Highly Visible Light Responsive, Narrow Band gap TiO2 Nanoparticles Modified by Elemental Red Phosphorus for Photocatalysis and Photoelectrochemical Applications

Abstract

This paper reports that the introduction of elemental red phosphorus (RP) into TiO2 can shift the light absorption ability from the UV to the visible region, and confirmed that the optimal RP loading and milling time can effectively improve the visible light driven-photocatalytic activity of TiO2. The resulting RP-TiO2 nanohybrids were characterized systematically by a range of techniques and the photocatalytic ability of the RP-TiO2 photocatalysts was assessed further by the photodegradation of a model Rhodamine B pollutant under visible light irradiation. The results suggest that the RP-TiO2 has superior photodegradation ability for model contaminant decomposition compared to other well-known photocatalysts, such as TiO2 and other reference materials. Furthermore, as a photoelectrode, electrochemical impedance spectroscopy, differential pulse voltammetry, and linear scan voltammetry were also performed in the dark and under visible light irradiation. These photoelectrochemical performances of RP-TiO2 under visible light irradiation revealed more efficient photoexcited electron-hole separation and rapid charge transfer than under the dark condition, and thus improved photocatalytic activity. These findings show that the use of earth abundant and inexpensive red phosphorus instead of expensive plasmonic metals for inducing visible light responsive characteristics in TiO2 is an effective strategy for the efficient energy conversion of visible light.

Figure 1

Proposed schematic diagram of the high energy ball milling synthesis mechanism for the fabrication of RP-TiO₂ nanohybrids.

Figure 2

XRD patterns of the pure TiO₂ (P-TiO₂), commercially available pure red phosphorus (P-RP), TiO₂ with 10 wt. % of RP (RP-TiO₂-1), TiO₂ with 20 wt. % of RP (RP-TiO₂-2), TiO₂ with 50 wt. % RP milled for 6 h (RP-TiO₂-6 h), TiO₂ with 50 wt. % RP milled for 12 h (RP-TiO₂-12 h), TiO₂ with 50 wt. % RP milled for 24 h (RP-TiO₂-24 h), and TiO₂ with 50 wt. % RP prepared by hand grinding (RP-TiO₂-mix). The inset shows the shifting and broadening of the peak.

Figure 3

(a) TEM image and inset shows the SAED pattern, (b) HR-TEM image and inset shows the lattice fringe, (c–f) scanning transmission electron microscopy elemental mapping, and (g) EDX of the RP-TiO₂-12 h nanohybrid.

Figure 4

(a) Comparative UV-visible diffuse absorbance spectra of the P-RP, P-TiO₂ and RP-TiO₂-12 h, (b,c) plots of (αhν)^1/2 vs. the energy of absorbed light of P-TiO₂ and RP-TiO₂-12 h.

Figure 5

XPS valence band spectra of (a) P-TiO₂, (b) RP-TiO₂-12 h, and (c) schematic diagram of the DOS of RP-TiO₂-12 h and P-TiO₂.

Figure 6

(a) Ti 2p photoelectron peak of P-TiO₂ and RP-TiO₂-12 h, (b) P 2p photoelectron peak of RP-TiO₂-12 h, (c) O1 s photoelectron peak of P-TiO₂, and (d) O 1 s photoelectron peak of RP-TiO₂-12.

Figure 7

(a) Photodegradation kinetic plot of RhB as a function of the illumination time over P-TiO₂ and RP-TiO₂-12 h nanohybrid photocatalyst, (b) proposed schematic diagram of the photoexcited electrons-holes generation, separation, and their transport process over the RP-TiO2-12 h photocatalyst interface under visible light illumination, and (c) Nyquist plots.

Figure 8

(a) Linear scan voltammograms and (b) DPV voltammograms obtained for P-TiO₂ and RP-TiO₂-12 h nanohybrid photoelectrodes in the dark and under visible light illumination.