**5. Methods for chalcogen materials characterization**

impact due to its role in bigger molecular mobility. The chalcogen bonds also form eclipse (*cis*) and staggered (*trans*) configurations, enable chalcogen elements to develop many types

The intra-molecular chalcogen bonds were obtained from the X-ray diffraction results and quantum chemical calculations, such as in thioindirubin [52], indicating advanced methodology in modern analysis of big molecules. Chalcogen bonds were also studied with the aid of computational programs, as the development of old theories, especially in the debates around the energetic significance and physicochemical origins of the so-called class *σ*-hole interaction [53], continue to fuel scientific discussions. These approaches have served as important steps

of materials for specific application, from real crystals to real amorphous substances.

**4. Challenges in analytical methods for chalcogen compounds and** 

Natural chalcogen compounds which are present in certain matrix in perfect blends, and separation procedure must be conducted first, followed by purification prior to analysis. There are several parameters must be taken into account for the correctness of measurements. In the extraction of sulfur-containing compounds in plants, one would normally use phytochemical procedures [44] by considering the separation of constituent analytes according to the

The development of new materials from chalcogen compounds due to the functionality of the materials must be supported by a better analytical methodology. When materials are dedicated to a specific purpose, then the proof to that claim might be assigned analytically too. In this case, there are two types of analytical chemistry for the assignment of chalcogen materi-

**1.** The need of pretreatment method prior to analysis, for the natural compounds in natural

**2.** The need of methods for characterization of the chalcogen compounds, as well as new

**3.** The methods to describe the application for the new materials which include several other

**4.** The need of methods validation involving more than one method and instrumentation.

All of the human efforts in the laboratory as well as in computational analysis are based on the four types of analytical objectives listed above. The emphasis must depend on the purpose or in one segment of a longer process and all point of views can count. Therefore, any bigger steps can be started from chemistry discussion and developed into a wider investigation

als. In short, the need of analytical method can cover four types of analysis:

toward the synthesis, analysis, and designing of new materials [51].

**materials**

12 Chalcogen Chemistry

matrix.

perspective.

fields of disciplines.

similarities of the compounds.

materials, in practice and theoretically.

Characterization method is the backbone of chalcogen chemistry material description, as it always accompanies the explanation of material properties [54–57]. There are still some divisions in chalcogen material characterization, which includes the analysis for the main material itself and characterization of the impurities. The presence of impurities will decrease the quality to some extent [58, 59]. Notwithstanding, there have been too few reports discussing the impurities aspect if not related to their main functionalities.

Basic spectroscopy methods, especially X-ray methods, are the main tool for material characterization, including chalcogen materials. The methods are based on incoming X-ray beam that undergoes some natural phenomena like absorption, emission, fluorescence, and diffraction, then scattering with many possibilities to explore the chemical composition and properties of the sample. The crystallinity of materials can be derived from the X-ray diffraction patterns and the crystal database from instrument companies. In this case, the X-ray penetrates through the materials, and a number of particles can be expected to be oriented in such a way as to fulfill the Bragg's law. Almost all crystalline compounds analysis rely on XRD spectra, such as analysis of metal complexes of metal-thiourea and metal phenyl-thiourea [60, 61] after several steps of synthesis, to determine the coordination sphere on the metal Zn(II), Co(II) and Cu(II) and the possible crystal structures. More study of the spectra confirmed the shape of crystalline compounds together with UV-Visible and infrared as well as magnetic susceptibility measurement. In other synthesis the X-ray spectra were used to calculate reactive tendency as well as shape of molecules [23–25] besides characterization. Another X-ray technique is the energy dispersive analysis by X-rays (EDX) which intensity is proportional to the amount of the elements. The method is commonly combined with scanning electron microscopy (SEM) to get pre-experimental data, before the variables are given [22, 24, 26] or to characterize and give elemental confirmation [62].

Nuclear magnetic resonance for solid sample can be powerful to characterize chalcogen materials. It is based on the impact of radiofrequency irradiation on specific nuclei in certain field strength of the magnet (FT-NMR), causing the nuclei to spin resulting in resonance frequency which is an indication of the atom (e.g., 77Se and 125Te) [28]. NMR for chemist such as 1 H or 13C NMR is usually the most important method for chemical structure elucidation. However, material scientists need solid-state NMR with its magic angle spinning (MAS). Moreover, when other nuclei of resonance are used, one must swing the magnetic field according to selected NMR active probe nuclei, such as 77Se and 125Te NMR [28, 29] to describe and confirm newly synthesized octahedral coordination compounds. NMR method assisted the description of how chalcogen elements (Se and Te) that can replace halogen as inner ligands in forming cluster cores in octahedral cluster complexes. This action reduces the symmetry and makes distortion on the metallic cluster as well as their isomers and of course changes the properties of the whole materials. Proton and (1 H) carbon (13C) NMR of complex protein molecules were employed to investigate *Se-*(2-aminoalkyl)selenocysteines as biochemical redox agents [63]. In this case, chalcogen-biochemical substance was investigated through the behavior of its protons and carbons. The similar proton, carbon as well as 77Se spectra recorded, was also the main method of metal chalcogenides characterization synthesized from single source molecular precursor, besides X-ray diffraction [64]. This NMR method extended to confirm the presence of chalcogen atoms in the complex molecular building through 195Pt NMR. There was also a good NMR result which suggested the coordination of thiones to Zink(II) although the sulfur (S) atoms indicated an up-field shifting of = C=S resonance of 13C NMR as well as a downfield N-H resonance in <sup>1</sup> H NMR [65]. Similar analysis was done for the characterization of complex coordination compound using thiourea derivative for biochemistry research purposes [66]. The proton NMR was also used to analyze the stability of intermolecular coordination as one good aspect is organo-chalcogen compound [5]. One can see that NMR is very useful in describing molecular properties and mobility.

samples together with the elastic and inelastic scattering of the electrons. Thicker samples result in decreasing energy of the electron beams and increasing the scattering as well as complexity from the bigger distribution of energy and at the end declined resolution is obtained. Most of the synthetic chalcogen materials or metal chalcogenides are firstly being visualized with the aid of SEM and TEM before any other theoretical modeling or use of methods for

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Dispersive infrared (IR) spectroscopy has been in use and became more popular with the modern nondispersive Fourier transform-infrared (FT-IR) systems to probe the presence of certain functional groups. From the energy point of view, vibrational frequencies are the base of most analyses, and rotational frequencies also count. Raman spectroscopy, on the other hand, from very different principles of spectroscopy, the scattered intensity of the absorbed energy informs the same energy absorbed by vibration. Sulfur is observable by infrared spectroscopy [66, 71, 72] by their vibrational modes, especially stretching and bending vibrational modes in solid, liquid, or gaseous phases. Fingerprint region is also important. Bulk characterization by IR was employed to analyze synthetic compounds to prove the presence of thionyl vibrational mode (*ν*(C-S)) with frequency band shifts to lower values after coordination with metallic atoms [66]. Similar recording of infrared spectra were found in metal complexes of thiourea and phenylthiourea crystals [60, 61]. The C-H stretching of the components overlapped with N-H stretching of the thiourea, but both can be differentiated since N-H is not directly involved in bond formation with the metals. The metal ion complexation on the ligands is more pronounced on N-C and C=S bond which is shifted after the complex is formed. It is also confirmed that the phenylthiourea is coordinated to metal *via* the sulfur with

Surface characterization modes use additional probes such as attenuated total reflectance (ATR) or diffuse reflectance infrared Fourier Transformed spectroscopy (DRIFT) [9] or reflection absorption infrared spectroscopy (RAIRS). Sorption study of selenium(IV) solution on natural zeolites was done by infrared spectroscopy [73]. In this case, pH and concentration of sodium selenite solution onto shabazite, analcime, stilbite, mesolite from volcanic fields were studied. Some new absorption bands from Se-O as well as Se-O-Se bridges were observed, different from original infrared spectra recorded before. The strongest changes due to the highest pH of sodium selenite were the shifted absorption of tetrahedral Al-Si-O of the natural zeolite framework downfield in alkaline situation and another band appeared that confirms the absorption state of the ions. The partial desilylation of zeolite in alkaline medium as well as dealumination of zeolites occurring in acidic solution were observed in the infrared spectrum. In thin film form materials, near infrared (NIR) analysis was used to compare the photo-response of silicon doped with Se and Te via laser irradiation [15]. In this case, surface morphology and optical properties were accessed by NIR spectroscopy, as well as the stability of Si-chalcogen interaction. In other discussion, the vibration-rotation spectrum informs the bond length of the molecules being investigated [71] and process chemistry can be followed. A very minute detail of absorption energy can be useful, making this method valuable from time to time during synthesis, for calculating and determining the crystal building of the structure. Moreover, some workers have used IR spectroscopy to complement the computa-

tional calculation of new inorganic complex cluster with chalcogen elements [29].

analysis [15, 20, 24, 50, 56, 62, 68–70].

a reduction in π-electron density of the C=S bond.

Thermogravimetric analysis is a method in which changes in physical and chemical properties of materials are measured as a function of increasing temperature (with constant heating rate) or as a function of time (with constant temperature and mass loss). The important method of differential thermal analysis (DTA) one in which the mass changes is related to specific heat capacity. The increase of temperature is programmed to be linear and the heat flow to both sample and reference. Solid decomposition will occur due to phase changes, and this process can be endothermic or exothermic. One example of the methods on chalcogen compound was reported in the discussion of TGA of zinc and cadmium thiolate and selenolate complexes that showed the formation of metal sulfide (MS) and metal selenides (MSe) [M = Zn, Cd], while the mercury complexes showed complete weight loss in this temperature range [5]. TGA is mentioned as a good method to characterize metal chalcogenides [67] and has become a key analytical information generating technique together with X-ray diffraction data to validate the formation on chalcogen arsenide clusters in the iron with carbonyl functional groups [13]. In studying the degradation of palladium thiolate and selenolate, TGA was also used, to confirm the formation of Pd<sup>4</sup> S and Pd15Se17 which was then characterized by XRD and EDX [64].

Optical microscopy is used after the synthesis steps of chalcogen materials, which aids the visual characterization of materials. This method is improved continuously and became the earlier stage of today's electron microscopy. Reflection mode of the instruments is preferable, and this method is called episcopic light differential interference contrast (DIC) microscopy, which enables imaging of polymer, glasses, semiconductor, metals and minerals sample with various reflective properties. DIC microscopy also has its limitation, as it gives experimental uncertainties during measurement, which is discussed in several numerical methods to minimize them [57].

Scanning electron microscopy (SEM), which is a technique used for a better description of materials' surface textures up to nanometer scale, is a good way of visualizing chemistry by secondary electrons. The ability to focus the extremely small incident wavelength of the energetic electrons to resolve object in extraordinary spatial resolution, makes the method popular for nanotechnology's purposes. Electrons are scattered very intensively compared to X-rays in both elastic and inelastic ways for both organic and inorganic materials, in dimensions less than 1 nm. While transmission electron microscopy (TEM) is a similar microscopy electron, but the image is formed from the passage of some electrons passing through thin sliced samples together with the elastic and inelastic scattering of the electrons. Thicker samples result in decreasing energy of the electron beams and increasing the scattering as well as complexity from the bigger distribution of energy and at the end declined resolution is obtained. Most of the synthetic chalcogen materials or metal chalcogenides are firstly being visualized with the aid of SEM and TEM before any other theoretical modeling or use of methods for analysis [15, 20, 24, 50, 56, 62, 68–70].

the main method of metal chalcogenides characterization synthesized from single source molecular precursor, besides X-ray diffraction [64]. This NMR method extended to confirm the presence of chalcogen atoms in the complex molecular building through 195Pt NMR. There was also a good NMR result which suggested the coordination of thiones to Zink(II) although the sulfur (S) atoms indicated an up-field shifting of = C=S resonance of 13C NMR as well as

tion of complex coordination compound using thiourea derivative for biochemistry research purposes [66]. The proton NMR was also used to analyze the stability of intermolecular coordination as one good aspect is organo-chalcogen compound [5]. One can see that NMR is very

Thermogravimetric analysis is a method in which changes in physical and chemical properties of materials are measured as a function of increasing temperature (with constant heating rate) or as a function of time (with constant temperature and mass loss). The important method of differential thermal analysis (DTA) one in which the mass changes is related to specific heat capacity. The increase of temperature is programmed to be linear and the heat flow to both sample and reference. Solid decomposition will occur due to phase changes, and this process can be endothermic or exothermic. One example of the methods on chalcogen compound was reported in the discussion of TGA of zinc and cadmium thiolate and selenolate complexes that showed the formation of metal sulfide (MS) and metal selenides (MSe) [M = Zn, Cd], while the mercury complexes showed complete weight loss in this temperature range [5]. TGA is mentioned as a good method to characterize metal chalcogenides [67] and has become a key analytical information generating technique together with X-ray diffraction data to validate the formation on chalcogen arsenide clusters in the iron with carbonyl functional groups [13]. In studying the degradation of palladium thiolate and selenolate,

Optical microscopy is used after the synthesis steps of chalcogen materials, which aids the visual characterization of materials. This method is improved continuously and became the earlier stage of today's electron microscopy. Reflection mode of the instruments is preferable, and this method is called episcopic light differential interference contrast (DIC) microscopy, which enables imaging of polymer, glasses, semiconductor, metals and minerals sample with various reflective properties. DIC microscopy also has its limitation, as it gives experimental uncertainties during measurement, which is discussed in several numerical methods to mini-

Scanning electron microscopy (SEM), which is a technique used for a better description of materials' surface textures up to nanometer scale, is a good way of visualizing chemistry by secondary electrons. The ability to focus the extremely small incident wavelength of the energetic electrons to resolve object in extraordinary spatial resolution, makes the method popular for nanotechnology's purposes. Electrons are scattered very intensively compared to X-rays in both elastic and inelastic ways for both organic and inorganic materials, in dimensions less than 1 nm. While transmission electron microscopy (TEM) is a similar microscopy electron, but the image is formed from the passage of some electrons passing through thin sliced

H NMR [65]. Similar analysis was done for the characteriza-

S and Pd15Se17 which was then character-

a downfield N-H resonance in <sup>1</sup>

14 Chalcogen Chemistry

useful in describing molecular properties and mobility.

TGA was also used, to confirm the formation of Pd<sup>4</sup>

ized by XRD and EDX [64].

mize them [57].

Dispersive infrared (IR) spectroscopy has been in use and became more popular with the modern nondispersive Fourier transform-infrared (FT-IR) systems to probe the presence of certain functional groups. From the energy point of view, vibrational frequencies are the base of most analyses, and rotational frequencies also count. Raman spectroscopy, on the other hand, from very different principles of spectroscopy, the scattered intensity of the absorbed energy informs the same energy absorbed by vibration. Sulfur is observable by infrared spectroscopy [66, 71, 72] by their vibrational modes, especially stretching and bending vibrational modes in solid, liquid, or gaseous phases. Fingerprint region is also important. Bulk characterization by IR was employed to analyze synthetic compounds to prove the presence of thionyl vibrational mode (*ν*(C-S)) with frequency band shifts to lower values after coordination with metallic atoms [66]. Similar recording of infrared spectra were found in metal complexes of thiourea and phenylthiourea crystals [60, 61]. The C-H stretching of the components overlapped with N-H stretching of the thiourea, but both can be differentiated since N-H is not directly involved in bond formation with the metals. The metal ion complexation on the ligands is more pronounced on N-C and C=S bond which is shifted after the complex is formed. It is also confirmed that the phenylthiourea is coordinated to metal *via* the sulfur with a reduction in π-electron density of the C=S bond.

Surface characterization modes use additional probes such as attenuated total reflectance (ATR) or diffuse reflectance infrared Fourier Transformed spectroscopy (DRIFT) [9] or reflection absorption infrared spectroscopy (RAIRS). Sorption study of selenium(IV) solution on natural zeolites was done by infrared spectroscopy [73]. In this case, pH and concentration of sodium selenite solution onto shabazite, analcime, stilbite, mesolite from volcanic fields were studied. Some new absorption bands from Se-O as well as Se-O-Se bridges were observed, different from original infrared spectra recorded before. The strongest changes due to the highest pH of sodium selenite were the shifted absorption of tetrahedral Al-Si-O of the natural zeolite framework downfield in alkaline situation and another band appeared that confirms the absorption state of the ions. The partial desilylation of zeolite in alkaline medium as well as dealumination of zeolites occurring in acidic solution were observed in the infrared spectrum. In thin film form materials, near infrared (NIR) analysis was used to compare the photo-response of silicon doped with Se and Te via laser irradiation [15]. In this case, surface morphology and optical properties were accessed by NIR spectroscopy, as well as the stability of Si-chalcogen interaction. In other discussion, the vibration-rotation spectrum informs the bond length of the molecules being investigated [71] and process chemistry can be followed. A very minute detail of absorption energy can be useful, making this method valuable from time to time during synthesis, for calculating and determining the crystal building of the structure. Moreover, some workers have used IR spectroscopy to complement the computational calculation of new inorganic complex cluster with chalcogen elements [29].

Most of organic and inorganic compound or ions adsorb radiation in the ultraviolet and visible region (UV-Vis) (180–750 nm). Part of chalcogen materials also produces electronic spectra that show shape of molecules or crystal as a result of the frequency absorption bands from ligands, especially for the bands near the visible region as expected [60, 61]. Furthermore, the electronic transition in *d-*orbitals also provides strong evidence for complex compounds containing transition metals. Examples of similar complex metal chalcogenides follow the same principles for different shapes of molecules, together with analysis of magnetic susceptibility, which suggests the shape of environment of the central metal ions with the presence of chalcogen ligands.

**6. Methods for testing the applications of chalcogen materials**

alternative device for use in infrared spectroscopy [80].

**Methods Characterization Application Explanation**

XRD [13, 20, 21, 23, 24,

NMR [2, 5, 13, 27, 28,

61–64]

51–53, 60, 61, 73]

materials.

The method of applications will depend on the field of applications. The difficult part of it is to find a probe or indicator for the desired properties needed to be performed by the materials. This includes more analytical chemistry, with biological capacity or computer calculations. Suitable characterization is also essential to correlate the application and the properties of the

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Application of many types of chalcogen materials for environmental purposes employs infrared sensing for chalcogenides fibers [72] and also extended to other signal in infrared region can be utilized for environmental sensors. The manufacturing and testing of optical fiber sensors made from transparent chalcogen compounds for environmental can also be an

For agricultural or medicinal application, bioassay is mostly used. New materials for antimicrobial properties are tested using qualitative or quantitative microbial assay [60, 61], in which pathogenic microbials were used to test the biological potentials of the compounds synthesized, as seen from disc diffusion method. The cultured microorganism in petri dishes would give clear inhibition zones around a spot of medium impregnated with stock solution of the synthesized complexes during incubation under certain conditions. Potential antibacterial activity can be further traced quantitatively. Usually, several methods are used together for the specific area of applications. Moreover, many methods can be compared one to other

Uv visible [60, 61] Characterization of compounds structures.

IR [13, 15, 24, 66, 68, 73] For the characterization of functional groups after

Luminescence [27] Characterization of chalcogen compounds by

SOx

and NH3

silicon. Sorption study of

[15] Investigation on fabrication of silicon based

the molecules

spectral luminescence study.

[81–85] For the mobility of small molecules in porous oxide

[68] For the analysis of colored compound when probing

reduction.

selenium(IV) solution on natural zeolites

new material by near infrared analysis.

reactions, characterization of chalcogen dopants on

Determination of crystallinity of materials and its combination, for characterization of intermolecular interactions, the effect of chalcogen substitution

Characterization of hydrogen and carbon- containing groups, also Se and Te NMR for chalcogen elements in

materials, testing of chalcogen material application

One other important analytical method for both characterization as well as application of chalcogen materials is electrochemistry [74]. The role of electrochemistry in synthesis, development, as well as characterization, up to applications, is obvious. This method is based on electron transfer in chemical reactions, in which metals have the most possible elements for electron storage systems. In photo-electrochemical systems, in which electron from the reaction is to be stored as energy or used for the next reaction.

Characterization of magnetic properties is also important in the study of chalcogen materials. Before, vibrating sample magnetometer was used to get information about the magnetization of samples when vibrated in a uniform magnetizing field. Magnetization is therefore induced, the product of magnetic susceptibility and the applied magnetic field provide chemical information of the materials. The specific techniques include: magnetic separation, magnetic spectroscopy, magnetic susceptibility measurement, magneto-relaxometry, magnetic particle spectroscopy, and rotating magnetic field. Some magnetic properties can be changed due to chalcogen substitution to metal iron complexed compounds [55, 75].

Dynamics in chalcogen materials is also trending in the field since it is crucial to describe the desired properties of the materials. In addition, the dynamics of materials are now core in understanding conductivity and diffusivity of the materials [76]. Materials with ion dynamics of different substructures enable phonon scattering process in their solid state. Actually, NMR relaxation and diffusion experiments are powerful tools used to describe molecular mobility, no matter what the nuclei probe is [77, 78]. The same method can be applied to characterize chalcogen materials as well as metal chalcogenides.

Since computer is involved in most of the modern chemical analysis, the chalcogen bonds in protein are one example in this field [79]. The computational analysis needs some unfamiliar tools and methods; however, it provides a lot of information about how molecules bind together naturally. Sulfur, selenium and tellurium are the probes of energetically favorable trends in the synthesis of chalcogen complex structures [29], following modeling by computational analysis. In addition, while there are more types of interaction occurring together in the biomolecules, which one cannot resolve them one by one, nonionic and noncovalent bonds are usually resolved by computational calculation. The intramolecular forces between sulfur and oxygen was also reported as the chalcogen bonds which is responsible for many bond formations in bigger molecules [52] is often being modeled by means of computation.
