Polymorph and Phase Discrimination of Lead Chromate Pigments by a Facile Room Temperature Precipitation Reaction Jun-Hua Xiang,† Shu-Hong Yu,*,† and Zili Xu‡
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1311-1315
Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, Structural Research Laboratory of CAS, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China, and Department of Materials Engineering, Beijing Institute of Petro-chemical Technology, 102617, Beijing, P. R. China Received July 8, 2004;
Revised Manuscript Received August 5, 2004
ABSTRACT: Monoclinic and orthorhombic PbCrO4, Pb2CrO5, and K2Pb(CrO4)2 nanocrystallites with different shapes can be selectively synthesized by a room temperature solution reaction without adding any additives. The results demonstrated that the manipulation of pH value and the molar ratio of Pb(Ac)2 and K2CrO4 can effectively affect the formation of polymorphs and phases. This facile room temperature approach provides a useful route for the selective synthesis of lead chromate pigments with different phases and polymorphs, a method which could be extended for other inorganic systems. 1. Introduction PbCrO4 is called crocoite and is usually used as a yellow pigment, which is usually monoclinic P21/n structure and is also used as photosensitizer.1 Dilead pentaoxochromate Pb2CrO5 called phoenicochroite, in the system PbO-chromium oxide,2 is described as a photoconductive dielectric material because of its centrosymmetry in the monoclinic C2/m space group. Pb2CrO5 has been found to have a wide band gap energy Eg (∼2.1-2.3 eV), a large absorption coefficient R (∼104-105 cm-1), and a high photoresponse speed in this spectral region. The observed data on the photoresponse and performance of Pb2CrO5 devices suggest that Pb2CrO5 may be categorized as a new type of optoelectronic dielectric material that could be utilized potentially in room temperature photoconductors in the visible and ultraviolet regions of the optical spectrum.3 In the previous work, some methods have been employed to synthesize lead chromate. By solid solution reaction of PbO and Cr2O3, Pb2CrO5 thin films can be prepared,4 and Pb2CrO5 ceramic disks have been synthesized by calcination of a mixed powder composed of PbO and Cr2O3.5 PbCrO4 nanocolloids have been prepared in H2O/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane water-in-oil (w/o) microemulsion medium.6 Recently, PbCrO4 nanorods and Pb2CrO5 microparticles were separately synthesized by a hydrothermal method in the presence of surfactant of poly(vinyl pyrrolidone) at pH 7 and greater than 14 at 140 °C for 20 h.7 Manipulation of the thermodynamic and kinetic control processes plays a key role in crystal growth, which determines the final crystal habit, phase, shape, and structures.8,9 Recently, we have shown that a mild solution method can be adopted for the selective syn* To whom correspondence should be addressed. E-mail: shyu@ ustc.edu.cn; fax: 0086 551 3603040. † University of Science and Technology of China. ‡ Beijing Institute of Petro-chemical Technology.
thesis of orthorhombic and hexagonal CeOHCO3, and cubic CeO2 crystals in an ethanol-water mixed solution.10 To our knowledge, a simple solution method has not been used to discriminate different phases and polymorphs of lead chromate at room temperature. In this paper, we report a systematic synthesis of lead chromates nanocrystals with different polymorphs and phases such as monoclinic PbCrO4, orthorhombic PbCrO4, Pb2CrO5, and K2Pb(CrO4)2 by a facile precipitation reaction without any additives at room temperature. The results show that the pH value and the molar ratio of the Pb2+ and CrO42- have a significant influence on the discrimination of polymorphs and phases of the lead chromate. 2. Experimental Section All chemicals were analytically pure and were used as received. A total of 5 mL of 0.2 M K2CrO4 aqueous solution was separately added into 5, 25, and 50 mL of 0.2 M Pb(Ac)2 aqueous solutions under magnetic stirring, which made the molar ratio of Pb2+ to CrO42- of 1, 5, and 10, respectively. The pH values of the mixtures were adjusted to 5 or 11 using HAc and NaOH (1 mol L-1). Similarly, 5 mL of 0.2 M Pb(Ac)2 aqueous solution was separately added into 5, 25, and 50 mL of 0.2 M K2CrO4 aqueous solution under magnetic stirring, which made the molar ratio of CrO42- to Pb2+ of 1, 5, and 10. The pH values of the mixtures were also adjusted to 5 or 11 using HAc and NaOH (1 mol L-1). The resulting mixtures were aged without stirring at room temperature for one month. Then the obtained products were centrifuged and washed several times with distilled water and absolute ethanol and finally dried in a vacuum at 60 °C for 6 h. The products were characterized by their X-ray diffraction (XRD) pattern, recorded on a MAC Science Co. Ltd. MXP 18 AHF X-ray diffractometer with monochromatized Cu KR radiation (λ ) 1.54056 Å). The TEM images of the samples were obtained from a Hitachi (Tokyo, Japan) H-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV.
3. Results and Discussion Polymorph Selective Synthesis of PbCrO4. The simple mixing of two solutions containing the same
10.1021/cg049777k CCC: $27.50 © 2004 American Chemical Society Published on Web 09/15/2004
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Figure 1. XRD patterns of PbCrO4 with different polymorphs obtained at pH 5 and at room temperature after aging the mixing solution for one month. Initial concentration of [Pb(Ac)2], and [K2CrO4] was 0.2 M, respectively. The molar concentration of [Pb2+]/[CrO42-] was kept as (a) [Pb2+]/[CrO42-] ) 1:1; (b) [Pb2+]/[CrO42-] ) 5:1; (c) [Pb2+]/[CrO42-] ) 10:1.
molar concentration of Pb2+ and CrO42- at pH 5 results in the formation of pure phase of PbCrO4 as confirmed by the XRD pattern in Figure 1a. All reflection peaks in Figure 1a can be easily indexed as a monoclinic phase (space group P21/n (14)) of PbCrO4 with cell parameters a ) 7.15 Å, b ) 7.42 Å, c ) 6.80 Å, and β ) 103.1°, which fit well with the literature value (JCPDS Card number: 08-0209). The equation of the reaction between Pb(Ac)2 and K2CrO4 can be expressed as follows:
Pb(Ac)2 + K2CrO4 f PbCrO4V + 2KAc
(1)
A TEM image (Figure 2a) showed that the products with an orange color collected after aging for two weeks are elongated rods with a length about 450-700 nm and diameter about 200-300 nm. Prolonging the aging time up to 28 days results in the formation of hollow structures as demonstrated in Figure 2b. The electron diffraction pattern in Figure 2c indicates that the particles are well-crystallized single crystals. When the molar ratio of Pb2+ to CrO42- increases to 5 or 10 and the pH value was still kept as 5, the products were also pure phase as shown in Figure 1b,c, respectively. All reflection peaks in Figure 1b,c can be
easily indexed as a pure orthorhombic phase (space group Pnma (62)) of PbCrO4 with cell parameters a ) 8.66 Å, b ) 5.58 Å, and c ) 7.11 Å (JCPDS Card number: 74-0812). A TEM image of the products obtained at the molar ratio of 5 for one month (Figure 3a) showed that the products were PbCrO4 nanocrystals with an average size of about 50-100 nm. The products obtained at the molar ratio of 10 were PbCrO4 nanocrystals with an average size of about 40-80 nm as the TEM image showed (Figure 3b). The color of orthorhombic PbCrO4 is bright yellow. The results underline that the key role of the molar ratio played in the polymorph discrimination of PbCrO4. Phase Selective Synthesis of Lead Chromates. The variation of the molar ratio of CrO42- to Pb2+ and the control of the pH value can effectively result in the formation of different phases of lead chromates. When the molar ratio of CrO42- to Pb2+ was 5 or 10 at pH 5, the yellow products were a mixture of monoclinic PbCrO4 and hexagonal K2Pb(CrO4)2 phase as shown in Figure 4. The reaction equation can be expressed as follows:
2Pb(Ac)2 + 3K2CrO4 f PbCrO4V + K2Pb(CrO4)2V + 4KAc (2) TEM images (Figure 5) showed that the products were nanocrystals with an average size of about 200 nm, which are attached on the nanorods with a length of about 2-5 µm and a diameter of about 100-1000 nm when the molar ratio of CrO42- to Pb2+ was 5 at pH 5. Obviously, increasing CrO42- concentration will favor the formation of the K2Pb(CrO4)2 phase. When the equal molar concentration of CrO42- and Pb2+ was used at pH 11, the product was a mixture of monoclinic Pb2CrO5 and hexagonal K2Pb(CrO4)2 (Figure 6a). Increasing the pH value to 12, the products were pure monoclinic Pb2CrO5 (Figure 6b). The reflection peaks in Figure 6b can be indexed as a monoclinic phase (space group C2/m (12)) of Pb2CrO5 with cell parameters a ) 13.94 Å, b ) 5.664 Å, c ) 7.092 Å, and β ) 114.96° (JCPDS Card number: 84-0678). TEM images in Figure 7 showed that the product was composed of nanorods with a diameter of about 50-80 nm and a length of about 150-300 nm, which are attached on the hexagonal nanoflakes. The
Figure 2. TEM images of monoclinic PbCrO4 nanocrystals obtained at pH 5 and at room temperature. Initial concentration of [Pb(Ac)2] and [K2CrO4] was 0.2 M, respectively. The molar concentration of [Pb2+]/[CrO42-] was kept as 1:1. (a) Two weeks; (b) four weeks; (c) electron diffraction pattern of PbCrO4 nanocrystals shown in (b). The scale bar is, respectively, 300 and 500 nm.
Polymorph Discrimination of Lead Chromate Pigments
Figure 3. TEM images of orthorhombic PbCrO4 nanocrystals obtained at pH 5 and at room temperature for one month. Initial concentration of [Pb(Ac)2] and [K2CrO4] was 0.2 M, respectively. The molar concentration of [Pb2+]/[CrO42-] was kept as (a) [Pb2+]/[CrO42-] ) 5:1; (b) [Pb2+]/[CrO42-] ) 10:1. The scale bar is, respectively, 100 and 125 nm.
Figure 4. XRD patterns of monoclinic PbCrO4 and hexagonal K2Pb(CrO4)2 phases obtained at pH 5 and at room temperature after aging the solution for one month. Initial concentration of [Pb(Ac)2], and [K2CrO4] was 0.2 M, respectively. The molar concentration of [CrO42-]/[Pb2+] was kept as (a) [CrO42-]/[Pb2+] ) 5:1; (b) [CrO42-]/[Pb2+] ) 10:1; 9: K2Pb(CrO4)2.
color of the product is salmon pink. The reactions at pH 11 and 12 were illustrated in eqs 3 and 4, respectively.
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Figure 5. TEM images of the product composed of monoclinic PbCrO4 and hexagonal K2Pb(CrO4)2, which is obtained at pH 5 and at room temperature for one month. Initial concentration of [Pb(Ac)2] and [K2CrO4] was 0.2 M, respectively. The molar concentration of [CrO42-]/[Pb2+] was kept as 5:1. The scale bar is 500 nm.
Figure 6. XRD patterns of Pb2CrO5 and K2Pb(CrO4)2 obtained at room temperature for one month. Initial concentration of [Pb(Ac)2] and [K2CrO4] was 0.2 M, respectively. The molar concentration of [CrO42-]/[Pb2+] was kept as (a) [CrO42-]/[Pb2+] ) 1:1, Pb2CrO5 + K2Pb(CrO4)2, pH 11; (b) [CrO42-]/[Pb2+] ) 1:1, Pb2CrO5, pH 12; (c) [CrO42-]/[Pb2+] ) 5:1, K2Pb(CrO4)2, pH 11; (d) [CrO42-]/[Pb2+] ) 10:1, K2Pb(CrO4)2, pH 11; +: K2Pb(CrO4)2; 9: Pb2CrO5.
3Pb(Ac)2 + 3K2CrO4 + 2NaOH f Pb2CrO5V + K2Pb(CrO4)2V + 4KAc + 2NaAc + H2O (3)
8b). When the molar ratio of Pb2+ to CrO42- was 5:1 at pH 11, the reaction occurred can be expressed as follows:
2Pb(Ac)2 + K2CrO4 + 2NaOH f Pb2CrO5 + 2KAc + 2NaAc + H2O (4)
5Pb(Ac)2 + 10NaOH + 2CO2 f PbO‚H2OV + PbOV + Pb3(CO3)2(OH)2V + 10NaAc + 3H2O (5)
When the molar ratio of Pb2+ to CrO42- was kept as 5:1 at pH 11, the obtained product was a mixture of lead oxide hydrate (PbO‚H2O), lead carbonate hydroxide (Pb3(CO3)2(OH)2), and lead oxide (PbO) as shown in Figure 8a. The formation lead carbonate hydroxide is due to the long-term (one month) exposure in air and the dissolution of CO2 from air into the solution. When the molar ratio of Pb2+ to CrO42- was 10 at pH 11, the product was a mixture of lead hydroxide oxide (Pb3O2(OH)2), lead carbonate hydroxide, and lead oxide (Figure
When the molar ratio of Pb2+ to CrO42- was 10:1 at pH 11, the reaction can be expressed as eq 6:
4Pb(Ac)2 + K2CrO4 + 6NaOH f PbCrO4V + Pb3O2(OH)2V + 6NaAc + 2KAc + 4H2O (6) Pure K2Pb(CrO4)2 phase can be obtained when the molar ratio of CrO42- to Pb2+ was 5 and 10 at pH 11 (Figure 6c,d), which has a bright yellow color. All
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Figure 7. TEM images of Pb2CrO5 nanorods obtained at pH 11 and at room temperature for one month. Initial concentration of [Pb(Ac)2] and [K2CrO4] was 0.2 M, respectively. The molar concentration of [CrO42-]/[Pb2+] was kept as 1:1. The scale bar is, respectively, 250 nm, 1 µm, and 200 nm.
Figure 8. XRD patterns of the crystals obtained at pH 11 and at room temperature after aging for one month. Initial concentration of [Pb(Ac)2] and [K2CrO4] was 0.2 M, respectively. The molar concentration of [Pb2+]/[CrO42-] was kept as (a) [Pb2+]/[CrO42-] ) 5:1; (b) [Pb2+]/[CrO42-] ) 10:1; #: PbO‚ H2O; 9: Pb3(CO3)2(OH)2; *: Pb3O2(OH)2; +: PbO.
Figure 9. TEM images of K2Pb(CrO4)2 nanocrystals obtained at pH 5 and at room temperature for one month. Initial concentration of [Pb(Ac)2], and [K2CrO4] was 0.2 M, respectively. The molar concentration of [CrO42-]/[Pb2+] was kept as (a) [CrO42-]/ [Pb2+] ) 5:1; (b) [CrO42-]/[Pb2+] ) 10:1. The scale bar is 500 nm.
reflection peaks in Figure 6c,d can be easily indexed as a pure hexagonal phase (space group R3m (166)) of K2Pb(CrO4)2 with cell parameters a ) 5.72 Å and c ) 21.05 Å (JCPDS Card number: 29-1007). The reaction equation was as follows:
Scheme 1. Sketch Map of the Reaction between Pb(Ac)2 and K2CrO4 at Room Temperaturea
Pb(Ac)2 + 2K2CrO4 f K2Pb(CrO4)2V + 2KAc (7) TEM images (Figure 9) showed the obtained K2Pb(CrO4)2 was nanocrystals with an average size of about 250 nm when the molar ratio of CrO42- to Pb2+ was 5 (Figure 9a) and about 400 nm when the molar ratio of CrO42- to Pb2+ was 10 (Figure 9b). On the basis of above experimental results, the general schematic illustration of the polymorph and phase synthesis of the various lead chromate compounds can be expressed in Scheme 1. Above results demonstrated that the manipulation of the relative molar ratio of Pb2+ and CrO42-, and the pH value of the reaction solution, provides an efficient way to synthesize lead chromate pigments with different polymorphs and phases. Further optimization of the reaction conditions could further improve the quality of these nanoparticles, which could find potential applications in the pigment industry and the photoconductor field.
a The ratios on the map denote the mol ratio of Pb(Ac) to 2 K2CrO4.
The optical properties of the lead chromate crystals were studied by UV-vis absorption spectroscopy as shown in Figure 10. The spectra showed a typical broad absorption band in the range from 400 to 700 nm, with the strongest peak located at 512, 494, and 484 nm for the monoclinic and orthorhombic phase lead chromate, respectively (Figure 10a,b,c). The slight blue shift of the adsorption band in Figure 10b,c could be related to the
Polymorph Discrimination of Lead Chromate Pigments
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reaction solution can selectively synthesize other phases of lead chromates such as Pb2CrO5, K2Pb(CrO4)2. These pigments could find applications in color decoration or photoconductors. The present route represents a useful way for the selective synthesis of various chromate compounds, which could be extended for controlling the formation of other phases or polymorphs in other inorganic systems. Acknowledgment. This work was supported by the special funding support from the Centurial Program of the Chinese Academy of Sciences, the Distinguished Youth Fund (Contract No. 20325104), the Distinguished Team (Grant No. 20321101), and Contract No. 50372065 from the Natural Science Foundation of China. Figure 10. UV-vis absorption spectra of the products obtained under different conditions at pH 5. (a) [Pb2+]/[CrO42-] ) 1:1, monoclinic PbCrO4; (b) [Pb2+]/[CrO42-] ) 5:1, orthorhombic PbCrO4; (c) [Pb2+]/[CrO42-] ) 10:1, orthorhombic PbCrO4; (d) [Pb2+]/[CrO42-] ) 1:5, a mixture of monoclinic PbCrO4 and K2Pb(CrO4)2; (e) [Pb2+]/[CrO42-] ) 1:10, a mixture of monoclinic PbCrO4 and K2Pb(CrO4)2.
size effect of the orthorhombic nanocrystals. The mixed phases of monoclinic PbCrO4 and K2Pb(CrO4)2 showed similar absorption characteristics as that of pure monoclinic PbCrO4 (Figure 10d,e). 4. Conclusions In summary, it has been demonstrated that the polymorphs and phases of lead chromates can be selectively synthesized by a simple precipitation reaction at room temperature. The relative concentration of Pb2+ and CrO42- plays a key role in the selective synthesis of the monoclinic phase orthorhombic phase of PbCrO4. The further control of the molar ratio of Pb2+ to CrO42- as well as the control of the pH value of the
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