Structure Of Amorphous Nickel Borate,aqueous Nickel Sulfate And Nickel Nitrate Solutions

At present,although the crystalline nickel borate structure is well developed,the structure of amorphous hydrated nickel borate has not been reported yet.In this work,two amorphous hydrated nickel borates NiO·0.8B₂O₃·4.5H₂O and NiO·B₂O₃·3H₂O were synthesized.The compositions of NiO and B₂O₃ for samples were determined through chemical analysis,thermogravimetric determined the number of water molecules,X-ray diffraction confirmed that samples are amorphous,and the main anions for NiO·0.8B₂O₃·4.5H₂O and NiO·B₂O₃·3H₂O are B₃O₃?OH?5^2- and B₂O?OH?₆^2-,respectively,analyzed by IR and Raman spectra.The local structure of these samples was studied by synchrotron radiation extended X-ray absorption fine structure?EXAFS?.The neighboring coordination atoms type,interatomic distance and the atom-pair numbers of Ni atom are given by EXAFS data fitting.The results show that the nearest neighbor coordination atom of Ni is O atom,and the nearer neighbor atoms are B and Ni atom.For NiO·0.8B₂O₃·4.5H₂O,the interatomic distances for Ni-O,Ni-B and Ni-Ni are 2.08,2.63 and 3.11 ?,and the atom-pair numbers are 5.7,3.8 and 3.8,respectively.Meanwhile,the interatomic distances of Ni-O,Ni-B and Ni-Ni for NiO·B₂O₃·3.0H₂O are 2.07,2.62 and 3.10 ?,and the atom-pair numbers are 6.0,4.0 and 4.0,respectively.The results of the EXAFS spectra for NiO·0.8B₂O₃·4.5H₂O and NiO·B₂O₃·3.0H₂O reveal that the first shell of Ni atoms is an octahedral structure with six oxygen atoms,and the second and third shells are a plane quadrilateral structure with four boron atoms and four nickel atoms,respectively.On the surface,NiSO₄ and Ni?NO₃?₂ solutions are simple electrolyte solution.In fact,there are many micro interactions including ion-associated interaction between anion and cation,the solvation of anion and cation,the competition coordination of anion and water with cation,and the hydrogen bonds among water molecules in NiSO₄ and Ni?NO₃?₂ solutions,which results in its complicated micro-structures and properties.Accordingly,adopts the density functional theory and EXAFS experimental to study the micro hydrated structure of NiSO₄ and Ni?NO₃?₂ solution in our work.A detailed investigation of [Ni?H₂O?_n]²⁺ hydration clusters in this work has been carried out with DFT and the relativistic effective core potentials basis set Lanl2DZ.The energy parameters,atomic charge density distribution and bond parameters are given.The results of DFT calculation indicate that the structure of first solvation shell for Ni²⁺ is an octahedral sphere with six water molecules,which is almost unaffected by the water molecules of outer solvation sphere and there is some relatively certainty for the second hydration shell structure of Ni²⁺.For the DFT calculations of [SO₄?H₂O?n]^2-and [NO₃?H₂O?_n]^- anion hydration clusters,the basis set is B3LYP/aug-cc-pVDZ.The results show that the structure of first hydration layer for SO₄^2- is instable influenced easily by outside hydration layer water molecules.For [NO₃?H₂O?_n]^-hydration clusters,the number of water molecules of first hydration layer for NO₃^-is 4⁶,and the structure is relatively stable.By comparing the energy parameters of [Ni?H₂O?_n]²⁺ and [SO₄?H₂O?_n]^2-,[NO₃?H₂O?_n]^-hydration clusters indicate that water molecules prefer to Ni²⁺ and form interaction,next is SO₄^2- or NO₃^-in aqueous NiSO₄ and Ni?NO₃?₂ solutions.A systematic theoretical investigation of [NiSO₄?H₂O?_n]⁰ clusters are realized by DFT calculation.The geometry optimization and frequency analysis of [NiSO₄?H₂O?_n]⁰ clusters are calculated by DFT with B3LYP/gen custom basis set?aug-cc-p VDZ basis set generally was used for O,H and S atoms,while for Ni atom the basis set Lanl2DZ was used?.The results of hydration energy calculation reveal that the six-coordinated contact ion pair?CIP?is the stable configuration in concentrated and super saturation solutions,and the solvent-shared ion pair?SSIP?is the favorable structure in dilute solutions,while in extreme and infinite dilute aqueous solutions isolated ion hydration clusters are the stable forms.In addition,the calculation was also found that NiSO₄ will hydrolyze easily in solution and generates HSO₄^-.Using the same calculation method and basis set for [Ni?NO₃?₂?H₂O?_n]⁰ and [NiNO₃?H₂O?_n]⁺ clusters,the results of structure optimization and energy calculation indicate that the CIP configuration is the stable structure when the number of water molecules is small in cluster.However,the SSIP configuration begins to appear and become to be the stable isomer gradually when n?3.Therefore,the free Ni²⁺ and NO₃^-are turn into the main species in solution with the increase of the number of water molecules.That is to say,the isolated [Ni?H₂O?_n]²⁺ and [NO₃?H₂O?_n]^- hydration clusters prove to be the stable structure of main species in solution.In addition,Ni?NO₃?₂ is prone to hydrolysis when n is small according to calculation results.EXAFS detection of aqueous NiSO₄ and Ni?NO₃?₂ solutions confirms that Ni²⁺ is surrounded by six water molecules tightly formed an octahedral structure in the first hydration shell.The Ni-O distance of NiSO₄ and Ni?NO₃?₂ are about 2.044 ?,and the coordinated number are around 6.0.Even in saturated NiSO₄ and Ni?NO₃?₂ solutions,also no obvious proof is existed for outer-shell SO₄^2-or NO3-ions approximating extremely the inner sphere in aqueous NiSO₄ and Ni?NO₃?₂ solutions.The conclusions of EXAFS experiments and DFT calculation are in a good agreement with each other.