Microwave-assisted synthesis and enhanced photocatalytic performance of Bi 2 O 2 CO 3 nanoplates

In this research, Bi 2 O 2 CO 3 (BOC) nanoplates as a semiconductor-based photocatalyst composed of [Bi 2 O 2 ] 2+ configuration layers were used for environmental treatment. Highly crystalline and pure phase of BOC nanoplates were successfully synthesized by a microwave-assisted method. Effect of irradiation time and microwave heating power on phase, purity, crystallinity, particle size and morphology of the as-synthesized products was investigated. The as-synthesized BOC nanoplates were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) spectroscopy, Brunauer-Emmett-Teller (BET) surface area analysis, photoluminescence (PL) spectroscopy and UV-visible spectroscopy. Upon increasing microwave heating power, purity and crystallinity of the as-synthesized products were improved. Due to the existence of double internal electric field, the separation of photo-induced charged carriers was enhanced, leading to promote photocatalytic activity of the BOC using the BOC nanoplates synthesized by 600 W microwave for 60 min. A formation mechanism of BOC nanoplates was also proposed and discussed according to the experimental results. The crystallite best semiconducting photocatalytic decompose The photocatalytic performance of BOC photocatalysts SEM of The particle size distribution by dynamic light Malvern The specific surface area was measured on a Nova surface area analyzer (Quantachrome Instruments) by nitrogen adsorption through the Brunauer-Emmett-Teller (BET) method. Ultraviolet visible spectra were recorded on UV-visible spectrophotometer


Introduction
Semiconductor-based photocatalysts are considered as promising materials in purification and disintegration of pollutants containing in wastewater, remedy for hazardous waste and wastewater treatment as compared with traditional strategies [1][2][3][4][5][6]. They are harmless to surrounding environment and can be used to degrade organic pollutants to non-hazardous products through photo-induced oxidation reaction [1][2][3][4][5][6]. However, the properties of conventional semiconductor photocatalysts are severely limited by several factors: low utilization of visible -light and fast recombination of photogenerated charge carriers [4][5][6][7][8][9]. The crystalline structure and surface of semiconducting materials are able to play the role in their physical and chemical properties [1, 4,6,9,11]. Thus, many efforts have been devoted to modify semiconductors and to explore for new UV-visible light responsive photocatalysts.
Among the investigated photocatalysts, bismuth-based semiconductors, particularly Sillén-type structure BiOX (X = Cl, Br, I) [1-4], Aurivillius structure Bi2XO6 (X = W, Mo) [5][6][7][8][9][10], Sillen-structure-related Bi2O2[BO2(OH)] [11][12][13] and Bi2O2(OH)(NO3) [14] have been intensively studied due to their unusual layered structure, adequate chemical stability and superior photocatalytic performance. These materials are composed of [Bi2O2] 2+ fluorite structure layers and interleaved halide ions, perovskite-like (Am-1BmO3m+1) 2+ or BO3 3+ /NO3ionic blocks, which can induce strong internal static electric field. Consequently, the separation of electron-hole pairs and diffusion of photoinduced charges are promoted. Comparing to bulk photocatalysts without layered structure, the layered structure photocatalysts are mor e favorable environment for diffusion and separation of photoexcited electron-photoinduced hole pairs because the oxidation and reduction sites exist on the surfaces and edges of the sheet units. Holes generated in the layered photocatalysts diffuse to a very short distance before reaching the surface of the sheet units. They can be trapped by interlayer water molecules. The rapid hole-trapping process allows electrons to freely and sufficiently diffuse through the sheet units until reaching the sheet edges.
Microwave is a good method used to generate interaction between chemicals occupying electric charges in order to create internal heat during the proceed reaction. The resulting volumetric heating can lead to homogeneous nucleation, high reproducibility and well-defined particle size with narrow size distribution. Moreover, it is much rapid process, shorter crystallization time, high yield product by decreasing the formation of impurities during the reaction and enhanc ing the reaction rate comparing with other conventional heating method [33][34][35][36][37].
In the present study, the effect of irradiation time and microwave power on the morphology and optical properties of BOC nanostructure obtained by a microwaveassisted aqueous solution method was studied. The as-synthesized products were characterized by different techniques to determine phase, morphology, crystallite size, atomic vibration and optical properties. The best semiconducting materials were used for photocatalytic process in order to decompose organic pollutants induced by UV light. The photocatalytic performance of BOC photocatalysts was evaluated 4 through the decomposition of methyl orange (MO) under UV light irradiation . In addition, a possible mechanism for photodegradation of organic pollutants by BOC photocatalysts was proposed.

Preparation of Bi2O2CO3 nanostructure
The Bi2O2CO3 (BOC) nanoplates were synthesized by a microwave assisted synthesis method. All the reagents used in the experiment were analytical-grade and were used without any further purification. In a typical synthetic procedure, 4.8507 g of Bi(NO3)3·5H2O was first dissolved in 10 ml of 1 M HNO3, labeled as solution A.
Concurrently, 8.4792 g of Na2CO3 was dissolved in 90 ml deionized water, labeled as solution B. When the above solutions were clear, the solution B was dropped into solution A with 30 min stirring and plenty of white precipitates formed. Then, the whole system was processed at different synthetic conditions by varying of microwave power from 100 W to 600 W and irradiation time of 15-60 min. In the end, the as-synthesized products formed, and were washed with deionized water and ethanol three times to remove some residual reactants. The final products were dried in air at 70 ºC for 12 h for further characterization.

Characterization
The as-synthesized products were characterized by different techniques to identify phase, morphology, structure and constituent. Phase of the samples was

Photocatalysis
The photocatalytic activities of Bi2O2CO3 were evaluated through the (1) C0 is the initial dye concentration and C is the dye concentration after photocatalytic test for a period of time (t).

Phase and chemical analyses
Crystalline structures of the as-prepared samples were examined by X-ray  7 The SEM images of BOC synthesized by a 600 W microwave power for 15 min, 30 min, 45 min and 60 min are showed in Figure 5. At 600 W microwave and 15 min, the product was incomplete nanoplates. They became complete plates when the length of microwave exposure time was as long as 60 min. By using the length of time of 60 min at different microwave powers (Figure 6), the as-synthesized products with different morphologies were synthesized. Obviously, increasing in the heating power can lead to generate more defined structure with more homogeneous particle size and

BET surface area and particle size distribution
The BET specific surface areas of the BOC nanoplates synthesized at different microwave exposure periods and microwave heating powers showed that the BOC sample at 100 W exhibited specific surface area larger than that at 600 W. The morphology and specific surface area of the BOC nanoplates were controlled by microwave power. The surface area of the BOC sample decreases significantly with increasing in the microwave power and the opposite is true for the average grain size. and 450 W were uniform nanoplates with narrow particle size distribution. The BOC sample at 600 W shows the highest mean particle size. Upon increasing microwave power, the higher amount of energy is supplied to the reaction medium. This energy is able to readily stimulate nucleation rate, thus resulting in large BOC nanoparticles.
These unstable nanoparticles tend to agglomerate and become more stable nanoplates.

Growth of BOC nanoplates by microwave-assisted method
Based on the experimental results, a formation mechanism of the BOC nanoplates is probably related to the dissociation of Bi(NO3)3 in HNO3 solution to produce BiONO3 intermediates. Upon mixing the solution with an aqueous Na2CO3 solution, H + rapidly reacted with CO3 2and CO2 was released. Concurrently, the asobtained BiONO3 reacted with excessive CO3 2ions to form Bi2O2CO3 with reduced solubility. A mechanism for the formation of BOC nuclei is as follows.
In this case, a possible formation mechanism of BOC nanoplates by varying microwave irradiation power is shown in Figure 9. In the early stage, the initial BOC nuclei acted as dipoles in the mixture solution under the electric field of microwave.
Upon illumination by microwave with different powers, the obtained BOC nuclei polarize and aggregate to form BOC particles. In the next step of reaction, the adjoining nanoparticles grew along the oriented direction, and agglomerated nanoplates formed. The sample processed at 100 W contains irregular plates. The morphology changed to plate-like structure at 180 W, 300 W, 450 W and 600 W. The plate like particles did not completely maintain at 300 W and 450 W comparing to those at 600 W. These plate-like particles formed by stacking of self-assembled spherical nanoparticles in a proper orientation of a particular direction. Significant enhancement of the crystallization was detected with increasing in the applied microwave power and 9 a little influence of reaction time. High microwave power with high electric field is likely to create more stable nanoplates than low microwave power with low electric field.

Light absorption, charge separation and photoluminescence
The optical properties of as-prepared BOC nanoplates synthesized by different microwave exposure periods of time and microwave heating powers are shown in (αhν) = A(hν -Eg) n/2 (6) , where α is the absorption coefficient, hν is the photon energy, A is a constant and Eg is the energy band gap [39]. Among them, n is controlled by the transition characteristic in a semiconductor: direct transition (n=1) or indirect transition (n=4 Photoluminescence spectra were investigated to determine the recombination of electron-hole pairs, including the charge separation efficiency. Photoluminescence of the as-synthesized BOC nanoplates synthesized at different microwave exposure periods and microwave heating powers are shown in Figure 12. For the as-prepared BOC nanoplates at different microwave exposure periods and microwave heating powers, the spectra are very similar in shape. The emission is around 350-600 nm, and BOC 600 W at microwave exposure time of 60 min is the lowest. This suggests that the photogenerated charge carriers with the lowest irradiative recombination and efficient charge separation can be improved by the use of BOC at 600 W and 60 min.

Photocatalysis
The plate-like BOC samples were used for photocatalytic test. The photocatalytic activities of the as-prepared BOC nanoplates synthesized at different microwave exposure periods and microwave heating powers were measured through the degradation of MO solutions under UV light irradiation. Figure 13 shows  min and BOC 60 min within 120 min irradiation, respectively. The degradation rate can be described by the pseudo-first-order kinetics expressed as follows [40].
C0 is the initial concentration of MO solution, C is the MO concentration within the irradiation time (t) and the slope k is the apparent reaction rate constant [40]. The firstorder kinetic plots for MO degradation photocatalyzed by the BOC samples synthesized at different microwave exposure periods are shown in Figure 14

Recyclability of BOC nanoplates
It is well acknowledge that heterogeneous catalysts are superior to the recyclability and stability process. During the photocatalytic process, photo-corrosion of catalysts can lead to significant decrease activity or even complete loss of catalytic activity. Thus, the photostability of the BOC samples was investigated using the degradation of MO as a probe reaction. Aqueous MO with initial concentration of 1.0x10 -5 M was almost completely removed in each run and there is no obvious decrease in activity after the end of cycle five ( Figure 18). The final catalyst collected by centrifugation was characterized by XRD and FTIR which demonstrate no significant change in BOC structure ( Figure 19). The results confirm that the BOC nanoplates show high photostability and have good potential in wastewater treatment.

Conclusions
A simple, fast, cost effective microwave-assisted aqueous solution method was used to synthesize BOC nanostructure with high crystallinity and purity. They were found that effect of microwave power and microwave heating time played the role in both morphology and crystallinity of the BOC nanostructure. A possible formation mechanism of the BOC nanoplates was proposed based on the interaction of microwave with materials. In this research, the BOC sample prepared at 600 W microwave for 60 min has the highest photodegradation of MO.