Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

TiO2 photocatalysis and related surface phenomena

[Liu, Chang; Li, Feng; Ma, Lai-Peng; Cheng, Hui-Ming] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China.;Cheng, HM (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;cheng@imr.ac.cn

Advanced Materials for Energy Storage

Metal hydride materials for solid hydrogen storage: a review

Hexagonal boron nitride (h-BN) is a layered material with a graphite-like structure in which planar networks of BN hexagons are regularly stacked. As the structural analogue of a carbon nanotube (CNT), a BN nanotube (BNNT) was first predicted in 1994; since then, it has become one of the most intriguing non-carbon nanotubes. Compared with metallic or semiconducting CNTs, a BNNT is an electrical insulator with a band gap of ca. 5 eV, basically independent of tube geometry. In addition, BNNTs possess a high chemical stability, excellent mechanical properties, and high thermal conductivity. The same advantages are likely applicable to a graphene analogue-a monatomic layer of a hexagonal BN. Such unique properties make BN nanotubes and nanosheets a promising nanomaterial in a variety of potential fields such as optoelectronic nanodevices, functional composites, hydrogen accumulators, electrically insulating substrates perfectly matching the CNT, and graphene lattices. This review gives an introduction to the rich BN nanotube/nanosheet field, including the latest achievements in the synthesis, structural analyses, and property evaluations, and presents the purpose and significance of this direction in the light of the general nanotube/nanosheet developments.

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Boron Nitride Nanotubes and Nanosheets

The pursuit of a clean and healthy environment has stimulated much effort in the development of technologies for the utilization of hydrogen-based energy. A critical issue is the need for practical systems for hydrogen storage, a problem that remains unresolved after several decades of exploration. In this context, the possibility of storing hydrogen in advanced carbon materials has generated considerable interest. But confirmation and a mechanistic understanding of the hydrogen-storage capabilities of these materials still require much work1,2,3,4,5. Our previously published work on hydrogen uptake by alkali-doped carbon nanotubes cannot be reproduced by others6,7,8. It was realized by us and also demonstrated by Pinkerton et al.8 that most of the weight gain was due to moisture, which the alkali oxide picked up from the atmosphere. Here we describe a different material system, lithium nitride, which shows potential as a hydrogen storage medium. Lithium nitride is usually employed as an electrode, or as a starting material for the synthesis of binary or ternary nitrides9,10. Using a variety of techniques, we demonstrate that this compound can also reversibly take up large amounts of hydrogen. Although the temperature required to release the hydrogen at usable pressures is too high for practical application of the present material, we suggest that more investigations are needed, as the metal–N–H system could prove to be a promising route to reversible hydrogen storage.

Interaction of hydrogen with metal nitrides and imides

Pure lithium amide (LiNH2) decomposes to lithium imide and ammonia at temperatures above 300 °C. Lithium hydride, on the other hand, liberates hydrogen at temperatures above 550 °C. By thoroughly mixing these two substances and conducting temperature-programmed desorption (TPD), we noticed that hydrogen was produced at temperatures around 150 °C. Combined thermogravimetric (TG), X-ray diffraction (XRD), and infrared (IR) analysis revealed that lithium amide would react with lithium hydride and convert to hydrogen and lithium imide (or Li-rich imide). The reaction mechanism was investigated by isotopic exchange.

Interaction between Lithium Amide and Lithium Hydride

TiO(2) nanotubes can reproducibly store up to approximately 2 wt % H(2) at room temperature and 6 MPa. However, only about 75% of this stored hydrogen can be released when the hydrogen pressure is lowered to ambient conditions, suggesting that both physisorption and chemisorption are responsible for the hydrogen uptake. FTIR spectroscopy, temperature-programmed desorption (TPD), and pressure-composition (P-C) isotherms suggest that 75% of the H(2) is physisorbed and can be reversibly released upon pressure reduction. Approximately 13% is weakly chemisorbed and can be released at 70 degrees C as H(2), and approximately 12% is bonded to oxide ions and released only at temperatures above 120 degrees C as H(2)O.

Room-Temperature Hydrogen Uptake by TiO2 Nanotubes

Synthesis of boron nitride nanotubes and its hydrogen uptake

To seek new potential materials for hydrogen storage, an arc-discharge method was employed to prepare nanosized nickel(or cobalt)/graphite composites, in which the nickel (or cobalt) particles were highly dispersed in a carbon matrix with particle size between 20 and 70 nm (or 5-20 nm). Quantitative TPD measurements showed that at about 500 °C and 30-50 atm these nanosized composites could uptake up to 2.8 wt % H 2 , which can be released at 500 °C and 1 atm. The addition of Ni (or Co) in C was found to largely enhance the H? adsorption, with the optimal amount of Ni being 20 wt %. In-situ FTIR showed that hydrogen was dissociatively adsorbed only in the presence of a transition metal and bonded to carbon atoms forming C-H bond. The hydrogen adsorption/desorption could be recycled. However, the capacity decreased to 1.6 wt % after 5 cycles. TEM, XPS, and BET surface-area and pore-volume measurements revealed that some of the transition metal particles migrated out from the carbon matrix and agglomerated after the H 2 adsorption/ desorption cycles, which may reduce the transition metal-carbon synergism and thus the H 2 storage capacity. Under low temperatures below -120 °C and moderate pressures above 6 atm hydrogen storage by these Ni(or Co)/C composites could be detected. Storage capacity up to 2.7 wt % for Ni/C was measured by PCI at 77 K and 70 atm.

Jizhong Luo

Papers

Interaction of hydrogen with metal nitrides and imides

Interaction between Lithium Amide and Lithium Hydride

Room-Temperature Hydrogen Uptake by TiO2 Nanotubes

Synthesis of boron nitride nanotubes and its hydrogen uptake

Nanosized Nickel(or Cobalt)/Graphite Composites for Hydrogen Storage