TiO2 Heterogeneous Hybrid for Noble-Metal-Free

Aug 28, 2017 - The Ti–C bond as reactive sites, such as platinum behavior, makes it an interesting potential substitue for noble metals in hydrogen ...
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Research Article pubs.acs.org/acscatalysis

Two-Dimensional C/TiO2 Heterogeneous Hybrid for Noble-Metal-Free Hydrogen Evolution Song Ling Wang,† Jing Li,† Shijie Wang,‡ Ji’en Wu,† Ten It Wong,‡ Maw Lin Foo,† Wei Chen,† Kai Wu,§ and Guo Qin Xu*,† †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore Institute of Materials Research and Engineering, A*STAR, 3 Research Link, 117602, Singapore § College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Developing catalysts to improve excitonic chargecarrier transfer and separation properties is critical for solar energy conversion through photochemical catalysis. Layer staking of two-dimensional (2-D) materials has opened up opportunities to engineer heteromaterials for strong interlayer excitonic transition. However, scalable fabrication of heteromaterials with seamless and clean interfaces remains challenging. Here, we report an in situ growth strategy for synthesizing a 2-D C/TiO2 heterogeneous hybrid. Layered structure of TiO2 and chemically bonded Ti−C between graphitic carbon and TiO2 generate synergetic effects, promoting interfacial charge transfer and separation, leading to more electrons participating in photoreduction for hydrogen evolution. The Ti−C bond as reactive sites, such as platinum behavior, makes it an interesting potential substitue for noble metals in hydrogen evolution. In the absence of noble metals, the C/TiO2 hybrid exhibits a significant enhancement of hydrogen evolution from water splitting using solar light, ∼3.046 mmol h−1 g−1. The facile and scalable fabrication of 2-D heterogeneous hybrid with enhanced interfacial charge transfer and separation provides perspectives for the creation of 2-D heteromaterials in optoelectronics and solar-light-harvesting applications. KEYWORDS: hydrogen evolution, photocatalysis, water splitting, TiO2, graphitic carbon, heterogeneous hybrid

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INTRODUCTION Hydrogen evolution from photocatalytic water splitting provides an exciting platform for transformation and storage of solar energy to meet environmental and energy demand.1−3 The ability to control charge-transfer events of catalysts is related to driving the proton reduction to generate hydrogen.4 To achieve high activities of hydrogen evolution, the excited charge carriers (electrons and holes) must migrate from the interior to the surface of catalysts with a low recombination possibility.5 In principle, shrinking catalyst size in geometric dimension can promote charge transfer to surfaces by shortening migration routes, hence improving photocatalytic efficiency. Beyond traditional semiconductor catalysts, ultrathin sheets are highly desirable for improving charge transfer, because of their fast carrier mobility,6 such as two-dimensional (2-D) transitionmetal dichalcogenides (TMDs).7,8 Generally, transition-metal oxides materials (TMOs) are photochemically stable. Engineering bulk TMOs into ultrathin layers is a novel strategy to promote excited charge transfer and separation.6,9,10 For example, long-distance diffusion of excited electrons and holes in K4Nb6O17 is greatly reduced, because of the ultrathin layered structure.11 Recently, interesting advances © 2017 American Chemical Society

have been made in creating heterostructures of 2-D materials as basic building blocks. Such heterostructures could be formed via van der Waals stacking, where multiple layers are vertically stacked with each other.12,13 More importantly, the stacked heterostructures can accelerate charge transfer and facilitate electron−hole separation.10,12,14,15 Titania (TiO2), as one of the TMO catalysts, has been extensively investigated for its applications in photocatalytic water splitting, because of its favorable band-edge positions, stability, and low cost.4,16,17 Thus, developing a graphitic carbon-TiO2 heterogeneous hybrid could further improve the efficiency of charge-carrier transfer and separation, hence enhancing H2 evolution from water splitting. Although mechanical transfer techniques had been applied for fabricating heterostructures by stacking different 2-D materials with van der Waals stacking, some limitations should be considered, including poor adhesion, the lack of precise control of stacking orientation, possible contamination of interfaces between layers, Received: July 15, 2017 Revised: August 24, 2017 Published: August 28, 2017 6892

DOI: 10.1021/acscatal.7b02331 ACS Catal. 2017, 7, 6892−6900

Research Article

ACS Catalysis and challenging mass fabrication.18−20 Direct and scalable growth of heterogeneous hybrids, in particular, has yet to be achieved under mild conditions. Here, we report a graphitic carbons−layered TiO2 (GC-LT) heterogeneous hybrid synthesized via an in situ method. Furthermore, such graphitic carbon can effectively control the formation of 2-D TiO2 layers. The photoinduced charge transfer and separation of GC-LT hybrid can be greatly accelerated as a result of the layered structure of TiO2 and interfacial interaction between graphitic carbon and TiO2 layers. Most importantly, despite the absence of noble metals (Pt, Au, Ag, etc.), the GC-LT hybrid catalyst gives rise to a significantly enhanced efficiency of H2 evolution from water splitting and photocurrent response with solar light.

water (30 mL) and ethanol (15.0 mL). When the solution became homogeneous after ultrasonicating for 5 h, the commercial TiO2 (P25) powder (321.3 mg) was added and subjected to vigorous magnetic stirring for 2 h. Then, the homogeneous suspension was transferred to the 50 mL Teflon-sealed autoclave and maintained at 120 °C for 24 h. Finally, the rGOP25 composite, consisting of ∼93.2 wt % TiO2 and 5.86 wt % rGO, was achieved by this hydrothermal treatment, after washing with water, centrifuging, and drying at 100 °C in oven for overnight. (2) To synthesize a rGO-TiO2 nanoparticle composite,22 the TiO2 colloidal suspension was first obtained by the hydrolysis of titanium isopropoxide (0.68 mL) in ethanol (20 mL) under magnetic stirring. After continuous stirring at room temperature for 12 h, 22.8 mg of the rGO was added into the suspension. The uniform distribution of TiO2 nanoparticles and rGO in the suspension was ensured by 30 min of ultrasonication. The resulting rGO-TiO2 powder, with ∼93.2 wt % TiO2 nanoparticles and 5.86 wt % graphene, was collected after washing with water, centrifuging, and drying in an oven. Photocurrent Responses Measurement. A three-electrode system was used to measure the photocurrent. The working electrode is the GC-LT sample coated on ITO (1 cm × 0.5 cm). Platinum gauze and Ag/AgCl serve as the counter and reference electrodes, respectively. The NaOH aqueous solution (1 M) was employed as the electrolyte in a quartz reaction cell. A 300 W xenon lamp provides simulated sunlight (AM 1.5). The chronoamperometric measurement of GC-LT hybrid was obtained at 0.0 V while the simulator was switched on and off every 10 s manually, using a shutter to record the change of photocurrent. The P25, rGO-P25, and rGO-TiO2 composites were tested with the same experimental procedure for comparison. Photocatalytic H2 Generation. The photocatalytic H2 evolution through water splitting was carried out in a sealed gas circulation system at a pressure of 101 kPa and room temperature (298 K). The quartz reaction cell (150 mL) and pump were connected into this circulation system. The 5 mg of GC-LT hybrid was suspended in 50 mL of aqueous methanol solution (40 mL H2O, 10 mL methanol) in quartz reaction cell. Methanol serves as the sacrificial reagent. A 300 W Xe lamp was employed as a light source, which is the AM 1.5 simulated solar power system. The amount of H2 generated was determined by gas chromatography with the flow of argon as carrier gas. The photocatalytic H2 evolution activities of rGO, P25, rGO-P25, rGO-TiO2 composites were performed with the same procedure for comparison. The main oxidation product of sacrificial methanol reagent is CO2. The hydrocarbons (CH4, C2H4, and C2H6) and CO were also detected, possibly attributable to the CO2 reduction. The apparent quantum efficiency (QE) of hydrogen production was tested by using the same experimental setup for the hydrogen production above. To be more accurate measurement of QE, 50 mg of catalyst powder was used and the irradiation light (350 nm) was obtained by fixing a band-pass filter (350 ± 10 nm) to the 500-W Xe lamp. The light intensity was 6.5 mW cm−2, and the irradiation area was 15.2 cm2. The QE was calculated based on the following equation:

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EXPERIMENTAL SECTION Characterizations. Field-emission scanning electron microscopy (FE-SEM) images were obtained (JEOL, Model JSM-6701F). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) studies were performed (JEOL, Model 2010 scanning TEM system). X-ray diffraction (XRD) patterns were recorded on a Bruker D8 high-resolution XRD system featuring a four-bounce Ge(022) incident beam monochromator and highly monochromatic X-rays with Cu radiation (Cu Kα = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurement was conducted using an ESCALAB Mk2 system (Vacuum Generators) with Mg Kα. All the binding energy positions were referenced to the C 1s peak of the surface adventitious carbon (at 284.6 eV). Thermogravimetric analysis (TGA) was made on a DTA-TGA system (TA Instruments, Model 2960). The solid carbon and liquid H nuclear magnetic resonance (13C NMR and 1H NMR, respectively) spectra were collected using a Bruker DRX400 system with CPMAS and a Bruker AV500 system with autotune 5 mm BBO probes, respectively. Electron spin resonance (ESR) experiments were performed on JEOL FA200 (X-band) spectrometer at room temperature, centered at 331.756 mT with a sweep width of 50 mT. The elemental analysis (EA) for the accurate quantitation of carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents was carried out by the Elementar Vario MICRO cube. The metal was determined via inductively coupled plasma−optical emission spectrometry (ICP), using the PerkinElmer Optima 5300 DV system. Synthesis of Graphitic Carbon-Layered TiO2 (GC-LT) Hybrid. We first prepared the transparent solution by mixing 1 mL of titanium(IV) isopropoxide (99.9%) and oleylamine (5 mL) at room temperature. Five milliliters (5 mL) of 1,2-ethanedithiol (99.5%) was added into the above mixture under stirring. The color of the solution immediately changed from transparent to red. After stirring for ∼1 h, this precursor mixture was transferred to a Teflon autoclave with a capacity of 50 mL for solvothermal reaction at 200 °C for 24 h. Upon completion, the autoclave was cooled to room temperature and the black precipitate (GC-LT) was collected after washing with ethanol/hexane and freeze-drying for 2−3 days, followed by drying in a tubular furnace with an argon flow, at 200 °C, for 24 h. Preparation of rGO, rGO-P25, and rGO-TiO2 Composites. The rGO was synthesized by hydrothermal reduction of graphene oxide (GO).2 The rGO-P25 and rGO-TiO2 composites were fabricated by integrating 93.2 wt % TiO2 nanoparticles with 5.86 wt % rGO. (1) For the synthesis of rGO-P25 nanoparticles composite,21 20.0 mg of rGO was suspended in a mixed solution, including

QE (%) = = 6893

number of reacted electrons × 100 number of incident photons number of evolved H 2 molecules × 2 × 100 number of incident photons DOI: 10.1021/acscatal.7b02331 ACS Catal. 2017, 7, 6892−6900

Research Article

ACS Catalysis

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RESULTS AND DISCUSSION We developed a lamellar organic micelles strategy for controlling the fabrication of the 2-D graphitic carbon-layered TiO2 hybrid (GC-LT). The oleylamine (OA) molecules can electrostatically coordinate to Ti4+ and self-assemble to form lamellar micelles of OA-Ti complex.23−25 This is due to the binding of OA’s weakly cationic amphiphile and lone pairs of nitrogen centers with Ti4+. 1,2-Ethanedithiol (ED) as linkers can also coordinate with Ti4+ to form an organotitanium complex precursor, hence producing isopropanol reacting with residual ED molecule to generate H2O at 200 °C (see Figure S1 in the Supporting Information). For this process, we can see the color change from transparent to red (see Figures 1a and 1b). Meanwhile, the hydrolysis of Ti4+ can occur on the micelles and consequently form 2-D TiO2 layers. In this process, the graphitic carbon was synergistically formed in situ and incorporated to TiO2 layers with black color (Figure 1c). Thus, the 2-D GC-LT heterogeneous hybrid does not roll into nanotubes or rods. Further interpretation on the formation of GC-LT hybrid is described with typical characterizations (see Figures S1−S4 in the Supporting Information). In Figure 2a, as well as Figures S5 and S6 in the Supporting Information, TEM images of the GC-LT hybrid show ultrathin sheet morphology. The XRD patterns of GC-LT hybrid (Figure 2b) present the diffraction peaks of graphitic carbon and TiO2 phases. The TiO2 largely resembles to the anatase phase (JCPDS No. 21-1272). Furthermore, the AFM image and corresponding height profile (Figure 2c) indicate the GC-LT hybrid sheets with a thickness of ∼3.0 nm. To gain more insight into the structure of the hybrid, we performed HR-TEM studies. The HR-TEM images clearly display a crystal structure in Figure 2d. Based on the inserted fast Fourier transform (FFT) patterns, we found that the hexagonal and tetragonal patterns are assigned to the graphitic carbon and TiO2, respectively. Interestingly, the GC-LT heterogeneous hybrid was also found to have layered structures, as indicated in magnified

TEM images (see Figure 3a, as well as Figures S5 and S6). By examining the edge configuration (e.g., A and B in Figure 3a), we found different spacing or thickness for the sheets. In Figures 3b and 3c, the spacing distances of ∼0.7 and ∼0.76 nm are attributed to the TiO2 monolayer based on the literature.7,26 Because of the existence of chemical bonds (e.g., Ti−C, Ti−O−C bonds), the spacing of graphitic carbon mononlyer is close to that of graphene oxide (∼1.0 nm).27,28 Consequently, the two thicknesses of ∼1.0 and ∼2.0 nm are assigned to the single and double layers of graphitic carbon, respectively. The presence of graphitic carbon and 2-D TiO2 layers in GC-LT heterogeneous hybrid material was also evidenced by Raman shifts, Fourier tranform infrared (FTIR) spectroscopy, gas chromatography−mass spectroscopy (GC-MS), solid 13 C NMR, and XPS measurements. Apart from the signals of TiO2 layers in the Raman shift spectrum (see Figure 4a), such as Eg (1), B1g (1), A1g + B1g (1), and Eg (2), two Raman shift peaks can be related to the D-band (∼1383 cm−1) and G-band (∼1570 cm−1) of graphitic sheets, which are ascribed to the symmetric breathing (A1g of sp3 carbon and in-plane E2g mode vibration of sp2 carbon, respectively).29,30 The FTIR spectrum (Figure 4b) shows that the CC vibration from aromatic zooms of the graphitic carbon can be directly identified at 1623 cm−1.31−33 More importantly, the graphitic sp2 carbon (at 122.5 ppm) was also found in solid 13C NMR spectrum (Figure 4c), further indicating the existence of graphitic carbon.33,34 By comparing with reduced graphene oxide (rGO) (120.8 ppm), the slightly left-shifted and broadened resonance peak of GC-LT hybrid suggests the existence of graphitic carbon flake domains or nonuniformity plausibly due to impure honeycomb lattice incorporating five-, six-, and seven-membered carbon rings.35 Therefore, these results, together with the 2-D layer morphology, strongly suggest the in situ formation of graphitic carbon-layered TiO2 heterogeneous hybrid. The TGA, ICP, and EA characterizations provide the information on the relative amount of graphitic carbon in GC-LT hybrid. As shown in TGA curve (Figure 4d), the weight loss is

Figure 1. Schematic synthesis of graphitic carbon-layered TiO2 (GC-LT) heterogeneous hybrid: (a) transparent solution containing titanium(IV) isopropoxide and oleylamine molecules; (b) red precursor solution obtained by adding 1,2-ethanedithiol into the transparent solution in panel (a) under magnetic stirring, at room temperature; and (c) black GC-LT hybrid product after solvothermal reaction. 6894

DOI: 10.1021/acscatal.7b02331 ACS Catal. 2017, 7, 6892−6900

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ACS Catalysis

Figure 2. Morphology and structure characterizations of GC-LT heterogeneous hybrid: (a) low-magnified TEM image; (b) XRD patterns, indicating diffraction peaks of TiO2 and the circled graphitic carbon (GC) phases; (c) AFM image and height profiles obtained along the line across the sheet edge; (d) HR-TEM image and the inserted fast Fourier transform (FFT) patterns, indicating crystal structure of the TiO2 and GC phases.

Figure 3. Layer-structure characterization of GC-LT hybrid: (a) TEM image, showing layered structure at the sheets edges of A and B parts circled; (b and c) magnified-TEM images of A and B parts in panel (a), respectively, indicating different spacing distances of graphitic carbons and TiO2 by examining the edge configuration.

∼6 wt % at 450−700 °C, which is assigned to the removal of carbon skeletons, forming CO or CO2 by oxidation.36 The ICP

and EA analysis (Table S1 in the Supporting Information) shows Ti (40.28 wt %), C (5.86 wt %), and H (0.94 wt %) in 6895

DOI: 10.1021/acscatal.7b02331 ACS Catal. 2017, 7, 6892−6900

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ACS Catalysis

To study the interaction between graphitic carbon and TiO2 layers, XPS spectra was measured over the GC-LT heterogeneous hybrid and reduced graphene oxide (rGO). The rGO was used for comparison and fabricated by reducing graphene oxide based on previous literature.21 The core-level XPS C 1s spectra of both GC-LT hybrid and rGO (see Figure 4e and Figure S7 in the Supporting Information) display a main peak with a binding energy of 284.6 eV, which is attributable to the nonoxygenated sp2 carbon (CC) ring in graphitic carbon.32,37,38 Furthermore, the peak centered at the binding energy of 285.9 eV is probably ascribed to the C−O oxygenated carbon band.38,39 Two shoulder peaks positioned at 282 and 283.6 eV are possibly related to the C−Ti and Ti−O−C bonds, respectively.40,41 In addition, the chemical bond (C−Ti) was further examined by the high-resolution of XPS Ti 2p spectrum of GC-LT heterogeneous hybrid. As shown in Figure 4f, the binding energies located at 458.5 (Ti 2p3/2) and 464.6 (Ti 2p1/2) eV are assigned to the lattice Ti−O bond in TiO2 layers.42 Apart from these bands, another set with binding energies at 455.5 (Ti 2p3/2) and 461.6 (Ti 2p1/2) eV is attributable from the C−Ti bond formed at the interface between graphitic carbon and TiO2.38 Such Ti−C chemical bond was formed during the in situ synthesis of graphitic carbon−TiO2 heterogeneous hybrid. Whereas the Ti−C bond was not detectable for rGO-P25 composites obtained via the mechanical mixing of rGO and P25 particles (see Figure S8 in the Supporting Information). More importantly, the presence of Ti−C chemical bond can significantly enhance charge transfer and separation in TiO2 layers. Thus, graphitic carbon may act as noble metals (e.g., Pt, Au, etc.) for hydrogen evolution from water splitting. We further studied the optical absorption spectra and valence band position of the GC-LT in order to identify whether the band structure was changed, because of the presence of graphitic carbon. As shown in Figure 5a, the GC-LT and rGO powders with black color inserted display the similar UV-vis

the GC-LT hybrid. Thus, the GC-LT hybrid contains 93.2 wt % TiO2 and 5.86 wt % graphitic carbon.

Figure 4. Characterizations of GC-LT heterogeneous hybrid: (a) Raman shift spectrum; (b) FTIR spectrum; (c) solid-state 13C NMR spectra; (d) TGA curve measured in air, from 30 to 1000 °C, with a 10 °C/min rate; (e and f) core-level XPS C 1s and Ti 2p spectra, respectively. The rGO sample was selected for comparison.

Figure 5. Optical absorption spectra and photocatalytic activities of GC-LT sample: (a) UV-vis diffuse reflectance (DRS) spectra and the corresponding inserted pictures of powder samples; (b) XPS valence band spectra; (c) photocurrent density responses; (d) comparison of noblemetal-free hydrogen evolution from water splitting; (e) comparison of H2 production quantum efficiency (QE) of noble-metal free graphene/ reduced graphene oxide/TiO2-based composites (P25, rGO-P25, and rGO-TiO2 NP, rGO-TiO2 NS, GR/TiO2/MoS2, TiO2/Cu2(OH)2CO3 materials; and (f) stability evaluation of the GC-LT hybrid (cycling reproducibility test of H2 production with multiruns in a five-day period). 6896

DOI: 10.1021/acscatal.7b02331 ACS Catal. 2017, 7, 6892−6900

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ACS Catalysis

The excellent photocatalytic performances of the GC-LT hybrid are possibly attributed to the layered structure of twodimensional TiO2 layers and interfacial interaction between graphitic carbon and TiO2. XPS spectra (Figures 4e and 4f) show the existence of Ti−O, Ti−C, Ti−O−C, and CC bonds in GC-LT hybrid material. However, the peak intensities of Ti−O and CC bonds are much stronger than those of Ti−C and Ti−O−C bonds. This suggests that a small amount of C in graphitic carbon is covalently bonded to Ti or O in the form of Ti−C and Ti−O−C. Consequently, the interface between TiO2 layers and graphitic carbon possibly includes two modes: noncovalent contact (Van der Waals force) and covalent contact (Ti−C/Ti−O-C bonds). A specific mechanism upon the charge transfer and separation of the GC-LT heterogeneous hybrid is schematically depicted, according to the specific interfacial interaction and layered structure of TiO2, in Figure 6a. First, the extent of interaction contact can determine the electron transportation.38,47 For example, graphene/BiOCl enhanced photocatalytic performance, as a result of the special charge transportation of graphene and the chemically bonding C−Bi, allowing for faster charge transfer and separation.38 Herein, the chemical bonding Ti−C on the surface of TiO2 layers can cause interfacial electron transfer from TiO2 to graphitic carbon, efficiently facilitating electron−hole separation. Consequently, the GC-LT hybrid exhibits enhanced photocatalytic hydrogen evolution from water splitting, compared to the mechanically mixed rGO-P25 composite. To evidence this possibility, the electron spin resonance (ESR) studies of the GC-LT hybrid (Figure 6b) and rGO-TiO2 composite were conducted to detect the electronic state localized at room temperature. Compared to rGO-TiO2 composite, the GC-LT hybrid presents a stronger ESR intensity with a g-value of 2.003, which is ascribed to the electrons localized at graphitic carbon.48 These carbon-centered single electrons are originated from the chemically bonding Ti−C formed on the surface of the GC-LT hybrid. Such single

diffuse reflectance (DRS) spectra, significantly different from that of white P25. Furthermore, GC-LT and P25 materials exhibited the same valence band maximum (VBM) positions verified by XPS valence band measurement (Figure 5b), indicating that the layered TiO2 was not lattice-doped by graphitic carbon. As such, C in Ti−C may be attributable to C at the surface linking with other graphitic carbon atoms at the edges or defective sites. This is consistent with the XPS result (Figure 4f) that reduced Ti3+ or Ti2+ species were not observed in the GC-LT sample. As a result, graphitic carbon did not lead to narrowing the band gap of the GC-LT sample. We performed noble-metal-free hydrogen evolution from water splitting over GC-LT hybrid under solar light irradiation. The rGO, rGO-P25, and rGO-TiO2 materials were utilized for comparison. Figure S9 in the Supporting Information presents XRD patterns of rGO-P25, rGO-TiO2 composites, which are similar to that of the GC-LT hybrid. Interestingly, the GC-LT catalyst gave a much greater photocurrent response than P25, rGO-P25, and rGO-TiO2 materials, as shown in Figure 5c. The H2 evolution efficiency (Figure 5d) for the GC-LT hybrid catalyst can achieve a value of ∼3046 μmol h−1 g−1, which is much higher than those of rGO-TiO2 (687.6 μmol h−1 g−1), rGO-P25 (355.3 μmol h−1 g−1), and P25 (83 μmol h−1 g−1). Pure rGO seems to be inactive for H2 generation, which is consistent with the report.43 We further evaluated the QE of the GC-LT hybrid under light irradiation 350 nm. Figure 5e shows the comparison of QE values over graphene/TiO2-based composites: 0.36% (P25) < 1.52% (rGO/P25)21 < 2.95% (rGO-TiO2 NP)22 < 3.1% (rGO-TiO2 NS)44 9.7% (GO/TiO2/ MoS2)45 < 15.4% (TiO2/Cu2(OH)2CO3)46 < 33.1% (GC-LT). Thus, our GC-LT presents an excellent QE value, compared to other graphene/TiO2-based composites. Furthermore, the GC-LT hybrid exhibited strong stability for the reproducibility of H2 evolution. Herein, no obvious reduction of H2 evolution rate (Figure 5f) was observed within 5 days.

Figure 6. (a) Schematic charge transfer and separation associated with interfacial interaction and layered structure for H2 evolution from water splitting over GC-LT hybrid catalyst; (b) ESR spectra of rGO, GC-LT, and rGO-TiO2 samples; and (c) photoluminescence emission spectra of rGO-P25 and GC-LT materials. 6897

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ACS Catalysis electrons can react with H+ to generate H2 from water splitting. Thus, the graphitic carbon not only exhibits the metallic properties to transport electrons, but also provides reduction sites for H2 evolution. Apart from the particular interfacial interaction, the layered structure of 2-D TiO2 also plays an important role in promoting charge transfer and separation, similar to K4Nb6O7.11 The photoinduced electrons move toward graphitic carbon, because of the presence of Ti−C chemical bonds, while the holes are left in the interlayers of TiO2 layers. Herein, the electrostatic gradient in TiO2 layers can be formed and act as a driving force for migration of holes to the edge plane. In this case, electrons and holes can be automatically separated into opposite sides of TiO2 layers, producing H2 and O2 in different parts of the GC-LT hybrid. In other words, the H2 has a tendency to be directly evolved at graphitic carbon into solution or gas phase under the existence of sacrificial reagent for O2 evolution (i.e., methanol). However, for TiO2 particles, the excited electrons transfer from one TiO2 grain to the neighboring one should get through a potential barrier, limiting the fast mobility of charge carriers.49 This case is consistent with the photocatalytic performance that the GC-LT heterogeneous hybrid gave a higher H2 evolution, even though more Ti−C bonds were formed in rGO-TiO2 composite (see XPS Ti 2p in Figure S8). In addition, the Schottky barrier in the GC-LT hybrid probably can be formed at the interface between graphitic carbon and n-type TiO2 layers, such as carbon nanotubes−TiO2 and TiO2/graphene systems.40,50 This Schottky barrier can also promote the photoinduced charge-carrier separation.51 In this case, the graphitic-carbon-like noble metals can act as a sink for the excited electrons, thus retarding the electron−hole recombination on the surface of 2-D TiO2 layers. The excitonic charge-carrier separation of the GC-LT hybrid was demonstrated by the photoluminescence (PL) emissions (Figure 6c) with an excitation wavelength at 320 nm. The peaks at ∼452 and 469 nm arise from the free excitons at the band edge.52 The bound excitons were observed at 483 and 494 nm.52 The band gap transition of anatase TiO2 is seen as a shoulder emission peak at ∼439 nm. The remaining minor intensities are most likely ascribed to the surface defects of GC-LT hybrid. The PL intensities present a clear trend of GC-LT < rGO-P25. This is consistent with a higher separation rate of photoexcited charges in GC-LT hybrid than that in rGO-P25 composite.52−54 The difference in charge recombination can account for the observed photocatalytic H2 evolution rates: GC-LT (3046 μmol g−1 h−1) > rGO-P25 (355.3 μmol g−1 h−1). We also compared the photocatalytic water splitting for hydrogen production over the GC-LT heterogeneous hybrid and reported TiO2-based catalysts44−46,55−68 by modifying or using either graphene or noble metal (e.g., Pt, Au, Ag) as additives or co-catalysts in Table 1. GC-LT hybrid can be seen to have an impressive hydrogen evolution performance. In summary, the enhanced noble-metal-free hydrogen evolution from water splitting over the GC-LT hybrid can be presumably related to (1) layered structure of TiO2, (2) interfacial interaction between graphitic carbon and TiO2, and (3) Schottky contact effect. Our study provides new insights into the development of 2-D hybrid catalysts for photocatalytic applications.

Table 1. Comparison of TiO2-Based Catalysts for Photocatalytic Water Splitting Using Solar Light with/ without Noble Metals catalysta GR-TiO2 NS rGO-TiO2 (001) TiO2/MoS2/GR P25 rGO-P25 GR-P25 Ag/GR/P25 GR/TiO2 3D GR/TiO2 Pt−P25 Pt/N/TiO2 NTs Pt-TiO2 NTAs Cu(OH)2/ TiO2NTAs ZnS−In2S3−Ag2S/ TiO2NTAs GR/TiO2−x GO/TiO2−x GR/Au GOD/TiO2 3D GR/TiO2 GC-LT hybrid

light source 350 W Xe arc lamp 300 W Xe arc lamp 350 W Xe arc lamp Xe arc lamp 200 W Xe arc lamp Xe lamp Xe lamp 500 W Xe lamp 300 W Xe lamp 300 W Xe arc lamp AM 1.5 G 250 W Hg lamp solar light AM 1.5 300 W Xe lamp 500 W Xe arc lamp 300 W Hg lamp 300 W Xe lamp 150-W xenon lamp UV-vis light 200 W Xe arc lamp 300 W Xe lamp

H2 evolution rate (μmol g‑1 h−‑1)

ref

736 169 165.3 33 740 210 353 86 58 2580

44 44 45 46 55 56 56 57 58 59

1508 25 6.5

60 61 62

25.02 1166 1756