Surface analysis of electrochromic CuxO films in their colored and bleached states

Abstract

CuxO is known as an electrochromic material with a possible applicability for solar-light modulation. The reversible transition between the two different oxidation states, CuO and Cu2O, is responsible for the visible-light switching ability. CuxO films in their as-prepared, colored and bleached states were subjected to a surface analysis in order to relate the bleaching/coloring effects to the quantified Cu-oxide transition. An XPS analysis on the Cu2p electrons of the as-prepared, bleached and colored films showed that the Cu-ion quantity reversibly turning from CuO to Cu2O during the electrochromic cycling was about 3.4 %. An analysis of the XRD patterns of the film's three states confirmed that a small portion of the surface Cu-atoms probably participate in the coloration/bleaching process. Scanning electron microscopy (SEM) images revealed obvious changes in the surface morphology due to bleaching and coloration transitions, particularly in the grain size and porosity of the CuxO films. The surface morphology of the films was also studied with the atomic force microscopy (AFM). This technique allowed significant conclusions to be derived relating to the surface roughness as well as the compositional homogeneity of the films before and after electrochemical treatments. These results appeared to be complementary to those derived from the X-ray diffraction patterns. One may assume that the coloration centers are located at very few film's monolayers of the interface with the electrolyte. Keywords: electrochromism, Cu 2 O, CuO, XPS, XRD, SEM, AFM CuxO je poznan kot elektrokromizni material z mo`nostjo uporabe za modulacijo son~ne svetlobe. Reverzibilni prehod med dvema oksidacijskima stanjema, CuO in Cu2O, vpliva na mo`nost preklapljanja vidne svetlobe. Tanke plasti CuxO v izhodnem, obarvanem ali obeljenem stanju so bile analizirane na povr{ini, da bi ugotovili u~inke barvanja/beljenja pri kvantificiranem prehodu Cu-oksidov. XPS-analiza z elektroni Cu2p pripravljene, obeljene in obarvane plasti je pokazala, da se okrog 3,4 % koli~ine Cu-ionov med elektrokromizno obdelavo reverzibilno odmika od CuO proti Cu2O. Analiza XRD-sledov treh stanj traku je potrdila, da verjetno majhen dele`Cu-atomov sodeluje v postopku obarvanje/beljenje. Vrsti~na elektronska mikroskopija (SEM) je potrdila ob~utne spremembe v morfologiji povr{ine zaradi prehodov obarvanja in obeljenja, posebno velikost zrn in poroznost plasti CuxO. Morfologija povr{ine plasti je bila pregledana z mikroskopijo na atomsko silo (AFM). Ta tehnika omogo~a pomembne sklepe glede povr{inske hrapavosti, kot tudi homogenosti sestave plasti pred elektrokemijsko obdelavo in po njej. Ti rezultati se ujemajo s tistimi, dobljenimi z XRD-posnetkov. Lahko sklepamo, da so centri obarvanja locirani v nekaj monoplasteh na stiku z elektrolitom.