Although several groups have reported the synthesis of ZnS quantum dots, only a few have developed methods to prepare potentially nontoxic, noble metal loaded ZnS QDs. In this study, we devised a gram scale, environmentally benign, room temperature, aqueous solution based method for the synthesis of renewable and eco-friendly, well dispersed ZnS quantum dots along with noble metal loaded ZnS QDs (loading amount 4 wt%). The properties of the nanophotocatalysts were determined by using XRD, XPS, TEM and SEM techniques. The ZnS QDs were found to have small sizes of ca. 4.5 nm and to display quantum effects in terms of blue shifts in their absorption maxima associated with an optical band gap, Eg, of 4.63 eV. The photocatalytic activities of ZnS QDs, Au/ZnS and Ag/ZnS were assessed using photodegradation of methylene blue. The results demonstrate that a significant increase in the photocatalytic efficiency takes place upon the loading of Ag and Au nanoparticles on ZnS QDs. An in-depth investigation was carried out to uncover information about the effects of noble metals on band gap energies and surface charges of the QDs. Finally, a new surface reactivation procedure was developed for the reactivation of nanophotocatalysts. Consequently, the newly synthesized photocatalysts are renewable, a property that should make their use in practical applications cost effective.
© 2016 Optical Society of America
Pollutants in industrial wastewater are a major environmental concern. Particularly problematic in this regard are dyes that are present in wastewater emanating from the textile, paper and plastics industry. Importantly, about 15% of the dyes employed in industrial processes is lost [1, 2]. Because many of the dyes are toxic and carcinogenic, they pose serious threats to living organisms . Hence, an urgent need exists to develop new methods to remove toxic dyes from wastewater. Heterogeneous photocatalyzed reactions are arguably among the most promising methods for eliminating organic pollutants from water under ambient conditions [4,5]. As a result of their unique physical properties and ready availability, II-VI semiconductors have been studied extensively as photocatalysts employed for this purpose . Following photoexcitation of a semiconductor, an electron moves into the conduction band concurrent with a hole (positive charge) being generated in the valence band. The created positive hole and negative conduction band electrons, called charge carriers, have the capability of oxidizing and reducing many types organic compounds adsorbed on the semiconductor surface. These processes result in mineralization of the compounds through their conversion to CO2, H2O and mineral acids . However, fast recombination of an electron and a hole limits the efficiencies of these redox reactions because it results in non-productive deactivation of the excited state of the photocatalyst .
It is widely believed that modification of the surface of a semiconductor with noble metals leads to improved photocatalytic efficiency . Several mechanisms, which are governed by the operational conditions, are responsible for the efficiency enhancements. Specifically, noble metals can inhibit electron/hole recombination by acting as electron traps and, therefore, they extend the lifetimes of the charge carriers. In addition, owing to a surface plasmon resonance effect these metals can cause light absorbed by semiconductors to extend into the visible range. Finally, the surface properties of semiconductors can be altered by coating them with noble metals [10–13]. As a consequence of these features, ZnS loaded by noble metals have important environmental applications as photocatalysts in wastewater treatment processes [14–17]. Many reports have appeared describing investigations of the effects of doping ZnS with Mn, Cu and Ni. In contrast, only a few studies have been carried out probing the effects of doping ZnS with Ag and Au nanoparticles.
Although many reports exist describing the synthesis of ZnS quantum dots, only a few concern have focused on the preparation of potentially nontoxic ZnS QDs and/or the development of synthetic methods that utilize one-step, aqueous, room temperature protocols . In the investigation described below, we developed a facile, one-pot procedure for the synthesis of monodispersed unloaded and noble metal loaded ZnS QDs, which utilizes a PVP/PVA polymer blend as the growth media. The optoelectronic properties of the nanoparticles in terms of band structure were determined. The photocatalytic properties of the synthesized nanoparticles were assessed using photodegradation of methylene blue. Finally, a simple washing procedure for reactivation of the photocatalyst was devised.
2. Experimental section
Reagent grade zinc nitrate Tetra-hydrate (Fluka, cryst. >98.0%), sodium sulphide (AnalaR, cryst. 32-38%), PVP (Sigma, Av. Mol. Wt. 10,000) and PVA (Fluka, Av Mol. Wt. 72,000), gold chloride (HAuCl4●3H2O, Sigma Aldrich, 99.9%) and silver nitrate (AgNO3, Sigma Aldrich, 99.0%) were purchased and used without further purification. All solutions were prepared using DI water.
2.2. Preparation of the catalysts
For ZnS QDs synthesis, 0.2 M Zinc nitrate is added to a stirred 5% aqueous solution of PVP/PVA (50/50% wt). An equal volume of a 0.2 M aqueous solution of Na2S as the sulfur source is then added The resulting solution is stirred to obtain homogeneity and then transferred to a 300 mL Teflon lined stainless-steel autoclave and let stand at 110 °C for 5 h. Centrifugation of the mixture gives a precipitate of ZnS QDs that is washed several times with doubly distilled water and ethanol, and dried overnight at 70 °C. For the synthesis of Ag/ZnS and Au/ZnS heterostructures, a solution of polyvinyl alcohol (Au: and Ag:PVA = 1.5:1) are added as protecting agent to aqueous solutions of gold chloride and silver nitrate (10 mg/100 mL) at room temperature followed by stirring for 10 min. An aqueous solution of sodium borohydride (Au: or Ag: NaBH4 = 2.5:5) is then added to each solution followed by stirring for 30 min. To the outstanding solutions of Ag and Au, an amount of the synthesized ZnS QDs is added with loading 4 wt% followed by stirring for 90 min. Finally, nanoparticles generated using this procedure is washed 4 times with DI water followed by centrifugation (12, 000 rpm) and drying at 90 °C.
2.3. Characterization of nanophotocatalysts
The bulk and surface characteristics of the photocatalysts were determined by evaluating their X-ray Powder diffraction (XRD) patterns, obtained by using a Bruker, D8 ADVANCE diffractometer with Cu Kα (λ = 0.154 nm) radiation under 40 kV, 40 mA and a scanning range of 10–80° 2θ. The morphological properties of the photocatalysts were determined using transmission electron microscopy (TEM) with a JEOL JEM 1230 (JEOL Ltd. Japan) operating at 120 KV. Surface elemental analyses were carried out by using X-ray photoelectron spectroscopy (XPS) with a VG ESCALAB2 XPS spectrometer and an Al Kα monochromatic source and a charge neutralizer.
Binding energies are referenced to C 1s peak at 284.5 eV. Optical properties of the samples were determined by using UV–Vis spectroscopy. Diffuse reflectance spectra (DRS) were recorded on a Varian Cary 5000 UV–Vis spectrometer. N2 adsorption-desorption isotherms were measured on test samples at −195 °C using a model ASAP 2010 automatic Micromeritics sorptiometer (USA), equipped with an out gassing platform. Zeta potential (ζ) measurements for isoelectric point (IEP) determinations were made (Zeta sizer Nano ZS, Malvern Instruments Ltd, Malvern, UK) on loaded and unloaded ZnS QDs dispersed in water. The determined electrophoretic mobilities were converted to ζ-potentials by using Dispersion Technology Software, version 5.03, Malvern Instruments Ltd. UK. The ICP-OES GBC Quantima was used to determine the noble metal loading on ZnS QDs. Before analysis, 300 mg of each nanoaprticle was digested with 10 mL nitric acid until complete digestion was achieved in a microwave digestion Teflon tube. The digested sample after suitable dilution (up to 50 mL by using 1% nitric acid) was then injected into the ICP machine equipped with a 1200 W plasma power.
2.4. Photocatalytic measurements
The photocatalytic efficiencies of unloaded and noble metal loaded ZnS QDs (5mg/100ml) were assessed by using photodegradation of methylene blue (MB, 3 mg) in an aqueous solution (100 mL) at room temperature using ultraviolet irradiation. MB solutions (pH 6.3) containing suspended QDs were first magnetically stirred in the dark for 0.5 h to achieve homogeneity. Irradiation was performed using an ultraviolet light (6W, at 356 nm, Boitton Instruments), positioned parallel to and at a fixed distance of 20 cm from the surface of the suspension in a 10 cm in diameter beaker. At fixed time intervals, irradiation was terminated and 3 mL aliquots of the suspensions were removed and analyzed to determine MB concentrations by using UV–Vis spectroscopy. Following immediate return of the aliquots to the reaction container irradiation was begun.
3. Results and discussion
3.1 Characterization of nanophotocatalysts
XRD patterns of loaded and unloaded ZnS QDs are displayed in Fig. 1. The pattern for unloaded ZnS QDs contains diffraction peaks at 2θ values of 29.33, 48.80 and 57.61 corresponding to the respective diffraction planes (111), (220) and (311) . The XRD patterns for Ag/ZnS QDs contain diffraction peaks at 2θ values of 38.01, 44.2°, 64.4° and 77.4° corresponding to metallic silver . The diffraction peaks of Au/ZnS QDs pattern at 2θ = 38.31°, 44.4°, 64.5° and 77.6°, these peak positions are characteristics of Au NPs [21, 22]. The crystallite sizes of the loaded QDs (Table 1) were estimated using the full width at half-maximum of the peaks at 2θ = 38.31° for Au, 2θ = 38.01° for Ag and 2θ = 27.4° for ZnS and the Scherrer equation, d(Å) = Kλ / β cos θ, where d is the average crystallite size, the coefficient K is 0.9, the X-Ray wavelength λ is 0.1541 nm, β is the full width at half maximum of the catalyst, θ is the diffraction angle .
TEM was employed to characterize the size and morphology of each nanophotocatalyst. The images (Fig. 2) show that bare ZnS and noble metal loaded ZnS QDs have uniform, nearly spherical ball-like shapes arranged in the form of clusters, and that their surfaces are for the most part homogeneous. Moreover, the TEM images show that the sizes of individual loaded and unloaded ZnS QDs are in the range of 4.5–5.5 nm (Table 1). To quantify the actual loading of Au and Ag on the ZnS QDs, the actual noble metal loading was determined by using ICP-OES and the data is tabulated in Table 1 to be 3.91% and 3.87% for Au and Ag NPs which are very close to the theoretical loading value (4.0 wt.%).
The Zn 2p1/2 and 2p3/2 binding energies of unloaded ZnS, determined by using X-Ray photoelectron spectroscopy, were found to be 1045.0 and 1022.1 eV respectively (Fig. 3). The spin–orbit splitting of Zn 2p3/2 and Zn 2p1/2 is 23.0 eV, which is characteristic for ZnS . In addition, the observed S 2p1/2 binding energy of 162.0 eV is in accord with previously reported value . In Fig. 3 is displayed the Ag 3d X-Ray photoelectron spectrum of Ag/ZnS, which consists of two sets of peaks. The first set is comprised of two strong peaks at 372.8 and 366.8 eV, which correspond to the respective binding energies of Ag 3d3/2 and Ag 3d5/2 of Ag+. The other set is comprised of two weak peaks at 373.9 and 367.9 eV, which correspond to the binding energies of Ag 3d3/2 and Ag3d5/2, respectively . In the spectrum of Au/ZnS, the Au 4f7/2 and Au 4f5/2 peaks are positioned at 83.2 eV and 87.28 eV, respectively, indicating the presence of metallic Au .
Analysis of the absorption spectra of bare and noble metal loaded ZnS QDs, given in Fig. 4A, shows that unloaded ZnS nanoparticles do not absorb light in the visible region and have an absorption onset (λos) at 268 nm. In contrast, light absorption by the loaded analogues, Ag/ZnS and Au/ZnS, extends into the visible region. The plasmon absorption maxima for the respective Ag and Au NPs occur at 405 and 518 nm. By using these data, the band gap energies (Eg) of unloaded and noble metal loaded ZnS QDs (Fig. 4(b)) were calculated using plots based on the Tauc equation, (αhʋ)1/m = k (hʋ − Eg), where k is a constant and m = 1/2. Specifically, the Eg values are obtained by extrapolating the linear portion of plots of (αhν)2 versus hν to the ordinate (Fig. 4B) . By using this technique, the calculated band gap energy of unloaded ZnS QDs was found to be 4.63 eV, while those for the respective Ag/ZnS and Au/ZnS are red-shifted to 3.06 and 2.76 (Table 1).
These observations suggest that quantum confinement has a large influence on band gap energies of noble metal loaded ZnS QDs. The excitonic Bohr radius for bulk ZnS is 2.5 nm and quasi quantum confinement is typically observed for ZnS particles having diameters up to 5 nm . The VB edge potential (EVB) of a ground-state semiconductor can be empirically determined by using the equation, EVB = χSemiconductor − Ee + 0.5Eg, derived using Mulliken electronegativity theory , where χSemiconductor is the electronegativity of the semiconductor (in this case 5.26 eV) ), Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and Eg is the band gap energy of the semiconductor (calculated by the method described above). The CB edge potential can be determined using the relationship ECB = EVB – Eg . Because Eg of unloaded ZnS QDs is 4.63 eV, the estimated EVB of unloaded ZnS QDs is 3.08 eV, and the corresponding ECB is −1.56 eV. In contrast, the EVB and ECB values for Ag/ZnS and Au/ZnS QDs (Table 1) are influenced by the work function associated with the noble metals.32] in terms of the optical band gap value (EQD) by employing the relationship given in Eq. (1). The results show that the generated ZnS QDs have estimated diameters (2r) of 4.2 nm.
3.2. Photocatalytic degradation efficiency
In one pathway for photocatalyzed degradation of organic pollutants, electron transfer takes place between the catalyst and pollutant to promote redox and subsequent chemical reactions. In order for this process to occur, the light employed must have an energy that is compatible with the optical band gap of the photocatalyst . However, the efficiency of the degradation process promoted by photocatalyst is not only governed by its band gap structure. Another critical factor is the transport and lifetime of the photogenerated electron, which are controlled by the conduction and valence band positions . Under continuous irradiation conditions, the photodegradation reaction follows a pseudo-first-order rate profile that can be expressed in the form of Ln (Co/C) = kt, where Co and C are the respective initial concentration and concentration at time t of the pollutant, and k is the apparent first-order rate constant. As a result, a plot of ln Co/C versus time should be linear and the slope of the line should provide the apparent first-order rate constant k. The rates and efficiencies determined for methylene blue photodegradation reactions promoted by these photocatalysts are presented in Table 1 and Fig. 5. As shown in Table 1, the photodegradation rate obtained in case of Au,Ag/ZnS QDs is regarded to be very high as they revealed photodegradation rates of 17.3.10−3 and 18.3.10−3 min−1 for Ag/ZnS and Au/ZnS, respectively. By comparison, the Au:TiO2 and Ag:TiO2 photocatalysts were disclosed previously  and exhibited MB photodegradation rates of 11.4.10−3 and 7.1.10−3 min−1 respectively. Also in a recent study  MB photodegradation rates of 3.5·10−3 min−1, 7.2·10−3 min−1 and 10.2·10−3 min−1 were disclosed for the Au, Pt and Pd decorated CdSe nanoheteroplatelets, respectively. Therefore, our obtained data is much high compared to those published in the literature which confirms the enhanced photoreactivity of our synthesized photocatalysts.
The results demonstrate that lowering the band gap of the nanophotocatalyst brings about a lower efficiency of MB photodegradation process because it enables more facile electron/hole recombination in the excited state of the catalyst. The Au and Ag NPs were found to have enhanced photocatalytic efficiencies relative to that of unloaded ZnS owing to their lower electron/hole recombination rates . Specifically the Fermi levels of Au and Ag NPs are lower than that of ZnS. As a result, in the noble metal loaded ZnS QDs, photo-excited electrons can be transferred to the conduction bands of Au and Ag located on the surface. In contrast, photo-generated valence band holes remain on ZnS, creating a charge separation that diminishes the probability of electron-hole recombination and enhances the photocatalytic efficiency [38–40]. Conduction and valence band energies of p-type semiconductor bend upwards (or downwards) in the direction of those of the metal dopants to a degree that is dependent on their relative work functions. Band bending is caused by the transfer of electrons between the semiconductor and metal at the interface [41, 42]. Using a potential scale that is relative to the standard hydrogen electrode (NHE), the work functions of the noble metals, Ag and Au, and the band structure of pure ZnS can be compared. Based on the data shown in the inset in Fig. 4A, it appears that Au/ZnS photocatalyst has the smallest work function and thus should be more catalytically active than the unloaded and Ag loaded analogues. Owing to the fact that different parameters (e.g. loading amount) were used, it has not been possible thus far to determine which noble metals metal dopant leads to the highest level of photocatalyst performance . We believe that the Fermi level energy of the noble metal used as dopant will correlate with photocatalytic efficiencies. Considering the electrochemical reduction potentials for Au and Ag [E°reduction (Au3+ + 3e− → Au) = 1.5 V and E°reduction = (Ag+ + e− → Ag) = 0.8 V], it can be seen that the reduction potential of Au is closer to the valence band of ZnS. As a result, the MB degradation reaction photocatalyzed by Au/ZnS should be the most efficient , a prediction that matches the experimental results. One more interpretation for the enhanced photocatalytic activity of ZnS QDs upon loading noble metals could be attributed to the photocorrosion effect. Due to the poor stability of ZnS in the aqueous photocatalytic processes as a result of its photocorrosion. It is well known that ZnS undergoes photocorrosion upon irradiation because of oxidizing holes which causes semiconductor decomposition into sulfur and metal ions . This photocorrosion effect could be suppressed by raising valence band (VB) position. In this aspect, the loading of noble metal nanoaprticles on the surface of ZnS QDs serves this function as shown in Fig. 4A inset. The increase of the VB potential enhanced the photostability of ZnS QDs and in turn results in superior photocatalytic activity .
3.3 Effect of pH on photocatalytic activity
The efficiency of the ZnS promoted, photocatalytic MB degradation reaction was found to be dependent on the pH of the medium. This is a consequence of the general amphoteric behavior of semiconductors. Accordingly, the surface charges of loaded and unloaded ZnS QDs change with changes in pH . As a result of this phenomenon, it is necessary to determine the pH-photoactivity relationship for the photocatalyst. The pH values of the photodegradation reaction mixtures were adjusted by using aqueous HCl and NaOH solutions. The data collected in these experiments (Fig. 5(F)) clearly show that the photodegradation efficiency increases upon increasing the pH up to 11, after which it decreases with further increases in pH.
An explanation of this finding requires a consideration of the isoelectric point pHiep of the photocatalyst. The results of zeta potential (ζ) measurements show that the pHiep of a ZnS semiconductor is ca. 4.5, which is close to previously reported values . This finding shows that the surface of ZnS QDs is positively charged below pH of 4.5 and negatively charged above this pH value. Because it is a cationic dye, MB should be more highly attracted to the surface of ZnS QDs when the pH is higher than the pHiep value. Thus, the higher pH/higher efficiency trend seen experimentally is consistent with the higher degree of adsorption of MB on the surface of the photocatalyst. The deviation in the trend seen at high pH values is likely caused by the presence of excess hydroxide ions which hinder adsorption of the dye . The pHiep values of the Ag/ZnS and Au/ZnS were found to be 3.8 and 3.5, respectively. The lower pHiep values for the noble metal loaded ZnS QDs are associated with a more highly acidic surface, which corresponds to a lower affinity for protons and more negative charge character . Thus, the pHiep values contribute in part to the enhancements observed in photocatalytic activity caused by loading Ag and more so by Au at a fixed pH value.
3.4. Repeatability and regeneration of nanophotocatalysts
The repeatability of photodegradation reactions of MB promoted by unloaded and noble metal loaded ZnS QDs was examined. Also, another challenging point in photocatalysis besides regeneration is its separation which represents a drawback that hinders the industrial scale applicability of photocatalysis. The most common disclosed separation technique is coagulation . In this study, the photocatalyst was collected following each reaction cycle by centrifugation, washed using doubly distilled water and then dried before being reused. The experimentally determined MB degradation efficiencies through five cycles (Fig. 6) show that, as expected, deactivation of photocatalysts occurs in a continuous manner as the catalysts are reused.
Reactivation of ZnS, Au/ZnS and Ag/ZnS photocatalysts was carried out after the 5th cycle by washing with dilute acetic acid solution and then an alkaline solution. The final pH value always adjusted to exceed the corresponding isoelectric point for each photocatalyst. The washing procedure is performed for two main purposes. The first is to remove excess hydroxide ions, which block the active sites on the photocatalyst and hinder the dye adsorption. The second is to make the surface sufficiently anionic to enable electrostatic attraction of the dye . The results show that the washing procedure leads to regeneration of nearly all of the photocatalytic activities of the unloaded and noble metal loaded ZnS QDs. Specifically, the respective regenerated activities of ZnS, Au/ZnS and Ag/ZnS are 67%, 87% and 83% (6th cycle, Fig. 6). Comparing these percentages with those cited in Table 1 which corresponds to the 1st cycle, we can conclude that we reached a recovery percentage of 92.5%, 90.0% and 89.6% of the original catalyst reactivity for ZnS, Au/ZnS and Ag/ZnS, respectively.
In this study, we developed a new method to prepare well dispersed unloaded ZnS QDs and Ag and Au loaded ZnS QDs on a large scale in an eco-friendly manner. The photocatalytic activities of unloaded and noble metal loaded ZnS QDs were assessed by their utilization to promote photodegradation of methylene blue. The results demonstrate that the photodegradation efficiency increases upon loading noble metal nanoparticles. The superior activities of the noble metal loaded ZnS QDs as photocatalysts are explained in terms of the Schottky barrier of noble metals, which prevents recombination of the charge carriers and, as a result, prolongs the lifetime of the photogenerated electrons. Stated in an alternative manner, the closer the reduction potential of the noble metal is to the valence band of ZnS, the more active is the loaded photocatalyst. This conclusion is confirmed by the observation that Au/ZnS is more active than Ag/ZnS in catalyzing photodegradation of MB. Finally, an investigation was carried out to probe the regeneration of the photocatalysts so that they can be employed for long-term use. The results show that simple washing steps can be used to reactivate the catalysts by removing hydroxide ion and raising the surface pH to values that are higher than the isoelectric point. The general conclusion that arises from this effort is that both the optoelectronic characteristics and isoelectric point need to be considered in designing ideal nanophotocatalyst.
Authors acknowledge the financial support of Kuwait University Research Administration, Project No. SC 13/14.
The authors gratefully acknowledge the support of Kuwait University Research Administration, Project No. (SC 13/14) and SAF Facilities No. (GS 01/01, GS 01/05 and GS 02/08 and GE 03/08). In addition, Nanoscopy Science Centre is highly acknowledged.
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