An Efficient p-n Heterojunction Copper Tin Sulfide/g-C3N4 Nanocomposite for Methyl Orange Photodegradation

Abstract

The discharge of toxic dye effluents from industry is a major concern for environmental pollution and toxicity. These toxic dyes can be efficiently removed from waste streams using a photocatalysis process involving visible light. Due to its simple synthesis procedure, inexpensive precursor, and robust stability, graphitic carbon nitride (g-C₃N₄, or CN) has been used as a visible light responsive catalyst for the degradation of dyes with mediocre performance because it is limited by its low visible light harvesting capability due to its wide bandgap and fast carrier recombination rate. To overcome these limitations and enhance the performance of g-C₃N₄, it was coupled with a narrow bandgap copper tin sulfide (CTS) semiconductor to form a p-n heterojunction. CTS and g-C₃N₄ were selected due to their good stability, low toxicity, ease of synthesis, layered sheet/plate-like morphology, and relatively abundant precursors. Accordingly, a series of copper tin sulfide/graphitic carbon nitride nanocomposites (CTS/g-C₃N₄) with varying CTS contents were successfully synthesized via a simple two-step process involving thermal pyrolysis and coprecipitation for visible-light-induced photocatalytic degradation of methyl orange (MO) dye. The photocatalytic activity results showed that the 50%(wt/wt) CTS/g-C₃N₄ composite displayed a remarkable degradation efficiency of 95.6% for MO dye under visible light illumination for 120 min, which is higher than that of either pristine CTS or g-C₃N₄. The improved performance is attributed to the extended light absorption range (due to the optimized bandgap), effective suppression of photoinduced electron-hole recombination, and improved charge transfer that arose from the formation of a p-n heterojunction, as evidenced by electrochemical impedance spectroscopy (EIS), photocurrent, and photoluminescence results. Moreover, the results of the reusability study showed that the composite has excellent stability, indicating its potential for the degradation of MO and other toxic organic dyes from waste streams.

Figure 1

Schematics illustrating the synthesis of (a) CN, (b) CTS, and (c) CTS/CN composites.

Figure 2

(a) Powder XRD patterns of CN, CTS, and 50CTS/CN with standard diffraction patterns of petrukite phase orthorhombic CTS and rhombohedral CTS. (b) FT-IR spectra of pristine CN, CTS, and the CTS/CN nanocomposite.

Figure 3

SEM images of (a) pristine CTS, (b) pristine CN, (c) 50CTS/CN composite, (d) TEM image, (e–h) HRTEM image and IFFT profile, and (i–n) STEM-EDS elemental mapping images of the 50CTS/CN composite.

Figure 4

XPS spectra of 50CTS/CN composite: (a) survey, (b) Cu 2p, (c) Sn 3d, (d) S 2p, (e) N 1s, and (f) C 1s.

Figure 5

Ultraviolet–visible absorption spectra of (a) CN, (b) 50CTS/CN, and (c) CTS materials with insets of Tauc plots and the estimated bandgaps.

Figure 6

MS plots of (a) CTS, (b) CN, and (c) 50CTS/CN composite obtained from EIS measurements carried out at 100 Hz.

Figure 7

(a) PL spectra, (b) transient photocurrent response, and (c) EIS Nyquist plot of CN, CTS, and 50CTS/CN composite samples, respectively.

Figure 8

(a) Adsorption and photocatalytic degradation of MO over the synthesized materials under visible light irradiation; (b) pseudo-first-order kinetic fitting of the photodegradation of MO; (c) fitted pseudo-first-order kinetic constants for MO photodegradation; and (d) stability test of 50CTS/CN for MO photodegradation.

Figure 9

Schematics illustrating the possible charge transfer mechanism before and after contact for enhanced photocatalytic degradation of MO by a p–n heterojunction CTS/CN composite photocatalyst.