Hydrothermal Synthesis of Transition Metal Oxides



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2.2 Reagents and Solvents

Boric acid (H3BO3, Alfa Aesar 99.99%) was used as the solvent in the reactions. Sodium vanadate (NaVO3, Fluka), and potassium metavanadate (KVO3, Aldrich 98%) were used as transition metal source. Lead chloride (PbCl2, Riedel-de Haen) was used. Distilled water and acetone were used to wash products.



2.3 Characterization Techniques

The simplest and most obvious first question about an inorganic substance is ‘What is it?’. The methods that are used to answer this come into two main categories depending on whether the substance is molecular or non-molecular. If the substance is molecular (whether it is solid, liquid or gaseous), identification is usually carried out by some combination of spectroscopic methods and chemical analysis. If the substance is non-molecular and crystalline, identification is usually carried out by X-ray powder diffraction supplemented, where necessary, by chemical analysis.

Each crystalline solid has its own characteristic X-ray powder pattern which may be used as a ‘fingerprint’ for its identification. The powder patterns of most known inorganic solids are included search procedure, unknowns can usually be identified rapidly and unambiguously.

Once the substance has been identified, the next stage is to determine its structure, if it is not known already. For molecular materials, further spectroscopic measurements can be used to study the details of the molecular geometry. If the substance is crystalline, X-ray crystallography may be used, in which case information is also obtained on the way in which the molecules pack together in the crystalline state [22].

No single technique is capable of providing a complete characterization of a solid. There are three main categories of physical technique that may be used to characterize solids; these are diffraction, microscopic and spectroscopic techniques. In addition, other techniques such as thermal analysis, physical property measurements may give good information.

2.3.1 X-ray Powder Diffraction

X-ray diffraction methods are some of the most powerful characterization tools known by scientists. In chemistry concerning solids, the two primary pieces of information most often sought are the structure of the material and its reactivity. X-ray diffraction methods can be used to study single crystals, powders and other forms of solids [42].

An X-ray powder diffraction pattern is a set of lines or peaks, each of which are in different intensity and position (d-spacing or Bragg angle, θ) on a strip of photographic film or chart paper. For a given substance the line positions are fixed and characteristic for that substance. The intensities may vary from sample to sample depending on the method of sample preparation and the instrument conditions.

If the substance is a common type then the experimental X-ray powder diffraction pattern can be compared to known published patterns such as those found in the ASTM (American Society for Testing and Materials) tables [42]. Standard patterns of crystalline substances are given in the Powder Diffraction File, JCPDS (Joint Committee on Powder Diffraction Standards) or ASTM File. The inorganic section of this file now contains over 35000 entries and is increasing at a rate of about 2000 per year [22]. If a powder pattern has never been collected before, analogies to known structural types can be made.

X-ray powder diffraction pattern of the compound synthesized were obtained by using a Philips X’pert Pro X-ray diffractometer. Samples were placed on a zero-background silicon sample holder. Data was collected by using CuK ( =1.5406 Å) radiation at settings of -45 kV and 40 mA for 30 minutes. The scan rate was 0.1o/sn and the data was collected for 2 values of 5 to 65o.

2.3.2 Single Crystal X-Ray Diffraction

Single crystal X-ray diffraction methods have several applications such as determination of unit cell and space group, crystal structure determination, electron distribution, atom size and bonding, crystal defects and disorder. The role that crystal structure determination by X-ray diffraction has played in inorganic and solid state chemistry cannot be exaggerated. Virtually all our knowledge of crystal structures has been obtained by this method over a period of some sixty years. This knowledge has been essential to understand crystalline materials, their structures, properties and applications.

Crystal quality is probably the single most important factor in determining the final precision for a given structural investigation. High-precision structural results require high-quality crystals. In general, crystals should meet the following criteria:


  1. They must be single. The crystal should be a single entity which has no smaller crystals or powder attached to it.

  2. They must be of the proper size and shape. The crystal should be 0.1 to 0.6 mm. on an edge and as equidimensional as possible. For substances which do not contain highly absorbing elements (e.g. most organic molecules), the optimum sample would be a ~0.5mm. diameter sphere. Needle- or plate-shaped crystals can usually be used if they are at least 0.1mm thick.

  3. Good single crystals usually have well-defined and lustrous faces; they are of uniform color and contain no cracks or fracture lines.

  4. They must be ordered and diffract to reasonably high scattering angles.

  5. They must have reasonably uniform and small mosaic spreads.

Suitable single crystals were mounted in epoxy, and placed in a capillary. Single crystal X-ray diffraction data were collected on a Bruker Smart 1000 CCD diffractometer under following conditions. A full reciprocal sphere corresponding to a total of 3x606 frames collected (-scan, 15 s per frame, 0.3o oscillations for 3 different values of j). Monochromatic MoK (=0.71073 Å) was employed. Cell refinement and data reduction were carried out with the use of the program SAINT [43]. Face-indexed absorption corrections were made with the program XREP [44]. The structures were solved by direct methods with the program SHELXS and refined by full-matrix least squares techniques with the program SHELXL in the SHELXTL-97 [44] suite.



2.3.3 Infrared Spectroscopy

The basis of the IR experiment is to pass infrared radiation through a thin sample of compound and measure which energies of the applied infrared radiation are transmitted by the sample [45].

The infrared absorption spectra of both single crystals and KBr pellets were studied in the range of 4000-400cm-1 using a FTIR spectrometer (Nicolet Magna-IR 550). A few milligram of sample was mixed with KBr in approximately a 1:10 ratio to prepare the pellet. The mixture was ground by using mortar and pestle and placed inside a die. Then the die was pressed in hydraulic press for 2 minutes under 6000-psi pressure.

Pellets were then placed inside the spectrometer port and measurements were done. Resolution was optimized to 8cm-1 and 32 scans were done. Samples were typically run in transmittance mode.



2.3.4 Electron Microscopy (SEM/EDX)

The Scanning Electron Microscope (SEM) has become one of the most widely utilized instruments for material characterization. The SEM is a microscope that uses electrons rather than light to form an image. The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time. The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Preparation of the samples is relatively easy since most SEMs only require the sample to be conductive

The most common accessory equipped with a SEM is the energy dispersive x-ray detector or EDX. This type of detector allows a user to analyze a sample molecular composition.

Qualitative analysis of the heavy elements in single crystal samples was obtained using a Philips XL 30S FEG Scanning Electron Microscope. The accelerating voltage for each scan was 5 kV with an accumulation time of 30 seconds. Spot was 3, and magnification was 1200. The detector type was Secondary Electron (SE) or Through the Lens (TLD). The results of EDX analysis are usually presented as a spectrum. In this graphical representation the X -axis represents the energy level - and therefore identifies the elements, and the Y-axis provides the number of counts of each element detected.



2.3.5 Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC)

Change in the weight and enthalpy changes of the synthesized compounds with temperature were examined using thermogravimetric analysis (Shimadzu TGA-51), and differential scanning calorimetry (Shimadzu DSC-50), respectively.

For the DSC measurements, powder samples typically in the size of 10-25mg were placed in an aluminum pan. The samples were analyzed at 5oC/min from room temperature to 500oC and then cooled to room temperature with nitrogen flow. Flow rate was 40 mL/min.

For the TGA measurements, powder samples in the size of 10-25mg were placed in a platinum pan. The samples were analyzed at 5oC/min from room temperature to 1000oC with nitrogen flow. Flow rate was 40 mL/min.




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