Bulk synthesis of long nanotubes of transition metal chalcogenides
Nanotubes of transition metal chalcogenides as long as 0.2-20 microns or more, perfect in shape and of high crystallinity, are synthesized from a transition metal material, e.g. the transition metal itself or a substance comprising a transition metal such as an oxide, water vapor and a H 2 X gas or H 2 gas and X vapor, wherein X is S, Se or Te, by a two-step or three-step method including first producing nanoparticles of the transition metal as long as 0.3 microns, and then annealing in a mild reducing atmosphere of the aforementioned gas or gas mixture. The transition metal chalcogenide is preferably WS 2 or WSe 2 . Tips for scanning probe microscopy can be prepared from said long transition metal chalcogenide nanotubes.
1. A method for bulk synthesis of long nanotubes of a transition metal chalcogenide from a transition metal material, water vapor and H 2 X gas or H 2 gas and X vapor, wherein X is S, Se or Te, comprising:
a) either heating a transition metal material in the presence of water vapor in a vacuum apparatus or evaporating a transition metal material in the presence of water vapor by an electron beam, at a suitable pressure, to obtain nanoparticles of the transition metal oxide;
b) elongating the transition metal oxide nanoparticles of step (a); and
c) annealing the transition metal oxide nanoparticles obtained in step (b) in a reducing atmosphere with a H 2 X gas or H 2 gas and X vapor, wherein X is S, Se or Te, at a suitable temperature, thus obtaining long nanotubes of the transition metal chalcogenide.
2. A method according to claim 1 , wherein the elongation of the transition metal oxide nanoparticles is carried out by heating the oxide under reducing conditions for a few minutes, or by electron beam irradiation of the oxide in high vacuum conditions.
3. A method according to claim 2 , wherein said reducing conditions consist in heating the oxide nanoparticles under the flow of a gas comprising from 0.05 to 1% H 2 and from 99.95 to 99% N 2 at a flow rate of 110 ml per minute for up to 10 minutes.
4. A method according to claim 1 for the synthesis of long nanotubes of a mixture of transition metal chalcogenides, wherein the annealing step is carried out by alternating the annealing atmosphere of H 2 X gas or of H 2 gas and X vapor.
5. A method according to claim 4 , wherein the chalcogenides are sulfides and selenides and the annealing step is carried out by alternating the annealing atmosphere of H 2 S and H 2 Se gas or of H 2 gas and S and Se vapor.
6. A method according to claim 1 , wherein the reducing atmosphere for annealing the oxide nanoparticles includes chalcogenation, under the flow of a gas comprising 1% H 2 and 99% N 2 at a flow rate of about 110 ml/min, and H 2 X at a flow rate of about 1 ml/min.
7. The method according to claim 1 , wherein said transition metal is selected from the group consisting of Mo, W, V, Zr, Hf, Pt, Re, Nb, Ta, Ti, and Ru.
8. The method according to claim 1 , wherein the nanotubes obtained are 0.2-20 micron long.
9. The method according to claim 1 , for bulk synthesis of long nanotubes of WS 2 and/or WSe 2 which comprises:
a) either heating W in the presence of water vapor in a vacuum apparatus or evaporating W or WO 3 in the presence of water vapor by an electron beam, at a pressure of 1-20 Torr, thus obtaining WO 3 nanoparticles having a length of up to 0.3 microns;
b) heating the WO 3 nanoparticles having a length of up to 0.3 microns under reducing conditions at 800-850° C., for about 10 minutes to obtain WO 3 nanowhiskers having a length of up to 10 microns; and
c) annealing the WO 3 nanoparticles obtained in step (b) in a reducing atmosphere with H 2 S or H 2 Se gas or with H 2 gas and S or Se vapor, or by alternating the annealing atmosphere with H 2 S and H 2 Se or with H 2 gas and S or Se vapor, at 800-850° C., thus obtaining relatively long and hollow WS 2 and/or WSe 2 nanotubes having a length from 10 microns to more than 20 microns.
10. A method according to claim 9 , wherein the pressure in step a) is between 8 and 12 Torr.
11. A method according to claim 9 , wherein in step b) the temperature is between 835 and 840° C.
12. A method according to claim 9 , wherein in step c) the temperature is between 835 and 840° C.
13. A method according to claim 6 , wherein said chalcogenation is sulfidization or selenization.
14. A method according to claim 6 , wherein said H 2 X is H 2 S or H 2 Se.
15. A method according to claim 13 , wherein said H 2 X is H 2 S or H 2 Se.