Perovskite-type KTaO₃–reduced graphene oxide hybrid with improved visible light photocatalytic activity

Abstract

Novel rGO–KTaO₃ composites with various graphene content were successfully synthesized using a facile solvothermal method which allowed both the reduction of graphene oxide and loading of KTaO₃ nanocubes on the graphene sheets. The as-prepared photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) specific surface area, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), UV-Vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) emission spectroscopy. The obtained rGO–KTaO₃ composites showed greatly improved photocatalytic performance for degradation of phenol under visible light irradiation (λ > 420 nm) over pristine KTaO₃ which could be related to the photosensitizer role of graphene in the rGO–KTaO₃ composites as well as the formation of p–n heterojunctions between KTaO₃ nanocubes and rGO sheets. The highest photocatalytic activity in phenol degradation reaction was observed for rGO–KTaO₃ hybrid with 30 wt% graphene. The enhanced photoactivity of this composite could be attributed to the synergistic effect of several factors such as: small crystallite size, extended absorption range in the visible spectrum and intimate contact between graphene and KTaO₃ cubes.

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1. Introduction

Pollution of the natural environment due to the rapid development of science and technology is currently an important issue. In particular, pollution of water is a significant health risk and continues to affect both human quality of life and the ecosystem.^1–3 In recent years, semiconductor materials have been of great research interest due to their novel properties and potential photocatalytic applications for the degradation of toxic organic pollutants in the wastewater.^4–6 Perovskite-type oxides are a particularly important group of semiconducting materials because their crystal structure is stable as well as very flexible toward substitutions that may induce interesting properties such as electrical, magnetic and optical behaviour.^7–9

Among perovskites, potassium tantalate (KTaO₃) is one of the most excellent functional material due to its outstanding dielectric properties and cubic perovskite structure at all temperatures.^10,11 Moreover, tantalates possess conduction bands consisting of a Ta 5d orbital located at a more negative position than that of titanates (Ti 3d). As a result, the high potential of the conduction band of tantalates could lead to being more advantageous in the photocatalytic reaction.¹² KTaO₃ semiconductor is known as a promising photocatalyst for the reaction of photocatalytic hydrogen generation,^13,14 photodegradation of pollutants in both the gas^15,16 and aqueous¹⁵ phases as well as photoreduction of CO₂.¹⁷ However, perovskite-type potassium tantalate is a semiconductor material with a wide band gap and can efficiently absorb UV light, but absorbs visible light relatively inefficiently. Therefore it is necessary to develop a high-performance photocatalysts that can utilize a wide range of visible light.

Graphene, a two-dimensional, single-layer sheet of carbon atoms that are closely packed into a hexagonal lattice structure, has attracted tremendous attention and research interest, because of its excellent thermal conductivity, excellent mobility of charge carriers, high specific surface area, locally conjugated aromatic system, very good chemical and electrochemical stability, excellent transparency and flexibility.^18–22 Owing to its extraordinary advantages, graphene is also considered as an outstanding support or promoter for photocatalytic applications.^23–25 Combination of different types of semiconductors with a graphene has been suggested as a promising method for obtaining an enhanced photocatalytic performance.^26–29 Importantly, some wide band gap semiconductors such as TiO₂, ZnO could exhibit visible light activity after composited with graphene.^30–33 Khalid and co-workers³⁴ synthesized graphene–TiO₂ composite photocatalysts which showed enhanced photocatalytic activity as compared with bare TiO₂ under visible-light irradiation for methylene blue degradation. It was attributed to giant two-dimensional planar structure of graphene and possibility of more π–π interaction between composite and organic compound.³⁴ Li et al.³⁵ demonstrated graphene–TiO₂ nanotubes prepared by using a hybrid synthetic strategy. Graphene–TiO₂ nanotubes showed higher photocatalytic efficiency than graphene–TiO₂ nanofibers and the bare TiO₂ nanomaterials under visible light irradiation for degradation of rhodamine B (RhB), which was arising from the light-trapping effect and the incorporation of graphene.³⁵ In other study, Ashkarran presented an innovative approach for synthesis of zinc oxide–graphene hybrid nanostructures through combination of improved hummer and arc discharge methods in liquid. The results indicated that the ZnO–graphene composites unusually enhanced the photodegradation of standard dyes under visible-light irradiation which was attributed to the effect of electron transport among ZnO nanoparticles and graphene sheets.³⁶ However, to the best of our knowledge, there is no report on the design and fabrication of graphene–KTaO₃ composites and their photocatalytic performances.

In this paper, a novel graphene–KTaO₃ photocatalysts has been synthesized by a facile solvothermal method for the first time. The effects of graphene loading ratio on the morphology, surface area, absorption properties and structure of the composites were systematically studied and further correlated with the photocatalytic performance. Photocatalytic properties under visible light have been studied by employing photodegradation of phenol in the aqueous phase as a model pollutant.

2. Experimental

2.1 Materials and instruments

Tantalum(V) oxide (>99% Aldrich, Poznan, Poland) and potassium hydroxide (Chempur, pure p.a.) were used as precursors for the preparation of KTaO₃. Ethanol, polyethylene glycol 400 (PEG-400) were purchased from POCH S.A. (Gliwice, Poland). Deionized water was used for all reactions and treatment processes. All chemicals were of analytical reagent grade and were used as received without further purification.

Nitrogen adsorption–desorption isotherms at −196.15 °C were measured using a Micromeritics Gemini V (model 2365) physisorption analyzer (Micromeritics Instrument, Norcross, GA, USA). Specific surface areas were calculated following typical Brunauer–Emmett–Teller (BET) method using the adsorption data in the relative pressure (p/p₀) range from 0.05 to 0.3. Prior to adsorption measurements the samples were degassed under vacuum at 200 °C for 2 h. Diffuse reflectance spectra (DRS) of the synthesized materials were characterized using the Thermo Scientific Evolution 220 UV-Visible spectrophotometer (Thermo Scientific, Waltham, MA, USA) equipped with ISA-220 integrating sphere accessory. The UV-Vis DRS spectra were recorded in the range of 200–800 nm using a barium sulfate reference. Powder X-ray diffraction (PXRD, Philips/PANalytical X'Pert Pro MPD diffractometer, (Cu Kα radiation λ = 1.5418 Å) was used to determine the phase composition and calculate lattice parameters of polycrystalline samples. The morphology of the semiconductor composites was investigated with FEI Quanta 250 FEG scanning electron microscope (SEM; FEI, Hillsboro, OR, USA) working in high vacuum mode as well as transmission electron microscopy (STEM-EDX, FEI Europe, model TecnaiF20 X-Twin) and selected area electron diffraction (SAED). Energy-dispersive X-ray spectroscopy (EDS) measurements were carried out using SEM-integrated EDAX Apollo-SDD detector (EDAX Inc., Mahwah, NJ, USA). Accelerating voltage was set to 30 kV. Standardless analysis was conducted with the EDAX TEAM software with eZAF quantization method. X-ray photoelectron spectroscopic (XPS) measurements were performed using the a PHI 5000 VersaProbe (ULVAC-PHI) spectrometer with monochromatic Al Kα radiation (hν = 1486.6 eV) from an X-ray source operating at 100 μm spot size, 25 W and 15 kV. The high-resolution (HR) XPS spectra were collected with the hemispherical analyzer at the pass energy of 23.5 eV, the energy step size of 0.1 eV and the photoelectron take off angle 45° with respect to the surface plane. The CasaXPS software (version 2.3.16) was used to evaluate the XPS data. Deconvolution of HR XPS spectra were performed using a Shirley background and a Gaussian peak shape with 30% Lorentzian character. Charge compensation was achieved using a low energy electron flood gun. FT-IR spectra were carried out on a Bruker model IF S66 FTIR spectrometer using potassium bromide discs. Thermogravimetric analysis (TGA) was performed in argon atmosphere using a Netzsch TG 209 thermogravimetric analyzer at a heating rate of 10 °C min⁻¹ from 20 to 900 °C. The photoluminescence (PL) emission spectra were recorded using a Perkin-Elmer Luminescence Spectrometer LS 50B. The samples were excited with 325 nm wavelength light at room temperature and the emission was scanned between 350–700 nm.

2.2 Preparation of graphene oxide

The graphene oxide (GO) nanoplatelets were synthesized in Institute of Electronic Materials Technology with chemical exfoliation method. First step of obtaining this material was oxidizing graphite flakes, a natural, low cost and broadly available material, with the mixture of sulphuric acid H₂SO₄, sodium nitride NaNO₃ and KMnO₄. This method is well known as a Hummer's and Offerman method. As a result a graphene oxide was obtained in thermal expansion, which has been further ultrasonically exfoliated with Vibra-Cell VCX 750 high intensity ultrasonic processor. Average thickness of platelets was in range of 10–15 nm with average diameter of 25–50 μm.

2.3 Preparation of perovskite-type potassium tantalate

The KTaO₃ semiconductor was prepared by the hydrothermal method based on our previous work with a little modification.¹⁵ In a typical procedure for the preparation of potassium tantalate, KOH (30 g) was dissolved in deionized water (60 mL), then Ta₂O₅ (11 g) and PEG-400 (1 mL) were added. This mixture was stirred for 1 h before it was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and got heated at 200 °C for 24 h. After cooling naturally to room temperature, the resulting powder was washed several times by centrifugation with distilled water and ethanol respectively and dried in an oven at 70 °C for 8 h. Finally, some white powder was obtained.

2.4 Preparation of rGO–KTaO₃ photocatalysts

The reduced graphene oxide (rGO)–KTaO₃ photocatalysts with different weight ratios of GO were obtained via the hydrothermal method. First, appropriate amount of GO was dispersed in a mixing solution of H₂O (80 mL) and ethanol (40 mL) by ultrasonic treatment for 2 h. Then, 0.2 g of KTaO₃ powder was added to the above GO solution. The mixing solution was stirred for another 2 h to obtain homogenous suspension. The suspension was transferred to Teflon-sealed autoclave and maintained at 120 °C for 12 h. The resulting photocatalyst was washed several times by centrifugation with distilled water and dried in an oven at 60 °C for 12 h. In this work, a series of graphene–KTaO₃ photocatalyst with different GO content of 5, 10, 15, 20, 25 and 30 wt% (weight ratio of GO to KTaO₃) were prepared.

2.5 Measurement of photocatalytic activity

The photocatalytic activity of obtained photocatalysts in the visible light (Vis) was estimated by monitoring the decomposition rate of 0.21 mM phenol in the aqueous solution. Phenol was selected as a model contaminant because it is a non-volatile and common organic pollutant found in various types of industrial wastewater. Photocatalytic degradation runs were preceded by blind tests in the absence of a photocatalyst or illumination. The aqueous phase containing the photocatalyst (125 mg), deionized water (24 mL) and phenol (1 mL, c = 500 mg L⁻¹) was placed in a photocatalytic reactor (V = 25 mL) equipped with a 30 mm-thick quartz window. The temperature of the aqueous phase during the experiments was maintained at 10 °C by an external circulating water bath. The prepared suspension was stirred using magnetic stirrer and aerated (V = 5 dm³ h⁻¹) for 30 min in the dark to reach the adsorption equilibrium and then the content of the reactor was photoirradiated with a 1000 W Xenon lamp (Oriel Instruments, Stratford, CT, USA) which emitted both UV and Vis irradiation. The optical path included a water filter and glass filters (GG420, Optel, Opole, Poland) to cut off wavelengths shorter than 420 nm. During the irradiation, 1 mL of suspension sample was collected at regular time periods and filtered through syringe filters (Ø = 0.2 μm) to remove the photocatalyst particles. Phenol concentration was estimated by colorimetric method after derivatization with diazo-p-nitroaniline using UV-Vis spectrophotometer (DU-7, Beckman, Warsaw, Poland).

2.6 Measurement of the formation of hydroxyl radicals

The production of hydroxyl radicals (˙OH) in the presence of visible light illuminated rGO–KTaO₃ composite was detected by a photoluminescence (PL) method using coumarin as a probe molecule which can easily react with hydroxyl radical to produce highly fluorescent product 7-hydroxycoumarin. In the detection experiment, photocatalyst powder was added to an aqueous coumarin solution (14 mL, 10⁻³ mol L⁻¹) and the resulting suspension was magnetically stirred in dark for 0.5 h prior to irradiation. The mixture was then irradiated under visible light (>420 nm), samples were withdrawn at regular intervals and filtered through syringe filters. P25 irradiated by UV-Vis light was used as a reference system. The filtrate was analyzed on a Perkin-Elmer Luminescence Spectrometer LS 50B by the excitation with the wavelength of 330 nm.

3. Results and discussion

3.1 Optical properties and BET surface area

DRS UV-Vis absorption spectra in the wavelength range of 200–800 nm of KTaO₃ and rGO–KTaO₃ samples with different amount of rGO (from 5 to 30 wt%) were investigated and the results are presented in Fig. 1. The absorption edge of KTaO₃ nanocubes is at about 306 nm, which coincides with our previous study (310 nm) and is much bigger than in the case of KTaO₃ nanooctahedra reported by Zou (265 nm).³⁷ It can be clearly seen that the incorporation more than 5 wt% of graphene into KTaO₃ significantly affects the optical absorption properties for the rGO–KTaO₃ nanocomposites. As compared to the pristine KTaO₃, the absorption background in the visible region is enhanced for the rGO–KTaO₃ nanocomposites and is increasing with the amount of graphene incorporated to the composite. Reduced graphene oxide itself, similar to other carbon materials, has good absorption characteristics, and due the range of available light wavelengths was extended after the incorporation of rGO.³⁸ Moreover, it can be observed a red-shift to higher wavelength in the absorption edge of the rGO–KTaO₃ nanocomposites. The results show the significant influence of reduced graphene oxide on the optical properties of as-prepared composites, suggesting that addition of graphene enhances the visible-light absorption and is expected to improve photocatalytic activity under visible light irradiation.

Fig. 1 The UV-Vis diffuse reflectance spectra of pristine KTaO₃ and rGO–KTaO₃ nanocomposites with different amount of reduced graphene oxide.

The specific BET surface areas of pristine KTaO₃ and the rGO–KTaO₃ composites are listed in Table 1. The surface area of as-prepared samples fluctuated from 1.6 to 3.6 m² g⁻¹ and is dependent on amount of rGO in the composites. As is clearly seen, the BET specific surface area gradually increases with increasing content of graphene and all nanocomposites show higher BET surface area as compared with bare potassium tantalate. It can be explained by the average densities of samples.³⁹ The planar density of graphene is 0.77 mg cm⁻², and the density of KTaO₃ is 7.015 g cm⁻³. Thus, the average densities of nanocomposites decrease with increasing graphene content, resulting in the increase of the BET surface area. Similar results were recently reported for B-doped graphene–TiO₂ (ref. 40) and CdS–graphene composites.³⁹ The BET surface areas of rGO–KTaO₃ hybrid composites are very low and it would not have a big impact on photocatalytic activity.

Table 1 Sample label, BET surface area and photoactivity of KTaO₃ semiconductor and rGO–KTaO₃ semiconductor composites

Sample label	Graphene content (wt%)	BET surface area (m^2 g^−1)	Phenol degradation rate under visible light (μmol dm^−3 min^−1)
KTaO3	0	1.6	0.14
5 wt% rGO–KTaO3	5	1.9	0.31
10 wt% rGO–KTaO3	10	2.1	0.17
15 wt% rGO–KTaO3	15	2.6	0.37
20 wt% rGO–KTaO3	20	3.0	0.38
25 wt% rGO–KTaO3	25	3.2	0.73
30 wt% rGO–KTaO3	30	3.6	0.79

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3.2 Morphology

The morphology of as-synthesized photocatalysts was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As clearly shown (Fig. 2a), the KTaO₃ have a well-developed cube-like shape with various sizes. The widths of KTaO₃ nanocubes is in the range of 0.1–1.5 μm which is smaller as compared with our previous study (0.2–8 μm).¹⁵ It may be caused by addition lower amount of PEG in the present synthesis. Fig. 2b–g shows SEM images of rGO–KTaO₃ composites with different amount of reduced graphene oxide. It can be seen that cubic structure of KTaO₃ is retained and the KTaO₃ particles are dispersed on the graphene sheets. However, in the case of composites 5 wt% rGO–KTaO₃ and 10 wt% rGO–KTaO₃, containing lower amount of rGO, it can be observed that rGO sheets are not connected well to KTaO₃ cubes which may cause lower photocatalytic activity of these composites. Additionally, to determine the distribution of these samples, element mapping analyses were conducted and shown in the inset of Fig. 2b and c. In the case of composites containing highest amount of rGO (15–30 wt%), we can observed better dispersion of KTaO₃ nanocubes on the rGO – as the content of the rGO in the composite increased, more KTaO₃ nanocubes are loaded on the rGO sheets. Fig. 3a demonstrates TEM image of rGO which presents a transparent thin paper-like structure with typical wrinkles, probably indicating the presence of oxygen-containing functional groups and the resultant defects during the preparation of rGO.^41,42 The high resolutions TEM image displays that rGO is composed of a few layers and indicates that the rGO sheet was efficiently reduced from exfoliated GO (Fig. 3b). A selected-area electron diffraction (SAED) pattern of the rGO is shown in the inset of Fig. 3b. The diffraction dots indicate the highly crystalline structure of rGO.⁴³ Moreover, the low-magnification TEM image of 30 wt% rGO–KTaO₃ composite (Fig. 3c) shows a few aggregations of potassium tantalate cubes on graphene sheets. The high-magnification TEM image of 30 wt% rGO–KTaO₃ composite clearly displays combination of KTaO₃ nanocube and rGO sheet with a good contact (Fig. 3d) This contact allows the electronic interaction between KTaO₃ nanocubes and graphene sheets and could be potentially beneficial for effective separation of charge carriers and improving the photocatalytic activity.

Fig. 2 SEM images of (a) pristine KTaO₃ nanocubes and rGO–KTaO₃ composites with (b) 5 wt% rGO, (c) 10 wt% rGO, (d) 15 wt% rGO, (e) 20 wt% rGO, (f) 25 wt% rGO and (g) 30 wt% rGO.

Fig. 3 (a) TEM image of the rGO, (b) HRTEM image of rGO (SAED pattern of rGO has been shown in inset), (c) low-magnification TEM image of 30 wt% rGO–KTaO₃ composite (d) high-magnification TEM image of 30 wt% rGO–KTaO₃ composite.

3.3 FT-IR spectroscopy

The FT-IR spectra obtained for GO, KTaO₃, 5 wt% rGO–KTaO₃ and 30 wt% rGO–KTaO₃ nanocomposites are shown in Fig. 4. It is clearly seen that GO (graphene oxide) presented several absorption peaks that correspond to various oxygen functional groups, such as: carbonyl stretching vibration at 1727 cm⁻¹, hydroxyl OH bending vibration from molecular water and C=C vibration in aromatic ring at 1617 cm⁻¹, alcoholic C–OH bending vibration at 1379 cm⁻¹ and phenolic C–OH stretching vibration at 1273 cm⁻¹ as well as C–O stretching vibrations or epoxy C–O–C vibrations at 1053 cm⁻¹. Moreover, it can be seen that all samples have a broad peak at 3400 cm⁻¹ which corresponds to the stretching vibration of –OH groups and the physically adsorbed water.^44,45

Fig. 4 FT-IR spectra of graphene oxide (GO), pristine KTaO₃ as well as 5 wt% rGO–KTaO₃ and 30 wt% rGO–KTaO₃ composites.

After the solvothermal treatment of GO, the intensity of all absorption peaks corresponding to oxygen functional groups (C=O, C–OH, C–O, C–OH, C–O–C) has a significant decrease which indicates an effective reduction of GO to rGO and is in good agreement with the results obtained from XRD analysis. Furthermore, the 30 wt% rGO–KTaO₃ composite has characteristic peak at 1561 cm⁻¹ corresponds to the skeletal vibration of the graphene sheets.⁴⁶ In the case of 5 wt% rGO–KTaO₃ composite this peak is absent which was attributed to the relatively low (5 wt%) graphene loading on KTaO₃.

For the KTaO₃ and rGO–KTaO₃ nanocomposites the broad Ta–O bands in the region from 790 to 550 cm⁻¹ are clearly visible. The spectrum of pristine KTaO₃ display also peak at 1628 cm⁻¹ ascribed to the bending vibration of H₂O, which results from the KBr disc.⁴⁷

3.4 Thermogravimetric analysis (TGA)

Thermal properties of the as-prepared GO, KTaO₃ and rGO–KTaO₃ composites under argon atmosphere were studied and the TG curves are depicted in Fig. 5. As can be seen from this figure, GO is thermally unstable and there are three obvious weight loss stages. The first weight loss stage is between 40 °C to 140 °C which is attributed to the dehydration process of water molecules in the interlayer of graphene oxide.⁴⁸ The second is from 140 °C to 440 °C owing to the burning decomposition of oxygen-containing groups attached to graphene oxide layers. The third stage is from 440 °C to 660 °C which can be assigned to the burning decomposition of carbon skeleton.⁴⁹

Fig. 5 TGA curves of GO, KTaO₃ as well as 5 wt% rGO–KTaO₃ and 30 wt% rGO–KTaO₃ composites.

The rGO–KTaO₃ composites exhibit higher thermal stability as compared with GO which could be explained by a smaller amount of oxygen functional groups in the structure of rGO–KTaO₃ composites. In the case of both 10 wt% rGO–KTaO₃ and 30 wt% rGO–KTaO₃ samples, the main mass loss at about 500 °C corresponds to the thermal decomposition of residual labile oxygen-containing functional groups.

3.5 XRD analysis

The sample purity of the products was checked by powder X-ray diffraction (PXRD). Fig. 6 shows the PXRD patterns of the GO, KTaO₃ and rGO–KTaO₃ composites. It can be seen that GO shows a single and broad diffraction peak at a 2θ value of 11.4 deg, having an interlayer distance of ∼7.7 Å, which indicates that most of the graphite powder was oxidized into GO. The KTaO₃ and rGO–KTaO₃ samples present almost the same profiles and all the diffraction peaks match perfectly to the cubic perovskite KTaO₃. For rGO–KTaO₃ composites, diffraction peak at 11.4 deg disappears indicating the successful reduction of GO to reduced graphene (rGO) in the final composites. Moreover, lattice parameters for KTaO₃ were refined by using the LeBail method. The lattice constants a of KTaO₃ and rGO–KTaO₃ composite with 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt% and 30 wt% rGO were calculated to be 3.9920(1) Å, 3.9904(1) Å, 3.9902(1) Å, 3.9903(1) Å, 3.9898(1) Å, 3.9895(1) Å, 3.9895(1) Å, respectively (Table 2). The largest lattice parameter was estimated for pristine KTaO₃ (open circle on Fig. 7) and subsequent heat treatment with rGO causes shrinkage of the unit cell. Smaller but noticeable decrease in the lattice constant a is observed with increasing graphene concentration from 5 wt% to 30 wt% in the rGO–KTaO₃ composite as presented in Fig. 7. This behavior might be due to subtle change of the potassium concentration in KTaO₃. It is worth noting that substantial decrease of the unit cell was observed for potassium deficient K_0.94TaO₃, for which compound estimated a = 3.9877(3) Å.^50,51 It was previously reported that temperature and pressure of hydrothermal precipitation has an effect on lattice parameters of carbonated calcium hydroxyapatite⁵² and antimony doped tin oxide.⁵³

Fig. 6 XRD patterns of GO and rGO–KTaO₃ composites with different amount of rGO. The vertical tick marks correspond to the expected Bragg reflections of KTaO₃. The intensity of GO sample was multiplied by a factor of 10.

Table 2 Crystallite size and lattice constant of KTaO₃ and rGO–KTaO₃ composites

Sample label	a (Å)	Crystallite size (nm)
KTaO3	3.9920(1)	130
5 wt% rGO–KTaO3	3.9904(1)	120
10 wt% rGO–KTaO3	3.9902(1)	160
15 wt% rGO–KTaO3	3.9903(1)	160
20 wt% rGO–KTaO3	3.9898(1)	120
25 wt% rGO–KTaO3	3.9895(1)	180
30 wt% rGO–KTaO3	3.9895(1)	110

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Fig. 7 The reduced graphene oxide content dependence of the lattice constant a of rGO–KTaO₃ composites.

Increasing graphene content in rGO–KTaO₃ composites causes the value of the lattice parameter a to decrease gradually which suggests that incorporation of reduced graphene oxide into potassium tantalate has an influence on the formation of the KTaO₃ crystal structure. Moreover, the average crystallite size of the KTaO₃ and rGO–KTaO₃ composite materials was calculated using the Scherrer equation and was found to be in the range of 110–180 nm (Table 2). The smallest particle size (110 nm) obtained for 30 wt% rGO–KTaO₃ sample could be beneficial for its photocatalytic performance.

3.6 XPS analysis

Table 3 shows the composition and chemical characters of elements formed in the surface layer of GO, KTaO₃ and rGO–KTaO₃ composites with various graphene content. The presented XPS data were obtained after analysis of high-resolution (HR) XPS spectra of Ta 4f, K 2p, O 1s and C 1s for all detected elements; tantalum, potassium, oxygen and carbon, respectively. The deconvoluted HR spectra of all elements are collected in Fig. 8. The fitting procedure of C 1s and O 1s XPS spectra recorded on GO sample was the same as previously reported.⁵⁴ Five peaks identified in C 1s spectra (Fig. 8B) at binding energy (BE) of 284.6, 285.7, 286.7, 287.7 eV and 288.7 eV can be assigned to sp² graphite component, hydroxyl (C–OH), epoxide (C–O–C), carbonyl ([double bond splayed left]C=O) and carboxyl (COOH) groups, respectively.^54–56 Deconvolution of O 1s spectra (Fig. 8C) reveals three main peaks at BE of 531.0, 532.4 and 533.6 eV assigned to C=O, C–O and phenolic groups, respectively.^54,56

Table 3 Elemental composition (in at%) and chemical characters of tantalum, potassium, oxygen and carbon states in the surface layer of graphene oxide (GO), KTaO₃ and rGO–KTaO₃ photocatalysts, evaluated by XPS analysis

Sample label	∑Ta (at%)	Fraction Ta 4f7/2 state (%)		∑K (at%)	Fraction K 2p3/2 state (%)		∑O (at%)	Fraction O 1s state (%)			∑C (at%)	Fraction C 1s state (%)
		KTaO3	Ta-Ox		KTaO3	K-Ox		KTaO3,–C=O	–C–O	Phenolic		sp^2	C–OH	C–O–C	[double bond splayed left]C=O	COOH
		27.1 eV	28.1 eV		292.6 eV	293.9 eV		531 eV	532.4 eV	533.6 eV		284.6 eV	285.7 eV	286.7 eV	287.7 eV	288.7 eV
GO	—	—	—	—	—	—	35.4	6.8	77.2	16.0	64.6	31.0	16.8	28.5	15.3	8.4
KTaO3	19.2	100	0	23.9	73.5	26.5	53.0	81.2	18.8	0	3.9	0	0	79.4	0	20.6
5 wt% rGO–KTaO3	16.3	77.0	23.0	14.9	76.2	23.8	48.7	74.8	22.0	3.2	20.1	51.7	26.9	13.4	7.1	0.9
10 wt% rGO–KTaO3	10.9	95.9	4.1	10.1	96.7	3.3	36.7	75.9	15.0	9.1	42.3	64.2	27.1	3.7	4.1	0.9
15 wt% rGO–KTaO3	7.8	100	0	6.8	98.0	2.0	30.7	72.6	18.7	8.7	54.7	68.5	20.3	6.2	2.6	2.4
20 wt% rGO–KTaO3	4.4	100	0	4.0	95.1	4.9	25.8	59.2	26.1	14.7	65.8	67.2	20.3	5.9	2.9	3.7
25 wt% rGO–KTaO3	3.3	100	0	2.8	99.1	0.9	24.5	50.2	32.2	17.6	69.4	65.1	18.7	7.0	3.8	5.4
30 wt% rGO–KTaO3	5.4	100	0	5.1	90.5	9.5	25.7	67.3	17.8	14.9	63.8	69.8	20.6	3.4	3.1	3.1

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Fig. 8 High-resolution XPS spectra of Ta 4f (A), K 2p and C 1s (B) and O 1s (C) monitored on rGO–KTaO₃ photocatalysts with 5, 10, 15, 20, 25 and 30 wt% rGO content. Deconvoluted peaks present various chemical states of tantalum, potassium, carbon and oxygen species.

The deconvoluted spectra of Ta 4f, K 2p and O 1s recorded on KTaO₃ sample are shown in Fig. 8A–C, respectively. The Ta 4f spectrum exhibits a single doublet corresponding to Ta ⁵⁺ (BE of Ta 4f_7/2 = 27.1 eV) in KTaO₃.^57–59 The K 2p line consists of two doublets at BE of K 2p_3/2 at 292.6 and 293.9 eV, respectively. The first doublet, with higher intensity peaks, relates to KTaO₃ crystal lattice.^57,58 The second one can be attributed to K–O complexes,⁵⁷ which can be formed at the surface during preparation of KTaO₃. The O 1s spectrum reveals two components at BE 531.0 and 532.4 eV, related to lattice oxygen and C–O adsorbate compounds.^57,59

Inspection of the XPS spectra presented in Fig. 8 clearly show that the relative intensities of all HR spectra are significantly changed after hydrothermal mixing of GO and KTaO₃. The progress in interaction of GO component with KTaO₃ can be well observed on XPS HR spectra of K 2p, C 1s (Fig. 8B) and O 1s (Fig. 8C) monitored on rGO–KTaO₃ photocatalysts with growing rGO content. As contribution of rGO increase, the C 1s and O 1s spectra exhibit the characteristic features of both rGO and KTaO₃ but intensity of peaks originated form rGO becomes more pronounced. The XPS data, collected in Table 3, corresponds well with this observation. The total surface content of Ta, K and O becomes smaller and the concentration ratio of all carbon–oxygen surface species identified in the C 1s spectra by functional groups (C–OH, C–O–C, [double bond splayed left]C=O, COOH) to sp² graphite component was estimated to be 2.23 for GO and 0.50 ± 0.05 for 10–30% rGO–KTaO₃ photocatalysts. The slightly larger value of this ratio for 5% wt% rGO–KTaO₃ sample (0.93) is probably resulted from relatively large content of tantalum oxide species in surface region of this sample (Fig. 8A, Table 3). The presented XPS data confirm the effective reduction of GO to rGO as a result of hydrothermal interaction between GO and KTaO₃, what agree well with the results obtained from XRD and FT-IR spectroscopy analysis.

3.7 Photoluminescence properties

Photoluminescence (PL) spectroscopy is an effective technique to analyze the transfer behavior of the photoinduced electrons and holes, therefore it can reflect the separation and recombination of photoinduced charge carriers.^60,61 The higher the PL emission intensity indicates the higher the recombination efficiency of the photogenerated carriers and therefore the lower the photocatalytic activity.⁶²Fig. 9 presents the room temperature PL spectra of KTaO₃ and rGO–KTaO₃ nanocomposites at an excitation wavelength of 325 nm. The rGO–KTaO₃ samples exhibit much lower PL intensities as compared with pristine KTaO₃ in the range of 350–700 nm, which could indicate effective suppressing of charge carriers recombination in the rGO–KTaO₃ composites due to incorporation of reduced graphene oxide. The PL intensities of rGO–KTaO₃ composites decrease with an addition of higher amount of graphene. However, it can be also seen that composites containing 15–25 wt% rGO give almost similar photoluminescence. The lowest PL intensity was observed for the 30 wt% rGO–KTaO₃ composite, which could be attributed to the most efficient separation of electron–hole pairs among all obtained rGO–KTaO₃ composites. Enhanced separation efficiency of electron–hole pairs of the composites can be beneficial especially in the case of rGO–KTaO₃ photocatalysts excited by UV irradiation, because in this system graphene is supposed to behave as an electron reservoir to capture the electrons photogenerated from KTaO₃.

Fig. 9 The PL spectra of KTaO₃ and rGO–KTaO₃ composites. The excitation wavelength is 325 nm.

3.8 Photocatalytic activity

Photocatalytic activity of as-prepared KTaO₃ and rGO–KTaO₃ nanocomposites was evaluated by examining the reaction of phenol degradation in the presence of visible light irradiation (λ > 420 nm). Firstly, the control experiment of direct photolysis was conducted under visible light without photocatalyst. After 60 min of photolysis about 2% of phenol was removed, indicating that only visible light cannot efficiently degrade the phenol. Moreover, to investigate the adsorption behavior of photocatalyst, adsorption experiment was performed in the dark condition. The results demonstrated that the percentage of the phenol adsorbed on the surface of the photocatalysts increased during the first 30 min and then remained almost constant. In the presence of the sample containing the highest amount of graphene (30 wt% rGO–KTaO₃) 45% of phenol was adsorbed on the surface of the photocatalyst after first 30 min in the dark, and then remained almost constant. Therefore, prior to photocatalytic irradiation, the mixed solution of phenol and the photocatalyst was stirred in the dark for 30 min to establish the equilibrium adsorption state. Photocatalytic activity under visible light is presented as phenol degradation rate (Table 1) and as efficiency of phenol removal after 60 min of irradiation (Fig. 10). Fig. 10 shows a comparison of the visible-light photocatalytic activity of the rGO–KTaO₃ composites with various rGO content (from 5 wt% to 30 wt%) and pristine KTaO₃ semiconductor. As can be seen from this figure, all the rGO–KTaO₃ samples exhibit higher photocatalytic activity as compared with bare KTaO₃. It indicates that incorporation of graphene into KTaO₃ enhances its photoactivity for the degradation of phenol. In the presence of 5 wt% rGO–KTaO₃ and 10 wt% rGO–KTaO₃ composites, the percentage degradation of phenol after 60 min was about 8% and 5%, respectively, which could be ascribed to the lower crystallite size (120 nm) of 5 wt% rGO–KTaO₃ sample as compared with 10 wt% rGO–KTaO₃ sample (160 nm). However, the further observation shows that the photocatalytic activity increase with increasing rGO content for composites containing higher amount of graphene (from 15 to 30 wt%) probably owing to the enhanced absorbance for visible light, higher surface area and more intimate interfacial contact between graphene sheets and KTaO₃ nanocubes. In particular, the 30 wt% rGO–KTaO₃ sample exhibit the highest photocatalytic activity among all obtained samples. The photocatalytic activity of 30 wt% rGO–KTaO₃ composite reached 43% after 60 min of visible light irradiation and phenol degradation rate was ∼6 times higher than the rate of bare KTaO₃ (0.14 μmol dm⁻³ min⁻¹). The highest photocatalytic performance obtained for 30 wt% rGO–KTaO₃ composite can be explained by synergistic effect of the smallest crystallite size and the most enhanced absorption range in the visible spectrum among all obtained samples.

Fig. 10 The percentage degradation of phenol at various time intervals under visible light in the presence of pristine KTaO₃ and rGO–KTaO₃ composites with various graphene content.

In general, the visible-light photocatalytic activity of rGO–KTaO₃ composites is remarkably improved due to the following reasons: (i) graphene, owing to its unique the two dimensional structure with exceptional high surface, was used as the supporting material which can offer more active photocatalytic sites; (ii) incorporation of graphene to the perovskite-like KTaO₃ particles shifted absorption edge to longer wavelengths and increased the absorbance in the visible region of the spectrum which resulted in more efficient utilization of the solar energy; (iii) as the content of the rGO in the composite increased, more KTaO₃ nanocubes are loaded on the rGO sheets. This intimate contact between KTaO₃ nanocubes and graphene accelerated the transfer of photogenerated electrons and thus enhanced photocatalytic activity.

3.9 Proposed mechanism

Based on the above results, a tentative mechanism for the photocatalytic process is proposed and illustrated in Fig. 11. In the rGO–KTaO₃ system, perovskite-type KTaO₃ cannot be photoexcited under visible light irradiation due to its wide band (3.4 eV). However, since the graphene is a zero band gap semiconductor (both the CB and VB are −4.42 eV),⁶³ it can act as a photosensitizing material under visible light irradiation. Therefore, under visible light reduced graphene oxide in the rGO–KTaO₃ composite is photoexcited from the ground-state (rGO) to the excited-state (rGO*) during which electrons are photogenerated. The rGO* in the excited state injects electrons into the conduction band (CB) of KTaO₃. The photoexcited electrons react with the dissolved oxygen in water on the graphene surface to generate peroxide radical anions (˙O₂⁻) and other oxidative species. The active oxygen species can cause mineralization and oxidize phenol into CO₂ and H₂O. Furthermore, hydroxyl radicals (˙OH) generated during the photocatalytic process were detected using photoluminescence technique. After 60 min of visible light irradiation, formation of hydroxyl radicals was not detected which indicates that ˙OH are not responsible for the degradation of phenol under visible light illumination in the presence of rGO–KTaO₃ composites. In the case of reference system (P25 irradiated by UV-Vis light), intensive band related to the presence of 7-hydroxycoumarin was observed (Fig. 12). These results are in good agreement with proposed mechanism in which the main active oxygen species are peroxide radical anions (˙O₂⁻).

Fig. 11 Schematic illustration of the photocatalytic mechanism of rGO–KTaO₃ composite.

Fig. 12 PL spectra for 30 wt% rGO–KTaO₃ composite under visible light and P25 under UV-Vis light (as a reference system) dispersed in coumarin aqueous solution, PL spectra for 30 wt% rGO–KTaO₃ composite under visible illumination obtained after various times (0, 30 and 60 min) has been shown in inset.

In addition, graphene oxide is a mixture of sp² and sp³ hybridized carbon atoms with a very high amount of oxygen bonding on the sp³ hybridized carbon. Oxygen atoms have a higher electronegativity than carbon atoms, and consequently graphene oxide becomes a p-type semiconductor material where its conduction band is the antibonding π* orbital and its valence band mainly the O 2p orbital.^64,65 The reduced graphene oxide was obtained by removing most of the oxygen-containing functional groups from the surface of GO using hydrothermal route. However, oxygen remaining on the carbon surface caused p-type behavior of the rGO.⁶⁶ When rGO sheets are combined with n-type KTaO₃ nanocube semiconductor,⁶⁷ a p–n junction composite photocatalyst can be obtained. There are some benefits of the formation of p–n heterostructure, such as: (a) a more effective charge separation; (b) a rapid charge transfer to the catalyst; (c) a longer lifetime of the charge carriers; and (d) a separation of locally incompatible reduction and oxidation reactions in nanospace.⁶⁸ In this regards, rGO–KTaO₃ composites are also supposed to be efficient photocatalysts excited by ultraviolet light irradiation where rGO does not work as a photosensitizer but as an electron acceptor. As a result, the enhancement on photocatalytic activity of rGO–KTaO₃ composites both under UV and visible light could be ascribed to the formation of p–n heterojunctions between KTaO₃ nanocubes and rGO sheets.

4. Conclusions

In summary, a series of rGO–KTaO₃ nanocomposites with various graphene content have been successfully synthesized by a facile solvothermal route for the first time. The all prepared rGO–KTaO₃ nanocomposites presented higher photocatalytic activity under visible light irradiation as compared with pristine KTaO₃ which could be ascribed to the photosensitizer role of graphene in the rGO–KTaO₃ composites as well as the formation of p–n heterojunctions between p-type rGO and n-type KTaO₃ nanocubes. Furthermore, the results indicate that incorporation of graphene into KTaO₃ nanocubes enhanced visible light absorption and greatly improved separation of photogenerated carriers. We found that the sample with a content of 30 wt% graphene exhibited the best photocatalytic performance in the phenol degradation among all obtained photocatalysts – the activity reached 43% under visible light 60 min irradiation. The enhanced photoactivity of 30 wt% rGO–KTaO₃ composite can be attributed to the small crystallite size, enhanced absorption range in the visible spectrum as well as superior intimate contact between KTaO₃ nanocubes and rGO sheets. Our results may provide useful guide for designing efficient photocatalysts based on KTaO₃ and reduced graphene oxide for the environmental purification of organic pollutants in both the gas and aqueous phases.

Acknowledgements

This work was supported by National Centre for Research and Development (Third generation photoactive materials and materials-based system for photocatalytic air treatment, PHOTOAIR, Pol-Nor/207686/18/2013). The project was also co-financed by the European Union within the European Social Fund.

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