First, core-shell nanocrystals of Au-CdS were prepared in a hydrothermal process by using the pre-synthesized Au colloids and Cys/Cd complexes as the starting materials.
The formation of Au-CdS nanocrystals involved the binding of Cys/Cd complexes toward Au nanoparticles, followed by the decomposition of Cys/Cd in the hydrothermal reaction and the subsequent growth of CdS onto the surfaces of Au. By suitably modulating the experimental parameters such as the volumes of Cys/Cd added, a controllable shell thickness of Au-CdS nanocrystals can be achieved. In this work, Au-CdS nanocrystals with three different shell thicknesses (9.0, 14.0 and 18.6 nm) were prepared and compared. The present Au-CdS nanocrystals provide an ideal platform to study the interfacial charge carrier dynamics for metal-semiconductor core-shell heterostructures. Due to the difference in band structures between Au and CdS, a pronounced photoinduced charge separation took place at the interface of Au and CdS, resulting in the electron-charged Au core and the hole-enriched CdS shell. The electron-charging of Au core in Au-CdS nanocrystals can be revealed with the corresponding XPS analysis and photocurrent measurement. As shown by the XPS spectra of Figure 1(a), a binding energy of 84.1 eV of Au 4f7/2 peak was found for pure Au colloid sample, which is in good agreement with the value of bulk metallic Au.17 However, a negative binding energy shift of around 0.5 eV of Au 4f7/2 peak was observed for Au-CdS nanocrystals, indicating a significant difference in electronic structures between Au and CdS and a strong electronic interaction therein.18 Similar phenomenon was ever reported in Au-SnO2 core-shell nanocrystal system, in which the binding energy shift of Au 4f was attributed to the effective electron transfer from SnO2 to Au.19 Here we ascribed the negative binding energy shift of Au 4f observed in Au-CdS nanocrystals to the electron-charging of Au core that resulted from the occurrence of
charge separation. To further elucidate the effect of Au on the charge separation of CdS for the present core-shell nanocrystals, we compared the photocurrent response of Au-CdS nanocrystal and CdS counterpart electrodes by inserting them in a photoelectrochemical cell.
Note that CdS counterpart was composed of CdS hollow structures, which were prepared by dissolving the Au core of Au-CdS nanocrystals.15 Figure 1(b) depicts the photocurrent generation for Au-CdS nanocrystal and CdS counterpart electrodes subjected to the white light irradiation. Both electrodes showed prompt response to the on/off cycles of light illumination, demonstrating the effective charge transfer and successful electron collection for the samples within the photoelectrochemical cell. More importantly, Au-CdS nanocrystals exhibited lower photocurrents than CdS counterpart upon light irradiation. We believed that the significant electron transfer from CdS shell to Au core accounted for such an evident photocurrent depression found in Au-CdS nanocrystals.
If the observed photocurrent depression as well as the XPS binding energy shift for Au-CdS nanocrystals indeed involved the electron transfer from CdS to Au, we should be able to reveal this event in the excitonic emission decay profile of CdS. Figure 2 represents the time-resolved PL spectra for two Au-CdS samples with different shell thicknesses. The emission decay data were analyzed with biexponential kinetics in which two decay components were derived. For Au-CdS nanocrystals with the shell thickness of 14.0 nm, emission lifetimes of both components were shorter than those of the corresponding CdS counterpart (τ1=0.40 ns,
τ
2=2.95 ns for Au-CdS versus τ1=0.53 ns,τ
2=2.98 ns for CdS counterpart). The intensity-average lifetime was then calculated to make an overall comparison of the emission decay behaviour.20 The difference in the average emission lifetime between Au-CdS (<τ> = 0.57 ns) and CdS counterpart (<τ> = 1.06 ns) indicates the emergence of a nonradiative pathway from the interaction between CdS and Au. This proposition can beconfirmed by the emission quenching of CdS observed for Au-CdS sample.15 Such difference became more noticeable as the shell thickness of Au-CdS nanocrystals further increased to 18.6 nm (<τ> = 0.48 ns for Au-CdS versus <τ>
= 1.51 ns for CdS counterpart), inferring a much more significant electronic interaction between CdS and Au. If electron transfer from CdS to Au was the predominant process that dictated the emission quenching of CdS, we can then estimate the electron-transfer rate constant (ket) of Au-CdS nanocrystals from the emission lifetime data by the following equation:
Using the lifetime values listed in Table 1, we obtained the electron-transfer rate constants as 0.36×109, 0.51×109, and 1.42×109 s-1 for Au-CdS nanocrystals with the shell thickness of 9.0, 14.0 and 18.6 nm, respectively.
It should be noted that the electron-transfer rate constant of Au-CdS nanocrystals increased with increasing shell thickness. The less pronounced interaction between electrons and holes in the thicker CdS shell may contribute to such an increase in electron-transfer rate constant for Au-CdS nanocrystals with increasing shell thickness.
We noticed that the emission lifetimes of the three CdS counterpart samples were substantially different, with CdS of larger characteristic size (thickness) showing longer emission lifetime, as can be seen in Table 1.
This size-dependent correlation of exciton lifetime has been widely reported for CdS nanocrystals.21 It is generally believed that the significant interaction between electrons and holes, which is due to the confinement of electrons and holes in a particle of reduced size, may induce additional pathways for nonradiative recombination.22 A shortened exciton lifetime would consequently be observed for CdS nanocrystals with reduced size. The electron-hole interaction in small particles is related to the trapping of excitons by the abundant surface states that may further act as alternative sites for nonradiative charge recombination.23 For the present Au-CdS
nanocrystals, it is reasonable to presume a less pronounced electron-hole interaction for Au-CdS with thicker CdS shell since they possessed a larger characteristic size of CdS and thus a less amount of surface states. Such less pronounced electron-hole interaction in fewer surface states prohibited charge carriers from being consumed in nonradiative recombination, which further enabled a fuller extent of participation of photoexcited electrons in the charge separation process. Accordingly, an increase in the electron-transfer rate constant was observed for Au-CdS nanocrystals with increasing shell thickness.
Since Au-CdS nanocrystals exhibited pronounced charge separation upon light illumination, it is worth studying the potential application that this property may bring. Owing to the effective electron transfer from CdS shell to Au core, photogenerated holes with an abundant amount were existent in CdS shell and would transfer to the surfaces of Au-CdS nanocrystals. These highly reactive holes could oxidize water to produce hydroxyl radicals that can further decompose organic pollutants through an oxidation process. A spectacular capability of photocatalytic oxidation is therefore expected at the surfaces of Au-CdS nanocrystals. A series of photocatalysis experiments were performed in this work to investigate the photocatalytic properties of the as-synthesized Au-CdS nanocrystals. RhB, a typical dye that can be decomposed by hydroxyl radicals,24 was used as the test pollutant to monitor the photocatalytic oxidation progress for Au-CdS nanocrystals.
The time-dependent UV-visible spectra of RhB solutions under visible light illumination in the presence of Au-CdS nanocrystals with a shell thickness of 14.0 nm were first shown in Figure 3(a). It can be seen that the intensity of the characteristic absorption peak at 553 nm decreased dramatically with the irradiation time.
Besides, a concomitant blue shift in the absorption maximum was observed after the solution was irradiated for 20 min. It is well known that the photodegradation of RhB undergoes two competitive processes.25 One is the destruction of dye chromogen, which is characteristic of the loss of absorbance at 553
nm. The other is the N-demethylation reaction that produces a series of N-demethylated intermediates, accompanied by a blue shift in the absorption maximum from 553 to 498 nm.
In the current case, RhB concentration was determined by referring to the absorbance of the characteristic peak at 553 nm. To quantitatively understand the reaction kinetics of RhB photodegradation for our samples, we analyzed the normalized concentration of RhB (C/Co) as a function of irradiation time. As shown in the inset of Figure 3(a), an exponential decay of RhB concentration with the irradiation time was evident for Au-CdS
nanocrystal photocatalyst. The photodegradation process was then fit to
pseudo-first-order reaction, in which the value of the apparent rate constant (kRhB) is equal to the slope of the fitting line according to the following expression:26
ln(C/C
o) = - kRhBt, where C
o and C are the concentrations of RhB at initial and at a certain irradiation time t, respectively.For Au-CdS nanocrystals with a shell thickness of 14.0 nm, kRhB is found to be 0.026 min-1. The mechanism for RhB photodegradation by using Au-CdS nanocrystal photocatalyst can be described by the following four pathways:
Au–CdS + hν Æ Au(e–)–CdS(h+) (1) Au(e–)–CdS(h+) + H2O Æ Au(e–)–CdS + H+ + ·OH (2) RhB + ·OH Æ oxidation products (3) Au(e–)–CdS + O2 Æ Au-CdS + ·O2– (4)
Under visible light illumination, charge separation occurred within Au-CdS nanocrystals, resulting in an electron-charged Au core and a hole-enriched CdS shell (1).
Subsequently, the photogenerated holes transferred to the surfaces of nanocrystals and reacted with water to produce hydroxyl radicals (2). RhB molecules were then decomposed by hydroxyl radicals through an oxidation process (3). Once the photogenerated holes were depleted in photocatalysis, Au-CdS nanocrystals underwent a Fermi level
equilibration due to the accumulation of photoexcited electrons.27 Note that experiments of RhB photodegradation were conducted in air.
The exposure to air during the operation of photocatalysis can discharge the accumulating electrons of Au-CdS nanocrystals to the dissolved oxygen,5a,28 which resulted in a neutralized state of nanocrystals that are allowed for further photoexcitation (4).
The photocatalytic performance of Au-CdS nanocrysyals with three various shell thicknesses was then compared in Figure 3(b).
It should be noted that experiment in the absence of photocatalyst showed almost no RhB photodegradation, implying that the self-photolysis of RhB is negligible under visible light illumination. For Au-CdS nanocrystals with a shell thickness of 9.0 nm, about 50% of RhB was degraded after 40 min of irradiation. A higher extent of RhB photodegradation to around 70% at the same irradiation time was achieved when using Au-CdS nanocrystals with a thicker shell of 14.0 nm. For Au-CdS nanocrystals with the shell thickness of 18.6 nm, RhB was almost completely decomposed after 40 min of irradiation. From the above observations, we concluded that the photocatalytic activity of Au-CdS nanocrystals was enhanced with increasing shell thickness. This is mainly a result of the extensive growth of CdS shell in Au-CdS nanocrystals. With increasing shell thickness, a raised ratio in the amount of CdS to Au was attained, leading to a greater capability of light absorption for Au-CdS nanocrystals and thus the generation of more charge carriers. This argument can be verified by the fact that the excitonic absorption of CdS for Au-CdS nanocrystals turned significant with increasing shell thickness.15 Consequently, a higher amount of photoexcited charge carriers was expected for Au-CdS nanocrystals with larger shell thickness, which in turn promoted the resulting photocatalytic efficiency toward RhB photodegradation. It is worth noting that the increase in photocatalytic activity of Au-CdS nanocrystals with the increasing shell
thickness corresponded well with the result of electron-transfer rate constant variation, which could be realized by the causal relation between electron transfer and hole generation. As the shell thickness of Au-CdS nanocrystals increased, more and more photoexcited electrons transferred from CdS shell to Au core, simultaneously leaving photogenerated holes of an increased amount in CdS shell. The rise in the number of photogenerated holes further led to the enhancement in the resulting photocatalytic performance as observed. For more clarity on this relation, we depicted the correlations of electron-transfer rate constant (ket) and rate constant of RhB photodegradation (kRhB) with the shell thickness of Au-CdS nanocrystals in Figure 4.
Further comparative experiments were conducted to demonstrate the superior photocatalytic performance for the present Au-CdS nanocrystals. Four kinds of photocatalysts including N-doped P-25 TiO2, commercial CdS powders, CdS counterpart and Au-CdS nanocrystals were used in the photodegradation of RhB under the same experimental conditions. The comparative results were shown in Figures 5(a), from which several points can be observed. First, as compared to the relevant commercial products like N-doped P-25 TiO2 and CdS powders, Au-CdS nanocrystals exhibited superior photocatalytic performance under visible light illumination, demonstrating their potential as an efficient visible-light-driven photocatalyst in relevant redox reactions. Second, Au-CdS nanocrystals performed better toward RhB photodegradation than CdS counterpart, which can be accounted for by the pronounced charge separation that occurred at the interface of Au and CdS. This demonstration addresses the benefit of the present metal-semiconductor core-shell nanocrystals to photocatalytic applications. To further explore the applicability of the as-synthesized Au-CdS nanocrystals in a more practical situation, their photocatalytic performance under natural sunlight was also evaluated. As illustrated in Figure 5(b), after exposure to 150 min of daytime sunlight, 90% of RhB was degraded by using Au-CdS nanocrystals, accompanied with
an evident decoloration of the resultant solution.
This result shows that the present Au-CdS nanocrystals can be used as highly efficient photocatalysts which may practically harvest energy from sunlight. As a final note, the as-prepared Au-CdS nanocrystals did not suffer significant photocorrosive oxidation as deduced from the XPS data. In Figure 5(c), both Cd and S XPS spectra of Au-CdS sample exhibit signals corresponding to the bulk CdS with the binding energies of 404.8, 411.6 and 161.8 eV for Cd 3d5/2, Cd 3d3/2 and S 2p core levels, respectively.29 For the S 2p XPS spectrum, note that no sulphate-related peak at around 168.4 eV, which originated from the photocorrosive oxidation of CdS with oxygen and water,30 was observed. The absence of sulfate-related peak in S 2p spectrum indicates that the present Au-CdS nanocrystals exhibited considerably high stability in air, which is important to the durability performance during their use as photocatalysts.
4. Conclusions
In conclusion, the interfacial charge carrier dynamics for core-shell Au-CdS nanocrystals with various shell thicknesses were investigated and presented. Due to the difference in band structures between Au and CdS, a pronounced photoinduced charge separation took place at the interface of Au and CdS, resulting in the electron-charged Au core and the hole-enriched CdS shell.
The electron-charging of Au core in Au-CdS nanocrystals was revealed with the corresponding XPS analysis and photocurrent measurement. Time-resolved PL data showed that a higher electron-transfer rate constant was observed for Au-CdS nanocrystals with thicker CdS shell. On the other hand, the hole-enriched CdS shell of Au-CdS nanocrystals upon light illumination was characterized with a photocatalytic process. The photocatalytic activity of Au-CdS nanocrystals was found to increase with increasing shell thickness, consistent with the result of electron-transfer rate constant variation. The current Au-CdS nanocrystals may find promising photocatalytic applications, especially in the
photooxidative decomposition of various organic pollutants such as aldehydes31 and chlorinated hydrocarbons.32 The present study also gives rise to a new class of highly efficient metal/semiconductor hybrid photocatalysts which may effectively utilize the solar power.