Crystal Morphology Control of Synthetic Giniite by Alkaline Cations

Aug 8, 2017 - Crystal morphology was studied by scanning electron microscopy (SEM) using a FEI Nova 200 (FEG/SEM). Thermogravimetric analyses (TGA) we...
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Crystal morphology control of synthetic giniite by alkaline cations and pH variations Renato Gonçalves, Ricardo Martins, Carlos M. Costa, Stanislav Ferdov, and Senentxu Lanceros-Mendez Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00590 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Crystal morphology control of synthetic giniite by alkaline cations and pH variations Renato Gonçalves,†,‡ Ricardo Martins,† Carlos M. Costa,†,‡ Stanislav Ferdov,†* Senentxu Lanceros-Méndez§,& †

Centro/Departamento de Física, Universidade do Minho, 4710-057 Braga, Portugal



Centro/Departamento de Química, Universidade do Minho, 4710-057 Braga, Portugal

§

Basque Center for Materials, Applications and Nanostructures (BCMaterials), Parque Tecnológico de Bizkaia, Ed. 500, Derio 48160, Spain.

&

IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain

Abstract: The synthetic analogue of the mineral giniite is a promising material for capture of phosphopeptides from complex tryptic digests, photocatalysis and cathode for lithium battery applications. In all these applications, the crystal morphology of giniite shows to be extremely important, however, the reported synthesis and crystal morphologies are scattered among a variety of approaches. Here we report a hydrothermal synthesis that allows achieving five different morphologies of giniite only by changing the pH by addition of three different alkaline cations (Na+, Li+ and K+) in form of hydroxides. At pH 2, micrometric sphere-like assemblies of nanoparticles were obtained. At pH 4, fractal dendritic crystals as well as intergrown flower-like and asterisk-like crystals were synthesized. At pH 6, the synthesis resulted in dagger-like crystals of dissolved dendritic structures.

Introduction 1 ACS Paragon Plus Environment

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Control of the morphology of crystalline materials is essential in order to tune their properties.1,

2

It is well known that specific combinations between particle size and

shape, lead to important variations of the physical and chemical properties of solids with the same structure.3, 4 Thus, understanding the synthesis conditions that lead to a specific crystal growth is essential for microstructure design and for tailoring of solids with particular applications.5 The most used methods for materials preparations are solid state reaction, hydrothermal, sol-gel, co-precipitation, polyol, electrochemical syntheses and laser pyrolysis, among others.6-10 From those methods, the hydrothermal synthesis is one of the most attractive techniques for particles preparation. It is a simple method that allows scaled up production of particles from nm to µm sizes with controlled morphology at low cost.11 Among the materials prepared under hydrothermal conditions, iron phosphates are interesting for different applications such as catalysis and cathodes for lithium battery.12 In particular, iron hydroxyl phosphates are known by their oxidative dehydrogenation catalysis and interesting crystal chemistry.13,

14

The basic iron phosphates include

analogues of the minerals dufrenite, rockbridgeite, laubmannite, beraunite, cacoxenite14 and giniite that is an iron hydroxyl phosphate discovered in 1980 in Namibia by Paul Keller15 with the (Fe2+, Fe3+)4(PO4)4(OH)2·2H2O chemical structure. The crystal structure of giniite is monoclinic, pseudo-orthorhombic,16 although the literature shows two dimensions for the giniite unit cell.15, 16 The synthetic giniite is used for photocatalysis,17-19 as cathodes in lithium-ion battery applications20 and in life science for capture of phosphopeptides.21 Giniite is prepared is prepared hydrothermally using inorganic precursors17, 21, 22 or by the combination of inorganic and organic reagents16,

18

testing different pH, initial

stoichiometric ratios, temperature and time for synthesis. As a result, wide variety of

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crystal morphologies have been reported. Giniite crystals are synthesized in form of dendrites17, 21, spheres22, ceroid-asteroid particles,17 bipyramids, plates, hollow spheres, tetragonal hexadecahedra, and carved bipyramids.18 In this work, the influence of pH and the addition of alkaline cations (K+, Li+, Na+) in the initial synthesis solution on the giniite crystal morphology is investigated. As a result, five different morphologies of synthetic giniite were demonstrated. Experimental The synthesis procedure included mixing of three individual solutions: (1) 0.29 ml of H3PO4 (85% Aldrich) dissolved in 5 g of H2O; (2) 0.85 g of FeNO3 (99% Panreac) dissolved in 10 g of H2O and (3) 0.44 g of NaOH, LiOH or KOH (97% Aldrich) dissolved in H2O for pH adjustment. The mixing of three solutions followed the order 1+2+3. Twelve synthesis divided in 3 series were carried out. Each series contained only one alkaline cation and a pH of 2, 4, 6 and 7 adjusted by the addition of solution of NaOH, LiOH or KOH (g per g of H2O), respectively. After that, the obtained mixture was homogenized by magnetic stirring (700 rpm) for 5 min, placed in a Teflon-lined stainless steel autoclave and heated at 230 ºC during 72 h. The obtained powders were filtered, washed with distilled water and dried overnight at 60 °C. Phase composition of each sample was determined by powder X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer. The powder XRD patterns were collected using the θ-2θ scan mode, range 5-60°, step scan 0.06° and 1 s per step. Crystal morphology was studied by scanning electron microscopy (SEM) using a FEI Nova 200 (FEG/SEM). Thermogravimetric analyses (TGA) were performed with a TGA/SDTA 851e Mettler Toledo apparatus by heating from 25 to 900 ºC at 10 ºC min-1 under a high purity nitrogen atmosphere (99.99 % minimum purity; 50 mL.min-1 flow rate).

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Fourier transformed infrared spectroscopy (FTIR) was carried out from 1500 to 600 cm−1 with a resolution of 4 cm−1 and 64 scans with a Jasco FT/IR-4100 system in the attenuated total reflectance (ATR) mode. Results and discussion X-ray diffraction. Figure 1 shows Le Bail fits of the powder XRD patterns of samples synthesized at different pH (2, 4, 6 and 7) adjusted by the addition of NaOH, LiOH or KOH. Independently of the nature of the alkaline cation, at pH 2 and 4 the materials crystallize as Fe5(PO4)4(OH)3·2H2O, which is analogue of the mineral giniite (JCPDS Card number: 45-1436).

Figure 1. Le Bail fits of the phases obtained at (a) pH 2, (b) pH 4, (c) pH 6 and (d) pH 7 using different alkaline cations. Аsterisks denote admixture of secondary phases, as listed in Table 1.

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Increasing the pH of the solution to 6 leads to a distinct difference between the influence of Na, Li and K cations. When Na cations are present, the obtained phase is the same (Fe5(PO4)4(OH)3·2H2O) as the one obtained from solutions at low pH (2-4). However, the synthesis in the presence of Li cations results in a mixture of synthetic analogues

of

the

minerals

tavorite

((LiFe(PO4)(OH))

and

rockbridgeite

(Fe5(PO4)3(OH)5). When potassium ions are introduced in the initial solution, a different mixture of Fe5(PO4)4(OH)3·2H2O and Fe(H3O)(HPO4)2 is obtained. The results from the XRD data are summarized in Table 1. Table 1. Phases obtained at different pH and alkaline cations. Phase

pH

Alkaline cation

PDF - No

Fe5(PO4)4(OH)3·2H2O

2

Na

45-1436

Fe5(PO4)4(OH)3·2H2O

2

Li

45-1436

Fe5(PO4)4(OH)3·2H2O

2

K

45-1436

Fe5(PO4)4(OH)3·2H2O

4

Na

45-1436

Fe5(PO4)4(OH)3·2H2O

4

Li

45-1436

Fe5(PO4)4(OH)3·2H2O

4

K

45-1436

Fe5(PO4)4(OH)3·2H2O

6

Na

45-1436 41-1376

LiFe(PO4)(OH) & 6

Li

Fe5(PO4)3(OH)5

34-0150

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Fe5(PO4)4(OH)3·2H2O &

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45-1436 6

K

Fe(H3O)(HPO4)2

96-202-0344*

Li(Na)Fe(PO4)(OH) &

41-1376 7

Na

Fe2O3

96-900-0140*

LiFe(PO4)(OH) &

41-1376 7

Li 96-900-0140*

Fe2O3 Fe2O7P2 & unknown phase

7

K

96-400-0292*

*entry number from Crystallography Open Database Powder XRD data show that with increasing pH the influence of the alkaline cations on the final phase become stronger as their concentration increase and they can be incorporated in the crystal structure. When the synthesis is carried out with sodium cations, the resulting material crystallizes in a phase with tavorite structure and small amount of hematite (Fe2O3). Considering that no lithium has been used, the obtained phase should be identified as sodium tavorite (NaFe(PO4)(OH)). It is to notice that the presence of Li also favors the crystallization of tavorite and a small amount of hematite. On the other hand, the inclusion of the K cations favors the crystallization of a mixture of Fe2P2O7 and an unidentified secondary phase. Crystal morphology. Figures 3 to 5 show representative SEM images of the iron phosphates obtained at different pH of the initial solution using three different alkaline cations, Na, Li and K, in the form of hydroxides, respectively. Crystal morphology in the presence of Na cations. Figure 3 shows giniite crystals obtained at different pH and in the presence of Na cations in the initial solution. Morphologies such as spheres (Figure 3a), flower-like (Figure 3b) 6 ACS Paragon Plus Environment

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Figure 3. SEM images of the synthetized samples with NaOH precursor at different pH values. and relicts of dendrite-like structures (Figure 3c, d) were obtained only by change of the pH and amount of Na cations, respectively. Sphere-like particles crystallize when the pH of the initial solution is 2. The size of these particles vary from 1 to 8 µm. Closer look at surface of the particles reveals that they are composed of assemblies of nanoplarticles (10-20 nm) (Figure 3a). When the pH is increased to 4 the morphology of giniite change to flower-like particles. These particles show more uniform size (5-6 µm) when compared to the sphere-like ones. Each particle is composed of pyramid-like intergrowths which is clear indication of crystal twinning. When the pH is further increased to 6 another morphological type is observed. Dagger-like crystals attached to 7 ACS Paragon Plus Environment

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elongated trunks are formed. This morphology is similar to previously reported dendrite-like structures17, 21 composed of ordered branches distributed on both sides of a trunk. However, in our particles there are clear signs of dissolution that starts from the edges of the branches. This suggests that at the given conditions this morphology is unstable. The evolution of the crystal dissolution is clearly seen when the synthesis is carried out at pH 7 (Figure 3d) where only the trunks of the attached dagger-like particles are partly preserved. Crystal morphology in the presence of Li cations. Figure 4 shows SEM images of the samples synthetized with the presence of Li cations at different pH values.

Figure 4. SEM images of the samples synthetized with LiOH precursor at different pHvalues. 8 ACS Paragon Plus Environment

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Similar to the sodium system at pH 2 crystallizes giniite in form of micrometric spherelike particles composed of self-attached nanoparticles. However, the presence of Li cations seems to narrow the particles size variations (1-5 µm). At pH 4 the crystal morphology changes to asterisk-like particles composed of four or five orthogonally arranged segments. These segments are carved, with irregular surface are clear signs of selective dissolution and dynamic processes of the morphology evolution. At pH 6 the crystal morphology appears as remnants of dendrite-like structures that are completely dissolved at pH 7, and likewise the sodium system only fragments of elongated particles are left. Crystal morphology in the presence of K cations. The crystal morphologies obtained at the presence of K cations and different pH values are shown in Figure 5.

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Figure 5. SEM images of the samples synthetized at different pH-values with KOH as precursor. At pH 2 the giniite phase crystallize as sphere-like particles formed by the assembly of nanoparticles. The spheres show similar size (1-4 µm) variation when compared to the same particles obtained from solution containing Li cations. However, the nanoparticles that compose the spheres obtained from the potassium system are relatively bigger (70 – 100 nm) than the nanoparticles assembled in the sodium and lithium systems. The common sphere-like morphology of giniite samples obtained from the sodium, lithium and potassium systems indicates that at the particular synthesis conditions the nature of the cation plays little role when compared to the pH. The only influence of the alkaline cation seems to be on the average size variations of the nanoparticles that compose the bigger sphere-like entities. When the synthesis is carried out at pH 4, the use K cations results in significant change in the morphology when compared with the sodium and lithium systems at the same pH. Well-formed dendrites of orthogonally arranged braches are formed. Unlike the sodium and lithium systems this fractal structure appears at lower pH and show no signs of dissolution. In the particular system, this result reveals the morphology-directing role of the K cation. Increasing the pH to 6 and 7 leads to plate-like particles decorated with randomly distributed nanoparticles that are indirect clue of the phase mixture detected by XRD patterns. For all three series of experiments, the influence of different alkaline cations on the crystal morphology increase with increasing pH. This influence is related with the dynamic affinity of each alkaline cation to cap certain surface of growing atomic planes. Small cations as Li and Na tend to favor the growth of smaller particles than the giniite crystals obtained in the presence of K. Similar influence of non-structure forming ions during hydrothermal synthesis has been previously demonstrated.23, 24

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Thermal and spectroscopic properties of giniite. Figure 6a,b shows TGA, DTG curves and FTIR spectra of Fe5(PO4)4(OH)3·2H2O produced in the presence of Na cations and pH value of 2. The other samples with the same structure showed TG curves similar to the one presented here.

Figure

6.

(a)

TGA

and

DTG

curves

and

(b)

FTIR

spectrum

of

the

Fe5(PO4)4(OH)3·2H2O pure phase. TG and DTG analyses. TGA curve shows that the weight loss up to 900 ºC is 7.1 % which indicates the presence of less hydroxyl species than the expected 9.6 % for Fe5(PO4)4(OH)3·2H2O. Similar observation has been reported before.25 We consider that the reduced amount of water in TG curve is due the existence of Fe2+ unlike the expected sole Fe3+ in the ideal formula (Fe5(PO4)4(OH)3·2H2O) which can decrease the required number of negatively charged hydroxyl groups in the structure. Furthermore, the thermogram shows two main stages of weight loss: the first between 0 and around 500 ºC that corresponds to a release of the weakly connected water molecules (~ 3 %) and the second between 500 ºC and 560 ºC (~3 %) that correspond to the release of the strongly connected hydroxyl groups. These stages of weight loss are accompanied by two well-defined endothermic effects followed by the DTG curve. Further weight loss

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(1.1 %) between 560 and 900

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o

C is attributed to O2 release from phosphate

decomposition.26 FTIR spectroscopy. FTIR transmittance spectrum (600 – 1500 cm-1) of the synthetic giniite (Figure 6b) is composed of broad and well-defined vibrational bands at 751, 775, 943, 998, 1021 and 1074 cm-1. The band broadening in the infrared spectra is usually assigned to the polycrystalline nature of the samples.27 The vibrational modes between 850 and 1100 cm-1 belong to the asymmetric P-O stretching in the PO4 tetrahedra22, 27, 28 while the lower frequency band split into two maximums at 751 and 775 cm-1 is attributed to asymmetric O-P-O bending vibrations. Conclusions This work shows the dependence between the pH adjusted by three different alkaline hydroxides (NaOH, LiOH and KOH) and the crystal morphology of the synthetic giniite. At pH 2 the presence of any alkaline cations has little impact on giniite morphology and sphere-like assemblies of nanoparticles are formed. At pH values of 4 and 6 the different type of alkaline cations reveals a wide variety of crystallization patterns while pH 7 favors structural incorporation of alkaline cations and formation of phases that are different than giniite. These results offer an easy synthesis of giniite with desired morphology for potential applications such as cathode electrodes in lithium-ion batteries and photocatalytic degradation that will be focus on our future work. Corresponding Author *E-mail: [email protected] Acknowledgements This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013. The authors 12 ACS Paragon Plus Environment

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thank FEDER funds through the COMPETE 2020 Programme and National Funds through

FCT

under

UID/CTM/50025/2013

the and

projects grants

PTDC/CTM-ENE/5387/2014 SFRH/BD/88397/2012

and (R.G.),

SFRH/BPD/112547/2015 (C.M.C.) and IF/01516/2013 (S.F.). SLM thanks financial support from the Basque Government Industry Department under the ELKARTEK Program. References (1)

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Crystal morphology control of synthetic giniite by alkaline cations and pH variations R. Gonçalves,†,‡ R. Martins,† C. M. Costa,†,‡ S. Ferdov,†* S. Lanceros-Méndez§,&

Sphere-like oriented assembly of nanoparticles, dendritic, intergrown flower-like crystals as well as asterisk-like and dagger-like crystals of the synthetic analogue of the mineral giniite have been synthesized at hydrothermal conditions by control of pH by addition of three different alkaline cations as Na, Li and K.

16 ACS Paragon Plus Environment