Interfacial Engineering\nover Pt–Calcium Ferrite/2D\nCarbon Nitride Nanosheet p–n Heterojunctions for Superior Photocatalytic\nProperties

The present study\ndiscloses the fabrication of efficient\np–n\nheterojunctions using n-type polymeric bulk carbon nitride (b-CN, <i>E</i><sub>g</sub> = 2.7 eV) or exfoliated nanosheets of carbon\nnitride (NSCN, <i>E</i><sub>g</sub> = 2.9 eV) with p-type\nspinel ferrite CaFe<sub>2</sub>O<sub>4</sub> (CFO, <i>E</i><sub>g</sub> = 1.9 eV) for photocatalytic hydrogen generation. A\nseries of p–n combinations were fabricated and characterized\nby various techniques. The oxide–carbon nitride interactions,\nlight absorption, band alignment at the interface, and water/H<sub>3</sub>O<sup>+</sup> adsorption capability were elucidated over heterojunctions\nand correlated with the photocatalytic hydrogen yield. The main developments\nin the present study are as follows: (1) All heterojunctions were\nmore active than pure phases. (2) The photocatalytic activity trend\nvalidated an increase in the lifetime of charge carriers from TRPL.\nPt(1 wt %)–CFO(1 wt %)/NSCN (481.5 μmol/h/g under ultraviolet\n(UV)–visible-simulated light, 147.5 μmol/h/g under CFL\nillumination for 20 h, τ<sub>avg</sub> = 10.33 ns) > Pt–NSCN\n> Pt–CFO/b-CN > CFO/NSCN > CFO/b-CN > NSCN >\nPt/b-CN > mechanical\nmixture (MM) of 1 wt �O + NSCN–MM > 1 wt �O + b-CN–MM\n> CFO > b-CN (τ<sub>avg</sub> = 4.5 ns). (3) Pt–CFO/NSCN\nwas most active and exhibited 250 times enhanced photocatalytic activity\nas compared to parent bulk carbon nitride, 6.5 times more active than\nCFO/NSCN, and twice more active than Pt–NSCN. Thus, enhanced\nactivity is attributed to the smooth channelizing of electrons across\np–n junctions. (4) NSCN evidently offered improved characteristics\nas a support and photocatalyst over b-CN. The exfoliated NSCN occupied\na superior few-layer morphology with 0.35 nm width as compared to\nparent b-CN. NSCN allowed 57% dispersion of 6 nm-sized CFO, while\nb-CN supported 14% dispersion of 7.8 nm-sized CFO particles, as revealed\nby small-angle X-ray scattering spectroscopy (SAXS). Sizes of 2–4\nnm were observed for Pt nanoparticles in the 1 wt %Pt/1 wt % CFO/NSCN\nsample. A binding energy shift and an increase in the FWHM of X-ray\nphotoelectron spectroscopy (XPS) core level peaks established charge\ntransfer and enhanced band bending on p–n contact in Pt–CFO/NSCN.\nFsTAS revealed the decay of photogenerated electrons via trapping\nin shallow traps (τ<sub>1,</sub> τ<sub>2</sub>) and deep\ntraps (τ<sub>3</sub>). Lifetimes τ<sub>1</sub> (3.19 ps,\n42%) and τ<sub>2</sub> (187 ps, 31%) were higher in NSCN than\nthose in b-CN (τ<sub>1</sub> = 2.2 ps, 42%, τ<sub>2</sub> = 30 ps, 31%), which verified that the recombination reaction rate\nwas suppressed by 6 times in NSCN (<i>k</i><sub>2</sub> =\n0.53 × 10<sup>10</sup> s<sup>–1</sup>) as compared to\nb-CN (<i>k</i><sub>2</sub> = 3.33 × 10<sup>10</sup> s<sup>–1</sup>). Deep traps lie below the H<sup>+</sup>/H<sub>2</sub> reduction potential; thus, electrons in deep traps are not\navailable for photocatalytic H<sub>2</sub> generation. (5) The role\nof CFO in enhancing water adsorption capability was modeled by molecular\ndynamics. NSCN or b-CN both showed very poor interaction with water\nmolecules; however, the CFO cluster adsorbed H<sub>3</sub>O<sup>+</sup> ions very strongly through the electrostatic interaction between\ncalcium and oxygen (of H<sub>3</sub>O<sup>+</sup>). Pt also showed\na strong affinity for H<sub>2</sub>O but not for H<sub>3</sub>O<sup>+</sup>. Thus, both CFO and Pt facilitated NSCN to access water molecules,\nand CFO further sustained the adsorption of H<sub>3</sub>O<sup>+</sup> molecules, crucial for the photocatalytic reduction of water molecules.\n(6) Band potentials of CFO and NSCN aligned suitably at the interface\nof CFO/NSCN, resulting in a type-II band structure. Valence band offset\n(VBO, Δ<i>E</i><sub>VB</sub>) and conduction band\noffset (CBO, Δ<i>E</i><sub>CB</sub>) were calculated\nat the interface, resulting in an effective band gap of 1.41 eV (2.9\n– Δ<i>E</i><sub>VB</sub> = 1.9 – Δ<i>E</i><sub>CB</sub>), much lower than parent compounds. The interfacial\nband structure was efficient in driving photogenerated electrons from\nthe CB of CFO to the CB of NSCN and holes from the VB of NSCN to the\nVB of CFO, thus successfully separating charge carriers, as supported\nby the increased lifetime of charge carriers and favorable photocatalytic\nH<sub>2</sub> yield.