Effects of electronic structure and magnetic performance at the surface/interface of r-GO and TiO₂ in r-GO/TiO₂ composite thin films: X-ray absorption near-edge structure and x-ray photoelectron spectroscopy Open Access

Among the different phases of titanium dioxide (TiO₂), the anatase phase has been considered a potential material for several applications due to its promising optical and electrical properties,¹ nontoxicity,² high oxidative,³ chemical stability,^1,2,4 cost-effective,^1–4 and higher photocatalytic (PC) activities.⁵ The photocatalytic (PC) activity of TiO₂ within the bandgap of ≈3.0–3.2 eV absorbs only ultraviolet light,⁶ but the absorption edge could be shifted into the visible/red region by the fabrication of composite materials using GO/r-GO with TiO₂ that could enhance the PC activity by improving the electronic structure of the composite materials. The GO/r-GO is the derivative of graphene material, which is associated with different O-moieties that promote the isotropic growth of TiO₂ crystals and widen their practical technological applications.^7–11 The oxygenated groups, such as –COOH and C=O groups, exist at the edge of the graphene structure¹² that are responsible for the incorporation of TiO₂,^13,14 including higher solubility and the effectiveness of surface modification,¹⁰ which enhances the PC activities and magnetic performance of (r-GO)/TiO₂ composite materials^6,15–19 after tuning the electronic structure. The r-GO enhances these properties higher than GO because r-GO is a high specific surface area. A large number of methods are adopted to fabricate the r-GO/TiO₂ composite material, viz., sol–gel,²⁰ thermal,^15,21–23 and radiation²⁴ process.

In this study, TiO₂ and GO are synthesized separately by radio frequency (RF) reactive magnetron sputtering technique on the Si-substrate (TiO₂/Si) as a thin film and the improved Hummers method as a GO-powder form, respectively. The r-GO is then produced from GO by the hydrazine hydrate treatment. Furthermore, for making r-GO/TiO₂ composite thin films, an aqueous solution of r-GO is deposited onto the TiO₂/Si thin film by spin coating process [(r-GO/TiO₂)/Si]. To determine the degree of incorporation of r-GO onto TiO₂, the (r-GO/TiO₂)/Si is characterized using diverse and more advanced spectroscopic techniques, such as XANES and XPS.

Our findings contribute to understanding the mechanism to tune the electronic structure and magnetic performance of the (r-GO/TiO₂)/Si, which will be the important key points for the development of promising optoelectronic, spintronic, and diverse technological applications. Thus, the role of the interfacial defects, vacancies in the (r-GO/TiO₂)/Si composite, and the mechanism of the enhancement of magnetic properties are elucidated comprehensively.

TiO₂ thin films are prepared by the radio frequency (RF) reactive magnetron sputtering technique using a titanium target (99.99% purity) with a constant frequency of ≈13.56 MHz and a working pressure of chamber ≈10⁻² mbar. The films were deposited on a n type-Si (100) substrate at room temperature for 1 h with a constant Ar:O₂ ≈ 70:30 in sccm and RF power supply of ∼200 W. Before deposition, the substrate surface was cleaned by sputtering of Ar⁺-ion. The deposited films are then annealed at different temperatures varied from ≈110 to 250 °C in an argon and oxygen (70:30) in sccm atmosphere. After annealing of TiO₂ at different temperatures, Raman and XPS characterization have been conducted from where the films annealed at 250 °C were observed to be good crystalline in nature with TiO₂ anatase phase. Hence, in this study, we used the deposited TiO₂ thin films annealed at ≈250 °C. The thickness of the TiO₂ thin film used in this study is ≈200 nm. The details of the synthesis method and their characterization could be found elsewhere.²⁵ 

GO is synthesized by the improved Hummers method, and the r-GO is then produced from GO by the hydrazine hydrate treatment. The details of the synthesis method could be found elsewhere.²⁶ 

An aqueous solution of r-GO is deposited on the annealed (250 °C) TiO₂/Si by spin coating process to prepare r-GO/TiO₂ composite thin films on the Si-substrate [(r-GO/TiO₂)/Si] and then dried overnight at a temperature of ≈50 °C, just above the room temperature. It is estimated that the thickness of r-GO is less than ≈100 nm. It is further noted that the deposited TiO₂ on the Si-substrate annealed at 250 °C has been used in this study due to its good crystallinity and having an anatase phase.

XRD, TEM, Raman spectroscopy, XANES, and XPS techniques were used to study the effects at the interface of TiO₂ and r-GO on the surface defects; vacancy; and the microstructural, surface morphological, and electronic properties; and finally all the properties are correlated with the magnetic properties/behaviors of the (r-GO/TiO₂)/Si composite thin-film materials measured by the superconducting quantum interference device (SQUID)-type magnetometer for the uses of future technological applications. XANES spectra C K-edge, O K-edge, and Ti L_3,2-edge were obtained at BL-20A in surface-sensitive TEY-mode, respectively in Taiwan light source Hsinchu, Taiwan. The details of the characterization techniques could be found elsewhere.²⁵ 

Surface morphology with the microstructure of TiO₂ and r-GO/TiO₂ composites is observed by the TEM, XRD, and Raman spectroscopy. TEM images, presented in the inset of Fig. 1(a), show that small and uniform TiO₂-particles are distributed uniformly throughout the surface, indicating that TiO₂ is completely crystalline, whereas Fig. 1(b) shows that the TiO₂-particles are detected on ultrathin r-GO nanosheets and allocated uniformly throughout the surface. The distribution of TiO₂-particles over the ultrathin r-GO increases the specific surface area compared to TiO₂. The average diameter of TiO₂ has been estimated at ≈25–30 nm, as seen in the TEM image [Fig. 1(a)], and the interplanar spacing of ≈0.352 nm is assigned to the (101) planes of anatase TiO₂ [Fig. 1(b)]. XRD patterns of r-GO, TiO₂, and r-GO/TiO₂ composites are shown in Fig. 1(c). Wider peaks at ∼26.0° and ∼43.3° correspond to the (002) and (100) characteristic diffraction graphite peaks of r-GO caused by the disorder in the graphene sheets.^27–29 The diffraction peaks of TiO₂ and r-GO/TiO₂ composites at 25.5°, 38.0°, 48.3°, 55.2°, and 63.0° are described in the crystal planes (101), (004), (200), (211), and (204) of anatase TiO₂, respectively.³⁰ The TiO₂ diffraction peak at ∼25.5° is merged with the wider r-GO peak at ∼26.0° confirms that the chemical interaction between TiO₂ and r-GO exists undoubtedly and retains the anatase phase of TiO₂ in the r-GO/TiO₂ composite structure. Different Raman vibration modes at ≈144 (1 E_g), 396 (B_1g), 513 (A_2g), and 640 cm⁻¹(2 E_g) confirm the anatase phase of TiO₂,^31–34 whereas the peaks at ≈1340, 1585, and 2688 cm⁻¹ correspond to the well-documented D band—the out of plane vibrations attributed to the presence of structural defects and disorder of the sheet; G band—the in-plane vibration of sp² hybridization of carbon atoms; and 2D band (second-order D-band)—related to the low-intensity Raman band second-order effect of phonons,³⁵ respectively, and confirm the presence of carbon-based r-GO materials³⁵ as shown in Fig. 1(d). In the case of the r-GO/TiO₂ composite, D and G peaks are shifted at ≈1348 and 1593 cm⁻¹ along with several peaks at the low-frequency region, indicating that the TiO₂ is in the anatase phase. In addition, the 2D peak (≈2688 cm⁻¹) disappeared, and a new peak appeared at ≈2935 cm⁻¹ ,³⁶ which is described as the graphitic (D + G) band,³⁷ the combination of D-band ∼1348 cm⁻¹ and G-band ∼1593 cm⁻¹, i.e., very near to ≈2935 cm⁻¹ (6 cm⁻¹ lower position), that originated from r-GO in the r-GO/TiO₂ composites probably due to the TiO₂ and r-GO interaction process.^30,36,38 To make clear the change of I_D/I_G, we have overlaid the two spectra of r-GO and r-GO/TiO₂ after baseline background subtraction [see the inset in Fig. 1(d)]. It is observed that the I_D/I_G ratio of r-GO ≈1.2 is reduced to ≈0.95 in r-GO/TiO₂ composites implying that the graphitic nature is drastically reduced, i.e., the sp²-rich nature of r-GO is becoming sp³-rich nature of r-GO/TiO₂ composites. Similar phenomena were also observed by How et al.³⁹ in the r-GO/TiO₂ composites. The decrease of I_D/I_G ratio and shift of Raman peak in r-GO/TiO₂ composite from r-GO with the TiO₂ content are due to induced stress by TiO₂ grown on the surface of r-GO as also observed by Wang et al.³⁰ and Perera et al.,³⁸ respectively, in the GO/TiO₂ composites.

The C 1s, O 1s, and Ti 2p core-level deconvoluted XPS of r-GO, TiO₂, and r-GO/TiO₂ composite thin film surfaces are shown in Figs. 2(a)–2(c). Elemental compositional and quantification data and their deconvoluted peak positions are tabulated in Table I. In r-GO, three deconvoluted spectra of C 1s are ≈283.2 (C–C), 284.1 (C=C), and 287.0 eV (C–H/C–O) are shifted to 284.8 (C–C/Ti–C), 286.1 (C–O/Ti–C–O), and 288.5 eV (Ti–O–C/OC=O) when r-GO is deposited on TiO₂, as shown in Fig. 2(a). In the O 1s, the spectral features are different from each other. In r-GO, three convoluted spectra at 529.8, 531.2, and 532.7 eV are assigned as different oxygen-containing groups, viz., C–O, C=O, and C–O–O. In TiO₂, deconvoluted two spectra at 529.8 and 531.2 eV are changed into three spectra at 529.0, 530.5, and 532.4 eV for r-GO/TiO₂ composites described as oxygens bound to Ti⁴⁺ ions in TiO₂, Ti–O–C, and oxygen-containing groups in reduced graphene oxides, respectively,⁶ as shown in Fig. 2(b), which confirms the interaction of r-GO with TiO₂ in the r-GO/TiO₂ composites. These three peak positions are different than r-GO, which implies that the r-GO/TiO₂ composite bonding structure is different than r-GO. In Ti 2p, the two main peaks of TiO₂ arise at ∼458.6 and 464.3 eV and are shifted at ∼458.7 and 464.4 eV when r-GO is deposited on TiO₂, as shown in Fig. 2(c). These two main peaks are ascribed as Ti 2p_3/2 and Ti 2p_1/2, respectively, and their energy differences (ΔE_BE) remain unchanged and are ≈5.7 eV, indicating the anatase crystal phase kept unchanged. We have deconvoluted these two peaks into four peaks as shown in Fig. 2(b) and found that the peaks of TiO₂ at 458.3, 458.9, 463.9, and 464.7 eV are shifted to 457.9, 459.0, 463.5, and 464.9 eV in the r-GO/TiO₂ composites, indicating electronic with bonding structural change. The shifting of peak positions of C 1s, O 1s, and Ti 2p indicates that the interactions between r-GO and TiO₂ have occurred for r-GO deposition on TiO₂. A peak at ∼532.4 eV describes as Ti–O/Ti–O–C and/or Ti–OH bond.^40,41 Reduction of O 1s peak in the r-GO/TiO₂ composites has occurred after recombination of r-GO and TiO₂ on the deposition of r-GO on the TiO₂ thin film.

Figure 3(a) shows the C K-edge XANES spectra that are plotted from 270 eV to 315 eV to study the electronic structure of r-GO/TiO₂ composites. Spectral features of r-GO and r-GO/TiO₂ composites are different from each other. In r-GO, the two main features are observed at ≈284.6 and 292.2 eV, respectively,^42–44 that are assigned to the unoccupied π* (C=C), and σ* (C–C) bands, respectively. On r-GO deposition on TiO₂, π* and σ* are slightly shifted to ≈284.4 (sp²) and 291.5 eV (sp³), respectively,^42–44 indicating that the structural behaviors are changed in the r-GO/TiO₂ composites. In between π* and σ* features, three peaks, marked as a, b, and c, are observed approximately at ≈286.4 (285.8), 288.2 (287.4), and 289.8 (289.5), respectively, and these peaks are ascribed as C–H, O–C=C, and C=O bonding structures. The π* region is subtracted using a best-fitting Gaussian line and shown in the inset of Fig. 3(a). Considering the intensity of the peaks, it shows that the intensity of C=C is increasing and O–C=C decreases for r-GO/TiO₂ composites, compared to r-GO. These indicate an increase in the graphitization of r-GO/TiO₂ composites that give rise to sp² hybridization as seen in Raman spectroscopy, after deposition of r-GO on TiO₂ to make r-GO/TiO₂ composites. The O K-edges of r-GO, TiO₂, and r-GO/TiO₂ composites are presented in Fig. 3(b). The spectral features are different from each other. It is observed that the r-GO has a wide structure π* peak at ≈529.3 eV, whereas TiO₂ has a double structure with two peaks at ≈530.2 and 532.6 eV, respectively.^45–47 In the r-GO/TiO₂ composites, those two peaks of TiO₂ have reduced their intensity by shifting the second peak from 532.6 to 532.0 eV, implying that the structural changes occur after the hybridization of O 2p to Ti 3d states⁴⁸ during the formation of r-GO/TiO₂ composite thin film. The Ti 3d splits into two peaks 530.2 (t_2g) and 532.6 eV (e_g) due to crystal field effects.⁴⁶ Again, two peaks that appeared at ∼538 and 544 eV within the σ* region are the hybridization of O 2p to Ti 4sp state.^49,50 Figure 3(c) shows the Ti L_2,3-edge XANES of r-GO/TiO₂ composites and TiO₂ thin film, where L₃-edge and L₂-edge regions correspond to O 2p_3/2 → Ti 3d and O 2p_1/2 → Ti 3d due to crystal field splitting of 3d band into t_2g (≈456.3 and 458.5 eV) and e_g (≈461.6 and 463.9 eV) bands,^51,52 respectively. Figure 3(c) shows the variation among the intensities of the TiO₂ and r-GO/TiO₂ composites, suggesting that oxygen and/or carbon are substituted for the Ti-atom in the r-GO/TiO₂ composites. The spectrum of Ti L_3,2-edge XANES spectrum of TiO₂ has two pre-edge resonance features, marked as A and A′, along with a shoulder-like feature, marked as B′, which is the trace of the anatase structure of TiO₂.⁵³ It is noted that the shoulder-like feature marked as B′ becoming smooth, broadened, and the intensity is decreased in the r-GO/TiO₂ composites as shown in the inset of Fig. 3(c), which indicates a decrease of Ti 3d unoccupied states. These events further imply the formation of defects and/or vacancy of Ti 3d orbitals through the interaction with r-GO. The change of oxygen vacancies in the system directly influences the fundamental interactions of the electronic structure by disrupting the Ti–O network.

From the XPS (XANES) results discussed earlier, it is very clear that, in the deposition of r-GO on TiO₂, the electronic structure and bonding properties of r-GO/TiO₂ composites are changed from r-GO as well as TiO₂. As evidenced in the XPS (XANES), the peaks of C 1s (C K-edge), Ti 2p (Ti L_3,2-edge), and O 1s (O K-edge) shifted along with reduced their peaks intensities of the r-GO/TiO₂ composites compared to r-GO and TiO₂ confirmed the rearrangement/redistribution of Ti, O, and C related bonds that form interfacial defects and vacancies in the interface of r-GO/TiO₂ composites. In addition, the decrease of I_D/I_G ratio and peak shift of Raman spectra in the r-GO/TiO₂ composite from r-GO with the TiO₂ content further confirms the microstructural change. These are the strong evidence for the chemical interactions that change the electronic and bonding structure that occurred due to the presence of oxygenated groups such as, –COOH and C=O groups, at the surface/edge of the r-GO structure that responds to incorporating TiO₂ on the r-GO surface. A schematic diagram of the r-GO/TiO₂ composites and energy levels of TiO₂ and the localized sp² domains of r-GO with respect to reduction and oxidation are shown in Figs. 4(a) and 4(b). We believe that the interface between TiO₂ and r-GO with a separation distance of ≈1.5 nm⁵⁴ can facilitate efficient charge transfer across the interfaces. In the rGO/TiO₂ interface, the chemical reduction will elevate the inter-connectivity of localized sp² sites as well as increase the percentage of zero-gap regions on the carbon sheets. The O 2p level is lifted with the decreasing oxygen content as observed in XPS/XANES resulting in the migration of carriers out of the TiO₂ and r-GO energy levels that are expedited, leading to a change in the electronic structure of the GO/TiO₂ composites.

Figure 5(a) shows the magnetization vs applied magnetic field (M-H) hysteresis loops of r-GO, TiO₂, and r-GO/TiO₂ that are measured at room temperature (∼300 K) and below the room temperature (∼40 K), whereas the M-T curve during FC and zero-field cooling (ZFC) are shown in Figs. 5(b)–5(d). From the M-H hysteresis loops and the M-T curve during field cooled (FC) and ZFC, it is very clear that magnetization is enhanced when r-GO is deposited on the TiO₂ thin film. For a comparative study, we have obtained different magnetic parameters, viz., the saturation magnetization (Ms) and coercivity (Hc) values from the M-H hysteresis curve of r-GO, TiO₂, and r-GO/TiO₂, and are tabulated in Table I. These enhanced magnetic performances of r-GO/TiO₂ come from the vacancies/defects that induce magnetic moment at the interface of TiO₂ and r-GO. The ferromagnetism in the r-GO/TiO₂ composites is associated with various structural defects (O-/Ti-vacancies and O-/Ti-interstitials) present in the interface–lattice sites that induce magnetic moments. We believe that the magnetic performances are enhanced due to interactivity among magnetic Ti⁺ and O-/Ti-vacancies and/or O-/Ti-interstitial present at the interface of the r-GO/TiO₂ composite. It is attributed to the cutting of r-GO crystal during the oxidation/reduction process of various oxygen functional groups (–OH, –O–, –COOH, and C=O) that break the π bond network at grain boundaries⁵⁵ that induce magnetic domain. Figures 5(b)–5(d) show the dependence of magnetic susceptibility on temperature for a composite r-GO/TiO₂ in the ZFC and FC modes at an applied magnetic field H = 500 Oe. There is a small difference in magnetic susceptibility in TiO₂ and r-GO/TiO₂ due to the possibility of setting some anti-parallel magnetic moments.

In summary, the magnetic anatase-based defective r-GO/TiO₂ composite thin film has been successfully synthesized and investigated the electronic and magnetic performance. The various oxygen functional groups at the interface of r-GO/TiO₂ play a key role in tailoring the magnetic behavior of the r-GO/TiO₂ composite thin film. The assessment of electronic and magnetic results shows that the coupling of anatase with r-GO ensures efficient magnetic performance. The composite materials could also be developed for other technological applications, such as photocatalyst activity under sunlight irradiation.

W.F.P. acknowledges the Ministry of Science and Technology (MoST) of Taiwan for providing financial support for research under Project Nos. MoST 110-2112-M-032-017 and 110-2112-M-032-018. S.C.R. acknowledges the NRF South Africa (Grant No. EQP13091742446). D.K.M. acknowledges by Dr. Atala Bihari Panda for the synthesis of TiO₂ thin films using the radio frequency (RF) reactive magnetron sputtering technique.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding authors upon reasonable request.