Hybrid PDI/BiOCl heterojunction with enhanced interfacial charge transfer for a full-spectrum photocatalytic degradation of pollutants

Xiaoming Gao *a, Kailong Gao a, Xibao Li b, Yanyan Shang a and Feng Fu *a
aDepartment of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan'an University, Yanan, 716000, P. R.China. E-mail: dawn1026@163.com; yadxfufeng@126.com
bSchool of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, P. R. China

Received 27th August 2019 , Accepted 11th November 2019

First published on 12th November 2019


A full-spectrum (300–750 nm) responsive hybrid PDI/BiOCl photocatalyst was successfully constructed. The hybrid structure of the PDI/BiOCl photocatalyst was first examined by surface photovoltage measurements and DFT theoretical calculations. Due to the O atoms bonding to Bi atom, PDI was successfully anchored on the surface of rod-like BiOCl. Owing to the strongly coupled heterojunction interface and conjugated structure of PDI, a rapid interfacial charge transfer was allowed from PDI to BiOCl across the interface. The hybrid PDI/BiOCl photocatalyst presented the best full-spectrum photocatalytic degradation activity for organic pollutants, which was 2.2 and 1.6 times higher than that of BiOCl and PDI, respectively, for the photocatalytic degradation of phenol. The enhanced photocatalytic ability was not only due to the formation of the hybrid PDI/BiOCl composites, which could produce more active species, but also since PDI/BiOCl possessed remarkable full spectrum light-harvesting and excellent interfacial charge separation. The results of the free radical measurements showed that the main active radicals were ˙O2 and ˙OH in the full-wavelength photocatalytic oxidation degradation.


1. Introduction

With the development of society, the increasingly serious water pollution has greatly affected and endangered the life of human beings; especially, the harm caused by numerous organic compounds is persistent. Therefore, efficient and advanced treatment of organic pollutants has important practical significance for water purification. Among numerous treatment methods, photocatalytic oxidation technology has many advantages,1–4 such as mild conditions, no secondary pollution, and deep degradation. Nevertheless, photocatalysts with excellent light utilization efficiency and high quantum efficiency are necessary for the potential application of photocatalytic oxidation technology.5–7

Because of its diversity in crystal and electronic structure, BiOCl has both a suitable energy band structure and high charge fluidity.8,9 Therefore, BiOCl has received extensive research attention.10,11 However, it also inevitably possesses some shortcomings,12–14 such as a high recombination rate of photogenerated electron–holes and a narrow range of light-harvesting. In fact, the spectral response range of BiOCl reported is no more than 450 nm, which cannot effectively absorb visible light, and limits its potential applications as a visible photocatalyst.15,16 Recently, organic semiconductors, such as perylenediimide (PDI),17,18 porphyrin (PP),19–21 diketopyrrolopyrrole (DPP),22–24 and perylene monoimide (PMI),25 which are only composed of C, H, O and N without any metals, have been precisely designed by π–π interactions and hydrogen bonds. With regard to PDI, the microstructures of organic semiconductors are diverse due to their controllable molecular structures. Moreover, the electronic structures of organic semiconductors are readily tunable and the valence band potential is deeper, so they have strong oxidation and mineralization abilities. Meanwhile, most organic semiconductors exhibit a broad range of light absorption covering the ultraviolet and visible light region. Namely, they have the potential for visible photocatalytic performance. In previous works, our team designed and constructed a variety of organic semiconductors, and explored their applications in environmental purification and energy utilization.26–28 However, those organic semiconductors also had an inevitable disadvantage of relatively poor crystallinity.29–31 In addition, because of the existence of the large conjugated benzene ring in the PDI structure, their solubility in most solvents was relatively low, especially in aqueous media, which limits their application. So, to broaden the light-harvesting range of Bi-based materials and improve the crystallinity of organic semiconductors, hybrid inorganic–organic composites were fabricated, which could not only effectively complement each other's shortcomings, but also promote the dissociation of excitons and improve the separation efficiency of photogenerated carriers, thus improving the photocatalytic activity.32,33 Although much research has been done on inorganic–organic composite photocatalysts,34,35 there are some aspects that remain unexplored. For instance, the mechanism of photocatalytic enhancement in inorganic–organic composites remains unclear. Specifically, the combination mode, charge transfer path, and interface interaction of the inorganic–organic composites have not been fully clarified.

Herein, an inorganic–organic hybrid heterojunction PDI/BiOCl photocatalyst was prepared by a co-deposition method. The combination mode of inorganic–organic hybrid composites was clarified by experimental and theoretical calculations. BiOCl particles participated in the self-assembly of PDI. The composite had remarkable UV-vis light-harvesting ability, good thermal stability and excellent carrier mobility. Compared with BiOCl and self-assembled PDI, the ability of PDI/BiOCl to remove organic pollutants was significantly improved.

2. Experimental section

2.1 Preparation

2.1.1 Preparation of BiOCl. 1 mmol Bi (NO3)35H2O, 1 mmol NH4Cl and 0.4095 g PVP were dissolved in 50 mL distilled water. The pH of the resulting solution was adjusted to 11.0 by a 2 M NaOH solution. Subsequently, the solution was magnetically stirred for 1 h at room temperature and transferred to a 100 mL Teflon-sealed autoclave with a filling volume of 70%. Afterwards, the autoclave was heated at 160 °C for 14 h. The yellow precipitate was washed repeatedly with distilled water and dried under vacuum at 70 °C for 8 h. Then, the BiOCl sample was obtained.
2.1.2 Preparation of PDI. Perylene diimide (PDI) was prepared by following previous work.26,27 First, 2.5205 g 3,4,9,10-perylenetetrahydride (PDTA), 5.0387 g β-alanine and 40 g imidazole were heated at 140 °C for 6 h under an argon atmosphere. Afterwards, the reactant was washed with 10 mL ethanol and 300 mL 2 M HCl. Then, PDI was obtained by washing with distilled water and dried under vacuum at 70 °C for 8 h.
2.1.3 Preparation of PDI/BiOCl. A stock solution was prepared by dispersing 0.54 g PDI in 200 mL distilled water and adding 834 μL trimethylamine (TEA) (1 mL 2 M NaOH solution). The solution was magnetically stirred for 30 min. Subsequently, different ratios of BiOCl were added to the solution under continuous stirring. Afterwards, the solution was heated in a water bath at 60 °C, and simultaneously 25 mL 4 M HCl was added while stirring for 1 h. The sample was centrifuged and washed till neutral and dried under vacuum at 70 °C for 8 h. The amounts of BiOCl were wBiOCl:wPDI = 0.1, 0.3, 0.5, 0.7, 0.9, 0, the sample was named as 10% BiOCl, 30% BiOCl, 50% BiOCl, 70% BiOCl, 90% BiOCl, and PDISA.

2.2 Characterization

A SmartLab X-ray diffractometer (Rigaku, Japan) was used to determine the phase composition of the samples under 40 kV, 30 mA and Kα radiation of nickel-filtered copper (λ = 0.15418 nm). UV-vis diffuse reflectance spectra (UV-Vis-DRS) and UV-vis absorption spectra of the samples were collected on a U-3900 UV-vis spectrophotometer (Hitachi, Japan) with BaSO4 as the reference. Photoelectrochemical properties of the samples were collected at a CHI660D electrochemical workstation (Chenhua, China). The surface elemental composition of the samples was analyzed by a X-Max 20 X-ray energy spectrometer (Oxford, British). The binding energy of the elements in samples was measured by a PHI Quantera X-ray photoelectron spectrometer (PHI, American), Al-Kα (1486.6 eV) was used as the excitation source and the binding energy (284.6 eV) of C 1s was used as the standard for error correction. The surface morphology of the samples was observed by a SU-8010 field emission scanning electron microscope (Hitachi, Japan). The morphology and micro-structure of the samples were determined by a JEOL-2100 transmission electron microscope (JEOL, Japan). The fluorescence excitation emission spectra of the samples were measured by a FS5 fluorescence spectrophotometer (Edinburgh, British). The infrared absorption spectra of the samples were collected using a V70 Fourier transform infrared spectrometer (Bruker, Germany). The thermogravimetric curves of the samples were measured by a TGA/DSC1 simultaneous thermal analyzer (Mettler Toledo, Switzerland). The zeta potential of the samples was measured by a SZ-100 Zeta potential analyzer (Horiba, Japan). The active radicals of the samples were determined by electron paramagnetic resonance (ESR) spectroscopy (JES-FA200, Japan). The surface photovoltage (SPV) measurements were carried out with a 500 W xenon lamp, lock-in amplifier and photovoltaic cell. The monochromatic light was provided by a double-prism monochromator. The total organic carbon (TOC) was measured by a Muti N/C 2100 total organic carbon/total nitrogen analyser.

2.3 The evaluation of photocatalytic activity

The photocatalytic activity was evaluated by a XPA-7 photo-reactor at room temperature and pressure. 20 mg of photocatalyst was placed in a quartz tube containing 20 mL of a 5 mg L−1 phenol aqueous solution. 300 W xenon lamps were used as the light source. The concentration of phenol was determined by LC-20A high performance liquid chromatography.

3. Result and discussion

3.1 Controllable synthesis of PDI/BiOCl

To elucidate the micro-structure of inorganic–organic hybrid heterojunction PDI/BiOCl and illustrate the assembly pathway of BiOCl and PDI, TEM was used to characterize and analyze the micro-morphologies of BiOCl, PDISA and PDI/BiOCl. From Fig. 1a and b, PDISA with an irregular sheet structure was assembled with rod structured BiOCl to form inorganic–organic hybrid composites. The inorganic–organic hybrids could also be clearly seen from HR-TEM images (Fig. 1c and d). The ordered lattice structure was observed in the region marked by red dotted lines, and the lattice fringes are clear. Consequently, it could be inferred that the region was located by the rod-like BiOCl. However, a disordered lattice structure was observed outside this region, which was the sheet structure PDISA. It could be concluded that BiOCl participated in the self-assembly of PDI. Furthermore, the average zeta potential of BiOCl and PDI was around 24.1 eV and −52.9 mV, respectively (Fig. 2), indicating that the surface of BiOCl was positively charged, and the surface of PDI was negatively charged. As a result, it was easy for the positively charged BiOCl to surround the surface of PDI via electrostatic interactions, which facilitated the anchoring of PDI on the surface of the rod-like BiOCl. The average zeta potential of 30% BiOCl was around −28.3 mV, which is more positive than that of PDI, indicating that BiOCl successfully anchored on the surface of the rod-like BiOCl to some extent with the O atoms bonding to neighboring Bi atoms.36 This structure not only improved the crystallinity of the organic photocatalyst, but also favored the formation of an interface for carrier transport and migration.37 Therefore, photogenerated electrons and holes were easily transported at the inorganic–organic hybrid interface, which improved the separation efficiency of the photogenerated carriers.
image file: c9cy01722d-f1.tif
Fig. 1 (a and b) TEM images of 30% BiOCl, (c and d) HR-TEM images of 30% BiOCl. The region marked by red dotted lines was rod-like BiOCl with an ordered lattice structure, while outside this region was sheet structure PDISA with a disordered lattice structure.

image file: c9cy01722d-f2.tif
Fig. 2 Zeta potential of BiOCl (a), PDISA (b), 30% BiOCl (c).

Generally, macrostructures with π–π stacking enhance the photocatalytic activity of PDI-based materials by improving the electron delocalization and transfer. The XPS spectra of the samples are shown in Fig. 3 and S3. As shown in Fig. 3c and S3d, the peak at 289.0 eV can be attributed to π electron excitation, indicating the self-assembly structure of π–π stacking. It is worth mentioning that the binding energy of Bi4f of 30% BiOCl shifted slightly to higher binding energies in comparison with that of BiOCl.38,39 This was due to the change in the chemical environment from the formation of the inorganic–organic hybrid PDI/BiOCl composites. Upon interaction with other elements, the outer electron density of Bi in BiOCl decreased, the shielding effect weakened, and thus the binding energy increased, which favored the efficient transfer of carriers. Comparing the O 1s, C 1s and N 1s spectra of PDISA and 30% BiOCl allowed us to observe a similar trend. The shift to higher binding energies indicated that the change in the outer electron density was due to the strong interaction between PDI and BiOCl.26,37,38,40,41


image file: c9cy01722d-f3.tif
Fig. 3 HR-XPS spectra of (a) Bi of BiOCl and 30% BiOCl; and (b) O, (c) C, (d) N of PDISA and 30% BiOCl.

From the XRD patterns (Fig. 4a and S4), the peak at 2θ = 25.8° implied that spatial diffraction occurred due to the π–π stacking spacing (0.32–0.35 nm).31 The characteristic diffraction peaks of BiOCl were not obvious when the amount of BiOCl was small. The intensity of the PDISA diffraction peak increased as the amount of BiOCl increased (less than 30%), indicating that BiOCl could strengthen the self-assembly of PDI. However, when BiOCl amount was more than 30%, the intensity of the PDISA diffraction peak decreased with increasing BiOCl, but the diffraction peak at 2θ = 25.8° still existed, indicating that the self-assembled structure is present in PDI. This might be due to the poor crystallinity of PDISA, which interfered with the detection of the diffraction peak of PDI. Moreover, the intensity of the diffraction peak of 30% PDI/BiOCl at 2θ = 25.8° increased, which indicated that it was due to the strong π–π stacking interactions. Furthermore, the diffraction peak at 2θ = 25.8° in PDI/BiOCl shifted slightly to larger angles as compared to that of PDISA, indicating that PDI/BiOCl possessed a smaller d distance of the π–π stacking, which could be attributed to the interface interaction between PDI and BiOCl.31,42 When the BiOCl amount was more than 30%, the characteristic diffraction peaks of BiOCl began to appear at 2θ = 33.8°. With increasing BiOCl, the other characteristic diffraction peaks were obvious and the intensity was enhanced. These phenomena implied that BiOCl particles were involved in the self-assembly of PDI. The existence of π–π stacking was further confirmed by Raman measurements of the samples (Fig. 4b). The peaks located at 1291 cm−1 and 1577 cm−1 were ascribed to the C–H in-plane bending and the stretching vibration of C[double bond, length as m-dash]C, which implied that π–π stacking exists in both PDISA and PDI/BiOCl. However, compared to PDISA, the Raman peaks at 1291 cm−1, 1374 cm−1 and 1577 cm−1 of PDI/BiOCl underwent slight shifts, indicating the interaction between PDI and BiOCl.26 The chemical bond function was of great significance for the carrier transfer and the stability of the hybrid PDI/BiOCl composites. The hybrid structure of the chemical anchoring of PDI to BiOCl was also determined by UV-vis absorption measurements (Fig. 4c). The absorption peaks of BiOCl located at 229 nm were ascribed to BiOCl nanorods. The absorption peaks of PDISA and 30% BiOCl located at 499 nm and 539 nm, respectively, were assigned to the electronic transitions of π–π stacking in PDISA and PDI/BiOCl.43,44 However, the absorption maximum peak of 30% BiOCl at 216 nm revealed an apparent blue shift in comparison with that of BiOCl, which was due to the anchoring of carboxylic acid end groups of PDI on the surface of BiOCl.


image file: c9cy01722d-f4.tif
Fig. 4 (a) XRD patterns, (b) Raman spectra, (c) UV-vis absorption spectra of BiOCl, PDISA, and 30% BiOCl (0.1 μM, sample in H2O solution).

The thermal stability of as-prepared samples was evaluated by thermogravimetric analysis (TGA). From the TGA spectra of BiOCl, PDISA and PDI/BiOCl (Fig. 5a), at 400 °C, the weight loss was 3%, 25%, and 16% for pure BiOCl, PDISA, and 30% BiOCl, respectively. Therefore, the addition of BiOCl can significantly improve the thermal stability and crystallinity of PDISA. It was beneficial for the fabrication of the hybrid phase interface favored in the efficient carrier transfer. Furthermore, the weight loss of all the as-prepared samples was about 2–5% from the evaporation of adsorbed water at room temperature to 300 °C. As shown in Fig. 5b, the peaks located at 1677 cm−1 and 1585 cm−1 belonged to the stretching vibration of the C[double bond, length as m-dash]C bond in the benzene ring. The peaks at 1337 cm−1 and 1237 cm−1 corresponded to the stretching vibration of C–N or protonated C–N in PDISA. The peaks at 1160 cm−1 and 1028 cm−1 could be assigned to the in-plane bending and out-of-plane bending of the C–H bond, respectively, indicating the existence of π–π stacking.45 Due to the increase of PDI amount, the intensities of the main characteristic peak increased. As shown in Fig. 5c, all samples could harvest light in the ultraviolet region and part of the visible light region and there were obvious light absorption boundaries caused by the band gap transitions. BiOCl could absorb light in the ultraviolet region and under 536 nm visible light, while PDISA exhibited a remarkable light-harvesting range over the ultraviolet and visible region. Compared to BiOCl, the visible light absorption ability of PDI/BiOCl was improved significantly.


image file: c9cy01722d-f5.tif
Fig. 5 (a) TGA of BiOCl, PDISA and PDI/BiOCl, (b) FT-IR spectra, (c) UV-vis DRS of as-prepared samples.

3.2 Photocatalytic oxidation performances

The full spectrum photocatalytic activity of BiOCl, PDISA and 30% BiOCl is shown in Fig. 6c. From Fig. 6c, the removal rates of phenol over 30% BiOCl were respectively higher than that of BiOCl and PDISA. After irradiation for 180 min, the degradation rate of phenol over 30% BiOCl was 87%, which was 2.2 times than that of BiOCl, and 1.6 times than that of PDISA. The kinetics of the photocatalytic degradation of phenol by BiOCl, PDISA and 30% BiOCl is shown in Fig. 6d. The apparent reaction rate constant of BiOCl was 0.0035, 0.0043 for PDISA, and 0.011 for 30% BiOCl. This indicated that the apparent reaction rate constant of 30% BiOCl was 3.1 and 2.6 times greater than that of BiOCl and PDISA, respectively. The photocatalytic performance under ultraviolet light (UV light) of BiOCl, PDISA and 30% BiOCl is shown in Fig. 6a. Evidently, 30% BiOCl exhibited remarkable UV light photocatalytic properties and the apparent reaction rate constant was 1.8 and 2.6 times greater than that of BiOCl and PDISA (Fig. 6d), respectively. The same phenomenon was also observed in the photocatalytic degradation of phenol under visible light (λ > 420 nm) over BiOCl, PDISA and 30% BiOCl (Fig. 6b). The apparent reaction rate constant of 30% BiOCl was 1.8 and 1.6 times greater than that of BiOCl and PDISA (Fig. 6d).
image file: c9cy01722d-f6.tif
Fig. 6 The degradation effect of phenol over BiOCl, PDISA and PDI/BiOCl under (a) ultraviolet irradiation, (b) visible light irradiation and (c) full spectrum irradiation; (d) apparent reaction rate constant over BiOCl, PDISA and PDI/BiOCl.

From Fig. 7a, the degradation rate was remarkable during the recycling process. With increasing recycling times, the degradation rate decreased slightly, but it was not obvious. After 3 uses, the degradation rate still reached more than 80%. This was owing to the uniform dispersion of PDI/BiOCl in the aqueous solution during the photocatalytic process; thus, the high activity of ultrafine 30% BiOCl was lost in the recovery process, which led to the decline of the degradation rate. Fig. 7b and c are the XRD and FT-IR spectra of 30% BiOCl before and after use, respectively. The positions of the main characteristic diffraction peaks and characteristic infrared peaks of 30% BiOCl had not changed after the reaction, which indicated that there were no changes in the crystalline phase and composition after the reaction took place.


image file: c9cy01722d-f7.tif
Fig. 7 (a) Effect of recycling use of 30% BiOCl for phenol degradation, (b) XRD patterns and (c) FT-IR patterns of before-and-after 30% BiOCl.

3.3 Enhanced interfacial charge mobility

The excellent photocatalytic activity of 30% BiOCl was attributed to the effective interfacial charge separation. As shown in Fig. 8a, under the electron-pull effect of the conjugated structure of PDI, the free charge carriers were generated by the transfer of the photoinduced electron from the organic donor (PDI) to the inorganic acceptor (BiOCl) across the interface. According to the theoretical calculation of the total SCF density (Fig. 8b), the carboxylic acid groups in PDI had significant negative electron distributions, while BiOCl presented positive electron distributions. Therefore, the strong interaction between BiOCl and PDI preferentially formed. The effective interfacial charge transfer could be confirmed by the photoelectrochemical properties. The assumption of the interfacial carrier transport pathway could be supported by the SPV measurements. As shown in Fig. 8c, under irradiated light of 300–800 nm, BiOCl presents a negative SPV signal; nevertheless, PDISA possessed a positive SPV signal. It demonstrated that the main carriers of BiOCl and PDISA were photogenerated electrons and photogenerated holes, respectively.46 When the hybrid PDI/BiOCl composites were formed, under light irradiation, the photoinduced electrons were injected into BiOCl from PDI, and the photogenerated holes were accumulated in PDI.
image file: c9cy01722d-f8.tif
Fig. 8 (a) Schematic graph of the charge transfer process in hybrid PDI/BiOCl, (b) the electron density from total SCF density of PDI/BiOCl (carried out with the G09 program), (c) surface photovoltage spectrum of the as-prepared samples, (d) transient photocurrent responses of the as-prepared samples with an on/off 300 W xenon lamp, (e) photoluminescence (PL) spectra of the as-prepared samples with an excitation wavelength of 220 nm, (f) Nyquist plots of the as-prepared samples with a 300 W xenon lamp.

As for the charge transfer strength, the transient photocurrent, photoluminescence and photoimpedance of the as-prepared samples were measured. The transient photocurrent response curves of BiOCl, PDISA and 30% BiOCl are shown in Fig. 8d. The photocurrent intensity of 30% BiOCl was 4.17 μA cm−2, which was 11.27 times higher than that of pure BiOCl (0.37 μA cm−2), and 5.58 times higher than that of PDISA (0.76 μA cm−2). It might be due to the “donor to acceptor” nature of PDI and BiOCl that was beneficial for the separation of photogenerated carriers. Fluorescence photoluminescence (PL) spectra could be also used to measure the separation efficiency of photogenerated carriers. The higher PL intensity implied the lower separation efficiency of electron–hole pairs. Fig. 8e shows the PL spectra of BiOCl, PDISA and PDI/BiOCl. The fluorescence emission peak was observed at 350 nm, which was caused by the bandgap transition and intrinsic luminescence of the sample. However, 30% BiOCl had a lower intensity PL peak, which indicated that 30% BiOCl had a higher separation efficiency of photogenerated carriers. The combination of BiOCl and PDISA could effectively inhibit the recombination of photogenerated carriers and improve the photocatalytic performance. The electrochemical impedance spectroscopy (Nyquist fitting curve) of the sample is shown in Fig. 8f. The arc radius of the Nyquist curve of 30% BiOCl was obviously smaller, indicating that the charge transfer resistance of 30% BiOCl was smaller and the charge transfer efficiency had improved.

3.4 The photocatalytic mechanism

The role of active radicals was explored by adding ammonium oxalate (TEOA), benzoquinone (BQ) and isopropanol (IPA) to a 5 mg L−1 phenol solution, which is used to capture h+, superoxide radical ˙O2 and hydroxyl radical ˙OH, respectively. As shown in Fig. 9a, after adding TEOA, BQ and IPA and irradiating for 180 min, the degradation rate of phenol decreased from 87% to 32%, 28% and 20%, respectively. Namely, upon the addition of radical scavengers, the degradation rate of phenol decreased significantly. This indicated that the main active radicals were ˙OH and O2 in the photocatalytic oxidative degradation. This result was further confirmed by the detection of ˙O2 and ˙OH with ESR measurements. As shown in Fig. 9b, there were no ESR signals of ˙OH in the dark for 30% BiOCl; nevertheless, an obvious ESR signal of ˙OH was observed after light illumination, and the signal intensity increased with the continuous light illumination, indicating that ˙OH was generated in the photocatalytic process. Simultaneously, according to Fig. 9c, the same phenomenon was found in the ESR signals of ˙O2, demonstrating that ˙O2 was also a dominant active species. It agreed with the results of the active radical trapping experiment, in which ˙OH, O2 and h+ were the main active species in the photocatalytic degradation.
image file: c9cy01722d-f9.tif
Fig. 9 (a) Effect of radical scavengers on the photocatalytic activity of PDI/BiOCl, ESR spectra for DMPO–˙OH (b) and DMPO–˙O2 (c) of 30% BiOCl under the irradiation of full-spectrum light.

Based on the tangent intercept of the Mott–Schottky curve, the flat-band potential (Efb) of BiOCl and PDI could be estimated to be −0.21 eV and −0.25 eV, respectively47,48 (Fig. 10c and d). However, BiOCl and PDISA are n-type semiconductors because of the positive value of the Mott–Schottky curves of BiOCl and PDISA. The Efb (vs. NHE) of an n-type semiconductor was about 0.2 eV higher than that of ECBM.49 Consequently, the ECB of BiOCl and PDISA could be calculated as −0.41 eV and −0.45 eV, respectively. According to the UV-vis DRS, the band gap energy (eV) can be calculated to be 2.55 eV and 1.72 eV for BiOCl and PDISA by the equation Ahν = c(Eg)n from the diffuse reflection spectral data50 (Fig. 10a and b). Therefore, the conduction bands of PDISA and BiOCl were more negative than the generation potential of O2/˙O2 (−0.33 V vs. NHE), indicating that PDI/BiOCl had the ability to generate ˙O2 in the photocatalytic process. Meanwhile, the valence band of BiOCl was more positive than the redox potential of OH/˙OH (1.99 V vs. NHE), indicating that PDI/BiOCl could oxidize water to produce ˙OH. It was consistent with the results of the active radical trapping experiment and ESR measurements. Because the conduction band energy of PDISA was more negative than that of BiOCl, and the valence band energy of BiOCl was more positive than that of PDISA, there are two types of possible patterns (Fig. 11) of separation and transfer of photogenerated carriers: type II heterojunction and direction Z-scheme system. As shown in Fig. 11a, since the EVB of PDISA was more negative than E0(OH/˙OH), the holes enriched in the VB of PDISA could not reduce OH to produce ˙OH. Thus, the carrier transfer mode of a type II heterojunction was probably not in effect. However, the VB of BiOCl was more positive than E0(OH/˙OH); consequently, the direction Z-scheme system (Fig. 11b) was likely to be energetically feasible. This assumption was in accordance with the active species capturing experiments that showed ˙O2 and ˙OH were the active species in the photocatalytic degradation. The direction Z-scheme system not only increased the quantity of carriers, but also extended the transmission path of carriers. Thus, the photocatalytic activity was improved by isolating the photogenerated electron–hole pairs and inhibiting the recombination of the photogenerated carriers. Simultaneously, the staggered energy band structure enabled the holes of the valence band of BiOCl to inject into the conduction band of PDISA, which also was consistent with the “donor to acceptor” nature of the hybrid PDI/BiOCl composites.


image file: c9cy01722d-f10.tif
Fig. 10 (a) UV-vis DRS of as-prepared samples, (b) the plots of (Ahv)2vs. Eg of as-prepared samples, Mott–Schottky plots of (c) PDISA and (d) BiOCl.

image file: c9cy01722d-f11.tif
Fig. 11 The photocatalytic degradation mechanism over PDI/BiOCl (a) type II heterojunction, (b) Z-scheme system.

4. Conclusion

In summary, hybrid PDI/BiOCl composites were prepared by a deposition method with BiOCl and PDI. PDI/BiOCl was formed by the structures of sheet PDI and rod BiOCl through the successful anchoring of PDI on the surface of rod-like BiOCl via the O–Bi bond. Benefiting from improved thermal stability, significant crystallinity, remarkable full spectrum light-harvesting, and enhanced interfacial charge separation, PDI/BiOCl composites were able to more efficiently remove organic pollutants under full spectrum light, which was 2.2 times and 1.6 times greater than that of BiOCl and PDI for the photocatalytic degradation of phenol, respectively. The main active radicals in the full-wavelength photocatalytic degradation were ˙OH, O2 and h+, which agreed with the energy band alignment of the hybrid PDI/BiOCl composites formed with BiOCl and self-assembled PDI.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has received the finical support from the National Natural Science Foundation of China (21766039, 51962023), the project of Graduate Student Office of Yan'an University (YDYJG2018021), and the graduate Education Innovation Project of Yan'an University (YCX201997).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cy01722d

This journal is © The Royal Society of Chemistry 2020