The degradation of organic pollutants by bismuth vanadate - graphene oxide nanocatalyst under visible light

Abstract

Water is known as an essential resource for human life and socioeconomic development. However, the quality and quantity of water resource have been decreasing rapidly because of many factors such as global warming, over-exploitation, pollution and global population growth in recent years. Therefore, water scarcity is an inevitable problem at the present. In order to solve this issue, the wastewater treatment is considered as an effective solution to retain the water resource. However, the traditional treatment technologies such as chemical, biological and physical processes are not able to meet the increasingly stringent standard of water quality. Because these technologies can only transfer pollutants to other phases or generate toxic by–products that need further treatment processes.

Therefore, advanced oxidation processes (AOPs) as alternative techniques have been developed to remove completely organic pollutants in recent years. Briefly, the AOPs generate powerful oxidizing species such as hydroxyl radical (OH·) super–dioxide (O2-) and hole (h+) to mineralize completely organic compounds to carbon dioxide (CO2) and water (H2O). Among AOPs, photocatalytic process is a highly effective method using semiconductors to degrade completely organic pollutants. Several semiconductors have been reported to be good photocatalysts, such as titanium dioxide (TiO2), zinc oxide (ZnO), zinc sulfide (ZnS), tungstate (WO3), iron oxide (Fe2O3), cerium oxide (CeO2) and so on. However, the use of these photocatalysts is mainly limited by their wide bandgap or corrosion restriction.

Therefore, bismuth vanadate (BiVO4) has been recently emerged as a viable candidate for photocatalytic process due to its excellent properties such as narrow bandgap (2.4 eV), good dispersibility, resistance to corrosion, ferro–elasticity and non–toxicity. However, the photocatalytic activity of BiVO4 is limited by the weak absorption and the difficult migration of photogenerated carriers. In order to improve the photocatalytic activity of BiVO4, many efforts have been conducted such as using nanosized particles, obtaining hetero–junction structure, and controlling morphology by doping and coupling with other materials.

Among them, the combination of SiO2 and BiVO4 has showed significant potential due to the useful properties of SiO2 such as cheap cost, large specific surface area, well biocompatibility, and easy functionalization. Besides, graphene oxide (GO) is a 2D material which has the outstanding properties such as high specific surface area, high conductivity, and high mobility. Therefore, GO can be used as a supporting material for photocatalyst. As a result, it is expected that a nanocomposite with a better photocatalytic activity can be obtained from the combination of BiVO4, SiO2 and GO. However, the results of this nanocomposite are seldom reported.

Therefore, the objective of present work was to synthesize BiVO4–based nanocomposite by hydrothermal method. The influence of synthetic conditions including pH value, hydrothermal temperature and reaction time were examined in this work. Besides, the characterizations of as–prepared samples were investigated by various techniques including X–ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Brunauer–Emmett–Teller (BET) to determine the morphology and the surface of samples. The photocatalytic activity was also evaluated via the photodegradation of organic dyes (including methylene blue and rhodamine B) and phenol under visible light irradiation. In addition, the active species trapping test was also conducted to determine the main active species during photocatalytic process by using isopropanol (C3H8O), benzoquinone (C6H4O2), and ammonium oxalate (C2H8N2O4) as scavengers to trap OH·, O2·- and h+ respectively.

The results showed that BiVO4, BiVO4/SiO2 and BiVO4/SiO2/GO nanocomposites were synthesized successfully by using hydrothermal method. In which, pure BiVO4 was composed of tetragonal phase (JCPDS cards No. 14–0133) and monoclinic phase (JCPDS cards No. 14–0688) at pH 1. However, the tetragonal phase disappeared when pH value was increased, and BiVO4 became a single monoclinic structure. Besides, the further increase from pH 5 to pH 9 caused the decrease of the crystallinity and the crystal gain of BiVO4. In the meantime, the hydrothermal temperature and the reaction time had no effect on the crystalline phase of BiVO4. As a result, the optimal condition of BiVO4 fabrication was pH 5, 180oC and 10 h. The same XRD peaks of monoclinic BiVO4 were observed in the case of BiVO4/SiO2 core–shell and BiVO4/SiO2/GO nanocomposite due to the lower content and the reduction of GO to graphene.

The SEM and TEM results showed that the as–prepared BiVO4 was the constitution of amorphous nanoparticles with a size of several nanometers. Similarly, BiVO4/SiO2 core–shell was also fabricated from the different aggregation of amorphous nanoparticles. In the case of BiVO4/SiO2/GO nanocomposite, BiVO4 and SiO2 nanoparticles were deposited uniformly on the GO nanosheets because the reduction to graphene and the functional groups of GO nanosheets facilitated to the adhesion of BiVO4 and SiO2 nanoparticles. For the evaluation of the photocatalytic activity, all of samples exhibited the good efficiency for the photocatalytic degradation of MB. In which, 35% of MB concentration was removed in the dark condition, and 70% was degraded further after 30 min of visible light irradiation in the presence of pure BiVO4. By adding SiO2, the adsorptive capacity was increased significantly contributing to 84% of the removed MB concentration. In the meantime, BiVO4/SiO2/GO nanocomposite showed the highest efficiency for the adsorption and the photocatalytic degradation of MB with 82% and 94% respectively. This was because the specific surface area and the pore size were increased significantly by coupling with SiO2 and GO nanosheets.

In addition, the influence of GO dosage on the photocatalytic activity of nanocomposite was also examined in this work. The increase of GO dosage from 1 to 1.5% improved more the adsorptive and the photocatalytic capacity of nanocomposite. However, the further increase of GO dosage could limit the active surface of photocatalyst and the light exposure, leading to the decrease of photocatalytic activity of nanocomposite. In addition, it was found that nanocomposite exhibited too high adsorption instead of showing the high photodegradation rate for MB. This meant that MB molecules were not photodegraded completely to CO2 and H2O. As a result, the combination of BiVO4, SiO2 and GO made the best adsorbent with the high removal efficiency in this study.

In the case of RhB dye, only 19% of RhB concentration was removed in the presence of BiVO4 nanoparticle. The removal efficiency was increased slightly for BiVO4/SiO2 core–shell, and 30% of RhB was removed by BiVO4/SiO2/GO nanocomposite under visible light irradiation. The reason was that the dissociation of carboxyl acid group in the structure of RhB depend on the pH value of the reaction media leading to the formation of negatively charged molecule (R–COO–), hence causing the electrostatic repulsion and reducing the photodegradation rate.

In addition to the sensitization of the dye, phenol as a typical colorless pollutant was also chosen to evaluate the photocatalytic activity of as–prepared samples. Due to the extremely stable structure of phenol, the photocatalytic experiment of phenol was assisted by adding 1 mL of hydrogen peroxide (H2O2, 30%) as electron acceptor. No adsorption was observed for all of photocatalyst because phenol existed as phenoxide ions with negative charge in aqueous environment. Unexpectedly, pure BiVO4 showed the highest efficiency for phenol because H2O2 reduced the recombination rate of electron – hole pairs leading to more formation of OH·, thereby increased much the photocatalytic activity of BiVO­4.

In the meantime, the lowest efficiency was obtained by BiVO4/SiO2 core–shell because SiO2 could play as a barrier which adsorbed or reacted with H2O2, thus reducing the formation OH· and decreasing the photocatalytic degradation. This was improved by the addition of GO nanosheets due to the enhancement of the migration and the separation of electron–hole pairs, leading to the increase of photodegradation of phenol over nanocomposite. In active species trapping test, results revealed that the recombination rate of electron–hole pairs of pure BiVO4 was too fast, leading to less formation of OH·. Therefore, scavengers could react as an intermediate center which reduced the recombination rate and then increased the photocatalytic activity of pure BiVO4. In the meantime, scavengers could be adsorbed by SiO2 and GO, hence no trapping reaction occurred for BiVO4/SiO2 core–shell and BiVO4/SiO2/GO nanocomposite. As a result, the active species trapping test was useless to determine the main species during the photocatalytic activity of composite systems in this work.

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