Bulk, morphology, vibration and optical characteristics
The XRD results of synthesized catalysts consisting of GO and x TNT-y GO are illustrated in Fig.1. The GO pattern shows main peaks at 2q= 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 2q =25° (101), 38.08° (004), 48.32° (200), 54.88° (105), 63.36° (204) are distinguished characterizing the anatase TiO2 structure (anatase, JCPDS: 99-0008) together with a tail peak positioned at 2q =5° (200) describes the hydrogen titanate nanotube [14]. 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 2q 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 TiO2 nanotubes. The 30TNT-70GO and 10TNT-90GO samples also revealed broad peaks at 2q= 14.8° (200) ad 44.32° (204) never seen in 50 TNT-50 GO ascribed to the titanate nanotube [15]. 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. 2. 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) anatase planes covered by the (200) plane of the RG, which carefully matches the corresponding XRD profile. This estimates the intimate interaction between the anatase TNT and RG. The HRTEM inset image verifies the SAED results via exposing 0.352 nm and 0.19 nm lattice spacings in which the former agrees with either the (101) anatase plane or the (200) of the RG plane where the spacing of 0.19 nm verifies the (200) plane of titanate. This proves the presence of surface contacts between RG and TNT.
For the 50TNT-50GO sample, the nanotube structure was entirely damaged to display the evolution of an aggregated nanoparticles array of 7 nm dimension. Similarly, the HRTEM shows the presence of lattice spacings correspond to either (101) of anatase or (200) of RG at 0.352 nm beside the lattice spacing 0.19 nm consistent with the (200) plane of titanate. This manifests the contact of the intersected area between the two components. The SAED circles are perfectly correlated to the crystalline mixed phases of TNT and RG attained via the notification of the planes at (101), (440) and (200), respectively.
The FT-IR spectra measured for evaluating the variations in the functional groups following the hybrid interaction between titanates and graphene are seen in Fig. 3. Apparently, GO displays many absorption peaks comparable to oxygen-containing functional groups including 1750, 1642, 1394, 1261, 1067 and 3450 cm-1 assigned respectively to C=O, C=C, C-OH, C-O, C-O-C and –OH groups. On the other hand, the TNT shows a hydroxyl stretching vibrational band at 3466 cm-1 beside peaks at 1645, 1394, 1002, 797 and 508 cm-1 endorsed respectively to Ti-OH stretching, C-OH and Ti-O-Ti vibrations in the titanate structure (1002-508 cm-1). The spectra of the hybrid indicate the typical feature of the titanate spectrum except shifting the 508 cm-1 band of the [TiO6] octahedron vibration into 488 cm-1 in TNT50-GO50 and 472 cm-1 in TNT30-GO70, configuring the TNT distortion. This reflects that the interaction takes place mainly on the latter octahedron via involving of the RG (Ti-O-C). Also, some GO oxygenated bands are disappeared (such as 1750, 1261 and 1067 cm-1) pointing out the importance of C=O and C-O moieties during the interaction with titanates. The stabilization of the C=C bonds bring around the fruitful reduction of GO.
The UV–visible spectra of the synthesized samples are presented in Fig. 4A. 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. 4B). 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 TiO2 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 TNT. 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 [16].
Photoelectrochemical water splitting
The PEC performance of the plain TNT and GO incorporated TNT samples stacked on FTO were examined to display their photocatalytic activities. The current density vs. potential (J–V) curves performed under replicated solar irradiation were evaluated in 1.0 M KOH, as depicted in Fig. 5. The photoelectrochemical consequence of the 50TNT-50GO shown in Fig. 5a upon sweeping the potential cathodically, reveals a sharp rise in the current density comprised of 9.2 mA cm-2 and at an onset potential equal 0.2 V vs. RHE. On the other hand, the 30TNT-70GO and 10TNT-90GO samples showed smaller photocurrent densities, with comparable values at 4.0 mA/cm2 and 3.7 mA/cm2; parallel to onset potential of 0.385 V and 0.4 V, respectively. However, the TNT photocathode shows an inferior photocurrent value (0.3 mA cm2) following illumination of the solar emulator and rather perceived at higher potential equivalent to 0.43V vs. RHE. Boosting the current density of the 50TNT-50GO sample; compared to other samples, that acquired at 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 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 [17].
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. 5B). 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 photocathode shows the highest photocurrent density (about 9.2 mA/cm2) compared with the other photoanodes and after 6 repeated cycles it signifies a decline in the current density comprised of 7.9 mA/cm2. 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. 5C. 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 SBET, 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 5D 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 catalytic 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).
Volmer; H+ + e- ® Had (1)
Heyrovsky; Had + e- + H+ ® H2 (2)
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 Had more on their surfaces [18] thus promoting slow electrochemical desorption step and thus the slow kinetics. during performing the electrochemical reactions (Table 1).
Table 1: Surface properties of the studied photoelectrocatalysts
Material name
|
SBET surface area (m2/g)
|
Pore volume Vp (cm3/g)
|
Pore diameter- BJH
r (nm)
|
TNT
|
240
|
1.35
|
19.5
|
50 TNT-50 GO
|
185
|
1.85
|
17.8
|
30 TNT-70 GO
|
201
|
1.41
|
18.5
|
10 TNT- 90 GO
|
220
|
1.38
|
19.2
|
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% = 1240Jph/ Iincλ × 100%; where Jph is the measured photocurrent density (mA/cm2), Iinc is the incident light power intensity (mW/cm2) at the electrode surface where λ is the wavelength (nm) of the emitted light. The calculated IPCE values was maximized for the 50TNT-50GO photocatalyst giving a value of 45% at 400 nm that dropped off into 25% and 10% for 30TNT-70GO and 10TNT-90GO, respectively. Boosting the photocurrent of the former is likely caused by decreasing the band gap, enhancing the visible light absorption range, high charges separation and deep pore volume.
Constructively, our cost effective photocatalyst 50TNT-50GO has shown superior photoelectrocatalytic water splitting performances (9.2 mA cm-2) compared to others in literatures such as various black titania either produced via reduction by CaH2 (1.99 mA cm-2) or Mg metal (3.1 mA cm-2) [19-20]. It also exceeds those of the ternary structured Ag/TiO2/CNT (0.9 mA cm-2), TiO2@RG (6.0 mA cm-2), reduced graphene-graphene oxide nanohybrid (61.35 μA/cm2) and controlled film of TiO2 nanotubes (175 µA cm−2) [21-24] all performed in the same electrolyte.