Materials Research Bulletin 45 (2010) 739–743

Synthesis of shape-controlled Nb3O7F/NbB2 heterostructure: A new idea tosynthesize binary hybrid materials by incomplete reaction

Fei Huang a, Zhengyi Fu a,*, Weimin Wang a, Hao Wang a, Yucheng Wang a, Jinyong Zhang a,Qingjie Zhang a, Soo Wohn Lee b, Kochi Niihara c

a State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR Chinab Department of Materials Engineering, SunMoon University, Asar, ChungNan 336-708, Republic of Koreac Extreme Energy Density Research Institute, Nagoka University of Technology, 1603-1, Kamitomioka, Nagoba, Niigata 940-2188, Japan

A R T I C L E I N F O

Article history:

Received 5 July 2009

Received in revised form 24 January 2010

Accepted 4 February 2010

Available online 10 February 2010

Keywords:

A. Inorganic compounds

B. Chemical synthesis

B. Crystal growth

D. Optical properties

A B S T R A C T

Niobium oxide fluoride/niobium diboride (Nb3O7F/NbB2) heterostructures with urchin-like and

nanowall-like morphologies were synthesized by a facile hydrothermal approach. The high-density

one-dimensional Nb3O7F nanoneedle arrays and two-dimensional Nb3O7F nanosheets stand on the

surface of NbB2 cores. Here a new idea is proposed to synthesize binary heterostructure by in situ

‘‘incomplete reaction’’. Ultraviolet–visible spectra showed that such heterostructure has a wide

absorption peak at around 270 nm and the absorption edge of the products synthesized at higher

temperature shifts to longer wavelength because of stronger nanometric effect.

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1. Introduction

Considerable efforts are made to synthesize binary, triple ormultiple hybrid materials with hierarchical architectures in thepast decade [1–3]. These materials could provide a greatopportunity to enhance their properties or to explore their novelproperties. Among them, niobium-containing compounds haveattracted much attention because of their outstanding chemicaland physical properties, which makes them promising candidatesin the fields of gas sensor, electron emitter, photoelectrochemicaldevice and catalyst [4–7]. However, conventional physicalmethods, such as chemical vapor deposition (CVD), pulsed laserdeposition, solid state reaction, milling–annealing, etc., areinvolved in expensive apparatus and complex process. Conven-tional chemical methods, such as coprecipitation, freeze drying,sol–gel, etc., are involved in weak combination and nonhomoge-neous distribution between each unit [8–14]. This inspires manyresearchers to develop simple methods to improve the combina-tion and the distribution.

Recently, several groups reported that the heterostructurecould markedly improve the distribution of binary hybrid systemand increase the active surface of effective composition in

* Corresponding author. Tel.: +86 27 87865484; fax: +86 27 87215421.

E-mail address: zyfu@whut.edu.cn (Z. Fu).

0025-5408/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2010.02.005

comparison to simple hybridization [15,16]. However, up to date,few reports were referred to the niobium-containing compoundheterostructure, as well as the investigation of their potentialproperties, such as electric property and catalytic property.Moreover, it is hard for traditional niobium-containing materialsto satisfy the demand of catalytic application due to hard control ofactive sites. Therefore, the niobium-containing heterostructuremay potentially improve the catalytic properties.

In this paper, we developed a simple method to synthesize anew class of Nb3O7F/NbB2 heterostructure in situ using niobiumdiboride as the raw materials. To the best of our knowledge, thereis no report about such system. Our facile synthesis provides adirect route for controlling the morphology of heterostructure anda potential opportunity for selecting the binary heterostructure.UV–vis spectra show that the as-synthesized Nb3O7F/NbB2

heterostructure may have potential application in the field ofphotocatalysis.

2. Experimental

NbB2 (99.5%) and hydrofluoric acid (HF, AR) were purchasedfrom Alfa Aesar Inc (US) and Sinopharm Chemical Reagent Co., Ltd(China), respectively. All the chemicals were used without furtherpurification. In a typical procedure, NbB2 (0.859 g) and HF (30%,2 g) were added into an aqueous solution (40 mL), respectively.Then the mixed solution was transferred into a 50 mL Teflon-lined

F. Huang et al. / Materials Research Bulletin 45 (2010) 739–743740

autoclave. Subsequently, the autoclave was sealed and maintainedat 80 8C, 120 8C and 160 8C for 24 h, respectively, followed bynatural cooling to room temperature. Afterwards, the productswere centrifugated and washed with deionized water andanhydrous ethanol for five times, respectively. The final productswere dried under vacuum at 80 8C for 12 h. In order to investigatethe influence of oxygen on the products, argon (Ar) gas was used asprotective atmosphere during the experiment.

The morphologies of the final samples were characterized byfield emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan). The composition of the products was analyzed by anenergy dispersive X-ray detector (EDX, Thermo Noran VANTAG-ESI, US), X-ray diffraction (XRD, Rigaku Ultima A, Japan) with CuKa irradiation, high-resolution transmission electron microscopy(HRTEM, JEOL 2100F, Japan) and X-ray photoelectron spectroscopy

Fig. 1. SEM images of the products with different morphologies synthesized at di

(XPS, Thermoelectron Corporation, VG Multilab 2000, US).Ultraviolet–visible (UV–vis) spectrum was recorded on a Lambda35 UV–vis spectrophotometer at room temperature (PerkinElmerInc., USA). For comparison, commercial NbB2 was also adopted forreference under the same experimental conditions. Beforemeasurement, BaSO4 was used as referenced datum mark with100% reflection.

3. Results and discussion

3.1. Morphology and structure

The SEM images of the samples in Fig. 1 illustrate the interestingmorphological evolution of the samples synthesized at differenttemperatures. It is clearly observed that the temperature has great

fferent temperatures: (a) and (b) 80 8C; (c) and (d) 120 8C; (e) and (f) 160 8C.

Fig. 2. TEM images of the samples under strong ultrasonic treatment for a long time: (a) shell part and (b) core part.

F. Huang et al. / Materials Research Bulletin 45 (2010) 739–743 741

effect on the morphology of the products. At 80 8C, the productkeeps the same morphology with NbB2 raw powders except somesmall particles on the surface (Fig. 1a and b). The surface is found tobe uniformly covered with oriented nanoneedles when thetemperature arrives at 120 8C (Fig. 1c and d). The nanoneedlesare 50–70 nm in diameter with a sharp tip of size typically in therange of 10–20 nm. Interestingly, further increasing the tempera-ture to 160 8C results in the formation of nanosheets (Fig. 1e and f).Clearly, one can obtain a desired morphology from nanoparticles,nanoneedles to nanosheets by simply controlling the temperature.Further experiments show that the samples synthesized at 120 8Cand 160 8C are actually core/shell structure under strong ultrasonicwave (Fig. 2a and b). That is, the nanoneedles and nanosheets growout from the core.

Fig. 3. (a) XRD patterns of the samples synthesized at different tempera

Fig. 4. (a) HRTEM image of the edge of an individual nanosheet

To determine the chemical composition and structure of theproducts, XRD, EDX, HRTEM and XPS were used. Fig. 3a presentsthe XRD pattern of the samples. It is revealed that all the samplescan be indexed as hexagonal NbB2 (JCPDS Card No. 75-1048) andorthorhombic Nb3O7F (JCPDS Card No. 74-2363). The diffractionpeaks at 2u = 22.68, 23.61, 25.78, 47.39, and 57.18, correspond to(0 0 1), (1 1 0), (6 0 0), (0 2 0) and (5 1 2) of Nb3O7F, while the otherdiffraction peaks at 2u = 27.02, 33.34, 43.50, 59.62, 66.54, 66.94,69.8, 76.48, and 86.31 correspond to (0 0 1), (1 0 0), (1 0 1), (1 1 0),(1 0 2), (1 1 1), (2 0 0), (2 0 1) and (1 1 2) of NbB2. That is, theproducts are actually a kind of hybrid material. Another feature ofthe reaction is that the main phase for all samples is NbB2 withminor Nb3O7F. Moreover, the peak broadens slightly with theincrease of temperature, which can be attributed to the stronger

tures and (b) EDX spectrum of the samples synthesized at 160 8C.

and (b) XP spectrum of the sample synthesized at 160 8C.

Fig. 5. The schematic illustration for the formation mechanism of Nb3O7F/NbB2 heterostructures.

Fig. 6. UV–vis absorption and reflection spectra of NbB2 materials and Nb3O7F/NbB2

heterostructures.

F. Huang et al. / Materials Research Bulletin 45 (2010) 739–743742

nanometric effect of thin nanosheets. EDX results indicate that thehybrid materials only contain B, O, F and Nb elements, which is inagreement with the results of XRD (Fig. 3b).

To further determine the detailed microstructure of theNb3O7F/NbB2 hybrid materials, the sample with nanowall struc-ture was characterized by HRTEM. As shown in Fig. 4a, the distancebetween the lattice planes perpendicular to nanosheet is 0.374 nm,which corresponds to (1 1 0) d-space of Nb3O7F (JCPDS Card No.74-2363). Further analysis indicates that the space distance alongthe nanosheets is 0.391 nm, which is corresponded to (0 0 1) d-space of Nb3O7F. The results could consist of two points as follows.Firstly, it indicates the projected periodic growth of the crystalstructure along the [0 0 1] direction. Secondly, it is reasonable tosuppose that the outer shell is Nb3O7F and the inner core is NbB2. Inanother word, the urchin-like and nanowall-like structures areactually a kind of heterostructure.

The above conclusion is also supported by the XP spectrum. Itcan be seen that Nb atom has two obvious binding energies of209 � 0.47 and 201 � 0.47 eV, corresponding to Nb3d3/2 and Nb3d5/2,respectively (Fig. 4b). That is, Nb atoms have the +2 and +5 oxidationstate. F1s and O1s are also observed at 686 � 0.47 and 532 � 0.47 eV,respectively. It should be noted that B1s valence state is absent in thespectrum. One possible explanation is that NbB2 is nearly embeddedby the Nb3O7F nanosheets. As a surface analytic apparatus, XPScannot detect the weak information of B element from inner NbB2,which further proves the result of HRTEM.

3.2. Formation mechanism

Judging from XRD, EDX, HRTEM, and XPS, it can be concludedthat the synthesized samples are actually a kind of Nb3O7F/NbB2

heterostructure. In order to form such heterostructure under hightemperature and high pressure, the whole process should be basedon ‘‘incomplete reaction’’. In other words, the whole reactionshould be limited to a slow reaction rate so that NbB2 cannot beconsumed completely during the process. Therefore, one possibleformation process for these heterostructures could be regarded asthe following classical theory: dissolution, nucleation, and growthprocess, as shown schematically in Fig. 5. Firstly, NbB2 with smallsize or the surface of NbB2 with large size are slowly oxidized toNb2O5 and B2O3 [Eq. (1)], then Nb2O5 and B2O3 can further reactwith HF to form soluble Nb3O7

+ ion and BF4� ions at a certain

temperature according to Eqs. (2) and (3) [17–20]. It should bestressed that the oxygen comes from the residual oxygen in theTeflon-lined container or a small amount dissolved oxygen in thesolution. Then the nuclei of Nb3O7F precipitates on the surface ofunreacted NbB2 core from homogeneous supersaturated solution[21,22]. Consequently, Nb3O7F grows out from the unreacted NbB2

core and formed such heterostructure. The morphology differencebetween different temperatures can be ascribed to the reactionrate and the concentration of Nb3O7

+ ion in the solution. It is wellknown that NbOx octahedron in niobium-containing oxides is

prone to form 1D and 2D oxides through a shared corner or ashared face [5,23]. The reaction rate and the concentration ofNb3O7

+ ion increase with the increase of temperature, whichresults in rapid supersaturation of Nb3O7

+ ion and precipitation.Consequently, enough Nb3O7F can fully grow along two-dimen-sion. More research work on the formation mechanism is currentlybeing performed in our lab.

4NbB2þ11O2¼ 2Nb2O5þ4B2O3 (1)

B2O3þ8HF ¼ 2HBF4þ3H2O (2)

3Nb2O5þ2HF ¼ 2Nb3O7F þ H2O (3)

F. Huang et al. / Materials Research Bulletin 45 (2010) 739–743 743

3.3. UV–vis spectra

The UV–vis absorption spectrum and reflection spectrum of thesamples synthesized at different temperatures are shown in Fig. 6.As anticipated, the structural evolution illustrated above isaccompanied by remarkable changes in the optical properties ofthe products. Compared to NbB2, there appears a wide absorptionand reflection band for Nb3O7F/NbB2 heterostructures from 200 to370 nm. Moreover, the absorption edge for nanowall sample is red-shifted by 18 nm compared to that of urchin structure, which couldbe attributed to stronger nanometric effect of thin nanosheets.Another feature for the spectra is that the absorption intensityincreases with the increase of temperature, which could becontributed to the higher content of Nb3O7F at higher temperature.

4. Conclusions

In summary, a new class of Nb3O7F/NbB2 heterostructure wassynthesized in situ by a facile hydrothermal method. The high-density Nb3O7F nanoneedles and Nb3O7F nanosheets stand on theunreacted NbB2 cores. The formation mechanism of the hetero-structure is based on ‘‘incomplete reaction’’. UV–vis spectrumshows that Nb3O7F/NbB2 heterostructure could be a candidate forphotocatalyst.

Acknowledgements

This work was financially supported by the National NatureScience Foundation of China (50772081, 50821140308) and theMinistry of Education of China (PCSIRT0644).

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