Nanoscale Kirkendall Effect Driven Au Decorated CdS/CdO Colloidal Nanocomposites for Efficient Hydrogen Evolution, Photocatalytic Dye Degradation and Cr (VI) Reduction
Graphical abstract
Introduction
Nanocomposites (NCs) or nano heterostructure (NHs) containing multiple semiconductors are one of the emergent candidates for solar light absorption and consecutive conversion to chemical energy and enhancement of photo physical properties [1,2]. Light absorption, separation of electron and hole and finally transportation to initiate chemical redox reaction are the basic events in photocatalysis process. Single semiconductor cannot optimize each step to obtain a desirable photocatalytic efficiency as they have large bandgap and improper band alignment [[3], [4], [5], [6]]. A combination of two or more semiconductors with proper band alignment can reduce electron-hole recombination and easier charge transfer to reactant molecules. The semiconductor heterostructure can be classified as Type –I, Type-II and Z scheme depending on relative positions of conduction band (CB) and valence band (VB) edges [7]. In Z-scheme, photogenerated electrons in the CB of one semiconductor are migrated directly or indirectly to the VB of another semiconductor. The photogenerated electrons in higher CB are more reductive while holes in lower VB are more oxidative giving rise to superior photocatalytic performance compared to other configurations [[8], [9]].
The energy band gaps of semiconductors should match the visible (39%) and infrared light (54%) of solar spectrum for maximum efficiency of photocatalysis process. Moreover redox potential of reactant should lie within VB and CB edges of photocatalyst. Semiconductor nanocrystals with size comparable or smaller than exciton Bohr radius conventionally called as quantum dots (QD) reveal an increase of band gap due to quantum confinement effect. The positions of VB and CB edges can be tuned with the variation of size of nanocrystals to improve the efficiency of photocatalytic activity [10]. CdO with a narrow band gap of 2.3 eV is one of the earth abundant cheap oxide which can absorb visible light due to high absorption coefficient (1.3-5.3×104 cm-1) [[11], [12]]. Unique electronic and optical properties of CdS with band gap of 2.4 eV is also a promising candidate for absorbing solar light in visible region owing to its high absorption coefficient [12]. High electron-hole recombination and instability due to photocorrosion limits the applications of CdS QDs. Such difficulties can be removed by forming nanocomposite or heterostructure with suitable semiconductors for efficient and long life applications. The composite, CdS/BiOI enhances the performance of photocatalytic hydrogen evolution under visible light irradiation due to type II band alignments[13]. The combination of graphene like C3N4 and CdS yields a higher rate of hydrogen production and exhibits excellent stability [14]. The easiest transfer of photoexcited electrons from CdS to wide band gap TiO2 increases the hydrogen production rate. [15] The decoration of CdS QDs on TiO2 nanorods enhances photocurrent density suitable for Cu2+ detection [16]. These reports demonstrate that the efficiency of catalytic activity can be improved by the formation of composite based on CdS.
The band gaps of CdO and CdS do not differ significantly whereas the CB edge of CdS is more negative than that of CdO as shown in Scheme I. Thus the band alignments of CdO and CdS are appropriate to design Z-type heterostructure for superior photocatalytic applications. Moreover, CdS-Au hybrid nanostructure facilitates separation of photo induced electron and hole suggesting a further improvement of photocatalytic performance [[17], [18]]. Thus incorporation of Au into Z-type heterostructure can efficiently promote charge separation when Au selectively deposited over CdS and can enhance photocatalytic activity. The positions of VB and CB of semiconductors and Fermi level of Au versus normal hydrogen electrode potential are illustrated in Scheme I for easier understanding of sequential charge transfer process.
Reports on synthesis of CdS-CdO heterostructure are very limited. CdCO3-CdS core-shell nanocubes were prepared through microwave assisted in situ sulfidation process. Thermal decomposition of CdCO3-CdS at 450 °C transforms to CdO-CdS core-shell nanoboxes [19]. Li et al first synthesized Cd(OH)2 nanorods by electrochemical deposition on F-doped SnO2 (FTO) substrate [20]. CdO-CdS nanorods were obtained by immersing Cd(OH)2 nanorods into thioacetamide aqueous solution and annealing at 350 °C. CdS nanorods were grown on FTO substrate by solution-liquid-solid method. Then dip coating method was followed to decorate CdO on CdS nanorods [21]. Recently Li et al prepared CdS on FTO substrate by hydrothermal method. A thin layer of CdO was generated by annealing in air at 450-550 °C on the surfaces of CdS nanorods to form CdO-CdS core-shell structure [22]. These methods are based on multi-step reaction mixing to fabricate heterointerface between CdO and CdS. The charge separation and migration are not efficient across such interface. In-situ chemical reaction to synthesize heterointerface by chemical phase transformation from one component to another is the most convenient way to improve the charge transfer process.
Recent progress in anion and cation exchange chemistry in colloidal synthesis provides a wide opportunity to synthesize metal oxide-metal sulphide core shell heterostructure, segmented metal oxide-metal sulphide heterostructure and void nanoparticles based on nanoscale Kirkendall effect [[23], [24], [25]]. The chemical phase transformation occurs via the ionic diffusion in such effect. The imbalance between inward and outward diffusion of ions generates cavities/voids inside nanocrystals. The chemical composition of heterostructure can be controlled by monitoring concentration of reactants, reaction temperature and time. Hollow nanoparticles containing multiple voids are a very interesting system to adsorb more dye molecules and toxic metal ions due to large surface area which can enhance photocatalytic activity [26].
In this article we demonstrate the synthesis of spherical mono disperse CdO nanoparticles of average size 29.7 ± 0.18 nm. CdO spherical nano particles have been selectively converted to hollow CdS/CdO nanocomposite. Time dependent sulfidation reaction established formation of CdS/CdO nanocomposite with different amount of CdS with increasing void space in single nanoparticle and finally of a CdS nanoring. The conversion chemistry based on nanoscale Kirkendall effect has been studied by HR-TEM analysis. The modification of optical properties was studied by UV-VIS and room temperature PL spectrum analysis. As formed CdS/CdO nanocoposite was found very efficient for Cr (VI) reduction, multiple dye degradation and photocatalytic H2 production from water.
Section snippets
Experiments
Cadmium acetylacetonate [Cd(C2H7O2)2], Sulfur powder, Oleylamine [(OLAM) 70%, tech] and Oleic acid (OLAC, 99%) were purchased from Aldrich. 1-octadecene (ODE, 90%) and cetyltrimethylammonium bromide (CTAB) were received from Alfa Aesar. Ethyl alcohol (GR), n-Hexane (GR) and Methanol (GR) were purchased from Merck. All chemicals were used as received from respective suppliers.
Synthesis of Hollow Nanocrystals
In this synthesis protocol we took 1 mmol of [Cd(C2H7O2)2] and 5 mmol OLAC in a 25 ml three neck round flux fitted with a reflux condenser. The mixture was vacuumed for 1 hr at room temperature to remove dissolved gases. The cadmium precursor Cd(C2H7O2)2 reacts with oleic acid to form intermediate cadmium oleate. Next the mixture was heated to 150 °C under vacuum and kept for 1 hr for complete conversion of cadmium oleate. Temperature of yellow coloured transparent solution was raised to 300 °C
Characterization
Crystallinity of as synthesized product was determined by X-ray powder diffraction (XRD) by utilizing Bruker AXS D8SWAX diffractometer with Cu Kα radiation (λ = 1.54 Å), employing a scanning rate of 0.5 °S-1 in the 2θ range from 20° to 80°. For XRD measurement the hexane solution of the nanocrystals (NCs) was drop cast on amorphous silicon wafer till a naked eye visible moderate thin layer was formed. Transmission electron microscopy (TEM) images, high angle annular dark field scanning TEM
Photocatalytic Hydrogen Generation
Photocatalytic hydrogen production experiments were explored in 200 ml Pyrex round bottom flask. Neck of the flask was sealed with silicon rubber septum. Ambient temperature of flux was maintained by continuous flow of water around the flask. A 300 W xenon arc lamp equipped with a UV cut-off filter (< 390 nm) was used as a proto type visible light source to trigger up the photocatalytic reaction, keeping the light source 20 cm apart from reaction chamber. The intensity of irradiated light was
Photocatalytic dye degradation measurement
10 mg of CTAB capped nanocomposite was added in 50 ml 0.01 mM aqueous solution of RhB and MB dye and the total combination was stirred under dark environment for 1.5 hr. Continuous stirring was performed to disperse the catalyst properly in dye solution and adsorb dye molecule on the surface of catalyst also. After the 1.5 hr stirring, 5 ml of solution was centrifuged for catalyst separation. This catalyst free solution gives the initial concentration of dye before photo degradation (C0). After
XRD
The crystallinity of as prepared samples were investigated by XRD analysis. Fig. 1 shows the XRD pattern of pure CdO and different time varying sulfidised CdS/CdO nanocomposites. Peak positions of pure samples confirm the face cantered cubic CdO lattice system (JCPDS No.750592) stabilized in this synthesis protocol. The characteristic (100), (002), (101), (110) and (112) peaks of CdS were started to become gradually prominent when sulfidation reaction performed on pure CdO system. Enhancement
Conclusions
In conclusion, we have successfully synthesized new photocatalysts based on CdO, CdS and Au by precise controlling the nanoscale Kirkendall effect starting from CdO nanoparticles. High light absorption due to hollow morphology, efficient separation of photogenerated electrons and holes in Z-type/Au system result in very fast degradation of Dyes (RhB and MB), removal of Cr(VI) from waste water and photo catalytic H2 production from water in presence of visible light. Excellent catalytic activity
Acknowledgement
Author Manas Saha sincerely acknowledges Indian Association for the Cultivation of Science for providing experimental facilities. The authors thank Prof. Abhisek Dey, School of Chemical Science, Indian Association for the Cultivation of Science for Hydrogen gas detection.
References (54)
- et al.
Applied Catalysis B: Environmental
(2017) - et al.
Sensors & Actuators: B. Chemical
(2018) - et al.
Electrochimica Acta
(2018) - et al.
Sensors & Actuators: B. Chemical
(2018) - et al.
Nano Energy
(2017) - et al.
Journal of Colloid and Interface Science
(2018) - et al.
Journal of Alloys and Compounds
(2014) - et al.
Chemical Engineering Journal
(2017) - et al.
Nanoscale
(2015) - et al.
Cryst. Eng. Comm.
(2016)
J. Mater. Chem. A
J. Mater. Chem. A
Adv. Mater.
Adv. Mater.
Adv. Sci.
Int. J. of Hydrogen Energy
Small
J. Phys. Chem. C
Int. J. Hydrogen Energy
J. Am. Chem. Soc.
Adv. Funct. Mater.
J. Mater. Chem.
Cryst. Eng. Comm
Y. Yin Chem. Mater.
Nanoscale
Small
Cited by (30)
Enhanced photocatalytic performance of S-scheme CdMoO<inf>4</inf>/CdO nanosphere photocatalyst
2024, Journal of Materials Science and TechnologyZIF-67-derived hollow CoS and Mn<inf>0·2</inf>Cd<inf>0·8</inf>S to form a type-II heterojunction for boosting photocatalytic hydrogen evolution
2024, International Journal of Hydrogen EnergyAn overview and recent progress in photocatalytic Cr(Ⅵ) reduction and hydrogen evolution
2024, International Journal of Hydrogen EnergyPhotocatalysis for synergistic water remediation and H<inf>2</inf> production: A review
2023, Chemical Engineering JournalMOFs derived CdS/CdO heterojunction photoanode for high-efficient water splitting
2022, Applied Surface ScienceCitation Excerpt :In order to better understand the mechanism of this PEC process, the conduction and valence band positions of CdO-M and CdS were estimated according to the bandgaps and Mott-Schottky (M−S) plots. As revealed by the M−S plots (Fig. 6B), the CdO-M and CdS showed positive slopes, indicating that both CdO-M and CdS are n-type semiconductors [45]. The flat band potentials obtained by M−S plots can be approximately deemed to the conduction band of CdO-M and CdS.