Reduced Graphene Oxide / Strontium Titanate—Investigation of Improved Photoactivity

Strontium titanate is a ternary n-type semiconductor that has recently been identified as a promising material for many photocatalytic applications such as degradation of organic pollutants or oxygen evolution reaction, especially because of high durability. However, SrTiO3 has not demonstrated high photoactivity and therefore many approaches have been studied towards improving its photo-efficiency. One of them is combining it with graphene oxide, which demonstrated huge improvement in overall photocatalytic activity, and the reasons of this effect have not yet been fully identified. In this work, we characterize SrTiO3/ graphene oxide photomaterials synthesized by a straightforward hydrothermal procedure and for the first time report many of the quintessential material features that are relevant to improved photocatalytic activity towards photocatalytic degradation of toluene. Our results provide important insights into the efficiency of charge separation, carrier transport, and photostability.


Introduction
Metal titanates are a class of materials that are well-known for their photocatalytic activity. Titanates are characterized by high durability under light and low cost. Moreover, titanium-based photocatalysts attract attention because the multi-metals materials offer a broad range of possibilities in the area of band-engineering as well as wide choice for further surface and bulk modifications [1][2][3][4][5]. Among them, a SrTiO 3 seems to be a photocatalyst offering the highest activity, in particular towards degradation of various organic contaminants or the hydrogen evolution from water reduction [6][7][8][9][10][11]. Strontium titanate (STO) is a n-type semiconductor with an indirect band gap energy of ca. 3.2-3.5 eV depending on the morphology and structure. The conduction band (formed by Ti 3d states) potential is −0.4 eV vs. NHE, while valence band (formed by O 2p and Sr 4p states) potential is 2.8 eV [10]. Its photocatalytic activity and durability can be improved by various approaches described in Tomasz Baran biuro@satom.pl (potential organic contaminants in the purification of water or air) can be easily adsorbed on GO via π-π interaction [28]. Adsorption of reagents on photocatalyst's surface is also facilitated by a large specific surface area of GO.
Since numerous articles have reported the superior photoactivity of composites based on titanates and graphene oxide, it was necessary to search for bases of this phenomenon. In the following paper, we investigated the reasons for improved photoactivity of strontium titanate -reduced graphene oxide composites. By using series of spectroscopic and electrochemical methods we studied the effect of the coupling of SrTiO 3 with GO, in principle, focusing on the charge separation, trapping and recombination processes. Moreover, we demonstrated the photodegradation of toluene in the gas phase under ultraviolet and visible light irradiation.

Materials and Methods
Synthesis of materials Graphene oxide was obtained from graphite using a modified Hummer's procedure described in literature [29]. Briefly, graphite (0.5 g), sodium nitrate (0.5 g) and sulfuric acid (25 mL) were stirred for 1 h in an ice bath. Subsequently, 2.5 g of KMnO 4 was added and the solution was stirred at 50 °C for next 10 h. The reaction mixture was cooled to room temperature and then 150 mL of water was added to stop the reaction. Subsequently, the excess of hydrogen peroxide (35%) was slowly added to reduce manganese ion. The solution was filtered and a filter cake was washed with 5% HCl solution. Finally, the product was dried and calcined at 250 °C for 2 h to form RGO.
For synthesis of STO-RGO, reduced graphene oxide (36,7 or 18,4 mg) was dispersed in water (20 mL) and sonicated for 20 min. Subsequently, 0.532 g of strontium hydroxide (Alfa Aesar) was dissolved in 20 mL of water. The Sr(OH) 2 solution was poured into RGO suspension. Ethylene glycol (5 mL) was added to the mixture. Then, 0.568 g of titanium(IV) isopropoxide, (98+%, ACROS Organics) was dissolved in 10 mL of isopropyl alcohol. Titanium solution was added dropwise into RGO suspension under vigorous stirring condition. As obtained mixture was stirred for 10 h and the mixture was then transferred to a Teflon lined autoclave with capacity of 150 mL and heated at 180 °C for 8 h. The precipitate was separated by centrifugation, washed with water and methanol and dried under vacuum.
Materials were deposited on a carbon tape and pressed to obtain flat surface of sample. Scanning Electron Microscope (SEM) SU3500 was used to collect SEM images. Raman spectra were recorded using Renishaw inVia Raman microscope. UV-vis diffuse reflectance spectra of the solid samples were recorded using a UV-vis spectrophotometer (UV-2600 Shimadzu) equipped with an integrating sphere. BaSO 4 was employed as the reference material as well as to dilute samples. Nitrogen adsorption isotherms measurements were performed in -195℃ using a Micromeritics apparatus.
Electrochemical studies. STO-RGO materials were mashed with water until homogeneous pastes, which were used to produce a photoelectrodes onto fluorine doped tin oxide glass (Sigma Aldrich, ~ 8 Ω/sq) via a doctor-blade technique using the scotch tape to set the area (1 cm 2 ), and thickness of films (ca. 0.05 mm). Electrodes were subsequently dried on a hot plate. Photocurrent measurements were performed using a typical 3-electrodes setup controlled by BioLogic SP-150 potentiostat. LED matrix equipped with electronic light shutter was used as a light source. Phosphate buffer (0.1 M K 2 HPO 4 and KH 2 PO 4 , pH = 7) was used as an electrolyte. Electrolyte was bubbled with nitrogen or oxygen prior to the measurements. Potentials in the manuscript are referred to the Reversible Hydrogen Electrode (RHE). All of the photoelectrochemical analyses were carried out at room temperature.
Photovoltage measurements were performed under open-circuit conditions. Working electrodes were illuminated with increasing power of light using home-made setup described in Supporting Information. Irradiance of light (mW/cm 2 ) was measured using radiometer SL-3201 (Solar Light).
EIS (electrochemical impedance spectroscopy) was performed in 0.1 M phosphate buffer (pH = 7) in a typical 3-electrodes configuration: working electrode -material deposited on FTO (fluorine-dopped tin oxide) glass; saturated calomel electrode -reference electrode; spiral platinum wire -counter electrode. Measurements were performed using BioLogic SP-150 instrument. Measurement were performed in dark, with absence of scattered daily light.

Photocatalytic tests
The photocatalytic experiments of VOC degradation were performed in a gastight reactor of total volume 3 L equipped with LED matrix (λ = 420 nm). Toluene was used as model VOC contaminant. The STO-RGO materials (10 mg) were sonicated in 2 mL of methanol and subsequently suspensions were spin-coated on the glass plate (area 20 cm 2 ) and dried. Then the photocatalyst was placed into reactor. Appropriative amount of toluene has been injected into reactor and evaporated, so its initial concentration of was ca. 300 ppm. Concentration of VOC and products of its degradation were monitored (once per hour) by gas chromatography (GC-2030 Nexis, Shimadzu with FID and TCD detectors; column -Zebron ZB-5; carrier gas -He). After the completion of the first round of the experiment, the catalyst was collected, washed, dried in 110℃ for 2 h and used in the next cycles of experiment. Blank tests were performed in the dark, in absence of photomaterial, and in absence of toluene.

Results and Discussion
Strontium titanate-reduced graphene oxide (STO-RGO) composites have been prepared on a hydrothermal route. As prepared materials, containing 5% or 10% of RGO (by weight), have been characterized by using spectroscopic, microscopic and electrochemical techniques. Figure 1 shows XRD patterns of STO and STO-RGO samples. All the diffraction peaks for the neat STO particles can be index to the cubic perovskite structure of strontium titanate SrTiO 3 (JCPDS card 35-0734), and no traces of impurity phases are observed. Miller's indexes hkl are given in Fig. 1. Moreover, no apparent diffraction peaks of reduced graphene oxide in the STO-RGO are observed, which is due to the weak diffraction intensity of the RGO. This indicates that the SrTiO 3 particles undergo no structural change due to coupling with reduced graphene oxide. Such observation was reported previously [25]. Crystallite size (D -the size of the ordered (crystalline) domains) calculations are given by Scherrer's equation: where, K is a shape factor; λ is the X-ray wavelength (0.15418); β is the line broadening at half the maximum intensity (FWHM); θ is the Bragg angle. Details of calculation are given in Table S1 in Supporting Information. The average size of the crystallites is 34.03 nm. Raman spectra were measured to prove the presence of RGO in STO-RGO composites, as shown in Fig. 2. The Raman bands at 1332 cm − 1 (D-band) and at 1588 cm − 1 (G-band) confirm the existence of RGO in the composite. Band at Raman shift lower than ca. 900 cm − 1 are assigned to STO, as indicated in the literature [30].
The morphological structure of composite was investigated by SEM. As shown in Fig. 3, the material is composited of large particles of reduced graphene oxide covered by significantly smaller particles of strontium titanate. EDS (Energy Dispersive Spectroscopy) analysis proves homogenous distribution of SrTiO 3 on graphene oxide. Elemental analysis, given in supplementary file ( Figure S1), shows the following composition of materials ( Table 1).
Modification with RGO increased the specific surface area of the sample: 29.3 m 2 /g -STO, 111.2 m 2 /g -STO-5RGO, 136.3 m 2 /g -STO-10RGO. This effect can be clearly ascribed to the large specific surface area of RGO. All materials showed the nitrogen adsorption-desorption isotherm with distinct hysteresis loops (a typical example of type IV) as shown in Figure S2 in the supplementary file.  Figure S3) are in accordance with the previous study on graphene oxide [31,32]. Lower photocurrent density observed in the case of neat STO is attributed to the inefficient charge separation in STO or high rate of charge recombination. The improved photocurrent generation efficiency may be a result of an interaction between excited STO and RGO which leads to electron transfer from STO to RGO. The ability of reduced graphene oxide to accept electrons from excited semiconductors has been demonstrated previously for TiO 2 -RGO electrodes [33,34].
The open-circuit photovoltages have been measured as a function of light power, which is shown in Fig. 5C using LED l = 400 nm as a light source, with home-made instrumentation for power control (supplementary file Figure S4). It can be seen that the photovoltage increases with the light intensity. The saturated light intensity was higher than 0.25 mW for both materials. Upon illumination with this light open circuit photovoltage reached 50 mV and 60 mV for STR and STO-5RGO, respectively.
As already proved, the photocurrent density strongly depends on the incident light wavelength. The IPCE characterization (Incident Photon to Current Efficiency) of materials were measured at 0.96 V vs. RHE as chronoamperometric measurements for each wavelength at pH = 7. IPCE was calculated using the following Eq.
where λ is the wavelength of the incident light, I ph and J are the measured photocurrent density and the measured irradiance at the selected wavelength, respectively. As shown The presence of RGO in composite significantly affects the UV-V is spectrum of STO. Diffuse reflectance spectra of samples grounded with BaSO 4 have been measured. The absorbance data, in terms of Kubelka-Munk function, are presented in Fig. 4. Strontium titanate, as a typical widebandgap semiconductor, shows an absorption peak below ca. 400 nm. Coupling with RGO results in a significant broadening of absorption range towards visible light. Band gap energy was calculated according to the Tauc method, considering an indirect band gap transition in STO. Determination of band gap in STO is shown in Fig. 4 -inset. The calculated band gap energy of STO was 3.19 eV. Coupling with reduced graphene oxide does not change band gap of STO -it simply shows additional absorption in broad range of visible light.
The density of generated photocurrent is a simple measure of charge separation efficiency. STO and STO-RGO were studied for their ability to generate photocurrent as a function of electrode potential under ultraviolet (λ = 350 nm) or visible light (λ = 420 nm) irradiation. Unmodified strontium titanate generates anodic photocurrent within a wide potentials window, upon UV illumination (Fig. 5). In contrast, under visible light only slight photocurrents are generated. Coupling the STO with RGO resulted in improved efficiency of photocurrent generation under UV as well as under visible light. It has been noted, that STO-RGO showed also a higher "dark current" that may suggest oxidation processes occurring within material. Blank  to expectation, STO is active only under ultraviolet irradiation. To perform optimization of material composition, the composite loaded with 15% of graphene oxide was also studied, however, its photoactivity is lower than material with 10% of RGO, which showed the highest photoactivity under irradiation with all wavelengths (Fig. 6B).
The durability of the photocatalysts increased with the increasing amount of graphene oxide. As shown in Fig. 7, all studied materials decomposed toluene during the 6 photocatalytic runs, however, the neat STO lost a noticeable part of its initial activity. In contrast, the efficiency of toluene degradation in the presence of STO loaded with graphene oxide was almost unchanged during all cycles. These results suggest high chemical stability of composite that will be further investigated in the next paragraph.
As a familiar air pollutant, toluene was used as a model VOC. Photocatalytic degradation of toluene was performed under ultraviolet or visible light irradiation, in the presence of neat SrTiO 3 as well as STO-RGO composite photocatalysts. Results are shown in Fig. 6, while example chromatographic data are given in supporting information (supplementary file Figure S5). Under UV, toluene decomposition efficiency is being achieved at 96.7% or 90.2% in 4 h, for STO-10RGO and STO-5RGO, respectively. In contrast, the neat STO decomposes ca. 80% of initial toluene within 4 h of irradiation. In turn, under visible light (λ = 633 nm) the activity of STO-RGO material is noticeably lower but it still reaches up to 55%. For comparison, the neat STO is almost inactive under visible light -according the binding energies of Ti 2p3/2 and Ti 2p1/2 respectively, which suggests that the titanium exists in the state of Ti(IV). Figure 8C presents the strontium spectra, where the peaks at 132.0 eV and 134.5 eV proves the binding energies of Sr 3d5/2 and Sr 3d3/2, respectively, confirming the presence of Sr 2+ . Further, as shown in Fig. 8D the oxygen peak is partially divided into two peaks 531.2 and 529.8 eV suggesting the two different O chemical states of photocatalyst. Finally, To study the chemical composition and photostability of the materials, XPS spectra were taken for STO-RGO samples both before and after the photocatalytic test. Figure 8A shows the XPS survey spectrum of pristine STO-RGO and the STO-RGO used for 24 h (6 photocatalytic runs for 4 h each), revealing the existence of Ti, Sr, O, and C elements in both composites. Figure 8B shows the spectrum of titanium, where the peaks at 458.2 eV and 464.1 eV prove   [35].
Both spectra, spectrum of pristine material and spectrum after the photocatalytic test performed for 24 h are very similar and both showed the same peaks. The most noticeable changes occurred within titanium peaks. The binding energies of Ti 2p3/2 and Ti 2p1/2 slightly shift to lower values, indicating the presence of Ti(III) [36]. Deconvolution of high-resolution spectrum of Ti leads to a peak at 457.5, typical binding energy of Ti(III), as shown in Fig. 9. Judging by this result, it is reasonable to assume that after conducting the PEC test for 24 h, some amount of Ti(IV) in STO is reduced to Ti(III), and it further demonstrates the electron transfer towards titanium, which can be considered formally as a conduction band (by analogy with TiO 2 ) [37].
Charge recombination has been studied using photoluminescence measurements. The spectrum of STO showed  by electrochemical impedance spectroscopy (Fig. 10B). Obtained results suggest that the composites STO-RGO possesses better separation efficiency of light induced electron-hole pairs.

Conclusion
In this work, we characterized strontium titanate/ reduced graphene oxide photomaterials synthesized by a straightforward hydrothermal procedure. STO-RGO showed a significantly improved photocatalytic activity towards toluene degradation in a gas phase, as well as enhanced stability and durability under light conditions. For the first time we report many of the quintessential material features that are relevant to the improved photocatalytic activity. Our results provide important insights into the efficiency of charge separation, carrier transport, and photostability. First of all, we demonstrated how the modification by RGO affects the charge separation efficiency. Functionalized materials demonstrated a higher density of generated photocurrent under UV and visible light, as well as an enhanced incident photon to current efficiency (IPCE). On the other hand, the presence of RGO prevents a charge carrier recombination and the STO-RGO materials exhibit smaller charge transfer resistance, as demonstrated by EIS. Under irradiation, photogenerated electrons can be transferred towards RGO, improving the photocatalytic efficiency. Finally, we discovered that 24 h long irradiation leads only to minor chemical changes within STO-RGO composite, in particular reduction of Ti(IV) to Ti(III).

Author Contributions
Authors contributed equally to the study conception, design, analysis, data collection and writing. All authors read and approved the final manuscript.

Data Availability Not applicable.
Code Availability Not applicable.

Declarations
Conflict of interest No conflict of interest. two overlapping bands: ''blue band'' at about 420-440 nm and ''green band'' at 495-510 nm (Fig. 10A). According to literature, the "green band" is associated with a triplet-singlet optical transition of a self-trapped exciton, while "blue" emission is ascribed to a transition from the conduction band edge to the in-gap level of a self-trapped hole, or due to oxygen vacancies [38]. As shown in Fig. 10A, the luminescence intensity of the neat STO is much higher than that of STO-RGO composites. These results can indicate the depressed recombination of the photogenerated electron-hole pairs, as suggested in literature in the case of TiO 2 /graphene composites [39,40]. Therefore, photoluminescence experiment can be considered as evidence to prove the enhanced photocatalytic performance of the strontium titanate-graphene oxide composite photocatalysts, at least under UV irradiation, because of the wide band gap of SrTiO 3 . Additionally, compared with the neat STO, the STO-RGO materials exhibit smaller charge transfer resistance, as demonstrated