Graphene oxide and Ag engulfed TiO2 nanotube arrays for enhanced electrons mobility and visible-light-driven photocatalytic performance

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Introduction
−9 TNTs possess larger surface area, vectorial charge transfer, long term stability to photo and chemical corrosion. 10,11However, the photocatalytic reactions of TNTs are limited by the low absorption capability in visible light region and the high recombination rate of photogenerated electron-hole pairs formed in photocatalytic activity.Consequently, significant efforts have been devoted to improve the photocatalytic efficiency of TNTs, such as doping metal ions, 10,12,13 non metal 5,14 and coupling with semiconductor nanoparticles. 15,16oble metals, such as gold (Au) and silver (Ag), possess an additional ability to absorb visible light due to the existence of a localized surface plasmon resonance (LSPR). 17Moreover, these metals function as electron donors to promote electron transfer from metal to TiO 2 and act as electron traps in the metal-TiO 2 nanostructures minimizing the surface charge recombination in TiO 2 . 18,19Alternately, conducting carbon materials are incorporated into TiO 2 to promote the electron transport.
Graphene is a two-dimensional sp 2 -hybridized carbon nanosheet which possesses high specific surface area with a large interface, high electron mobility and tunable band gap. 20Metal or metal 50 oxides (Ag, Au, TiO 2 and SnO 2 ) are combined with graphene and the resultant hybridized materials exhibit superior photocatalytic properties than the bulk metal or metal oxide. 21,22Nevertheless, hydrophobic graphene is not compatible with hydrophilic metal/metal oxides, which makes graphene difficult to deposit on 55 the surface of metal/metal oxides. 23n the other hand, graphene oxide (GO) is a layer-structured graphite compound built up by hydrophilic, stacked graphene sheets bond to oxygen in the form of carboxyl, hydroxyl or epoxy groups. 24,25Better solubility of GO in water and other solvents 60 allows to ease their deposition onto the surface of metal/metal oxides.Song et al. demonstrated methylene blue (MB) removal by GO/TNTs with a 15 times increase in the photoconversion efficiency. 26Gao et al. reported that GO-TiO 2 hybrids exhibit higher adsorption ability for methyl orange (MO) than GO, and 65 about ~55% of MO was absorbed by the GO-TiO 2 hybrids at the beginning of the photodegradation. 23Utilising the similar hybrid materials, Jiang et al. showed that the photo-oxidative degradation rate of MO and the photo-reductive conversion rate of Cr (VI) over the hybrids were as high as 7.4 and 5.4 times that enhancing effect of GO on the photocatalytic properties of TiO 2 was attributed to a large surface area, adsorption capacity, and strong electron transfer ability of the GO in the hybrid materials.
Most of the findings on graphene-semiconductor composites are of nanoparticles while TNTs are scant.Indeed, TNTs can be combined with GO for practical application.In this work, we attempt to enhance photocatalytic performance of TNTs by using Ag and GO as an electron transfer channel or electron sink to reduce the recombination of photogenerated electron-hole pairs.Similarly, Tang et al. showed that the composite of Ag, reduced graphene oxide (RGO) and TNTs exhibited 2,4dichlorophenoxyacetic acid removal efficiency of almost 100%, much higher than 49% over Ag-TNTs. 28The Ag particles were respectively deposited onto the surface of TNTs and RGO, forming Ag/TNTs and Ag/RGO-TNTs.However, it is inappropriate to compare the removal efficiency of both photocatalysts because the degradation mechanism might act differently according to the different location of Ag NPs.Many cases have mentioned that GO served as an electron sink to hinder electron-hole pairs recombination, 23,26,29,30 but indeed the role of GO could change in different pollutant model.The present work offers several advantages over previously reported ones, including (1) deposition of Ag particles onto the surface of TNTs instead of GO to draw more conclusive results of comparison, (2) a low cost and a facile assembly method to deposit GO onto Ag/TNTs, (3) the photocatalytic activities were examined by comparing the photocatalytic degradation between methylene blue (MB) and 2-chlorophenol (2-CP), considering that photocatalyst could respond differently to different types of pollutants.

Experimental Preparation of GO
Graphite oxide was synthesized through simplified Hummers method, 31 3 g of nature graphite powder (99.99%,Sigma-Aldrich) were oxidized by a mixture of 400 mL of H 2 SO 4 and 18 g of KMnO 4 .The mixture was stirred for three days to ensure complete oxidation of the graphite.Then, H 2 O 2 solution was added to stop the oxidation process.The graphite oxide was washed with 1 M of HCl and DI water until a pH 4−5 was achieved.During the washing process, the graphite oxide underwent exfoliation to form GO gel.It was then vacuum dried at 60 °C for 24 h to obtain brownish GO solid.

Preparation of Ag/TNTs and GO-Ag/TNTs
Ti foil (99.7%, Sigma-Aldrich) was first anodized in ethylene glycol (anhydrous, 99.8%) electrolyte containing 0.3 M ammonium fluoride (NH 4 F, 98%) and 2 vol % water (H 2 O) with graphite rod as the counter electrode under 50 V for 3 h.After annealing at 450 °C for 1 h, the anodized sample was sonicated for 30 min and then annealed for 2 h.Photodeposition of Ag on TNTs were carried out by dipping TNTs in an equal volume ratio of methanol-water mixture containing 1 mM of AgNO 3 .The surface of the TNTs was exposed to 400 W high pressure Hg lamp for 1 h under nitrogen atmosphere with sonication.The resulting product was designated as Ag/TNTs.The procedures for the preparation of GO-Ag/TNTs are illustrated in Fig. 1.Thus obtained Ag/TNTs were immersed in a 0.5 mg mL -1 aqueous GO suspension for 5 h and vacuum dried.The modified composite material was denoted as GO-Ag/TNTs.

Characterization
The phase composition of the synthesized photocatalysts were obtained using X-ray diffractometer (XRD, D8 Advance, Bruker) 65 operated in the reflection mode with Cu Kα radiation (λ = 1.54 Å).The morphologies of samples were examined by a field emission scanning electron microscope (FESEM, SU8000, Hitachi) equipped with an EDS (energy dispersive X-ray spectroscopy) detector.The images were taken at an accelerating 70 voltage of 20 kV.High resolution transmission electron microscope (HRTEM, JEM-2100F, Jeol) images were obtained at 200 kV.A Micro-PL/Raman spectroscope (Renishaw, inVia Raman Microscope) was used to acquire the Raman and photoluminescence (PL) spectra.Fourier transform infrared 75 (FTIR) spectra were obtained on a Perkin Elmer Spectrum 400 spectrophotometer with scan range of 4000−450 cm -1 .UV-vis diffuse reflectance spectra (UV-DRS) were measured using UVvis spectrophotometer (UV-2600, Shimadzu) equipped with an integrating sphere attachment.The spectra were collected with 80 BaSO 4 as a reference.The surface chemical composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS, Omicron, Germany) with Al Kα radiation source.

Photocatalytic experiment
The photocatalytic activities were evaluated based upon the 85 removal of methylene blue (MB) and 2-chlorophenol (2-CP) in aqueous solutions.For comparison, the photocatalytic activities of TNTs, Ag/TNTs and GO/TNTs were also studied.The prepared photocatalysts were immersed in a glass beaker containing 100 mL aqueous solutions for MB (5 mg L -1 ) and 200 90 mL aqueous solutions for 2-CP (10 mg L -1 ), respectively.Prior to photodegradation, the solutions were magnetically stirred in a dark for 1 h to establish an adsorption-desorption equilibrium.A 500 W tungsten-halogen lamp was used as visible light source, with any UV light below 400 nm was removed with a high-pass 95 filter (FSQ-GG400, Newport Corp.).For the degradation of MB, the samples were collected at regular interval, analyzed for residual MB concentration with visible spectrometer (Spectroquant ® Pharo 100, Merck) at λ max = 664 nm.Similarly, 2-CP samples were withdrawn at regular interval, centrifuged and

Results and discussion
The FESEM images show that the synthesized TNTs are uniformly stacked in tubular structure and vertically orientated with a tube diameter ranging from 100−120 nm and wall thickness of 15 nm.The cross-sectional image in Fig. 2a reveals that the tube length ranging from 8−9 µm.As shown in Fig. 2b, the photodeposited Ag particles have a wide range of sizes and shapes.After the photodeposition with a sonication process, the surface of TNTs is covered by Ag NPs with a near smaller average particle size of 100 nm (Fig. 2c).GO-Ag/TNTs sample was obtained with a sheet of GO coating the most surface of the TNTs, as shown in Fig. 2d.It is observed that GO has a flake-like structure with wrinkles and folds, which is consistent with the earlier reports. 21,32,33It is the characteristic features of GO when it is not conformally coated on the surface of TNTs. 34The inset in Fig. 2d is the corresponding EDX spectrum, confirming the presence of C, Ag, O and Ti in GO-Ag/TNTs.As illustrated in Fig. 2e, the Ag NPs were deposited onto the surface of the TNTs and even inside the tubes.The synthesized Ag and TiO 2 can be clearly identified by the lattice fringes shown in the HRTEM image of GO-Ag/TNTs (Fig. 2f).The lattice fringes with 0.24 nm and 0.35 nm spacing are attributed to Ag (1 1 1) and anatase TiO 2 (1 0 1) planes, respectively. 35,36ig. 3 depicts the XRD of graphite, GO, TNTs, Ag/TNTs and GO-Ag/TNTs.Pure anatase TiO 2 phase is observed in TNTs, Ag/TNTs and GO-Ag/TNTs.The two obvious peaks of tetragonal TiO 2 anatase phase (JCPDS no.21-1272) appeared at  10.6° was observed for GO, indicating most of the natural graphite was oxidized into GO by expanding the d-spacing from 3.37 Å to 8.6 Å.This indicates the introduction of oxygencontaining groups on the GO sheets. 21However, there is no peak ascribed to GO can be observed in the sample of GO-Ag/TNTs 55 due to the low amount of GO which is below the detection limit of XRD. 37,38The average crystallite sizes of TiO 2 anatase and Ag particles were calculated using Scherrer equation: where β is the full width half maximum (FWHM) θ peak, K is the 5 shape factor taken as 0.89 for calculations, λ is the wavelength of X-ray (0.154 nm), and θ is the diffraction angle.There is no significant change in the crystallite size of anatase TiO 2 in pure TNTs (33.81 nm) and GO-Ag/TNTs (33.12 nm), proving that a large part of Ag particles with crystallite size of 45.16 nm were 10 not incorporated in TiO 2 lattice, but deposited on the surface of the matrix instead.
The UV-vis diffuse reflectance spectra (UV-DRS) of TNTs, Ag/TNTs and GO-Ag/TNTs are shown in Fig. 4. As expected, the sample of TNTs shows an absorption band lower than 380 nm 15 (UV region) due to the charge transfer from O 2p valence band to Ti 3d conduction band. 39A broad absorption peak at approximately 460 nm is observed for Ag/TNTs, which is attributed to the surface plasmon absorption of Ag NPs. 40This pristinely shows the presence of metallic Ag NPs on the surface 20 of TNTs.In addition, GO-Ag/TNTs shows higher light absorption capacities in the entire visible region due to the presence of GO.Raman spectra of graphite, GO, TNTs, Ag/TNTs and GO-Ag/TNTs are depicted in Fig. 5. Four distinct Raman peaks of anatase TiO 2 can be observed at 145 (E g ), 399 (B 1g ), 519 25 (A 1g + B 1g ) and 639 cm -1 (E g ) for the samples of TNTs, Ag/TNTs and GO-Ag/TNTs.It further proved that all the combinations of synthesized samples resulted in 100% anatase phase.It is expected that GO and GO-Ag/TNTs have two peaks at around 1595 cm -1 and 1350 cm -1 , corresponding to the G-and D-bands, 30 respectively.The G-band appearing around 1595 cm -1 is the significant characteristic of sp 2 hybridized carbon materials, which can provide information on the in-plane vibration of sp 2bonded carbon domains. 41,42Whereas, the D-band appears at around 1350 cm -1 indicate the presence of sp 3 defects within the 35 hexagonal graphitic structure 43 and can be associated with the amorphous carbon, or edges that break the symmetry and The D-band and G-band of GO-Ag/TNTs were roughly at the 50 similar position to that of GO.However, the ratio of I D /I G for GO-Ag/TNTs is 0.95 which is slightly higher than GO, showing a marginal decline of graphitic domains.
FTIR spectra were employed to characterize the carbon species in the prepared samples.Fig. 6 shows the FTIR spectra of GO, 55 GO-Ag/TNTs, TNTs and Ag/TNTs.GO exhibits many strong absorption peaks corresponding to the stretching of hydroxyl group (3300 cm -1 ), C=O groups in carbonyl and carboxyl moieties (1720 cm -1 ), C=C skeletal vibration bands from unoxidized graphitic domains or contribution from the stretching 60 deformation vibration of intercalated water (1620 cm -1 ), carboxyl group (1375 cm -1 ), epoxide C−O−C or phenolic C−O−H stretching vibrations (1220 cm -1 ), and C−O stretching vibrations in epoxy or alkoxy groups (1045 cm -1 ). 35,46,47For GO-Ag/TNTs, most of these groups are retained with a significant decrease in 65 the peak intensity due to the lower GO dosage in the synthesis.The disappearance of C−O stretching band at 1220 cm -1 suggests that epoxide or phenolic groups in GO react with the surface hydroxyl groups of Ag/TNTs and finally form the Ti−O−C bonds in the GO-Ag/TNTs composite.The absorption peaks appear at 70 800 cm -1 can be assigned as a combination of Ti−O−Ti vibration in crystalline TiO 2 and Ti−O−C vibration. 48he PL spectra in Fig. 7a were obtained to understand the emission mechanism of the prepared samples.PL emission intensity is related to the recombination rate of excited electron- Ag/TNTs are obviously quenched as compared to that of TNTs.The quenching behavior revealed that both the GO and Ag trap electron or transfer electron to suppress electron-hole pairs recombination.The effective charge carrier separation could extend the reactive electron-hole pairs lifetimes and enhance the photocatalytic activity of GO-Ag/TNTs.High-resolution XPS was performed to determine the chemical composition and the oxidation state for GO-Ag/TNTs.As shown in Fig. 7b, there are two peaks observed at 459 eV (Ti 2p 3/2 ) and 464.6 eV (Ti 2p 1/2 ), both correspond to Ti 4+ in pure anatase.The presence of Ag NPs can be detected at two peaks centered at 368.2 eV and 374.2 eV, which is assigned to Ag 3d 5/2 and Ag 3d 3/2 , respectively (Fig. 7c).As shown in Fig. 7d, the C 1s XPS signals were deconvoluted into three components.The peak at 284.5 eV is assigned to the sp 2 carbon atoms of GO.The peaks at higher binding energies are assigned to the oxygenated carbon species of GO, such as C-OH, C=O and COOH. 21,35The contact between GO and TNTs can be proved by the presence of Ti-C (281 eV) and Ti-O-C (288.7 eV) signals.The former one is attributed to the formation of Ti-C bond in the interface between GO and TNTs.The coordination between carboxyl groups of GO and Ti(OH) x form Ti-O-C bond. 49The XPS results show that oxygenated groups of GO were retained in GO-Ag/TNTs and the formation of Ti-O-C bond, which is in good agreement with the FTIR results.
The photocatalytic activity of the prepared GO-Ag/TNTs sample was evaluated by the degradation of MB and 2-CP under visible light irradiation as depicted in Fig. 8a and Fig. 8b.For the adsorption process in the dark, both of GO/TNTs and GO-Ag/TNTs exhibited the adsorption capacity of almost 34% for MB and 12% for 2-CP, which is higher than the other samples.
The reason for the high adsorption capacity of MB on the surface of GO is attributed to the strong π-π stacking interactions between the benzene rings of MB and the surface of GO. 50A significant decrease in the adsorption capacity of MB is observed for GO-Ag/TNTs after the first run, while it remains almost unchanged from the second to sixth run (Fig. 8c).It can be explained that the chemisorptions which is irreversible plays a dominant role at    2-CP since there is no significant loss in the adsorption capacity after many runs.The initial concentration (C 0 ) is considered as the concentration of MB and 2-CP after adsorption-desorption equilibrium.As shown in Fig. 8a, the degradation efficiency of MB follows an order of GO-Ag/TNTs (68.3%) > GO/TNTs (57.2%) > Ag/TNTs (37.6%) > TNTs (27.9%).Fig. 8b shows the degradation efficiency of 2-CP follows an order of GO-Ag/TNTs (66.8%) > Ag/TNTs (57.7%) > GO/TNTs (56.2%) > TNTs 15 (42.6%).These results exhibited that the degradation efficiency of both MB and 2-CP is comparable in the first run and also improved remarkably in the presence of GO, particularly with the coexistence of Ag and GO.In most cases, GO sheets were used as an electron sink to facilitate photogenerated electrons 20 separation and store the separated electrons. 51The degradation mechanism of MB in Fig. 9a shows that GO can accumulate the electrons injected from the photogenerated MB because of the πconjugated network and higher work function of GO than that of the excited MB.However, the injected electron could recombine 25 with the surface adsorbed MB˙+ to lower the degradation efficiency.Besides that, the direct transfer of photogenerated electrons from TNTs to GO is restricted by the limited contact between GO and TNTs.Therefore, the photocatalytic activity of GO/TNTs is lower compared to that of GO-Ag/TNTs in the 30 degradation of MB and 2-CP, respectively.Ag NPs were deposited onto the surface of TNTs prior to the decoration of GO to overcome these limitations.Ag NPs able to absorb visible light due to the existence of a localized surface plasmon resonance (LSPR), 17 resulted in a better degradation efficiency for Ag/TNTs 35 (37.6 % for MB and 57.7 % for 2-CP) compared to that of TNTs (27.9% for MB and 42.6% for 2-CP).−54 Band gap energy (E g ) of GO is mainly formed by the anti-bonding π* orbital as a 40 conduction band with a higher energy level and the O 2p orbital as a valence band. 55,56It has been reported that Ag + can be reduced in graphene/TiO 2 photocatalytic systems. 57This shows that the energy level of the anti-bonding π* orbital is higher than that of Ag NPs, and thus the electrons transfer from graphene to Ag + .In this case, it is appropriate to conclude that electrons can be injected from the excited GO to Ag NPs. 58hen Ag NPs and TNTs are in contact, a Schottky barrier is formed at the interface of Ag NPs and TNTs.−64 Herein, the electrons generated by the LSPR effect in Ag NPs diffuse into the CB of TNTs.The TNTs function as an electron reservoir by capturing the electrons transferred from the 55 GO and Ag to further increase the degradation efficiency of MB.GO served as an electron-accepting mediator between the MB and Ag NPs, which is consistent with the previous studies. 65,66lternatively, the excited MB can also transfer electrons to TNTs and Ag due to its lower work function (3.81) than Ag (4.26) and 60 lying above the conduction band of TNTs (4.2 eV).However, the electron transfer rate is slower because the deposited GO blocked the tube openings as visualised in FESEM, and perhaps decreased the effective area of Ag/TNTs for the electron transfer.
For the degradation of MB, GO-Ag/TNTs demonstrated a 65 tremendous decreasing trend by 27% after the first run and followed by a significant loss of 36.7% after the sixth run (Fig. 8c).It can be speculated that the active sites of the GO can be undesirably occupied by the adsorbed MB through chemisorption which cannot be eluted, resulting in decreased photocatalytic 70 activity after the first run.The involvement of some functional groups on the surface of GO in the adsorption of MB is shown in Fig. S1, ESI †.In contrast, GO-Ag/TNTs showed a greater stability in the reuse study of 2-CP with a total loss of 19.5% as the physically adsorbed 2-CP can be eluted and more active sites 75 in GO are available for the degradation or charge transfer.It has been demonstrated previously that the 4-chlorophenol (4-CP) and other phenolic compounds can be degraded under visible irradiation due to the charge transfer surface complex formation between the phenolic compound and TNTs. 67Such a surface 80 complex enables the excitation by visible light through ligand-tometal charge transfer (LMCT) between the 2-CP (ligand) and the Ti 4+ site on the surface. 68,69Since 2-CP is one of the phenolic compounds, the surface complex formation is taken into account when degrading 2-CP (Fig. 9b).This explains for higher 85 degradation efficiency of 2-CP (42.6%) than MB (27.9%) for TNTs.The electrons are transferred from TNTs/2-CP surface complex to conduction band of TNTs.These electrons were subsequently injected to Ag NPs and finally to GO which served as an electron sink to facilitate the separation of the excited  S1, ESI † and Table S2, ESI †, respectively.

Conclusions
We have successfully deposited GO onto the surface of Ag/TNTs using a simple impregnation method to synthesize GO-Ag/TNTs.The hydrophilic behaviour of GO enabled its deposition onto the surface of Ag or TNTs.Ag NPs with average size of 100 nm were deposited onto the surface of TNTs and inside the tubes.A series of characterization works including FTIR, XPS and XRD confirmed the presence of oxygenated groups in GO after the oxidation of graphite.PL spectra clearly portrait the excellent electron-hole pairs separation performance of TiO 2 rendered by both Ag and GO deposition.The prepared composite photocatalyst displayed superior photocatalytic activity under visible light irradiation.A duality contribution was unveiled by 15   the GO where it acts not only as an electron sink for 2-CP degradation but also as an electron-accepting mediator for MB degradation.GO-Ag/TNTs well acted upon 2-CP than MB with higher repeatability and stability.The photocatalytic degradation mechanism clearly shows that every specific pollutant has a 20 unique mechanism.Thus it presents a new insight on the utilization of Ag and GO for efficient visible light driven photocatalytic systems for pollutants removal.

Fig. 1
Fig. 1 Schematic diagram of the procedures for the preparation of GO-60

Fig. 2
Fig. 2 FESEM images of the (a) cross-section of GO-Ag/TNTs, top view of Ag/TNTs (b) with no sonication, (c) with sonication, (d) top view of GO-Ag/TNTs.The inset of (d) is the EDX of GO-Ag/TNTs and (e−f) HRTEM images of GO-Ag/TNTs.

Fig. 5
Fig. 5 Raman spectra of (a) TNTs (b) GO-Ag/TNTs and (c) Ag/TNTs.The inset is the D and G band of graphite, GO and GO-Ag/TNTs.

Fig. 9
Fig. 9 Schematic diagrams of electron transfer and degradation mechanism of (a) MB and (b) 2-CP.

90 electrons. 2 −.
On the other hand, the LSPR effect in the Ag NPs provides electrons to the CB of TNTs.59−64The electrons react with O 2 to produce superoxide radical anion • O While the photogenerated holes oxidize the organic molecule in MB or 2-CP to form R + , or react with OH -or H 2 O and then further 95 oxidizing them into • OH radicals.The resulting • OH radicals are strong oxidizing agent to oxidize MB dye or 2-CP to endproducts.The photocatalytic degradation of MB and 2-CP followed pseudo second-order reaction kinetics (Fig.S2, ESI †and Fig.S3, ESI †).The kinetic parameters of both pollutants are tabulated in Table