Efficient photocatalytic reduction of chromium (VI) using photoreduced graphene oxide as photocatalyst under visible light irradiation

doi:10.1016/j.jmst.2021.02.051

Abstract

Abstract:

Graphene oxide (GO), a new and promising material, has been widely used as a co-catalyst in photocatalytic reactions; however, its capacity as a sole photocatalyst has rarely been investigated. In this study, ultraviolet (UV) light irradiation was used as a modification method to obtain reduced GO (rGO) samples. The samples were used as photocatalysts to examine their visible light photocatalytic activity toward hexavalent chromium (Cr(VI)) removal. Atomic force microscopy (AFM), X-ray diffraction (XRD), UV-vis spectrophotometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR) spectroscopy were applied to interpret the surface and structure changes with UV irradiation. The oxygen-containing functional groups (OFGs) on the GO surface were reduced to defective carbons and π-conjugated C=C (sp2 domains) under UV light; this led to a decrease in the interlayer distance between GO sheets, GO fragmentation, and increased disorder on the GO surface. The restoration of sp2 domains led to a narrower band gap of GO, which favored the rGO excitation by visible light to generate electron-hole pairs. The rGO pre-irradiated with UV for 1h (rGO-1), possessing the highest defect density and electron generation efficiency, exhibited the best Cr(VI) reduction efficiency, which was about three times that of the GO sample; moreover, it outperformed most of the reported GO-based nanomaterials. In addition, low pH and the addition of citric acid as a hole scavenger could further improve the photocatalytic activity. This study proves that GO or rGO can be used as a sole photocatalyst under visible light to remove environmental pollutants such as heavy-metal ions, and it paves the way for the development of this kind of material and its UV-irradiation modification for further applications.

Key words: Graphene oxide, Chromium (VI), Defect density, UV-irradiation modification, Photocatalytic reduction

Mei Yu, Jing Shang, Yu Kuang. Efficient photocatalytic reduction of chromium (VI) using photoreduced graphene oxide as photocatalyst under visible light irradiation[J]. J. Mater. Sci. Technol., 2021, 91: 17-27.

Figures/Tables 11

Fig. 1. Experimental setup for (a) preparation of rGO-X suspension samples and (b) photoreduction of Cr(VI).

Fig. 2. Photos of (a) GO, (b) rGO-1, (c) rGO-3, (d) rGO-6, (e) rGO-9, and (f) rGO-12. AFM images of (g) GO, (h) rGO-1, (i) rGO-3, (j) rGO-6, (k) rGO-9, and (l) rGO-12.

Fig. 3. (a) UV-vis spectra (normalized) and (b) Tauc plots of GO and rGO-X; the inset is the corresponding band gaps.

Fig. 4. (a) Oxygen/carbon ratio (RO/C) of GO and rGO-X samples with reduction time. (b) Peak deconvolution of XPS C1s spectra of GO and rGO-12. (c) Atomic percentage of carbon compositions of GO and rGO-X samples with reduction time.

Fig. 5. (a) Raman spectra and (b) ID/IG ratio of GO and rGO-X with reduction time.

Fig. 6. (a) ESR spectra of GO and rGO-sX. (b) Deconvolution of the ESR spectra of rGO-12 and GO. (c) Total PFR contents of GO and rGO-X.

Table 1 PFR content in ×1013 spins (mg mL-1)-1 of GO and rGO-X (X = 0.25, 1, 2, 3, 6, 9, 12).

Sample	GO	rGO-0.25	rGO-1	rGO-2	rGO-3	rGO-6	rGO-9	rGO-12
PFR₁	0.353	1.534	2.716	3.716	5.398	7.015	6.558	6.151
PFR₂	0	0	0	0	0	0	3.192	4.008

Fig. 7. (a) ESR spectra of TEMPO for rGO-1 suspension irradiated for 0, 10, and 15 min under visible light. (b) Trend of the slope of the decay curve of TEMPO content in GO and different rGO-X samples with reduction time.

Fig. 8. (a) Photocatalytic reduction of aqueous Cr(VI) by GO/rGO-X and TiO2 at pH 2. (b) The rate constant k of GO and rGO-X at pH 2. (c) Recycling performance of rGO-1 with adding CA at pH 2. (d) The rate constant k of rGO-1 at different pH. (e) Reduction kinetics of Cr(Ⅵ) in the presence or absence of hole trapping agent (CA) at pH 2. (f) Influences of initial concentration of Cr(VI) solution on the Cr(VI) removal rates by rGO-1 at pH 2.

Table 2 . Comparison of removal efficiency of photocatalytic reduction of Cr(VI) using GO-based materials either as co-catalyst or catalyst.

Systems	pH	[Cr(Ⅵ)](mg L^-1)	Catalyst dosage(mg)	The Role of GO/rGO	Removal Efficiency (L^-1 min^-1)	Light Source	Ref.
UiO-66(NH₂) -rGO2%	2	10	20	co-catalyst	5.0	Xe lamp (λ ≥ 420 nm).	[3]
ZnO /rGO-1%		10	60	co-catalyst	2.7	500 W high pressure Hg lamp)365nm(	[6]
CQDs-TiO_2-_x /rGO	5.38	10	100	co-catalyst	1.0	300 W Xe lamp (λ > 420 nm(	[9]
Al₄SiC₄ /rGO-2.5%	3	10	70	co-catalyst	2.3	300 W Xe lamp (λ > 400 nm)	[49]
TiO₂ /rGO-5%	3	10	100	co-catalyst	0.5	300 W Xe lamp )λ> 400 nm(	[50]
CdS /rGO-1.5%		10	100	co-catalyst	0.4	Visible Light	[51]
ZnO S₁ /rGO -5%	6.8	10	30	co-catalyst	4.1	UV light produced by 300 W Xe lamp	[52]
M₅₃-rGO	4	20	40	co-catalyst	6.3	300 W Xe lamp (420 nm≤λ ≤ 760 nm(	[53]
3D CuS /rGO aerogel	3	20	80	co-catalyst	4.1	Xe lamp )λ >420 nm(	[54]
GO	3	9.36	80	catalyst	3.3	1500 W Xe lamp )λ >400 nm(	[21]
rGO-1	2	10	10	catalyst	6.4	500 W Xe lamp (420 nm≤λ ≤ 760 nm(	Our work

Fig. 9. Structures of (a) GO and (b) rGO. (c) Schematic diagram of the photocatalytic reduction of Cr(VI) on rGO samples.

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