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二维材料最新大作

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Received8Aug2013|Accepted7Apr2014|Published12May2014

DOI:10.1038/ncomms4813 Generalized self-assembly of scalable

two-dimensional transition metal oxide nanosheets

Ziqi Sun1,*,Ting Liao1,2,*,Yuhai Dou1,Soo Min Hwang1,Min-Sik Park3,Lei Jiang4,Jung Ho Kim1&Shi Xue Dou1

Two-dimensional(2D)transition metal oxide systems present exotic electronic properties

and high speci?c surface areas,and also demonstrate promising applications ranging from

electronics to energy storage.Yet,in contrast to other types of nanostructures,the question

as to whether we could assemble2D nanomaterials with an atomic thickness from molecules

in a general way,which may give them some interesting properties such as those of gra-

phene,still remains unresolved.Herein,we report a generalized and fundamental approach to

molecular self-assembly synthesis of ultrathin2D nanosheets of transition metal oxides by

rationally employing lamellar reverse micelles.It is worth emphasizing that the synthesized

crystallized ultrathin transition metal oxide nanosheets possess con?ned thickness,high

speci?c surface area and chemically reactive facets,so that they could have promising

applications in nanostructured electronics,photonics,sensors,and energy conversion and

storage devices.

1Institute for Superconducting and Electronic Materials,Australian Institute for Innovative Materials,University of Wollongong,Innovation Campus, North Wollongong,New South Wales2500,Australia.2Australian Institute for Bioengineering and Nanotechnology,University of Queensland,St Lucia, Queensland4072,Australia.3Advanced Batteries Research Center,Korea Electronics T echnology Institute,68Yatap-dong,Bundang-gu,Seongnam463-816, Republic of Korea.4Beijing National Laboratory for Molecular Sciences,Key Laboratory for Organic Solids,Institute of Chemistry,Chinese Academy of Sciences,Beijing100190,China.*These authors contributed equally to this work.Correspondence and requests for materials should be addressed to J.H.K. (email:jhk@https://www.wendangku.net/doc/fc16917956.html,.au).

U ltrathin two-dimensional(2D)nanostructures are highly desirable for obtaining superior catalytic,photovoltaic

and electrochemical performances,due to their large surface-to-volume ratio and con?ned thickness on the atomic scale1–8.For example,the2D carbon network—graphene—features extremely high carrier mobility,mechanical?exibility, optical transparency and chemical stability,which provide a great opportunity for developing new electronic materials,novel sensors and metrology facilities,and superior energy conversion and storage devices4–6.Although transition metal oxides have been widely studied as functional materials so far,they are mainly obtained in the form of0D(nanodots and?ne nanoparticles),1D (nanowires and nanotubes)and3D mesoporous structures and nanoclusters9–12.In contrast,2D nanomaterials,especially those with con?ned thickness,have remained conspicuously absent until the recent emergence of delamination of layered compounds into single layers,referred to as exfoliation or top–down synthesis3,7–8,13–18.Only a few2D metal oxides that have suitably layered host crystals,unfortunately,in which2D platelets are weakly stacked to form3D bulk materials,can be obtained via the exfoliation method3,15–18.Moreover,the preparation of homogeneous2D nanosheets on a large scale by

exfoliation is also a big challenge.Thus,a generalized bottom-up strategy for controlled synthesis of metal oxides in2D nanostructures from precise molecular self-assembly is extremely desirable to meet the growing demand for such2D metal oxides in various applications related to sensing,catalysis, energy storage and electronic devices.Moreover,the bottom-up wet chemistry synthesis of2D metal oxide nanosheets can meet the demand for homogeneous production on a large scale for practical applications.Thus,a novel synthesis strategy that has ?exible capabilities for2D nanostructure fabrication would be a great breakthrough in nanotechnology,allowing us to take advantage of the high speci?c surface area,con?ned atomic level thickness and salient surface chemical states,as well as possible unique mechanical,thermal,electronic and optical properties of the2D nanostructured metal oxides.

Herein,we provided a generalized synthesis strategy for surfactant self-assembly or bottom-up growth of various ultrathin 2D transition metal oxide nanostructures from molecular precursors,in which amphiphilic block copolymers and short-chain alcohol co-surfactants are intelligently employed as structure-directing agents to con?ne the stacking and growth of the metal oxide along the chosen direction.Surfactant self-assembly has been successfully employed in the synthesis of0D, 1D and3D metal oxide nanostructures or mesoporous structures; however,the synthesis of2D metal oxides still lacks systematic studies19–23.Our concept for surfactant self-assembly of ultrathin 2D metal oxide nanosheets,as shown in Fig.1,is to form inverse lamellar micelles of polyethylene oxide–polypropylene oxide–polyethylene oxide(PEO20–PPO70–PEO20,Pluronic P123) surfactant together with ethylene glycol(EG)co-surfactant in ethanol solvent.The hydrated inorganic oligomers are then con?ned inside the inverse lamellar micelles,leading to the formation of layered inorganic oligomer agglomerates.In this step,the water content that is used to form the hydrated precursor oligomers is controlled in the phase area to form inverse lamellar micelles based on the phase behaviour of the surfactant–water–oil equilibrium system(Supplementary Fig.1)24,25.It has been reported that EG can work in the role of both co-surfactant and co-solvent in the surfactant–water system26.Here,the addition of EG is very crucial in the formation of ultrathin2D metal oxide nanosheets,as it can lead to a more stable lamellar phase compared with the binary surfactant–water system25,26.Hydrothermal or solvothermal treatment is then carried out to improve the organization and induce complete condensation and crystallization.Finally,well-crystallized ultrathin2D transition metal oxide nanosheets are collected after the removal of the surfactant https://www.wendangku.net/doc/fc16917956.html,pared with exfoliation synthesis,the biggest merits are that the self-assembly synthesis of metal oxide nanosheets is not restricted by the limited numbers of layered host materials,and synthesis of homogeneous nanomaterials in large quantities is possible. Surfactant self-assembly thus can provide a?exible and general way to synthesize2D metal oxide nanostructures.This synthesis procedure is successfully applied to the preparation of2D nanostructures of some typical transition metal oxides such as TiO2,ZnO,Co3O4,WO3,Fe3O4and MnO2.

Results

Synthesis and microscopy characterizations of2D nanosheets. In a typical synthesis protocol,Pluronic P123was dissolved in ethanol to form a transparent surfactant solution.To this solu-tion,metal alkoxides or inorganic metal salts in the desired molar ratio were added under vigorous stirring.The resulting sol solution was then mixed with EG.Solvothermal treatment was then carried out on the as-prepared or the well-aged solutions to form crystallized metal oxide2D nanosheets.Via this synthesis strategy,ultrathin2D nanostructures of the typical transition metal oxides,such as TiO2,ZnO,Co3O4,WO3,Fe3O4and MnO2, were successfully synthesized.Details of the synthesis procedure for the individual samples can be found in the Methods section and the Supplementary Information.

We performed scanning electron microscopy(SEM)and transmission electron microscopy(TEM)analyses on the synthesized ultrathin2D metal oxide nanostructures.The ultrathin2D nanosheets of TiO2,ZnO,Co3O4and WO3are shown in Fig.2.These nanosheets can be clearly distinguished in low-magni?cation SEM images(Fig.2a,d,g,j)and low-magni?ca-tion TEM images(Fig.2b,e,h,k).They exist in the form of groups of nanosheets.More detailed electron microscopy characteriza-tions of these ultrathin2D nanosheets can also be found in Supplementary Figs2–5.From these microscope images,it can be seen that the products are all nanosheets with their edges rolled up due to surface tension,which is similar to the general behaviour of graphene.The sizes of the nanosheets vary with the chemical composition:around200nm for TiO2and1–10m m for ZnO,Co3O4and WO3.In addition,the TiO2,ZnO,Co3O4

and Figure1|Schematic drawing of self-assembly of2D metal oxide nanosheets.The schematic shows the concept of molecular assembly of ultrathin2D metal oxide nanosheets from liquid solutions,where metal oxide precursor oligomers are strategically and collaboratively self-assembled into lamellar structures with polymer surfactant molecules, before they are condensed,polymerized and crystallized into2D metal oxide nanosheets with atomic thickness.

WO3nanosheets present well-crystallized anatase,wurtzite,cubic and monoclinic phase,respectively,as demonstrated by the high-resolution TEM(HRTEM)characterizations(insets in Fig.2c,f,i,l) and the corresponding X-ray diffraction(XRD)patterns (Supplementary Fig.6).The electron microscope characteriza-tions of the ultrathin2D Fe3O4and MnO2nanosheets are shown in Supplementary Figs7and8,where the features of the2D nanosheets are also clearly exhibited.Supplementary Fig.9 presents the corresponding XRD patterns,approving their characteristics of crystalline.The uniform morphologies of all the obtained ultrathin2D transition metal oxide nanosheets demonstrate the general and effective nature of the proposed synthesis method,and indicate its feasibility to be extended to the synthesis of other2D nanostructures.

The thickness of the synthesized ultrathin2D metal oxide nanosheets was determined based on the conventional used methods employing the TEM and atomic force microscopy (AFM)techniques(Supplementary Figs10and11)27,28.For TiO2 nanosheets,as shown in Supplementary Fig.10,it was determined that the monolayer thickness of the self-assembled TiO2nanosheets is around0.62nm by observing the edge con?guration.The ultrathin TiO2nanosheets synthesized by the present method have a thickness of around3.3nm,which implies that they are stacks consisting of4–5monolayers.The thickness of the ZnO,Co3O4and WO3ultrathin2D nanosheets were also determined by AFM(Supplementary Fig.11).The thicknesses of these metal oxide nanosheets varied between1.6and5.2nm, corresponding to2–7stacking layers of the monolayer (Supplementary Table1).In general,all the thicknesses of these 2D nanosheets are thin enough to present quantum con?nement effects(Supplementary Fig.12),demonstrating their potential applications in some novel electronic devices.

The roles of the added P123surfactant,EG and water in the formation of the ultrathin2D nanosheets were investigated by performing the synthesis without one chemical or by changing the amount to be added.Supplementary Fig.13presents the morphologies of the nanostructures of TiO2(Supplementary Fig.13a,c,e,g)and ZnO(Supplementary Fig.13b,d,f,h)that were

(

)

(

)

(

)

(22

(02

(010

)

Figure2|Electron microscope images of2D transition metal oxide nanosheets.SEM images(a,d,g,j),low-magni?cation TEM images(b,e,h,k)and high-magni?cation TEM images(c,f,i,l)of ultrathin2D nanosheets of TiO2(a–c),ZnO(d–f),Co3O4(g–i)and WO3(j–l).The insets in c,f,i and l are high-resolution TEM images of the crystal lattice structure of the nanosheets.Scale bar,200nm(a,d,g,j),50nm(b,e,h),20nm(k),5nm(c,f,i,l)and1nm(insets in c,f,i,l).

synthesized from the solutions:(I)without the addition of P123 (Supplementary Fig.13a,b),(II)without the addition of EG (Supplementary Fig.13c,d),(III)with the addition of300wt.% water(Supplementary Fig.13e,f)and(IV)with the addition of 700wt.%water(Supplementary Fig.13g,h).The morphologies of the nanostructures with modi?ed recipes showed huge deviation from that of the2D nanosheets.Without P123,even though the 2D morphology is still present in disguised form,the products are mainly in the form of large agglomerates,indicating that the P123 might be absorbed on the surface of the oligomers to resist the contact and growth of2D nanosheets in a thin dimension. Without EG,almost no nanosheets can be found,demonstrating the key role of EG in the formation of inverse micelles as co-surfactant and co-solvent.EG presents similar behaviour to that of water for non-ionic surfactants in the role of co-solvent.In addition,EG that behaves as co-surfactant facilitates the separation of lamellar phase,leading to the formation of more stable lamellar structures25,26.In the present study,therefore,the formation of ultrathin2D transition metal oxide nanosheets should be resulted from a synergetic effect of P123and EG.The synergetic effect of water-soluble short-chain alcohol and water has been well studied25,26.It is suggested that utilizing the synergetic effect of the alcohol and surfactant should be effective in constructing desirable aggregate architectures in water–alcohol–surfactant system via adjusting the ratios of alcohol/ surfactant,alcohol/water or alcohol/alcohol25.Obviously,the role of each chemical is very crucial in the synthesis of2D nanosheets. Even a small change in the chemical contents of the solution could result in a large deviation from the desirable area in the phase diagram.

Surface properties of2D nanosheets.Unlike the bulk crystals, the ultrathin metal oxide nanosheets have an extremely high percentage of surface(nearly100%).It has been reported that the exposed surfaces present different chemical states from the inner bulk atoms29,30.The surface chemical states of the ultrathin metal oxide nanosheets were examined by the X-ray photoelectron spectroscopy(XPS)technique(Fig.3a–d and Supplementary Table2).It is of interest that the core levels of the metal elements, Ti2p,Zn2p,Co2p and W4f,all show a0.5–2eV shift to lower binding energy compared with the corresponding stoichiometric bulk crystals29,30.The lower binding energy for surface atoms has been reported to be the result of electron gain(reduction)of metal atoms from surrounding oxygen atoms,for example,from the Ti4tto the Ti3tstate30.We suspect that the chemical state shifting of the transition metal atoms,or the electron transfer from O atoms to metal atoms,probably has resulted from the dramatic distortion of the crystal structure that occurs in the surface-dominated ultrathin metal oxide nanosheets28.To clarify this,density functional theory(DFT)calculations were employed to analyse the crystal structure distortion and the electronic density distributions in the free-standing ultrathin2D metal oxide nanosheets and the corresponding bulk inner atoms,respectively (Fig.3e–l).To give a quantitative estimation of the electron transfer between the metal and oxygen atoms,Lo¨wdin atomic population analysis was also performed for the2D nanosheets and bulk materials(Table1).Population analysis is the study of charge distribution and electron transfer within molecules.The intention is to accurately model partial charge magnitude and location within a molecule31–33.The DFT calculations reveal that the metal atoms in the ultrathin2D nanosheets have higher surrounded electron densities that resulted in reduction states compared with the corresponding bulk materials,accompanying with new bond formation,bond breaking or bond distortion between the metal–oxygen atoms of the crystal structures,which coincides well with the XPS chemical state shifts.Therefore, compared with the corresponding inner atomic planes of the bulk materials,the2D nanosheets present signi?cant lattice distortion and extra chemical bond formation,breaking and distortion, which accompany the redistribution of electron density and signi?cantly less electron transfer from metal atoms to O atoms, which,in turn,results in reduced chemical states of the transition metal atoms in the2D metal oxide nanosheets.These results demonstrate the unique chemical states of the surface atoms in the ultrathin2D metal oxide nanosheets,which will introduce

Table1|Electron charge distribution of the2D transition metal oxide nanosheets and the corresponding bulk materials as quanti?ed by Lo¨wdin atomic populations.

Materials Atoms Total

charge

(e)

T otal

charge

transfer

(e)

Partial charge(e)

s p d

Bulk TiO2Ti10.3881 1.6119 2.3988 5.9904 1.9988

O 6.7640à0.7640 1.8036 4.9604/

2D TiO2Ti10.4640 1.5360 2.3644 5.9844 2.1132

nanosheets O 6.7222à0.7222 1.8210 4.9012/

Bulk ZnO Zn10.5350 1.46500.59870.00009.9362

O7.2161à1.2161 1.9076 5.3086/

2D ZnO Zn10.5836 1.41640.61370.00009.9699

nanosheets O7.2425à1.2425 1.8729 5.3696/

Bulk Co3O4Co7.9562 1.04380.43480.00007.5214

O 6.6997à0.6997 1.8067 4.8930/

2D Co3O4Co7.9933 1.00670.45660.00007.5366

nanosheets O 6.7155à0.7155 1.8022 4.9134/

Bulk WO3W12.9011 1.0989 2.3614 6.7901 3.7497

O 6.3826à0.3826 1.6166 4.7659/

2D WO3W12.9404 1.0596 2.3689 6.7842 3.7872

nanosheets O 6.3130à0.3130 1.5950 4.7180/

Figure3|Surface chemical states of2D transition metal oxide nanosheets.(a–d)XPS spectra of the ultrathin2D metal oxide nanosheets:(a)TiO2, (b)ZnO,(c)Co3O4and(d)WO3,where the binding energies show0.5–2eV shifting to the low binding energy range compared with the corresponding bulk material surface states;(e,f)atomic structures and slices of the total charge density distribution in the(010)plane of bulk anatase TiO2(e)and the TiO22D monolayer nanosheets(f),showing the redistribution of electron densities and the extra bonding formed between Ti–Ti in the TiO22D nanosheets; (g,h)atomic structures and slices of the total charge density distribution in the(001)plane of bulk wurtzite ZnO(g)and the ZnO2D bilayer nanosheets (h),showing the redistribution of electron densities and the interlayer bond breaking between Zn–O–Zn in the ZnO2D nanosheets;(i,j)atomic structures and slices of the total charge density distribution in the(010)plane of bulk Co3O4(i)and the Co3O42D monolayer nanosheets(j),showing the redistribution of electron densities and the extra bonding formed between Co–Co in the Co3O42D nanosheet;(k,l)atomic structures and slices of the total charge density distribution in the(100)plane of bulk monoclinic WO3(k)and the WO32D bilayer nanosheets(l),showing the redistribution of electron densities and the crystal structure distortion in the WO32D nanosheets.The arrows in(e–l)indicate the structural differences between the ultrathin2D nanosheets and the corresponding bulk inner atoms:new bond formation between Ti–Ti(e,f),bond breaking between Zn–O(g,h),new bond formation between Co–Co(i,j)and bond distortion between W–O(k,l).

some anticipated physical and chemical properties that differ from those associated with the conventional morphologies.

It is interesting that some ultrathin 2D TiO 2nanosheets present unusual exposed https://www.wendangku.net/doc/fc16917956.html,ttice images of the anatase ultrathin TiO 22D nanosheets can be seen in Supplementary

Fig.2d.By combining the selected area electron diffraction patterns with the HRTEM images (Supplementary Fig.2c),it is easy to determine that the exposed surfaces of the anatase TiO 2nanosheets are high-energy {010}facets.It has been demonstrated that exposed high-energy surfaces are crucial for improving the

C o u n t s (a .u .)

466Ti 2p 1/2

Zn 2p 1/2

Zn 2p 3/2

Satellite peaks

5p 3/2

W 4+p 3/2W 6+

5p 3/2

0 4f 5/24+

4f 7/2

4+ 4f 5/2Co(OH)2

+8/3

Co +2

Ti 2p 3/2

Ti 2p

2p ref

1,0501,0451,0401,0351,0301,0251,0201,015

7924442

403836343230

790788786784782780778Co°

3/2Co 2p

W 4f

4f 5/2 ref

W 6+ 4f W 6+ 45/2

4f 7/2 ref

776

464

462460458456454

O Zn Zn

Zn Zn

W W Co Co

Ti Ti

Binding energy (eV)

Zn 22p ref 2p ref

Binding energy (eV)

C o u n t s (a .u .)

chemical activity of TiO 2(refs 34,35).Although the surface energy of anatase {010}(0.53J m à2)was calculated to be between those of {001}(0.90J m à2)and {101}(0.44J m à2),no {010}appears in the equilibrium shape of anatase crystals or nanostructures 35.The presence of the high percentage of exposed high-energy anatase {010}surfaces in the ultrathin 2D

TiO 2nanosheets would endow them with high chemical activity 35.For the exposed surfaces of ZnO,Co 3O 4and WO 3,however,no such high-energy exposed surfaces were observed.It is certain that the special shapes and electronic structures will induce some salient properties in the ultrathin 2D nanosheets.One of the most obvious features is the high speci?c surface area

PET support

Cu etchant

Graphene on PET support

Single-layer graphene

Flexible, transparent photodetector electrode I

(2D)/I (G) ~ 2.65I n t e n s i t y (a .u .)

P h o t o c u r r e n t d e n s i t y (n A c m –2)

1,200

1,600

2,000

2,400

–1.0–1,500

1,0001,0001,0001,0005005005005000000

50 n m

100150200

WO 3

Co 3O 4

ZnO

TiO 2

O N O N

O F F Time (s)

0–1,000–5000500TiO 2

ZnO WO 3Co 3O 4ZnO_dark

1,0001,500

–0.8–0.6–0.4–0.20.2–80

–0.8–0.40.4

0.8

0.0Voltage (V)

P h o t o c u r r e n t d e n s i t y (n A c m –2

)

–40408000.40.60.8 1.0

0.0Voltage (V)

2,800

Raman shift (cm –1

)

Spin coating 2D metal oxide nanosheets

Figure 4|Fabrication and performance of transparent and ?exible 2D nanosheet photodetector.(a )Schematic diagram of the fabrication of the photodetector electrode,consisting of one single-layer graphene sheet on PET substrate and a thin layer of spin-coated 2D metal oxide nanosheets;(b )Raman spectrum of the as-prepared graphene on PET ?lm,demonstrating the single-layer feature,where the 2D-band is much more intense and sharper compared with the 2D-band in multilayer graphene,and with an intensity ratio of 2D/G peaks of around 2.65(ref.41);(c )the fabricated 2D nanosheet layers on the graphene-PET photodetector electrode show its high transparency and (d )?exibility,with the inset containing an SEM cross-section image of the photoelectrode in d showing that the thickness of the deposited 2D metal oxide nanosheets on the single-layer graphene-PET substrate was around 50nm;(e )I –V characteristics of the transparent,?exible photodetector devices constructed from the 2D nanosheets of TiO 2,ZnO,Co 3O 4and WO 3,respectively,under 325nm ultraviolet light (67mWcm à2)irradiation,with the inset presenting the I –V characteristic of the dark photocurrent of the ZnO 2D nanosheet photoanode;and (f )the photoresponse behaviour of the transparent,?exible photodetectors under alternating on and off 325nm ultraviolet light (67mWcm à2),with an interval of 10s and a bias of 0.5V.

(Supplementary Fig.14).An enhancement of the speci?c surface area by3–10times compared with nanoparticles prepared via the conventional synthesis strategies is achieved.The ultrathin TiO2 nanosheets exhibit an ultrahigh speci?c surface area of 298m2gà1,which is almost sixfold higher than that of the commercial P25TiO2nanoparticles(B50m2gà1).Besides the high surface area of2D TiO2nanosheets,the speci?c surface area of the ZnO2D nanosheets is up to265m2gà1,the Co3O42D nanosheets of246m2gà1and the WO32D nanosheets of 157m2gà1.It is known that the higher the speci?c surface area is,the larger the amount of active reaction sites on the surface. The high speci?c surface areas of the2D metal oxide nanosheets would guarantee their high chemical reaction activity. Another feature is the shifting in the band structure aroused by the con?nement,which is due to the atomic scale thickness of the nanosheets.All the ultraviolet–visible(UV–vis)adsorption spectra of the TiO2,ZnO,Co3O4and WO3nanosheets (Supplementary Fig.12)show an obvious blue shift compared with the bulk materials,demonstrating their signi?cant quantum con?nement effect in the thinnest dimension.It is a general understanding that smaller size of transitional metal oxide nanoparticles raises the conduction band and lowers the valence band,and results in the observation of a blue shift in the absorption edge,or we called quantum size effect.On the basis of the study by Saton et al.36,however,the signi?cant quantum size effect can only be observed for the nanoparticles with diameters o2nm,in the case of TiO2.The commonly used transitional metal oxide nanoparticles usually have diameters much larger than2nm.For example,one of the typical TiO2nanoparticles, P25,has an average diameter of20nm.In the present work,the thicknesses of the ultrathin2D nanosheets were in the range of 1.5–5.2nm in the form of stacks and below1nm for monolayers, which are in the range for the size with signi?cant quantum size effect.Thus,these ultrathin2D nanosheets would have much more signi?cant quantum size effect than those of the normally used nanoparticles.Owing to the high surface area,chemically active exposed surface and con?ned thickness,it is expected that these unique ultrathin2D nanosheets would be attractive in a variety of applications,from nanostructured electronic or photonic devices to energy generation and storage systems.

Performance of transparent and?exible ultraviolet photo-detectors.To explore the applicability of2D nanostructures,we examined the performance of transparent,?exible ultraviolet (UV)photodetectors by depositing the ultrathin2D metal oxide nanosheets on a single-layer graphene back electrode(Fig.4). Visible-light-blind ultraviolet(UV-A band at320–400nm)pho-todetectors have attracted intensive interest for their wide appli-cations in healthcare,climate control,agriculture,astronomy and aeronautics37.The ultrathin2D metal oxide nanosheets prepared in the present work feature widened bandgaps,which allow the transmission of visible light but high responses to ultraviolet light, and are the most promising candidates for high-performance ultraviolet photodetectors.

Figure4shows the preparation of the single-layer graphene back electrode for the photodetector,deposition of ultrathin2D transition metal oxide nanosheets on the single-layer graphene and the performance of the transparent,?exible ultraviolet photodetector.As schematically illustrated in Fig.4a,the single-layer graphene on the188-m m thick polyethylene terephthalate (PET)substrate was prepared by transferring the chemical vapour-deposited graphene on Cu foil to PET via a roll-to-roll production method described elsewhere38–40.The Raman spectrum clearly proves that the obtained graphene is single layer(Fig.4b),based on the method to determine the number of layers of graphene from the2D-band,as the intensity ratio of the 2D-band to the G-band is B2.65(ref.41).Then,well-dispersed diluted ultrathin2D metal oxide nanosheets in ethanol suspensions were spin-coated onto the graphene substrate to form a transparent and?exible photoelectrode(Fig.4c,d).To ensure signi?cant signal intensities in the photoelectrode,the thickness of the deposited2D metal oxide nanosheet layer was kept at around50nm.After assembly with a graphene-PET counter electrode,the transparent,?exible ultraviolet photodetectors were tested under325nm ultraviolet light (67mW cmà2)or an alternating on and off ultraviolet source with an interval of10s.Figure4e presents the I–V characteristics of the transparent,?exible ultraviolet photodetectors.The photocurrent densities of the photodetectors made from the2D transition metal oxide nanosheets reach the order of mA cmà2. Except for the WO3photodetector,which presents an obvious p-type Schottky barrier contact,the others show ideal linear I–V responses under the ultraviolet irradiation and good ohmic behaviours.The linear I–V response,or the ohmic behaviour,is very desirable because it enhances the photosensitive properties of the devices.The I–V characteristics demonstrate that the TiO2, ZnO and Co3O42D ultrathin nanosheets are promising candidates due to their ohmic behaviour and the large output signal.Figure4f shows the photoresponses of the ultraviolet photodetectors under the alternating on and off ultraviolet light with an interval of10s and a bias of0.5V.All of the time responses of the devices are highly stable and reproducible.No degradation was found in tens of on–off switching cycles, con?rming the excellent stability and fast response speed of our device.It is interesting that the WO3photoelectrodes presented increasing photocurrents with prolonged irradiation time,due to its photochromic effect under ultraviolet irradiation42.These ?ndings show the promising potential of these ultrathin2D metal oxide nanostructures for practical applications such as photoelectric or photochemical devices.

Discussion

In this work,we demonstrate that the generalized bottom-up synthetic method for ultrathin2D transition metal oxide nanosheets is of great interest for making various functional nanostructures with high surface area,high chemical activity and a quantum con?nement effect.Such unique ultrathin2D nanostructures could be the fundamental basis for emerging applications in not only energy generation/storage systems,but also electronic devices and sensors.Moreover,this kind of nanostructure may also provide unique mechanical,thermal and anti-corrosive properties that would be useful in transparent protective coatings,barrier?lms and reinforcement materials.

Methods

Synthesis of ultrathin2D TiO2nanosheets.Titanium isopropoxide(TTIP, 1.05g)was added into0.74g concentrated HCl solution during vigorous stirring (bottle A);and0.2g polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO20–PPO70–PEO20,Pluronic P123)was dissolved in3.0g ethanol(EtOH) (bottle B).After stirring for15min,the solution in bottle B was added into bottle A and stirred for another30min.Then,2.5ml TTIP solution with20ml EG was transferred into a45-ml autoclave and heated at150°C for20h.The products of the hydrothermal reaction were washed with ethanol three times,and the white powders were collected after washing/centrifugation and drying at80°C for24h.

Synthesis of ultrathin2D ZnO nanosheets.Pluronic P123(0.2g)was added into 3g EtOH with0.45g H2O to form a clear solution with stirring for15min,and then0.1g ZnAc2á2H2O and0.045g hexamethylenetetramine(HMTA)were added into the ethanol solution.After stirring for around15min,a clear solution was obtained.Then,46ml EG was added into the ethanol solution.After stirring for 30min,a transparent solution was obtained.The obtained EtOHtEG precursor solution was statically aged for7days.After aging,the transparent precursor solution was divided into two parts,transferred into two45ml autoclaves and

heated at110°C for15h.The products of the solvothermal reaction were washed with distilled water and ethanol three times,and the light yellow powders were collected after washing/centrifugation and drying at80°C for24h.

Synthesis of ultrathin2D Co3O4nanosheets.Pluronic P123(0.2g)was added into13g EtOH with1g H2O to form a clear solution with stirring for15min,and then0.125g Co(Ac)2á4H2O and0.07g HMTA were added into the ethanol solution.After stirring for around15min,a purple solution was obtained.Then, 13ml EG was added into the ethanol solution.After stirring for30min,a trans-parent solution was obtained.The obtained EtOHtEG precursor solution was statically aged for1day.After aging,the transparent precursor solution was transferred into a45ml autoclave and heated at170°C for2h.The products of the solvothermal reaction were washed with distilled water and ethanol three times, and the brown powders were collected after washing/centrifugation and dehy-dration at150–400°C for2h.

Synthesis of ultrathin2D WO3nanosheets.Pluronic P123(0.2g)was added into13g EtOH with0.45g H2O to form a clear solution with stirring for15min, and then0.4g WCl6was added into the ethanol solution.After stirring for around 15min,a yellow solution was obtained.Then,the precursor solution was trans-ferred into a45ml autoclave and heated at110°C for2h.The products of the solvothermal reaction were washed with ethanol three times,and the white-blue powders were collected after washing/centrifugation and drying at80°C for24h.

Characterization.The composition and phase of the as-prepared products were evaluated using a powder X-ray diffractometer(XRD,MMA,GBC Scienti?c Equipment LLC,Hampshire,IL,USA)with Cu K a radiation.The chemical states of the nanostructures were determined by XPS(PHOIBOS100Analyser from SPECS;Al K a X-rays,Berlin,Germany).The morphology of the samples was observed with a SEM(JSM-7500FA,JEOL,Tokyo,Japan).HRTEM observation was carried out using a JEM-2011F(JEOL)operated at200kV.The thickness of the 2D ultrathin nanosheets was determined by AFM(MPF-3D,Asylum Research, Santa Barbara,USA).For AFM observation,a nanosheet specimen was prepared by quickly dipping single-crystal silicon substrate with a100-nm-thick amorphous SiO2top layer into diluted acetone-dispersed2D metal oxide nanosheet solution. Surface area was measured by a surface area analyzer(NOVA1000,Quantachrome Co.,FL,USA)at77K.Before the measurement,the sample was heated at120°C for5h.

Photocurrent performance tests.Single-layer graphene on188-m m thick PET substrate was prepared by transferring chemical vapour-deposited graphene on Cu foil to PET based a roll-to-roll production method described elsewhere38–40. Brie?y,coarse-grain copper foil was heated to1,000°C,and then a?owing gas mixture of CH4and H2was introduced at460mtorr and8sccm for30min, followed by rapid cooling to room temperature under?owing H2.The graphene grown on copper foil was attached to a188-m m thick PET supporting?lm and passed through two heated rollers for soft pressing.After removing the copper substrate by electrochemical reaction with aqueous0.1M ammonium persulphate solution(NH4)2S2O8,the single-layer graphene was transferred onto the PET supporting?lm.Transparent,?exible photoelectrode was prepared by spin-coating techniques.A well-dispersed,0.1mg mlà12D transition metal oxide nanosheet ethanol solution was dropped onto sliced,single-layer,graphene-PET substrate and spin-coated at2,000r.p.m.for30s.To reach the target thickness,2–5repetitions of the coating process were needed.The coated photoelectrodes were then aged in covered petri dishes at room temperature for24h.After aging,the photoelectrodes were dried at80°C for12h.A photosensitive detector was fabricated by attaching the photoelectrode made from2D metal oxide nanosheets with another uncoated ?exible single-layer graphene-PET counter electrode.The photocurrent was examined by a Keithley2400source meter under a325-nm ultraviolet(UV)light source(67mW cmà2)or an alternating on and off ultraviolet source

(67mW cmà2)with an interval of10s.

DFT calculations.The?rst-principle calculations were carried out within the framework of spin-polarized plane-wave DFT,implemented in the Quantum-Espresso package43.The Perdew–Burke–Ernzerhof variant of the generalized gradient approximation was used for the exchange-correlation potential44.The valence-ion interaction was described by ultrasoft pseudopotentials45.A plane-wave cutoff energy of40Ry was adopted for the plane-wave expansion of the wave function to converge the relevant quantities.For Brillouin-zone integrations,a Monkhorst-Pack mesh46of5?5?1k-points was used for structural optimization, while a denser mesh of15?15?1was used for electronic structure analysis.In our calculations,TiO2,ZnO,Co3O4and WO3nanosheets have been described using supercells,respectively,by monolayer TiO2sheets parallel to(010),bilayer ZnO sheets parallel to(001),monolayer Co3O4sheets parallel to(001)and bilayer WO3sheets parallel to(100).A large vacuum spacing of15?between2D nanosheets was selected to prevent spurious interactions between the periodic layers.All atomic positions and lattice constants were optimized by using the Broyden–Fletcher–Goldfarb–Shanno algorithm47.The convergence for the residual forces is o10à3Ry per Bohr(2.6?10à2eV?à1)on ionic relaxation. References

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Acknowledgements

This work was supported by Australian Research Council Discovery Project DP1096546. Z.S.was supported by an APD Research Fellowship and a UOW VC Research Fellow-ship.T.L.was supported by a UQ Postdoctoral Research Fellowship and a UOW VC Research Fellowship.We are grateful to Dr Yi Du for his help on AFM characterization. T.L.appreciates the generous grants of CPU time from both the University of Queensland and the Australian National Computational Infrastructure Facility. Author contributions

Z.S.,J.H.K.,S.X.D.and L.J.designed the experiments,T.L.performed the DFT theoretical calculations,Z.S.carried out the sample synthesis,microscopy characterizations and measurements,Y.D.helped to synthesize Co3O4,Fe3O4and MnO2nanosheets,M.-S.P. carried out the XPS measurements and analysis.S.M.H.prepared and characterized the large-size single-layer graphene on PET?lms.Z.S.and J.H.K.analysed the data and wrote the paper.

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How to cite this article:Sun,Z.et al.Generalized self-assembly of scalable

two-dimensional transition metal oxide https://www.wendangku.net/doc/fc16917956.html,mun.5:3813

doi:10.1038/ncomms4813(2014).