TiO2@Carbon Photocatalysts: The Effect of Carbon Thickness on

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TiO2@Carbon Photocatalysts: The Effect of Carbon Thickness on Catalysis Jianming Zhang,† Mitra Vasei,† Yuanhua Sang,‡ Hong Liu,‡,§ and Jerome P. Claverie*,† †

NanoQAM, Department of Chemistry, Quebec Center for Functional Materials, UQAM Succ Centre Ville, CP8888, Montreal, Quebec, H3C 3P8, Canada ‡ State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China § Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Science, Beijing, 100083, China S Supporting Information *

ABSTRACT: Nanocomposites composed of TiO2 and carbon materials (C) are widely popular photocatalysts because they combine the advantages of TiO2 (good UV photocatalytic activity, low cost, and stability) to the enhanced charge carrier separation and lower charge transfer resistance brought by carbon. However, the presence of carbon can also be detrimental to the photocatalytic performance as it can block the passage of light and prevent the reactant from accessing the TiO2 surface. Here using a novel interfacial in situ polymer encapsulation−graphitization method, where a glucose-containing polymer was grown directly on the surface of the TiO2, we have prepared uniform TiO2@C core−shell structures. The thickness of the carbon shell can be precisely and easily tuned between 0.5 and 8 nm by simply programming the polymer growth on TiO2. The resulting core@shell TiO2@C nanostructures are not black and they possess the highest activity for the photodegradation of organic compounds when the carbon shell thickness is 1−2 nm, corresponding to ∼3−5 graphene layers. Photoluminescence and photocurrent generation tests further confirm the crucial contribution of the carbon shell on charge carrier separation and transport. This in situ polymeric encapsulation approach allows for the careful tuning of the thickness of graphite-like carbon, and it potentially constitutes a general and efficient route to prepare other oxide@C catalysts, which can therefore largely expand the applications of nanomaterials in catalysis. KEYWORDS: carbon, TiO2, RAFT polymerization, hybrid, photocatalysis, degradation NPs)21−23 and carbon materials (i.e., carbon nanotubes, graphene, or graphite).7,24−26 Among those, the decoration of the TiO2 surface with carbon nanomaterials is emerging rapidly due to its considerably enhanced photoactivity.24−33 As graphene/graphite is highly conducting, the photoexcited charges of TiO2 can be separated rapidly and the electrons can be transported efficiently.11,31,33 Additionally, compared to other hybridizing materials, carbon offers the inherent advantage of being stable, inexpensive, and prepared from sustainable/environmentally friendly raw materials (e.g., glucose).34,35 Thus, endowed by a high specific surface available for substrate adsorption and a high electrical conductivity, TiO2−C hybrids are promising highly active photocatalysts. The interfacial area between the carbon material and TiO2 also strongly affects the performance of the photocatalyst.24 For example, TiO2 NPs distributed at the surface of carbon nanotubes or graphene sheets7,12,24,29,33,36,37 have only a small overall contact area between the two materials, which acts as a

1. INTRODUCTION The efficient removal of dissolved organic compounds (DOCs) is a major challenge in the treatment of wastewater.1,2 Heterogeneous photocatalysis is one of the most promising methods to remove DOCs from wastewater, as organic pollutants can potentially be fully decomposed to small molecules with no need for a subsequent separation or concentration process.2 Among all photocatalyts, TiO2-based nanomaterials are highly scrutinized because of their ability to promote the catalytic photodecomposition of DOCs under ultraviolet (UV) illumination.3−6 Besides, TiO2 nanomaterials are very stable, nontoxic, and commercially available at low cost.7,8 Nonetheless, the very low absorption outside the UV region and the high recombination rate of photogenerated electron−hole (e−/h+) pairs9−13 severely decrease their photoactivity. The doping of the TiO2 lattice by elements such as N, B, and S is a prevailing method to shift the band gap energy of TiO2 to a lower level and, hence, to increase its solar light absorption.9,11,14−20 However, the distortion of the TiO2 lattice induced by these dopants may favor e−/h+ pair recombination.19,20 Another approach consists of hybridizing TiO2 with various materials, such as metallic nanostructures (e.g., Pt or Au © 2015 American Chemical Society

Received: October 21, 2015 Accepted: December 30, 2015 Published: December 30, 2015 1903

DOI: 10.1021/acsami.5b10025 ACS Appl. Mater. Interfaces 2016, 8, 1903−1912

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ACS Applied Materials & Interfaces

Scheme 1. (a) Synthetic Procedure of the TiO2@C Core@Shell Structure Using an In Situ Polymer Encapsulation− Graphitization Method; (b) RAFT Polymerization and Graphitization Process

uniform carbon layers, and whereby the layer thickness can be simply programmed by experimental design. We report here a novel and facile method for the preparation of TiO2@C core@shell nanostructures using an interfacial in situ polymer encapsulation−graphitization technique based on Reversible Addition−Fragmentation chain Transfer (RAFT) polymerization (Scheme 1). Using this method, the thickness of the uniform carbon layer can be precisely controlled by experimental design. Photocatalytic degradation assays of various organic compounds reveal that the TiO2@C with a graphite-like carbon shell thickness of ∼1−2 nm (corresponding to 3−5 graphene layers) possesses the highest activity.

bottleneck for the transport of charge carriers from one material to another. One solution to this problem consists in tightly wrapping the TiO2 by one or several carbon layers to form TiO2@C core@shell structures.13,24,25,27,29,33,38,39 However, the preparation of such TiO2@C structures present inherent practical difficulties. For example, hydrothermal−carbonization (HTC) of biomass (e.g., glucose)25,34,35,40 necessitates high pressure conditions; graphitization of polyacrylonitrile generates toxic nitriles and hydrogen cyanide (HCN) byproducts41 and reduction of graphene oxide (GO)13,24,27,29,33,38 is usually achieved under high pressure conditions or prolonged UV light irradiation. In many cases, product purification can also be time-consuming.27,29,33,39 Such methods are generally not suitable to achieve precise spatial distribution of carbon (i.e., control on the carbon thickness and on the location of the carbon layers). Although these photocatalysts only contain a small amount of carbon, they have a black color that indicates significant light absorption, an unfavorable configuration for the photocatalytic depollution of water as the carbon prevents light from accessing the photoactive portion of the material. Interestingly, a single graphene layer absorbs only 2.3% of the incident white light (corresponding to an absorbance of 0.01) and 10 graphene layers absorb 70% of the light (corresponding to an absorbance of 0.5).42 Therefore, the black color of these photocatalysts is indicative of the presence of occasional thick graphene (or graphite) layers or particles within the sample. Thus, in order to study the influence of the carbon thickness on photoactivity, it is necessary to develop an efficient approach that is facile to implement, that leads to

2. RESULTS AND DISCUSSION 2.1. In Situ Polymer Encapsulation−Graphitization to Synthesize TiO2@C Nanobybrid. The encapsulation method was implemented on the one-dimensional (1D) TiO2 nanobelt (NB) structures (see below for structure characterization), since these 1D single-crystalline TiO2 nanostructures are expected to act as electron highways with superior electron transport capability and with lower charge carrier recombination rate in photocatalysis.18,39,43−46 As demonstrated in Scheme 1a, the encapsulation of TiO2 begins with the adsorption of a low molecular weight dispersant polymer (poly(acrylic acid), PAA) prepared by RAFT polymerization.42,47−49 Colloidally stable TiO2 dispersions (c = 10 g·L−1 in water) are achieved by adsorbing the negatively charged PAA (c = 3 g·L−1) on the positively charged TiO2 surface (zetapotential: ∼11 mV).42,48 The second step is the in situ chain growth/encapsulation process on the TiO2 surface via RAFT 1904


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ACS Applied Materials & Interfaces polymerization.41,47−50 As the RAFT polymerization is a wellcontrolled polymerization process, the chains of the RAFT dispersant are extended upon adding monomers, and the chain length of the final block copolymers can be easily tuned by varying the amount of added monomers. The RAFT dispersant is adsorbed at the surface of the TiO2; therefore, the second block of the polymer effectively forms a continuous polymer shell around the TiO2, resulting in a complete engulfing of the TiO2 NBs by a polymeric shell. In this work, a novel sugar monomer of N-acryloyl-D-glucosamine (AGA) was utilized, as it possesses a glucose unit that is a good carbon precursor for the subsequent graphitization step. However, the polymer synthesized with AGA monomer alone tends to desorb from the TiO2 surface due to its high solubility in water; therefore, the polymeric shell was prepared by copolymerizing AGA with hydrophobic monomer of butyl acrylate (BA; Scheme 1b). The formation of the copolymer, P(AGA) (BA), was confirmed by Fourier transform infrared (FT-IR) spectroscopy and proton nuclear magnetic resonance (1H NMR) spectroscopy (Figures S1 and S2 in the Supporting Information), as well as transmission electronic microscopy (TEM). Furthermore, the in situ polymerization process occurs in very high yield, as shown by the absence of residual monomer in the collected polymer (Figure S2, 1H NMR spectra). The encapsulation step is highly efficient, as every TiO2 NB is trapped in a continuous shell of hydrophobic copolymer to form a TiO2@P(AGA) (BA) core@shell structure without any free TiO2 NBs or free polymer particles (Figure S4). Simple mixing of the P(AGA) (BA) polymer with TiO2 does not result in the formation of a core@shell structure, as the hydrophobic P(AGA) (BA) cannot efficiently diffuse through the aqueous phase to reach the TiO2 surface. Table 1 lists the composition of the samples prepared

Figure 1. TEM images of TiO2 NBs (a), and P(AGA) (BA) encapsulated TiO2 NBs before (b) and after graphitization (c). (d) HR-TEM image of TiO2@C-1 (see Table 1 for composition). Inset shows the TEM image of TiO2 NBs at higher magnification.

the polymer is converted into carbon (mostly due to the thermal decomposition of the PAGA units, Figure S5). The encapsulation−calcination sequential method we devised offers several important advantages. Compared to other preparative approaches, that is, HTC method and adsorption-reduction of GO,39 this encapsulation step proceeds under mild, easily scalable, environmentally friendly reaction conditions (∼70 °C in aqueous solution, atmospheric pressure), and the product does not require any time-consuming purification. Furthermore, in contrast to the popular HTC methods, the carbohydrate derivative in our method is only located at the surface of the TiO2, with no free carbohydrate polymer in the medium. Therefore, after calcination, carbon is only found at the surface of the TiO2 and nowhere else, which is important due to the deleterious effect of free carbon particles for light transmission (see below). As ∼35 wt % of the polymer is converted to carbon, the thickness of the carbon layer is directly proportional to the thickness of the polymer coating, which, in turn, depends on the amount of monomers (AGA + BA) used during the encapsulation step. Therefore, it is remarkably easy to adjust the thickness of the carbon layer. Finally, as shown below, this method yields highly uniform carbon thicknesses (see Figure S6, TEM images for large area observation), thus, allowing the precise construction of nanocomposites. 2.2. Characterization of TiO2@C Nanohybrids. The morphology of the TiO2 NBs was characterized by TEM at each step. As shown in Figure 1a, the 1D structure of the pristine TiO2 materials is visible. The smooth and clean surface of the NBs is quite apparent at a higher magnification (inset of Figure 1a). Scanning electron microscopy (SEM) observations confirm that TiO2 adopts a belt-like structure with a length around 2−10 μm, a width ∼20−300 nm, and a thickness a few tens of nanometers (see Figure S3). Figure 1b shows the TEM image of the TiO2 NBs after polymer encapsulation (TiO2@ P(AGA) (BA); see TEM images in Figure S4 for more

Table 1. Basic Features of the TiO2@C Samples Prepared in This Worka

sample

mass of AGA used for reaction (mg)

P(AGA) (BA) Mnb (g/mol)

carbon coating thicknessc (nm)

carbon contentd (mass %)

BET surface area (m2/g)

TiO2 NBs TiO2@C-1 TiO2@C-2 TiO2@C-3 TiO2@C-4

0 100 160 240 390

10300 16600 25500 35500

0 0.9−1.2 1−2 4−5 8−9

0 2.1 2.7 4.6 7.2

22.2 22.9 23.8 30.7 45.7

a All TiO2@C samples were synthesized using 50 mg TiO2 NBs under the same reaction conditions. bDetermined by GPC. cDetermined by TEM. dDetermined by TGA on TiO2@C.

in this study as well as the corresponding molecular weight (Mn, measured using a gel permeation chromatography (GPC)) of the P(AGA) (BA) copolymer present at the TiO2 surface. The Mn of the copolymer increases from 10300 to 35500 g/mol when the amount of AGA is increased from 100 to 390 mg, which is consistent with the controlled nature of the RAFT polymerization. This simple and mild encapsulation process yields a freeflowing dispersion of TiO2@P(AGA) (BA) in aqueous solution, which is sufficiently concentrated (c > 13 g·L−1) to envisage the preparation of large quantities of TiO2@C structures. Following the encapsulation, a calcination step is then conducted under an inert atmosphere to yield TiO2@C structures (Figure 1c,d). Thermogravimetric analysis (TGA) of the polymer in the absence of TiO2 indicates that ∼35 wt % of 1905


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Figure 2. XRD (a) and Raman (b−d) spectra of TiO2 NBs (lower, black curve) and graphite-like, carbon-coated, TiO2 NBs (TiO2@C-1, upper, red curve).

spectroscopy. Figure 2a shows the XRD patterns of the TiO2 NB (lower, black curve) and the TiO2@C samples (upper, red curve). The diffractograms of both samples are similar, with diffraction peaks corresponding to the {101}, {004}, {200}, and {116} planes of the tetragonal phase of anatase TiO2 (JCPDS card No. 21−1272) but without any peaks belonging to the rutile phase. A further study reveals that the anatase phase in TiO2@C is conserved even for calcinations performed at ∼750 °C (see Figure S9), which is consistent with the previous report that the thermal stability of TiO2 can be enhanced by the introduction of a thin carbon coating.25 No peaks related to carbon could be observed for TiO2@C-1, which is expected, in view of the low carbon content in this sample. It has been reported that the treatment at high temperature of amorphous carbon thin films eventually leads to the formation of disordered graphite nanocrystals.51 In order to confirm the chemical nature of the carbon shell, the TiO2 NB samples were also characterized by Raman spectroscopy. Figure 2b−d show the Raman spectra of TiO2 NBs (lower, black curve) and TiO2@C (upper, red curve) with a carbon coating thickness of ∼1 nm. The intense peaks between 100 and 800 cm−1 (Figure 2b,d) correspond to the typical optical modes of anatase TiO2.25 Furthermore, in the TiO2@C sample, two intense broad peaks are observed at 1334 and 1583 cm−1 (Figure 2c), which are respectively assigned to the D and G band of the carbon coating.38,39 The G band corresponds to sp2 planar and conjugated structures, while the D band corresponds to sp2 units adjacent to structural defects.38,39 The presence of G and D bands at these wavenumbers is characteristic of a disordered graphitic structure (amorphous carbons have distinct Raman spectra).40,51 The D/G ratio (∼0.86) is slightly smaller than the reported values of the TiO2−C nanohybrid prepared by the HTC of biomass,25 which is indicative of the

observations). The undulating polymer coating with a thickness ∼3−10 nm can be clearly observed at the NB surface, indicating a successful encapsulation. It will be seen below that the undulations disappear after calcination, as expected for a nonflat polymeric surface that is heated above its Tg. As shown in Figure 1c and Figure S6, the structure of the TiO2@ P(AGA) (BA) sample after thermal treatment (550 °C for 4 h) confirms that the overall morphology of the NB has not been affected by the encapsulation−graphitization process. The surface of the calcined samples was further characterized using high resolution TEM (HR-TEM). As shown in Figure 1d, the surface of the NB is covered by a continuous carbon coating with a uniform thickness of ∼1 nm (see TEM and SEM images in Figure S6 for more observations). The lattice fringes of the TiO2 and the layer spacing of the carbon coating are clearly observed, with a lattice spacing of ∼0.35 nm corresponding to TiO2 {101} planes (JCPDS card No. 21−1272) and ∼0.34 nm matching the {112} facet of graphite (JCPDS card No. 41− 1487). By contrast, for the pristine TiO2 NBs, such an interfacial carbon layer is not observed (Figure S3f). The presence of the thin carbon coating was further verified by Xray energy dispersive spectroscopy (EDS). The signal assigned to the carbon element can only be detected in the TiO2@C sample (Figure S7). These results confirm that the TiO2@C core@shell structure is successfully obtained using a polymer encapsulation technique followed by thermal treatment, and the carbon shell is constituted of graphite-like carbon (corresponding to ∼2−3 graphene layers). Similar distorted graphite-like carbon structure was also observed for the sample with thicker carbon shell of ∼2 nm (see Figure S8 in Supporting Information). The structure of the TiO2@C sample with ∼1 nm carbon coating was further analyzed using X-ray diffraction (XRD) 1906


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Figure 3. TEM images [(a) TiO2@C-2, (b) TiO2@C-3, (c) TiO2@C-4 as listed in Table 1) and the appearance of the water suspension (d) of the TiO2@C samples with variable carbon coating thicknesses and of a sample containing TiO2 and free carbon black (e) UV−vis diffuse reflection spectra of TiO2@C samples and of pure TiO2 NBs.

surface area increases from 22.9 to 45.7 m2/g. The latter is about twice higher than that of the pure TiO2 NBs. Since the thin coating does not bring a significant size enlargement of the TiO2 NB structure, this surface area increase is highly likely due to the presence of pores in the carbon coating which usually form during the thermal carbonization process of the polymer.34,35 The TiO2@C samples can be easily dispersed in aqueous solution to form stable suspensions. Zeta-potential measurement (Figure S12) indicates that the surface of the TiO2@C samples is negatively charged in the 2−10 pH range. This is due to the partial oxidation of the carbon coating and possibly incomplete decomposition of the acrylic polymer, both yielding carboxylic groups34,36 during the thermal decomposition process of the P(AGA) (BA) polymer. The visual appearance of the TiO2@C suspensions with increasing carbon coating thicknesses is presented in Figure 3d. The TiO2@C samples are opaque with a gray tinge which becomes more pronounced as the carbon shell thickness increases. While the sample TiO2@ C-1 is off-white, a sample containing the same amount of TiO2 NBs (c = 8.3 g·L−1) and carbon (2.1 wt % relative to TiO2) under the form of free carbon particles is black (right vial). Thus, free carbon particles (which are absent in our samples) have a negative impact for the transmission of light, highlighting the necessity to achieve spatial control on carbon layering to attain high activity. The optical properties of TiO2@C samples were further characterized by UV−visible (UV−vis) diffuse reflectance spectroscopy (Figure 3e). As expected, all the samples show the characteristic absorption feature of TiO2 with a cutoff at ∼380 nm, implying an identical band gap energy (∼3.2 eV) for TiO2 and TiO2@C structures. By contrast, carbon-doped TiO2 is reported to have a decreased band gap energy due to the formation of Ti−O−C-like microstructures within the TiO2 lattice.9 In our study, the carbon is simply covering the surface of the TiO2 NBs, and it does not work as

good quality of the carbon coating. Raman results in conjunction with HR-TEM analysis suggest that the wellcontrolled polymer encapsulation−graphitization technique favors the growth of nanocrystalline carbon on TiO2 surface. Moreover, the Raman peaks at ∼142 and 197 cm−1 assigned to the Eg anatase optical mode in TiO2 are shifted to 146 and 199 cm−1, respectively, in TiO2@C (Figure 2d). This shift could originate from the strong surface interaction developed between the carbon layer and TiO2 NBs.25 The TEM images of TiO2@C core@shell structures synthesized with various monomer-to-TiO 2 mass ratios (Table 1) are shown in Figure 3a−c. Clearly, the surfaces of the NBs are coated by a continuous carbon layer with a highly uniform thickness. As can be seen from the TEM images at higher magnification (insets), the thickness of the shell is estimated to be ∼2 (a), ∼4 (b), and ∼8 nm (c), respectively. When increasing the mass ratio of monomers to TiO2, the molecular weight of the polymeric shell increases (Table 1), and therefore, the amount of the carbonaceous source (glucose unit in AGA) increases. These data indicate that by simply adjusting the mass ratio of monomer-to-TiO2 NB in the in situ polymerization process, precise control of the carbon shell thickness is simply and successfully achieved. Individual graphene-like layers can be observed by HR-TEM for samples TiO2@C-1 (Figure 1d, ∼2−3 graphene layers) and TiO2@C-2 (Figure S8, ∼3−5 graphene layers). The carbon content of the samples with different carbon thickness was also assessed using TGA. Table 1 shows that the carbon content varies from ∼2.1 to 7.2% as the carbon thickness increases from ∼1 to 8 nm, which is in good agreement with TEM observations (Figure 3a−c). Moreover, concomitantly to an increase of carbon shell thickness, an increase of the specific surface area, as measured by Brunauer− Emmett−Teller (BET) method, is observed. As a result of the increase of the carbon coating thickness from ∼1 to ∼8 nm, the 1907


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Figure 4. C/C0 (a) and −ln(C/C0) (b) vs time for the degradation of MO under UV light irradiation. Inset of (a) shows a typical evolution of UV− vis absorption spectra during the photodegradation of MO using TiO2@C-2 as catalyst. The gray area of (a) shows the C/C0 in darkness (MO adsorption process). The solid lines in (b) are the linear extrapolations. (c) Photocatalytic degradation of several organic compounds (rhodamine B, RhB; methylene blue, MB; and 4-chlorophenol, 4-CP) under UV light. (d) Repeated runs for the decomposition of MO using TiO2@C-2 catalyst under UV light.

dopant to vary the lattice structure of TiO2.9 The presence of the carbon shell is however evidenced in the diffuse reflectance spectrum by the more or less flat absorption in the 450−800 nm region, as expected for a zero band gap energy material (this flat region is not due to scattering, as TiO2 of same size and same surface concentration does not scatter significantly in Figure 3e). 2.3. Photocatalytic Property of TiO2@C Samples. The effect of the carbon shell thickness on photocatalytic activity of TiO2@C samples was assessed through a methylene orange (MO) photodegradation experiment, an experiment that has been widely used for the assessment of the catalytic activity of TiO2 photocatalysts.39,52 The concentration of MO is quantified by separating the TiO2@C particles and by monitoring the UV−vis absorbance of the MO solution at 464 nm (Abs464 nm). The plots of C/C0 versus time (t < 0, in darkness; t > 0, under UV light irradiation), where C0 and C are the concentrations of MO at initial reaction time (t = 0 min) and at time t, respectively, are shown in Figure 4a. Prior to each photocatalysis test, the MO solution is contacted with the catalyst under darkness until an adsorption−desorption equilibrium is attained.39,52 This process (gray area, Figure 4a) reached equilibrium in ∼20 min (t = −40 min). Thus, by extending the equilibration period by a 12 h under darkness, the MO concentration remains unchanged (Figure S13), indicating that the dye is not degraded by the catalyst in the absence of light. The amount of adsorbed dye increases with increasing carbon thickness (larger C/C0 decay), in agreement with BET results of Table 1. Once adsorption−desorption

equilibrium was reached, UV light irradiation was initiated (t = 0 min). As shown in the inset of Figure 4a, the Abs464 nm value drops rapidly as the reaction proceeds from t = 0−30 min under UV light irradiation, reflecting the decrease of the MO concentration in solution due to the MO degradation. In the absence of catalyst, the MO solution shows only ∼17% degradation in 30 min. In the presence of the TiO2 NBs (black, Figure 4a) or TiO2@C samples with shell thickness of ∼1−2 nm (green and red), the photodegradation reaction is rapid, and the degradation of MO is almost complete in 30 min. However, with a thicker carbon shell (∼4 and ∼8 nm), the degradation reaction is significantly slower. The photodegradation reaction kinetics follows a first-order rate law,39,52 as shown by a highly linear −ln(C0/C) vs time plot (R2 > 0.992, Figure 4b), with the slope corresponding to the reaction rate constant k. The degradation of MO catalyzed by TiO2@C, with a shell thickness of ∼1−2 nm (∼3−5 graphene layers), has the highest reaction rate (k = 1.47 × 10−1 min−1), which is almost 3.5× higher than the rate for pure TiO2 NBs (k = 4.3 × 10−2 min−1). When the thickness of the carbon shell is greater than 2 nm, the reaction rate is even slower than for pure TiO2. The catalytic efficiency of the photocatalysts was verified again for the photodegradation of other organic compounds of rhodamine B (RhB),52 methylene blue (MB),53 and 4-chlorophenol (4-CP)54 (see Figure S14 for corresponding UV−vis spectra evolution). Figure 4c illustrates the concentration variation (C/ C0) of these compounds as a function of carbon coating thickness after 30 min photoreaction under UV light. For these four organic compounds (RhB, MB, 4-CP, and MO), the 1908


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ACS Applied Materials & Interfaces photodegradation results are similar in essence, with the fastest decay observed for the samples with 1−2 nm carbon coating. We can exclude that this effect is due to a change of the surface area, as TiO2@C with ∼4 and ∼8 nm carbon layers have the highest BET surface area, and therefore the largest capacity to adsorb reactants, but the lowest activity for photodegradation. Furthermore, the carbon layer remains unchanged (and, therefore, it is not oxidized) during the photocatalysis, as confirmed by comparing the samples before and after reaction using TEM and Raman technique (see Figure S15). Thus, the optimal carbon shell thickness for TiO2@C is 1−2 nm (corresponding to ∼3−5 graphene layers). Although reported for the photodegradation of several dyes molecules,55 a sensitization mechanism whereby the dye absorbs the light and transfers the electrons to TiO2 is most likely not at stake here because the degradation of the four organic compounds we assessed proceeds smoothly in all cases although they have widely different extinction coefficients (4-CP is not a dye). This study thus confirms that the TiO2@C with an optimal carbon coating thickness of 1−2 nm can be used as a highly efficient and versatile photocatalyst for the degradation of dissolved organic compounds. The long-time stability/durability of the catalyst is of high importance for its actual application, thus the catalytic activity of TiO2@C sample with carbon coating 1−2 nm and pure TiO2 NBs was examined in six successive MO photodegradation reactions. As presented in Figure 4d, the degradation of MO catalyzed by the TiO2@C sample remains unchanged in the six cycles, indicating a stable catalytic activity for this carboncoated TiO2. In contrast, an obvious activity decrease for pure TiO2 was seen in the third cycle (Figure S16). It has been reported that photogenerated holes can react with the surface oxygen atoms of an inorganic oxide photocatalyst, leading to an activity decrease.52,56 At the oxide−carbon interface, interfacial C−O bonds can prevent the activation of the surface oxygen atoms by the holes, therefore, leading to an improved photostability.52,56 Accordingly, the good catalytic stability for the TiO2@C sample is likely to be attributed to the strong interfacial interaction between TiO2 and C layer, as implied in its Raman spectra (Figure 2d). By comparing the activity of the TiO2@C catalyst with ∼1−2 nm carbon thickness (assessed by k, the kinetic rate constant) with the one of pristine TiO2 (k0), an enhancement of k/k0 ∼ 1.9−3.4 is observed (depending on the dye). This enhancement is comparable to the highest reported enhancements for TiO2−C hybrids (Table S2) prepared by other techniques, indicating that indeed a 1−2 nm carbon thickness is optimal. Efficient charge separation is critical to achieve high photocatalytic activity. In order to probe the extent of charge separation in our catalysts, photoluminescence (PL) spectroscopy was conducted, as PL emission originates from the recombination of electrons and holes (Figure 5a).11,12 Although the TiO2 NBs show a strong PL emission band at ∼460 nm, for the TiO2@C sample, this PL band is completely quenched, which is due to an efficient electron transfer from the conduction band (CB) of TiO2 to carbon coating and, consequently, an efficient suppression of charge recombination.11,12 Photocurrent and electrochemical impedance measurements using a photoelectrochemical cell (PEC) were conducted to explore the electronic interaction of carbon material and TiO2. The chronoamperometric curves (I−t) of TiO2 NB and TiO2@ C (∼1−2 nm carbon thickness) electrodes at 0 V versus Ag/

Figure 5. (a) PL spectra of TiO2 and TiO2@C (∼1−2 nm carbon thickness) using excitation λ = 320 nm. PEC I−t curves (b) and electrochemical impedance spectra Nyquist plots (b) of the TiO2 NBs and TiO2@C electrodes under UV light irradiation.

AgCl, are presented in Figure 5b. A rapid and stable photocurrent response was collected for each UV light on− off event in both electrodes. The current generated using TiO2@C electrodes is about 3.5× greater than that of TiO2 NB one, indicating that the separation efficiency of photoexcited electron−hole pairs is increased considerably due to the strong electronic interaction between zero-band gap carbon and TiO2. Figure 5c displays the Nyquist plots obtained by electrochemical impedance spectroscopy (EIS). As expected, the diameter of the semicircles for TiO2@C catalyst are much smaller than for pure TiO2 NBs, indicating a smaller charge transfer resistance on the surface.25 Although the investigation on the exact mechanism of the charge separation in this system is ongoing, a plausible mechanism that accounts for the improved photocatalytic activity of the TiO2@C catalyst can be proposed based on above observations (Scheme S1). During the photocatalytic degradation process with TiO2@C (1−2 nm thickness), the light can efficiently pass through the ultrathin graphitic-like carbon layer to reach the TiO2 surface. Subsequently, e−/h+ 1909


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magnetic resonance (1H NMR (D2O, 300 MHz, δ)) analysis: 6.37− 6.08 (m, 2H), 5.74 (dd, J = 9.8, 1.8 Hz, 1H), 5.17 (d, J = 3.5 Hz, 1H), 4.08−3.26 (m, 8H). 2.3. Synthesis of TiO2@C. Polymer encapsulation of TiO2 NBs. Typically, TiO2 NBs (50 mg) and of RAFT agent PAA (∼15 mg) were first dispersed into 5 mL of water. This dispersion was sonicated using a Fisher scientific sonic dismembrator model 500 for 5 min at 30% amplitude (400 W) to yield a white stable TiO2−PAA dispersion. In a separate vial, AGA (100 mg), BA (110 mg) monomer, and ABV initiator (∼5 mg) were dissolved in 15 mL of water/ethanol solution (1:2), which was degassed with N2 for 30 min. This mixture was transferred to a syringe (50 mL) that was fixed to a syringe pump connected to the TiO2−PAA suspension. The TiO2−PAA suspension was heated at 70 °C under magnetic stirring and the monomers were added at 5 mL/h rate under a N2 atmosphere. Once monomer addition was finished, the polymerization was continued for 3 additional hours. The product was collected by centrifugation. Graphitization: The encapsulated TiO2 samples was dried in a vacuum and calcined in a furnace under nitrogen (temperature ramp of 5 °C/ min from room temperature to 550 °C, followed by a 4 h isotherm at 550 °C). 4.4. Photocatalyzed Organic Compound Degradation. Typically, a TiO2@C sample (10 mg) was dispersed in 20 mL of 1× 10−5 M of dye (MO, RhB and MB) aqueous solution under stirring in darkness for ∼1 h to reach an equilibrium adsorption for dye molecules.52 During the adsorption process, the suspension was purged with N2 to remove dissolved O2. The solution was then irradiated at room temperature by a Mercury lamp (Newport Inc.) with an average light intensity of ∼16 mW/cm2. At regular intervals, aliquots were removed and analyzed by UV−visible spectroscopy. For the photocatalytic degradation of phenol chemicals, the same amount and procedures were applied to a 4-CP solution with concentration of 0.15 mM. 4.5. Electrode Preparation and Photoelectrochemical Cell (PEC) Test. The photocatalyst (5 mg) was suspended in 5 mL of ethanol and sonicated in a sonication bath for 5 min. The suspension was spin-coated at 2400 rpm for 1 min on an ITO substrate (1.2 × 1.2 cm). The electrode was dried in air at 60 °C for 2 h and then heated at 350 °C for 30 min under N2. A standard three-electrode electrochemical cell was assembled using Ag/AgCl electrode and Pt wire as reference and counter electrodes, respectively. The potential of the working electrodes against Ag/AgCl reference electrode was set to be 0.0 V. To measure the photocurrent, a degassed Na2SO4 electrolyte solution (0.5 M, pH = 2.6 adjusted with H2SO4) was used as electrolyte, and the PEC cell was irradiated under UV light with 30 s on−off cycles. A Metrohm Autolab PGATAT302N with an AC perturbation signal of 1 V over the frequency of 1 MHz to 100 mHz was used to to capture the impedance spectra of the coated electrodes. 4.6. Characterization. (i) The polymer coating was analyzed using a Nicolet 6700 FT-IR spectroscopy equipped with an ATR accessory. (ii) The crystal structure of the as-synthesized materials was examined by a D8 Advance (Bruker, Billerica, MA) XRD using CuKα radiation. (iii) The morphology of the samples was analyzed using a JEOL JSM7600F SEM and JEM-2100F TEM. (iv) 1H NMR spectra of the monomer and copolymer were recorded with a Bruker 300 (300 MHz) instrument using Deuterium oxide (D2O) as solvent. (v) Raman spectra were acquired with a WITEC Alpha300 Raman microspectrometer equipped with a 10× objective. The spot size focused on the sample surface with a 532 nm incident laser for signal collection is ∼25 × 25 μm. The laser power was adjusted to be low enough to avoid thermal damage to the samples. (vi) UV−vis diffuse reflection spectra were measured using a PerkinElmer Lambda 1050 spectrometer equipped with an integrating sphere. UV−vis absorption spectra were collected using a Varian Cary 100Bio spectrometer. (vii) Molecular weight of the polymers was determined using a GPC with water as the mobile phase and equipped with a Wyatt Dawn 18 angle light scattering detector and a Dawn DSP refractometer. (viii) TGA was performed in air or He with a TA Instrument TGA Q500 thermogravimetric analyzer coupled with MS. (ix) BET surface area test is conducted using Quantachrome Autosorb-1. (x) PL properties

pairs are generated upon the absorption of the UV light, and the electrons are excited to the CB of TiO2, with a hole left at the valence band (VB). Although charge recombination is often dominant in TiO2, the presence of a highly conductive carbon layer facilitates charge separation, as the carbon can act as an electron acceptor.11,27,39 Furthermore, the continuous graphitelike carbon shell serves as a good electron transporter due to its excellent conductivity and partial 2D π-conjugated planar structure.31 The separated electrons can react with the adsorbed O2 to form •O2− for the photo-oxidation of the dye molecules. The remaining holes in TiO2 can also take part in the redox reactions by forming a •OH for dye degradation.12,27 Thus, thanks to the highly efficient e−/h+ separation promoted by the carbon coating, the organic compounds are decomposed at a higher reaction rate.

3. CONCLUSION In conclusion, an in situ RAFT polymer encapsulation− graphitization method is reported to synthesize the TiO2@C core@shell structure with a graphite-like carbon shell. The thickness of the carbon shell can be precisely architected by merely adjusting the concentration of each polymeric building block during encapsulation. The TiO2@C catalyst with ∼1−2 nm shell thickness demonstrates the highest activity for the photocatalytic degradation of various organic compounds, ∼2− 3.5× higher than the TiO2 NBs alone. The 1−2 nm carbon coating is thin enough not to prevent light from accessing the oxide, but conductive enough to accept electrons and to transport them, resulting in a greater efficiency for e−/h+ pair separation. As this in situ RAFT polymer encapsulation− graphitization is highly versatile, it can be easily scaled-up for mass production and used to prepare more complex hierarchical structures using different oxide materials. Thus, the unique polymeric encapsulation−graphitization technique offers an ideal approach for the preparation of high performance photocatalysts. 4. EXPERIMENTAL SECTION 4.1. Materials and Characterization. TiO2 powder (P25), Dglucosamine hydrochloride, acryloyl chloride, tert-butyl acrylate (BA), 4,4-azobis (4-cyanovaleric acid) (ABV), potassium carbonate (K2CO3), sodium nitrite (NaNO2), methylene orange (MO), rhodamine B (RhB), methylene blue (MB), and 4-chlorophenol (4CP) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl), absolute ethanol, and methanol were purchased from Fisher Scientific. Indium tin oxide coated glass substrate (ITO glass, 8−12 Ω/sq) was graciously offered from the Pilkington Company. The poly(acrylic acid) (PAA) RAFT agent was prepared previously in our research group.57 BA was passed through a basic alumina (50−200 μm, Acros) column prior to use. Other chemicals were used without further purification. Water was Nanopure grade (18.2 MΩ·cm at 25 °C). See Supporting Information for details of the synthesis of TiO2 nanobelts (NBs). 2.2. Synthesis of N-Acryloyl-D-glucosamine (AGA) Monomer.58 D-Glucosamine hydrochloride (8.60 g) and NaNO2 (0.14 g) were dissolved in a K2CO3 aqueous solution of (2 M, 20 mL). The reaction mixture was cooled to ∼0 °C in a ice bath. The mixture was vigorously stirred under N2 protection. Acryloyl chloride (4.00 g) was added dropwise over 1 h. While stirring was maintained, the reaction solution was kept below 5 °C for ∼3 additional hours. After warming to room temperature and stirred for 24 h, the suspension was poured into 200 mL of cold absolute ethanol, refrigerated overnight, and the precipitated salts were filtered off. The solution was concentrated under vacuum, and the product was purified by recrystallization with methanol (75%). The product yield was ∼47%. Proton nuclear 1910


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ACS Applied Materials & Interfaces were measured using a Varian Cary Eclipse fluorescence spectrophotometer.

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Enhanced Solar Energy Unitilization. Nanoparticles. Energy Environ. Sci. 2014, 7, 967−972. (12) Liu, J.; Bai, H.; Wang, Y.; Liu, Z.; Zhang, X.; Sun, D. D. SelfAssemlbling TiO2 Nanorods on Large Graphehe Oxide Sheets at a Two-Phase Interface and Their Anti-Recombination in Photocatalytic Applications. Adv. Funct. Mater. 2010, 20, 4175−4181. (13) Lim, D. K.; Barhoumi, A.; Wylie, R. G.; Reznor, G.; Langer, R. S.; Kohane, D. S. Enhanced Photothermal Effect of Plasmonic Nanoparticles Coated with Reduced Graphene Oxide. Nano Lett. 2013, 13, 4075−4079. (14) Tang, X.; Li, D. Sulfur-Doped Highly Ordered TiO 2 Nanotubular Arrays with Visible Light Response. J. Phys. Chem. C 2008, 112, 5405−5409. (15) Chen, D.; Yang, D.; Wang, Q.; Jiang, Z. Effects of Boron Doping on Photocatalytic Activity and Microstructure of Titanium Dioxide Nanoparticles. Ind. Eng. Chem. Res. 2006, 45, 4110−4116. (16) Gopal, N. O.; Lo, H.-H.; Ke, S.-C. Chemical State and Environment of Boron Dopant in B,N-Codoped Anatase TiO2 Nanoparticles: An Avenue for Probing Diamagnetic Dopants in TiO2 by Electron Paramagnetic Resonance Spectroscopy. J. Am. Chem. Soc. 2008, 130, 2760−2761. (17) Wang, Y.; Zhang, Y.-Y.; Tang, J.; Wu, H.; Xu, M.; Peng, Z.; Gong, X.-G.; Zheng, G. Simultaneous Etching and Doping of TiO2 Nanowire Arrays for Enhanced Photoelectrochemical Performance. ACS Nano 2013, 7, 9375−9383. (18) Wang, J.; Tafen, D. N.; Lewis, J. P.; Hong, Z.; Manivannan, A.; Zhi, M.; Li, M.; Wu, N. Origin of Photocatalytic Activity of NitrogenDoped TiO2 Nanobelts. J. Am. Chem. Soc. 2009, 131, 12290−12297. (19) Zhao, X. P.; Yin, J. B. Preparation and Electrorheological Characteristics of Rare-Earth-Doped TiO2 Suspensions. Chem. Mater. 2002, 14, 2258−2263. (20) Wang, J. A.; Limas-Ballesteros, R.; López, T.; Moreno, A.; Gómez, R.; Novaro, O.; Bokhimi, X. Quantitative Determination of Titanium Lattice Defects and Solid-State Reaction Mechanism in IronDoped TiO2 Photocatalysts. J. Phys. Chem. B 2001, 105, 9692−9698. (21) Liu, L.; Ouyang, S.; Ye, J. Gold-Nanorod-Photosensitized Titanium Dioxide with Wide-Range Visible-Light Harvesting Based on Localized Surface Plasmon Resonance. Angew. Chem., Int. Ed. 2013, 52, 6689−6693. (22) Bian, Z.; Tachikawa, T.; Zhang, P.; Fujitsuka, M.; Majima, T. Au/TiO2 Superstructure-Based Plasmonic Photocatalysts Exhibiting Efficient Charge Separation and Unprecedented Activity. J. Am. Chem. Soc. 2014, 136, 458−465. (23) Pu, Y.-C.; Wang, G.; Chang, K.-D.; Ling, Y.; Lin, Y.-K.; Fitzmorris, B. C.; Liu, C.-M.; Lu, X.; Tong, Y.; Zhang, J. Z.; Hsu, Y.-J.; Li, Y. Au Nanostructure-Decorated TiO2 Nanowires Exhibiting Photoactivity across Entire UV-Visible Region for Photoelectrochemical Water Splitting. Nano Lett. 2013, 13, 3817−3823. (24) Xie, G.; Zhang, K.; Guo, B.; Liu, Q.; Fang, L.; Gong, J. R. Graphene-Based Materials for Hydrogen Generation from LightDriven Water Splitting. Adv. Mater. 2013, 25, 3820−3839. (25) Zhang, L.-W.; Fu, H.-B.; Zhu, Y.-F. Efficient TiO2 Photocatalysts from Surface Hybridization of TiO2 Particles with Graphitelike Carbon. Adv. Funct. Mater. 2008, 18, 2180−2189. (26) Chang, K.; Mei, Z.; Wang, T.; Kang, Q.; Ouyang, S.; Ye, J. MoS2/Graphene Cocatalyst for Efficient Photocatalytic H2 Evolution under Visible Light Irradiation. ACS Nano 2014, 8, 7078−7087. (27) Wang, P.; Wang, J.; Wang, X.; Yu, H.; Yu, J.; Lei, M.; Wang, Y. One-Step Synthesis of Easy-Recycling TiO2-rGO Nanocomposite Photocatalysts with Enhanced Photocatalytic Activity. Appl. Catal., B 2013, 132−133, 452−459. (28) Fang, Z.; Liu, Z.; Wang, Y.; Ajayan, P. M.; Nordlander, P.; Halas, N. J. Graphene-Antenna Sandwich Photodetector. Nano Lett. 2012, 12, 3808−3813. (29) Fan, W.; Lai, Q.; Zhang, Q.; Wang, Y. Nanocomposites of TiO2 and Reduced Graphene Oxide as Efficient Photocatalysts for Hydrogen Evolution. J. Phys. Chem. C 2011, 115, 10694−10701. (30) Long, R.; English, N. J.; Prezhdo, O. V. Photo-Induced Charge Separation across the Graphene-TiO2 Interface Is Faster than Energy

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10025. FT-IR and 1H NMR spectra of the TiO2@P(AGA) (BA); TGA spectra; XRD pattern of TiO2@C; TEM images of TiO2@C; UV−vis spectra of MO solution in darkness; UV−vis spectra of RhB, MB and 4-CP after photodegradation; cyclic run of MO photodegradation of TiO2; additional synthetic details and data about TiO2@ C; thermal and solvent stability of TiO2@C (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 1 (514) 987-3000, ext. 6143. Fax: 1 (514) 987-4054. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge funding from the Natural Sciences and Engineering Research Council of Canada and Fonds de la Recherche du Québec sur la Nature et les Technologies (FRQNT) and NanoQuébec (Major Infrastructures). J.Z. would like to thank FRQNT for a postdoctoral fellowship.

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TiO2@Carbon Photocatalysts: The Effect of Carbon Thickness on

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