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The rapid synthesis of nanostructured orthorhombic KNbO3 particles by a microwave-assisted hydrothermal method and their characterization
Author’s Accepted Manuscript The rapid synthesis of nanostructured orthorhombic KNbO3 particles by a microwave-assisted hydrothermal method and their characterization Tiago Bender Wermuth, Mario Norberto Baibich, Tania Maria Hubert Ribeiro, Carlos Pérez Bergmann www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)32756-6 https://doi.org/10.1016/j.ceramint.2017.12.060 CERI16940
To appear in: Ceramics International Received date: 11 November 2017 Revised date: 5 December 2017 Accepted date: 7 December 2017 Cite this article as: Tiago Bender Wermuth, Mario Norberto Baibich, Tania Maria Hubert Ribeiro and Carlos Pérez Bergmann, The rapid synthesis of nanostructured orthorhombic KNbO3 particles by a microwave-assisted hydrothermal method and their characterization, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The rapid synthesis of nanostructured orthorhombic KNbO3 particles by a microwave-assisted hydrothermal method and their characterization Tiago Bender Wermutha*; Mario Norberto Baibichb; Tania Maria Hubert Ribeiroa; Carlos Pérez Bergmanna a
Laboratório de Materiais Cerâmicos, Departamento de Materiais, Universidade Federal do Rio Grande do Sul – UFRGS. Osvaldo Aranha 99, Porto Alegre, RS, 90035-190, Brazil b Laboratório de Supercondutividade e Magnetismo, Instituto de Física, Universidade Federal do Rio Grande do Sul – UFRGS. Avenida Bento Gonçalves 9500, Porto Alegre, RS, 91501-970, Brazil a
* e-mail address: [email protected]
Abstract We studied the molar ratio effects of niobium and potassium precursors on the structure and morphology of potassium niobate powders prepared via microwave-assisted hydrothermal synthesis (MaHS). KNbO3 nanostructures in the form of nanotowers and nanocubes were obtained at reduced synthesis times (30 min to 240 min). The products were characterized via XRD, Raman spectroscopy, SEM, and TEM; band gap calculations used diffuse reflectance data. The results indicate that KNbO3 nanostructures were obtained with crystallite sizes ranging from 33 to 52 nm. An orthorhombic crystalline structure was formed from the increase of KOH at a molar ratio Nb2O5:KOH (1:8 to 1:16 M). The band-gap of 3.1 eV to 3.3 eV has potential use in photodegradation applications.
Keywords: Fast synthesis; potassium niobate; nanostructured, microwaveassisted hydrothermal synthesis; morphology
Introduction Niobium perovskites (ANbO3, A = Na, K, Ag, Cu) and especially potassium niobate (KNbO3) have attracted great interest due to their nonlinear optical properties, ferroelectricity, piezoelectricity, pyroelectricity, and photocatalytic properties[1-17]. Numerous methods of synthesis have been used to obtain
KNbO3, including the sol-gel method[18-22], solid-state reaction[21,
23-26]
,
hydrothermal synthesis[8, 9, 21, 27-32]. The hydrothermal synthesis enables the production of binary oxides (ZnO, CuO, MgO, TiO2, SnO2), ternary compounds (BaTiO3, PbTiO3, BiFeO3, KNbO3), and complex
compounds
[33] 300Ca0.375MnO3)
(Ba1-xSrxTiO3,
La0.5Ca0.5MnO3,
La0.325Pr0.
at low temperatures. It also allows the synthesis of materials
with high crystallinity, high purity, and low particle aggregation[34]. Recent studies reported different morphologies (nanowires, nanotowers, nanocubes, and nanotubes) of KNbO3 (orthorhombic and tetragonal structure) via the hydrothermal method. In these works, KNbO 3 was obtained by varying the precursor quantity, duration time (12 hours to 7 days), and synthesis temperature (150 to 250ºC)[21,
35-37]
. Although the hydrothermal method is
already being studied for the synthesis of these materials, one of its main limitations is the long time required (many hours or days)[34]. On the other hand, microwave-assisted hydrothermal synthesis (MaHS) can produce materials with similar characteristics to those obtained by conventional hydrothermal synthesis, but in a much shorter time. This method offers a high heating rate, which leads to a better precursor dissolution. The microwave energy promotes non-uniform distribution of energy inside the reactor to heat the system efficiently and rapidly. Consequently, the reaction rate increases[38] and accelerates the crystallization process[39,
40]
. This method allows greater
control of the products’ microstructural characteristics and final properties[34]. Studies involving KNbO3 MaHS are rare in the literature
[41]
, and additional
studies are critical. For the first time, this work offers a systematic investigation of the molar ratio of precursors (KOH and Nb2O5) and how this ratio affects the
morphology of KNbO3 obtained by MaHS. The crystallinity and morphology of KNbO3, obtained via this rapid synthesis were evaluated through reaction time studies. In addition, this study offers important information for promising applications in photocatalysis and/or other relevant areas. Material and methods The raw materials were niobium pentoxide(i), potassium hydroxide(ii), Milli-Q water(iii), and ethyl alcohol(iv). Stoichiometric molar ratios (Nb2O5:KOH – 1:2) and non-stoichiometric molar ratios (Nb2O5:KOH – 1:4; 1:8; 1:12; 1:16) were studied. The potassium hydroxide was slowly dissolved in Mili-Q H2O preheated at 50°C, and the mixture was homogenized for 30 min using a magnetic stirrer. Subsequently, the Nb2O5 was added and stirred for another 30 min for complete homogenization. The resulting suspension was microwaved(v) in a Teflon-coated flask. The total volume of the suspensions prepared was 20 mL. Preliminary studies showed that KNbO3 does not form below 200 °C. The synthesis was performed at different times (30, 60, 120 and 240 min) at 200 °C. After the reaction, the system was cooled to room temperature, and the product was centrifuged(vi) and washed with Mili-Q water and ethanol until the pH was stabile. The product was then dried(vii) at 60 °C for 12 hours.
(i)
Nb2O5, 99,8% of purity, CBMM – Companhia Brasileira de Metalurgia e Mineração KOH, 85% min., K2CO3 2,0% Max., Dinâmica (iii) σ = 0,054 μs/cm a 25°C (iV) 99,5%, Dinâmica, Brasil (v) MDS-8G, Shanghai Sineo Microwave Chemistry Technology Co., Ltda (vi) 9000 Rpm, 10 min., H2050R, Quimis, Brasil (vii) Jung, JV 0990 (ii)
Crystalline phases were determined via X-ray diffraction (XRD) with Philips equipment(viii) using Cu Kα radiation (λ = 1.54184 A) operating at 40 kV and 40 mA with a 2θ of 10-90°. The average crystallite size was calculated via the Scherrer equation[42]. Raman spectroscopy measurements were performed (ix) using a laser wavelength of 532 nm. The morphological characteristics of the synthesized KNbO3 were evaluated with scanning electron microscopy(x) (SEM) and transmission electron microscopy(xi) (TEM) with an intensity of 80 kV and high-resolution transmission electron microscopy(xii) at 200 kV. Light absorption curves of the KNbO3 were obtained using diffuse reflectance spectrophotometry(xiii) (UV-Vis) with an integrating sphere(xiv). The band gap energy was determined using the Kubelka – Munk function [43]. Results and Discussion Figure 1 shows XRD patterns of the products from MaHS under both stoichiometric [1:2] and non-stoichiometric conditions as function of KOH concentrations [1:4], [1:8], [1:12], and [1:16] after 120 min of MaHS. At the stoichiometric reaction (molar ratio [1:2]), only Nb2O5 is seen(xv). Upon increasing the molar ratio to [1:4] (Figure 1b), the Nb2O5 phase peaks decrease, and the KNbO3 phase appears. This fact is associated with a larger quantity of K+ and OH- ions available to react with the Nb2O5.
(viii)
Philips equipment (model X'Pert MPD) Renishaw equipment (Invia Spectrometer System) (x) EVO MA10, Zeiss, Germany (xi) JEM - 1200 Exll (JEOL, USA) (xii) HRTEM, JEM 2010 (xiii) Cary 5000 equipment (Agilent, USA) (xiv) DRA - 1800 (xv) (JCPDS 016-0053) (ix)
The Nb2O5 solubility is increased under extreme alkalinity conditions. Pure orthorhombic KNbO3(xvi) is formed at a molar ratio of [1:8] (Figure 1c). At [1:12] (Figure 1d) and [1:16] (Figure 1e), the presence of a single KNbO3 phase is seen with an orthorhombic crystalline structure(xvii).
KNbO3 Nb2O5
Synthesis duration: 120 min
Intensity (a.u.)
[1:16]
[1:12]
[1:8] [1:4] [1:2]
20
30
40
50
60
70
80
2(°) Figure 1: XRD patterns of KNbO3 synthesized by MaHS at 200 °C, after 120 minutes, as function of molar ratios ([1:2], [1:4], [1:8], [1:12] and [1:16]).
Figure 2 shows the formation of a pure phase of sub-stoichiometric KNbO3 at molar ratios (a) [1: 8], (b) [1:12], and (c) [1:16] for different synthesis durations (30, 60, 120, and 240 mins).
(xvi)
(JCPDS 01-071-0946, space group Amm2)
(xvii)
(JCPDS 01-032-0822, spatial group Cm2m)
An orthorhombic KNbO3 phase is formed after 30 min. This indicates that the synthesis of KNbO3 via MaHS is mainly associated with the Nb2O5:KOH molar ratio. The system supersaturation favors the material’s rate of nucleation—thus, the KNbO3 crystals form[44]. The results strongly suggest that the synthesis duration influences the crystallinity of the phases due to increasing crystallization kinetics via microwaves. In the conventional hydrothermal synthesis, the main factor influencing formation of the desired crystalline phases is the time of synthesis[21, . (a) [1:8]
Intensity (a.u.)
240 min
(b) [1:12]
KNbO3
120 min 60 min
240 min
40
50 2 ()
Intensity (a.u.)
30
60
70
120 min 60 min 30 min
30 min
20
KNbO3
Intensity (a.u.)
35]
20
80
(c) [1:16]
30
40
50 2 ()
60
70
80
KNbO3
240 min 120 min
60 min
30 min
20
30
40
50 2 ()
60
70
80
Figure 2: Evolution of the crystallinity of the sub-stoichiometric KNbO3 by using of molar ratios (a) [1:8], (b) [1:12] and (c) [1:16] as function of synthesis duration (30, 60, 120, and 240 min).
Figure 3 shows the KNbO3 Raman spectra for different synthesis durations (30, 60, 120 and 240 min) at different molar ratios (a) [1:8], (b) [1:12], and (c) [1:16]. All samples have characteristic orthorhombic bands from KNbO3[45]. The literature[46] reported that the band located at 191 cm-1 is attributed to the internal vibration modes of the NbO6 octahedron, while the band at 280 cm -1 indicates a folding mode. The bands at 532 cm -1 and 592 cm-1 correspond to stretching modes, while the band at 830 cm -1 results from band combination at 532 and 592 cm-1. The bands at 246, 570, and 832 cm-1 indicate that KNbO3 is orthorhombic [47].
82 9
Intensity (a.u.)
83 0
59 1
52 7 58 9
18 7 24 9
(b) [1:12]
27 8
53 0
Intensity (a.u.)
18 5 24 1
(a) [1:8]
240 min 120 min 60 min
120 min 60 min
30 min
30 min
200
400 600 -1 800 Raman shift ( cm ) (c) [1:16] 83 0
1000 53 0 58 7
400 600 -1 800 Raman shift ( cm ) 18 8 24 5
200
240 min
1000
240 min
Intensity (a.u.)
120 min 60 min
30 min
200
400 600 -1 800 Raman shift ( cm )
1000
Figure 3: Raman spectra of KNbO3 synthesized by MaHS at 200 °C, as function of both molar ratios (a) [1:8]; (b) [1:12]; (c) [1:16]), and synthesis duration (30, 60, 120, and 240 min).
The morphology of the synthesized KNbO3 powders as function of molar ratios and synthesis duration is presented in Figure 4. For the [1:8] molar ratio, towerlike particles of KNbO3 are obtained (Figures 4 (a-d)). This might be associated with a non-stoichiometric molar ratio or a higher proportion of KOH ions, which increases the reaction rate for nucleation and growth processes of KNbO 3 particles in a specific direction. Recent works in which KNbO3 is obtained by conventional hydrothermal synthesis (12 h at 200ºC) report the formation of KNbO 3 with nanotower morphology when the molar ratio of KOH in the system is increased. Those nanotowers consist of small particles preferentially stacked in the form of different sized cubes[35]. These materials clearly have a surface/volume relation that corresponds to a lower surface free energy relative to small particles. The system tends to reduce its free energy. The ions on the surface of the smaller particles (higher energy) tend to diffuse through the solution and are deposited on the surface of larger ones[48,
49]
. At a molar ratio to [1:12], larger KNbO3
cubes form (Figure 4 (e-h)). The formation of these cubes is fully in accordance with the Ostwald ripening maturation theory in which larger particles grow at the expense of smaller particles[48]. At [1:16], new particles form with a cubic geometry on the surface of larger cubic particles (Figure 4 (i-m)). At [1:16], the increased availability of K+ and OH-—as well as the microwave irradiation— increases the crystalline nucleation rate. These have a smaller final size when the nucleation becomes faster than the crystal growth [48].
Figure 4: SEM images of KNbO3 powders synthesized by MaHS at 200°C, as function of both synthesis duration and KOH concentrations: [1:8] (a) 30 min, (b) 60 min, (c) 120 min, (d) 240 min; [1:12] (e) 30 min, (f) 60 min, (g) 120 min, (h) 240 min; [1:16] (i) 30 min, (j) 60 min, (l) 120 min and (m) 240 min.
The average crystallite size for the KNbO3 powders in each condition was calculated by the Scherrer equation (from XRD results) is shown in Figure 5. Crystallite size increases at [1:8] when the synthesis duration increases. Values range from 33 to 46 nm. In this case, the synthesis times increases the crystallite size. When the molar ratio of KOH is [1:12], there was an increase in the average sizes of the crystallites compared to the [1:8] molar ratio (46 to 47
nm). The crystallite size decreased to 33 to 35 nm when the molar ratio was increased further. This decrease in crystallite size might be associated with the formation of new crystallites of KNbO3 at [1:16]. These results agree with the data in Figure 4.
60
[1:8] [1:12] [1:16]
Crystallite size (nm)
50 40 30 20 10 0 0
30
60
90
120
150
180
210
240
270
Time (min)
Figure 5: Crystallite size calculated by Scherrer equation for KNbO3 synthesized by MaHS at 200 °C, as function of both synthesis duration (30, 60, 120, and 240 min) and molar ratios ([1: 8], [1:12], [1:16]).
Figure 6 shows the TEM images of KNbO3 powders synthesized with a molar ratio of [1:8]. TEM micrograph (a) is 30 min and TEM micrograph (d) is 240 min. The images show tower-like microstructures with different crystallite sizes. Furthermore, longer synthesis duration favored ordered growth of the KNbO3 structure. Increasing the KOH concentration to a molar ratio of [1:12] indicates that one can see non-uniform growth of a new morphology in the form of cubes with well-defined faces. It is important to note that longer synthesis durations
increase the crystallinity, i.e., XRD in Figure 2b. Further increases in the molar ratio [1:16] resulted in new KNbO3 nanocrystals for both synthesis durations. In addition, large agglomerates of KNbO3 composed of smaller nanocube crystallites appear (Figure 6c). The crystallite sizes measured by TEM confirm those from the Scherrer equation.
Figure 6: TEM images of KNbO3 powders synthesized by MaHS at 200 °C, as function of both synthesis duration and KOH concentrations: [1:8] (a) 30 min (e) 240 min; [1:12] (b) 30 min (f) 240 min; [1:16] (c) and (d) 30 min (g) 240 min.
The MaHS product at a molar ratio of [1:16] with 30 minutes synthesis had an interplanar distance of 0.4243 nm, which corresponds to the distance between [110] planes in a KNbO3 orthorhombic structure(xviii). The HRTEM images are detailed in Figure 7. Figure 7c shows that the chemical composition of the KNbO3 crystallites consists mainly of niobium (~ 2.1 keV), potassium (~ 3.3 keV) and oxygen (~ 0.5 keV) as expected from the XRD analysis (Figure 2).
(xviii)
(JCPDS 01-032-0822, spatial group Cm2m)
Figure 7: (a) TEM images of KNbO3 nanocubes obtained by MaHS, using a molar ratio of [1:16], after 30 minutes; (b) interplanar distances for the obtained KNbO3 nanocubes; (c) EDX analysis.
Table 1 shows the band gap energies (Eg) of products from MaHS at 200 °C as a function of both synthesis duration and molar ratios. The calculated band-gap for all products are 3.14 to 3.38 eV range (~ 390 to 367 nm). The [1:8] molar ratio sample has a small variation in the band-gap over the time of synthesis, while the [1:12] sample has values that are nearly constant (3.20 to 3.21 eV). When the molar ratio increased to [1:16], a larger band gap value was observed compared to previous molar ratios with nearly constant values but at a slightly different level (3.36 to 3.37 eV). Furthermore, the band-gap increase of the KNbO3 nanoparticles is linked to the decrease in crystallite size—this is due to quantum confinement effects[21, 50, 51].
The results show that the KNbO3 powders have a band-gap value similar to those found in the literature [21, 46, 52]. In addition, the values are close to TiO2 (Eg between 3.0 and 3.2 eV), which is used in photodegradation [53, 54]. These values suggest that KNbO3 nanoparticles could be used in effluent treatment. Table 1: Determination of KNbO3 band-gap (eV) synthesized by the microwaveassisted hydrothermal method at 200 °C for the synthesis duration of 30, 60, 120 and 240 min with molar ratios [1: 8], [1:12], and [1:16]. Band-gap (eV)
Synthesis duration (min)
[1:8]
[1:12]
[1:16]
30
3.14 ±0.50
3.21 ±1.03
3.37 ±0.30
60
3.15 ±0.26
3.20 ±0.71
3.35 ±0.25
120
3.16 ±0.27
3.23 ±0.84
3.38 ±0.49
240
3.15 ±0.01
3.20 ±0.87
3.36 ±0.29
Conclusions We synthesized nanostructured KNbO3 via a hydrothermal assisted microwave method. This synthesis time was very short (30 min to 240 min) at 200 ºC. The synthesis duration influences the crystallinity of the product because of increased crystallization kinetics caused by the microwaves. The characteristics of the synthetized KNbO3 with orthorhombic symmetry depended on both the synthesis duration and the molar ratio of Nb2O5: KOH (1:8 to 1:16M). The crystallite size varied from 33 to 52 nm from the Scherrer equation and TEM. The band-gap energy for the different molar ratios were 3.1 eV to 3.3 eV indicating their potential use in different applications involving photodegradation.
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List of Figure Caption: Figure 1: XRD patterns of KNbO3 synthesized by MaHS at 200 °C, after 120 minutes, as function of molar ratios ([1:2], [1:4], [1:8], [1:12] and [1:16]). Figure 2: Evolution of the crystallinity of the sub-stoichiometric KNbO3 by using of molar ratios (a) [1:8], (b) [1:12] and (c) [1:16] as function of synthesis duration (30, 60, 120, and 240 min). Figure 3: Raman spectra of KNbO3 synthesized by MaHS at 200 °C, as function of both molar ratios (a) [1:8]; (b) [1:12]; (c) [1:16]), and synthesis duration (30, 60, 120, and 240 min). Figure 4: SEM images of KNbO3 powders synthesized by MaHS at 200°C, as function of both synthesis duration and KOH concentrations: [1:8] (a) 30 min, (b) 60 min, (c) 120 min, (d) 240 min; [1:12] (e) 30 min, (f) 60 min, (g) 120 min, (h) 240 min; [1:16] (i) 30 min, (j) 60 min, (l) 120 min and (m) 240 min. Figure 5: Crystallite size calculated by Scherrer equation for KNbO3 synthesized by MaHS at 200 °C, as function of both synthesis duration (30, 60, 120, and 240 min) and molar ratios ([1: 8], [1:12], [1:16]). Figure 6: TEM images of KNbO3 powders synthesized by MaHS at 200 °C, as function of both synthesis duration and KOH concentrations: [1:8] (a) 30 min (e) 240 min; [1:12] (b) 30 min (f) 240 min; [1:16] (c) and (d) 30 min (g) 240 min. Figure 7: (a) TEM images of KNbO3 nanocubes obtained by MaHS, using a molar ratio of [1:16], after 30 minutes; (b) interplanar distances for the obtained KNbO3 nanocubes; (c) EDX analysis.
Table
Table 1: Determination of KNbO3 band-gap (eV) synthesized by the microwaveassisted hydrothermal method at 200 °C for the synthesis duration of 30, 60, 120 and 240 min with molar ratios [1: 8], [1:12], and [1:16]. Band-gap (eV)
Synthesis duration (min)
[1:8]
[1:12]
[1:16]
30
3.14 ±0.50
3.21 ±1.03
3.37 ±0.30
60
3.15 ±0.26
3.20 ±0.71
3.35 ±0.25
120
3.16 ±0.27
3.23 ±0.84
3.38 ±0.49
240
3.15 ±0.01
3.20 ±0.87
3.36 ±0.29
PII: DOI: Reference:
S0272-8842(17)32756-6 https://doi.org/10.1016/j.ceramint.2017.12.060 CERI16940
To appear in: Ceramics International Received date: 11 November 2017 Revised date: 5 December 2017 Accepted date: 7 December 2017 Cite this article as: Tiago Bender Wermuth, Mario Norberto Baibich, Tania Maria Hubert Ribeiro and Carlos Pérez Bergmann, The rapid synthesis of nanostructured orthorhombic KNbO3 particles by a microwave-assisted hydrothermal method and their characterization, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The rapid synthesis of nanostructured orthorhombic KNbO3 particles by a microwave-assisted hydrothermal method and their characterization Tiago Bender Wermutha*; Mario Norberto Baibichb; Tania Maria Hubert Ribeiroa; Carlos Pérez Bergmanna a
Laboratório de Materiais Cerâmicos, Departamento de Materiais, Universidade Federal do Rio Grande do Sul – UFRGS. Osvaldo Aranha 99, Porto Alegre, RS, 90035-190, Brazil b Laboratório de Supercondutividade e Magnetismo, Instituto de Física, Universidade Federal do Rio Grande do Sul – UFRGS. Avenida Bento Gonçalves 9500, Porto Alegre, RS, 91501-970, Brazil a
* e-mail address: [email protected]
Abstract We studied the molar ratio effects of niobium and potassium precursors on the structure and morphology of potassium niobate powders prepared via microwave-assisted hydrothermal synthesis (MaHS). KNbO3 nanostructures in the form of nanotowers and nanocubes were obtained at reduced synthesis times (30 min to 240 min). The products were characterized via XRD, Raman spectroscopy, SEM, and TEM; band gap calculations used diffuse reflectance data. The results indicate that KNbO3 nanostructures were obtained with crystallite sizes ranging from 33 to 52 nm. An orthorhombic crystalline structure was formed from the increase of KOH at a molar ratio Nb2O5:KOH (1:8 to 1:16 M). The band-gap of 3.1 eV to 3.3 eV has potential use in photodegradation applications.
Keywords: Fast synthesis; potassium niobate; nanostructured, microwaveassisted hydrothermal synthesis; morphology
Introduction Niobium perovskites (ANbO3, A = Na, K, Ag, Cu) and especially potassium niobate (KNbO3) have attracted great interest due to their nonlinear optical properties, ferroelectricity, piezoelectricity, pyroelectricity, and photocatalytic properties[1-17]. Numerous methods of synthesis have been used to obtain
KNbO3, including the sol-gel method[18-22], solid-state reaction[21,
23-26]
,
hydrothermal synthesis[8, 9, 21, 27-32]. The hydrothermal synthesis enables the production of binary oxides (ZnO, CuO, MgO, TiO2, SnO2), ternary compounds (BaTiO3, PbTiO3, BiFeO3, KNbO3), and complex
compounds
[33] 300Ca0.375MnO3)
(Ba1-xSrxTiO3,
La0.5Ca0.5MnO3,
La0.325Pr0.
at low temperatures. It also allows the synthesis of materials
with high crystallinity, high purity, and low particle aggregation[34]. Recent studies reported different morphologies (nanowires, nanotowers, nanocubes, and nanotubes) of KNbO3 (orthorhombic and tetragonal structure) via the hydrothermal method. In these works, KNbO 3 was obtained by varying the precursor quantity, duration time (12 hours to 7 days), and synthesis temperature (150 to 250ºC)[21,
35-37]
. Although the hydrothermal method is
already being studied for the synthesis of these materials, one of its main limitations is the long time required (many hours or days)[34]. On the other hand, microwave-assisted hydrothermal synthesis (MaHS) can produce materials with similar characteristics to those obtained by conventional hydrothermal synthesis, but in a much shorter time. This method offers a high heating rate, which leads to a better precursor dissolution. The microwave energy promotes non-uniform distribution of energy inside the reactor to heat the system efficiently and rapidly. Consequently, the reaction rate increases[38] and accelerates the crystallization process[39,
40]
. This method allows greater
control of the products’ microstructural characteristics and final properties[34]. Studies involving KNbO3 MaHS are rare in the literature
[41]
, and additional
studies are critical. For the first time, this work offers a systematic investigation of the molar ratio of precursors (KOH and Nb2O5) and how this ratio affects the
morphology of KNbO3 obtained by MaHS. The crystallinity and morphology of KNbO3, obtained via this rapid synthesis were evaluated through reaction time studies. In addition, this study offers important information for promising applications in photocatalysis and/or other relevant areas. Material and methods The raw materials were niobium pentoxide(i), potassium hydroxide(ii), Milli-Q water(iii), and ethyl alcohol(iv). Stoichiometric molar ratios (Nb2O5:KOH – 1:2) and non-stoichiometric molar ratios (Nb2O5:KOH – 1:4; 1:8; 1:12; 1:16) were studied. The potassium hydroxide was slowly dissolved in Mili-Q H2O preheated at 50°C, and the mixture was homogenized for 30 min using a magnetic stirrer. Subsequently, the Nb2O5 was added and stirred for another 30 min for complete homogenization. The resulting suspension was microwaved(v) in a Teflon-coated flask. The total volume of the suspensions prepared was 20 mL. Preliminary studies showed that KNbO3 does not form below 200 °C. The synthesis was performed at different times (30, 60, 120 and 240 min) at 200 °C. After the reaction, the system was cooled to room temperature, and the product was centrifuged(vi) and washed with Mili-Q water and ethanol until the pH was stabile. The product was then dried(vii) at 60 °C for 12 hours.
(i)
Nb2O5, 99,8% of purity, CBMM – Companhia Brasileira de Metalurgia e Mineração KOH, 85% min., K2CO3 2,0% Max., Dinâmica (iii) σ = 0,054 μs/cm a 25°C (iV) 99,5%, Dinâmica, Brasil (v) MDS-8G, Shanghai Sineo Microwave Chemistry Technology Co., Ltda (vi) 9000 Rpm, 10 min., H2050R, Quimis, Brasil (vii) Jung, JV 0990 (ii)
Crystalline phases were determined via X-ray diffraction (XRD) with Philips equipment(viii) using Cu Kα radiation (λ = 1.54184 A) operating at 40 kV and 40 mA with a 2θ of 10-90°. The average crystallite size was calculated via the Scherrer equation[42]. Raman spectroscopy measurements were performed (ix) using a laser wavelength of 532 nm. The morphological characteristics of the synthesized KNbO3 were evaluated with scanning electron microscopy(x) (SEM) and transmission electron microscopy(xi) (TEM) with an intensity of 80 kV and high-resolution transmission electron microscopy(xii) at 200 kV. Light absorption curves of the KNbO3 were obtained using diffuse reflectance spectrophotometry(xiii) (UV-Vis) with an integrating sphere(xiv). The band gap energy was determined using the Kubelka – Munk function [43]. Results and Discussion Figure 1 shows XRD patterns of the products from MaHS under both stoichiometric [1:2] and non-stoichiometric conditions as function of KOH concentrations [1:4], [1:8], [1:12], and [1:16] after 120 min of MaHS. At the stoichiometric reaction (molar ratio [1:2]), only Nb2O5 is seen(xv). Upon increasing the molar ratio to [1:4] (Figure 1b), the Nb2O5 phase peaks decrease, and the KNbO3 phase appears. This fact is associated with a larger quantity of K+ and OH- ions available to react with the Nb2O5.
(viii)
Philips equipment (model X'Pert MPD) Renishaw equipment (Invia Spectrometer System) (x) EVO MA10, Zeiss, Germany (xi) JEM - 1200 Exll (JEOL, USA) (xii) HRTEM, JEM 2010 (xiii) Cary 5000 equipment (Agilent, USA) (xiv) DRA - 1800 (xv) (JCPDS 016-0053) (ix)
The Nb2O5 solubility is increased under extreme alkalinity conditions. Pure orthorhombic KNbO3(xvi) is formed at a molar ratio of [1:8] (Figure 1c). At [1:12] (Figure 1d) and [1:16] (Figure 1e), the presence of a single KNbO3 phase is seen with an orthorhombic crystalline structure(xvii).
KNbO3 Nb2O5
Synthesis duration: 120 min
Intensity (a.u.)
[1:16]
[1:12]
[1:8] [1:4] [1:2]
20
30
40
50
60
70
80
2(°) Figure 1: XRD patterns of KNbO3 synthesized by MaHS at 200 °C, after 120 minutes, as function of molar ratios ([1:2], [1:4], [1:8], [1:12] and [1:16]).
Figure 2 shows the formation of a pure phase of sub-stoichiometric KNbO3 at molar ratios (a) [1: 8], (b) [1:12], and (c) [1:16] for different synthesis durations (30, 60, 120, and 240 mins).
(xvi)
(JCPDS 01-071-0946, space group Amm2)
(xvii)
(JCPDS 01-032-0822, spatial group Cm2m)
An orthorhombic KNbO3 phase is formed after 30 min. This indicates that the synthesis of KNbO3 via MaHS is mainly associated with the Nb2O5:KOH molar ratio. The system supersaturation favors the material’s rate of nucleation—thus, the KNbO3 crystals form[44]. The results strongly suggest that the synthesis duration influences the crystallinity of the phases due to increasing crystallization kinetics via microwaves. In the conventional hydrothermal synthesis, the main factor influencing formation of the desired crystalline phases is the time of synthesis[21, . (a) [1:8]
Intensity (a.u.)
240 min
(b) [1:12]
KNbO3
120 min 60 min
240 min
40
50 2 ()
Intensity (a.u.)
30
60
70
120 min 60 min 30 min
30 min
20
KNbO3
Intensity (a.u.)
35]
20
80
(c) [1:16]
30
40
50 2 ()
60
70
80
KNbO3
240 min 120 min
60 min
30 min
20
30
40
50 2 ()
60
70
80
Figure 2: Evolution of the crystallinity of the sub-stoichiometric KNbO3 by using of molar ratios (a) [1:8], (b) [1:12] and (c) [1:16] as function of synthesis duration (30, 60, 120, and 240 min).
Figure 3 shows the KNbO3 Raman spectra for different synthesis durations (30, 60, 120 and 240 min) at different molar ratios (a) [1:8], (b) [1:12], and (c) [1:16]. All samples have characteristic orthorhombic bands from KNbO3[45]. The literature[46] reported that the band located at 191 cm-1 is attributed to the internal vibration modes of the NbO6 octahedron, while the band at 280 cm -1 indicates a folding mode. The bands at 532 cm -1 and 592 cm-1 correspond to stretching modes, while the band at 830 cm -1 results from band combination at 532 and 592 cm-1. The bands at 246, 570, and 832 cm-1 indicate that KNbO3 is orthorhombic [47].
82 9
Intensity (a.u.)
83 0
59 1
52 7 58 9
18 7 24 9
(b) [1:12]
27 8
53 0
Intensity (a.u.)
18 5 24 1
(a) [1:8]
240 min 120 min 60 min
120 min 60 min
30 min
30 min
200
400 600 -1 800 Raman shift ( cm ) (c) [1:16] 83 0
1000 53 0 58 7
400 600 -1 800 Raman shift ( cm ) 18 8 24 5
200
240 min
1000
240 min
Intensity (a.u.)
120 min 60 min
30 min
200
400 600 -1 800 Raman shift ( cm )
1000
Figure 3: Raman spectra of KNbO3 synthesized by MaHS at 200 °C, as function of both molar ratios (a) [1:8]; (b) [1:12]; (c) [1:16]), and synthesis duration (30, 60, 120, and 240 min).
The morphology of the synthesized KNbO3 powders as function of molar ratios and synthesis duration is presented in Figure 4. For the [1:8] molar ratio, towerlike particles of KNbO3 are obtained (Figures 4 (a-d)). This might be associated with a non-stoichiometric molar ratio or a higher proportion of KOH ions, which increases the reaction rate for nucleation and growth processes of KNbO 3 particles in a specific direction. Recent works in which KNbO3 is obtained by conventional hydrothermal synthesis (12 h at 200ºC) report the formation of KNbO 3 with nanotower morphology when the molar ratio of KOH in the system is increased. Those nanotowers consist of small particles preferentially stacked in the form of different sized cubes[35]. These materials clearly have a surface/volume relation that corresponds to a lower surface free energy relative to small particles. The system tends to reduce its free energy. The ions on the surface of the smaller particles (higher energy) tend to diffuse through the solution and are deposited on the surface of larger ones[48,
49]
. At a molar ratio to [1:12], larger KNbO3
cubes form (Figure 4 (e-h)). The formation of these cubes is fully in accordance with the Ostwald ripening maturation theory in which larger particles grow at the expense of smaller particles[48]. At [1:16], new particles form with a cubic geometry on the surface of larger cubic particles (Figure 4 (i-m)). At [1:16], the increased availability of K+ and OH-—as well as the microwave irradiation— increases the crystalline nucleation rate. These have a smaller final size when the nucleation becomes faster than the crystal growth [48].
Figure 4: SEM images of KNbO3 powders synthesized by MaHS at 200°C, as function of both synthesis duration and KOH concentrations: [1:8] (a) 30 min, (b) 60 min, (c) 120 min, (d) 240 min; [1:12] (e) 30 min, (f) 60 min, (g) 120 min, (h) 240 min; [1:16] (i) 30 min, (j) 60 min, (l) 120 min and (m) 240 min.
The average crystallite size for the KNbO3 powders in each condition was calculated by the Scherrer equation (from XRD results) is shown in Figure 5. Crystallite size increases at [1:8] when the synthesis duration increases. Values range from 33 to 46 nm. In this case, the synthesis times increases the crystallite size. When the molar ratio of KOH is [1:12], there was an increase in the average sizes of the crystallites compared to the [1:8] molar ratio (46 to 47
nm). The crystallite size decreased to 33 to 35 nm when the molar ratio was increased further. This decrease in crystallite size might be associated with the formation of new crystallites of KNbO3 at [1:16]. These results agree with the data in Figure 4.
60
[1:8] [1:12] [1:16]
Crystallite size (nm)
50 40 30 20 10 0 0
30
60
90
120
150
180
210
240
270
Time (min)
Figure 5: Crystallite size calculated by Scherrer equation for KNbO3 synthesized by MaHS at 200 °C, as function of both synthesis duration (30, 60, 120, and 240 min) and molar ratios ([1: 8], [1:12], [1:16]).
Figure 6 shows the TEM images of KNbO3 powders synthesized with a molar ratio of [1:8]. TEM micrograph (a) is 30 min and TEM micrograph (d) is 240 min. The images show tower-like microstructures with different crystallite sizes. Furthermore, longer synthesis duration favored ordered growth of the KNbO3 structure. Increasing the KOH concentration to a molar ratio of [1:12] indicates that one can see non-uniform growth of a new morphology in the form of cubes with well-defined faces. It is important to note that longer synthesis durations
increase the crystallinity, i.e., XRD in Figure 2b. Further increases in the molar ratio [1:16] resulted in new KNbO3 nanocrystals for both synthesis durations. In addition, large agglomerates of KNbO3 composed of smaller nanocube crystallites appear (Figure 6c). The crystallite sizes measured by TEM confirm those from the Scherrer equation.
Figure 6: TEM images of KNbO3 powders synthesized by MaHS at 200 °C, as function of both synthesis duration and KOH concentrations: [1:8] (a) 30 min (e) 240 min; [1:12] (b) 30 min (f) 240 min; [1:16] (c) and (d) 30 min (g) 240 min.
The MaHS product at a molar ratio of [1:16] with 30 minutes synthesis had an interplanar distance of 0.4243 nm, which corresponds to the distance between [110] planes in a KNbO3 orthorhombic structure(xviii). The HRTEM images are detailed in Figure 7. Figure 7c shows that the chemical composition of the KNbO3 crystallites consists mainly of niobium (~ 2.1 keV), potassium (~ 3.3 keV) and oxygen (~ 0.5 keV) as expected from the XRD analysis (Figure 2).
(xviii)
(JCPDS 01-032-0822, spatial group Cm2m)
Figure 7: (a) TEM images of KNbO3 nanocubes obtained by MaHS, using a molar ratio of [1:16], after 30 minutes; (b) interplanar distances for the obtained KNbO3 nanocubes; (c) EDX analysis.
Table 1 shows the band gap energies (Eg) of products from MaHS at 200 °C as a function of both synthesis duration and molar ratios. The calculated band-gap for all products are 3.14 to 3.38 eV range (~ 390 to 367 nm). The [1:8] molar ratio sample has a small variation in the band-gap over the time of synthesis, while the [1:12] sample has values that are nearly constant (3.20 to 3.21 eV). When the molar ratio increased to [1:16], a larger band gap value was observed compared to previous molar ratios with nearly constant values but at a slightly different level (3.36 to 3.37 eV). Furthermore, the band-gap increase of the KNbO3 nanoparticles is linked to the decrease in crystallite size—this is due to quantum confinement effects[21, 50, 51].
The results show that the KNbO3 powders have a band-gap value similar to those found in the literature [21, 46, 52]. In addition, the values are close to TiO2 (Eg between 3.0 and 3.2 eV), which is used in photodegradation [53, 54]. These values suggest that KNbO3 nanoparticles could be used in effluent treatment. Table 1: Determination of KNbO3 band-gap (eV) synthesized by the microwaveassisted hydrothermal method at 200 °C for the synthesis duration of 30, 60, 120 and 240 min with molar ratios [1: 8], [1:12], and [1:16]. Band-gap (eV)
Synthesis duration (min)
[1:8]
[1:12]
[1:16]
30
3.14 ±0.50
3.21 ±1.03
3.37 ±0.30
60
3.15 ±0.26
3.20 ±0.71
3.35 ±0.25
120
3.16 ±0.27
3.23 ±0.84
3.38 ±0.49
240
3.15 ±0.01
3.20 ±0.87
3.36 ±0.29
Conclusions We synthesized nanostructured KNbO3 via a hydrothermal assisted microwave method. This synthesis time was very short (30 min to 240 min) at 200 ºC. The synthesis duration influences the crystallinity of the product because of increased crystallization kinetics caused by the microwaves. The characteristics of the synthetized KNbO3 with orthorhombic symmetry depended on both the synthesis duration and the molar ratio of Nb2O5: KOH (1:8 to 1:16M). The crystallite size varied from 33 to 52 nm from the Scherrer equation and TEM. The band-gap energy for the different molar ratios were 3.1 eV to 3.3 eV indicating their potential use in different applications involving photodegradation.
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List of Figure Caption: Figure 1: XRD patterns of KNbO3 synthesized by MaHS at 200 °C, after 120 minutes, as function of molar ratios ([1:2], [1:4], [1:8], [1:12] and [1:16]). Figure 2: Evolution of the crystallinity of the sub-stoichiometric KNbO3 by using of molar ratios (a) [1:8], (b) [1:12] and (c) [1:16] as function of synthesis duration (30, 60, 120, and 240 min). Figure 3: Raman spectra of KNbO3 synthesized by MaHS at 200 °C, as function of both molar ratios (a) [1:8]; (b) [1:12]; (c) [1:16]), and synthesis duration (30, 60, 120, and 240 min). Figure 4: SEM images of KNbO3 powders synthesized by MaHS at 200°C, as function of both synthesis duration and KOH concentrations: [1:8] (a) 30 min, (b) 60 min, (c) 120 min, (d) 240 min; [1:12] (e) 30 min, (f) 60 min, (g) 120 min, (h) 240 min; [1:16] (i) 30 min, (j) 60 min, (l) 120 min and (m) 240 min. Figure 5: Crystallite size calculated by Scherrer equation for KNbO3 synthesized by MaHS at 200 °C, as function of both synthesis duration (30, 60, 120, and 240 min) and molar ratios ([1: 8], [1:12], [1:16]). Figure 6: TEM images of KNbO3 powders synthesized by MaHS at 200 °C, as function of both synthesis duration and KOH concentrations: [1:8] (a) 30 min (e) 240 min; [1:12] (b) 30 min (f) 240 min; [1:16] (c) and (d) 30 min (g) 240 min. Figure 7: (a) TEM images of KNbO3 nanocubes obtained by MaHS, using a molar ratio of [1:16], after 30 minutes; (b) interplanar distances for the obtained KNbO3 nanocubes; (c) EDX analysis.
Table
Table 1: Determination of KNbO3 band-gap (eV) synthesized by the microwaveassisted hydrothermal method at 200 °C for the synthesis duration of 30, 60, 120 and 240 min with molar ratios [1: 8], [1:12], and [1:16]. Band-gap (eV)
Synthesis duration (min)
[1:8]
[1:12]
[1:16]
30
3.14 ±0.50
3.21 ±1.03
3.37 ±0.30
60
3.15 ±0.26
3.20 ±0.71
3.35 ±0.25
120
3.16 ±0.27
3.23 ±0.84
3.38 ±0.49
240
3.15 ±0.01
3.20 ±0.87
3.36 ±0.29