Facile synthesis of nitrogen self-doped rutile TiO 2 nanorods{ Shuan Wang, Junmin Xu, Hualin Ding, Shusheng Pan, Yunxia Zhang and Guanghai Li * Received 25th May 2012, Accepted 26th June 2012 DOI: 10.1039/c2ce25827g Nitrogen doping is a promising method to enhance the visible light absorption and photo-catalytic activ ity of TiO 2 . A new method is reported for the synthesis of nitrogen self-doped rutile TiO 2 nanorods, along with the formation study of V-shaped N-doped TiO 2 nanorods, using TiN as a precursor and using a hydrothermal method. Our synthesis method gives a facile and easy way to control nitrogen doping in a TiO 2 lattice. Two types of the V-shaped nanorods, with a (101) coherent boundary of either 114.4 or 134.9 inner angle, were observed. The N-doped TiO 2 nanorods exhibit an enhanced visible light absorption and red-shift in band gap in comparison with pure rutile TiO 2 nanopowders. The mechanisms of N doping and the formation of the V-shaped nanorods are analyzed and discussed. The oriented attachment and Ostwald ripening are considered responsible for the formation and growth of the straight and V-shaped N-doped TiO 2 nanorods. Introduction Titanium dioxide (TiO 2 ), with a wide band gap of 3.2 eV for ana tas e or 3.0 eV for rut il e, is an imp ort ant semic ond uc tor phot ocat alys t and has attr acte d cons ider able atte nti on. The abi lit y to con trol the band gap of TiO 2 nano cryst als and to enhance the utilization rate of the solar spectrum is essential for applicati ons in fields such as dye-sensit ized solar cells 1 and photo- catalysis. 2 Many methods have been explored to reduce the band gap of TiO 2 , such as doping transition-metal (iron, 3 vanadium, 4 nickel 5 and chro miump 6 ) and non-meta l (nitr oge n, 7 sulfur, 8 fluorine, 9 and carbon 10 ) into the TiO 2 host lattice. Among them, the nitrogen doping is an effective method to enhance visible light absorptio n and photo-cata lytic activity. 7,11–13 Di ff er ent methods have be en develope d to incorporate nitrogen in TiO 2 , and those methods can be generally classified into three categories: (1) sputtering and implantation techniques, mai nl y use d to pre par e sin gle crysta lli ne or pol ycr yst alline N-dop ed TiO 2 thin films ; 7,14 (2) high temperature annea ling treat ment under a N-con tain ing atmos phere ; 15 a nd (3) w et metho ds, incl udin g sol–g el, 16 solvo thermal and hydro thermal methods. 17–19 T he fi rs t two me t ho ds need either a h ig h tempe ratur e or compl icate d and expen sive equi pment , whil e the wet chemical method is simple and effective in controlling bot h the nit rog en dop ing con ten t and TiO 2 nanocrystal size, through changing the experimental parameters, such as reaction temperature, solution pH value and solvent system. It is well known that titanium nitride (TiN) is a metallic conductor with a partially filled band and a chemical bond of simultaneously met all ic, cov ale nt and ion ic cha ract ers, 20 wh ic h has a NaC l-li ke cu bi c crystal lizati on in the rock-sa lt structure, with N atoms occupying interstitial positions in a close-packed arrangement of Ti atoms. 21 The fact that TiO 2 can be preparedviaa simple oxidation process ofTiN 22 promises an opportunity for nitrogen self doping. TiO 2 n ano stru ctu res with differe nt morp hol ogies, suc h as nanowires, 23 nanorods, 24 nanotubes, 19 and nanofl owers, 25 hav e bee n prepared in recent years. The V-shaped nanostructures have been obs erve d in SnO 2 , 26 RuO 2 , 27 and ZnS e. 28 Ho we ve r, th ere is no repo rt about the V-shaped N-doped TiO 2 nanostruc tures. The V-shap ed structures are of part icu lar interest because the sud den bre ak dow n in lattice periodicity at the junction offers a good lateral confinement, and thus can enhance the excitonic optical response. 29 It is believed th at th is comp le x nano ro d de ri ve d structure co ul d of fe r ne w opportunities in tailoring the properties of discrete 1D nanostruc- tures and in 3D organization of nanostructured materials. 30 Here, a new method for the synthesis of nitrogen self-doped rutile TiO 2 nanorods is reported, along with the formation study of V-shaped N-doped TiO 2 nanorods, using TiN as a precursor, and using hydrothermal method. The separation of N from TiN provided a self dopi ng in the format ion proc ess of Ti O 2 nano rods. An enha nced visible ligh t absorp tion and red-s hift of the optical band gap were observed. Experimental Materials and synthesis Ti N nan opo wde rs, pro vid ed by Hef ei Kai er Nano Ene rgy Science and Technology Co., Ltd., Hefei, China, were produced Key Laboratory of Materials Physics, Anhui Key Lab of Nanomaterials and Nanostructure, Institute of Solid State Physics, Chinese Academy ofSciences, Hefei 230031, P.R. China. E-mail: [email protected]; Fax: +86-551-5591437; Tel: +86-551-5591437{ Elect ronic Supplement ary Info rmation (ESI ) ava ilable: [FES EM images of TiN precursor powders, HRTEM image of the tapered tip in N-dop ed TiO 2 nanoro d and the corresponding schematic orie ntat ion rela tions of the {111} facets on the tapere d tip, HRTEM imag es ofN-doped TiO 2 nanocrystals at the early stage of hydrothermal treatment, FESEM image and XRD pattern of the product hydrothermally treated without HCl and the determination of the band gap from the plots of(ahn) n vs. hn: withn=1/2 and n =2]. See DOI: 10.1039/c2ce25827g CrystEngComm Dynamic Article Links Cite this: CrystEngComm, 2012, 14, 7672–7679 www.rsc.org/crystengcomm PAPER 7672 | CrystEngComm, 2012, 14, 7672–7679 This journ al is The Royal Society of Chemistry 2012 P u b l i s h e d o n 2 6 J u n e 2 0 1 2 . D o w n l o a d e d b y P o n d i c h e r r y U n i v e r s i t y o n 1 0 / 1 0 / 2 0 1 3 1 0 : 1 0 : 1 6 . View Article Online / Journal Homepage / Table of Contents for this issue
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Facile synthesis of nitrogen self-doped rutile TiO2 nanorods{
Shuan Wang, Junmin Xu, Hualin Ding, Shusheng Pan, Yunxia Zhang and Guanghai Li *
Received 25th May 2012, Accepted 26th June 2012
DOI: 10.1039/c2ce25827g
Nitrogen doping is a promising method to enhance the visible light absorption and photo-catalytic
activity of TiO2. A new method is reported for the synthesis of nitrogen self-doped rutile TiO2
nanorods, along with the formation study of V-shaped N-doped TiO2 nanorods, using TiN as a
precursor and using a hydrothermal method. Our synthesis method gives a facile and easy way to
control nitrogen doping in a TiO2 lattice. Two types of the V-shaped nanorods, with a (101) coherent
boundary of either 114.4u or 134.9u inner angle, were observed. The N-doped TiO2 nanorods exhibit
an enhanced visible light absorption and red-shift in band gap in comparison with pure rutile TiO2
nanopowders. The mechanisms of N doping and the formation of the V-shaped nanorods are
analyzed and discussed. The oriented attachment and Ostwald ripening are considered responsible for
the formation and growth of the straight and V-shaped N-doped TiO2 nanorods.
Introduction
Titanium dioxide (TiO2), with a wide band gap of 3.2 eV for
anatase or 3.0 eV for rutile, is an important semiconductor
photocatalyst and has attracted considerable attention. The
ability to control the band gap of TiO2 nanocrystals and to
enhance the utilization rate of the solar spectrum is essential for
applications in fields such as dye-sensitized solar cells1 and photo-
catalysis.2 Many methods have been explored to reduce the band
gap of TiO2, such as doping transition-metal (iron,3 vanadium,4
nickel5 and chromiump6) and non-metal (nitrogen,7 sulfur,8
fluorine,9 and carbon10) into the TiO2 host lattice. Among them,
the nitrogen doping is an effective method to enhance visible light
absorption and photo-catalytic activity.7,11–13
Different methods have been developed to incorporate
nitrogen in TiO2, and those methods can be generally classified
into three categories: (1) sputtering and implantation techniques,
mainly used to prepare single crystalline or polycrystalline
N-doped TiO2 thin films;7,14 (2) high temperature annealing
treatment under a N-containing atmosphere;15 and (3) wet
methods, including sol–gel,16 solvothermal and hydrothermal
methods.17–19 The first two methods need either a high
temperature or complicated and expensive equipment, while
the wet chemical method is simple and effective in controlling
both the nitrogen doping content and TiO2 nanocrystal size,
through changing the experimental parameters, such as reaction
temperature, solution pH value and solvent system.
It is well known that titanium nitride (TiN) is a metallic conductor
with a partially filled band and a chemical bond of simultaneously
metallic, covalent and ionic characters,20 which has a NaCl-like cubic
crystallization in the rock-salt structure, with N atoms occupying
interstitial positions in a close-packed arrangement of Ti atoms.21
The fact that TiO2
can be prepared via a simple oxidation process of
TiN22 promises an opportunity for nitrogen self doping.
TiO2 nanostructures with different morphologies, such as
nanowires,23 nanorods,24 nanotubes,19 and nanoflowers,25 have been
prepared in recent years. The V-shaped nanostructures have been
observed in SnO2,26 RuO2,27 and ZnSe.28 However, there is no report
about the V-shaped N-doped TiO2 nanostructures. The V-shaped
structures are of particular interest because the sudden break down in
lattice periodicity at the junction offers a good lateral confinement,
and thus can enhance the excitonic optical response.29 It is believed
that this complex nanorod derived structure could offer new
opportunities in tailoring the properties of discrete 1D nanostruc-
tures and in 3D organization of nanostructured materials.30
Here, a new method for the synthesis of nitrogen self-dopedrutile TiO2 nanorods is reported, along with the formation study
of V-shaped N-doped TiO2 nanorods, using TiN as a precursor,
and using hydrothermal method. The separation of N from TiN
provided a self doping in the formation process of TiO2
nanorods. An enhanced visible light absorption and red-shift
of the optical band gap were observed.
Experimental
Materials and synthesis
TiN nanopowders, provided by Hefei Kaier Nano Energy
Science and Technology Co., Ltd., Hefei, China, were produced
Key Laboratory of Materials Physics, Anhui Key Lab of Nanomaterialsand Nanostructure, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China. E-mail: [email protected];Fax: +86-551-5591437; Tel: +86-551-5591437 { Electronic Supplementary Information (ESI) available: [FESEMimages of TiN precursor powders, HRTEM image of the tapered tip inN-doped TiO2 nanorod and the corresponding schematic orientationrelations of the {111} facets on the tapered tip, HRTEM images of N-doped TiO2 nanocrystals at the early stage of hydrothermal treatment,FESEM image and XRD pattern of the product hydrothermally treatedwithout HCl and the determination of the band gap from the plots of (ahn)n vs. hn: with n=1/2 and n=2]. See DOI: 10.1039/c2ce25827g
CrystEngComm Dynamic Article Links
Cite this: CrystEngComm, 2012, 14, 7672–7679
www.rsc.org/crystengcomm PAPER
7672 | CrystEngComm, 2012, 14, 7672–7679 This journal is The Royal Society of Chemistry 2012
View Article Online / Journal Homepage / Table of Contents for this issue
disappeared if the N-doped TiO2 nanorods were annealed at
500 uC in air. As both XPS and Raman analyses denied the
existence of TiN in the N-doped TiO2 nanorods, it is thus
considered the tail-up is due to the relative high N doping content
in TiO2.
It was found that by adjusting the hydrothermal solvent and
pH value of the precursor solution, N-doped anatase or anatase/
rutile TiO2 nanopowders, nanorods and even nanospheres can be
obtained.
The decrease in band gap and the increase in visible light
absorption are beneficial in enhancing the performance in dye-
sensitized solar cells and photo-catalysis, further work isunderway.
Conclusion
N-doped TiO2 nanorods have been synthesized directly from a
TiN precursor by a facile hydrothermal method in the presence
of HCl solution. The nanorods are highly crystalline with a rutile
phase, and exhibit both straight and V-shaped morphologies. It
was found that the lower the HCl concentration, the higher the
N doping content and 1.09 at% N doping can be obtained for
2.0 M HCl concentration. There are two types of the V-shaped
N-doped TiO2 nanorods, one with a 114.4u
inner angle andanother with an angle of 134.9u, and these two types of V-shaped
nanorods have the same coherent boundary of the (101) plane.
The size and shape of the N-doped TiO2 nanorods can be
controlled by the hydrothermal conditions. The oriented
attachment and Ostwald ripening are considered responsible
for the formation and growth of the straight and V-shaped
N-doped TiO2 nanorods. The band gap decreases and visible
light absorption increases with increasing N doping content for
the N-doped TiO2 nanorods. Our results not only add a new
number in the V-shaped nanorod family but provide a simple
route to prepare N-doped TiO2 nanostructures, which will
benefit both basic research and practical applications.
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (2012CB932303), and innovation
project of the Chinese Academy of Sciences (KJCX2-YW-H2O).
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Fig. 12 UV-vis diffuse reflectance spectra of N-doped TiO2 nanorods
with different HCl concentrations and pure rutile TiO2 nanopowders.
The inset is a plot of absorption versus energy in the absorption edge
region.
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