Facile synthesis of meso-porous ZnO nano-triangular prisms with enhanced photocatalytic activity

Abstract

We report a simple route for the synthesis of ZnO nano-triangular prisms (ZnO-nt) with excellent photocatalytic activity. The activity of ZnO-nt was superior for the degradation of rhodamine-B (Rh-B) dye, when compared to ZnO-nanorods (ZnO-nr) and ZnO nano-particles (ZnO-np). The enhancement in photocatalytic activity of ZnO-nt is attributed to its special morphology, good surface area, high number of oxygen vacancies and suppression of electron–hole recombination. The results testify that ZnO-nt could act as a promising photocatalyst for the remediation of dye contaminated water under solar light irradiation.

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1. Introduction

Semiconductor photocatalysis is a low cost technology to solve the energy and environmentally related problems with the help of inexhaustible solar energy.^1,2 Natural sunlight consists of both UV and visible light. Hence, the photocatalyst must be active under UV and visible light for practical applications. Recently, the measurement of photocatalytic activity under simulated solar light or UV-vis irradiation has been a hot topic of research. The key factors in the design of a photocatalyst are morphology, surface area, visible light absorption capacity, electron mobility, catalytic efficiency and stability.³ In recent years, the design of photocatalysts with novel architectures is of great interest because of their unique chemical and physical properties.⁴ Zinc oxide (ZnO) is an inexpensive and wide band gap semiconductor photocatalyst with similar characteristics to those of TiO₂.⁵ The photocatalytic applications of ZnO with different nano morphologies such as rods,⁶ wires,⁷ belts,⁸ sheets⁹ and tubes¹⁰ have been reported. The photocatalytic activity of ZnO nano-triangular prisms (ZnO-nt) has not been reported so far. M. Salavati-Niasari et al.¹¹ studied the synthesis, characterization and optical properties of ZnO nano-triangles.

Herein, we report the synthesis, characterization and photocatalytic activity of ZnO-nt. ZnO-nt was characterized using X-ray diffraction (XRD), transmission electron microscope (TEM), high resolution TEM (HR-TEM), BET surface area analysis, UV diffuse reflectance spectroscopy (UV-DRS) and photoluminescence (PL) techniques. The photocatalytic activity was evaluated for the degradation of rhodamine-B dye (Rh-B) under simulated solar light irradiation. The results of this research work were also compared with some recently reported catalysts.

2. Experimental

All the chemicals used were of analytical grade and used as received without further purification.

ZnO-nt was synthesized using a hydrothermal method as follows: 0.1 M of zinc acetate di-hydrate solution was prepared using ethanol. Then, the pH was adjusted to 3 using acetic acid and the solution was stirred for 3 h at room temperature. The final mixture was transferred into a Teflon coated stainless steel autoclave and heated at 120 °C for 3 h. The product was centrifuged, washed with double distilled water, ethanol, dried at 100 °C for 24 h. For comparison, ZnO-nr and ZnO-np were synthesized by the reported procedures. ZnO-nr was prepared from the calcination of zinc acetate at 350 °C.⁶ ZnO-np was also prepared from zinc acetate using precipitation method.¹²

The detailed characterization techniques and procedure for the measurement of photocatalytic activity was already given in our previous report.¹³ The Rh-B dye with 10 ppm of initial concentration was taken for the photocatalytic measurements. The solar light irradiation was simulated by 150 W Xe arc lamp. Chemical oxygen demand (COD) of Rh-B after photoreaction was determined by the dichromate oxidation method.¹⁴

3. Results and discussion

3.1 Characterization

The crystalline phases of the synthesized samples were analyzed using XRD and the results are shown in Fig. 1(a). The diffraction peaks of ZnO-nt, ZnO-nr and ZnO-np are perfectly matched with the standard JCPDS pattern of ZnO (36-1451). No other impurity phases are detected, which indicates the high purity of the products. It is also noted that the intensity of the diffraction peaks are differed with respect to their morphology (see Fig. S1 and Table S1 in ESI†). The differences in peak intensity could be explained as follows: ZnO consists of a polar zinc terminated plane (001), polar oxygen terminated plane as (001) and non-polar plane as (100).¹⁵ The growth of ZnO is different in solvent with different polarities. It is ascribed to the interaction between solvent and polar/non-polar planes of ZnO. The morphology of ZnO is also related to the difference in surface free energies of the main crystallographic planes [(100), (002) and (101)] of ZnO.¹⁶

Fig. 1 (a) XRD patterns of ZnO-nt, ZnO-nr and ZnO-np. (b), (c) and (d) TEM images of ZnO-nt (e) and (f) HR-TEM images of ZnO-nt.

TEM images of ZnO-nt are shown in Fig. 1(b–d). It can be observed that ZnO is composed of more number of nano-triangular prisms with a length range from 50 nm to 60 nm. Some nano-hexagonal shaped ZnO are also formed in addition to the nano-triangular prisms. HR-TEM was taken to investigate the existence of meso-pores in ZnO-nt. The HR-TEM images of ZnO-nt are displayed in Fig. 1(e and f). The results suggest that ZnO-nt has some meso-pores in its structure.

The N₂ adsorption–desorption isotherms of ZnO-nt, ZnO-nr and ZnO-np are shown in Fig. S1.† The isotherm of ZnO-nt is identified as type IV, according to IUPAC data, which is the characteristic of meso-porous materials. The isotherms of ZnO-nr and ZnO-np are classified as type III with non-porous nature. The specific surface area of ZnO-nt, ZnO-nr and ZnO-np is found to be 19.83 m² g⁻¹, 7.66 m² g⁻¹ and 2.96 m² g⁻¹, respectively. The average pore diameter of ZnO-nr (68.87 nm) and ZnO-np (155.78 nm) are higher than that of ZnO-nt (32.89 nm). The high surface area of ZnO-nt could make it more photoactive than ZnO-nr and ZnO-np.

Fig. 2 displays the UV-DRS of ZnO-nt, ZnO-nr and ZnO-np. There is no remarkable difference in the absorption edge of ZnO-nt, ZnO-nr and ZnO-np. The band gap energies were estimated by applying Tauc method.¹³ The band gap energies of ZnO-nt, ZnO-nr and ZnO-np are found to be 3.07 eV, 3.09 eV and 3.10 eV, respectively.

Fig. 2 UV-DRS of ZnO-nt, ZnO-nr and ZnO-np.

3.2 Photocatalytic activity

The photocatalytic activity of the samples was examined for the degradation of Rh-B (10 ppm) under simulated solar light irradiation. The concentration of the catalyst was fixed at 1.25 g L⁻¹. Fig. 3(a) illustrates the photo-degradation of Rh-B as a function of irradiation time over ZnO-nt, ZnO-nr and ZnO-np. The degradation efficiency is expressed as C/C₀, where C and C₀ are the final and initial absorbance of Rh-B, respectively. The order of photocatalytic activity at 90 min of light irradiation is summarized as follows: ZnO-nt > ZnO-nr > ZnO-np.

Fig. 3 (a) Photodegradation curves of Rh-B (10 ppm) over ZnO-nt, ZnO-nr and ZnO-np under simulated solar light irradiation. (b) Cycling runs of ZnO-nt, ZnO-nr and ZnO-np for the degradation of Rh-B under simulated solar light irradiation. (c) PL spectra of ZnO-nt, ZnO-nr and ZnO-np.

The photocatalytic efficiency of ZnO-nt is 94% whereas that of ZnO-nr is 68% and ZnO-np is 50%. The high performance photocatalytic activity of ZnO-nt is mainly caused by its special morphology with meso-porous nature, high specific surface area and efficient transport of photo generated electron–holes. The activity of ZnO-nt is compared with some recently reported ZnO based photocatalysts and the results are given in Table 1. It can be clearly seen that ZnO-nt showed excellent photocatalytic activity for the degradation of Rh-B than other catalysts. The maximum degradation process is achieved using ZnO-nt and minimum intensity lamp (150 W of Xe) within 90 min. The results also suggest that ZnO-nt based photocatalyst could produce excellent activity.

Table 1 A Comparison of photocatalytic activity of ZnO-nt with recently reported ZnO based catalysts for the degradation of Rh-B

S. no.	Catalyst	Lamp source	Irradiation time (min)	Percentage of degradation (%)	Reference
1	ZnO-nt	150 W of Xe	90	94	Present work
2	GR–ZnO–CoPC	Sun light	130	100	19
	ZnO	Sun light	130	78
3	Ag/AgBr/ZnO	300 W of iodine tungsten (400 nm to 800 nm)	180	100	20
	ZnO	300 W of iodine tungsten (400 nm to 800 nm)	180	38
4	Ag/ZnO	300 W of Xe (λ > 420 nm)	180	80	21
	ZnO	300 W of Xe (λ > 420 nm)	180	10
5	ZnO/ZnFe2O4	300 W of Xe (λ > 400 nm)	180	89	22
	ZnO	300 W of Xe (λ > 400 nm)	180	29
6	Leaf-like ZnO	175 W of Hg	120	80	23

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The photolysis and dark adsorption experiments were carried out to evidence the role of light and catalyst in the photo-degradation process. The photolysis test reveals that only 21% of Rh-B is degraded in the absence of catalyst. The dark adsorption capacity of ZnO-nt was also evaluated for 90 min. The adsorption of Rh-B on ZnO-nt is limited as soon as the adsorption–desorption equilibrium is reached. The difference between photocatalysis, photolysis and dark adsorption results clearly testify the contribution of light and catalyst in the degradation of Rh-B.

COD test was performed to find out the conversion of organic molecule (Rh-B) to CO₂ and H₂O. The COD of Rh-B after 90 min of irradiation using ZnO-nt, ZnO-nr and ZnO-np are decreased to 49%, 56% and 61% respectively. This indicates that Rh-B is completely converted to CO₂ and H₂O only after long time of irradiation.

The cycling experiments for photo-degradation of Rh-B were performed to check the reusability of catalysts and the results are shown in Fig. 3(b). After each cycle, the catalyst was collected, washed with double distilled water, dried overnight and then reused. The experimental conditions are: Rh-B = 10 ppm, catalyst concentration = 1.25 g L⁻¹ and irradiation time = 90 min. The results inferred that the photocatalytic activity of all the three catalysts (ZnO-nt, ZnO-nr and ZnO-np) is dropped after the first use. This is attributed to the photo-dissolution of ZnO during long time of light irradiation. Therefore, ZnO-nt must be coupled with suitable surface modifier for its cyclic use.

The electron–hole separation process was studied with the help of PL. Fig. 3(c) shows the PL spectra of ZnO-nt, ZnO-nr and ZnO-np. Three peaks at 385 nm, 441 nm and 600 nm are observed for ZnO-nt. The peaks of ZnO-nr are located at 382 nm, 443 nm and 470 nm. ZnO-np has four peaks centered at 385 nm, 442 nm, 470 nm and 616 nm. The appearance of peak in the UV region (380 nm) is ascribed to the band gap emission. The peak located around 440 nm is attributed to the intrinsic transition of zinc ions from excited state to the ground state. The visible emission peaks may result from the surface oxygen vacancies and defects in the crystal during the crystal growth.^17,18 The emission intensity peak of ZnO-nt at 441 nm is lower than that of ZnO-nr and ZnO-np. It indicates that the life time of charge carriers is increased and the electron–hole transport is improved in the nano-triangles.

The electron–hole transfer process of ZnO was discussed in many papers. The conduction band (CB) and valence band (VB) positions of ZnO-nt at the point of zero charge were calculated using Butler and Ginley equation. The CB and VB positions of ZnO-nt are found to be −0.24 eV and 2.82 eV, respectively. The electron–hole transfer process in ZnO-nt was proposed and schematically shown in Fig. 4. When the ZnO-nt is being irradiated with simulated solar light, it is easily excited and electrons and holes are generated. The electrons accumulated in the CB of ZnO-nt are readily trapped by the dissolved oxygen to produce super oxide radical (O₂˙⁻), similarly, the VB holes of directly decompose the Rh-B or react with water to form ˙OH. The degradation of Rh-B may be caused by the reactive oxygen species (O₂˙⁻, ˙OH, h⁺) formed in the photocatalysis process.

Fig. 4 The schematic diagram of electron–hole transfer process in ZnO-nt under simulated solar light irradiation.

It is believed that the triangular prism morphology of ZnO can promote the effective conduction and mobility of photo-generated electron–hole pairs on the catalyst surface, which leads to the significant enhancement in the photocatalytic activity. The high specific surface area of ZnO-nt can supply more active sites for the photodegradation of Rh-B and promote the diffusion of reactants/products during photo-reaction.

4. Conclusion

In summary, a simple and convenient method has been developed for the synthesis of ZnO-nt. The crystal structure, morphology, surface area and optical properties were measured using XRD, TEM, HR-TEM, BET, UV-DRS and PL techniques. Nearly 94% of Rh-B degradation is achieved by ZnO-nt within 90 min of simulated solar light irradiation. The activity of ZnO-nt is superior to that of ZnO-nr and ZnO-np. The results prove that the morphology of ZnO plays an important role in its photocatalytic activity. Therefore ZnO-nt is holding great promise for the practical development of solar light active photocatalysts.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2012R1A1A3005043), for which the authors are very grateful.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02042e

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