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Crystals 2017, 7(9), 266; doi:10.3390/cryst7090266

Article
Ultrathin g-C3N4 Nanosheet-Modified BiOCl Hierarchical Flower-Like Plate Heterostructure with Enhanced Photostability and Photocatalytic Performance
Tiekun Jia 1,*, Jili Li 1, Fei Long 2, Fang Fu 1, Junwei Zhao 1, Zhao Deng 3,*, Xiaohui Wang 4 and Ying Zhang 1
1
School of Materials Science and Engineering, Luoyang Institute of Science and Technology, Luoyang 471023, China
2
School of Materials Science and Engineering, Guilin University of Technology, Guilin 532100, China
3
State Key Lab of Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
4
Luoyang Orthopedic Hospital of Henan Province, Luoyang 471013, China
*
Correspondence: Tel./Fax: +86-379-65928196 (T.J.)
Academic Editor: Julien Haines
Received: 22 July 2017 / Accepted: 30 August 2017 / Published: 31 August 2017

Abstract

:
A novel ultrathin g-C3N4 nanosheet-modified BiOCl hierarchical flower-like plate heterostructure (abbreviated as BC/CN) was constructed via a thermal polymerization of urea precursor followed with hydrolysis route. The as-prepared samples were well characterized by various analytical techniques. The morphological observation showed that hierarchical flower-like BiOCl nanoplates were discretely anchored on the surface of ultra-thin C3N4 nanosheets. The photocatalytic performance of the as-prepared photocatalysts was evaluated by degradation of methylene blue (MB) under visible-light irradiation. The results showed that BC/CN photocatalyst exhibited enhanced photostability and photocatalytic performance in the degradation process. On the basis of experimental results and the analysis of band energy structure, it could be inferred that the enhanced photocatalytic performance of BC/CN photocatalyst was intimately related with the hybridization of hierarchical flower-like BiOCl nanoplates with ultrathin g-C3N4 nanosheets, which provided good adsorptive capacity, extended light absorption, suppressed the recombination of photo-generated electron–hole pairs, and facilitated charge transfer efficiently.
Keywords:
BiOCl/g-C3N4; heterostructure; visible light; photocatalytic performance

1. Introduction

Environmental pollution and energy crisis are becoming a highly serious threat to humanity’s existence and development, while photocatalysis is promising to be one of the most economic and ecologically safe approaches for settling the above-mentioned problems. For this purpose, many studies have been devoted to developing modified traditional TiO2 or TiO2-alternative photocatalysts for improving their photocatalytic performance [1,2,3,4,5]. As an important representative of BiOX (X = Cl, Br, I) species (p-type semiconductors), BiOCl has unique layered structure built by interlacing (Bi2O2) slabs with double chlorine slabs, leading to the formation of self-built internal static electric fields. Besides above, it also has the merits of good chemical stability, excellent resistance to photocorrosion, non-toxicity and earth abundance [6,7,8]. Previous study reported that the two dimensional (2D) BiOCl showed layered structure, which is conductive to the transfer and separation of the photo-generated electron–hole pairs [9]. Thereby, BiOCl exhibits good photocatalytic performance for degrading organic pollutants in wastewater, such as Rhodamine B, acetophenone, etc. [10,11,12]. However, bare BiOCl can only absorb a small portion of the total irradiated natural sunlight due to its wide bandgap (3.19–3.60 eV), which limits the efficient utilization of solar energy. Nowadays, wide bandgap and low quantum efficiency of BiOCl based photocatalysts are regarded as the main cause of low visible-light photocatalytic efficiency. Consequently, great efforts have been made to enhance the photocatalytic performance by modifying BiOCl with other semiconductors, e.g., Bi2S3/BiOCl [13], Bi2WxMo1−xO6/BiOCl [14], BiOI/BiOCl [15], BiVO4/BiOCl [16], WO3/BiOCl [17], Bi2O2CO3/BiOCl [18], and BiOCl/Bi2O3 [19]. All of the above-mentioned reports are intriguing and impressive, however, it still remains a great challenge to design facile and effective approaches for synthesizing novel BiOCl based photocatalysts with wide spectral response range and high photocatalytic efficiency.
Graphitic carbon nitride (g-C3N4), a polymeric metal-free n-type semiconductor with an indirect narrow bandgap of 2.7 eV, has become a hotspot in photocatalysis due to its facile synthesis, high thermal and chemical stability, excellent optical characteristics and low cost, thus it has been extensively studied as a visible light driven photocatalyst for organic pollutant photodegradation and hydrogen production from water photo-splitting [20,21,22,23,24,25,26,27,28,29,30,31,32]. However, bare C3N4 has the intrinsic drawbacks of poor mass diffusion, low quantum efficiency and high recombination rate of photo-generated electron–hole pairs, leading to the fact that its photodegradation rate is far from optimal level [24,25,26,27,28,29,30,31,32,33,34,35,36,37]. According to previous reports [27,28,29,30,31,32,33,34,35,36,37,38], constructing heterostructures with two well matched energy level semiconductors is an effective strategy to extend absorption range and achieve a rapid and effective separation of electron–holes pairs. Feng et al. [39] reported the preparation of Bi@BiOCl/g-C3N4 with excellent photocatalytic efficiency, using a solvothermal method followed by a surface oxidation treatment. In this hybrid structure, Bi spheres were deposited on porous C3N4 and BiOCl was on the surface of Bi spheres except for the interface between g-C3N4 and metallic Bi. Shan et al. [40] prepared g-C3N4/bismuth-based oxide composites, using Bi2O3 or CH5N3·HCl and g-C3N4 as starting materials. The results showed that some thin flakes of Bi2O2CO3 or BiOCl sprouted from the g-C3N4 bulk, and the degradation rate of MO over 75% g-C3N4/BiOCl photocatalyst reached 100% after 5 h visible light irradiation. Aghdam et al. [41] reported the synthesis of BiOI/BiOCl/g-C3N4 ternary photocatalyst and investigated the proposed photocatalytic mechanism for the degradation of acid orange 7. However, the above obtained g-C3N4 based photocatalysts presented a low BET surface area (6.7–32 m2/g), which probably limited their absorption capability involved in the photodegradation reactions [42,43,44,45]. In addition, it has confirmed that large surface area and the p-n heterostructure are beneficial for achieving excellent photocatalytic activity of g-C3N4 based photocatalysts [42,43,44,45,46,47,48,49]. Based on the consideration mentioned above, provided that 2D BiOCl nanoplates combine with g-C3N4 ultrathin nanosheets to form plate-on-sheet heterostructure with large surface contact, this novel p-n BC/CN heterostructure photocatalyst is promising to exhibit significantly enhanced visible light photoactivity for degrading pollutants. Unfortunately, limited research work is focused on addressing this issue in BC/CN system up to now. Therefore, it is of great interest to explore facile approaches for the synthesis of novel p-n BC/CN heterostructure and investigate the proposed mechanism of the enhanced photoactivity.
In this work, hierarchical flower-like BiOCl nanoplates/ultrathin g-C3N4 nanosheet heterostructure was constructed via a thermal polymerization followed by hydrolysis method. First, ultrathin g-C3N4 nanosheets were prepared by one-step thermal polymerization of urea precursor. Subsequently, hierarchical flower-like BiOCl nanoplates were formed in situ on the n-g-C3N4 ultrathin nanosheets via a simple hydrolysis route. The phase, surface composition, morphologies and optical property of the as-prepared samples were well characterized, and the visible-light photocatalytic performance of photocatalysts was examined for the degradation of methylene blue (MB). The separation and migration of the photoexcited electron–hole pairs of the photocatalysts were investigated by photoluminescence (PL) spectroscopy measurement, while the active species involved in the photodegradation process were investigated by free radicals and hole scavenging experiments. The proposed mechanism of the enhanced photocatalytic performance over BC/CN heterostructure was examined based on experimental results and the analysis of band energy structure.

2. Results and Discussion

2.1. Phase Structure and Morphology

XRD was utilized to identify the crystal phase structure of the as-prepared samples. Figure 1 shows the XRD patterns of the g-C3N4, N0.45B0.55 and BiOCl samples. As is seen in Figure 1, the pronounced diffraction peak at 27.7° of g-C3N4 is indexed to (002) planes, which is a characteristic inter-planar stacking reflection of conjugated aromatic systems. The weak diffraction peak at 13.1° is indexed to (100) diffraction planes of g-C3N4, corresponding to the interlayer structural packing. With respect to BiOCl, all the diffraction peaks are in good accordance with those of tetragonal-phase BiOCl reported in JCPDS card (06-0249). The diffraction peaks with 2θ values of 11.98°, 24.10°, 25.86°, 32.50°, 33.45°, 34.48°, 36.54°, 40.89°, 46.64°, 48.35°, 49.70°, 54.10°, 55.14°, 58.48°, 60.67°, 68.16°, 70.06°, and 77.58° can be indexed to (001), (002), (101), (110), (102), (111), (003), (112), (200), (201), (113), (211), (104), (212), (114), (220), (214) and (310) planes of BiOCl, respectively. In the case of N0.45B0.55, the diffraction peaks of BiOCl are obviously observable, and the typical pattern for g-C3N4 (002) is also found despite of its weak diffraction peak. No other impurity phase is detectable in patterns, suggesting the high purity of BC/CN heterostructure photocatalyst.
In this work, XPS analysis was conducted to further investigate the surface compositions and chemical status in the as-obtained g-C3N4, N0.45B0.55 and BiOCl samples. The results of binding energies spectra are shown in Figure 2. The survey spectra in Figure 2a show that the as-prepared N0.45B0.55 sample consists of C, N, O, Cl, and Bi. The high resolution spectra of C1s, N1s, O1s, Cl2p, and Bi 4f are shown in Figure 2b–f. In Figure 2b, we can find that C1s peak centered at about 284 eV can be assigned the adventitious carbon depositing on the samples in the measurement. As for bare g-C3N4, the major peak of C1s centered at about 287.4 eV is ascribed to the coordination between carbon atoms and three nitrogen atoms in the g-C3N4 lattice [50,51]. Simultaneously, in Figure 2c, the asymmetrical N1s peaks could be fitted into three peaks centered at 398.2 eV, 399.6 eV, and 400.2 eV, indicating that there probably exist three kinds of coordination related with N species in the g-C3N4 sample. According to previous literature [52], the peaks centered at about 398.4 eV and 399.7 eV are corresponding to the pyridinic-like (N-sp2C) nitrogen and pyrrolic-like (N-sp3C) nitrogen, respectively, whereas the peak with higher binding energy centered at about 400.2 eV corresponds to graphitic nitrogen. Compared with those of pure g-C3N4, the C1s and N 1s peaks of N0.45B0.55 shift to higher binding energies by ca. 0.2 eV. The high resolution O1s spectrum of N0.45B0.55 shown in Figure 2d could be fitted well with two peaks centered at about 529.6 eV and 530.4 eV, relating to the lattice oxygen O2− from a Bi-O bond and chemisorbed H2O molecules or OH group on the surface, respectively [46], whereas these peaks in pure BiOCl shift to 529.3 eV and 530.1 eV. Two peaks with binding energies of 197.0 eV and 198.6 eV, corresponding to Cl 2p3/2 and Cl 2p1/2, respectively, are observed in the high resolution Cl 2p spectrum of N0.45B0.55 (Figure 2e), while these peaks in pure BiOCl shift to 197.7 eV and 199.2 eV. It can also be seen from Figure 2f that two strong peaks centered at 159.4 eV and 164.7 eV in N0.45B0.55 are attributed to Bi 4f5/2 and Bi 4f7/2, respectively, which are regarded as the characteristic of Bi3+ species in BiOCl network. Moreover, the peaks of Bi 4f5/2 and Bi 4f7/2 in pure BiOCl are also shifted up by 0.7 eV. Briefly, the above results indicate that both BiOCl and g-C3N4 with graphite-like sp2-bonded structure exist in the heterostructure photocatalyst. Furthermore, the shifts of the peaks in the heterostructure photocatalyst, compared with those in the pure BiOCl and g-C3N4, are predominantly caused by the interaction between the BiOCl and g-C3N4 [53].
Figure 3 shows the FESEM images of the as-prepared pure g-C3N4, BiOCl, and N0.45B0.55 samples. In Figure 3a,b, numerous distorted nanosheets with coarse surface and ultrathin thickness are clearly observed in g-C3N4, indicating that the obtained g-C3N4 ultrathin nanosheets can be served as a substrate for loading cocatalysts. As is seen in Figure 3c,d, pure BiOCl exhibits rose flower-like aggregate microstructure with sizes in the range of several nanometers to 1 μm, which are assembled with lots of nanoplates. Figure 3e shows hierarchical flower-like BiOCl nanoplates dispersing on the surface of g-C3N4 nanosheets. Furthermore, the detailed morphology and microstructure of the as-prepared bare g-C3N4, BiOCl, and N0.45B0.55 samples were examined by TEM and HRTEM, as shown in Figure 4. It is clear in Figure 4b that hierarchical flower-like BiOCl nanoplates anchored on the surface of g-C3N4 nanosheets discretely. Moreover, the magnified TEM image shown in the inset of Figure 4b indicates that BiOCl nanoplates were stacked and interconnected with each other randomly, the thickness of irregular plates were about 10 nm. Furthermore, the width and thickness of BiOCl seemingly decreased after being hybrid with g-C3N4. From the HRTEM image (Figure 4c), the interplanar lattice of 0.343 nm between adjacent lattice fringes of top surface in BiOCl can be indexed to (011) plane. Consulting previous study [54], the top and bottom surface are determined to be (001) facets. As indicated by HRTEM image (Figure 4d), a heterojunction was formed along the interface between BiOCl nanoplate and poorly crystallized g-C3N4 nanosheet, demonstrating that the two components are in close contact. Thereby, the large interface of 2D layered structure is constituted by (001) facets of BiOCl and (002) facets of g-C3N4, which favors the efficient transfer of photogenerated electron-hole pairs.

2.2. BET Surface Area

The specific surface area of the as-prepared g-C3N4 and N0.45B0.55 samples was investigated by nitrogen adsorption-desorption isotherm analysis. Figure 5 depicts the nitrogen adsorption–desorption isotherms of pure g-C3N4 and N0.45B0.55 samples. The isotherms of g-C3N4 and N0.45B0.55 are similar and both of them can be characterized as classical type IV, revealing the nature of mesoporous materials [55]. The hysteresis loops can be categorized as typical H3 hysteresis loop for the relative pressure P/P0 in the range of 0.7–1. According to Brunauer-Emmett-Teller method, the specific surface areas of g-C3N4 and N0.45B0.55 are calculated to be 95.5 m2/g and 44.2 m2/g, respectively. The BET surface area of the N0.45B0.55 nanocomposites decreases in comparison with pure g-C3N4, however, the N0.45B0.55 nanocomposites still have larger surface area and porous structure, which could contribute to the enhanced visible-light photocatalytic performance.

2.3. Optical and Electronic Properties

Figure 6 shows the optical absorption properties of the as-prepared samples, which were measured by using the UV-vis diffuse reflectance spectra (DRS). As is evident in Figure 6, the optical absorption band of pure BiOCl presents a steep absorption edge in the UV region of 368 nm, agreeing well with previous reports [12,13]. Compared with pure BiOCl, the N0.45B0.55 nanocomposites extend their light absorption to visible light region. That is to say, after being coupled with g-C3N4, N0.45B0.55 nanocomposites exhibit enhanced visible light absorption ability owing to the fact that the heterostructure formed between BiOCl and g-C3N4 changes the optical properties of the photocatalysts. Based on absorption results above, the optimal bandgap energy (Eg) of a semiconductor can be determined from the well-known equation: A(hν-Eg)n = αhν, where α, A, and hν represent the adsorption coefficient, proportionality and photon energy, while the value of n is dependent on the intrinsic characteristic of semiconductor, n = 2 for an indirect transition semiconductor and n = 1/2 for a direct transition semiconductor [56]. For both g-C3N4 and BiOCl, the value of n is equal to 2. By extrapolating the linear portion of hν against (αhν)1/2 curves, the band gaps of BiOCl and g-C3N4 are estimated to be about 3.3 eV and 2.6 eV, respectively.
It is vital to investigate the transportation and recombination of processes of photo-induced electron-hole pairs for exploring the main cause of the enhanced photocatalytic activity. As is well known, PL spectra of the as-prepared samples are valuable to explain the migration and recombination processes of photo-induced electron-hole pairs. After being activated by the light, photocatalysts can generate electrons and holes, while the photoinduced holes and electrons recombine together, resulting in the energy release in the form of fluorescence emission. Accordingly, it can be inferred that lower fluorescence emission intensity indicates lower recombination rate of electron-hole. The fluorescence emission spectra of the as-prepared photocatalysts excited at a wavelength of 310 nm are shown in Figure 7a. All of photocatalysts exhibit a broad emission peak centered at about 480 nm, which could be ascribed to the band-band PL phenomenon of the photo-induced charge carriers for g-C3N4. Bare g-C3N4 exhibits the highest PL emission intensity among these photocatalysts, indicating that the charge carrier recombination in bare g-C3N4 is the fastest of all photocatalysts. The N0.45B0.55 nanocomposites exhibit the lowest PL emission intensity, indicating that the recombination of photo-generated charge carriers is suppressed and the efficient separation of photo-generated charge carriers is successfully achieved. The photoluminescence decay kinetics was further investigated by time-resolved spectra. As shown in Figure 7b, the N0.45B0.55 nanocomposites have the longer lifetime corresponding to the slower charge recombination of the excited state in comparison with g-C3N4. According to previous study [57], the average decay lifetimes of carriers in the sample the N0.45B0.55 nanocomposites and g-C3N4 were estimated to be about 3.47 ns and 0.42 ns, respectively. The significantly prolonged life time of the charge carriers indicates that a lower recombination and a higher separation of charge carriers exist in the N0.45B0.55 nanocomposites catalyst. Thus, the high separation efficiency can enhance the probability of their involvement in the photocatalytic reaction before recombination, hence improving the photoactivity.

2.4. Photocatalytic Performance

To evaluate the photoactivity of the as-prepared BiOCl, g-C3N4 and CN-BC nanocomposites, the photodegradation of MB in water was investigated under visible light irradiation in this work. The changes in the characteristic absorption band of MB at 662 nm were used to monitor the photocatalytic performance. The photodegradation efficiency (%) of MB was expressed by Ct/C0, in which Ct represents concentration of the MB solution at a certain irradiation time t, while C0 is the initial concentration of MB solution after adsorption–desorption equilibrium. For comparison, the photocatalytic reactions induced by different photocatalysts were carried out with otherwise identical conditions under visible light irradiation.
Figure 8a illustrates the time-dependent absorption spectra of MB aqueous solution degraded by N0.45B0.55 nanocomposites under visible light irradiation. As shown in Figure 8a, the intensity of the characteristic peak decreased very quickly with the irradiation time prolonging. Correspondingly, the color of the suspension gradually becomes lighter. Figure 8b illustrates the effects of the amount of g-C3N4 on the photocatalytic degradation of MB solution. As shown in Figure 8b, pure BiOCl and g-C3N4 exhibited generally low photocatalytic performance. After being irradiated for 30 min, only about 56% and 69% of MB can be degraded by bare BiOCl and g-C3N4, respectively. As for BiOCl, the photodegradation of MB under visible light irradiation could be ascribed to the indirect dye photosensitization procedure. The above low photocatalytic performance is most likely related with the intrinsic spectral response and fast recombination rate of photogenerated electron-hole pairs. Comparably, the as-prepared CN-BC nanocomposites showed higher photocatalytic performance under visible light irradiation than BiOCl and g-C3N4. Among these nanocomposites, N0.45B0.55 nanocomposites have the highest photocatalytic efficiency, and an almost complete photodegradation of MB was achieved after visible light irradiation for 30 min. Thus, it can be inferred that the optimum amount of loaded g-C3N4 nanosheets is 45%. When the content of loaded g-C3N4 nanosheets in CN-BC nanocomposites increases higher than the optimal amount, the photocatalytic performance decreases contrarily. The above result indicates that the mass ratio of g-C3N4 in the as-prepared nanocomposites is crucial to the synergistic effect during the photodegradation process [58]. In order to quantitate the MB photodegradation rate, the reaction kinetics curves is approximated as a pseudo-first-order process by linear transforms ln(C0/Ct) = kt, where Ct and C0 represent MB concentration at time t (min) and t = 0; k (min−1) is the photodegradation reaction rate constant. The corresponding kinetic constant (k) for MB photodegradation over different photocatalysts is shown in Figure 8c. The rate constant (k) of the MB photodegradation over N0.45B0.55 was estimated to be ca. 0.157 min−1, which was about four times higher than those over BiOCl (0.0275 min−1) or g-C3N4 (0.0393 min−1).

2.5. Catalyst Photostability and Photocatalytic Mechanism

Photostability is one of the significant properties for an excellent photocatalyst. Thus, the repeatability experiments of MB degradation over the N0.45B0.55 photocatalyst were performed to evaluate the reusability and stability, and the results are shown in Figure 9. It is very clear in Figure 9 that the degradation degree is approximate to 99% after the fifth cycling run, indicating that the N0.45B0.55 photocatalyst exhibits adequate photostability for degrading MB pollutant.
It is essential to investigate the proposed mechanism for a heterostructure photocatalyst system. In the photocatalytic degradation process of the pollutants, some reactive species, such as •OH, •O2, and h+, are available for involving oxidation-reduction reactions. To evaluate the effect of the above species on the degradation process, different scavenges of EDTA disodium (Na2-EDTA 4 mmol L−1), methanol (ME 2 mmol L−1), and benzoquinone (BQ 2 mmol L−1) were employed to capture h+, •OH, and •O2, respectively. Figure 10 shows the effect of various scavengers on the visible light photocatalytic performance of N0.45B0.55 toward the degradation of MB. Compared with scavenger-free N0.45B0.55 photocatalytic system, the degradation rate of MB exhibited a slight decrease after the addition of ME (2 mmol L−1) or BQ (2 mmol L−1) in the reaction system. On the contrary, the photocatalytic performance of N0.45B0.55 was significantly suppressed upon the introduction of Na2-EDTA (4 mmol L−1) to the above system, indicating that the photo-generated holes separated in the heterostructure contribute overwhelmingly to the highly enhanced photocatalytic performance.
As is well known, the p-n heterostructure in a photocatalytic system plays a great role in accelerating the efficient separation of photogenerated electron–hole pairs and enhancing the photocatalytic activity [46,47,48,49]. Theoretically, adequate amounts of p-n heterojunctions between (001) facets of BiOCl and (002) facets of g-C3N4 are formed during the synthetic process, which facilitates electrons from the n-type C3N4 in the heterostructure interfaces transferring to the p-type BiOCl. Actually, the holes are inclined to stay at lower energy states, leading to the fact that the status of positively charged holes in the p-type BiOCl and extra negatively charged electrons in the n-type C3N4 will be achieved [59]. Consequently, an internal electric field is simultaneously formed with a positively charged portion at the interface of p-type BiOCl nanoplates and a negatively charged portion at the interface of n-type g-C3N4 nanosheets, and the direction of the internal electric field is from n-type g-C3N4 to p-type BiOCl. Additionally, the photocatalytic performance of BC/CN nanocomposites is closely related with the band structure. The potentials of the valence band (VB) and conduction band (CB) of the two semiconductors at the point of zero charge can be calculated according to the two equations as follows:
ECB = X − Ee − 0.5Eg
EVB = ECB + Eg
where X, Ee and Eg represent the absolute electronegativity of the corresponding semiconductor, the energy of free electrons on the hydrogen scale (4.5 eV) and band gap of the corresponding semiconductor, respectively. The X values of BiOCl and g-C3N4 are calculated to be 6.33 eV and 4.72 eV, respectively. Through theoretical calculation, the CB potentials and VB potentials for BiOCl and g-C3N4 are estimated to be about 0.18 eV and 3.48 eV, and −1.08 eV and 1.52 eV, respectively. Under visible-light irradiation, photogenerated electron-hole pairs in g-C3N4 are obtained due to its narrow bandgap. Afterwards, the electrons in the CB region of the n-type g-C3N4 can easily transfer to the p-type BiOCl due to the well-matched band structure and closely contacted interface, and the holes in the VB region of the n-type BiOCl transfer to the p-type g-C3N4 contrarily. Furthermore, the formation model for the heterostructure photocatalyst and the schematic drawing of electron–hole separation process is illustrated in Figure 11 on the basis of the analysis mentioned above. Therefore, it can be inferred that an effective separation of photogenerated electron–hole pairs in heterostructure photocatalyst are mainly ascribed to the inner electric field assisted charge transfer at the junction interfaces constituted by (001) facets of BiOCl and (002) facets of g-C3N4.

3. Experimental Section

3.1. Materials

All the chemicals, including urea ((NH2)2CO), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), potassium choloride (KCl), methylene blue (MB), ethylene glycol, glycerine and ethanol were purchased from Sinopharm Chemical Reagent CO., Ltd., Shanghai, China. All of them were of analytical grade, and used as received without further purification. Deionized water was used throughout the study.

3.2. Synthesis of g-C3N4/ BiOCl Heterostructure

3.2.1. Synthesis of Ultrathin g-C3N4 Nanosheets

Ultrathin g-C3N4 nanosheets were prepared by a one-step polymerization method, which was similar to the previous report [22]. In detail, 20 g urea powder was put into an alumina crucible with a cover in air. Subsequently, the crucible containing 20 g urea powder was heated in a muffle furnace at 580 °C for 3 h with a heating rate of 3 °C /min. The obtained product was washed with dilute hydrochloric acid (0.1 M), deionized water and ethanol to remove any residual species adsorbed on the surface, followed by drying at 60 °C for 12 h.

3.2.2. Synthesis of g-C3N4 /BiOCl Heterostructure

The typical preparation of BC/CN was carried out through hydrolysis of bismuth salt in ethylene glycol solvent. First, 2 mmol of Bi(NO3)3·5H2O was added to 100 mL mixed solvents (containing 80 mL ethylene glycol and 20 mL glycerine) and stirred for 20 min to form a transparent solution, and then a certain amount of g-C3N4 nanosheets was completely dispersed into the above solution under vigorous magnetic stirring. After that, 2 mmol of KCl was added to the mixture solution and stirred for 1 h to obtain a stable and homogeneous suspension. Subsequently, 50 mL deionized water was slowly dropped into the above suspension under vigorous magnetic stirring. The mixture was then stirred for 3 h at room temperature to make the hydrolysis reaction complete. Finally, the obtained precipitate was collected, centrifuged, and washed with distilled water and ethanol, respectively, followed by drying at 60 °C for 12 h. A series of BC/CN heterostructure photocatalysts with different ratio of 0.15:0.85, 0.35:0.65, 0.45:0.55, 0.55:0.45, and 0.75:0.25 was prepared via the similar synthetic process. Briefly, the as-obtained BC/CN heterostructure photocatalysts were denoted as N0.15B0.85, N0.35B0.65, N0.45B0.55, N0.55B0.45, and N0.25B0.75, respectively. For comparison, bare BiOCl was also prepared via the similar procedure when no g-C3N4 nanosheets were added.

3.3. Characterization

Crystal structure identification was carried out on a Bruker D8 (Bruker, Billerica, MA, USA) advance X-ray diffractometer (XRD) equipped with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 40 mA over the 2θ range of 10°–80°. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, NewYork, NY, USA) equipped with monochromatized Al-Kα radiation, and the binding energy positions were calibrated against the C1s at 284.6 eV. The morphologies of the as-prepared BC/CN heterostructure photocatalysts were observed by a Hitachi S-4800 field emission scanning electron microscope (Hitachi, Toyko, Japan) and a JEM 2100F microscope (JEOL Ltd., Tokyo, Japan) with a field gun, operated at an acceleration voltage of 200 kV, respectively. The Brunauer-Emmett-Teller (BET) specific surface area of the as-prepared BC/CN heterostructure photocatalysts was evaluated based on nitrogen adsorption and desorption isotherms at 77 K using a Quantachrome NOVA 2000e sorption analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). Using BaSO4 as the reference, UV-vis diffuse reflectance spectra were measured by a UV-vis spectrophotometer (TU 1901, Puxi, Beijing, China). Photoluminescence (PL) spectra of the as-prepared products were recorded on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Toyko, Japan) using an excitation wavelength of 310 nm. Time-resolved fluorescence measurement of the as-prepared samples was conducted on a fluorescence spectrophotometer (Horiba Fluorolog 3-22, JY Horiba, Madison, WI, USA).

3.4. Photocatalytic Experiments

The visible-light photocatalytic performance of the as-prepared samples was evaluated by the degradation of MB aqueous solution under visible-light irradiation. As reported in the previous work [32,60,61], 60 mL MB aqueous solution (1.0 × 10−5 M) was added into a culture dish with a diameter of 70 mm which was coated with 80 mg prepared catalyst on the surface. Two daylight lamps (60 W, λ ≥ 400 nm) were used as a light source to trigger the photocatalytic reaction. The two lamps was placed parallel and hung vertically above the culture dish, and the distance between the dish and the lamps was about 10 cm. The average light intensity striking the surface of the reaction solution was about 20 mW/cm2. Prior to light illumination, the solution was allowed to reach an adsorption-desorption equilibrium among the catalyst, MB and water. At given irradiating time intervals, 5 mL reacted solution was taken from the suspension, and then was centrifuged for the concentration measurement. The concentration of MB aqueous solution was determined by using an UV-vis spectrophotometer (TU 1901, Puxi, Beijing, China) at its maximum absorbance wavelength (662 nm).

4. Conclusions

In summary, the novel p-n heterostructure photocatalyst of BC/CN was successfully synthesized via a thermal polymerization of urea precursor followed by hydrolysis route. Compared with pure g-C3N4 and BiOCl, the BC/CN heterostructures exhibit high photocatalytic efficiency. The effective separation of photo-generated electron and the intimately contacted large interfaces between g-C3N4 and BiOCl are the main causes for the enhancement of photocatalytic performance, according to energy band theory and the formation of an internal electrostatic field. This study provides a simple and economical strategy for the design and development of high-efficiency visible-light-driven p-n heterostructure photocatalysts for the applications in environmental remediation and clean energy production.

Acknowledgments

The authors express their grateful thanks to the National Natural Science Foundation of China (Grant U1304520), Education Department of Henan Province for the fund support (18A 430020), and Hubei Provincial Natural Science Foundation (2016CFB337). We also express our thanks to Xiaoqing Liu from Wuhan University of Technology for assistance with the HRTEM characterizations.

Author Contributions

Tiekun Jia conceived and designed the experiments; Jili Li, Fang Fu and Ying Zhang performed the synthetic experiment; Junwei Zhao and Xiaohui Wang performed the photo-catalytic activity of the as-prepared samples; Fei Long gave basic and important ideas as well as precise instruction; Zhao Deng analyzed the data; and Tiekun Jia wrote the paper.

Conflicts of Interest

The authors declared that we do not have any direct financial relation with the commercial identities mentioned in this paper that might lead to any conflict of interest for any of the authors.

Abbreviations

The following abbreviations are used in this manuscript
XRDX-ray diffraction
SEMScanning electron microscopy
TEMTransmission electron microscopy
HRTEMHigh resolution transmission electron microscopy
XPSX-ray photoelectron spectroscopy
BETBrunauer–Emmett–Teller
DRSDiffuse reflectance spectrum
PLPhotoluminescence
MBMethylene blue

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Figure 1. XRD patterns of the as-obtained BiOCl, g-C3N4, and N0.45B0.55 samples.
Figure 1. XRD patterns of the as-obtained BiOCl, g-C3N4, and N0.45B0.55 samples.
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Figure 2. XPS spectra of the as-obtained BiOCl, g-C3N4 and N0.45B0.55 samples: (a) survey; (b) C1s; (c) N1s; (d) O1s; (e) Cl 2p; and (f) Bi 4f.
Figure 2. XPS spectra of the as-obtained BiOCl, g-C3N4 and N0.45B0.55 samples: (a) survey; (b) C1s; (c) N1s; (d) O1s; (e) Cl 2p; and (f) Bi 4f.
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Figure 3. (a,b) Low and high magnification FESEM images of the as-obtained g-C3N4 sample; (c,d) low and high magnification FESEM images of the as-obtained BiOCl sample; and (e) high magnification FESEM image of N0.45B0.55 sample
Figure 3. (a,b) Low and high magnification FESEM images of the as-obtained g-C3N4 sample; (c,d) low and high magnification FESEM images of the as-obtained BiOCl sample; and (e) high magnification FESEM image of N0.45B0.55 sample
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Figure 4. (a) TEM image of the as-obtained g-C3N4 sample; (b) TEM image of N0.45B0.55 sample; and (c,d) HRTEM images of N0.45B0.55 sample
Figure 4. (a) TEM image of the as-obtained g-C3N4 sample; (b) TEM image of N0.45B0.55 sample; and (c,d) HRTEM images of N0.45B0.55 sample
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Figure 5. Nitrogen adsorption-desorption isotherms of the as-obtained g-C3N4 and N0.45B0.55 samples.
Figure 5. Nitrogen adsorption-desorption isotherms of the as-obtained g-C3N4 and N0.45B0.55 samples.
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Figure 6. (a) UV-vis diffuse reflectance spectra of BiOCl, g-C3N4, and N0.45B0.55 samples; (b) the corresponding (αhν)1/2 vs. photon energy (hν) plot of BiOCl sample; and (c) the corresponding (αhν)2 vs. photon energy (hν) plot of bare g-C3N4 sample.
Figure 6. (a) UV-vis diffuse reflectance spectra of BiOCl, g-C3N4, and N0.45B0.55 samples; (b) the corresponding (αhν)1/2 vs. photon energy (hν) plot of BiOCl sample; and (c) the corresponding (αhν)2 vs. photon energy (hν) plot of bare g-C3N4 sample.
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Figure 7. (a) PL spectra of g-C3N4 and BC/CN nanocomposites; and (b) time-resolved photoluminescence spectra monitored at 480 nm of g-C3N4 and N0.45B0.55 samples
Figure 7. (a) PL spectra of g-C3N4 and BC/CN nanocomposites; and (b) time-resolved photoluminescence spectra monitored at 480 nm of g-C3N4 and N0.45B0.55 samples
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Figure 8. (a) Time-dependent absorption spectra of MB aqueous solution degraded by N0.45B0.55 nanocomposites under visible light irradiation; (b) Photocatalytic degradation curves; and (c) photodegradation kinetics of MB in presence of different photocatalysts under visible light irradiation.
Figure 8. (a) Time-dependent absorption spectra of MB aqueous solution degraded by N0.45B0.55 nanocomposites under visible light irradiation; (b) Photocatalytic degradation curves; and (c) photodegradation kinetics of MB in presence of different photocatalysts under visible light irradiation.
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Figure 9. Cycling runs in the photocatalytic degradation of MB in the presence of N0.45B0.55 sample.
Figure 9. Cycling runs in the photocatalytic degradation of MB in the presence of N0.45B0.55 sample.
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Figure 10. Effect of various scavengers on the visible light photocatalytic performance of N0.45B0.55 sample toward the degradation of MB.
Figure 10. Effect of various scavengers on the visible light photocatalytic performance of N0.45B0.55 sample toward the degradation of MB.
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Figure 11. Schematic illustration of the charge transfer process over g-C3N4/BiOCl heterostructure.
Figure 11. Schematic illustration of the charge transfer process over g-C3N4/BiOCl heterostructure.
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