Toheed Ahmed*abc,
Muhammad Ammar*cd,
Aimen Saleeme,
Hong-ling Zhangb and
Hong-bin Xub
aDepartment of Applied Chemistry, Government College University, Faisalabad 38000, Pakistan. E-mail: toheedahmed@gcuf.edu.pk
bKey Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
cUniversity of Chinese Academy of Sciences, Beijing 100049, China
dDepartment of Chemical Engineering Technology, Government College University, Faisalabad 38000, Pakistan. E-mail: mammar@gcuf.edu.pk; Fax: +92419203027; Tel: +92419203027
eBiotechnology and Fermentation Group, Department of Animal Sciences, The Ohio State University, QARDC, Wooster, Ohio 44691, USA
First published on 20th January 2020
For economical water splitting and degradation of toxic organic dyes, the development of inexpensive, efficient, and stable photocatalysts capable of harvesting visible light is essential. In this study, we designed a model system by grafting graphitic carbon nitride (g-C3N4) (g-CN) nanosheets on the surface of 2D monoclinic bismuth vanadate (m-BiVO4) nanoplates by a simple hydrothermal method. This as-synthesized photocatalyst has well-dispersed g-CN nanosheets on the surface of the nanoplates of m-BiVO4, thus forming a heterojunction with a high specific surface area. The degradation rate for bromophenol blue (BPB) shown by BiVO4/g-CN is 96% and that for methylene blue (MB) is 98% within 1 h and 25 min, respectively. The 2D BiVO4/g-CN heterostructure system also shows outstanding durability and retains up to ∼95% degradation efficiency for the MB dye even after eight consecutive cycles; the degradation efficiency for BPB does not change too much after eight consecutive cycles as well. The enhanced photocatalytic activities of BiVO4/g-CN are attributed to the larger surface area, larger number of surface active sites, fast charge transfer and improved separation of photogenerated charge carriers. We proposed a mechanism for the improved photocatalytic performance of the Z-scheme photocatalytic system. The present work gives a good example for the development of a novel Z-scheme heterojunction with good stability and high photocatalytic activity for toxic organic dye degradation and water splitting applications.
Among the photo-driven semiconductors, m-BiVO4 has attracted more attention due to nontoxicity, relatively high photocatalytic activity and chemical stability for the degradation of organic compounds and water splitting,13–15 but BiVO4 alone cannot degrade dyes and split water efficiently because its conduction band is located at a more positive potential than the potential of water reduction [0 eV vs. NHE; H+/H2].
In order to solve this problem, many strategies such as doping with nonmetallic elements,16 combined with graphene17 and constructing heterojunctions with two semiconductors, have been employed.18 Among the variety of photocatalytic systems, the direct solid-state Z-scheme system has attracted considerable attention because it not only enhances the spatial separation efficiency of photo-induced electron–hole pairs but also reduces the undesirable backward reaction of the photocatalytic process due to two different redox sites.19,20 Furthermore, maximum overpotentials can be achieved with this unique Z-scheme system, which is beneficial for the effective utilization of a high conduction band from a semiconductor and a low valence band from another semiconductor.21–23 Li et al. synthesized a Z-scheme BiVO4/g-CN photocatalytic system by thermal annealing and hydrothermal method.24 The development of a reliable and efficient methodology to obtain reusable and stable heterostructure photocatalysts with optimized activity is still a great challenge. Based on the afore-mentioned results, controllable surface coverage is significant in the Z-scheme system to achieve highly efficient photocatalysts. So far, there are few reports on the construction of m-BiVO4-based Z-scheme ternary heterostructures for the photodegradation of bromophenol blue (BPB).
Herein, we demonstrate the construction of an efficient 2D heterojunction BiVO4/g-CN photocatalyst model system with 2D-m-BiVO4 nanoplates covered by discrete g-CN nanosheets with controllable surface coverage. To construct the heterojunction photocatalyst, g-CN and m-BiVO4 were selected with the following considerations: primarily, both g-CN and BiVO4 have been proven to be promising visible-light photocatalysts with desirable chemical stability. Second, the proper energy-band alignments at the heterojunction interface are crucial and beneficial for the light-induced separation of charge carriers in the as-synthesized heterojunction photocatalyst system. Predominantly, the heterojunction structures of BiVO4/g-CN can be simply modified to attain a controllable coverage of g-CN on the surface of BiVO4 by the hydrothermal process. These features provide us with a good platform to get insights into the significance of heterostructure engineering for fabricating heterojunction photocatalysts.
In order to investigate the surface chemistry and the presence of various groups in the prepared samples, the relative FTIR spectra are displayed in Fig. 1(b). For pure BiVO4, a peak at 740 cm−1 can be attributed to the ν3 asymmetric stretching vibration of the VO4 unit, while the other peak at 843 cm−1 can be identified as the ν1 symmetric stretching vibration of the VO4 unit.30 For pure g-CN, the typical FTIR spectrum of g-CN can be observed. A series of peaks in the region of 1200–1800 cm−1 belong to the typical stretching modes of heterocyclic CN. For example, the peak locates at 807 cm−1 corresponds to the out-of-plane bending vibration of the g-CN triazine unit.31,32 The FTIR spectra of all the BiVO4/g-CN nanocomposites contain almost all characteristic peaks of BiVO4 and g-CN, indicating the co-existence of both structures. By increasing the amount of g-CN, the peak intensities of g-CN increased for the BiVO4/g-CN nanocomposite samples.
Fig. 2(a) depicts the morphology of the g-CN nanosheets having ∼5 nm thicknesses. The pristine BiVO4 sample exhibits homogeneous nanoplate morphology (Fig. 2(b)). The average diameter of the nanoplates was ∼100 nm with an average thickness of ∼15 nm. With the increase in the initial amount of g-CN from 3 to 6 mg in the BiVO4/g-CN composite, the nanoplates became thinner and smaller in diameter. Numerous nanosheets can be clearly seen covering the BiVO4 nanoplates, which may be due to g-CN (Fig. 2(c and d)). The particle size of BiVO4 is much smaller and the particles are distributed between the nanosheets of g-CN, inferring that the agglomeration of BiVO4 is well prevented by the nearby g-CN nanosheets. To confirm the phase distribution and the network morphologies, a high-resolution transmission electron microscopy (HRTEM) image of the BiVO4/g-CN (6 mg) composites is shown in Fig. 2(e). It is apparent that the phase with a nanoplate shape shows a high degree of crystallinity for a pure lattice m-BiVO4 phase. Conversely, the g-CN nanosheets covering the BiVO4 networks showed low crystallinity with fuzzy lattices. It is noteworthy that the two phases are in contact with each other through a well-defined sharp interface boundary, which indicates the formation of a high-quality BiVO4/g-CN heterojunction structure. Specifically, g-CN formed a continuous sheet on the surface of BiVO4 with a thickness from 7 nm to 5 nm. Furthermore, the scanning transmission electron microscopy (STEM) elemental mapping confirmed the discrete dispersion of the g-CN phase on the BiVO4 surface (Fig. 2(g–k)).
Moreover, Fig. 2(f) and HRTEM image (Fig. S1 in ESI†) prove that the top surface of BiVO4 corresponds to the (121) facet. Fig. S1† displays that the nanosheets and nanoplates attached to each other, with the (002) facets of g-CN and the (121) facets of BiVO4, indicating that the coupling between g-CN and BiVO4 may happen on the (002) facets of g-CN and the (121) facets of BiVO4.
The BET surface areas of the pure BiVO4 and BiVO4/g-CN (6 mg) composite samples were investigated by nitrogen (N2) adsorption. The inset in Fig. S2† shows the N2 sorption isotherms and the corresponding distribution of pore size curves for the BiVO4/g-CN (6 mg) composite and pure BiVO4. The N2 adsorption–desorption isotherms for the afore-mentioned samples exhibit type-IV BDDT (Brunauer–Deming–Deming–Teller) classification, which indicates the existence of mesopores (widths of 24 and 32 nm). These isotherms showed H3 hysteresis loops related to the mesopores existing in aggregates composed of primary particles. In addition, the adsorption branch of the N2 isotherms firmly increased when P/P0 proceeded towards unity, indicating the formation of small macropores and large mesopores. Very broad pore size distributions of samples (inset in Fig. S2†) indicate the existence of both macropores and mesopores. The BiVO4/g-CN (6 mg) sample has a larger surface area than that of pure BiVO4 (Table S1†). The larger surface area of the composite is due to the existence of g-CN (∼31 m2 g−1) in the composite. It can be concluded that the specific surface area increase on increasing the amount of g-CN contributes in enhancing the photocatalytic performance of the composites.
The electronic interactions between BiVO4 and g-CN in the hybrid and the chemical states of the elements were studied by using X-ray photoelectron spectroscopy (XPS). Fig. S3(a and b)† demonstrate the survey spectra of pure BiVO4 and the BiVO4/g-CN (6 mg) composites, demonstrating the presence of all the elements of g-CN and BiVO4 in the composite material. The characteristic peak for NC–N2 in g-CN is observed at 288.2 eV in the C 1s spectrum. The N 1s (Fig. S3(c)†) spectrum was deconvoluted into three typical peaks at 398.3, 399.2 and 400.7 eV due to the N 1s core level in the C–NC bond, the bridging N atoms bonded to three C atoms (N–[C]3) and the N atoms in the C–NH bonds, respectively. The C 1s peaks (Fig. S3(d)†) at 284.8 eV and 288.6 eV can be attributed to the adventitious carbon on the surface of g-CN.33 These results further confirmed that both BiVO4 and g-CN exist in the composite.
Fig. 3 presents the diffuse reflectance spectra in the range of 400–700 nm for all samples. By increasing the amount of g-CN, the absorption of the BiVO4/g-CN samples in the visible light region probably the same. The steep shape of the curves indicated that the band gap transition was responsible for the visible light absorption. The optical absorption near the band edge is calculated using the equation αhν = A(hν − Eg)n, where A is a constant, α is the absorption coefficient, Eg represents the band gap, ν is the light frequency and the value of n depends on the transition characteristics of the semiconductor (n = 2 for indirect transition and n = ½ for direct transition). For our system, m-BiVO4, the value of n is 2.34 The plot of photon energy (αhν)1/2 versus hν for m-BiVO4 and the BiVO4/g-CN composite yields a straight line (inset in Fig. 3) and the intercept of the tangent to the X-axis used for the approximation of its band gap. As listed in Table 1, the approximate Eg values of m-BiVO4 and the BiVO4/g-CN composite vary from 2.22 eV to 2.28 eV. BiVO4/g-CN has a low Eg value and higher absorption, which is beneficial for enhancing its photocatalytic degradation performance. On the contrary, the band gap of pure g-CN exhibits a higher value of 2.78 eV, and it is not very efficient under visible light due to the high band gap value.
Fig. 3 UV-visible spectra of the prepared samples and the corresponding plots of (αhν)1/2 versus photon energy (hν). |
Sample | Temperature | Phasea | Surface area (m2 g−1) | Degradation (%) MB | Degradation (%) BPB | k (min−1) MB | k (min−1) BPB |
---|---|---|---|---|---|---|---|
a M = monoclinic, k = rate constant. | |||||||
BiVO4 | 100 °C | M | 11 | 82 | 85 | 0.028 | 0.032 |
g-CN | 100 °C | M | 31 | 40 | 15 | 0.011 | 0.002 |
BiVO4/g-CN (3 mg) | 100 °C | M | 19 | 84 | 86 | 0.042 | 0.033 |
BiVO4/g-CN (6 mg) | 100 °C | M | 28 | 98 | 95 | 0.128 | 0.048 |
In order to further depict the photocatalytic reaction, the photocatalytic degradation route of MB and BMB was fitted to a pseudo-first-order kinetics model ln(C0/C) = kt (Fig. 4(b) and 5(b)), where k is the reaction rate constant. The value of k, which is equal to the corresponding slope of the fitting line, was calculated and listed in Table 1. The value of k for an efficient photocatalyst is usually high. A sharp increase in k was observed on increasing the amount of g-CN present in the BiVO4/g-CN composite. When the amount of g-CN reached up to 6 mg in the BiVO4/g-CN composite, it exhibited the maximum photocatalytic activity. The rate constant of the BiVO4/g-CN (6 mg) composite was 0.128224 min−1 for MB, which was 4.5-fold higher than that of pure BiVO4; for BMB, the rate constant was 2.02 min−1, which was 1.5-fold higher than that of pure BiVO4. The interaction with the g-CN nanosheet layers formed in the presence of the BiVO4 nanoplates at the interface generated sufficient nano-junctions to hinder the re-stacking of the 2-D nanosheets. Therefore, the enlarged specific surface area of the as-obtained composite due to better surface coverage not only allowed access for the molecules of the dye to the surface but also promoted the transfer of surface carriers across the interfaces. Combined with the above-mentioned photo-electron chemical and optical analyses, it can be deduced that the efficiency enhancement in the photocatalytic degradation performance can be attributed to the better surface coverage of g-CN on the surface of BiVO4, improved charge separation by the well-matched band structure and enlarged specific surface area by the well contacted interface.
The pollutant concentration is a very important parameter in wastewater treatment. The effects of various initial dye concentrations (MB and BPB) on photocatalytic decolorization investigated from 5 to 15 mg L−1 against fixed amounts (0.1 g) of BiVO4 and BiVO4/g-CN (6 mg) were recorded and shown in Fig. 4(c) and 5(c), respectively. By the successive increment in the concentration, the degradation time increased linearly. However, superior degradation up to 96% could still be achieved in about 1 hour for up to 15 mg L−1 of MB by using the BiVO4/g-CN (6 mg) photocatalyst and for up to 15 mg L−1 of BPB, 94% degradation was achieved in about 3 hours using the same photocatalyst.
The reusability and stability of the BiVO4 and BiVO4/g-CN photocatalysts were assessed by recycling photodegradation experiments and the results for the degradation of MB and BMB over different photocatalyst samples are shown in Fig. 6(a and b). No significant loss in the photocatalytic activity of the BiVO4/g-C3N4 (6 mg) photocatalyst was observed after successive cycles; it showed 98% to 95% degradation activity after 8 cycles for MB and 95% to 86% degradation activity after 8 cycles for BMB, suggesting that the strong interfacial interaction between BiVO4 and g-CN benefits the stability of the heterostructure. Even after 8 successive photodegradation experiments, the BiVO4/g-C3N4 (6 mg) photocatalyst did not show any obvious decrease in the photocatalytic degradation activity under visible-light irradiation, implying that the BiVO4/g-CN photocatalyst is sufficiently stable for photocatalytic degradation. On the contrary, the efficiency of pure BiVO4 decreased after 5 successive runs against both dyes.
Fig. 6 Recycle experiments of BiVO4 and BiVO4/g-CN (6 mg) nanocomposites for the photocatalytic degradation of (a) MB (b) BPB. |
As further evidence, the XRD and FTIR patterns of the fresh and used hybrid photocatalyst are compared in Fig. 7 (a–d). All peak intensities decrease in the XRD spectra after 8 cycles, as shown in Fig. 7 (a and c). The FTIR spectra (Fig. 7 (b and d)) of BiVO4/g-CN (6 mg) show that the intensities at 740 cm−1, 843 cm−1 and 1200–1800 cm−1 decrease, which might be due to the increased surface strain of BiVO4/g-C3N4 caused by the adsorption of dye molecules. In contrast, no significant difference could be observed in absorption bands, which demonstrated the stable chemical structure during the whole reaction.
Fig. 7 (a) The XRD pattern (MB), (b) FTIR pattern (MB) and (c) XRD pattern (BPB), (d) FTIR pattern (BPB) of BiVO4/g-CN (6 mg) before and after eight runs. |
Reactive oxygen species, especially hydroxyl radicals (·OH) and superoxide anion radicals (·O2−), have been considered to be the key species in the photocatalytic decomposition of many hazardous organic compounds due to their high reaction activity. PL of a photocatalyst has been defined as the measure of the charge separation efficiency. The charge separation efficiency greatly increased in the BiVO4/g-CN samples, which was proved by the decreased PL spectrum intensity, as shown in Fig. 9(a). This increased charge separation efficiency plays a key role in improving the photocatalytic activities because both g-CN and BiVO4 suffer from the fast recombination of electron–hole pairs, which severely limits their photocatalytic activities. Typically, three scavengers (t-BuOH, BQ and AO) were used to estimate the hydroxyl radicals (·OH), superoxide radicals (·O2−) and holes (h+), respectively.38,39 The above-mentioned three scavengers (1 mmol) were added to the reaction system along with BiVO4/g-CN (6 mg) or pure BiVO4.
As displayed in Fig. 9(b), when AO is added into the reaction system as an h+ scavenger along with BiVO4/g-CN (6 mg) or pure BiVO4, the photodegradation process was inhibited significantly, indicating that h+ can dominantly affects the decomposition of MB. The rate of MB removal was also significantly suppressed with the addition of the BQ scavenger for BiVO4/g-CN (6 mg) and on the other hand, it was not suppressed for pure BiVO4. This result demonstrated that the ·O2− radical species significantly played a crucial role towards the degradation of MB by BiVO4/g-CN (6 mg). In addition, when the t-BuOH scavenger was added to capture ·OH species, the MB removal efficiency was reduced to some extent for BiVO4/g-CN (6 mg) but significantly suppressed for pure BiVO4, implying that to some extent, ·OH was also produced and participated in the photodegradation process by BiVO4/g-CN (6 mg).
ESR is usually used to explore the reactive oxygen species evolved during the photocatalytic process. Fig. 10 shows the ESR spin-trap signals (with DMPO) of the samples in two different dispersions. From Fig. 10(a), the characteristic peaks of the DMPO–˙O2− adducts are found for the visible-light-irradiated BiVO4/g-CN (6 mg) suspension in methanol. As shown in Fig. 10(b), four separate characteristic peaks with intensity 1:2:2:1 for the DMPO–˙OH adduct are observed for the suspension of BiVO4/g-CN (6 mg) in water under visible-light illumination, showing that the ·OH radicals are produced efficiently. However, in the dark and under identical conditions, no such signals were detected.
Fig. 10 ESR spectrum of BiVO4/g-CN samples under irradiation: (a) in methanol dispersion for DMPO–˙O2− and (b) in aqueous dispersion for DMPO–˙OH. |
On the basis of all the above-mentioned results, we can preliminarily conclude that ·O2− and h+ are the leading species in this photocatalytic reaction system for the BiVO4/g-CN (6 mg) heterojunction system. Meanwhile, the ·OH species also plays a certain role. It is commonly accepted that the overall photocatalytic performance is dependent on several factors, such as the specific surface area, photogenerated electron–hole transportation rate and photoresponse ability. The evaluation of the photocatalytic activity indicated that the composite with the optimal amount of g-CN (6 mg) showed remarkably enhanced performance due to the enlarged surface area (25 m2 g−1).
Thus, based on the afore-mentioned results, a proposed pathway mechanism for the photocatalytic reactions occurring on BiVO4/g-CN (6 mg) is demonstrated in Fig. 11. Fig. 9(c) demonstrates that the two constituent semiconductors show a well-staggered band alignment. The positions of the valence band (VB) and conduction band (CB) were calculated by the following empirical equation: EVB = ECB + Eg; here, ECB corresponds to the CB edge potential, EVB corresponds to the VB edge potential and Eg represents the band gap of the samples. The VB and CB of the as-prepared m-BiVO4 were calculated to be +2.71 eV and +0.43 eV, while those of g-CN were +1.54 eV and −1.24 eV, respectively. The VB of g-CN was not much +ve than that of ·OH\OH− (2.4 eV) and the CB of m-BiVO4 was not much −ve than that of ·O2−\O2 (−0.33 eV);40 this means that the ·OH and ·O2− species cannot be generated based on the traditional heterojunction-transfer process. However, it was demonstrated that the ·OH and ·O2− species were generated and participated in this photocatalytic degradation reaction system. Therefore, the Z-scheme mechanism was more suitable to describe the photocatalytic reaction process. In other words, the photogenerated e− generated on BiVO4 (CB) prefers to transfer to g-CN (VB) and recombine with the remaining holes. Then, the e− in g-CN (CB) reacts with the dissolved O2 to generate ·O2− for dye (MB) degradation. The holes in BiVO4 (VB) would participate in the photocatalytic reaction process via dual pathways: on one hand, the holes can react directly with MB; on the other hand, the holes can also react with water (H2O) to generate ·OH and then as a degradation product degrade MB. Meanwhile, due to the lack of the transfer process, the remaining holes and electrons in the VB of BiVO4 and the CB of g-CN influenced strong redox ability, which is an important factor to enhance the photocatalytic performance based on the prepared BiVO4 and g-CN hybrid composites. Furthermore, the intrinsic material properties, i.e., the effective surface coverage of the g-CN nanosheets on the m-BiVO4 networks leads to more reactive sites by completely exposing the adjacent area of layer interfaces to the solution. Similarly, the separation of electron–hole pairs is facilitated by the small size of the g-CN nanosheets with a decreased charge transport distance.41
Fig. 11 Schematic illustration of the proposed reaction mechanism in the BiVO4/g-CN (6 mg) nanocomposite-based reaction system towards dye (MB, BPB) degradation under visible light irradiation. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra09473c |
This journal is © The Royal Society of Chemistry 2020 |