**4.4 Phosphorous doping**

Kumar et al. [22] used solid thermal condensation of urea, citric acid, and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) in a reactor at 200°C to make phosphorus-doped carbon nitride quantum dots (CNPQDs). **Figure 4a** depicts the fabrication of carbon nitride-doped quantum dots (CNPQDs). By condensation polymerization, urea and citric acid produce a basic carbonitride skeleton, and

## **Figure 3.**

*The preparation of oxygen-doped g-C3N4 nanospheres. Reproduced with permission [18]. Copyright 2018, Royal Society of Chemistry.*

## *Graphite Carbon Nitride DOI: http://dx.doi.org/10.5772/intechopen.104976*

BMIM-PF6 serves as a phosphorus doping source in this synthesis. **Figure 4b** depicts the steady-state photoluminescence (PL) spectra of phosphorus-doped carbon nitride quantum dots (CNPQDs) and carbon nitride quantum dots (CNQDs) in water. 15 N and 31P are two of the most common elements found in nature. **Figure 4c** shows the CPMAS NMR spectra of CNPQDs. **Figures 4d** and **e** depict the process of (CNPQDs) alteration in a titanium dioxide array.
