One Pot Microwave Irradiation Synthesis of Spherical and Nanotube Titanates Incorporated Reduced Graphene for Efficient Hydrogen Production Photo-Electrocatalytically

Nanosphere and titanate nanotube (TNT) amalgamated with different graphene oxide (GO) ratios synthesized by one pot microwave irradiation route have presented excellent potential towards hydrogen production photoelectrochemically under solar irradiation. The hybrid nanospherical array of the 50TNT-50GO photocatalyst showed a current density equal 9.2 mA cm−2 at a bias of 0.20 V vs. RHE exceeding those of the nanotubes 30TNT-70GO (4.0 mA cm−2 at 0.38 V) and 10TNT-90GO (3.7 mA cm−2 at 0.4 V). The former electrode also exhibits small Tafel slope value (40 mV dec−1), decreased particle diameter (7 nm), decreased band gap (2.6 eV) and high rate of charges transfer. The hybrid structure elucidation carried out using TEM-SAED, XRD, N2 adsorption, UV–Vis, FTIR and PL techniques approved the interfacial interaction between TNT and GO(RGO) networks that was responsible for the high quantum yield, delay of charges recombination beside the increase in the pore volume.


Introduction
As a result of recognizing the characteristics of titanate nanotubes that combine those of the traditional TiO 2 nanoparticles with the properties of the 1D layered titanates, many applications have been disclosed [1][2][3]. On the other hand, many approaches have been presented for improving the TiO 2 nanotubes photo-activity to be sensitive towards visible light irradiation including nonmetal doping, noble metals doping, as well as incorporation with dyes and polymers [4][5][6][7]. However, their limited wide scale applicability were mainly relied on the electron-hole pair recombination, high price, and the feasible split-up.
The established routes for synthesizing TiO 2 nanotubes were alkaline hydrothermal, template assisted, and anodic oxidation. However, they suffer morphology control drawback beside declining their properties during applications [8,9]. Thus, to tackle the mentioned shortcoming and to improve the properties of titanate structure, a modification with graphene has been proposed [10]. Researchers on the other hand focused lately on the electrospun produced TiO 2 via using various organic/polymer additives during the fabrication alongside its quantum dots manufacturing and thus demonstrates an improve in the photocatalysis management [11,12]. Graphene incorporation is an emerging approach because of its large surface area, optical transmittance, and electronic conductivity [13,14]. However, the way of dispersing graphene into titanates is the most important issue. Many routes have been applied such as hydrothermal and soft chemical approaches where the incorporation of GO is primarily used. Indeed, graphene is merged with titanates after the GO reduction that used to be accomplished via using applied chemical constituents, thermal and microwave treatment routes [15]. This indeed reduces the heterojunction expected to be formed between the nanotubes and graphene and thus produces weak or deficient interfaces. Thus, we tried to use a facile efficient method to boost the water splitting slow kinetics via building up an efficient interface capable of transferring electrons via employing a simple microwave irradiation approach. This is to excellently improve the charge separation/transportation courses and rather facilitate the electrolyte diffusion. Thus, we have synthesized the hybrid TiO 2 nanotube-graphene that unveiled an effective visible light harvesting via executing large wavelength range beside facile PEC surface reaction. This was unlike several photocatalysts reported on the visible-light assisted water splitting systems [16]. Consequently, the photocatalytic activities of the TiO 2 nanotubegraphene hybrids synthesized by one step microwave route are assessed using hydrogen generation via water splitting. This photoelectrochemical reaction is carried out for sake of designing, developing and optimizing a low priced electrocatalyst with a noble metal free and high stability for an efficient water reduction.

Synthesis of Graphene Oxide Modified TiO 2 Nanotube
The microwave-irradiation route was exploited to synthesize sodium titanate nanotubes. A digester CEM type operated at a power equal 450 W was used. A 1.0 g of TiO 2 -powder (Degussa) was added to 10 mol L −1 NaOH so as to achieve a suspension volume of 60 mL. The synthesis condition was fixed at 150 °C for 1 h. The obtained suspension was filtered, washed with 0.1 M HCl several times then dried at 100 °C. For GO modified titanates samples, definite amounts of GO to give various weight percentage ratios were immersed into the TiO 2 Degussa followed by NaOH addition and then placed in the closed Teflon container to be treated as above except extending the time into 2 h to accomplish reduction of GO. A general scheme for the titanate-graphene hybrid synthesis route is presented in Fig. 1.

Photo-and Electro-Chemical Measurements
The photo-current study was performed using a workstation (CHI 660B) containing three-electrode design with X GO-Y TNT electrodes (of area 1.0 cm 2 ) as working electrodes. The carbon electrode and reversible hydrogen electrode (RHE) were respectively worked as counter and reference electrodes, and 1.0 M KOH was utilized as an electrolyte.
The carbon counter electrode of good adhesion was synthesized following the procedure of the Q. Meng et al. [17,18]. The working electrode was prepared using a paste of the X GO-Y TNT hybrid using 1.0 mL of the Triton X-100 on the FTO conducting glass (resistance 19 Ω cm −1 ). Impedance spectroscopy (EIS) justified using Kramers-Kronig transformation was verified with an ac disconcertion indicator of 5 mV in the frequency limit of 0. KOH electrolyte together with the commercial 20% Pt/C for comparative purposes. Transient photocurrent was determined on the same potentiostat at zero bias voltage versus RHE utilizing the referred light source.

Characterization Techniques
The morphology of the samples was measured using both transmission electron microscope (TEM, S-4700) and selected area electron diffraction (SAED) technique. The X-ray diffraction (XRD) patterns were conducted on a PANalytical PRO construction with Cu Kα radiation (k = 1.54056 Å) controlled at 40 kV and 30 mA. The UV-Vis diffuse reflectance spectra were accomplished on a Shimadzu UV-3150, spectrophotometer. The FT-IR spectra were confirmed on a Nicolet Magna-550 spectrometer. The surface area, pore volume and pore radius of the samples were quantified via adsorption of the N 2 gas at − 196 °C using Micromeritics 3020, subsequent outgassing at 120 °C. The Photoluminescence spectra were carried out using FLS980, Edinburgh Instruments Ltd. with an incited wavelength of 330 nm.

Bulk, Morphology, Vibration and Optical Characteristics
The XRD results of synthesized catalysts consisting of GO and x TNT-y GO are illustrated in Fig. 2. The GO pattern shows main peaks at 2θ = 10.4° (001) and 43.5° (102) with a small broad peak at 20.3° ascribed to the presence of reduced graphenes (RG) at a minor ratio equivalent to 15% of the GO. In the 50TNT-50GO pattern, pronounced peaks at 2θ = 25° (101), 38.08° (004), 48.32° (200), 54.88° (105), 63.36° (204) are distinguished characterizing the anatase TiO 2 structure (anatase, JCPDS: 99-0008) together with a tail peak positioned at 2θ = 5° (200) describes the hydrogen titanate nanotube [19]. In the latter pattern, GO peaks are totally disappeared toward the construction of the RG. However, the RG main peak is probably obscured by the anatase strong peak accustomed to be located at the same 2θ value (~ 25). In case of GO addition at the ratios of 70 and 90%, a marked decrease in crystallinity as well as sizes of the anatase structure is perceived via vanishing some bands (such as those of the facets (004) and (105)) and shifting others into lower theta values like the interlayer spacing of (101), where the rest suffers a marked decrease in intensities. This latter shift emphasizes the increase in the d values proposing an expansion in the nanotubes lattice spacings and thus comprehends the successful incorporation between RG and TNT. This advocates the sensitivity of the latter planes when GO interacts with the TiO 2 nanotubes. The 30TNT-70GO and 10TNT-90GO samples also revealed broad peaks at 2θ = 14.8° (200) ad 44.32° (204) never seen in 50 TNT-50 GO ascribed to the titanate nanotube [20]. This highlights that the GO excess oxygen helps establishing the structure of the hydrogen titanate. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images seen for 10GO-90TNT as well as 50GO-50TNT catalysts are depicted in Fig. 3. The TEM picture seen for the former shows overlaying of the interconnected TNTs; of an average inner diameter of 25 nm and length equal 500 nm, over the RG that composed of ~ 5 layers. The inset SAED rings certified the presence of (101) and (004)  The FT-IR spectra measured for evaluating the variations in the functional groups following the hybrid interaction The UV-visible spectra of the synthesized samples are presented in Fig. 5A. Undeniably, the UV-visible spectrum of 50TNT-50GO exhibited a superior absorbance across the whole range including the visible region (400-800 nm) than the other samples. Enhancing the absorption edge to an extended long absorption for all GO incorporated TNT especially the latter sample suggests their intimate interaction. The optical band gaps anticipated using the Tauc relation indicate values comprised of 2.6, 2.75, 2.88 and 3.1 for 50TNT-50GO, 30TNT-70 GO, 10TNT-90 GO and TNT, respectively.
To examine the charge carrier's transfer as well as their recombination and separation, a photoluminescence emission was analyzed for all the samples. The fluorescence spectrum of 50TNT-50GO owns the lowest intensity followed by 30TNT-70GO and 10TNT-90GO where the TNT acquires the highest intensity (Fig. 5B). Hence, the GO incorporated TNT samples verified the low recombination rate to be in the order; 50TNT-50GO > 30TNT-70GO > 10TNT-90GO. The TNT sample shows a peak at about 475 nm ascribed to the surface oxygen vacancies from the TiO 2 defects. Different emission peaks localized at ~ 600 nm emitting photons in the visible range were shown for all GO incorporated TNT samples proposing the presence of defects in TNTs. This observation has been ascribed to the presence of an impurity band prompted from the RG addition that helps the energy transfer from the RG to TNT or vice versa [21].

Photoelectrochemical Water Splitting
The PEC performances of the plain TNT and GO incorporated TNT samples stacked on FTO were examined to display their photocatalytic activities. The current density vs.  The photoelectrochemical consequence of the 50TNT-50GO electrode shown in Fig. 6a upon sweeping the potential,  On the other hand, the 30TNT-70GO and 10TNT-90GO electrodes showed smaller photocurrent densities, with comparable values at 4.0 mA/cm 2 and 3.7 mA/cm 2 ; parallel to onset potential of 0.385 V and 0.4 V, respectively. However, the TNT photoanode shows an inferior photocurrent value (0.3 mA cm 2 ) following illumination of the solar emulator and rather perceived at a higher potential equivalent to 0.43 V vs. RHE. Boosting the current density of the 50TNT-50GO sample; compared to other samples, that acquired at a lower onset potential proposes extra effective charge separation and compilation than the other catalysts, as confirmed from the PL results. Also, the great negative shift of the onset potential of the former sample necessitates an intimate interaction between the TNT and GO; as confirmed from IR and TEM results. This indicates that the photo-generated electrons in TNT conduction band are conveyed to the graphene oxide of lower work function [22].
Furthermore, the physicochemical stability while performing the PEC water splitting is evaluated via drawing the time-dependent current density at a constant bias equal 1.0 V against RHE (Fig. 6B). As exhibited in the Figure, the current densities of all samples are jumped to their maximum values following illumination and instantaneously decreased after light turning off, signifying their rapid photoelectric response power. The TNT50-GO50 electrode shows the highest photocurrent density (about 9.2 mA/cm 2 ) compared with the other photoanodes and after 6 repeated cycles it signifies a decline in the current density comprised of 7.9 mA/cm 2 . This distinctly indicates that the TNT-GO heterojunction possesses superior borderline charge-transfer efficacy resulted from the exhibited considerable interaction between the two components. Exhibition of the rectangular shapes in the J − t investigations reveal not only a decrease in the electron back recombination but also an effective reproducibility of the photo-anodic actions. To emphasize the facility of the charge transfer and to what extent the internal resistance can affect the capacity of the samples, the impedance spectra of the samples were measured and shown in Fig. 6C. The semicircle diameter of the 50TNT-50GO photocatalyst was smaller in sequence than 30 TNT-70 GO, 10 TNT-90 GO and TNT ones, advocating higher heterojunction charge transfer as well as efficient electron/hole separation following the amalgamation with GO.
Important factors such as surface texturing (pore radius, pore volume and surface area) properties are very important factors to affect the electrolyte absorbability and the amount of solar light absorbed. Table 1 shows the S BET , Vp and r values, in which the surface area values were in the sequence of TNT > 10 TNT-90 GO > 30 TNT-70 GO > 50 TNT-50 GO whereas the pore volume data states that the 50 TNT-50 GO sample exhibits the highest value exploring higher electrolyte adsorption. Increasing the pore volume of the latter sample proposes the strong interaction between TNT and GO components, in which GO is deeply pushed inside the pore causing pore volume enlargement. This is highly ascertained via decreasing the pore diameter; to be the smallest, to elucidate the crowding of GO at the TNT pore mouth affecting its dimension. Figure 6D shows the Tafel plots of all the photocatalysts. The slope of 50TNT-50GO (40 mV dec −1 ) was much lower than those of 30 TNT-70 GO (73 mV dec −1 ) and 10 TNT-90 GO (71 mV dec −1 ) and comparable to that of Pt/C (38 mV dec −1 ). This reveals more promising photocatalytic kinetics on the former catalyst. This superior photocatalytic activity of the 50TNT-50GO catalyst is ascribed to the synergism between TNT and GO besides the high electronic conductivity offered from the produced reduced graphene.
The lower value of 50TNT-50GO is indeed designs that the discharging mechanism (Tafel and/or Heyrovsky) is the principal route, realizing that the hydrogen evolution is the rate determining stage (combination reaction).
This ascribes that 50TNT-50GO has a potentiality for electrons transmission thus enhances the hydrogen evolution densities that was in accord with diminishing the electrochemical resistance. However, in case of 30 TNT-70 GO and 10 TNT-90 GO, a Volmer reaction is counted as the rate limiting step owing to sticking the H ad more on their surfaces [23] thus promoting slow electrochemical desorption step and thus the slow kinetics.
The photocatalysts quantum efficiencies were evaluated at 0.5 V vs. RHE from utilizing a monochromator with a power meter and photo-diode, assumed by the equation: IPCE% = 1240J ph /I inc λ × 100%; where J ph is the measured photocurrent density (mA/cm 2 ), I inc is the incident light power intensity (mW/cm 2 ) at the electrode surface where λ is the wavelength (nm) of the emitted light. The calculated (1) Volmer; H + + e− → H ad (2) Heyrovsky; H ad + e − +H + → H 2  [26][27][28][29]. This indeed is correlated to the synergy between the TNT morphology and reduced graphene in enhancing both the active sites and conductivity together with the pore volume expansion that facilitates the ions passage. However, it gives lower HER activity compared to some perovskite ferrite electrodes such as BiFeO 3 /TiO 2 [30] as a result of the CB BFO is more -ve compared to H 2 /H 2 O redox potential used for hydrogen creation. Accordingly, the low coast coupling of reduced graphene with TNTs manufactured using microwave irradiation modified not only the electronic structure and morphology but also improved water splitting with appreciable stability. It also produces H 2 at a much lower overpotential than comparable catalysts [24][25][26][27][28][29][30] elaborating its liberation ease based on diminishing internal resistances and adequately proposes no barrier at the interface contacts.

Conclusion
The nanohybrid 50TNT-50GO synthesized by microwave irradiation approach verified its ability to accomplish high performance PEC water splitting under solar illumination with exhibiting high current density (9.2 mA cm −2 ) at low bias (0.2 V) and small Tafel slope values (40 mV dec −1 ). The heterojunction photoanode 50TNT-50GO of nanospherical array facilitates chargers transfer via the constructed interface formed between TNT and RG, and thus improved the visible light absorptivity. Lowering the energy gap (2.6 eV) and charges separation efficacy as well as their transport feasibility, beside the high quantum yield and pore volume are all share in attaining efficient hydrogen production from water via solar energy conversion.