C H A P T E R
4 Nanoscale Characterization Srabanti Ghosh and Rajendra N. Basu Fuel Cell & Battery Division CSIR-Central Glass and Ceramics Research Institute, Kolkata, West Bengal, India
4.1 INTRODUCTION Noble metal metal oxide hybrid nanoparticles (HNPs) represent an important class of nanomaterial for the tuning of the optical, electrical, magnetic, and catalytic properties of nanocrystals [1 4]. In HNPs, two different functional materials (i.e., metal/magnetic oxides or metal/semiconductor oxides) are combined through surface reconstruction around the junction , lattice mismatch-induced crystal strain , and electron interaction/ transfer across the interface , etc. These multifunctional nanomaterials can exhibit novel physical and chemical properties, which are essential for future technological applications. For example, core shell [8,9], yolk shell [10,11], and heterodimers [12,13], which can enable electronic [14,15] and magnetic  coupling between the constituent domains and therefore allow for multifunctionalities that are not possible in single-component systems. Broad range of applications of HNPs originated from strong interaction between metal and metal oxides, which facilitate the dispersion of small particles hereby promoting utilization of expensive noble metal and also improved the catalytic properties of metal NPs [17,18]. Control of the dimension of each component of HNPs permits the widespread engineering of electronic energy state configuration within the nanoscale architecture, which makes them promising materials for a wide range of applications in biomedical imaging , cancer treatment , solar-energy harvesting [21,22], heterogeneous catalysis [23,24], photonics , and optoelectronics . Moreover, a controlling mechanism for improved physicochemical properties can be achieved by precisely manipulating the charge transfer, charge carrier dynamics, and electron hole separation within the HNPs. Up to now, the synthesis of HNPs has been accomplished by various approaches, most of which have been generally based on seeded growth techniques which allowing magnetic, metallic, and fluorescent spherical components to be combined into binary (such as of Fe3O4 Ag ,
Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00004-8
© 2019 Elsevier Inc. All rights reserved.
4. NANOSCALE CHARACTERIZATION
Au Fe3O4, , Ni Fe2O3 , Pd Cu2O , Fe3O4 Pt [31,32], Au TiO2 , Ag TiO2 , Pt, Pd, Ag, Au Cu2O ) and ternary HNPs (such as ZnO/Pt/Cd12xZnxS , Au Fe3O4 Au , Fe3O4 Au PbSe  Pt Fe3O4 MnOx , Ag Pt Fe3O4 [38,39]). Further developments have demonstrated the possibility to grow foreign material sections onto selected locations of anisotropically shaped NCs. Examples include ZnO decorated with Ag , Au ZnO hybrid nanopyramids , etc. Over the past decades, a number of excellent reviews describing the various aspects of metal metal oxides HNPs have been published [42 44]. In particular, methods for characterization of hybrid nanomaterials, such as preparations, final compositions, or large-scale products, generally are in the stage of development. This chapter provides an overview of recent progress in the characterization of metal oxide noble metal nanohybrids, particularly highlighting the general characterization techniques to understand the morphology controlled formation of metal metal oxides HNPs that contain noble metal and magnetic or semiconductor nanoparticles, and illustrating the interesting optical and magnetic properties found in these hybrid particles determined through various advanced characterization tools. For example, integration of noble metals (e.g., Au, Ag, Pt, Pd, etc.) and metal oxides (e.g., Fe3O4, TiO2, ZnO, CeO2, ZrO2, etc.) into the core shell or yolk shell single nanostructures can be studied by measuring the metal oxide shell of a certain thickness via transmission electron microscopy. Electron and atomic force microscopy, optical spectroscopy, and radiation scattering techniques are widely used. Applying these techniques to measure nanoparticle size, structure, and composition can help to understand the underlying synthesis mechanism. High resolution transmission electron microscopy (HRTEM) coupled with energy dispersive X-ray spectroscopy (EDS) are essential tools for the structural characterization of the HNPs providing lattice parameters and revealing the crystal structure . Aberration-free high-angle annular dark field (HAADF)  and scanning TEM (Z-STEM)  are also attractive tools to analyze the chemical composition of the HNPs as they allow different elements to be imaged separately and it is possible to have a clear understanding of the structural basis of the hybrid structures, especially the core/multishell structures. Z-STEM is highly sensitive to the atomic number and can thus be exploited to achieve atomic resolution elemental mapping of the hybrid structures. All of these measurements give valuable information regarding the nanoparticle’s physical behavior, but it is important to realize under which conditions the measurements have been performed. Moreover, combining metal and a semiconductor nanoparticle has been found to be interesting as the metal can provide an anchor point for electrical and chemical connections to the functional semiconductor part. Significant changes of the optical properties of the semiconductor material upon coupling with the metal nanoparticles have also been observed. The optical properties of these HNPs are determined by the complex interaction between the enhancement of the local excitation field and the modification of radiative and nonradiative exciton decay rates which includes a shift in the plasmon resonance of noble metal nanocrystals or changes in the photoluminescence intensity of semiconductor nanocrystals . These structures are found to be photocatalytically active because of their appropriate band alignment for water photolysis. Hence, it is deemed important to characterize a final HPNs product to obtain better insight into the design and application of well-defined nanohybrids in both the energy and environmental fields (Fig. 4.1).
4.2 MORPHOLOGICAL CHARACTERIZATION
FIGURE 4.1 Schematic representation of metal metal oxides hybrid nanoparticles.
4.2 MORPHOLOGICAL CHARACTERIZATION 4.2.1 Transmission Electron Microscopy The most widely used technique for characterizing nanoparticles is transmission electron microscopy (TEM) or high resolution TEM (HR-TEM), which provide direct visual information on the size, dispersity, structure, and morphology of nanoclusters [49 51]. TEM has been used for materials characterization for a long time but its need has increased after the realization of the vast possible scope of property tailoring with the decreasing length scale of materials in various dimensions like thin-films, nanotubes/nanorods/quantum wires, nanoparticles/quantum dots, etc. Manna and coworkers  provided a study on the formation of Au FexOy heterostructures in which spinel ferrite (FexOy) was grown on a spherical gold (Au) from the TEM images. Fig. 4.2A demonstrates a typical sample of Au FexOy heterostructures, the FexOy nanorod section grown on the Au seeds in the presence of dodecyl dimethyl ammonium bromide (DDAB) has diameters in the 5 6 nm range and is 60 80 nm long and in each nanoparticle the Au seed could be located at any position along the rod section. HRTEM analysis of these nanostructures give clear evidence that the rods always grew along the [2 2 0] direction of spinel ferrite (Fig. 4.2B). In addition, a full preferential orientation relationship is determined between the gold and the spinel ferrite nanocrystals, i.e., Au(0 0 2)//spinel ferrite(1 1 1) and Au[2 2 0]//spinel ferrite[2 2 0].
4. NANOSCALE CHARACTERIZATION
FIGURE 4.2 (A) TEM image of Au FexOy HSs. (B) HRTEM image of Au FexOy HSs with the epitaxial relationship between Au NCs and FexOy HSs, the lower inset sketching the growth direction of the FexOy nanorod on the Au seed. (C) Atomic models sketching the atomic periodicity along one couple of gold and FexOy facets that are in contact in the Au FexOy HSs. For iron oxide only the Fe tetrahedral atomic sub-lattice is shown. Source: Reproduced from C. George, A. Genovese, F. Qiao, K. Korobchevskaya, A. Comin, A. Falqui, et al., Optical and electrical properties of colloidal (spherical Au) (spinel ferrite nanorod) heterostructures, Nanoscale 3 (2011) 4647 4654 with permission from The Royal Society of Chemistry, 2011.
This symmetry relation can be rationalized by considering that eight surface unit cells of the (0 0 2) gold facets match with one surface unit cell of the (1 1 1) spinel ferrite facet (Fig. 4.2C). Such periodicity leads to a commensurate epitaxy with low lattice mismatch values (m) along two directions lying in the interfacial plane, the former parallel to the rod elongation direction (m 5 1.03%) and the latter perpendicular to it (m 5 1.73%). Sun et al.  developed a general method for coating oxides having NPs, nanowires (NWs), and nanotubes of different compositions combined with noble metal to create a
4.2 MORPHOLOGICAL CHARACTERIZATION
69 FIGURE 4.3 TEM images of the [email protected]
NPs (dAu 5 40 nm) with different kinds of oxide shells: (A) [email protected]
, (B) [email protected]
, (C) [email protected]
, (D) [email protected]
. Insets show magnified views of typical NPs. Source: Reproduced from H. Sun, J.T. He, J.Y. Wang, S.-Y. Zhang, C.C. Liu, T. Sritharan, et al., Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation, J. Am. Chem. Soc. 135 (2013) 9099 9110 with permission from American Chemical Society, 2013.
large variety of core shell nanostructures. Fig. 4.3 shows TEM images of four distinct nanohybrids that are generated by growing Au nanoparticles using different metal oxides. Notably, a common problem in generating these oxide shells is that their rates of reactions can be quite different. In contrast to pure oxide NPs, a large amount of oxide is obtained in the product. For oxide shells, the reaction needed to be slowed to reduce homogeneous oxide nucleation; in fact the most effective way is to use less reactive reactants, and also change the solvent, lower the reactant concentration, or increase the seed concentration. The sample shown in Fig. 4.3A represents a typical [email protected]
, which is formed using ethanol as the solvent, which reduced the hydrolysis rate of TiF4 and led to successful TiO2 encapsulation. Fig. 4.3B shows a different sample of Au Fe3O4 core shell synthesized using FeCl2 which reacted slower, allowing the formation of uniform oxides shells, that can be suitably oxidized to give Fe3O4, and while FeCl3 can be used to generate iron oxides, it leads to fast reaction, causing the homogeneous nucleation of pure α-Fe2O3 spindles and therefore the presence of FeCl2 was a significant criteria for the synthesis of the Au Fe3O4 core shell structures. Further, Mn(CH3COO)2 is used as the Mn source for generating MnO shells on Au in place of using MnCl2 which leads to fast hydrolysis (Fig. 4.3C). Fig. 4.4D shows [email protected]
core shell structures synthesized from a rare earth oxide of Eu2O3 nanoparticle seeds. Sun et al.  further extended this study to developed a general method for coating of ZnO on different seeds. Citrate-stabilized Au, Ag, and Pt nanospheres can be easily coated with ZnO shells using 4-mercaptobenzoic acid as ligand (Fig. 4.4A C). Moreover, Pd nanospheres, Ag nanocubes, and Ag nanowires that were stabilized by polyvinylpyrrolidone (PVP) can be directly coated with ZnO without addition
4. NANOSCALE CHARACTERIZATION
FIGURE 4.4 TEM images and photographs of [email protected]
NPs that were synthesized from different noble metal cores: citrate-stabilized NPs, including (A) Au nanospheres (dAu 5 15 nm); (B) Ag nanospheres (dAg 5 60 nm); and (C) Pt nanospheres (dPt 5 40 nm); and PVP-stabilized NPs, including (D) Pd nanospheres (dPd 5 20 nm); (E) Ag nanocubes (dAg 5 150 nm); and (F) Ag NWs (dAgNW 5 120 nm, lAgNW 5 3 5 μm). Insets show magnified views of typical NPs. Scale bar: 200 nm. Source: Reproduced from H. Sun, J.T. He, J.Y. Wang, S.-Y. Zhang, C.C. Liu, T. Sritharan, et al., Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation, J. Am. Chem. Soc. 135 (2013) 9099 9110 with permission from American Chemical Society, 2013.
of a ligand as shown in TEM images (Fig. 4.4D F). Particularly, for the Ag nanocubes and nanowires, it can be observed that the uniform ZnO shells conformed to the shape of the seeds.
4.2.2 Atomic Number Contrast Scanning Transmission Electron Microscopy Atomic number contrast scanning transmission electron microscopy (Z-STEM) provides an exceptional ability to achieve structural and chemical information from individual nanostructures at the atomic level . Principally, Z-STEM uses a HAADF detector to collect an incoherent image, a direct image of the object’s structure [55 57]. In contrast to traditional HRTEM that uses phase-contrast imaging to gain insight into the crystalline nature of the particles, the intensity seen in the Z-STEM images depends on the scattering power of the atom being imaged, yielding chemical information simultaneously with structural position, which makes Z-STEM an ideal tool for studying core shell structures at the atomic level. The intensity difference between the core and shell or heterodimer or heterotrimer seen in Z-STEM images allows precise characterization of shell shape, coverage, chemical composition, and the presence of any extended defects. For example, ZSTEM tomography was used to probe the chemoselective addition of Ag to Pt Fe3O4 heterodimer seeds to form Ag Pt Fe3O4 heterotrimers . Fig. 4.5A C displays HAADF STEM images of Ag Pt Fe3O4 samples at different time interval (A) 15 min aliquot, (B) 60 min aliquot, and (C) the final product after the 14-h reaction, respectively. Fig. 4.5A and B demonstrates small particles decorating both the Pt and Fe3O4 surfaces
4.2 MORPHOLOGICAL CHARACTERIZATION
FIGURE 4.5 Representative HAADF STEM images of aliquot samples taken from the Ag Pt Fe3O4 synthesis at (A) 15 min and (B) 60 min into the reaction, as well as (C) the final Ag Pt Fe3O4 heterotrimer product isolated after 14 h. The corresponding EDS elemental maps are shown in panels D G, H K, and L O, respectively, and indicate that Ag indiscriminately nucleates on the Pt Fe3O4 heterodimers to form various Ag (Pt Fe3O4) intermediates, followed by coalescence onto the Pt domain to form Ag Pt Fe3O4. Source: Reproduced from J.M. Hodges, J.R. Morse, M.E. Williams, R.E. Schaak, Microscopic investigation of chemoselectivity in Ag Pt Fe3O4 heterotrimer formation: mechanistic insights and implications for controlling high-order hybrid nanoparticle morphology, J. Am. Chem. Soc. 137 (2015)15493 15500 with permission from American Chemical Society, 2015.
of the Pt Fe3O4 heterodimer seeds while, after 14 h, the expected final nanoparticle product has Ag Pt Fe3O4 heterotrimer architecture as in Fig. 4.5C. Further STEM EDS is used to create elemental maps of each sample. The STEM EDS maps for the aliquot taken after 15 min (Fig. 4.5D G), which show small Ag nanoparticles attached to both the Pt and Fe3O4 surfaces of the Pt Fe3O4 heterodimer seeds (marked as Ag (Pt Fe3O4)), as well as Pt Fe3O4 heterodimers having no detectable Ag. After 60 min, the number and size of the Ag NPs growing on the Pt Fe3O4 seeds has increased, and the formation of Ag Pt Fe3O4 heterotrimers has initiated, as indicated by the STEM EDS maps shown in Fig. 4.5H K. The STEM EDS maps illustrated in Fig. 4.5L O confirm that after 14 h the final nanoparticle product is Ag Pt Fe3O4 heterotrimer configuration.
4. NANOSCALE CHARACTERIZATION
The formation of the Ag Pt Fe3O4 heterotrimers initiates with indiscriminate Ag nucleation onto both the Pt and Fe3O4 surfaces of Pt Fe3O4, followed by surface diffusion and coalescence of Ag onto the Pt surface to form the Ag Pt Fe3O4 product. These results provide exceptional microscopic insights into the pathway by which Ag Pt Fe3O4 heterotrimer NPs form and controlling nucleation and growth therefore permits the development of high-order HNPs with precisely targeted morphologies and properties. Ortalan and coworkers  identified a strong metal support bonding in nanoengineered Au Fe3O4 dumbbell-like NPs by in situ TEM where the average diameters of the Au NPs and the Fe3O4 NPs are 5.0 and 10.4 nm, respectively. Drastic morphological changes of Au NP from a spherical NP to Au thin films (i.e., complete wetting) on Fe3O4 during the vacuum annealing directly indicates the presence of strong bonds between Au and Fe3O4 during the in situ annealing experiment. Further, in situ STEM-coupled with electron energy loss spectroscopy (EELS) as well as in situ electron diffraction results indicates that the core part of the final state still has a form of unreduced Fe3O4 and this suggests that the drastic morphology change of the dumbbell NPs is due to the interaction between the Au thin film and the iron oxides. Very recently, Lord et al.  tested electrical contacts with multiprobe electrical transport measurements and correlated this behavior directly to aberration-corrected scanning transmission electron microscopy (ac-STEM) for the Au catalyst ZnO nanowire system which is the only known material that exhibits quantummechanical edge tunneling in such a way that the effect can be used to modulate the transport properties from Schottky to ohmic. The ac-STEM analysis shows atomic- and nanoscale modifications to the interface edge can entirely alter the transport properties rather than the less-influential central zone of the circular Au ZnO interfaces.
4.2.3 Scanning Tunneling Microscopy Scanning tunneling microscopy is the most suitable technique to obtain a single-dot image at room temperature (TE 300K) as well as low temperatures (T 5 4.2 100K) . The atomic-resolution of STM is attributed to the imaging mechanism based on the quantum tunneling phenomena and STM measures the tunneling current I that is generated by a bias voltage V between the atomically sharp STM tip and the material surface [61,62]. ˚ reduction in disThe tunneling current increases by an order of the current for every 1 A tance. The distance in the xyz-directions is also controlled by a piezoelectric scanner, which provides angstrom-order changes in the distance, and a feedback loop, which controls the z-direction, is installed to keep the tunneling current constant. By observing the tunneling current as the tip scans a surface, the morphology of the surface can be precisely detected. Measurements can be carried out at room temperature in solution as well as at low temperature under ultrahigh vacuum (UHV) conditions. For example, Rieboldt et al.  utilized STM measurement to study the nucleation and growth of Pt NPs on TiO2 (1 1 0) surfaces of different oxidation state such as with O on-top atoms (oxidized TiO2 represented as o-TiO2), surface O vacancies (represented as r-TiO2), and H adatoms, respectively (reduced TiO2 represented as h-TiO2). At room temperature, Pt is found to be trapped at O on-top atoms and surface O vacancies, leading to rather small Pt NPs. In contrast, on surfaces with H adatoms the mobility of Pt is much larger and large Pt NPs are found at room temperature on TiO2 (1 1 0) surfaces with H adatoms. Fig. 4.6 illustrates
4.3 QUANTIFICATION OF METAL CONTENT IN NANOHYBRIDS
FIGURE 4.6 STM images of the r-TiO2 (A and D), h-TiO2 (B and E), and o-TiO2 (C and F) surfaces after evaporation of Pt at RT. The STM images were acquired at RT. Source: Reproduced from F. Rieboldt, L.B. Vilhelmsen, S. Koust, J.V. Lauritsen, S. Helveg, L. Lammich, et al., Nucleation and growth of Pt nanoparticles on reduced and oxidized rutile TiO2 (1 1 0), J. Chem. Phys. 141 (2014) 214702 with permission from American Institute of physics, 2014.
STM images acquired following the evaporation of B2.5% ML Pt at room temperature (RT). In case of the r-TiO2 (Fig. 4.6A and D) and o-TiO2 (Fig. 4.6C and F) surfaces, high densities (0.07 6 0.01 nm22) of small Pt NPs with homogeneous NP distributions (15% 20% of Pt NPs at step edges) are obtained. In contrast, larger Pt NPs are found on the h-TiO2 surface and their density was at 0.030 6 0.005 nm22 rather low (Fig. 4.6B and E). Hence, the STM results indicate that Pt NPs or Pt atoms on the h-TiO2 surface have a higher mobility at RT than Pt on r- and o-TiO2 surfaces and thus larger NPs are formed on h-TiO2.
4.3 QUANTIFICATION OF METAL CONTENT IN NANOHYBRIDS The mass percentages of metal and metal oxides in HNPs are measured by inductively coupled plasma optical emission spectra (ICP-OES) analysis or inductively coupled plasma mass spectrometry (ICP-MS) . ICP-MS has become the technique of choice for detection and characterization of nanoparticles in solution. Compared with other techniques, ICPMS is unique in its ability to provide information on elemental composition, with high sensitivity, multielement capability, wide linear dynamic range, high sample throughput, and ability to discriminate between isotopes [65,66]. ICP-MS is capable of scanning
4. NANOSCALE CHARACTERIZATION
mass-to-charge (m/z) range 5 240 amu with a minimum resolution of 0.9 amu at 10% peak height. In principle, ICP-MS can be used to directly detect and quantify the signal generated from the atomization and ionization of a single particle introduced to the plasma, known as “single particle mode.” If the masses and densities of the elemental constituents of the particle are known along with the elemental response factor based on an ionic calibration standard, then the theoretical size of the particle, calculated as a sphere, can be determined. If the transport efficiency from the nebulizer to the plasma is also known, then the particle number concentration can be determined. For example, the mass percentages of Pd and ZnO in four Pd/ZnO composites are determined by ICP-OES analysis. The measured mass percentages of Pd in each composite (2.67% 3.5%) were generally larger or close to the theoretical mass percent of Pd (B2.9%) based on the experimental dosages of Pd(acac)2 and Zn(acac)2, indicating that the addition of Pd(acac)2 to the precursor solution is almost completely transferred into Pd NPs and incorporated into the metal oxide support reported by Bao et al. . Recently, single-particle or particle-mode ICPMS (spICP-MS) has been considered as a novel nanoparticle characterization technique, which is used in the time-resolved mode for the measurement of dilute NP dispersions (particle concentrations less than 105/mL are adequate) [68,69]. After the statistical evaluation of the signal time profile and assuming a spherical NP geometry, information can be obtained about not only the elemental (isotopic) composition of the NPs, but also their characteristic size and distribution, as well as the particle concentration. For metallic NPs, size detection limits ranging from 10 to 30 nm and upper detectable size limits around or above a few hundred nm are typically reported . The spICP-MS technique has been used to determine the concentration of Pt in a Pt/silica nanocomposite by Sa´pi et al. . Fig. 4.7A shows that the lognormal Pt NP peak in the signal histogram is well resolved
Pt spICP-MS signal histogram recorded for the Pt/SiO2 nanocomposite. Please note that the lognormal NP signal peak is well separated from the solution background signal. The characteristic intensity of the NP peak in the histogram corresponds to an equivalent NP size of 20.4 nm in the spICP-MS size calibration curve. Source: Reproduced from A. Sa´pi, A. Ke´ri, I. Ka´lomista, D.G. Dobo´, A. Szamosvo¨lgyi, K.L. Juha´sz, et al., Determination of the platinum concentration of a Pt/silica nanocomposite decorated with ultra-small Pt nanoparticles using single particle inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 32 (2017) 996 1003 with permission from The Royal Society of Chemistry, 2017.
4.4 CRYSTAL PHASE CHARACTERIZATION THROUGH X-RAY TECHNIQUES
from the background peak. The mode of the particle peak was found to be equivalent to the signal from a 20.4 nm diameter spherical Pt particle. Considering the size and density of the support and the load particles, the Pt wt.% concentration is calculated to be 0.0966 wt.%, with a standard deviation of 0.0025 based on three repeated measurements.
4.4 CRYSTAL PHASE CHARACTERIZATION THROUGH X-RAY TECHNIQUES XRD is powerful technique that has long been used to address all issues related to the crystal structure of solids, including lattice constant, geometry, identification of unknown materials, orientation of single crystal, preferred orientation of polycrystals, defect, etc., and can also be used to determine the interfacial strain by comparing and calculating the shift from the spectra of the individual component [72,73]. The diffraction pattern is used to identify the crystalline phases and measure its structural properties. The broadening of the XRD peaks reflects either crystallinity or the size of the nanocrystal. Assuming that the crystallinity of nanoparticles is not too different, the broadening of the XRD peaks reflects the size of nanocrystals only: smaller nanocrystals have a wider reflection peak. However, the nanoparticles often form twinned structures; therefore, the Scherer’s formula may produce results different from the true particle size. In addition, X-ray diffraction only provides the collective information of the particle sizes and usually requires a sizable amount of powder. Compared to electron diffraction, the low intensity of diffracted X-rays is obtained particularly for low Z-materials, i.e., XRD is more sensitive to high Z-materials. Bao et al.  developed a one-pot synthetic methodology of noble metal/zinc oxide composites with controllable morphology and high catalytic performance controllable morphology including tube-like, flower-like, star-like, and skin needling-like. The crystal structures of the Pd/ZnO composites are examined by the XRD patterns (Fig. 4.8). All XRD diffraction peaks of the four Pd/ZnO composites could be indexed as a combination of the typical wurtzite structure of ZnO (JCPDS 36-1451) and the face centered-cubic structure of Pd (JCPDS 46-1043). FIGURE 4.8 XRD patterns of the tube-like (A), flower-like (B), star-like (C), and skin needlinglike (D) Pd/ZnO composites. The red color belongs to the wurtzite structure of ZnO and the blue color belongs to the face-centered-cubic structure of Pd, respectively. Source: Reproduced from Z. Bao, Y. Yuan, C. Leng, L. Li, K. Zhao, Z. Sun, One-pot synthesis of noble metal/zinc oxide composites with controllable morphology and high catalytic performance, ACS Appl. Mater. Interfaces 9 (2017) 16417 16425 with permission from American Chemical Society, 2017.
4. NANOSCALE CHARACTERIZATION
However, it is difficult to observe the signal from Pd NPs in the composites due to the small size of the Pd NPs in the four composites compared to ZnO particles. Tremel and coworkers  used Rietveld refinements to the powder XRD data to characterize directly the epitaxial growth of γ-Fe2O3 nanorods to the Pd nanotetrahedra ([email protected]
) and nanoplates ([email protected]
). Crystalline phases were identified according to the PDF-2 database using Bruker diffractometer. Full profile fits (Le Bail/Pawley/ Rietveld) were performed with TOPAS Academic version 4.1 by applying the fundamental parameter approach [75,76]. Fig. 4.9A shows the X-ray diffractogram of the Pdnth@γFe2O3 superparticles. The reflections can be assigned to Pd as well as γ-Fe2O3. The γ-Fe2O3 nanorods show a (1 1 1) orientation on the Pd(1 1 1) surface. The intensity of the Pd reflections is much lower compared to corresponding reflections for the Pd nanotetrahedra due to the absorption by the surrounding γ-Fe2O3 nanorods (Fig. 4.9A). Hence, the maghemite [Fe0.67(1)O] content with a crystallite size of 13(1) nm could be refined. For [email protected]
superparticles (Fig. 4.9B), a FIGURE 4.9 Rietveld refinements to the powder XRD data of (A) [email protected]
superparticles and (B) [email protected]
superparticles. Red dots mark the experimental data; the black line corresponds to the calculated pattern, and the red line shows the difference between the experimental and calculated data. Black ticks mark reflections of Pd. Q 5 [4π sin(Θ)]/λ is the scattering vector. Source: Reproduced from M. Kluenker, M.N. Tahir, R. Ragg, K. Korschelt, P. Simon, T.E. Gorelik, et al., [email protected]
superparticles with enhanced peroxidase activity by solution phase epitaxial growth, Chem. Mater. 29 (2017) 1134 1146 with permission from American Chemical Society, 2017.
4.5 SURFACE CHARACTERIZATION
Pd content of 11(1) wt.% could be extracted from the Rietveld refinement. Hence, the presence of a Pd core, selectively overgrown with maghemite nanorods could be established for nanohybrids.
4.5 SURFACE CHARACTERIZATION The surface characterization of the HNPs involve common techniques like X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES), and they have penetration depths comparable to the typical dimensions of the HNPs [77,78]. The electronic structure of the constituent elements in the HNPs can be well-defined by XPS, giving further composition information of these NPs. The depth of the photoemitted electrons escaping from the very top surface of samples by the Al Kα X-ray source (1486.6 eV photons) is usually in the 0.5 3 nm range, comparable to half of the diameters of a NPs (for example, B5 nm). Thereby, XPS can help us to obtain the electronic structures of elements in cores and coatings, or whether they are metallic or oxidized. For example, XPS has been used to analyze surface composition of Au/ZnO nanowire hetero-nanoarrays, in which C 1s (284.8 eV) is used to calibrate the binding energies . The full XPS spectra confirm the existence of Zn, O, Au, and C elements and also reveal that the cross-linked Au/ZnO nanowire arrays are successfully formed without any existing impurity (Fig. 4.10A). The high resolution XPS spectrum for Zn element (Fig. 4.10B) in Au/ZnO nanowire arrays shows two broad peaks centered at 1021.8 eV and 1044.8 eV, which can be indexed as the signals from Zn 2p3/2 and Zn 2p1/2, respectively the binding energy of Zn oxides. Remarkably, there is a distinct peak located at about 530.6 eV which is associated with the lattice oxygen of ZnO, whereas the weaker shoulder peak at about 531.9 eV can be attributed to chemisorbed oxygen caused by the surface hydroxyl groups, as shown in Fig. 4.10C. The high energy resolution XPS spectrum for Au element (Fig. 4.10D) shows noticeable peaks centered at 83.6 eV and 87.3 eV that are attributed to Au 4f7/2 and 4f5/2, which exhibit a negative shift of 0.2 eV in comparison to 83.8 eV of the bulk Au. This minor shift is caused by electron transfer from plasmonic Au thin nanowires to ZnO nanowire arrays due to the strong electronic interaction between the Au and oxide support [80,81]. XANES spectroscopy is a well-established technique providing information on the electronic and structural properties of materials. X-ray absorption occurs in the region of approximately 40 eV above the edge and is sensitive to the treatment of interactions between the photoelectron and the core hole. In XANES, a photon is absorbed and electron is excited from a core state to an empty state and photon energy has to be equal or higher than the binding energy of this core level. Hence, the energy of an absorption edge corresponds to the core-level energy, which is characteristic for each element, making XANES an element selective technique. The evolution of the phases involved during the annealing can also be followed through the semiquantitative analysis of XANES spectra. The thermal evolution of Pt-Rich FePt/Fe3O4 heterodimers during the annealing under an inert atmosphere is followed by in situ time-resolved Pt L3 and Fe K edges XANES spectroscopy experiments . Fig. 4.11A shows the Fe K edge and Pt L3 XANES spectra of Pt-Rich FePt/Fe3O4 heterodimers. XANES features at the Fe Kedge similar to the magnetite (Fe3O4). The main component of the pre-edge peaks of Fe3O4 arises from tetrahedrally
4. NANOSCALE CHARACTERIZATION
Zn LMM Zn LMM O 1s Zn LMM
Intensity (a.u.) Zn 3d Au 4f Zn 3p
Binding Energy (eV)
Binding Energy (eV)
(D) Au 4f7/2
Binding Energy (eV)
Binding Energy (eV)
FIGURE 4.10 (A) Wide XPS spectrum and high resolution XPS spectra of (B) Zn 2p, (c) O 1s, and (D) Au 4f of cross-linked Au/ZnO nanowire arrays. Source: Reproduced from T. Wang, B. Jin, Z. Jiao, G. Lu, J. Ye, Y. Bi, Photodirected growth of Au nanowires on ZnO arrays for enhancing photoelectrochemical performances, J. Mater. Chem. A 2 (2014) 15553 15559 with permission from The Royal Society of Chemistry, 2014.
coordinated Fe3þ, and the shoulder corresponds to the octahedrally coordinated Fe2þ Fe3þ ions. The average oxidation state of iron from these XANES data is 2.5(1)þ, which is slightly less than expected for Fe3O4 (2.67þ), indicating the major iron phase is magnetite, and also contains a small percentage of a metallic phase (γ-Fe2O3). Fe K edge EXAFS are suitably fitted to a cubic spinel structure and the Fe-O distances are close to ˚ as expected for the tetrahedral and octahedral sites in Fe3O4, the values of 1.89 and 2.06 A respectively. A reduction of the amplitude of the second main peak is observed in comparison to Fourier transform of bulk Fe3O4, which is associated to the local structural disorder due to the higher percentage of atoms at the particle surface layer in the NPs. Further detailed analysis can be obtained from the Fourier transforms χ(R) of the EXAFS spectra at the Pt L3 edge shown in Fig. 4.11C. ˚ , which is shorter than The nearest neighbor distance Pt Pt (or Pt Fe) is close to 2.74 A ˚ the value expected for pure platinum (2.77 A). This indicates the incorporation of low quantity of iron into platinum forming a FePt alloy. The inhomogeneity in the Pt
4.6 SPECTROSCOPIC CHARACTERIZATION
FIGURE 4.11 XANES spectra at the (A) Pt L3 and (B) Fe K edges of as-made Pt-rich FePt/Fe3O NPs. Spectra of Fe and Pt reference foils are also shown for comparison. (C) Fourier transforms of k3χ(k) EXAFS at Pt L3 edge and at Fe K edge of the as-made Pt-rich FePt/Fe3O4 NPs. Solid lines correspond to the fitting results. XANES of bulk Fe3O4 is also shown for comparison. Source: Reproduced from M. Ahmad, S. Yingying, A. Nisar, H. Sun, W. Shen, M. Wei, et al., Synthesis of hierarchical flower-like ZnO nanostructures and their functionalization by Au nanoparticles for improved photocatalytic and high performance Li-ion battery anodes, J. Mater. Chem. 21 (2011) 7723 7729 with permission from American Chemical Society, 2011.
concentration lies in the different nucleation velocity, Pt atoms nucleate faster than the Fe ones, leading to a Pt concentration that decreases from inside to outside of the NP.
4.6 SPECTROSCOPIC CHARACTERIZATION 4.6.1 UV Vis and Photoluminescence Spectroscopy To unravel the photophysical properties of the HNCs, a combination of spectroscopic techniques are needed. Absorption, photoluminescence (PL), and PL excitation (PLE) provide the basic information about the exciton energy level structure. PLE spectra are
4. NANOSCALE CHARACTERIZATION
particularly better suited than the absorption spectra for the identification and assignment of absorption transitions since only the emitting states contribute to this spectra. Timeresolved PL spectroscopy serves as a quantitative tool for the analysis of photoexcitation dynamics in HNCs yielding information about both radiative and nonradiative exciton recombination channels. To probe the dynamics of other ultrafast processes, such as intraband relaxation, multiexciton generation, and exciton spatial separation, several advanced spectroscopic techniques like transient absorption, femtosecond fluorescence upconversion, and THz time domain spectroscopy can be used which provide complimentary information regarding the fast relaxation of electrons and holes. Optical spectroscopic technique can be generally categorized into two groups • Absorption and emission spectroscopy—determines the electronic structure of atoms or molecules or crystals through exciting electrons from the ground to excited states, i.e., absorption, and relaxing from the excited to ground states, i.e., emission. • Vibrational spectroscopy—involving the interaction of photons with the species that results in energy transfer via vibrational excitation or de-excitation and provides useful information about molecular structure. UV visible absorption spectroscopy is the easiest tool available to characterize nanocrystals . In addition one can get an estimate of the size distribution and concentration from the sharpness of the absorption peak. In the case of the combination of the metal and semiconductor, hybrid structures exhibit significant changes in the absorption spectrum which are not typical linear additions of the absorbance of the individual components. Amalgamation of the electronic states of metal and semiconductor results in modified density of states which in turn affects the absorption spectrum. Typically, upon the growth of the metal domains onto semiconductor substrate, the excitonic peak and the fine structure of the semiconductor component becomes less pronounced with the increase and shifting of the plasmonic band of the metals. Fig. 4.12 demonstrates the UV vis diffuse reflectance spectra of the pure porous TiO2 and the Aux/TiO2 nanohybrids with different Au loading . The pure porous TiO2 shows strong absorption in the UV region, whereas Aux/TiO2 nanohybrids reveals a broad absorption feature at B600 650 nm, which is assigned to the localized surface plasmon resonance (LSPR) of Au NPs supported on TiO2. The intensity of Au LSPR increases as the Au loading increased from 2 to 10 wt.%. The wavelength and intensity of the Au LSPR signal both depend on the Au particle size and shape, as well as the surrounding medium. The broadening of LSPR peaks of Au/TiO2 hybrid materials has been observed due to the broad size distribution Au NPs or located at different positions in the porous TiO2 support. Fluorescence spectroscopy is one of the most widely used spectroscopic techniques in the fields of material chemistry . Although fluorescence measurements do not provide detailed structural information, the technique has become quite popular because of its acute sensitivity to changes in the structural and dynamic properties of nanomaterials. The fluorescence spectroscopic studies can be carried out at many levels, ranging from simple measurement of steady-state emission intensity to quite sophisticated timeresolved studies. The fluorescence spectrum which owes its origin to the semiconductor component, in the presence of metal domains leads to interplay between fluorescence quenching and enhancement effects [86,87]. Fluorescence quenching may arise from
4.6 SPECTROSCOPIC CHARACTERIZATION
FIGURE 4.12 UV vis diffuse reflection spectra (DRS) of (i) pure porous TiO2, (ii) Au2/TiO2, (iii) Au5/TiO2, and (iv) Au10/TiO2. Source: Reproduced from B. Li, Y. Hao, X. Shao, H. Tang, T. Wang, J. Zhu, et al., Synthesis of hierarchically porous metal oxides and Au/TiO2 nanohybrids for photodegradation of organic dye and catalytic reduction of 4-nitrophenol, J. Catal. 329 (2015) 368 378 with permission from Elsevier, 2015.
energy transfer from the exciton in the semiconductor to the metal. Alternatively, enhancement has been observed by chemically linking gold nanocrystals to the surface of semiconductor nanowires. Such an enhancement is often seen in cases in which the semiconducting and metallic domains are separated by a small distance and a large potential barrier is present between the two domains. Kostopoulou et al.  reported the synthesis of anisotropic HNPs that are individually comprised of a single rod-shaped ZnO section, ubiquitously decorated with multiple nearly spherical [email protected]
domains. Fig. 4.13A and B shows TEM images of two representative examples of HNPs which are distinguished by a relative high and low surface coverage of the relevant ZnO NR cores, respectively (referred to as HNC-1 and HNC-2). The variable-dimension of ZnO NRs are studied by PL spectroscopy as shown in Fig. 4.13C and D. The nanorods exhibit a pronounced near band-edge (NBE) UV emission located at 3.2 eV for the seeds of the HNC-1 sample (Fig. 4.13A, upper curve) and at 3.25 eV for ZnO seeds used in the growth of the HNC-2 sample (Fig. 4.13B, upper curve). While the difference in the NBE between the two batches is less as the ZnO NR dimensions are much larger than the exciton Bohr radius (B2.34 nm) due to quantum confinement effects. Moreover, a broader and much weaker band emission was observed in the visible spectral region (B2.4 meV) for all samples, related to such deep level defects. It is important to note that the NBE emission is shifted to the blue spectral region in comparison to the parent NR seeds (Fig. 4.13). The NBE spectral shift upon coverage is ΔNBE 5 175 meV for the HNC-1 sample, while it is moved by ΔNBE 5 128 meV for the HNC-2 sample. The modifications in the optical properties of the ZnO should be a consequence of the coverage of its surface by the [email protected]
nanodomains. Time-resolved photoluminescence spectroscopy was applied to study the distinct differences between magnetic plasmonic heterodimers, [email protected]
and [email protected]
altered by the variation of the electronic structure of the metal oxides by Tremel and coworkers .
4. NANOSCALE CHARACTERIZATION
FIGURE 4.13 PL at 300K of the HNC-1 (A) and HNC-2 (B) samples compared to the corresponding ZnO NRs seeds (top spectra). The vertical dash line shows the blue shift of the NBE emission due to the [email protected]
coverage attained over ZnO. Each spectrum has been normalized to the associated NBE UV emission max intensity. Source: Reproduced from A. Kostopoulou, F. The´tiot, I. Tsiaoussis, M. Androulidaki, P.D. Cozzoli, A. Lappas, Colloidal anisotropic ZnO [email protected]
nanoarchitectures with interface-mediated exchange-bias and bandedge ultraviolet fluorescence, Chem. Mater. 24 (2012) 2722 2732 with permission from American Chemical Society, 2012.
Fig. 4.14 displays the fluorescence spectra and decay dynamics of the pristine Au NPs in comparison to [email protected]
and [email protected]
heterodimers. Thiol-functionalized Au NPs and [email protected]
heterodimers show a maximum of photoluminescence at 481 and 475 nm, respectively, and it is shifted to 463 nm for [email protected]
with an additional peak at 632 nm as shown in the time-integrated fluorescence spectra (Fig. 4.14A). The emission spectrum of Au NPs is almost corresponding with the one of [email protected]
heterodimers, which suggests the Au domains to be the origin of the fluorescence.
4.6.2 Fourier Transforms Infrared Spectroscopy Infrared spectrometry is a vibrational technique that involves coupling of highfrequency infrared (IR) electromagnetic radiation ranging from 1012 to 1014 Hz (3 300 μm wavelength), with vibration of chemical bond . In the infrared spectroscopy, the intensity of a beam of infrared radiation is measured before and after it interacts with the sample as a function of light frequency. A plot of relative intensity versus frequency is the “infrared spectrum.” As the interferogram is measured, all frequencies are being measured simultaneously. Thus, the use of the interferometer results in extremely fast measurements. Then the intensity time output of the interferometer is subjected to a well-known mathematical technique called the Fourier transformation to convert it to the familiar infrared spectrum. This transformation is performed by the computer, which then presents
4.6 SPECTROSCOPIC CHARACTERIZATION
FIGURE 4.14 (A) Time-integrated emission spectra of pure Au nanoparticles, [email protected]
and [email protected]
heterodimers. The samples were excited at 400 nm by a 100-fs laser pulse. (B) Photoluminescence dynamics monitored at the emission peak wavelength and stretched-exponential fits using the parameter inverse decay rates τ and stretching exponent β. Source: Reproduced from I. Schick, D. Gehrig, M. Montigny, B. Balke, M. Pantho¨fer, A. Henkel, et al., Effect of charge transfer in magnetic plasmonic [email protected]
(M 5 Mn, Fe) heterodimers on the kinetics of nanocrystal formation, Chem. Mater. 27 (2015) 4877 4884 with permission from American Chemical Society, 2015.
FIGURE 4.15 PSD DRIFT spectra collected at 498K during a MES experiment for both Au0.75Pd0.25/Al2O3 and Au0.80Pd0.20 FexOy/Al2O3; inset: magnification of the peak, indicating the evolution of the band components at increasing ϕdelay (from the weakest green spectrum to the weakest orange spectrum). Source: Reproduced from C. George, A. Genovese, A. Casu, M. Prato, M. Povia, L. Manna, et al., CO oxidation on colloidal Au0.80Pd0.20 FexOy dumbbell nanocrystals, Nano Lett. 13 (2013) 752 757 with permission from American Chemical Society, 2013.
the user with the desired spectral information for analysis. The surface active sites can be identified by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) . The enhanced catalytic activity of CO oxidation by the dumbbell (Au0.80Pd0.20 FexOy) nanocrystalline catalyst in comparison to Au0.75Pd0.25 NPs can be determined by using time-resolved diffuse reflectance infrared Fourier transform spectroscopy coupled with modulation excitation spectroscopy (MES) (Fig. 4.15). The kinetic
4. NANOSCALE CHARACTERIZATION
information provided by phase sensitive detection (PSD) processed spectra (Fig. 4.15A and B) shows that the formation of CO surface species, in the spectral range 2100 1900 cm21, and the formation of gaseous CO2, in the spectral range 2400 2300 cm21, are evidently faster in the case of the dumbbell catalyst than for the metal “only” catalyst (phase delay, ϕdelay 5 260 , Fig. 4.15B, vs ϕdelay 5 280 , Fig. 4.15A, for CO adsorption; ϕdelay 5 290 , Fig. 4.15B, vs ϕdelay 5 2120 , Fig. 4.15A, for CO2 formation). The surface species are detected on both catalysts (Au0.75Pd0.25/Al2O3 and Au0.80Pd0.20 FexOy/Al2O3) by IR spectra. This could be correlated to the presence of the epitaxial connection between the FexOy and the Au0.80Pd0.20 domains, resulting in an electron flow from the FexOy domain to the Au0.80Pd0.20 domain and influence favorably the nature and composition of the catalytically active surface sites of the dumbbells. In fact, when the metal alloy domain is attached to the metal oxide domain, surface Pd species are more active compared to the noble metal Au0.75Pd0.25 domain and also Auδ2 sites are formed that are not present on the initial Au0.75Pd0.25 NCs.
4.6.3 Nuclear Magnetic Resonance Spectroscopy Nuclear magnetic resonance (NMR) serves as a powerful tool for assigning MRI commercial tomographs dedicated to clinical use and laboratory research . In modern diagnosis where highly accurate information is desired, single-mode contrast agents are not always sufficient. Dual-mode T1 T2 contrast agents, combining the advantages of positive and negative contrasts, may allow for improved diagnosis by sharpening anatomical details in the MR image [92,93]. However, development of dual-mode agents with strong T1 T2 contrast effects is very challenging. 1H NMR relaxometry characterizations have been used to measure the longitudinal and the transverse nuclear relaxation times, T1 and T2, respectively, in the 5 212 MHz frequency range, which corresponds to an external magnetic field spanning from 0.15 to 5 T. The superparamagnetic metal oxides NPs have the ability to enhance the image contrast in MRI techniques by modifying the proton relaxation rates in different tissues. The NPs induce magnetic field inhomogeneities in the surrounding medium that significantly decrease the transverse relaxation time (T2) of the protons and. the shortening in T2 leads to a signal loss and, in turn, to negatively contrast images. Figuerola et al. explored bimagnetic hybrid nanocrystals, comprising size-tuned FePt and inverse spinel iron oxide domains epitaxially arranged in a heterodimer configuration as MRI contrast agents . The NMR dispersion (NMRD) profile allows measuring the frequency dependence of the longitudinal R1 and transverse R2 nuclear relaxivities. Fig. 4.16 displays the longitudinal R1 and transverse R2 relaxivities (panels A and B, respectively) as a function of the frequency for four FePt iron oxide HNP samples with different dimensional features. The Endorem contrasting agent (commercial material) is used as the reference. The HNPs mainly behaved as T2 relaxing MRI contrast agents and the effects of HNP dimensions and geometry on their longitudinal relaxivity (R1) are not significant, as observed in Fig. 4.16A. While, a variation in the proton transverse relaxivity as a function of the HNC dimensions is noticeably observed. The R2 values for each sample remained constant over the whole frequency range investigated, as shown in Fig. 4.15B. The preliminary studies on the proton nuclear relaxation in the presence of the
4.7 ELECTROCHEMICAL CHARACTERIZATION
FIGURE 4.16 Plots of roomtemperature 1H NMRD relaxivities R1 (A) and R2 (B) vs frequency for the various HNC samples dissolved in water. The FePt and iron oxide domain dimensions were, respectively: (a) 10.0 and 16.0 nm, (b) 4.0 and 11.1 nm, (c) 6.2 and 15.4 nm, (d) 8.9 and 12.0 nm. For a comparison, the relaxivity values for Endorem contrasting agent are also shown. Source: Reproduced from A. Figuerola, A. Fiore, R.D. Corato, A. Falqui, C. Giannini, E. Micotti, et al., One-pot synthesis and characterization of size-controlled bimagnetic FePt iron oxide heterodimer nanocrystals, J. Am. Chem. Soc. 130 (2008)1477 1487 with permission from American Chemical Society, 2008.
FePt iron oxide HNCs have proven the possibility to reach relaxivity values comparable or even higher compared to the commercial Endorem contrasting agent. According to the NMRD profiles, the transverse relaxation becomes progressively faster with comparatively larger heterodimers, whereas, such effect cannot be obtained for FePt seeds. The enhanced performances of the HNPs may associate with the iron oxide component in the heterostructures. The experimental result suggests that the proton relaxation rate scales up with the overall dimensions of HNPs, so that an improvement with respect to Endorem contrasting agent can be achieved for heterodimers larger than B20 nm.
4.7 ELECTROCHEMICAL CHARACTERIZATION Photoelectrochemical cell is a photocurrent-generated device composed of an electrolyte, a photoactive semiconductor electrode . Under irradiation of the interface electrolyte semiconductor with an energy level greater than the band gap of the
4. NANOSCALE CHARACTERIZATION
semiconductor, electron hole pairs are generated. The charge in an oxides-based semiconductor is distributed creating a space charge region that enables the separation of the electron hole pairs. Photoelectrochemical (PEC) characterizations can be conducted in a single compartment cell with a pyrex window, using a three-electrode configuration system where the prepared samples are the working electrodes, a Pt wire is the counter electrode, and Ag/AgCl (saturated KCl) is a reference electrode in the presence of aqueous electrolyte (NaOH, KCl, or Na2SO4 aqueous solution). A low power UV-LED (365 nm) can be used as a light source. Linear scan voltammetry (LSV) has been carried out using a Potentiostat. It is generally known that transient photocurrent always reflects the transfer and separation of photoinduced charge carriers under intermittent illumination. As the light is turned on, the photocurrent values increase while the photocurrent values decrease rapidly as the light is turned off. Photocurrent measurements of an anatase/rutile mixedphase titanium dioxide (TiO2) hierarchical network deposited with Au nanoparticles (Au/ TiO2 ARHN) are investigated in Fig. 4.17 . The measured photocurrent was normalized to the sample area to obtain the photocurrent density for comparison. Fig. 4.17A shows the LSV curves of the different samples in the dark and under light irradiation. Remarkably, a 4.5-fold enhancement of the photocurrent for Au/TiO2 ARHN was observed as compared to that for TiO2 under AM 1.5G solar illumination, suggesting its potential application in PEC cells. The photocurrents of the Au/TiO2 ARHN samples improved compared with that of bare TiO2, suggesting that the ARHN revealed a stronger ability to separate photogenerated electron hole pairs. The low photocurrent density is observed due to the limit of the wide bandgap characteristics of TiO2 (3.2 eV for anatase and 3.0 eV for rutile), which allows only UV light absorption. The generation of photocurrent for the samples is observed via many on off cycles which indicate that the electrodes are stable and the photocurrent is quite reversible. In Fig. 4.17B all electrodes show a good reproducibility and stability as the illumination was turned on and off.
4.8 OTHER TECHNIQUES Elam and coworkers  examined the atomic layer deposition of Pd and Pt films onto a variety of metal oxide surfaces including Al2O3, ZrO2, and TiO2 using in situ quartz crystal microbalance (QCM) and quadrupole mass spectrometry (QMS) to explore the nucleation and growth of the Pd and Pt on the different metal oxide surfaces. QCM measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to oxide growth/decay or film deposition at the surface of the acoustic resonator. QMS provides intensity of a specific fragment where particular ions of interest are being studied, as it can stay tuned on a single ion for extended periods of time. Fig. 4.18A displays QCM measurements of Pd atomic layer deposition (ALD) on an Al2O3 surface. The Pd deposition can be divided into two stages: nucleation (below B100 cycles) during which the Pd film thickness changes very slowly, and growth (above B100 cycles) during which the Pd film thickness increases linearly with the number of cycles. This transition occurs at a Pd film thickness of B1 Pd monolayer as indicated in Fig. 4.18A. Both HCOH and hydrogen gas (H2) were used as the reducing agent for Pd ALD. It is not possible to
4.8 OTHER TECHNIQUES
FIGURE 4.17 (A) Linear sweep voltammograms and (B) amperometric I t curves of TiO2 NW, TiO2 ARHN and Au/TiO2 ARHN photoelectrode. Source: Reproduced from Y.-C. Yen, J.-A. Chen, S. Ou, Y.-S. Chen, K.-J. Lin, Plasmon-enhanced photocurrent using gold nanoparticles on a three-dimensional TiO2 nanowire-web electrode, Sci. Rep. 7 (2017) 42524 with permission from Nature Publishers, 2017.
FIGURE 4.18 Pd nucleation and growth on Al2O3 at 200 C examined by (A) QCM and (B, C) QMS. Source: Reproduced from J.W. Elam, A.V. Zinovev, M.J. Pellin, D.J. Comstock, M.C. Hersam, Nucleation and growth of noble metals on oxide surfaces using atomic layer deposition, ECS Trans. 3 (2007) 271 278 with permission from The Electrochemical Society, 2007.
nucleate the Pd ALD on Al2O3 surfaces using H2, however, once a film has been nucleated using HCOH, then continued deposition of the Pd is possible using H2. Further, QMS has been used to monitor the HCOH (m 5 30) and H2 (m 5 2) signals during the HCOH exposures for Pd deposition on Al2O3 (Fig. 4.18B and C). The HCOH signal decreases while the H2 signal increases during the Pd nucleation, and both of these signals remain constant during the Pd growth which may be explained by the decomposition of HCOH to form H2 that occurs on Pd. The rates of H2 production and HCOH consumption are low initially because the Pd coverage is low. Both of these rates increase and then level off as the Pd nucleates and grows to cover the entire Al2O3 surface.
4.9 CONCLUSION In this chapter, we describe the recent progress made in the study of metal metal oxides nanohybrids characterized by microscopy, X-ray techniques, spectroscopy, and electrochemical measurements. Basic principles of such measurements for HNPs are summarized. We highlight the results of optical and photoelectrochemical properties of HNPs studied by UV vis and photoluminescence spectroscopy and LSV and chronoamperometric measurements. Photocurrent from HNPs is also introduced, based on PEC measurements. 1H NMR relaxometry characterization are presented to measure the longitudinal and the transverse nuclear relaxation times for MR images. This chapter provides an overview of HNPs characterization methods, which include the detailed understanding of formation and the optoelectronic properties of HNPs. A deep insight into the synthesis and mechanism of formation of HNPs enable the tuning of the physicochemical properties or imparting of multiple functionalities to HNPs for a broad range of applications. However, theoretical calculation and prediction for hybrid nanostructures and their mutual interaction have received little attention, which limits the rapid growth process of the field.
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