el-esonanceue.ygen

ine for the

Journal of Colloid and Interface Science 275 (2004) 596–600www.elsevier.com/locate/jcis

Photochromic inorganic–organic multilayer films based onpolyoxometalates and poly(ethylenimine)

Min Jiang,a Enbo Wang,a,∗ Gang Wei,b Lin Xu,a and Zhuang Lib

a Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of Chinab State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Jilin 130022, People’s Republic of China

Received 19 November 2003; accepted 25 February 2004

Available online 9 April 2004

Abstract

Novel photochromic inorganic–organic multilayers composed of polyoxometalates and poly(ethylenimine)have been prepared by thlayer-by-layer (LbL) self-assembly method. The growth process, composition, surface topography, and photochromic properties of the mutilayer films were investigated by UV–visible and Fourier transform infrared spectroscopy, atomic force microscopy, electrospin r(ESR), and X-ray photoelectron spectroscopy (XPS). Irradiated with ultraviolet light, the transparent films changed from colorless to blMoreover, the blue films showed good reversibility of photochromism and could recover the colorless state gradually in air, where oxplays an important role in the bleaching process. On account of the ESR and XPS results, parts of W6+ in multilayers were reduced to W5+,which exhibited a characteristic blue; a possible photochromic mechanism can be speculated. This work provides basic guidelassembly of multilayers with photochromic properties. 2004 Elsevier Inc. All rights reserved.

Keywords: Polyoxometalates; Layer-by-layer multilayer films; Photochromism

es. Aslay

ewm-rage

lassthy

of

s,ox-tonsor

o-

tionte-de-

su-ultre.

lies,con-B)-anyelf-,me-bLatile

ularand

1. Introduction

Recently, considerable attention has been focused on thstudy of advanced materials preparation and applicationa kind of advanced materials, photochromic materials pan important role in some highly technological fields in viof their potential applications in information displays, cheical sensors, modified electrodes, and holographic stodevice[1–5].

Polyoxometalates (POMs) represent a well-known cof structurally well-defined metal oxide clusters with wealtopological, chemical, and physical properties and arewide application in different fields such as catalysis[6],medicine[7], and materials sciences[8]. As versatile inor-ganic entities for construction of functionally active solidone of the most important properties of these metalide clusters is that they can accept electrons or proto become mixed-valency colored species (“poly blue”“heteropoly blue”), which make them suitable for ph

* Corresponding author. Fax: 86-431-568-4009.E-mail address: wangenbo@public.cc.jl.cn (E. Wang).

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.02.072

tochromic and electronchromic materials[9,10]. However,these potential applications require successful fabricaof thin films. In previous years, thin films of these marials were mainly produced by physical or chemicalposition techniques such as spray pyrolysis[11], chemicalvapor deposition (CVD)[12], electrodeposition[13], andsol–gel methods[14]. However, these techniques are ually both expensive and complicated, and it is also difficto form large-area films with a highly ordered structuTo choose a rational immobilization method for improvingthe ordering and properties of such molecular assembClemente–Lenon et al. prepared magnetic multilayerstaining heteropolyanions by the Langmuir–Blodgett (Ltechnique[15]. Unfortunately, with the LB method it is difficult to construct larger size and thickness films. Due to madvantages of the electrostatic layer-by-layer (LbL) sassembly technique[16], such as simplicity in fabricationindependence of substrate size and topology and goodchanical and chemical stability of the prepared film, the Lassembly technique has proved to be a powerful and versalternative for assembling multicomposite supramolecstructures, with good control over the layer composition

M. Jiang et al. / Journal of Colloid and Interface Science 275 (2004) 596–600 597

lf-g ons.ho-

in-Ms

ic–ionlat-ners

e toer-canor-

-ore

lti-

mic

theas

odumte),hyl-ndgenta

erft,ldxi-to

es,rre-tra

s

d atthe.m is

rstion

ar-

gte

the

duregd.ly

artzm-

thickness[17]. Based on such principles of molecular seassembly and self-organization, we have been focusinfabrication of functional multilayer films containing POMTo date, our group has reported the construction of ptoluminescent POM-based LbL multilayer films[18] andthe preparation of nonlinear optical ultrathin films containg POMs[19]. Latterly, we also in situ synthesized POnanoparticles in polyelectrolyte multilayers by means of thimethod[20].

Very recently, we found that photochromic inorganorganic multilayer films could be obtained by incorporatof POMs[21]. Chen et al. fabricated photochromic supertice films by the LbL technique[22], which was based ometal oxides and organic molecules. As we know, polymare versatile for producing self-assembled thin films dutheir varied structural properties and functionality. Numous publications further indicate that polyelectrolytesbe widely applied to self-assemble multilayer films. Inder to further confirm the photochromic feasibility of POMbased multilayers and their reasonable mechanism in mdetail, we systematically investigated photochromic mulayers of POMs ([NaP5W30O110]14− and [W10O32]4−) andpoly(ethylenimine), discovered their excellent photochroproperties and good photochromic reversibility, and founddifferences between the two systems, which could paveway for applications in optical data storage, especiallyholographic storage media.

2. Materials and methods

2.1. Materials

Preyssler tungstophosphate, K12.5Na1.5[NaP5W30O110](NaP5W30), was prepared according to the literature meth[23], and the product was recrystallized twice. Soditungstate, Na2WO4·2H2O (used to prepare decatungstawas purchased from Beijing Chemical Products. Poly(etenimine) (PEI; MW 50,000) was obtained from Aldrich aused as received. All of the other chemicals are of reagrade. The water used in all experiments was deionized toresistivity of 17–18 M� cm.

2.2. Preparation of decatungstate

An aqueous solution of decatungstate, [W10O32]4− (W10),was prepared by the acidification of 0.1 M Na2WO4·2H2Oto pH 2.5 by the dropwise addition of 1 M HCl undvigorous stirring [24]. Fig. 1 shows the UV spectra othe same Na2WO4 solution at different pH values. FirsNa2WO4·2H2O was dissolved in distilled water to yiea 0.1 M solution. The pH of the solution was appromately 10.0 (curve a). When 1 M HCl was dropped inthe above solution, the solutions with different pH valuwhich show obvious differences (curves b, c, and d cospond to pH 7.0, 5.0, 2.5, respectively) in the UV spec

Fig. 1. UV–vis spectra of Na2WO4·2H2O solution at different pH value(pH values for a, b, c, and d are 10.0, 7.0, 5.0, and 2.5, respectively).

Scheme 1.

could be obtained. Curve d exhibits an absorption banapproximately 325 nm, characteristic of the formation of[W10O32]4− structure[25], which is dominant in solutionFor spectra c and d, the broad absorption band at 260 nassigned to the formation of other polytungstates[26].

2.3. Multilayer assembly

The substrate (quartz slide or silicon wafer) was fithoroughly cleaned by treatment with Piranha solut(H2O2:H2SO4 = 3:7 v/v) at 80◦C for 30 min, followed byrinsing with deionized water. Further purification was cried out by immersing in NH4OH:H2O2:H2O (1:1:5 v/v)solution at 70◦C for 30 min and then extensively washinwith water and drying with N2. Then the cleaned substrawas immersed in 10−2 M PEI solution for 20 min and aprecursor layer of PEI was modified on the surface ofsubstrate. Next, the precursorfilm was alternately dippedinto the 10−2 M POMs solution and 10−2 M PEI solu-tion (pH 9.0 for NaP5W30/PEI and pH 2.5 for W10/PEILbL films) for 20 min, rinsed with deionized water, andried in a N2 stream after each dipping. The procedresults in the build-up of the multilayer films containinPOMs (POMs= W10 or NaP5W30), which can be expresseas PEI/(POMs/PEI)n, wheren is the number of bilayersScheme 1shows a schematic illustration of LbL assemband photochromism of multilayers.

2.4. Characterization

UV–vis absorption spectra were recorded on a quslide using a 765 CRT UV–vis spectrophotometer. AFM i

598 M. Jiang et al. / Journal of Colloid and Interface Science 275 (2004) 596–600

ntsithfer

erIR

SRromriedlight0 cm

as

rbed

eof

multhelots

de-andhatfirst

ingEIhe-

oolf

hee

ks:h-.ionIstertheul-

bridredhe

lues as

areMs.

437oyan-ontic

r-ited

PEI-essulti-

ongageheg-

rnalndouldbe

e ob-crved

ages were taken on a silicon slide by a Digital InstrumeNanoscopeXa instrument operating in contact mode wsilicon nitride tips. XPS was performed on a quartz wausing an ESCALAB-MK� photoelectronic spectrometwith an MgKα (1253.6 eV) achromatic X-ray source. FTspectra were recorded on a Bruker IFS 66V instrument. Espectra were recorded on a Bruker ER200-D-SRC specteter in the X-band. Photochromic experiments were carout using a 500-W high-pressure mercury lamp as thesource. The distance between the lamp and sample is 1

3. Results and discussion

Electrostatic LbL self-assembly of POM anions wmonitored by UV–vis spectroscopy.Fig. 2 shows UV–visspectra of (W10/PEI)n multilayer films with n = 0–12 de-posited on the precursor PEI film. Since PEI is not absoabove 200 nm, the film exhibits characteristic bands of W10at 325 nm corresponding to the oxygen→ tungsten chargtransfer (CT) transition, which confirms the incorporationPOMs into multilayer films. Moreover, the inset inFig. 2presents the plots of the absorbance values for thesetilayer films at 196, 260, and 325 nm as a function ofnumber of deposition cycles. This linear nature of the palso indicates that an equivalent amount of W10 is adsorbedafter each deposition cycle and further confirms that theposition process is very consistent from layer to layerhighly reproducible. Moreover, it is interesting to note tthe absorbance increment at 196 and 260 nm for theW10/PEI bilayer is larger than the later bilayers, indicatthat a greater amount of W10 penetrated the precursor Pfilm, as explained by Kurth, who observed a similar pnomenon in preparing LbL films consisting of Mo57 clustersand polyelectrolytes[27].

FTIR spectroscopy is an effective experimental twidely used for the characterization and determination ostructure and composition of multilayer films.Fig. 3presentsthe FTIR spectra of NaP5W30/PEI and W10/PEI multi-layer films. The FTIR spectrum (Fig. 3a) recorded forNaP5W30/PEI multilayer films has regions as follows: tvibration bands at 911 and 780 cm−1 are ascribed to thvibration modes of W=Od and W–Oc–W, and three well-resolved bands at 1165, 1082, and 1018 cm−1 correspond tothe P–Oa stretching band[28]. The FTIR spectrum (Fig. 3b)of W10/PEI multilayer film also reveals three clear pea960, 890, and 796 cm−1, which are attributed to the stretcing modes of W=Od, W–Ob–W, and W–Oc–W, respectivelyFurthermore, the multilayer films also exhibit absorptbands at 1450–1600 cm−1 (νC–C, νC–N) due to the PEcomponent. These results demonstrate that POM cluhave been incorporated into the multilayer films andbasic structure of POMs is still preserved inside the mtilayers, although the bands in the inorganic–organic hyfilms associated with PEI are slightly shifted when compato the spectrum of POMs in the KBr pellet. Obviously, t

-

.

-

s

Fig. 2. UV–vis spectra of PEI/(W10/PEI)n films with n = 0–12 on quartzsubstrates. The insets show plots of the absorbance values at peak vaa function of the number of bilayers.

Fig. 3. FTIR spectra of (a) NaP5W30/PEI and (b) W10/PEI multilayer films.

bands in the composite film associated with the anionsslightly shifted when compared to the spectrum of POIn both W10/PEI and NaP5W30/PEI multilayer films, thewavenumber of N–H stretching band decreased from 3to 3432 cm−1 after the multilayers formation, which alsproved that the hydrogen bond was formed between polions and PEI[29]. In a word, the electrostatic anion–catiinteraction lead to an increase or a decrease in characterisfrequencies and present a little blue-shift or red-shift.

In order to obtain further information involving the suface morphology and the homogeneity of the deposfilms, AFM image of PEI/(W10/PEI)3 (seeFig. 4) multi-layer films was investigated. In general, the precursorfilm is fairly uniform and smooth[18]. However, after adsorption of W10 and PEI layer, the mean surface roughnof multilayers increased, and the surface consists of a mtude of small domains with diameter of ca. 40–60 nm althe horizontal axis (see cross section of topographic imshown belowFig. 4). These domains are attributed to taggregation of POMs. Although AFM is a surface imaing technique, these data provide insight into the intestructure of the multilayers, because both topography apacking of the POMs repeat in subsequent layers. It shbe pointed out that the multilayer containing POMs canconsidered as a composite material, which does not havvious interfaces between the polyelectrolyte and inorganicomponent. Similar surface morphology was also obsein NaP5W30/PEI multilayers[21].

M. Jiang et al. / Journal of Colloid and Interface Science 275 (2004) 596–600 599

s

o-if-

nrsi-e

,

he

EI

c-

ncehelorigh,n for

nlor

ol-

r

an-

udyes ofrig-

redgain.

fhet

lec-s

ho-

be-

of

re-EIthe

Fig. 4. AFM image of PEI/(W10/PEI)3 multilayers on a silicon wafer. Crossection of the topographic image is also shown below figure.

3.1. Photochromic behavior and mechanism

The fresh multilayer films exhibit interesting photchromic properties in the region above 400 nm with dferent UV irradiation time (seeFig. 5). The (W10/PEI)30films turn slightly blue when subjected to UV irradiatioin air. This is characteristic ofcolored-reduced moleculaspecies, withd–d band charge transfer intervalence trantions W5+ → W6+ in the visible region. The change in thabsorbance of (NaP5W30/PEI)30 film (0.022 unit at 800 nmRef. [21]) is larger than that of (W10/PEI)30 film (0.015 unitat 800 nm,Fig. 5b) after UV irradiation for 10 min. Thedifferences might boil down to the following reasons: tcharge density of NaP5W30 is higher than that of W10, whichcauses more NaP5W30 clusters to be adsorbed onto Psurface, and the surface area of NaP5W30 cluster is largerthan that of W10, which results in more adequate interation with PEI. Therefore, the NaP5W30/PEI multilayers haveenhanced photochromic properties compared with W10/PEImultilayers. With prolonged irradiation time, the absorbaof multilayer films reached saturation gradually. After tUV light was turned off, the blue films began to discogradually in air. The response speed of decoloration is hand the bleaching process is nearly over after 5 and 3 mi(NaP5W30/PEI)30 film and (W10/PEI)30 film, respectively.The reason is probably that the quantity of NaP5W30 (orW10) and PEI is very low in the 30-bilayer films. Whethe blue films are stored in nitrogen, they do not deco

Fig. 5. Absorption spectra of PEI/(W10/PEI)30 multilayers as a func-tion of irradiation time. The insets show their reversibility in the coration–decoloration process.

Fig. 6. ESR spectra of the NaP5W30/PEI and W10/PEI samples at roomtemperature. A 500-W high-pressuremercury lamp irradiated samples fo10 min. Curve a, b are due to the photoreduced NaP5W30 and W10.

for a long time. The results indicate that oxygen playsimportant role in the bleaching process, which can support a possible photochromic mechanism. In order to stthe reversibility of the coloration–decoloration process, thwavelength of 800 nm was used to observe the changeabsorbance of multilayers. First, the absorbance of the oinal film was measured. Then the film was irradiated 10 minand the absorbance also measured immediately. In the fol-lowing step, the blue film was stored in air and decoloafter 5 min, and the absorbance was measured once aBy repeating the process, the inset ofFig. 5can be obtainedfor the reversibilityof the coloration–decoloration cycle o(W10/PEI)30 multilayer films. The absorption spectra of tcompletely bleached (W10/PEI)30 film are almost consistenwith that of the film without UV irradiation.

To explain the photochromicbehavior of the multilayerfilms, it is necessary to investigate the variation of the etronic structure of the component in the multilayer filmduring the photochromic process to make clear the ptochromic mechanism. NaP5W30/PEI and W10/PEI samplesexhibit no significant ESR signals at room temperaturefore UV irradiation. After UV irradiation, both blue W10/PEIand NaP5W30/PEI samples exhibit typical ESR signalsW5+ (seeFig. 6) at g = 1.868 andg = 1.857 at 298 K,respectively. It is concluded that photoexcitation of O=Win WO6 ligand-to-metal charge transfer (LMCT) bandssults in transferring a hydrogen from the nitrogen of Pto the bridge oxygen atom at the photoreduced site in

600 M. Jiang et al. / Journal of Colloid and Interface Science 275 (2004) 596–600

ens-ft at

gen,usedns-

ngardan

ha-PS

-

-

V,rts

l-Fur-terinenicde-

de

icte

ferxy-nset-ibit

nat

is-he

sys-ring

at-

223.3)

02)

91)

)

-

140

)

E.14.

02)

03)

27

o,

18

r, In-

g-

Scheme 2.

edge-shared WO6 octahedral lattice. This is followed by thinteraction of one electron with the proton which was traferred to the oxygen atom. Simultaneously, the hole lethe oxygen atom as a result of the O→W LMCT transferinteracts with nonbonding electrons on the amino nitroatom to form a charge-transfer complex[9]. The bleachingwhich occurs in the presence of oxygen molecules, is caby the back reaction, which is triggered by an electron trafer from the W5+ atom to the oxygen molecules. Accordito the above results and the similar conclusion put forwby Yamase[30], the possible photochromic mechanism cbe speculated asScheme 2.

In order to further demonstrate photochromic mecnisms and the variation of the valence state of W, the Xspectrum of the blue W10/PEI multilayer films was investigated. It is found that not only the W4f doublet of W6+but also the W4f doublet of W5+ are detected for the irradiated film. The binding energy values of the W4f doubletof W6+ and W5+ are 35.4 and 37.5 eV, 34.0 and 36.2 erespectively. This implies that for the irradiated film, paof W6+ are reduced to W5+ during the photochromic cooration process, which is in agreement with ESR result.thermore, the peaks of N1s appeared a few changes afirradiation. It indicates that the proton of quaternary amgroup is transferred to the oxygen molecule of inorgacomponents and the content of nitrogen atom with protoncreased after coloration process. These results also provicogent evidence for possible photochromic mechanism.

4. Conclusions

This paper reported the fabrication of photochrominorganic–organic multilayer films containing polytungsta([NaP5W30O110]14− and [W10O32]4−) and poly(ethylenimi-ne) (PEI) constructed by LbL method. A charge-transbridge is built between the nitrogen of PEI and the ogen of the WO6 octahedral lattice in POMs by hydrogebonding. When exposed to UV light, the multilayer filmshowed blue color due to the formation of poly blue and heropoly blue species. Moreover, POM-based films exhgood reversibility of photochromism,and oxygen plays aimportant role in the bleaching process. We anticipate th

a comparison study between W10/PEI and NaP5W30/PEIwill be helpful and constructive for the forthcoming dcussion about the formation of the photochromic film. Tresults obtained in this paper indicate that such thin filmtems would open a straightforward route toward prepapromising materials in optical data storage in the future.

Acknowledgment

The work was financially supported by the National Nural Science Foundation of China (20171010).

References

[1] D.E. Katsoulis, Chem. Rev. 98 (1998) 359.[2] J.N. Yao, K. Hashimoto, A. Fujishima, Nature 355 (1992) 624.[3] G.H. Brown, Photochromism, Wiley, New York, 1971.[4] J.N. Yao, P. Chen, A. Fujishima, J. Electroanal. Chem. 406 (1996)[5] C. Bechinger, G. Definger, S.Herminghaus, J. Appl. Phys. 74 (199

4527.[6] R. Ben-Daniel, L. Weiner, R. Neumann, J. Am. Chem. Soc. 124 (20

8788.[7] Y. Inouye, Y. Tokutake, T. Yoshida, Chem. Pharm. Bull. 39 (19

1638.[8] L. Cheng, J.A. Cox, Chem. Mater. 14 (2002) 6.[9] T. Yamase, Chem. Rev. 98 (1998) 307.

[10] S.Q. Liu, D.G. Kurth, H. Mohwald, D. Volkmer, Adv. Mater. 14 (2002225.

[11] J.B. Schlenoff, S.T. Dubas, T. Farhat, Langmuir 16 (2000) 9968.[12] A.M. Wrobel, A. Walkiewicz-Pietrzykowska, Y. Hatanaba, S. Wickra

manayaka, Y. Nakanishi, Chem. Mater. 13 (2001) 2742.[13] D. Ingersoll, P.J. Kulesza, L.F. Faulkner, J. Electrochem. Soc.

(1994) 141.[14] D. Avnir, S. Braun, O. Lev,M. Ottolenghi, Chem. Mater. 6 (1994

1605.[15] M. Clemente-León, B. Agricole, C. Mingotand, C. Gómez-García, J.

Coronado, P. Delhaes, Angew. Chem. Int. Ed. Engl. 36 (1997) 11[16] G. Decher, Science 277 (1997) 1232.[17] X. Zhang, J.C. Shen, Adv. Mater. 11 (1999) 1139.[18] L. Xu, H.Y. Zhang, E.B. Wang, D.G. Kurth, J. Mater. Chem. 12 (20

654.[19] L. Xu, E.B. Wang, Z. Li, D.G. Kurth, New J. Chem. 26 (2002) 782.[20] M. Jiang, E.B. Wang, Z.H. Kang, S.Y. Lian, J. Mater. Chem. 13 (20

647.[21] M. Jiang, E.B. Wang, G. Wei, L. Xu, Z.H. Kang, New J. Chem.

(2003) 1291.[22] Z.H. Chen, Y. Ma, T. He, R.M. Xie, K. Shao, W.S. Yang, J.N. Ya

New J. Chem. 26 (2002) 621.[23] M. Filouitz, R.K.C. Ho, W.G. Klemperer, W. Shum, Inorg. Chem.

(1979) 93.[24] A. Chemseddine, C. Sanchez, J. Jivage, J.P. Launay, M. Fournie

org. Chem. 23 (1984) 2609.[25] I. Moriguchi, J.H. Fendler, Chem. Mater. 10 (1998) 2205.[26] S. Termes, M. Pope, Inorg. Chem. 17 (1978) 500.[27] F. Caruso, D.G. Kurth, D. Volkmer, M.J. Koop, A. Müller, Lan

muir 14 (1998) 3462.[28] S. Lis, J. Alloys Compounds 300–301 (2000) 88.[29] C.S. Ha, H.D. Park, C.W. Frank, Chem. Mater. 12 (2000) 839.[30] T. Yamase, J. Chem. Soc. Dalton Trans. 11 (1991) 3055.