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Elovikov D.P., Osminina A.A. Nanosystems: Phys. Chem. Math., 2024,15 (3), 361-368.
http://nanojournal.ifmo.ru
Original article
DOI 10.17586/2220-8054-2024-15-3-361-368
The role of the reaction medium pH in the formation of nanocrystalline phases in the Bi2O3-P2O5-H2O system
Dmitry P. Elovikov", Alena A. Osminina6
Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia asyncdima@mail.ru, balenaosminina3001@gmail.com Corresponding author: Dmitry P. Elovikov, syncdima@mail.ru
Abstract The work is devoted to studying the influence of pH values in an aqueous-salt medium on the formation of compounds in the Bi2O3-P2O5-H2O(OH-, H+) oxide system. It has been shown that in an acidic environment (pH = 2) at a temperature of 298 K, hexagonal BiPO4 forms, while at pH values of 8 and 12, X-ray amorphous substances are produced. After hydrothermal treatment at 473 K in an aqueous-salt environment, a monoclinic modification of bismuth phosphate forms from hexagonal bismuth phosphate in an acidic environment, and nanometer-sized particles of crystalline compounds Bi3O(OH)(PO4)2 (with a crystallite size of about 62 nm) and Bi2O3 (with a crystallite size of about 70 nm) form in weakly alkaline and alkaline media. Using the method of thermodynamic calculation, the dependences of the equilibrium molar solubility of these crystalline compounds on the pH value of the aqueous-salt suspension were obtained. Thermodynamic calculations showed that the BiPO4 compound is stable in the pH range from 0 to 5.8 at temperatures of 298 and 473 K. The pH range from 5.8 to 9.8 is characterized by the formation of the Bi3O(OH)(PO4)2 compound at 298 K, and a further increase in the pH value leads to the precipitation of Bi2O3, BiOOH or Bi(OH)3, which are similar in solubility, at 298 and 473 K. The data obtained from thermodynamic calculations are consistent with experimental data on the stability boundaries of BiPO4, Bi3O(OH)(PO4)2, and Bi2O3 compounds. Keywords nanocrystals, BiPO4, Bi3O(OH)(PO4)2, influence of pH value, solubility, thermodynamic calculation, hydrothermal synthesis
Acknowledgements The work was supported by state assignment No. 1023032900322-9-1.4.3. Elemental analyses of samples were performed employing the equipment of the Engineering Center of the St. Petersburg State Institute of Technology. The X-ray structural studies of the samples were carried out using the DRON-8 diffractometer at the loffe Institute.
For citation Elovikov D.P., Osminina A.A. The role of the reaction medium pH in the formation of nanocrystalline phases in the Bi2O3-P2Os-H2O system. Nanosystems: Phys. Chem. Math., 2024,15 (3), 361-368.
1. Introduction
Low-temperature chemistry encompasses a wide variety of methods and approaches for synthesizing compounds and producing functional materials based on them [1-6]. Among the widely used and promising methods are precipitation and co-precipitation techniques [7-9], sol-gel methods [10-15], and hydrothermal treatment in various modifications [16-24]. Water and aqueous solutions often serve as the medium for heat and mass transfer during synthesis using these approaches. It is known that the composition of such a medium can significantly influence on the processes of compound formation [25-30], their structure and polymorphic transformations [31-35], the morphology of the resulting particles [36-39], crystallite sizes [40-43], and, consequently, the physicochemical and functional properties of the obtained compounds and materials. In this regard, it is pertinent to study the influence of various factors on the processes of compound formation, such as the composition of the aqueous solution, the concentration of soluble precursors, mineralizing additives, the sequence and conditions of mixing solutions, the presence and state of amorphous precursors [44-49], and the use of specific additional conditions: such as limiting mass transfer [6,50], various physical impacts [51,52], and the organization of reagent solution flows [53-55].
One of the important synthesis parameters reflected in this work is the pH value of solutions and suspensions, as well as its influence on the processes of oxide compound formation. For many low-temperature processes involving bismuth cation and phosphate anions, it is characteristic that a stable bismuth phosphate (with a hexagonal structure) precipitates at certain stages [56]. In some cases, associated with the formation of complex oxide compounds based on bismuth and phosphorus, this leads to kinetic hindrances (undesirable limiting factors of mass transfer) and the stable existence of BiPO4 over a wide pH range [57]. Several studies have focused on the polymorphic transformations of simple compounds in the Bi2O3-P2O5 system [56,58,59] and the targeted synthesis of individual compounds: BiPO4 or Bi2O3. There are mentions of a new and promising compound for use in photocatalysis, Bi3O(OH)(PO4)2, obtained in this system [60,61].
© Elovikov D.P., Osminina A.A., 2024
However, little attention has been paid to the thermodynamic analysis of the formation of complex compounds in the Bi2O3-P2O5-H2O(OH-, H+) system.
Due to the reasons listed above, this work is devoted to studying the influence of the pH value of the solution in the reaction medium on the formation of compounds in the Bi2O3-P2O5-H2O system, as well as the stability boundaries of these compounds, particularly, by using the thermodynamic calculations.
2. Experimental
For the synthesis, two solutions were initially prepared. The first solution was an acidic aqueous solution of bismuth nitrate, obtained by dissolving Bi(NO3)3 • 5H2O (analytical grade) in a 6 M aqueous solution of HNO3 (analytical grade). The second solution was prepared by dissolving a specified amount of NaOH (analytical grade) in 50 ml of distilled water, followed by the addition of a measured amount of (NH4)2HPO4 (analytical grade). The mixing of the two solutions was carried out by the dropwise addition of the acidic bismuth nitrate solution into the alkaline ammonium phosphate solution under continuous stirring. The resulting suspensions with pH values of 2, 8, and 12 were stirred for 10 minutes at T = 298 K.
The hydrothermal treatment of the suspensions obtained at room temperature was carried out in a stainless steel autoclave with a Teflon liner (filling coefficient 0.8) at T = 473 K for 20 hours.
The samples, obtained both by precipitation and after hydrothermal treatment, were decanted by centrifugation (3500 rpm) for 30 minutes, washed with distilled water until pH = 7, dried at T = 353 K for 24 hours, and ground in an agate mortar.
The samples were designated in the work according to the pH of the aqueous-saline suspension during precipitation: those obtained by the precipitation method were labeled as "pH = 2", etc.; those obtained after hydrothermal treatment were labeled as "pH = 2(HTS)", etc.
X-ray diffraction studies of the obtained samples were performed using the DRON-8 (8H) X-ray diffractometer (Russia) with CuKa radiation in the 20 range of 10 - 80°, with a step size of 0.0142° and a scanning speed of 3 °/min. Qualitative analysis of the samples was performed using the ICSD PDF-2 database. The distribution of crystallites by size and the average weighted values of crystallite sizes were determined for the reflections with the maximum intensity.
For X-ray spectral microanalysis of the samples' composition, a scanning electron microscope Tescan Vega 3 SBH (Tescan, Czech Republic) with an Oxford Instruments INCA x-act attachment (United Kingdom) was used. Measurements were conducted in the range up to 20 keV.
3. Calculation
In aqueous-salt solutions saturated with relatively sparingly soluble solids ^-BiPO4, c-Bi2O3, Bi(OH)3, BiOOH, and Bi3O(OH)(PO4)2, the following heterogeneous equilibria are established:
BiPO4(s) ^ Bi3+)+P04(og), (1)
0.5Bi203(s) + l.5H20(0 ^ Bi3+)+30H(oq), (2)
Bi(0H)3(s) ^ Bi3+)+30H(oq), (3)
Bi00H(s)+H20(1) ^ Bi3aq) +30H(aq)' (4)
Bi30(0H) (P04)2(s)+H20(1) ^ 3Bi3+q)+30H(aq)+2P04(aq)- (5)
The thermodynamic constants of the specified heterogeneous equilibria are determined by the expressions:
KS(1) = [aBi3+ • XBi3+ • apo3- • Xpo3-]t = const (T),
KS(2 - 4) = [aBi3+ • Xb
KS (5) = [aBi3+ • X3
3+ • a3OH-]t = const (T),
'3oh- • apO3- • XpO3-]t = const(T),
Bi
where KSS is the thermodynamic solubility constant, XBi3+ and XPO3- - are the mole fractions of free cations Bi3+ and
anions PO4-, respectively. In an aqueous solution, depending on the pH value, the following homogeneous equilibria of water-soluble weak acids and bases are established:
Bi3a+q)+OH(aq) ^ [BKOH)]^) , (6)
Bi3+)+ 2OH(aq) ^ [Bi(OH)2](10+q), (7)
Bi30+q)+3OH(0q) ^ [Bi(OH)3]0oq), (8)
Bi3a+q)+4OH-aq) ^ [Bi(OH)4]1aq), (9)
H3PO4(aq) ^ H(aq) +H2PO— ^ 2H+og) +HPO2(0q) ^ 3H+og) +P°4(og). (10)
Equilibria (6) - (9) are described by the corresponding stability constants of mononuclear hydroxo complexes of Bi(III) in an aqueous medium: pi, p2, P3, P4 = f (T).
Equilibria (10) are described by the corresponding constants of the three-step dissociation of phosphoric acid: Ki, K2, K3 = f (T).
The necessary values of the Gibbs free energy of formation for compounds were taken from the HSC 6.0 database. The Gibbs free energy of formation of the compound Bi3O(OH)(PO4)2 was calculated using the increment method based on data from reference [62]. Using these values, changes in the Gibbs free energy of processes were calculated, and subsequently, equilibrium constants, stability constants of mononuclear hydroxo complexes, and dissociation constants were calculated. By means of mathematical transformations described earlier in reference [63], the equilibrium molar solubility (S, mol/L) of sparingly soluble BiPO4, Bi2O3, Bi(OH)3, BiOOH, and Bi3O(OH)(PO4)2 in the aqueous-salt suspension was calculated. The calculation was performed taking into account the mole fractions of water-soluble bound and free ions (equilibria (6) - (10)), the ionic strength of the solution (calculated for each composition point), the temperature dependence of the water autoprotolysis constant, and the temperature dependence of equilibria (1) - (4) and (6) - (10). For the calculation at 473 K, it was assumed that water is only in the liquid state.
4. Results and discussion
The results of X-ray diffraction studies of the obtained samples showed that the pH value and temperature play an important role in the formation processes of compounds in the Bi2O3-P2O5-H2O system. Thus, under precipitation conditions at room temperature (298 K) and a pH value of 2 of the aqueous-salt suspension, the appearance of reflections of crystalline BiPO4 with a hexagonal ximengite-like structure can be observed (Fig. 1). With an increase in pH to 8 and 12, well-defined reflections of the crystalline compound are absent (Fig. 1a). This indicates the X-ray amorphous state of the obtained samples. Hydrothermal treatment at 473 K for 20 h of the aqueous-salt suspensions obtained at room temperature leads to the formation of new crystalline compounds (Fig. 1b). The results of elemental analysis of the obtained samples (Table 1) satisfy the stoichiometry of the crystalline compounds within the error of determination of Bi and P atoms.
Fig. 1. Diffraction patterns obtained under different conditions (pH, t) of samples. a - 298 K, b - 473 K
During isothermal treatment under hydrothermal conditions of the aqueous-salt suspension with a pH of 2, containing particles of sparingly soluble crystalline BiPO4, a restructuring of the structure from hexagonal to a low-temperature monoclinic modification is observed, which is reflected in the change in the topology of the reflections (Fig. 1).
It is worth noting that the average crystallite sizes (weighted average) increase from 75 to 200 nm, while the average sizes of aggregated particles (length) increase from 1 to 1.5 ^m. This indicates that even at room temperature under these conditions, there is active crystallite growth and the formation of large aggregates, the treatment of which under hydrothermal conditions leads to further growth in crystallite sizes. The similarity in morphology and particle sizes (on average 0.3 and 0.4 ^m in width) may indicate that the polymorphic transformation h-BiPO4 ^ m-BiPO4 occurs via a martensitic mechanism. The subsequent prolonged particle growth (on average 0.03 ^m/h, Fig. 2) primarily affects the change in particle length and may be due to mass transfer processes limited in speed by the low ion concentration in the solution. This polymorphic transition was also noted in paper [59].
Hydrothermal treatment of aqueous-salt suspensions containing amorphous substances, obtained at room temperature under precipitation conditions at pH 8 and 12, leads to the formation of nanocrystalline compounds Bi3O(OH)(PO4)2 with a petitjeanite-like structure and c-Bi2O3 doped with phosphorus, respectively (Fig. 1). The weighted average particle sizes for the Bi3O(OH)(PO4)2 compound are approximately 200 nm for particles and approximately 62 nm for crystallites
D.P. Elovikov, A.A. Osminina Table 1. Results of X-ray diffraction studies and elemental analysis
Designation of samples Crystalline phase / Chemical formula in oxide form Elemental analysis Chemical formula in oxide form according to elemental analysis
Bi P
pH = 2 h-BiPO4 (Ximengite-like structure) 0.5[Bi2O3-P2 O5 ] 50±1 50±2 0.5[Bi2 O3-P2O5 ]
pH = 8 X-ray amorphous 70±1 30±2 O.7B12O3-O.3P2O5
pH = 12 X-ray amorphous 88±1 12±2 0.9Bi2O3-O.IP2O5
pH = 2(HTS) m-BiPO4 0.5[Bi2O3-P2 O5 ] 51 ± 1 49±2 0.5[Bi2 O3-P2O5 ]
pH = 8(HTS) Bi3O(OH)(PO4 )2 (Petitjeanite-like structure) 1.5[Bi2 O3-O.7P2 O5-O.3H2 O] 63±1 37±2 1.5[Bi2O3-O.6P2O5 ]
pH = 12(HTS) C-Bi3.69P0.31 O6.31 1.8[Bi2O3-O.O9P2O5 ] 87±1 13±2 1.8[Bi2O3-O.3P2O5 ]
Fig. 2. Microphotographs of particles and lognormal dependences of the size distribution of particles and crystallites
(Fig. 2), indicating that the vast majority of particles are aggregates consisting of 30 - 40 interconnected crystallites. Spherical particles of the c-Bi2O3 compound, obtained by hydrothermal treatment of the X-ray amorphous precursor (Fig. 1b), are comparable in size to the crystallite sizes (weighted average is about 70 nm).
Thermodynamic calculations (Fig. 3) show that the compound BiPO4 is formed and stable in the oxide system Bi2O3-P2O5-H2O(OH-, H+) within the pH range of 0 - 5.8 at temperatures of 298 and 473 K. This is consistent with the results obtained at pH = 2 in this study and with the results of obtaining BiPO4 in acidic environments in several other studies [56,57,64,65]. It also aligns with the findings of study [66] where the formation of BiPO4 was considered in the pH range of 0.5 - 4.0 at 298 K (mention of a white homogeneous mass) and 433 K. From pH 5.8 to 9.8, the most stable compound at 298 K is Bi3O(OH)(PO4)2. In this study, the compound Bi3O(OH)(PO4)2 was obtained by isothermal treatment under hydrothermal conditions of an amorphous substance with the following composition: 0.7Bi2O3-0.3P2O5 in an aqueous-salt environment with a pH of 8. In the work of other authors, this compound was obtained at pH = 10, and in another study [61], the authors mention that they obtained it at a pH greater than 10. In this case, the calculation agrees with the data obtained in this study and satisfactorily agrees with the data from the works [60,61] within the error of calculating the Gibbs free energy of formation of Bi3O(OH)(PO4)2. The dashed line in Fig. 3 shows the region where freshly precipitated amorphous bismuth hydroxide, which is thermodynamically unstable, forms. At pH values greater than 10, bismuth oxides, hydroxides, and oxyhydroxides are approximately equally stable. However, according to the calculation, the lowest solubility is characteristic of bismuth hydroxide. In this study, at pH = 12, bismuth oxide doped with phosphorus of the composition 1.8Bí203-0.5P205 was obtained. Apparently, this compound forms because in these conditions, the solution is dominated by water-soluble unbound phosphate anions. This fact may facilitate the incorporation of phosphorus into the structure. It is worth noting that the amorphous precursors also contain phosphorus atoms (Table 1).
5. Conclusions
It has been shown that the pH value of the medium significantly influences the formation of compounds in the aqueous-salt oxide system Bi2O3-P2O5-H2O(OH-, H+). Thus, at low pH values (pH = 2) and room temperature, hexagonal BiPO4 is formed, while weakly alkaline and alkaline conditions promote the formation of X-ray amorphous substances. Hydrothermal treatment at 473 K leads to the polymorphic transformation of bismuth phosphate from hexagonal to monoclinic modification, as well as the formation of nanosized particles of crystalline compounds Bi3O(OH)(PO4)2 (with a crystal size of about 62 nm) and Bi2O3 (with a crystal size of about 70 nm) from amorphous precursors. Elemental analysis confirmed the correspondence of the chemical composition of the obtained crystalline compounds. By thermodynamic calculation, dependencies of the equilibrium molar solubility of identified compounds on the pH value of the aqueous-salt suspension were obtained, and for the compound Bi3O(OH)(PO4)2, such a dependence was obtained for the first time. Thermodynamic calculation showed that the compound BiPO4 is stable in the pH range from 0 to 5.8 at temperatures of 298 and 473 K. The pH range from 5.8 to 9.8 is characterized by the stability of the compound Bi3O(OH)(PO4)2 at 298 K, while further increasing the pH leads to the precipitation of compounds with similar solubility, such as Bi2O3, BiOOH, or Bi(OH)3 at 298 and 473 K. Overall, the thermodynamic calculation is in satisfactory agreement with the experimental data obtained.
Fig. 3. Dependence of the equilibrium molar solubility (S, mol/L) of solid BiPO4, Bi(OH)3, BiO15, BiOOH and Bi3O(OH)(PO4)2 in a aqueous-salt suspension on pH at temperatures 298 and 473 K
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Submitted 29 April 2024; revised 30 May 2024; accepted 3 June 2024
Information about the authors:
Dmitry P. Elovikov - Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia, 197022, St. Petersburg, Russia; ORCID 0000-0003-4345-6086; syncdima@mail.ru
Alena A. Osminina - Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia, 197022, St. Petersburg, Russia; ORCID 0009-0008-8804-4732; alenaosminina3001@gmail.com
Conflict of interest: the authors declare no conflict of interest.
