The Effect of Fabrication Conditions on the Morphology and Photocatalytic Activity of Bismutite Bi 2 O 2 CO 3 Particles

In this study, three-dimensional bismutite Bi2O2CO3 nanoparticels (BSC NPs) have been synthesized through a facile hydrothermal treatment under mild conditions. The reaction temperatures and NaOH concentration have a vital influence on the physical and photocatalytic properties of the obtained BSC NPs. The crystal structure, morphology, chemical composition, specific surface area and photoresponse of as-obtained catalysts were characterized by X-ray diffraction, Scanning electron microsprctrosopy, Energy-dispersive spectroscopy, N2 adsorption/desorption isotherms and UV-Vis spectra, respectively. Furthermore, Rhodamine 6G was used as model reaction to evaluate the photocatalytic activity of BSC NPs. As a result, there was no obvious effect of hydrothermal reaction temperature and NaOH concentration on phase structure and UV-visible light response; while the morphology, BET surface area and photoactivity were affected by hydrothermal reaction temperature and NaOH concentration.


Introduction
With the increasingly serious environmental problems and dispassion of fossil fuels, the elimination of hazardous chemicals through photochemical degradation technology has attracted enormous research interest [1].In terms of photo-oxidation approach, it has many unique advantages including mild reaction conditions, low-energy consumption and environmental friendly.However, the photo-oxidation reactions suffer from sluggish kinetics and great energy barrier need to be overcome to achieve a appreciate conversion efficiency.
In recent years, the bismuth-based compounds have attracted much research interest and show excellent performance in a variety of fields such as biomedical fields, solid oxide fuel cells, oxygen sensors, varistors or electrochromic devices as well as environmental science [2]- [8].Specifically, BSC show significant potential in photocatalysis owing to its excellent photochemical properties according to early reports.For example, Zhang et al. have synthesized 3D BSC nanocrystals for selective adsorption and separation of dyes [9].Nevertheless, the effect of fabrication conditions of BSC on the photocatalysis performance has been investigated yet [10].Therefore, the study of fabrication conditions which may have crucial effect on the morphology and photocatalytic properties is necessary [11].About 1% -20% of the total world textile dyes and other industrial dyes are released to waste water [12].Dye wastewater is typically characterized by strong color, stability and, biotoxicity and carcinogenic effects even at very low concentration, which is harmful to our health and environment [13] [14].The removal of dyes from waste water has received considerable attention.Compared with traditional physical techniques such as adsorption, ultrafiltration, reverse osmosis and ion exchange that would result in secondary pollution, photocatalytic oxidation can degrade dyes to CO 2 and H 2 O [12].Rhodamine 6G (R6G) is a typical dye that is chemically stable and poorly biodegradable in waste water.In this paper, we therefore selected R6G as model pollutant to determine the photocatalytic activity.The influence of fabrication conditions on the physical properties and photocatalytic activity of BSC were investigated.

Catalyst Preparation
Bismuth nitrate (Bi(NO 3 ) 3 ), urea (CO(NH 2 ) 2 ) and sodium hydroxide (NaOH) were of analytical grade and used without any further purification.Deionized water was used in the whole fabricating process.Three-dimensional bismutite Bi 2 O 2 CO 3 (BSC) was synthesized by a hydrothermal reaction under mild conditions.10 mmol of bismuth nitrate and 40 mmol of urea were dissolved in 30 ml of 0.5 -2 M sodium hydroxide solution.The mud mixture was stirred for 2 h at room temperature with a stirring rate of 600 rpm, and then poured into a Teflon autoclave.The autoclave was put in an oven and allowed to be heated at 120˚C -160˚C for 6 h under autogenous pressure, and then cooled to room temperature.
The precipitates were collected and washed with deionized water and absolute ethanol for 3 times each to remove any residual ions.The final product was dried at 80˚C for 6 -12 h for further use.The BSC samples synthesized using 0.1 M NaOH and at 120˚C, 140˚C, and 160˚C were noted BSC1, BSC2, BSC3, respectively; and BSC4, BSC5, BSC6 were prepared at hydrothermal temperature of 120˚C using 0.5, 1, 2 M NaOH respectively.

Characterizations
X-ray diffraction (XRD, D8 ADVANCE) with Cu Kα radiation (25 mA and 40 kV) was used to investigate the structure of the BSC samples.The morphology and composition of the samples were characterized by a scanning electron microscope (SEM, JEOL JSM-7001F) and Energy dispersive spectrometer (EDS) that is attached to JEOL JSM-7001F.UV-Vis absorption spectra of BSC were scanned by a spectrophotometer in the range of 200 -800 nm (UV-VIS, SHIMADZU UV-2600).The specific surface area, pore size distribution and pore volume were measured by Tristar II3020 and the specific surface area was calculated with the Brunauer-Emmett-Teller (BET) method (Analysis Adsorptive: liquid N 2 ).The adsorbed amount at a P/P 0 value of 0.99 was used to estimate the total pore volume.The Barrett-Joyner-Halenda (BJH) method was utilized to calculate the average pore diameter from desorption branch of the isotherm.The as-obtained catalysts were used for photodegradation of R6G and the corresponding photocatalytic activity was investigated by UV-vis spectroscopy.

Surface Area and Pore Size Distribution
BET surface area, pore size distribution and pore volume of all as-obtained sample are listed in Table 1.Apparently, the BET surface area and pore volume decrease with the increase of hydrothermal temperature, which is in well agreement with the result observed in SEM images.However, there is no distinct change in BET surface area and pore volume of BSC1 and BSC4 with the change of NaOH concentration (from 0.1 to 0.5 M).However, the BET surface area decreases sharply when NaOH concentration increases to 1 and 2 M, which attributes to the particle aggregation along with NaOH concentration increasing.This further confirms that NaOH plays dual function.The average pore diameter, calculated by the BJH method based on the desorption branch, shows that the BSC samples have a very clear mesoporous structure with pore size centered at 13.4 and 42.8 nm.The photodegration reaction was carried out at room temperature and the pH was adjusted by adding 0.1 M HCl.As reported in our previous work [15] R6G is positively charged in the pH range of below 6.0, BSC surfaces are negatively charged with a zeta potential of about −40 mV.There is bigger binding force between R6G and BSC when the pH is below 6.The results in Figure 5 indicate that all photocatylysts show high photoactivity.However BSC3 and BSC6 have a little lower photoactivity.This is attributed to the low BET surface area of BSC3 and BSC6, which would decrease the contact surface between catalyst and reactant.The photocatalyst reacts with water to form hydroxyl radical (•OH), which can rapidly attacks pollutants at the surface and in solution as well.Hydroxyl radical (•OH) is a very powerful oxidant with the oxidation potential of 2.8 V (NHE).Photocatalyst can mineralize organic pollutants into CO 2 , H 2 O and so on [16] [17].

Conclusion
Three-dimensional bismutite BSC were prepared through a hydrothermal synthesis with bismuth nitrate, urea and sodium hydroxide under mild conditions.There are no obvious effects of hydrothermal reaction temperature and NaOH concentration on the structure and light response of BSC.However, the particle size, BET surface area, pore volume and photoactivity decrease with the increase of hydrothermal reaction temperature and NaOH concentration.It suggestes that the concentration of NaOH plays dual role in the fabrication of BSC.

Figure 2
Figure2shows the morphology of different BSC samples.Similarly, no obvious

Figure 2 .
Figure 2. SEM images of BSC samples at different fabrication conditions.

Figure 4
Figure 4 shows UV-Vis diffuse reflectance spectra of different photocatalysts BCS1 -BSC6.The results indicate that different BSC samples have almost same light response.The photodegration reaction was carried out at room temperature and the pH was adjusted by adding 0.1 M HCl.As reported in our previous work[15] R6G is positively charged in the pH range of below 6.0, BSC surfaces are negatively charged with a zeta potential of about −40 mV.There is bigger binding force between R6G and BSC when the pH is below 6.The results in Figure5indicate that all photocatylysts show high photoactivity.However BSC3 and BSC6 have a little lower photoactivity.This is attributed to the low BET surface area of BSC3 and BSC6, which would decrease the contact surface between catalyst and reactant.The photocatalyst reacts with water to form hydroxyl radical (•OH), which can rapidly attacks pollutants at the surface and in solution as well.Hydroxyl radical (•OH) is a very powerful oxidant with the oxidation potential of 2.8 V (NHE).Photocatalyst can mineralize organic pollutants into CO 2 , H 2 O and so on[16] [17].
for sponsoring her visiting research at Monash University through "Jiangsu Overseas Research & Training Program for University Prominent Young & Middleaged Teachers and Presidents".

Table 1 .
Macroscopic physical properties of BSC.
a Volume of pores between 1.7 nm and 300 nm.