We probe the local and global structure of spin-coated colloidal crystals via laser diffraction measurements and scanning electron and atomic force microscopies, and find that they are unique three-dimensional orientationally correlated polycrystals, exhibiting short-range positional order but long-range radial orientational correlations, reminiscent of-but distinct from-two-dimensional colloidal hexatic phases. Thickness and symmetries are controllable by solvent choice and spin speed. While the polycrystallinity of these colloidal films limits their applicability to photonics, we demonstrate their feasibility as templates to make crack-free magnetic patterns. DOI: 10.1103/PhysRevE.77.050402 PACS number͑s͒: 82.70.Dd, 64.70.pv, 64.75.Yz The self-assembly of colloidal microspheres has been used to address the fundamental questions of how materials crystallize ͓1-6͔ or fail to crystallize ͓7-9͔. Micrometer-scale colloidal crystals can be used as a template that, using further processing methods, can be used to create photonic materials ͓10-12͔, optical sensors ͓13͔, and antireflection coatings ͓14͔. However, the high density of missing-sphere defects and cracks in photonic crystals produced via self-assembly ͓15͔ remains a serious limitation, and thus the study of colloidal defects ͓16͔ is an active area of research. Spin-coating of colloidal suspensions is the quickest and most reproducible method to make large-area colloidal crystals. While spin-coating has been proposed to fabricate single crystals for photonic applications ͓17,18͔, the symmetric radial optical interference patterns observed are unexpected for single crystals. We find here that spin-coated colloidal films are indeed neither single crystals nor powder polycrystals, but are in fact a unique polycrystal phase. While true singledomain sizes are ϳ10 m, there is orientational correlation on the centimeter scale. Our results demonstrate a novel crystal packing strategy by which long-range orientational order develops in the absence of long-range positional order, reminiscent of two-dimensional colloidal hexatic phases ͓19,20͔, and leading to crack-free crystals. Distinct from colloidal hexatic phases, our polycrystals exhibit centimeterscale orientational order, which arises due to the spinning axis and can be produced with fourfold, sixfold, or mixed symmetries for a range of thicknesses as a function of spin speed. The electrodeposition of magnetic material through colloidal polycrystals demonstrates their feasibility for material templating applications.The standard technique to make large-area close-packed crystals is controlled ͑vertical͒ drying, utilizing capillary forces ͓21-23͔ to direct self-assembly. Other external shear ͓24͔, electric ͓25͔, electrohydrodynamic ͓26,27͔, and gravitational forces ͓28͔ have also been used. Making dried colloidal crystals with these methods is slow, taking from hours to days. Spin-coating has been shown to be a robust technique ͓17,18,29,30͔ to make large-area colloidal crystals in minutes. In this work, we ...

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Removal of color from dye-bearing water has been a bothering task from the health point of view and in the aesthetic sense as well. Finding out the effectiveness of Remazole Red RGB dye removal is the aim of the present work using banana stem, an agricultural waste, as an activated carbon. The preparation of the banana stem activated carbon was done in the laboratory by carbonization followed by activation. To assess the impact of varied exploratory variables such as pH of adsorbate, contact time, initial dye concentration and dosage of adsorbent on the removal of Remazole Red RGB dye from aqueous solution adopting batch studies were performed. The Freundlich and Langmuir isotherm models were used to predict the adsorption capacity of adsorbent at equilibrium conditions, the kinetic parameters were also determined. The concentration of dye remaining in the system was measured using a UV visible spectrometer. The batch test resulted in the removal of the maximum of 87% of dye when the dye concentration was 2 mg/L, at an adsorbent dose of 0.5 gm/L and dye pH 4 in 35 minutes. It is established from the studies that the equilibrium data suited excellently both Freundlich and Langmuir isotherms. The maximum adsorption capacity of 14.28 mg/L was established from the Langmuir isotherm model. To state the sorption kinetics, pseudo-first-order and pseudo-second-order kinetic models were evaluated. The intraparticle diffusion model was checked for the adsorption data and it was observed that intraparticle diffusion is not the only rate-limiting step. The regression analysis results indicated that the linear regression model gave the best results. The above detection exhibits that banana stems can be used productively for the removal of color from dye-containing wastewater.

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This research work goals at searching the effectiveness of Malachite Green dye removal using banana stem, an agricultural waste as an activated carbon. The banana stem activated carbon was made ready in the laboratory by carbonization followed by activation. Adsorption studies were carried out to check the effect of various experimental conditions like different pH values, varying contact time, initial concentration of dye and changing banana stem carbon dosage on the removal of Malachite Green dye from aqueous solution at constant Temperature and agitation speed. The equilibrium experimental data were used for applicability of Langmuir and Freundlich isotherm models and the kinetic models. Batch test showed that maximum of 99% of dye was removed when the dye concentration was 2 mg/L, at an adsorbent dose of 0.75 gm/L at dye pH 8 in 45 minutes. From the obtained results it is validated that the equilibrium data’s favorable for both Freundlich and Langmuir isotherms. Maximum adsorption capacity of banana stem carbon on malachite green dye was found to be 8.29 mg/g. It was prevailing that the adsorption process followed the pseudo-second-order rate kinetics. It was observed that intra particle diffusion is not the only rate-limiting step in this adsorption system but also regression results indicate that the linear regression model gives the best results. The above observations recommend that Banana stem carbon can be competently implemented for removal of malachite green dye from aqueous solution in the adsorption treatment processes.