Enhanced Photocatalytic Performance of Carbon-Coated TiO2-x with

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Enhanced Photocatalytic Performance of CarbonCoated TiO with Surface Active Carbon Species 2-x

Fen Liu, Ningdong Feng, Longxiao Yang, Qiang Wang, Jun Xu, and Feng Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02716 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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The Journal of Physical Chemistry

Enhanced Photocatalytic Performance of Carbon-Coated TiO2-x with Surface Active Carbon Species Fen Liua,b, Ningdong Fenga,*, Longxiao Yanga,b, Qiang Wanga, Jun Xua, Feng Denga,* a

State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,

National Center for Magnetic Resonance in Wuhan, Key Laboratory of Magnetic Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China. b

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

* To whom correspondence should be addressed. E-mail: [email protected], [email protected], Fax: +86-27-87199291.

ACS Paragon Plus Environment

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ABSTRACT Carbon (C) coating on the TiO2 surface has attracted extensive research interest due to the unique properties of the conjugated materials in electron transport and photo-electronic coupling ability. However, owing to the complexity of surface C species, there is no experimental study on its structure and property. Although the C-coated TiO2−x photocatalyst (C/TiO2-x) and its corresponding acid-washed sample (C*/TiO2-x) exhibit similar visible-light absorption, their catalytic activity is quite different. According to HRTEM, XPS, ESR, and NMR results, the only structural difference between C/TiO2-x and C*/TiO2-x lies in the surface C species. Our NMR experimental results show that several C species (including alkoxy and carboxylate, and macromolecular graphite-like C) are present in C/TiO2-x, while only macromolecular graphite-like C exists in C*/TiO2-x. Combined with the photocatalytic activity measurements, it can be deduced that the surface graphite-like C should be the active C sites, which facilitate the separation of photoinduced electron and hole, and lead to the exceptionally high photocatalytic activity for C*/TiO2-x, while the alkoxy and carboxylate C species that should be the recombination centers would poison seriously the surface of C/TiO2-x. Accordingly, the hole and electron transfer mechanism in the C-coated TiO2-x photocatalyst is proposed.


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1. INTRODUCTION Utilization of solar energy for hydrogen production and water/air decontamination has attracted extensive research interest. Since the pioneered discovery of photocatalytic water-splitting on titanium dioxide (TiO2)1, this semiconductor photocatalyst has been extensively studied and recognized as one of the most promising photocatalysts. However, the wide band gap of pure TiO2 (ca. 3.2 eV) renders it only active in the ultraviolet irradiation range. In order to significantly enhance its solar-driven photocatalytic efficiency, band gap engineering is highly desirable to improve the visible light harvesting of TiO2. A useful strategy is to incorporate Ti3+ into TiO2 itself. It has been reported that the sufficient Ti3+ doping should induce a continuous electronic state just below the conduction band minimum of TiO2, which can enhance the visible light adsorption, and further leads to a high electron mobility and suppress the rapid combination of photogenerated carriers (electrons and holes) in the bulk of TiO2.2-5 Besides the efficiency of the light harvesting and the separation of photogenerated carriers in TiO2 bulk, the efficiency of the carrier separation and transfer for photocatalytic reaction on TiO2 surface is also crucial to enhance the solar-driven photocatalytic activity. Much effort has been made to improve the efficiency of the carrier separation and transfer of TiO2 surface, including the deposition of plasmonic metal nanoparticles6-8, the formation of composite materials with other inorganic semiconductors9-11, and the coating of carbon (C) nanomaterials12-15. Among of them, coating the TiO2 surface with carbon nanomaterials has been received much attention because of the unique



properties of the conjugated materials in electron transport and photo-electronic coupling ability14,16-18. Additionally, carbon offers the inherent advantage of being stable, inexpensive and environmentally friendly. For instance, the surface carbon can expand the light absorption range via a photosensitizing effect19-21, and can efficiently reduce the recombination of photogenerated carriers by accelerating surface electron transfer21,22. To data, almost all studies on the optimization of carbon loaded materials focus on the regulation of surface capacity and thickness of load layer. It has been found that the excessive thickness of carbon layers shields the absorption of light.23 Thus, a coating of surface carbon with a thickness of a few organic molecules (monolayer or island type) on TiO2 surface should be essential to enhance the photocatalytic activity, which, however, does not warrant enhanced activity of the carbon-coated TiO2 under the visible irradiation. To solve this problem, it is highly desirable to reveal the structure and property of the surface carbon. Owing to the complexity of the surface carbon, which mainly consists of alkoxy, carboxylate, aromatics, and graphite-like carbon on the carbon coated TiO2, there is no experimental study to address which is the active carbon species to trap the photogenerated electrons, or which can poison the TiO2 surface to decrease the photocatalytic activity. Solid-state NMR spectroscopy is a powerful tool for exploring the local environments of active sites in various photocatalysts24-27. However, it is difficult to study the structure and distribution of surface carbon species on the surface of TiO2 and other oxides by

13

C MAS NMR techniques unless a large amount of carbon is


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loaded or 13C labeled28,29. By far, little attention has been paid to the roles of various surface carbon species in the photocatalytic reaction. Herein, a simple one-step method was used to synthesize the TiO2 photocatalysts with thin layer carbon and Ti3+ (denoted as C/TiO2−x). The resultant photocatalysts exhibit efficient visible optical absorption and extremely high photocatalytic activity for degradation and hydrogen production. To gain insight into the active center structures and plausible mechanisms associated with the photocatalysts, High Resolution Transmission Electron Microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) and solid-state nuclear magnetic resonance (NMR) spectroscopy have been utilized. 2. Experimental Sections 2.1 Sample Preparation. 2.5 ml of nitric acid was dissolved in 50 ml of a methanol/water (1:1) mixture. Subsequently, 3.2 ml of titanium(IV) tetrabutoxide was added dropwise to the solution mixture. The solution mixture was continuously stirred for about 2.5 h at room temperature (295 K), and then the mixture was transferred into a Teflon bottle for further hydrothermal treatment at 443 K for 3 h. After cooling for an additional 12 h, the reagent was centrifuged, dried at 353 K to obtain a white sample. The white sample was then calcined in vacuum (10-3Pa) at 673 K for 5 h to obtain the black C-coated and self-doped TiO2 (C/TiO2-x). The C/TiO2-x was treated by HCl and washed by deionized water. After drying at 333K, the black C*/TiO2-x sample was obtained. The white sample was calcined in O2 atmosphere at 673 K to obtain white



TiO2 sample. The white TiO2 sample was calcined in vacuum (10-3Pa) at 673 K to form the TiO2-x sample. 2.2 Photocatalytic activity Photocatalytic degradation. Catalyst powders (100 mg) were dispersed into the rhodamine B (RhB) aqueous solution (10 mg/L, 100 mL). A Xe lamp (300 W) with a visible light filter (λ > 420 nm) was used. Before illumination, the mixture was magnetically stirred in the dark for 60 min to ensure adsorption-desorption equilibrium of the RhB dye on the surface of the catalyst. After illumination for given time intervals, the reaction solution (1 mL) was taken out and centrifuged to remove the catalyst. The obtained filtrate was subjected to an analysis of MO concentration on a UV-Vis spectrometer (Agilent Technologies Cary 4000). Photocatalytic H2 production. After loading a sample (50 mg) with 0.6% Pt (0.3 mg), the photocatalysts were placed into an aqueous methanol solution (50 ml, 5.0 volume %) in a Quartz glass container. The simulated solar light generated by Xe lamp (300 W) was used as the excitation source, which produced a power of ca. 100 mW/cm2. For Pt deposition, 200 mg of as-prepared TiO2 photocatalysts were dispersed in a 10 mL aqueous solution of H2PtCl6•6H2O (0.618 mM) by stirring. The mixture was irradiated using a 300 W Hg lamp for 2 h. 2.3 Characterization Methods The crystalline structures of various photocatalysts were determined by X-ray diffraction (XRD) on a Bruker D/max2550 instrument using Cu KR radiation (40 kV, 40 kmA). All UV-Vis diffuse reflectance spectroscopy (DRS) measurements were


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carried out on an Agilent Technologies Cary 4000/5000 spectrophotometer using BaSO4 as the reference. The high resolution transmission electron microscope (HRTEM) images were taken using a JEOL 2010 electron microscope operating at 200 kV. Samples of each catalyst were prepared for the HRTEM by dispersing the photocatalyst powder in high-purity ethanol. A drop of the suspension was then allowed to evaporate on a holey-carbon film supported by a 300-mesh copper TEM grid. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis Ultra delay line detector (DLD) spectrometer equipped with a monochromatic Al Kα X-ray source (hʋ = 1486.6 eV), hybrid (magnetic/electrostatic) optics with a multichannel plate, and DLD. All XPS spectra were recorded using an aperture slot of 300 × 700 µ m. Survey spectra were recorded with an energy of 160 eV, as compared to the high resolution spectra (40 eV). The accuracy of the XPS binding energies (BE) is 0.1 eV. ESR measurements were carried on a JEOL JES-FA200 ESR spectrometer at room temperature. The TiO2 photocatalysts were transferred into ESR tube in the air atmosphere, followed by degassing (to remove gaseous O2) at room temperature on a vacuum line. TEMPO (2, 2, 6, 6 - Tetramethyl - 1 - piperidinyloxy) was used as an external reference for quantification of Ti3+ concentration. Solid-state

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C one-dimensional (1D) magic-angle-spinning (MAS) NMR

experiments were performed by using a 4 mm double-resonance probe on a Bruker-AdvanceIII 500 spectrometer with Larmor frequencies of 500.58 and 125.87



MHz for 1H and 13C respectively. The π/2 pulse length is 4.0 µs and 4.0 µs of for 1H and

13

C respectively. 1H→13C CP MAS NMR experiments were carried out with a

contact time of 7 ms, and a total of 300000 free-induction-decay (FID) signals were accumulated with a repetition time of 2 s. The chemical shifts for the 13C resonances were referred to hexamethylbenzene (HMB). All experiments were carried out with a MAS frequency of 10 kHz. 3. RESULTS AND DISCUSSION Solar absorption and photocatalytic activity. The optical response of as-prepared pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x was evaluated by the diffusive reflectance spectroscopy, as shown in Figure 1. Compared to pure TiO2, the absorption curve observed for TiO2-x extends toward the visible-light region due to the introduction of Ti3+ species and/or oxygen vacancies. For C/TiO2-x and C*/TiO2-x photocatalysts, their visible-light absorption is similar, and the intensity of visible-light absorption further grows up. However, as will be seen in the following, the photocatalytic activities of C/TiO2-x and C*/TiO2-x are quite different. The photocatalytic activities of various catalysts were firstly evaluated by monitoring the photodegradation of a typical dye, namely rhodamine B (Rh B), under visible-light irradiation. (Figure 2a) The photocatalytic activity of TiO2-x reaches a conversion efficiency of ca. 70% after 45 min irradiation, much better than that of pure TiO2 (ca. 4%). Upon the carbon coating, although higher visible-light absorption is present, the photocatalytic activity of C/TiO2-x (ca. 16%) largely declines instead. After the C/TiO2-x was washed with HCl solution to generate C*/TiO2-x, a much higher


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photocatalytic activity, corresponding to a conversion efficiency of ca. 100% after 30 min of visible-light irradiation, is observable. The activity is even better than that of a commercial P25 under simulated solar light (including 5.0% of ultraviolet, 39% of visible light, and 56% of infrared light) as shown in Figure 2a. It is noteworthy that C*/TiO2-x also exhibits exceptionally high activity in the photocatalytic H2 production. Figure 2b shows the time course of H2 evolution for the different TiO2 photocatalysts. The photocatalytic activity of C*/TiO2-x is much better than those of pure TiO2, TiO2-x, and C/TiO2-x samples. We found that 1 hour of solar irradiation generates 0.43 mmol of H2 using 0.05 g of C*/TiO2-x (ca. 8.6 mmol/hour/g), and the reaction rate is ca. 3.0 times as much as that of the commercial P25. Structural Characterization. To establish the structure-activity relationship, the detailed structural characteristics of carbon-coated TiO2-x photocatalysts was investigated with various analytical techniques. As revealed by X-ray diffraction and high-resolution transmission electron microscopic (TEM) (Figure 3a, S1–S2 in supporting information), all the TiO2 photocatalysts exists mainly in form of anatase phase, but contains a small amount of rutile and brookite. The size of individual TiO2 nanocrystals is ca. 17.0 nm in diameter. The C/TiO2-x and C*/TiO2-x exhibit similar structure, but the size of individual TiO2 nanocrystals decreases to ca. 10.0 nm in diameter, and the carbon layer coating is ca. 1.0 nm thick. The presence of C dopants is clearly confirmed by the energy-dispersive X-ray spectroscopy (EDX) mapping. (Figure 3 and S2 in supporting information). The C content of C/TiO2-x and C*/TiO2-x is ca. 1.6 and 1.5 wt.% respectively, indicating that a small amount of coated C



vanishes

after

the

HCl

washing.

Additionally,

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C/TiO2-x

possesses

a

Brunauer−Emmett−Teller (BET) specific surface area of 197.9 m2/g, which is similar with that of C*/TiO2-x (190.2 m2/g), but slightly larger than those of TiO2 (159.7 m2/g) and TiO2-x (157.2 m2/g). XPS and ESR analysis. Figure 4a shows Ti 2p XPS spectra of pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples. The Ti 2p3/2 XPS spectra of these TiO2 photocatalysts exhibits a typical peak at 459.0 eV in addition to the 2p1/2 peak centered at ca. 464.9 eV, which is responsible for lattice Ti4+ species,30, 31 indicating that there are no Ti3+ species on the surface of these TiO2 photocatalysts. The O 1s XPS spectra of the different TiO2 samples are shown Figure 4b. It can be found that all the O 1s XPS spectra consist of two peaks at 530.7 and 532.5 eV. The former peak can be attributed to lattice O (Ti–O–Ti) species, and the latter peak should correspond to surface O species (hydroxyl groups, and/or Ti–O–C etc).32 As shown in Figure 4b, compared to pure TiO2, the surface O species of TiO2-x decreases due to the vacuum treatment. Upon the carbon coating, the O 1s XPS signal at 532.5 eV increases due to the introduction of C=O and C–O groups, and C/TiO2-x and C*/TiO2-x exhibit similar O 1s XPS spectra. According to the C 1s XPS spectra in Figure 4c, three signals are present at 287.3, 285.9, and 284.8 eV, which can be ascribed to –COO-, C–O, and C-C groups, respectively33,34. When C/TiO2-x was washed with HCl solution to obtain C*/TiO2-x, the amount of all these carbon species declined. Since Ti3+ (or oxygen vacancy) species is paramagnetic, its existence can be confirmed by ESR experiments. As shown in Figure 5, an ESR signal due to Ti3+ with


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a g-value of 1.987 is observable.5, 35 Combined with the Ti 2p XPS results, it can be deduced that the Ti3+ species mainly exists in the bulk of TiO2-x, C/TiO2-x, and C*/TiO2-x since the surface Ti3+ species usually gives rise to a Ti 2p peak at 457.5 eV5 which is absent in the corresponding Ti 2p3/2 XPS spectra (Figure 4a). The ESR results also indicate that these TiO2 photocatalysts have similar Ti3+ content. Surface carbon species studied by solid-state NMR techniques. The detailed structural changes of surface C species have been studied by

13

C MAS NMR

experiments on TiO2-x, C/TiO2-x, and C*/TiO2-x samples as shown in Figure 6. There is no C species in TiO2-x (Figure 6a). Upon the C coating, up to four different C species can be identified in C/TiO2-x (Figure 6b). According to previous reports,29,36 the signal I at 182.9 ppm is associated with –COO- of carboxylate. The broad peak II at ca. 129 ppm and narrow peak II at 128.3 ppm can be assigned to graphite-like C and aromatics C, respectively. The signal III at 58.3 ppm is due to the methylene of −CH2COO− and/or −CH2O−, while the signal IV around 7.6 ~ 29.8 ppm should result from the methylene and methyl bonded to other alkyl C, graphite-like C, and aromatics C. Noted that the narrow peak is due to fast motion of low molecular weight C species, while the broad signal is due to slow motion of high molecular weight C species. When the C/TiO2-x was washed with HCl solution to obtain C*/TiO2-x, most of the narrow peaks at 182.9, 128.3, 58.3, 22.2, 18.4, and 7.6 ppm arising from low molecular weight carboxylate, alkoxy, and aromatics (adsorbed on C/TiO2-x surface) vanish, while the broad signals arising from high molecular weight graphite-like C remain on C*/TiO2-x. The graphite-like C is likely bonded to the TiO2



surface via the –COOTi (182.9 ppm) and −CH2O Ti (74.8 ppm) groups (Figure 6). Photocatalytic mechanism. According to the experimental results, the only structural difference between C/TiO2-x and C*/TiO2-x lies in the surface C species. Since the two catalysts show quite different photocatalytic activity, the surface C species should be directly related to their photocatalytic activity. It has been found that although the visible-light absorption of C/TiO2-x is much higher than that of TiO2-x and its BET surface area is also slightly larger than that of TiO2-x, the photocatalytic activity of the former is much lower than that of the latter. Interestingly, after C/TiO2-x was washed with HCl solution to obtain C*/TiO2-x, although C*/TiO2-x exhibits similar visible-light absorption and BET surface area, but it possesses a much higher photocatalytic activity compared to C/TiO2-x. Additionally, compared with TiO2-x and TiO2, the increase in the BET surface area of C*/TiO2-x cannot compensate for the enhancement of photocatalytic activity. Our NMR results show that after the acid washing, most of the low molecular weight C species (carboxylate, alkoxy and aromatics) are removed, while the high molecular weight graphite-like C species are mainly present on the surface of C*/TiO2-x. This suggests that the C coating leads to the introduction of both separation sites (active C sites, graphite-like C species) of photoinduced carriers and recombination sites (carboxylate, alkoxy, and aromatics) of photoinduced carriers. After the acid washing, the latter vanishes, while the former remains. This should be the reason that C*/TiO2-x exhibits a higher photocurrent than C/TiO2-x. (Figure 7). Therefore, on the basis of our experimental results, the hole (h) and electron (e) transfer mechanism in the carbon-coated TiO2-x photocatalysts can be proposed


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(Figure 8). According to the previous report37, the graphite-like C could improve the transfer of photoinduced electrons, and then decrease possibility of recombination of electron and hole pairs. As such, upon the irradiation, the photoinduced electron transfers and concentrates on the surface graphite-like C that can facilitate the separation of photoinduced electron and hole due to the formation of heterojunction between graphite-like C and TiO2. According to our previous work,27 the surface bridging hydroxyl (OH) groups act as the channel for the transfer of photoinduced holes to reactants in the photocatalytic reaction. Since carboxylic acid and alcohol can react with the surface OH groups to form carboxylate and alkoxy species (Figure 8), this would hinder the transfer of holes, and further generate recombination centers of photoinduced electrons and holes, in consistence with the previous observation that the existence of alkoxy could hinder effectively the photocatalytic reaction of hydrogen production.27 Thus, the exceptionally high photocatalytic activity observed for C*/TiO2-x under visible-light irradiation can be attributed to both the existence of surface thin layer carbon (graphite-like C) and the removal of recombination centers (carboxylate and alkoxy).

CONCLUSIONS In summary, C-coated TiO2−x photocatalysts with a thin C layer (ca. 1.0 nm) and

Ti3+ species have been prepared by vacuum calcination technique. It has been found that the Ti3+ species can improve the visible-light adsorption and photodegradation activity of TiO2-x catalysts. For C/TiO2-x and C*/TiO2-x, the visible-light absorption is further enhanced, while their catalytic activity is quite different. HRTEM, XPS, ESR,



and NMR spectroscopy techniques have been applied to gain insight into the origin of their different activity. The only structural difference between C/TiO2-x and C*/TiO2-x lies in the surface C species. Our XPS and NMR experimental results show that after acid washing, the low molecular weight carboxylic acid, alcohol and aromatics vanish, while the high molecular weight graphite-like C retains on the surface of C*/TiO2-x. Combined with the studies of photocatalytic activity of C/TiO2-x and C*/TiO2-x, the exceptionally high photocatalytic activity observed for the C*/TiO2-x is ascribed to the synergistic effect of surface graphite-like carbon and Ti3+ species, while the small molecules of alkoxy and carboxylate can poison seriously the surface of C/TiO2-x. The experimental and theoretical calculation results presented herein should not only facilitate a better understanding of the photocatalytic mechanism at the atomic level but also be helpful for the rational design of highly active titania-based photocatalysts.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21673283, 21473246 and 21733013).

ASSOCIATED CONTENT

Supporting Information: XRD spectra of pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples; HRTEM image, Electron image, and EDS elemental mappings of C and Ti on the C/TiO2-x. This material is available free of charge via the Internet at http://pubs.acs.org.


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Figure Captions

Figure 1. UV–Vis absorption spectra of pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples. Figure 2. (a) Photodegradation curves of rhodamine B on pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples upon irradiation with visible light (λ > 420 nm). The concentration of rhodamine B was determined by monitoring the variation of the optical intensity at λ = 553 nm; (b) The photocatalytic activity of the various photocatalysts in water splitting hydrogen production under simulated solar-light irradiation. Figure 3. (a) HRTEM image, (b) Electron image, and EDS elemental mappings of C (c) and Ti (d) on C*/TiO2-x. Figure 4. Ti 2p (a), and O 1s (b) XPS spectra of pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples, and C 1s (c) XPS spectra of C/TiO2-x, and C*/TiO2-x samples. Figure 5. ESR spectra of pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples. Figure 6. 13C MAS NMR spectra of TiO2-x, C/TiO2-x, and C*/TiO2-x samples. Figure 7. Photocurrent−time profiles of C/TiO2-x and C*/TiO2-x samples. Figure 8. Proposed hole and electron transfer mechanism in the carbon-coated TiO2-x photocatalyst upon visible-light irradiation.


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Figure 1. UV–Vis absorption spectra of pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples.



Figure 2. (a) Photodegradation curves of rhodamine B on pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples upon irradiation with visible light (λ > 420 nm). The concentration of rhodamine B was determined by monitoring the variation of the optical intensity at λ = 553 nm; (b) The photocatalytic activity of the various photocatalysts in water splitting hydrogen production under simulated solar-light irradiation.


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Figure 3. (a) HRTEM image, (b) Electron image, and EDS elemental mappings of C (c) and Ti (d) on C*/TiO2-x.



Figure 4. Ti 2p (a), and O 1s (b) XPS spectra of pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples, and C 1s (c) XPS spectra of C/TiO2-x, and C*/TiO2-x samples.


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Figure 5. ESR spectra of pure TiO2, TiO2-x, C/TiO2-x, and C*/TiO2-x samples.



Figure 6. 13C MAS NMR spectra of TiO2-x, C/TiO2-x, and C*/TiO2-x samples.


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Figure 7. Photocurrent−time profiles of C/TiO2-x and C*/TiO2-x samples.



Figure 8. Proposed hole and electron transfer mechanism in the carbon-coated TiO2-x photocatalyst upon visible-light irradiation.


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The TOC graphic for the manuscript, “Enhanced Photocatalytic Performance of Carbon-Coated TiO2-x with Surface Active Carbon Species”


Enhanced Photocatalytic Performance of Carbon-Coated TiO2-x with

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