Low-temperature Synthesis of TiB2 Nanocrystals Using TiO2, LiBH4 and Mg

SHEN Ye，LIU Binhong，LI Zhoupeng

(1.School of Materials Science & Engineering，Zhejiang University, Hangzhou 310027, China；2.School of Chemical & Biological Engineering, Zhejiang University，Hangzhou 310027, China)

【Abstract】 TiB2 is an ultra-high temperature ceramic(UHTC)material.It is usually synthesized at high temperatures through carbothermal or borothermal reactions, making it difficult to obtain nanosized crystals in a controllable way.In this work, we successfully synthesized and purified TiB2 nanocrystals of around 10-20 nm through a solid state reaction route using TiO2, LiBH4 and Mg.During the reaction, LiBH4 and Mg collaboratively reduced TiO2 and transformed it completely into TiB2 below 600 ℃.The growth of TiB2 nanocrystallites was fulfilled at a higher temperature of 800 ℃, resulting in TiB2 nanocrystals with a homogeneous size distribution.The present synthesis method ensured not only desirable nanosizes, but also well-crystallized structure for TiB2 nanoparticles in a facile and controllable way.

【Key words】 Titanium diboride；Nanocrytals；Titanium oxide；Lithium borohydride；Magnesium

1 Introduction

Titanium diboride TiB2is a well-known ultra-high temperature ceramics as it has a high melting point(3230 ℃), high hardness, as well as good thermal and electric conductivities[1].TiB2is a well known material for evaporation boats.It is also well-used as the electrode material for electrolytic production of aluminum due to its wettability by molten aluminium.Recently, new applications have been found for TiB2.For example, TiB2is a potential anode material for primary batteries due to its large discharge capacity of 2314 mAh·g-1in alkaline media[2-5].

TiB2is usually prepared at temperatures much higher than 1000 ℃ through carbothermal, borothermal or magnesiothermal reactions[6-9].In these synthesis reactions, titanium oxide and/or boron oxide are reduced by carbon, boron, B4C or Mg at high temperatures with significant exothermal effects that can generate self-propagating high-temperature synthesis reactions(SHS).The high temperature preparation methods for TiB2make it difficult to obtain nanocrystals and nanoparticles.Other synthesis approaches including chemical vapor deposition(CVD)and mechanochemical methods[10-20]are thus tried.For example, Ti(BH4)3was used as the precursor to synthesize TiB2film[11].Or metal chlorides such as TiCl4were used instead to react with NaBH4to form Ti(BH4)3intermediate and then synthesize TiB2nanocrystallites[12].Another attempt used TiO2, B2O3and Mg to synthesize nanosized TiB2by high-energy ball milling[13].Kimetal.[15]used TiCl3, LiBH4and LiH to synthesize TiB2through ball milling, producing TiB2nanocrystallites of 3-5 nm.On the other hand, nanometric diboride particles were reported to be prepared by pulverization of large particles through high-energy mechanical methods such as ball milling or attrition milling[21-22].However, a size reduction limit of submicron was found for these mechanical methods[23].Moreover, these pulverizations usually result in surface oxidation and contamination by milling media[24-25].

Although several attempts have been made to prepare nanometric TiB2as mentioned above, there is not yet a well-established synthesis method that can be able to control its size in a facile and reliable manner[1].On the other hand, nanometric TiB2is expected to demonstrate significantly enhanced properties, especially in electrochemical applications[3].

In a previous study[26], we have found that TiB2can be formed below 600 ℃ through the reaction between TiO2with LiBH4, in which LiBH4acted not only as a reducing agent but also an active boron source.However, the reduction was found to be incomplete and TiB2crystallites of a few nanometers were formed in a TiOxmatrix, resulting in difficulties in isolating and purifying TiB2nanocrystals.Thus in this study, we report our enhancements in achieving pure TiB2nanocrytals with high yields and a good control of size and morphology by using Mg as a co-reductant.

2 Experimental details

2.1 Materials

The chemicals and raw materials used in this study were commercially purchased as follows: TiO2(99.8%), LiBH4(95%), Mg(200 mesh, 99.9%).

2.2 Synthesis procedures of nanosized TiB2

The powders of TiO2, LiBH4and Mg were mixed at a molar ratio of 1∶2∶x(x=0, 1, 2)and ball milled at 400 r/min for 3 h in a planetary mill.The stainless steel vessel for ball milling was 100 ml and the weight ratio of ball to sample was 180∶1.The ball milled mixture was then introduced into a stainless steel reactor.The handling of the samples was undergone in a glove box filled with high purity argon gas.The sealed reactor was then set up onto a Sievert’s apparatus.The reactor was first evacuated and then heated from room temperature to 800 ℃ at a rate of 2 ℃·min-1, and finally held at 800 ℃ for several hours.The pressure in the system was recorded during the experiments.The reactor was then cooled down to room temperature and opened slightly to let the product expose to air slowly.The air-stable powder was then washed with a dilute HCl solution by controlling the pH between 3 and 7.After being rinsed by deionized water for 4-5 times, the powder was centrifugally separated and dried in a vacuum oven at 60 ℃.

2.3 Instrumental characterizations and analyses

X-ray diffraction(XRD)analysis was conducted on a X’Pert PRO using the CuKα radiation.In some cases, a special sample stand was used to protect the samples from air exposure during XRD measurements.X-ray photoelectron spectroscopy(XPS)analysis was performed on ESCALAB 250Xi system equipped with an Al Kα(1486.6 eV)X-ray source.High resolution transmission electron microscopy(HRTEM)observations were performed on Tecnai G2 F30 S-Twin.Differential Fourier transform infrared spectrometry(FTIR)analysis was carried out on Tensor 27.About 1mg of the sample was mixed with 100 mg KBr in a mortar in the glove box, and the mixture was then pressed into a pellet under 10 MPa.The pellet was introduced into the cell and the test was finished within 1min.Samples were examined within the wave number range of 400-4000 cm-1.

3 Results and Discussion

3.1 TiB2 synthesis and product characterizations

In a typical synthesis process, the powders of TiO2, LiBH4and Mg at a molar ratio of 1∶2∶2 were ball milled at 400 rpm for three hours, and the mixture was then transferred to a reactor in the glove box.The reactor was heated from room temperature to 800 ℃ and then held at 800 ℃ for several hours.Fig.1 reveals the XRD pattern of the product after being held at 800 ℃ for 9h.Only three phases were identified in the product: TiB2, MgO and Li2O.All three phases demonstrated well defined diffraction peaks, indicating that they are all in good crystalline states.Based on the XRD pattern of(001)peak and the Scherrer equation, TiB2was calculated to have a crystallite size of around 15 nm.MgO had a larger particle size by showing stronger and sharper peaks than TiB2.No apparent peaks from Ti oxides were detected, suggesting a complete transformation from TiO2to TiB2.The HRTEM images shown in Fig.2 demonstrated clearly the morphologies of TiB2nanoparticles.At first sight, these TiB2nanoparticles looked like nanorods.However, a close observation suggested that the rod-like appearance might be the side view of hexagonal nanoplates because the hexagonal plate is the most common morphology observed for TiB2crystals due to its hexagonal structure[11-12].A typical TiB2nanocrystal is shown in Fig.2(b), for which its(001)plane is parallel to the surface.The thickness of the nanoplate is of 13.3 nm and its diameter is of several tens nanometers.This size agrees well with the crystallite size derived from the XRD result.Moreover, this nanoparticle is in an impressively well-crystallized state by showing one single crystal.On the whole, these TiB2nanoparticles revealed a homogenous size distribution and a low degree of coalescence.

Fig.1 XRD patterns of(a)TiO2+2LiBH4+2Mg heated at 800 ℃ for 9 h;(b)the powder after being washed and dried

Fig.2 HRTEM images showing the morphologies of TiO2+2LiBH4+2Mg heated at 800 ℃ for 9 h

As the product only contained TiB2, MgO and Li2O, TiB2can be easily separated and purified through washing the powder product with a weak HCl solution.As can be seen in Fig.1(b), a well-defined pure TiB2pattern was observed for the sample after washing and rinsing, indicating that pure TiB2nanocrystals of around 16 nm were successfully separated.The slightly larger size after washing was mainly due to the loss of extremely fine particles during leaching.

The XPS analysis was also performed to examine the surface state of the washed sample.As can be seen in Fig.3, the purified TiB2sample revealed relatively strong Ti 2p peak.Three detected peaks at 454.0, 458.6 and 464.2 eV for Ti 2p, and two peaks at 187.0 and 191.6 eV for B 1s indicate a partially oxidized TiB2surface.But the equally strong peaks at 187.0 and 191.6 eV for B 1s suggest a low degree of boron oxidation after the acidic washing.Both the XRD and XPS results suggest that pure, crystalline and nanosized TiB2was obtained after washing and isolation.

Fig.3 XPS analysis results of the washed and dried sample

3.2 Elucidation of TiB2 synthesis process and reaction mechanism

3.2.1The role of Mg Based on results of the previous study[26], the gas emitted during heating the mixtures containing LiBH4was predominately H2gas.Thus the release of gaseous hydrogen during the heating process is presented in Fig.4.It can be seen that TiO2+2LiBH4mixture started to release hydrogen apparently at around 150 ℃, about 50 ℃ earlier than other two samples with Mg additions.The results shown in Fig.4 imply that the reaction route was most probably altered after Mg addition.As shown in Fig.5(a), the XRD analysis demonstrated that Li0.5TiO2was formed in ball milled TiO2+2LiBH4+Mg and TiO2+2LiBH4+2Mg.In contrast, no such new phases were detected after the extended ball milling of TiO2+2LiBH4for 16 hours.This result indicates that Mg promoted the reduction of Ti+4in TiO2to Ti+3.5in Li0.5TiO2even during the ball milling.The formation of Li0.5TiO2indicates that some LiBH4decomposed during ball milling.However, the chemical state of boron from this part of LiBH4remained unknown mainly because it was present in amorphous states.Fig.5(b)compares the XRD patterns of three samples heated to 800 ℃.For TiO2+2LiBH4, the final product contained some LiTiO2other than TiB2, confirming that some Ti oxides were not yet fully transformed to TiB2.Though there were no apparent peaks detected from TiOxin the TiO2+2LiBH4+Mg sample, the TiB2peaks were found to be weak and broad, indicating that TiB2was in a size of a few nanometers.It can be seen in Fig.5(b)that a complete transformation to TiB2with a good crystallinity was realized only in TiO2+2LiBH4+2Mg.It is thus concluded that Mg acted as a co-reductant and assisted LiBH4in the reduction of TiO2.The combination of Mg and LiBH4could then reduce TiO2completely and achieved its full transformation into TiB2.

Fig.4 Comparison of hydrogen release behaviors during heating for the TiO2+2LiBH4+xMg(x=0, 1, 2)samples

Fig.5 XRD patterns for the TiO2+2LiBH4+xMg(x=0, 1, 2)samples(a)after being ball milled at 400 rpm;(b)after being heated at 800 ℃ for 9 h

According to the detected final products, it is reasonable to suppose that the synthetic reaction of TiB2was as follows:

TiO2+2LiBH4+Mg→TiB2+Li2O+MgO+4H2

(1)

According to the above reaction，the stoichiometric molar ratio of TiO2∶LiBH4∶Mg=1∶2∶1 should be enough for TiB2fabrication.However, it was found experimentally that adding some excess amounts of Mg would be more secure to achieve a desirable quality of TiB2nanocrystals.It is supposed that the presence of excess Mg can not only protect TiB2from being re-oxidized at 800 ℃, but also favor for TiB2crystal growth as a small amount of excess Mg in a liquid state may promote the diffusion in TiB2.

3.2.2Effects of synthesis temperature and holding time To further elucidate the reaction mechanism of the TiB2synthesis, the reaction products heated to different temperatures were examined by XRD and FTIR.Fig.6 demonstrates the XRD patterns and FTIR spectra of TiO2+2LiBH4+2Mg mixtures ball milled and heated to different temperatures.As described above, a new phase Li0.5TiO2was detected in the mixture right after ball milling.Also in the FTIR spectra of the ball milled sample, the characteristic peaks of LiBH4at 1120 cm-1due to B-H bending and at 2216.1, 2283.6 and 2351.1 cm-1due to B-H stretching can be apparently observed.On the other hand, Ti-O at 621 cm-1can also be observed.No MgO was detected both in the XRD pattern and FTIR spectra.When the temperature was raised up to 400 ℃, new intermediate phases, which seemed like LixTiOyphases, were detected in the XRD pattern.Metallic Mg was also detected in the mixture.At this temperature, the peaks from B-H stretching mode in FTIR shifted to 2326.0, 2356.9, and 2420.5 cm-1, corresponding to a borohydride intermediate Li2B12H12.When the temperature was further increased to 600 ℃, peaks from TiB2appeared in the XRD pattern.Also MgO was detected both in the XRD pattern and FTIR spectra.Meanwhile the peaks from B-H disappeared in the FTIR spectra.It is thus concluded that TiB2was formed within the temperature range of 400 ℃ and 600 ℃, during which LixTiOywas reduced by Mg and borohydride intermediates to form TiB2, MgO and Li2O.When the temperature was further increased from 600 to 800 ℃, it can be seen in Fig.6 that there were no apparent changes both in XRD patterns and FTIR spectra, suggesting that the reaction was already completed at 600 ℃.Further heating only resulted in the growth of nanocrystallites and particles.As shown in Fig.7, the crystallite size of TiB2increased with the holding time at 800 ℃.After holding at 800 ℃ for 9 h, TiB2crystallites finally grew to around 15 nm.

Fig.6 XRD patterns and FTIR spectra for the TiO2+2LiBH4+2Mg sample after being heated to different temperatures(a)XRD patterns;(b)FTIR spectra

Fig.7 XRD patterns for the TiO2+2LiBH4+2Mg(x=0, 1, 2)sample after being holding at 800 ℃ for different periods

The above reaction mechanism revealed that the formation of TiB2nanocrystals can be divided into two stages.At the first stage below 600 ℃, small TiB2crystallites of less than 10 nm were formed.Then at the second stage beyond 600 ℃, the growth of TiB2crystallites and particles took place, and TiB2became well crystallized at this stage.The growth of TiB2crystallites to a size larger than 10 nm is found to be important for its separation and purification because the crystallites smaller than 10 nm would be very sensitive to air and aqueous solutions.

4 Conclusion

In this work, we successfully synthesized and separated TiB2nanoparticles of around 10-20 nm through a solid state reaction route using TiO2, LiBH4and Mg.In this reaction, Mg as a co-reductant assisted LiBH4to reduce TiO2and transform it completely into TiB2below 600 ℃.Then at a higher temperature of 800 ℃, TiB2crystallites grew to a desirable size and achieved a good crystallinity.Finally, pure TiB2nanocrystals were separated from other metal oxide products through washing by a dilute HCl solution.The final size of TiB2nanocrystals can be easily tuned through adjusting the annealing temperature and period, making the approach a reliable, controllable and facile method for synthesizing nanometric transition metal diborides.