Improved Synthesis of Reduced Graphene Oxide-Titanium Dioxide Composite with Highly Exposed { 001 } Facets and Its Photoelectrochemical Response

1 Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia 2 Department of Chemistry, Faculty of Science, University Putra Malaysia, 43400 UPM, Serdang, Selangor (Darul Ehsan), Malaysia 3 Department of Physics, Masjed-Soleiman Branch, Islamic Azad University (I.A.U.), Masjed-Soleiman 64914, Iran 4Department of Physics, Ahwaz Branch, Islamic Azad University, Ahwaz 63461, Iran


Introduction
Titanium dioxide (TiO 2 ) has been widely studied owing to its nontoxic, chemically inert, photostable characteristics, and cheap production cost, which make it a promising candidate for energy and environmental applications [1][2][3].Over the past decade, significant progress has been made in crystal facet engineering of TiO 2 crystals, to enhance performance for applications, such as photocatalysis, lithium storage, dye sensitised solar cells, and gas sensors [4][5][6][7].Generally, TiO 2 crystals can be described in terms of three main structures: anatase, rutile, and brookite.In recent years, focus has been directed toward anatase TiO 2 due to its higher activity, especially as a heterogeneous photocatalyst.The anatase phase of TiO 2 has a truncated tetragonal bipyramid structure with two {001} facets and eight {101} facets [8].According to the Wulff construction method for determining equilibrium crystal shapes [9], anatase TiO 2 crystals are usually dominated by {101} facets (more than 90% of the total number of facets), which have a lower surface energy (0.44 J m −2 ) compared to the reactive {001} facets (0.96 J m −2 ).Under equilibrium conditions, the lower surface energy plane is favourable during crystal growth to minimise the total surface energy, causing reactive {001} facets to diminish quickly, while encouraging the dominance of the thermodynamically stable {101} facets [10].Hence, the percentage of exposed reactive {001} facets is greatly reduced, resulting in loss of performance in value-added applications [11].This field was started by Yang and coworkers, who produced crystals consisting of approximately 50% reactive {001} facets by using 2 International Journal of Photoenergy hydrofluoric acid (HF) as a capping agent [12].Consequently, more effort is being directed toward improving the synthesis of TiO 2 structures with a high percentage of exposed {001} facets, including control of the concentration of precursor [13], selective use of solvents [14,15], and use of fluorinesourced chemicals (HF, NH 4 F), in which fluorine species were discovered to have the potential to stabilise {001} facet growth [16,17].
The performance efficiency of TiO 2 crystals can also be enhanced by doping with metal ions or nonmetal ions, noble metal loading, or attaching the crystals onto large surface area materials such as graphene sheets [18][19][20].Recently, graphene, a two-dimensional sheet of sp 2 bonded carbon atoms densely packed in a honeycomb crystal lattice structure, has attracted much attention due to its exceptional electronic, biological, mechanical, and thermal properties [21][22][23][24].The efficiency of graphene-based semiconductor photocatalysts is vastly increased compared to that of pure TiO 2 , which was evident in terms of extended light adsorption range, enhanced charge separation, and high dye adsorption [25].These beneficial characteristics of the composite materials would have great potential in enhancing photoelectrochemical performance applications.In addition, the role of the increased exposed {001} facets of TiO 2 on graphene could be demonstrated in its photoelectrochemical performance properties.
In this work, a fluorine-free solvothermal route is reported to produce highly exposed {001} facets in anatase TiO 2 nanosheets patterned on reduced graphene oxide (RGO) sheets.Photoelectrochemical behaviour was investigated based on the materials photocurrent response for various electrodes sintered at different temperatures.

Preparation of Graphene Oxide (GO)
. GO aqueous suspension was synthesized using a simplified Hummers method [26].Graphite flakes (3 g) were slowly added into a H 2 SO 4 and H 3 PO 4 (9 : 1) solution.Following this, 18 g of KMnO 4 powder was gradually added into the mixture and stirred for 3 days to allow oxidation to occur.The obtained solution was transferred into a beaker containing ice, and 27 mL of 30% H 2 O 2 was added.A yellowish-brown solution was observed and washed with 1 M HCl solution and deionised water.

Preparation of RGO-TiO 2
Composite.The RGO-TiO 2 composite was synthesized using a solvothermal route.GO aqueous suspension was freeze-dried for 24 h using a freeze dryer (Martin Christ, ALPHA 1-2/LD plus).Then, 40 mg of freeze-dried GO was dispersed in a solution containing 40 mL of isopropyl alcohol and gently sonicated for 1 hour.Following this, 0.824 mL of TIPT was added drop-wise into the GO solution (14 : 1), and then 30 L of DETA was added and stirred for a few minutes.This step is crucial as it ensures the formation of the desired morphology.The solution was then transferred into a Teflon-lined autoclave and heated at 200 ∘ C for 24 h.The resulting black precipitate was washed with ethanol and left to dry in an oven at 60 ∘ C overnight.Finally, the obtained black powder was calcined at 400 ∘ C in air, with a heating rate of 1 ∘ C min −1 for 2 h.Pure TiO 2 and RGO were synthesized for comparison purposes.

Fabrication of RGO-TiO 2
Electrodes.The samples were ground and mixed with absolute ethanol to obtain a slurry paste.The paste was painted onto taped-down clean indium tin oxide (ITO) glass and then spread over using a glass rod.To study the effect of the sintering temperature on the photocurrent response, the ITO glass was sintered in a furnace under purified N 2 gas flow at different temperatures: 300, 400, and 500 ∘ C. The fabricated electrodes were labeled as G-TiO 2 300, G-TiO 2 400, and G-TiO 2 500, and a TiO 2 500 electrode was fabricated for comparison purposes.

Characterisation.
The crystallographic phases of the samples were analysed using X-ray diffraction (XRD, D5000, Siemens), with Cu K radiation ( = 1.5418Å) and a scan rate of 0.02 ∘ s −1 .The morphology and structural properties were investigated using a field emission scanning electron microscope (FESEM, JEOL JSM-7600F) and a high resolution transmission electron microscope (HRTEM, JEOL JEM-2100F).X-ray photoelectron spectroscopy (XPS) measurements were performed using synchrotron radiation from beamline number 3.2 at the Siam Photon Laboratory in the Synchrotron Light Research Institute, Thailand.Raman and photoluminescence (PL) spectra were obtained using 514 nm and 325 nm laser beams, using a Renishaw inVia Raman microscope system.Electrochemical impedance spectra (EIS) measurements were carried out using a VersaSTAT 3 potentiostat (Princeton Applied Research) in a 0.1 M KCl solution containing 1 mM K 4 [Fe(CN) 6 ].

Photocurrent Response Measurements.
Photocurrent response measurements were carried out using a VersaSTAT 3 potentiostat and recorded in VersaStudio v2.2 data acquisition software.In a three-electrode cell configuration, the fabricated electrodes were employed as the working electrode a saturated calomel electrode was used as the reference electrode, and a Pt wire was used as the counter electrode.A 150 W Xenon lamp was used as a light source and all electrodes were dipped in 0.5 M KCl electrolyte.All  the photoelectrochemical experiments were carried out in the presence of N 2 atmosphere.The CA measurements were carried out at applied potential of 0.8 V under light "on-off" condition using Newport solar simulator with manual shutter.

Results and Discussion
The RGO-TiO 2 composite was synthesized by mixing GO, TIPT, and DETA in a solution.Figure 1 illustrates two preparation routes to obtain RGO-TiO 2 composites, using different mixing sequences.The mixing sequence of TIPT and DETA was found to influence the final product of synthesis.Figure 1(a) shows an initial synthesis route similar to Chen et al. [27], first mixing GO with DETA, and then adding TIPT.However, the stirred solution was found to be nonhomogenously dispersed and yielded irreversible agglomerations, as mixing GO with DETA causes flocculation of the GO sheets.Thus, after centrifugation, three different distinct layers were formed that can be attributed to TiO 2 , RGO-TiO 2, and RGO.By altering the mixing sequence, using TIPT first instead of DETA, as shown in Figure 1(b), a homogeneously dispersed solution was observed, in which a single homogenous layer of RGO-TiO 2 composite, without any evidence of pure TiO 2 or RGO, was easily obtained.The distinguishable end-products of Figures 1(a) and 1(b) were clearly influenced by the sequence in which the chemicals were mixed.In Figure 1(a), GO could plausibly be reduced by the amine groups of DETA, not unlike the reduction of GO with NaOH.This gives rise to certain degree of aggregation caused by the - restacking interaction of RGO.When TIPT was added into the coagulate, only the exposed active sites of GO, which had not formed complexes with DETA, interacted with TIPT to form TiO 2 on RGO during the solvothermal reaction.On the contrary, the order of addition of chemicals in Figure 1(b) allows complete reaction between GO and TIPT.After TIPT was introduced to GO, the network of TIPT molecules expanded, forming a cage-like continuous network structure that enabled maximum electrostatic interaction with GO.The addition of DETA stabilised the complex scaffold, leading to 100% formation of RGO-TiO 2 .Throughout this paper, the characterisation results are given for samples obtained using Figure 1(b) mixing sequence.Figure 2 depicts the XRD patterns of graphite, GO, RGO, TiO 2, and the RGO-TiO 2 composite.The peak position at 2 = 26 ∘ of the (002) graphite plane corresponds to a dspacing value of 3.4 Å.It was observed that the graphite peak completely disappeared for GO, and a new peak position emerged at 9.7 ∘ with an increase of the d-spacing to 9.1 Å, which was thought to be due to exfoliation and oxidation of graphite to GO.On the other hand, a similar peak position to graphite was shown for RGO, but with a much broader peak, indicating the successful dispersion of GO sheets after the solvothermal reaction [28].Clearly, RGO was successfully synthesized through this route, by reduction and removal of the oxide functional groups from GO to RGO.Both TiO 2 and RGO-TiO 2 characteristic peaks at 25.2 ∘ , 37.8 ∘ , 48.1 ∘ , 53.9 ∘ , and 55.1 ∘ corresponded well to the (101), (004), ( 200), (105), and (211) crystal planes of anatase TiO 2 , with lattice constants of  =  = 3.7852 Å and  = 9.5139 Å (JCPDS 21-1272) [29].Therefore, no other TiO 2 phases were detected.Additionally, the absence of RGO peak at 25.2 ∘ in the RGO-TiO 2 composite was probably due to the relatively low diffraction intensity of RGO that was shielded by the (101) crystal peak of TiO 2 [30].
Morphological features of the as-prepared samples were obtained using FESEM imaging.Pure TiO 2 nanoparticles can be observed in Figure 3(a), agglomerating together, with a surface that appears rough.Increased magnification (Figure 3(b)) of the surface reveals the presence of randomly assembled TiO 2 nanosheets, which will be discussed in the HRTEM section.Similar features were observed for the RGO-TiO 2 composite (Figure 3(c)), as the agglomeration of TiO 2 nanosheets was greatly reduced limiting them to the surface of RGO, indicating the successful anchoring of TiO 2 nanosheets onto RGO sheets.With higher magnification, Figure 3(d) reveals that the surface of RGO-TiO 2 is composed of two different types of TiO 2 features: TiO 2 nanoparticles and TiO 2 nanosheets (white worm-like features due to folding of the TiO 2 nanosheet) that cover the entire surface of the RGO.
HRTEM characterisation was employed to further determine the structure and planes of the samples.In Figure 4  were consistent with the FESEM results; such a structure formed to minimise the surface energy [31].A magnified view, shown in Figure 4(b), reveals that the outer region was populated with thin layers of TiO 2 that resemble sheet-like features.However, the entire inner sphere was populated with densely packed TiO 2 nanoparticles.The output of these two different types of TiO 2 nanostructures could be attributed to the Oswald ripening process and the use of DETA as the morphology controlling agent [32].At high magnification, the image of a TiO 2 nanosheet edge (Figure 4(c)) reveals a lattice with fringe spacing of 0.19 nm that corresponds to either the (020) or (200) planes of anatase TiO 2 .According to Chen et al. [27], the TiO 2 spheres formed have almost 100% of their {001} planes exposed.It was noted that DETA played a key role, as the growth along the (001) direction was retarded, stabilising the growth of the exposed {001} facets and eventually leading to a higher amount of exposed {001} facets.Similarly, the RGO-TiO 2 composite (Figure 4(d)) reveals the same structure.Two different structures of TiO 2 nanoparticles and nanosheets were wrapped around the RGO sheet (Figure 4(e)).Therefore, this exhibits two different fringe spacings, 0.19 nm and 0.351 nm in Figure 4(f), which correspond to the {001} planes of TiO 2 nanosheets and the thermodynamically stable {101} planes of TiO 2 nanoparticles [17,33].
To investigate the chemical state of the as-prepared samples, XPS measurements was carried out; the results are shown in Figure 5.In Figure 5(a), a considerable degree of oxidation for the peaks at binding energies of 284.5 eV, 286.2 eV, 287.8 eV, and 289 eV in the C 1s of GO could be attributed to C-C, C-O, C=O, and O=C-O bonds [34,35].However, in comparison with RGO-TiO 2 , the oxide functional groups' peak intensities were greatly reduced, which confirms that reduction from GO to RGO was successful.In addition, the small peak at 282.4 eV could be attributed to the Ti-C bond, which reflects the chemical bonding between titanium and carbon.For the Ti 2p peaks in Figure 5(b), two peaks centred at 453.5 eV and 459.2 eV were observed for TiO 2 , which correspond to the Ti 2p 3/2 and Ti 2p 1/2 spinorbital splitting photoelectrons in the Ti 4+ state [36].These peaks redshift to 453.8 eV and 459.6 eV for RGO-TiO 2 due to the interaction between Ti and the oxygen groups of RGO.Since oxygen is much more electronegative than carbon, the TiO 2 electron density is attracted more strongly, which then increases the binding energy of Ti in the composite [37].In addition, the calculated spin orbit splitting between Ti 2p 3/2 and Ti 2p 1/2 was 5.7 eV in both samples, which indicates the presence of normal states of Ti 4+ [38].
Raman spectroscopy was employed for molecular morphology characterisation of carbonaceous materials.As International Journal of Photoenergy Raman shift (cm −1 ) Raman shift (cm −1 ) 100 200 300 400 500 600 700 800 shown in Figure 6, two peaks were located at 1353 cm −1 and 1597 cm −1 for the fabricated electrodes.These peaks correspond well to the documented D band, which is attributed to sp 3 defects, and to the G band, which is due to in-plane vibrations of sp 2 carbon atoms and a doubly-degenerated phonon mode ( 2g symmetry) at the Brillouin zone centre [39].In addition, shifts at 142, 395, 514, and 637 cm −1 can be attributed to the  g ,  1g ,  1g , and  g modes of anatase TiO 2 (inset) and are consistent with the XRD results [19].This also suggests that the fabricated G-TiO 2 electrodes' TiO 2 anatase phase was retained, even with increased sintering temperature.
PL spectra of various samples were recorded, which reveal the surface structure and excited state of the semiconductor [40].In Figure 7, the PL intensity was greatly affected by the recombination rate of electron-hole pairs.A lower PL intensity indicates an enhanced charge separation of electronhole pairs.A low PL emission intensity was depicted for the RGO-TiO 2 composite, which was attributed to RGO sheets acting as an electron shuttle for TiO 2 to hinder electronhole recombination.Thus, the recombination rate was greatly suppressed.The inset of Figure 7 shows the PL spectra of the fabricated electrodes sintered at various temperatures.As shown, TiO 2 500 has the highest emission intensity, greater than G-TiO 2 500 or any of the composite electrodes, which is consistent with the previous explanation.The PL intensity decreases with increasing sintering temperatures, with G-TiO 2 500 being the lowest.Wang   annealing of photocatalysts under N 2 gas reduces the density of surface defects [41].Hence, the decrease of recombination points for the electron-hole pairs enhances charge separation efficiency.
In order to confirm the charge recombination kinetics of the TiO 2 and RGO-TiO 2 composite electrodes, EIS measurements were applied to understand the interfacial charge transfer process.The Nyquist plots of TiO 2 and RGO-TiO 2 composite electrodes were plotted in Figure 8 as the arc shape reveals information of the charge transport and recombination properties and a depressed semicircle was observed for RGO-TiO 2 composite compared to TiO 2 .The charge transfer resistance ( CT ) was determined by the diameters of the semicircles and RGO-TiO 2 composite (48.244Ω) has a lower  CT value than TiO 2 (83.192Ω) indicating increase charge transport and reduced recombination rate that is in good agreement with the PL results [42,43].Photocurrent response measurements of the photogenerated electron-hole interaction of the fabricated electrodes were carried out under repeated ON-OFF cycles, and the results are shown in Figure 9.In this case, the photocurrent density is greatly affected by two significant factors: the time taken for the photo-induced electrons to withdraw from the TiO 2 to the ITO substrate and the rate of electronhole recombination within the film and the film/electrolyte interface [44].In the results shown, the photocurrent density under dark conditions is relatively small and negligible.A rapid current rise and decay was observed when the light was switched on and off.The effect of the sintering temperature of the RGO-TiO 2 films on the photocurrent response was investigated.The output of the photocurrent density was observed to increase with higher sintering temperatures.The highest recorded photocurrent response was that of the G-TiO 2 500 electrode (104.4 A cm −2 ) which is around ten times higher than that of the G-TiO 2 300 electrode (9.6 A cm −2 ).The dramatic increase could be attributed to a delayed recombination rate and longer electron lifetime, as shown in the PL spectra [45,46].When comparing both the TiO 2 500 and G-TiO 2 500 electrodes, the photocurrent response of the G-TiO 2 was observed to be better than that of the TiO 2 500 electrode.This suggests that the presence of RGO sheets increases the photocurrent response due to their extensive two-dimensional - conjugation structure.Hence, photo-induced electrons from TiO 2 can be accepted by RGO and transferred to the external circuit more quickly [47,48].Moreover, the charge separation efficiency increases due to the electronic interaction between TiO 2 and RGO in the composite [49].The highly exposed {001} facets of the TiO 2 nanosheets in a hierarchical structure contribute to the photocurrent response by favouring electron transport, which minimises the grain interface effect and improves the light harvesting efficiency [50,51].

Conclusions
RGO-TiO 2 composite composed of highly exposed {001} TiO 2 facets was successfully synthesized through a solvothermal route with 100% yield of composite material.The advantage of this method harnesses the benefits of both the highly exposed {001} facets for higher reactivity and RGO as an excellent platform with a large surface area, by using DETA instead of the commonly-used HF, which is hazardous and toxic to the environment.The role of the sintering temperature on the characteristics of the fabricated electrodes was remarkably interesting, as the highest photocurrent density, 104.4 A cm −2 , was observed for a sintering temperature of 500 ∘ C, which is attributed to the reduced recombination of electron-hole pairs and improved light harvesting efficiency.We anticipate that this study of the RGO-TiO 2 composite with highly exposed {001} facet features would aid in further improving photoelectrochemical performance applications.

Figure 1 :
Figure 1: Schematic illustration of the preparation of RGO-TiO 2 composite in sequence, producing, after centrifugation, (a) three different layers (RGO, TiO 2 and RGO-TiO 2 ), and (b) a single homogeneous layer of the RGO-TiO 2 composite.
(a), the morphological similarities of agglomerated TiO 2 crystals

Figure 4 :
Figure 4: ((a), (b)) Low magnification TEM images of TiO 2 and (c) high magnification TEM image of the edge of a TiO 2 nanosheet.((d), (e)) Low magnification TEM image of the RGO-TiO 2 composite and (f) high magnification TEM image of the surface of the composite.

Figure 6 :
Figure 6: Raman spectra of G-TiO 2 electrodes sintered at various temperatures.The inset shows the typical peak positions for anatase TiO 2 .

Figure 7 :Figure 8 :
Figure 7: PL spectra of the as-prepared TiO 2 and RGO-TiO 2 composite.The inset shows the PL spectra of fabricated TiO 2 and various G-TiO 2 electrodes.