CdS/ZnO/GO
Scheme 5.1 Scheme 5.1 Schematic illustration of the charge separation and the transfer of photo-induced charge carriers in CdS/ZnO/GO system under visible light irradiation; the photo-excited electrons
transfer from the CB of CdS to CB of ZnO and from CB of ZnO to the carbon atoms on the graphene oxide sheets, which are easily, react with H+ to produce H2
Under visible light irradiation, the electrons (e-) get excited from the valance band (VB) to the CB of CdS; nevertheless, no charge formation takes place in the CB of ZnO due to its inactivity under visible light. The CB of CdS (−0.88 eV) is in close proximity with the CB of ZnO (−0.22 eV) which facilitates efficient transfer of reductive electrons
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form the CB of CdS to the CB of ZnO. The reductive electrons in ZnO CB further transfers to the GO because of its high charge carrier mobility, thereby leading the mobilization of these reductive electrons onto the graphene oxide sheet which reduces the recombination rate of photoinduced electrons and holes. Additionally, the unique feature of graphene oxide permits the photocatalytic reaction to take place not only on the surface of semiconductor photocatalyst, but also on the graphene oxide sheet causing enhanced reaction space. Eventually, these reductive electrons react with the adsorbed H+ ions to produce H2. Meanwhile, the remaining holes are scavenged by the sacrificial reagents (mixture of Na2S and Na2SO3) present in the solution.
The apparent quantum yield (AQY) for hydrogen generation using CdS/ZnO/GO, CdS/Al2O3/GO and CdS NPs was calculated to be around 30%, 14% and 4%. The AQY for CdS/ZnO/GO was almost two times higher than that of hierarchical CdS@ZnO reported in chapter 3A section 3A.3.6. Enhanced hydrogen production rate for CdS/ZnO/GO is mainly attributed to effective charge separation and extended photo- response in visible region due to the presence of GO sheet. The advantage of CdS/Metal Oxide/GO photocatalyst is the enhanced hydrogen production rate without addition of any noble metal co-catalyst such as Pt, Pd, Ru, Rh etc, which makes photocatalytic process inexpensive. Co-catalyst has low band positions due to which the addition of any co- catalyst assists effectual charge separation of electron and holes thereby making the photocatalytic process more efficient. Based on this conviction, CdS/Oxide/GO has been synthesized which has been proven as an efficient photocatalyst by providing large surface area for effective charge transfer and less recombination rate of photoinduced charge carriers due to the presence of GO sheet.
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5.3.5. Photocatalytic Dye Degradation
Along with hydrogen production, CdS/Metal Oxide/GO exhibited excellent photocatalytic activity for the degradation of organic dye. The dye degradation experiments were performed using CdS NPs, GO and CdS/Metal Oxide/GO photocatalysts with methyl orange as a reference dye. The degradation of dye with these photocatalysts was studied by measuring the concentration of the dye after irradiation with visible light for varying time periods. The concentrations of dye were measured by monitoring the absorption peak at 464 nm. The absorption spectra of dye with CdS/Al2O3 and CdS/ZnO in presence of GO at different time interval are shown in Figure 5.10 whereas with CdS NPs and GO are shown in Figure 5.9. The concentration of un-degraded dye relative to its initial concentration (C/C0) after visible light irradiation in presence of GO, CdS NPs and CdS/Metal Oxide/GO at different time intervals is shown in Figure 5.11.
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Wavelength (nm)
(a) (b)
Figure 5.9 UV-Vis absorption spectra of methyl orange with (a) CdS nanoparticles and (b) graphene oxide in 60 min interval
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0.00 0.03 0.06 0.09 0.12 0.15 0.18
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(c) (d)
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Figure 5.10 UV-Vis absorption spectra of methyl orange with (a) CdS/Al2O3/GO and (b) CdS/ZnO/GO in 60 min interval while (c) CdS/Al2O3/GO and (d) CdS/ZnO/GO in 10 min interval
From Figure 5.11, highest degradation of dye was achieved for CdS/ZnO/GO compared to CdS/Al2O3/GO, CdS NPs and GO. Photodegradation of the dye follows a first order decay kinetics with an apparent first order linear transform ln(C/C0) = −kappt,18 as shown in Figure 5.11, where C0 and C represent the concentration of dye before and after the irradiation respectively, kapp is the apparent first-order reaction rate constant representing the reaction rate and calculated from the slope of the plot (Figure 5.11). The degradation efficiencies (Eff) for GO, CdS/ZnO/GO and CdS/Al2O3/GO were calculated based on the initial and final concentration of the dye.19 It was found that within 10 min of irradiation, 85% and 51% of dye degradation was achieved (Figure 5.12) whereas, 99%
and 90% degradation occurred within 60 minutes in presence of CdS/ZnO/GO and
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CdS/Al2O3/GO respectively. The improved efficiency of dye degradation could be mainly attributed to enhanced surface area for mass transfer and facile charge transfer due to the presence of GO sheet which is also responsible for the increased hydrogen production rate.
y ====e-0.091x y ====e-0.044x
y ====e-9E-04x y ====e-0.011x
0.001 0.01 0.1 1
0 10 20 30 40 50 60
Graphene Oxide
CdS/Al2O3/GO CdS/ZnO/GO CdS NPs
Irradiation time (min)
C/C0
Figure 5.11 Evolution of apparent rate versus irradiation time in the presence of GO, CdS NPs, CdS/Al2O3/GO and CdS/ZnO/GO photocatalyst at 60 min time interval
y ====e-0.082x
y ====e-0.243x
0.01 0.1 1
0 1 2 3 4 5 6 7 8 9 10
CdS/Al2O3/GO CdS/ZnO/GO
C/C0
Irradiation time (min)
Figure 5.12 Evolution of apparent rate versus irradiation time in the presence of CdS/Al2O3/GO and CdS/ZnO/GO photocatalyst at 10 min time interval
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The proposed schematic of the mechanism for the degradation of methyl orange by CdS/ZnO/GO is shown in scheme 5.2.
e−−−− e−−−− e−−−− e−−−− e−−−−
h+ h+ h+
e
-O2 O2-.
H2O
OH.
OH.
Visible
Light
Dye + •OH CO
2+ H
2O
CdS
Scheme 5.2 Schematic illustration of the charge separation, transfer of electron to graphene oxide