Suqin Liu*,
Huizhen Liu,
Guanhua Jin and
Hao Yuan
Central South University, College of Chemistry & Chemical Engineering, Changsha, Hunan, China. E-mail: sqliu2003@126.com
First published on 15th May 2015
A novel flower-like MnO2/BiOI composite has been fabricated by a simple and cost-effective approach. Multiple experiments are carried out to optimize the molar ratio of MnO2/BiOI for the decomposition of methyl orange (MO) and Rhodamine B (RhB). The adsorption property of the as-prepared samples in dark conditions and the photocatalytic activity under visible light and simulated solar light irradiation are investigated in detail. When the MnO2/BiOI molar ratio is 4:
5, the highest photocatalytic activity for degradation of MO (97.8%) and RhB (91.7%) is achieved under visible light irradiation for 40 min as well as simulated solar light irradiation (93.4% for MO and 88.2% for RhB within 40 min). The excellent photocatalytic performances are attributed to the synergistic effect of BiOI and MnO2 and the unique flower-like morphology of the MnO2/BiOI composite as verified by relative experiments.
Semiconductor photocatalysis has great potential to decompose organic water pollutants into CO2 and H2O using solar light.7,11–13 As a representative of traditional photocatalytic materials, TiO2 is widely studied by virtue of its nontoxicity, high chemical stability, low cost and outstanding oxidative power.14 However, TiO2 could only respond to ultraviolet radiation because of its large band gap (3.0–3.2 eV), which leads to low photocatalytic efficiency. Recently, bismuth oxyhalides (BiOX, X = F, Cl, Br, and I) with layered structures have been put forward as promising candidate photocatalysts under both ultraviolet and visible light irradiation.6,15,16 Among them, BiOI exhibited the best photocatalytic activity because of its smallest band gap (1.77–1.92 eV).15,17,18 Even so, the photocatalytic activity of BiOI has been still restricted by some disadvantages, such as low efficiency of light absorption, slow rate of charge transfer and high recombination probability of the photogenerated electron–hole pairs.19,20 In the past few years, coupling with second semiconductors, such as Bi2S3/BiOI,6 AgI/BiOI,18 ZnO/BiOI,21 TiO2/BiOI,22 BiOI/BiOX,23–25 and MnOx–BiOI,26 has been proposed to notably reduce the recombination possibility and accelerate the separation rate of photogenerated charge carries, thus effectively improve the photodegradability of semiconductor oxide photocatalyst. As a result, an increasing number of studies have focused on the coupled semiconductor systems.
MnO2 is of considerable importance in technological applications including electrochemical supercapacitor, molecular adsorption, catalysis and ion-exchange due to its masses of merits like inexpensive, non-toxic and ease of fabrication. As a typical catalyst, MnO2 is extensively investigated for oxidation of organic pollutants in wastewater because of its strong adsorption and oxidation ability.9,27–30 Dong et al. found that MnO2 possesses high adsorption capacity to MO with a removal efficiency of 78.6%.28 It is reported that MnO2 has a narrow band gap of 0.25 eV, which enable it to absorb the light at infrared wavelengths in theory and increase the utilization rate of solar energy when combination with other type photocatalyst such as TiO2.31–33 Recently, to improve the photocatalystic activity of BiOI, MnOx is considered as a deriving-hole-type cocatalyst to enhance the transfer of the photo-induced holes.26 Ye et al. reported that MnOx–BiOI can deliver higher photocatalytic activity than BiOI for the degradation of RhB dye under visible light irradiation. However, the reported highest photocatalytic degradation efficiency of the prepared MnOx–BiOI via photo-deposition method for RhB under visible light in 30 min is only 78.8%.26
As is well known, the photocatalytic activity is much depended on the morphology, fabrication strategy and the component of cocatalyst. In general, the catalyst with a morphology which has larger surface area, trends to show better photocatalytic activities due to the masses of exposed active sites. In addition, the combination structure also remarkably affects the photocatalytic performance. Based on these, in this manuscript, a novel flower-like MnO2/BiOI hybrid composite is fabricated by a simple two-step method. And the catalytic activity of MnO2/BiOI composites with different molar ratio is investigated by decomposing MO and RhB under dark condition, visible light and simulated solar light irradiation at room temperature. As expected, the as-prepared MnO2/BiOI composite demonstrates much higher photocatalytic activity than the bare BiOI and MnO2. Furthermore, the possible reasons and photocatalytic mechanism have been put forward based on the investigation of reactive species.
In a typical experiment, 316 mg of KMnO4 was dissolved in 112 mL of deionized water by strong magnetic stirring. Then, 11 mL of 6 mol L−1 HCl solution was slowly dropped into the previous solution under stirring. The as-obtained MnO2 precipitates were collected by centrifugation, after washing several times with distilled water and ethanol and dried in an oven overnight at 60 °C finally.
The pure BiOI was synthesized according to ref. 34. 0.728 g of Bi(NO3)3·5H2O was added in 20 mL of absolute ethanol under stirring at room temperature for 30 min. Then, 40 mL of NaI solution (0.225 g of NaI dissolved in distilled water) was added dropwise into the previous solution. The pH of the mixture was adjusted to 7 by adding 1.5 mol L−1 NH3·H2O. Then, the mixture was transferred to a 250 mL flask and maintained at 80 °C for 3 h by oil bath heating. After completion of the reaction, the precipitates were collected by centrifugation, washed several times with distilled water and ethanol and dried in an oven overnight at 60 °C finally.
In order to synthesize MnO2/BiOI composite, the as-prepared MnO2 was well dispersed by ultrasound for 1 hour in ethanol solution containing the same molar of Bi(NO3)3·5H2O. The subsequent operations are the same as the preparation of BiOI. For comparison, MnO2/BiOI composites with molar ratio of MnO2 to Bi(NO3)3·5H2O (2/7, 3/5, 4/5, 4/4, 6/3) are designed.
To further confirm the existence of MnO2 in the composite, X-ray photoemission spectroscopy (XPS) (Fig. 2) is applied to investigate the surface composition and chemical state of the MnO2/BiOI (4/5) composite and the physical mixed MnO2 and BiOI (marked as MnO2 + BiOI (4 + 5)). Elements of Bi, O, I and Mn are clearly observed in the survey spectra (Fig. 2a). The high resolution XPS of Mn 2p3/2 is displayed in Fig. 2b, from which two peaks positioned at 642.2 eV and 654.2 eV, which are ascribed to the Mn 2p3/2 state and Mn 2p1/2, are observed.26 The two strong peaks at 159.0 eV and 164.4 eV in the high resolution XPS of Bi 4f (Fig. 2c) are assigned to Bi 4f7/2 and Bi 4f5/2 peaks of Bi3+, in good agreement with the valence state of Bi in BiOI.26 As shown in Fig. 2d, the two peaks of I region at 619.0 eV and 630.5 eV relate to the I 3d5/2 and I 3d3/2, respectively, corresponding to the characteristics of I− in the BiOI. The O 1s core level spectrum (Fig. 2e) of MnO2/BiOI (4/5) could be fitted to three peaks at 529.7 eV, 530.4 eV and 531.8 eV,26 which could be assigned to the Mn–O–Mn37 bonds in MnO2, Bi–O bonds22 in [Bi2O2] slabs of BiOI layered structure, and hydroxyl groups, respectively. Note that the peaks of Mn–O–Mn bonds and Bi–O bonds in MnO2/BiOI (4/5) has a slight shift compared with those of physical sample MnO2 + BiOI (4 + 5) (Fig. 2f), implying the interaction between MnO2 and BiOI.
![]() | ||
Fig. 2 The survey XPS of MnO2/BiOI (4/5) (a), high resolution XPS of Mn 2p (b), Bi 4f (c), I 3d (d), O 1s (e) in MnO2/BiOI (4/5), and O 1s (f) in MnO2 + BiOI (4 + 5). |
The morphology features of the as-prepared products were characterized by SEM, TEM, and HRTEM. Fig. 3a shows that the pure BiOI is composed of smooth flakes with uneven size less than 500 nm. The pure MnO2 is of microsphere with a diameter of 500 nm, but an aggregation takes place (Fig. 3b). The MnO2/BiOI (4/5) composite demonstrates a flower-like morphology structure, which is constructed by many straight flakes (Fig. 3c and d). The diameter of the flower is about 1 μm, reveal a possible combination of MnO2 and BiOI. Note that, in the preparation process, the MnO2 was dispersed in the Bi(NO3)3 solution by ultrasound firstly. Then, I− was added. After adjusting the pH to 7, the primary BiOI crystals were formed firstly and prefer to grow in the surface of MnO2 when precipitating from solution owing to the tendency to reduce the total surface energy. As a result, a different flower-like morphology was formed. In such a unique flower-like structure, the BiOI could contact with MnO2 intimately, which will facilitate the fast transfer of photo-induced carriers. Additionally, as seen from the TEM picture (Fig. 3d), the flakes stack and intercross with one another, forming a lot of pores in the surface. The abundant voids could not only enrich the active sites of BiOI, but also increase the contact between catalyst and MO solution, which will benefit to the photocatalytic activity.
![]() | ||
Fig. 3 SEM images of BiOI (a) and MnO2 (b); SEM (c), TEM (d) and HRTEM (e and f) images of MnO2/BiOI (4/5) composite; inset is EDS image of MnO2/BiOI (4/5) composite. |
The HRTEM images of the MnO2/BiOI composite are given in Fig. 3e and f. Three sets of different fringes are clearly found. The fringe spacing of 0.305 nm, 0.224 nm and 0.199 nm agree well with the spacing of the (102), (004) and (200) lattice planes of BiOI, respectively. In Fig. 3e, lattice fringes with spacing of 0.240 nm and 0.718 nm are found, corresponding to the (112) fact of BiOI and the (001) facet of MnO2, respectively.38 The well-defined fringes and the high crystallinity of the MnO2/BiOI composite will facilitate the separation of the photo-induced carrier and thus improve the corresponding catalytic activities.18 In the EDS image (Fig. 3f inset), elements of Mn, I, Bi and O are clearly displayed, further confirming the co-existence of MnO2 and BiOI.
αhν = A(hν − Eg)n/2 | (1) |
![]() | ||
Fig. 4 (a) UV-vis diffuse reflectance spectra (DRS) of as-prepared MnO2/BiOI samples; (b) the plotting of (αhν)1/2 vs. photon energy. |
Although the band edge positions of the BiOI are not easily determined experimentally, a theoretical prediction is possible using concepts of electronegativity. The conduction band (CB) and valence band (VB) potentials can be calculated by the following equations:41
EVB = X − Ec + 0.5Eg | (2) |
ECB = EVB − Eg | (3) |
Efp = E0fp − 0.05915 pH | (4) |
As seen from the adsorption data, the concentration of the MO and RhB solution shows negligible decrease after 30 min in the darkness. Therefore, we irradiated the samples with visible or simulated solar light after treating the solution under dark conditions for 30 min. As shown in Fig. 6a and b, the concentration of dyes decrease slightly in the absence of catalyst, which confirm the good chemical stability of MO and RhB. For MnO2, about 60.5% of MO and 43.4% of RhB are removed in 40 min under visible light irradiation, closing to its adsorption capacity (59.5% for MO and 42.2% for RhB). The poor photocatalysis activity may have relationship with its narrow band gap of 0.25 eV, which enable it to absorb the light at infrared wavelengths.31–33 It is worth noting that, the photocatalytic conversion of MO degradation over the MnO2/BiOI (4/5) sample reach the max value of 94% after just 10 min irradiation. As is shown in Fig. 6c, the main peak of MO (λ = 464 nm) completely disappears only after 10 min irradiation. Moreover, the color of the supernatant changes from yellow to colorless observed by the naked eye, verifying the excellent photocatalytic activity of the MnO2/BiOI (4/5) under visible light irradiation. Such result is much higher than that of the bare BiOI (44.4% in 10 min). Significantly, the as-obtained MnO2/BiOI (4/5) also shows a relatively high photocatalytic performance for RhB with a degradation ratio of 91.7% after 40 min of visible light irradiation. At the same time, it can be seen from Fig. 6d that most the fuchsia color of the starting RhB solution fades just after visible light irradiation for 10 min. And an evident decrease from 2.109 to 0.083 in RhB absorption at λ = 554 nm is observed. Obviously, the MnO2/BiOI (4/5) composite demonstrates much enhanced catalytic activity than the bare BiOI and MnO2. The highest photocatalytic activity of MnO2/BiOI (4/5) is mainly attributed to the synergistic effect between MnO2 and BiOI. It was reported that MnO2 could act as a deriving-hole type cocatalyst to enhance the separation efficiency of the photoinduced carriers of BiOI.26 The detailed photocatalytic mechanisms are further discussed in the photocatalytic mechanism part. Moreover, the unique flower-like morphology will also contribute to the outstanding photocatalytic performance. In this structure, the intimate conduct contact between MnO2 and BiOI is conducive to the charge transfer from BiOI to MnO2. This could be confirmed by the low photocatalytic activities of the physical mixed MnO2 and BiOI (marked as MnO2 + BiOI (4 + 5)) under both dark condition and visible light irradiation. Note that a simple physical mixing could not provide a close combination of MnO2 and BiOI, thus the charge transfer is not fluent.5,20 In addition, the flower-like morphology has a relative high SBET which increase the surface active sites as well as the contact area between the dye solution and catalyst.1,45,46
![]() | ||
Fig. 6 Photodegradation of MO (a) and RhB (b) as a function of irradiation time; UV-vis spectral changes of MO (c) and RhB (d) as a function of irradiation time over MnO2/BiOI (4/5). |
To further reveal the excellent photocatalytic performance of the MnO2/BiOI (4/5) composite, we also evaluated the photocatalytic activity using MO and RhB under simulated solar light irradiation. As Fig. 7 displayed, the highest photocatalytic activity is still achieved over the MnO2/BiOI (4/5) sample, inducing 93.4% for MO and 88.2% for RhB degradation within 40 min. The cause of the slight decrease in activity for the MnO2/BiOI (4/5) sample compared to the degradation performance (97.8% for MO and 91.7% for RhB) under visible light is that the photodegradation experiments under simulated solar light irradiation were performed without constant stirring.
![]() | ||
Fig. 7 Photocatalytic activity of the bare MnO2, BiOI, and MnO2/BiOI samples with different molar ratios (MnO2/BiOI) under simulated solar light irradiation. |
Fig. 8 shows the variation of MO and RhB degradation with different quenchers. Both the MO and RhB solution added with IPA affect the photocatalytic activity of MnO2/BiOI (4/5) slightly, suggesting that ˙OH does not play a key role for the degradation of the two dyes. On the contrary, the photocatalytic degradation of MO or RhB is obviously inhibited after the addition of BQ or EDTA, which implies that h+ and ˙O2− played major role in MnO2/BiOI (4/5) under visible light irradiation. This corresponds well with the previous reported results.
![]() | ||
Fig. 8 Effects of different scavengers on the degradation of MO (a) and RhB (b) over MnO2/BiOI (4/5) composite. |
Based on above results, the coupling effects of MnO2 and BiOI was proposed to explain the enhanced photocatalytic activity of MnO2/BiOI (4/5) composite. The proposed schematic mechanism of MnO2/BiOI composite is shown in Fig. 9. It is well known that the photodegradation processes are depended on charge carrier generation and separation. As shown in Fig. 9, under visible excitation, the electron of BiOI can be promoted from the valence band to the conduction band, leaving behind a hole in the valence band. Then the photoinduced holes transfer to MnO2 and largely reduce the recombination of photogenerated electrons and holes, thus more effective electrons and holes are taking part in the photodegradation process.26
The unique flower-like morphology of the composite also attributes to the remarkable photocatalytic performance. Firstly, the relative high SBET and massed of pores increase the surface active sites as well as the contact area between the dye solution and catalyst. Moreover, the enhanced sensitization effect between the dyes and BiOI may also be beneficial to the degradation efficiency. In the process of dye photosensitization, the electrons on the highest occupied molecular orbital (HOMO) of dye molecules will be excited to the lowest unoccupied molecular orbitals (LUMO) under visible light, and subsequently would be injected to the CB of BiOI. Then the adsorbed molecular oxygen would capture the electrons in the CB of BiOI to form ˙O2− radicals, which degrade the adsorbed MO or RhB dye further.5,6 That is why the ˙O2− scavenger suppresses dye degradation as shown in Fig. 8. At the same time, the photoinduced holes on the surface of BiOI can also directly oxidize the organic pollutants. That is to say, the photocatalytic process and photosensitization process jointly involve in the dye degradation.
This journal is © The Royal Society of Chemistry 2015 |