Effects of Morphology and Zr Doping on Structural, Optical, and

Article pubs.acs.org/IECR

Effects of Morphology and Zr Doping on Structural, Optical, and Photocatalytic Properties of ZnO Nanostructures N. Clament Sagaya Selvam,† J. Judith Vijaya,*,† and L. John Kennedy‡ †

Catalysis and Nanomaterials Research Laboratory, Department of Chemistry, Loyola College, Chennai 600 034, India Materials Division, School of Advanced Sciences, VIT University, Chennai Campus, Chennai 600 127, India

‡

ABSTRACT: ZnO nanomaterials with different morphologies such as nanoflakes, spherical nanoparticles (SNPs), and nanorods have been synthesized via a simple low temperature coprecipitation method. The concentration of the capping agent is a key factor in the morphological control of ZnO nanostructures. Triton X-100 micelles were found to be single molecules at low concentration but spherical and rod-like shaped micellar aggregation at higher concentrations. The formation of different morphologies of ZnO was confirmed by HR-SEM and HR-TEM. XRD data showed the formation of single-phase ZnO with the wurtzite crystal structure. The influence of Zr contents on the structure, morphology, absorption, emission, and photocatalytic activity of ZnO SNPs was investigated systematically. The influence of the ZnO morphologies on the photocatalytic degradation (PCD) of resorcinol as a model reaction is evaluated and discussed in terms of particle size, surface area, crystal growth habits, and oxygen defects. The results indicated that the particle size is an important factor for the PCD, and thus, the 1.5 wt % Zrdoped ZnO SNPs show superior performance toward PCD of resorcinol than other samples due to the small particle size distribution. Furthermore, the effect of different photocatalytic reaction parameters on the resulting PCD efficiency of ZnO SNPs was investigated.

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INTRODUCTION The shape control of semiconductor nanostructures has attracted considerable attention due to the fact that they play very important roles in determining their physical and chemical properties.1−4 Nanostrucutured zinc oxide (ZnO) is a versatile and interesting semiconductor material to study because it possesses very attractive physical properties such as a wide direct band gap (3.37 eV), a large exciton binding energy of 60 meV at room temperature, and unique electronic, catalytic, optoelectronic, and photocatalytic properties.5−9 A wide variety of synthetic routes have thus been proposed to prepare specific nanostructures of ZnO, including nanorods,10 nanowires,11 nanobelts,12 tetrapods,13 and many other anisotropic prototypes in order to further the development of ZnO nanostructures. Among the many synthetic approaches, wet chemistry is an effective way for the production of ZnO nanostructures. In wet-chemistry processes, capping reagents are often required to control the crystal growth of the materials to enable the formation of nanostructures. Meanwhile, by suitably modulating the reactant concentrations,14 reaction temperature, solvent,15 organic additives,16 and quenching treatments17 during the reaction, ZnO nanostructures, with controllable dimensions, have been obtained. Most of the above operations, however, involve complicated synthetic procedures, which may hinder the applicability of the products. Therefore, development of a simple, environmentally friendly method to prepare ZnO nanostructures with controllable morphology is crucial to their practical applications and has thus become an important topic of investigation. The precipitation approach compared with other traditional methods provides a facile way for low cost and large-scale production, which does not need expensive raw materials and complicated equipment.18 In this work, a series of ZnO nanoparticles with different morphologies © 2012 American Chemical Society

were prepared via a simple precipitation route at low temperature. Our results show that the variation of concentration of the capping agent provides an easy way to prepare ZnO with novel morphologies. Among the various applications of ZnO nanostructures, photocatalysis is the most important application for the environmental protection. Li et al.19−21 have investigated the relationship between photocatalytic activity, crystallinity, surface area, and morphology of ZnO particles in detail. In addition, it is well-known that the existence of defects in ZnO would influence the photocatalytic properties.22,23 It has been reported24,25 that the absorption edge of ZnO shifted to longer wavelengths with the increase of the cobalt concentration to the optimum level and the presence of Co also increases the surface oxygen vacancies. This counteracts the increased photocatalytic activity of ZnO. Addition of Ag deposits on the ZnO surface acted as electron sinks, which hindered the recombination of photoinduced electrons and holes. This enhanced the photocatalytic activity of ZnO photocatalysts.26−29 When La was doped into ZnO, more surface defects were produced, which hindered the recombination of photoinduced electron−hole pairs. This contributed to the improvement of the photocatalytic activity of La-doped ZnO.30,31 The studies concerning the effect of metal (M = Ce, Al, Cr) doped ZnO for photocatalysis is very scarce. Ce-doped ZnO32 and Al-doped ZnO33 showed enhanced photocatalytic activity as compared with pure ZnO, due to the increase in oxygen vacancies and surface area, respectively, as dopants increase. However, the Received: Revised: Accepted: Published: 16333

June 26, 2012 November 2, 2012 November 23, 2012 November 23, 2012 dx.doi.org/10.1021/ie3016945 | Ind. Eng. Chem. Res. 2012, 51, 16333−16345

Industrial & Engineering Chemistry Research

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enhanced photocatalytic activity of Cr-doped ZnO34 has been explained by its enhanced visible light absorption ability. Ganesh et al.35 reported that doping ZnO with Li exhibits ptype semiconducting behavior. This could be the reason for the noted stability against photocorrosion and the high photocatalytic activity of ZnO. It has been reported16,36−40 that the ZnO crystals with different morphology have been prepared by controlling the reaction conditions and they affect their photocatalytic activity. However, the studies concerning the effect of ZnO morphology and metal doping on its photocatalytic activity are still of great importance and challenge to explore as it plays an important role in determining the extent of their photocatalytic activity. Therefore, the optical, photoluminescence, and photocatalytic activity of ZnO nanostructures with different morphology and Zr doping were explored in the present study. A plausible mechanism for the growth of the various ZnO nanostructures is also proposed.

Photocatalytic Reactor Setup and Degradation Procedure. PCD experiments were carried out in a selfdesigned photocatalytic reactor, as shown in Figure 1. The

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EXPERIMENTAL SECTION Preparation of Photocatalysts. All the chemicals were obtained from Merck, India (Analytical grade). The commercially available TiO2 (Degussa P-25) was obtained from Degussa Chemical, Germany. The typical synthesis procedure for pure and Zr-doped ZnO is as follows: Zinc acetate dihydrate (Zn(Ac)2·2H2O) and zirconyl nitrate monohydrate ZrO(NO3)3·H2O were taken as the precursors of zinc and zirconium, respectively. Zn(Ac)2·2H2O and NaHCO3 were dissolved separately in double-distilled water to obtain 0.1 mol/ L solutions. Zinc acetate solution (250 mL of 0.1 mol/L) was slowly added into vigorously stirred NaHCO3 (250 mL of 0.1 mol/L) and Triton-X 100 mixed solution. The Triton X-100 concentrations in the mixed solutions differed within the range 0−2 mmol/L, which correspond to the concentration for structural changes of surfactant from premicelle concentration (PMC) to critical micelle concentration (CMC) such as spherical micelle (CMC1) and spherical to rod-like micelle concentration (CMC2). On the basis of this, PMC, CMC1, and CMC2 in Triton X-100 aqueous solution were obtained. The values are respectively close to 2.1 × 10−4, 3.2 × 10−4, and 1.3 × 10−3 mol/L. Zirconyl nitrate in the required stoichiometry was slowly added into the above solution, and a white precipitate was obtained. The precipitate was filtered, repeatedly rinsed with distilled water, and then washed twice with ethanol. The resultant solid product was dried at 70 °C for 2 h and calcined at 200 °C for 3 h. Pure ZnO was also prepared by the same procedure without the addition of zirconyl nitrate solution. The doping concentrations of zirconium are expressed in wt %. Characterization of Photocatalysts. The structural characterization of pure and Zr-doped ZnO was performed using a Philips X’pert X-ray diffractometer with Cu Kα radiation at λ = 1.540 Å. The particle size and morphology of pure and Zr-doped ZnO samples were observed using a high resolution scanning electron microscope (HR-SEM) (Stereoscan LEO 440) and a high resolution transmission electron microscope (HR-TEM) (JEOL JEM 3010). The diffuse reflectance UV−visible spectra of pure and Zr-doped ZnO samples were recorded using a Cary100 UV−visible spectrophotometer to estimate their energy band gap. The emission spectra of the pure and Zr-doped ZnO photocatalyst samples were recorded using a Varian Cary Eclipse Fluorescence Spectrophotometer at an excitation wavelength of 372 nm.

Figure 1. Schematic diagram of the photocatalytic reactor.

cylindrical photocatalytic reactor tube was made up of quartz/ borosilicate with a dimension of 36−1.6 cm (height-diameter). The top portion of the reactor tube has ports for sampling, gas purging, and gas outlet. The aqueous resorcinol solution containing an appropriate quantity of either pure ZnO or Zrdoped ZnO was taken in the quartz/borosilicate tube and subjected to aeration for thorough mixing. This was then placed inside the reactor setup. The lamp housing has low pressure mercury lamps (8 × 8 W) emitting either 254 or 365 nm with polished anodized aluminum reflectors and black cover to prevent UV leakage. The PCD was carried out by mixing 100 mL of aqueous resorcinol solution and a fixed weight of pure ZnO or Zr-doped ZnO photocatalysts. Prior to irradiation, the slurry was aerated for 30 min to reach adsorption equilibrium followed by UV irradiation. Aliquots were withdrawn from the suspension at specific time intervals and centrifuged immediately at 1500 rpm. The extent of resorcinol degradation was monitored by using a UV−vis spectrophotometer (PerkinElmer, Lamda 25) and a high performance liquid chromatograph (HPLC) (Shimadzu LC10 ATVP series equipped with aUV−vis detector). The effect of the pH of the solution was studied by adjusting the pH of the resorcinol solution containing the catalyst, using dilute HCl and NaOH (both from Merck, India). The pH of the solution was measured using a HANNA Phep (Model H 198107, 0.2−0.5 pH unit accuracy) digital pH meter. The intermediates were identified using a gas chromatograph coupled with a mass spectrometer (GC−MS) (Perkin-Elmer Clarus 500). The temperature of the column was programmed as follows: the initial column temperature was held for 2 min at 70 °C, ramped at 10 °C/min to 280 °C, with a final hold for 2 min at 280 °C. The extent of mineralization was determined using a total organic carbon analyzer (TOC) (Shimadzu VCPN). The PCD efficiency (η) was calculated from the following expression: η = Ci − Ct/Ci × 100

or

η = TOCi − TOCt/TOCi × 100

where Ci or TOCi is the initial concentration of resorcinol and Ct or TOCt the concentration of resorcinol after “t” minutes. 16334

dx.doi.org/10.1021/ie3016945 | Ind. Eng. Chem. Res. 2012, 51, 16333−16345


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Figure 2. HR-SEM images of the as-synthesized ZnO structures in different capping agent (Triton X-100) concentrations: (a) ZnO nanoflakes (PMC - 2.1 × 10−4 mol/L); (b) ZnO SNPs (CMC1 - 3.2 × 10−4 mol/L); (c) ZnO nanorods (CMC2 - 1.3 × 10−3 mol/L); (d) 1.5 wt % Zr-doped ZnO SNPs (CMC1 - 3.2 × 10−4 mol/L).

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RESULTS AND DISCUSSION

tures with different morphology, HR-TEM analysis was carried out. A HR-TEM image of typical ZnO nanoflakes is presented in Figure 3a, indicating that the nanoflakes are self-assembled. The inset of Figure 3b shows the corresponding selected area electron diffraction (SAED) pattern. The pattern implies that the ZnO nanoflakes are a good crystalline material with single crystalline nature. A HR-TEM image of ZnO SNPs is presented in Figure 3c,d, indicating that the SNPs are self-assembled without agglomeration. The inset of Figure 3d shows the corresponding SAED pattern. The pattern implies that the ZnO SNPs are a good crystalline material with single crystalline nature. A HR-TEM image of ZnO nanorods is presented in Figure 4a,b, indicating that the nanorods are self-aggregated. The corresponding SAED pattern is shown as the inset in Figure 4b, indicating the single crystalline nature of ZnO nanorods. In addition, the Zr doping on ZnO SNPs results in the formation of elongated nanoparticles, as shown in Figure 4c,d. The inset of Figure 4d shows the corresponding SAED pattern. The pattern implies that the Zr-doped ZnO spherical nanoparticles have single crystalline nature. Possible Formation Mechanism of ZnO Nanostructures. Controlling the morphologies is very important because the morphology of the nanostructures strongly affects their

Size and Morphology of ZnO Nanocrystals. HR-SEM observations confirm the morphology of ZnO nanocrystals prepared under different concentrations of capping agent, as presented in Figure 2a−c. It is obvious that the morphology of the nanocrystals changes with the increase in the concentration of capping agent. When the concentration of capping agent as 2.1 × 10−4 mol/L (PMC) is adopted, a high yield of ZnO nanoflakes is obtained with diameters of 35−40 nm, as shown in Figure 2a. When the concentration of capping agent is increased to 3.2 × 10−4 mol/L (CMC1), the morphology of ZnO changes to spherical shaped particles with diameters of 10−12 nm, as shown in Figure 2b. When the concentration of capping agent is further increased to 1.3 × 10−3 mol/L (CMC2), the morphology of ZnO becomes nanorods with diameters of 12−15 nm, as shown in Figure 2c. Therefore, it is concluded that the morphology of the ZnO sample evolves gradually from nanoflake-like morphology to nanorod-like morphology with an increase in the concentration of capping agent. In addition, the Zr doping on ZnO SNPs results in the formation of elongated ZnO SNPs, as shown in Figure 2d. To provide further evidence in the formation of ZnO nanostruc16335



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Figure 3. HR-TEM images and corresponding SAED pattern of the as-synthesized ZnO nanostructures: (a, b) ZnO nanoflakes; (c, d) ZnO SNPs.

optical, luminescence, and photocatalytic properties.41−43 To control the desired nanostructure morphology, it is very important to understand the factors that determine the rate of growth along the various orientations. Furthermore, there are various external factors that can have a strong impact on the morphology of nanostructures. These are the choice of precursors, capping agent, solvents, and the experimental conditions such as time, temperature, and pressure, which can affect the reaction mechanisms. 44,45 It has also been reported 46,47 that the concentration of capping agent determines the size and morphology of nanoparticles. In the present study, the concentration of capping agent has been varied and the reaction conditions (concentration of reactants, temperature, and time) were kept the same for all the reactions. Therefore, the differences in the morphologies of ZnO nanostructures can only be due to the effect of different concentrations of capping agent. Moreover, the concentration where aggregation of surfactant and monomers into micelles occurs is called the critical micelle concentration (CMC). CMC is a key parameter for the optimization of capping agent in morphology formulations. When the concentration of a capping agent is increased, its structure may change from single molecules to spherical and rod-like,48 as shown in Scheme 1. Therefore, three concentrations of capping agent (Triton X-100), which represent premicelle, spherical micelle, and rod-like micelle, respectively, are selected for the present study. A schematic diagram of the possible formation mechanism of ZnO nanostructures with different morphology has been shown in Scheme 2. In the case of premicelle concentration (PMC), a high yield of ZnO nanoflakes is

obtained. This is because, during the particle formation, the monomers (capping agent) are adsorbed onto preferred planes and alter the growth kinetics. Therefore, in this case, in the nucleation stage, growth along all preferred directions might have retarded to produce nanoflake shaped morphology. The nanoflakes are aggregated due to their nanoscale forces. Spherical micelles are formed at the first critical micelle concentration (CMC1), which results in the formation of ZnO SNPs. Due to the smaller dimension of the spherical shaped granular particles, the polar fields generated in each particle were weaker.49 Consequently, a less pronounced tendency of agglomeration between single particles was expected, leading to an unagglomerated assembly of ZnO nanoparticle formation as shown in HR-TEM images. The second critical micelle concentration (CMC2) indicates the structural transition from spherical micelles to rod-like ones, which results in the formation of ZnO nanorods. The collective behavior of van der Waals forces and electrostatic interactions would have favored the self-aggregation of the ZnO nanorods,50 as observed from the electron microscopy studies. The presence of small amounts of Zr ions on ZnO SNPs alters the growth rate and resulted in the elongated nanocrystals, as shown in Figure 4c,d. Crystal Structure of ZnO Nanocrystals. The XRD patterns of the ZnO nanostructures with different morphologies are shown in Figure 5a. All the diffraction peaks match with the standard data for a hexagonal ZnO wurtzite structure (JCPDS 36-1451), and no characteristic peaks of any impurities are detected in the patterns, which indicates that all the samples have high phase purity. The XRD pattern of ZnO SNPs clearly shows that the shifting of diffraction peaks indicates an 16336



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Figure 4. HR-TEM images and corresponding SAED pattern of the as-synthesized ZnO nanostructures: (a, b) ZnO nanorods; (c, d) 1.5 wt % Zrdoped ZnO SNPs.

Scheme 1. Formation of Single Molecules to Spherical and Rod-Like Micelles at Different Concentrations of Capping Agent

of Zr4+ is larger (0.84 Å) than that of Zn2+ (0.74 Å), as reported in the literature51,52 for Zr-doped ZnO. UV−vis Absorption and Photoluminescence Spectroscopy. The present study investigated the UV−vis absorption and PL measurements to verify how the morphology of ZnO and Zr doping on ZnO affects their absorption and emission properties. This relationship has already been investigated for a number of ZnO nanostructures in order to understand if the substantially increased surface to volume ratio in a nanostructure leads to significant rearrangement of absorption and PL emission characteristics.53,54 For the ZnO samples taken for this study, a correlation between morphology, Zr doping, absorption, and emission properties of ZnO was illustrated in Figures 6 and 7. Figure 6 shows diffuse reflectance spectra of the pure ZnO nanostructures with varying morphology. ZnO nanoflakes (Figure 6c) exhibit a sharp absorption edge at about 372 nm. ZnO nanorods and ZnO SNPs exhibits a sharp absorption edge at about 370 nm (Figure

expansion of unit cell as a result of size effect. In addition, the peak width broadens due to the smaller particle size distribution. The XRD pattern of the Zr-doped ZnO SNPs (Figure 5b) clearly shows the shifting of diffraction peaks, slightly toward lower angle, on Zr doping in comparison to that of pure ZnO SNPs. Furthermore, the intensity of the diffraction peaks decreases, and the width broadens due to the formation of smaller average diameters of ZnO SNPs as a result of Zr doping. The shifting and broadening of XRD lines with doping strongly suggest that Zr4+ were successfully substituted into the ZnO host structure at the Zn2+ site. The crystal size and unit cell parameter are given in Table 1. It is clearly seen from Table 1 that the lattice constants of ZnO SNPs were found to be slightly larger than those of other ZnO nanostructures due to the crystal size effect. The lattice constants of Zr-doped ZnO samples were also found to be slightly larger than those of pure ZnO SNPs. This is consistent with the fact that an ionic radius 16337



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Scheme 2. Formation Mechanism of ZnO Nanostructures with Different Morphologies

Table 1. Physical Characteristics of Pure ZnO and Zr-Doped ZnO catalyst ZnO nanoflakes ZnO nanorods ZnO SNPs 0.5 wt % Zr−ZnO 1.0 wt % Zr−ZnO 1.5 wt % Zr−ZnO 2.0 wt % Zr−ZnO

crystal size (nm)

lattice parameter (a) (nm)

lattice parameter (l) (nm)

λmax (nm)

band gap (eV)

36.5

0.3241

0.5201

372

3.33

36.5

0.3246

0.5206

370

3.35

36.5 18.2

0.3251 0.3253

0.5219 0.5222

367 366

3.37 3.38

15.6

0.3259

0.5228

364

3.40

12.5

0.3300

0.5236

362

3.42

11.5

0.3310

0.5241

361

3.43

Figure 5. X-ray diffraction patterns of ZnO: (a) different morphologies of pure ZnO; (b) pure and Zr-doped ZnO SNPs.

Figure 6. UV−vis absorption spectra of ZnO nanostructures: (a) ZnO SNPs; (b) ZnO nanorods; (c) ZnO nanoflakes.

6b) and 365 nm (Figure 6a), respectively. It was interesting that the absorption peaks became sharper and blue-shifted for ZnO SNPs due to the quantum size effect. Moreover, as the size of ZnO SNPs falls below the critical radius, the charge confinement leads to a series of discrete electronic states. As a

result, there is an increase in the effective band gap and a shift in the band edges. Thus, by varying the size of the semiconductor particles, it is possible to enhance the redox potential of the valence-band holes and the conduction-band electrons. With the decreasing of the particle size, the diffusion of the photoinduced electrons or holes from bulk to surface 16338



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It has been reported58,59 that doping ZnO with Zr increases substantially the energy required for making an oxygen vacancy at the surface of the oxide. When dopants with higher valence replace a Zn atom, at the surface of ZnO, Zn2+ does not satisfy their valence. This drives the oxygen atoms to bind with near dopants more strongly, indicating that the dopant is undercoordinated. The undercoordinated dopant will take an oxygen atom from the surface of the ZnO, leaving behind an oxygen vacancy in the surface layer. Therefore, oxygen vacancy increases as Zr doping increases, as shown in the inset of Figure 7. Influence of the Morphologies on the Photocatalytic Performance of the ZnO Nanostructures. The influence of morphologies on the photocatalytic degradation efficiency is shown in Figure 8 .The morphologies of ZnO catalysts play an

Figure 7. Room temperature PL spectra of ZnO nanostructures: (a) ZnO nanoflakes; (b) ZnO nanorods; (c) ZnO SNPs. The inset is for pure and Zr-doped ZnO SNPs.

becomes fast, which may lead to an enhancement of the photocatalytic activity. The inset of Figure 6 shows diffuse reflectance spectra of Zr-doped ZnO SNPs. The figure indicates that the maximum absorbance band shifts toward lower wavelength by increasing the Zr loading. Consequently, the band gap of Zr-doped ZnO increases gradually with an increase in the Zr loading and is much higher as compared to that of pure ZnO SNPs, as shown in Table 1. This is mainly attributed to the reduction of size as the Zr content increases. Thus, the use of size-quantized doped semiconductor particles may result in increased photocatalytic activity. It is interesting to investigate the PL spectra of the asprepared ZnO nanostructures with varying morphology, as shown in Figure 7. The weak UV band gap emission (392 nm) results from the radiative recombination of an excited electron in the conduction band (eCB−) with the valence band hole (hVB+). The visible or deep trap state emissions (403, 423, 432, 460, 485, 504, and 537 nm) are commonly defined as the recombination of the electron−hole pair from localized states with energy levels deep in the band gap, resulting in lower energy emission. These dominant deep-trap emissions indicate high defects or oxygen vacancy of ZnO nanostructures.55 However, the PL intensity of ZnO has been changed by varying the morphology, as shown in Figure 7. The emission bands were slightly blue-shifted with higher PL intensity for ZnO with spherical shaped morphology, indicating that the smaller dimensions of ZnO SNPs have more surface defects. There is literature reported56,57 that smaller sized ZnO nanostructures might favor high-level surface defects, which account for the increase of the defect emission relative to the UV emission, as seen in the present case of ZnO SNPs. The inset of Figure 7 shows PL spectra of Zr-doped ZnO SNPs. All the emission bands were broadened with higher PL intensity due to the increase in Zr content. This can be attributed to the increased density of surface defect states because of the presence of dopants. Therefore, the band gap and PL of these ZnO nanostructures can be manipulated by controlling their morphologies and doping with metal ions, indicating the capabilities of these nanostructures for application in various fields.

Figure 8. Influence of morphologies on the photocatalytic degradation efficiency (experimental conditions: catalyst = 30 mg/100 mL, resorcinol = 200 ppm, pH of suspension = 6−6.5 (natural pH), λ = 365 nm).

important role in the photocatalytic activity.60,45 This arises from differences in particle sizes, surface areas, polar planes, or oxygen vacancies, as discussed below. The particle sizes of ZnO catalysts play an important role in the photocatalytic activity. Dodd et al.61 reported that smaller ZnO particle size is attributable to an increase in the photocatalytic activity. John Becker et al.62 reported that the ZnO sample with small crystallite size showed a higher photocatalytic activity. Moreover, as the particle size decreases, the number of active surface sites increases. Thus, it is expected that ZnO SNPs with a very small particle size distribution would be a potentially efficient photocatalyst. Moreover, among the ZnO nanostructures with different morphology synthesized in this study, the ZnO with spherical morphology had a smaller particle size distribution, as indicated by the XRD, HR-SEM, and HR-TEM data, which showed considerably higher photocatalytic activity for the degradation of resorcinol (Figure 8) than other ZnO morphologies. Usually, a high specific surface area has a beneficial effect on the activity for catalysts. High-surface-area ZnO nanostructure63 showed considerably higher photocatalytic activity for the degradation of dyes than commercial ZnO. In this work, the surface areas of the ZnO with spherical, rod, and flake shaped morphologies are 45.64, 39.26, and 32.16 m2/g, respectively. The photocatalytic performance followed the order of spherical shaped morphol16339



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ogy > rod-like morphology > flake-like morphology. However, in other studies,42,45 higher surface areas of the catalysts did not result in higher photocatalytic performance. Catalysts with higher surface energy show better catalytic performance.64,42 Nonfaceted particles have higher surface energies than the faceted ones.65 In the present work, spherical shaped ZnO morphology showed the best photocatalytic activity among all the ZnO nanostructures synthesized, presumably because of the formation of nonfaceted morphologies with higher surface energies. The rod-like morphology showed better photocatalytic activity than the flake-like morphology. The reason is that the Zn-terminated (001) and O-terminated (001) polar faces are facile to adsorb oxygen molecules and OH− ions, resulting in a greater production rate of H2O2 and OH• radicals, hence enhancing the PCD efficiency. In addition, the surface energies (E) of the facets in ZnO crystals follow the sequence E (0001) > E (1011) > E (1010) > E (10 11) > E (0001). Therefore, the rod-like morphology, having a 001 plane, with high surface energy showed better photocatalytic activity than the flake-like morphology. ZnO with flake-like morphology showed the least catalytic activity. This is due to the formation of nanoparticles with smooth (1101) and (1010) facets, which did not have higher surface energy, resulting in poor activity. As reported in the literature, differences in photocatalytic activity are highly related to the concentration of defects on the surface of the nanomaterials.45,66 Different effects of native defects have been proposed in the literature for the enhanced photocatalytic activity of ZnO. Usually, higher photocatalytic activity of ZnO nanostructures has been attributed to a high concentration of surface donor defects (oxygen vacancies, zinc interstitials).67,68 In this case, the higher activity in the presence of more surface defects (oxygen vacancies) was attributed to lower recombination between photogenerated electrons and holes with oxygen vacancies serving as electron traps.69 ZnO nanostructures with different morphologies (spherical shaped morphology, rod-like morphology, and flake-like morphology) synthesized in this study showed the presence of oxygen vacancies, as indicated by the photoluminescence data. However, ZnO with spherical shaped morphology showed the best activity as compared with the other ZnO morphologies, as shown in Figure 8. The reason is that the ZnO spherical morphology with a smaller particle size distribution has high-level surface defects, attributed to the increased concentration of both electron traps (oxygen vacancies) and hole traps (oxygen interstitials), as indicated by the photoluminescence data. However, as the particle size decreases, surface defects increase and thus the charge carrier recombination rate decreases. This counteracts the increased photocatalytic activity. In addition, when the size of ZnO particles decreases, the amount of dispersion of particles per volume in the solution will increase, resulting in the enhancement of photon absorbance. Therefore, the photocatalytic activity of ZnO SNPs is high due to the smaller particle size distribution and the photocatalytic efficiency of different ZnO morphologies is in the order of spherical shaped morphology > nanorod-like morphology > nanoflake-like morphology. Therefore, it is concluded that the particle size is an important factor for governing the enhanced photocatalytic activity. Thus, ZnO SNPs were selected as the best catalyst for the study of the other parameters. Effect of Reaction Parameters on the Photocatalytic Degradation. Blank experiments were carried out without photocatalyst to examine the extent of degradation (photol-

ysis). There was no evidence of degradation of resorcinol in aqueous solution in the absence of ZnO SNPs at 365 nm wavelength of light irradiation. When an aqueous solution of resorcinol (200 mg/L) containing ZnO SNPs was irradiated with UV light, PCD of resorcinol was observed. The PCD of resorcinol was found to increase with an increase in the amount of ZnO SNPs up to 30 mg/100 mL. Further increase in photocatalyst amount showed a negative effect, as illustrated in Figure 9. The reason for this is that the increase in the amount

Figure 9. Effect of the amount of photocatalyst on PCD efficiency (experimental conditions: catalyst = pure ZnO SNPs/100 mL, resorcinol = 200 ppm, pH of suspension = 6−6.5 (natural pH), λ = 365 nm).

of catalyst increases the number of active sites on the photocatalyst surface, which in turn increases the number of hydroxyl and superoxide radicals to degrade resorcinol. However, when the concentration of the catalyst increases above the optimum level, the degradation decreases, which is due to an increase in the turbidity of the suspension, which affects the penetration of UV light as a result of increased screening effect and scattering of light.70 Sun et al.71 and Huang et al.72 added that, as excess catalysts prevent the penetration of light in the photocatalytic degradation process, the efficiency of the photocatalytic degradation reduces accordingly. Thus, 30 mg/100 mL was selected as the optimum amount for the study of other parameters. The effect of initial concentrations on PCD of resorcinol at different initial concentrations in the range 50−250 mg/L was investigated at the natural pH of the suspension (6.2, without adjustment). The results are illustrated in Figure 10. It was found that the PCD efficiency of resorcinol was decreased from 99 to 76% with the increase of the initial resorcinol concentration from 50 to 250 mg/L, respectively, after 210 min. This may be due to the fact that, as the initial concentration of resorcinol increases, more and more resorcinol molecules are adsorbed on the surface of ZnO SNPs, but the number of •OH and •O2− radicals formed on the surface of ZnO and the irradiation time are constant. Therefore, the relative number of •OH and •O2− radicals available for attacking resorcinol molecules becomes less, and consequently the PCD efficiency decreases.70 However, under the given set of conditions, the maximum concentration of resorcinol that could be degraded by 30 mg/100 mL ZnO SNPs is found to be 16340



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point charge of ZnO) is equally favorable for generation of •OH radicals and adsorption of substrate molecules on the surface of ZnO, which is attributed to the increase in PCD efficiency. On the other hand, above pH 9, the catalyst surface is negatively charged by means of metal-bound OH− ions. Consequently, the surface concentration of the resorcinol is low. Thus, the surface concentration of the resorcinol decreases, which results in the decrease of PCD at pH 11. Therefore, pH 8 was selected as the optimum pH for the study of other parameters. Effects of Zr Doping on the Photocatalytic Activity. The influence of Zr doping on ZnO SNPs on the PCD efficiency is evaluated, as shown in Figure 12. The PCD

Figure 10. Effect of the concentration of resorcinol on PCD efficiency (experimental conditions: pure ZnO SNPs = 30 mg/100 mL, initial pH of suspension = 6−6.5 (natural pH), λ = 365 nm).

200 mg/L. Thus, 200 mg/L resorcinol was selected as the optimum concentration for the study of other parameters. The pH of the reaction medium has a significant effect on the surface properties of semiconductor oxide particles, including the surface charge and adsorption−desorption characteristics. The point of zero charge (PZC) of ZnO is 9.3. Therefore, the catalyst surface is positively charged when the pH is lower than the respective PZC value and negatively charged when the pH is higher. As observed, the PCD of resorcinol is low at pH 2 and 11, which may be due to the substantial loss of ZnO (acidic and basic dissolution) at exceedingly low and higher pH.73 The degradation rate increases up to pH 6 as the ZnO stability is less disturbed. However, the extent of PCD of resorcinol was found to increase with an increase in the initial pH of the suspension exhibiting maximum PCD at pH 8 (alkaline medium) and remains nearly the same at pH 9, as illustrated in Figure 11. In alkaline medium, an excess of hydroxyl anions facilitates the photogeneration of •OH radicals, which is accepted as the primary oxidizing species responsible for PCD.74 The reason is that the suspension pH up to 9 (pH zero

Figure 12. Effect of Zr doping on the PCD efficiency (experimental conditions: resorcinol = 200 ppm, ZnO SNPs = 30 mg/100 mL, λ = 365 nm).

efficiency of commercial TiO2 (Degussa P-25) is also evaluated for the purpose of comparison, as shown in Figure 12. The PCD efficiency of ZnO SNPs increases with an increase in the Zr loading and shows a maximum activity at 1.5 wt % and then decreases on further Zr doping to 2 wt %. The reason can be explained as follows: As demonstrated by HRTEM and XRD, the synthesized Zrdoped ZnO catalyst possesses a smaller particle size distribution than pure ZnO SNPs. Apart from their small size, as Zr4+ was doped in ZnO, more surface defects are produced, as demonstrated in PL spectra, and a space charge layer could be formed on the surface. Consequently, the migration of the photoinduced electrons and holes toward surface defects is reasonable.75 Thus, the separation efficiency of the electron− hole pairs of Zr-doped ZnO with more oxygen defects should be more than that of the pure ZnO SNPs. Therefore, the enhancement in the PC activity of ZnO as Zr doping increases was attributed to the small particle size and higher defect concentration. However, excessive amounts of dopants can retard the photocatalysis process, because an excess amount of dopants deposited on the surface of ZnO increases the recombination rate of free electrons and energized holes, thus inhibiting the photodegradation process.66 Hence, further increase in Zr doping to 2 wt % results in the decrease of PCD efficiency. Photocatalytic Mineralization of Resorcinol. Photocatalytic mineralization of resorcinol at optimized conditions (resorcinol = 200 mg/L, photocatalyst = 30 mg/100 mL, pH =

Figure 11. Effect of pH on PCD efficiency (experimental conditions: resorcinol = 200 ppm, pure ZnO SNPs = 30 mg/100 mL, λ = 365 nm). 16341



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was exposed to UV light alone (photolysis) at 254 nm, the solution turned slightly brownish yellow in color with the wavelength shifting slightly toward the red region (bathochromic shift) in absorbance in the UV region, due to the inclusion of chromophore groups as a result of ring opening.76 This is because the substance can absorb light at 254 nm wavelength to a significant extent (λmax of resorcinol = 274 nm), which breaks down the resorcinol molecules into stable intermediates (p-benzoquinone, o-benzoquinone), as shown in Scheme 3. This cannot be further degraded at 254 nm even

8, 1.5 wt % Zr-doped ZnO SNPs) at wavelengths of 254 and 365 nm was evaluated using a TOC analyzer. The excitation of electrons in Zr-doped ZnO with 254 nm can promote the electrons to the conduction band with high kinetic energy. The electrons may reach the solid−liquid interface easily and thereby suppress electron−hole recombination in comparison to 365 nm. Thus, the rate of mineralization was very high in the initial period (inset of Figure 13) followed by the slow

Scheme 3. Photocatalytic Degradation Mechanism of Resorcinol

after 240 (Figure 13). However, when the catalyst was used in addition to UV light, the color of resorcinol solution turned in to dark reddish brown and the red shift was very prominent in absorbance in the UV region as shown in Figure 15. However,

Figure 13. Comparison of photocatalytic mineralization of resorcinol and kinetic fit for the mineralization of resorcinol (experimental conditions: resorcinol = 200 ppm, 1.5 wt % Zr−ZnO SNPs = 30 mg/ 100 mL).

degradation at 254 nm, as illustrated in Figure 13. This slow degradation indicates the formation of a few long-lived intermediates. Therefore, the present work focused to analyze the development of the dark color observed in the solution under photocatalytic treatment and also formulate a reaction mechanism to explain the formation of intermediates responsible for the dark color. The experiments were carried out following the batch-wise procedure and analyzed using a UV−visible spectrophotometer and GC-MS spectrometer. The UV−visible spectra are shown in Figure 14. When resorcinol

Figure 15. UV−vis absorption spectra of the photocatalytic degradation of resorcinol (experimental conditions: irradiation wavelength λ = 254 nm, resorcinol = 200 ppm, catalyst = 30 mg/100 mL).

the complete mineralization was not achieved even after 280 min at 254 nm. The reason is competitive absorption of light by resorcinol and by the ZnO photocatalyst, which are able to scatter part of the light.77 Hence, low absorption and wasting of light at 254 nm by photocatalyst might be the actual cause for less rate of degradation as reported earlier.78 On the other hand, the complete mineralization of resorcinol is achieved in 150 min at 365 nm, as shown in Figure 13. It is observed experimentally that the brown color formed during the reaction became faint and finally turned into colorless after 180 min at 365 nm. This is because resorcinol does not absorb light at 365 nm wavelength to a significant extent. The complete light absorption by the Zr-doped ZnO semiconductor results in the generation of a greater number of hydroxyl radicals and

Figure 14. UV−vis absorption spectra of the photodegradation of resorcinol (experimental conditions: irradiation wavelength λ = 254 nm, resorcinol = 200 ppm). 16342



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superoxide free radicals. Therefore, during the first steps of the reaction, resorcinol may be degraded to p-benzoquinone and obenzoquinone quickly. In the next stage, the rings open to form carboxyl acids, which were further degraded to water and carbon dioxide, as shown in Scheme 3.

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CONCLUSION In summary, ZnO nanomaterials with a range of different morphologies have been synthesized by a facile, low-cost, coprecipitation approach. It is found that the structural, optical, and PC properties are sensitively dependent on the ZnO morphology and the incorporation of Zr4+ ions. ZnO morphology with spherical shaped nanocrystals showed enhanced photocatalytic activity as compared with the other ZnO morphologies due to the small crystal size distribution, high surface area, and greater amount of oxygen vacancies. However, 1.5 wt % Zr-doped ZnO shows superior performance toward degradation and mineralization of resorcinol than the other doped ZnO, pure ZnO, and commercial TiO2 (Degussa P-25). The study thus concludes that novel self-assembled morphology, smaller particle size, more surface defects, and increase in the band gap value of ZnO upon Zr loading have a significant influence on the enhanced photocatalytic activity of 1.5 wt % Zr-doped ZnO NPs.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-44-28178200. Fax: +91-44-28175566. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors duly acknowledge the financial support rendered by University Grants Commission (UGC) (Ref. F. No. 38-118/ 2009 (SR)), New Delhi.

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REFERENCES

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