Higher NaOH Concentrations Research Articles

Optimized Process for Separating NaOH from Sodium Aluminate Solution: Coupling of Electrodialysis and Electro-Electrodialysis

Most bauxite-produced alumina is obtained by the Bayer process, but the production efficiency is limited by the slow gibbsite crystal growth due to the high concentration of NaOH in sodium aluminate solution. Here electrodialysis (ED) and electro-electrodialysis (EED) are coupled to separate NaOH from the sodium aluminate solution so as to enhance the gibbsite crystal growth rate and achieve high alumina production efficiency. The ED or EED process is also investigated before the coupling process to find the optimal operating conditions. The ED process indicates that the optimized current density is in the range of 45–60 mA cm–2 and the optimal membranes are CMV/AMV. The current density of 60 mA cm–2 can achieve a high recovery ratio (ηOH– 92.6%), low energy consumption (2.38 kW h kg–1), but a relatively high Al(OH)4– leakage ratio (ηAl(OH)4– 15.1%). The EED process indicates that with the optimized current density of 30 mA cm–2 and membrane CMV, the ηAl(OH)4– can be “zero” and the energy consumption can be as low as 2.07 kW h kg–1, but the treatment capability is low since OH– ions cannot be recovered directly and a single cation exchange membrane is used. The coupling process can combine the advantages of ED and EED, so that the ηOH– can keep a high value of 90.9%, the ηAl(OH)4– decreases to a low value of ∼5%, and the energy consumption remains low at 2.25 kW h kg–1. Overall, the coupling process of ED and EED is an excellent method to separate NaOH from the sodium aluminate solution.

Polypropylene composites reinforced with untreated and chemically treated coir: Effect of the presence of compatibilizer

The effect of coir surface modification on the compatibility of polypropylene (PP)/coir fiber (CF) composites, in the presence and absence of compatibilizer (maleic anhydride grafted polypropylene, PP-g-MA) was assessed. Chemical pulping of the fibers was performed with 2, 4, 8, and 12% NaOH solutions for a period of 2 h at (100 ± 4)°C. Pressed composite samples were subjected to tensile testing, scanning electron (SEM) and atomic force microscopy (AFM). Lignin and holocellulose concentrations of untreated and treated coir were determined. Pulping resulted in increased tensile strength of the composites containing coir treated with up to 2% NaOH, due to increased fiber roughness as evidenced by AFM. This property decreased when higher NaOH concentrations were used, likely due to increased deterioration of coir. The presence of compatibilizer in the PP composites containing treated coir altered adhesion due to chemical changes of the fiber surface. At high NaOH concentrations increased delignification and therefore increased exposure of hydroxyls favors reaction between the fiber hydroxyls and the carboxyl acids of the hydrolyzed maleic anhydride, present in the composites. POLYM. ENG. SCI., 55:2050–2057, 2015. © 2014 Society of Plastics Engineers

Falha por stress cracking em híbridos PET/argila

O presente estudo investiga a falha por stress cracking em polímeros contendo argila montmorilonítica. Ensaios de tração e de relaxação de tensão foram conduzidos para avaliar a resistência ao stress cracking do PET puro e dos híbridos PET/argila em contato com soluções aquosas de hidróxido de sódio. As análises por difratometria de raios-X evidenciaram que a argila adicionada não gerou uma estrutura esfoliada e sim a obtenção de microcompósitos. Os resultados mostraram que a presença de argila dispersa no PET causa concentração de tensão, o que exerce forte influência no comportamento de stress cracking, com os efeitos sendo afetados pela ordenação lamelar e ao teor de carga, resultando em maiores taxas de relaxação de tensão. A argila com menor regularidade no empilhamento lamelar, embora ocasione menor concentração de tensão, favorece mais o fissuramento superficial da matriz, sugerindo que o efeito de barreira ao fluido não foi efetivo. Medidas de massa molar evidenciaram que a argila acelera o ataque químico da matriz quando elevadas concentrações de solução são utilizadas, porém diminui o efeito da tensão mecânica na degradação.

Template-Free Hydrothermal Synthesis of Octahedral Fe<sub>3</sub>O<sub>4</sub> Microcrystals and its Magnetic Property

Well-defined Fe3O4 octahedrons have been readily realized in high yield via a simple and general hydrothermal synthesis technology free of any surfactant or template. Detailed investigations indicated that the octahedral Fe3O4 microcrystals are single crystal and were dominated by (110) surfaces. The influences of the amount of NaOH on the morphologies of the final products were investigated. The experimental results show that the high concentrations of NaOH play vital roles for the formation of octahdral Fe3O4 microcrystals. Besides, the magnetic property of the as-obtained Fe3O4 sample was evaluated as potential materials for electromagnetic device application on a vibrating sample magnetometer (VSM) at room temperature. The values of saturation magnetization and coercivity of octahedral Fe3O4 are approximately 95 emu g-1 and 197 Oe, respectively.

Properties and controlled release of chitosan microencapsulated limonene oil

Chitosan microcapsules containing limonene essential oil as active ingredient were prepared by coacervation using three different concentrations of NaOH (0.50, 1.00, 1.45 wt%) and fixed concentrations of chitosan and surfactant of 0.50 wt%. The produced microcapsules were fully characterized in their morphology and chemical composition, and the kinetic release analysis of the active ingredient was evaluated after deposition in a non-woven cellulose fabric. The concentration of 1.00 and 1.45 wt% clearly show the best results in terms of dimension and shape of the microcapsules as well as in the volatility results. However, at the concentration of 1 wt% a higher number of microcapsules were produced as confirmed by FTIR and EDS analysis. Free microcapsules are spherical in size with disperse diameters between 2 and 12μm. Immobilized microcapsules showed sizes from 4 to 7μm, a rough surface and loss of spherical shape with pore formation in the chitosan walls. SEM analysis confirms that at higher NaOH concentrations, the larger the size of the microcapsules. This technique shows that by tuning NaOH concentration it is possible to efficiently control the release rate of encapsulated active agents demonstrating great potential as insect repellent for textiles.

Non-monotonic influence of a magnetic field on the electrochemical behavior of Fe78Si9B13 glassy alloy in NaOH and NaCl solutions

Abstract The corrosion behavior and microstructure of Fe78Si9B13 glassy alloy in NaOH and NaCl solutions under a 0.02-T magnetic field were investigated through electrochemical testing and scanning electron microscopy (SEM). The current-density prepeak (PP) in the anodic polarization curves in low-concentration NaOH solutions (classified as type I) tends to disappear when the NaOH concentration is increased to 0.4 mol/L and the magnetic field is applied. Under the magnetic field, the height of the second current-density peak is increased in low-concentration NaOH solutions (type I) but decreased in high-concentration NaOH solutions (type II). The non-monotonic effect of the magnetic field was similarly observed in the case of polarization curves of samples measured in NaCl solutions. Ring-like corroded patterns and round pits are easily formed under the magnetic field in NaOH and NaCl solutions. These experimental results were discussed in terms of the magnetohydrodynamic (MHD) effect.

Morphology-controlled synthesis of orthorhombic LuFeO3 particles via a hydrothermal route

LuFeO3 particles were synthesized via a hydrothermal route, where the influence of various experimental parameters on the products was systematically investigated. XRD analysis reveals that single o-LuFeO3 samples can be synthesized at reaction temperatures of 160–260°C, reaction times of 24–60h, NaOH concentrations of 0.625–6.25M, and Lu3+/Fe3+ ion concentrations of 0.025–0.075M. SEM observation shows that LuFeO3 particles prepared at relatively low temperatures (160–220°C) and low NaOH concentrations (0.625–3.75M) exhibit bar-like shape, and the aspect ratio of the bars decreases with increasing the temperature and NaOH concentration. At relatively high temperatures (240–260°C) and high NaOH concentrations (5–6.25M), the resulted LuFeO3 particles are shaped like spheres, polyhedrons or cubes. The samples prepared at the temperature of 220°C and the NaOH concentration of 2.5–3.75M have a relatively large particle size. All the particles exhibit a layered structure, indicating that they are mainly constructed from irregular sheets. Compared to the reaction temperature and NaOH concentration, the reaction time and Lu3+/Fe3+ concentration has a relatively small effect on the morphology and size of LuFeO3 particles. The as-prepared LuFeO3 samples have a similar bandgap energy of 2.14eV and exhibit a week ferromagnetic behavior at room temperature.

Potential utilization of class C fly ash-based geopolymer in oil well cementing operations

The early age compressive strength development of class C fly ash-based geopolymers under high pressure and high temperatures of curing is considered as an alternative to well cements. Uniaxial compressive strength (UCS) results show how the curing temperature affects the early compressive strength development. As the temperature rises from 87 to 125°C, a consecutive reaction seems to take place at the higher concentrations of NaOH, which decrease the compressive strength at the higher temperature. The taken scanning electron microscope (SEM) images show a change in the morphology of the samples at 125°C with the higher concentrations of NaOH. Ultrasonic cement analyzers (UCA) were employed to investigate the instantaneous strength development of the geopolymeric slurries. As the common cement models were not able to assess the compressive strength development, the custom algorithm option in the UCA software was applied. The developed empirical correlations were not able to accurately estimate the sonic strength of the slurries remarkably at 125°C. The rheological measurements of the prepared geopolymeric slurries showed a Newtonian like behavior.

Enhancement of nickel biosorption on fungal biomass by enzymatic and alkali pretreatments

Enzymatic and alkali pretreatments were employed to improve nickel biosorption capacity of Rhizomucor pusillus biomass. Pretreatment with 0.002–80 g l−1 NaOH and 0.0001–0.1 Anson Unit (AU) g−1 protease enhanced the biosorption capacity of fungal biomass. Increasing the concentration of NaOH from 0.002 to 5 g l−1 improved nickel removal from 93.2 to 100.0 % while untreated biomass showed 64.6 % Ni(II) removal. Pretreatment with higher concentrations of NaOH, 5–80 g l−1 resulted in nearly complete removal of nickel ions. Pretreatment of the biomass with 0.0001 AU g−1 protease improved the nickel removal to over 91 %, while increasing the enzyme loading to 0.1 AU g−1 improved the removal to 93 %. Untreated biomass removed 78.4, 63.0, and 96.3 % of chromium, copper, and lead ions, respectively, from a mixture solution of the ions. Respective metal removals were increased to 100, 98.9, and 100 % after pretreatment with 0.2 g l−1 NaOH solution and to 87.8, 86.7, and 100 % after the enzymatic pretreatment with 0.1 AU g−1 protease. Scanning electron microscopy analysis indicated that alkali and enzymatic pretreatments enhanced the porosity of the biomass. Furthermore, compositional analysis showed that both of the pretreatments removed a major part of fungal proteins (2.1–95.8 % removal). Glucosamine, N-acetyl glucosamine, and phosphates were the major ingredients of the pretreated biomass.

Application of Design of Experiments to the Alkaline Treatment in Mordenite Zeolite: Influence on Si/Al Ratio

This paper presents the application of Design of Experiments (DOE) to an alkaline treatment in mordenite zeolite, in order to identify the optimum settings of the variables (temperature and concentration of NaOH) that promote greater desilication. Samples of commercial mordenite with Si / Al ratio = 9 have been treated with aqueuos NaOH solution of various concentration (0,1;0,5;1,0) and temperature (30,50,70°C). Statistical analysis was carried out based on a 22 factorial design with center point replicates. Seven experiments were executed and the data of Si/Al ratio obtained. The resulting samples were characterized by x-ray difraction (XRD) and energy dispersive x-ray (EDX), to identify the structures and crystalline phases, and to determine the percentages of silicon and aluminum, respectively. In all samples, a decrease in the Si / Al ratio was observed, keeping the crystalline structure. The treatment was most effective, especially, at high temperature and NaOH concentration (experiment 4). DOE was shown to be a powerful techinique for the study of the variables, allowing determining which variables and value ranges have more influence on response, enabling optimization.

Lime-calcined clay materials with alkaline activation: Phase development and reaction transition zone

The research aim was to investigate phase development and reaction transition zone of alkaline activated lime-calcined clay materials. Locally available china clay mainly supplied metakaolin (MK) after calcination. MK and calcium hydroxide (CH) were used to supply SiO2 and CaO, respectively. The CH to MK ratio of 0.4 with CaO/SiO2 of 1.18 was selected to coincide with the theoretical CaO/SiO2 of calcium silicate hydrate in tobermorite group (Ca5Si6O16(OH)2). Phase development was investigated using X-ray diffraction and Fourier transform infrared spectroscopy. Compressive strength and density were also investigated to confirm binding properties of the pozzolanic reaction products. The reaction of mixture was dependent on the NaOH concentration. At low concentration of NaOH (<1M), the pozzolanic reaction was dominant while the zeolitic reaction and geopolymerization became dominant at medium NaOH concentration (>1M) and at high NaOH concentration, respectively. The activations of CH:MK mixtures with 0.01 and 0.1M NaOH promoted semicrystalline calcium silicate hydrate (CSH (I)) and crystalline calcium aluminosilicate hydrate (CASH) formations. The mixture with 0.01M NaOH gave the highest compressive strength of 19.0MPa. With 3 and 5M NaOH activations, sodium aluminosilicate hydrate (NASH) and sodium calcium silicate hydrate (NCSH) compounds were formed instead of CSH (I) gel and crystalline CASH phase. At 10M NaOH, NCSH disappeared and only NASH was formed and resulted in a relatively low compressive strength of 6.7MPa.

Spectroscopic investigation of hydrothermally synthesized zeolites from expanded perlite

The results of vibrational spectroscopy studies of zeolites synthesized from expanded perlite using hydrothermal method were presented. The starting material used was a waste obtained during the production of expanded perlite. The effects of synthesis temperature and time, as well as NaOH concentration and solid to liquid ratio on the obtained products were determined. The resulting materials were analyzed regarding phase composition. In particular, the structures of materials were examined using FT-IR spectroscopy in the middle and far infrared ranges as well as Raman spectroscopy. The results were compared to the XRD and XRF measurements, as well as SEM observations.It has been found that by using a sufficiently high concentration of NaOH, it is possible to efficiently synthesize zeolite at temperatures above 50°C. The presence of zeolite phase was confirmed by the measurement of spectra. Pseudolattice range, i.e. 800–400cm−1, was analyzed in detail. In this range, bands associated with the ring vibrations occur. They are characteristic for secondary building units (SBU) existing in zeolite structure. Depending on time, temperature and NaOH concentration, zeolite X, A, Na-P1 and hydroxysodalite have been identified as synthesis products in autogenous pressure.

Influence of Sonomechanical Treatment on the Structure of Cellulose Micro/Nano Fibrils

The fast-growing Eucalyptus (E. Europhylla) was used as the raw materials to prepare for micro/nanocellulose fibrils. The morphology changes of cellulose by sodium hydroxide linkage ultrasonic energy treatment was discussed. The properties of treated cellulose was evaluated by X-ray , scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy. By the degree of crystallinity of the experimental test results showed that: a high concentration (17.5%, mass fraction) NaOH solution swelling with ultrosonication chemical pretreatment of cellulose prepared micro/nanofibrils change in crystal form, that is transformed cellulose I into cellulose II. However, the cellulose micro/nanofibrils remained crystalline cellulose I type after treated by a low concentration (2%, mass fraction) NaOH solution swelling with ultrosonicaion chemical pretreatment. High alkali activation sound chemical pretreatment increased the crystallinity of obtained micro/nanofibrils, the corresponding values were 89.2% and 86.3%. Observed by the scanning electron microscope that: a low concentration alkaline with ultrosonication chemical pretreatment increased the degree of sub-wire broom, the fiber surface area increased accordingly, and the fiber is more "open", so that the reaction activity of the cellulose fibers improved. The infrared spectrum showed that: the chemical changes between cellulose micro/nanofibrils and NaOH occurred after mercerization.

Real-Time PCR 기법을 이용한 반추위 섬유소분해 박테리아의 부착과 조사료 분해에 관한 연구

The comparisons between cellulolytic bacteria adhesion on rice straw and fiber digestion in time course during rumen fermentation were studied in situ. The adhesions of cellulolytic bacteria, F. succinogenes. R. albus and R. flavefaciens, were measured by RT-PCR. When the rice straws were incubated at 0. 2, 4, 8, 12 and 24 hours of the in situ rumen, straw was degraded with increasing speed during the incubation and showed the highest disappearance increasing rate (DM g/h) from 8 to 12 hour. The adhesions of F. succinogenes, R. flavefaciens and R. albus were achieved above 80% in 1 hour of in situ rumen fermentation and then keep adhesive population up after the time of fermentation. When the in situ samples were collected at 0, 5, 10, 30 and 60 min to detect the early stages of adhesion on the rice straws ingested into rumen, the numberous adhesive colony of F. succinogenes, R. flavefaciens and R. albus were detected in 5 min. In case of rice straw treated with 0, 2, 4 and 8% NaOH, all of three cellulolytic bacteria showed the increasing trends of adhesion with increasing DM disappearance of rice straw by higher concentration of NaOH at 12 hour of in situ. However, there were showed respectively difference at 24 hour. The present results gave certain evidence that adhesion of cellulolytic bacteria is definitely achieved in early stage of roughage ingestion into rumen, their colony develop the stable communities on roughage in process of rumen fermentation and then fiber degradation is accelerated. (Key words : F. succinogenes, R. albus, R. flavefaciens, Adhesion, RT-PCR)

도시쓰레기 소각재 슬래그로 제조된 지오폴리머의 특성

본 연구에서는 도시 쓰레기 소각재 슬래그에 알칼리 활성화제로서 NaOH를 첨가하여 지오폴리머를 합성하고 그 물성을 평가하였다. 특히 NaOH의 몰농도, 원료의 입도 그리고 액체/고체 비율이 제조된 지오폴리머의 압축강도에 미치는 영향을 조사하였다. 원료의 입도가 미세할수록 합성된 지오폴리머의 강도는 증가하였으며, 액체/고체 비율의 최적 값은 0.13으로 나타났다. 합성된 지오폴리머의 압축강도는 첨가된 NaOH의 몰농도가 증가함에 따라 함께 증가하는 경향을 나타내었으나, 20 M 이상의 농도에서는 일정 값에 수렴하였다. 20 M 이상의 NaOH 농도로 제조된 지오폴리머에는 sodium aluminum silicate 및 sodium aluminum silicate hydrate 형태의 2종류 zeolite 결정상이 생성되었다. 20 M NaOH 및 <TEX>$70^{\circ}C$</TEX> 양생조건으로 제조된 시편에서 가장 높은 압축강도, 163 MPa이 발현되었으며, 이것은 고농도의 NaOH가 지오폴리머 반응 및 치밀한 미세구조 형성을 촉진시켰기 때문인 것으로 사료된다. 본 연구에서 제조된 고강도의 지오폴리머는 향후, 도시쓰레기 소각재 슬래그의 재활용율 제고는 물론 시멘트 대체 분야 발전에 일조할 것으로 기대된다. In this research, the geopolymer was fabricated using municipal solid waste incineration ash (denoted as MSWIA) slag and alkali activator, NaOH and its properties were analyzed. Particularly, the effects of NaOH molarity, particle size of MSWIA, and liquid/solids ratio on the compressive strength of geopolymers were investigated. The compressive strength of geopolymers fabricated increased with finer grain size of MSWIA, and optimum value of the liquid/solids ratio was identified as 0.13. As the molarity of the NaOH increased, the compressive strength of geopolymers was increased. Even more the 20 M of NaOH, but the strength was not increased. The calcium aluminum silicate and calcium aluminum silicate hydrate zeolites were generated in the geopolymer fabricated with more than 20 M of NaOH, with some unreacted silica and unknown crystals remained. The highest compressive strength, 163 MPa, of geopolymer was appeared at conditions of curing temperature <TEX>$70^{\circ}C$</TEX>, and 20 M of NaOH, indicating that the high concentration of NaOH accelerates the geopolymer reaction and dense microstructure. The high-strength geopolymer fabricated in the present study is expected to contribute significantly to develop the field of cement alternative substances and to improve the recycling rate of MSWIA slag.

Catalytic behavior of alkali-treated Pt/HMOR in n-hexane hydroisomerization

Bifunctional Pt-HMOR catalysts were prepared by incipient wetness impregnation of various desilicated MOR obtained by alkaline treatment using NaOH concentrations ranging from 0.1 to 0.5M. The zeolite structural changes upon modification were investigated by several techniques including powder X-ray diffraction, 27Al and 29Si MAS-NMR spectroscopy, N2 adsorption, pyridine adsorption followed by infrared spectroscopy and the catalytic model reaction of m-xylene transformation. For low alkaline concentration the zeolite acidity is preserved, along with a slight increase of the volume correspondent to the larger micropores due to the removal of extra-framework debris already existent at the parent zeolite. At higher NaOH concentrations there is a significant loss of crystalinity and acidity as well as the formation of mesoporosity. The characterization of the metal function shows similar patterns for Pt-HMOR and Pt-M/0.1 samples, with Pt particles located mainly inside the inner porosity. In contrast, large Pt particles become visible at the intercrystalline mesoporosity of MOR crystals developed during the desilication treatments at severe alkaline conditions. The catalytic results obtained for n-hexane hydroisomerization showed an improved selectivity for dibranched over monobranched isomers for Pt-M/0.1 sample, likely due to the preservation of the support acidity and the slight enlargement of the micropores. This work is a new example in which the mesoporous development does not improve the catalytic efficiency of the zeolites, whereas mild alkaline desilication might be considered as an effective solution to produce customized catalysts with enhanced performance for a given application.

One-step transformation of Cu to Cu2O in alkaline solution

Cu can be directly transformed to Cu2O before forming CuO in high concentration NaOH aqueous solution by facile surface oxidation reaction without additional oxidant. By using different Cu substrates, pure Cu2O films with different dominant facets can be obtained.

Structural and optical properties of Gd2SiO5 prepared from hydrothermally synthesized powder

Cerium doped Gd2SiO5 (GSO) is an important scintillator material due to its high density and non-hygroscopic nature. GSO powder of uniform size and morphology synthesized by chemical method can be used as a source material to grow single crystals and to prepare transparent ceramics. In this work, rod-like Gd2SiO5 powder has been synthesized by hydrothermal reaction of precursors followed by a thermal treatment at 1400°C for 20h. Effect of various growth parameters on phase and morphology of the synthesized Gd2SiO5 has been investigated. The desired Gd2SiO5 phase is obtained for an appropriate concentration (∼0.5M) of the precipitating agent (NaOH) and molar ratio of precursors (Gd (NO3)3⋅6H2O: Si (OC2H5)4=2:1). A higher NaOH concentration and lower molar ratio of precursors (Gd:Si) result in the formation of additional phases. For the Gd:Si=1:1 ratio, a leaf-like structure of hexagonal Gd4.67 (SiO4)3O is obtained. Evolution of the desired phase GSO has been studied by the non-resonant Raman spectroscopy. Substitutional cerium doping up to 1at.% in the GSO initiates a preferred growth towards the [021] direction. The rod-shaped Ce doped GSO has an emission at 425nm for excitations at 280 and 345nm near room temperature.

Preparation and characterization of cellulose nanofibers from partly mercerized cotton by mixed acid hydrolysis

Cellulose nanofibers with a diameter of 70 nm and lengths of approximately 400 nm were fabricated from partly mercerized cotton fibers by acid hydrolysis. Morphological evolution of the hydrolyzed cotton fibers was investigated by powder X-ray diffraction, Fourier transform infrared analysis and field emission scanning electron microscopy. The XRD results show that the cellulose I was partially transformed into cellulose II by treatment with 15 % NaOH at 150° for 3 h. The crystallinity of this partially mercerized sample was lower than the samples that were converted completely to cellulose II by higher concentrations of NaOH. The intensities of all of the diffraction peaks were noticeably increased with increased hydrolysis time. Fourier transform infrared results revealed that the chemical composition of the remaining nanofibers of cellulose I and II had no observable change after acidic hydrolysis, and there was no difference between the hydrolysis rates for cellulose I or II. The formation of cellulose nanofibers involves three stages: net-like microfibril formation, then short microfibrils and finally nanofibers.

Facile solution-based synthesis and optical properties of Co3O4 nanoparticles at low-temperature

Cobalt oxide(Co3O4) with different morphologies was achieved by a simple solution-based method. Various parallel experiments show that several experimental parameters, such as the concentrations of NaOH and ethylene glycol(EG), play important roles in the morphological controlling of Co3O4 nanoparticles. A lower concentration of NaOH favors quasi-spherical product with a uniform size of about 15 nm, whereas a higher concentration of NaOH generally leads to the formation of nanoplates with wide size distribution. In addition, Co3O4 nanorods were also obtained partially by introducing a certain amount of EG. A possible mechanism was proposed for the selective formation of Co3O4 with various morphologies. X-Ray diffraction(XRD), infrared(IR) spectrometry, scanning electron microscopy(SEM), transmission electron microscopy(TEM) and UV-Vis spectrometry were used to characterize the samples.