Substrate-free 2D tellurene

1. A method of preparing substrate-free 2D tellurene crystals, wherein the method comprises reacting sodium tellurite and hydrazine in an alkaline condition with the presence of polyvinylpyrrolidone (PVP) at a reaction temperature of 120-250° C., thus synthesizing substrate-free 2D tellurene crystals, wherein the 2D tellurene crystals are characterized by an X-ray diffraction pattern (CuKα radiation, λ=1.54056 A), wherein the X-ray diffraction pattern consists of five peaks from a range of 23.79 (29±0.1°) to 64.43 (29±0.1°) within an intensity range of 7.8% to 100%, and wherein the five peaks are ranked in an order of intensity from high to low by 23.79 (29±0.1°), 47.79 (29±0.1°), 41.26 (29±0.1°), 64.43 (29±0.1°), and 50.41 (29±0.1°).

2. The method of claim 1 , wherein a mole ratio of Na 2 TeO 3 /PVP is 10-100:1.

3. The method of claim 2 , wherein acetone is added to the as-synthesized 2D tellurene crystals to provide 2D tellurene crystals with a thickness of 0.5-10 nm.

4. The method of claim 2 , wherein NaOH is added to the as-synthesized 2D tellurene crystals to provide 2D tellurene crystals with a thickness of 0.5-10 nm.

5. The method of claim 1 , wherein the reaction time is at least 1 hours.

6. The method of claim 1 , wherein the 2D tellurene crystals are characterized by an X-ray diffraction pattern (CuKα radiation, λ=1.54056 A) comprising a peak at 23.79 (2θ±0.1°) and optionally one or more peaks selected from the group consisting of 41.26, 47.79, 50.41, and 64.43 (2θ±0.1°).

7. The method of claim 1 , wherein the alkaline condition has a pH>7.