Journal of Materials Chemistry AMaterials for energy and sustainability

rsc.li/materials-a

Volume 8Number 97 March 2020Pages 4581–4964

ISSN 2050-7488

PAPERSajjad S. Mofarah et al. Assembly of cerium-based coordination polymer into variant polycrystalline 2D–3D CeO2−x nanostructures

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Assembly of ceri

aSchool of Materials Science and Engineerin

Australia. E-mail: s.seimofarah@unsw.edubResearch Center for Functional Materials (R

Science (NIMS), Tsukuba, Ibaraki 305-0047cSchool of Chemistry, UNSW Sydney, SydneydLaboratory of Advanced Catalysis for Sus

University of Sydney, Sydney 2006, AustralieElectron Microscopy Unit (EMU), Mark

Sydney, Sydney, NSW 2052, AustraliafCenter for Green Research on Energy an

National Institute for Materials Science (

305-0044, Japan

Cite this: J. Mater. Chem. A, 2020, 8,4753

Received 31st October 2019Accepted 29th December 2019

DOI: 10.1039/c9ta11961b

rsc.li/materials-a

This journal is © The Royal Society o

um-based coordination polymerinto variant polycrystalline 2D–3D CeO2�x

nanostructures†

Sajjad S. Mofarah, *a Esmaeil Adabifiroozjaei, b Yuan Wang, c

Hamidreza Arandiyan, d Raheleh Pardehkhorram, c Yin Yao, e

M. Hussein N. Assadi, af Rashid Mehmood, ag Wen-Fan Chen, ah

Constantine Tsounis,i Jason Scott, i Sean Lim, e Richard Webster, e

Vicki Zhong,a Yuwen Xu, a Pramod Koshy a and Charles C. Sorrell a

Precise control over the morphology of nanomaterials is critical yet challenging. The present work reports

an efficient approach to tailor the architecture of nanostructures. The process involves rapid disassembly/

reassembly of an unstable metal-based coordination polymer (MCP) by controlling the kinetics of the

reassembly process. The synthesis procedure delivers unprecedented polycrystalline nanostructures, e.g.,

holey 2D CeO2�x nanosheets, with precisely tailored thicknesses in the range of 10–100 nm, and hollow

3D pseudo-octahedra and spheres. The consequent high surface areas and pore volumes, short

diffusion distances, and high defect densities of the holey 2D CeO2�x indicate significant densities of

active sites. This holey architecture exhibits rapid CO conversion and outstanding solar light

photocatalytic performance. This approach of directed assembly offers a template-free, controllable, and

cost-effective approach to achieve engineered CeO2�x architectures, which are nearly impossible

through existing approaches.

Introduction

The architecture of nanostructures is of great importance forboth fundamental and practical applications.1–3 This impor-tance originates from the direct impact of nanoscale geometryand the associated surface chemistry on the intrinsic propertiesof nanomaterials. These characteristics include exposed facets,adsorption energies, and density of active sites, which arerelevant to energy and environmental applications.4 Although,some conventional strategies have recently been applied to tunethe nanostructures of materials, these approaches typicallyinvolve multistep processes that require suitable surfactants,modulators, templates, and specic fabrication parameters.5–8

These complexities, their associated costs, and poor

g, UNSW Sydney, Sydney, NSW 2052,

.au

CFM), National Institute for Materials

, Japan

, NSW 2052, Australia

tainability, School of Chemistry, The

a

Wainwright Analytical Centre, UNSW

d Environmental Materials (GREEN),

NIMS), 1-1 Namiki, Tsukuba, Ibaraki

f Chemistry 2020

reproducibility have led to the need to develop alternativematerials and processes capable of overcoming these short-comings. A novel tactic that is gaining increasing attentioninvolves the post-treatment transformation of precursor-template coordination polymers (CP), including metal–organicframeworks (MOF).9–12 Using this method, metal oxides (MO) orcarbon-based nanostructures can be obtained.13,14 Although thisyields MOs with improved functionalities, the existing methodis costly, multistep, and complex in terms of processing andchemicals, and requires high temperatures. More importantly,the architectures of the resultant MO are limited to those of theprecursor CP, which are retained following post-treatment.9,15,16

Thus, only a single MO nanostructure can be obtained fromeach morphology of the CP precursor.

gDepartment of Physics and Astronomy, ARC Centre of Excellence for Nanoscale

BioPhotonics, Macquarie University, Sydney, NSW 2109, AustraliahInstitute of Medical Science and Technology, National Sun Yat-Sen University,

Kaohsiung 80424, TaiwaniParticles and Catalysis Research Group, School of Chemical Engineering, UNSW,

Sydney, NSW 2052, Australia

† Electronic supplementary information (ESI) available. See DOI:10.1039/c9ta11961b

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The present work is established on the basis of tuning thehighly weak bonds between metallic nodes/clusters and theircoordinated linkers to synthesise unprecedented MO architec-tures from a single precursor. The success of this method isconrmed by the synthesis of a new and unstable cerium-basedcoordination polymer (Ce-CP) that can undergo controllabledisassembly/reassembly in a polar solvent (ethanol). This allowsfor the formation of distinctive Ce-CP nanostructures throughcontrol of the kinetics of the reassembly process. Simple post-treatment of the Ce-CP nanostructures by low-temperaturepyrolysis and/or ageing in an alkaline solution resulted in theformation of defect-rich CeO2�x in the form of 2D and 3Dnanostructures. This approach provides a rapid, simple,template-free, precisely controllable, and economical approachto synthesise MCPs of specic architectures. This process alsohas the potential to be applied to numerous metal-basedcomplexes that can be designed to engineer novel functionalmaterials for energy and catalysis applications.

Results and discussionElectrochemical fabrication of Ce-CP

To synthesise CeO2�x nanostructures, the novel Ce-CPprecursor was fabricated using a modied anodic chro-nopotentiometric deposition (MACE) in aqueous solution atroom temperature. The schematic of the synthesis process(Scheme 1a) indicates that free-standing Ce-CP hexagonal rodsare grown on uorine-doped tin oxide (FTO) substrate. This isshown by Scanning electron microscopy (SEM) images as indi-cated in Fig. S1.† A critical factor in deposition of the Ce-CPprecursor was the application of a high current density withinthe water oxidation range such that vigorous oxygen bubblingresults in an oxidised atmosphere and the formation of acidicpH both at the surface of the working electrode and its vicinity.

Scheme 1 Schematics of (a) chronopotentiometric electrodepositionof solid Ce-CP hexagonal rods under electrolysis conditions; (b)dissolution of Ce-CP hexagonal rods and recrystallisation of Ce-CPinto hollow pseudo-octahedra. (c) Simplified molecular structures ofhexagonal Ce-CP rod, (d) schematic of solutes in ethanol solution and(e) corresponding molecular structure, (f) schematic of recrystallisedCe-CP and (g) corresponding molecular structure. Large yellowspheres¼ Ce4+, small green spheres¼ C4+, small blue spheres¼O2�,small red spheres ¼ Cl�.

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The details of the synthesis mechanism and characterisation ofthe Ce-CP are provided in Fig. S1–S7.† The X-ray diffraction(XRD) pattern of the Ce-CP powder was indexed to triclinicCe(OH)2(C2O2Cl3)2$2H2O, space group P�1, a ¼ 1.31 nm, b ¼1.32 nm, c¼ 1.10 nm, a¼ 81�, b¼ 93�, and g¼ 112� (Fig. S6 andS7†).17 The Ce-CP structure was analysed for its stability and itwas observed to be fairly stable upon exposure to air for 90 daysaer the deposition as determined from the X-ray diffraction(XRD) patterns of the corresponding samples (Fig. S8†).However, the Ce-CP exhibited high instability on exposure toethanol, which is a polar solvent, and this is shown in Scheme1b, step 1.

The simplied molecular structure of the hexagonal Ce-CProds consists of eightfold-coordinated cerium ions (Scheme1c), where the coordinating oxygen ions are linked by tri-chloroacetic acetate (TCA) ligands (four), hydroxyl ions (two),and water molecules (two). Additionally, cerium ions arebridged together by covalent bonding with carboxylic groups ofthe TCA ions, hence forming a two-dimensional (2D)substructure. However, there are weak electrostatic interactionsat the interlayer spaces of the 2D Ce-TCA substructure leadingto the formation of a stratied structure (Fig. S9†).

The Ce-CP structure, upon exposure to ethanol is dis-assembled readily forming a pale-yellow transparent solution(Scheme 1d and e). This is illustrated by an optical microscopyrecording movie obtained for 80 seconds, as provided in ESIMovie 1.† The high instability and resultant rapid disassemblyof the Ce-CP is largely due to the retention of the Ce ions in the+4 valence state.18 From thermodynamic perspective, the Ce4+

ion is of higher eld strength compared to the Ce3+ ion.19

Consequently, as shown in the Pourbaix diagram, Ce4+ hasa greater tendency to attract surrounding OH�, even in acidicpH, while Ce3+ tends to remain in the cationic state.

The aqueous solution conditions that were used to fabricatethe Ce-CP resulted in highly acidic conditions of pH < 2.3, whichyielded Ce(OH)2

2+ as the predominant species. From the rele-vant speciation diagram, it is seen that this species is a solutethat is stable at these pH values but becomes unstable at highervalues.18,19 Further, the Pourbaix diagram (ESI, Fig. S2†) showsthat Ce(OH)2

2+ exists only in the region of water instability, soits presence requires the application of an external bias andsuitably low pH. Application of external bias causes exit fromwater stability range, resulting in rapid proton formation andlocal pH decrease. Therefore, Ce(OH)2

2+ is not formed undertypical aqueous processing conditions, which are used in thepresent work. The coexistence of unsaturated coordinationbonds in positively-charged Ce(OH)2

2+ along with thenegatively-charged bidentate TCA as the organic linker resultsin the formation of Ce-CP with a unique layered structure. Thisaqueous chemistry of Ce4+ differentiates it from that of Ce3+, forwhich there are coordination polymers that cannot be dis-assembled/reassembled.20

In the recrystallisation of the Ce-CP from ethanol, this localbonding conguration and thus the presence of Ce4+ isretained, thereby enabling the re-formation of the Ce-CP(Scheme 1f and g). The design of the nal architecture can betailored through control of the kinetics of the solvent

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evaporation and the concentration of the Ce-CP solute. Thisnovel technique yields a precise controllable assembly ofnanostructures at room temperature or at even lower tempera-tures without using a template. This method can thus be used tofabricate unique architectures that are very difficult to preparethrough pre-existing techniques.

Although the creation of nanoholes in transition-metal (TM)-based 2D materials has been shown to considerably improvetheir functionality via the enhancement of the number of activesites and short diffusion pathways,21 there has been very limitedwork on the fabrication of cerium-based holey nanosheets,17

and more importantly, such structures with controllablethicknesses.

2D Ce-CP nanosheets and derived holey CeO2�x

In this work, holey CeO2�x nanosheets with various thicknesseswere fabricated successfully by imposing the conditions of slowkinetics of ethanol evaporation at the low temperature of�10 �C and vapour pressure (VP) of 0.744 kPa. As illustrated in

Fig. 1 (a) Schematic showing the formation of Ce-CP monolayer atethanol/air interface: Ce4+ (green), –OH group of ethanol (purple),–COO� group of TCA (blue), and –CCl3 group of TCA (red). (b)Schematic of monolayer and stacking arrangement (residual –OH andH2O in are omitted from Ce-CP and solution volume for simplicity). (c)Optical microscopy image of Ce-CP nanosheets. (d) AFM image of Ce-CP nanosheet and index corresponding to height profile. (e) A lowmagnification TEM image of Ce-CP nanosheets; inset: SAED pattern ofCe-CP nanosheet. (f–k) EDS mapping of the Ce-CP nanosheetshowing maps for (g) Ce; (h) O; (i) Cl; (j) C; and (k) Sn.

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the schematic of a Ce-CP monolayer in Fig. 1a, these conditionsresulted in the formation of individual Ce-CP layers by effec-tively Langmuir–Blodgett deposition.22 A relevant bottom-up 2Dgrowth mechanism has been proposed by Wang et al.,23,24 inwhich zinc hydroxyl dodecylsulfate nanosheets were syn-thesised at the water/air interface using the surfactant sodiumdodecylsulfate in aqueous solution. The polarity of the surfac-tant caused the positively-charged Zn ions in solution to beattracted electrostatically to the negatively-charged hydrophilic–SO3

� group of the surfactant, while the hydrophobic –CH3

group of the surfactant is projected upwards in air. In contrastto the work by Wang et al.,23,24 which used separate solvent andsurfactant, the mechanism illustrated in Fig. 1b involvessurface-assembly of Ce-CP at the ethanol/air interface, whereethanol shows dual functionality as both the solvent andsurfactant in this bottom-up 2D process.3,4 The aligned projec-tion of the positively-charged hydrophobic –CH3 groups ofethanol in air establishes a negatively-charged layer consistingof hydrophilic –OH groups of ethanol at the surface. Theformation of this layer provides the polar attraction to Ce4+ ionsin solution and thus forms the basis for the development ofa cerium-enriched electrostatic double layer. The commensu-rately aligned –COO� groups attached to the Ce4+ each containa negative hydrophobic tail of a –CCl3 group, the layer of whichterminates the Ce-CP monolayer. This terminal layer providesthe structural and charge neutrality requirements for electro-static bonding to the positive –CH3 groups of ethanol on theopposite terminal layer of the Ce-CP monolayer. Continualevaporation of ethanol provides the driving force for themigration of more Ce4+ ions toward the surface irrespective ofwhether the monolayer is permeable or not. In this way,multiple monolayers can stack together to form sheets witha wide range of thicknesses. This is shown in Fig. S10,† whereCe-CP sheets with varying thickness ranging from extremelythin (10 nm) to slightly thick (100 nm) were synthesised duringreassembly over 12–72 h. The variation of thickness as a func-tion of evaporation time is plotted in Fig. S10f† providinga semi-linear trend for the controllable fabrication of nano-sheets with precisely tailored thickness. Further, changes in theCe-CP concentration, as a precursor, at constant reassemblytime of 48 h results in the formation of Ce-CP nanosheets withdifferent thicknesses (Fig. S11†).

Fig. 1c shows the optical image of the fragmented Ce-CPnanosheets with lateral sizes of a few hundred microns.Fig. 1d shows an atomic force microscopy (AFM) image ofa representative nanosheet collected from the ethanol/airinterface aer 48 h of ethanol evaporation at �10 �C. Theassociated height prole shown in the inset of Fig. 1d revealsa consistent thickness of �48 nm. The transmission electronmicroscopy (TEM) and corresponding selected area diffraction(SAED) patterns conrm the presence of Ce-CP nanosheets withpolycrystalline structure, as illustrated in Fig. 1e and the cor-responding inset, respectively. Elemental mapping done byenergy dispersive spectroscopy (EDS) (Fig. 1f–k) shows thepredominant elements to be Ce and Cl. These nanosheets canbe transferred easily to a glass substrate using van der Waalsexfoliation technique.25

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Fig. 2 (a and b) HAADF images and (b, inset) SAED image of the ofholey CeO2�x nanosheet. (c) HRTEM image of the holey CeO2�x

nanosheet. (d) XPS spectra of Ce 3d orbital of Ce in holey CeO2�x

nanosheet. (e) AFM image of holey CeO2�x nanosheet. (f) AFM heightprofile of CeO2�x nanosheet.

Fig. 3 (a) SEM image and (b) schematic of as-recrystallised Ce-CP. (c)Corresponding XRD pattern. (d) SEM image and (e) schematic ofNaOH-aged CeO2�x pseudo-octahedron. (f) Corresponding XRDpattern. (g) SEM image and (h) schematic of CeO2�x pseudo-octahe-dron. (i) Corresponding XRD pattern. (j) Dark field TEM and SAED(inset), (k) dark field HRTEM image of CeO2�x pseudo-octahedron.

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The Ce-CP transformation into CeO2�x was carried out byageing the Ce-CP nanosheets in strongly basic solution (6 MNaOH) at room temperature followed by heating at 200 �C. Asa result, the 2D morphology was retained along with wide-spread nanohole formation. Fig. 2a and b show high angleannular dark-eld (HAADF) images of the holey CeO2�x

nanosheet. The polycrystalline nature of the CeO2�x isconrmed by the SAED pattern in inset (Fig. 2b). The high-resolution TEM (HRTEM) image of the nanosheet (Fig. 2c)illustrates crystallites with sizes in the ranges of 4–8 nm andintercrystallite holes of up to 10 nm. In addition, there arestrong chemical bonds between the single crystallites owing tothe cross-fringed lattices.26 Fig. 2d shows the X-ray photo-electron spectroscopy (XPS) spectra of the holey CeO2�x

nanosheet that indicates the coexistence of both Ce3+ and Ce4+

oxidation states in the CeO2�x. The presence of Ce3+ reectsthe oxygen vacancy defects ðV ��

O Þ, which is considered as anactive site in catalysts.18,27 The concentration of oxygenvacancies ð½V ��

O �Þ was quantied indirectly from the amount ofCe3+ and this is discussed later. Fig. 2e and f show the AFMimage (e) and the corresponding height prole (f) of a highlyporous CeO2�x nanosheet derived from a Ce-CP nanosheetcollected aer 12 h of evaporation.

3D Ce-CP hollow pseudo-octahedra and derived CeO2�x

It was shown previously that the ethanol evaporation at �10 �Cresulted in very slow formation of Ce-CP nanosheets. In order tohighlight the signicant role of the kinetics of recrystallisation(KR), rapid recrystallisation of the Ce-CP was done at roomtemperature, while the concentration remained unchanged([Ce-CP] ¼ �8 M). Fig. 3a shows the scanning electronmicroscopy (SEM) image of a free-standing Ce-CP pseudo-octahedron. The pseudo-octahedra with variable c axislength, terminated by positive and negative pyramids, shownin Fig. 3b, is a common crystal form for minerals crystallisingin the triclinic system.28 The XRD pattern of the Ce-CP pseudo-octahedra is identical to that of the Ce-CP rods (Fig. S12a†),

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conrming that the crystal structure remained unchangedand is unaffected by the disassembly/reassembly process.However, the peaks for the pseudo-octahedra were broadenedrelative to those of the rods. The difference in full-width ofhalf-maximum (FWHM) of the XRD patterns can be rational-ised by the smaller crystallite size of the pseudo-octahedra,relative to the Ce-CP precursor (Fig. S12b†). For furtherconrmation, identical chemical structures of the Ce-CPprecursor and pseudo-octahedra are shown by laser Ramanmicrospectroscopy (Raman) and Fourier transform infraredspectroscopy (FTIR) (Fig. S12c and d†).

The transformation of pseudo-octahedral Ce-CP intoCeO2�x without morphological change was carried out byageing the Ce-CP in the 6 M NaOH solution at room temper-ature. This is illustrated by the SEM image and the corre-sponding schematic in Fig. 3d and e, respectively. A similartechnique has been reported previously by Zeng et al.,29 whoprepared hierarchical dumbbell-shaped CeO2 derived froma Ce-based MOF.29 The XRD pattern of the CeO2�x that wasderived from the Ce-CP (Fig. 3f) was indexed to the cubicuorite structure of CeO2 with space group Fm3m.30 Generally,the transformation of a CP into a metal oxide is attributed tothe replacement of weakly-bonded organic linkers by the OH�

and/or H2O in aqueous solution. For the Ce-CP in aqueoussolution, the relatively high eld strength of Ce4+ enhances itsability to form Ce(OH)4, which readily converts to CeO2�x upondrying.19 The transformation can be achieved by pyrolysis attemperatures of >200 �C. Although this results in concavedistortion of the facets owing to the removal of the residualOH� ions and H2O molecules (Fig. 3g and h), this results inincreasing crystallinity (Fig. 3i). Further, from the SEM imageof the CeO2�x pseudo-octahedron, pores are seen on thestructures (shown by magenta circles in Fig. 3g). This isconrmed by dark eld HRTEM imaging (Fig. 3j and k), inwhich the pore clusters of �10 nm size are identied. Thediffuse rings in the selected area diffraction (SAED) pattern(Fig. 3j inset) show the randomly orientated structure of the

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polycrystalline CeO2�x. The Brunauer–Emmett–Teller (BET)surface area of the hollow pseudo-octahedra was measured tobe 47.18 m2 g�1 with a pore size of 6.86 nm and pore volume of0.42 cm3 g�1 (Table S1†).

3D Ce-CP hollow spheres and derived CeO2�x

Another factor is the degree of electrolytic dissociation (a) of theCe-CP; this value is constant at �1 owing to full disassembly ofthe Ce-CP in ethanol:

a ¼ (CTCA/CCe-CP)

A third key factor controlling the structural reassembly is theCe-CP concentration. In principle, the concentrations of theions in solution determine the supersaturation factor (S)according to the following equation:

S ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCCe � ðCTCAÞd

Ksp

s

where CCe, CTCA, Ksp, d are dened as the concentrations ofcerium cations and dissociated TCA anions, solubility productconstant, and number of ions in the complex anion (TCA),respectively. Increasing the value of Smay result in a shi in thecrystallisation towards 3D structure,2 while lower value of Scould form structures with lower dimensions, e.g., 2D. Accord-ing to the constant Ksp for the Ce-CP, considerable increase theCe-CP concentration is expected to lead to the formation of 3Darchitectures.

The effect of S was foreshadowed by KR through focusing onthe kinetics of Ce-CP recrystallisation by tailoring the vapourpressure of the ethanol solvent. This was shown previouslythrough the fabrication of different morphologies at 25 �C(pseudo-octahedra) and �10 �C (holey nanosheets). That is, the

Fig. 4 SEM images of Ce-CP morphologies synthesised at 0 �C: (a)[Ce-CP] ¼ 4 M, (b) [Ce-CP] ¼ 16 M. (c) Hollow spheres being liberatedfrom nanosheets. (d) Schematic showing the formation of Ce-CPhollow spheres through bubbling of the stacked nanosheets as a resultof ethanol evaporation. (e) 3D AFM image of the Ce-CP nanosheetssynthesised by two-stage evaporation at �10 �C (12 h) and 15 �C (0.5h). (f) AFM height profile (black dotted line).

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signicantly different VPs of ethanol at these two temperatures,i.e., 7.830 kPa and 0.744 kPa, respectively, indicate the presenceof signicant Ce-CP concentration gradients during evapora-tion. Consequently, the intermediate temperature of 0 �C (VP ¼1.568 kPa) was selected as the basis for the examination of theeffect of concentration and the corresponding results areillustrated in Fig. 4. Fig. 4a and b show that increasing the [Ce-CP] by four times (from 4 M to 16 M) causes the resultantmorphologies to change from nanosheets to purely hollowspheres. This transformation is conrmed by SEM image of thespheres being liberated from the stacked nanosheets (Fig. 4c).

The proposed formation mechanism of the spheres is basedon bloating of the nanosheets during the evaporation of inter-layer adsorbed ethanol and this is schematically shown inFig. 4d. To conrm the proposed mechanism, the experimentalconditions were designed to accelerate the evaporationresponsible for the formation of hemispheres prior to theformation of spheres and their eventual detachment. The 3DAFM image and corresponding height prole are shown in

Fig. 5 Characterisation of hollow CeO2�x spheres. (a) Low-magnifi-cation and (b) high-magnification SEM images of hollow CeO2�x

spheres. (c) SEM image of broken hollow spheres. (d) Low magnifi-cation TEM image of the hollow CeO2�x spheres. (e and f) High-magnification TEM image of the hollow CeO2�x spheres. (g) SAEDpattern of the hollow CeO2�x spheres (the planes are in the order fromcentre to perimeter). (h and i) EDS elemental mapping of Ce and O inthe hollow CeO2�x spheres. (j) Raman spectra of Ce-CP before andafter NaOH ageing.

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Fig. 4e and f, respectively. The hemispheres were of diameters�600–700 nm (heights � 10–23 nm) and these are larger thanthose of the spheres at �200–400 nm, as shown in Fig. 5. Thissize difference is attributed to the gradual contraction of theformer during the formation of the spheres.

Similar to other nanostructures, the NaOH ageing andpyrolysis at 200 �C were used to transform the Ce-CP intoCeO2�x spheres. The SEM images of the CeO2�x are shown inFig. 5a–c revealing the hollow spheres with sizes between 200and 400 nm. The TEM images in Fig. 5d and e show the hollowstructure of the CeO2�x, while the wall thicknesses of thespheres were in the range of �28–40 nm. These thicknesses areassumed to be approximately half the thickness of the originalnanosheets. HRTEM image of an individual hollow sphere(Fig. 5d) is shown in Fig. 5e in which the crystallites withexposed facets of (111) and (100) are identied. The SAEDpattern of the hollow spheres, as shown in Fig. 5g, was indexedto CeO2 and the rings conrm the polycrystalline structure.Further, Fig. 5h and i show EDS mapping of Ce and O in theCeO2�x hollow spheres.

Fig. 5j shows Raman spectra of the Ce-CP and the effects ofthe ageing process on the CeO2�x derived Ce-CP. Aer the NaOHageing, the peak at 455 cm�1 was indexed to the F2g vibrationmode of Ce and O. However, the asymmetric nature and red shiof the peak is attributed to the presence of the V ��

O in the structure.This is conrmed by the two broad lower intensity peaks, whichare indicative of charge-compensating ðV ��

O Þ.31–33

Overall Ce-CP formation mechanism

Fig. 6 illustrates various nanostructures obtained at roomtemperature (constant KR), while the S value is variable(different [Ce-CP]), where Fig. 4a–d show the Ce-CP nano-structure and Fig. 4e–h show the CeO2�x obtained throughNaOH ageing and heating at 200 �C. An architectural alterationof the Ce-CP and consequently CeO2�x as a function ofincreasing [Ce-CP] follows the order of nanosheets, hollowspheres, hollow pseudo-octahedra, hollow elongated octahedra,and dense leaves. Themodel for the stacking of the multiple atnanosheets suggested in Fig. 1 is supported by the presence of

Fig. 6 SEM images of the Ce-CP nanostructures synthesised at 25 �Cby using varying Ce-CP concentrations of (a) 4 M, (b) 8 M, (c) 40 M, (d)120M; (e–h) SEM images of the correspondingCeO2�x nanostructuresderived from the Ce-CP by ageing in NaOH (6 M) at 25 �C followed bysubsequent heating at 200 �C.

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the ridges clearly apparent in Fig. 6f and g and faintly visible inFig. 6h and S13k.† Finally, the dense leaf morphology shown inFig. 6h is formed as a result of the collapse of the elongatedoctahedral morphology shown in Fig. 6g. This density derivesfrom the greater [Ce] and the consequently reduced diffusiondistance. All the nanostructures in Fig. 6 were generated at lowtemperature (25 �C) and thus the driving forces for diffusionwere low. This favoured the formation of polycrystalline struc-tures rather than single-crystals. Consequently, the exibility ingenerating the various nanostructures suggests structuralalteration through low-energy displacive rather than high-energy reconstructive phase transformations.34

Further morphological analyses of the CeO2�x are providedin Fig. S13.† Identication of the profusion of single CeO2�x

pyramids in Fig. S13h,† in combination with the ridges presentin the pseudo-octahedra suggest that these forms were gener-ated from the mated hemispheres still being attached todiametral nanosheets (Fig. 6a). The process can be proposed tooccur by early faceting via planarisation of the rounded hemi-spheres, where the ridges are formed from the fracture of theexible Ce-CPmonolayers. As suggested in Fig. 6a, the pyramidsare formed before separation from the nanosheets owing to thepresence of themaximal diametral stress at the circle of greatestsheet misalignment. While the individual pyramids would haveformed by complete delamination, the nanosheets and theelongated octahedra are formed by a different mechanism.These structures are likely to have been generated by cyclicevaporation of and backlling by ethanol when the two hemi-spheres remained in close proximity, causing chemical gradientuctuations. Closer inspection of Fig. 6g supports this notion inthat the central ridges of the elongated octahedra are the mostmisaligned, suggesting the closure of the two pyramids at thenal stage of evaporation–condensation.

It is known that recrystallisation of assemblies from anelectrolyte containing both component cations and anions isdetermined by electrolytic dissociation (a), kinetics of recrys-tallisation (KR), and supersaturation factor (S),2 which aredescribed in the preceding equations. In terms of a, the solventethanol, which has a high dissociation degree, and the soluteCe-CP are single variables and so the electrolytic dissociationfactor in principle is xed. Despite this, the kinetics of recrys-tallisation (KR) was varied through controlling temperaturevariation, which altered the ethanol evaporation rate. Similarly,in terms of S, the changes in Ce-CP concentration duringevaporation also resulted in variations in the S value. Conse-quently, the formation of polycrystalline 2D and 3D structuresdepends on the synergetic control of both the KR and S factors.Such control through a single experimental variable allows thesystematic and precise variation of the morphology from 2D to3D. More specically, evaporation kinetics characterised by lowKR and S factors result in the formation of 2D Ce-CP nanosheets.When a low KR factor is retained but the S factor is increased,increasing nanosheet thickness occurs. When the KR factor isincreased through evaporation at room temperature, increasingS results in alteration to 3D structures in the progression hollowspheres, hollow pseudooctahedra, hollow elongated

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Fig. 7 Formation mechanism for the Ce-CP nanostructures.

Fig. 8 (a) CO conversion rate and TOF values for CO oxidation ob-tained using different nanostructured morphologies of CeO2�x, (b)Arrhenius plots for the oxidation of CO over the samples.

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pseudooctahedra, and nally solid leaf. This general formationmechanism of various nanostructures is shown in Fig. 7.

Fig. 9 Photocatalytic performances of CeO2�x: (a) UV-vis absorptionspectra of MB dye solution following 160 min irradiation for differentmorphologies, (b) 664 nm peak intensities based on UV-vis absorptionspectra of MB dye solution at different irradiation times for differentmorphologies, (c) plots of absorbance (At/A0, at time t vs. initial time)and extent of the dye degradation as a function of irradiation time forholey nanosheet, (d) comparison of the photocatalytic performancesobtained in this work and that reported from surveyed published dataunder similar testing conditions (Table S1†).

Catalytic and photocatalytic applications

The CO oxidation was tested in order to assess the catalyticperformance of the CeO2�x nanostructures. The results in Fig. 8show that the CO conversion rates decrease in the order nano-sheet > pseudo-octahedron > sphere > leaf. For example, at400 �C, these values are 21.1, 12.8, 1.93 and 0.0 mol g�1 s�1,respectively. The turnover frequency (TOF) values calculated onthe basis of the CO molar ratio for each of the catalysts at thistemperature show that the holey nanosheet (surface area ¼ 81m2 g�1, pore volume ¼ 0.32 cm3 g�1) exhibited the highest TOFvalue of 4.42 � 10�3 mol mol�1 s�1, which is 1.5 times higherthan that of the pseudo-octahedron (surface area ¼ 47 m2 g�1,pore volume ¼ 0.42 cm3 g�1). This value for the nanosheet isalso 5 times that of the sphere (surface area ¼ 53 m2 g�1, porevolume ¼ 0.15 cm3 g�1) and nearly 50 times that of the leaf(surface area ¼ 6 m2 g�1, pore volume z 0 cm3 g�1). Theseresults conrm that the combined surface and pore volumesreect the density of active sites, which consist of unsaturatedcoordination bonds that enhance CO adsorption.35 Further, thepolycrystalline nature of the nanostructures is importantbecause V ��

O as point defects have been shown to be present athigh concentrations along the grain boundaries.17,36

The kinetics of catalysis also were characterised throughArrhenius plots in order to determine the activation energies(Ea) for CO oxidation for the different nanostructures. As ex-pected, these follow in the same relative order as the CO

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conversion rates and TOF values increase for the nanosheet,pseudo-octahedron, sphere, and leaf: 47 < 58 < 115 <134 kJ mol�1, respectively. It is signicant to note that high ½ V ��

O �of the CeO2 samples, obtained from quantitative analysis of XPSresults in Fig. S14 and S15,† play an important role in thecatalytic activity by facilitating CO adsorption and acceleratingthe mobility of lattice oxygen to enhance the desorption ofCO2.37,38

Further, the photocatalytic performance of the CeO2�x

morphologies was investigated initially by photodegradation ofa standard dye (methylene blue (MB)), which is a benchmark forquantication,17,39 during 100 mW cm�2 of irradiance at AM 1.5G solar illumination. Themaximal intensities of the absorbancepeaks, at 664 cm�1, were used as the bases for the comparativeassessment, the data for which are shown in Fig. 9a and tabu-lated in Table S1.† Fig. 9b reveals that there are three levels ofperformance for the dye degradation: high (84% holey nano-sheet), medium (55% pseudo-octahedron, 40% sphere), and low(16% leaf). These data are in agreement with the CO oxidationactivities, suggesting the predominant roles of surface area andpore volume in catalytic activities.

The kinetics of degradation by the holey nanosheets, plottedin terms of the ratio of absorbance at time t (At) to the absor-bance at the initial time (A0) against the irradiation time areshown in Fig. 9c. The rate constant (k) of the degradation wasdetermined to be 0.014 min�1, which can be contrasted with theonly other published values obtained under similar test condi-tions, namely 0.003 min�1 (ref. 40) and 0.012 min�1.41 Theobserved high efficiency for pure CeO2�x is attributed to twoprincipal factors. First, the holey and thin nanostructureprovided high accessibility of the charge carriers to the activesites owing to the short diffusion distances from the bulk to the

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surfaces. This positive characteristics resulted in reducedcharge carrier recombination times, as has been reportedpreviously for catalysts, such as holey nanosheets of Ru3Al42 andNi(OH),43 that showed enhanced hydrogen and oxygen evolu-tion reactions (HER and OER, respectively). Second, the XPSdata reveal the high areal densities of active sites through thehigh calculated ½V ��

O � values (Fig. S14†), which have been deter-mined to be the relevant active sites for reactions.44

Fig. 9d illustrates a range of published values for photo-degradation tests conducted for different pure and hybridCeO2�x morphologies of variable sizes (Table S1†). The superi-ority of the holey nanosheet morphology is demonstrated by theextent of degradation. Further, analysis of the collected data, inTable S1† reveals that different CeO2 morphologies with crys-tallite sizes #20 nm, exhibited BET specic surface areas in therange of 2–65 m2 g�1, and photodegradation extents in therange of�4–70%. These valuesmay be contrasted with those forthe holey nanosheet, which exhibited crystallite sizes in therange of 4–8 nm, specic surface area 81 m2 g�1, andoutstanding performance of 77% photodegradation. The latteris the best performance for CeO2�x yet reported. Comparison ofthe data for the present work in Table S1† highlights thedominance of the effect of the accessible active sites as revealedmost distinctly by the coupled specic surface area and porevolume; these are the predictors of performance.

The impact of the architecture, defect equilibria, and nano-structure on the catalytic and photocatalytic performances ofCeO2�x is summarised in Fig. 10, which plots the oxygenvacancy concentrations ð½V ��

O �Þ, specic surface areas, and porevolumes for the four morphologies. These data are compared tothe tabulation of the CO conversion rates, turnover frequencies,activation energy (Ea) required for CO conversion, and photo-degradations. These data show that the predominant factorcontrolling the performances is the specic surface area, whichreects the density of active sites. However, the inconsistenttrend for the pseudo-octahedra and spheres shows that thisparameter is mitigated by the effects of the oxygen vacancy

Fig. 10 Effects of structural and physical properties of the morphol-ogies on their catalytic and photocatalytic performances.

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concentration and the pore volume. Finally, there is no directcorrelation between the morphologies and the oxygen vacancyconcentrations, which are concentrated at the crystallite andgrain boundaries.18 While such correlations have been observedfor single-crystal CeO2�x before,27 the disagreement with thepresent work highlights the effect of the polycrystalline natureof these architectures.

Conclusions

This Ce-CP processing approach represents a simple, cost-effective, template-free, and low-temperature method (#25 �C)for the fabrication of CeO2�x with unprecedented architectures.This approach involves oxidation of Ce-CP, which allows rapiddisassembly/reassembly in the polar solvent ethanol and soyields well-dened holey 2D and hollow 3D CeO2�x nano-structures with high functionalities.

Further, this work reports the fabrication of holey 2D CeO2�x

with precisely controlled thicknesses by manipulation of thekinetics of nucleation/growth of the Ce-CP. The outstandingphotocatalytic performances of the holey 2D CeO2�x nano-structures derive from the short charge carrier diffusiondistances and low recombination density that result from thethin, holey, and polycrystalline nanosheets, which contain highconcentrations of active sites. This work has the potential toform the basis for a new route for the architectural tuning ofinorganic nanostructures of many metals.

ExperimentalReagents and materials

Ce(NO3)6$6H2O (99.0%), trichloroacetic acid (Cl3CCOOH)($99.0%), NaCl ($98.0%), and NaOH ($98.0%) were purchasedfrom Sigma Aldrich. For electrochemical setup, platinum wire(Basi Inc., Evansville, IN, USA, L¼ 23 cm, D ¼ 0.5 mm), and 3 MAg/AgCl (Basi Inc., Evansville, IN, USA) were used as the counterand reference electrodes, respectively. Further, uorine-dopedtin oxide on glass (FTO; Wuhan Geao Scientic EducationInstrument, Wuhan, China; 3.0 cm � 1.5 cm; lm resistivity �16 U sq�1) was used as the substrates. Ethyl alcohol (ethanol)with 96.0–97.2% purity and methylene blue dye (M9140) withdye content of $82 wt% were purchased from Sigma-Aldrich.

Characterisation

Electron microscopy (EM). Dry powder of the specimens wassuspended in water and drop-cast onto a Cu grid with a laceycarbon support, followed by air-drying at room temperature.The prepared samples were used for transmission electronmicroscopy (TEM), scanning electron microscopy (SEM), scan-ning transmission electron microscopy (STEM), high angleannular dark-eld (HAADF), energy dispersive spectroscopy(EDS), and electron energy loss spectroscopy (EELS) analyses.High-resolution transmission TEM (HRTEM) images and EDSanalyses of the nanostructures were obtained using a PhilipsCM 200 TEM (Eindhoven, the Netherlands); the HAADF imagesand EELS analyses were done using a JEOL JEM-ARM200F TEM

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(Tokyo, Japan). Both instruments were operated at an acceler-ating voltage of 200 kV. SEM images were obtained using a FEINova NanoSEM (Hillsboro, OR, USA) in rst and secondaryelectron emission mode, with accelerating voltage 5–10 kV.

X-ray photoelectron spectroscopy (XPS). Surface analysis ofthe samples was carried out using a Thermo Fisher ScienticESCALAB 250Xi spectrometer (Loughborough, Leicestershire,UK) equipped with a monochromatic Al Ka source (1486.6 eV)hemispherical analyser. The chamber pressure during theanalysis was kept constant at <8–10mbar. The acquired bindingenergies were referenced to the C 1s signal corrected to 285 eVand the spectra were tted using a convolution of Lorentzianand Gaussian proles.

X-ray diffraction (XRD). Mineralogical data for the nano-structures were obtained using a Philips X'Pert Multipurpose X-ray diffractometer (Almelo, Netherlands) with CuKa radiation,40 kV, 20 mA, scan range 20–60� 2q, step size 0.02� 2q, and scanspeed 5.5� 2qmin�1. The peaks were analysed using X'Pert HighScore Plus soware (Malvern, UK).

Laser Raman microspectroscopy (Raman). Raman data werecollected using a Renishaw inVia confocal Raman microscope(Gloucestershire, UK) equipped with a helium-neon green laser(514 nm) and diffraction grating of 1800 grooves per mm. AllRaman data were recorded for the range 100–3200 cm�1 (withresolution 1 cm�1), laser power 35 mW, and spot size � 1.5 mm.The data analyses were performed using Renishaw WiRE 4.4soware and the spectra were calibrated against the siliconpeak at �520 cm�1.

Thermogravimetric analysis (TGA). The decomposition ofthe Ce-CPs was assessed using thermogravimetric analysis(TGA; TA Instruments, Q5000, 20–1000 �C, 10 �C min�1 heatingrate) under both nitrogen and air.

Fourier transform infrared spectroscopy (FTIR). Attenuatedtotal reectance FTIR (ATR-FTIR; Spotlight 400 FTIR, Perki-nElmer (Waltham, MA, USA)) in the wavelength range 400–4000 cm�1 was used to determine the chemical species presentin the Ce-CP.

Density functional calculations. Density functional calcula-tions were performed using augmented plane wave pseudopo-tentials45 with the Perdew–Burke–Ernzerhof functional46 asimplemented in the VASP code.47 For the electronic settings,a neMonkhorst–Pack k-point grid with spacing of 0.05 A�1 andcut-off energy of 520 eV were used. In order to determine theground-state conguration, quenching ab initio moleculardynamics simulations were run using a micro-canonicalensemble with a target temperature of 20 K, with steps of 0.1fs for 10 ps. Full geometry optimisation then was carried out onthe equilibrated structure, with convergence criteria for theenergy and forces of 10�6 eV and 10�2 eV A�1, respectively. Thenal geometry optimisation run was conducted with van derWaals correction (vdw-DFT) using the approach formulated byKlimes et al.48 Further geometry optimisation was once againperformed with the spin–orbit interaction, which was found tohave negligible effect on the structure.

Atomic force microscopy (AFM). The thickness of nano-sheets was measured by atomic force microscopy (AFM; BrukerDimension Icon SPM, PeakForce Tapping mode). A ScanAsyst-

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Air probe (Bruker AFM probes) was installed in the AFMholder and used for all measurements. The samples were touch-printed49 on either glass or silicon substrate. The pixel resolu-tion was 512 samples per line. A slow scan rate of 0.195 Hz wasused to ensure accuracy. The peak force was minimised in orderto avoid sample deformation and the feedback gain settingswere optimised accordingly. The thicknesses of the thin lmswere determined using height prole with line scanning.

Methods

Synthesis of Ce-CP precursors. The synthesis was carried outby modied anodic chronopotentiometric deposition (MACE)using an electrochemical station (Ezstat Pro, Crown Point, IN,USA, with a resolution of 300 mV and 3 nA in the �100 mA range)in an undivided three-electrode conguration system. The systemincluded FTO, 3 M Ag/AgCl, and a platinum wire as working,reference, and counter electrodes, respectively. The electrolytewas prepared by mixing TCA and Ce(NO3)3$6H2O, both atconcentrations of 0.05 M in deionized (DI) water. While the pH ofthe as-prepared aqueous solution was measured to be�3, the pHwas increased using 1 M NaOH solution to slightly less than 6while magnetic stirring at 500 rpm. Prior to electrodeposition,each FTO substrate was cleaned stepwise by ultrasonication inethanol and acetone each for 5 min, followed by activation byimmersion (1 cm) in 40% nitric acid for 2 min and drying withcompressed nitrogen. The anodic electrodeposition was carriedout at room temperature over 50min by applying the high voltageof 1.2 V vs. Ag/AgCl; critically, this is in the water oxidation region.Consequently, the electrodeposition involved oxygen bubbling atthe FTO working electrode and hydrogen bubbling at the Ptcounter-electrode. The depositions were rinsed by gentle sprayingwith DI water and dried at room temperature in air.

Synthesis of Ce-CP and derived CeO2�x morphologies. Inorder to synthesise the 3D CeO2�x morphologies, differentconcentrations of Ce-CP precursors in the range of 4 M to 120 M(the full range of concentrations, temperatures, and resultantmorphologies are given in Table S2 in the ESI†) were added topure ethyl alcohol as solvent, followed by stirring at roomtemperature for 5min. The resultant yellow solutions, which areindicative of the Ce4+ ion,50 were evaporated at different rates byadjusting the temperature in the range of 0 �C to 25 �C, whichresulted in recrystallisation of the Ce-CP in various morphol-ogies. Temperatures less than 25 �C (room temperature) wereachieved with the use of a freezer with an inserted temperatureprobe.

The transformation of Ce-CP into CeO2�x was affected byimmersing the Ce-CP morphologies in 6 M NaOH aqueoussolution and oxidising for 30 min followed by rinsing byspraying with DI water and complete drying by heating in anoven at 200 �C.

The synthesis of the 2D CeO2�xmorphologies was done in anidentical manner with the following exceptions. The evapora-tion temperatures were in the range of �10 �C to 0 �C; thecorresponding vapour pressures are given in ESI, Table S2.† Forthickness measurements as a function of drying time, the Ce-CPnanosheets were deposited on glass substrates using the touch-

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print technique.39 Nanosheets of varying thicknesses were ob-tained by controlling the evaporation time for 12 h to 72 h; theresultant data are given in ESI Fig. S10.† Further, nanosheetswere obtained at constant temperature of �10 �C but differentCe-CP concentrations, the AFM results of which are shown inESI Fig. S11.†

Air purication. CO oxidation catalytic activity was evaluatedusing a xed-bed quartz tubular microreactor (ID ¼ 6.0 mm) atatmospheric pressure. 50 mg of catalyst were pre-treated in pureoxygen (30 mL min�1) at 300 �C for 2 h and then in purenitrogen (30 mL min�1) during cooling to room temperature.Then, the reactant gas mixture, which consisted of 5 vol% CO +30 vol% O2 + 65 vol% N2, was introduced to the reactor at a owrate of 40 mL min�1, giving a gas hourly space velocity of�48 000 mL$(g h)�1. The concentrations of the reactants andproducts in the reactor effluent were monitored on-line by a gaschromatograph (GS; Young Lin 6500, Gyeonggi-do, Republic ofKorea) equipped with a thermal conductivity detector (TCD) anda Carboxen-1010 PLOT column. CO oxidation was determinedquantitatively using 100(cinlet � coutlet)/cinlet, where cinlet andcoutlet are the CO concentrations in the inlet and outlet feedstreams, respectively.

Photocatalytic activity. The photocatalytic activity wasstudied by analysis of photodegradation of MB solutions (50 mLof MB [1 � 10�6 M]) containing 30 mg of CeO2�x photocatalyst.Each suspension was magnetically stirred for 15–20 min ina light-free environment prior to irradiation in order to maxi-mise MB adsorption, followed by illumination using a solarsimulator equipped with 300 W m�2 xenon lamp at 100 mWcm�2 irradiance power (1 sun AM 1.5 light) for 0–160 min at40 min intervals. Centrifugation at 5000 rpm for 10 min wasdone in order to separate the CeO2�x photocatalyst. Thedegradation of the MB solutions was assessed by ultraviolet-visible absorbance spectrophotometry (UV-vis, PerkinElmerLambda 35 UV-visible spectrometer, aperture 20 mm � 10mm), with quantication being based on the absorptiondetermined by the peak intensity at 664 nm.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work has been supported by the Australian ResearchCouncil (DP170104130). The authors are grateful for access tothe characterisation facilities provided by the Mark WainwrightAnalytical Centre, UNSW Sydney. S. S. M. is pleased toacknowledge UPA and RTP scholarship support from UNSWSydney. E. A. acknowledges the nancial support (JSPSKAKENHI Grant Number: 18F18064) provided by the JapanSociety for the Promotion of Science.

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