Accepted Manuscript A facile one-step solvothermal synthesis of BiPO4-graphene nanocomposites with enhanced photocatalytic activity Chao Wang, Gehong Zhang, Chao Zhang, Weiqiang Fan, Weidong Shi PII: DOI: Reference:
S0021-9797(14)00450-0 http://dx.doi.org/10.1016/j.jcis.2014.06.031 YJCIS 19654
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
8 March 2014 13 June 2014
Please cite this article as: C. Wang, G. Zhang, C. Zhang, W. Fan, W. Shi, A facile one-step solvothermal synthesis of BiPO4-graphene nanocomposites with enhanced photocatalytic activity, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.06.031
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A facile one-step solvothermal synthesis of BiPO4-graphene nanocomposites with enhanced photocatalytic activity Chao Wang, Gehong Zhang, Chao Zhang, Weiqiang Fan, Weidong Shi* School of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301, Zhenjiang, 212013, PR China.
*Corresponding author: Tel.: +86 511 8879 0187
Fax. : +86 511 8879 1108
E-mail address: [email protected]
Abstract A facile one-step solvothermal approach was developed to synthesize BiPO4-graphene (BP-RGO) nanocomposites using ethylene glycol/water as the solvent and reducing agent. During the solvothermal reaction, both the effective reduction of graphene oxide (GO) and the growth of rod-shaped BiPO4 as well as its deposition on graphene occurred simultaneously. The as-obtained BP-2%RGO nanocomposite
photodegradation of methyl orange (MO), which was about 2.0 and 1.5 times as high as that of pure BiPO4 and physical mixture of BiPO4 and graphene, respectively. The enhanced photocatalytic activity of BP-2%RGO nanocomposite is attributed to a larger surface area, much increased adsorption capacity, and more effective charge transportations and separations arisen from the introduction of graphene along with the intimate interfacial contact between BiPO4 and graphene. This work highlights the
significant effect of solvothermal method and introduction of graphene on the photoactivity of graphene-based nanocomposites. It is expected that this method could aid to fabricate more efficient graphene-based photocatalysts with improved interfacial contact and photocatalytic performance for environmental remediation. Keywords: BiPO4; graphene; nanocomposites; one-step; solvothermal; photocatalysis
1. Introduction Graphene, a new carbon material with a monolayer of sp2-hybridized carbon atoms in a dense honeycomb crystal structure, has attracted great interest in various fields, such as composite materials, batteries, solar cell, and photocatalysis [1,2], due to its high specific surface areas of ~2600 m2/g, high thermal conductivity, and excellent electronic mobility . Recently, it was demonstrated that the reduction of graphene oxide (GO) nanosheets was an effective route to produce high-quality graphene. However, preparing from graphite via strong oxidation process, the resultant GO nanosheets usually contain various oxygen-containing functional groups and defects, resulting in a low electron transport property [4,5]. Therefore, it is highly desirable to develop various strategies to reduce GO nanosheets to recover its high electron mobility. Up to now, various methods such as chemical reduction by using reductants (N2H4  and NaBH4 ), hydrothermal [8,9], thermal , and
ultraviolet-assisted reduction  have been used to prepare graphene to increase the electronic
shortcomings, such as the use of toxic or hazardous chemical agents, the additives of acid or alkali, requiring high temperature, and the residual of oxygen-containing functional groups. We notice that the solvothermal reduction is more effective than the above-mentioned reduction methods in removing the oxygen and defect levels and increasing the conductivity of graphene nanosheets [12–14]. Bismuth phosphate (BiPO4), an oxoacid salt photocatalyst, has exhibited potential applications in catalysis, ion sensing, and separating radioactive elements because of its exceptional optical properties, nontoxicity, low cost, and high catalytic efficiency [15–17]. It has been well reported that BiPO4 exhibits high photocatalytic oxidative ability for organic dye decomposition under UV light irradiation [17,18]. Further research indicates that the nonmetal oxy-acid PO43− promotes the separation of photoinduced electron-hole pairs . However, it should be noted that there are still some limitations in the present BiPO4 photocatalytic system, such as poor adsorptive performance, large size, and rapid recombination of photoinduced electron-hole pairs [19,20]. To improve the photocatalytic activity of BiPO4, various strategies have been proposed to modify bare BiPO4, such as ion doping, surface modification by noble metals, and coupling with other semiconductors [21–23]. Generally, there are three approaches to improve the photocatalytic activity of BiPO4: band-gap tuning, minimizing charge carrier recombination, and promotion of the forward reaction and adsorption of reactants on the surface of photocatalysts.
Obviously, the combination of BiPO4 with graphene will meet two approaches. Because the two-dimensional platform of graphene with outstanding electric conductivity and high surface area can sufficiently contact with target pollutants to provide plenty of reactive sites as well as efficiently accelerate the photoinduced electrons transfer process that from the inside of photocatalyst to reactive sites to suppress the recombination of photoinduced electron-hole pairs . Thus, an appropriate integration of BiPO4 and graphene can form a nanocomposite that combines the desirable properties of BiPO4 and graphene. As a result, the photocatalytic activity of BiPO4 system will be remarkably improved. However, to the best of our knowledge, few studies on the synthesis of BP-RGO nanocomposites have been reported to date. Recently, Shen et al. prepared BP-GO nanocomposites by a two-step procedure in which Oleylamine-capped BiPO4 was firstly synthesized and then assembled on the GO nanosheets at the water/toluene interface by a second step . More recently, Wang et al. synthesized the quasi-core-shell BP-RGO cuboids with low OH-related defects via a two-step hydrothermal approach with the assistance of DMF . Unfortunately, while they have prepared the BP-GO or BP-RGO nanocomposites successfully, there are still several drawbacks in their experimental procedures: (1) the synthesis of BP-GO or BP-RGO nanocomposites contains two- or multi-step which is tedious and time-consuming [26,27]; (2) hazardous reducing agents (Oleylamine), toxic organic solvents (toluene), and other additives (DMF) may bring about great difficulties to the product post-treatment and environmental protection; (3) weak interaction between the interface of BiPO4 and graphene
nanosheets results from two- or multi-step synthetic routes ; (4) GO is not reduced or is partially reduced to reduced graphene oxide instead of graphene which will influence its conductivity. Therefore, it is still a great challenge to develop a facile, green, and effective way to prepare high-quality BP-RGO nanocomposites. Herein, we report the synthesis of BP-RGO nanocomposites via a facile, green, and effective approach, in which the formation of BiPO4 nanorods, complete reduction of GO into graphene, and sufficient mixing between these two materials are accomplished in a one-step synthetic route. Notably, besides being a solvent, the ethylene glycol (EG) also plays the role of a mild agent for GO reduction, and therefore there is no need for adding additional agents. Moreover, the usage of EG is completely compatible with the synthesis of BiPO4 nanorods. The as-obtained BP-2%RGO sample exhibits much higher photocatalytic activity than that of pure BiPO4 and physical mixture of BiPO4 and graphene towards the photodegradation of MO under UV light irradiation, which is attributed to a larger surface area, much increased adsorption capacity, and more effective charge transportations and separations arisen from the introduction of graphene along with the intimate interfacial contact between BiPO4 and graphene. It is hoped that this work could provide a facile, green, and effective strategy for the preparation of graphene-based composite materials. 2. Experimental 2.1. Materials Flake graphite, H2SO4, NaNO3 and KMnO4 were purchased from Sinopharm (Beijing,
China). H2O2, Bi(NO3)3⋅5H2O and NaH2PO4⋅2H2O were purchased from Aladdin (Shanghai, China). All reagents were of analytical grade without further purification and the deionized water was used in all experiments. 2.2. Preparation of graphene oxide sheets GO was synthesized via the oxidation of graphite using the improved Hummers’ method . Briefly, 10.0 g graphite powder and 5.0 g NaNO3 were added to 200 mL 98% H2SO4 under the condition of ice-bath, then 30.0 g KMnO4 was gradually added into the above mixture under vigorous stirring. After the obtained mixture was stirred at 35 °C for 4 h, 230 mL deionized water was added, followed by vigorous stirring at 98 °C for 15 min. Then the suspended mixture was further diluted to 700 mL by deionized water and stirred for 30 min. The reaction was ended by adding 20 mL H2O2 (35%) under stirring at room temperature. Finally, the resulting product was washed with deionized water and ethanol, and dried at 40 °C in vacuum for 3 days. 2.3. Preparation of BiPO4-RGO nanocomposites In a typical synthesis of BP-RGO nanocomposite with 2 wt% graphene, 6 mg GO in 30 mL ethylene glycol/water mixed-solvent (the volume ratio is 2:1) was ultrasonicated for 1 h to give a homogeneous suspension, followed by adding 1 mmol Bi(NO3)3⋅5H2O. After the addition, the mixed suspension was kept stirring for further 6 h to ensure the electrostatically driven assembly of positively charged Bi3+ on the surface of negatively charged GO sheets. Then 1 mmol NaH2PO4⋅2H2O was added to the mixture, and the reaction mixture was kept stirring for 2 h. Finally, the above mixed suspension was transferred into a Teflonlined stainless steel autoclave and
maintained at 160 °C for 18 h. The autoclave was left to cool naturally. The obtained precipitate was collected by centrifugation, and washed several times with distilled water and absolute ethanol. The obtained product was finally dried at 60 °C overnight. The designed mass ratio of graphene to BiPO4 is 0, 1, 2, 3, and 5 wt%, and the corresponding samples are denoted as BP-x%RGO, where x is 0, 1, 2, 3, and 5, respectively. For comparison, a sample with 2 wt% graphene is also prepared by a simple physical mixing of BiPO4 and graphene, which is noted hereafter as “Mixing”. 2.4. Characterization Powder X-ray diffraction (XRD) patterns were obtained on a D/MAX-2500 diffract meter (Rigaku, Japan) using Cu Kα radiation source (λ=1.54056 Å) at a scan rate of 7° min-1. The accelerating voltage and the applied current were 50 kV and 300 mA, respectively. The Scanning electron microscope (SEM) images and energy-dispersive X-ray spectrum (EDS) analysis were taken with a Hitachi S-4800 scanning electron microscope (SEM, 5 kV) equipped with a Thermo Scientific energy dispersion X-ray fluorescence analyzer. Transmission electron microscopy (TEM) images were obtained with a JEOJ-2100F transmission electron microscope with an acceleration voltage of 200 kV. Fourier transform infrared spectra (FTIR) were recorded on an IRAffinity-1 FTIR spectrometer (Shimadzu, Japan). Raman spectra were recorded on a Raman spectrometer (WITEC Spectra Pro 2300I) operating with 532 nm laser. X-ray photoelectron spectroscopy (XPS) data were obtained by an ESCALa- b220i-XL electron spectrometer (VG Scientific, England) using 300 W Al Kα radiation. The photoluminescence spectra were obtained on a F4500 (Hitachi,
Japan) photoluminescence detector. Specific surface areas were calculated using the Brunauere-Emmette-Teller (BET) model via Micromeritics ASAP-2000 nitrogen adsorption apparatus. 2.5. Photocatalytic experiments. The photocatalytic activity of as-obtained samples was evaluated by the degradation of MO under UV light irradiation of a 300 W high-pressure mercury lamp. MO solution (10 mg/L) containing 0.05 g sample was put in a quartz vessel. The solution was stirred for 1 h to ensure adsorption/desorption equilibrium prior to UV light irradiation. 4 mL of the suspension was taken at given time intervals and separated through centrifugation (8000 rpm, 5 min). The concentration of the solution was determined by measuring the absorbance of MO (the maximum absorption peak 464 nm) using a UV–vis spectrophotometer (Shanghai TU-1800). 3. Results and discussion 3.1. The formation mechanism of BiPO4-RGO nanocomposites The one-step synthetic route of BP-RGO nanocomposites is illustrated in Scheme 1. Firstly, because of the electrostatic interaction between Bi3+ and the plenty of functional groups (hydroxyl, carboxyl and epoxy groups) of GO sheets in aqueous solution, the positively charged Bi3+ ions can easily absorb onto the surface of negatively charged GO sheets [29,30]. After the addition of H2PO4–, ion-exchanged reaction between H2PO4– and Bi3+ anions of GO sheets took place leading to the formation of BiPO4 nuclei at the interaction sites between Bi3+ and GO sheets. As the reaction proceeded, these nuclei slowly grew and were transformed into BiPO4
nanorods during the solvothermal process. During this course, the color of the mixture changed from bright yellow to black which indicated the reduction of GO. The elimination of the oxygen-containing functional groups on GO nanosheets is primarily ascribed to the reduction capacity of EG [31,32]. On the other hand, high temperature, to some extent, also plays a role in decreasing the oxygenic groups, which can lower the nucleation energy, control the nucleation sites and density .
Scheme 1. The XRD patterns of GO and graphene obtained through solvothermal reduction of GO with the reaction temperature and time are shown in Fig. 1a. It can be seen that GO shows a sharp (001) diffraction peak at 2θ value of ~10.1°. For the graphene obtained from the solvothermal reduction, the diffraction peak at ~10.1° disappears while a broad (002) diffraction peak at ~25.0° appears, which means that GO sheets have been effectively exfoliated from the raw graphite, and after solvothermal reduction, almost all the GO sheets have been transformed to graphene sheets with a
random packing and significantly less functionalities [34,35]. The digital images of GO and graphene suspensions are also shown in the inset of Fig. 1a, the color of suspensions shifts from brown (GO) to black (RGO) after the solvothermal reaction, which further confirms the successful reduction of GO. For the BP-RGO nanocomposites with different weight ratios of graphene (Fig. 1b), they show similar XRD patterns to blank BiPO4 and all of the diffraction peaks can be readily indexed into the hexagonal phase of BiPO4, which are in good agreement with the standard card (JCPDS No. 15-0766). With the increase of the amount of graphene, no obvious change is found in these patterns, which suggests that the introduction of graphene has no obvious influence on the BiPO4 crystalline structure. Notably, no diffraction peaks for graphene can be observed in the BP-RGO nanocomposites, which may be due to the low amount of graphene in these nanocomposites. On the other hand, the main characteristic peak of graphene at ~25.0° may be overlapped with the strong (110) peak of BiPO4 at 25.4°.
Fig. 1. It is shown in Fig. 2a that the as-prepared BiPO4 displays the rod nanostructure
with an average diameter of ca.100 nm. After the solvothermal reaction, it can be seen from Fig. 2b that the BiPO4 nanorods are densely anchored onto the graphene nanosheets. Compared with the original BiPO4 nanorods, the morphology of BiPO4 ingredient in the BP-2%RGO nanocomposite is almost unchanged, which indicates that the presence of graphene has no obvious effect on the morphology of BiPO4. Moreover, the graphene sheets can serve as a supporting material to suppress the aggregation of BiPO4 nanorods. The chemical composition of BP-2%RGO was further determined by the EDS (Fig. 2c) attached to the SEM, which revealed the presence of Bi, P, O and C elements with the molar ratio of Bi/P/O close to 1:1:4 (Au signals come from the plated element for SEM measurement). Fig. 2d shows the only graphene obtained by the solvothermal reduction. The obvious wrinkles and folds exhibit clearly the 2-D structure of the transparent graphene sheets. As shown in Fig. 2e and f, the ripple of graphene sheets can be explicitly identified as compared with Fig. 2d, convincing us the presence of graphene in the BP-2%RGO nanocomposite. Moreover, it can be seen that BiPO4 nanorods are anchored on the surface of graphene or wrapped by the graphene sheets even after long time sonication during the preparation of the TEM characterization. The joint characterization from SEM and TEM suggests a good interfacial contact between BiPO4 and graphene.
Fig. 2. Fig. 3 shows the FTIR spectra of GO, BiPO4, and BiPO4-2%RGO samples. It is clear that GO shows many strong absorption peaks corresponding to various oxygen functional groups, such as C–O (νC–O at 1042 cm-1), C–O–C (νC–O–C at 1220 cm-1), C–OH (νC–OH at 1400 cm-1), C=O (νC=O at 1724 cm-1), and a broad peak in the range of 3000–3500 cm-1 that is attributed to the O–H stretching vibrations of the C–OH group and water . For the pure BiPO4, the absorption peaks at 3496 and 1603 cm-1
are due to the O–H stretching and H–O–H bending vibrations of crystal water molecules on the surface of BiPO4, and the absorption peak appearing at 1023 cm-1 corresponds to the ν3 stretching vibration of PO4 groups. The other absorption peaks at 595 and 540 cm-1 are assigned to δ (O–P–O) and ν4 (PO4), respectively . Furthermore, compared with the peaks of GO, the sample of BP-2%RGO has a similar spectrum of pure BiPO4, but with the absence of the peaks at 1220, 1400, and 1724 cm-1 for the oxygen-containing functional groups. This result also indicates the effective reduction of GO to graphene during the solvothermal reaction.
In order to further affirm the presence of graphene and intimate interaction between BiPO4 and graphene in the BP-2%RGO nanocomposite, Raman spectra were characterized in particular, as illustrated in Fig. 4. The recorded spectrum of GO shows two characteristic peaks at 1342 and 1585 cm-1, corresponding to the D and G 13
band, respectively . The D band corresponds to the disorder band associated with structural defects created in graphene during the reduction of GO. On the other hand, the G band corresponds to the first order scattering of the E2g phonon of sp2 carbon atoms of graphene . Of particular note is the intensity ratio of D band and G band, ID/IG, which is a measure of the relative concentration of local defects or disorders compared to the sp2 hybridized graphene domains . It can be seen that the ID/IG is 0.99 for GO. After the solvothermal reaction, a significant increase in ID/IG from 0.99 to 1.13 suggests that smaller in-plane sp2 domains are formed during the solvothermal reaction, further demonstrating the effective reduction of GO to graphene . Furthermore, compared with the G band position (1585 cm-1) of GO, the 6 cm-1 shift of the G band peak position to lower wavenumber could be assigned to the result of the intimate interaction between BiPO4 and graphene in the BP-2%RGO nanocomposite [42,43]. In addition, peaks at 205, 401, 541, 590, 968 and 1056 cm-1 correspond to the different vibration modes of BiPO4 . After coupling with graphene, the intensity of the characteristic peaks of BiPO4 show a significant decrease, which further proves that the surface of BiPO4 is wrapped by graphene sheets in the sample of BP-2%RGO . More importantly, the band at 1056 cm-1 of BP-2%RGO was broadened and shift to a lower frequency in comparison with that of BiPO4 spectrum (1603 cm-1). This Raman shift in the BP-2%RGO composite could be induced by the doping effect and/or bonding formation [25,46 ].
In summary, Raman characterization not only demonstrates that BiPO4 nanorods have been successfully attached onto graphene sheets, but also indicates that strong 14
interaction exists between BiPO4 and graphene in the BP-2%RGO sample.
Fig. 4. The X-ray photoelectron spectra (XPS) were further employed to elucidate the surface elemental composition of the sample. In the XPS survey spectrum of BP-2%RGO nanocomposite (Fig. 5a), the signals of Bi 4p, Bi 4d, Bi 4f, Bi 5d, O KLL, O 1s, P 2s, P 2p, and C 1s can be clearly observed, indicating the definite existence of Bi, O, P, and C elements in this sample. In the high-resolution spectrum of C 1s of BP-2%RGO (Fig. 5b), only one peak exists at 284.57 eV, which is ascribed to the sp2 bonded carbon in graphene. Peaks for oxygen-containing functional groups at 286.6 eV (epoxy/hydroxyls, C–O), 287.8 eV (carbonyls, C=O), and 288.9 eV (carboxyl, O–C=O) are all absent in this spectrum, indicating the sufficient removal of oxygen-containing functional groups and the high-degree reduction of GO into graphene during the solvothermal reaction . 15
3.2. Enhanced Photocatalytic activity and stability
The photocatalytic activity of as-prepared samples was evaluated by measuring the degradation of MO in an aqueous solution under UV light irradiation. Fig. 6(a) represents the variation of MO relative concentration C/C0 with irradiation time over samples under UV light irradiation (where C is the real concentration at different time and C0 is the original concentration of MO). For comparison, direct photolysis of MO solution was also tested under identical experimental conditions without photocatalyst. It is demonstrated that the photolysis of MO is negligible without a photocatalyst under UV light irradiation. As shown in Fig. 6a, all of the BP-RGO nanocomposites exhibit higher photocatalytic activity than the pure BiPO4 under UV light irradiation. This comparison suggests that the introduction of graphene can actually enhance the photocatalytic activity of BiPO4. Generally, the photocatalytic efficiency is maximized at the optimal graphene content in the catalyst. Obviously, the catalyst with a graphene-concentration of 2 wt% shows the best catalytic activity. After 60 16
min, nearly 97% of MO is photodegraded by this catalyst. However, further increasing the proportion of graphene, the degradation activity decreased slightly although it remained higher than that of pure BiPO4. It is reasonable because the introduction of a large percentage of graphene leads to shielding of the active sites on the catalyst surface and also rapidly decreases the intensity of light through the depth of the reaction solution, which is called a “shielding effect” . Moreover, the photodegradation reaction of MO with the catalysts agrees well with the pseudo-first-order kinetics (considering the first four points). An integrated rate equation is suggested as follows: ln (C0/Ct) = kt, where C0 and Ct are the initial concentration and concentration at time t of MO and k is the apparent degradation rate constant. The apparent rate constant k, which is equal to the corresponding slope of the fitted line, is shown in Fig. 6b inset in the form of a histogram graph. The apparent rate constant of BP-RGO nanocomposites first increased and then decreased with the increase of graphene content. Among them, BP-2%RGO photocatalyst possessed the highest removal constant, which was about 5 times as high as that of pure BiPO4.
In order to substantiate the contribution of intimate contact between BiPO4 and graphene on the photocatalytic activity, a reference experiment on the physical mixture of BiPO4 and graphene with the same graphene content of 2 wt% was also performed. As shown in Fig. 6c, the activity of the Mixing sample shows higher than that of pure BiPO4, but still much lower than that of BP-2%RGO prepared via one-step solvothermal method. The result indicates that our synthetic route is beneficial to the formation of good contact between BiPO4 and graphene which is 17
significant for the enhancement of photocatalytic activity [48,49]. In addition, cycling photodegradation experiments were also conducted to investigate the stability of as-prepared samples. Fig. 6d displays the data of cycling experiments of degradation of MO over the BP-2%RGO photocatalyst under UV light irradiation. The result indicates that no noticeable activity change is observed during four successive recycles, suggesting that the as-prepared photocatalyst is stable during the photodegradation. Additionally, XRD pattern and TEM image (Fig. S1) clearly reveal that the morphology and fine structure of the nanocomposites are preserved quite well after degrading MO dye molecules, further verifying their excellent durability in the process of photodegradation.
Fig. 6. 18
To explore and understand the origins of the enhanced photocatalytic activity over BP-2%RGO nanocomposite as compared with BiPO4 and Mixing samples, a series of characterization measurements have been performed. As shown in Fig. S2, Nitrogen sorption was used to measure the surface areas of BiPO4, Mixing, and BP-2%RGO samples. It was found that the BET surface area of BP-2%RGO (63.7 m2/g) and Mixing sample (63.1 m2/g, close to BP-2%RGO) was higher than that (59.6 m2/g) of bare BiPO4. In addition, adsorption experiments in the dark for MO over BiPO4, BP-2%RGO, and Mixing samples also have been performed. As displayed in Fig. S3, the results suggest that there is an obviously enhanced adsorption capacity of BP-2%RGO and Mixing samples in comparison with that of bare BiPO4. Generally, a greater BET surface area of photocatalysts is beneficial to achieve better adsorption of dye molecules and can also supply more surface active sites for the photocatalytic reaction , which is in accordance with the absorption measurements and photocatalytic experiments. However, the BET surface area of Mixing sample is very close to that of BP-2%RGO, revealing that the much higher photocatalytic activity of BP-2%RGO than the counterpart mixture is not attributed to the surface area enhancement. So, what is the reason for their large difference between BP-2%RGO and Mixing in their photocatalytic activity? The possible reasons, accounting for the different photocatalytic performance over these nanocomposites especially for BP-2%RGO and Mixing, could be ascribed to the different interfacial contact between BiPO4 and graphene. Direct evidence to prove the different interfacial contact of BiPO4 with graphene
between the samples of BP-2%RGO and Mixing is that Mixing sample prepared via the “hard” integration method has an obvious stratification phenomenon while the suspension of BP-2%RGO obtained from solvothermal treatment is uniform and homogeneous (Fig. S4). Moreover, as we can see in Fig. 2b and f, for the BP-2%RGO sample, the graphene sheets are densely covered with BiPO4, which clearly implies that BiPO4 and graphene have been integrated with an intimate interfacial contact via such a one-step solvothermal treatment. More importantly, Raman characterization also indicates the bonding information between BiPO4 and graphene in the BP-2%RGO composite. However, as displayed in Fig. S5, most of the graphene surface does not effectively integrate with BiPO4, suggesting that the interfacial contact between BiPO4 and graphene is relatively poor and ineffective in the Mixing sample. Thus, it is reasonable to deduce that BP-2%RGO and Mixing samples differ in their photocatalytic activity due to the intensity of interaction between BiPO4 and graphene. If so, in what manner should BiPO4 integrate with graphene for the sample of BP-2%RGO? During the synthesis progress, the Bi atoms bound with the O atoms of the oxygen-containing functional groups of GO are homogenously precipitated with the addition of NaH2PO4, accompanied by an in situ reduction of GO. Therefore, it can be considered that both the nucleation and growth of BiPO4 are aided by the oxygenated functional groups either on the basal planes or at the edges of the GO sheets. During the reaction, BiPO4 atoms may form bonds with O atoms of the functional groups such as –OH, –O–, C=O, and –COOH groups via a covalent coordination bond, acting
as anchor sites for the BiPO4 crystals to grow gradually. This is similar to the previous studies on the combination of semiconductor and graphene via a covalent coordination bond [51–54]. It is worthy noting that –OH and –O– groups are the major force in that –OH and –O– groups are the major part, existing on the basal plane of GO sheets, while carboxyl groups are the minor part, occurring on the edge of GO sheets . And it seems more BiPO4 nanorods are observed on the basal plane than along the edge from SEM and TEM images. Finally, during the solvothermal reaction, GO sheets are reduced and the BiPO4 nanoparticles grow larger and still remain on the surface of graphene via a covalent coordination bond. This intense binding remains stable even after solvothermal reaction to form BP-2%RGO composite, resulting in the intimate interaction between BiPO4 and graphene.
The enhanced efficiency in charge transportations and separations of BiPO4 by coupling with graphene is supported by the photoluminescence (PL) spectra measurements, which can give information about the photoexcited energy/electron transfer and recombination process of electron-hole pairs . PL spectra for BiPO4, BP-2%RGO, and Mixing samples under excitation at 360 nm in the range of 440−600 nm are given in Fig. 7. Obviously, two distinct emission peaks at about 468 and 545 nm can be observed in the spectra of pure BiPO4, which is consistent with the previous reports [57,58]. The lowest PL intensity for BP-2%RGO indicates that recombination of photoinduced electron-hole pairs is most efficiently inhibited, which is consistent with the highest photocatalytic activity of this sample. Thus, it 21
demonstrates that the introduction of graphene can inhibit the recombination of photoinduced charge carriers and promote the separation of photoinduced electron-hole pairs, which is beneficial to the enhancement of photocatalytic activity. In particular, Mixing sample also exhibits a slightly lower PL intensity than BiPO4, but is much higher than that of BP-2%RGO sample. This is mainly attributed to the physical mixing process not being able to create effective interfacial contact between BiPO4 and graphene, whereas the BP-2%RGO sample prepared by one-step solvothermal reaction causes an intimate interaction between BiPO4 and graphene. In this regard, an appropriate introduction of graphene in the BP-RGO nanocomposite, to some extent, can inhibit the recombination of photoinduced charge carriers, while the graphene loading with close interfacial contact will strength the interaction between BiPO4 and graphene. As a consequence, the efficiency of charge transportations and separations can be amplified remarkably.
Fig. 7. 22
Based on the above results, it can be concluded that the enhanced photocatalytic performance of BP-2%RGO nanocomposite should be mainly attributed to the introduction of graphene along with the intimate interfacial contact between BiPO4 and graphene, which thus significantly improve the efficiency of charge transportations and separations. In addition, the introduction of graphene also improves the adsorption capacity of BP-2%RGO towards reactants due to a larger surface area. These two integrative factors result in the significant enhancement of photocatalytic activity of BP-2%RGO nanocomposite towards the photodegradation of MO.
4. Conclusion In summary, we have presented a facile, green, and effective solvothermal method to prepare a series of BP-RGO nanocomposites using ethylene glycol/water as the solvent and reducing agent. During the synthesis, both the effective reduction of GO and the growth of rod-shaped BiPO4 as well as its deposition on graphene were accomplished in this one-step synthetic route. The as-obtained BP-2%RGO nanocomposite
photodegradation of MO, which was about 2.0 and 1.5 times as high as that of bare BiPO4 and physical mixture of BiPO4 and graphene, respectively. The enhanced photocatalytic activity of BP-2%RGO nanocomposite is attributed to a larger surface area, much increased adsorption capacity, and more effective charge transportations and separations arisen from the introduction of graphene along with the intimate interfacial contact between BiPO4 and graphene. This work highlights the significant 23
effect of introduction of graphene and solvothermal method on the photoactivity of graphene-based nanocomposites. We hope this simple method can enrich the preparation of graphene-based photocatalysts with improved interfacial contact and photocatalytic performance. Acknowledgements The work was supported by National Natural Science Foundation of China (Nos. 21001086, 21276116), the Nature Science Foundation of Jiangsu Province (No. BK2012701, BK2011528 and BK2010340), the Postgraduate Research Foundation of Jiangsu Province (No. 1102123C), National Postgraduate Research Foundation of China (No. 2011M500853), Jiangsu University (No. 10JDG070). References
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Scheme 1. Schematic illustration of the formation process of BiPO4-RGO nanocomposites.
Fig. 1. XRD patterns of the samples of (a) GO and graphene, (b) bare BiPO4 and BiPO4-RGO nanocomposites with different weight ratios of graphene; the inset is the digital images of GO and graphene suspensions.
Fig. 2. SEM images of (a) BiPO4 nanorods and (b) BiPO4-2%RGO; (c) EDS spectrum of BiPO4-2%RGO; TEM images of (d) graphene, (e and f) BiPO4-2%RGO.
Fig. 3. FT-IR spectra of GO, BiPO4, and BiPO4-2%RGO.
Fig. 4. Raman spectra of GO, BiPO4, and BiPO4-2%RGO.
Fig. 5. XPS spectra of BiPO4-2%RGO sample: (a) full survey spectrum and (b) the high-resolution XPS spectrum of C 1s.
Fig. 6. (a) Photocatalytic activity of samples under UV light irradiation; (b) the corresponding kinetics of MO degradation. Insert: degradation rate constant k of samples; (c) comparison of the photocatalytic degradation in the presence of BiPO4-2%RGO and Mixing; (d) cycling runs in photocatalytic degradation in the presence of BiPO4-2%RGO.
Fig. 7. Photoluminescence (PL) spectra of BiPO4, Mixing, and BiPO4-2%RGO.
Research highlights ●
nanocomposites. ● The BiPO4-RGO nanocomposites exhibited enhanced photocatalytic activity. ● Reasons for the enhanced photocatalytic activity were discussed. ● Good interaction between BiPO4 and graphene for effective charge pairs separation.