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N-doped graphene composites and mechanism insight
Accepted Manuscript Title: Enhanced photocatalytic property of BiFeO3 /N-doped graphene composites and mechanism insight Author: Pai Li Lei Li Maji Xu Qiang Chen Yunbin He PII: DOI: Reference:
S0169-4332(16)32411-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.052 APSUSC 34356
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-7-2016 4-11-2016 7-11-2016
Please cite this article as: Pai Li, Lei Li, Maji Xu, Qiang Chen, Yunbin He, Enhanced photocatalytic property of BiFeO3/N-doped graphene composites and mechanism insight, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.052 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced photocatalytic property of BiFeO3/N-doped graphene composites and mechanism insight Pai Li, Lei Li, Maji Xu, Qiang Chen, Yunbin He Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory of Green Preparation and Application for Functional Materials, Faculty of Materials Science & Engineering, Hubei University, Wuhan 430062, China
Corresponding author. Tel./fax: +86 27 88661803. E-mail address: [email protected] (Y. He). 1
Graphical abstract
Highlights:
A hydrothermal process was used to prepare BiFeO3/N-doped graphene
composites.
BiFeO3/N-doped graphene exhibits superior photocatalytic activity and stability.
The energy band of BiFeO3 bends downward by ~1.0 eV at the composite
interface.
Downward band bending leads to rapid electron transfer at the composite
interface.
Holes and •OH are predominant active species in the photo-degradation process.
2
Abstract：
A series of BiFeO3/(N-doped) graphene composites are prepared by a facile hydrothermal method. BiFeO3/N-doped graphene shows photocatalytic performance superior to that of BiFeO3/graphene and pristine BiFeO3. The enhanced photo-degradation performance of BiFeO3/N-doped graphene are mainly attributable to the improved light absorbance of the composite, abundant active adsorption sites and high electrical charge mobility of N-doped graphene, and the downward band bending of BiFeO3 at the composite interface. In particular, X-ray photoelectron spectroscopy analyses reveal that the electron energy band of BiFeO3 is downward bent by 1.0 eV at the interface of BiFeO3/N-doped graphene, because of different work functions of both materials. This downward band bending facilitates the transfer of photogenerated electrons from BiFeO3 to N-doped graphene and prompts the separation of photo-generated electron-hole pairs, leading eventually to the enhanced photocatalytic performance.
Keywords：BiFeO3; N-doped graphene; band bending; photocatalytic activity
1. Introduction As the only room-temperature multiferroic material known so far, BiFeO3 appears profound with amazing potential applications [1, 2]. Recent research has revealed that BiFeO3 could be a good photocatalyst for water splitting [3] and degradation of organic pollutants [4]. Compared with traditional TiO2-based photocatalysts with large band gap around 3.2 eV, BiFeO3 exhibits higher photocatalytic activity in the 3
visible-light range due to its lower band gap of ~2.2 eV [5-8]. However, the overall photocatalytic activity of BiFeO3 is still too low for commercial applications by now. Researchers have focused on enhancing the photocatalytic performance of BiFeO3 by tailoring its size and shape [4, 9], doping it with various metal elements [10-15] as well as designing relevant nanocomposites [16-18]. It was demonstrated that Pt/BiFeO3 heterostructure photocatalysts [16], g-C3N4/BiFeO3 nanocomposites [19], and BiFeO3@carbon core/shell nanofibers [20] exhibited improved visible-light photocatalytic performance relative to pristine BiFeO3. In particular, the combination of BiFeO3 with carbon materials [18-20] could greatly enhance its photocatalytic performance. Among the carbon materials, graphene was most popularly introduced to form semiconductor/graphene composites [21-26], including BiFeO3/graphene [18-20] in order to develop high-performance photocatalysts. In semiconductor/graphene composites, graphene usually serves as an electron acceptor and can thus suppress the recombination of photo-generated electrons and holes [27, 28]. Moreover, because of its large surface area and exposure of abundant functional groups on surface, graphene always supply plenty of active adsorption sites for the organic pollutants [29-33] leading to enhanced photocatalytic activity. However, graphene prepared by the common hydrothermal method always contains various defects, which degrade its performance. Various means have been applied to modify graphene to improve its physical and chemical properties. Among them, nitrogen (N) doping of graphene [34-37] can effectively increase the electrical mobility and decrease the work function of pristine graphene [38], and leads to 4
abundant functional groups with excellent adsorption capability towards organic dyes for water pollutants treatment [39]. Therefore, it is highly interesting to explore whether the N-doping of graphene can benefit the photocatalytic performance of BiFeO3/graphene composite. We attempted to address this issue since an earlier study [40]. In that work, through a facile hydrothermal process we succeeded in preparing bismuth ferrite/ (N-doped) graphene composites. However, due to different alkalinities of the aqueous solution in the synthesis processes involving pristine and N-doped graphene, we ended up respectively with Bi25FeO40/graphene and BiFeO3/N-doped graphene composites, which led to difficulty for a direct clarification of the effect of N-doping on the photocatalytic activity of BiFeO3/graphene. In the present study, via a same process with careful control on the aqueous-solution alkalinity in the synthesis process, we obtained a series of perovskite BiFeO3-containing products, namely, pristine BiFeO3, BiFeO3/graphene and BiFeO3/N-doped graphene composites. This allows a direct comparison between products to pinpoint the role of N-doping in affecting the photocatalytic performance of BiFeO3/graphene composite system. It turns out that the photocatalytic properties of BiFeO3/N-doped graphene are greatly superior to those of BiFeO3/graphene and pristine BiFeO3. In addition to photoluminescence (PL), electrochemical impedance spectroscopy (EIS) and N2 adsorption analysis, X-ray photoelectron spectroscopy (XPS) was performed to clarify the mechanism responsible for the enhanced photocatalytic performance of BiFeO3/N-doped graphene. 5
2. Experimental 2.1. Composites synthesis In this study, chemical reagents for the synthesis experiments were of analytic grade and used as-purchased. Graphene oxide (GO) was synthesized by using the Hummers method [41]. N-doped graphene (GN) was prepared by a one-pot hydrothermal process [42], with urea/graphene mass ratios of 0:1, 100:1, 200:1, 400:1 and 600:1, which were denoted as G, GN1, GN2, GN3 and GN4, respectively. A reverse co-precipitation method was applied to prepare the precursors of BiFeO3 [43]. To prepare BiFeO3/ (N-doped) graphene composites, the BiFeO3 aqueous dispersions were first mixed with G, GN1, GN2, GN3 and GN4, respectively. Next, 30 ml of each mixture was loaded in a Teflon-lined stainless autoclave, adding 10 mol/L KOH solution until 70% volume of the autoclave. Then the autoclaves containing the new mixtures were sealed and heated at 180 °C for 9 h. Upon completion of the hydrothermal reaction, we collected and washed the products with deionized water (until pH value around 7), and then dried them in vacuum at 60 °C for 12 h. Finally, we obtained six samples which were denoted as BFO (pristine BiFeO3), BG (BiFeO3/graphene), BGN1, BGN2, BGN3 and BGN4, respectively.
2.2. Characterization Crystalline phases of the composites were analyzed by X-ray diffraction (XRD) using a powder diffractometer (Bruker D8 Advance, GmbH) with Cu Kα (λ = 0.15418 nm) radiation. The morphology of the composites was inspected using a 6
scanning electron microscope (SEM; JSM-6510, JEOL, Japan). Raman tests were carried out with a radiation of Ar+ laser at 633 nm using a LabRAM HR 800UV Raman
spectrometer
(HORIBA
JobinYvon,
France).
The
BET
(Brunauer-Emmett-Telter) specific surface area and corresponding pore size distribution were obtained by N2 adsorption isotherms examined at 77 K by Micromeritics ASAP 2020 apparatus. The flat band potential of BiFeO3 was determined by the Mott-Schottky method via the CHI660C electrochemical workstation. In detail, BiFeO3 was deposited on fluorine-tin oxide and served as the working electrodes, while Pt was employed as counter electrode; Ag/AgCl was served as reference electrode; and 0.5 M potassium nitrate was employed as electrolyte. UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS) were performed with a SHIMADZU UV-3600 UV-Vis spectrophotometer. Electrochemical impedance spectra (EIS) were carried out with a computer-controlled IM6 electrochemical workstation (ZAHNER, Germany). The electrolyte for EIS was 0.01 mol L−1 Na2SO4 solution. Working electrodes were prepared by casting pristine BiFeO3 or BiFeO3/ (N-doped) graphene composites onto the Pt/ITO electrode. Nyquist plots were recorded over a frequency range of 0.1~100 kHz. Photoluminescence spectra (PL) were obtained by a Hitachi F-4500 (excited at λ =375 nm) fluorescence spectrophotometer. Composition and binding energy shift analyses were carried out by X-ray photoelectron spectroscopy (XPS; PHOIBOS 150, SPECS) at photon energy of 1486.6 eV (Al Kα radiation).
7
2.3. Photocatalytic performance measurements Photocatalytic performance of pristine BiFeO3 and BiFeO3/(N-doped) graphene composites was evaluated via photo-degradation of Congo red (CR) under visible-light in a hollow-cylindrical-vessel photo-reactor. A 300 W xenon lamp was used to generate the visible light with the help of UV cut-off filters. Via circulating water into the cylindrical-vessel, the photocatalytic reactions were ensured to occur at room temperature, avoiding the thermal disturbance. For the photocatalysis experiments, we added 0.02 g photocatalyst (BiFeO3 or BiFeO3/(N-doped) graphene composite) into 80 ml CR solution (10 mg L-1) to form suspension. The suspension solutions were first kept in dark for half an hour while being stirred to establish the adsorption-desorption equilibrium. Then the photocatalytic reaction started by exposure of the suspension solutions to visible-light. At selected time intervals, the suspension solutions were taken out for analysis after removing the catalyst particles by centrifugation. The CR degradation was evaluated by measuring the absorption intensity of CR at wavelength of 495 nm relative to its initial value (C/C0) via a spectrophotometer.
3. Results and discussion XRD patterns of the photocatalysts BFO, BG, BGN1, BGN2, BGN3 and BGN4 are shown in Fig. 1. All the diffraction peaks of BFO are assigned to the perovskite phase of BiFeO3 (JCPDS 71-2494). BG, BGN1, BGN2, BGN3 and BGN4 composites maintain the pure perovskite phase of BiFeO3, implying that the introduction of 8
(N-doped) graphene has no effects on the phase structure and crystallinity. Figure 2 shows SEM images of BFO, BG, BGN1, BGN2, BGN3, BGN4 and N-doped graphene (GN) powders. All BiFeO3-based composites consist of grains of similar spherical shape. GN consists of stacked flakes with wrinkles and crumples all over itself. As shown in the enlarged part of BGN4, the sphere grains of BiFeO3 and crumpling N-doped graphene could be clearly distinguished. The BiFeO3-grain diameters of BFO, BG, BGN1, BGN2, BGN3 and BGN4 are about 13 μm, 10 μm, 10 μm, 15 μm, 7 μm and 10 μm, respectively. The grain sizes of BiFeO3 in all BiFeO3/(N-doped) graphene composites are mostly smaller than that of pristine BiFeO3. The decreased grain size could be attributed to the existence of (N-doped) graphene, which contained partly graphene oxide (Fig. 3b). It was reported that graphene oxide can act as a surfactant in the preparation process [44], which may disperse the particles in the hydrothermal solution and then lead to small grain size of the sample. Generally, the photocatalytic activity rises with decreasing size of the spherical-particles [4]. In our case, although BGN2 has the largest particle size, its photocatalytic activity is much higher than that of BFO and BG, as will be seen in Fig. 7a. This indicates that there exist other mechanisms responsible for the superior photocatalytic activity. XPS survey spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 are shown in Fig. 3a, which demonstrate the existence of Bi, Fe, O, C and N elements in the BiFeO3/N-doped graphene composites. Due to air exposure of the samples, C 1s signal is observed in the spectrum of BFO, while C 1s peaks of BG, BGN1, BGN2, 9
BGN3 and BGN4 originate mainly from (N-doped) graphene. From energy dispersive X-ray spectroscopy analyses (data not shown), the atom contents of N in BGN1, BGN2, BGN3 and BGN4 composites are deduced to be 2.62%, 3.48%, 4.82% and 6.22%, respectively. Fig. 3b shows the XPS survey spectra of graphene (G) and N-doped graphene (GN). The presence of O 1s peak implies incomplete reduction of the graphene oxide. The peak at 400 eV in the spectrum of GN is attributed to the N 1s, indicating successful doping of N into graphene. In addition, Fig. 3b inset displays the high-resolution N 1s spectrum of GN, which can be decomposed into three components: graphitic-N (401.9 eV), pyrrolic-N (399.8 eV), and pyridinic-N (398.2 eV) [45]. Usually, graphitic-N refers to the N atoms which substitute for the C atoms in plane, and each graphitic-N atom bonds with three sp2 hybridized C atoms, while each pentagonal pyrrolic-N and each hexagonal pyridinic-N just bonds with two C atoms in a π conjugated configuration, leading to lower binding energy than that of graphitic-N. The fractions of different N components representing the different N bonding configurations in BGN1, BGN2, BGN3 and BGN4 (obtained from N 1s peak fitting) are given in Table 1. It is clearly seen that the dominant N bonding configuration in all samples is pyrrolic-N, which is more stable at the edge of graphene according to theoretical prediction, and can provide more active adsorption sites for pollutants. This could be one of the reasons for the enhanced photocatalytic performance of the N-doped graphene containing composites. It should be noted that the content of graphitic-N is obviously reduced in BGN4, which may be part of the cause for the 10
lower photocatalytic performance of BGN4 compared with BGN3 (cf. Fig. 7a). It was reported that graphitic-N and pyrrolic-N could make a synergistic effect for the enhancement of catalytic performance [45], and thus the reduced fraction of graphitic-N in BGN4 would affect the synergistic effect leading to degraded catalytic performance. Figure 4 displays the Raman spectra for BFO, BG and BGN3. All the A and E active modes for BFO are in good correspondence with the perovskite structure of BiFeO3. As for BG and BGN3, two distinct peaks located at 1345 cm−1 and 1585 cm−1 are observed. The former one is D band in graphene, reflecting the structure defects and disorder, while the latter one is G band in graphene, revealing the ordered E2g phonon scattering of sp2 carbon atoms. Usually, the intensity ratio of D band to G band (ID/IG) is used to evaluate the structural defects and disorder of graphene sheets. As seen from the inset of Fig. 4, the ID/IG ratio of GN (1.16) is larger than that of G (0.96). The larger ID/IG ratio for GN suggests that N-doping affects some sp2 graphene carbon bonds and creates more structural defects, which could supply more active adsorption sites for the pollutants, leading to enhanced catalytic performance of BiFeO3/N-doped graphene relative to BiFeO3/graphene. Fig. 5 presents the nitrogen adsorption-desorption isotherms of BFO, BG and BGN3 at 77 K and their BJH (Barrett-Joyner-Halenda) pore size distribution curves (Fig. 5 inset). For all the samples, the isotherms can be classified as Type Ⅳ, indicating existence of mesopores in the samples. As shown in the inset of Fig. 5, the pore size distribution of our samples ranges from 10 to 50 nm, which confirms the 11
presence of mesopores in the samples. The narrow adsorption hysteresis loops of the nitrogen sorption isotherms for BG and BGN3 indicate the appearance of slit-like pores caused by the aggregation of graphene sheets. Moreover, the steep increase of the isotherms in high relative pressures is related to macropores, which may be formed by the stacking of BiFeO3 grains. The specific surface area of BGN3 calculated from the BET analysis is 4.55 m2 g-1, which is higher than that of BG (3.91 m2 g-1) and much higher than that of BFO (1.01 m2 g-1). The increased specific surface area caused by (N-doped) graphene could provide more active adsorption sites, which are beneficial for enhancing the photocatalytic performance. Figure 6 displays UV-Vis absorption spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. Since BiFeO3 is a direct band gap material, near the band edge its optical absorption coefficient obeys the equation (ahv)2 = B(hv - Eg), where a is the absorption coefficient, hv the photon energy, Eg the optical band gap and B is a constant. Based on this equation the Eg of BiFeO3 can be determined via extrapolation of the linear part of (ahv)2 vs hv plot to the point a = 0, which leads to a band gap of about 2.0 eV, as shown in the inset of Fig. 6. Because graphene is half-metallic, BiFeO3/ (N-doped) graphene composites all show improved absorption towards visible light relative to pristine BiFeO3, as seen in Fig. 6. Therefore, the photocatalytic activities of BG, BGN1, BGN2, BGN3 and BGN4 are expected to be higher than that of BFO. The photocatalytic performance of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites towards CR degradation under visible-light is presented in Fig. 7a. 12
As seen, CR degradation without photocatalyst (i.e., self-photolysis of aqueous CR) is negligible under visible-light irradiation. With BiFeO3 as a photocatalyst, nearly 52% of CR is oxidized within 3 h. Upon incorporation of (N-doped) graphene, all composite catalysts exhibit obviously improved photo-degradation efficiencies; about 85%, 88%, 90%, 92% and 86% of CR are degraded with BG, BGN1, BGN2, BGN3 and BGN4, respectively, in the same period of 3h. With increasing amount of N (≤ 4.82%) in the composites, the photocatalytic activity increases, and BGN3 delivers the best performance. However, with further increasing N content up to 6.22%, BGN4 shows contrarily degraded catalytic performance as compared with other N-doped composites. On the one hand, this may be attributed to the reduced fraction of graphitic-N in BGN4 compared with BGN3 as discussed previously. On the other hand, the excess amount of N may create extra defects and impurities that act as recombination centers for the photo-generated electron-hole pairs, thereby leading to a lowed photocatalytic performance of BGN4 [16]. Overall, the above results suggest that the combination of BiFeO3 with graphene modified by an appropriate amount of N could benefit the separation of photo-induced electron–hole pairs, thus resulting in improved photocatalytic performance of the composite. Fig. 7b shows kinetic plots for all the catalysts towards CR photo-degradation as a function of irradiation time (t), from which corresponding reaction rate constants (k) can be deduced based on the equation of –ln(C0/C)= k × t. Clearly, all N-doped composites exhibit higher k than the pristine BFO. Especially the k for BGN3 amounts to 0.04 min-1, which is 4 times that for the pristine BFO (0.01 min-1). In order to clarify the active species in the 13
photocatalytic process, trapping experiments of reactive oxygen species were carried out with BGN3 as the catalyst. Ammonium oxalate (AO), Tert-Butyl alcohol (TBA) and 1, 4-Benzoquinone (BQ) were added as scavengers for the photo-generated hole, •OH and •O2-, respectively. The results in Fig. 7c show that both hole and •OH are the primary reactive species while •O2- plays unimportant role in the photo-degradation of CR. The photocatalytic stability of the composite catalysts was examined by repeating photocatalytic experiments. Fig. 7d displays the results of photo-degradation of CR with BGN3 for five successive cycles. It is clearly seen that the BiFeO3/N-doped graphene photocatalyst is quite stable in performance, beneficial to future practical applications. Photoluminescence (PL) spectra are generally useful for the investigation of transfer and recombination processes of the photo-induced electron-hole pairs in semiconductors. The PL emission primarily originates from the recombination of photo-generated electrons and holes in semiconductors [16, 17, 46-49]. Fig. 8 illustrates PL spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 composites measured with excitation wavelength of 375 nm. As seen, the emission of BFO centers at 560 nm, and the PL spectra of all composites show a similar trend with varying intensity, which indicates a dominant role of BiFeO3 in PL generation. Compared with BFO and BG, BiFeO3/N-doped graphene (BGN1, BGN2, BGN3 and BGN4) composites exhibit much weaker fluorescence signals, implying greatly suppressed recombination of electrons and holes. This implies that the photo-excited electrons in BiFeO3 are instantly transferred into N-doped graphene upon generation, 14
leading to reduced charge recombination and thus enhanced photocatalytic performance. The PL results well accord with the photocatalytic activities shown in Fig. 7. Electrochemical impedance spectra (EIS) of BFO, BG and BGN3 in the dark and under visible-light illumination are presented in Fig. 9. The Nyquist plot, which represents the charge transfer resistance [50, 51], exhibits only one capacitive arc for each sample. Generally, in EIS Nyquist plot a smaller arc radius indicates a more efficient charge separation and transfer. Interestingly, the arc radius of the Nyquist plot of BG is smaller than that of BFO, and BGN3 exhibits the smallest arc radius, regardless under illumination or in the dark. This implies an effective separation of photo-excited electron–hole pairs and swift interfacial charge transfer over BiFeO3/N-doped graphene, leading to the greatly improved photocatalytic performance. This result is also consistent with the photocatalytic performance presented in Fig. 7. Figure 10a-c shows respectively high-resolution x-ray photoelectron spectra of Bi 4f7/2, Fe 2p3/2 and O 1s from BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. Bi 4f7/2 peak is observed at binding energy of 158.6 eV, indicative of Bi3+ in pristine BiFeO3, and is shifted to 159.4 eV in BiFeO3/graphene and 159.6 eV in all BiFeO3/N-doped graphene composites (Fig.10a). Interestingly, both Fe 2p3/2 and O 1s peaks show similar shifts. More specifically, compared with BiFeO3, all peaks of Bi 4f7/2, Fe 2p3/2 and O 1s are shifted upward in binding energy by same values of 0.8 eV for BiFeO3/graphene, and 1.0 eV for various BiFeO3/N-doped 15
graphene composites (Fig. 10a-c). For semiconductor heterojunctions, if the core-level electrons of all positive and negative ions in the semiconductor show the same trend of shift in binding energy after calibration (e.g., by C 1s at 284.8 eV), the shift is usually caused by band bending of the semiconductor at the interface [52-54]. In contrast, chemical shift often exhibits opposite trend for positive and negative ions in a material. In the case of BiFeO3/N-doped graphene composites, the same trend shifts of Bi 4f7/2, Fe 2p3/2 and O 1s can be attributed to the band bending of BiFeO3 at the interface between BiFeO3 and (N-doped) graphene. The energy band diagram of BiFeO3/ (N-doped) graphene composites can be derived based on the XPS results, as illustrated in Fig. 11b. A downward band bending of BiFeO3 by 0.8 (1.0) eV occurs at the interface of BiFeO3/ (N-doped) graphene, which facilitates the transfer of electrons in the conduction band of BiFeO3 to the surface of (N-doped) graphene, and thus helps separate the photo-induced electrons and holes. The band bending of BiFeO3 at the BiFeO3/N-doped graphene interface is by 0.2 eV more than at the BiFeO3/graphene interface, indicating a faster electron transfer at the former than at the latter. This well accords with and rationalizes the ESI results presented above. Therefore, compared with BiFeO3/graphene, BiFeO3/N-doped graphene composites have higher ability for separating photo-induced electrons and holes (i.e., better charge transfer ability at the interface) and hence show higher photocatalytic activity. It should be noted, however, that in the present study we did not detect any difference for the band bending of the composites with different N doping contents. More details about the band alignment and interaction at the interface need further investigation. 16
Figure 10d displays the Mott-Schottky plot of pristine BiFeO3, base on which the flat-band potential of BiFeO3 can be estimated from linear part of the plot [23, 55, 56]. The negative slope of the linear plot suggests that the BiFeO3 synthesized in this work is a p-type semiconductor, and its flat-band potential (close to valance band maximum) is 2.53 eV (vs. Ag/AgCl). Based on the concepts of electronegativity, the valance band maximum (VBM) energy of a semiconductor can be calculated using the following equation [57] 𝐸VBM = 𝑋 − 𝐸 e + 0.5𝐸g
(1)
where 𝑋 is the Mulliken electronegativity of the semiconductor defined as the geometric mean of the electronegativities of its constituent atoms, 𝐸 e is the energy of free electrons on the hydrogen scale (≈ 4.5 eV), and 𝐸g is the band gap of the semiconductor. The conduction band minimum (CBM) energy 𝐸CBM can be derived by 𝐸CBM = 𝐸VBM - 𝐸g . The X value of BiFeO3 is ca. 6.09 eV, and the band gap of pristine BiFeO3 deduced from the optical absorption measurement is 2.0 eV (Fig. 6). The 𝐸VBM calculated according to equation (1) amounts to 2.59 eV, which accords well with the measured flat band potential of 2.53 eV (vs. Ag/AgCl ) (Fig. 10d). Further, 𝐸CBM is deduced to be 0.59 eV. As shown in the inset of Fig. 10d, the VBM of BiFeO3 extrapolated from the XPS valance-band spectrum is 0.71 eV (vs. Femi level), which means the Femi energy of BiFeO3 is 1.88 eV. The band diagram of BiFeO3 derived from above calculations is illustrated in Fig. 11a. According to a recent research that focused on tuning the work function of graphene by nitrogen doping [38], we assume the work function of graphene as 4.91 eV and N-doped 17
graphene as 4.37 eV. Therefore, the Femi energy locates at 0.41 eV (vs. NHE) for graphene, and -0.13 eV (vs. NHE) for N-doped graphene. As seen in Fig. 11a, the Femi level of (N-doped) graphene is much higher than that of pristine BiFeO3. When both comes to contact forming composite, electrons transfer from (N-doped) graphene to BiFeO3 until both Fermi levels equalize, leading to a downward band bending of BiFeO3 at the interface. This downward band bending was revealed by XPS analyses as presented above (Fig. 10a-c and 11b). As previously discussed, this downward band bending is beneficial to the separation of photogenerated holes and electrons at the side of BiFeO3, thereby enhancing the photocatalytic performance of the composites. Moreover, as seen from Fig. 11a, the VBM of BiFeO3 is more positive relative to the •OH/H2O potential, which means the photogenerated holes at the VBM prefer to turn the absorbed H2O into •OH. Therefore, both the holes at the VBM of BiFeO3 and hydroxyl radicals •OH contribute substantially to the photocatalytic activity. On the other hand, the CBM of BiFeO3 is lower than the potential of O2/•O2-, implying difficulties in the generation of •O2-. Hence, the superoxide radicals •O2- should have minor effect on the photocatalytic process. These results accord well with the trapping experiments presented in Fig. 7c, which identified photogenerated holes and hydroxyl radicals •OH as the predominant active species in the CR degradation with BiFeO3/(N-doped) graphene as the photocatalyst.
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4. Conclusions In conclusion, BiFeO3/(N-doped) graphene composites were successfully synthesized via a facile hydrothermal process. Compared with BiFeO3/graphene and pristine BiFeO3, BiFeO3/N-doped graphene shows much superior photocatalytic performance. Core-level XPS measurements revealed that a downward band bending of BiFeO3 (about 1.0 eV) occurred at the BiFeO3/N-doped graphene interface, which benefited the separation of photogenerated electron-hole pairs at the BiFeO3 side. Combined with the high electron mobility of N-doped graphene, this downward band bending leads to rapid transfer of photogenerated electrons from BiFeO3 to N-doped graphene and thus suppresses the recombination of photogenerated electrons and holes, as indicated respectively by ESI and PL measurements. The greatly enhanced photo-degradation performance to Congo red are principally attributed to the improved light absorbance of the composite, abundant active adsorption sites and excellent electrical transport properties of N-doped graphene, and the downward band bending of BiFeO3 at the composite interface. Both photogenerated holes and hydroxyl radicals •OH were identified as the predominant active species in the photo-degradation
process.
The
superior
photocatalytic
performance
of
BiFeO3/N-doped graphene composites lend them great potential in photocatalytic water splitting and photo-degradation of organic pollutants under visible-light irradiation.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51202062, 61274010 and 51572073), Research Fund for the Doctoral Program of Higher Education of China (20124208120005), Scientific Research Foundation for Returned Scholars, Ministry of Education of China, and the Natural Science Foundation of Hubei Province (2015CFA038).
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(2013) 9706-9712. [50] H. Liu, S. Cheng, M. Wu, H. Wu, J. Zhang, W. Li, C. Cao, Photoelectrocatalytic degradation of sulfosalicylic acid and its electrochemical impedance spectroscopy investigation, J. Phys. Chem. A 104 (2000) 7016-7020. [51] W.P. Gomes, D. Vanmaekelbergh, Electrochemical impendance spectroscopy at semiconductor electrodes: review and recent developments, Electrochim. Acta 41 (1996) 967-973. [52] Z. Zhang, J.T. Yates, Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces, Chem. Rev. 112 (2012) 5520-5551. [53] Y.Y. Mi, S.J. Wang, Y.F. Dong, J.W. Chai, J.S. Pan, A.C.H. Huan, C.K. Ong, Evolution of Fermi level position and Schottky barrier height at Ni/MgO(0 0 1) interface, Surf. Sci. 599 (2005) 255-261. [54] Y. He, D. Langsdorf, L. Li, H. Over, Versatile model system for studying processes ranging from heterogeneous to photocatalysis: Epitaxial RuO 2(110) on TiO2(110), J. Phys. Chem. C 119 (2015) 2692-2702. [55] K. Gelderman, L. Lee, S.W. Donne, Flat-band potential of a semiconductor: using the Mott–Schottky equation, J. Chem. Educ. 84 (2007) 685. [56] M.A. Butler, D.S. Ginley, Prediction of flatband potentials at semiconductor ‐ Electrolyte interfaces from atomic electronegativities, J. Electrochem. Soc. 125 (1978) 228-232. [57] Y.I. Kim, S.J. Atherton, E.S. Brigham, T.E. Mallouk, Sensitized layered metal oxide semiconductor particles for photochemical hydrogen evolution from nonsacrificial electron donors, J. Phys. Chem. 97 (1993) 11802-11810.
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Figure Captions: Fig. 1 XRD patterns of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. Fig. 2 SEM images of BFO, BG, BGN1, BGN2, BGN3, BGN4, GN and enlarged part of BGN4 microcrystallites. Fig. 3 (a) XPS survey spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. (b) XPS survey spectra of G and GN, and high-resolution XPS spectrum of N 1s (inset) fitted by three peaks: graphitic N (401.9 eV), pyrrolic N (399.8 eV), and pyridinic N (398.2 eV). Fig. 4 Raman spectra of BFO, BG and BGN3 between 100 and 2000 cm−1, and Raman spectra of G and GN between 800 and 2000 cm−1 (inset). Fig. 5 Nitrogen sorption isotherms of BFO, BG and BGN3. The inset shows the corresponding pore-size distribution. Fig. 6 UV-Vis absorption spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. The inset shows the estimation of band gap of BiFeO3. Fig. 7 (a) The variation of CR concentration vs. visible-light irradiation time, and (b) the rate constant (k: slope of the ln(c/c0) - t plot) for CR photo-degradation with BFO, BG, BGN1, BGN2, BGN3 and BGN4 photocatalysts. For easier comparison, CR concentrations in all blank samples are normalized to 1, ignoring tiny CR degradation in the absence of photocatalyst. (c) Reactive species trapping experiments for BGN3. (d) Cycling degradation of CR with BGN3 photocatalyst. Fig. 8 PL spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. Fig. 9 Electrochemical impedance spectra of BFO, BG and BGN3 microcrystallites (a) in the dark and (b) under illumination. 25
Fig. 10 (a)-(c) High-resolution XPS spectra of Bi 4f7/2, Fe 2p3/2 and O 1s for BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. (d) The Mott-Schottky plot of BiFeO3 for the determination of its flat-band potential. The inset shows valance-band XPS spectrum of BiFeO3. Fig. 11 (a) Energy band configurations of BiFeO3, graphene and N-doped graphene before contact. (b) Schematic band alignment at the interface of BiFeO3/ (N-doped) graphene composites.
26
Figure 1
27
Figure 2
28
Figure 3
29
Figure 4
30
Figure 5
31
Figure 6
32
Figure 7
33
Figure 8
34
Figure 9
35
Figure 10
36
Figure 11
37
Table 1. N 1s peak area fractions of different N configurations in BGN1, BGN2, BGN3 and BGN4 (obtained from XPS data).
Sample
Graphitic N
Pyrrolic N
Pyridinic N
(401.9 eV)
(399.8 eV)
(398.2 eV)
BGN1
18.6%
58.1%
23.3%
BGN2
21.2%
61.2%
17.6%
BGN3
28.6%
61.0%
10.4%
BGN4
17.8%
61.1%
21.1%
38
S0169-4332(16)32411-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.052 APSUSC 34356
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-7-2016 4-11-2016 7-11-2016
Please cite this article as: Pai Li, Lei Li, Maji Xu, Qiang Chen, Yunbin He, Enhanced photocatalytic property of BiFeO3/N-doped graphene composites and mechanism insight, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.052 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced photocatalytic property of BiFeO3/N-doped graphene composites and mechanism insight Pai Li, Lei Li, Maji Xu, Qiang Chen, Yunbin He Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory of Green Preparation and Application for Functional Materials, Faculty of Materials Science & Engineering, Hubei University, Wuhan 430062, China
Corresponding author. Tel./fax: +86 27 88661803. E-mail address: [email protected] (Y. He). 1
Graphical abstract
Highlights:
A hydrothermal process was used to prepare BiFeO3/N-doped graphene
composites.
BiFeO3/N-doped graphene exhibits superior photocatalytic activity and stability.
The energy band of BiFeO3 bends downward by ~1.0 eV at the composite
interface.
Downward band bending leads to rapid electron transfer at the composite
interface.
Holes and •OH are predominant active species in the photo-degradation process.
2
Abstract：
A series of BiFeO3/(N-doped) graphene composites are prepared by a facile hydrothermal method. BiFeO3/N-doped graphene shows photocatalytic performance superior to that of BiFeO3/graphene and pristine BiFeO3. The enhanced photo-degradation performance of BiFeO3/N-doped graphene are mainly attributable to the improved light absorbance of the composite, abundant active adsorption sites and high electrical charge mobility of N-doped graphene, and the downward band bending of BiFeO3 at the composite interface. In particular, X-ray photoelectron spectroscopy analyses reveal that the electron energy band of BiFeO3 is downward bent by 1.0 eV at the interface of BiFeO3/N-doped graphene, because of different work functions of both materials. This downward band bending facilitates the transfer of photogenerated electrons from BiFeO3 to N-doped graphene and prompts the separation of photo-generated electron-hole pairs, leading eventually to the enhanced photocatalytic performance.
Keywords：BiFeO3; N-doped graphene; band bending; photocatalytic activity
1. Introduction As the only room-temperature multiferroic material known so far, BiFeO3 appears profound with amazing potential applications [1, 2]. Recent research has revealed that BiFeO3 could be a good photocatalyst for water splitting [3] and degradation of organic pollutants [4]. Compared with traditional TiO2-based photocatalysts with large band gap around 3.2 eV, BiFeO3 exhibits higher photocatalytic activity in the 3
visible-light range due to its lower band gap of ~2.2 eV [5-8]. However, the overall photocatalytic activity of BiFeO3 is still too low for commercial applications by now. Researchers have focused on enhancing the photocatalytic performance of BiFeO3 by tailoring its size and shape [4, 9], doping it with various metal elements [10-15] as well as designing relevant nanocomposites [16-18]. It was demonstrated that Pt/BiFeO3 heterostructure photocatalysts [16], g-C3N4/BiFeO3 nanocomposites [19], and BiFeO3@carbon core/shell nanofibers [20] exhibited improved visible-light photocatalytic performance relative to pristine BiFeO3. In particular, the combination of BiFeO3 with carbon materials [18-20] could greatly enhance its photocatalytic performance. Among the carbon materials, graphene was most popularly introduced to form semiconductor/graphene composites [21-26], including BiFeO3/graphene [18-20] in order to develop high-performance photocatalysts. In semiconductor/graphene composites, graphene usually serves as an electron acceptor and can thus suppress the recombination of photo-generated electrons and holes [27, 28]. Moreover, because of its large surface area and exposure of abundant functional groups on surface, graphene always supply plenty of active adsorption sites for the organic pollutants [29-33] leading to enhanced photocatalytic activity. However, graphene prepared by the common hydrothermal method always contains various defects, which degrade its performance. Various means have been applied to modify graphene to improve its physical and chemical properties. Among them, nitrogen (N) doping of graphene [34-37] can effectively increase the electrical mobility and decrease the work function of pristine graphene [38], and leads to 4
abundant functional groups with excellent adsorption capability towards organic dyes for water pollutants treatment [39]. Therefore, it is highly interesting to explore whether the N-doping of graphene can benefit the photocatalytic performance of BiFeO3/graphene composite. We attempted to address this issue since an earlier study [40]. In that work, through a facile hydrothermal process we succeeded in preparing bismuth ferrite/ (N-doped) graphene composites. However, due to different alkalinities of the aqueous solution in the synthesis processes involving pristine and N-doped graphene, we ended up respectively with Bi25FeO40/graphene and BiFeO3/N-doped graphene composites, which led to difficulty for a direct clarification of the effect of N-doping on the photocatalytic activity of BiFeO3/graphene. In the present study, via a same process with careful control on the aqueous-solution alkalinity in the synthesis process, we obtained a series of perovskite BiFeO3-containing products, namely, pristine BiFeO3, BiFeO3/graphene and BiFeO3/N-doped graphene composites. This allows a direct comparison between products to pinpoint the role of N-doping in affecting the photocatalytic performance of BiFeO3/graphene composite system. It turns out that the photocatalytic properties of BiFeO3/N-doped graphene are greatly superior to those of BiFeO3/graphene and pristine BiFeO3. In addition to photoluminescence (PL), electrochemical impedance spectroscopy (EIS) and N2 adsorption analysis, X-ray photoelectron spectroscopy (XPS) was performed to clarify the mechanism responsible for the enhanced photocatalytic performance of BiFeO3/N-doped graphene. 5
2. Experimental 2.1. Composites synthesis In this study, chemical reagents for the synthesis experiments were of analytic grade and used as-purchased. Graphene oxide (GO) was synthesized by using the Hummers method [41]. N-doped graphene (GN) was prepared by a one-pot hydrothermal process [42], with urea/graphene mass ratios of 0:1, 100:1, 200:1, 400:1 and 600:1, which were denoted as G, GN1, GN2, GN3 and GN4, respectively. A reverse co-precipitation method was applied to prepare the precursors of BiFeO3 [43]. To prepare BiFeO3/ (N-doped) graphene composites, the BiFeO3 aqueous dispersions were first mixed with G, GN1, GN2, GN3 and GN4, respectively. Next, 30 ml of each mixture was loaded in a Teflon-lined stainless autoclave, adding 10 mol/L KOH solution until 70% volume of the autoclave. Then the autoclaves containing the new mixtures were sealed and heated at 180 °C for 9 h. Upon completion of the hydrothermal reaction, we collected and washed the products with deionized water (until pH value around 7), and then dried them in vacuum at 60 °C for 12 h. Finally, we obtained six samples which were denoted as BFO (pristine BiFeO3), BG (BiFeO3/graphene), BGN1, BGN2, BGN3 and BGN4, respectively.
2.2. Characterization Crystalline phases of the composites were analyzed by X-ray diffraction (XRD) using a powder diffractometer (Bruker D8 Advance, GmbH) with Cu Kα (λ = 0.15418 nm) radiation. The morphology of the composites was inspected using a 6
scanning electron microscope (SEM; JSM-6510, JEOL, Japan). Raman tests were carried out with a radiation of Ar+ laser at 633 nm using a LabRAM HR 800UV Raman
spectrometer
(HORIBA
JobinYvon,
France).
The
BET
(Brunauer-Emmett-Telter) specific surface area and corresponding pore size distribution were obtained by N2 adsorption isotherms examined at 77 K by Micromeritics ASAP 2020 apparatus. The flat band potential of BiFeO3 was determined by the Mott-Schottky method via the CHI660C electrochemical workstation. In detail, BiFeO3 was deposited on fluorine-tin oxide and served as the working electrodes, while Pt was employed as counter electrode; Ag/AgCl was served as reference electrode; and 0.5 M potassium nitrate was employed as electrolyte. UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS) were performed with a SHIMADZU UV-3600 UV-Vis spectrophotometer. Electrochemical impedance spectra (EIS) were carried out with a computer-controlled IM6 electrochemical workstation (ZAHNER, Germany). The electrolyte for EIS was 0.01 mol L−1 Na2SO4 solution. Working electrodes were prepared by casting pristine BiFeO3 or BiFeO3/ (N-doped) graphene composites onto the Pt/ITO electrode. Nyquist plots were recorded over a frequency range of 0.1~100 kHz. Photoluminescence spectra (PL) were obtained by a Hitachi F-4500 (excited at λ =375 nm) fluorescence spectrophotometer. Composition and binding energy shift analyses were carried out by X-ray photoelectron spectroscopy (XPS; PHOIBOS 150, SPECS) at photon energy of 1486.6 eV (Al Kα radiation).
7
2.3. Photocatalytic performance measurements Photocatalytic performance of pristine BiFeO3 and BiFeO3/(N-doped) graphene composites was evaluated via photo-degradation of Congo red (CR) under visible-light in a hollow-cylindrical-vessel photo-reactor. A 300 W xenon lamp was used to generate the visible light with the help of UV cut-off filters. Via circulating water into the cylindrical-vessel, the photocatalytic reactions were ensured to occur at room temperature, avoiding the thermal disturbance. For the photocatalysis experiments, we added 0.02 g photocatalyst (BiFeO3 or BiFeO3/(N-doped) graphene composite) into 80 ml CR solution (10 mg L-1) to form suspension. The suspension solutions were first kept in dark for half an hour while being stirred to establish the adsorption-desorption equilibrium. Then the photocatalytic reaction started by exposure of the suspension solutions to visible-light. At selected time intervals, the suspension solutions were taken out for analysis after removing the catalyst particles by centrifugation. The CR degradation was evaluated by measuring the absorption intensity of CR at wavelength of 495 nm relative to its initial value (C/C0) via a spectrophotometer.
3. Results and discussion XRD patterns of the photocatalysts BFO, BG, BGN1, BGN2, BGN3 and BGN4 are shown in Fig. 1. All the diffraction peaks of BFO are assigned to the perovskite phase of BiFeO3 (JCPDS 71-2494). BG, BGN1, BGN2, BGN3 and BGN4 composites maintain the pure perovskite phase of BiFeO3, implying that the introduction of 8
(N-doped) graphene has no effects on the phase structure and crystallinity. Figure 2 shows SEM images of BFO, BG, BGN1, BGN2, BGN3, BGN4 and N-doped graphene (GN) powders. All BiFeO3-based composites consist of grains of similar spherical shape. GN consists of stacked flakes with wrinkles and crumples all over itself. As shown in the enlarged part of BGN4, the sphere grains of BiFeO3 and crumpling N-doped graphene could be clearly distinguished. The BiFeO3-grain diameters of BFO, BG, BGN1, BGN2, BGN3 and BGN4 are about 13 μm, 10 μm, 10 μm, 15 μm, 7 μm and 10 μm, respectively. The grain sizes of BiFeO3 in all BiFeO3/(N-doped) graphene composites are mostly smaller than that of pristine BiFeO3. The decreased grain size could be attributed to the existence of (N-doped) graphene, which contained partly graphene oxide (Fig. 3b). It was reported that graphene oxide can act as a surfactant in the preparation process [44], which may disperse the particles in the hydrothermal solution and then lead to small grain size of the sample. Generally, the photocatalytic activity rises with decreasing size of the spherical-particles [4]. In our case, although BGN2 has the largest particle size, its photocatalytic activity is much higher than that of BFO and BG, as will be seen in Fig. 7a. This indicates that there exist other mechanisms responsible for the superior photocatalytic activity. XPS survey spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 are shown in Fig. 3a, which demonstrate the existence of Bi, Fe, O, C and N elements in the BiFeO3/N-doped graphene composites. Due to air exposure of the samples, C 1s signal is observed in the spectrum of BFO, while C 1s peaks of BG, BGN1, BGN2, 9
BGN3 and BGN4 originate mainly from (N-doped) graphene. From energy dispersive X-ray spectroscopy analyses (data not shown), the atom contents of N in BGN1, BGN2, BGN3 and BGN4 composites are deduced to be 2.62%, 3.48%, 4.82% and 6.22%, respectively. Fig. 3b shows the XPS survey spectra of graphene (G) and N-doped graphene (GN). The presence of O 1s peak implies incomplete reduction of the graphene oxide. The peak at 400 eV in the spectrum of GN is attributed to the N 1s, indicating successful doping of N into graphene. In addition, Fig. 3b inset displays the high-resolution N 1s spectrum of GN, which can be decomposed into three components: graphitic-N (401.9 eV), pyrrolic-N (399.8 eV), and pyridinic-N (398.2 eV) [45]. Usually, graphitic-N refers to the N atoms which substitute for the C atoms in plane, and each graphitic-N atom bonds with three sp2 hybridized C atoms, while each pentagonal pyrrolic-N and each hexagonal pyridinic-N just bonds with two C atoms in a π conjugated configuration, leading to lower binding energy than that of graphitic-N. The fractions of different N components representing the different N bonding configurations in BGN1, BGN2, BGN3 and BGN4 (obtained from N 1s peak fitting) are given in Table 1. It is clearly seen that the dominant N bonding configuration in all samples is pyrrolic-N, which is more stable at the edge of graphene according to theoretical prediction, and can provide more active adsorption sites for pollutants. This could be one of the reasons for the enhanced photocatalytic performance of the N-doped graphene containing composites. It should be noted that the content of graphitic-N is obviously reduced in BGN4, which may be part of the cause for the 10
lower photocatalytic performance of BGN4 compared with BGN3 (cf. Fig. 7a). It was reported that graphitic-N and pyrrolic-N could make a synergistic effect for the enhancement of catalytic performance [45], and thus the reduced fraction of graphitic-N in BGN4 would affect the synergistic effect leading to degraded catalytic performance. Figure 4 displays the Raman spectra for BFO, BG and BGN3. All the A and E active modes for BFO are in good correspondence with the perovskite structure of BiFeO3. As for BG and BGN3, two distinct peaks located at 1345 cm−1 and 1585 cm−1 are observed. The former one is D band in graphene, reflecting the structure defects and disorder, while the latter one is G band in graphene, revealing the ordered E2g phonon scattering of sp2 carbon atoms. Usually, the intensity ratio of D band to G band (ID/IG) is used to evaluate the structural defects and disorder of graphene sheets. As seen from the inset of Fig. 4, the ID/IG ratio of GN (1.16) is larger than that of G (0.96). The larger ID/IG ratio for GN suggests that N-doping affects some sp2 graphene carbon bonds and creates more structural defects, which could supply more active adsorption sites for the pollutants, leading to enhanced catalytic performance of BiFeO3/N-doped graphene relative to BiFeO3/graphene. Fig. 5 presents the nitrogen adsorption-desorption isotherms of BFO, BG and BGN3 at 77 K and their BJH (Barrett-Joyner-Halenda) pore size distribution curves (Fig. 5 inset). For all the samples, the isotherms can be classified as Type Ⅳ, indicating existence of mesopores in the samples. As shown in the inset of Fig. 5, the pore size distribution of our samples ranges from 10 to 50 nm, which confirms the 11
presence of mesopores in the samples. The narrow adsorption hysteresis loops of the nitrogen sorption isotherms for BG and BGN3 indicate the appearance of slit-like pores caused by the aggregation of graphene sheets. Moreover, the steep increase of the isotherms in high relative pressures is related to macropores, which may be formed by the stacking of BiFeO3 grains. The specific surface area of BGN3 calculated from the BET analysis is 4.55 m2 g-1, which is higher than that of BG (3.91 m2 g-1) and much higher than that of BFO (1.01 m2 g-1). The increased specific surface area caused by (N-doped) graphene could provide more active adsorption sites, which are beneficial for enhancing the photocatalytic performance. Figure 6 displays UV-Vis absorption spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. Since BiFeO3 is a direct band gap material, near the band edge its optical absorption coefficient obeys the equation (ahv)2 = B(hv - Eg), where a is the absorption coefficient, hv the photon energy, Eg the optical band gap and B is a constant. Based on this equation the Eg of BiFeO3 can be determined via extrapolation of the linear part of (ahv)2 vs hv plot to the point a = 0, which leads to a band gap of about 2.0 eV, as shown in the inset of Fig. 6. Because graphene is half-metallic, BiFeO3/ (N-doped) graphene composites all show improved absorption towards visible light relative to pristine BiFeO3, as seen in Fig. 6. Therefore, the photocatalytic activities of BG, BGN1, BGN2, BGN3 and BGN4 are expected to be higher than that of BFO. The photocatalytic performance of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites towards CR degradation under visible-light is presented in Fig. 7a. 12
As seen, CR degradation without photocatalyst (i.e., self-photolysis of aqueous CR) is negligible under visible-light irradiation. With BiFeO3 as a photocatalyst, nearly 52% of CR is oxidized within 3 h. Upon incorporation of (N-doped) graphene, all composite catalysts exhibit obviously improved photo-degradation efficiencies; about 85%, 88%, 90%, 92% and 86% of CR are degraded with BG, BGN1, BGN2, BGN3 and BGN4, respectively, in the same period of 3h. With increasing amount of N (≤ 4.82%) in the composites, the photocatalytic activity increases, and BGN3 delivers the best performance. However, with further increasing N content up to 6.22%, BGN4 shows contrarily degraded catalytic performance as compared with other N-doped composites. On the one hand, this may be attributed to the reduced fraction of graphitic-N in BGN4 compared with BGN3 as discussed previously. On the other hand, the excess amount of N may create extra defects and impurities that act as recombination centers for the photo-generated electron-hole pairs, thereby leading to a lowed photocatalytic performance of BGN4 [16]. Overall, the above results suggest that the combination of BiFeO3 with graphene modified by an appropriate amount of N could benefit the separation of photo-induced electron–hole pairs, thus resulting in improved photocatalytic performance of the composite. Fig. 7b shows kinetic plots for all the catalysts towards CR photo-degradation as a function of irradiation time (t), from which corresponding reaction rate constants (k) can be deduced based on the equation of –ln(C0/C)= k × t. Clearly, all N-doped composites exhibit higher k than the pristine BFO. Especially the k for BGN3 amounts to 0.04 min-1, which is 4 times that for the pristine BFO (0.01 min-1). In order to clarify the active species in the 13
photocatalytic process, trapping experiments of reactive oxygen species were carried out with BGN3 as the catalyst. Ammonium oxalate (AO), Tert-Butyl alcohol (TBA) and 1, 4-Benzoquinone (BQ) were added as scavengers for the photo-generated hole, •OH and •O2-, respectively. The results in Fig. 7c show that both hole and •OH are the primary reactive species while •O2- plays unimportant role in the photo-degradation of CR. The photocatalytic stability of the composite catalysts was examined by repeating photocatalytic experiments. Fig. 7d displays the results of photo-degradation of CR with BGN3 for five successive cycles. It is clearly seen that the BiFeO3/N-doped graphene photocatalyst is quite stable in performance, beneficial to future practical applications. Photoluminescence (PL) spectra are generally useful for the investigation of transfer and recombination processes of the photo-induced electron-hole pairs in semiconductors. The PL emission primarily originates from the recombination of photo-generated electrons and holes in semiconductors [16, 17, 46-49]. Fig. 8 illustrates PL spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 composites measured with excitation wavelength of 375 nm. As seen, the emission of BFO centers at 560 nm, and the PL spectra of all composites show a similar trend with varying intensity, which indicates a dominant role of BiFeO3 in PL generation. Compared with BFO and BG, BiFeO3/N-doped graphene (BGN1, BGN2, BGN3 and BGN4) composites exhibit much weaker fluorescence signals, implying greatly suppressed recombination of electrons and holes. This implies that the photo-excited electrons in BiFeO3 are instantly transferred into N-doped graphene upon generation, 14
leading to reduced charge recombination and thus enhanced photocatalytic performance. The PL results well accord with the photocatalytic activities shown in Fig. 7. Electrochemical impedance spectra (EIS) of BFO, BG and BGN3 in the dark and under visible-light illumination are presented in Fig. 9. The Nyquist plot, which represents the charge transfer resistance [50, 51], exhibits only one capacitive arc for each sample. Generally, in EIS Nyquist plot a smaller arc radius indicates a more efficient charge separation and transfer. Interestingly, the arc radius of the Nyquist plot of BG is smaller than that of BFO, and BGN3 exhibits the smallest arc radius, regardless under illumination or in the dark. This implies an effective separation of photo-excited electron–hole pairs and swift interfacial charge transfer over BiFeO3/N-doped graphene, leading to the greatly improved photocatalytic performance. This result is also consistent with the photocatalytic performance presented in Fig. 7. Figure 10a-c shows respectively high-resolution x-ray photoelectron spectra of Bi 4f7/2, Fe 2p3/2 and O 1s from BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. Bi 4f7/2 peak is observed at binding energy of 158.6 eV, indicative of Bi3+ in pristine BiFeO3, and is shifted to 159.4 eV in BiFeO3/graphene and 159.6 eV in all BiFeO3/N-doped graphene composites (Fig.10a). Interestingly, both Fe 2p3/2 and O 1s peaks show similar shifts. More specifically, compared with BiFeO3, all peaks of Bi 4f7/2, Fe 2p3/2 and O 1s are shifted upward in binding energy by same values of 0.8 eV for BiFeO3/graphene, and 1.0 eV for various BiFeO3/N-doped 15
graphene composites (Fig. 10a-c). For semiconductor heterojunctions, if the core-level electrons of all positive and negative ions in the semiconductor show the same trend of shift in binding energy after calibration (e.g., by C 1s at 284.8 eV), the shift is usually caused by band bending of the semiconductor at the interface [52-54]. In contrast, chemical shift often exhibits opposite trend for positive and negative ions in a material. In the case of BiFeO3/N-doped graphene composites, the same trend shifts of Bi 4f7/2, Fe 2p3/2 and O 1s can be attributed to the band bending of BiFeO3 at the interface between BiFeO3 and (N-doped) graphene. The energy band diagram of BiFeO3/ (N-doped) graphene composites can be derived based on the XPS results, as illustrated in Fig. 11b. A downward band bending of BiFeO3 by 0.8 (1.0) eV occurs at the interface of BiFeO3/ (N-doped) graphene, which facilitates the transfer of electrons in the conduction band of BiFeO3 to the surface of (N-doped) graphene, and thus helps separate the photo-induced electrons and holes. The band bending of BiFeO3 at the BiFeO3/N-doped graphene interface is by 0.2 eV more than at the BiFeO3/graphene interface, indicating a faster electron transfer at the former than at the latter. This well accords with and rationalizes the ESI results presented above. Therefore, compared with BiFeO3/graphene, BiFeO3/N-doped graphene composites have higher ability for separating photo-induced electrons and holes (i.e., better charge transfer ability at the interface) and hence show higher photocatalytic activity. It should be noted, however, that in the present study we did not detect any difference for the band bending of the composites with different N doping contents. More details about the band alignment and interaction at the interface need further investigation. 16
Figure 10d displays the Mott-Schottky plot of pristine BiFeO3, base on which the flat-band potential of BiFeO3 can be estimated from linear part of the plot [23, 55, 56]. The negative slope of the linear plot suggests that the BiFeO3 synthesized in this work is a p-type semiconductor, and its flat-band potential (close to valance band maximum) is 2.53 eV (vs. Ag/AgCl). Based on the concepts of electronegativity, the valance band maximum (VBM) energy of a semiconductor can be calculated using the following equation [57] 𝐸VBM = 𝑋 − 𝐸 e + 0.5𝐸g
(1)
where 𝑋 is the Mulliken electronegativity of the semiconductor defined as the geometric mean of the electronegativities of its constituent atoms, 𝐸 e is the energy of free electrons on the hydrogen scale (≈ 4.5 eV), and 𝐸g is the band gap of the semiconductor. The conduction band minimum (CBM) energy 𝐸CBM can be derived by 𝐸CBM = 𝐸VBM - 𝐸g . The X value of BiFeO3 is ca. 6.09 eV, and the band gap of pristine BiFeO3 deduced from the optical absorption measurement is 2.0 eV (Fig. 6). The 𝐸VBM calculated according to equation (1) amounts to 2.59 eV, which accords well with the measured flat band potential of 2.53 eV (vs. Ag/AgCl ) (Fig. 10d). Further, 𝐸CBM is deduced to be 0.59 eV. As shown in the inset of Fig. 10d, the VBM of BiFeO3 extrapolated from the XPS valance-band spectrum is 0.71 eV (vs. Femi level), which means the Femi energy of BiFeO3 is 1.88 eV. The band diagram of BiFeO3 derived from above calculations is illustrated in Fig. 11a. According to a recent research that focused on tuning the work function of graphene by nitrogen doping [38], we assume the work function of graphene as 4.91 eV and N-doped 17
graphene as 4.37 eV. Therefore, the Femi energy locates at 0.41 eV (vs. NHE) for graphene, and -0.13 eV (vs. NHE) for N-doped graphene. As seen in Fig. 11a, the Femi level of (N-doped) graphene is much higher than that of pristine BiFeO3. When both comes to contact forming composite, electrons transfer from (N-doped) graphene to BiFeO3 until both Fermi levels equalize, leading to a downward band bending of BiFeO3 at the interface. This downward band bending was revealed by XPS analyses as presented above (Fig. 10a-c and 11b). As previously discussed, this downward band bending is beneficial to the separation of photogenerated holes and electrons at the side of BiFeO3, thereby enhancing the photocatalytic performance of the composites. Moreover, as seen from Fig. 11a, the VBM of BiFeO3 is more positive relative to the •OH/H2O potential, which means the photogenerated holes at the VBM prefer to turn the absorbed H2O into •OH. Therefore, both the holes at the VBM of BiFeO3 and hydroxyl radicals •OH contribute substantially to the photocatalytic activity. On the other hand, the CBM of BiFeO3 is lower than the potential of O2/•O2-, implying difficulties in the generation of •O2-. Hence, the superoxide radicals •O2- should have minor effect on the photocatalytic process. These results accord well with the trapping experiments presented in Fig. 7c, which identified photogenerated holes and hydroxyl radicals •OH as the predominant active species in the CR degradation with BiFeO3/(N-doped) graphene as the photocatalyst.
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4. Conclusions In conclusion, BiFeO3/(N-doped) graphene composites were successfully synthesized via a facile hydrothermal process. Compared with BiFeO3/graphene and pristine BiFeO3, BiFeO3/N-doped graphene shows much superior photocatalytic performance. Core-level XPS measurements revealed that a downward band bending of BiFeO3 (about 1.0 eV) occurred at the BiFeO3/N-doped graphene interface, which benefited the separation of photogenerated electron-hole pairs at the BiFeO3 side. Combined with the high electron mobility of N-doped graphene, this downward band bending leads to rapid transfer of photogenerated electrons from BiFeO3 to N-doped graphene and thus suppresses the recombination of photogenerated electrons and holes, as indicated respectively by ESI and PL measurements. The greatly enhanced photo-degradation performance to Congo red are principally attributed to the improved light absorbance of the composite, abundant active adsorption sites and excellent electrical transport properties of N-doped graphene, and the downward band bending of BiFeO3 at the composite interface. Both photogenerated holes and hydroxyl radicals •OH were identified as the predominant active species in the photo-degradation
process.
The
superior
photocatalytic
performance
of
BiFeO3/N-doped graphene composites lend them great potential in photocatalytic water splitting and photo-degradation of organic pollutants under visible-light irradiation.
19
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51202062, 61274010 and 51572073), Research Fund for the Doctoral Program of Higher Education of China (20124208120005), Scientific Research Foundation for Returned Scholars, Ministry of Education of China, and the Natural Science Foundation of Hubei Province (2015CFA038).
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Figure Captions: Fig. 1 XRD patterns of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. Fig. 2 SEM images of BFO, BG, BGN1, BGN2, BGN3, BGN4, GN and enlarged part of BGN4 microcrystallites. Fig. 3 (a) XPS survey spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. (b) XPS survey spectra of G and GN, and high-resolution XPS spectrum of N 1s (inset) fitted by three peaks: graphitic N (401.9 eV), pyrrolic N (399.8 eV), and pyridinic N (398.2 eV). Fig. 4 Raman spectra of BFO, BG and BGN3 between 100 and 2000 cm−1, and Raman spectra of G and GN between 800 and 2000 cm−1 (inset). Fig. 5 Nitrogen sorption isotherms of BFO, BG and BGN3. The inset shows the corresponding pore-size distribution. Fig. 6 UV-Vis absorption spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. The inset shows the estimation of band gap of BiFeO3. Fig. 7 (a) The variation of CR concentration vs. visible-light irradiation time, and (b) the rate constant (k: slope of the ln(c/c0) - t plot) for CR photo-degradation with BFO, BG, BGN1, BGN2, BGN3 and BGN4 photocatalysts. For easier comparison, CR concentrations in all blank samples are normalized to 1, ignoring tiny CR degradation in the absence of photocatalyst. (c) Reactive species trapping experiments for BGN3. (d) Cycling degradation of CR with BGN3 photocatalyst. Fig. 8 PL spectra of BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. Fig. 9 Electrochemical impedance spectra of BFO, BG and BGN3 microcrystallites (a) in the dark and (b) under illumination. 25
Fig. 10 (a)-(c) High-resolution XPS spectra of Bi 4f7/2, Fe 2p3/2 and O 1s for BFO, BG, BGN1, BGN2, BGN3 and BGN4 microcrystallites. (d) The Mott-Schottky plot of BiFeO3 for the determination of its flat-band potential. The inset shows valance-band XPS spectrum of BiFeO3. Fig. 11 (a) Energy band configurations of BiFeO3, graphene and N-doped graphene before contact. (b) Schematic band alignment at the interface of BiFeO3/ (N-doped) graphene composites.
26
Figure 1
27
Figure 2
28
Figure 3
29
Figure 4
30
Figure 5
31
Figure 6
32
Figure 7
33
Figure 8
34
Figure 9
35
Figure 10
36
Figure 11
37
Table 1. N 1s peak area fractions of different N configurations in BGN1, BGN2, BGN3 and BGN4 (obtained from XPS data).
Sample
Graphitic N
Pyrrolic N
Pyridinic N
(401.9 eV)
(399.8 eV)
(398.2 eV)
BGN1
18.6%
58.1%
23.3%
BGN2
21.2%
61.2%
17.6%
BGN3
28.6%
61.0%
10.4%
BGN4
17.8%
61.1%
21.1%
38