Photochemically-assisted synthesis of layered birnessite (MnO

1. A method of forming birnessite δ-MnO 2 nanosheets, the method comprising oxidizing manganese (Mn 2+ ) in the presence of a source of nitrate and a light source, the light source comprising one or more of a UV lamp containing UV light above 300 nm, or a Xe lamp.

2. The method of claim 1 , further comprising:

irradiating an aqueous solution comprising Mn 2+ (aq) and a nitrate solution with the light source;

oxidizing the Mn 2+ (aq) to form Mn(IV) in the nitrate solution;

creating superoxide from photodecomposition of nitrate; and

generating disordered δ-MnO 2 nanosheets.

3. The method of claim 2 , the step of oxidizing Mn 2+ to Mn(IV) comprising oxidizing Mn 2+ to form Mn(III) and oxidizing the Mn(III) to form the Mn(IV).

4. The method of claim 2 , further comprising increasing a concentration of the nitrate solution to accelerate the formation of δ-MnO 2 nanosheets.

5. The method of claim 2 , wherein the concentration of the nitrate solution is at least 0.1 mM.

6. The method of claim 2 , wherein the formation of δ-MnO 2 nanosheets takes a time ranging from 0.5 hrs to 6 hrs.

7. The method of claim 2 , wherein the abiotic formation rate of the δ-MnO 2 nanosheets is comparable to the formation rate of δ-MnO 2 in microbial processes.

8. A method of oxidizing manganese (Mn 2+ ) to Mn(IV), the method comprising contacting Mn 2+ to a source of nitrate and a light source, the light source comprising one or more of a UV lamp containing UV light above 300 nm, or a Xe lamp.

9. The method of claim 8 , further comprising: preparing an aqueous solution comprising Mn 2+ (aq), nitrate, and pyrophosphate (PP) having a concentration of at least 0.3 mM; irradiating the aqueous solution using a light source; oxidizing the Mn 2+ (aq) to form Mn(IV); creating superoxide from photodecomposition of nitrate; and synthesizing disordered δ-MnO 2 nanosheets, wherein the stacking of δ-MnO 2 nanosheets is enhanced by changing the birnessite structure from hexagonal to orthogonal in the presence of PP.

10. The method of claim 9 , wherein the PP is up to 2 mM.

11. The method of claim 9 , wherein the proportion of Mn(III) in δ-MnO 2 nanosheets increases with the PP concentration.

12. The method of claim 9 , wherein when the PP concentration is between 1 mM and 2 mM, orthogonally structured δ-MnO 2 nanosheets are formed to have Mn(III) dominantly in layers with fewer vacant sites than that when the PP concentration is between 0.3 mM and 0.5 mM, and when the PP concentration is between 0.3 mM and 0.5 mM, hexagonal structured δ-MnO 2 nanosheets are formed to have Mn(III) dominantly in the interlayers with more vacant sites than that when the PP concentration is between 1 mM and 2 mM.

13. The method of claim 9 , the step of oxidizing Mn 2+ to form Mn(IV) comprising oxidizing Mn 2+ to form Mn(III) and oxidizing the Mn(III) to form the Mn(IV).

14. The method of claim 9 , wherein the formation of δ-MnO 2 nanosheets takes a time ranging from 0.5 hrs to 6 hrs.