Showing papers on "Electrochromism published in 2005"

Increasing the conductivity of crystalline polymer electrolytes

Abstract: Polymer electrolytes consist of salts dissolved in polymers (for example, polyethylene oxide, PEO), and represent a unique class of solid coordination compounds. They have potential applications in a diverse range of all-solid-state devices, such as rechargeable lithium batteries, flexible electrochromic displays and smart windows1,2,3,4,5. For 30 years, attention was focused on amorphous polymer electrolytes in the belief that crystalline polymer:salt complexes were insulators. This view has been overturned recently by demonstrating ionic conductivity in the crystalline complexes PEO6:LiXF6 (X = P, As, Sb); however, the conductivities were relatively low6,7. Here we demonstrate an increase of 1.5 orders of magnitude in the conductivity of these materials by replacing a small proportion of the XF6- anions in the crystal structure with isovalent N(SO2CF3)2- ions. We suggest that the larger and more irregularly shaped anions disrupt the potential around the Li+ ions, thus enhancing the ionic conductivity in a manner somewhat analogous to the AgBr1-xIx ionic conductors8. The demonstration that doping strategies can enhance the conductivity of crystalline polymer electrolytes represents a significant advance towards the technological exploitation of such materials.

Novel Aromatic Poly(Amine-Imide)s Bearing A Pendent Triphenylamine Group: Synthesis, Thermal, Photophysical, Electrochemical, and Electrochromic Characteristics

Abstract: A new triphenylamine-containing aromatic diamine, N,N-bis(4-aminophenyl)-N‘,N‘-diphenyl-1,4-phenylenediamine, was synthesized from the amination reaction between 4-aminotriphenylamine and 4-fluoronitrobenzene and subsequent reduction of the dinitro intermediate. A series of novel aromatic poly(amine-imide)s with pendent triphenylamine units were prepared from the newly synthesized diamine and various tetracarboxylic dianhydrides by either a one-step or a conventional two-step polymerization process. All the poly(amine-imide)s were amorphous and readily soluble in many organic solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide, and chloroform. These polymers could be solution cast into transparent, tough, and flexible films with good mechanical properties. They had useful levels of thermal stability associated with relatively high glass transition temperatures (264−352 °C), 10% weight-loss temperatures in excess of 568 °C, and char yields at 800 °C in nitrogen higher than 63%. These polym...

Electrochromic windows and method of making the same

Abstract: An electrochromic window assembly is disclosed that includes a first substrate and a second substrate that is maintained in a parallel and spaced relation from the first substrate by means of a window frame and spacer. The window assembly further includes an electrochromic device mounted within the airtight chamber formed between the first and second substrates. Electrochromic device 22 may be mounted so as to provide an air chamber between first substrate 12 , which is the external substrate, and electrochromic device 22 . At least one of the chambers formed between substrates 12 and 14 and electrochromic device 22 may be filled with an insulating gas such as argon. Also disclosed are novel methods for manufacture of an electrochromic device for incorporation into such a window assembly. A novel electrochromic device is also disclosed having electrical bus clips secured about the entire periphery of the electrochromic device.

A Processable Green Polymeric Electrochromic

Abstract: We report electrochemical and optical properties of the first electrochemically and chemically prepared green, soluble conducting polymer in its neutral form. Oxidative electrochemical and chemical polymerization of dioctyl-substituted 2,3-di(thien-3-yl)-5,7-di(thien-2-yl)thieno[3,4-b]pyrazine (3b) results in a soluble polymer (4b) with novel optical properties. The neutral polymer 4b absorbs both blue (above 600 nm) and red light (below 500 nm), reflecting a saturated green color. In the oxidized form, these absorptions are depleted, resulting in a transmissive pale brown polymer with very strong absorption in the near-infrared. Electrochemical and spectroelectrochemical semiconductor band gaps of 4b were found to be ca. 1.3 eV. Molecular weight analysis showed that soluble polymers have different conjugation length from oligomers to polymers. Since the monomer 3b has low oxidation potential (0.48 V vs Ag/Ag+), mild oxidizing agents (i.e., CuCl2) initiate chemical polymerization. The oxidant/monomer rati...

Switchable window based on electrochromic polymers

Abstract: Syntheses of a new blue EC monomer (ProDOT-MePro), and a new red EC monomer (ProDOP-Et2) are described. Two additional new types of EC monomers based on 3,4-alkylenedioxythiophene include fluorinated EC monomers and an EC monomer including silicon. EC polymer devices having more than one different color EC polymer to enable additional colors to be provided using subtractive color mixing are also described, as well as EC polymer devices incorporating a logo, image, or text, are generally obscured when the device is colored, but become visible when the device is not colored. Also described are EC polymer devices that include a cathodic EC polymer layer, a gel electrolyte, a counter electrode, and a reference electrode. Working prototypes of such devices exhibit significant increases in the speed of transition of the EC device from a colored state to a transparent state.

Atmospheric Pressure Chemical Vapor Deposition of Crystalline Monoclinic WO3 and WO3-x Thin Films from Reaction of WCl6 with O-Containing Solvents and Their Photochromic and Electrochromic Properties

Abstract: The atmospheric pressure chemical vapor deposition (CVD) reaction of WCl6 with a variety of reactants (ethanoic anhydride, ethanoic acid, ethyl ethanoate, methanol, ethanol, 2-propanol, 2-methyl-2-propanol, and water) was examined on glass substrates. The deposited films displayed strikingly different morphologies with differing reactants under otherwise identical conditions. Full characterization revealed that the films were all monoclinic WO3, but the different morphologies lead to significant differences in the functional properties of the deposited films, such as photochromism and photocatalysis. The effects of the CVD parameters on deposited films were extensively studied for the reaction of WCl6 with ethanoic anhydride.

Water dispersible polypyrroles made with polymeric acid colloids for electronics applications

Abstract: Compositions are provided comprising aqueous dispersions of at least one polypyrrole and at least one colloid-forming polymeric acid. The colloid-forming polymeric acid may be fluorinated. The new compositions are useful in electronic devices including organic electronic devices such as organic light emitting diode displays, memory storage, electromagnetic shielding, electrochromic displays, and thin film transistors, field effect resistance devices.

Understanding electrochromic processes initiated by dithienylcyclopentene cation-radicals.

Abstract: Simple photochromic dithienylethylenes with either a perfluoro or a perhydro cyclopentene ring, and a variety of substituents (chlorine, iodine, trimethylsilyl, phenylthio, aldehyde, carboxylic acid, and ethynylanisyl), have been prepared and their electrochemical behavior was explored by cyclic voltammetry. All dithienylethylenes present two-electron irreversible oxidation waves in their open form, but the cation-radical of the open isomers can follow two different reaction pathways: dimerization or ring closure, whereas the halogen derivatives follow a dimerization mechanism, the presence of donor groups, such as the phenylthio-substituted compound, promote an efficient oxidative ring closure following an ECE/DISP mechanism. Electrochromic properties are also found in the corresponding ring-closed isomers. Depending on the substituents on the thiophene ring, and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding rin...

Growth and electrochromic properties of single-crystal V2O5 nanorod arrays

Abstract: This paper reports a study on the template-based growth and electrochromic properties of single-crystal vanadium pentoxide nanorod arrays by sol electrophoretic deposition The formation of single-crystal vanadium pentoxide nanorods is attributed to the homoepitaxial aggregation of nanoparticles Nanorod arrays have demonstrated significantly enhanced electrochromic properties; both the larger change of transmittance intensity and more rapid response upon applying external electric field are attributed to the large surface to volume ratio and improved transport kinetics for electrochemical intercalation

In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue

Abstract: An in situ colorimetric method, based on the CIE (Commission Internationale de l'Eclairage) system of colorimetry, has been successfully applied to the study of electrochromism in electrochemically deposited films of Prussian blue (iron(III) hexacyanoferrate(II), PB) on transmissive ITO/glass substrates for the first time. On electrochemical reduction of PB to Prussian white (iron(II) hexacyanoferrate(II), PW), sharp and reversible changes in the hue and saturation occur, as shown by the track of the CIE 1931 xy chromaticity coordinates. For PB, the CIELAB 1976 colour space coordinates were L* = 73, a* = −26 and b* = −33, with a dominant wavelength calculated as 488 nm. Concurrently, as the intensely absorbing PB mixed-valence chromophore is ‘bleached’ to the transparent PW, a large increase in the relative luminance of the electrochromic film is observed. On oxidation of PB, the CIELAB 1976 colour space coordinates show the transition through intermediate green to the Prussian yellow (iron(III) hexacyanoferrate(III), PY) state (L* = 94, a* = 2 and b* = 18), with a steady increase in relative luminance. To reliably compare the power requirement of PB films with other electrochromic systems, composite coloration efficiencies (CCE's) have been calculated, using a tandem chronoabsorptometry/chronocoulometry method, as previously developed for organic polymer systems. Using 95% of the total transmittance change at λmax as reference point, coloration efficiencies, η = ΔA(λmax)/Q, were calculated as 143 and 150 cm2 C−1 respectively for the PB/PW and PW/PB electrochromic transitions.

Electrochromic properties of atmospheric CVD MoO3 and MoO3–WO3 films and their application in electrochromic devices

Abstract: The transition metal oxides exhibit electrochromism, defined as color change caused by applied voltage. The electrochromic (EC) effect can be stronger in mixed oxides because of the higher optical absorption, due to electron transitions between two kinds of metal sites with different valences. Thin films of MoO3 and mixed Mo/W oxides were obtained by atmospheric pressure chemical vapor deposition (CVD), using pyrolytic decomposition of metal hexacarbonyls. IR and Raman spectra are presented and compared with data for WO3 from previous investigation. Sputter auger spectroscopy was used to determine the chemical composition of the mixed films. The visible transmittance of films deposited on glass substrate was about 80%; on conductively coated glass (i.e. SnO2:Sb/glass) the visible transmittance drops to around 60%. After annealing, the optical transmittance of the single oxide films slightly improves, while for the mixed films it slightly decreases. The electrochromic properties of MoO3 and mixed Mo/W oxide films were investigated by cyclic voltammetry performed in a conventional potentiostatic three-electrode configuration. Current and transmittance were measured simultaneously during the potential sweep at different wavelengths over the spectral range of 400–800 nm. Deep blue coloring of all the electrochromic films (MoO3, WO3 or Mo/WO3) appears only in the cathodic range, minimizing visible light transmittance. At positive potentials, the EC film starts bleaching (for about 3 min) and the film returns its initial transparency. It was found that the CVD deposited MoO3 and Mo0.07W0.93O3 thin films performed well as electrodes in electrochromic devices. We made devices based on Mo0.07W0.93O3/LiClO4 + propylene carbonate + polyvinyl alcohol + PVA/SnO2:Sb. The devices exhibited good cyclic reversibility (colored/bleached state) and good coloration efficiencies. We measured a transmittance change (600 nm) of 48.4% switching to 17.4% for our best device.

Welded Electrochromic Conductive Polymer Nanofibers by Electrostatic Spinning

Abstract: With the availability of electrochromic conjugated polymers [1] exhibiting transitions of the three primary colors, full-color electrochromic displays could be realized in the near future. [2] Although many, but not all, of the color transitions have been addressed, there is a need for rapid and wide-area processing methods as well as materials with response times within the millisecond regime in order to obtain an active display. Recently, the electrochromic phenomenon for poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes [3] prepared using a porous membrane [4] has been reported to be on the order of 5 ms, rapid enough for active-display applications. Other techniques for producing one-dimensional (1D) nano-structures of conjugated polymers of similar promise include interfacial polymerization, [5] templated synthesis, [6] and phase separation of block copolymers. [7] Apart from electrochro-mics, conjugated polymer nanostructures could find application as electronic interconnects, optoelectronic device components , chemical and biological sensors, catalysts, membranes, and drug-delivery systems. Electrostatic spinning (e-spinning) offers a practical means to prepare nanostructured conjugated polymers described above by producing high volumes of conjugated polymer nanofibers continuously, even to lengths of several kilometers. However, the intractability of conjugated polymers has prohibited their processing by e-spinning in pure inherently conductive polymer form. Instead, researchers have reported the blending of conjugated polymers with more readily electro-spun polymers such as poly(ethylene oxide) or poly(vinyl pyrrolidone). Most reports of inherently conductive polymer nanofibers formed in this way have utilized polyaniline, [8] although a report on the e-spinning of poly(phenylene vinylene) and a thiophene-based conjugated polymer has recently appeared. [9] To our knowledge, there has not been a report of an electrospun conjugated polymer exhibiting elec-trochromic function. Herein we describe a technique by which we electrospin a precursor polymer that can later be oxidized either chemically or electrochemically in the solid state [10] to produce a cross-linked conjugated polymer. Since solvent is used in the conversion process to swell the precursor polymer, the nanofibers naturally weld together during conversion to the crosslinked conjugated polymer, of benefit to multiple properties. In contrast , flash welding of polyaniline (PANI) nanofibers has been reported to give continuous films. [11] Further, we demonstrate here the capability of a 5 lm thick non-woven mat of this conjugated polymer nanofiber to electrochromically switch between two colored states on the timescale of a few seconds. The precursor polymer, 1, used for e-spinning was a 50:50 (mol/mol) random copolymer (Fig. 1) prepared via ring-opening metathesis polymerization (ROMP) of …

Soluble Variable Color Carbazole-Containing Electrochromic Polymers

Abstract: Three new carbazole-containing polymers were synthesized through Suzuki cross-coupling between N-octylcarbazole-3,6-bis(ethyleneboronate) (NOctCz(BO2Et)2) and several arylene dibromides. 2,5-Dibromothiophene (DBT), 2,2‘-dibromo-5,5‘-bithiophene (DBBT), and 4,7-dibromo-2,1,3-benzothiadiazole (DBBTD) gave the three polymers PCzTh, PCzBi, and PCzBTD, respectively. Cyclic voltammetry showed two separate redox processes for each polymer, PCzTh (E1/2 = 0.33 V and E1/2 = 0.65 V vs Fc/Fc+), PCzBi (E1/2 = 0.32 V and E1/2 = 0.60 V vs Fc/Fc+), and PCzBTD (E1/2 = 0.56 V and E1/2 = 0.84 V vs Fc/Fc+). The two redox couples signify the presence of stable radical cation and dication oxidation states. In addition, the three materials were observed to display electrochromic properties. UV/vis/NIR absorbance spectroscopy revealed that each of the polymers displayed a unique absorbance maximum (345 nm for PCzTh, 375 nm for PCzBi, and 460 nm for PCzBTD) in their neutral states. Spectroelectrochemical analyses demonstrated tha...

Alkoxysulfonate-functionalized PEDOT polyelectrolyte multilayer films : Electrochromic and hole transport materials

Abstract: Water-soluble poly(4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl-methoxy)-1-butanesulfonic acid (PEDOT-S) has been synthesized both chemically and electrochemically, and the properties of the resultant films have been examined and compared. In particular, it was found that PEDOT-S, deposited in conjunction with poly(allylamine hyrochloride) PAH in the form of polyelectrolyte multilayer (L-B-L) films of PEDOTS/PAH, possesses highly advantageous properties that allow for applications involving aqueous-based electrolytes. Electrochemically prepared films of PEDOT-S and L-B-L prepared films of PEDOT-S/PAH were characterized by electrochemical, spectroelectrochemical, and colorimetric methods illustrating reversible redox states at −0.2 and +0.2 V, respectively (vs Ag/Ag+ in organic media), which resulted in distinctive color changes between light blue (oxidized) and purple/pink (neutral). The change in color states was evident by a change in the electronic absorbance in the 500 nm region (neutral state) which w...

Near-infrared electrochromic materials for optical attenuation based on transition-metal coordination complexes

Abstract: Redox-active transition-metal complexes which show strong changes in their electronic spectra in the near-infrared (NIR) region in different oxidation states are attracting increasing interest as NIR electrochromic dyes for electro-optic switching at the wavelengths used for silica-based optical telecommunications networks. This brief review surveys recent work in the field based on ruthenium(II)-dioxolene, tris(pyrazolyl)borato-molybdenum(V) and mixed-valence Ru(II)-Ru(III) complexes, which all show strong NIR electrochromism and have been incorporated into prototypical devices such as the variable optical attenuator by attachment to transparent electrodes, either by adsorption or polymerisation.

Flexible foils with electrochromic coatings: science, technology and applications

Abstract: This paper covers a number of aspects of a novel flexible electrochromic foil capable of varying its optical transmittance. The foil includes thin films of tungsten oxide and nickel oxide and an intervening polymer electrolyte serving as lamination material. Concerning scientific aspects, we discuss the prevalent defects in amorphous tungsten oxide and how they lead to a consistent picture of the optical properties of tungsten oxide films as a function of non-stoichiometry and ion intercalation. We also present a refined model for the coloration/bleaching due to proton extraction/insertion in surface sheaths of nano-crystallites of nickel oxide. We then turn to aspects of technology and treat ways to enhance the bleached-state transmittance by mixing the nickel oxide with another oxide having a wide band gap, pre-assembly charge insertion/extraction by facile gas treatments of the films and practical device manufacturing. The final part covers some applications with emphasis on architectural “smart windows” capable of achieving improved indoor comfort together with significant energy savings due to lowered demands for space cooling. We also touch upon applications concerning eyewear.

Electrochemical synthesis and fast electrochromics of poly(3,4-ethylenedioxythiophene) nanotubes in flexible substrate

Abstract: We describe a new electrochemical synthetic method and characterization of poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes in a polycarbonate membrane. The very thin nanotubular structures in the highly flexible template provide fast electrochromic response (20 ms for oxidation or decoloration) even in a rolled state.

Electrochromic multilayer films of tunable color by combination of copper or iron complex and monolacunary dawson-type polyoxometalate.

Abstract: Electrochromic multilayer films consisting of polyoxometalate (POM) cluster α-K10[P2W17O61]·17H2O (P2W17), copper(II) complex [CuII(phen)2](NO3)2 (phen = 1,10-phenithroline), and iron complex [FeII(phen)3](ClO4)2 were fabricated on silicon, quartz and ITO substrates by layer-by-layer self-assembly method. The multilayer films, PSS/CuII(phen)2/[(P2W17/CuII(phen)2)]n and PSS/FeII(phen)3/[(P2W17/FeII (phen)3)]n were characterized by UV−vis spectra, X-ray photoelectron spectra, cyclic voltammetry (CV), chronoamperometric (CA) and in-situ spectral electrochemical measurements. The interesting feature of the electrochromic film is its adjustable color by reduction of both transition metal complex and polyoxometalate at different potentials. The multilayer films also exhibit high optical contrast, suitable response time and low operation potential due to the presence of mono-lacunary-substituted polyoxometalate and transition metal complex. This is the first example that the color of electrochromic film can be a...

Tunable electrochromic photonic crystals

Abstract: Photonic crystals based on the electrochromic phenomenon have been fabricated and proposed for band gap tuning. Electrochromic tungsten trioxide (WO3) inverse opals have been fabricated by polystyrene colloidal crystal templating. The WO3 matrix was obtained through a dip-infiltrating sol-gel process, with subsequent removal of the polymer microspheres by calcination. Scanning electron micrographs confirm the ordering of the hexagonal macroporous structure. The reflection spectra show two pronounced Bragg diffraction peaks. By inserting lithium into the crystals, the first reflection peak shifts gradually toward shorter wavelength for 36 nm, while the second reflection peak shifts toward longer wavelength for about 28 nm. This should be of great interest for photonic device applications.

Multicolored Electrochromism of a Poly{1,4-bis[2-(3,4-ethylenedioxy)thienyl]benzene} Derivative Bearing Viologen Functional Groups†

Abstract: 1,4-Bis[2-(3,4-ethylenedioxy)thienyl]-2,5-bis[6-(1'-butyl-4,4'-bipyridyl)hexyloxy]benzene tetrahexafluorophosphate (BEDOT-Ph-2V) was synthesized and electropolymerized to form a viologen-bearing conductive polymer [P(BEDOTPh-2V)] film on an electrode surface. This polymer exhibits multicolor electrochromic behavior: it is highly transparent light-blue P(BEDOT-Ph 1 . 7 + -2V 2 + ) at 1.0 V (vs. Ag/Ag + ), pale-bisque P(BEDOTPh 0 . 6 + -2V 2 + ) at 0.1 V, magenta P(BEDOTPh 0 -2V 2 + ) at -0.5 V, purple P(BEDOTPh 0 -2V_ + ) at -0.9 V, and crimson P(BEDOT 0 -2V 0 ) at -1.4 V. Since both the viologen pendant and the polymer backbone have cathodic coloring characteristics and their redox-potential ranges do not overlap each other, the dual electrochromic action provides more plentiful electrochromic colors compared to simple poly{1,4-bis[2-(3,4-ethylenedioxy)thienyl]-2,5-dialkoxybenzene}.

Electrochromic Polymer‐Dispersed Liquid‐Crystal Film: A New Bifunctional Device

Abstract: Polymer-dispersed liquid crystals (PDLCs) are liquid-crystal dispersions within a polymer matrix. These films can be changed from an opaque to a transparent state by applying a suitable alternating-current electric field. PDLCs have attracted the interest of researchers for their applications as light shutters, smart windows, and active displays. For such applications, electrochromic devices, which change color as a result of electrochemical reactions, have also become a recent focus of research. Herein, we report our preliminary results on bifunctional devices based on PDLCs that host electrochromic guest molecules. Such devices allow both an independent and fast switching from a scattering opaque state to a transmissive transparent state owing to liquid-crystal reorientation and a color change from white (pale yellow) to dark blue, due to either oxidation or reduction of the electrochromic molecules.

Synthesis, luminescence and electrochromism of aromatic poly(amine–amide)s with pendent triphenylamine moieties

Abstract: A new triphenylamine-containing aromatic dicarboxylic acid monomer, N,N-bis(4-carboxyphenyl)-N′,N′-diphenyl-1,4-phenylenediamine (2), was synthesized from the amination reaction between 4-aminotriphenylamine and 4-fluorobenzonitrile and subsequent alkaline hydrolysis of the dinitrile intermediate. A series of novel aromatic poly(amine–amide)s with triphenylamine units in the main chain and as the pendent group were prepared from the newly synthesized dicarboxylic acid and various aromatic diamines. These poly(amine–amide)s were amorphous and readily soluble in many organic solvents. All the polymers could be solution-cast into flexible films with good mechanical properties. They had excellent levels of thermal stability associated with high glass-transition temperatures (226–261 °C). These polymers exhibited strong UV–vis absorption bands at 350–365 nm in NMP solution. Their photoluminescence spectra showed maximum bands around 512–543 nm in the green region. The hole-transporting and electrochromic properties are examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the poly(amine–amide) 5a prepared from the dicarboxylic acid monomer 2 with a structurally similar diamine monomer N,N-bis(4-aminophenyl)-N′,N′-diphenyl-1,4-phenylenediamine (4a) exhibited four reversible oxidation redox couples in acetonitrile solution at E1/2 = 0.60, 0.80, 0.97 and 1.13 V, respectively. All the poly(amine–amide)s exhibited excellent reversibility of electrochromic characteristics by continuous five cyclic scans between 0.0 to 1.30 V, with a color change from the original pale yellowish neutral form to the green and then to blue oxidized forms.

Electrochromic Films Composed of MnO2 Nanosheets with Controlled Optical Density and High Coloration Efficiency

Abstract: Electrochemical and electrochromic properties of MnO 2 nanosheet electrodes prepared by layer-by-layer assembly on transparent conductive indium-tin oxide substrates were examined. Cyclic voltammograms (CVs) of multilayerfilm electrodes of MnO 2 nanosheets in nonaqueous Li + -containing electrolyte showed well-defined redox peaks which are accounted for by the reduction/ oxidation of Mn 4 + /Mn 3 + of the nanosheets associated with the insertion/extraction of Li + ions into/from the nanosheet galleries. The magnitude of the redox peaks was linearly enhanced as the number of layers increased. The nanosheets themselves were stable against the CV cycles, as revealed by in-plane X-ray diffraction, although the regular stacking structure of nanosheets was lost in the initial several cycles of potential sweeps. The redox processes induced changes in the UV-visible absorption spectra of MnO 2 nanosheet electrodes, exhibiting electrochromism. The films were bleached with the electrochemical reduction of Mn 4 + and reverted to their original state by the reoxidation of Mn 3 + of the nanosheets. The coloration efficiency was estimated to be 36.7 cm 2 C-1 at 385 nm. The absorbance changes were proportional to the number of layers of MnO 2 nanosheets, suggesting that appropriate changes in optical density can be easily achieved by controlling the number of deposition cycles.