Yi Feng,
Chunbo Liu,
Jibin Chen,
Huinan Che,
Lisong Xiao,
Wei Gu and
Weidong Shi*
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China. E-mail: swd1978@ujs.edu.cn; Fax: +86 511 8879 1108; Tel: +86 511 8879 0187
First published on 3rd February 2016
In this work, a series of novel BiOI/CdWO4 p–n junction photocatalysts were successfully fabricated via a facile ultrasonic and stirring process. The photodegradation tests showed that the BiOI/CdWO4 p–n junction photocatalysts show enhanced efficiencies compared to pure BiOI and CdWO4. The best photocatalytic performance was obtained for the BC-2.0 sample, and the as prepared samples were studied by XRD, TEM, HRTEM, XPS, UV-vis DRS and photoluminescence (PL) spectroscopy. This enhancement may be predominantly attributed to the improvement of the photogenerated electron–hole separation and migration efficiency of the synergy impact between BiOI and CdWO4. The efficient separation of electron–hole pairs because of the synergy impact between BiOI and CdWO4. Radical scavenger experiments and ESR indicated that holes (h+) and superoxide radicals (˙O2−) were the main active species in the photocatalytic process.
In recent years, bismuth oxyhalides (BiOX, X = Cl, Br, and I) have received considerable attention due to their potential use in photocatalysts.13–15 Among them, BiOI, which is a typical narrow-bandgap (1.91 eV) semiconductor photocatalyst in the Aurivillius family composed of alternating Bi2O22+ and I− layers, has received much attention due to its good visible light absorption ability.16–18 However, it is a pity that high recombination rates of photogenerated charge carriers leads to the pure BiOI with low photocatalytic activity.19,20 Therefore, further study is required to enhance the photocatalytic activity of BiOI. As a typical p-type semiconductor, BiOI can serve as an efficient visible-light photosensitizer for n-type TiO2 with a large band gap to greatly enhance its photocatalytic efficiency.21 CdWO4 is also an n-type semiconductor with a band-gap energy (about 3.2 eV) similar to that of TiO2. Some studies reported that CdWO4 exhibits better efficiency than TiO2 in the photocatalytic degradation of organic pollutants and photoelectric conversion.22,23 However, although much work has been done, it is still a challenge to develop an inexpensive and facile method for the fabrication BiOI-based heterojunction photocatalysts with high visible-light photocatalytic ability. To the best of our knowledge, there is no report on the design and fabrication of coupled BiOI/CdWO4 heterostructures and their photocatalytic performances. Based on the reports in the literature about the synthetic method using ultrasonic agitation,24–27 we first report the preparation and characterization of BiOI/CdWO4 composite photocatalysts.
Herein, a series of BiOI/CdWO4 composites were prepared at low-temperature and were successfully used as photocatalysts for the photocatalytic degradation of two organic pollutants, tetracycline (TC) and rhodamine B (RhB) under visible-light (λ ≥ 420 nm) irradiation. The results show that the BiOI/CdWO4 heterojunction efficiently improved the photocatalytic activity compared with pure BiOI and CdWO4. The activity enhancement was mainly attributed to the p–n heterojunction, which facilitated interfacial charge transfer and improved the photogenerated electron–hole pair separation. Moreover, the possible mechanism for the enhanced photocatalytic activity was also proposed based on the obtained experimental results.
Degradation (%) = (C0 − C)/C0 × 100%, | (1) |
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Fig. 1 The X-ray powder diffraction patterns of CdWO4, BiOI and BiOI/CdWO4 photocatalysts with different BiOI amounts. |
The samples were analyzed by SEM and EDS, and the results are shown in Fig. 2. Fig. 2a and b show CdWO4 at different magnifications, indicating that it is made up of regularly shaped agglomerated nanorods with an average diameter of approximately 300 nm. The SEM image of the BiOI/CdWO4 heterojunction obtained after chemical etching is shown in Fig. 2c; the structure is tight and jagged. The BiOI particles randomly cover the surface of the CdWO4 nanorods, indicating the integration of BiOI and CdWO4. In addition, the EDS spectrum of the BiOI/CdWO4 composite indicates the presence of O, Bi, I, W, and Cd, confirming that CdWO4 was successfully combined with BiOI (Fig. 2d).
To further detect the BiOI/CdWO4 heterostructure structure, BC-2.0 was characterized by TEM and HRTEM. Fig. 2e confirms that the product is assembled by BiOI nanoparticles supported on the surfaces of the CdWO4 nanorods. Fig. 2f shows the HRTEM image of sample BC-2.0; two sets of lattice fringes are found in the spectrum. The clear lattice fringe with an interval of 0.3012 nm can be indexed to the (122) lattice plane of tetragonal BiOI, while the fringe with an interval of 0.1319 nm corresponds to the (110) lattice plane of CdWO4, further demonstrating that the BiOI/CdWO4 heterostructure structure was successfully synthesized. The results were consistent with the XRD results shown in Fig. 1.
X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface compositions and chemical states of samples. Fig. 3a shows that Bi, O, I, Cd, and W are clearly observed in the survey spectrum. The peaks (Fig. 3b) with binding energies of 159.2 and 164.5 eV in the high-resolution XPS Bi 4f spectrum are attributed to Bi 4f7/2 and Bi 4f5/2, respectively, demonstrating that the main chemical state of Bi in the sample is +3.29 In Fig. 3c, the two peaks of I at 619.0 and 630.5 eV correspond to I 3d5/2 and I 3d3/2, respectively, indicated that the state of I in the sample is −1,30 in good agreement with other references. The W 4f XPS spectrum illustrated in Fig. 3d shows two peaks at 35.37 and 37.62 eV, indicated that the valence of W in the sample is +6.31 The XPS spectrum of O is shown in Fig. 3d, O is primarily from BiOI, CdWO4 and H2O absorption on the surface of the sample.32 The high-resolution C 1s spectrum is shown in Fig. 3f. The XPS results further demonstrate the coexistence of BiOI and CdWO4 in the BC-2.0 sample, which is consistent with the EDS analysis.
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Fig. 3 XPS spectra of the BC-2.0 BiOI/CdWO4 composite. (a) Survey of the sample; (b) Bi 4f; (c) I 3d; (d) O 1s; (e); Cd; and (f) W 4f. |
DRS is often used to detect the optical absorption and energy band characteristics of semiconductors. Fig. 4a shows that BiOI has strong absorption in the visible light range, with an absorption edge approaching 570 nm. However, CdWO4 shows no absorption in the visible light region, which has also been reported in other studies.33 In contrast to CdWO4, the BiOI/CdWO4 heterostructures exhibit obvious absorption in the visible light region; the DRS spectrum is similar to that of BiOI due to the high BiOI loading in BiOI/CdWO4. These results imply that the BiOI/CdWO4 heterostructures can respond to visible light. Thus, a lower band gap is beneficial for electronic transitions and subsequently results in enhanced photocatalytic activity.
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Fig. 4 (a) UV-vis diffuse reflectance spectra of the as-prepared samples and (b) (αEphoton)1/2 vs. Ephoton curves of the as-prepared samples. |
The band gap energy (Eg) of samples were calculated by the following formula based on the DRS results:34
αhν = A(hν − Eg)n/2 |
The valence band (VB) and conduction band (CB) edge positions of BiOI and CdWO4 were also estimated according the following empirical formulas:37
EVB = X − Ee + 0.5Eg |
ECB = EVB − Eg |
In the empirical formulas, EVB is the VB of the semiconductor, ECB is the CB of the semiconductor, Ee is the energy of free electrons (about 4.5 eV), Eg is the band gap energy of the semiconductor, and X is the electronegativity of the semiconductor. According to previous studies, the X values of BiOI and CdWO4 are 5.94 and 6.28 eV, respectively.38,39 The EVB values of BiOI and CdWO4 were calculated to be 2.39 and 3.45 eV, respectively, and their K homologous ECB values were estimated to be 0.49 and 0.15 eV, respectively. These results confirmed that BiOI and CdWO4 formed a perfectly composite structure, which was beneficial for the separation of the electron–hole (e−–h+) pairs.
The photocatalytic activities of the BiOI, CdWO4 and heterostructure samples were evaluated for the degradation of TC and RhB under visible light irradiation. Fig. 5a shows the photocatalytic efficiencies of the BiOI, CdWO4 and BiOI/CdWO4 heterostructure samples for the photodegradation of RhB. After 90 min of irradiation under visible light, the degradation of RhB by pure CdWO4 was negligible because CdWO4 does not responding under visible light. Pure BiOI exhibited a weak degradation efficiency of 49.8% after 90 min of visible light illumination. All of the BiOI/CdWO4 heterostructures exhibited superior photocatalytic activities compared to pure BiOI and CdWO4. For comparison, the BT-2.0 sample was evaluated for the degradation of RhB under visible light irradiation, and the results are shown in Fig. S2.† Among all the samples, the BC-2.0 film showed the highest photocatalytic activity. As shown in Fig. 6a, the photodegradation efficiency of RhB reaches nearly 92% after 90 min irradiation for BC-2.0. The maximum peak at k = 553 nm does not shift, indicating that no intermediate was formed during the reaction.
Fig. 5c shows the degradation of TC with different samples under visible light irradiation. Pure BiOI and CdWO4 exhibit very low photocatalytic activities, while coupling the two semiconductors together to construct a heterojunction significantly enhanced the photocatalytic efficiency. The enhanced photocatalytic activity of BiOI/CdWO4 heterostructures was further verified for the photodegradation of TC. According to previous studies, the photocatalytic degradation kinetics of TC follow a pseudo-first-order reaction with a Langmuir–Hinshelwood model. When the initial concentration (C0) was below 10 mg L−1 for TC in the present experiment, the rate complies with a first-order reaction kinetics model as shown in Fig. 5d:
ln(C/C0) = kt |
In the above equation, C is the concentration of TC at time t, C0 is the initial concentration of TC, and the slope k is the apparent reaction rate constant. A higher k value indicates a faster degradation rate. The BC-2.0 sample has a higher rate constant than pure BiOI and CdWO4. The maximum rate of the BC-2.0 sample was almost five times that of pure BiOI and 20 times that of CdWO4.
The above results indicate that a synergistic effect exists in the BiOI/CdWO4 heterostructure photocatalysts. This effect should be attributed to the effective interfacial charge transfer resulting from the fabrication of a heterojunction, which will be discussed in detail in the following section on the photocatalytic mechanism. Therefore, these results demonstrate that our photocatalyst is a good candidate for applications in environmental purification.
As is well known, in the photocatalytic oxidation process (POC), a series of reactive oxygen species such as h+, ˙OH and ˙O2− are supposed to be involved. In order to investigate the main species involved in RhB photodegradation over the BiOI/CdWO4 composite, 1,4-benzoquinone (BQ), disodium ethylene diaminetetraacetate (EDTA-2Na) and tert-butyl alcohol (IPA) were added into the POC as scavengers for ˙O2−, h+ and ˙OH to test the catalyst activity.40–42 As shown in Fig. 6a and b, the photocatalytic degradation of RhB is obviously inhibited after the addition of BQ or EDTA. In contrast, it is only slightly suppressed upon the addition of IPA to the POC. The reactive species results indicate that h+ and ˙O2− play important roles in the photocatalytic degradation reaction.
ESR using dimethyl pyridine N-oxide (DMPO) as a trapping agent was used to more precisely verify the generation of radicals; the results are shown in Fig. 6c and d. Before the measurement, the samples (10 mg) and DMPO (30 μL) were dispersed in deionized water and methanol, respectively. In Fig. 6c, the black line is irradiated under faint natural light, and the red line is irradiated under the instrumental light. The six characteristic peaks of the DMPO–˙O2− adducts indicate that superoxide radicals (˙O2−) were formed during the photocatalytic reaction. The ˙O2− signal intensity under the instrumental light is obviously stronger than that under the natural light, suggesting that the ˙O2− radicals are generated under a strong light. DMPO–˙OH was also investigated; no ˙OH signal was observed under irradiation with the instrumental light. Thus, we may conclude that ˙O2− plays a major role in the reaction.
The Mott–Schottky equation was used to verify the flat band potential of the as-prepared CdWO4 and BiOI. Based on the Mott–Schottky equation, a linear relationship of 1/C2 versus applied potential can be obtained, and negative and positive slopes correspond to p- and n-type conductivities, respectively. From previous reports, we know that CdWO4 is an n-type semiconductor, and BiOI is a p-type semiconductor.43,44 Thus, BiOI/CdWO4 can be seen as a p–n heterogeneous structure, which would facilitate the separation of photogenerated charges. As reported earlier, the flat band potential represents the apparent Fermi level of a semiconductor in equilibrium with a redox couple.45–47 Therefore, we show the change in the energy band structure of the two semiconductors before and after contact in Fig. 7a and b. The Fermi levels of CdWO4 and BiOI are 0.17 and 1.74 V (vs. AgCl), respectively. When the BiOI are assembled on CdWO4 to construct the p–n heterojunction, the energy levels of BiOI shift upward, whereas the energy band of CdWO4 shifts downward until the EF of BiOI and CdWO4 reaches an equilibrium. The excited electrons on the CB of p-type BiOI effectively transfer to n-type CdWO4 under the effect of the internal electric field and then react with O2 adsorbed on the surfaces of the catalysts to produce reactive species ˙O2−. However, the VB potential of BiOI is higher than that of CdWO4; thus, h+ in the CdWO4 can transform to the VB of BiOI and directly oxidize organic pollutants (Fig. 8). In this case, the formation of the p–n heterojunction (BiOI/CdWO4) could effectively separate the photo-excited electron–hole pairs and remarkably reduce the recombination of photogenerated charge carriers. As a result, the BiOI/CdWO4 heterostructures exhibit better photocatalytic activities for the degradation of organic pollutants under visible light irradiation compared to pure CdWO4 and BiOI. The enhancement in the separation of photogenerated electrons and holes can be verified by photoluminescent and photocurrent results.
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Fig. 8 Schematic of the separation and transfer of photogenerated charges in the BiOI/CdWO4 combined with the possible reaction mechanism of photocatalytic procedure. |
According to the above discussion, the probable reactions that occur during the photodegradation of RhB are the following:
BiOI → BiOI (e− + h+) |
CdWO4 (e−) + O2 → ˙O2− |
e− + ˙O2− + 2H+ → H2O2 |
H2O2 + organics → … → degradation product |
BiOI (e− + h+) → BiOI (e−) + BiOI (h+) |
BiOI (h+) + organics → … → degradation products |
It is well known that a low PL intensity results in a better charge separation of semiconductor materials.48 The better photocatalytic activity of the semiconductor is discussed and confirm by PL analysis. Fig. 9a shows that CdWO4 and BC-2.0 exhibit an intense emission peak at about 470 nm. Compared with CdWO4, the emission peak intensity of BC-2.0 is obviously decreased, which suggests that the recombination of photogenerated charge carriers is extremely inhibited. Therefore, the PL results verify the efficient separation of the photoinduced charge carriers during the photocatalytic reactions. Fig. 9b shows the PL spectra of the BC-2.0 and BT-2.0 samples under the excitation wavelength of 390 nm; all the catalysts exhibit similar PL spectra, which can be attributed to the band–band PL phenomenon with the energy of light. Obviously, BT-2.0 shows a higher PL intensity than BC-2.0, indicating that when BC-2.0 is compared with BT-2.0 the recombination of electron–hole pairs is efficiently proven, as this results in a higher photodegradation efficiency. In order to compare the PL quenching results of the BiOI/CdWO4 and BiOI/TiO2 heterojunctions and the possible reasons for the differences, the PL of the samples have been investigated and the results shown in Fig. 9. The relative positions of the VBs and CBs of CdWO4, TiO2, and BiOI were investigated, and the results are shown in Fig. S2.† Fig. S2a† shows that the ΔCB(BT-2.0) = 0.79 eV and the ΔCB(BC-2.0) = 0.34 eV. After the p–n junctions were formed, ΔCB(BT-2.0) < ΔCB(BC-2.0); thus, the BC-2.0 recombination of electron–hole pairs is efficiently proved for the PL quenching efficiently. At the same time, the electric potential difference of VB can explain the comparative degree of PL quenching in BiOI/CdWO4 and BiOI/TiO2 heterojunctions.
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Fig. 9 (a) PL emission spectra of CdWO4 and BC-2.0 samples. (b) PL emission spectra of BC-2.0 and BT-2.0 samples. (c) Transient photocurrent response for the pure BiOI and BC-2.0 composite. |
To further demonstrate that the high photocatalytic activity is derived from the efficient separation of electron–holes, photocurrent measurements were carried out. Fig. 9c shows the high photocurrent intensity of BC-2.0 compared to that of the pure BiOI sample. The photocurrent analysis demonstrated that the high separation of photo-induced electrons and holes results in higher photocatalytic activity.49
Semiconductor stability is widely known to be very important for practical applications. Fig. 10a shows that the catalyst exhibits a slight loss of activity after four cycles of photo-degradation reaction. Due to the loss of catalyst during the filtering and transferring processes, some amount of catalyst is not recovered. At the same time, p–n heterojunctions can efficiently decrease the photo-corrosion of the catalysts. The results indicated that the BiOI/CdWO4 p–n heterostructure has a good photostability. In order to further test the durability and stability of the catalysts, the catalysts were characterized by XRD before and after photocatalysis, and the results are shown in Fig. 10b. There was no obvious change in intensity after the photocatalytic reaction, indicating that the p–n heterojunction of BiOI/CdWO4 has good stability and recyclability for practical applications.
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Fig. 10 (a) Repeated photocatalytic degradation of RhB over BC-2.0 under visible light irradiation. (b) XRD patterns of BC-2.0 before and after 6 h irradiation. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23383f |
This journal is © The Royal Society of Chemistry 2016 |