Journal of Non-Crystalline Solids 357 (2011) 1768–1773

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Journal of Non-Crystalline Solids

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A non-template hydrothermal route to uniform 3D macroporous films withswitchable optical properties

Yong Liu a,⁎, Hai Wang a,c, Yong-cai Wang a, Wen-xia Zhao b, Hong Huang b, Chao-lun Liang b, Hui Shen a

a School of Physics and Engineering, Institute for Solar Energy Systems, State Key Laboratory of Optoelectronic Materials and Technologies,Sun Yat-sen University, Guangzhou 510275, Chinab Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou 510275, Chinac Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education, Guilin University of Technology, Guilin 541004, China

⁎ Corresponding author. Tel.: +86 020 39332863.E-mail address: liuyong7@mail.sysu.edu.cn (Y. Liu).

0022-3093/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.jnoncrysol.2011.01.031

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 November 2010Received in revised form 4 January 2011Available online 24 February 2011

Keywords:Macroporous;Template;Hydrothermal;Smart window;Optical switching glazing

Here we report the direct synthesis of long-range uniform 3D macroporous film on the surface of glass via anon-template strategy by reacting silica with a mixture solution of HCl and H2O2 under hydrothermalconditions. The size of macropores can be tuned from 1 μm to 7 μm by changing the hydrothermaltemperature. Followed by the impregnation or release of water, the 3D porous films showed a high-contrastoptical switchable property. The maximum optical transmittance modulation of these 3D porous films was upto 50% over the visible and near IR wavelength range. Furthermore, liquid-induced optical switching devicewere designed as a sandwich structure. The average transmittance of the device was varied between 62.3%(from 350 to 1100 nm) at the transparent state and 13.2% (from 350 to 1100 nm) at the opaque state,respectively. Such a wide modulation range of optical transmittance is suitable for application of this porousfilm to energy saving smart windows.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Porous materials are of great scientific and technological interestbecause of their potential applications in catalysis, separations, ionexchange, sensing, and optical devices, etc. [1,2]. Controlled pore sizeand uniformity of porous material would provide the desired functionin a specific application. To direct the formation of pores, mostprevious studies have mainly focused on using templates such ascolloidal crystals or organic templates [3–8], since template methodcan readily provide complex order pore structures with more lengthscales (micro, meso, and macro-scales). However, to date, there stillexist some problems for the template methods. One is that the use oftemplate has to be removed via chemical or thermal treatments,which may cause the damage of the inorganic shell. The other is thatthe pore size of porous materials cannot be continuously tuned due tothe limitation of the selected templates [9–14]. Furthermore, three-dimensional (3D) porous structures are usually built up from the 2Dtemplates. These fabrication procedures involve tedious and compli-cated manipulations, which lead to expensive implement on a largescale and usually build few layers into the third dimension [3]. Mostprevious studies have focused on micropores and mesopores,materials with uniform macropores are predicted to have unusualproperties and be utilized for a broad range of applications [3,4].

However, the application of template methods to the synthesis ofuniform macroporous structures with large size is rarely reported inprevious literatures since a complete filling of the empty space in atemplatewould bemore difficult while the size of template gets larger[15]. Therefore, it is desirable for synthesizing macropores by a non-template strategy since the template removal methods are usuallyenvironment-unfriendly and complicated. Recently, a few fabricationmethods have been proposed on the synthesis of macropores by thenon-template route [16,17]. However, to the best of our knowledge,the non-template formation of 3D macroporous structure withuniform pores of arbitrary size and shape at larger length scalesremains a challenge.

Here we report a non-template strategy to create the long-rangeuniform 3D macroporous film on glass by reacting silica with amixture solution of HCl and H2O2 under hydrothermal conditions. Thesize of macropores can also be tuned by changing the hydrothermaltemperature. These 3D porous films exhibit unique high-contrastoptical switching properties induced by impregnation or release ofliquid such as water, which is different from the mechanisms of othertraditional switchable chromomeric materials [18–21]. The advantageof the 3D porous film for switchable chromomeric device applicationis that it has the low transmittance in opaque state due to a reflectivecontrol rather than an absorptive control [22]. It is expected toprovide an alternative route to smart windows application that canreversibly control the amount of solar radiation and heat passingthrough a window to maximize the energy savings in buildings andautomobiles [23].

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2. Experimental

2.1. Preparation of 3D macroporous films on the surface of glass

In a typical synthesis, 29 ml of 37% (w/w) hydrochloric acid (A.R.,Beijing Beihua Fine Chemicals Co., Ltd.) and 21 ml of 30% (w/w)hydrogen peroxide (A.R., Guangzhou Chemical Reagent Factory) weremixed. Then, this mixed solution was placed in a static Teflon-linedstainless steel autoclave (100 ml) after air bubble was released fromthis mixture solution for 20 min. Subsequently, a piece of fluorinedoped tin oxide (FTO) conductive glass (Nippon Sheet Glass Co., Ltd,2.2 mm thick) was placed against the wall of the autoclave with aninclined angle of 45°. Then the autoclave was heated at temperaturefrom 180 °C to 230 °C. After reacting silica with the mixture solutionof HCl and H2O2 under hydrothermal conditions, the obtained 3Dmacroporous film is only formed on no-conductive side of FTO glass.

2.2. Characterizations

The morphology of samples was observed by thermal fieldemission environment scanning electron microscope (SEM; Quanta400). The crystal structure of the samples was analyzed using X-raydiffraction (XRD, Bruker) with Cu Kα radiation and high-resolutiontransmission electron microscopy (HRTEM; JEOL JEM-2010). Thetransmittance and reflectance spectra were obtained using an UV-VIS-NIR spectrometer (Hitachi U-4100) equipped with an integratingsphere and vertical 5° specular reflectance accessory. The charactersof the macroporous film were investigated by a mercury porosimeter(Autopore IV, Micromeritics Instrument Corporation), while that ofthe mesopores distributed in the macropore skeletons was analyzed

Fig. 1. SEM images of 3D macroporous film. Top view of porous film synthesized at hydrrespectively. And the corresponding pore sizes are about 7 μm, 5 μm, 3 μm, 1 μm, respectivelpore walls (f) were obtained at hydrothermal temperature of 230 °C for 4 days.

by nitrogen adsorption–desorption apparatus (Belsorp mini II, BelJapan, Inc, Japan).

3. Results

Fig. 1 shows SEM images of 3D macroporous films. Fig. 1a–d revealthat the resulting pores have uniform pore size over large areas, andthe size of pores decrease from 7 μm to 1 μm with increasinghydrothermal temperature from 180 °C to 230 °C. Therefore, it isprospective that the size of pores can be continuously tuned bychanging the hydrothermal temperature. Fig. 1e show cross-sectionSEM image of 3D porous film built up by uniform cavities, and porousfilm may reach several tens of micrometers in thickness. Themagnified SEM indicates that a typical macropore wall, synthesizedat 230 °C, is formed in a concave-lens-shaped structure, as shown inFig. 1f. Transmission electron microscopy (TEM) image furtherconfirms this concave-lens-shaped structure (the insert in Fig. 2).Furthermore, the HRTEM image reveals that a lot of mesopores withaverage pore size of ca. 4 nm were distributed on the wall ofmacropores (Fig. 2).

The porous characters measured by mercury porosimetry showthat a narrowed peak for the macropores centers at 835.5 nm andporosity is 70.1% (Fig. 3a). The N2 adsorption–desorption isotherm(77 K) exhibits a type-II isotherms according to the IUPAC classifica-tion (Fig. 3b). A steep increase in the amount of nitrogen adsorbed isclearly observed at high relative pressure (P/P0N0.8), suggesting thepresence of macroporous structure, in which multi-layer adsorptionforms due to a strong interaction between adsorbed molecules.Besides, the presence of hysteresis loops is associated with capillarycondensation in mesoporous solid distributed on the wall of

othermal temperature of (a) 180 °C, (b) 190 °C, (c) 200 °C and (d) 230 °C for 4 days,y. The cross section SEM image of 3D porous structure (e) and magnified SEM image of

Fig. 2. HRTEM images of mesopores distributed on the wall of macropores. Insert: TEMimage of a typical macropore wall achieved at hydrothermal temperature of 230 °C for4 days.

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macropores. The porous frameworks have BET surface area of10.4 m2/g. Additionally, the average pore size of these mesoporescalculated by the BJH model is about 8.0 nm.

To further illuminate the formation mechanism of the 3Dmacroporous films, we collected the resultant liquid after thehydrothermal reaction at 230 °C, and then dried them by a rotaryevaporator at 60 °C. Finally, the brown samples were obtained. Fig. 4aand the inset display HRTEM and TEM images of the obtained brownsamples. Interestingly, the samples were assemblies of good crystalstructure distributed in the amorphous SiO2 matrix. The fringespacing of 2.82 Å corresponded to the (200) planes of SiCl4. Thecrystal SiCl4 phase contained in the amorphous SiO2 matrix could be

Fig. 3. Pore characteristics in the 3D macroporous films obtained at hydrothermaltemperature of 230 °C for 4 days. (a) Pore size distribution curve of 3D macroporousfilm obtained from mercury porosimetry. (b) Nitrogen adsorption–desorptionisotherms of macroporous samples.

Fig. 4. (a) HRTEM and TEM (the inset) of the crystal SiCl4 phase contained in theamorphous SiO2 matrix. The fringe spacing of 2.82 Å corresponded to the (200) planesof SiCl4. (b) EDS spectrum of same sample, as denoted by the white circle in the inset.

further confirmed by the energy dispersive X-ray spectrometer (EDS),as shown in Fig. 4b. The corresponded element ratio of Si, O and Clinferred from the EDS spectrum was very high, and Cu element wasmeasured from copper grids for TEM support, whereas other elementswere from the impurities in glass.

X-ray diffraction (XRD) analyses were also performed on the samebrown samples, as shown in Fig. 5. The result indicated that thediffraction peaks of the sample matched well with the crystalstructure of the SiCl4 phase, which verified well with the HRTEMand EDS analysis. According to above measurements, we proposed apossible formation mechanism of 3D macroporous structure. Usually,glass would not react with hydrochloric acid solution at normaltemperature. With the aid of high temperature and high pressurecondition, silica reacted with HCl to form SiCl4 phase, and thengenerated SiCl4 was bubbled immediately from the glass surface toform 3D macroporous structure by oxygen released from H2O2.

Finally, the surface of SiCl4 phase reacted with H2O to generate SiO2

matrix. Then, the liquid crystal SiCl4 phase was enclosed in theamorphous SiO2 matrix and protected well by the SiO2 matrix. Toillustrate the effect of H2O2 on the formation of 3D porous, only 50 mlhydrochloric acid (37%) was added in Teflon lined autoclave to react

Fig. 5. XRD pattern of the crystal SiCl4 phase that enclosed in the amorphous SiO2

matrix.

Fig. 7. Optical properties of 3D porous films. (a) Transmittance spectra of 3D porousfilm before (opaque state) and after (transparent state) impregnation with water.

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with the glass at same hydrothermal condition, but the obtained poresin film are very deep, as shown in Fig. 6.

Fig. 7a shows the transmittance spectra of 3D porous film beforeand after being impregnated with water, corresponding to the opaquestate and transparent state, respectively. In the opaque state (withoutwater impregnation), 3D porous film reduces light transmissionuniformly up to 23.9% in the wavelength range of 350–1100 nm,showing white color. After being impregnated with water (thetransparent state), the film became highly transparent immediately,and the average transmittance value is as high as 74.1% within thewavelength range of 350 to 1100 nm. The relatively low BET surfacearea of this 3D porous film (Fig. 3) indicates that there are very fewmesopores distributed on the macropore wall. It is well known thatthe larger pores release water more readily than smaller pores.Therefore, the water retained in macroporous cavities can be easily

Fig. 6. SEM images of 3D macroporous film reacted with the solution containing onlyhydrochloric acid (37%) at hydrothermal condition. (a) Top view of porous film withpore size of about 1 μm. (b) The cross section of porous structure.

(b) Diffuse reflectance and specular reflectance spectra of 3D porous film on glass. Themeasured 3D porous samples are obtained at hydrothermal temperature of 230 °C for4 days.

released by nature air drying within about 5 min or hot air dryingimmediately. Thus, the transmittance state could be reversiblychanged to the opaque state after release of water from the porousfilm.

Furthermore, the low transmittance in opaque state shown inFig. 7a is attributed to the diffuse reflection of light rather than theabsorption of light for this 3D porous film. As shown in Fig. 7b, theaverage value of diffuse reflection was up to 54.37% within thewavelength range of 260 to 1100 nm. Interestingly, its weaktransmission below 350 nm in is also due to diffuse reflection ratherthan absorption of 3Dmacroporous film. Comparedwith transmissioncurve in the opaque state of Fig. 7a, the reflectivity increases as thetransmittance decreases over whole wavelength range. However, thevalue of specular reflectivity was almost close to zero. We attributehigh diffuse reflectivity to the structure of lens-shaped cavities withinthe film, as identified by Fig. 1f and the insert of Fig. 2. When lightstrikes these lens-shaped cavities, it would be continuously reflectedsince it cannot transmit along straight line within film due tocountless diverging of these cavities.

Photographs of a 15-cm2 optical switching porous film in theopaque, partially transparent and transparent states are shown inFig. 8. The macroporous film firstly showed the white color in theopaque state because of its uniform reduction of light intensity of allwavelengths (Fig. 8a). Subsequently, only a part of the porous filmwas impregnated with water, the wetted part became transparentwhile the unwetted part still remained white color (Fig. 8b). Afterwetting the whole porous film, it became completely transparent, andthe flowers behind it could be seen clearly (Fig. 8c). The video of the

Fig. 8. Photographs of liquid-induced 3D porous film switchable glazing. (a) The opaquestate. The sample is white color due to the reduction of light transmission of allwavelengths uniformly. (b) The samples showing partially transparent state byimpregnating only this area with water. (c)The transparent state obtained byimpregnating porous film with water entirely.

Fig. 9. Transmittance of 3D porous films (opaque state) obtained under thetemperature of 230 °C at different hydrothermal time from 2 h to 96 h.

Fig. 10. (a) The schematic diagram of liquid-induced optical switching device thatdesigned as a sandwich structure. (b) The transmittance spectra of the opticalswitchable device before and after being impregnated with water corresponded to theopaque state and transparent state, respectively.

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whole process changing from the opaque state to the transparentstate of this liquid-induced optical switching glazing is provided inSupplementary movie 1.

Additionally, the transmissivity of 3D porous films in the opaquestate can be controlled by different hydrothermal times, as shown inFig. 9. The porous film has been obviously formed on glass after 2 hhydrothermal reaction, while the light transmission has decreaseduniformly up to 60.1% in the wavelength range of 400 nm to 1100 nm.Furthermore, the transmittance of 3D porous films decreases with theincreasing of hydrothermal time. Therefore, the modulation range ofliquid-induced optical switching glazing could be selected accordingto the actual requirements.

The application of liquid-induced optical switching device could beeasily designed as a sandwich structure, as shown in Fig. 10a. Twopieces of glass with 3D porous film were face to face fabricated as asandwich device by using 2 mm thick hollow plastic sheet as spacer,then sealed with epoxy resin. After the water were injected into thedevice through the small entry hole, the 3D porous film turnedtransparent immediately. Subsequently, the opaque state could bereversibly realized by releasing water out from the exit hole under thecombined action of self-weight and hot wind from the hair dryerwithin 20 s. Fig. 10b shows the transmittance spectra of this sandwichoptical switchable device before and after being impregnated withwater, corresponded to the opaque state and transparent state,respectively. The average transmissivity of the opaque state andtransparent state was 13.2% and 62.3% in the wavelength range of 350to 1100 nm, respectively. The modulation range of this liquid-inducedoptical switching device also shows about 50% in visible and nearinfrared wavelength range.

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4. Discussion

For most chromogenic-materials, the low transmittance in opaquestate is due to the light absorption control. For example, electrochro-mic devices have the radiation absorption values of about 80% in theirlow-transparent state [24], which causes the temperature of electro-chromic elements to be much higher than the ambient air.Accordingly, the absorbed heat would still enter into the room dueto the temperature difference between the electrochromic pane andindoor air [25]. Therefore, the reflection control of 3D porous filmwould have advantages over the absorption control of electrochromicdevices in energy savings, which would completely prevent heat toenter the room in the opaque state. Besides, one of themost importantfactors in the evaluation of smart windows is their transmittancemodulation range in the visible and whole solar spectrum. So far, thelargest modulation ranges found in literatures are from WO3-basedelectrochromic devices, but transmittance modulation ranges higherthan 50% in the visible spectrum or complete solar spectrum are rare[23]. Whereas, the best modulation range of our liquid-inducedoptical switching glazing shows more than 50% in visible and nearinfrared wavelength range. Such a wide optical switching modulationrange is promising for application of this porous film to energy savingsmart windows.

5. Conclusion

In conclusion, the long-range uniform 3D macroporous films havebeen prepared on glass without using structure-directing templates.The size of macropores can be controlled by changing the hydrother-mal temperature. These 3D porous films exhibit unique high-contrastoptical switching properties induced by impregnation or release ofwater. Unlike the traditional absorption based chromogenic materials,the low transmittance in opaque state is attributed to the diffusereflection of light rather than the absorption of light for this 3D porousfilm. Furthermore, liquid-induced optical switching device weredesigned as a sandwich structure. The practical operation of thesandwich structure liquid-induced optical switching device using thismaterial has been tested, and the possibility of realizing a high-contrast optical switching window with potentially low cost andenvironment-friend liquid has been demonstrated.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.jnoncrysol.2011.01.031.

Acknowledgements

We gratefully acknowledge financial support from the NationalNatural Science Foundation of China (grant no. 50702079), theFundamental Research Funds for the Central Universities (grant no.10lgpy17), and Science and Technology Planning Project of Guang-dong Province, China (grant no. 2008A080800007).

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