Article pubs.acs.org/IECR
Photocatalytic Properties of CdS Nanoparticles Synthesized under Various Ultrasonic Operating Conditions Krishnamurthy Prasad and Muthupandian Ashokkumar*,# School of Chemistry, University of Melbourne, Parkville, Victoria 3010, Australia ABSTRACT: Photocatalytic activities of nanosized CdS particles synthesized using ultrasound (US) assistance were compared with those synthesized using mechanical agitation. The mode of US input had an effect on the properties and catalytic activity of CdS nanoparticles. The CdS samples synthesized using an US horn (USH) showed lower photocatalytic degradation of an organic dye, Reactive Blue 4 (72−80%), as compared to those synthesized in an US bath (USB; 94%) and using mechanical stirring (NUS; 86%). Samples synthesized using a high-shear Ultra-Turrax (HSH) showed a similar degradation (85%) to that of the NUS sample. However, when the power input per unit volume (W/L) was considered, the degradation obtained with the HSH sample (0.22%/W/L) was inferior to the NUS sample (0.57%/W/L). The shear intensive HSH and USH processes showed higher percent degradation per unit surface area. However, the percent degradation per W/L was lower for these compared to the NUS (no shear) and USB (low shear) processes. Overall, the sample prepared using the USB process showed the highest percent degradation per W/L (6%/W/L).
1. INTRODUCTION Band-gap excitation of semiconductor materials leads to the formation of electron−hole pairs that drive photocatalytic processes. The most widely used material for photocatalysis is TiO2 owing to its high activity, chemical resistance, and low toxicity. However, TiO2 may be utilized as a catalyst only in the presence of UV light,1 and this reduces its potential use as a visible (solar)-light catalyst owing to the low proportion of UV light in solar radiation (180 °C) and removal of SDS (>200 °C).
Figure 14. State of Al foil after ∼15 s of sonication in the ultrasound bath used in the current study. 720
dx.doi.org/10.1021/ie4039373 | Ind. Eng. Chem. Res. 2014, 53, 715−722
Industrial & Engineering Chemistry Research
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indicating the similarity among the products synthesized using a high-shear process. Overall, there was not a significant change in D1 across the various syntheses. The individual electric power input (P), measured using an Arlec energy meter, in each synthesis is shown in Table 6. The W/L was calculated considering the reaction volume (75 mL). However, for the USB system, in contrast to the other syntheses, the electrical power (188 ± 2 W) input was to the whole bath, so to estimate power input per unit volume we need to take into account the volume of water in the bath (V = 12 000 mL) and the volume of reaction solution processed (v = 75 mL). Thus, the W/L for the USB process is given by
monitored by UV−vis spectroscopy, and the drop in concentration of the dye for the various samples is shown in Table 5. As can be seen, the NUS and USB samples show the largest dark adsorption values. This is consistent with their increased surface area values. The dark adsorption values observed were quite high, and such an observation with CdS photocatalysts was also made by Zhu et al.27 The observation is attributed to the presence of a large number of small-sized pores on the surface of the synthesized samples.27 Gas sorption analysis can also provide information regarding the pore-size distribution in the samples. When the pore-size distributions of the samples were compared (Figure 8), it was found that both the NUS and USB samples had the largest abundance of smaller pores. Thus, the extent of adsorption that was seen for the NUS and USB systems is also the highest. The natural pH of the solution containing the dye and catalyst solution was found to be 4 ± 0.1. To see the effect of solution pH on the adsorption and catalytic activity, the pH of the solution was adjusted to 11 using a 0.1 M NaOH solution. The dark adsorption value for the USB sample was then measured. It was found that the value dropped from 52 (under no pH control) to 10%. At high pH values, both the dye and the surface of the catalyst would be negatively charged,28,29 causing electrostatic repulsion and leading to a lower adsorption. After stirring in the dark, the photocatalytic activities of the samples were investigated, and the results are shown in Figure 9. The values of absorbance obtained after 1 h of dark adsorption was considered the zero point. All of the photocatalytic reactions showed an initial increase in degradation followed by a plateau. The reason may be the reduction in dye concentration with increasing reaction time. The overall degradation values after 4 h ranged from 72 (USH L) to 94% (USB). The USH- and HSH-synthesized samples showed a lower extent of degradation as compared to the USB and NUS samples, and this could be on account of the reduced surface area of the samples. Photocatalytic degradations of the dye solution at pH 11 and that observed under the no pH control are shown in Figure 10. The extent of degradation observed at pH 11 is much lower than that when the no pH control was exerted. This is due to the electrostatic repulsion between the dye and the catalyst surface, as discussed earlier. As all reactions were carried out using the same amount (0.07 g) of catalyst, an effective way of estimating the catalyst efficacy would be the effective degradation per unit surface area (A) of catalyst given by
PUSB =
P V+v
= 16 W/L
(5)
(4)
The trend of D2 versus synthesis method is shown in Figure 12. It can be seen that the sample synthesized using the US bath (6%/W/L) seemed to be effective as compared to the NUS (0.57%/W/L), HSH (0.22%/W/L), and USH (0.09%/ W/L, 0.07%/W/L, and 0.06%/W/L for USH L, USH H, and USH 2, respectively) samples. Thus, for this particular system, a reaction technique that has a relatively low power density and shear was found to be the most effective, and the high shear in the USH and HSH processes seemed to have a rather detrimental effect on the system, as seen from the reduced specific surface area and catalytic activity. However, for the USB process, there is still a significant amount of electrical power input into the system and thus carrying out just a single synthesis is not viable. To prove the advantage of using a US bath as a reaction medium, it is necessary to carry out multiple syntheses simultaneously. To this end, two syntheses of CdS (labeled CdS S1 and CdS S2) were carried out in the US bath simultaneously by positioning two reaction cells at two different active locations of the US bath and compared to the averaged USB sample values. The photocatalytic activities obtained, shown in Figure 13, were found to be similar to what was obtained when just a single synthesis is carried out. This seems to indicate that the overall energy required for synthesizing an effective photocatalyst by this technique is quite low and that there is no need to utilize an energy-intensive synthetic technique, such as a US horn. In addition, the ability to carry out multiple syntheses makes the US bath an efficient system for larger-scale operations. However, a US bath always has a significant amount of dead (unutilized) volume even if all of the active zones in the bath are used for carrying out reactions. This can be visually observed in Figure 14. The holes on the Al foil seen are the active spots (where the majority of cavitation occurs) of the bath. It can be concluded that cavitation is not generated in all regions of the liquid. Only certain regions are active. Thus, taking this into account along with the fact that this particular system requires only mild cavitation to show improvements, a setup like a low-intensity flow-through ultrasonic unit could have potential for use, which will be the focus of our future investigation.
In terms of effective degradation per unit area (D1) shown in Figure 11, the sample USH 2 showed the highest value (48%/ m2) as compared to the NUS (32%/m2) and USB (36%/m2) samples. The value shown by the HSH sample (38%/m2) seemed to be comparable to that shown by the US horns,
5. CONCLUSIONS By changing the mode of ultrasonic input and energy density, it was possible to synthesize CdS possessing varying catalytic activity. The US horn (USH H, USH L, and USH 2) syntheses
D1 =
total degradation after 4 h (%) 0.07A
(3)
In addition, the power input per unit volume (W/L; Table 6) into the system also needs to be taken into account when considering the most effective catalyst. The percent degradation per W/L (D2) was calculated as D2 =
total degradation after 4 h (%) power input per unit volume (W/L)
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resulted in a product with a reduced specific surface area and an improved degradation per unit area but a very low degradation efficiency per unit electric power input, affirming them as power-intensive processes. The NUS process produced a catalyst with a relatively high surface area but with a lower D1 than the high-shear HSH process. However, in terms of D2, the HSH process was inferior compared to the NUS process and was comparable to the USH processes. The USB sample possessed a high specific surface area and showed the highest values of D2 among all of the synthesized samples. Hence, a mild cavitation environment, observed in the USB process, is effective in generating an efficient photocatalyst. In addition, for this system, the success of multiple simultaneous syntheses made the USB process an ideal technique for producing a product with high catalytic efficacy at low energy levels.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +61 3 83447090. Fax: +61 3 93475180. Present Address #
Adjunct Professor, Chemistry Department, King Abdulaziz University, Jeddah, Saudi Arabia. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Dr. Meifang Zhou for the TEM images. K.P. acknowledges the Melbourne International Fee Remission and Melbourne International Research Scholarships provided by the University of Melbourne.
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REFERENCES
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