General molten-salt route to three-dimensional porous transition metal nitrides as sensitive and stable Raman substrates

Transition metal nitrides have been widely studied due to their high electrical conductivity and excellent chemical stability. However, their preparation traditionally requires harsh conditions because of the ultrahigh activation energy barrier they need to cross in nucleation. Herein, we report three-dimensional porous VN, MoN, WN, and TiN with high surface area and porosity that are prepared by a general and mild molten-salt route. Trace water is found to be a key factor for the formation of these porous transition metal nitrides. The porous transition metal nitrides show hydrophobic surface and can adsorb a series of organic compounds with high capacity. Among them, the porous VN shows strong surface plasmon resonance, high conductivity, and a remarkable photothermal conversion efficiency. As a new type of corrosion- and radiation-resistant surface-enhanced Raman scattering substrate, the porous VN exhibits an ultrasensitive detection limit of 10−11 M for polychlorophenol.


Adsorption experiment
During the adsorption test, 50 mg of the 3D porous TMNs were added into the solvent to be adsorbed (100 mL) at room temperature and one atmosphere. After 10 min of full contact, the 3D porous TMNs were quickly separated by vacuum filtration.
Weight measurements should be made as soon as possible to avoid evaporation of organic liquids with low boiling points. The weight of 3D porous TMNs before and after adsorption was recorded, and the weight increment was calculated.

Photothermal Test
50 mg of 3D porous VN was dispersed well in 10 mL distilled water, under the assistance of ultrasonic bath. The dispersed mixture was deposited on the foam rubber membrane under vacuum. The formed VN/foam rubber film was placed on a heat plate and the temperature was kept at 70 o C for 12 min. Foam rubber was chosen as a bottom supporting layer because of its unique inner microporous structure and hydrophilicity. The microporous structure of the cellulose membrane enables efficient absorption of water through capillary effect. This effect enables more rapid replenishment of surface water after evaporation, while the hydrophilicity would benefit the water adhesion and speed up the water transfer upward. Under illumination of a solar simulator at power density of 2 KW m -2 , the light-thermal system can be quickly heated up in 10 s and generates visible steam flow on top of the water surface.

Hydrophobicity Measurement
The contact angle was measured using a contactangle measurement system (Contact Angle System OCA 20, Dataphysics). For the test of hydrophobic property, a water droplet was placed on the top by a syringe needle. The droplet keeps a round shape and merges into the structure. The picture of the formed droplet was taken by an optical microscope from the side view and was then inserted into an "Image Software". The contact angle was calculated by the Software for different substrates.

Enhanced Factor Calculation
To calculate the EF of the 3D porous VN samples, the ratio of SERS to normal Raman spectra (NRS) of R6G was determined by using the following calculating Å, in good agreement with the experimental result of 4.14 Å 6 .

Calculation of photothermal conversion efficiency
According to the Beer-Lambert Law, the mass extinction coefficient α of the 3D porous VN microparticles can be calculated using equation (1): (1) In equation (1), A refers to the absorbance of 3D porous VN microparticles at 532 nm, α is the mass extinction coefficient of 3D porous VN microparticles (L g -1 cm -1 ), L is the optical length of the quartz cuvette (cm), and C is the mass concentration (g L -1 ). According to Figure S20a, the calculated mass extinction coefficient of 3D porous VN is 0.91 L g -1 cm -1 .
The photothermal conversion efficiency (η) of the 3D porous VN microparticles was determined by equation (2) Following the previous reports, the value of hS can be calculated by equation (3)- (5).
The T, Tmax and Tsur are random temperature, the maximum temperature after irradiation and the surrounding temperature. In this work, we measured the temperature change at concentration of 1 mg/mL, the Tmax-Tsur = 64.6°C, and the s was calculated to be 378.64 s by equation (3) and (4)