Results Of N2 Adsorption Research Articles

Effect of pyrolysis temperature and hydrothermal activation on structure, physicochemical, thermal and dye adsorption characteristics of the biocarbons.

In this study, the possibility of using sawdust from mixed woods to obtain biocarbons in the pyrolysis and steam activation using microwave radiation as an energy source was investigated. Biocarbons were characterized based on the results of N2 adsorption, thermogravimetric analysis (TGA), Fourier spectroscopy (ATR-FTIR), and Raman spectroscopy. Surface characterization was performed by Boehm titration and pHpzc determination. The adsorption capacity of methylene blue (MB) and methyl orange (MO) was studied. It was proved that the pyrolysis temperature and hydrothermal activation increased the biocarbons structural, surface and adsorption parameters of the obtained biocarbons. The amorphous materials of small specific surface area with a significant share of micropores were obtained. The steam activation resulted in significant development of the porous structure (to SBET =729.2 m2 /g and Vp =0.36 cm3 /g). The presence of functional groups, mainly phenolic ones, and pHpzc values indicating a potential for adsorption applications regarding MO and MB were found. The highest sorption capacity concerning MB (qe =122.8 mg/g) and MO (qe =99.1 mg/g) was observed for the steam-activated biocarbon BC-800MW . Using microwave energy hydrothermal activation proved very efficient, resulting in significant surface area and porosity development with a small loss of carbon matter.

Activity Enhancement of Ferrierite in Dimethyl Ether Carbonylation Reactions through Recrystallization with Sodium Oleate.

Methyl acetate (MA) has a wide range of applications as an important industrial chemical. Traditional MOR zeolite for carbonylation of DME to MA accumulated carbon easily because of a 12-membered ring (12 MR) channel. In this work, we innovatively developed the method of recrystallization ferrierite (FER) zeolite using special chelating ligand sodium oleate which can affect ions other than alkali metals. The characterization results of N2 adsorption, transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR) show that hydrothermal recrystallization of ferrierite using sodium oleate resulted in a higher Si/Al ratio, a bigger specific surface area and a larger number of Brønsted acid sites in the eight MR channels, which was more efficient in the reaction of carbonylation of dimethyl ether than ordinary alkali treatment.

Investigation on the mechanism of Nb and Si co-doping on low SO3 generation from V-based catalyst during NH3-SCR process

Generally, SO3 tends to be formed in the process of selective catalytic reduction (SCR), and SO3 has negative effects on atmospheric environmental production and the safe operation of coal-fired power plants. In this study, a series of Nb and Si codoped V-based catalysts were prepared via the impregnation method, and Nb and Si species were applied to reduce the SO3 generation ratio from the catalyst. Moreover, the controlled condensation method was used to collect the SO3, which formed during the NH3-SCR reaction process. The results indicated that the Nb and Si codoped catalysts exhibited the best NO reduction performance with the lowest SO3 generation ratio. The SO3 generation ratio and the NO conversion ratio for the VW2NbTi-10Si catalyst were 0.39% and 92%, respectively. The physicochemical properties of the catalysts were analysed by a series of characterization techniques. The X-ray diffraction (XRD) results revealed that the Nb and Si species could inhibit the crystallization of TiO2, and the X-ray photoelectron spectroscopy (XPS) results indicated that Nb and Si could decrease the ratio of Oα and V5+ on the surface of the catalyst. From the CO2 and NH3 temperature programmed desorption (CO2 and NH3-TPD) results, the Nb and Si species could reduce the number of alkaline sites on the catalyst surface, and the Si species could significantly enhance the acidity of weak and strong acid sites over the catalyst. Combined with the results of N2-adsorption and desorption, CO2-TPD, H2 temperature programmed reduction (H2-TPR) and XPS experiments, the addition of Nb and Si species was beneficial to inhibiting the adsorption and oxidation of SO2 on the surface of catalysts, resulting in a significant decrease in SO3 emissions. Therefore, in-situ diffuse reflective infrared Fourier transform spectroscopy (DRIFTS) analysis demonstrated that the Nb and Si species could restrict the reaction between SO2 and V species, decreasing the formation of the intermediate product VOSO4. Furthermore, the enhancement of reaction temperature and NH3 volumetric fraction would significantly promote the formation of SO3. However, the variations in the volumetric fraction of O2 and NO expressed ignorable effects on the SO3 generation ratio.

Achieving efficient toluene mineralization over ordered porous LaMnO3 catalyst: The synergistic effect of high valence manganese and surface lattice oxygen

To elucidate the role of high valence manganese and surface lattice oxygen in catalyst for catalytic combustion of toluene, herein, a porous LaMnO3 catalyst was firstly fabricated using ordered mesoporous carbon (OMC) as a hard-template. Compared with pristine LaMnO3-bulk, the optimal porous LaMnO3 exhibited boosted catalytic performance for toluene combustion, leading to a ∼50 ℃ lower T90% (required temperatures for 90% toluene conversions). The results of N2 adsorption–desorption measurement, X-ray photoelectron spectroscope, O2 temperature-programmed desorption and H2 temperature-programmed reduction indicated that the optimal porous LaMnO3 displayed high molar ratios of surface Mn4+/Mn3+ and Olatt/Oads, which exposed abundant active sites and promoted the preferential low-temperature catalytic oxidation of toluene. Simultaneously, the increased content of high valence manganese (Mn4+) and surface lattice oxygen (Olatt) in porous LaMnO3 catalyst accelerated the cycle of toluene deep oxidation. Combined with the analysis of toluene intermediates by in situ diffuse reflectance infrared Fourier transform spectroscopy, the toluene degradation mechanism over porous LaMnO3 was also explored. This work provides a feasible strategy for design of porous Mn-based perovskite catalyst with a high efficiency and stability for catalytic combustion of VOCs.

Potassium-doped carbon nitride supported on SBA-15 for enhanced catalytic Knoevenagel condensation under mild conditions

Knoevenagel condensation reaction is an important reaction for the manufacture of α,β-unsaturated carbonyl compounds. Traditional base catalysts suffer from the disadvantage in catalystproduct separation. Herein, potassium-doped carbon nitride materials supported on SBA-15 (CN/SBA15-KCl) have been prepared through a one-pot calcination method. The characterization results of N2 adsorption–desorption, small-angle XRD, TEM, and SEM revealed that the ordered mesostructures of SBA-15 had been preserved after the loading of carbon nitride and KCl, and nitrogen and potassium species were dispersed on the surface of SBA-15 support. As solid bases, the CN/SBA15-KCl materials demonstrated high and steady catalytic performances in Knoevenagel condensation. Under 50 °C of reaction temperature, the optimized conversion of benzaldehyde was up to 74% at 4 h of reaction time, much higher than other carbon-nitride-based materials. The incorporation of potassium has been proved to enhance the basic strength of CN/SBA15, and therefore improved the catalytic activity.

In-situ synthesis of metal-organic framework embedded in ordered mesoporous silica functionalized with carboxyl groups for 4-nitrophenol to 4-aminophenol

Herein, the –COOH functionalized ordered mesoporous silica (OMS-C) was successfully synthesized using 3-phosphonopropionic acid as a functionalized reagent via one-pot co-condensation method. Then Cu2+ ions were adsorbed by the –COOH groups present on the OMS surface. Finally, the composite material (MOF-199@OMS-C) functionalized by Metal-Organic Framework (MOF-199) was in-situ prepared through the Cu2+ ions adsorbed by the OMS-C. The experimental results of N2 adsorption–desorption, low-angle XRD, and TEM showed that MOF-199@OMS-C composites possessed an ordered 2D hexagonal mesoporous structure, high specific surface area and the formation of MOF-199 in the pore of OMS-C. Moreover, FT-IR, Raman, XPS, and TG analysis demonstrated the carboxylation on the surface of OMS, successful synthesis, and immobilization of MOF-199 within the pores of OMS-C through the –COOH of OMS, as well as the interaction between OMS-C and MOF-199. The MOF-199@OMS-C composites exhibited good catalytic activity in the reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The apparent kinetic constant of the reduction reaction of 4-NP was 0.26 min−1 under the optimum reaction condition. The MOF-199@OMS-C composites prevented the loss and structural damage of MOF-199, and its catalytic activity even remained stable after 4 cycles. In addition, the kinetic curves of reduction reaction of 4-NPs were consistent with first-order kinetics.

Cyclodextrin-based metal–organic framework nanoparticles as superior carriers for curcumin: Study of encapsulation mechanism, solubility, release kinetics, and antioxidative stability

In this paper, we propose a facile program of preparing nanoscale γ-cyclodextrin-based metal–organic frameworks (Nano-CD-MOFs) for the encapsulation of curcumin. Such Nano-CD-MOFs not only possess excellent mono-dispersity and crystalline structure, but also perform superior loading capacity. The results of N2 adsorption–desorption, XRD, DSC, and microtopography are utilized to confirm the presence status of encapsulated curcumin and further reveal the encapsulation mechanism of Nano-CD-MOFs. Curcumin-loaded Nano-CD-MOFs (Cur-Nano-CD-MOFs) dramatically increase curcumin solubility and a top-down uniform dispersion in the dissolution process. The perfect fitting of First-order and Korsmeyer-Peppas models suggests that the release performance of Nano-CD-MOFs is controlled by the loaded quantity of curcumin and related to Fickian diffusion. Moreover, the antioxidative stability of Cur-Nano-CD-MOFs is considerably enhanced even after 120 min of persistent ultraviolet irradiation. Therefore, we suggest that such Nano-CD-MOFs can be promoted as an advanced carrier for the delivery of curcumin or other nutraceuticals.

Synthesis, characterization and photocatalytic activity of a new type of high-efficiency polyacid composite

In order to achieve the degradation of Congo red dye in wastewater, a new type of three-dimensional porous composite catalyst PW7Mo3Cu2/PANI/MnO2 was prepared by using heteropoly acid [TBA]4H3[PW7Mo3Cu2O38(H2O)2] doped intermediate PANI/MnO2. Using IR, UV, SEM, XPS and other characterization techniques, it was confirmed that the heteropoly acid [TBA]4H3[PW7Mo3Cu2O38(H2O)2] was successfully doped into the intermediate PANI/MnO2 to form a three-dimensional porous structure. The results of N2 adsorption–desorption experiment indicated that the composite catalyst belongs to the type IV (a) mesoporous structure material and has a large pore size and specific surface area. Then, the composite catalyst PW7Mo3Cu2/PANI/MnO2 was used to photocatalyze the degradation of Congo red dye. Under the best photocatalytic conditions, the decolorization rate of Congo red dye reached 93.84%. After recovering and repeating the photocatalysis experiment three times, the decolorization rate of Congo red dye was 73.18%. The experimental results proved that the novel composite catalyst has a strong ability to degrade Congo red dye and reusability, and has potential application value.

Phosphorus and nitrogen-doped palladium nanomaterials support on coral-like carbon materials as the catalyst for semi-hydrogenation of phenylacetylene and mechanism study

In this work, two types of polyporous and coral-like materials (CN) with high specific surface area are prepared using sodium glutamate as a carrier. At the same time, a CN-supported phosphorus-nitrogen-doped palladium nanomaterial CN-P-Pd is synthesized and applied to the preparation of styrene by selective hydrogenation of phenylacetylene under mild conditions. As shown in the TEM images, Pd nanoparticles with a particle size of about 4.4 nm are uniformly dispersed on the surface of the carrier. The results of N2 adsorption–desorption reveal that the surface area of the prepared catalyst (CN-P-Pd) is 1307 m2g−1. In addition, the experimental exploration shows the intervention of P in carbon-nitrogen materials can contribute to improve the selectivity of the reaction, which can be attributed to the fact that P element can change the electron density of Pd. Meanwhile, it is found that the solvent not only affects the activity of catalyst, but also the selectivity of the reaction. Kinetic study shows the activation energy of the reaction is 4.5 kJ/mol. With the increase of the reaction temperature, the dissolution rate of hydrogen in the solvent gradually slows down, which inhibits the progress of the reduction reaction. Mechanistic studies demonstrate that the carbon-nitrogen materials have strong adsorption capacity for substrates, and also provide more adsorption sites for phenylacetylene. Additionally, the optimal catalyst (CN-P-Pd) also has high reaction activity to other alkynes and the conversion can reach at 95%. Moreover, the optimal catalyst can be reused several times without significant reduction in reaction activity.

Novel CuO@TiO2 Core–Shell Nanostructure Catalyst for Selective Catalytic Reduction of NOx with NH3

CuO@TiO2 core–shell nanostructure catalyst with CuO core and TiO2 shell was prepared by a two-step method and then used in NH3-SCR reaction. FE-SEM, TEM and element mapping were used to investigate the morphologies of the sample. Their results indicated CuO nanorods were coated with TiO2 and thus forming a core–shell nanostructure. According to the results of N2 adsorption–desorption, XRD, Raman, XPS, NH3-TPD, H2-TPR and in situ DRIFTS, the core–shell nanostructure would benefit the formation of better redox ability, more oxygen vacancies, more acid sites and abundant adsorbed NOx species on the catalyst. Thus, the core–shell nanostructure catalyst presented significantly higher activity than pure CuO and TiO2. NH3-SCR reaction on CuO@TiO2 core–shell catalyst should follow both Eley–Rideal (E-R) mechanism and Langmuir-Hinshewood (L–H) mechanism. These findings may promote the development of the new effective non-vanadium-based SCR catalysts.

Sisal xylem fibre-based activated carbon fibres for fuel adsorption: effect of thermal stabilization of diammonium phosphate

Activated carbon fibres (ACFs) are considered as the next generation of activated carbon products. However, lack of structural diversity in pore structure and high prices of raw materials for ACFs has restrained the development of ACF materials. In this paper, a sisal-based activated carbon fibre (SACF) material was prepared from sisal wastes with a unique thermal stabilization treatment to maintain fibrous shapes of SACFs while dispersing in solutions, and the SACFs were prepared as raw fibre materials for fuel evaporation emissions controlling products. Experimental results of N2 adsorption showed that SACF has a typical I-type adsorption isotherm, with specific surface area of SACF samples of approximately 1200 m2 g−1, and mainly microporous pore structure. Compared with commercial samples (specific surface area, 1841.29 m2 g−1), the butane working capacities of SACF for advanced fuel evaporation emissions controlling product was 0.4 g/100 ml higher. Furthermore, two dynamic models, Thomas model and Clark model, were applied to adsorption breakthrough data, which showed excellent fit. And it indicated from the adsorption breakthrough curves and parameters of both models that the SACF has better performance in fuel adsorption and desorption process than the commercial samples.

Electrochemical performance of porous carbons derived from needle coke with different textures for supercapacitor electrode materials

The porous carbons with high specific surface area and excellent electrochemical properties were prepared using three types of green needle coke as raw materials. Electrochemical performances of the porous carbons derived from different microstructure green needle coke were investigated. The XRD and Raman spectra demonstrated that the content of the ordered carbon microcrystals were decreased and the content of amorphous and cross-linked structure were increased in the porous carbons with comparison to the raw materials. The results of N2 adsorption–desorption analysis verified that the content of ordered microcrystalline structure in the raw materials evidently influence the specific surface area and pore size distribution of the porous carbons. The porous carbon with 1665 m2 g−1 specific surface area and 2.89 nm average pore size has shown that the specific capacitance was 288 F g−1 at the current density 1 A g−1. Furthermore, the capacity retention was 94.93% and the Coulombic efficiency was 92.87% after 5000 charge/discharge cycles.

Tobacco stem-derived N-enriched active carbon: efficient metal free catalyst for reduction of nitroarene

A series of N-enriched active carbons (NACs) were prepared from carbonization of tobacco stem with KOH as activating agent. The obtained NACs were used as efficient metal free catalysts for reduction of nitroarene. The results of N2 adsorption–desorption indicate that NACs have a large number of mesopores with the increase of mass ratio of KOH to the precursor. NACs-3800 has high surface area (SBET = 2938 m2 g−1) and a large pore volume (Vtotal = 1.60 cm3 g−1) with a nitrogen content (1.32%). NACs-3800 showed a high catalytic activity (99.9% conversion, nearly 100% selectivity to aniline) in the reduction of nitrobenzene. Moreover, NACs-3800 also exhibits good catalytic activity in the reduction of substituted nitroarenes. Furthermore, the NACs-3800 presented remarkable activity and durability for nitrobenzene reduction to the corresponding aniline. The recyclability of the catalyst and its cheap feedstock makes the overall process much simple, cost-efficient and environment-friendly.

Ultrafine MnO2/graphene based hybrid nanoframeworks as high-performance flexible electrode for energy storage applications

Pristine MnO2 and MnO2/graphene (abbreviated as Mn and MnG) nanocomposite has been prepared through a facile hydrothermal method under mild conditions. The morphology and nanostructure of the prepared composite are individualized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman microscope, Brunauer–Emmet–Teller and X-ray photoelectron spectroscopy (XPS). XRD results reveal the pure tetragonal phase of α-MnO2 with the space group of I4/m. TEM images show spherical morphology for pure Mn and layered structure for MnG. The results of N2 adsorption–desorption analysis indicates the specific surface area of the prepared nanomaterials and it was found to be 92 m2/g and 134 m2/g for the pure Mn and MnG nanocomposites. The XPS spectrum shows the confining states of different elements present in the composites. Electrochemical execution of the synthesized composite electrodes was evaluated using both two and three-electrode system. We have achieved maximum specific capacitance of 1537 Fg−1 at the charging current of 20 Ag−1 with high steadiness was observed up to 6000 cycles. The fabricated ASC devices manifest a maximum energy density of 22 Wh kg−1 with long standing cyclic stability of 90.5% capacitance retention after 5000 cycles.

Extraction of highly pure silica from rice husk as an agricultural by-product and its application in the production of magnetic mesoporous silica MCM–41

Rice husk is one of the agricultural by-products that is found abundantly around the world. After utilization of its organic matters as a source of energy in the combustion process, the remaining ash is rich in silica, which is a valuable precursor for production of silica materials. In this study, in the first step, the extraction of silica from rice husk was optimized by leaching with four acids of HCl, HNO3, H2SO4, and H3PO4, and in the second and third step, the mesoporous silica MCM–41 and the magnetic mesoporous silica MCM–41 (magMCM–41) were produced by the extracted silica. The produced materials were characterized by the XRF, XRD, FT-IR, N2 adsorption–desorption, VSM (vibrating sample magnetometry), TGA, SEM, and TEM analyses. The results of XRF analysis showed that the silica was extracted by 95.48% purity by leaching with HCl. The XRD and TEM analyses results revealed that the magMCM–41 was produced with a highly ordered hexagonal structure. The results of N2 adsorption–desorption analysis indicated that the magMCM–41 specific surface area, total pore volume, average pore diameter, and wall thickness were 695.7 m2 g−1, 0.65 cm3 g−1, 2.71 nm, and 1.59 nm, respectively. The magnetic property of the magMCM–41 was determined to be 10.61 emu g−1 at optimum conditions by the VSM analysis. The results of TGA analysis proved the thermal stability of the magMCM–41 by low total weight loss of 1.97%.

The sludge-based adsorbent from oily sludge and sawdust: preparation and optimization

ABSTRACT Sludge-based adsorbent (S-AB) converted by oily sludge can make full use of the precious resource. In this paper, oily sludge and discarded sawdust are used to prepare adsorbent through chemical activation. The adsorbent prepared is used to adsorb raw petroleum. Firstly, the most reasonable chemical activator ZnCl2 is ascertained through parallel comparative experiments. The characterization results of N2-adsorption are consistent with adsorption experiment results, which shows that higher mesopore surface area and volume are benefitted by the adsorption process. Secondly, the optimization of preparation technology is investigated through orthogonal experiments after parallel comparative experiments. The adsorption capacity of S-AB-ZnCl2 is stronger when the preparation conditions are as follows: an activation temperature of 550°C, an activation time of 3.5 h, a solid–liquid ratio of 1:1.5, a sludge-sawdust ratio of 1:0.5 and the heating rate of 15°C/min. The maximum quantity adsorbed Q 0 = 434.78 mg/g, calculated through the Langmuir adsorption isothermal models, of S-AB-ZnCl2 prepared under optimized condition is higher than that before optimization. In addition, the most reasonable kinetics fits were of the second-order model.

Aerobic Oxidation of Fluorene to Fluorenone over Copper-Doped Co3O4 with a High Specific Surface Area

Copper-doped Co3O4 with a high specific surface area has been prepared by using mesoporous silica KIT-6 as a hard template, which showed exceptional catalytic performance (99% conversion of fluorene and 99% selectivity of fluorenone) for catalytic oxidation of fluorene to fluorenone using molecular oxygen as the oxidant in the absence of a radical initiator or other cocatalysts. Characterization results of N2 adsorption–desorption, X-ray diffraction, and transmission electron microscopy of the samples revealed the ordered mesoporous structure and increased specific surface area. X-ray photoelectron spectroscopy and O2 temperature-programmed desorption results showed the increased surface cobalt, Co3+/Co2+ ratio, and oxygen species. The H2 temperature-programmed reduction also indicated that the redox ability could be improved by doped copper in Co3O4, which all facilitated the aerobic oxidation reaction.

Tailoring a Thermally Stable Amorphous SiOC Structure for the Separation of Large Molecules: The Effect of Calcination Temperature on SiOC Structures and Gas Permeation Properties

A SiOC membrane with high oxidative stability for gas separation was tailored by utilizing vinyltrimethoxysilane, triethoxysilane, and 1,1,3,3-tetramethyldisiloxane as Si precursors. Amorphous SiOC networks were formed via the condensation of Si–OH groups, the hydrosilylation of Si–H and Si–CH=CH2 groups, and a crosslinking reaction of Si–CH3 groups, respectively. The crosslinking of Si–CH3 groups at temperatures ranging from 600 to 700 °C under a N2 atmosphere was quite effective in constructing a Si–CH2–Si unit without the formation of mesopores, which was confirmed by the results of N2 adsorption and by the gas permeation properties. The network pore size of the SiOC membrane calcined at 700 °C under N2 showed high oxidative stability at 500 °C and was appropriate for the separation of large molecules (H2/CF4 selectivity: 640, H2/SF6: 2900, N2/CF4: 98). A SiOC membrane calcined at 800 °C showed H2/N2 selectivity of 62, which was approximately 10 times higher than that calcined at 700 °C because the SiOC networks were densified by the cleavage and redistribution reactions of Si–C and Si–O groups.

One-step strategic synthesis of x%Ni–AlSBA-15 sorbents and properties of high adsorption desulfurization for model and commercial liquid fuels

Hexagonally mesoporous 5–30%Ni-AlSBA-15 (30) sorbents were prepared by one-step strategy synthesis and the adsorptive desulfurization performances were investigated in a fixed bed adsorber via model and commercial liquid fuels at ambient pressure. The highest sulfur capacity (17.46 mg-S/g-sorbent) over 20%Ni-AlSBA-15 (30) correlated closely with high Lewis acid sites and larger SBET of 20%Ni-AlSBA-15 (30). The characterization results of N2 adsorption, FT-IR, Py-FT-IR, ICP, HRTEM, EDXA, small- and wide-angle XRD revealed that aluminum or nickel atoms could enter effectively the Si-O-Al (or Ni) framework of SBA-15 and were randomly dispersed highly on the hexagonal pore walls of SBA-15. The hexagonal p6mm symmetry mesoporous structure of 20%Ni-AlSBA-15 (30) still remained intact after four successive desulfurization/regeneration cycles, indicating that 20%Ni-AlSBA-15 (30) presents ultrahigh structural stability and regenerable ability. As comparison, SBA-15 and AlSBA-15 (SiO2/Al2O3 = ∞, 100, 50, 30, 20, 10) were also prepared and evaluated on physical properties.

Insight into the Organic–Inorganic Hybrid and Microstructure Tailor Mechanism of Sol–Gel ORMOSIL Antireflective Coatings

Having a deep insight into the organic–inorganic hybrid (OIH) and microstructure tailor mechanism of sol–gel ORMSOSIL antireflective (AR) coating is of great interest in the field of sol–gel science and technology. VTES/TEOS ORMOSIL sols were prepared via a sol–gel process by using vinyltriethoxysilane (VTES) and tetraethoxysilane (TEOS) as co-precursors. 29Si MAS NMR was applied to deduce the OIH mechanism between VTES and TEOS. The results of N2 adsorption–desorption analysis indicated that OIH could effectively tailor the pore structure. The OIH of VTES and TEOS significantly increased the porosity. With this convenient sol–gel process, the refractive index of VTES/TEOS ORMOSIL AR coatings could be regulated gradually between 1.12 and 1.21 by varying the VTES/TEOS molar ratio. The OIH mechanism and the drying process of these VTES/TEOS ORMOSIL AR coatings were proposed after a detailed discussion on the relationship between microstructure and optical properties.