Solution combustion synthesis of ZnO powders using mixture of fuels in closed system

Author’s Accepted Manuscript Solution combustion synthesis of ZnO powders using mixture of fuels in closed system Z. Kalantari Bolaghi, M. Hasheminiasari, S.M. Masoudpanah www.elsevier.com/locate/ceri

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S0272-8842(18)30929-5 https://doi.org/10.1016/j.ceramint.2018.04.069 CERI17986

To appear in: Ceramics International Received date: 18 March 2018 Revised date: 6 April 2018 Accepted date: 8 April 2018 Cite this article as: Z. Kalantari Bolaghi, M. Hasheminiasari and S.M. Masoudpanah, Solution combustion synthesis of ZnO powders using mixture of fuels in closed system, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.04.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Solution combustion synthesis of ZnO powders using mixture of fuels in closed system Z. Kalantari Bolaghi, M. Hasheminiasari, S.M. Masoudpanah* School of Metallurgy & Materials Engineering, Iran University of Science and Technology (IUST), Tehran, Iran * Corresponding author: e-mail: [email protected], Phone: +98 21 77240540, Fax:+98 21 77240480

Abstract Single phase ZnO powders with wurtzite structure were synthesized by solution combustion method using various amounts of mixed glycine-citric acid fuel in the presence (open system) and absence (closed system) of air oxygen. Phase evolution, microstructure and optical properties were investigated by thermal analysis, X-ray diffractometry, electron microscopy and Raman, photoluminescence (PL) and diffuse reflectance spectrometry techniques. Rapid combustion reaction in closed system led to weak crystallinity, as confirmed by deep-level emissions in PL spectra. Larger spherical particles (~200 nm) were synthesized in open system at =1. The as-combusted ZnO powders in closed system exhibited higher photocatalytic activity under ultraviolet irradiation, due to their higher adsorption capacity of methylene blue on ZnO surface. Photodegradation rate increased with the increase of fuel content in as-combusted ZnO powders produced by open route as a result of the reduction of particle size and band gap energy. Keywords: ZnO; Solution combustion synthesis; Mixture of fuels, Photocatalytic activity;

1. Introduction

Solution combustion synthesis (SCS) has attracted a great attention for mass production of nanomaterials such as oxides, metals, alloys, and sulfides owing to its simplicity, versatility, time- and energy-efficiency [1, 2]. SCS involves a self-sustained exothermic reaction between oxidants (e. g. nitrates, acetates, sulfates) and organic fuels (e. g. urea, glycine, citric acid, etc.), releasing a great amount of thermal energy and gaseous products [3]. Enough released heat results in solid state reaction between intermediate phases to form final products by calcination at low temperatures or with no need of further calcination [4]. Moreover, the as-combusted powders exhibit spongy structures with high specific surface area, due to the liberation of a large amount of gases during organic fuel’s burning [5]. Powder characteristics such as phase, particle size, morphology, surface areas are mainly dependent on fuel type and content, metal precursor, pH, etc. [6-8]. Urea, glycine and citric acid are popular organic fuels thanks to their low decomposition temperature, availability and low cost [9]. However, the mixture of various fuels has received significant interests on account of its ability to control over combustion temperature and the amount of various released gaseous products [10]. Moreover, the combustion behavior of a single fuel can be modified by its combination with other fuels through adjustment of the decomposition temperature [11]. Ianos et al. [12] showed that urea or glycine fuels alone did not yield SrAl2O4 following combustion reaction, while the direct formation of (, )-SrAl2O4 powders was possible using fuel-mixture approach (urea and glycine) being attributed to the increase of released hypergolic gases, triggering the ignition of a flaming combustion reaction. Naveenkumar et al. [13] also prepared the SrFeO3- catalysts with small particle size, minimum agglomeration of the particles and high specific surface area using the mixture of citric acid, oxalic acid and glycine fuels. Some researchers hypothesized that combustion reaction occurs by exothermic reaction of flammable gases (NOx, NH3, CO, etc.) originated from the decomposition of the starting raw materials (nitrates and fuels) [14-16]. This hypothesis was also confirmed by occurrence of combustion reaction in the absence of oxygen [17]. Therefore, the combustion behavior can be dependent on the amount and nature of hypergolic gases, tunable by fuel and combustion method. The combustion reaction can be executed in open or closed system which may introduce different combustion behaviors and combustion temperatures. For example, Ianos et al. [18] synthesized magnetite (Fe3O4) powders by combustion of precursor solution including iron (III) nitrate and glycine in closed system. While, hematite (-Fe2O3) powders were formed during combustion in an open system. Furthermore, physicochemical properties of the combusted products are dependent on the combustion atmosphere [19, 20]. Cross et al. [19] investigated the effects of combustion atmosphere on phase evolution of Ni/NiO system. They showed that the nickel nanoparticles formed in combustion front were rapidly oxidized in the presence of air oxygen, while the inert atmospheres such as Ar, He, etc. prevented the oxidation process.

In this work, ZnO was selected as a model material because of its interesting optical properties such as wide band gap (3.27 eV) and large exciton binding energy (60 meV), thus encouraging its usage in many applications as gas sensors, ultraviolet photodetectors, photocatalysis, etc. [2123]. Phase evolution, microstructure, optical properties and photocatalytic properties of the

solution combusted ZnO powders were investigated as a function of the amount of glycine-citric acid mixed fuels. Moreover, the physicochemical properties of ZnO powders combusted in open and closed system were compared. 2. Experimental procedures Required amounts of zinc nitrate (Zn(NO3)2), glycine (C2H5NO2) and citric acid (C6H8O7) were dissolved in distilled water at various fuel to oxidant ratios. The precursor solution was slowly evaporated at 80 °C until a viscous solution was formed. A part of the solution was poured into a dish (A approach) and heated till transformed into a gel while further heating up to 250 °C on a hot plate, ignited a reaction starting from a point and propagated spontaneously. Another part was poured into a round bottom flask (B approach) whereby the liberated gases were bubbled in a large beaker filled with water, as schematically shown in Fig. 1. The resulted powders were hand-crushed with a pestle. Thermal decomposition of the dried gel at 80 °C was examined by simultaneous differential thermal and thermogravity analysis (DTA/TGA) in air with the heating rate of 5 °C/min on the STA BäHR 503 instrument. Raman analysis was performed on powders by TEKSAN N1-541 instrument (Nd:YAG laser source: = 532 nm and 0.7 Mw power, and range: 50–1400 cm-1). Phase evolution was analyzed by DRON-8 X-ray diffractometer (XRD) using monochromatic CuKα radiation. The XRD patterns were also submitted to a crystal structure analysis by the Rietveld method using MAUD program. Morphology and microstructure of the powders were observed by TESCAN Vega II field emission scanning electron microscopy. Diffuse reflectance spectra were recorded on a Shimadzu UV-Vis-52550 spectrophotometer in the wavelength range of 200–700 nm. Photoluminescence (PL) spectra were measured by using excitation wavelength of 325 nm in an Agilent G9800A fluorescence spectrophotometer. Photocatalytic activity was determined by the degradation of methylene blue (MB) in aqueous solution under radiation of two ultraviolet (UV) lamps (8 W) as light source. In each experiment, 0.1 g of photocatalyst was added into 100 mL of methylene blue solution with a concentration of 15 mg/L at pH of 2. Before illumination under UV irradiation, the suspension was stirred in dark for 60 min to establish the adsorption/desorption equilibrium. At appropriate time intervals, the concentration of each degraded solution was monitored on PG Instruments Ltd T80-UV/Vis spectrophotometer following separation of the solid phase by centrifugation at 4000 rpm for 20 min. 3. Results and discussion Considering CO2, N2, and H2O as byproducts, the redox reaction can be written as follows [24]: (

(1)

)

(

)

(

)

→

(

)

Herein, (=1) is related to the stoichiometric mixture, while the >1 (or <1) corresponded to the fuel-rich (or fuel-lean) condition. TGA/DTA curves of the gel prepared at =1 are presented in Fig. 2. Slow weight loss of about 12 % up to ~200 °C is originated from the gel’s dehydration. A sharp weight loss of ~46 % along with exothermic peak at 210 °C can be attributed to the combustion reaction between nitrate ions and polymeric chains formed between organic fuels [25]. The liberation of gaseous products is the cause of huge weight loss. High combustibility and low decomposition temperature of glycine and citric acid fuels result in a fast combustion reaction [26]. Burning and slow oxidation of residual organics are accompanied by slow weight loss (~10 %) above 220 °C. XRD patterns of the as-combusted ZnO powders using A and B approaches are shown in Figs. 3 and 4, respectively. Crystallite size, lattice strain and lattice parameters of solution combusted ZnO powders are also presented in Table 1. Single phase ZnO powders were directly formed at all fuel contents by A approach, being confirmed by (100), (002), (101), (102), (110), (103), (112) and (201) peaks related to wurtzite structure with P63mc space group symmetry. The crystallite size increases from 15 to 26 nm up to =1 and decreases to 10 nm with the increase of fuel content, similar to the dependence of adiabatic temperature on  values. The stoichiometric mixture (=1) has maximum combustion temperature because of the completion of fuel oxidation without any need of atmospheric oxygen, while the need to heat up the excess products (or reactants) in fuel-rich (-lean) mixture results in the lower combustion temperatures [27]. The lattice parameters of solution combusted ZnO powders are in agreement with the bulk values (a=b=3.2498 Å and c= 5.2066 Å). Furthermore, the as-combusted ZnO powders by A approach have insignificant lattice strains, indicating their higher crystallinity. However, there are some un-burnt residues along with ZnO phase in the as-combusted powders using B approach at =0.5. The residues are disappeared with the increase of fuel content in which single phase ZnO is directly formed without any following calcinations. The crystallite size decreases from 26 to 10 nm with the increase of  values. Moreover, the crystallite size by the B approach is smaller than that of the A approach at =1 and 1.5, as a result of its lower combustion temperature. At  value of 2, however, the as-combusted ZnO powders have smaller crystallite size with noisy diffraction patterns. As proposed, the combustion reaction proceeds by exothermic reaction between released hypergolic gases such as NH3, HNO3, NOx, etc. [16, 28]. In closed system (approach B), larger entrapment of hypergolic gases in combustion front results in a highly intensive combustion reaction. Accordingly, the existence of residual organics may be due to the incomplete combustion reaction caused by the absence of air oxygen and less hypergolic gases released during combustion in closed system. In contrast to white color of ZnO, the black colored powders can be attributed to the deposition of organic residues, increasing their ability to absorb organic molecules. Lower lattice parameters and higher lattice strains in approach B may be due to its fast combustion reaction with lower combustion temperature [19], thus preventing the formation of well crystalline ZnO powders. A proper amount of structural defects by trapping of electrons and holes can avoid charge recombination and then improve the photocatalytic activity [29].

Figs. 5 and 6 show SEM micrographs of the solution combusted ZnO powders using A and B approaches, respectively. At =0.5, the powders synthesized by A approach are composed of spherical particles (~50 nm), growing to ~200 nm with the increase of fuel content up to =1. However, the as-combusted ZnO powders at >1 have smaller spherical particles (~30 nm) agglomerated to larger irregular particles. In B approach, the combusted powders at =0.5 have bulky microstructure with indiscernible particles. However, the crystallinity and particle size initially rise with fuel content and then decline with  values. Particle size of the as-combusted powders strongly depend on the adiabatic combustion temperature, being controllable via fuel nature and fuel content [30]. In spite of increase of released thermal energy with fuel content, the exhaustion of great amount of gaseous products outperforms heat amount thus decrease the combustion temperature preventing further particle growth and sintering. Raman spectra of the solution combusted powders using A and B approaches at =1 are given in Fig. 7. The wurtzite structure of ZnO with P63mc space group shows the following optical modes [31]: =A1+2B1+E1+2E2 (2) which A1 and E1 modes are polar and have transverse (TO) and longitudinal (LO) optical components. E2 mode corresponds to the vibration of zinc sublattice and oxygen atoms, leading to the low and high frequency phonons (E2 (low) and E2 (high)), respectively [32]. Strong bands at 99 and 426 cm-1 are related to E2 (low) and E2 (high) modes, respectively, confirming crystallinity of as-combusted ZnO powders. The acoustic and optical phonon overtone for A1 symmetry locate at 204 and 323 cm-1, respectively, while the band at 1110 cm-1 can be assigned to the acoustic combination of A1 and E2 modes [33]. Also the bands at 540 and 570 cm-1 are due to the A1 (LO) and E1 (LO) modes, respectively [34]. However, the broad peak at 645 cm-1 corresponds to the multiphonons process [31]. Fig. 8 compares PL spectra of ZnO powders synthesized using A and B approaches at fuel to oxidant ratio of 1. The free excitons recombine via an exciton–exciton collision process, leading to the near band-edge emission in the UV region (~ 395 nm) [35]. However, the emissions in the visible region (400-600 nm) are known to be deep-level emission originated from the structural defects and impurities [36, 37]. The as-combusted ZnO powders using B approach show the broad peaks in visible emission, confirming their weak crystallinity with many structural defects such as oxygen and zinc vacancies, zinc and oxygen interstitial and antisites [38]. For A approach, however, PL spectrum shows the strong UV emission together with very weak deeplevel emission, indicating their good crystalline structure with excellent optical properties. UV–Vis diffuse reflectance spectra of as-combusted ZnO powders using A and B approaches at =1 are illustrated in Fig. 9. The UV–Vis diffuse reflectance spectra for other  values are given in the supporting information. The absorption increases at <400 nm on account of the intrinsic absorption edge of zinc oxide. Tauc’s plots ((h)2 vs. h) are also shown as inset in Fig. 9. The band gap energies calculated by extrapolation of the tangent line to (αhν)2=0 on the base of Tauc’s equation: ( ) ( ) are shown in Table 1. The as-combusted ZnO powders

using A approach have lower band gap energies compared to that of B approach at all  values. However, the absence of clear band-edge for ZnO powders prepared at =2 by means of B approach can be ascribed to their very weak crystallinity. The optical band gap energy (Eg) depends on the microstructural and surface defects [29]. Therefore, the slightly wider band gap for B approach may be certified to its weaker crystallinity [39]. UV–Vis spectra of MB solution after different irradiation times in the presence of the ascombusted ZnO powders are given in supporting information. The weakening of main absorption peak at 665 nm demonstrates the degradation of MB molecules with the increase of UV irradiation time. Relative concentrations (C/C0) of MB dye in solution versus exposure time are presented in Fig. 10. For A approach, ZnO powders prepared at =0.5 photodegraded ~30 % of MB molecules during 3 h irradiation. The amount of photodegradation increases to ~96 % with the increase of  value up to 2. The MB dye is not photodegraded in the presence of ZnO powders prepared by B approach at =0.5 and 2, confirming photo stability of MB dye in the absence of ZnO photocatalyst. However, the as-combusted ZnO powders at =1 can photodegrade the MB molecules in two steps which ~ 85 % are quickly degraded at initial 60 min and the rest are slowly degraded up to 100% following 240 min irradiation. About 98 % of MB are linearly degraded during 150 min irradiation by ZnO powders synthesized at =1.5. The photocatalytic process is based on the photogeneration of electron-hole pairs in ZnO catalyst . under UV irradiation, leading to the formation of active species such as , , HOO / OH by reacting electrons and holes with O2, OH- and H2O [40]. The active species oxidize dye molecules adsorbed on catalyst surface to CO2 and H2O species. Therefore, photocatalytic activity depends on the optical properties such as light absorption coefficient, band gap energy, etc. [41]. Moreover, the photocatalytic efficiency is related to adsorption capacity of molecule dyes on the catalyst surface, depending on physicochemical properties of particle surface, surface ligands, specific surface areas, etc. [29]. The role of adsorption of MB dye on ZnO powders on the photodegradation are illustrated in Fig. 11. It is obvious that photodegradation increases with the increase of adsorption of MB molecules on the as-combusted ZnO powders. The increase of adsorption with fuel content can be attributed to the increase of specific surface area. However, despite of higher adsorption of MB on the as-combusted ZnO powders using B approach at =2, MB dye could not be photodegraded under UV irradiation. The electron-hole pairs are not generated for =2 on account of their weak crystallinity. In other hand, the deposited carbon on the particle surface decreases the absorption of UV light and then prevents further photogeneration of electrons and holes, leading to lower photodegradation. 4. Conclusions Single phase ZnO powders were prepared by solution combustion synthesis method in open and closed systems. The as-combusted ZnO powders in open system had higher crystallinity and larger crystallite size. The dependence of particle size on fuel content was similar to that of combustion temperature, increased up to =1 and then decreased for >1. The broad peak in the range of 400-600 nm for ZnO powders synthesized in closed system was attributed to their

crystal defects. About 85 % of MB dye was photodegraded during 60 min UV irradiation by the as-combusted ZnO powders at =1 in closed system, as a result of their higher ability for the adsorption of MB molecules on the particle surface. Due to the reduction of particle size and band gap energy with higher  values, the photodegradation rate increased for as-combusted ZnO powders in the open system. References [1] F.-t. Li, J. Ran, M. Jaroniec, S.Z. Qiao, Solution combustion synthesis of metal oxide nanomaterials for energy storage and conversion, Nanoscale, 7 (2015) 17590-17610. [2] H.H. Nersisyan, J.H. Lee, J.-R. Ding, K.-S. Kim, K.V. Manukyan, A.S. Mukasyan, Combustion synthesis of zero-, one-, two- and three-dimensional nanostructures: Current trends and future perspectives, Progress in Energy and Combustion Science, 63 (2017) 79-118. [3] A. Varma, A.S. Mukasyan, A.S. Rogachev, K.V. Manukyan, Solution Combustion Synthesis of Nanoscale Materials, Chemical Reviews, 116 (2016) 14493-14586. [4] S.T. Aruna, A.S. Mukasyan, Combustion synthesis and nanomaterials, Current Opinion in Solid State and Materials Science, 12 (2008) 44-50. [5] K.V. Manukyan, Y.-S. Chen, S. Rouvimov, P. Li, X. Li, S. Dong, X. Liu, J.K. Furdyna, A. Orlov, G.H. Bernstein, W. Porod, S. Roslyakov, A.S. Mukasyan, Ultrasmall α-Fe2O3 Superparamagnetic Nanoparticles with High Magnetization Prepared by Template-Assisted Combustion Process, The Journal of Physical Chemistry C, 118 (2014) 16264-16271. [6] T.K. Pathak, A. Kumar, C.W. Swart, H.C. Swart, R.E. Kroon, Effect of fuel content on luminescence and antibacterial properties of zinc oxide nanocrystalline powders synthesized by the combustion method, RSC Advances, 6 (2016) 97770-97782. [7] P.R. Potti, V.C. Srivastava, Comparative Studies on Structural, Optical, and Textural Properties of Combustion Derived ZnO Prepared Using Various Fuels and Their Photocatalytic Activity, Industrial & Engineering Chemistry Research, 51 (2012) 7948-7956. [8] N. Srinatha, V. Dinesh Kumar, K.G.M. Nair, B. Angadi, The effect of fuel and fuel-oxidizer combinations on ZnO nanoparticles synthesized by solution combustion technique, Advanced Powder Technology, 26 (2015) 1355-1363. [9] K. C. Patil, M. S. Hegde, T. Rattan, S.T. Aruna, Chemistry of Nanocrystalline Oxide Materials (Combustion Synthesis, Properties, and Applications), World Scientific Publishing Co., Singapore, 2008. [10] H. Parnianfar, S.M. Masoudpanah, S. Alamolhoda, H. Fathi, Mixture of fuels for solution combustion synthesis of porous Fe3O4 powders, Journal of Magnetism and Magnetic Materials, 432 (2017) 24-29. [11] H. Fathi, S.M. Masoudpanah, S. Alamolhoda, H. Parnianfar, Effect of fuel type on the microstructure and magnetic properties of solution combusted Fe3O4 powders, Ceramics International, 43 (2017) 7448-7453. [12] R. Ianos, R. Istratie, C. Pacurariu, R. Lazau, Solution combustion synthesis of strontium aluminate, SrAl2O4, powders: single-fuel versus fuel-mixture approach, Physical Chemistry Chemical Physics, 18 (2016) 1150-1157. [13] A. Naveenkumar, P. Kuruva, C. Shivakumara, C. Srilakshmi, Mixture of Fuels Approach for the Synthesis of SrFeO3−δ Nanocatalyst and Its Impact on the Catalytic Reduction of Nitrobenzene, Inorganic Chemistry, 53 (2014) 12178-12185. [14] A. Kumar, E.E. Wolf, A.S. Mukasyan, Solution combustion synthesis of metal nanopowders: Nickel—Reaction pathways, AIChE Journal, 57 (2011) 2207-2214.

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A approach

Fig. 1. Experimental setup for A and B approaches.

B approach

80

4

46 %

60

2

40

10 %

DT (mV)

Weight (%)

12 %

210 °C

6 100

0 20 0

-2 0

100

200

300 400 500 Temperature (°C)

600

700

Fig. 2. TGA/DTA curves of the dried gel prepared by mixture of glycine and citric acid fuels at =1.

(112) (201)

(103)

(102)

Intensity (arb. units)

(110)

(101) (002)

(100)

=2

=1.5

=1

=0.5

20

30

40

50 2 theta (°)

60

Fig. 3. XRD patterns of the as-combusted ZnO powders using A approach.

70

(112) (201)

(103)

(102)

(110)

(101) (002)

(100)

=2

Intensity (arb. units)

=1.5

=1

=0.5

20

30

40

50 2 theta (°)

60

70

Fig. 4. XRD patterns of the as-combusted ZnO powders using B approach ( residues). Table 1. Crystallite size, lattice parameters, lattice fuel content. A approach  Crystallite size (nm) a=b (Å) 0.5 17 3.2478 1 45 3.2490 1.5 20 3.2491 2 15 3.2482 B approach 0.5 15 3.2475 1 26 3.2488 1.5 12 3.2483 2 10 3.2468

: Un-burnt

strain and band gap energy as a function of

c (Å) Strain (%) Eg (eV) 5.2050 0.1 3.10 5.2062 0.00 3.18 5.2064 0.01 3.16 5.2058 -0.07 3.14 5.2060 5.2056 5.2048 5.205

0.09 0.03 0.1 -0.7

3.19 3.20 3.21 -

(a)

(b)

(c)

(d)

Fig. 5. SEM micrographs of the as-combusted ZnO powders using A approach; (a) =0.5, (b) =1, (c) =1.5 and (d) =2.

(a)

(b)

(c)

(d)

Fig. 6. SEM micrographs of the as-combusted ZnO powders using B approach; (a) =0.5, (b) =1, (c) =1.5 and (d) =2. .

426

200

540 570 645 400

600 800 1000 Raman shift (cm-1)

1110

323 204

99

Intensity (arb. u.) 0

A B

1200

1400

Fig. 7. Raman spectra of the as-combusted ZnO powders using A and B approaches at =1.

PL Intensity (arb. u.)

1000 A B

800 600 400 200 0 350

400

450 500 Wavelength (nm)

550

600

Fig. 8. PL spectra of the as-combusted ZnO powders using A and B approaches at =1.

(h)2

Absorbance (arb. u.)

200

3.1

3.2 3.3 h (eV)

3.4

A B

300

400 500 Wavelength (nm)

600

700

Fig. 9. Diffuse reflectance spectra of the as-combusted ZnO powders using A and B approaches at =1 (The inset shows Tauc’s plots).

1.2 (a) 1

C/C0

0.8 0.6 0.5 1 1.5 2

0.4 0.2 0 0

30

60

90 120 Time (min)

150

180

1.2 (b) 1 0.5 1 1.5 2

C/C0

0.8 0.6 0.4 0.2 0 0

30

60

90 120 Time (min)

150

180

Fig. 10. C/C0 vs. irradiation time for photodegradation of MB dye under ultraviolet light irradiation by the as-combusted ZnO powders using (a) A and (b) B approaches.

120 Removal efficiency

100 80

Adsorption-A Adsorption-B Photodegradtion-A Photodegradation-B

60 40 20 0 0.5

1



1.5

2

Fig. 11. Removal efficiency of MB dye by the as-combusted ZnO powders using A and B approaches.