Graphene Grown on Anatase-TiO2 Nanosheets - ACS Publications

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C: Physical Processes in Nanomaterials and Nanostructures 2

Graphene Grown on Anatase-TiO Nanosheets: Enhanced Photocatalytic Activity on Basis of a Well-Controlled Interface Huihui Liu, Zongwei Chen, Lei Zhang, Dongbo Zhu, Qun Zhang, Yi Luo, and Xiang Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12305 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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The Journal of Physical Chemistry

Graphene Grown on Anatase-TiO2 Nanosheets: Enhanced Photocatalytic Activity on Basis of a Well-Controlled Interface Huihui Liu,1,† Zongwei Chen,1,† Lei Zhang,1 Dongbo Zhu,1 Qun Zhang,1,2,4,* Yi Luo,1,2,4 and Xiang Shao1,3,4,* 1

Department of Chemical Physics, 2Hefei National Laboratory for Physical Sciences at the Microscale,

3

CAS Key Laboratory of Urban Pollutant Conversion, and 4Synergetic Innovation Center of Quantum

Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China †

These authors contributed equally to this work

* To whom correspondence should be addressed: [email protected]; [email protected].

ABSTRACT: To achieve a best promotion effect of photocatalytic activity, an optimized interface between graphene and TiO2 is highly desirable, which is, however, difficult to realize via conventional preparation of the graphene/oxide composites. To this end, we here develop a novel strategy to directly grow graphene on the anatase TiO2 (a-TiO2) nanosheet via an ambient pressure chemical vapor deposition (AP-CVD) method using acetylene as the precursor. Such a recipe successfully avoids the intercalating contaminations, meantime ensures the contact uniformity of graphene over a-TiO2 (G/a-TiO2). The ultrafast transient absorption spectroscopy clearly reveals the efficient charge transfer from TiO2 to graphene in assisting the separation of photo-induced electron-hole pairs. Moreover, the measurements of photocurrent and the photocatalytic degradation of methyl orange turn out that the composite with double layer graphene shows the best performance, which can possibly result from the competition of interlayer interactions within the multilayer graphene and the interactions between graphene with TiO2.

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1. INTRODUCTION Because of the increasing global concerns of the environmental and energy issues, photocatalysis has received more and more attention since it directly utilizes the perpetual sunlight as the energy source.1-6 TiO2 is usually considered one of the most promising photocatalysts due to its superior photocatalytic performance in the removal of pollutants in water and air7-11 and meanwhile a number of other advantages such as low price, low toxicity, and long-term stability.12 The practical problems of using TiO2 are mainly the relatively low absorption of visible light as well as the inevitable consumption of precious metals in order to achieve acceptable performance for practical applications.13,14 Recently, various two-dimensional materials, in particular graphene, have been found to exhibit a significant promotion effect on the photocatalytic activity of TiO2.15-21 Graphene is a special carbon sheet with superior properties in thermal stability, transparency, and electric conductivity.22-25 Theoretically, when combined with TiO2, it would serve as an effective electron sink and hence render the quick separation of photogenerated charge carriers across the interface and enhance the redox reactions of the chemisorbed molecules.26-28 However, the theoretical predictions are drawn on an ideal model of contamination-free, intimate interface between graphene and TiO2. Nevertheless, experimentally the promotion effect of graphene was only examined on the composites obtained by either physical mixing or one-pot hydrothermal synthesis.7,10,29,30 In both cases, the TiO2 surface is covered with various chemically bound adsorbates prior to its contact with graphene. On the other hand, the applied graphene oxide usually contains a large number of hydroxyl and carboxyl groups.31-33 These species inevitably constitute an undesired intercalation layer between the graphene and TiO2, which impedes the efficiency of electron transfer between the two functional materials. Moreover, the contact area between graphene and TiO2 is usually rather limited as the mechanical intermixing approach can hardly achieve a uniform coating of graphene onto the surface of TiO2 particles. These factors


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obviously weaken the promotion effect of graphene for the photocatalytic activity of oxide semiconductors. Therefore, a lot of work has been conducted aiming at solving these problems. For instance, Rümmeli et al. has successfully grown nanographene on MgO nanocrystals via chemical vapor deposition (CVD) method.34 Choi and co-workers used a solvothermal method to realize a “quasi” coating of graphene oxide films over ZnO quantum dots.35 Notably, under their conditions the ZnO surface turned out to be only partially covered by graphene. Very recently, our group have developed a recipe to fabricate polycrystalline graphene layers directly on different rutile TiO2 single crystal surfaces via an ambient pressure chemical vapor deposition (AP-CVD) method using acetylene as the carbon source.36 With such strategy, the formation of interlayer species can be largely avoided due to the operating condition, thus achieving a truly seamless interface with intimate and uniform contact between graphene and TiO2 surface. Fitri et al. has synthesized polycrystalline graphene on commercial P25 nanoparticles but using methanol and propene as the carbon sources of a similar CVD process.37 However, the authors mostly focused on the fabricating recipe instead of the mechanism of charge transfer occurring at the graphene/TiO2 interface as well as of the relationship of photocatalytic activity versus graphene thickness. In this work, the uniform graphene adlayers have been successfully synthesized on the well-shaped anatase-TiO2 (a-TiO2) nanosheets through a similar AP-CVD method as we reported before.36 Under the controlled atmosphere and the elevated temperature of the CVD process, the intimate contact between graphene and TiO2 surface can be achieved, while the thickness of graphene can be manipulated by changing the growth parameters. On the obtained samples, synchronous blue shifts of the binding energies of Ti 2p and O 1s were observed and assigned to the interfacial dipole field induced by charge transfer from graphene to TiO2 at the ground state. Ultrafast transient absorption spectroscopy measurements reveal that under the excitation of light irradiation, the well-established graphene adlayers open a quick channel for separating the photo-exited charge carriers. Photocatalytic degradation of methyl orange (MO) demonstrate that ACS Paragon Plus Environment


the fabricated G/a-TiO2 has markedly enhanced activity than the conventional nanocomposite obtained by physical mixing of GO and a-TiO2. Moreover, the optimized performances were found pinned to the double layer graphene, which can be explained by the just established π-π stacking in bilayer graphene as well as the reduced graphen-TiO2 interactions for thick graphene adlayers.

2. METHODS 2.1.Materials Titanium butoxide (Ti(OBut)4, Shanghai Macklin Biochemical Co., Ltd, AR grade), hydroflupric acid (HF, Sinopharm Chemical Reagent Co., Ltd, 40%), deionized water (Millipore, 18.2 MΩ·cm), anhydrous ethanol (99.99%, Sinopharm Chemical Reagent Co., Ltd). Hydrogen (99.999%, Linde Industrial Gases), argon (99.999%, Linde Industrial Gases) and acetylene (99.5%, Linde Industrial Gases) were all used as received without further purification. 2.2.Preparation of anatase-TiO2 nanosheets In a typical process, 2.6 mL of Ti(OBut)4 was added dropwise into 20 mL of anhydrous ethanol solution and sonicated afterwards for 30 min before mixing with 0.4 mL HF. The obtained solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave and heated to 200 oC in an electric oven for 24 h. After that the autoclave was taken out and cooled down to room temperature. The precipitate was separated by centrifugation and washed thoroughly with ethanol and deionized water. Finally, the product was fully dried at 60 oC in a vacuum oven.38 2.3. Preparation of G/a-TiO2 nanocomposites 2.3.1 CVD growth of graphene on anatase-TiO2


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The CVD synthesis of graphene on anatase-TiO2 nanocrystals was performed in a horizontal quartz tube mounted inside a furnace (Thermo Fisher Sci.). The typical procedures are as follows: (1) Load the as-prepared TiO2 nanosheets (60 mg) into the quartz tube and heat to ~650 oC under an argon (Ar) flow of 100 sccm to remove the surface fluorite adsorbates. (2) Feed in 5 sccm H2 and 0.5 sccm C2H2 together with 100 sccm Ar while keeping the temperature constant at 650 oC for a controlled period of time. This time length defines the growth time. (3) To finish the growth, close H2 and C2H2, and anneal the sample at 650 oC in Ar for another 60 min. (4) Cool down the sample to room temperature in Ar before taking out from the furnace. Notice here that the growth temperature is selected by referring to our previous work of fabricating graphene on rutile-TiO2 single crystals.36 The thickness of graphene adlayer can be controlled by varying the growth time at the second stage. For the sake of simplicity, we use the notion G-t/a-TiO2 to define the specific sample that has undergone CVD growth for t minutes. For instance, G-60/a-TiO2 corresponds to the sample grown for 60 min while G-0/a-TiO2 represents the blank sample without acetylene and hydrogen fed into the oven. 2.3.2 Physical mixture of graphene oxide with calcined TiO2 For a comparative study, we also prepared G/TiO2 composites by a traditional physical intermixing method. We firstly calcined the as-prepared a-TiO2 nanosheets at 650 oC in Ar for 60 min. Then, typically 63 mg of the calcined TiO2 (termed as CT) was dispersed in 60 mL deionized water together with 1–15.8 mg of GO (Aladdin), in order to get different mass fractions of GO (1.5–20%). After 60 min sonication, the mixtures were centrifuged and dried in a vacuum oven. The dried samples were further calcined at 300 oC in Ar for 2 h.39 These samples are termed as “GO+CT” and were subjected to the same photocatalytic reaction as those prepared via CVD method for comparison. 2.4. Photocatalytic activity measurement



For the photocatalytic activity tests, typically 10 mg photocatalyst (G-t/a-TiO2 or other reference samples) was added into 100 mL solution of 12.5 mg/L methyl orange (MO) in water. Before irradiation, the suspension was sonicated for 5 min and stirred for 1 h in the dark to achieve the adsorption–desorption equilibrium. Under ambient pressure and continuous stirring, the solution was then irradiated using a 250-W high-pressure mercury lamp (central wavelength ~365 nm). A 3-mL sample solution was taken every 10 min after the reaction starts. The sample solution was centrifuged to remove the catalyst completely and then subjected to analysis with a UV–vis spectrophotometer (PERSEE). The specific absorption of MO at around 463 nm was used to quantify the concentration of the unreacted MO in the solution. In this way, the degradation rate of MO can be expressed by Ct/C0, where Ct is the absorption at a certain reaction time t while C0 is the absorption of the initial MO solution before irradiation. Furthermore, the reaction rate coefficient, k, can be derived by fitting the MO conversion curves to the first-order reaction kinetics. 2.5.Characterization methods Transmission electron microscopy (TEM) samples were prepared by depositing a drop of diluted suspensions in ethanol onto a micro-grid copper network. All TEM images were taken with an accelerating voltage of 200 kV (JEM-2100F). X-Ray diffraction (XRD) patterns of the samples were collected with TTR-III (Rigaku). X-Ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 spectrometer (VG Co., UK) with Al Kα source for excitation. Raman analysis was carried out on a LabRamHR Raman spectrometer (Horiba) using a 514.5-nm laser source. The ultrafast transient absorption (TA) measurements were performed, under ambient conditions, on a Helios pump–probe system (Ultrafast Systems LLC, USA) combined with an amplified femtosecond laser system (Coherent, USA). The 320-nm pump pulses (~100 nJ/pulse at the sample) were delivered by an optical parametric amplifier (TOPAS-800-fs), which was


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excited by a Ti:sapphire regenerative amplifier (Legend Elite-1K-HE; 800 nm, 35 fs, 3 mJ/pulse, 1 kHz) seeded with a mode-locked Ti:sapphire laser system (Micra 5) and pumped with a Nd:YLF laser (Evolution 30). The stable white-light continuum (WLC) probe pulses (in the visible region 420–650 nm) were generated by focusing the 800 nm beam (split from the regenerative amplifier, ~ 450 nJ/pulse) onto a CaF2 crystal plate. A reference beam split from the WLC was used to correct the pulse-to-pulse fluctuation of the WLC. The time delays between the pump and probe pulses were varied by a motorized optical delay line. The instrument response function (IRF) was determined to be ~100 fs by a routine cross-correlation procedure. A mechanical chopper operating at 500 Hz was used to modulate the pump pulses such that the TA spectra with and without the pump pulses can be recorded alternately. The temporal and spectral profiles (chirp corrected) of the pump-induced differential transmission of the WLC probe light (i.e., absorbance change) were visualized by an optical-fiber-coupled multichannel spectrometer (with a CMOS sensor) and further processed by the Surface Xplorer software.

3. RESULTS AND DISCUSSION 3.1. Synthesis and characterizations of the G/a-TiO2 nanocomposites Before the growth of graphene, well-defined a-TiO2 nanocrystals have to be firstly prepared for serving as the substrate. By following the reported recipe,38 but alternatively using ethanol as the promoter and hydrofluoric acid as the capping agent, we successfully synthesized a-TiO2 nanosheets with mainly (001) facets exposed. As shown by the high-resolution TEM images in Figure S1, the fabricated a-TiO2 nanosheets have a lateral size of 20–40 nm and thickness of 5– 10 nm. The as-grown a-TiO2 nanosheets were then subjected to the CVD growth of graphene. The TEM images are shown in Figure 1 as a function of growth time. One can see from Figure 1a-d that after CVD process the TiO2 nanosheets have grown larger and been transformed into more ACS Paragon Plus Environment


round shapes, possibly due to the removal of fluorite ligands as well as the aging effect under high-temperature treatments. However, the crystalline structure of a-TiO2 remains unchanged, as demonstrated by the XRD measurements shown in Figure S2. The statistical histograms in Figure 1e-h reveals that the averaged crystal sizes are more or less the same for the series of samples grown for different time, indicating that the acetylene cracking reaction may have little influence on the particle surfaces.

Figure 1. TEM images of graphene adlayers grown on anatase TiO2 by APCVD for different time. (a–d) Low magnification images, corresponding size distributions (e–h) and high resolution (i-l) images of the G-20/a-TiO2, G-30/a-TiO2, G-60/a-TiO2, and G-180/a-TiO2 samples, respectively.

The high-resolution TEM images in Figure 1i–l highlighted the side edges of the CVD samples, clearly revealing the gradual formation of graphene adlayers with the extension of growth time. Particularly, Figure 1j demonstrates that a monolayer of graphene can be formed on the TiO2 surface after about 30 min of growth. Growth times shorter than 30 min would only lead ACS Paragon Plus Environment

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to formation of graphene fragments (Figure 1i); whereas, those longer than 30 min obviously resulted in the formation of additional graphene layers. For example, the double layer formed at 60 min and multilayers (3~5 layers) at 180 min. Importantly, in all of the cases, the graphene films attach seamlessly on the surfaces of the a-TiO2 nanosheets. The resolved lattice fringes of the G/a-TiO2 particles, as shown in Figure 1i–l, were determined to be 0.35, 0.24, 0.23, and 0.17 nm, respectively, coinciding perfectly with the (101), (004), (112), and (105) atomic planes of the a-TiO2 crystals.40 This result also indicates that the graphene adlayers have uniformly covered the whole surface of a-TiO2 nanocrystals regardless of the exposed distinct facets.

Figure 2. (a) Raman spectra recorded on the series of G/a-TiO2 samples prepared at different growth times. (b) Expanded view of (a) in the range of 1100–3300 cm-1. (c) Plot of FWHM of the deconvoluted G-band versus the growth time.

In addition to the direct observation of graphene formation with TEM, we also conducted Raman spectroscopy measurements on our samples, considering its wide utilizations in disclosing the microscopic structures of various carbon materials, in particular graphene. On the blank a-TiO2 samples, as shown in Figure 2a, the Raman spectra show three prominent peaks at 140, 393, 516, and 637 cm-1 which are characteristic of the Eg, B1g, A1g or B1g, and Eg vibration modes of a-TiO2, respectively.41,42 These vibrations are reproduced nicely on the G/a-TiO2 samples, once again demonstrating that a prolonged thermal treatment did not change the anatase phase of TiO2. On the other hand, as shown in Figure 2a and b, along with the CVD growth additional ACS Paragon Plus Environment


vibrations in the range of 1000–3500 cm-1 gradually develop, which are characteristic of the carbon materials. More specifically, the symmetry-allowed E2g mode of sp2-bonded carbon atoms appears at ~1601 cm-1, which is commonly associated with the G-band. Meanwhile the D-band at ~1358 cm-1 increases in parallel, and is related to the vibration of sp3-hybridized carbon atoms near the K-point of graphene.43 These two vibrations grow synchronously with the extension of CVD time and show the same frequency as those found in GO and polycrystalline graphene,34,44,45 demonstrating the stepwise accumulation of graphene adlayers on the a-TiO2 nanosheet surfaces. The plot in Figure 2c shows that the full-width half-maximum (FWHM) of the deconvoluted G-band (see Figure S3 for the deconvolution of the D and G bands), which is reciprocal with the in-plane crystallinity,46 decreases sharply from the beginning to around 30 min and then enters a flat basin. The sharp transition at 30 min may be attributed to the formation of a complete graphene monolayer, after which the in-plane ordering would not change drastically any further. In addition to the prominent D and G bands, another two weak but discernable vibrations at around 2710 and 2950 cm-1 also develop gradually along with the CVD synthesis, which are assigned to the 2D and (D + G) modes, respectively. Their appearance further evidence the increasing degree of graphitization.47 It is noted that the Raman spectra of our samples are very similar to those of typical polycrystalline graphene, reflecting the relatively low quality of the fabricated graphene adlayers. This can be reasonably attributed to the low reaction temperature (~650oC) in our recipe, which is chosen for feasibly maintaining the surface structures as well as the crystallinity of the a-TiO2 nanocrystals.36


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Figure 3. XPS analyses of the G/a-TiO2 nanocomposites. (a) C 1s XPS spectrum with deconvolutions of the G-60/a-TiO2 sample. (b) High-resolution Ti 2p and (c) O 1s XPS spectra were shown as a function of CVD growth times.

To disclose the chemical state of the G/a-TiO2 nanocomposites, XPS measurements have also been performed. Full range X-ray photoelectron spectrum of a typical sample in Figure S4 yields no clues of any metallic species (e.g., Ni, Cu, and Pt) or fluorine, indicating that the entire process is completely free of metal and the final samples have no residual fluorine at the surface. Moreover, as shown in Figure 3a, the high-resolution C 1s XPS spectrum of a typical sample prepared by 60 min CVD growth shows an intense and sharp peak at 284.7 eV that corresponds to the C=C bond of a conjugated honeycomb lattice.48 Other binding energies (B.E.) with much weaker signals can be found at 285.7, 286.9, and 289.4 eV, which are assigned to the C–H, C–O, and O–C=O species, respectively.49-51 These species can be reasonably related to the defective sites of graphene that are formed either during CVD or upon exposure to air. The high-resolution XPS spectra of Ti 2p and O 1s are shown in Figure 3b and c, respectively. One can clearly see that the Ti and O signals gradually decrease along with the CVD growth, concomitantly accompanied by the stepwise blue-shift of the binding energies. Taking the blank sample as a reference, the G-20/a-TiO2 sample shows no change at all for both Ti 2p and O1s transitions. While for the G-40/a-TiO2 sample, where a complete graphene layer may have been formed on the a-TiO2 nanocrystal, a blue-shift of ~0.4 eV can be seen for both Ti 2p and O ACS Paragon Plus Environment


1s. Such a blue-shift rises to ~0.9 eV for the G-60/a-TiO2 sample on which a nominal bilayer graphene has been formed, as estimated from the TEM images shown in Figure 1. However, for even thicker graphene films grown for longer times, the Ti 2p and O 1s are stiffly fixed to 459.5 and 530.7 eV (Figure 3b and c), respectively. Similar core level shifts of TiO2 has been frequently reported for the GO/TiO2 nanocomposites prepared by variously other methods.52-57 They were generally attributed to the formation of new chemical bonds between GO and TiO2 considering the abundant hydroxyl and carboxyl groups in GO and the TiO2 surface. However, our synthetic condition is highly reductive and severely lacking oxygen which is a prerequisite for the formation of the above-mentioned species. Moreover, the identical shapes of both the Ti 2p and O 1s peaks throughout all the samples (see Figure S5) manifest the negligible correlation of the amounts of Ti3+ and OH species with the graphene formation. In fact, the systematic shifts of both Ti 2p and O 1s more likely suggest a delocalized field effect instead of the local effect of any chemical bonding. Graphene has a relatively lower work function compared with TiO2 (~4.5 eV versus ~5.1 eV).58,59 When both materials come to contact, the electrons immediately transfer from graphene to a-TiO2 in order to align the fermi level. Thus, a dipole field pointing from graphene toward the a-TiO2 is established which causes the downward band-bending of the core levels of all the detected atoms. Along this lead, the gradually changed B.E. shifts in Figure 3b and c actually reflect the amount of the electrons that have transferred from graphene to TiO2, which changes with the stepwise development of the atomic structure as well as electronic property of the graphene film. It continues growing after the completion of first layer and saturates until the formation of double layer graphene. This is because further growth of more graphene layers would not largely change the crystallinity as well as the electronic structure of the graphene adlayers (as dictated by the Raman spectra in Figure 2), and hence results in the saturation of the dipole field at the interface. On the basis of the above characterizations, we may briefly discuss the mechanism of graphene formation over the a-TiO2 surfaces. Generally on the oxide substrates, both the surface ACS Paragon Plus Environment

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cations and anions are considered playing important roles in catalyzing the carbon source molecules as well as stabilizing the graphene fragments. For C2H2, the precursor in our recipe, it can be readily dehydrogenated and graphitized under the catalysation of unsaturated metal ions.60,61 It has been well documented that the stoichiometric TiO2 surfaces are extremely inert to the acetylene molecules.62,63 However, on the reduced surfaces with highly unsaturated Ti3+ or Ti2+ cations in presence, the binding strength to acetylene molecules can be significantly increased and the cyclotrimerization reactions can take place. In fact, two decades ago Barteau et al. conducted temperature-programmed desorption (TPD) measurements of acetylene on the Ar-sputtered rutile-TiO2(001) surface and observed sufficient yields of ethylene and benzene.64 Recently, Jain et al. concluded from their combination investigations of Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and electron paramagnetic resonance (EPR) measurements, that acetylene would undergo a heterolytic dissociative chemisorption followed by cyclization reactions on the TiO2 surfaces at room temperature.65 In our CVD process, the highly reductive atmosphere would inevitably create a certain number of oxygen vacancies at the a-TiO2 surfaces where low-coordinated Ti ions are bound. These surface sites can facilitate the dissociative chemisorption and catalyzing cyclotrimerization of acetylene molecules to form various aromatics. Meantime, the elevated temperature of 650oC also provides a thermodynamic drive for the extension of small graphene fragments into continuous polycrystalline film. Further illustrations of the whole mechanism would certainly require more systematic spectroscopic studies based on plane surfaces of single crystals, which is beyond the scope of this paper.

3.2. Electron dynamics at the seamless interface of G/a-TiO2 As is well known, the photocatalytic activity is closely correlated with the interfacial dynamics of photo-induced charge carriers.66-68 In the current case, the charge transfer across the interface of graphene and a-TiO2 under the light irradiation plays a vital role in the related surface



reactions. We thus conducted ultrafast transient absorption (TA) spectroscopy experiments (using a femtosecond time-resolved pump–probe configuration) on the series of G/a-TiO2 samples to monitor the dynamics of photo-excited electrons therein. As shown in Figure 4a, all of the G/a-TiO2 samples (including the blank one) show a negative TA signal manifested as probe bleaching via a stimulated emission process. By fitting the spectral profiles with a double-exponential function,69 the average lifetime of the electrons at the transient excited states (termed as τavg), can be obtained and are presented in each spectrum. Markedly, the average lifetimes of the photo-excited electrons exhibit a volcano-shaped relationship with the CVD growth time, as plotted in Figure 4b. Such a volcano-shaped profile reaches its maximum at 30 min, corresponding to the formation of a complete monolayer of graphene (see the TEM image in Figure 1). While for longer growth times (i.e. thicker graphene), the lifetimes tend to get shortened.


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Figure 4. (a) Ultrafast TA spectra (pump at 320 nm; probe at 500 nm) recorded on the series of G/a-TiO2 samples. (b) The average lifetime of the photo-excited electrons as a function of the graphene growth time. (c) Schematic illustration of the charge transfer mechanism in the G/a-TiO2 composite system. The TA signals (i.e., the absorbance changes, or ∆Abs.) are given in units of mOD (OD, optical density).

A general picture describing the involved mechanisms is depicted in Figure 4c. As for the blank a-TiO2, upon photoexcitation the valence-band (VB) electrons are promoted to the conduction band (CB), which then quickly relax into certain band-gap defect states followed by recombination with the VB holes. However, when graphene is intimately attached to the TiO2 surface, the photo-excited electrons can find a new relaxation channel via graphene that acts as an effective electron retainer. In the early stage of graphene growth, mainly many small carbon clusters are formed on the surface of TiO2 accompanied by the increase of the defect concentration of the TiO2 bulk. Both may function as the trapping centers (band-gap states) of the charges thus account for the elongation of the electron lifetimes as compared to the blank TiO2. Such effect would dominate until the formation of a complete layer of graphene. After that, further growth of more graphene adlayers on the TiO2 surface may not significantly modify the density of defect states in TiO2, while bring on the gradual improvement of the crystallinity as well as the electron capacity of graphene layer. In this manner, the graphene-mediated channel become more and more predominant, and the electrons tend to transfer to the graphene film. As a result, the lifetimes of the photo-excited electrons get shorter and shorter, as observed in Figure 4b. Therefore, the ultrafast TA spectroscopy results has unambiguously revealed the role of graphene in distributing the photo-induced charge carriers, which in turn rises the reaction efficiency of the molecules in contact with the surfaces of the G/TiO2 nanocomposite.



3.3. Photocatalytic activity tests of the G/a-TiO2 nanocomposites After unveiling the uniform and seamless interface of the CVD fabricated graphene with the a-TiO2 nanosheets, we finally subjected the G/a-TiO2 nanocomposite to photocatalytic reactions to examine the promotion effect. Figure 5 shows the degradation rates of methyl orange measured in water using various G/a-TiO2 samples as the photocatalysts, the reaction being routinely adopted to evaluate the photocatalytic performance of materials. As shown in Figure 5a, in the absence of TiO2 catalyst MO does not degrade at all under UV irradiation. Moreover, test experiments conducted under visible light irradiations excluded the self-sensitization effects of MO in the presence of either bare TiO2 or G/a-TiO2 composites, as can be found in Figure S6 of the supporting information. In contrast, under the UV irradiations the inclusion of graphene adlayers drastically improved the reaction efficiency of the bare a-TiO2 nanosheets even for the smallest coverage of graphene on the surface (i.e., G-10/a-TiO2). Moreover, the promotion effect gets enhanced with increasing the amount of graphene until the formation of double layer coverage, i.e., the a-TiO2 subjected to 60 min of CVD growth. After that, more graphene covering on the surface does not bring about more pronounced promotion effect, but rather contributes negatively to the reaction, despite more graphene may cause gradually increased adsorption of visible light as indicated by the UV-Vis diffuse reflectance measurements (Figure S7). For photocatalytic activity comparison, we also recorded the reaction rates by using commercial P25 (mixture of rutile- and anatase-TiO2 particles with an average size of ~10 nm) as well as the physical mixture of calcined a-TiO2 (CT) nanosheets and graphene oxide (GO) with optimized composition (~ 3% mass fraction for GO) as the photocatalysts (see Figure S8 for the optimization of the GO+CT mixture). The two reference catalysts turned out to degrade MO with much lower efficiency compared to the G-60/a-TiO2 catalyst, as shown in Figure 5a and b. These results clearly demonstrate that the seamless contact of graphene and the a-TiO2 nanosheets is the optimal configuration for achieving the best photocatalytic activities.


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Figure 5. (a) Liquid-phase photocatalytic degradation of MO as a function of the UV-irradiation time. (b) Plot of the deduced reaction rate coefficient k versus the graphene growth time.

The noteworthy phenomenon is the volcano-shaped relationship of the photocatalytic activity versus the graphene coverage, as reflected by the plot of the reaction rate coefficient (k) versus CVD growth time in Figure 5b which peaks at 60 min (corresponding to double-layer graphene). Such a trend also occurs in our photocurrent measurements of the G/a-TiO2 samples, as shown in Figure S9, despite the relative instability of the transient current signals at the early cycles. These results indicate that the density of the photo-excited electrons grows along with the coverage of graphene and reaches the maximum for the case of double layer. We notice that a recent theoretical study reported the significant difference between single layer and bilayer graphene over MgO(111) surface.70 It was demonstrated that single layer graphene would form a number of chemical bonds with the substrate. Whereas only for the bilayer graphene the π–π stacking is established, which is quite beneficial for the charge transfer across the graphene/MgO interface. We postulate that in our G/a-TiO2 system a similar situation occurs. The single layer graphene may contain a considerable number of defects, interlinking the TiO2 surface via the C– O bonds (as revealed by the XPS data in Figure 3). Such chemical bonds directly strengthen the interaction of graphene with the TiO2 surface, yet also set up barriers for the smooth flow of photo induced electrons between graphene and TiO2 which accumulate to the peak point until the ACS Paragon Plus Environment


formation of full monolayer graphene, as also evidenced by the ultrafast TA spectroscopy measurements in above. With additional graphene layer was grown, the concentration of the defects at the first layer may be reduced. The well-structured graphene also establishes an efficient channel for separating the photo-excited electrons and holes. Furthermore, the electron capacity should increase significantly owing to the established π–π stacking between the graphene layers. As such, the electron transfer reaches the saturation for the double layer graphene over a-TiO2. For even thicker graphene films, the electronic properties of the adlayers have maintained more or less the same as the double layer graphene, whereas the graphene-TiO2 interactions may be slightly reduced due to the perfection of the graphene lattice as well as the increased interlayer interactions. Therefore the effect on the separation of the photo-induced electron-hole pairs may be slightly reduced. In addition, with more graphene covered, the available hot sites for the molecular reactions may be gradually decreased at the surface. All these factors together lead to the slowing down of the photocatalytic degradation of MO along with the further thickening of the graphene films. Consequently, the overall effect shows a volcano-shaped behavior peaking at the coverage of double layer graphene.

4. CONCLUSIONS In conclusion, we have developed a novel recipe to grow graphene with controlled thickness directly over the surface of anatase-TiO2 nanosheets. This strategy enables us to maximize the photocatalytic activity of the G/TiO2 composite by avoiding the interfacial contaminates as well as ensuring the contact uniformity of G/TiO2, which thus constitutes a proper platform for investigating the interface dynamics of the photo induced charge carriers. Moreover, the double layer graphene has specially dictated the most proper interactions with the a-TiO2 surface hence leads to the extreme points of the a number of photo-induced properties. This work not only opens a fresh perspective for in-depth understanding of the interaction between graphene and


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oxide semiconductors, but also sheds new light on the graphene-functionalized catalytic reactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional experimental characterizations of the G/a-TiO2 samples including the TEM, XRD, XPS, Raman, and photocurrent measurements.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]. Telephone: +86-551- 63600765. E-mail: [email protected]. Telephone: +86-551- 63607736.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We are grateful to the financial support from the NSFC (91545128, 21333001, 21573211, 21633007), the MOST (2014CB932700, 2016YFA0200602), the Fundamental Research Funds for the Central Universities (WK2340000063), and the Thousand Talent Program for Young Outstanding Scientists of the Chinese government.



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CVD growth of graphene over anatase-TiO2 nanosheets ensures a clean and uniform interface and demonstrate an enhanced photocatalytic activity 163x100mm (300 x 300 DPI)


Graphene Grown on Anatase-TiO2 Nanosheets - ACS Publications

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