatively similar layer thickness for all samples on the substrate (Figure S5). Above-mentioned conclusions were corroborated by corresponding TEM (Figure 2d−f) and HR-TEM photographs (Figure S6). In good agreement with XRD results, collected HR-TEM images confirmed the presence of a TiO2 anatasecrystalline phase. Additional EDX spectra of mC−TiO2/UCN were collected to assess the chemical composition of hybrid nanostructures (Figure S7). Distinctive peaks ascribed to C, O, and Ti confirmed the presence of TiO2 doped with carbon moieties. The presence of Y, Yb, Tm, Gd, Na, and F in mC− TiO2/UCN was clear upon the incorporation of UCN. Corresponding weight percentages were determined for all present elements (Figure S7). In particular, the amount of carbon in the carbonized TiO2 architecture was ca. 3 wt %, in agreement with previous reports evidencing a corresponding maximum of 5 wt % following the carbonization of P123 under an argon atmosphere for the selected synthesis conditions. Xray elemental mapping analysis of mC−TiO2/UCN evidenced the presence and fine dispersion of C, Ti, and O in mC−TiO2 and F, Na, Y, Gd, Tm, and Yb from the UC nanorods (Figure S8). The XRD patterns of all samples agreed with the presence of a TiO2 anatase-crystalline phase (JCPDS 21-1272) (Figure 3a). The presence of XRD peaks for UCNs confirms the successful incorporation of these nanorods in the hybrid nanostructure following heat treatment at 450 °C. Raman spectroscopy was performed to assess the physicochemical nature of carbon moieties derived from calcination of P123 in the synthesized TiO2 mesostructure (Figure 3b). Obtained spectra of mC− TiO2 and mC−TiO2/UCN yielded two bands at around 1600 and 1350 cm−1. The two characteristic peaks were respectively assigned to the G band, corresponding to nanocrystalline carbon characteristics with a high content of sp2-hybridized carbon, and the D band, corresponding to the defects of disordered carbon. mC−TiO2 showed higher absorption across the visible region, with the incorporation of carbon moieties extending the absorption properties of the metal oxide (Figure 3c). Collected UV−vis diffuse reflectance spectra revealed strong reflectance in the visible range for all samples, with mC− TiO2/UCN exhibiting the highest reflectivity following the incorporation of UCN into the mC−TiO2 (Figure S9a). Estimated from the intercept of tangents with the x-axis of the Tauc plot (Figure S9b), relatively low band gap energy values were obtained for both mC−TiO2 (∼2.8 eV) and mC−TiO2/ UCN (∼2.9 eV), compared with ∼3.2 eV commonly reported for anatase-TiO2. The result served as further evidence for the incorporation of carbon moieties, which contributes to the visible light absorption of pristine TiO2. Through corresponding Mott−Schottky plots, the flat band potential of our mC−

Figure 2. SEM images of (a) mTiO2, (b) mC−TiO2, and (c) mC− TiO2/UCN, corresponding optical photographs of the resulting samples (insets), and representative TEM photographs (d−f, respectively).

images of as-cast P123/TiO2 precursor films with or without embedded UC nanorods and corresponding counterparts following carbonization at 450 °C under Ar (Figure S3). The heat treatment under Ar flow was suggested to yield the formation of carbon moieties by the variation of color of the resulting nanostructures (insets in Figure 2) and following a slight decrease of the high surface area attained with these mTiO2 porous systems (from 114 to 75 m2 g−1). In our previous studies, XPS spectra comparison from the C 1s core level has additionally underlined the extent of C−O−C species located at 286.2 eV in comparison to mTiO2 as supporting proof for the incorporation of carbon species in the oxide lattice (Figure S4).32 On the basis of the SEM photographs, no morphological changes could be evidenced in the embedded UC nanorods following the carbonization of mC−TiO2. Cross-

Figure 3. (a) XRD patterns of UCN, mTiO2, mC−TiO2, and mC−TiO2/UCN. (b) Collected Raman spectra of mC−TiO2 and mC−TiO2/UCN. (c) UV−vis absorbance spectra of the prepared materials. 1313

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presence of carbon species acting as a photosensitizer under visible irradiation was ascribed to the reduced band gap of the resulting hybrid materials. In good agreement, the incorporation of NIR-responsive UC nanorods remained ineffective under visible light irradiation, with mC−TiO2/UCN attaining similar conversion levels (57%). Similarly, to provide clear insight of the photoactivity enhancement upon incorporation of UCN, additional experimental data was collected using a 980 nm NIR laser (Figure 4c). Under NIR illumination, both mTiO2 and mC−TiO2 yielded similar photocatalytic activity after 3 h (32%). After the same time period, a notable increase in the photodegradation of NB up to 55% was obtained in the presence of mC−TiO2/UCN. The observed increase in activity is proposed to be the result of the absorption of NIR irradiation by the UC nanorods embedded in the hybrid nanostructure. Effective NIR absorption is believed to result in a subsequent transfer of highenergy UV irradiation to TiO2 and in an enhancement of the corresponding photocatalytic properties.34 Our novel nanohybrid structure was last evaluated under UV−vis−NIR irradiation to unveil the possibility of a cooperative effect for the incorporation of both UCN and C moieties in the high-surface TiO2 architecture (Figure 4d). In agreement with Figure 4b, which suggested that the incorporation of carbon moieties could be responsible for visible light-driven photocatalytic performance, under UV−vis− NIR irradiation, mC−TiO2 showed NB degradation up to 63% within 3 h compared with mTiO2 (50%). Most notably, the subsequent incorporation of UC nanorods in mC−TiO2/UCN resulted in a remarkable boost up to 83% in the degradation of NB within the same time period. In the presence of embedded NIR-responsive UC nanorods, strong UC luminescence emitted in the UV−vis range upon excitation could be transferred to C-doped mTiO2, as suggested in Figure 4c. The incorporation of UCN and carbon moieties in mC−TiO2/ UCN is confirmed to utilize a greater range of the solar spectrum in the degradation of NB. The dramatic photoactivity enhancement of TiO2 from 50 to 83% may further suggest a synergism established between the nanocomponents of our novel material. In an analogous optic, when the irradiated light was switched from solely vis (Figure 4b) to UV−vis−NIR (Figure 4d) over mC−TiO2/UCN, enhancement of the NB degradation from 57 to 83% was witnessed, in accordance with (1) a superior catalytic contribution ascribed to TiO2 under UV light and (2) a superior catalytic performance ascribed to NIRresponsive UCN incorporation. While in photocatalytic applications establishing a direct and unambiguous comparison of the reported system efficiencies remains of particular difficulty, we believe that our rationally designed architecture remains competitive in current representative reports.55,56,58,59 Conversely, superior activity was found following simple carbon incorporation during P123 carbonization, whereas NIRresponsive catalysts remain relatively unexplored in this application. The proposed mechanism of energy transfer between UCN and mC−TiO2 has been conveniently summarized in Scheme 2b. In the degradation of NB upon light irradiation, a number of reductive electrons and oxidative holes are generated in the conduction band (CB) and the valence band (VB) of TiO2. The excited electrons react with the oxygen adsorbed on the surface of TiO2 to form superoxide radical anions (•O2−), whereas the photogenerated holes can react with H2O to form hydroxyl radicals (•OH), which can oxidize the organic NB

TiO2 has been previously reported to be shifted to a more negative value by 0.4 V, indicating that the Fermi level of these hybrid materials could be displaced closer to the conduction band of TiO2.32 The creation of an additional electronic state above the valence band of TiO2 as represented in Scheme 2b is hence herein proposed. Photocatalytic Evaluation. To assess the potential interest of these hybrid materials, the photoactivity of mTiO2, mC− TiO2, and mC−TiO2/UCN under visible, NIR, and UV−vis− NIR illumination was herein evaluated. The decomposition of NB at room temperature was selected as a model reaction in the context of the degradation of organic pollutants.53−55 All samples were stabilized under dark conditions for 30 min prior to light irradiation. In each case, the absorption intensity of the characteristic peak of NB at 270 nm was observed to gradually decrease with increasing irradiation time (cf. Supporting Information). For a quantitative comparison, the photoconversion of each sample as a function of time under different light sources was calculated according to corresponding normalized absorbance values (Figure 4). The degradation of

Figure 4. Photocatalytic conversion (%) as a function of the irradiation time (min) for the degradation of NB at room temperature under (a) dark conditions, (b) visible light, (c) 980 nm laser, and (d) UV−vis−NIR light irradiation over mTiO2, mC−TiO2, and mC− TiO2/UCN.

NB over the prepared hybrid nanostructures in the dark was observed to attain maximal conversion levels of 18% after 3 h (Figure 4a). Results were tentatively ascribed to adsorption of the molecule on the surface of the catalyst, as highlighted in analogous reports.56,57 Under UV−vis−NIR light irradiation in the absence of any photocatalyst, the decomposition of NB reached 28% after the same time period (Figure S10). The photoactivity of the prepared nanostructures was first assessed under visible light irradiation at >420 nm (Figure 4b). For the degradation of NB after 3 h, conversion values of 46, 60, and 57% were obtained with mTiO2, mC−TiO2, and mC− TiO2/UCN, respectively. When compared with results under dark and under irradiation in the absence of any catalyst, mTiO2 revealed a negligible photocatalytic contribution due to the inherently wide band gap of the metal oxide. The incorporation of carbon moieties notably increased the catalytic activity of mC−TiO2. Excitation by low-energy photons in the 1314

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ACS Sustainable Chemistry & Engineering molecules through a photocatalytic-driven mechanism. Herein, the rational design of our TiO2 architecture reflects the interest of photon trapping toward an effective optical path length, triggered by scattering effects. In the presence of embedded NIR-responsive UC nanorods, strong UC luminescence in the UV−vis range upon excitation is emitted. According to a suitable band alignment, the emitted UV radiation can be directly transferred from 1D2 and 1I6 levels of Tm3+ into anatase TiO2, as depicted in Scheme 2b. The energy transfer additionally accelerates the relaxation of the excited states of Tm3+, increasing the overall transition rates. In this optic, UCN-embedded TiO2 could be activated to produce reductive electrons and oxidative holes in the CB and VB not only from directly irradiated UV light but also from additional UC luminescence triggered by the presence of UC nanorods. Strong UC emissions in the visible range are to be further taken into account upon incorporation of carbon moieties in the oxide lattice following P123 carbonization. Nonetheless, aside from decreasing the band gap of TiO2 and extending the absorption range into the visible region, the incorporated carbon moieties may also facilitate an effective charge separation. By accepting the electrons generated from the conduction band of TiO2, this process minimizes electron−hole recombination, therefore increasing the photoactivity of the resulting hybrid nanostructure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hannah Kwon: 0000-0003-2107-575X Filipe Marques Mota: 0000-0002-0928-3583 Kyungwha Chung: 0000-0002-6774-4720 Yu Jin Jang: 0000-0001-8116-3618 Jerome K. Hyun: 0000-0002-2630-5051 Jiseok Lee: 0000-0002-5762-6085 Dong Ha Kim: 0000-0003-0444-0479 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (2017R1A2A1A05022387; 2011-0030255), National Research Council of Science and Technology through the Degree & Research Center program (DRC-2014-1-KBSI), “The Project of Conversion by the Past R&D Results” through the Ministry of Trade, Industry and Energy (MOTIE), and the Korea Institute for Advancement of Technology (KIAT) (N0002202, 2016).

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CONCLUSIONS Herein, we demonstrated an effective strategy to enhance the photoactivity of TiO2 under UV−vis−NIR irradiation through simultaneous incorporation of UC nanorods and C moieties doped by facile carbonization of P123. UCN absorbing longwavelength radiation in the NIR region produced emission peaks in the UV−vis range. The resulting broad-band absorption showed a remarkable increase in the photoactivity of TiO2 under UV−vis−NIR in the degradation of NB (83%) compared to a pristine reference (50%). Results were attributed to a cooperative effect of the photon trapping TiO 2 architecture, improved light-harvesting efficiency upon UCN incorporation, and a simultaneous decrease in the band gap energy and increased visible light absorption over the fabricated C-integrated TiO2 structure. The possibility of harvesting lost photons in TiO2 through simultaneous carbon doping and UC strategies is reported herein for the first time and expected to underline the interest of enhancing the broad-band activity of photocatalysts for environmental remediation and in a number of chemical processes of current importance.

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degradation of NB under dark conditions, in the absence of a photocatalyst, and under visible, NIR (λ = 980 nm), and UV−vis−NIR light irradiation (PDF)

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REFERENCES

(1) Yoon, M.; Oh, Y.; Hong, S.; Lee, J. S.; Boppella, R.; Kim, S. H.; Marques Mota, F.; Kim, S. O.; Kim, D. H. Synergistically enhanced photocatalytic activity of graphitic carbon nitride and WO3 nanohybrids mediated by photo-Fenton reaction and H2O2. Appl. Catal., B 2017, 206, 263−270. (2) Yoon, M.; Lee, J.-E.; Jang, Y. J.; Lim, J. W.; Rani, A.; Kim, D. H. Comprehensive study on the controlled plasmon-enhanced photocatalytic activity of hybrid Au/ZnO systems mediated by thermoresponsive polymer linkers. ACS Appl. Mater. Interfaces 2015, 7, 21073− 21081. (3) Bizani, E.; Fytianos, K.; Poulios, I.; Tsiridis, V. Photocatalytic decolorization and degradation of dye solutions and wastewaters in the presence of titanium dioxide. J. Hazard. Mater. 2006, 136, 85−94. (4) Reijnders, L. Hazard reduction for the application of titania nanoparticles in environmental technology. J. Hazard. Mater. 2008, 152, 440−445. (5) Athanasekou, C.; Romanos, G. E.; Papageorgiou, S.; Manolis, G.; Katsaros, F.; Falaras, P. Photocatalytic degradation of hexavalent chromium emerging contaminant via advanced titanium dioxide nanostructures. Chem. Eng. J. 2017, 318, 171−180. (6) Wang, C.-C.; Du, X.-D.; Li, J.; Guo, X.-X.; Wang, P.; Zhang, J. Photocatalytic Cr (VI) reduction in metal-organic frameworks: A minireview. Appl. Catal., B 2016, 193, 198−216. (7) Chen, C.; Ma, W.; Zhao, J. Semiconductor-mediated photodegradation of pollutants under visible-light irradiation. Chem. Soc. Rev. 2010, 39, 4206−4219. (8) Gratzel, M. Photoelectrochemical cells. Nature 2001, 414, 338. (9) O’regan, B.; Grätzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737− 740. (10) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891−2959.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03658. Figures S1−S14 addressing additional characterization of interest and details regarding the photocatalytic tests, including the size distribution of the synthesized UC nanorods; SEM images of as-cast P123/TiO2 precursor; high-constant AFM images; XPS for mC−TiO2; crosssectional SEM images; HR-TEM photographs of mC− TiO 2 /UCN; energy-dispersive X-ray spectroscopy (EDX) spectra and corresponding elemental mapping analysis; UV−vis diffuse reflectance spectra and derived Tauc plots; and details regarding the photocatalytic 1315

DOI: 10.1021/acssuschemeng.7b03658 ACS Sustainable Chem. Eng. 2018, 6, 1310−1317

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ACS Sustainable Chemistry & Engineering (11) Wang, M.; Ioccozia, J.; Sun, L.; Lin, C.; Lin, Z. Inorganicmodified semiconductor TiO2 nanotube arrays for photocatalysis. Energy Environ. Sci. 2014, 7, 2182−2202. (12) Qin, W.; Zhang, D.; Zhao, D.; Wang, L.; Zheng, K. Nearinfrared photocatalysis based on YF3: Yb3+, Tm3+/TiO2 core/shell nanoparticles. Chem. Commun. 2010, 46, 2304−2306. (13) Boppella, R.; Kochuveedu, S. T.; Kim, H.; Jeong, M. J.; Marques Mota, F.; Park, J. H.; Kim, D. H. Plasmon-sensitized graphene/TiO2 inverse opal nanostructures with enhanced charge collection efficiency for water splitting. ACS Appl. Mater. Interfaces 2017, 9, 7075−7083. (14) Ha, S. J.; Kim, D. H.; Moon, J. H. N-doped mesoporous inverse opal structures for visible-light photocatalysts. RSC Adv. 2015, 5, 77716−77722. (15) Kannaiyan, D.; Kim, E.; Won, N.; Kim, K. W.; Jang, Y. H.; Cha, M.-A.; Ryu, D. Y.; Kim, S.; Kim, D. H. On the synergistic coupling properties of composite CdS/TiO2 nanoparticle arrays confined in nanopatterned hybrid thin films. J. Mater. Chem. 2010, 20, 677−682. (16) Lee, W. J.; Lee, J. M.; Kochuveedu, S. T.; Han, T. H.; Jeong, H. Y.; Park, M.; Yun, J. M.; Kwon, J.; No, K.; Kim, D. H.; et al. Biomineralized N-doped CNT/TiO2 core/shell nanowires for visible light photocatalysis. ACS Nano 2012, 6, 935−943. (17) Lee, S.; Lee, Y.; Kim, D. H.; Moon, J. H. Carbon-deposited TiO2 3D inverse opal photocatalysts: visible-light photocatalytic activity and enhanced activity in a viscous solution. ACS Appl. Mater. Interfaces 2013, 5, 12526−12532. (18) Yan, H.; Kochuveedu, S. T.; Quan, L. N.; Lee, S. S.; Kim, D. H. Enhanced photocatalytic activity of C, F-codoped TiO2 loaded with AgCl. J. Alloys Compd. 2013, 560, 20−26. (19) Kochuveedu, S. T.; Kim, D.-P.; Kim, D. H. Surface-plasmoninduced visible light photocatalytic activity of TiO2 nanospheres decorated by Au nanoparticles with controlled configuration. J. Phys. Chem. C 2012, 116, 2500−2506. (20) Wang, H.; Lewis, J. P. Effects of dopant states on photoactivity in carbon-doped TiO2. J. Phys.: Condens. Matter 2005, 17, L209−L213. (21) Smarsly, B.; Grosso, D.; Brezesinski, T.; Pinna, N.; Boissière, C.; Antonietti, M.; Sanchez, C. Highly crystalline cubic mesoporous TiO2 with 10-nm pore diameter made with a new block copolymer template. Chem. Mater. 2004, 16, 2948−2952. (22) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Block copolymer templating syntheses of mesoporous metal oxides with large ordering lengths and semicrystalline framework. Chem. Mater. 1999, 11, 2813−2826. (23) Kochuveedu, S. T.; Jang, Y. H.; Jang, Y. J.; Kim, D. H. Visible light active photocatalysis on block copolymer induced strings of ZnO nanoparticles doped with carbon. J. Mater. Chem. A 2013, 1, 898−905. (24) Zhou, W.; Sun, F.; Pan, K.; Tian, G.; Jiang, B.; Ren, Z.; Tian, C.; Fu, H. Well-Ordered Large-Pore Mesoporous Anatase TiO2 with Remarkably High Thermal Stability and Improved Crystallinity: Preparation, Characterization, and Photocatalytic Performance. Adv. Funct. Mater. 2011, 21, 1922−1930. (25) Zhou, W.; Li, W.; Wang, J.-Q.; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K.; Wang, L.; Fu, H.; Zhao, D. Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst. J. Am. Chem. Soc. 2014, 136, 9280−9283. (26) Jang, Y. J.; Jang, Y. H.; Kim, D. H. Nanostructured Carbon-TiO2 Shells Onto Silica Beads as a Promising Candidate for the Alternative Photoanode in Dye-Sensitized Solar Cells. Sci. Adv. Mater. 2015, 7, 956−963. (27) Kochuveedu, S. T.; Jang, Y. J.; Jang, Y. H.; Lee, W. J.; Cha, M.A.; Shin, H.; Yoon, S.; Lee, S.-S.; Kim, S. O.; Shin, K.; et al. Visiblelight active nanohybrid TiO2/carbon photocatalysts with programmed morphology by direct carbonization of block copolymer templates. Green Chem. 2011, 13, 3397−3405. (28) Jang, Y. J.; Jang, Y. H.; Steinhart, M.; Kim, D. H. Carbon/metal nanotubes with tailored order and configuration by direct carbonization of inverse block copolymer micelles inside nanoporous alumina. Chem. Commun. 2012, 48, 507−509.

(29) Jang, Y. H.; Kochuveedu, S. T.; Jang, Y. J.; Shin, H.-Y.; Yoon, S.; Steinhart, M.; Kim, D. H. The fabrication of graphitic thin films with highly dispersed noble metal nanoparticles by direct carbonization of block copolymer inverse micelle templates. Carbon 2011, 49, 2120− 2126. (30) Jang, Y. J.; Jang, Y. H.; Quan, L. N.; Kim, H. C.; Pyo, S.; Kim, D. H. A Soft-Template-Conversion Route to Fabricate Nanopatterned Hybrid Pt/Carbon for Potential Use in Counter Electrodes of DyeSensitized Solar Cells. Macromol. Rapid Commun. 2013, 34, 1487− 1492. (31) Quan, L. N.; Jang, Y. H.; Jang, Y. J.; Kim, J.; Lee, W.; Moon, J. H.; Kim, D. H. Mesoporous Carbon-TiO2 Beads with Nanotextured Surfaces as Photoanodes in Dye-Sensitized Solar Cells. ChemSusChem 2014, 7, 2590−2596. (32) Quan, L.; Jang, Y.; Stoerzinger, K.; May, K.; Jang, Y.; Kochuveedu, S.; Shao-Horn, Y.; Kim, D. H. Soft-template-carbonization route to highly textured mesoporous carbon-TiO2 inverse opals for efficient photocatalytic and photoelectrochemical applications. Phys. Chem. Chem. Phys. 2014, 16, 9023−9030. (33) Wang, W.; Huang, W.; Ni, Y.; Lu, C.; Xu, Z. Different upconversion properties of β-NaYF4:Yb3+, Tm3+/Er3+ in affecting the near-infrared-driven photocatalytic activity of high-reactive TiO2. ACS Appl. Mater. Interfaces 2014, 6, 340−348. (34) Tang, Y. N.; Di, W. H.; Zhai, X. S.; Yang, R. Y.; Qin, W. P. NIRResponsive Photocatalytic Activity and Mechanism of NaYF4:Yb,Tm@ TiO2 Core-Shell Nanoparticles. ACS Catal. 2013, 3, 405−412. (35) Li, X.; Zhang, F.; Zhao, D. Lab on upconversion nanoparticles: optical properties and applications engineering via designed nanostructure. Chem. Soc. Rev. 2015, 44, 1346−1378. (36) Jang, Y. J.; Kim, E.; Ahn, S.; Chung, K.; Kim, J.; Kim, H.; Wang, H.; Lee, J.; Kim, D.-W.; Kim, D. H. Upconversion-Triggered Charge Separation in Polymer Semiconductors. J. Phys. Chem. Lett. 2017, 8, 364−369. (37) Xu, Z.; Quintanilla, M.; Vetrone, F.; Govorov, A. O.; Chaker, M.; Ma, D. Harvesting Lost Photons: Plasmon and Upconversion Enhanced Broadband Photocatalytic Activity in Core@ Shell Microspheres Based on Lanthanide-Doped NaYF4, TiO2, and Au. Adv. Funct. Mater. 2015, 25, 2950−2960. (38) Yan, C.; Zhao, H.; Perepichka, D. F.; Rosei, F. Lanthanide ion doped upconverting nanoparticles: synthesis, structure and properties. Small 2016, 12, 3888−3907. (39) Feng, W.; Zhang, L.; Zhang, Y.; Yang, Y.; Fang, Z.; Wang, B.; Zhang, S.; Liu, P. Near-infrared-activated NaYF4:Yb3+, Er3+/Au/CdS for H2 production via photoreforming of bio-ethanol: plasmonic Au as light nanoantenna, energy relay, electron sink and co-catalyst. J. Mater. Chem. A 2017, 5, 10311−10320. (40) Chen, G.; Qiu, H.; Prasad, P. N.; Chen, X. Upconversion nanoparticles: design, nanochemistry, and applications in theranostics. Chem. Rev. 2014, 114, 5161−5214. (41) Naccache, R.; Vetrone, F.; Capobianco, J. A. Lanthanide-doped upconverting nanoparticles: harvesting light for solar cells. ChemSusChem 2013, 6, 1308−1311. (42) He, M.; Pang, X.; Liu, X.; Jiang, B.; He, Y.; Snaith, H.; Lin, Z. Monodisperse Dual-Functional Upconversion Nanoparticles Enabled Near-Infrared Organolead Halide Perovskite Solar Cells. Angew. Chem., Int. Ed. 2016, 55, 4280−4284. (43) Ye, X.; Collins, J. E.; Kang, Y.; Chen, J.; Chen, D. T. N.; Yodh, A. G.; Murray, C. B. Morphologically controlled synthesis of colloidal upconversion nanophosphors and their shape-directed self-assembly. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 22430−22435. (44) Suyver, J. F.; Grimm, J.; Van Veen, M. K.; Biner, D.; Krämer, K. W.; Güdel, H. U. Upconversion spectroscopy and properties of NaYF4 doped with Er3+, Tm3+ and/or Yb3+. J. Lumin. 2006, 117, 1−12. (45) Liang, L.; Liu, Y.; Bu, C.; Guo, K.; Sun, W.; Huang, N.; Peng, T.; Sebo, B.; Pan, M.; Liu, W.; Guo, S.; Zhao, X.-Z. Highly uniform, bifunctional core/double-shell-structured β-NaYF4:Er3+, Yb3+@SiO2@ TiO2 hexagonal sub-microprisms for high-performance dye sensitized solar cells. Adv. Mater. 2013, 25, 2174−2180. 1316

DOI: 10.1021/acssuschemeng.7b03658 ACS Sustainable Chem. Eng. 2018, 6, 1310−1317

Research Article

ACS Sustainable Chemistry & Engineering (46) Wang, F.; Liu, X. Upconversion multicolor fine-tuning: Visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles. J. Am. Chem. Soc. 2008, 130, 5642−5643. (47) Wang, F.; Liu, X. Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals. Chem. Soc. Rev. 2009, 38, 976−989. (48) Ye, Y.; Jo, C.; Jeong, I.; Lee, J. Functional mesoporous materials for energy applications: solar cells, fuel cells, and batteries. Nanoscale 2013, 5, 4584−4605. (49) Liang, X.; Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Synthesis of NaYF4 nanocrystals with predictable phase and shape. Adv. Funct. Mater. 2007, 17, 2757−2765. (50) Li, C.; Quan, Z.; Yang, J.; Yang, P.; Lin, J. Highly uniform and monodisperse β-NaYF4: Ln3+ (Ln= Eu, Tb, Yb/Er, and Yb/Tm) hexagonal microprism crystals: hydrothermal synthesis and luminescent properties. Inorg. Chem. 2007, 46, 6329−6337. (51) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y.; Wang, J.; Xu, J.; Chen, H.; Zhang, C.; Hong, M.; Liu, X. Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping. Nature 2010, 463, 1061. (52) Shi, F.; Zhao, Y. Sub-10 nm and monodisperse β-NaYF4: Yb, Tm, Gd nanocrystals with intense ultraviolet upconversion luminescence. J. Mater. Chem. C 2014, 2, 2198−2203. (53) Shen, X.-Z.; Liu, Z.-C.; Xie, S.-M.; Guo, J. Degradation of nitrobenzene using titania photocatalyst co-doped with nitrogen and cerium under visible light illumination. J. Hazard. Mater. 2009, 162, 1193−1198. (54) Tayade, R. J.; Kulkarni, R. G.; Jasra, R. V. Photocatalytic degradation of aqueous nitrobenzene by nanocrystalline TiO2. Ind. Eng. Chem. Res. 2006, 45, 922−927. (55) Nitoi, I.; Oancea, P.; Raileanu, M.; Crisan, M.; Constantin, L.; Cristea, I. UV−VIS photocatalytic degradation of nitrobenzene from water using heavy metal doped titania. J. Ind. Eng. Chem. 2015, 21, 677−682. (56) Jo, W.-K.; Won, Y.; Hwang, I.; Tayade, R. J. Enhanced photocatalytic degradation of aqueous nitrobenzene using graphitic carbon−TiO2 composites. Ind. Eng. Chem. Res. 2014, 53, 3455−3461. (57) Chen, X.; Kuo, D.-H.; Lu, D. N-doped mesoporous TiO2 nanoparticles synthesized by using biological renewable nanocrystalline cellulose as template for the degradation of pollutants under visible and sun light. Chem. Eng. J. 2016, 295, 192−200. (58) Zhang, L.; He, X.; Xu, X.; Liu, C.; Duan, Y.; Hou, L.; Zhou, Q.; Ma, C.; Yang, X.; Liu, R.; et al. Highly active TiO2/g-C3N4/G photocatalyst with extended spectral response towards selective reduction of nitrobenzene. Appl. Catal., B 2017, 203, 1−8. (59) Boxi, S. S.; Paria, S. Visible light induced enhanced photocatalytic degradation of organic pollutants in aqueous media using Ag doped hollow TiO2 nanospheres. RSC Adv. 2015, 5, 37657− 37668.

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DOI: 10.1021/acssuschemeng.7b03658 ACS Sustainable Chem. Eng. 2018, 6, 1310−1317