Comparative Studies on Structural, Optical, and Textural Properties of

Article pubs.acs.org/IECR

Comparative Studies on Structural, Optical, and Textural Properties of Combustion Derived ZnO Prepared Using Various Fuels and Their Photocatalytic Activity Parameswara Rao Potti and Vimal Chandra Srivastava* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, 247667 Uttarakhand, India S Supporting Information *

ABSTRACT: Pure mesoporous nanosized ZnO samples have been synthesized by combustion synthesis method using different type of fuels such as citric acid, dextrose, glycine, oxalyl dihydrazide, oxalic acid, and urea. All these samples were found to have the standard hexagonal wurtzite structure with the lattice constants a and c having values 3.25 Å and 5.21 Å, respectively. The diffuse reflectance spectra of the prepared samples have shown the maximum absorption of light in the UV region stating that these catalysts can be used as photocatalysts. The BET surface area measurements of the prepared catalysts were found to vary according to the equivalence ratio. The presence of pronounced hysteresis in N2 adsorption−desorption isotherm curves indicated the three-dimensional network arrangement of pores in the ZnO samples prepared using dextrose and urea as fuels. The photodegradation ability of the various synthesized photocatalysts was tested for the photodegradation of an azo dye, namely, orange G dye solution. ZnO photocatalyst prepared using oxalic acid as fuel showed the highest decolorization and degradation ability under UV light irradiation.

1. INTRODUCTION Photocatalytic degradation of water pollutants is one of the important techniques for the degradation of organic pollutants, dyes, etc. It requires a photocatalyst and a light source for photodegradation.1 The powerful hydroxyl radicals produced from the surface of the photocatalyst react with the organic matter to form simple harmless compounds. Photocatalysts are typically semiconductors having a wide band gap energy (Eg) such as TiO2 (3.18 eV),2 ZnO (3.37 eV; 3.2 eV),3,4 and WO3 (2.7 eV).5 Several studies have been reported in the literature using ZnO as a photocatalyst. ZnO has a stable hexagonal wurtzite structure, and it finds application in several fields, such as the electronic industry as transparent conducting oxide, UVsensitive and solar-blind photodetector, shield against highenergy radiation, organic light-emitting diodes (O-LED), and transparent thin-film transistors (TTFT), and as a conducting channel in field effect transistor.6 Several methods have been proposed in the literature to prepare the nanosized ZnO. These include sol−gel method,7−12 solvothermal method,13,14 flame spray pyrolysis,15 precipitation method,5,16−18 ultrasonic and boiling disperse method,19,20 hydrothermal method,4,21−23 chemical deposition method,24 thermal decomposition,25 solid state synthesis,26 solution combustion synthesis (SCS),27 etc. SCS method is a very good technique for the production of nanosized zinc oxide powder. This method is very fast, reliable, requires low cost raw materials, has no need of high temperature furnaces, and can be applied to produce any type of oxide material.28 Several investigators prepared ZnO nanoparticles using the SCS method and studied the structural and optical properties of the synthesized ZnO. Reddy et al.28 synthesized the ZnO nanoparticles using oxalyl dihydrazide (ODH) as fuel and carried out structural, optical, and Raman studies. Lin et al.29 prepared the ZnO nanoparticles using a mixture of metallic zinc powder and glycine. They performed © 2012 American Chemical Society

different morphological observations of as-prepared samples. Tarwal et al.30 used glycine alone as a fuel and investigated the optical behavior by performing photo luminescence study. Manoharan and Arora31 prepared Mg doped ZnO using urea as a fuel. Nagaraja et al.32 degraded the rhodamine B dye solution using ZnO synthesized through SCS using sucrose as a fuel under UV light irradiation. To the best of the knowledge of the authors, the effect of type of fuel on the structural, optical, textural, and photocatalytic degradation abilities of ZnO synthesized through the SCS route has not been investigated yet. An attempt has been made in the present study to synthesize ZnO nanoparticles through the SCS route using low-cost fuels such as citric acid, dextrose, glycine, ODH, oxalic acid, and urea. All the prepared various ZnO particles have been characterized to study the structural, optical, textural properties and photocatalytic degradation abilities toward orange G dye solution.

2. MATERIALS AND METHODS 2.1. Materials. All the chemicals used were of analytical reagent (AR) grade. Zinc nitrate hexa hydrate (Zn(NO3)2 6H2O), glycine (NH2CH2COOH), citric acid monohydrate (C6H8O7 H2O), oxalyl dihydrazide (C2H6N4O2), orange G dye (C.I. No. 16230, molecular weight: 452.4) were procured from Himedia Chemicals, India. Dextrose anhydrous extrapure (C6H12O6), urea extrapure (CH4N2O) and oxalic acid ((COOH)2) were purchased from SD Fine Chemicals, Received: Revised: Accepted: Published: 7948

February 23, 2012 May 5, 2012 May 7, 2012 May 7, 2012 dx.doi.org/10.1021/ie300478y | Ind. Eng. Chem. Res. 2012, 51, 7948−7956

Industrial & Engineering Chemistry Research

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Table 1. Reducing Valence and Synthesis Conditions of ZnO Using Different Fuels

a

fuel

molecular formula

catalyst code

reducing valence

equivalence ratio (Φ)

Ψa

furnace temp. (°C)

citric acid monohydrate dextrose glycine oxalyl dihydrazide oxalic acid urea

C6H8O7·H2O C6H12O6 NH2CH2COOH C2H6N4O2 C2H2O4 CH4N2O

ZC ZD ZG ZO ZOA ZU

18 09 10 24 06 02

0.5556 0.4167 1.111 1 5 1.667

0.5556 0.4167 1.111 1 5 1.667

400 400 300 350 450 350

Ψ is the molar ratio of fuel to the oxidizer in the synthesis reactions for preparation of various ZnO catalysts using different fuels.8

Table 2. Synthesis Reactions for Preparation of Various ZnO Catalysts Using Different Fuels catalyst ZC ZD ZG

reaction

5 10 79 ZnN2H12O12 + C6H10O8 → ZnO + N2 + CO2 + H 2O 9 3 9

5 5 17 C6H12O6 → ZnO + N2 + CO2 + H 2O 12 2 2 10 14 20 79 ZnN2H12O12 + C2H5NO2 → ZnO + N2 + CO2 + H 2O 9 9 9 9

ZnN2H12O12 +

ZO

ZnN2H12O12 + C2H6N4O2 → ZnO + 3N2 + 2CO2 + 9H 2O

ZOA

ZnN2H12O12 + 5C2H 2O4 → ZnO + N2 + 10CO2 + 11H 2O

ZU

ZnN2H12O12 +

5 8 5 28 CH4N2O → ZnO + N2 + CO2 + H 2O 3 3 3 3

hypergolic in nature27 and produce a spark with the help of the surrounding temperature. The fuel in the redox mixture catches the spark that appears at one corner and results in an incandescent flame that spreads throughout the mass giving a voluminous, fluffy, porous, solid product.33 In this typical synthesis, ZnO was prepared using various fuels like citric acid, dextrose, glycine, ODH, oxalic acid, and urea, and the prepared catalysts were named ZC, ZD, ZG, ZO, ZOA, and ZU, respectively (Table 1). Assuming complete combustion, the reactions depicting the entire synthesis for each fuel are given in the Table 2. 2.3. Characterization. For determining the crystal size, crystal structure, and lattice parameters, powder X-ray diffraction (XRD) analysis was carried out by using an X-ray diffractomer (Brueker AXS, Diffractometer D8, Germany). The analysis was done using Cu Kα radiation (1.542 Å) at an accelerating voltage and emission current of 40 kV and 30 mA, respectively. The samples were scanned at a rate of 1°/min and at a step size of 0.02. The crystal size was calculated by X-ray line broadening analysis using the Scherrer equation. All the data were recorded in the Bragg angle range of 5−100°. The compounds have been identified in accordance with the 2θ angle using the library of the international center for diffraction data (ICDD). The band gap energy (Eg) of the prepared ZnO samples was calculated using diffuse reflectance spectra recorded using UV− vis spectrophotometer equipped with an integrating sphere attachment (UV−2450 SHIMADZU). Spectra were recorded in the range from 200−800 nm using BaSO4 as a reference standard material. To understand the morphology of the ZnO prepared from various fuels, first the samples were gold sputter coated, and then, scanning electron microscopy (SEM) analysis was carried out using LEO, Model 438 VP, England. Textural characteristics such as surface area and pore size distribution were estimated for all the samples using the Micromeritics ASAP 2020 surface area and porosity analyzer. Prior to measurements,

Mumbai, India. Double-distilled water was used throughout the analysis. 2.2. Preparation of ZnO Photocatalyst through Combustion Synthesis Method. A similar procedure given by Nagaveni et al.33 was used. According to this method, stoichiometric redox mixture of zinc nitrate hexahydrate and the fuel were dissolved in minimum quantity of distilled water in a Petri-dish of 150 mL capacity. Citric acid, dextrose, glycine, ODH, oxalic acid, and urea were used as fuel for preparation of different catalysts. The stoichiometric redox mixture was calculated using the concept from propellant chemistry as oxidizing and reducing valencies of oxidizer and reducer, respectively. The oxidizing valency of zinc nitrate hexahydrate is −10 and the reducing valencies of various fuels used in the preparation and the conditions for the synthesis of ZnO have been listed in the Table 1.1 The redox mixture formed was stirred with the help of a magnetic stirrer for 0.5 h. Then, the Petri dish, along with the redox mixture, was kept in the muffle furnace at the preheated temperature. The temperature of the muffle furnace was different for different fuels, and it is given in Table 1. Generally, required furnace temperatures for different fuels can be roughly estimated from the thermogravimetric analysis (TGA) and differential thermal analysis (DTA). It can also be estimated based on the number of carbon atoms and the type of functional groups in the fuel. In the present study, high temperatures were used for citric acid and dextrose because both the compounds have a higher number of carbon atoms, which requires higher temperatures for removing the CO2 evolved during the combustion process. Although the fuel oxalic acid has a lesser fraction of carbon atoms, it still requires a higher temperature than the citric acid and dextrose, so as to decompose the intermediates formed during reaction of oxalic acid with zinc nitrate. Fuels such as oxalyl dihydrazide, glycine, and urea contain amine groups, and these compounds decompose at temperatures less than 350 °C. In the muffle furnace, the dehydration of the redox mixture and melting of nitrates takes place within 2−3 min. The nitrates are 7949

dx.doi.org/10.1021/ie300478y | Ind. Eng. Chem. Res. 2012, 51, 7948−7956


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synthesized ZnO compounds were compared and indexed to the joint committee on powder diffraction data (JCPDS) files. The samples ZO, ZC, ZD, ZG, and ZOA were mapped to the JCPDS file no. 36-1451 (zincite), whereas sample ZU is indexed to the JCPDS file no. 01-1136 (zincite). Thus, all the ZnO samples were found to possess hexagonal wurtzite structure. No more impurity phases were detected in all the ZnO samples. Hence, synthesized ZnO samples were in pure form. Figure 1 shows a series of characteristic peaks 2.81(100), 2.60(002), 2.47(101), 1.90(102), 1.62(110), 1.47(103), and 1.37(112) in all samples. The peak corresponding to 2.47(101) showed highest intensity. Figure 2 shows variation of the

the samples were degassed under vacuum for 2 h and at the temperatures where they were synthesized. The BET surface area for all the samples, and the micropore area and micropore volume were estimated by using the Brunauer−Emmett−Teller (BET) method;34 pore size distribution (PSD) was estimated using the Barrett−Joyner−Halenda (BJH) method.35 To determine the surface functional groups in the synthesized ZnO, Fourier transform infrared (FTIR) analysis (Thermo Nicolet, Model Magna 760) was carried out by preparing the pellet using a hand press pellet maker. KBr was used as a reference standard material. 2.4. Photoreactor and Experimental Conditions. 50 mg/L of orange G dye was taken as a test contaminant to assess the photocatalytic ability of the synthesized ZnO photocatalysts. A 250 mL borosilicate glass beaker was taken as the reaction chamber. The dye solution was irradiated for 3 h at a catalyst loading of 1 g/100 mL of solution, using a high pressure mercury vapor lamp emitting the light at 365 nm. This lamp was suspended in such a manner so that the distance from the top of the reaction solution to the tip of the light source was 8 cm. Prior to the irradiation, the reaction solution was stirred for 0.5 h in the dark to establish the adsorption−desorption equilibrium. The percentage degradation and decolorization of the dye was estimated by measuring the absorbance (UV−vis spectrophotometer, SHIMADZU) and as well as the color of the irradiated solution (Aqualytic, Germany). Percentage degradation and percentage decolorization were calculated using following equations: percentage degradation =

(Ai − A t )100 Ai

percentage decolorization =

(Ci − Ct )100 Ci

(1) Figure 2. Effect of equivalence ratio on crystallinity of ZnO. (2)

where Ai is the absorbance of the initial dye solution; At is the absorbance of the final dye solution measured after irradiation for 3 h; Ci is the color of the initial dye solution; and Ct is the color of the final dye solution measured after irradiation for 3 h.

intensity of peak 2.47(101) with the equivalence ratio (Φ), which is defined as the absolute value of the ratio of oxidizer valency to the reducer valency. It may be seen that the peak for Φ = 1, corresponding to sample ZO (Table 1), is highest. Thus, sample ZO seems to be most crystalline among all the samples. Table A1 (Supporting Information) provides the necessary information about the crystallographic orientations, which can be used for estimating various crystal parameters. Information pertaining to the crystallite orientation and the crystal’s number density in a given plane can be obtained by calculating the texture coefficient (TC(h k l)), which is given as28

3. RESULTS AND DISCUSSIONS 3.1. Structural Analysis. Figure 1 shows the XRD patterns of the ZnO synthesized from various fuels. All the peaks of as-

TChkl =

(Ihkl /Io)

(∑ ) Ihkl Io

100 (3)

From the Table A1 (Supporting Information), it is obvious that all the planes of all ZnO samples have good texture coefficient. TC(h k l) = 1 represents randomly oriented crystallite, while higher values indicate the abundance of grains/ crystallites oriented in a given (h k l) plane. The Debey− Scherrer formula for finding the crystallite size (D) is given as D=

Kλ β cos θ

(4)

where K is the Scherrer's constant and has the value 0.9 for hexagonal crystal structure, λ is the wavelength of the radiation (1.5418 Å), β is the forward width at half-maximum (fwhm) (radian), and θ is the half value of the Bragg diffraction angle in

Figure 1. XRD patterns of as-prepared ZnO samples. 7950



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3.2. Optical Studies. UV−vis diffuse reflectance spectra (DRS) can be used to determine the absorption ability of a semiconductor toward light.4 Figure 3 depicts the collected

the XRD pattern (degree). The lattice parameters (a, c) were calculated using the following equation:36 ⎡4 ⎛ a ⎞2 ⎤ 1 1 2 2 2⎜ ⎟ ⎥ ⎢ ( h hk k ) l = + + + 2 ⎝ c ⎠ ⎦ a2 dhkl ⎣3

(5)

where dhkl is the interplanar spacing for the plane (h k l), a and c are lattice parameters, and h, k, and l are the miller indices. The volume of the unit cell was calculated using the eq 6 given by Reddy et al.28 The strain associated with the nanocrystals was calculated using eq 7.37 V = 0.866a 2c

ε=

β 4 tan θ

(6)

(7)

Various crystal parameters including crystallite size, lattice parameters (a, c), volume (V) of the unit cell, and the strain (ε) associated with the crystal have been estimated using eqs 4−7. The estimated values of various crystal parameters are tabulated in Table A2 (Supporting Information). It may be seen that there is significant variation in the D and ε values among all the ZnO samples. From Tables A2 and A3 (Supporting Information), it is clear that an increase in the D values decreased the surface area. This may be due to the difference in crystal growth due to variation of temperature.14 A similar effect of temperature on the growth of crystal was also revealed by Reddy et al.28 in the synthesis of ZnO using ODH as fuel. Figure 1 clearly demonstrated the effect of fuel on the structural properties of synthesized ZnO. It is observed that samples ZC and ZD, having a fuel-to-oxidizer molar ratio (Ψ) < 1, exhibited low crystallite sizes with high strain. This may happen to samples for which smoldering combustion occurs during the synthesis. Smoldering combustion occurs when combustion temperature (Tc) is less than 650 °C.38 It causes improper growth of crystal, thus decreasing the intensity of peaks, but favors the formation of small crystals (having low D values), which in turn possess high surface area with high strain value. However, high specific surface area was observed only in the case ZD but not for ZC. It has been reported that no precipitation happens during the combustion process when citric acid is used as a fuel because of high complexing ability of citric acid.39 Therefore, ZC, which was made with citric acid as fuel, did not show as much surface area as ZD. High strains are often observed in nanocrystalline material due to the displacement of unit cells from their normal positions causing dislocation, surface restructuring, lattice vacancies, interstitials, substitutionals, etc. These lattice vacancies are beneficial for a photo catalyst where these may act as reaction sites. Sample ZO was synthesized with Φ = 1 resulting in maximum Tc, and the maximum amount of heat is produced, which helps in the proper growth of crystals there by exhibiting higher intensities. Samples ZG, ZOA, and ZU fall under the case of fuel lean category, that is, Φ > 1, and offer good combustion heat, which helps in better growth of crystal, as compared to ZD, resulting in higher D values, low specific surface area, and exhibit higher intensities.1 The estimated lattice parameters and the volume of the unit cell were compared with the standard JCPDS files (a = 3.2498 Å, c = 5.2066 Å, V = 47.62 (Å)3 for file no. 36-1451 and a = 3.2420 Å, c = 5.1760 Å, V = 47.11 (Å)3 for file no. 01-1136). The values for the synthesized ZnO are in good agreement, thus confirming the crystal structure as hexagonal.

Figure 3. DRS spectra of ZnO particles prepared from various fuels.

UV−vis DRS of ZnO samples prepared from various fuels. The maximum absorption was found at 370 nm, 351 nm, 366 nm, 371 nm, 371 nm, and 369 nm for ZC, ZD, ZG, ZO, ZOA, and ZU, respectively. Thus, maximum absorbance for all the samples was in the UV-region. In photocatalytic dye degradation, where a semiconductor is being used as a catalyst, determination of band gap of the semiconductor catalyst is necessary. This helps in choosing the right kind of light needed for the dye degradation. The band gap energy (Eg) of the various ZnO samples was calculated using the following relation:40,41 (hνα)1/ n = A(hν − Eg )

(8)

where h is the Plank's constant, α is the absorption coefficient, A is the proportional constant, ν is the frequency of the radiation, and the exponent n has the values 0.5, 1.5, 2, and 3 for direct allowed transition, direct forbidden transition, indirect allowed transition, and indirect forbidden transition, respectively. In the present study, the exponent n has the value of 0.5 because of the nature of direct allowed transition in ZnO.42 The obtained spectra were converted into a Kubelka− Munk function as discussed in the literature.8 The α in eq 8 was substituted with the remission function F(R∞) = (1 − R∞)2/ 2R∞. Where, R∞ is the diffuse reflectance based on the Kubelka−Monk theory of diffuse reflectance.42 The modified form of eq 8, thus, can be written as follows: (hνF(R ∞))2 = A(hν − Eg )

(9)

Eg values of the prepared ZnO particles were estimated by drawing the Tauc plot ((hνF(R∞))2 against hν). Eg value is equal to the intercept on X-axis, and it is obtained by extrapolating the linear region of the obtained curve on to the X-axis. The estimated Eg are given in Table A2. SEM analysis (Figure 4) showed that the particles are in agglomeration state. The agglomeration of particles caused a red shift in the spectrum of all ZnO samples. This shift lowered the Eg values of all the ZnO samples with respect to bulk ZnO. Since all the ZnO samples showed maximum absorption in the UV region and that all the samples had Egvalue less than that of the bulk ZnO, this indicates that the decrease in band gap 7951



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Figure 4. Scanning electron micrograph of various ZnO samples prepared using various fuels.

agglomerated state. The agglomeration of the particles may be due to very viscous nature of the citric acid solution used for ZnO preparation. The ZD sample prepared using dextrose as fuel was found to be irregular in shape with varied pore structure (Figure 4b). The size of pores in ZD seems to be large. The large pore size and high surface area of ZD may be due to the high volume of gases escaping during the combustion process. ZG and ZO particles, as shown in Figure 4c and Figure 4d, respectively, are found to be somewhat spherical in shape. The particle size distribution of ZG is less, as compared to that of ZO. ZO particles, similar to ZC, seem to be agglomerated with many fewer pores (Figure 4d). Sample ZOA prepared using oxalic acid as fuel was found to be cylindrical in shape (Figure 4e). ZOA particles, as shown in Figure 4e, are found to be crystalline in nature, although the number of pores observed in ZOA is fewer, as compared to ZD and ZG. Use of urea as a fuel produced the brick-red color zinc oxide particles (color not visible in the figure, although it is dull, as compared to that of ZOA) (Figure 4f). SEM of ZU reveal the flower-like structure. ZU particles do not show porosity on their surface. Overall, ZD seems to be most porous, whereas ZU seems to be least porous. Particles of ZC, ZD, ZG, and ZO are somewhat spherical in nature, though their particle size distribution seems to be varying. The structure of ZOA and ZU are entirely different, with the ZOA particle being cylindrical in nature. There seems to be some correlation between the structure type and equivalence ratio (Φ) (Table 1). ZOA and ZU have Φ

increases the chance of absorption of light of higher wavelength. Since the value of Eg depends on the particle size, the particle radius (r) can be estimated using Brus equation with the assumption that the particles are spherical in nature.43 ΔEg = |Egnano − Egbulk | =

h2π 2 ⎛ 1 1 ⎞ ⎛ 1.8e 2 ⎞ ⎟−⎜ ⎜ + ⎟ 2 mR* ⎠ ⎝ 4πεεor ⎠ 2r ⎝ me* (10)

Enano g

where is the band gap energy of the semiconductor nanoparticle, Egbulk is the band gap energy of the bulk semiconductor, h is the Plank's constant, r is the particle radius (m), m*e is the effective mass of electron, m*R is the effective mass of hole, ε is the dielectric constant of the sample, εo is the vacuum relative permittivity and has the value of 8.8542 × 10−12 (F/m), and e is the charge of the electron. For ZnO, m*e has value of 0.19 times the mass of the electron, m*R has value of 1.21 times the mass of the electron, and the value of ε is equal to 8.5.44 Calculated values r for different ZnO catalysts are given in Table A2 (Supporting Information). It can be seen that the samples ZG and ZO exhibited higher particle sizes as compared to other samples. 3.3. Morphological Observations. Scanning electron microscope (SEM) images of the synthesized ZnO samples from various fuels are shown in Figure 4. ZnO sample prepared using citric acid monohydrate as a fuel (ZC) produced particles with few surface pores. Particles, as shown in Figure 4a, are in 7952



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ratio of 5 and 1.667, respectively, whereas ZC, ZD, ZG, and ZO have Φ values in the range of 0.42−1.11. It seems that the ZnO particles with high Φ values have different structure, as compared to those having low Φ values ( 0.45). The adsorption and desorption branches of all the samples were inclined, and the samples ZD and ZU showed sharp slopes for the adsorption/desorption, as compared to other samples. This steep region may be due to tensile strength effect (TSE)46 which is attributed by sudden flow of the remaining condensate out of the pores. The presence of hysteresis in isotherms of ZD and ZU samples indicated presence of three-dimensional network of pores.47 Structural heterogeneity and solid internal structure both can be characterized in terms of PSD. For the use of industrial applications, PSD is closely related to both kinetic and equilibrium properties of the porous material.48 IUPAC has classified the pore sizes as micropores (diameter (d) < 20 Å), mesopores (20 Å < d < 500 Å), and macropores (d > 500 Å). Micropores are further divided into ultra-micropores (d < 7 Å) and super-micropores (7 Å < d < 20 Å). Since adsorption is preliminary step for the catalysis, the catalyst possessing the mesopores has great advantage in a solid−liquid system. The ZnO samples prepared from various fuels exhibited mesoporous nature (Figure 5). Table A3 shows that the BET surface area of the catalysts was in the following order: ZD > ZG > ZOA > ZC > ZO > ZU. The experimental points of t-plots (nitrogen adsorbed (Q) at different (P/Po) values as a function of thickness of adsorbed gas (t)) were found to give good agreement with the isotherm equation of Harkins and Jura.49 Experimental points fell in a straight line for all catalysts except the ZC. For all samples except ZC, this straight line intersected the negative Y-axis, thus indicating the porous nature of these samples. For ZC, the positive intercept indicated presence of micropores in ZC. The pore size calculation of different ZnO catalysts for determination of the mesopores size distribution was also performed on desorption branch of N2 adsorption−desorption isotherm by BJH method.35 The dV/dD pore volumes versus

Figure 5. Pore size distributions of different ZnO catalysts.

pore diameter curve of sample ZD showed two peaks at 83 Å and at 121.8 Å, whereas that for sample ZU showed two peaks at 56 Å and 70.6 Å. Thus, for ZD and ZU, the distribution curve was bimodal in nature.50 Furthermore, the BJH desorption PSD curves confirmed the predominance of mesopores in all the ZnO samples. The analysis of BJH adsorption PSD of the ZC catalyst showed that the micropores accounted for 10% the total pore area and the rest 90% of the area was accounted by mesopores. For ZD, ZG, ZO, ZOA, and ZU, the mesopores accounted for about 83%, 86.5%, 80.3%, 92%, and 82.5% of the total pore area, respectively. Thus, ZOA had the highest percentage of mesopores, although the actual amount of mesoporous area was in the following order: ZD (62.25 m2/g) > ZG (21.63 m2/g) > ZOA (14.45 m2/g) > ZC (11.7 m2/g) > ZO (6.44 m2/g) > ZU (3.3 m2/g). Another important factor associated with the catalysts is the fractal dimension, which is often used as an index for estimating the roughness or irregularity of the catalytic surface. In the present study, the fractal dimension for each ZnO catalyst was estimated by applying the Frenkel−Halsey−Hill (FHH) equation51 to the adsorption isotherm of N2:52 ⎛ P ⎞D−3 q = K ln⎜ o ⎟ ⎝P⎠ qe 7953

(11)



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Figure 6. FTIR spectra of different ZnO catalysts prepared using different fuels.

where q is the amount adsorbed at equilibrium pressure P, qe is the amount adsorbed filling micropore volume, Po is the saturated pressure, K a constant, and D is the fractal dimension. The logarithmic plot of (q/qe) versus ln(Po/P) (figure not shown) showed linear behavior, and D is calculated from the slope (D − 3) of the line. A surface, which is perfectly smooth, has the fractal dimension of 2 and a surface, which is very rough or irregular, has the fractal dimension of 3. For the prepared ZnO catalysts, the fractal dimension was found to be 2.598 for ZC, 2.525 for ZD, 2.5 for ZG, 2.53 for ZO, 2.53 for ZOA, and 2.43 for ZU. Thus, prepared ZnO catalysts have a maximum of 15% variation in surface irregularities. 3.5. FTIR Analysis. Figure 6 depicts the FTIR spectra of the different ZnO catalysts prepared using various fuels. The spectra of all the samples were recorded in the range 440−4000 cm−1. The presence of ZnO was confirmed with the characteristic peak of ZnO from 443−450 cm−1 and 1400 cm−1 in all the samples.5,10,18,53 The presence of surface hydroxyl groups in all the samples was confirmed by the peaks from 3420−3445 cm−1 and 1620−1640 cm−1, which can be ascribed to the basic hydroxyl groups of chemisorbed or physisorbed on the surface of the ZnO, and these are not the absorption peaks of zinc hydroxide.18,42 These hydroxyl groups react with the photogenerated hole forming the hydroxyl radicals.5 The stretching vibration of the C−H bond can be observed in all the samples at 3150−3000 cm−1. Sample ZU shows a small peak in the range 2200−2300 cm−1, which may be due to the stretching vibration of the CN.54 The samples ZU, ZC, ZG, ZD, and ZO also show peak at ≈2350 cm−1, and this may be due to the absorption of CO2, evolved during combustion, on the surface.55 In the samples ZC, ZO, and ZU, the intensity of CO2 peak was strong, where as the sample ZOA did not show any peak of presence of CO2. All samples were observed to be colored after combustion except ZOA, which was observed to be white in color. This may have happened due to nonabsorption of CO2 onto ZOA. 3.6. Photodegradability Studies. Various synthesized ZnO photocatalysts were tested for their ability to degrade the orange G dye solution, and the results are shown in Figure 7. All the synthesized photocatalysts showed higher degradation

Figure 7. Degradation and decolorization efficiencies of various synthesized photocatalysts.

efficiencies than decolorization efficiencies except the catalyst ZC. The point of zero charge (PZC) of ZnO is 9.0,56−59 and the nature of orange G dye is anionic, having natural pH of 5.1. In the aqueous solution, the dye is first dissolved and the sulfonate groups of the orange G dye (D-SO3Na) are dissociated and converted to anionic dye ions by following reaction:60 H 2O

D‐SO3Na ⎯⎯⎯→ D‐SO3− + Na +

(12)

At pH 5.1, a significant amount of dye molecules gets adsorbed onto the surface of ZnO. This adsorption ability further depends on the surface area and presence of mesopores. The excess decolorization was attributed to the presence of mesopores. The comparative studies among the synthesized photocatalysts showed that the photocatalysts ZC and ZOA exhibited higher photocatalytic degradation and decolorization activity than the other synthesized photocatalysts. Between ZC and ZOA, ZOA exhibited higher degradation and decolorization efficiency. This may be due to the lower band gap energy and higher BET surface area of ZOA (Eg = 3.16 eV, BET surface area = 18 m2/g) as compared to that of ZC (Eg = 3.23 eV, BET surface area = 13 m2/g). The lower band gap energy in ZOA increases the formation of OH• radicals, which attack the 7954



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chromophores of the dye molecules thereby causing higher degradation and decolorization. Other synthesized photocatalysts ZD (Eg= 3.25 eV, BET surface area = 75 m2/g), ZG (Eg = 3.21 eV, BET surface area =25 m2/g), and ZO (Eg = 3.20 eV, BET surface area = 7 m2/g) showed lower photocatalytic activity due to higher band gap energy. Photocatalyst ZD has highest BET surface area; still, because of its higher band gap energy, it shows lower degradation efficiencies as compared to ZC and ZOA. Similarly, ZU, despite its having the lowest band gap energy among all the prepared ZnO photocatalysts, shows lower degradation efficiencies due to its lower surface area. Thus, it seems that a balance of band gap energy and BET surface area are important for achieving higher degradation efficiencies.

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic orientations, their identification parameters, and texture coefficient of ZnO samples prepared using different fuels are given in Table A1. Band gap energy, particle size, crystallite size and crystal parameters of various ZnO samples prepared using different fuels are given in Table A2, whereas textural properties of the prepared ZnO catalysts are given in Table A3. Tauc plots of zinc oxide particles prepared using different fuels are given in Figure A1, and adsorption/ desorption isotherms of N2 at 77 K on different ZnO catalysts are given in Figure A2. This information is available free of charge via the Internet at http://pubs.acs.org/.

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REFERENCES

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4. CONCLUSION Various ZnO catalysts have been prepared through the solution combustion synthesis method using different fuels. XRD analysis confirmed that the synthesized catalysts possessed standard hexagonal wurtzite structure. Crystallite size of ZnO catalysts varied from 37−81 nm. Strain in the catalysts increased with decrease in crystallite size. No impurity phases were detected in all the samples. All the ZnO samples showed maximum DRS intensity in the UV region and that all the samples had Eg values in the range 3.13−3.25 eV. SEM analysis of different ZnO catalysts showed peculiar morphologies with respect to the fuel used. For use as a photocatalyst, ZnO should possess good crystallinity and high surface area. It was observed that the catalyst sample ZOA (Φ = 5, Eg = 3.16 eV, 18 m2/g), which was prepared using oxalic acid as fuel, showed the best photocatalytic activity toward the degradation and decolorization of an azo dye, namely, orange G.

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Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-1332-285889. Fax: +91-1332-276535/273560. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors are thankful to Prof. I. D. Mall and Prof. I. M. Mishra from Department of Chemical Engineering, IIT Roorkee, India for their suggestions, continuous help, and motivation. 7955



Article

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