Materials Sciences and Applications, 2011, 2, 1584-1592
doi:10.4236/msa.2011.211212 Published Online November 2011 (http://www.SciRP.org/journal/msa)
Solid State Reactions in Cr2O3-ZnO Nanoparticles
Synthesized by Triethanolamine Chemical
Precipitation
Imelda Esparza1,2, Myriam Paredes1, Roberto Martinez2, Adriana Gaona-Couto1,
Guadalupe Sanchez-Loredo1, Luisa M. Flores -V e lez 3, Octavio Dominguez1*
1Instituto de Metalurgia-UASLP, Sierra Leona, México; 2CIMAV, Cervantes, México; 3Facultad de Ciencias Químicas-UASLP,
Sierra Leona, México.
Received August 27th, 2011; revised October 1st, 2011; accepted October 16th, 2011.
ABSTRACT
The present work reports the preliminary results about solid state reactions of Cr-ZnO solid solutions and ZnCr2O4
nanometric particles obtained with triethanolamine (TEA). Different compositions were prepared from 0.65 to 33.3 at%
chromium, the last one corresponding to ZnCr2O4 cubic spinel composition. Fourier Transform Infrared Spectroscopy
(FTIR) together with X-ray diffraction (XRD) patterns of powders with Cr3+ between 0.65 and 16.0 at% were assigned
to Cr-ZnO solid solution due to the only presence of ZnO structure, FTIR spectra indicating that Cr-O bonding exists
even if there was no presence of ZnCr2O4. With low chromium atomic percent, lattice parameters increase, but as the
chromium content exceeds of 3 at%, there is basically no further expansion of the cell. From Williamson-Hall and
Rietveld methods the lattice dimensions were assigned to chromium incorporation in ZnO structure and the lattice con-
traction by particle size refinement. After annealing all samples from 0.65 to 16.0 at% at 400˚C in oxygen, the analysis
showed that nanoparticles of Cr-ZnO solid solution still remain.
Keywords: Nanometric ZrCr2O4 Spinel, Cr-ZnO Nanoparticles, Thermal Analysis, FTIR, Chemical Preparation
1. Introduction
Chromium (III) oxide has a wide range of applications
including pigments to reflect infrared radiation [1], het-
erogeneous catalysts [2], coating materials for thermal
protection, wear resistance [3], and so on. It has been
established that some spinels have advanced gas sensing
and catalytic properties [4], so ZnCr2O4 could be a suit-
able sensors for aggressive environments because chro-
mium improves the stability of ZnO films against diluted
hydrochloric and nitric acid media [5]. Besides Cr2O3-
ZnO materials, specially prepared in nanocrystalline state
are interesting systems for H2 production via photoelec-
trochemical splitting of water [6].
A number of synthetic routes have been employed to
synthesize ZnO-Cr2O3 nanoparticles such as chemical
vapor synthesis (CVS) [7], ball milling [8], spray pyroly-
sis [9], decomposition of coprecipitated hydroxides [10]
and calcinations of metallo-organic precursor solutions
[11]. Because triethanolamine is extensively used in in-
dustrial milling to avoid agglomeration and in cosmetic
and food products as dispersant agent, its present price is
low and quite competitive. Therefore, the present work
reports the preliminary results about a novel, technically
simple and more economical process for producing
ZnCr2O4 and high chromium-ZnO solid solution nano-
metric particles.
2. Experimental Procedure
Chromium (III) nitrate nonahydrate (Cr(NO3)3·9H2O,
Sigma-Aldrich 99%) and zinc (II) nitrate hexahydrate
(Zn(NO3)2·6H2O, Sigma-Aldrich 99%) were used as the
chromium and zinc ions sources respectively, and
triethanolamine N(CH2CH2OH)3 (TEA, Sigma-Aldrich
98%) was used as the base in a variation of the procedure
reported before for ZnO precipitation [12]. All experi-
ments were carried out by simultaneous addition at 25
mL/s of each metal ion solution, Zn2+(aq) and Cr3+(aq) at
different concentrations, simultaneously with the TEA
solution to a third water solution, all heated at 80˚C and
then mixed for 15 minutes at pH ≈ 9. After filtering, the

Solid State Reactions in CrO-ZnO Nanoparticles Synthesized by Triethanolamine Chemical Precipitation1585
2 3
samples were washed several times with acetone to re-
move TEA. When the concentration of Cr3+ reaches the
stoichiometric cationic ratio of Zn:Cr = 1:2, ZnCr2O4
spinel becomes the final product.
3. Instrumental Techniques
Crystal structure and phase composition of the samples
were determined by X-ray diffraction using a Rigaku
DMAX-1000 diffractometer with Cu Kα (λ = 1.54056 Å)
radiation. The XRD data was collected in the range 10 <
2θ < 80˚ with a step size of 0.01˚ and an integration time
of 2 seconds. Fourier Transform Infrared Spectroscopy
(FTIR) of the samples was carried out using a Perkin-
Elmer SPECTRUM-GX spectrophotometer scanning
between 3000 and 400 cm–1. Samples were prepared by
mixing about 2% of synthesized powder with KBr. The
TEM specimens were prepared by dispersing the powder
in n-hexane with the aid of ultrasonic agitation. Drops
were poured into a carbon supported copper grid, and
then dried in air. The TEM images were obtained using a
JEOL 1200 transmission electron microscope having an
EDS spectrometer and operated at an acceleration volt-
age of 120 kV. Thermal analysis was performed using a
Perkin-Elmer DTA7 differential thermal analyzer. The
instrument was operated using chromatographic O2
(Praxair, 99.999%) and a constant heating rate of 10˚C
/min.
4. Results and Discussion
4.1. As-Prepared Zinc Oxide and Chromium
Oxide
The aqueous reaction between Zn2+ and TEA always
produced ZnO samples presenting the wurtzite structure
(hexagonal phase, space group P63mc). The XRD pat-
terns of the as-prepared nanometric ZnO were compared
with a standard and are shown in Figure 1(a), all the
diffraction peaks being well assigned to hexagonal phase
ZnO as reported in JCPDS card No. 36-1451. The repre-
sentative TEM image of the as-prepared ZnO nanoparti-
cles is shown in Figure 1(b), indicating that particles
present spherical-like morphology and narrow size dis-
tribution, having a particle size between 30 and 50 nm.
The IR spectra of nanometric ZnO is exhibited in Figure
1(c). It shows the presence of several absorption bands
indicating a minor shift from those reported in the litera-
ture, 1633, 1555, 1384, 970, 833, 679 and 566 cm–1 [13].
On the other hand, Cr2O3 (rhombohedral phase, space
group 3
R
c) was obtained from the aqueous reaction
between Cr3+ and TEA solutions. After being washed and
filtrated, the powder was calcinated in electric oven at
400˚C to crystallize the chromium oxide.
The XRD pattern of the nanometric Cr2O3 is shown in
25 30 35 40 4550 55 6065 70 75
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
nano ZnO
(004)
(201)
(112)
(200)
(103)
(110)
(102)
(101)
(002)
R ela tive In ten s ity (a . u.)
Diffraction Angle (2  )
(100)
micro ZnO
(a)
(b)
3000 2500 2000 1500 1000500
0
10
20
30
40
50
60
70
80
90
100
675
1635
2095
875
985
1390
1555
TRANSMITTANCE (%)
WAVENUMBER (cm
-1
)
495
nano ZnO at 25 °C
(c)
Figure 1. (a) X-ray diffraction pattern of micrometric ZnO
and as-prepared zinc oxide nanoparticles; (b) TEM image
of the as-prepared nanometric zinc oxide particles; (c)
FTIR of the nanometric ZnO.

Solid State Reactions in CrO-ZnO Nanoparticles Synthesized by Triethanolamine Chemical Precipitation 1586 2 3
Figure 2(a), all diffraction peaks being assigned to
JCPDS card No. 38-1479. The representative TEM im-
age of the obtained Cr2O3 is shown in Figure 2(b) and
the corresponding IR spectra is shown in Figure 2(c).
The TEM image of the prepared Cr2O3 nanoparticles
indicated that particles present rhomboidal-like mor-
phology and particle size between 25 and 50 nm. The IR
spectra of nanometric Cr2O3 (Figure 2(c)) shows the pre-
sence of some absorption bands reported in the literature
(1633, 1505, 1024, 910, 800, 615 and 545 cm–1) [14].
4.2. Prep ared Z in c- Ch r omium Ox i de
Nanoparticles
Precipitation products from the reaction between Zn2+
and Cr3+ ions with TEA were obtained incorporating dif-
ferent amounts of the chromium salt. The obtained zinc-
chromium oxide powders were analyzed by XRD tech-
nique following Rietveld procedure using the MAUD
software [15] and some of their corresponding patterns
are shown in Figure 3. In these cases, the particle size
and the lattice strain contribution to the X-ray diffraction
peak broadening was estimated using Williamson-Hall
analysis [16]. Assuming that the particle size and strain
contributions to line broadening are independent from
each other and both have a Cauchy-like profile, the ob-
served line breadth is simply the sum of the two contri-
butions leading to the Williamson-Hall equation [16]:


cos sin
hkl hklhkl
KL



  (1)
hkl

corresponding to broadening and 4



,

being the root mean square value of microstrain. Plotting
the value of cos
hkl hkl

 as a function of sin hkl

the
microstrain may be estimated from the slope of the line
and the particle size L from the intersection with the ver-
tical axis.
Aqueous reaction experiments using different
zinc-chromium concentrations up to the stoichiometric
ratio [Cr3+] = 2[Zn2+] were performed using the same
conditions as described before. The obtained zinc-chro-
mium oxides were analyzed by XRD. Figure 3(a) shows
the corresponding XRD patterns where diffraction peaks
were completely assigned to the presence of ZnO for
those concentrations up to 16.0 at% of Cr3+. At a chro-
mium content of 16 at%, there is no more Cr2O3-ZnO
solid solution and some other CrxZnyO compound pre-
cipitates; at present there is no crystal structure linked to
such compound, but diffraction peaks could be assigned
to JCPDS card No. 11-0277 associated to amorphous
2ZnO·Cr2O3·H2O. When the spinel composition was
reached (33.3 at%), the new phase was evident by the
presence of peaks corresponding to the ZnCr2O4 cubic
structure with a space group 3
F
dm, all diffraction
peaks being assigned to ZnCr2O4 as reported in JCPDS
15 20 25 30 35 40 45 50 55 60 65 70
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
(113)
(300 )
(214)
(122)
(116)
(024)
(202)
(110)
(104)
(006)
R E lA T IVE IN T E N S IT Y (a . u.)
DIFFRACTIO N A N GL E (2  )
(012)
nano Cr
2
O
3
(a)
(b)
3000 2500 2000 1500 1000500
45
50
55
60
65
70
75
80
2885
2345
1550
1354
955
635
TRANSMITTANCE (%)
WA VE NUMBER (cm
-1
)
Cr
2
O
3
at 25 °C 569
(c)
Figure 2. (a) X-ray diffraction pattern of prepared chro-
mium oxide nanoparticles; (b) TEM bright field image and
(c) FTIR spectra of the nanometric Cr2O3 particles.

Solid State Reactions in CrO-ZnO Nanoparticles Synthesized by Triethanolamine Chemical Precipitation1587
2 3
25 30 35 40 455055 60 65 70 75
33.0%
(440)
(422)
(511)
(400)
(220)
(311)
(004)
(201)
(112)
(200)
(103)
(110)
(102)
(101) (002)
(100)
16.0%
8.00%
2.70%
0.65%
Relative Intensity (Arb. Units)
Diffraction Angle (2  )
(a)
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
B
hkl
cos (
hkl
) (radians)
sin ( hkl )
25 °C
0.00 at%
0.65 at%
2.70 at%
8.00 at%
33.3 at%
(b)
Figure 3. (a) XRD pattern and (b) Williamson-Hall plots of
the different nanometric ZnO/xCr2O3 particles.
card No. 22-1107. Because line broadening was impor-
tant, Rietveld and Williamson-Hall analysis were used to
get information about lattice parameter, strain distortion
and particle size in samples as a function of the chro-
mium content. Figure 3(b) shows the Williamson-Hall
plots obtained after using equation (1) on XRD data from
each sample at room temperature. According to the cal-
culated slope η and intersection with vertical axis (λ/L)
from Figure 3(b), slope augmented and particle size was
slightly reduced as Cr3+ was incorporated in the ZnO
structure (Table 1). Once the spinel composition was
reached, the particle refinement was notorious. Moreover,
Rietveld analysis was performed to each Cr-ZnO solid
solution to find the lattice parameters c and a (Table 1)
of the hexagonal ZnO structure. TEM bright field image
is shown on Figure 4(a) to illustrate a mean particle size
of 25 nm, corroborating the particle size calculated from
Williamson-Hall method. Figure 4(b) corresponds to the
Rietveld refinement of the same sample, proving that the
Table 1. Chemical Composition, Particle size (L), Lattice
Distortion (η) and Lattice Parameters (a and c) Obtained
from Nanometric ZnO-Cr2O3 Powders.
Chromium (at%)Williamson-Hall Analysis Rietveld Analysis
NominalEDS<L> (nm) η (%) a (Å) c (Å)
0.00 0.0045 0.87 3.24665.2044
0.65 0.5033 0.88 3.24895.2100
2.70 2.4925 1.22 3.25095.2094
8.00 7.8622 1.85 3.25155.2117
16.0 15.4015* ------ ----------------
33.3 34.1611 4.98 8.3327--------
*Estimated from TEM images.
(a)
20.0 40.0 60.0
2-Theta [degrees]
nano ZnO + 2.0 at % Cr
60.0
40.0
20.0
Intensity
1/2
[Count
1/2
]
(b)
Figure 4. (a) TEM of 8.0 at% as-prepared powder, and (b)
Rietveld XRD refi nement showing wurtzite structure.

Solid State Reactions in Cr2O3-ZnO Nanoparticles Synthesized by Triethanolamine Chemical Precipitation 1588
Consequently, as Cr3+ is incorporated in the lattice,
one should expect a small expansion in ZnO lattice pa-
rameters because of ionic radii differences.
measured x-ray pattern can be justified using only the
wurtzite structural parameters, so there is solid solution
even with 8 at% chromium.
Several studies have shown that micrometric ZnO can
take Cr2O3 in solid solution up to 7 mol%, this solubility
limit being modified by the presence of impurities [17].
One of the reasons of this considerable solubility can be
justified on the basis of ionic radii values [18] (rCr3+ =
75.5 pm and rZn2+ = 74 pm), hence it is concluded that
Cr3+ can be incorporated in the cationic sublattice. Nev-
ertheless, when Cr3+ is dissolved in the lattice instead of
Zn2+, it must be charge compensated by means of a point
defect, creating Zn2+ vacancies in the bulk, probably by
some mechanism like [17]
The presence of Cr-O bonding in ZnO wurtzite struc-
ture from synthesized Cr-ZnO solid solutions was deter-
mined by energy-dispersive x-ray spectroscopy (Table 1)
and additionally supported by FTIR spectra as shown in
Figure 5.
The FTIR spectra of all the ZnO-Cr2O3 samples
showed the same absorption bands as the ZnO sample
(Figure 1(c)) together with small absorption bands at
2360 and 2885 cm–1, both corresponding to some O-Cr
vibrational modes according to FTIR spectra of Cr2O3
(Figure 2(c)). When chromium content reaches 16.0 at%,
the FTIR spectrum presents the same ZnO absorption
bands (1350 to 1800 cm–1) and two more Cr2O3 absorp-

•
23ZnO Zn
ZnO
Cr O2Cr3OV  (2)
3000 2500 20001500 1000500
20
30
40
50
60
70
80
90
890
2870
2410
2095
1650
1050
1375
1470
710
TRANSMITTANCE (%)
WAVENUMBER (cm
-1
)
454
ZnO-2.7at%Cr at 25 °C
3000 2500 2000 1500 1000500
50
60
70
80
90
100
2400
2885
1645
2100
1765
1470
1375
1045
845
646
TRANSMITTANCE (%)
WA VENUMBER (cm
-1
)
453
ZnO-8.0at%Cr at 25 °C
(a) (b)
3000 2500 2000 1500 1000500
50
55
60
65
70
75
80
85
90
95
100
465
514 635
890
1032
1063 1159
1377
1470
1655
1760
2410
TRANSMITTANCE (%)
WA VENUMBER (cm
-1
)
2895
ZnO- 1 6.0 at% Cr at 2 5 °C
3000 2500 20001500 1000500
30
40
50
60
70
80
90
2900
2345
1623 1553
1427
1234
941
757
630
TRANSMITTANCE (%)
WAVENUMBER (cm-1)
520
ZnCr
2
O
4
at 25 °C
WAVENUMBER (cm
-
1
)
(c) (d)
Fig ure 5. FT IR spectr a at roo m te mperat ure of the nano metr ic Cr-Z nO particles having differe nt Cr co ntent. ( a) 0.65 at %;
(b) 8.0 at%; 16.0 at% and (c) 33.3 at%.

Solid State Reactions in CrO-ZnO Nanoparticles Synthesized by Triethanolamine Chemical Precipitation1589
2 3
tion bands at 514 and 635 cm–1. Finally, at the spinel-
composition (Figure 5(d)), FTIR spectrum changes con-
siderably due to the enhancement of the Cr2O3 absorption
bands at 520 and 630 cm–1, remaining the presence of
weak absorption bands of ZnO between 1350 and 1800
cm–1.
Figure 6 presents the progression between particle
size and lattice parameters c and a (of the ZnO hexagonal
cell), as a function of the chromium concentration. The
results suggest that the hexagonal cell of the nanometric
ZnO was, to a certain limit, modified by the Cr3+ ions
incorporated in the crystal lattice. Therefore, at low
chromium content both a and c parameters slightly aug-
mented as a consequence of Cr-O bonding inside the
ZnO structure. But as the chromium content exceeds of
2.7 at%, there is basically no further expansion of the cell
but FTIR spectra indicates that chromium associated to
Cr-O bonds exists without the presence of ZnCr2O4
structure (Figure 5). The size effect on structure and
lattice parameters has been widely documented and the
relative variation of lattice parameter with particle size is
given by [19]:
2
3
a
aR


 (3)
γ and R are the surface energy and the particle size;

and a being the compressibility factor and lattice pa-
rameter of the bulk solid respectively.
Therefore, there is a contraction of the crystal lattice
due to the pressure exerted toward the interior of the par-
ticle. This contraction is proportional to the surface en-
ergy and inversely proportional to the particle size, where
lattice contraction has been observed experimentally on
many solids with particle size reduction [20]. ZnO cell
expansion in both a and c lattice parameters have been
observed on samples prepared by solvothermal synthesis
0123456789
3.242
3.244
3.246
3.248
3.250
3.252
5.204
5.206
5.208
5.210
5.212
5.214
5.216
a
c
Cr
3+
Content (at%)
Lattice Parameter (Angstrom)
20
24
28
32
36
40
44
particle
Particle Size (nm)
Figure 6. Evolution of particl e size and lattice para meters a
and c of ZnO as a fu ncti on of the chromium content.
adding up to 8 mol% chromium [21] and in Cr doped
ZnO samples prepared by CVS techniques where no
second phases besides wurtzite structure have been ob-
served [7], indicating once again that the chromium ions
were incorporated in the ZnO structure. The fact that in
this process the incorporation of more than 3 at% Cr3+
ions does not modified any longer the ZnO lattice pa-
rameters could be associated to the size effect on lattice
parameters given by Equation (3). If chromium ions enter
in the cationic sublattice but parameters can not change
as a consequence of a smaller particle size, then lattice
strain must augment in the crystal structure, as shown
before in Williamson-Hall plots and the values reported
in Table 1 for parameter η.
4.3. The Thermal Evolution of Zinc-Chromium
Oxide Nanoparticles
Even if chemical precipitation using TEA was capable to
produce Cr-ZnO solid solution and ZnCr2O4 spinel as the
Cr content increases, almost certainly there will be an
inevitable need of heat processing to obtain well crystal-
lized strain-free nanometric zinc-chromium oxides. Of all
solid state reactions, the formation of oxide spinels are at
present the most systematically investigated compounds
[22]. The spinel ZnCr2O4 is commonly prepared by a
solid state reaction of zinc oxide and chromium oxide,
where the calcination temperature varies in a range of
800˚C to 1200˚C [23]. In order to assess the influence of
thermal treatment on the formation of Cr-ZnO (solid so-
lution) and spinel ZnCr2O4 nanoparticles, thermal analy-
sis of the different as-prepared compositions was carried
out.
We can not expect the same thermal evolution as those
reported in the literature by conventional processing [24],
for the reason that in this case samples already started
with solid solution, quasi-solid solution (2ZnO·Cr2O3·H2O)
or spinel composition at room temperature. Figure 7
presents the DTA curves plotted for each composition.
At low temperature, two endothermic reactions were
observed in all samples, the first one being at 122˚C and
corresponding to water desorption, the second one at
235˚C corresponding to TEA decomposition; both endo-
thermic peaks being independent of the chromium con-
tent. Afterward there was the presence of an exothermic
peak at 285˚C probably for all samples, but being noto-
rious for concentrations higher than 2.7 at% chromium.
Besides, at high temperature another endothermic proc-
ess appears at 430˚C, probably as a result of the forma-
tion of ZnCr2O4 from the Cr-ZnO solid. Finally, there
was a broad exothermic peak in all thermograms at ap-
proximately 550˚C, probably associated to particle growth.
Subsequently, the as-prepared samples were heat
treated in oxygen at two different temperatures (400 and

Solid State Reactions in CrO-ZnO Nanoparticles Synthesized by Triethanolamine Chemical Precipitation 1590 2 3
50100 150 200 250 300350 400450 500 550600 650700
-4
-2
0
2
4
6
8
10
235 °C
430 °C
16.0 at%
8.00 at%
550 °C
0.00 at%
285 °C
235 °C
2.70 at%
Heat Flow (Arb. U.)
TEMPERATURE (°C)
122 °C
<endo>
Figure 7. Thermodiagrams of the nanometric Cr-ZnO na-
noparticles obtained by DTA using a constant heating rate
of 10˚C/min i n oxygen.
700˚C), then rapidly cooled and compared by XRD in
order to relate the effect of Cr3+ during the calcination
process observed by DTA. XRD results are shown in
Figure 8, indicating that phase evolution depends on
Cr3+ content. Calcinations carried out on samples with
0.65, 2.7 and 8.0 at% at 400˚C (Figure 8(a)) lead to the
Cr-ZnO solid solution (wurtzite ZnO phase) and it was
not possible to distinguish ZnCr2O4 formation (cubic
spinel). On the other hand, in sample with 16 at% chro-
mium, only the Cr-ZnO solid solution was detected by
XRD, suggesting the reaction 2ZnO·Cr2O3 Cr-ZnO
(solid solution) taking place at 285˚C without formation
of ZnCr2O4. Finally, after annealing of all samples at
700˚C, XRD results showed the presence of ZnCr2O4
formation from Cr-ZnO solid solution. Probably, the
solid state reactions taking place during thermal evolu-
tion could be approximated to the following sequence:

 
 

23
285 C
23
285 C
ZnOCr OaCr-ZnOs
ZnOCr OcCr-ZnOs
Cr-ZnO ss





(4)
 
285 C
24
Cr-ZnO ssCr-ZnO s +ZnOCrOc

 (5)
where a: amorphous, s: solid solution, ss: Cr3+ over con-
centrated solid solution and c: crystalline solid. There-
fore, the exothermic peak at 285˚C probably corresponds
to both process, crystallization of the 2ZnO·Cr2O3 like
compound and formation of Cr3+ concentrated solid solu-
tion, followed of solid solution decomposition to
ZnCr2O4 at 430˚C.
Once again, Williamson-Hall analysis was used to get
information about strain distortion and particle size of
25 30 35 40 45 50 55 60 65 70 75
(620)
(440)
(511)
(422)
(400)
(222)(311)
(220)
33.3%
ZnCr
2
O
4
(004)
(112)
(200)
(103)
(110)
(102)
(101)
(002)
(100)
400 °C
Cr-ZnO (solid solutions)
16.0%
8.00%
2.70%
0.65%
Intensity (Arb. Units)
Di ffr a c ti o n Ang le (2  )
(a)
25 30 35 4045 50 55 60 65 70 75
Cr-ZnO (solid sol ution)
ZnCr
2
O
4
33.3%
(440)
700 °C
(533)
(620)
(511)
(422)
(400)
(222) (311)
(220)
(004)
(201)
(112)
(200)
(103)
(110)
(102)
(101)
(002)
(100)
16.0%
8.00%
2.70%
0.65%
Intensity (Arb. Units)
Diffraction A n
g
le
(
2 
)
(b)
Figure 8. XRD patterns of Cr-ZnO nanoparticles after heat
treatment at (a) 400˚C; (b) 700˚C.
ZnO phase in samples as a function of heat treatment and
chromium content. Figures 9(a) and 9(b) show the Wil-
liamson-Hall plots from XRD data of each sample at 400
and 700˚C. According to the calculated slope η and in-
tersection with vertical axis (λ/L) from Figure 9, the
slope was reduced and particle size was augmented (Ta-
ble 2) as temperature was increased. Nevertheless, in all
cases particle size remains in the nanometric domain by
means of Cr3+ incorporation. TEM bright field images of
two different compositions (0.65 and 16.0 at%) at 400
and 700˚C are shown on Figure 10 to corroborate that
Cr-ZnO solid solutions and ZnCr2O4 particle size re-
mains in the nanometric domain as calculated from Wil-
liamson-Hall method.
5. Conclusions
In this work, a technically simple and economical proc-
ess was used to produce at room temperature nanoparti-

Solid State Reactions in CrO-ZnO Nanoparticles Synthesized by Triethanolamine Chemical Precipitation1591
2 3
0.00 0.050.100.15 0.200.25 0.300.350.40 0.450.500.55 0.60
0.000
0.004
0.008
0.012
0.016
0.020
0.024
0.028
0.032
0.036
0.65 at%
2.70 at%
8.00 at%
16.0 at%
33.3 at%
B
hkl
cos (

hkl
) (radian s )
sin ( 
hkl
)
400 °C
(a)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400.45 0.50 0.55 0.60 0.6
5
0.000
0.002
0.004
0.006
0.008
0.010
16.0 at%
8.00 at%
2.70 at%
0.65 at%
B
hkl
cos (

hkl
) (radians)
sin ( 
hkl
)
700 °C
(b)
Figure 9. Williamson-Hall plots of the corresponding XRD
patterns of the different Cr-ZnO nanoparticles after heat
treatment at (a) 400˚C and (b) 700˚C.
Table 2. Mean particle size and lattice distortion obtained
from williamson-hall analysis.
Sample T = 400˚C T = 700˚C
Cr (at%) <L> (nm) (%) <L> (nm) (%)
0.00 57 0.31 95 0.14
0.65 40 0.35 62 0.40
2.70 38 0.54 60 0.76
8.00 29 0.76 55 0.88
16.0 28 0.85 45 0.96
33.3 15 1.85 30* ------
* Estimated from TEM images.
(a) (b)
(c) (d)
Figure 10. (a) and (b) TEM images of 0.65 and 16.0 at%
Cr3+ after heating at 400˚C. (c) and (d) TEM images of same
samples after heating at 700 ˚C.
cles of single phase Cr-ZnO solid solution and spinel
ZnCr2O4. It was observed that the lattice parameter of the
ZnO a-axis and c-axis slightly augment as the chromium
content increase up to 3 at%. Subsequently, adding more
Cr3+ into the ZnO structure does not substantially modify
the lattice parameters, probably as a consequence of cell
contraction effect from particle size reduction according
to Equation (3). Nevertheless, EDS and IR spectra indi-
cated that chromium atoms are incorporated during the
TEA aqueous precipitation in the wurtzite Cr-ZnO solid
solution up to 8 at% and do not forms a second phase in
the range of room temperature up to 400˚C. Besides, it
was observed at room temperature using a higher chro-
mium content (16.0 at%), the presence of a second com-
pound, probably of the form 2ZnO·Cr2O3·nH2O, trans-
forming at 400˚C into a Cr-ZnO solid solution incorpo-
rating 16.0 at% chromium in the wurtzite ZnO structure
and remaining in the nanometric domain (30 nm). The
most plausible solid state reactions involved during ther-
mal treatment were proposed according to reactions (4)
and (5). Finally, crystalline strain-free ZnCr2O4 nanopar-
ticles can be promoted with annealing treatment at 400˚C.
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