Cerâmica 61 (2015) 118-125  http://dx.doi.org/10.1590/0366-69132015613571853 118
Microstructural, structural and optical properties of nanoparticles of 
PbO-CrO3 pigment synthesized by a soft route
(Propriedades microestruturais, estruturais e ópticas de nanopartículas de 
pigmentos de PbO-CrO3 sintetizadas por rota soft)
V. D. Araújo1*, M. R. B. Andreeta2, L. J. Q. Maia3, R. M. Nascimento4, F. V. Motta4, 
M. R. D. Bomio4, C. A. Paskocimas4, M. I. B. Bernardi5
1Unidade Acadêmica do Cabo de Santo Agostinho, Universidade Federal Rural de Pernambuco - UFRPE, 
Recife, Brazil 52171-900 
2Departamento de Engenharia de Materiais, Universidade Federal de S. Carlos - UFSCar, P.O. Box 676, 
S. Carlos, SP, Brazil 13565-905
3Instituto de Física, Universidade Federal de Goiás, Campus Samambaia, C.P. 131, Goiânia, 
GO, Brazil 74001-970
4Laboratório de Síntese Química de Materiais LSQM, DEMat, Universidade Federal do Rio Grande do Norte - 
UFRN, P.O. Box 1524, Natal, RN, Brazil 59078-970
5Instituto de Física de São Carlos, Universidade de S. Paulo - USP, Av. Trabalhador São-carlense 400,  
S. Carlos, SP, Brazil  13560-970
*vicodantas@yahoo.com.br
Abstract
PbCrO4 and Pb2CrO5 particles were synthesized by the polymeric precursor method. Structural and microstructural properties of the 
particles were characterized by scanning electron microscopy with field emission gun, X-ray diffraction, and Raman spectroscopy 
techniques. The diffuse reflectance technique was employed to study the optical properties in the 400-700 nm range. The optical 
bandgap of the samples was obtained indirectly. Colorimetric coordinates L*, a*, b* were calculated for the pigment powders 
as a function of the heat treatment (400-700 oC). The powders displayed colors ranging from green to red. X-ray diffraction 
patterns showed the presence of monoclinic PbCrO4 phase in green samples, while red powders had a monoclinic Pb2CrO5 phase 
structure. The Raman spectra of the PbCrO4 and Pb2CrO5 powders were in good agreement with those reported in the literature. The 
synthesized compounds can be used as green and red pigments with high thermal stability. 
Keywords: nanoparticles, PbCrO4, structural properties, microstructural properties, optical properties.
Resumo
Partículas de PbCrO4 and Pb2CrO5 foram sintetizadas pelo método do precursor polimérico. Propriedades estruturais e 
microestruturais das partículas forma caracterizadas por técnicas de microscopia eletrônica de varredura, difração de raios 
X e espectroscopia Raman. A técnica de refletância difusa foi empregada para o estudo das propriedades ópticas na faixa 400-
700 nm. O bandgap óptico foi obtido indiretamente. As coordenadas colorimétricas L*, a*, b* foram calculadas para os pós de 
pigmento em função do tratamento térmico (400-700 oC). Os pós apresentaram cores do verde ao vermelho. Os difratogramas de 
raios X mostraram a presença da fase PbCrO4 monoclínica nas amostras verdes, enquanto os pós vermelhos apresentaram a fase 
Pb2CrO5 monoclínica. Os espectros Raman dos pós de PbCrO4 e Pb2CrO5 estão em concordância com os reportados. Os compostos 
sintetizados podem ser usados como pigmentos verdes e vermelhos com alta estabilidade térmica. 
Palavras-chave: nanopartículas, PbCrO4, propriedades estruturais, propriedades microestruturais, propriedades ópticas.
INTRODUCTION key role in crystal growth, which determines the final crystal 
habit, phase, shape, and structures [3, 4]. The added value 
In recent years, much attention has focused on of a compound depends on its structural, morphological and 
nanostructured systems applied in electronic devices or to optical properties. These properties depend directly on the 
improve mechanical, chemical, structural, optical, electrical crystal structure of the material. Thus, studies relating to 
and magnetic properties of materials [1, 2]. Manipulation the material’s structure, physical properties, and method of 
of the thermodynamic and kinetic control processes plays a synthesis are essential.
119 V. D. Araújo et al. / Cerâmica 61 (2015) 118-125
Lead chromate (PbCrO4) is an important solid material was added to polymerize the citrate by a polyesterification 
that is widely used as a photosensitizer and yellow pigment reaction. The citric acid:metal molar ratio was 6:1, while the 
with a monoclinic P21/n structure [5]. Pb2CrO5 was first used citric acid:ethylene glycol mass ratio was 60:40. The Pb:Cr 
as a dielectric material in 1968 [6]. Dilead pentaoxochromate ratio was adjusted in order to obtain two compositions: 
(Pb2CrO5) with a monoclinic C2/m structure has a large PbCrO4 and Pb2CrO5. The resulting polymeric resin was 
absorption coefficient and a high-speed photo-response then calcined to produce the desired oxide. The puff (first 
in the visible region of the electromagnetic spectrum. calcination) was made at 400 oC/4 h in an air atmosphere.
Therefore, it is a potential candidate for application in After annealing from 500 to 700 °C for 2 h, SEM 
photoconductors, optoelectronic devices, and reversible micrographs of the pigment powders were taken using a 
thermochromism materials [7]. Pb2CrO5 has been found to Zeiss (DSM-940A) scanning electron microscope equipped 
have wide band gap energy (Eg ~ 2.1-2.3 eV) and a large with a field emission gun (SEM-FEG), providing larger than 
absorption coefficient (µ ~ 104 cm-1). The photoresponse and 100,000 X magnification.
performance of Pb2CrO5 devices described in the literature Raman spectra was recorded with a confocal Raman 
suggest that Pb2CrO5 may be classified as a new type of microscope (WiTec, Alpha 300S – CRM 200) equipped 
optoelectronic dielectric material that could potentially be with a piezo scanner and 100 X microscopic objectives 
used in room temperature photoconductors for the visible (N.A.: 0.90; Nikon, Tokyo, Japan). The spatial resolution 
and ultraviolet regions [8]. The synthesis of PbCrO4 and in this experiment reached up to 400 nm. Samples were 
Pb2CrO5 crystals with well-controlled sizes and shapes is excited with an air-cooled Ar ion laser (Melles Griot, model 
crucial due to their potential applications. The literature 35-LAL – 515-230) operating at 514.5 nm with 1 mW of 
describes various procedures to synthesize PbCrO4 and power. Raman data were also obtained on a monochromator 
Pb2CrO5 crystalline phases into different shapes, such equipped with 1800-groove gratings.
as spherical nanoparticles, nanorods, microparticles, The powders were structurally characterized using an 
nanowires, rectangular nanorods and nanotubes [7, 9-15]. automatic X-ray diffractometer (Rigaku, Rotaflex RU200B) 
The reported methods are microemulsion [9], hydrothermal with Cukα radiation (50 kV, 100 mA, l = 1.5405 Å) and in 
[10, 11], self-seeding template growth (SSTG) process [12], a q–2q configuration using a graphite monochromator. The 
microwave-assisted ionic liquid method [7, 13], and room scanning range was between 10 and 75° (2θ), with a step 
temperature reaction of solutions without the additives [14]. size of 0.02° and a step time of 1 s. The Rietveld analysis 
The polymeric precursor route, the Pechini method, was performed with the Rietveld refinement program GSAS 
is a nonconventional method of synthesis that offers the [15]. A pseudo-Voigt profile function was used.
advantage of yielding homogeneous systems with a high The diffuse reflectance and colorimetric coordinates 
degree of purity, which can be calcined at relatively low of the pigments were measured using a spectrophotometer 
temperatures, allowing the synthesis of nanometric oxides (Minolta, CM2600d) in the 400 and 700 nm range, equipped 
with well defined and controlled properties. with standard D65 (daylight) light source, following the 
In this work, PbCrO4 and Pb2CrO5 crystalline particles, CIE-L*a*b* colorimetric method recommended by the 
known as crocoites, were synthesized via a soft chemical CIE (Commission Internationale de l’Eclairage) [16]. Other 
route, the polymeric precursor method, and the structural, optical property, such as the optical bandgap was obtained 
microstructural and optical properties of these crocoites indirectly.
annealed at different temperatures were examined to be used 
as pigments. RESULTS AND DISCUSSION
EXPERIMENTAL The powders obtained in this study presented a broad 
spectrum of colors ranging from green to red. It known that 
The polymeric precursor method is based on the the colors of some compounds containing transition metals 
polymerization of metallic citrate using ethylene glycol. A depend on both the structural arrangements of the elements 
hydrocarboxylic acid such as citric acid is normally used and on the shape and size of the particles. Herewith, we 
to chelate cations in an aqueous solution. The addition of a begin this section evaluating the grain size and morphology 
polyalcohol such as ethylene glycol leads to the formation of the synthesized powders by the SEM-FEG technique.
of an organic ester. Polymerization promoted by heating Fig. 1 presents the SEM-FEG images for the PbCrO4 
to around 100 ºC results a homogenous resin in which the and Pb2CrO5 compositions submitted to different annealing 
metal ions are distributed uniformly throughout the organic temperatures. Both compositions showed strong particle 
matrix. The resin is then calcined to produce the desired coalescence forming spherical agglomerates. In the case of 
oxides. Lead acetate trihydrate, (CH3CO2)2Pb·3H2O, and PbCrO4, the synthesized particles ranged in size from 500 to 
chromium trioxide, CrO3, were used as precursors. The lead 1200 nm, depending on the heat-treatment. The Pb2CrO5, in 
acetate and chromium trioxide were dissolved in water and contrast, displayed small grain sizes of 100 to 300 nm. These 
then mixed into an aqueous citric acid solution (100 °C) differences are due to the high reactivity and low sintering 
under constant stirring The pH of the solution was adjusted energy of PbCrO4 against Pb2CrO5, accelerating the grain 
to 1 with nitric acid. Next, ethylene glycol (HOCH2CH2OH) growth process for PbCrO4. 
V. D. Araújo et al. / Cerâmica 61 (2015) 118-125 120
Figure 1: FEG-SEM images of the PbCrO4 (a, b) and Pb2CrO5 (c, d) after calcination at 400 
oC (a, c) and 700 oC (b, d).
[Figura 1: Imagens obtidas em microscópio eletrônico de varredura de PbCrO4 (a, b) e Pb2CrO5 (c, d) após calcinação 
400 oC (a, c) e 700 oC (b, d).] 
Table I - Raman bands of the samples compared with those reported.
[Tabela I - Bandas Raman das amostras comparadas a valores reportados.]
PbCrO4 PbCrO4 Pb2CrO5 Pb2CrO5 Assignments 
[17] This work [18] This work [17, 18]
853 852 856 850 ν 3 
838 837 848 845 ν 1
- - 839 835 ν 3
825 829 826 823 ν 3
400 401 398 391 ν 4
377 375 377 377 ν 4
359 366 356 353 ν 4
336 344 339 336 ν 2
326 325 322 322 ν 2
Fig. 2 shows the Raman spectra of PbCrO4 and Pb2CrO5 results are presented in Table I, where ν1 and ν3 are the 
samples. According to Wilkins [17] and Frost [18], all symmetric and asymmetric stretching vibrations of CrO 2-4  
vibrational modes of crocoite are Raman-active (1A1 +1E respectively, ν2 and ν4 are the deformation modes for CrO
2
4 . 
+2T2), but only specimens with T2 symmetry are IR-active. Our results were consistent with those of Wilkins [17] and 
All degeneracies were removed from the crocoites, resulting Frost [18], demonstrating the efficiency of the polymeric 
in 9A vibrational modes, all Raman- and IR-active. The Cr-O precursor method to obtain pure PbCrO4 and Pb2CrO5 
distance in crocoite is around 1.65 Å, indicating a substantial phases. The vibrational modes of Pb2CrO5 showed lower 
amount of multiple-character bonds in the chromate ion. This frequencies than those of PbCrO4, indicating that the former 
leads to strong Raman interactions [17]. The comparative has a more compact structure which acts as a barrier against 
121 V. D. Araújo et al. / Cerâmica 61 (2015) 118-125
3.30 respectively, and for the Pb2CrO5 sample 6.74, 4.89 
and 1.92 respectively, where Rwp and Rp are indicators of 
the fit quality of the calculated pattern to the observed data 
and RBragg monitors the improvement in the structure model, 
while Table II compares the values of these parameters with 
those reported in the literature.
The low values of the reliability parameters Rwp and 
RBragg indicate the good quality of the refinements [19]. An 
excellent agreement was observed between the unit cell 
parameters and unit cell volume of the PbCrO4 and Pb2CrO5 
phases determined from the Rietveld refinements and the 
values reported in the literature [20, 21]. This statement is 
also consistent with the Raman results presented earlier.
300 600 900 Following, we present the results and discussions 
Raman Shift (cm-1) about optical properties of the nanopowders to be applied 
Figure 2: Raman shift spectra of the PbCrO  and Pb CrO  as pigments. One of the mechanisms responsible for color 4 2 5
nanometric pigment powders synthesized at 700 oC.
[Figura 2: Espectros Raman de pós de pigmentos 
nanométricos de PbCrO4 e Pb2CrO5 sintetizados a 700 
oC.]
uncontrolled grain growth. Pb2CrO5 therefore has smaller 
particles than PbCrO4, as illustrated in Fig. 1.
Fig. 3 presents the X-ray diffraction patterns of the 
PbCrO4 and Pb2CrO5 compositions calcined at 400 °C for 
4h. The samples annealead at 500 and 600 °C have similar 
diffraction patterns to the calcined at 400 °C and thus are 
not presented here. Figs. 4a and 4b show the Rietveld 
refinement of PbCrO4 and Pb2CrO5 samples, respectively. 
All the reflection peaks in Fig. 4(a) can be indexed easily as 
PbCrO4 monoclinic phase (space group P21/n (14)) (JCPDS 
73-2059), while the peaks in Fig. 4b correspond to Pb CrO  10 15 20 25 30 35 40 45 50 55 602 5
monoclinic phase (space group C2m (12)) (JCPDS 76- 2q (degree)
0861).
The refinement parameters, Rwp, Rp and RBragg for the 
PbCrO4 sample synthesized at 700 
oC are 9.71, 6.49 and 
10 15 20 25 30 35 40 45 50 55 60
2q (degree)
10 20 30 40 50 60
2  (degree) Figure 4: X-ray diffraction patterns and Rietveld q refinement of the (a) PbCrO4 and (b) Pb2CrO5 after 
Figure 3: X-ray diffraction patterns of the PbCrO4 and calcination at 700 ºC/2 h.
Pb2CrO5 after calcination at 400 
oC. [Figura 4: Difratogramas de raios X e refinamento de 
[Figura 3: Difratogramas de raios X de PbCrO4 e Pb2CrO5 Rietveld de (a) PbCrO4 e (b) Pb2CrO5 após calcinação a 
após calcinação a 400 oC.] 700 oC/2 h.]
Intensity Intensity
Intensity Intensity
V. D. Araújo et al. / Cerâmica 61 (2015) 118-125 122
Table II - Values of Rietveld refinement of PbCrO4 and 
Pb2CrO5 compared with those in the literature.
[Tabela II - Valores de refinamento de Rietveld de PbCrO4 e 
Pb2CrO5 comparados com od publicados.]
PbCrO4 PbCrO4 
Parameters Reference(JCPDS Refined
73-2059) (this work)
a  (Å) 7.12 7.12(7)
b  (Å) 7.43 7.43(6)
c  (Å) 6.79 6.79(8)
90.0o 90.0o 400 450 500 550 600 650 700
102.42o 102.48(9)o Wavelength (nm)
90.0o 90.0o
Unit Cell Volume (Å3) 350.8 351.79o
Z 4 4
Space group P 21/n P 21/n
Pb2CrO5 
Parameters Reference
Pb2CrO5 
(JCPDS Refined
76-0861) (This work)
a (Å) 14.001(7) 14.00(3)
b (Å) 5.675(3) 5.67(7)
c (Å) 7.137(5) 7.13(8)
90.0o 90.0o 400 450 500 550 600 650 700
115.22o 115.26(3)o Wavelength (nm)
90.0o 90.0o Figure 5: Diffuse reflectance spectra of a) PbCrO4, and b) 
Unit Cell Volume (Å3) 513.02 513.29 Pb2CrO5 calcined at different temperatures.
Z 4 4 [Figura 5: Espectros de refletância difusa de a) PbCrO4 e b) 
Space group C 2/m C 2/m Pb2CrO5 calcinados a diferentes temperaturas.]
The three absorption bands in Fig. 5a for PbCrO4 at around 
generation is known as charge transfer. This mechanism, 400, 500 and 600 nm are in agreement with those observed 
which consists of the motion of an electron from one by Reddy et al [24]. The first two bands are assigned to the 
transition metal ion to another, produced by the absorption transitions 1t1 g 2e, 6t2 g 2e respectively, and the band at 
of light energy [22], is responsible for the color of PbCrO  600 nm might be due to a forbidden transition.4
and Pb CrO  compounds. Highly charged ions such as It is well known that the morphology and size of materials 2 5
Cr6+ are not favored energetically. Cr6+ ions exert a strong have important effects on their color [25-27]. We believe that 
attraction on electrons, and the movement of some fraction the red shift in diffuse reflectance in Fig. 6 may be attributed 
of one or more electrons from the oxygen back to the central to small differences in particle size and shape with increasing 
ions produces a much more stable arrangement, resulting in annealing temperature as can be seen in Fig. 1.
ligand-to-metal (anion-to-cation) charge transfer transitions Color can be measured by several methods, but in 
and the color of both compounds. For PbCrO  and Pb CrO , ceramics, the most common method to determine the 4 2 5
the result is a broad absorption band at blue and high color of a product is the CIE-L*a*b* [17]. This method 
transmittance for the other wavelengths, leading to an green measures the intensity of diffuse reflectance in the visible 
color for PbCrO  and a red color for Pb CrO  [23]. region to obtain the three colorimetric coordinates L*, 4 2 5
Fig. 5 shows the diffuse reflectance of PbCrO  and a*, b*, yielding the black/white lightness, green/red and 4
Pb CrO  powders. The diffuse reflectance spectra indicate blue/yellow color intensities, respectively, as well as the 2 5
that absorption bands in the 560 nm region of PbCrO  (a total color difference, DE. This parameter DE is defined 4
green powder) and 400-550 nm of Pb2CrO  (a red powder) as the total color difference between the studied material 5
redshift as the annealing temperature increases, as illustrated and a reference sample, i.e., DE2 = DL*2 + Da*2 + Db*2. 
in Figs. 5a and 5b. For each correspondent composition, the sample calcined 
Diffuse reflectance Diffuse reflectance
123 V. D. Araújo et al. / Cerâmica 61 (2015) 118-125
2000 5000
4000
1500
3000
1000
2000
500 1000
0 0
2.20 2.25 2.30 2.35 2.40 2.55 2.50 1.9 2.0 2.1 2.2 2.3 2.4
hn (eV) hn (eV)
Figure 6: Absorption coefficient as a function of incident photon energy in the near band gap region of PbCrO4 (a) and 
Pb2CrO5 (b) powders calcined at different temperatures.
[Figura 6: Coeficiente de absorção em função da energia do fóton incidente na região próxima do bandgap de pós de PbCrO4 
(a) e Pb2CrO5 calcinados em diferentes temperaturas.] 
Table III - Colorimetric coordinates (a*, b* and L*), total color difference (DE) of PbCrO4 and Pb2CrO5, using type D65 
(daylight) light source, following the CIE-L*a*b* standard colorimetric method, and the optical bandgap of the powders as a 
function of the heat treatment.
[Tabela III - Coodenadas colorimétricas (a*, b* e c*) diferença de cor total (DE) de PbCrO4 e Pb2CrO5, usando fonte de luz (luz 
do dia) D65, segundo o método colorimétrico padrão CIE-L*a*b*, e o bandgap óptico dos pós em função do tratamento térmico.]
Sample – Calcination 
Temperature Light Source a* b* L* ΔE Eg (± 0.05 eV)
PbCrO o 2.324 – 400 C D65-10° 10.07 38.17 43.91 -
PbCrO4 – 500
oC D65-10° 11.06 33.75 42.73 4,68 3.31
PbCrO4 – 600
oC D65-10° 10.61 27.86 37.49 12,16 2.28
PbCrO4 – 700
oC D65-10° 10.26 26.85 37.70 12,91 2.27
Pb o 2.162CrO5 – 400 C D65-10° 42.25 49.93 44.92 -
Pb2CrO5 – 500
oC D65-10° 41.26 36.24 41.36 14,18 2.11
Pb o2CrO5 – 600 C D65-10° 35.99 31.17 40.41 20,28
2.06
Pb2CrO5 – 700
oC D65-10° 33.78 28.02 38.53 24,34 2.01
at 400 °C for 4 h was used as reference. Table III presents was obtained indirectly. Considering the high absorption 
the colorimetric coordinates (L*, a*, b*) and total color region, the transmittance T and reflectance R followed a 
difference (DE) of PbCrO  and Pb CrO  nanoparticles simplecorrelation with absorption coefficient:4 2 5
obtained in this work, using type D65-10° (day light) light 
source determined by the CIE-L*a*b* standard colorimetric T = A exp(-ld)    (A)  
method.
The samples’ colorimetric values a*, b* and L* reduced where A is approximately equal to the unity at the absorption 
as the calcination temperature was increased, indicating edge and d is the thickness of the sample. The relation 
that their red color enhanced by increasing the temperature. between the absorption coefficient l and incident photon 
We believe that these changes are related to differences energy hn for allowed direct transition can be written as [28, 
in particle size as the annealing temperature increases, as 29]
shown for the diffuse reflectance above. It is related to 
defects on the as prepared surface nanoparticles. The defects Dhn = A  (hn –E )1/21 g     (B)
on crystalline surface nanoparticles are reduced, once the 
crystallinity increased with the temperature as showed by where A1 is a constant and Eg is the direct bandgap.
DRX results. The (Dhn)2 vs. hn plots for the powders calcined at 
Other optical property, such as the optical bandgap different temperatures are shown in Fig. 6. A linear behavior 
(ahn)2 (eV)2
(ahn)2 (eV)2
V. D. Araújo et al. / Cerâmica 61 (2015) 118-125 124
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