Tailoring the heterojunction distance between ZnO and GNPs: Free capping hybrid nanoparticles

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New/Enhanced features brought by plasmonic-semiconductor heterojunction

The combination of two different materials into a single system enhances the characters of each component in addition to creating new traits. The difference in properties depends on the chemistry of the compounds in addition to the type of the interaction between them. Besides, inter-component structural or morphological characteristics of the constituents in the hybrid systems are also important in determining their features and performances. The SC characteristics depends on its energy band structures that will be affected after the combination with metals except at the M/SC interface where the band structures are bent together. Thus, the band gap energy of the SC could remain the same [22]. Contrarily, some main properties as emission and photocatalysis of the SC will be affected by SPR coupling. On the other hand, the hybridization of metals with a luminescent material will highly affect its plasmonic traits such as absorbance, scattering and catalysis. The M/SC combination leads to an important modification in their electronic, opto-electronic and magnetic properties [155–159]. Besides, inter-component structural or morphological features of the hybrid nanomaterials are also important in determining their properties and functionalities.

Heterojunction modified absorption of both materials

The creation of new electronic states in the M/SC interface and the electrodynamic effect will lead to the modulation in the absorption properties of both compounds. This property is highly affected by the distance between both constituents as described by He et al. in Figure 5 [160].
As a result of the doping plasmonic metals in the semiconductor forming the M/SC junction, broadening and shifting in the plasmonic peak of the metal [161], as well as the excitonic absorption peak of the SC were previously observed [42]. (Figure 6)
Figure 6 UV−vis absorption spectra of pure CdS nanorods (blue curve), Au nanoparticles (red curve), and high Au−CdS heterostructures (10.8 wt %, yellow curve). The green- and purple -dotted lines indicate the excitation source of a 532 nm laser and a 405 nm laser, respectively. Adapted from ref. [162].
In the ZnO QDs/Au system, Rahman et al. observed red shift in the absorbance peak of ZnO QDs in addition to the appearance of a weak peak in the visible region (Figure 7) [163]. The latter is attributed to the LSPR of the spherical GNPs [164]. The presence of this band affirms the presence of GNPs in the ZnO/Au hybrid system by which at smaller GNPs sizes this band becomes blue shifted. The red shift of the ZnO QDs with conjoining GNPs indicates the interaction between ZnO and GNPs at the interface of the created heterostructures [165].
As shown in the above figure, the PR peak of the GNPs was broadened and red shifted towards higher wavelengths as reported before in similar heterostructures [166,167]. Because metallic gold nanoparticles are considered as highly sensitive materials to the surrounding medium and the size of the NPs [168,169], the PR peak will be influenced with its size, and after adding the ZnO QDs over. Since the refractive index of ZnO NPs (nZnO=2) in the colloidal gold solution is higher than that of water (nwater=1.34), the red shift in the PR band is noticed.

Heterojunction modified SC’s fluorescence

The optical properties of the SC will be highly affected in the presence of nearby metallic nanoparticles in one system. The M/SC interface has an inevitable role on the optical response of the SC nanoparticles, by which PL enhancement or quenching occurs at the metal’s surface [170–174]. Mainly, there are two concepts stating the influence of the luminescence of the semiconductor when coupled to metals; the luminescence enhancement and quenching that has been generated since 1980’s [175–177]. Viste et al. had studied the effect of local surface plasmon resonance of GNPs on the emission wavelengths of quantum dots [178]. The SC emission properties depend on the characteristics of the adjoining MNPs [22]. However, this influence is confusing and still under debate.

Stimulating charge transfer after photo-induced charge separation

The M/SC hybrid nanosystems have gained much attention by several researchers, where the hybrid nanostructure will endorse fast charge transfer upon light excitation, resulting in the charge separation.
If the stimulating energy is in the visible range, the metallic nanoparticles will generate charges that will then transfer to the SC band gap [8,11,52,216,217]. Upon UV excitation, an opposite proposition occurs where the photoexcited charges will transport from the semiconductor to the plasmonic metal systems, where only the semiconductor is excited [78,218]. After simultaneous excitations of both components, the semiconductor will use the SPR of the metal to enhance its light absorption, which is a valuable concept in photovoltaic and photocatalytic applications. This will be illustrated in Scheme 5.

Engineering of plasmonic-semiconductor nanomaterials

As any given nanomaterial, hybrid nanoparticles are also synthesized by the two general approaches: the chemical bottom-up way, and physical top-down mode. As described above, based on the state of art about the plasmonic-semiconductor nanomaterials, three methods were mainly adapted to fabricate the M/SC hybrid systems [227–233]:
1. The first preparation is mixing the pre-grown semiconductor nanostructures into a metal salt solution, and then the addition of appropriate reducing agent is followed. By this strategy, the chemically reduced metal nanoparticles are supposed to be coupled to the semiconductor [17,65,232,234]. The shape and size of MNPs could be highly controlled through this process.
2. The second approach is the nucleation and growth of the oxide semiconductor (OSC) at the surface of the pre-existing metal nanoparticles. The oxide SC starting material is reduced, and then decomposed into OSC nanoparticles [155].
3. Another preparation method is the sequential approach, where coupling of the two constituents that are both already previously prepared is adapted. In some cases of this approach, additives as molecular linkers or stabilizers may be necessary to add [64,163].
On the basis of this chapter and particularly this final section that shows some disadvantages in the manufacturing of M/SC nanomaterials, we emphasize in the next chapters a new way to fabricate ZnO-Au NPs. Precisely, we describe a chemical approach allowing to control the coupling between Au and ZnO NPs in order to enhance both the optical and catalytic properties of ZnO nanostructures.

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Preparation of stock solutions (Zn and O sources)

The zinc nitrate stock solution and the sodium hydroxide stock solution were prepared in deionized water. Three molar concentrations (0.1, 0.5 and 1 M) of each stock solution was prepared. These aqueous solutions were prepared by constant stirring at 350 rpm using magnetic stirrer without any heating step for ~30 min to completely achieve the suspension of the precursors and to obtain clear and transparent solutions.

Preparation of ZnO QDs seeds

Equal volumes of equimolar stock solutions were added up together in a flacon, i.e. a volume of 0.1 M NaOH is added to the same volume of 0.1 M Zn(NO3)2.6H2O to form the seed solution A1’. The same process is repeated with the 0.5 M and 1 M for the seed solution A2’ and A3’, respectively, which contain equal molarities and volumes of Zn(NO3)2.6H2O and NaOH stock solutions. After the addition of NaOH to the zinc nitrate solution, a direct formation of white precipitate was observed. Then, the solution separated into two phases: a white bottom layer including ZnO clusters embedded in a complex structure, and an upper suspension that was nearly transparent including very small quantity of ZnO QDs. The quantity of the white precipitate in A3’ was larger than that in A2’ and A1’ where the latter had a very low quantity which was difficult to collect. This is related to the concentrations of the stock solutions used by which the highest concentrations lead to the high production of ZnO powders. The samples A1’, A2’, and A3’ were heated at 80°C for 20 min with stirring at 350 rpm on a normal heating plate. During the reaction, the solutions were covered to prevent the oxygen of the atmospheric air to contribute in the reaction, minimizing the excess formation of defects in the crystal structures. Annealing of the ZnO product after cooling to room temperature was indeed in order to crystallize ZnO. The post-thermal treatment was achieved for 1 hr at 150°C even before filtration of the obtained powders or after (see discussion part). The final obtained annealed raw material solutions (before filtration) of initial starting materials concentrations of 0.1, 0.5, and 1 M were named A1, A2, and A3, respectively. Figure 14 is shows an image of the suspension A3 obtained to clarify the different supernatant and precipitate phases obtained in the reaction.

Table of contents :

1. Chapter 1 State of Art
1.1 Introduction
1.2 Plasmonic-semiconductor hybrid nanomaterials
1.3 ZnO-Au: The studied plasmonic-semiconductor nanomaterial
1.3.1 ZnO nanostructures
1.3.2 Gold nanoparticles
1.4 New/Enhanced features brought by plasmonic-semiconductor heterojunction
1.5 Tunable optical properties (absorption and fluorescence)
1.5.1 Heterojunction modified absorption of both materials
1.5.2 Heterojunction modified SC’s fluorescence
1.5.3 SERS enhancement
1.5.4 Stimulating charge transfer after photo-induced charge separation
1.6 Applications of plasmonic-semiconductor heterojunctions
1.6.1 Solar cells
1.6.2 Catalysis-based applications
1.7 Engineering of plasmonic-semiconductor nanomaterials
1.8 Conclusion
1.9 References
2. Chapter 2 Synthesis and characterization of pure ZnO nanostructures highly efficient for photocatalysis
2.1 Introduction
2.2 Experimental part
2.2.1 Synthesis process
2.2.1.1 Preparation of stock solutions (Zn and O sources)
2.2.1.2 Preparation of ZnO QDs seeds
2.3 Results and discussion
2.3.1 Optimization of the synthesis reaction parameters
2.3.1.1 Effect of raw materials concentration
2.3.1.2 Role of synthesis time, temperature, and post-thermal treatment
2.3.1.3 Effects of synthesis time
2.3.1.4 Effects of synthesis temperature
2.3.2 Structural properties
2.3.2.1 TGA Analysis
2.3.2.2 XRD analysis
2.3.2.3 XPS analysis
2.3.2.4 SEM analysis
2.3.2.5 TEM analysis
2.3.3 Optical properties
2.3.3.1 Absorption measurements
2.3.3.2 Photoluminescence measurements
2.3.3.3 Photocatalytic activity
2.4 Conclusion
2.5 References
3. Chapter 3 ZnO-Au heterojunction enhanced the fluorescence and photocatalytic properties of ZnO NPs
3.1 Introduction
3.2 Experimental part
3.2.1 Reagents
3.2.2 Synthesis of gold nanoparticles (GNPs)
3.2.3 Coupling ZnO to GNPs
3.3 Results and discussion
3.3.1 Role of the semiconductor part (ZnO size) in the interaction with GNPs
3.3.2 Role of the plasmonic part (GNPs size or number, capping) in the interaction with ZnO nanocrystals
3.3.3 Photocatalytic performance of ZnO-GNPs
3.3.4 Other strategies of plasmon-exciton coupling
3.3.4.1 Real-time synthesis of GNPs over ZnO NPs: real-time interaction
3.3.4.2 Capping GNPs after reduction
3.3.4.3 Tailoring the heterojunction distance between ZnO and GNPs: Free capping hybrid nanoparticles
3.4 Conclusion
3.5 References
4. Chapter 4 Conclusions and perspectives
4.1 Conclusions
4.2 Future work and perspectives
4.3 References
5. Chapter 5 Supplementary information
5.1 Appendix-A
5.1.1 Effect of dilution in PL measurements
5.1.2 Reproducibility (Stability of dispersions)
5.1.3 Raw materials ZnO coupled into different CTAB-capped GNPs
5.1.4 Raw materials ZnO coupled into different CTAB-capped GNPs
5.1.5 CTAB-capped ZnO NPs (no GNPs)
5.1.6 ZnO NPs embedded PMMA holes
5.1.7 Role of ZnO mass embedded in PMMA holes
5.1.8 Different stabilizers mediated the heterojunction distance
5.1.9 Large area monolayer of ZnO NPs
5.2 Appendix-B
5.2.1 Characterization techniques:
5.2.1.1 Photoluminescence (PL)
5.2.1.2 UV-visible absorption spectroscopy
5.2.1.3 X-Ray diffraction (XRD)
5.2.1.4 Transmission electron microscopy (TEM)
5.2.1.5 Scanning electron microscopy (SEM)
5.2.1.6 Photocatalysis
5.2.1.7 Thermogravimetric analysis (TGA)
5.2.1.8 X-ray photoelectron spectroscopy (XPS)
5.2.1.9 Atomic Force microscopy (AFM)
5.2.1.10 Optical microscopy
5.3 References

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