solution combustion synthesis: role of oxidant to fuel ratio on powder properties

Solution Combustion Synthesis: Role of Oxidant to Fuel Ratio on Powder Properties

L. D. Jadhav1,*, S.P. Patil2, A.P. Jamale2 and A.U. Chavan2 1 Department of Physics, Rajaram College, Govt. of Maharashtra, India-416004.

2 Department of Physics, Shivaji University, Kolhapur, India-416004.

E-mail: [email protected] (corresponding author)

Keywords: Combustion synthesis, nano-materials, solid oxide fuel, oxidant to fuel ratio, ceria, copper.

Abstract:

Solution combustion synthesis technique is one of the novel techniques used to prepare nanoparticles, multi-component ceramic oxides and nanocomposites with properties better than conventionally prepared one and these materials have been used for various applications such as sensors, catalysts, and materials for solid oxide fuel cell (SOFCs). In the present work, the method has been used to prepare nanoparticles of 10 mol% Gd doped ceria (GDC) and Cu and its oxides. The oxidant to fuel (O/F) ratio is found to affect the powder properties and even compositional homogeneity. In glycine-nitrate combustion synthesis of GDC, as revealed by XRD studies, phase pure nanoparticles with crystallite size in the range 9-12nm were obtained for all the O/F ratios. TEM measurements of calcined powder showed hexagonal shaped particles of roughly 20nm size. The exothermicity was increased with the oxidant to fuel ratio resulting in high surface area and soft agglomerates. A slightly lean O/F ratio gives surface area of 73 m2/g and soft agglomerates (D50 = 5.34 µm), which eventually results into high sintering density at low temperature. Raman Spectra of GDC showed a sharp and intense peak at 467 cm−1 which corresponds to CeO2 due to F2g symmetry of the cubic phase. In combustion synthesis of copper nitrate and citirc acid, the compositional homogenity and phase purity was affected by the oxidant to fuel ratio. The combustion at stoichiometric O/F ratio gives Cu nano particles, lean O/F ratio gives nanoparticles of Cu, CuO and Cu2O and rich ratio gives pure CuO nanoparticles. These nanoparticles have been studied with different characterization techniques like XRD, TG-DTA, SEM, TEM, FT-IR and Raman.

1. Introduction

In the last decade, the nanoscience and nanomaterials have been identified worldwide as the key to

unlocking a new generation of devices with revolutionary properties and functionalities. The

various physical, chemical, mechanical and electrical properties have been changed drastically upon

switching from micro-particles to nanoparticles. Consequently, nanoparticles with unique properties

have become an emerging interdisciplinary field involving solid-state physics, chemistry, biology

and materials science. So nanomaterials have technological as well as fundamental importance.

There are a number of physical methods for preparing nanomaterials viz. inert gas condensation,

physical vapor deposition, laser ablation, chemical vapor deposition, sputtering, Molecular Beam

Epitaxy etc. Nonetheless, different chemical methods have came up as an alternative to the

expensive physical methods. These are sol–gel, spray- pyrolysis, co-precipitation,

mechanochemical, and solution combustion synthesis (SCS). Among these, the SCS has been

regarded as one of the effective, economic approaches owning to the convenient, simple

experimental setup and better homogeneity of final product. It has been gaining reputation as a

Materials Science Forum Vol. 757 (2013) pp 85-98Online available since 2013/May/27 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.757.85

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 129.186.1.55, Iowa State University, Ames, United States of America-30/09/13,10:05:18)

http://www.scientific.net

http://www.ttp.net

straightforward preparation process to produce very fine, crystalline and un-agglomerated powders.

The most striking feature is that it is a single step and short duration process and the gases evolved

during the process inhibit the particle size growth and favors the nano-sized particles. In SCS, the

exothermicity of the redox reaction between oxidizer and fuel has been used to produce varieties of

oxide, carbide and nitride materials such as Ce1-xGdxO2 [1-2], La1-xSrxMnO3 [3], Al2O3, SiC and

TiN [4-5]. Sharma et al. [6] have used the solution combustion method to prepare Ag nanoparticles

using glycine and urea as fuels.

The SCS has high exothermicity, which is manifested in the form of either smolder or fire or flame.

Hence the process is very often known as auto-ignition. In SCS, the mode of combustion i.e.

smoldering, volume and self-propagating can be controlled by controlling the SCS parameters i.e.

initial concentration of solution, ignition temperature, types of fuel and oxidant to fuel ratio. The

last two parameters are most significant and governs the mode of propagation as well as heat of

combustion and amount of gases evolved in the process and hence the final product properties. The

heat of combustion gives local sintering leading to crystallization while amount of gases evolved in

the process provides high surface area and also controls the rise of temperature.

The basic criterions the fuel should meet are: it should be water soluble and have lower ignition

temperature; the large amount of gas should be evolved during the combustion; reaction should not

be explosive; it should be low cost and readily available. Besides, fuel should be able to maintain

the compositional homogeneity among the constituents. In view of these, hydrazine, sucrose, urea,

citric acid and glycine were well considered as a fuel for SCS. Of these fuels, glycine serves two

purposes: a) it provides source of C and H, which on combustion form large amount of CO2 and

H2O with liberation of heat and b) it contains -NH2 and –COOH group, which on desolation

complexes various metal ions and prevent their selective precipitation to maintain compositional

homogeneity. Many times, with glycine, volume combustion is an explosive type with no flame but

results into high surface area dry powder with soft agglomerates [2]. We have used glycine in all

our works, except for Cu nano powders, where we have used citric acid as fuel and it gives higher

number of moles of gases compared to glycine during combustion, thus causing an improvement in

surface area. Also, the reaction with citric acid is wilder and can be controlled more easily as

compared to other organic fuels [7].

The other parameter that affects the powder properties is oxidant to fuel ratio. The fuel should be

added in such a way that the metal nitrate to fuel ratio be maintained at the molecular level. The

oxidant to fuel ratio should be unity according to the propellent chemistry [8] but by choosing the

proper fuel and keeping the initial solution concentration and ignition temperature constant, the

oxidant to fuel ratio should be varied from fuel deficient to fuel rich to study the mode of

propagation of SCS and powder properties. Further this precursor should give an intimate blending

of the starting constituents preventing the random redox reaction between a fuel and nitrates. The

oxidant to fuel ratio can be calculated by knowing the oxidizing and reducing valency of metal

nitrate and fuel respectively.

� = �� (1)

86 Engineering Applications of Nanoscience and Nanomaterials

The oxidant to fuel ratio thus can be fuel deficient (ψ<1), stoichiometric (ψ =1) or fuel rich (ψ>1)

quantifying the quantity of oxygen in-excess, appropriate or deficient. These respectively result into

slow and flameless reaction, explosive reaction throughout the volume and locally ignited reaction

propagating throughout the reacting mixture.

Several books and reviews have been published emphasizing the recipes and mechanism of

combustion synthesis and other fundamental issues related with it. Moreover, a research on use of

different fuels has also been reported. But a less has been reported on effect of oxidant to fuel ratio.

This manuscript focuses upon the role of oxidant to fuel ratio on properties of powder. Two systems

were selected for this study viz. 10 mo% gadolinium doped ceria (GDC) and Cu based nano-

particles. The combustion synthesized ash were characterized using various physio-chemical

techniques that include thermogravimetric analysis (TGA), powder X-ray diffraction, particle size

analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Subsequently, the nanoparticles were calcined and were characterized by XRD, TG/DTA, Fourier

transform infrared (FTIR) and Raman analysis. The effect of oxidant-to-fuel ratio (O/F) on

compositional homogeneity was also studied.

2. Experimental

The present work reports combustion synthesis of Gd doped ceria and Cu nanoparticles. The

precursors were obtained by mixing the corresponding metal nitrates and a suitable fuel. The

oxidant to fuel ratio was varied so as to tailor the powder properties and to study compositional

homogeneity. To prepare 10 mol% Gd doped ceria nanoparticles appropriate amounts of metal

nitrates Ce (NO3)3.6H2O and Gd (NO3)3.6H2O (ALFA AESAR, 99.9%) and glycine (C2H5NO2) as a

fuel were dissolved in deionised water. Different O/F ratios viz. fuel deficient (O: F = 1:1 and

1:1.3), optimum (O: F = 1:1.6) and fuel rich ratio (O: F =1:1.9) were studied. The details of the

experiment are reported in reference [1-2]. The stoichiometric amounts of metal nitrates and fuel

were dissolved in deionised water. This transparent solution was continuously stirred with heating

on a magnetic stirrer for the removal of moisture and hence to form gel. Further this gel was

allowed to combust on preheated (350 °C) hot plate in well designed combustion set up. After few

seconds, the gel auto ignites with evolution of voluminous gases. During this short exposure of high

temperature, formation of pale-yellow ash [1-2] i.e. solid solution Ce0.9Gd0.1O1.95 has formed.

Heating was continued for a while so that unburned part is completely burned. The powders

obtained after auto-ignition were calcined at 600 °C for 2hrs to obtain pure and well crystalline

powders and were sintered at different temperatures.

In another experiment, Cu (NO3)2.3H2O (ALFA AESAR, 99.9%) and citric acid (C6H8O7) were

used in different O/F ratios. The total oxidizing valency of fuel is 18 and the reducing valency of

copper nitrate is -10. So the stoichiometric composition of the redox mixture as calculated based on

the principles of propellant chemistry was 1: 0.55. The different molar ratios of O/F were 1: 0.5, 1:

0.45 (fuel lean) and 1: 0.62 and 1: 0.71 (fuel rich). The stoichiometric amounts of oxidant and fuel

were mixed in deionised water. The mixture was then kept on hot plate to evaporate excess water

till gel is formed. It was then allowed to auto-ignite with the rapid evolution of large volume of

gases to produce fine powder. As-prepared powder was heat treated at 600 oC for 2 hrs to remove

any carboneous impurity and hence pure and well crystalline powder was obtained. It was observed

Materials Science Forum Vol. 757 87

that different ratios give different compositions. The combustion at stoichiometric ratio gives pure

Cu nanoparticles; lean O/F ratios give the nanoparticles of Cu, Cu2O and CuO and rich O/F ratios

give almost phase pure CuO nanoparticles.

Prior to powder synthesis thermo gravimetric and differential thermal analysis (TG–DTA) of dried

gel was carried out using a Perkin Elmer TGA–DTA–DSC instrument with a heating rate of

10°C/min. in air to investigate the reaction temperature and to observe the behavior of material with

temperature. The prepared nanopowders were characterized by Fourier transformed infrared (FTIR)

spectroscopy using ‘Perkin Elmer, FTIR spectrum one’ for detection of various phases. Raman

spectroscopy was also performed and the spectra were collected at room temperature (FT-Raman,

Multi-RAM). The phase formation of samples was examined with powder X-ray diffractometer

(XRD- PHILIPS- PW- 3710). Transmission electron microscopic measurements of as synthesized

powder were carried out using TEM (PHILIPS CM-200, Operating voltage - 20 – 200 kV,

Resolution – 2.4 Å) to measure particle size. The particle size distribution and surface area of as-

synthesized powder were determined using laser particle size analyzer (Master-sizer 2000). It

measures the size of agglomerates/particles in the range of 0.1– 80 µm.

3. Results and Discussion:

3.1. Thermodynamic modeling:

The solution combustion reactions are exothermic in nature to provide high temperature, which is sufficient to propagate the combustion wave throughout the reacting mixture. The thermodynamic calculations for glycine-cerium nitrate system by considering the reaction temperature equal to 298 K are given below for the reaction:

Ce�NO��6H�O + 1.66C�H!NO� "#$!%%°&'((((() CeO� + 2.33N� + 3.32CO� + 10.15H�O (2)

In solution combustion reactions, extremely high temperatures (over 1550 °C) can be achieved within a very short duration. Therefore, a thermally isolated system exists as there would be very little time to disperse the heat to its surroundings. Accordingly, the maximum temperature obtained during the reaction is assumed to be adiabatic temperature (Tad). The heat liberated during the reaction is the enthalpy of the system and is a state function. It is expressed as

∆H% = . n∆C0123

�456 (Product) dT (3)

where n is the number of the moles, T is the adiabatic flame temperature and ∆H is enthalpy of

combustion, which is given as

∆H = ∆H789:;<=– ∆H?@A<=AB= (4) Table 1 lists the enthalpy and specific heat at constant pressure of different species involved in

reaction given by Eq. (2).


Table 1: Enthalpy and specific heat at constant pressure of different species involved in reaction

given by Eq. (2).

From Table 1 and Eq. (4), ∆H for reaction mentioned in Eq. (2) is

∆H = −1159.116 − (− 915.408268) = − 243.7074 Kcal/mol (5) Now,

( )pnC =∑ Summation of specific heats (at constant pressure) of reaction products

Again from Table 1

( )pnC =∑ 0.1517 Kcal /mol K (6)

Solving the Eq. (3) for Tad, we get

TA: = 298 + F �∆G∑�B&I�J = 1904.19K = 1631°C (7)

Fig.1. Theoretically calculated adiabatic temperature and enthalpy as a function of oxidant to fuel

ratio for Ce0.9Gd0.1O1.95 (GDC10) system

However, the actual flame temperatures will be much lower (±100 °C) than the theoretically

calculated values, as the radiative losses, incomplete combustion and heating of air may cause the

reduction. Despite of these losses, the adiabatic flame temperature at which the reactants are raised

during the combustion is much higher than the decomposition temperature (< 350 °C) of Ce

(NO3)3·6H2O [1]. Hence, glycine–nitrate combustion produces the nanopowders of ceria despite the

short duration (i.e. few seconds) of auto ignition. The Fig. 1 indicates the theoretically calculated

Compound ∆Hf ( Kcal/mole) Cp(Kcal/mole K) at 298 K

Ce(NO3).6H2O -729.14 - C2H5NO2 -112.21 - CeO2 -260.20 0.0147 N2 0 0.0067 CO2 -94.05 0.0112 H2O -57.80 0.0083


adiabatic temperature and enthalpy as a function of oxidant to fuel ratio for Ce0.9Gd0.1O1.95 (GDC10)

system. The enthalpy varies linearly while Tad gets deviated from linear one at the stoichiometric

ratio and then after becomes flat. This change in behavior is also explained by number of C, H and

O molecules available during the combustion for various ratios. As the oxidant to fuel ratio

increases, the number of C and H moles rises but the oxygen content required for combustion

reaction depleted beyond the stoichiometry and hence the reaction temperature. Therefore, in most

of the SCS, the fuel lean ratio was preferred as the particle size is significantly lower due to the

lower adiabatic temperature and also there may not have any probability of volatile residual carbon

as obtained in fuel rich.

3.2. Nanoparticles of Gd doped ceria:

The high oxygen ion conductivity of Gd doped ceria than that of its competent material has made it

a promising candidate for a variety of electrochemical devices, for example, as solid electrolytes for

intermediate temperature solid oxide fuel cell (IT-SOFCs) [9-11], three way catalysts for the

treatment of automotive exhaust gases [12], petroleum cracking catalyst [13], oxygen sensors [14],

UV filter [15], etc. Nevertheless, low sinterability and high density are of prime importance, which

could be achieved in solution combustion synthesis. The oxidant-to-fuel ratio was observed to affect

the thermogravimetric measurements, surface area, and the particles size distribution.

3.3. Thermogravimetric analysis:

The TGA curves of the dried gels for various oxidant-to-fuel ratios in oxygen atmosphere shown in

Fig. 2 reveal the effect of oxidant-to-fuel ratio on thermogravimetric plots. All the TG plots shows

two weight losses in the range, (1) room temperature to 200 °C and (2) 200–300 °C. The weight

loss in the TGA is accompanied by the presence of an endothermic peak and two exothermic peaks,

which are observed to be dependent on oxidant-to-fuel ratio. The gradual weight loss in the range

from room temperature to 200 °C and the presence of an endothermic peak in the range from 132 to

145 °C is attributed to the dehydration of gel. The steep reduction in sample weight, above 200 °C,

is due to decomposition of the gel. During this process, two exothermic peaks, a weak followed by

pronounced one, seen in DTA may correspond to the combustion of the residual organic matter in

the sample and to the change of cerium oxide structure from an amorphous state to a crystalline

state, respectively.

Fig.2. TGA of dried gel with various oxidant-to-fuel ratios and DTA for oxidant-to fuel

ratio of 1:1.9.


An absence of any weight loss in TGA and endothermic or exothermic peak in DTA curve above

300 °C confirms formation of fluorite type Ce0.9Gd0.1O1.95 at relatively lower temperature [2].

As we move from fuel lean to fuel rich ratio, the endothermic peak shifts from 145 to 132 °C while

the strong exothermic peak shifts from 305 to 275 °C and becomes more intense. As expected with

the increasing fuel amount, the exothermicity of the metal nitrates – glycine combustion is

increased, shifting the auto-ignition point to low temperature. Thus, all the oxidant-to-fuel ratio

results into formation of single phase material as evidenced in the XRD patterns.

3.4. Structural properties:

The oxidant-to-fuel ratio does not affect the structural properties of the material, as revealed in the

XRD patterns of as-synthesized powders (Fig. 3). Material crystallizes in fluorite type structure

with lattice parameter a = 5.418Å and no peak shift was observed.

Fig.3. XRD patterns of as-synthesized powder prepared from oxidant-to-fuel ratio of (a) 1:1, (b)

1:1.3, (c) 1:1.6 and (d) 1:1.9.

The crystallite size of as-synthesized powders was in the range of 9 – 12 nm while that of calcined

powder is in the range 13 – 17 nm, which was ascertained by TEM (Fig. 4). It clearly shows

formation of nanocrystalline GDC10 phase with uniform and compact distribution of fine particles

with average size of 20 nm. The observed particles are of roughly hexagonal shape. Further the

particles in as-synthesized powder are well dispersed compared to that of the calcined powder

where particles are more compact. This might be due to strong agglomerations during heat

treatment.

3.5. Surface area and particle size distribution:

The surface area of the combustion synthesized powder is found to vary significantly with oxidant-

to-fuel ratio (Table 2). The surface area is 39.58 m2/g for the oxidant-to-fuel ratio of 1: 1 which

increased to 118.02 m2/g for fuel rich ratio (O: F = 1: 1.9). The particle size distribution of as-

synthesized powder is shown in Fig. 5. The soft agglomerates are observed for all oxidant-to-fuel

ratios. The average agglomerate size (D50) of 4.67 and 5.34 µm was observed for oxidant-to-fuel

ratio of 1: 1.3 and 1: 1.6, respectively while it was 11.24 and 13.42 µm for oxidant-to-fuel ratio of

1: 1 and 1: 1.9, respectively. In view of the surface area calculations and average agglomerate size,


we confirmed that the oxidant-to-fuel ratio of 1:1.6, a slightly lower than the optimum value of

1:1.66, gives better particle properties. The large surface areas are observed for the oxidant-to-fuel

ratio of 1:1.6 and 1:1.9 which resulted into high sintered densities.

Fig.4. TEM of GDC10 (a) as-synthesized and (b) calcined powder

The green pellets sintered at low temperature as low as 1200 °C for different lengths of time

showed the density in the range of 85–95% of the theoretical value. The highest relative density of

93% and 95% is observed, respectively, for oxidant-to-fuel ratio of 1: 1.6 and 1: 1.9. The relative

densities close to theoretical one are possible if we sinter the green pellet at higher temperatures.

Table 2: Variation of average agglomerate size (D50) and surface area with different oxidant to fuel ratios

Oxidant to fuel

ratio

Average agglomerate size

D50 in µm

Surface

Area

m2/gm

1: 1.1 11.24 39.58

1 : 1.3 4.67 58.37

1 : 1.6 5.34 73.20

1 : 1.9 13.42 118.03

Fig.5. Particle size distributions for the GDC10 powder prepared from oxidant-to-fuel ratio 1: 1.6


3.6. Raman studies:

Fig. 6 depicts the Raman spectra of calcined powder of GDC10. A sharp and intense peak observed

at 467 cm−1 corresponds to CeO2 due to F2g symmetry of the cubic phase [16].

Fig.6. Raman spectra of Gd doped Ceria (GDC) nano powder

In addition to this, two weak peaks are observed at 550 and 600 cm−1. These vibrational modes are connected to the defect spaces related to the extrinsic and intrinsic oxygen vacancies, respectively, present in the structure of Gd doped ceria nano crystalline sample [17-18]. Further, the absence of the vibrational mode at 360 cm−1 corresponding to Gd2O3 cubic phase [19] confirms the formation of solid solution at room temperature.

3.7. Nanoparticles of Cu and CuO:

The Cu nanoparticles are the important part of nanomaterials, with the potential for broad range of

electronic, optical and communication applications. Therefore, the Cu nanoparticles were paid wide

attention on its application field in recent years. Similarly, oxides of copper find a variety of

applications such as nanoparticulate film fabrication [20], glucose sensor [21], both CuO and Cu2O

films are used in lithium ion batteries [22,23] and CuO nanowires are used for hydrogen detection

[24]. The Cu2O is used in DNA biosensors of Hepatitis B virus [25], as catalyst for synthesis of

carbon nanofibres [26] and it is also used for methane gas sensor and ethanol gas sensor [27-28].

In combustion synthesis of copper nitrate and citirc acid, the compositional homogenity and phase

purity is affected by the oxidant to fuel ratio. In the work, the stoichiometric ratio was 1: 0.55,

which was made lean up to 1: 0.45 and fuel rich ratio 1: 0.71. The combustion at stoichiometric

ratio gives Cu nano particles, lean O/F ratios give nanoparticles of Cu, CuO and Cu2O and rich ratio

gives pure CuO nanoparticles.

The crystallization and formation of desired phase is dependent on the combustion (flame)

temperature, which itself is dependent on nature of the fuel and oxidant to fuel ratio. Generally, in

solution combustion, metal nitrates decompose into oxygen containing species, which have high

temperature rapid interaction with the fuel to give oxide product. The citrate-nitrate combustion

results either in thermal decomposition of metal nitrates in to Cu nanoparticles at stoichiometric


ratio or incomplete combustion at fuel lean and complete combustion at fuel rich ratio. In citrate-

nitrate combustion, since more number of gases are evovled the combustion temperature reaches to

maximum value very slowly compared to glycine. Since the combustion is slow, it results into

thermal decomposition of metal nitrates before the ignition of the fuel. Due to this thermal

decomposition, the amount of oxidant at the time of combustion is decreased, which in turn results

in compositional inhomogeneity. The level of thermal decomposition is O/F ratio dependent. It is

maximum at stoichiometric ratio, almost absent for fuel rich ratio and intermediate for fuel lean

ratio. At rich O/F ratios, high exothermicity maintains the flame temperature high enough to give

phase pure CuO. Thus in citrate-nitrate combustion number gas molecules evoveled dominates over

the flame temperature. So precise control of O/F ratio is necessary to get required phase.

3.8. Thermogravimetric analysis:

The Cu nanoparticles obtained at stoichiometric ratio (1: 0.55) were characterized by XRD. This

shows the face-centered cubic (FCC) crystallinity of Cu with lattice constant 3.619Å and the

powder was subjected for TG-DTA analysis in oxygen atmosphere to see the oxidation behaviour of

Cu nanoparticles.

It is shown in Fig. 7 and can be divided into three regions viz: region I- room temperature to 250 oC; region II- 250 to 526 oC and region III- above 526 oC. A little weight loss observed in region I

with endothermic peak at 116.397 oC is attributed to dehydration of surface moisture. In region II,

oxidation of Cu to Cu2O begins slowly at 250 oC and becomes rapid after 300 oC. This change is

accompanied with a broad exothermic peak at 313.701 oC. Further weight gain is steep in the range

525-575 oC along with exothermic peak at 564.199 oC. At this, the level of cupric oxide (CuO)

formed is reached. The weight gain in region II would be 12%, if all Cu nanoparticles were

oxidized into Cu2O. These Cu2O nanoparticles and remaining Cu nanoparticles were then expected

to get oxidized to CuO in region III. So the total weight gain of 25% was expected. But it is

observed that above 565 oC, the level of Cu2O oxidation to CuO has not reached and only remaining

Cu nanoparticles were oxidized to CuO. This results into only 12.5% weight gain.

Fig.7. TG-DTA of Cu nano particles


3.9. Spectroscopic analysis:

To ascertain the absence of M-O type of bond in as synthesized (not shown here) and its presence in

heat treated samples (Fig. 8), FTIR measurements were performed.

Fig.8. FTIR of Cu nano particles heat treated at 600 oC for 2 hrs

No peak due to M-O type of bond was present in the FTIR of as synthesized powder revealing only

metallic presence. The bands below 600 cm-1 are assigned to Cu2O and CuO. The peak at 1631

cm-1 indicates the CO2 in air and hydrated sample, while the peak at 3409 cm-1 indicates the H2O in

air. The local atomic arrangement and vibrations of nanosized materials can be studied by using

Raman spectroscopy. For Raman only three Ag+2Bg modes are active.

Fig. 9 shows the Raman spectra of the (a) as synthesized and (b) Cu nanoparticles heat treated at

600 oC. From the Fig. 9 it is seen that no peaks of either Cu2O or CuO are observed in Raman

implying the presence of highly pure Cu nano particles in as synthesized nanoparticles and which is

confirmed by XRD as well as IR. These nanoparticles, as observed in TG-DTA, turned into oxides.

Fig. 9b revealed the presence of both CuO and Cu2O. The peaks at 291 cm-1, 340 cm-1 and 613 cm-1

corresponding to the Ag, Bg(1) and Bg(2) modes of bulk CuO particles, respectively, while the peaks

at 153 cm-1, 512 cm-1 and 635 cm-1 correspond to Cu2O.

Fig.9. FT-Raman of Cu nano particles a) as prepared powder b) Heat treated at 600 oC for 2 hrs.


3.10. CuO nanoparticles:

The fuel rich ratio gives CuO nanoparticles. A very small amount of Cu2O was observed for 1:0.62

ratio while phase pure CuO was obtained for the 1: 0.71 ratio. The phase purity of CuO

nanoparticles was examined by XRD and is as shown in fig. 10. The X-ray reflection peaks at 2θ =

53.38º and 58.37º can be indexed as (002) and (111) crystal planes of the CuO, respectively. The

CuO nano particles give the pure monoclinic structure (JCPDS card files of CuO (80-1917)) with

lattice parameters a = 4.68 Å, b = 3.42 Å and c = 5.12 Å and are in good conformity with the

reported data. No any impurity peak can be observed in XRD pattern. The average crystallite size

was calculated by using the Scherrer formula and it is 29-41 nm.

The phase purity of CuO nanoparticles is also ascertained by TG/DTA. The TG/DTA of the heat

treated powder of CuO nano particles are shown in Fig. 11. A little weight loss is observed in the

range of room temperature to 100 oC recognized due to dehydration of surface moisture. The broad

exothermic peak is observed at 248.88 oC accompanied with 0.2 % increase in weight up to 700 oC.

This slight change is due to oxidation of Cu to Cu2O and then to CuO. The weight calculations

showed that the Cu content would be less than 1% and hence were not detected in XRD.

Fig.10. X-ray Diffraction pattern of CuO heat treated at 600 oC for 2 hrs

Fig.11.TG-DTA of CuO nano particles


Fig.12 shows typical absorption spectra of CuO heat treated at 600 oC. IR spectra showed broad

absorption band in the region of 533.37 cm-1 and is assigned to Cu-O stretching vibration mode, the

broadness of the absorption band indicates that the CuO powders are nanocrystals. The absorption

bands in the region of 1000–1500 cm−1 are assigned to the O–C=O symmetric and asymmetric

stretching vibrations but the intensity of the band has weakened. The C-H stretching mode is

observed in the region 2800-3000 cm-1.

The CuO nano particles were well dispersed in ethanol to form a transparent solution by ultrasonic

vibration for 1 hr. The flat peaks near 401nm and 533 nm present in UV-visible absorption

spectrum indicates the formation of copper oxide nano particles as confirmed by XRD. The band

gap of CuO nano particles is calculated to be 2.32eV, which is high compared with reported values

(1.8eV).

Fig.12. FTIR of CuO nanoparticles

4. Conclusion:

In summary, solution combustion synthesis is a potential method for preparation of phase pure Gd

doped ceria nanoparticles. The O/F ratio has found to affect the exothermicity, surface area and

particle size. A slightly lean O/F ratio gives surface area of 73 m2/g and soft agglomerates (D50 =

5.34 µm), which eventually results into high sintering density at low temperature. Low temperature

sinterability enhances material properties and has been the bottleneck in many of the applications.

As the powder properties can be tuned by altering the O/F ratio, the method is very useful to

prepare phase pure nano-ceramics for variety of applications. The foregoing study also emphasizes

the ability of solution combustion synthesis to prepare nanoparticles of Cu. In nitrate-citrate

combustion, O/F ratio significantly affects the compositional homogeneity as well as phase purity.

So O/F ratio can be tailored to prepare either Cu or CuO or mixed phases of Cu2O and CuO

nanoparticles. The oxidation behavior of Cu nanoparticles was also studied. Thus solution

combustion synthesis is a versatile method of nanoparticle preparation and offers wide scope for

future research.


References:

[1] Jadhav LD, Chourashiya MG, Subhedar KM, Tyagi AK, Patil JY, J. Alloys Compd. 2009; 470 : 383–386.

[2] Jadhav LD, Chourashiya MG, Jamale AP, Chavan AU, Patil SP, J. Alloys Compd. 2010; 506: 739.

[3] Saha S, Ghanawat SJ, Purohit RD. J Mater Sci 2006; 41: 1939–43.

[4] Nair SR, Purohit RD, Tyagi AK, Sinha PK, Sharma BP. Mater Res Bull 2008; 43: 1572–82.

[5] Patil KC, Aruna ST, Mimani T. Curr Opin Solid State Mater Sci 2002; 6: 507–12.

[6] Sharma P., Lotey G., Singh S., Verma N.J. Nanoparticle Res., 2010, 13, (6), pp. 2553–2561.

[7] Xu H., Yan H., Chen Z., J. Power Sources, 2006, 163, pp. 409–414.

[8] Tyagi A., Chavan S., Purohit R., Ind. J. Pure Appl. Phys., 2006, 44, pp. 113–118.

[9] Inaba H., Tagawa H., Solid State Ionics 1996; 83: 1.

[10] Chourashiya MG, Patil JY, Pawar SH, Jadhav LD, Mater. Chem. Phys. 2008; 109: 39.

[11] Chourashiya MG, Pawar SH, Jadhav LD, Applied Surface Science 2008; 254: 3431–3435.

[12] Abdul H., Iskandar Y., Catal. Today 2004; 96: 165.

[13] Fathi M., Bjorgum E., Rokstad OA, Catal. Lett. 2001; 72: 25.

[14] Jasinski P., Suzuki T., Anderson HA, Sens. Actuators B 2003; 95: 73.

[15] Li R., Yabe S., Yamashita M., Momose S., Yoshida S., Yin S., Sato T., Solid State Ionics 2002; 151: 235.

[16] Matta J., Courcot D., Abiaad E., Aboukays A., Chem. Mater. 2002;14: 4118.

[17] DohcevicMitrovic ZD, GrujicBrojcin M., Scepanovic M., Popovic ZV, Boskovic S., Matovic B., Zinkevich M., Aldinger F., J. Phys.: Condens. Matter. 2006; 18: S2061.

[18] Nakajima A., Yoshihara A., Ishigame M., Phys. Rev. B. 1994; 50: 13297.

[19] Murillo A, Luyer CL, Garapon C., Dujardin C., Bernstein E., Pedrini C., J. Mugnier, Opt. Mater. 2002; 19: 161.

[20] Kida T., Oka T., Nagano M., Ishiwata Y., Zheng X.,J. Am. Ceram. Soc., 2007, 90, (1), pp. 107–110

[21] Zhang X.,Wang G., ZhangW., Hu N., Wu H., Fang B., J. Phys. Chem. C, 2008, 112, (24), pp. 8856–8862

[22] Wang H., Pan Q., Zhao J., Yin G., Zuo P, J. Power Sources, 2007, 167, pp. 206–211

[23] Fu L., Gao J., Zhang T., ET AL., J. Power Sources, 2007, 174, pp. 1197–1200

[24] Hoa N., Quy N., Jung H., Kim D., Kim H., Hong S. Sens. Actuators B, 2010, 146, pp. 266–272

[25] Zhu H., Wang J., Xu G., Cryst.Growth Des., 2009, 9, (1), pp. 633–638

[26] Zhang H., Ren X., Cui Z., J. Cryst. Growth, 2007, 304, pp. 206–210

[27] Jayatissa A., Samarasekara P., Kun G., Phys. Status Solidi A, 2009, 206, (2), pp. 332–337

[28] Guan L., Pang H., Wang J., Lu Q., Yin J., Gao F, Chem. Commun., 2010, 46, pp. 7022–7024


Engineering Applications of Nanoscience and Nanomaterials 10.4028/www.scientific.net/MSF.757 Solution Combustion Synthesis: Role of Oxidant to Fuel Ratio on Powder Properties 10.4028/www.scientific.net/MSF.757.85

http://dx.doi.org/www.scientific.net/MSF.757

http://dx.doi.org/www.scientific.net/MSF.757.85

solution combustion synthesis: role of oxidant to fuel ratio on powder properties

Documents