Photocatalytic properties of solution combustion synthesized ZnO powders using mixture of CTAB and glycine and citric acid fuels

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Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

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Original Research Paper

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Photocatalytic properties of solution combustion synthesized ZnO powders using mixture of CTAB and glycine and citric acid fuels

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H. Vahdat Vasei, S.M. Masoudpanah ⇑, M. Adeli, M.R. Aboutalebi

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School of Metallurgy & Materials Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

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a r t i c l e

i n f o

Article history: Received 2 March 2018 Received in revised form 2 October 2018 Accepted 4 November 2018 Available online xxxx Keywords: ZnO Solution combustion synthesis CTAB Mixture of fuels Photocatalytic activity

a b s t r a c t Zinc oxide (ZnO) powders have been prepared by solution combustion synthesis method using combination of cetyltrimethylammonium bromide (CTAB) with glycine and citric acid fuels. The combustion behavior, phase evolution, microstructure, optical properties and photocatalytic performance were compared by thermal analysis, X-ray diffractometry, electron microscopy, and diffuse reflectance and photoluminescence spectrometry techniques at various fuel to oxidant ratios (/ = 0.5, 0.75, 1 and 1.5). Single phase and well-crystalline ZnO powders were directly formed regardless of fuel type and fuel content. The higher specific surface area and pore volume of the as-combusted ZnO powders using mixture of CTAB and citric acid fuels increased with fuel content. The lower band gap energies (3.10–3.16 eV) of the as-combusted ZnO powders in the presence of glycine fuel were attributed to their lower crystallinity, as confirmed by higher visible emission in photoluminescence spectra. The higher photodegradation of methylene blue under ultraviolet light irradiation was achieved by the as-combusted ZnO powders by using CTAB-citric acid mixed fuels, due to their good crystallinity and higher specific surface area. Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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1. Introduction

46

Zinc oxide (ZnO) as an inexpensive n-type semiconductor has wide band gap energy (3.27 eV), high exciton binding energy (
60 meV) and high electron mobility (
115–155 cm-2V-1s1), making it as a suitable material for applications in gas sensors, biosensors, solar cells, varistors, ultraviolet photodiodes, electrical and optical devices [1–4]. ZnO nanoparticles have attracted considerable attention as semiconductor photocatalysts in environmental remediation by degradation of organic pollutants and hydrogen generation [5–7]. Photocatalytic activity of ZnO powders depends on composition, defect concentration, particle size and shape, crystallinity, specific surface area which can be controlled through synthesis method [5,8]. Wet chemical synthesis routes such as hydrothermal, precipitation, thermal decomposition, solution combustion synthesis (SCS), etc. can be efficiently employed for controlling crystallinity, specific surface area, defect concentration and particle size and shape of ZnO powders [9–11]. Solution combustion synthesis as a simple, time- and energy-efficient and inexpensive route can be used for production of well-crystalline ZnO powders with high specific surface area [12]. The high

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⇑ Corresponding author.

released thermal energy by exothermic reaction between oxidizers (e. g. metal nitrates) and organic fuels (e. g. urea, glycine, citric acid) leads to the formation of well-crystalline products [13]. The as-combusted powders has foamy and spongy structures due to the liberation of a large amount of gaseous products such as CO2, H2O, H2, N2, CO, etc. by burning of organic fuels [14]. Furthermore, the rapid cooling of as-combusted powders by outgoing gases prevents particle growth and sintering [15–17]. Structure, crystallinity, particle size and shape, specific surface area and optical properties of as-combusted ZnO powders depend on the adiabatic combustion temperature and the amount of released gaseous products which can be mainly tuned by nature of fuel and fuel to oxidant ratio [16–18]. Glycine, urea and citric acid are common fuels in SCS due to their low decomposition temperature, high solubility, readily available and low cost [19,20]. However, the approach of mixed fuels has been attracted a great interest in solution combustion synthesis, due to a good control over flame temperature and the type and amount of released gaseous products [21]. Cetyltrimethylammonium bromide (CTAB) as a cationic surfactant has been widely employed in solution combustion synthesis of oxide nanoparticles such as CuO [22], Co3O4 [23], SrFe12O19 [24], BiVO4 [25] to reduce particle size [25]. However, CTAB has high decomposition temperature and long polymeric chain, leading to bulky microstructure for as-combusted powders

E-mail address: [email protected] (S.M. Masoudpanah). https://doi.org/10.1016/j.apt.2018.11.004 0921-8831/Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article as: H. V. Vasei, S. M. Masoudpanah, M. Adeli et al., Photocatalytic properties of solution combustion synthesized ZnO powders using mixture of CTAB and glycine and citric acid fuels, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.004

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2. Experimental procedures

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Required amounts of zinc nitrate (Zn(NO3)2), cetyltrimethylammonium bromide ([(C16H33)N(CH3)3]Br), citric acid (C6H8O7) and/ or glycine (C2H5NO2) were dissolved in distilled water in which the different fuel to oxidant ratios (/ = 0.5, 0.75, 1 and 1.5) were used. After homogenization, the precursor solution was poured into a dish and heated till to transform into a gel while by further heating up to 250 °C on a hot plate, ignition reaction started from a point and propagated spontaneously. The resulted powders were hand-crushed with a pestle. IR spectra in the range of 400–4000 cm1 were measured by Fourier transform infrared (FTIR) spectrophotometer

100

433

(d)

1200

400

Fig. 1. FTIR spectra of the dried gels prepared using (a) the mixture of CTAB and glycine fuels and (b) the mixture of CTAB and citric acid fuels at / = 1 and the ascombusted ZnO powders using (c) the mixture of CTAB and glycine fuels and (d) the mixture of CTAB and citric acid fuels at / = 1.

(a)

176 °C 11 %

10

20

2

20

0 153 °C

φ=1.5

-2

200

300 400 500 Temperature (°C)

600

700

(b)

100

380 °C

18 %

(102)

100

10 8

80 62 %

4

280 °C

40

2

20

ΔT (μV)

6

60

(112)

(103)

(110)

50 2 theta ( )

60

70

(101)

40

(100)

4

ΔT (μV)

66 %

0

40

Fig. 3. XRD patterns of the as-combusted ZnO powders using the mixture of CTAB and glycine fuels at the various / values.

6

0

30

8

80 60

φ=0.75

φ=0.5

Intensity (arb. units)

100

(102)

644 551

825

800

(112)

2400 2000 1600 Wavenumber (cm-1)

(103)

2800

(110)

3200

φ=1

(002)

3600

Intensity (arb. units)

4000

590

(a)

(101)

(100)

φ=1.5

(b)

(002)

507 431

(c)

1081 1037

98

1112 1035 906 819

97

1640 1620 1570 1498 1357 1357

96

2340

95

2330

94

2927 2856

93

2921 2850

92

Transmition (%)

91

[26]. Therefore, it is better to combine the CTAB fuel with other fuels for preparation of spongy nanostructured powders. In the present work, the structure, microstructure, specific surface area, optical properties and photocatalytic efficiency of the ZnO powders synthesized by using the modern fuel-mixture approach at various fuel to oxidant ratios were reported. The ascombusted ZnO powders using mixture of CTAB and citric acid fuels showed higher photocatalytic efficiency than that for mixture of CTAB and glycine fuels, due to their higher specific surface area and crystallinity.

Weight (%)

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Weight (%)

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φ=1

φ=0.75

φ=0.5

0 158 °C

-2

0 0

100

200

300 400 500 Temperature (°C)

600

700

Fig. 2. TGA/DTA curves of the dried gels prepared by (a) the mixture of CTAB and glycine fuels and (b) the mixture of CTAB and citric acid fuels at / = 1.

20

30

40

50 2 theta ( )

60

70

Fig. 4. XRD patterns of the as-combusted ZnO powders using the mixture of CTAB and citric acid fuels at the various / values.

Please cite this article as: H. V. Vasei, S. M. Masoudpanah, M. Adeli et al., Photocatalytic properties of solution combustion synthesized ZnO powders using mixture of CTAB and glycine and citric acid fuels, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.004

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(8500S SHIMADZU). Thermal decomposition of the dried gels at 80 °C was examined by simultaneous differential thermal (DTA) and thermogravity analysis (TGA) in air atmosphere with the heating rate of 5 °C/min in a STA BäHR 503 instrument. Phase evolution was analyzed by PANalytical X-ray diffractometer (XRD) using monochromatic CuKa radiation. The XRD patterns were also submitted to a crystal structure analysis by the Rietveld method using MAUD program. The morphology and microstructure of the powders were observed by TESCAN Vega II field emission scanning electron microscopy. Brunauer–Emmett–Teller (BET) specific surface area of the powders was measured by the nitrogen gas adsorption technique

using a PHS-1020 instrument at 77 K, once the samples were degassed at 250 °C for 5 h. BJH (Barrett–Joyner–Halenda) cumulative volume of pores was calculated from the adsorption branch of the isotherms. UV–Vis diffuse reflectance spectra were recorded on a Shimadzu UV–Vis-52550 spectrophotometer in the wavelength range of 200–800 nm. Photoluminescence (PL) spectra were measured by using excitation wavelength of 325 nm in a Hitachi F-7000 Fluorescence spectrophotometer. Photocatalytic activity of the as-combusted ZnO powders was evaluated by degradation of methylene blue (MB) in aqueous solution under ultraviolet (UV) light radiation. Two UV lamps (8 W)

Table 1 Crystallite size, lattice parameters and strain, band gap energy and photodegradation rate constant of the as-combusted ZnO powders as a function of / values. /

Crystallite size (±3 nm)

a = b (±0.0002 Å)

c (±0.0002 Å)

Strain (%)

Eg (±0.01 eV)

k (min1)

CTAB + glycine 0.5 0.75 1 1.5

15 11 10 6

3.2594 3.2357 3.2392 3.2263

5.2180 5.1866 5.1806 5.1792

0.2 0.3 0.1 0.4

3.10 3.13 3.14 3.16

0.005 0.003 0.003 0.004

CTAB + Citric acid 0.5 18 0.75 14 1 10 1.5 9

3.2442 3.2437 3.2548 3.2483

5.2173 5.2161 5.2147 5.2106

0 0.1 0.1 0.2

3.15 3.17 3.18 3.15

0.028 0.033 0.035 0.016

Fig. 5. SEM micrographs of the as-combusted ZnO powders using the mixture of CTAB and glycine fuels at (a) / = 0.5, (b) / = 0.75, (c) / = 1 and (d) / = 1.5.

Please cite this article as: H. V. Vasei, S. M. Masoudpanah, M. Adeli et al., Photocatalytic properties of solution combustion synthesized ZnO powders using mixture of CTAB and glycine and citric acid fuels, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.004

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were employed as light source. In each experiment, 0.1 g of catalyst was added into 100 mL of methylene blue solution with a concentration of 15 mg/l. Furthermore, the pH of solution was adjusted to 2 by adding HCl. The suspension was stirred in dark for 60 min to establish the adsorption/desorption equilibrium, then the solution was illuminated under UV light irradiation. At appropriate time intervals, about 5 mL of suspension was sampled and the solid phase was separated from the solution with centrifugation at 4000 rpm for 20 min. The concentration of each degraded solution was monitored on PG Instruments Ltd T80-UV/Vis spectrophotometer.

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3. Results and discussion

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The redox reaction during combustion process by considering CO2, N2, Br2 and H2O as byproducts can be written as follows [27]:

135 136 137 138 139 140 141 142 143 144

148

149

151

1 10/ 5/ 5 C 19 H42 BrN þ C 6 H8 O7 Þ þ ð/  1ÞO2 ZnðNO3 Þ2 þ ð 2 118 9 2 2625/ 2125/ 1 5/ 5/ CO2 þ H2 O þ ð1 þ ÞN þ Br 2 ! ZnO þ 1062 1062 2 118 2 236

ð1Þ

152

154

1 10/ 10/ 5 ZnðNO3 Þ2 þ ð C 19 H42 BrN þ C 2 H5 NO2 Þ þ ð/  1ÞO2 2 118 9 2 2035/ 2420/ 1 635/ 5/ CO2 þ H2 O þ ð1 þ ÞN2 þ Br 2 ! ZnO þ 1062 1062 2 118 236

ð2Þ

Here, the stoichiometric mixture (/ = 1) means no need to atmospheric oxygen for completing fuel oxidation, while the / > 1 (or / < 1) corresponds to the fuel-rich (fuel-lean) condition. Fig. 1 shows FTIR spectra of the dried gels and as-combusted powders prepared by mixed fuels at / = 1. The hydroxyl groups originating from the organic phase and residual water lead to the broad band at
3200–3800 cm1 for the dried gel [28]. The bands at about 2920 and 2850 cm1 can be assigned to the stretching vibration of CAH bonds in CTAB molecules [29]. The absorption band at 2360 cm1 is due to the CO2 group on the metallic cations [29]. The existence of NO 3 groups in the structure of gel can be inferred from the absorption bands at about 1350, 905, 820 and 640 cm1 [30]. The CAO stretch associated with the formation of polymeric chain results in the absorption bands at about 1110 and 1035 cm1 [31]. The absorption bands at
1620 and 1498 cm1 can be attributed to the stretching vibrations of the coordinated carboxylate (COO) group for mixture of CTAB and glycine fuels, while the carboxylate groups for mixture of CTAB and citric acid fuels show the bands at 1640 and 1570 cm1, due to the various ligand type [30], leading to the various stretching vibration of metal-oxygen bonds at about 590 and 550 cm1 [29]. For the as-combusted powders, the absorption bands in the range of 400–600 cm1 are related to stretching vibration of ZnAO bonds which confirm the direct synthesis of ZnO following combustion reaction [32]. TGA/DTA curves of the dried gels prepared by the mixture of fuels at / = 1 are shown in Fig. 2. For mixture of CTAB and glycine

Fig. 6. SEM micrographs of the as-combusted ZnO powders using the mixture of CTAB and citric acid fuels at (a) / = 0.5, (b) / = 0.75, (c) / = 1 and (d) / = 1.5.

Please cite this article as: H. V. Vasei, S. M. Masoudpanah, M. Adeli et al., Photocatalytic properties of solution combustion synthesized ZnO powders using mixture of CTAB and glycine and citric acid fuels, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.004

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120 dVp/drp (cm3/(g-nm)

184

CTAB fuel as a cationic surfactant [38]. However, the appearance of foamy structure can be attributed to the increase of liberation of gaseous products at higher fuel contents in presence of glycine fuel. The slow combustion reaction rate in the presence of citric acid fuel may lead to the increase of porosities with fuel content. The nitrogen adsorption-desorption isotherms and pore size distribution plots of the as-combusted ZnO powders using mixture of fuels at various / values are shown in Fig. 7. The isotherms show IV type with H3 hysteresis according to IUPAC classification related to the fragile particle agglomerations with three-dimensional network of pores [39]. The dependence of specific surface area and pore volume of the as-combusted ZnO powders as a function of fuel type and fuel content is presented in Fig. 8. With the increase of fuel content, the as-combusted ZnO powders using mixture of CTAB and citric acid fuels show higher specific surface area than that for CTAB-glycine mixed fuels. The more gaseous products are generated during combustion reaction at higher fuel contents. These released gases disintegrate the large particles into smaller particles, leading to the higher specific surface area. The key role of released gases can be confirmed by higher pore volume for CTAB-citric acid mixed fuels, as shown in Fig. 8b. Furthermore, the as-combusted ZnO powders using mixture of CTAB and glycine fuels have micropores (1–2 nm), while there are mesopores (2– 12 nm) in the presence of higher amounts of citric acid fuel. The pores are mainly formed from the stacking of nanoparticles (Figs. 5 and 6). The mesopores have a great advantage in a solid-liquid system for the critical role of adsorption for catalysis. Fig. 9 shows the UV–Vis diffuse reflectance spectra of the ascombusted ZnO powders using mixture of fuels. The intrinsic absorption edge of ZnO leads to an increase in the absorption for k < 400 nm [40]. The direct band gap energy (Eg) can be calculated by the Tauc’s equation [6]:

100 80 60

10

0.75 1 1.5

8 6 4 2 0 0

40

2

4 6 8 10 12 14 Pore diameter (nm)

20

(a)

0 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Relative pressure, p/p0

1

250 dVp/drp (cm3/(g-nm)

183

fuels, the endothermic peak at 153 °C with the weight loss of
11% is due to the evolution of bromine gas [33]. A sharp exothermic peak at 176 °C along with the weight loss of
66% can be attributed to the exothermic reaction between nitrate anions and organic fuels [23]. Furthermore, the broad exothermic peak in the range of 250–600 °C with the continuous weight loss of
7% is due to the slow oxidation of solid organic residues [34]. In case of mixture of CTAB and citric acid fuels, the thermal decomposition occurs gradually in three steps. The evolution of yellow Br2 gas leads to the endothermic peak at 158 °C with weight loss of
18% [33]. Two exothermic peaks at 280 and 380 °C with total weight loss of
62% are due to slow decomposition. The exothermic peak at 280 °C may be related to the decomposition of citric acid, while the higher exothermic peak at 380 °C is for CTAB fuel, as shown in Supporting Information. The fast combustion reaction rate in the presence of glycine fuel can be ascribed to highly exothermic reaction of hypergolic (NOx, NH3, CO, etc.) formed during the decomposition of metal nitrate and fuel, while the existence of carboxylate (COO) groups postpone thermal decomposition [35]. Figs. 3 and 4 show XRD patterns of the as-combusted ZnO powders using mixture of CTAB and glycine fuels and mixture of CTAB and citric acid fuels, respectively. The indexed peaks as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) related to the hexagonal wurtzite structure (space group P63mc) confirm the direct synthesis of pure single phase ZnO powder following combustion reaction. The broad peaks are due to the small crystallites of solution combusted products. The crystallite size, lattice parameters and lattice strains are presented in Table 1. For mixture of CTAB and glycine fuels, the crystallite size decreases from 15 to 6 nm and from 18 to 9 nm for mixture of CTAB and citric acid fuels with fuel content. The decrease of crystallinity and crystallite size with fuel content can be attributed to the increase of exhausting gaseous products, outgoing thermal energy and preventing the particle sintering. The smaller crystallite size in the presence of glycine fuel may be attributed to its lower exothermicity of glycine fuel than that of citric acid fuel, releasing lower thermal energy during combustion reaction. The lower exothermicity can be confirmed by the calculation of adiabatic temperature by FactSage 6.1 software. The mixture of CTAB and glycine fuels shows the lower adiabatic temperature (2100 K) than that for combination with citric acid fuel (2200 K). The as-combusted ZnO powders using mixture of CTAB and glycine fuels have higher lattice strains, showing lower crystallinity on account of the lower adiabatic combustion temperature [28]. Furthermore, the lattice parameters of ZnO decrease with fuel content, due to the increase of concentration of defects such as vacancy, interstitial, etc. [36]. Figs. 5 and 6 show SEM micrographs of the as-combusted ZnO powders at various / values. In case of mixture of CTAB and glycine fuels, the as-combusted powders at / = 0.5 are composed of spherical and flake-like particles. With the fuel content, however, the compactly agglomerated powders were transformed to spongy and foamy structured powders along with disappearance of flake-like particles. The as-combusted ZnO powders using mixture of CTAB and citric acid fuels show spongy powders made of spherical particles (
50–35 nm) at all / values. The as-combusted morphologies strongly depend on the released heat and gases during combustion process which can be controlled through combustion reaction rate and the physicochemical properties of solution precursor such as molecular weight, surface tension, viscosity, etc. [12]. The released thermal energy leads to the particle growth and sintering, while the higher amounts of gaseous products are corresponding factor for the porous and foamy structures [37]. Furthermore, the combustion duration should be short to synthesis of nanostructured powders. The formation of flake-like particles at low fuel content can be attributed to the anisotropic growth of particles induced from lowering the surface tension in the presence of

Va/cm3(STP) g-1

182

0.75 1 1.5

12

200

Va/cm3(STP) g-1

181

150

8 4 0

100

0

4 8 12 Pore diameter (nm)

16

50

(b)

0 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Relative pressure, p/p0

1

Fig. 7. Adsorption (filled symbols) and desorption (open symbols) isotherms of the as-combusted ZnO powders using (a) the mixture of CTAB and glycine fuels and (b) the mixture of CTAB and citric acid fuels.

Please cite this article as: H. V. Vasei, S. M. Masoudpanah, M. Adeli et al., Photocatalytic properties of solution combustion synthesized ZnO powders using mixture of CTAB and glycine and citric acid fuels, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.004

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120 100

(a)

CTAB+Glycine CTAB+Citric acid

80 60 40 20 0 0.5

0.75

φ

1

0.6 0.5

1.5

200

0.4 0.3 0.2 0.1

200

0 0.75

φ

2.9

3

0.5 0.75 1 1.5

300

3.1 3.2 3.3 3.4 3.5 hν (eV)

(a) 400 500 600 Wavelength (nm)

700

800

(b)

CTAB+Glycine CTAB+Citric acid

0.5

(αhν)2

286

1

1.5

Fig. 8. Dependence of (a) the specific surface area and (b) the pore volume of the ascombusted ZnO powders as a function of the fuel type and fuel content.

(αhν)2

285

where hm is the photon energy in eV, a is the absorption coefficient, and A is a material constant. The extrapolation of the straight line in the inset of Fig. 9 to (ahm)2 = 0 gives the band gap energy. The calculated band gap energies are presented in Table 1. With the increase of fuel content, an obvious blue-shift occurs in the absorption edge for the microstructural effects and surface defects [38,41]. The band gap energy decreases for the particle size in the range of Bohr radius (1.8 nm). Therefore, the decrease of band gap energy cannot be explained by the quantum confinement effect [42]. The as-combusted ZnO powders using mixture of CTAB and glycine fuels exhibit the lower Eg due to their lower crystallinity and then the presence of more crystal defects. The defects generate energy levels between the valence band and conduction band, leading to the reduction of the band gap energy [43,44]. PL spectra of as-combusted ZnO powders using mixed fuels at / = 1 are presented in Fig. 10. ZnO nanoparticles exhibit distinctive electronic and optical behaviors on account of exciton quantum confinement phenomena [45]. The emission band in the UV region (
390 nm) is due to the near band-edge emission of ZnO because of the recombination of free excitons via an exciton–exciton collision process [40]. However, the emissions in the visible region are known to be deep-level emission originated from the structural defects and impurities in the structures [46,47]. The emission at 420 nm can be attributed to transition from zinc interstitial (Zni) to the valence band, while the transitions from extended zinc interstitial states to the valence band lead to the blue emissions at about 445, 460, and 490 nm [38]. Furthermore, the green emission at 505 nm is due to the transition between conduction band

and the energy levels originated from substitution of O at Zn (OZn) vacancies [48]. The different transitions are schematically shown in Fig. 10b. Therefore, the slightly lower UV emission and higher visible emission for the as-combusted ZnO powders using mixture of CTAB and glycine fuels can be due to their lower crystallinity with many crystal defects such as oxygen vacancies, zinc vacancies, oxygen interstitial and oxygen antisites. The crystal defects result in the decrease of band gap energy by generation the band tail energy between the valence band and the conduction band. However, the strong UV emission in comparison with that of visible emission for citric acid shows a good crystalline quality. The crystal defects result in the slightly decrease of band gap energy by generation the band tail energy between the valence band and the conduction band, as show in the inset of Fig. 10a. UV–Vis spectra of MB solution after different irradiation times in the presence of the as-combusted ZnO powders using the mixture of CTAB and citric acid fuels at / = 1 are given in Fig. 11a. The main absorption peak of MB molecules at 664 nm weakens with an increase in UV exposure time due to the destroying of MB structure. The photodegradation results of MB dye by the ascombusted ZnO powders as a function of light irradiation time are shown in Fig. 11b. About 99% of MB dyes are photodegraded during 90 min in the presence of the as-combusted ZnO powders prepared by mixture of CTAB and citric acid fuels at / = 0.75. However, the maximum MB photodegradation of 64% was obtained for the as-combusted ZnO powders prepared by using mixture of CTAB and glycine fuels at / = 0.5. Furthermore, the photodegradation rate constant of MB dye can be calculated by the modified Langmuir–Hinshelwood equation for the first-order reaction kinetics as follows [49]:

Absorbance (arb. u.)

284

ð3Þ

Absorbance (arb. u.)

283

2

BET Surface area (m2/g)

282

ðahmÞ ¼ Aðht  Eg Þ

Pore volume (cm3/g)

281

0.5 0.75 1 1.5

300

3

3.1

3.2

3.3 3.4 hν (eV)

3.5

(b) 400 500 600 Wavelength (nm)

700

800

Fig. 9. UV–Vis diffuse reflectance spectra of the as-combusted ZnO powders using (a) the mixture of CTAB and glycine fuels and (b) the mixture of CTAB and citric acid fuels.

Please cite this article as: H. V. Vasei, S. M. Masoudpanah, M. Adeli et al., Photocatalytic properties of solution combustion synthesized ZnO powders using mixture of CTAB and glycine and citric acid fuels, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.004

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(B)

0.8

Conduction Band

0.6

390

400

410

420

490 505

460

Wavelength (nm)

50

(a)

(b) 0 350

400

450 500 Wavelength (nm)

550

~490 nm

380

100

Ex-Zni ~445 nm

0

~460 nm

Zni

0.2

~505 nm

0.4

~420 nm

150

445

200

420

PL Intensity (arb. u.)

(A)

~390 nm

250

Normalized PL Intensity

H.V. Vasei et al. / Advanced Powder Technology xxx (xxxx) xxx

OZn

Valence Band

600

Fig. 10. (A) PL spectra of the as-combusted ZnO powders using (a) the mixture of CTAB and glycine fuels and (b) the mixture of CTAB and citric acid fuels at /=1 and (the inset shows normalized PL spectra) and (B) the energy band diagram.



344 345 346 347 348

 ¼ kt

ð4Þ 1

which rate constant (k (min )) values obtained from the plot of Ln(C/C0) vs. the irradiation time are presented in Table 1. UV light irradiation generates electron-hole (e-h+) pairs between the conduction (CB) and valence (VB) bands in the as-combusted ZnO powders. The photogenerated e- in CB migrates to the surface; scavenged by O2 to produce superoxide anion and simultaneous

(a) Increase of time

Absorbance

343

C C0

500

550

1.2

600 650 Wavelength (nm)

(b)

700

750

0.5 0.75 1 1.5

1 0.8 C/C0

342

Ln

0.6 0.4 0.2 0 0

30

60

90 120 Time (min)

150

180

Fig. 11. (a) UV–Vis spectra of MB solution in the presence of the as-combusted ZnO powders prepared by the mixture of CTAB and citric acid fuels at /=1 and (b) C/C0 vs. irradiation time for photodegradation of MB dye under visible light irradiation by the as-combusted ZnO powders at various / values (open symbols are for mixture of CTAB and glycine fuels and filled symbols are for mixture of CTAB and citric acid fuels).

protonation produces HOO. radicals. The photogenerated holes in VB reacts with either H2O or OH to yield an active species such : as O 2 , O2 , HOO/OH species which are responsible for degradation of MB into less harmful organic end products [5]. The higher rate of photodegradation by the as-combusted ZnO powders prepared by mixture of CTAB and citric acid fuels can be attributed to their good crystallinity and higher specific surface area, in spite of its higher band gap. The as-combusted ZnO powders with higher surface area can adsorb more MB dyes and provide more reactive sites for efficient degradation.

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4. Conclusions

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Single phase ZnO powders were directly synthesized by solution combustion method using mixture of fuels at various fuel contents. The particle size and crystallinity of the as-combusted ZnO powders decreased with the increase of fuel content, especially in the presence of glycine fuel. The higher specific surface area and pore volumes were obtained in the presence of citric acid fuel in addition to CTAB for exhausting of larger amount of gaseous products and slower combustion reaction rate. MB dye was completely photodegraded under UV light irradiation by the ascombusted ZnO powders using mixture of CTAB and citric acid fuels, due to their higher crystallinity and specific surface area in spite of their rather broad band gap in the range of 3.15–3.18 eV.

360

Appendix A. Supplementary material

372

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2018.11.004.

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