【Nano2014.3.14】Oriented Assembled TiO 2 Hierarchical Nanowire Arrays with Fast
Oriented Assembled TiO2Hierarchical Nanowire Arrays with Fast Electron Transport Properties
Xia Sheng,?Dongqing He,?Jie Yang,?Kai Zhu,*,?and Xinjian Feng*,?
?Suzhou Institute of Nano-Tech and Nano-Bionics,Chinese Academy of Sciences,Suzhou215123,Jiangsu,China
?National Renewable Energy Laboratory,Golden,Colorado80401,United States
*Supporting Information
(compared with1D nanowire arrays)but also exhibit fast charge
nanoarrays),leading to52%improvement in solar conversion
can be extended to assemble other metal oxides with one or
toward high-performance optoelectronics.
charge transport,solar cells
with rapid charge transport is essential for e?ective charge collection in solar cells,1,2arti?cial photosynthesis,3photo-catalysis,4and energy storage devices.5While the large surface area of electrode materials is normally associated with the high porosity and small size of building blocks2(e.g.,nanoparticles), the rapid charge transport property usually relies on straight conducting pathways.6As an example for achieving the latter goal,one-dimensional(1D)single-crystal rutile TiO2nanowire (NW)arrays have been reported.Also,1D rutile TiO2NW electrodes have been shown to markedly improve the electron transport property relative to nanoparticle(NP)counterparts.6d However,1D NW arrays usually have relatively low surface area because of the large volume of free space between the NWs. Substantial research e?orts have been recently focused on building one-dimensional(1D)nanoblocks with fast charge transport into three-dimensional(3D)hierarchical architec-tures.7?10Various techniques including pulsed laser deposi-tion,8vapor deposition,9and hydrothermal and solution methods10have been used to prepare3D hierarchical TiO2 NW arrays.For example,Lee10d and Wu et al10g,respectively, have reported a seed-induced multisteps and a one-step hydrothermal approaches for the fabrication of branched NWs.These assembly strategies have demonstrated enhanced light harvesting by enlarging the electrode surface area. However,they could in turn lead to the formation of grain boundaries and lattice defects between the branches and trunks that would limit charge transport within these hierarchical structures.Currently there is little experimental evidence of fast electron transport in these3D nanostructure-based solar cells. Herein,we describe the fabrication and fast charge transport properties of oriented assembled TiO2hierarchical nanoarrays consisting of1D branches epitaxially grown from the primary trunk.We?nd that charge transport in these single-crystal-like branched nanowire(B-NW)arrays is about200times faster than that in the nanoparticle(NP)?lms.We further demonstrate that the3D B-NW arrays have a larger surface area(71%higher)relative to the1D NW arrays,leading to52% improvement in solar conversion e?ciency without a?ecting the electron collection.In light of these results,we conclude that photoelectrodes composed of single-crystal-like3D hierarchical NW arrays are attractive for arti?cial photosyn-thesis,solar cells,and other optoelectronic applications. Figure1a,b show?eld emission scanning electron micros-copy(FE-SEM)top views of the as-assembled3D B-NW arrays on?uorine-doped tin oxide(FTO)coated substrates.The needlelike nanobranches grow in four symmetrical directions with a length of about70nm(Figure1b).The length of the nanobranch can be controlled between20and100nm by adjusting the growth time(Figure S1,Supporting Information).
Received:December13,2013
Revised:February18,2014
Published:March14,2014
The crystal phase of the primary 1D NW and 3D B-NW arrays were both identi ?ed as tetragonal rutile according to their X-ray di ?raction (XRD)patterns (Supporting Information Figure S2).Figure 1c shows that the well-separated 3D B-NW arrays grow vertically from the FTO substrate with an average thickness of about 3μm.The nanobranches grow uniformly and with a clear orientation along speci ?c side crystal facets of the trunk (Figure 1d).
Figure 2a shows a typical transmission electron microscopy (TEM)image of a single 3D branched nanowire.The central nanowire core has uniform,needlelike side branches.The branches grow either upward (Region 1)or downward (Region 2)and are both oriented at a speci ?c angle away from the trunk (Figure 2a,b (schematic illustration)).As shown in Figure 2d,e,the (110)lattice crystal plane with a fringe spacing of 0.325nm in the trunk and branch are both well-developed indicating that they are both single crystal and grow along the crystallographic c -axis.The crystallographic relationship between the trunk and branch was further analyzed by TEM and high-resolution transmission electron microscopy (HR-TEM)along the [010]zone axis (Figure 2c,f).Figure 2c is a typical high-magni ?cation TEM image of a trunk with one upward branch.The angle between the branch and the trunk is calculated to be about 65°,which is consistent with the angle between the crystallographic c -axis of the trunk and branch.The HR-TEM image taken from region f in Figure 2c is highlighted in Figure 2f.The (101)lattice plane of the trunk and the (101)lattice plane of the branch both have a fringe spacing of 0.249nm perfectly linked and form a {101}twinned structure.On the basis of the above TEM and HR-TEM analysis,the twinned structural relation-ship between the trunk and branch is illustrated schematically by the atomic model (Figure 2g,h).Thus,the nanobranches can be considered as epitaxial growth of the trunk along the (100)and (010)crystalline planes at four symmetrical directions with a typical angle of 65°between the c -axis of the trunk and
branch.Such orientated assembled 3D branched nanowire arrays with a twin-structured coherent interface should have few grain boundaries and lattice defects.Consequently,they should facilitate charge transport along the 3D TiO 2hierarchical nanowire arrays.
Figure 3shows the dependence of the electron di ?usion coe ?cients (D )on the photoelectron density (n )for 1D NW,3D B-NW,and NP rutile TiO 2?lms based dye sensitized solar cells (DSSCs).The values of D and n are determined from the transport time constants (τc )and ?lm thickness (d )using procedures described elsewhere.11The di ?usion coe ?cient in the 3D B-NW arrays is about 2orders of magnitude higher than that of randomly packed NP ?lms at the same photoelectron density (e.g.,1×1017cm ?3).12The fast charge transport property of 3D B-NW arrays can be attributed to the elimination of grain boundaries and lattice defects between the 1D NW trunks and the nanobranches.A closer examination of Figure 3indicates that the D value of the 3D B-NW arrays is about a factor of 3lower than that of the 1D single crystal NW arrays.Further increase in branch length results in a slower electron transport in the B-NW ?lm (Supporting Information Figure S3).This could be due to the higher density of
surface
Figure 1.FE-SEM images of the as-synthesized TiO 23D B-NW array on a transparent FTO-coated glass substrate.(a,b)Top views at low and high magni ?cations,respectively;(c,d)cross-sectional views with 45°titling angle at low and high magni ?cations,
respectively.
Figure 2.TEM and HR-TEM images of the as-synthesized 3D B-NW.(a,b)TEM image and schematic illustration,respectively,of a single 3D B-NW;(c)TEM image of a part of the 3D B-NW structure showing the trunk and one upward branch;(d,e)the HR-TEM images of a 1D single crystal trunk and a branch,respectively;(f)HR-TEM of the region f in panel (c)showing the interface between the trunk and the branch;(g)schematic illustration of a pro ?le atomic model along [010]zone axes of the trunk corresponding to the panel (f);(h)a 3D atomic model along the [001]zone axes of the trunk.The gray and red balls denote Ti and O,respectively.
states 13associated with the larger surface area of the 3D B-NW ?lm.
The dependence of photoelectron density on voltage is usually used to measure the distribution of sub-bandgap trap states of the photoelectrodes.Supporting Information Figure S4shows that for a given voltage,the photoelectron density of the 3D B-NW-based cell is about 40%higher than that of the 1D NW-based cells.This suggests that the total density of trap states (N tot )in the 3D B-NW sample is about 40%higher than that in the 1D NW one.We have shown previously that D is related to N tot by the expression D ∝(N tot )?1/α,where αis related to the shape of the distribution of the sub-bandgap trap states.13a Best ?ts to the data show that α=0.43for both the 3D B-NW and 1D NW ?lms.On the basis of this αvalue,we estimate that the 40%larger N tot should lead to about 2.2-fold slower transport for the 3D B-NW ?lm than the 1D NW arrays.This could account for the observed electron transport properties of the 3D B-NW and 1D single crystal NW arrays shown in Figure 3.
Figure 4shows the recombination lifetimes (τr )for DSSCs based on the 3D B-NW and 1D NW arrays as a function of the photoelectron density.Recombination is an interfacial charge transfer process that occurs mainly at the electrode/electrolyte interface 14,15and is thus in ?uenced by the number of surface states.16At a given photoelectron density,the lifetime of 3D B-NW array-based cells is about a factor of 2longer than that of 1D NW array-based cells because of its larger number of surface states as discussed above.The charge collection property of the DSSCs can be measured by the electron di ?usion length (L n )given by L n =(D τr )1/2.A longer L n usually leads to a higher charge-collection e ?ciency.L n is usually several times (e.g.,more than three times)greater than the ?lm thickness in order to obtain optimum cell performances.15Analyses of the data shown in Figures 3and 4yield an comparable electron di ?usion length for 3D B-NW (L n =30μm)and 1D NW (L n =38μm)based cells.Therefore,constructing orientated assembled hierarchical 3D B-NW arrays (up to 10μm)can increase the surface area of the 1D NW arrays without negatively a ?ecting the electron collection e ?ciency.
The J ?V characteristics of DSSCs based on 3D B-NW and 1D NW of similar NW thickness (3μm)are shown in Figure 5.
The DSSC based on the 3D B-NW arrays exhibits an open-circuit voltage (V oc )of 0.71V,a ?ll factor (FF)of 0.64,and a short-circuit photocurrent density (J sc )of 10.14mA/cm 2to give a solar conversion e ?ciency of 4.61%.In contrast,the cell based on 1D NW arrays shows a V oc of 0.73V,a FF of 0.63,and a J sc of 6.57mA/cm 2to yield an e ?ciency of 3.02%.The J sc value of the 3D B-NW based cell is about 54%higher than that of the 1D NW based cell,which is attributed to the larger surface area of the 3D B-NW ?lm than the 1D NW ?lm.These results are in agreement with the quantum e ?ciency (QE)spectra (Supporting Information Figure S5).According to dye desorption measurements as shown in Supporting Information Figure S6,the surface area of 3D B-NW arrays is found to be about 71%larger than that of the 1D NW arrays.Consistent with their di ?erence in surface areas,the dark current of 3D B-NW array-based DSSC is slightly higher than that of 1D NW array-based cells (Figure 5).It is also worth noting that the FF and V oc of 3D B-NW is comparable to that of 1D NW based cells,an observation consistent with their similar transport and recombination properties.Overall,the 3D B-NW arrays
based
Figure https://www.sodocs.net/doc/a83282305.html,parison of electron di ?usion coe ?cients as a function of the photoelectron density for 1D NW,3D B-NW,and NP rutile TiO 2?lms based
DSSCs.
Figure https://www.sodocs.net/doc/a83282305.html,parison of recombination lifetimes as a function of the photoelectron density for 3D B-NW and 1D NW rutile TiO 2based
DSSCs.
Figure https://www.sodocs.net/doc/a83282305.html,parison of the J ?V characteristics of the 3D B-NW and 1D NW array-based DSSCs under simulated AM 1.5sunlight and in the dark.
cells demonstrate about52%higher solar conversion e?ciency than the1D NW-based devices and thus should be attractive for organic/inorganic hybrid,QDs and perovskite-based solar cells,and other applications.
In conclusion,we have reported oriented assembled single-crystal-like TiO2hierarchical nanowire arrays grown on transparent conductive substrates.The1D nanobranches are formed via epitaxial growth from the trunk with few grain boundaries and provide a direct photoexcited carrier collection pathway in the3D architecture.In comparison to the1D NW arrays,the as-synthesized3D B-NW arrays not only demonstrate higher surface area but also exhibit comparable fast charge transport properties,leading to signi?cantly improved photovoltaic performance.Such3D nanostructure-based electrodes have great potential in optoelectronic devices, particularly in solid state solar cells where recombination is very fast.The orientated assembly strategy reported here can be extended to assemble other metal oxides with one or multicomponents into3D hierarchical nanostructures,and thus represents a critical avenue toward high-performance optoelectronics.
Experimental Section.A two-step procedure was employed to synthesize3D B-NW arrays on FTO glass (TEC-8)substrate.In the?rst step,aligned1D single crystal TiO2NW arrays were grown on FTO substrate through a solvothermal method adapted from a previous study.6d In a typical process,FTO substrates coated with a thin TiO2layer were loaded into a sealed Te?on-lined stainless steel reactor ?lled with6mL of2-butanone,6mL of37%hydrochloric acid, and0.4mL of tetrabutyl titanate,and then kept at200°C for 60min.For the growth of3D B-NW arrays,the as-obtained1D NW arrays were loaded into a sealed Te?on-lined stainless steel reactor?lled with10mL of DI water,0.1?0.3mL37% hydrochloric acid and0.1?0.2mL TiCl3solution(20wt%of TiCl3in2M HCl),and then kept at80°C for1?2h.The obtained3D B-NW samples were exposed to O2plasma at50 W for10min with an oxygen?ow rate of0.6L/min and then annealed at450°C for30min in O2.For comparison purpose, aligned1D TiO2NW arrays on FTO substrate were treated using the same method,that is,exposed to O2plasma at50W for10min and then annealed at450°C for30min in O2. Oxygen plasma treatment was carried out by using a plasma cleaner(Ming Heng,PDC-MG).The morphologies and microstructures of the samples were characterization using a FE-SEM(S4800,Hitachi,Tokyo,Japan).Average?lm thickness was determined using a surface pro?ler(KLA Tencor Alpha-Step500).The TEM and HR-TEM images were taken using a Tecnai F20(FEI,Hillsboro,OR,USA)microscope at an accelerating voltage of200kV.The crystal phase structures of the samples were investigated using an X-ray powder di?ractometer(X′Pert PRO,PANalytical,Almelo,The Nether-lands).
For use in DSSCs,NWs array samples were coated with dye by immersion overnight at ambient temperature in a0.5mM ethanolic solution of commercial N719dye.The electrolyte was composed of0.8M1-hexyl-2,3-dimethylimidazolium iodide and50mM iodine in methoxypropionitrile.The Pt counter electrode was prepared by spreading a thin layer of5-mM H2PtCl6isopropanol solution on a FTO substrate,which was followed by drying in air and then heating at400°C for20min. The thickness of the electrolyte layer between the NW array and counter-electrode was?xed by the use of a25μm thick Surlyn as the spacer.Three samples for each electrode type were measured.The photocurrent density and photovoltage of the DSSCs were measured with active sample areas of0.25cm2 using AM-1.5simulated sunlight(Oriel Sol3A Class AAA Solar Simulator).Electron transport and recombination properties of DSSCs were measured by intensity modulated photocurrent
and photovoltage spectroscopies as described previously.16■ASSOCIATED CONTENT
*Supporting Information
The time-dependent experimental growth process of nano-branches on the trunk,XRD patterns of3D B-NW and1D NW arrays,comparison of photoelectron density as a function of the voltage for1D NW and3D B-NW array-based cells, comparison of quantum e?ciency(IPCE)spectra for1D NW and3D B-NW array-based cells,and UV?vis spectra of dye solution desorbed from1D and3D nanowire arrays.This material is available free of charge via the Internet at http://
https://www.sodocs.net/doc/a83282305.html,.
■AUTHOR INFORMATION
Corresponding Authors
*E-mail:(X.F.)xjfeng2011@https://www.sodocs.net/doc/a83282305.html,.
*E-mail:(K.Z.)Kai.Zhu@https://www.sodocs.net/doc/a83282305.html,.
Author Contributions
X.S.is responsible for the experiments and preparation of the paper;D.H.performed the TEM and HRTEM analysis;J.Y. performed the SEM analysis;K.Z.performed the IMPS,IMVS, and IV measurements;X.F.provided guidance for the experiments and for editing the paper.
Notes
The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS
This work was?nancially supported by grants of Chinese Thousand Talents Program(YZBQF11001)and the National Natural Science Foundation of China(21371178).K.Z. acknowledges the support by the Division of Chemical Sciences,Geosciences,and Biosciences,O?ce of Basic Energy Sciences,U.S.Department of Energy,under contract No.DE-AC36-08GO28308with the National Renewable Energy
Laboratory.
■REFERENCES
(1)(a)Cai,N.;Moon,S.J.;Cevey-Ha,L.;Moehl,T.;Humphry-Baker,R.;Wang,P.;Zakeeruddin,S.M.;Gra t zel,M.Nano Lett.2011, 11,1452.(b)Jang,S.R.;Zhu,K.;Ko,M.J.;Kim,K.;Kim,K.C.;Park, N.G.;Frank,A.J.ACS Nano2011,5,8267.
(2)(a)Crossland,E.J.W.;Noel,N.;Sivaram,V.;Leijtens,T.; Alexander-Webber,J. A.;Snaith,H.J.Nature2013,495,215.
(b)O’Regan,B.;Gra t zel,M.Nature1991,353,737.
(3)(a)Habisreutinger,S.N.;Schmidt-Mende,L.;Stolarczyk,J.K. Angew.Chem.,Int.Ed.2013,52,7372.(b)Walter,M.G.;Warren,E. L.;Mckone,J.R.;Boettcher,S.W.;Mi,Q.X.;Santori,E.A.;Lewis,N. S.Chem.Rev.2010,110,6446.
(4)(a)Kubacka,A.;Ferna n dez-García,M.;Colo n,G.Chem.Rev. 2012,112,1555.(b)Hoffmann,M.R.;Martin,S.T.;Choi,W.; Bahnemann,D.W.Chem.Rev.1995,95,69.
(5)Xia,X.H.;Tu,J.P.;Zhang,Y.Q.;Wang,X.L.;Gu,C.D.;Zhao, X.B.;Fan,H.J.ACS Nano2012,6,5531.
(6)(a)Law,M.;Greene,L.E.;Johnson,J.C.;Saykally,R.;Yang,P.
D.Nat.Mater.2005,4,455.(b)Zhu,K.;Vinzant,T.B.;Neale,N.R.; Frank,A.J.Nano Lett.2007,7,3739.(c)Varghese,O.K.;Paulose,M.; Grimes,C.A.Nat.Nanotechnol.2009,4,592.(d)Feng,X.J.;Zhu,K.;
Frank,A.J.;Grimes,C.A.;Mallouk,T.E.Angew.Chem.,Int.Ed.2012, 51,2727.
(7)Wang,D.;Qian,F.;Yang,C.;Zhong,Z.H.;Lieber,C.M.Nano Lett.2004,4,871.
(8)Sauvage,F.;Fonzo,F.D.;Bassi,A.L.;Casari,C.S.;Russo,S.; Divitini,G.;Ducati,C.;Bottani,C.E.;Comte,P.;Gra t zel,M.Nano Lett.2010,10,2562.
(9)(a)Shi,J.;Hara,Y.;Sun,C.L.;Anderson,M.A.;Wang,X.D. Nano Lett.2011,11,3413.(b)Yang,X.F.;Zhuang,J.L.;Li,X.Y.; Chen,D.H.;Ouyang,G.F.;Mao,Z.Q.;Han,Y.X.;He,Z.H.;Liang,
C.L.;Wu,M.M.;Yu,J.C.ACS Nano2009,3,1212.
(10)(a)Wang,H.G.;Ma,D.L.;Huang,X.L.;Huang,Y.;Zhang,X.
B.Sci.Rep.2012,2,701.(b)Wu,W.Q.;Rao,H.S.;Xu,Y.F.;Wang, Y.F.;Su,
C.Y.;Kuang,
D.B.Sci.Rep.2013,3,1892.(c)Cho,I.S.; Chen,Z.B.;Forman,A.J.;Kim,D.R.;Rao,P.M.;Jaramillo,T.F.; Zheng,X.L.Nano Lett.2011,11,4978.(d)Lee,D.H.;Rho,Y.;Allen, F.I.;Minor,A.M.;Ko,S.H.;Grigoropoulos,C.P.Nanoscale2013,5, 11147.(e)Zhu,H.;Tao,J.;Wang,T.;Deng,J.Appl.Surf.Sci.2011, 257,10494.(f)Yin,Z.Y.;Wang,Z.;Du,Y.P.;Qi,X.Y.;Huang,Y.Z.; Xue,C.;Zhang,H.Adv.Mater.2012,24,5374.(g)Wu,W.Q.;Lei,B. X.;Rao,H.S.;Xu,Y.F.;Wang,Y.F.;Su,C.Y.;Kuang,D.B.Sci.Rep. 2013,3,1352.
(11)Zhao,Y.;Zhu,K.J.Phys.Chem.Lett.2013,4,2880.
(12)Park,N.G.;Schlichtho r l,G.;van de Lagemaat,J.;Cheong,H. M.;Mascarenhas,A.;Frank,A.J.J.Phys.Chem.B1999,103,3308.
(13)(a)Kopidakis,N.;Neale,N.R.;Zhu,K.;van de Lagemaat,J.; Frank,A.J.Appl.Phys.Lett.2005,87,202106.(b)Mohammadpour, A.;Farsinezhad,S.;Wiltshire,B.D.;Shankar,K.Phys.Status Solidi RRL 2014,DOI:10.1002/pssr.201308296.
(14)Hamann,T.W.;Ondersma,J.W.Energy Environ.Sci.2011,4, 370.
(15)Peter,L.M.J.Phys.Chem.C2007,111,6601.
(16)Zhu,K.;Kopidakis,N.;Neale,N.R.;van de Lagemaat,J.;Frank,
A.J.J.Phys.Chem.B2006,110,25174.