Journal of Nanomaterials
Volume 2019 (2019), Article ID 4869728, 9 pages
https://doi.org/10.1155/2019/4869728

Research Article

Hydrothermal Synthesis of rGO/PbTiO₃ Photocatalyst and Its Photocatalytic H₂ Evolution Activity

Chunyan Wang,¹ Lianwei Shan,¹ Dongyuan Song,¹ Yanwei Xiao,¹ and Jagadeesh Suriyaprakash²

¹School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China
²Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China

Correspondence should be addressed to Lianwei Shan; shlw0531@163.com and Jagadeesh Suriyaprakash; jaga788psg@gmail.com

Received 30 January 2019; Revised 10 September 2019; Accepted 9 December 2019; Published 24 December 2019

Academic Editor: Bhanu P. Singh

Copyright © 2019 Chunyan Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

In this letter, we investigated the photocatalytic activity of the newly formed rGO/PbTiO₃ composites, which are synthesized by a one-step hydrothermal route. By adjusting the amount of reduced graphene oxide (rGO) (0, 0.15, 0.30, 0.60, and 1.20 wt%) with the PbTiO₃, we constructed various photocatalysts for this investigation. The crystal structure and morphology of the various composites were studied by powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Photoelectron spectroscopic study revealed that the band structure of the newly formed composites and efficient charge separation can be obtained by the interfaces of various rGO content. In addition, the photocatalytic performance of the synthesized composites was explored by H₂ evolution and rhodamine blue (RhB) degradation. The obtained results indicated that the addition of the appropriate amount of rGO could improve the activity of pure PbTiO₃, significantly.

1. Introduction

The entire world suffers from serious energy crises and environmental problems, owing to the enormous usage of fossil fuels and the emission of poisonous gases associated with fuel combustion for the last eight decades [1–5]. Numerous techniques and approaches have been attempted to solve these problems; among them, the solar light-driven water splitting technique offers a promising future in terms of solar energy capture and storage [6–9]. A photoelectrochemical cell (PEC) has the advantages that the photogenerated electrons and holes are separated on the macroscale to different electrodes and decreased recombination as well as high efficiency [10, 11]. However, the cost of constructing the highly efficient and greatly durable PECs has been a significant barrier for practical application. To overcome this issue, fabrication of efficient photocatalyst by a facile route in a cost-effective way is crucial [12, 13]. In this context, scientists developed various types of photocatalysts, among them, nano-/microsized composites have the potential to be produced at much lower costs through easily scalable processes. So far, a group of perovskite-type photocatalysts including SrTiO₃, KTaO₃, CaTiO₃, and NiO/NaTaO₃:La have been synthesized and utilized successfully for the photocatalytic water splitting application. Recently, perovskite-type oxide materials based on transition metals with d (0) and d (10) electronic configuration such as Nb(V), Ti(IV), and In(III) were reported as efficient photocatalysts for overall water splitting with high quantum yields [14]. Moreover, various semiconductor photocatalysts, such as TaON [15], ZnO [16], and Ta₂O₅ [17], have been investigated for photocatalytic degradation of pollutants and hydrogen generation from water splitting. Nevertheless, the preparation in previous cases proceeded via the traditional solid-state synthesis technique which is lacking regular size control and homogeneity and has high preparation cost. To shed light on this issue and find a facile route to synthesize an efficient photocatalyst, we utilized reduced graphene-oxide (rGO) and PbTiO₃ as prototype materials.

It is well known that PbTiO₃ is an -type ferroelectric material with an internal dipolar field, which grants an excellent charge separation process inside the material. Because the photogenerated carriers could be separated by the internal polarization [18], they are less likely to recombine, and this may enhance photocatalytic efficiency [19]. In general, the semiconductor materials suffer from wide band gap, photocorrosion, and low separation efficiency of electron-hole pairs [20, 21]. To address these problems, many efforts have been made such as doping, composites, noble metal loading, and heterojunction fabrication (e.g., TiO₂/graphene [16], ZnO/rGO [17–19], and ZnCd1–S/rGO [20]). Among these methods, fabrication of graphene-based composites is considered as a promising approach to produce high performance photocatalysts because of the unique characteristics of rGO, i.e., electron collector and transporter, consequently inducing the improved charge separation and photocatalytic activity [22]. In the previous work, Gan et al. demonstrated that rGO plays an important role in the enhanced photocatalytic performance, which reveals the photothermal characteristic of the graphene-based nanocomposite [23].

In this work, we present a simple method to prepare rGO/PbTiO₃ composites in a cost-effective way. The purpose of this work is to show that the rGO can be formed an interface structure with the PbTiO₃ substrate by a one-step synthetic route and demonstrate the well interfacial coupling between rGO and PbTiO₃ followed by increase of photocatalytic efficiency. The photocatalytic activity of the various composites shows an obvious enhancement compared to bare PbTiO₃ as well as rGO.

2. Experimental

2.1. Preparation of rGO/PbTiO₃ Nanocrystals

The TiO₂ (it consists of anatase and rutile phases with 85% and 15% phase combination, respectively, which is calculated from the XRD dominant peaks of the corresponding planes (Figure 1(b))) and Pb(NO₃)₂ powder (the molar ratio of Ti to Pb is 1 : 1.25) were added in a 6 M KOH aqueous solution with continuous stirring. Later, the rGO was added into the above solution. This final mixture was then sealed in a 100 mL Teflon-lined stainless steel autoclave. Then, the hydrothermal treatment was performed in an autoclave at 200°C for 24 h. The resultant products were filtered and washed with 1% nitric acid, deionized water, and ethanol and subsequently dried at 80°C in air. According to the adding amount of rGO (0, 0.15, 0.30, 0.60, and 1.20%), the samples were named as PGO0, PGO015, PGO030, PGO060, and PGO120, respectively.

(a)

(b)

(c)

Figure 1: (a) XRD patterns for the PbTiO₃ and PbTiO₃/rGO composites: (A) standard ICSD of PbTiO₃, (B) PGO0, (C) PGO015, (D) PGO030, (E) PGO060, and (F) PGO120. (b) XRD pattern of TiO₂ for PbTiO₃ precursor. (c) The N₂ adsorption-desorption isotherm curve of TiO₂.

2.2. Characterization

The X-ray diffraction (XRD) patterns of the samples were measured on an X’Pert PRO X-ray powder diffractometer with Cu-Kα radiation ( Å). Their morphology was performed by means of a G³ F20 (FEI Tecnai) with operating voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Thermo ESCALAB 250 X-ray photoelectron spectrometer with an excitation source of Al Kα radiation ( eV). The optical properties of the samples were studied by measuring their UV-vis diffuse reflectance spectra (DRS) on a Shimadzu UV-2401PC. BaSO₄ was used as the reference standard. The emission spectra were recorded on a fluorescence spectrophotometer (Shimadzu, model RF-5301 PC). The Brunauer-Emmett-Teller (BET) method was used to determine the surface area of samples according to the nitrogen adsorption-desorption isotherms collected by a 3H-2000PS2 analyzer.

2.3. Photocatalytic Reactions

The photoelectrochemical properties were investigated in a conventional three-electrode cell arrangement using an electrochemical analyzer (Autolab, PGSTAT 302N). The corresponding powder samples were coated onto the fluorine-doped tin oxide (FTO) glass by using the electrophoretic deposition (EPD) technique. The prepared samples were used as the working electrode. They were irradiated from the FTO glass side (back light illumination). And a Pt foil was used as the counter electrode. Potentials were applied versus the Ag/AgCl reference electrode. Na₂SO₄ aqueous solution (0.5 mol L⁻¹, ) was used as the electrolyte. The measured potentials vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE) scale according to the following Nernst relation:

Photocatalytic hydrogen generation reactions were carried out in a top-irradiation vessel connected to a glass closed gas circulation system. The light source was a 500 W Xe lamp (Beijing Trusttech Co. Ltd). The amount of H₂ evolved was determined using gas chromatography (Agilent 6890). 100 mg photocatalyst powder was dispersed in 300 mL aqueous solution (containing 10 vol% triethanolamine scavenger). Total organic carbon (TOC) was measured using a TOC analyzer (TOC-VCPN). The experimental conditions of TOC are maintained with , .

The photocatalytic activity of the samples was examined by measuring the decomposition rate of rhodamine B (RhB) in aqueous solution. In a typical procedure, the as-prepared photocatalyst (0.1 g) was added into 100 mL RhB (15 mg·L⁻¹) aqueous solution. Prior to photoreaction, the mixture was maintained under dark conditions for 30 min to achieve adsorption-desorption equilibrium between dye and photocatalyst. Then, the mixture was exposed to a 500 W Xe arc lamp. At the given time intervals, about 5 mL of the suspension was taken for analysis after centrifugation. The supernatant solutions were then tested with an UV-vis spectrophotometer (UV757CRT) by measuring absorption spectra of RhB ( nm) as a function of irradiation time. The photocatalytic activity of the catalysts was calculated as , where is the concentration of the test solution of RhB before irradiation and is the concentration of RhB after irradiation. is initial RhB concentration (mg/L), and is the instantaneous concentration of RhB solution at time (mol/L).

3. Results and Discussion

X-ray diffraction (XRD) analysis was conducted to investigate the crystallographic structure of the prepared samples (PbTiO₃ and PbTiO₃/rGO composites) as shown in Figure 1(a). Figure 1(a), A line profile indicates the XRD pattern of standard PbTiO₃. As shown in Figure 1(a), B-F, all the peaks of synthesized rGO/PbTiO₃ composites with rGO exhibit similar peaks compared to that of the standard PbTiO₃. The main diffraction peaks of (101), (110), and (111) planes present in the patterns correspond to the PbTiO₃, which is well matched with the standard pattern (ICSD 78-0299). After introducing the rGO, there are no obvious peaks of impurities present in the rGO/PbTiO₃ samples compared with that of PbTiO₃. The diffraction peaks of PbTiO₃ and rGO/PbTiO₃ were sharp and intense, indicating the highly crystalline character of the samples. Figure 1(b) is the XRD pattern of the TiO₂ precursor; it exhibits a more dominant phase of anatase.

Generally, the electronic structure of the semiconductor plays an important role in its photocatalytic activity [24]. Therefore, investigating the optical band gaps and band structures is necessary to determine the band alignment in the synthesized composite rGO/PbTiO₃. Doing so, measurements of the UV-vis DRS were carried out for the PbTiO₃ and all rGO/PbTiO₃ composites as shown in Figure 2. For the pure PbTiO₃, the absorption edge occurs at about 440 nm, which well agrees with the reported case [25]. It is interesting to note that the rGO/PbTiO₃ samples exhibit a strong absorption in the visible light range at a wavelength of 440–800 nm. The band gaps of synthesized samples were calculated by Tauc plots, in order to estimate the allowed indirect and direct band gaps by plots of versus and versus , respectively. Particularly, the absorption edges of rGO/PbTiO₃ show slight redshift compared to that of pure PbTiO₃, indicating the enhanced visible light absorption ability. The band gaps for PbTiO₃ and rGO/PbTiO₃ are estimated to be 2.85 eV and 2.83 eV, respectively.

(a)

(b)

Figure 2: (a) UV-vis diffuse reflectance spectra of pure PbTiO₃ and rGO/PbTiO₃ with different rGO weight ratios. (b) Plots of versus photon energy () of PbTiO₃ and rGO/PbTiO₃ powders.

The morphology of the as-synthesized rGO/PbTiO₃ photocatalysts was displayed in Figure 3, which reveals a quasiplate shape with regular exposed facets as the reported case [26]. We observed that the synthesized composites consist of rGO and PbTiO₃, where PbTiO₃ present as a plate-like structure surrounded by the rGO chunk or layer. Due to the limitation of the 2D TEM image, we cannot confirm the exact morphology of the synthesized samples. However, by the deliberate analysis of XRD, UV-vis (variation in band gap), and TEM (different contrasts around the edges) results, we believe that there is a thin layer of rGO surrounding the PbTiO₃. These results demonstrate that PGO015 and PGO060 composites were successfully built between rGO and PbTiO₃ composition. It is expected that the spatial separation of charge carriers in PbTiO₃ particles could be obtained by the adding of rGO. This further suggests that there is a strong coupling between PbTiO₃ and added rGO, due to the formation of chemical/physical bonds. So, the coupling of rGO on the facet could significantly enhance the separation of photogenerated electrons in PbTiO₃.

(a)

(b)

Figure 3: Morphology of synthesized photocatalysts. TEM image of (a) PGO015 and (b) PGO060.

The photocatalytic performance of the as-prepared samples was evaluated by the degradation rate of RhB under simulated sunlight irradiation. As depicted in Figure 4(a) (black line), it can be seen clearly that the concentration of RhB is negligible without a photocatalyst (after an exposure time of 300 min), and also, we observe from Figure 4(a), A that the photolysis of RhB was very slow, and only about 3% was degraded after 300 min of illumination. When the pure PbTiO₃ was used as the photocatalyst, an RhB degradation of 23% is completed after 300 min irradiation. The degradation of RhB solution runs up to 65% over PGO030, indicating a much higher photocatalytic activity than the pure PbTiO₃ sample. However, the photocatalytic reaction for 93% RhB degradation in 300 min is achieved for the PGO060 sample. These results prove that an addition of rGO is a suitable strategy to optimize the photocatalytic activity of PbTiO₃. Generally, it has been assumed that the kinetics of photocatalytic decolorization follow a pseudo-first-order reaction model [13, 27]:

where is the rate constant (min⁻¹). The values of were derived from the slope of plots of versus time () shown in Figure 4(b). In contrast to PbTiO₃ (0.00079 min⁻¹), rGO/PbTiO₃ photocatalysts had much higher values, but the PGO060 (0.00847 min⁻¹) is the highest, indicating that the rGO coupling plays a key role in the strengthening of photocatalytic activity for PbTiO₃. The reaction kinetic rate constant of PGO060 is about 10.7 times as high as that of PbTiO₃. As we all know, for practical application of a photocatalyst, it needs to be stable. To evaluate the stability and reusability of the PGO060 for photocatalytic RhB degradation, the PGO060 was reused for three times, as shown in Figure 4(c). In each successive 300 min experiment, recycled sample PGO060 was collected by centrifugation and reused under the same reaction conditions. The photocatalytic activity of PGO060 is quite stable because no noticeable deactivation is observed after three cycles.

(a)

(b)

(c)

(d)

Figure 4: (a) Photocatalytic degradation of RhB using PbTiO₃ and rGO/PbTiO₃ composites as photocatalysts. (b) Simulated sunlight photocatalytic kinetic rates of RhB solution using the as-prepared PbTiO₃ and rGO/PbTiO₃ samples. (A) no photocatalyst, (B) PGO0, (C) PGO015, (D) PGO030, (E) PGO060, and (F) PGO120. (c) Recycling test of PGO060 photocatalyst. (d) Variation of TOC removal with reaction time for PGO060 at , .

To investigate the photocatalytic activity of the as-prepared photocatalyst further, TOC removal of PGO060 during discoloration of RhB was accomplished as shown in Figure 4(d). It was found that the rate of TOC removal is much slower than that of the rate of RhB discoloration. It was known that the chemical structure of RhB is very complicated; as a result, the complete discoloration of RhB does not equate to the chemical structure of RhB being totally mineralized into water and carbon dioxide. After 300 min reaction, the TOC removal percentage reached 65%, indicating that most of the RhB chemical structure can be destroyed. In terms of reaction, the h⁺ can oxidize H₂O [28] near the surface of the PbTiO₃ to create hydroxyl radicals:

On the other hand, the can reduce O₂ to form radicals in

Figure 5 illustrates the evolution of H₂ over the as-synthesized samples. There was weak H₂ evolution being detected with the PGO0, indicating the poor oxidizing power of the photogenerated holes in PbTiO₃. So far, there have been few reports on evolution of H₂ of PbTiO₃ in the presence of rGO. Here, the H₂ evolution reaction for the sample is as follows:

Figure 5: Photocatalytic hydrogen generation of samples from aqueous solutions (triethanolamine as a sacrificial agent) as a function of the irradiation time: (a) PGO0, (b) PGO015, (c) PGO030, (d) PGO060, and (e) PGO120.

Interestingly, a higher H₂ evolution was detected for the PGO060 (23.8 μmol after 6 h) compared to PGO0, representing the improved photocatalytic performance due to the addition of rGO. This enhancement of activity is related to the selective separation of electron by coupled rGO. In comparison, it suggests that the additional presence of rGO on the facet of PbTiO₃ can be beneficial in increasing electron-hole separation of PbTiO₃.

The charge transfer properties of the photocatalysts were further measured by means of electrochemical impedance spectrum (EIS) measurements. Based on the Nyquist plots, the charge transfer of the water splitting processes occurs at the interface between photoanodes and electrolytes as observed in Figure 6. And these processes can be interpreted by the simple Randles-Ershler (R-E) circuit model [29, 30] where is solution resistance, is charge transfer resistance, and is double layer capacitance. These data revealed that the charge transfer resistance on the photoanode surface was 42.2 and 28.3 kΩ for the PGO0 and PGO060 samples, respectively. The smaller semicircle of the PGO060 sample indicates that the PGO060 sample has a lower charge transfer resistance compared to the PGO0 sample. Thus, rGO could effectively increase the reaction rate of H₂ evolution of PbTiO₃. However, excess adding of rGO results into a decrease in H₂ evolution, which originates possibly from an increase in carrier recombination probability due to high carrier transporting ability of rGO. All results further confirm that the rGO coupling could facilitate the separation of electrons and holes, resulting in enhanced photocatalytic activity for the H₂ evolution.

Figure 6: Nyquist plots measured at 1.23 V (vs. RHE) in 0.5 M Na₂SO₄ solution. Several samples were irradiated with simulated sunlight from the back side of FTO. The frequency is in the range from 0.1 MHz to 0.01 Hz. The inset is the equivalent circuit model: (a) PGO0, (b) PGO015, (c) PGO030, (d) PGO060, and (e) PGO120.

X-ray photoelectron spectroscopy (XPS) spectrum was provided to characterize the surface, electronic, and chemical features of the samples (Figure 7). Figure 7(a) presents the O 1s photoelectron peaks. For pure PbTiO₃ (below spectrum), there exist only two oxygen signals, located at 529.0 and 530.8 eV, which are attributed to Ti-O (lattice O) and Ti-OH, respectively. The O 1s XPS spectrum could be resolved using the XPS peak-fitting program. It was found that the peak with binding energy of 529.0 eV is the characteristic O 1s peak of lattice for PGO060 (above spectrum). The O 1s binding energy of 530.9 eV can be attributed to the Ti-OH in PGO060, which are considered to favor photocatalytic reactions. could act as the hole-capturing agent in the photocatalytic process, producing OH⋅ [31, 32] as the following reaction:

(a)

(b)

(c)

(d)

Figure 7: (a) XPS spectra of O 1s PGO0 (below) and PGO060 (above). (b) XPS spectra of Ti 2p PGO060 (below) and PGO060 (above). (c) The comparison of O 1s levels in PbTiO₃ (below) and PGO060 (above). (d) The valence band (VB) spectra of the PGO060 and PGO060. The valence band maximum (VBM) value was determined by linear extrapolation of the leading edge to the baseline.

The Ti 2p core level spectrum from Figure 7(b) could be observed at the binding energies of around 464.3 eV (Ti 2p1/2) and 458.3 eV (Ti 2p3/2) for PbTiO₃ (below spectrum), in agreement with the reported work [33]. For the PGO060 (above spectrum), a pair of new peaks was found, which locates at 456.1 and 461.8 eV, respectively. The two lower binding energies may be related to the lower valence of Ti element. For PbTiO₃, the pair of strong peaks at 138.1 eV and 142.9 eV in the spectrum is attributed to the spin-orbit splitting of Pb 4f7/2 and Pb 4f5/2, respectively (Figure 7(c)). The peak difference between Pb 4f7/2 and Pb 4f5/2 signals is calculated to be 4.8 eV which indicates a +2 oxidation state of lead in the sample, and the peak values match well with those in the handbook of the XPS instrument [34]. It is worth noting that there are two peaks in higher binding energy, which located at 138.9 and 143.8 eV, respectively. It indicates that the addition of rGO has an obvious effect on the chemical valence of Ti and Pb.

To shed more light on the electronic structure of the synthesized photocatalyst, we employed XPS valence band (VB) spectra as illustrated by bottom and upper spectra in Figure 7(d); the valence band of PGO0 and PGO060 is 2.06 and 2.51 eV, respectively. The valence band maximum (VBM) positions in the VB spectra were determined according to linear extrapolation of the leading edges of the VB spectra recorded on the baselines. It is found that the valence band location of PGO060 is 0.45 eV lower than that of PGO0, due to the interface effect of rGO. It was reported that the hybrid hydrothermal method has a large sp² domain and thus more extended p-conjugation [35]. Under light irradiation, electron and hole pairs can be produced in PbTiO₃. By high electron mobility and strong light absorbance of rGO, it slows the recombination of electron-hole pairs and improves harvesting efficiency of simulated sunlight, therefore significantly increasing the photocatalytic activity of PbTiO₃.

4. Conclusions

In summary, we have successfully synthesized the rGO/PbTiO₃ composite photocatalysts by a simple hydrothermal method in a cost-effective way. Morphology and crystal structure characterizations reveal that the rGO surrounded the sheet like structured PbTiO₃. Absorption and photoelectron spectroscopy studies show that the coupling of rGO on the PbTiO₃ could enhance the harvesting ability of simulated sunlight. The photocatalytic activity enhancement mechanism was discussed by energy band positions. We found that the formation of the rGO/PbTiO₃ structure significantly enhanced the transfer and separation efficiency of photogenerated electron-hole pairs in this system.

Data Availability

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

Conflicts of Interest

The authors declare no competing financial interest.

Acknowledgments

The authors gratefully acknowledge financial supports from the Education Department of Heilongjiang Province (12541111) and the Postdoctoral Scientific Developmental Fund of Heilongjiang Province (LBH-Q16122).

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