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Many properties provided by supramolecular chemistry,nanotechnology and catalysis only appear in solids exhibitinglarge surface areas and regular porosity at the nanometre

scale1–4.In nanometre-sized particles,the ratio of the number of atomsin the surface to the number in the bulk is much larger than formicrometre-sized materials, and this can lead to novel properties.Here we report the preparation of a hierarchically structuredmesoporous material from nanoparticles of CeO2 of strictly uniformsize. The synthesis involves self-assembly of these 5-nm CeO2

pre-treated nanoparticles in the presence of a structure directing agent (poly(alkylene oxide) block polymer). The walls of thishexagonal structured CeO2 material are formed from the primarynanoparticles.The material possesses large pore volumes,high surfaceareas,and marked thermal stability,allowing it to be easily doped aftersynthesis whilst maintaining textural and mechanical integrity. It alsoexhibits a photovoltaic response, which is directly derived from thenanometric particle size—normal CeO2 does not show this response.We have constructed operational organic-dye-free solar cells usingnanometric ceria particles (in both mesostructured or amorphousforms) as the active component, and find efficiencies that depend onthe illuminating power.

Cerium oxide is being currently used for preparing high-temperature ceramics, catalysts5–7 and fuel cells8,9. For many suchapplications, it is desirable to prepare CeO2 samples with the highestpossible surface area.Thus,mesostructured CeO2 materials that presentlarge pore volume have been synthesized, but their thermal andhydrothermal stability is very low and the structure collapses uponsteaming at mild temperatures10,11.But numerous applications could bedeveloped on the basis of structural isotropic doping to modify theelectronic properties of the crystalline CeO2 parent materials—provided that high surface areas and mechanical and thermal integritycould be maintained.

We have been able to prepare a high-surface-area, thermallystable, nanostructured CeO2 by self-assembly of individual CeO2

nanoparticles in a liquid crystal phase. The required starting materialsare spherical crystalline CeO2 nanoparticles of 5 nm diameter and apoly(ethylene oxide) poly(propylene oxide) triblock copolymerdesignated EO20PO70EO20. Figure 1 shows the two-dimensional small-angle X-ray scattering (SAXS) pattern, displaying a single broaddiffraction peak centred at 15.5 nm,and suggesting a structure with welldefined hexagonal symmetry.

After calcination at 500 °C, a single low-angle diffraction peak at12.2 nm appears,corresponding to a contraction of the mesostructuredmaterial owing to the rearrangement of the individual nanoparticles withthe formation of strong covalent bridges between them. This covalentbonding between particles is responsible for the thermal stability of the calcined material. The high-angle X-ray diffraction pattern ofthe calcined sample shows well resolved peaks characteristic of theindividual CeO2 nanoparticles, with crystalline domains of 5 nm.Mesoscale hexagonal order in the calcined material is demonstrated bytransmission electron microscopy (TEM) of the large (2 ×0.7 µm) CeO2

crystals (Fig. 2), in which large channels organized in hexagonal arrays

Hierarchically mesostructured doped CeO2

with potential for solar-cell useAVELINO CORMA1*, PEDRO ATIENZAR1, HERMENEGILDO GARCÍA1 AND JEAN-YVES CHANE-CHING2

1Instituto de Tecnología Química,UPV-CSIC,Universidad Politécnica de Valencia,Avda.de los Naranjos s/n,46022 Valencia,Spain2Rhodia Recherches,52 Rue de la Haie Coq,93306 Aubervilliers Cedex, France*e-mail: acorma@itq.upv.es

Published online:16 May 2004; doi:10.1038/nmat1129

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Figure 1 Small-angle X-ray diffraction pattern of material formed by self-assembly of CeO2 nanoparticles under acidic conditions,before and aftercalcination.Wave vector q is defined as (4π/λ)sin(2θ/2), and 2θ is the scattering angle.Inset, the two-dimensional small-angle X-ray diffraction pattern of a selected area ofnon-calcined material, showing its hexagonal structure with a = 180 Å, illustratingliquid-crystal templating of CeO2 nanoparticles.

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can be observed. The BET surface area and pore volume of the calcinedmaterials are 160 m2 g–1 and 0.35 cm3 g–1, respectively. The SAXS and BET measurements are consistent with the TEM images, and from thepore diameter (7.5 nm) determined by N2 adsorption and the unit celldimensions (determined by SAXS), a wall thickness of ∼6.0 nm isinferred. The similarity of wall thickness and nanoparticle diameterindicates an assembly process involving a monolayer of individualnanoparticles (Fig.3).

The thermal and hydrothermal stability of the mesostructuredCeO2 material is remarkable, and considerably higher than relatedmesoporous silicas and other porous oxides. Even treatment of themesostructured CeO2 at 700 °C under steam for 5 h does not producenoticeable variations in the porosity of the material.

Zr4+and La3+doped CeO2nanostructured materials were prepared bya wet impregnation procedure,in which salts dissolved in water are placedin contact with the preformed mesostructured CeO2, followed bycalcination at 400 °C. An analogous impregnation procedure applied tonormal (micrometre-sized) particles leads to preferential surface doping.However, in the case of our nanometric CeO2 particles, an X-raydiffraction peak shift was observed upon doping,consistent with Vegard’slaw applicable for mixed oxides, even at the highest doping level.This indicates the formation of well crystallized Ce1–xZrxO2 (x=0–0.3) and Ce1–xLaxO2 (x=0–0.3) solid solutions in which the dopant has entered the nanoparticle and is uniformly distributed through it.Nitrogen adsorption–desorption isotherms and TEM confirmed that thedoped materials maintain the nanostructure of the original CeO2 sample.

In the context of replacing silicon as the photoactive material in theconstruction of solar cells, an intense line of research has been devotedto development of photovoltaic cells based on semiconductor metaloxides. Among these semiconductor oxides, titanium oxide is the most widely used12–14.A major achievement in this field has been the useof mesoporous titanium dioxide as photoactive material15. The large

surface area,porosity and electric contact of this special titanium dioxidefavours interfacial electron transfer and easy diffusion of electrolyte.

a b

Figure 2 Representative TEM images of CeO2 samples prepared by nanoparticle self-assembly under acidic conditions. a,Sample calcined at 500 °C for 6 h,and exhibiting anordered hexagonal structure.b,TEM image of material calcined at 500 °C,showing formation of a large self-assembled crystal up to 2 µm length,based on 5-nm CeO2 nanoparticlebuilding blocks.Scale bars:a,50 nm; b,100 nm.

5 nm

Figure 3 Nanostructured material.The assembly process, involving a monolayer ofindividual 5-nm nanoparticles.

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CeO2 displaying a bandgap of 3.2 eV (ref. 16) is commonly used asan ultraviolet blocking material17. Because this bandgap is usuallyattributed to an O2p → Ce4f transition,CeO2 is not generally considereda semiconductor and is not regarded as a photoactive material.Unexpectedly, the new non-doped and rare-earth-doped CeO2

nanoparticles exhibit a photovoltaic response derived directly from thenanometric structure of the constituent particles. Large-particle-sizeCeO2 does not possess photovoltaic response.

When developing a solar cell, one of the key issues is efficiency.The absorption spectrum of the photoactive components has to matchas closely as possible the solar emission spectrum. As TiO2, irrespectiveof its porosity,only absorbs light of wavelengths shorter than 350 nm,itsefficiency is low simply because the fraction of the solar emissionspectrum that it absorbs is low. To solve this problem, dye-sensitizedTiO2 solar cells have been developed15. In the case of the ceriananoparticles, the absorption spectrum is shifted about 80 nm ascompared to TiO2 (Fig.4),which gives a considerably better response inthe visible region of the solar spectrum. The bandgap of the CeO2

nanoparticles estimated from the onset of the absorption spectrum was3.0 eV. Ideally, quantum size effects arising from a reduction of theparticle size predict a bandgap increase that is the opposite effect to theone observed here. However, simple quantum size effect calculationsassume that no significant variation of the chemical structure of themetal oxide is occurring during the particle size reduction.In the case ofour CeO2,the presence of a significant fraction of Ce atoms (in either the3+ or 4+ state) on the external surface leads to oxygen vacancies anddefects whose influence on the bandgap overcomes the expectedinfluence of the regular quantum size effect18,19. On the other hand, thevalence band potential of regular CeO2 in vacuum has been theoreticallypredicted at –8.7 eV (ref. 20). It can also be anticipated that for CeO2

nanoparticles the position of the valence band will be somewhatdifferent,although it was not possible to determine directly the positionby voltamperometric measurements owing to the discharge of water.

Another aspect of particular importance when developing solarcells is the timescale over which photogenerated electrons (e–) and holes (h+) are present before undergoing charge recombination.Light absorption provokes, in the semiconductor, the promotion of anelectron from the valence band to an empty conduction band, with thecreation of a positive hole in the valence band (h+) and mobile ortrapped electrons in the conduction band (e–) (Fig. 5). A fast charge

recombination of h+ and e– will annihilate the charge-separated stateand will produce wasted energy without giving the possibility ofintroducing electrons into an external circuit.

Figure 6A shows the temporal profile of the voltage recorded on355-nm laser excitation for one selected example of doped ceria.For comparison, the temporal profile recorded for standard P-25 TiO2

under exactly the same experimental conditions is also included.As canbe seen, the temporal profile of the charge separation of ceria is muchlonger-lived than that of TiO2 under the same conditions.The photogenerated voltage spans seconds after a nanosecond laserflash, with a growth in the initial microseconds as compared to thesignificant voltage decay in the first hundreds of microseconds for TiO2

in an analogous cell.In view of the above results and the promising opportunities offered

by rare-earth-doped mesoporous ceria, in terms of the absorptionspectra and long lifetime of charge separation, a photovoltaic cell wasconstructed in which a thin film (50nm) of this solid was deposited on aplatinized aluminium electrode and a transparent conductive indium-tin oxide (ITO) electrode was used as the anode.A standard solution ofNaI and I2 in water was used as electrolyte (see Methods).

All the tested cells based on CeO2 nanoparticles (eithermesostructured or without periodicity) showed photocurrent underexcitationin the visible spectrum.The wavelengths at which a particularcell shows photocurrent corresponds to its response spectrum.We notethat CeO2 nanoparticles as well as rare-earth-doped ceria do not needphotosensitization to have photovoltaic activity in the visible region.Also, the excitation spectrum of the samples depends notably on thepresence of dopants. Figure 6B shows the incident photon to currentconversion spectra for some selected samples to illustrate this point.For the sake of comparison,the photocurrent response of TiO2 (withoutdye) recorded for a solar cell prepared in the same manner as that ofmesostructured ceria is also included. The presence of an organic ormetallic complex in a solar cell may limit the durability and the long-term stability of the device, as the organic components tend to becomedegraded and decompose on continuous operation of the cell.

The wavelength of the incident photon that produces maximumcurrent can be varied from 430 up to 560 nm in the CeO2 nanoparticlescells by simple doping, following the procedure described earlier.This large shift allows the tuning of the photovoltaic cell to match aparticular light source;this would enable the preparation ofcells for indoorapplications in which the ambient light produced by phosphorescentlamps is shifted compared to the solar emission spectrum.

The efficiency of a series of solar cells constructed using nanometricCeO2 and I2/I3

– electrolyte using ITO and platinized Al electrodes wastested under standard conditions; we used a solar simulator operatingfrom 400 to 900 W m–2 and AM1.5 filters. The voltage at open circuit(Voc) for a 1 × 1 cm2 cell of about 50 µm depth ranged from 0.75 to 0.9 V,and the current at short circuit (Isc) was between 0.7 and 2 mA.

300 400 500 600

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Figure 4 Optical spectra.Diffuse reflectance ultraviolet/visible spectra,plotted as theKubelka-Munk function (F ) of the reflectance,R.a,CeO2 nanoparticles (onset ofabsorption,430 nm); b,TiO2 (onset,350 nm).

CB

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Figure 5 Photogeneration of electrons and holes.Absorption of light by thesemiconducting nanoparticle promotes an electron from the valence band (VB) to theconduction band (CB).

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Fill factors obtained from the plot of current versus bias voltage alsovaried, between values of 0.25 and 0.5, depending on the sample ofnanometric CeO2 and preparation conditions. As expected, the actualefficiency increases as the light power decreases; the higher efficiencyvalues being measured for 400 W m–2.For one of the most efficient cells(prepared with colloidal non-structured La3+-doped CeO2 andirradiance of 400 W m–2), the values were Voc =0.9 V, Isc = 1.5 mA, fillfactor 0.41. The above values give an overall light to electrical energyconversion efficiency of 1.4%. Under 1,000 W irradiation with a solarsimulator using a 1.5 AM1 filter, a solar cell using a 50-µm film of La3+-and Zr4+-doped structured CeO2 exhibits 0.9% efficiency, Voc = 0.82 V,Isc = 3.4 mA and a fill factor of 0.32.It should be noted that the efficiencyvalues are not corrected for spectral mismatch. This is a good startingvalue for a new type of non-dye-sensitized solar cell, and it appears thatfurther optimization of these mesostructured materials, with adequatedoping,may provide a new family of efficient solar cells.

We note that the photovoltaic behaviour of nanocrystalline CeO2 isdifferent from that of bulk CeO2. A possible rationalization is that thehigh surface/grain-boundary area characteristic of nanocrystalsincreases considerably the presence of defects that enhance theelectronic transport properties of sintered nanocrystalline CeO2 withrespect to that of bulk ceria. In this regard, a related precedent that

would lend some support to this rationalization was the observation ofelectron conduction in 15-nm nanocrystalline CeO2 at 450 °C that is notobserved in bulk CeO2 (ref.21).

METHODSPREPARATION OF STRUCTURED CeO2

The synthetic method involves the functionalization of the nanoparticle surface with 6-aminocaproic

acid (ACA). The organic moiety was selected as it posses one terminal group which interacts specially

with the nanoparticle surface (acid) and a second function (amino) that interacts with the surfactant.

In order to do this, ACA was added to a colloidal dispersion of 5-nm CeO2 nanoparticles, with a molar

ratio ACA/CeO2 of 0.30. Thus 0.56 g EO20PO70EO20 poly(alkylene oxide) block polymer (Pluronics 123,

BASF) was dissolved in a mixture containing 10 g H2O and 5.8 ml of a 1 M CeO2 aqueous colloidal

dispersion pre-treated with ACA at pH 4.5. The resultant homogeneous dispersion was allowed to

evaporate in air: the as-synthesized samples were calcined at 500 °C for 6 h, yielding a nanostructured

material consisting of walls composed of a monolayer of CeO2 nanoparticles.

PHOTOVOLTAIC CELLSThe solar cell was prepared by suspending 10 mg of doped ceria in acetylacetone, and depositing the

suspension in a 1 × 1 cm2 square defined by adhesive tape on a transparent indium-tin oxide electrode.

After calcining at 300 °C for 2 h, a few drops of water solution containing 0.5 M LiI and 0.05 M I2 were

added. Platinized aluminium was used as counter electrode.

Measurement of the photovoltaic response of the cells was carried out in an Oriel Solar simulator at

an output power between 400 and 900 W m–2 and using a global AM1.5 filter. The performance of the

nanometric CeO2 cells was calibrated against a calibrated (Instituto de Energía Solar) standard solar cell.

The measuring equipment was a Digital multimeter 195A, a System DMM/Scanner 199 and a

voltage/current source (Keithley 228). Some of the measurements were performed at the Instituto de

Energía Solar at Madrid.

Received 10 November 2003; accepted 2 April 2004; published 16 May 2004.

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AcknowledgementsWe thank Comisión Interministerial de Ciencia y Tecnología, CICYT (MAT2003-07945-C02-01), for

financial support, and A. Luque and I. Tobias of the Instituto de Energía Solar de Madrid for technical

assistance and for providing calibrated solar cells.

Correspondence and requests for materials should be addressed to A.C.

Competing financial interestsThe authors declare that they have no competing financial interests.

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Figure 6 Comparison of the charge current and spectral response of CeO2

nanoparticles and TiO2. A,Temporal profile of the voltage measured for cells comprising afilm of semiconductor between a transparent conductive glass and an aluminium counterelectrode on 355-nm laser excitation (5 mJ per pulse,7-ns pulse width).a,TiO2; b,CeO2.The signals correspond to a single laser shot,and have not been averaged.B, IPCE versuswavelength for a solar cells based on La3+-CeO2 (a),Zr4+-CeO2 (b) and CeO2 (c).Forcomparison, the photoresponse of an analogous solar cell based on TiO2 (d) has also beenincluded. Inset,photocurrent–voltage plot of a solar cell based on La3+-CeO2.

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