A seeded heteroepitaxial growth of ZnO nanorods selectively on TiO2 nanorod tips was achieved by restricting crystal growth on highly hydrophobic TiO2 nanorod film surfaces. Intriguing light harvesting performance and efficient charge transport efficiency has been found, which suggest potential applications in photovoltaics and optoelectronics

Gai, Lili

Gu, Feng

Li, Chunzhong

Schmidt-Mende, Lukas

Shao, Wei

English

A seeded heteroepitaxial growth of ZnO nanorods selectively on TiO2 nanorod tips was achieved by restricting crystal growth on highly hydrophobic TiO2 nanorod film surfaces. Intriguing light harvesting performance and efficient charge transport efficiency has been found, which suggest potential applications in photovoltaics and optoelectronics.publishe

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Heteroepitaxial growth of ZnO branches selectively on TiO 2 nanorod tips with improved light harvesting performance

Open Access LMU ( Ludwig-Maximilians-Univ. München)

8400 Chem. Commun., 2011, 47, 8400–8402 This journal is c The Royal Society of Chemistry 2011Cite this: Chem. Commun., 2011, 47, 8400–8402Heteroepitaxial growth of ZnO branches selectively on TiO2 nanorodtips with improved light harvesting performancewFeng Gu,ab Lili Gai,a Wei Shao,a Chunzhong Li*a and Lukas Schmidt-Mende*bReceived 20th April 2011, Accepted 8th June 2011DOI: 10.1039/c1cc12309bA seeded heteroepitaxial growth of ZnO nanorods selectively onTiO2 nanorod tips was achieved by restricting crystal growth onhighly hydrophobic TiO2 nanorod film surfaces. Intriguing lightharvesting performance and efficient charge transport efficiencyhas been found, which suggest potential applications in photo-voltaics and optoelectronics.Recently, branched nanowires (NWs) have attracted muchinterest due to their special tree-like structures offering greatlyenhanced surface area and a direct conduction pathway for therapid collection of photogenerated electrons,1 and thereforepromise very attractive potential applications in optoelectronics,photocatalysis, photovoltaics and sensing.1d,2 One of the keyremaining issues in the field is the development of single-crystal nanobranches with an epitaxial relation to the trunkto diminish the possibility of charge recombination duringinterparticle percolation.3 Although there have been a fewreports on the formation of branched NWs with differentinorganic components fused together,2b,4 these systems imposeinterfaces between materials with large lattice mismatch, withthe formation of a large quantity of defects at grain boundariesas trapping sites, adversely affecting charge migration.5ZnO and TiO2 are the most investigated semiconductormaterials for optoelectronic applications. ZnO is a direct bandgap semiconductor with a wide band gap (Eg = 3.37 eV)and large binding energy, high electrical conductivity andtransparency in the visible region. After coupling with TiO2to form a ZnO–TiO2 heterostructure, faster electron transportwith reduced recombination loss can be expected because theelectron mobility of ZnO is higher by 2–3 orders of magnitudethan that of TiO2.1c,6 In this communication, a heteroepitaxialgrowth of ZnO nanorods selectively on the tips of TiO2nanorods to form special dandelion-like heterostructures wasachieved by restricting the seed planting and the followingcrystal growth on the film surface, based on the highly hydro-phobic property of the TiO2 nanorod film. Selective growth ofmetal or semiconductor dots on the secondary structure is ofparticular importance because of the strong coupling ofelectronic states and unusual properties.7 This strategy isan effective and simple attempt for the selective growth ofnanostructures with controlled density and location on thesecondary component by taking advantage of the macroscopicwettability of materials. Arising from this special branchedstructure with heteroepitaxial interfaces, intriguing lightharvesting performance and better electron transport efficienciesare exhibited, which imply potential applications in the fieldsof photovoltaics and optoelectronics.A key step enabling our design is to realize the selectivedeposition of a seed layer on the nanorod tips and prevent theprecursor solution penetrating the grooves during the followingcrystal growth process. A hydrothermally derived rutile TiO2nanorod film8 with rod length of B1.3 mm and diameter ofB90 nm is shown in Fig. 1a and b. The wettability wasevaluated by water contact angel (CA) measurement of theas-prepared film. Fig. 1c shows a spherical water droplet witha water CA of 131.31, indicating a highly hydrophobic surfaceof the rutile TiO2 nanorod film. Although rutile TiO2 is ahydrophilic material with a water CA of 741 on a smoothsingle crystal (001) surface,9 highly hydrophobic, even super-hydrophobic surfaces of TiO2 can be achieved in the case ofspecial geometrical morphology of the film surface.10 Therefore,when wetting the film with zinc acetate solution, the solutionwould not penetrate the grooves and is suspended on the surfaceof the nanorod films as droplets. After the decompositionFig. 1 (a, b) SEM images of rutile TiO2 nanorod film. (c) Photographof a water droplet on the TiO2 nanorod film. (d, e) SEM images of theZnO/TiO2 branched heterostructures.a Key Laboratory for Ultrafine Materials of Ministry of Education,School of Materials Science and Engineering, East China Universityof Science & Technology, Shanghai 200237, China.E-mail: czli@ecust.edu.cn; Fax: 86-21- 6425-0624;Tel: 86-21-6425-0949bDepartment of Physics and Center for NanoScience (CeNS),Ludwig-Maximilians University (LMU), Munich80799, Germany.E-mail: L.Schmidt-Mende@physik.uni-muenchen.de;Fax: 49-89-2180-17836; Tel: 49-89-2180-3443w Electronic supplementary information (ESI) available. See DOI:10.1039/c1cc12309bChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONDownloaded by Ludwig Maximilians Universitaet Muenchen on 25/04/2013 13:28:25. Published on 24 June 2011 on http://pubs.rsc.org | doi:10.1039/C1CC12309BView Article Online / Journal Homepage / Table of Contents for this issueThis journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 8400–8402 8401procedure, textured ZnO nanocrystals were stacked on theplatforms supplied by the TiO2 tips without slumping into thegrooves. Hydrothermal growth of ZnO nanorods on theseeded substrate was conducted according to the techniquereported previously.11 Fig. 1d and e show the scanning electronmicroscopy (SEM) images of the obtained heterogeneousnanostructures (HNs). Dense hexagonal ZnO nanorods withdiameter of 50 nm and length of 1000 nm grow divergently onthe TiO2 nanorod tips and form 3D branched NW hetero-structures. ZnO nanorod bundles were only decorated on theTiO2 nanorod tips with naked side surfaces, implying theselective anisotropic crystal growth on TiO2 nanorods. Largecorollas covering the entire TiO2 nanorod film surface, byinterweaving with neighbours, allow the incident light to bestrongly scattered and to improve the light harvesting efficiency.The crystallographic relationship between the TiO2 trunkand ZnO branch has been further analyzed by high-resolutiontransmission electron microscopy (HRTEM) measurements.Fig. 2a shows a HRTEM image taken at the junction of TiO2and ZnO, while insets of this figure are SAED patternsrecorded by focusing specifically at the TiO2 and ZnO regions,respectively. ZnO nanorods grew on the TiO2 rod tip along the002 c-direction with a lattice spacing of 0.26 nm and a cleaninterface can be observed clearly. Examination of the TiO2indicates that the single-crystalline TiO2 nanorods are of arutile phase with growth along the [001] direction as evidencedby the SAED pattern examined along the [1%10] zone axis.Lattice fringes with interplanar spacings, d110 = 0.32 nm andd001 = 0.29 nm, are clearly imaged. The HRTEM resultreveals that the ZnO nanorod is not growing perpendicularto the TiO2 nanorod surface but with an angle of 271, presumablybecause the ZnO (002) crystal planes (d002 = 0.26 nm) wouldgear at an angle to match the TiO2 (001) crystal planes(d001 = 0.29 nm) largely to relax strain (dZnO(002) = dTiO2(001)cosy,y= 271), confirming the epitaxial correlation for these HNs. Alattice-fringe-resolved HRTEM image is also shown in Fig. 2b,evidencing the crystallographic (002) planes of ZnO intercalatedTiO2 (001) planes with an angle of 271. A 2D atomic arrangementwith a twisted angle of 271 shown in Fig. S1w further demon-strates the epitaxial relationship between the TiO2 nanorodand ZnO branches by considering the observed (001)/(002)interface with zone axes of [1%10] and [010] for TiO2 and ZnO,respectively.The ZnO seeds, with a heteroepitaxial interface from whichthe heterogeneous nucleation takes place preferentially toform well-defined ZnO branches, is the essence to obtain thefinal interesting HNs. The lattice mismatch between the {001}planes of rutile TiO2 and the {002} planes of wurtzite ZnO issmall, noting that the interplanar distances of d001 (0.29 nm,TiO2) and d002 (0.26 nm, ZnO) is similar, which might presenta heteroepitaxial characteristic at the interface by simplemodulation of the lattices with a small angle to largely releasethe strain. ZnO seeds deposited on TiO2 tip with multiplelattice orientations and then the deposited ZnO extended toneutralize local polar charge and release the strain accumulated.By serving as starting positions, the epitaxial growth of ZnOtook place. Considering the strain induced at the interface,only minor strain can be detected, further confirming thelargely strain free relaxation due to congruent lattice fringes(Fig. S2w). Furthermore, the ZnO nanorods were found toanchor to TiO2 nanorod tips with point contacts inducedby the seeds (Fig. S3w). The HRTEM image shown in Fig. S4wevidenced the formation of ZnO seeds anchoring to the TiO2nanorod outer surface with lattice modulation, presumablyinduced by the strain from the lattice mismatch. Generally, akey step in obtaining single-crystal epitaxy is to remove thenative oxide layer to generate a clean interface between thedeposited amorphous material in a pore and the c-Sisubstrate.3 For Si NW-based epitaxial growth, if the nativeoxide was not removed completely, the crystal orientation inthe film was independent of the substrate orientation.3 It isalso reported that ZnO seeds failed to lead to a coating on a Sinanowire surface after removing native oxide.12 However, inour work, the decomposition of zinc salts was effective todeposit ZnO seeds onto the TiO2 nanorod tips without anyfurther surface modification process to the TiO2 film. Top andside views in Fig. 1a and b show that the top surface of theTiO2 nanorods appears to contain many step edges, while theside surface is smooth, as the crystal growth of rutile TiO2nanorods proceeds via ledge growth mechanism by addition oftitanium growth units (e.g., [Ti(OH)2Cl2(OH)2]0) at the stepedges.13 The step edges may allow for more seeds formed withdiverse directions and as a result dandelion-like ZnO/TiO2HNs were formed (Fig. 1d). Upon addition of polyethylenimine(PEI) as a retardant, ZnO nanorods with similar diameterswere obtained. PEI was adapted to affect the growth habit ofZnO nanorods by adsorption on the nonpolar facets of ZnOcrystal and by coordination to zinc ions.14 In this work, bychanging parameters such as addition of PEI or hexamethyl-enetetramine (HMT), or reaction time, ZnO nanorods withdifferent lengths of 200–1000 nm branched out on the TiO2 tip(Fig. S5w), which can interweave with neighbouring counter-parts to form a complex and dense canopy, implying anexcellent light scattering capability. Synthetic process andtypical morphologies of the HNs were given in the ESIw.Photoluminescence (PL) was used to study the structure anddefects of the ZnO/TiO2 NHs (Fig. 3). The HN films show astrong emission peak at B378 nm (UV emission) due to theexcitonic transition of ZnO15 with a weak broad emisson atB394 nm arising from the lowest indirect transition (X1- G1)of rutile TiO2.16 It should be noted that the known ‘‘green’’emission cannot be detected in the range of 420–600 nm,indicating very low residual defects in the structures, due tohigh crystallinity of the HNs, even after heteroepitaxialgrowth of ZnO rods. Increasing the ZnO rod length wouldresult in the PL intensity enhancement of the two components,not only the excitonic emission from ZnO but also that fromFig. 2 (a) HRTEM image at the interface with corresponding SAEDpatterns of the two components. (b) Lattice-fringe-resolved HRTEMimage showing the lattice spacings at the interface.Downloaded by Ludwig Maximilians Universitaet Muenchen on 25/04/2013 13:28:25. Published on 24 June 2011 on http://pubs.rsc.org | doi:10.1039/C1CC12309BView Article Online8402 Chem. Commun., 2011, 47, 8400–8402 This journal is c The Royal Society of Chemistry 2011rutile TiO2, indicating an efficient electron transfer occursupon excitation.Compared with the PL spectrum of ZnO nanorod film withsimilar length (Fig. S6w), the excitonic emission from ZnOweakened while the emission from TiO2 was improved afterforming the HNs, further proving the charge transfer behavior.ZnO has higher electron mobility (100–205 cm2 V1 s1) andcarrier concentration (B1018 cm3) than rutile TiO2.1c,6Therefore, due to the congruent interface and axial surfacefield within each nanorod, much more electrons photogeneratedin ZnO may swarm into the TiO2 rod with reduced recomb-ination loss (usually by residual defects located at the interface)and as a result the PL intensity from rutile TiO2 increasedgreatly. The efficient electron transfer and collection efficienciesenable the HNs to find possible uses in areas such as energygeneration and storage.Light scattering properties were studied by measuring thediffuse reflection spectra (Fig. S7w). Compared with TiO2nanorod film, the measured reflectance of ZnO/TiO2 HNs(branch lengths of 600, 1000 nm) was found to be remarkablyhigher than that of TiO2 nanorods, indicating a superior lightscattering effect. With increasing the length of ZnO branches,better reflection can be obtained due to the interwovencorollas constructed by ZnO nanorods, which can effectivelyconfine the incident light within the film. Assuming thecorollas as quasi-spheres, such scattering trend was found tobe similar to the results estimated from Mie resonance theory;17as the size of the corolla is comparable with the wavelength ofthe incident light, resonant scattering most likely occurs. Thecorolla was constructed by branched ZnO nanorods andfurther interwoven with the neighboring counterparts to forma dense scattering layer, such special structure can generatemultiple scattering to light and the formation of closed loopsfor light confinement,18 and as a result, to increased lightharvesting efficiency, especially when coupling with dyes andquantum dots in photovoltaic cells (inset of Fig. 3). The on-goingresearch is focusing on the photovoltaic properties of the HNs.In summary, we have reported a simple and effective solutionapproach for the selective growth of ZnO nanorods ontotip-seeded TiO2 nanorod arrays to form special dandelion-likeZnO/TiO2 heterogeneous nanostructures by exploiting thehigh hydrophobicity characteristics of TiO2 nanorod film.The branched ZnO nanorods were found to possess a strain-induced epitaxial relation to the TiO2 trunk, and therebyprominent charge transport, along with light harvestingefficiencies were exhibited. This strategy is a new concept forposition-selective growth of homo- or heterostructured nano-materials, which is certainly significant for future optoelectronicapplications.This work was supported by the National Natural ScienceFoundation of China (20925621, 81071994), ShanghaiRising-Star Program (09QH1400700), Program of ShanghaiSubject Chief Scientist (09XD1400800), Basic Research Programof Shanghai (10JC1403300) and Alexander von HumboldtFoundation.Notes and references1 (a) D. L. Wang and C. M. Lieber, Nat. Mater., 2003, 2, 355;(b) J. Zhu, H. L. Peng, A. F. Marshall, D. M. Barnett, W. D. Nixand Y. Cui, Nat. Nanotechnol., 2008, 3, 477; (c) S. H. Ko, D. Lee,H. W. Kang, K. H. Nam, J. Y. Yeo, S. J. Hong, C. P. Grigoropoulosand H. J. Sung, Nano Lett., 2011, 11, 666; (d) K. Sun, Y. Jing,N. Park, C. Li, Y. Bando and D. L. Wang, J. Am. Chem. Soc., 2010,132, 15465.2 (a) J. Y. Lao, J. G. Wen and Z. F. Ren, Nano Lett., 2002, 2, 1287;(b) S. Y. Bae, H. W. Seo, H. C. Choi and J. Park, J. Phys. Chem. B,2004, 108, 12318.3 H. Arora, P. Du, K. W. Tan, J. K. Hyun, J. Grazul, H. L. Xin,D. A. Muller, M. O. Thompson and U. Wiesner, Science, 2010,330, 214.4 (a) Y. Lei, G. Zhao, M. Liu, Z. Zhang, X. Tong and T. Cao,J. Phys. Chem. C, 2009, 113, 19067; (b) Y. K. A. Lau,D. J. Chernak, M. J. Bierman and S. Jin, J. Mater. Chem., 2009,19, 934.5 K. W. Kwon and M. Shim, J. Am. Chem. Soc., 2005, 127, 10269.6 D. C. Look, D. C. Reynolds, J. R. Sizelove, R. L. Jones,C. W. Litton, G. Cantwell and W. C. Harsch, Solid StateCommun., 1998, 105, 399.7 (a) X. H. Li, J. Lian, M. Lin and Y. Chan, J. Am. Chem. Soc.,2011, 133, 672; (b) W. S. Wang, J. Goebl, L. He, S. Aloni,Y. X. Hu, L. Zhen and Y. D. Yin, J. Am. Chem. Soc., 2010,132, 17316; (c) R. D. Robinson, B. Sadtler, D. O. Demchenko,C. K. Erdonmez, L. W. Wang and A. P. Alivisatos, Science, 2007,317, 355; (d) T. Mokari, E. Rothenberg, I. Popov, R. Costi andU. Banin, Science, 2004, 304, 1787.8 B. Liu and E. S. Aydil, J. Am. Chem. Soc., 2009, 131, 3985.9 (a) X. J. Feng, J. Zhai and L. Jiang, Angew. Chem., Int. Ed., 2005,44, 5115; (b) R. Wang, N. Sakai, A. Fujishima, T. Watanabe andK. Hashimoto, J. Phys. Chem. B, 1999, 103, 2188.10 (a) S. Minko, M. Muller, M. Motornov, M. Nitschke, K. Grundkeand M. Stamm, J. Am. Chem. Soc., 2003, 125, 3896; (b) X. J. Feng,L. Feng, M. H. Jin, J. Zhai, L. Jiang and D. B. Zhu, J. Am. Chem.Soc., 2004, 126, 62.11 L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger,G. Somorjai and P. Yang, Nano Lett., 2005, 5, 1231.12 D. Mudusu, K. R. Nandanapalli, P. Alexander and P. Fernando,ChemPhysChem, 2010, 11, 809.13 A. Pottier, C. Chaneac, E. Tronc, L. Mazerolles and J. P. Jolivet,J. Mater. Chem., 2001, 11, 1116.14 L. E. Greene, B. D. Yuhas, M. Law, D. Zitoun and P. D. Yang,Inorg. Chem., 2006, 45, 7535.15 G. Adamopoulos, A. Bashir, S. Thomas, W. P. Gillin,S. Georgakopoulos, M. Shkunov, M. A. Baklar, N. Stingelin,R. C. Maher, L. F. Cohen, D. D. C. Bradley andT. D. Anthopoulos, Adv. Mater., 2010, 22, 4764.16 F. X. Liu, M. Tang, L. Liu, S. Lu, J. Y. Wang, Z. Y. Chen andR. Ji, Phys. Status Solidi A, 2000, 179, 437.17 (a) Q. F. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe and G. Cao,Adv. Funct. Mater., 2008, 18, 1654; (b) S. M. Scholz, R. Vacassy,J. Dutta, H. Hofmann and M. Akinc, J. Appl. Phys., 1998,83, 7860.18 (a) D. S. Wiersma, P. Bartolini, A. Lagendijk and R. Righini,Nature,1997, 390, 671; (b) H. Cao, J. Y. Xu, D. Z. Zhang, S. H. Chang,S. T. Ho, E. W. Seelig, X. Liu and R. P. H. Chang, Phys. Rev. Lett.,2000, 84, 5584.Fig. 3 Photoluminescent spectra of ZnO/TiO2 HN films and TiO2nanorod film.Downloaded by Ludwig Maximilians Universitaet Muenchen on 25/04/2013 13:28:25. Published on 24 June 2011 on http://pubs.rsc.org | doi:10.1039/C1CC12309BView Article Online

Heteroepitaxial growth of ZnO branches selectively on TiO2 nanorod tips with improved light harvesting performance

Universität München: Elektronischen Publikationen

8400 Chem. Commun., 2011, 47, 8400–8402 This journal is c The Royal Society of Chemistry 2011
Cite this: Chem. Commun., 2011, 47, 8400–8402
Heteroepitaxial growth of ZnO branches selectively on TiO2 nanorod
tips with improved light harvesting performancew
Feng Gu,
ab
Lili Gai,
a
Wei Shao,
a
Chunzhong Li*
a
and Lukas Schmidt-Mende*
b
Received 20th April 2011, Accepted 8th June 2011
DOI: 10.1039/c1cc12309b
A seeded heteroepitaxial growth of ZnO nanorods selectively on
TiO2 nanorod tips was achieved by restricting crystal growth on
highly hydrophobic TiO2 nanorod film surfaces. Intriguing light
harvesting performance and efficient charge transport efficiency
has been found, which suggest potential applications in photo-
voltaics and optoelectronics.
Recently, branched nanowires (NWs) have attracted much
interest due to their special tree-like structures offering greatly
enhanced surface area and a direct conduction pathway for the
rapid collection of photogenerated electrons,1 and therefore
promise very attractive potential applications in optoelectronics,
photocatalysis, photovoltaics and sensing.1d,2 One of the key
remaining issues in the field is the development of single-
crystal nanobranches with an epitaxial relation to the trunk
to diminish the possibility of charge recombination during
interparticle percolation.3 Although there have been a few
reports on the formation of branched NWs with different
inorganic components fused together,2b,4 these systems impose
interfaces between materials with large lattice mismatch, with
the formation of a large quantity of defects at grain boundaries
as trapping sites, adversely affecting charge migration.5
ZnO and TiO2 are the most investigated semiconductor
materials for optoelectronic applications. ZnO is a direct band
gap semiconductor with a wide band gap (Eg = 3.37 eV)
and large binding energy, high electrical conductivity and
transparency in the visible region. After coupling with TiO2
to form a ZnO–TiO2 heterostructure, faster electron transport
with reduced recombination loss can be expected because the
electron mobility of ZnO is higher by 2–3 orders of magnitude
than that of TiO2.
1c,6 In this communication, a heteroepitaxial
growth of ZnO nanorods selectively on the tips of TiO2
nanorods to form special dandelion-like heterostructures was
achieved by restricting the seed planting and the following
crystal growth on the film surface, based on the highly hydro-
phobic property of the TiO2 nanorod film. Selective growth of
metal or semiconductor dots on the secondary structure is of
particular importance because of the strong coupling of
electronic states and unusual properties.7 This strategy is
an effective and simple attempt for the selective growth of
nanostructures with controlled density and location on the
secondary component by taking advantage of the macroscopic
wettability of materials. Arising from this special branched
structure with heteroepitaxial interfaces, intriguing light
harvesting performance and better electron transport efficiencies
are exhibited, which imply potential applications in the fields
of photovoltaics and optoelectronics.
A key step enabling our design is to realize the selective
deposition of a seed layer on the nanorod tips and prevent the
precursor solution penetrating the grooves during the following
crystal growth process. A hydrothermally derived rutile TiO2
nanorod film8 with rod length of B1.3 mm and diameter of
B90 nm is shown in Fig. 1a and b. The wettability was
evaluated by water contact angel (CA) measurement of the
as-prepared film. Fig. 1c shows a spherical water droplet with
a water CA of 131.31, indicating a highly hydrophobic surface
of the rutile TiO2 nanorod film. Although rutile TiO2 is a
hydrophilic material with a water CA of 741 on a smooth
single crystal (001) surface,9 highly hydrophobic, even super-
hydrophobic surfaces of TiO2 can be achieved in the case of
special geometrical morphology of the film surface.10 Therefore,
when wetting the film with zinc acetate solution, the solution
would not penetrate the grooves and is suspended on the surface
of the nanorod films as droplets. After the decomposition
Fig. 1 (a, b) SEM images of rutile TiO2 nanorod film. (c) Photograph
of a water droplet on the TiO2 nanorod film. (d, e) SEM images of the
ZnO/TiO2 branched heterostructures.
a Key Laboratory for Ultrafine Materials of Ministry of Education,
School of Materials Science and Engineering, East China University
of Science & Technology, Shanghai 200237, China.
E-mail: czli@ecust.edu.cn; Fax: 86-21- 6425-0624;
Tel: 86-21-6425-0949
bDepartment of Physics and Center for NanoScience (CeNS),
Ludwig-Maximilians University (LMU), Munich80799, Germany.
E-mail: L.Schmidt-Mende@physik.uni-muenchen.de;
Fax: 49-89-2180-17836; Tel: 49-89-2180-3443
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc12309b
ChemComm Dynamic Article Links
www.rsc.org/chemcomm COMMUNICATION
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View Article Online / Journal Homepage / Table of Contents for this issue
This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 8400–8402 8401
procedure, textured ZnO nanocrystals were stacked on the
platforms supplied by the TiO2 tips without slumping into the
grooves. Hydrothermal growth of ZnO nanorods on the
seeded substrate was conducted according to the technique
reported previously.11 Fig. 1d and e show the scanning electron
microscopy (SEM) images of the obtained heterogeneous
nanostructures (HNs). Dense hexagonal ZnO nanorods with
diameter of 50 nm and length of 1000 nm grow divergently on
the TiO2 nanorod tips and form 3D branched NW hetero-
structures. ZnO nanorod bundles were only decorated on the
TiO2 nanorod tips with naked side surfaces, implying the
selective anisotropic crystal growth on TiO2 nanorods. Large
corollas covering the entire TiO2 nanorod film surface, by
interweaving with neighbours, allow the incident light to be
strongly scattered and to improve the light harvesting efficiency.
The crystallographic relationship between the TiO2 trunk
and ZnO branch has been further analyzed by high-resolution
transmission electron microscopy (HRTEM) measurements.
Fig. 2a shows a HRTEM image taken at the junction of TiO2
and ZnO, while insets of this figure are SAED patterns
recorded by focusing specifically at the TiO2 and ZnO regions,
respectively. ZnO nanorods grew on the TiO2 rod tip along the
002 c-direction with a lattice spacing of 0.26 nm and a clean
interface can be observed clearly. Examination of the TiO2
indicates that the single-crystalline TiO2 nanorods are of a
rutile phase with growth along the [001] direction as evidenced
by the SAED pattern examined along the [1%10] zone axis.
Lattice fringes with interplanar spacings, d110 = 0.32 nm and
d001 = 0.29 nm, are clearly imaged. The HRTEM result
reveals that the ZnO nanorod is not growing perpendicular
to the TiO2 nanorod surface but with an angle of 271, presumably
because the ZnO (002) crystal planes (d002 = 0.26 nm) would
gear at an angle to match the TiO2 (001) crystal planes
(d001 = 0.29 nm) largely to relax strain (dZnO(002) = dTiO2(001)cosy,
y= 271), confirming the epitaxial correlation for these HNs. A
lattice-fringe-resolved HRTEM image is also shown in Fig. 2b,
evidencing the crystallographic (002) planes of ZnO intercalated
TiO2 (001) planes with an angle of 271. A 2D atomic arrangement
with a twisted angle of 271 shown in Fig. S1w further demon-
strates the epitaxial relationship between the TiO2 nanorod
and ZnO branches by considering the observed (001)/(002)
interface with zone axes of [1%10] and [010] for TiO2 and ZnO,
respectively.
The ZnO seeds, with a heteroepitaxial interface from which
the heterogeneous nucleation takes place preferentially to
form well-defined ZnO branches, is the essence to obtain the
final interesting HNs. The lattice mismatch between the {001}
planes of rutile TiO2 and the {002} planes of wurtzite ZnO is
small, noting that the interplanar distances of d001 (0.29 nm,
TiO2) and d002 (0.26 nm, ZnO) is similar, which might present
a heteroepitaxial characteristic at the interface by simple
modulation of the lattices with a small angle to largely release
the strain. ZnO seeds deposited on TiO2 tip with multiple
lattice orientations and then the deposited ZnO extended to
neutralize local polar charge and release the strain accumulated.
By serving as starting positions, the epitaxial growth of ZnO
took place. Considering the strain induced at the interface,
only minor strain can be detected, further confirming the
largely strain free relaxation due to congruent lattice fringes
(Fig. S2w). Furthermore, the ZnO nanorods were found to
anchor to TiO2 nanorod tips with point contacts induced
by the seeds (Fig. S3w). The HRTEM image shown in Fig. S4w
evidenced the formation of ZnO seeds anchoring to the TiO2
nanorod outer surface with lattice modulation, presumably
induced by the strain from the lattice mismatch. Generally, a
key step in obtaining single-crystal epitaxy is to remove the
native oxide layer to generate a clean interface between the
deposited amorphous material in a pore and the c-Si
substrate.3 For Si NW-based epitaxial growth, if the native
oxide was not removed completely, the crystal orientation in
the film was independent of the substrate orientation.3 It is
also reported that ZnO seeds failed to lead to a coating on a Si
nanowire surface after removing native oxide.12 However, in
our work, the decomposition of zinc salts was effective to
deposit ZnO seeds onto the TiO2 nanorod tips without any
further surface modification process to the TiO2 film. Top and
side views in Fig. 1a and b show that the top surface of the
TiO2 nanorods appears to contain many step edges, while the
side surface is smooth, as the crystal growth of rutile TiO2
nanorods proceeds via ledge growth mechanism by addition of
titanium growth units (e.g., [Ti(OH)2Cl2(OH)2]
0) at the step
edges.13 The step edges may allow for more seeds formed with
diverse directions and as a result dandelion-like ZnO/TiO2
HNs were formed (Fig. 1d). Upon addition of polyethylenimine
(PEI) as a retardant, ZnO nanorods with similar diameters
were obtained. PEI was adapted to affect the growth habit of
ZnO nanorods by adsorption on the nonpolar facets of ZnO
crystal and by coordination to zinc ions.14 In this work, by
changing parameters such as addition of PEI or hexamethyl-
enetetramine (HMT), or reaction time, ZnO nanorods with
different lengths of 200–1000 nm branched out on the TiO2 tip
(Fig. S5w), which can interweave with neighbouring counter-
parts to form a complex and dense canopy, implying an
excellent light scattering capability. Synthetic process and
typical morphologies of the HNs were given in the ESIw.
Photoluminescence (PL) was used to study the structure and
defects of the ZnO/TiO2 NHs (Fig. 3). The HN films show a
strong emission peak at B378 nm (UV emission) due to the
excitonic transition of ZnO15 with a weak broad emisson at
B394 nm arising from the lowest indirect transition (X1 - G1)
of rutile TiO2.
16 It should be noted that the known ‘‘green’’
emission cannot be detected in the range of 420–600 nm,
indicating very low residual defects in the structures, due to
high crystallinity of the HNs, even after heteroepitaxial
growth of ZnO rods. Increasing the ZnO rod length would
result in the PL intensity enhancement of the two components,
not only the excitonic emission from ZnO but also that from
Fig. 2 (a) HRTEM image at the interface with corresponding SAED
patterns of the two components. (b) Lattice-fringe-resolved HRTEM
image showing the lattice spacings at the interface.
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8402 Chem. Commun., 2011, 47, 8400–8402 This journal is c The Royal Society of Chemistry 2011
rutile TiO2, indicating an efficient electron transfer occurs
upon excitation.
Compared with the PL spectrum of ZnO nanorod film with
similar length (Fig. S6w), the excitonic emission from ZnO
weakened while the emission from TiO2 was improved after
forming the HNs, further proving the charge transfer behavior.
ZnO has higher electron mobility (100–205 cm2 V1 s1) and
carrier concentration (B1018 cm3) than rutile TiO2.
1c,6
Therefore, due to the congruent interface and axial surface
field within each nanorod, much more electrons photogenerated
in ZnO may swarm into the TiO2 rod with reduced recomb-
ination loss (usually by residual defects located at the interface)
and as a result the PL intensity from rutile TiO2 increased
greatly. The efficient electron transfer and collection efficiencies
enable the HNs to find possible uses in areas such as energy
generation and storage.
Light scattering properties were studied by measuring the
diffuse reflection spectra (Fig. S7w). Compared with TiO2
nanorod film, the measured reflectance of ZnO/TiO2 HNs
(branch lengths of 600, 1000 nm) was found to be remarkably
higher than that of TiO2 nanorods, indicating a superior light
scattering effect. With increasing the length of ZnO branches,
better reflection can be obtained due to the interwoven
corollas constructed by ZnO nanorods, which can effectively
confine the incident light within the film. Assuming the
corollas as quasi-spheres, such scattering trend was found to
be similar to the results estimated from Mie resonance theory;17
as the size of the corolla is comparable with the wavelength of
the incident light, resonant scattering most likely occurs. The
corolla was constructed by branched ZnO nanorods and
further interwoven with the neighboring counterparts to form
a dense scattering layer, such special structure can generate
multiple scattering to light and the formation of closed loops
for light confinement,18 and as a result, to increased light
harvesting efficiency, especially when coupling with dyes and
quantum dots in photovoltaic cells (inset of Fig. 3). The on-going
research is focusing on the photovoltaic properties of the HNs.
In summary, we have reported a simple and effective solution
approach for the selective growth of ZnO nanorods onto
tip-seeded TiO2 nanorod arrays to form special dandelion-like
ZnO/TiO2 heterogeneous nanostructures by exploiting the
high hydrophobicity characteristics of TiO2 nanorod film.
The branched ZnO nanorods were found to possess a strain-
induced epitaxial relation to the TiO2 trunk, and thereby
prominent charge transport, along with light harvesting
efficiencies were exhibited. This strategy is a new concept for
position-selective growth of homo- or heterostructured nano-
materials, which is certainly significant for future optoelectronic
applications.
This work was supported by the National Natural Science
Foundation of China (20925621, 81071994), Shanghai
Rising-Star Program (09QH1400700), Program of Shanghai
Subject Chief Scientist (09XD1400800), Basic Research Program
of Shanghai (10JC1403300) and Alexander von Humboldt
Foundation.
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nanorod film.
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Feng Gu

Lili Gai

Wei Shao

Chunzhong Li

Lukas Schmidt-Mende

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Heteroepitaxial growth of ZnO branches selectively on TiO2 nanorod tips with improved light harvesting performance

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