Electrochemical Reactivity at Free and Supported Gold Nanocatalysts Surface

This chapter presents an overview on size, structure, morphology, composition as well as the effect of the support on the electrocatalytic properties of gold nanoparticles (AuNPs). It was found that the electrocatalytic properties of unsupported AuNPs strongly depend on their size and shape. Consequently, the electrocatalytic properties of AuNPs can be tuned. Furthermore, to design high-performance electrocatalysts with minimal precious metal content and cost, the direct immobilization of metal NPs onto carbon-based substrates during their synthesis constitutes another elegant alternative and has been thoroughly examined. These “easy-to-use” supports as scaffolds for AuNPs, namely carbon black, carbon paper, etc., offer beneficial contributions. Indeed, thanks to their high available surface area, good electronic conductivity and synergis‐ tic effect between the chemical species present on their surface and the loaded NPs, carbon-based supports enable maximizing the efficient utilization of the catalysts toward drastic enhancement in both activity and durability. We also examined different judicious combinations of (electro)analytical techniques for the unambiguous determi‐ nation of the reaction product(s) over the Au-based nanocatalysts, using glucose as model molecule given its importance in electrocatalysis. The performances of carbon-supported AuNPs as anode materials in direct glucose fuel cell in alkaline medium were also discussed.


Introduction
During several decades, bulk gold has been considered as less active material, but at nanoscale, it exhibits surprisingly physicochemical and catalytic properties. Such properties depend on the size, the morphology, and the surface structure of the nanoparticles. Different synthesis approaches have paved the way of controlling the size, shape, and crystallographic structures of gold nanoparticles (AuNPs) in order to tune their (electro)catalytic activity. The control of these key parameters enables designing highly effective and durable gold catalysts for potential applications not only in sensors, electrochemical reactors, and fuel cells but also over electrochemical field. In electrocatalysis, the surface structure of the electrode material plays a key role. Thereby, the preparation of active and efficient nanomaterials becomes a challenge to be taken up. It is known that various gold nanomaterials exhibit relevant ability toward the oxidation of organic molecules. Therefore, the electrocatalytic activity of different AuNPs toward the glucose oxidation in alkaline electrolyte was described in the literature [1][2][3][4][5]. On AuNPs, this reaction is size, shape, and structure dependent. The goal of this chapter was to address the recent advances in the preparation of free and carbon supported AuNPs and their performance in electrocatalysis. Precisely, factors affecting the growth mechanism and the synthesis processes are presented. Furthermore, discussion on the electroactivity of synthesized AuNPs toward biomass-based compounds (glucose…) and their performances in fuel cells and/or the production of sustainable addedvalue chemicals from selective oxidation will be extensively reported.

Electrochemical reactivity at free and supported gold nanocatalysts surface 2.1. Growth of gold nanoparticles in solution
The preparation of metal nanoparticles in solution involves systematically two important processes, which are the nucleation followed by the growth of the nanoparticles.

Theoretical aspects: nucleation and reaction-limited growth
Key concepts that enable the understanding of reactions limiting the nucleation and the growth processes need to be reminded. On the kinetic point of view, it is recognized that the nucleation step is very fast and cannot be observed by a usual transmission electron microscope. In a typical metallic nanoparticles synthesis, the precursor compound containing the metal cation is reduced or decomposed to metallic atoms (zero oxidation state) that will coalesce to form nanoparticles. La Mer and Dinegar proposed a mechanism for explaining the nucleation process as follows [6] (Figure 1). According to this mechanism, the concentration of metal atoms increases rapidly during the nucleation, as regularly as the precursor is reduced. The reduction reaction can be promoted by ultrasonication or by raising the temperature. Once the concentration of nuclei reaches a critical point so-called point of supersaturation, they start aggregating into small clusters via a self-nucleation process in the case of homogeneous nucleation. Then, the nuclei grow rapidly by consuming the metal atoms present in the bulk solution. Thereby, a decrease in the concentration of the single atoms in the solution is observed. If the monomer concentration falls rapidly below the supersaturation, the nucleation ends, and only the nuclei already present in the reaction mixture will grow to nanoparticles with homogeneous size distribution. With the continuous supply of the atoms due to the reduction in the metal precursor, the nuclei grow to larger nanoparticles until the establishment of an equilibrium state between the atoms at the surface of the nanoparticles and those in the solution. Once the clusters reach a critical size, structural changes require significant energy input so that the clusters grow in a well-defined structure leading to the formation of seeds. The key strategy in synthesizing nanoparticles with controlled-shape is to ensure meticulous monitoring of the seeds population with different internal structures. Thermodynamic and kinetic factors are important for controlling the nucleation process. The heterogeneity of nanostructured seeds is determined by the surface free energies of the different species in combination with the kinetic effects on the generation and the addition of metal atoms to the nuclei. Different theories have been developed for understanding the mechanism of nanoparticle growth process [7,8]. Due to the small size of the clusters, they have high surface-to-volume ratio. As a result, very small particles exhibit a significant surface excess energy, which is a non-negligible percentage of the total energy. Therefore, the formation of larger particles results from a solution, which is not initially at thermodynamic equilibrium and leads to a decrease in the surface energy. Thereby, it plays a crucial role in the growth of nanocrystals. A colloidal particle grows by a sequence of monomer diffusion toward the surface followed by the reaction of the monomers at the surface of the nanocrystal. The classical theory by Lifshitz and Slyozov [9] and Wagner [10] (referred as LSW) has dominated researchers thinking about precipitate coarsening. In fact, the LSW theory driven by the reduction in the surface energy was proposed by Ostwald in 1901 [11]. A key idea of this theory is that coarsening or Ostwald ripening results from the interaction of particles embedded within a matrix phase. Coarsening effects, controlled by the mass transport (most frequently the diffusion), are often termed the Ostwald ripening process. The diffusion process is dominated by the surface energy of the nanoparticle. The interfacial energy is the energy associated with an interface due to the difference between the chemical potential of atoms in an interfacial region and atoms in neighboring bulk phases. The LSW theory, considering a diffusion-limited growth, was able to make quantitative predictions on the long-time behavior of the coarsening process. The LSW approach examines the growth of spherical particles in a supersaturated medium and is based on basic assumptions extensively described in the literature [7][8][9][10].

Size and shape-controlled gold nanoparticles
The electrochemical properties of metal NPs are strongly influenced by their size, shape, and structure. This dependence has motivated the development of a variety of synthesis methods for controlling their size and shape. Thus, the synthesis of most gold nanoparticles (AuNPs) is established, in terms of degree of control over the size, shape, monodispersion, and in the understanding of the growth mechanism.

• Spherical gold nanoparticles
In the early 1951, the most popular Turkevich synthesis method, later refined in the 1970s by G. Frens [12], was developed to yield quasi-spherical and mono-dispersed AuNPs in water through a small amount of hydrochloroauric acid as a precursor and sodium citrate solution [13]. The mean size of the particles ranged from 5 to 200 nm. The mixture was heated to boiling. In this synthesis, the citrate ions adsorb on the surface of the nanoparticles by creating a negatively charged layer. This stabilizes and prevents the nanoparticles from aggregation by providing a sufficient electrostatic repulsion between the particles. In the early 1990s, Brust et al. [14] have developed a method to synthesize AuNPs in organic medium based on the reaction of hydrochloroauric acid, the tetraoctylammonium bromide (TOAB), and the sodium borohydride in toluene [14]. The gold nanoparticles were small from 2 to 6 nm. NaBH 4 was the reducing agent, while TOAB played both the role of intermediary transfer phase and surfactant. After 24 h of stirring, two phases were separated. The organic phase was successively washed with dilute sulfuric acid and distilled water. These clusters have attracted much attention due to their small size. However, due to the toxicity of organic solvents as toluene and the surfactant or molecule-like properties (high affinity with catalytic actives sites), electrocatalytic applications are somewhat limited. To overcome this passivation effect due to the strong adsorption of surfactant at AuNPs surface, we have recently revisited the synthesis method initially developed by Slot and Geuze to yield spherical AuNPs [15]. Based on an empirical Taguchi model, Habrioux et al. [16] adapted this synthetic method to find experimental conditions and optimal synthesis parameters to yield spherical AuNPs, monodisperse in size ranging from 3 to 17 nm. In this synthesis, two solutions are separately prepared: one containing the gold salt solution and the second is a mixture of tannic acid and sodium citrate as reported [15,16]. Then, AuNSs-10 and AuNSs-4 samples (the number after ANSs represents the mean particles size) were obtained. Here, due to its long carbon chain, tannic acid acts as a surfactant for the control of gold particles shape and size, while a trisodium citrate solution serves as reducing agent. Slot and Geuze [15] indicated that the concentration of tannic acid in the reducing solution, the temperature and pH of the solution are important parameters in the size control. When the pH is adjusted between 7.5 and 8 by adding a few drops of 0.1 mol L −1 NaOH solution and the temperature is set at 60°C, monodispersed nanoparticles can be obtained by varying the amount of tannic acid [15].
In addition, spherical AuNPs can also be synthesized by the reduction in gold salts by sodium borohydride in the presence of CTAB as surfactant [3][4][5]. After the appropriated time and amount of chemicals as reported, AuNPs with different sizes can be obtained in particular AuNSs-6 and AuNSs-14 samples, respectively. It is important to notice that CTAB has strong interaction with AuNPs surface. However, we have shown that it is possible to remove it electrochemically. Figure 2 shows the TEM and HRTEM images and the corresponding particles size distribution of different synthesized spherical NPs. The AuNPs are homogeneous in size and shape. However, the AuNSs-14 sample is polydispersed in size due to the overgrowth effect of gold particles during the ageing time of this sample [17]. Characteristically, the average particle sizes are 4.2 ± 0.7, 6.2 ± 1.2, 10.3 ± 1.4, and 14.7 ± 2.9 nm for AuNSs-4, AuNSs-6, AuNSs-10, and AuNSs-14, respectively, from the gold solution during the synthesis; AuNSs-10 and AuNSs-4 are spherical AuNPs obtained from the Slot synthesis method with 8 and 12 mL of tannic acid, respectively.
As can be seen in Figure 2(A-D), the HRTEM images of all AuNSs samples present mostly (1 1 1) facets. In fact, one can consider that the nanoparticles obtained by the Slot method are mainly decahedron delimited by (1 1 1) facets. The kinetic growth in the {100} direction seems to be superior to that in the {1 1 1} direction, which promotes the development of (1 1 1) facets. The measured interplanar spaces for all lattice fringes from high-resolution transmission electron microscopy images correspond to the values obtained from electron diffraction and XRD data [18,19]. As an example, the measured interplanar space of 2.35 Å is in good agreement with the (1 1 1) lattice plane of face-centered-cubic (fcc) of gold [18,19]. The surface energy (γ) associated with different crystallographic facets of fcc metal types is described as follows: γ(1 1 1) < γ(1 0 0) < γ(1 1 0) [20]. This order fully justified from a thermodynamic point of view the preferential formation of (1 1 1) facets on the surface of seeds. In addition, the (1 0 0) facet is also observed in the AuNSs-14 sample.

• Anisotropic gold nanoparticles
Different approaches have been developed to yield anisotropic AuNPs by the wet chemical method. Among them, the seed-mediated growth was employed to synthesize a wide range of size and shape controlled nanoparticles [21][22][23]. The process is generally carried out in two steps: nucleation followed by the growth step. Both steps may be performed in the same or separate reactors. In the growth stage of NPs, the reaction conditions for the shape-controlled are less severe than those for the seeds preparation. The activation energy for the reduction in the metal precursor on a particle already formed is much lower than that required for homogeneous nucleation of seeds in the same solution [24]. Therefore, the control of the morphology can be considered as the process of proliferation of seeds. The growth of particles is progressively done by the reduction in the metal precursor to the surface of seeds. The metal atoms formed are deposited onto the surface of the seeds, then diffuse to reach a site where they will find atoms of low coordination leading to an increase in the particle size. This dynamic interaction of growth and dissolution conducts the evolution of seeds in nanocrystals. The general strategy in controlling the shape in the growth of nanoparticles is to stabilize a particular facet through a molecular interaction by using a surfactant. Chemisorption of a surfactant has a strong influence on the final shape and structure of the nanoparticle. Actually, it plays an important role in the growth kinetics of the facets since its adsorption at the surface of the seeds reduces the surface free energy and promotes the stabilization of the nanocrystal. Interestingly, surfactant has a preferential affinity with a particular surface orientation, which can promote interactions between atoms during the growth process [25]. During a synthesis for example, if the seeds are truncated octahedrons with crystallographic planes (1 1 1) and (1 0 0), the selective adsorption of a surfactant on the planes (1 0 0) will cause the decrease in the rate of growth of these facets and block their access.

• Case of gold nanorods
A very large number of articles cover the synthesis of gold nanorods (AuNRs) by the seedmediated growth [26][27][28]. The original idea was that surfactants such as cationic micelles could serve as a template mode to guide the growth and provide colloidal stability [23]. However, the role of the seeds is also critical. The aspect ratio can be precisely controlled by varying the amount of seed in the growth solution [26]. Furthermore, the presence of small amounts of silver nitrate in the synthesis has a dramatic effect on yield, and the final shape of the particle [29]. Many hypotheses have been advanced to explain the mechanism by which the Ag(I) ions react and alter the kinetics of growth, structure, and the formation of the AuNRs. A significant contribution for understanding the roles of the ions Ag(I) and the crystal structure of the seed was provided by Liu and Guyot-Sionnest [30]. They found that the AuNRs synthesis yield increases by regulating the pH value of the reaction between 2 and 4 and by increasing the reaction time from 1 to 2 h. Additionally, it was issued by Jana et al. [23], in the presence of CTAB, that the bromide anion and Ag(I) precipitate to form AgBr which will adsorb at the seeds surface during the growth process. When AgBr is adsorbed, the crystal facets are blocked, and therefore, their growth is restricted. Under acidic pH conditions, the adsorption of Ag(I) is favored and outweighs the reduction in silver atom [31]. Thereby, the aspect ratio of AuNRs can be controlled by adjusting the amount of Ag(I) in the growth solution [29,32]. Sau and Murphy [33] showed the influence of the amount of seeds on the aspect ratio and the diameter of AuNRs. Following these observations, Nikoobakht and El-Sayed [29] have proposed that CTAB forms a flexible template whose size is dependent on its concentration and ionic strength of the solution. Thus, the Ag(I) ion and the polar heads of CTAB may be considered as pairs of AgBr. In this way, the charge density of the bromide ions decreases. Therefore, repulsion between adjacent polar heads on the gold surface leads to the template elongation of CTAB [29]. Wang [34] showed that CTAB monomers had a higher affinity for the lateral facets which were particularly favored, compared to the facets at the ends due to the van der Waals interactions between the nonpolar channels tail of CTAB. Unlike the previous mechanism, Murphy's group [33,35] proposed a mechanism whereby a rigid structure of CTAB monomers helps to maintain a unidirectional growth by "zipping" mechanism. The presence of Ag(I) ions enables controlling the reduction kinetics of gold salt. Indeed, the adsorption of AgBr on the surfaces of gold nanocrystals promotes the development of high-energy sides composed of (1 1 0) facets and allows the growth of monocrystalline rods [30]. According to Liu and Guyot-Sionnest [30], in the synthesis conditions (acidic media), a silver monolayer can be deposited onto the lateral (1 1 0) facets driven poisoning of these surfaces. The growth kinetics of these facets is then slowed down, and the anisotropic growth might not be made from silver facets. This mechanism is the basis of unidimensional growth of monocrystalline nanocylinder.
Based on the synthesis procedure developed by Murphy's group [33], AuNRs were prepared at a constant temperature of 27°C by a modified seed-mediated growth method. The seed solution was prepared as described in the literature [5]. The AuNRs obtained have an aspect ratio of 3.3 ± 0.7 for an average length of 33.2 ± 6.0 nm. The AuNRs solution is called AuNRs-E ( Figure 2E).
Based on the seed mediated growth, we have synthesized AuNRs by mixing growth solution and NaBH 4 in the same reactor [5]. This permits to produce in situ the seeds followed by the growth of the AuNRs. The obtained AuNRs were named AuNRs-F. The AuNRs-F have an aspect ratio of 2.54 ± 0.68 for an average length of 14.18 ± 2.93 nm ( Figure 2F).
The AuNRs can be also synthesized by one-step method without any use of seeds solution. In fact, the seeds are generated in situ in the same solution. This method, developed by Tollan et al. [36], is based on the particular reducing and stabilizing properties of acetylacetone (acac) and stabilizing properties of CTAB. Thus, in the presence of CTAB, silver ions and at moderate pH, acac reduces the gold precursor and promotes the growth of nanorods. It is well known that acac is a good organic diketone chelate ligand [37]. In the presence of gold salt HAuCl 4 , it can complex to form a stable chelate Au(III)-acac which can decompose under ambient conditions to form spherical nanoparticles of variable size (10-40 nm). The kinetics of the reduction depends on the pH value. It is slower in high pH due to the ease of ligands transformation into enoles. Under these conditions, the chelates are stable and a smaller particles size is obtained. In the case of the AuNRs synthesis, chelates Au(III)-acac in a basic medium that provides enolates may reduce the gold salt. When the pH value is higher than 10, the nucleation is rapid, thereby the formed nuclei will aggregate fast, leading to the formation of spherical particles. It seems that the seeds generated in situ keeps getting bigger under the same conditions as those of the seed-mediated growth process in the presence of CTAB and Ag(I) in AuNRs [36]. The fabricated AuNRs were named AuNRs-G. The AuNRs-G has an aspect ratio of 4.90 ± 1.06 for an average length of 34.27 ± 7.28 nm (Figure 2G).
Gold nanocuboids (AuNCs) can be synthesized by seed-mediated growth process in the presence of copper ions. This method is based on the same procedure as for AuNRs-E preparation, but the Ag(I) cation is replaced by the Cu(II) one. Sun et al. [19] showed that the formation of AuNCs is related to both preferential (1 1 1) facets poisoning by the cations Cu(II) and CTAB. Indeed, the presence of these two species affects significantly the surface energy of the different facets of the crystal due to their preferential adsorption and therefore the growth kinetics of the different facets of the crystal. As described in the literature, the seeds produced in the presence of CTAB have (1 1 1) facets more accessible to the solvent as the (1 0 0) facets appear to have a greater affinity with the CTAB than the (1 1 1) facets [38]. When introduced into the growth solution, the Cu(II) cations are adsorbed preferentially on the readily accessible (1 1 1) facets and decrease the kinetics of growth in this direction in accordance with their concentration. Therefore, nanocuboids and nanodecahedrons are obtained based on the amount of Cu(II) ions. These authors showed that when the concentration of Cu(II) was 0.2 mmol L −1 , the copper ions were selectively adsorbed on the (1 1 1) facets and the kinetics of growth in the (1 1 1) direction is reduced but still remained higher than that in the (1 0 0) direction, which led to AuNCs. However, when the concentration of Cu(II) ions increases up to 1.6 mmol L −1 , the growth kinetics of the (1 1 1) direction is much slower. The average particle size is 29 ± 3 nm with mostly (1 0 0) facets at the edges and (1 1 1) facets at the corners ( Figure 2H).
With a procedure similar to that described above, we were able to obtain polyhedrons. Typically, 12.5 μL of 1.0 × 10 −2 mol L −1 silver nitrate and 12.5 μL of 1.0 × 10 −2 mol L −1 CuSO 4 were jointly added. Finally, 1.25 μL of the seed solution was added to the growth solution at 25°C. The combination of silver and copper ions promotes the polyhedrons formation with 37 nm in size. The average particle size is 36.8 ± 4.9 nm with mostly (1 1 1) facets ( Figure 2I).

Synthesis of gold nanoparticles supported on carbon substrates
Carbon-supported AuNPs can be synthesized from various bottom-up approaches, including the polyol [39], water-in-oil (w/o) microemulsion [40,41]. The w/o method has been initiated by Boutonnet et al. [41] in 1982 when they reported the successful preparation of Pt, Pd, Rh, and Ir NPs with sizes of 3-5 nm. Then, it has been successfully used to prepare various metallic nanomaterials such as Au [40] for electrocatalytic tasks. Unfortunately, the nature of the surfactants (Brij ® 30, PVP, etc.) and their strong adsorption at the NPs surface constitutes the main drawback of the w/o and other surfactant-based methods. As the majority of the methods for synthesizing noble metal NPs involve surfactants, which are undesired for electrocatalysis application because of their adsorption on catalytic sites, elegant methods for the direct "printing" of AuNPs onto carbon papers or fibers to be used directly in electrocatalysis have been initiated. Figure 3A shows the SEM image of AuNPs embedded in electrospun carbon fibers (CFs) at the metal loading of 26 wt.% [42]. In typical experiment, HAuCl 4 is first mixed into preheated N,N-dimethylformamide (DMF) at 70°C, followed by the slow addition of polyacrylonitrile (PAN, M W = 150,000) and stirred for 3 h. The obtained electrospun felts are stabilized in air at 250°C for 2 h and then carbonized at 1000°C for 1 h under N 2 . The resulting sample is composed of pure CFs ~240 nm diameter and Au@CFs ~700 nm. As displayed in Figure 3A, the gold particle size is heterogeneous from 50 to 250 nm. In addition, it was found that 12.2 wt.% of Au particles are located inside the fibers, thus inaccessible. Furthermore, the group of Hsin-Tien Chiu has developed several electrodeposition methods that enable the growth of Au nanostructures on carbon paper: nanoparticles ( Figure  3B), nanocorals (Figure 3C), and branched belt ( Figure 3D) [43,44]. The electrodeposition is achieved by applying a voltage of 1.6-1.8 V for at least 18 h in an aqueous mixture of HAuCl 4 , NaNO 3 , and cetyltrimethylammonium chloride (CTAC). The surfactant CTAC acts as a capping agent to reduce the surface energy and controls the growth and shape of nanostructures, while NO 3 − is expected to increase the conductivity of the solution and oxidize less stable Au facets back into AuCl 4 − [44,45]. This leads to a high Au amount (>2.5 mg cm −2 ) [44]. The electrochemical characterizations combined TEM ( Figure 3E) and selected area electron diffraction (SAED) pattern ( Figure 3F) suggested that the surface structure of the nanocorals resembles that of the branched belt, with highly exposed Au(1 1 0) planes [43]. The presence of exposed Au(1 1 0) surfaces is known to promote the glucose oxidation [2]. Overall, these methods allow passing through the intermediate carbon black that leads to NPs detachment during the reaction. However, it should be noticed that the particles size is relatively high and the remaining CTAC might limit the accessibility to some active sites.
Notwithstanding these successful demonstrations, carbon paper-based methods remain questionable since they give high loading of precious metals and larger particles size, which substantially decrease the effectiveness of the catalyst. On the other side, the retained molecules at the surface of NPs from chemical methods decrease notably the catalytic performances of the obtained electrodes due to the inaccessibility of some active sites that are obviously blocked. Therefore, the exploration of other alternatives to minimize the use of organic molecules that have an affinity with the NPs surface and decrease the noble metal content in the catalyst is desired. The so-called bromide anion exchange (BAE) method, a bottomup approach, has been initiated since 2012 to meet these requirements by fabricating advanced surfactant-free metal NPs for electrochemical energy conversion technologies [46][47][48]. The main feature of this method lies in its simplicity of implementation by using only potassium bromide (KBr) as surfactant/capping agent. Halide ions (Cl − , Br − , I − ) may serve as coordination ligands and thus play the role of capping agent for shape and size control of NPs [49]. In a standard procedure of BAE, Au precursor salt is dissolved in water at 25°C followed by the addition of KBr. Afterwards, a given amount of carbon black is added under ultrasonic homogenization for 45 min, followed by the dropwise addition of the reducing agent. Thereafter, the temperature is raised at 40°C for 2 h. Finally, metal NPs supported on carbon black are filtered, washed with ultra pure water, and dried in an oven at 40°C for 12 h. Before the reduction step, the change of the solution color (from a clear yellow to a deep yellow) can be observed upon the addition of KBr [48]. This change supported by UV-vis measurements [50] is assigned to the ligand-to-metal charge transfer transition phenomenon in metal complex ions because of the partial substitution of Cl − by Br − , yielding to [AuCl 4−x Br x ] − , 0 ≤ x ≤ 4. Br − being bigger than Cl − , a mixed complex ion [AuCl 4−x Br x ] − is expected to provide more steric environment than [AuCl 4 ] − . Hence, it could better control the particles size/shape growth after the reduction. Figure 4A shows the TEM image of Au/C and highlights well-dispersed 3-10 nm AuNPs. The HRTEM image shows an octahedron shape having different degrees, with crystallographic (1 1 1) and (2 0 0) facets. For supported NPs, the formation of facets (1 1 1) and (1 0 0) is thermodynamically more favorable since the surface energy (γ) associated with different crystallographic planes is γ(1 1 1) < γ(1 0 0) < γ (1 1 0). Thus, the polyhedron corresponding to the more stable thermodynamic morphology (Wulff's theorem) for a nanoparticle with face-centered cubic crystal symmetry is a truncated octahedron. Furthermore, the presence of the carbon support undoubtedly influences the final shape. Obtaining high Miller indices such as (2 0 0) instead of (1 0 0) suggests that the BAE synthesis method offer favorable thermodynamic conditions. Figure 4B displays the high-resolution X-ray photoelectron spectroscopy (XPS) spectrum of the Au 4f core level. The observed doublets are related to spinorbit splitting (3 ± 1/2) with binding energies of 83.9 (Au 4f 7/2 ) and 87.6 eV (Au 4f 5/2 ). The presence of AuO x is indicated by doublets at 85.5 (Au 4f 7/2 ) and 89.1 eV (Au 4f 5/2 ). Energydispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) have regardless shown that the oxide amount is negligible [51]. Indeed, upon exposure to ambient air, a thin protective layer safeguards the metal surface from deep oxidation. The total metal loading determined from the thermogravimetric analysis was 21 wt.% on the basis of 20 wt.% and Au/C was produced with a high synthesis yield greater than 94% [51].

Cyclic voltammetry in alkaline solution
Among the surface chemistry techniques, cyclic voltammetry is particularly an efficient, size, and structure sensitive tool in electrochemistry for analyzing and probing the electrode material surface. Figure 5 displays the typical cyclic voltammograms of the different synthesized AuNPs electrodes in 0.1 mol L −1 NaOH recorded at 20 mV s −1 and 20°C. As can be seen, each CV shows three main regions: a large double layer region followed by the adsorption of hydroxyl OH − species from 0.6 V vs. RHE. Afterwards, the oxidation and reduction regions of gold oxides can be observed.
During the positive scan, the large capacitive current associated with the double layer region is typical electrochemical behavior of gold material. This behavior of the gold electrode in the double layer region as well as the surface oxidation region is structure dependent as evidenced by Kokoh et al. [3,4] on size and shape controlled gold nanoparticles and Hamelin [52] on gold single-crystal Au (1 1 1) and Au (1 0 0).
During the backward potential scan, such oxides are reduced irreversibly from 1.4 to 0.8 V vs. RHE.
Interestingly, the particle morphology affects the profile of the double layer and oxides formation regions strongly. Indeed, the increase in size of gold particles diameter from 4 nm to the bulk leads to a thin double layer and well-defined oxide region on the CVs. Such features in the change of the CV profile indicate the increase in metallic trend as a function of the particle size. Additionally, small AuNSs (14.7 ± 2.9 and 6.2 ± 1.2 nm) exhibit high affinity with oxygen species, which is revealed by the oxygen evolution reaction observed at ca. 1.60 V vs. RHE, that is, an electrode potential lower than that on the bulk material [3].

Under-potential deposition of lead adatoms in alkaline solution
The under-potential deposition (UPD) of adatoms on noble metals such as platinum or gold is also a helpful tool for characterizing the surface structure of electrode materials [55,56]. In general, it involves the deposition of up to one monolayer of metal on a foreign substrate at potentials higher to the reversible thermodynamic potential. The positive overpotential at which this process occurs is considered to be a direct consequence of the high bonding energy of the metal being deposited at the substrate surface. It has been previously shown that the synthesis method affects the crystallographic structure of AuNPs [36,[55][56][57]. Because of its high sensitivity with the gold surface, the UPD of lead (Pb UPD ) was employed to characterize the crystallographic structure of the synthesized spherical AuNPs [3-5, 16, 56]. For the AuNSs materials (Figure 6), no desorption peak of lead on (1 0 0) facet was observed for the small AuNSs-4 (4.2 ± 0.7 nm), AuNSs-6 (6.2 ± 1.2 nm), and AuNSs-10 (10.3 ± 1.4 nm). However, an increase in the particle size leads to a formation of (1 0 0) facets for the large particles such as AuNSs-14. These results clearly show that the surface properties can be perfectly modeled by tailoring the particle size. The Pb UPD on the shape controlled AuNPs has been extensively discussed in our previous works [4,5]. Until now, the Pb UPD permitted to reveal only the low-index facets. Investigations on high-index facets are needed.

Glucose electrooxidation on gold-based catalysts
Carbohydrate-based energy converters are emerging as unavoidable powerful, durable, cheap, and environmentally friendly items. In addition, the selective electrochemical conversion of these highly functionalized organic molecules (glucose, lactose, etc.) may offer valuable benefits such as electricity, heat, and added-value chemicals. The total glucose electrooxidation involves 24 electrons per molecule and enables getting an open circuit voltage of 1.25 V, which represents a free energy of 2871 kJ mol −1 , that is, 4.430 kWh kg −1 [58]. For 2-electron process, it yields 1.43 V at pH = 13, that is, a specific energy of 0.435 kWh kg −1 . Gluconate is a high added-value product, and its derivatives, such as gluconolactone or sodium and calcium salts, are used in food, pharmaceutical, and cosmetic industries [59]. The effective development of efficient fuel cells relies on the kinetics of both reactions at the cathode and anode. Indeed, the anode catalyst must withstand the hard poisoning phenomenon due to strongly adsorbed intermediates. Up to now, glucose electrooxidation at the anode, or even with the most active nanocatalyst (Pt), largely occurs with overpotential ≥200 mV. Theoretically, at pH 13, glucose electrooxidation must start at −0.24 V vs. RHE [51]. Unfortunately, Pt the most dehydrogenation catalyst is rapidly deactivated. The best compromise relies on gold-based materials, which play a crucial role in the catalysis of hemiacetal compounds resulting in enhanced intrinsic activity and reaction turnover. The capability of gold in oxidizing carbohydrates can be tuned through its size and morphology during its synthesis. The following sections focus on the recent efforts that have been devoted for engineering advanced electrode materials, which can offer a great opportunity of achieving enhanced catalytic performances.

Size effect: case of spherical gold nanoparticles
The assessment of the determining parameters that influence the activity of catalysts is of paramount interest in electrocatalysis. Figure 7 highlights the effect of the particles size on the catalytic properties of the unsupported-gold electrode materials for the glucose oxidation in alkaline solution. On AuNSs-4, the oxidation of glucose begins at ca. 0.35 V vs. RHE followed by three oxidation peaks. The first one centered at 0.60 V is a broad oxidation peak (peak A) which maximum reaches j max = 2 mA cm −2 and assigned to the dehydrogenation of the anomeric carbon of glucose in C1-position leading to the formation of gluconolactone as intermediate or final product [1,2,60,61]. The second peak (B) centered at 1.00 V vs. RHE may correspond to the oxidation of adsorbed intermediates followed by the main oxidation peak at 1.20 V vs. RHE (peak C) as previously observed for other AuNPs, single or polycrystalline gold electrodes [2,4,5,62,63]. According to the literature, several products such as gluconate or glucuronate can be resulted in the electrochemical oxidation of the glucose molecule at the Au oxide/hydroxide species, gluconate being issued from hydrolysis of the lactone in the bulk solution [1,2,61,62,64]. Nanospheres having average size of 4.2 and 6.2 nm (AuNSs-4 and AuNSs-6) display similar electroactivities despite the slight difference in size due to the large presence of (1 1 0) facets for AuNSs-6 compared to AuNSs-4. The effect of the crystallographic structure will describe in details in the next section.
The current density corresponding to the dehydrogenation process of the glucose molecule (peak A) is 3.2-, 4.9-, and 11.0-fold higher for AuNSs-4 than those observed for AuNSs-10, AuNS-20, and the bulk electrode, respectively. Similar comparison shows current densities, respectively, for the same AuNSs, 2.0-, 2.2-, and 2.1-fold higher than the bulk gold electrode toward glucose oxidation at peak C.
Based on both the CV results, it can be concluded that the electrocatalytic activities of the different spherical gold electrodes toward glucose oxidation are size and structure dependent. The formation of (OH) ads on Au is crucial for the electrochemical oxidation of glucose [60]. Thereby, the oxidation of glucose is assumed to occur through the interaction between the adsorbed hemiacetal group and (OH) ads . It is generally accepted that the AuOH sites on the Au surface act as the active species for glucose oxidation [65]. Therefore, the oxidation of glucose strongly depends on the number of AuOH sites. The first step of the oxidation of glucose in alkaline media involves the adsorption of glucose on Au surface through the anomeric carbon leading to the dehydrogenation process via the formation of an adsorbed radical. This process leads to the formation of gluconolactone [62,64]. Furthermore, gluconolactone can be hydrolyzed in solution to give sodium gluconate or the gluconolactone adsorbates can interact with the metal center represented by these AuOH species to give more than two-electron oxidation products resulting from carbon-carbon bond cleavage [60,65]. The second step deals with the oxidation of adsorbed intermediates or gluconolactone with the adsorbed OH species. Several products can be obtained as indicated in the literature [62].

Morphology effect
The electrooxidation of glucose has intensively been studied on gold single crystals and gold nanoparticles in the 1990s [2,60,64,66]. More recently, we have investigated the dependence of the activity on low-index crystalline surface of AuNPs toward the glucose oxidation [4,5]. At this stage of our investigations, electrocatalytic activity of AuNPs is sensitive to their morphology [2,67]. It was mentioned that the oxidation of the glucose depends on the surface structures that determine the adsorbed intermediates [2,4,5,64]. It was concluded that the glucose oxidative conversion is enhanced according to the crystallographic orientation of the electrocatalyst as follows: (1 0 0) > (1 1 0) > (1 1 1). Figure 8 shows the positive potential going profile of different AuNPs recorded during the glucose oxidation. This reaction begins at ca 0.3 V vs. RHE for all materials followed by the first oxidation peak, which is associated with the formation of gluconolactone species. Surprisingly, AuNSs (dot line) and AuNCs (dashed dot line) show high activities for the oxidation of glucose. This sharp oxidation peak appears at 0.54 V vs. RHE for AuNCs, 100 mV lower than for AuNSs. Afterwards, the oxidation covers a large potential range from 0.8 to 1.3 V vs. RHE. AuNRs (solid line) and AuNPolys (dashed line) show an overlap shoulder followed by a maximum oxidation peak at 1.3 V vs. RHE. However, this peak is observed at 1.2 V vs. RHE for AuNSs and AuNCs. The high current density observed for AuNSs can be related to their small size (around 6 ± 2 nm) and their electronic surface structure. The catalytic effect on AuNCs depends on their structure, that is, the number of sites of low coordination. AuNCs exhibit the mean (1 0 0) and (1 1 0) facets that promote dehydrogenation of the molecule of glucose at low potential (0.54 V vs. RHE) compared to the other nanostructured materials. These facets are the most favorable for this reaction [64]. According to theoretical model developed by Hammer and Nørskov [68], and Bell et al. [69], the d-band of the orbital plays an essential role in the adsorption of reactants and intermediates on metal surfaces. As suggested, the d-orbital occupancy reflects the number of electrons available to participate in bonding between the metal center and the adsorbate. However, all these explanations do not elucidate the similar activity between the AuNSs and AuNCs whose surface structures are theoretically different. Consequently, more investigations combining different techniques are certainly required.

Support effect: performances of carbon supported gold nanoparticles
The direct immobilization of metal NPs onto carbon-based substrates induces a high improvement in their catalytic performances, assigned to better interaction between NPs and the support [70,71]. In addition, a support is needed to boost the current in real application such FCs. Free NPs in solution are used to find out the intrinsic activity of the catalysts, especially the structure sensitivity [70]. Thus, single crystals and shape-controlled NPs constitute cornerstones for the fundamental understanding of the catalytic activity and carbon-supported NPs serve as subgrades for practical use. The electrochemical behavior of different carbonbased Au electrodes is shown in Figure 9 in the absence and presence of glucose in alkaline medium. Considering the supporting electrolyte, there is no obvious evidence of Au in the case of Au@CFs despite the loading of 26 wt.% ( Figure 9A). Contrariwise, two major features during the positive scan (A′: metal oxidation) and the negative scan (C′: oxide reduction) typify gold at Au nanocorals ( Figure 9B) and Au/C (Figure 9C) electrodes. However, all electrodes show activity toward glucose electrooxidation, marked by several peaks during the forward (glucose dehydrogenation, oxidation) and only one main peak in backward. The reaction starts at ca. −0.5, −0.4, and −0.6 V vs. Ag|AgCl (at pH 13, Ag|AgCl|KCl sat = +0.96 V vs. RHE) on Au@CFs, Au nanocorals and Au/C, respectively. Otherwise, Au/C shows improved kinetics with ca. 100 mV shift toward lower potentials, which is auspicious for application as anode material. Furthermore, the coincidence of the peak labeled by "C1" in Figure 9C during the negative scan with the gold oxides reduction one indicates that the main phenomenon concerns the oxidation of new glucose molecules at the freshly released Au active sites. By taking into account Au content and the electrolyte composition (see caption), Au/C exhibits the best performance. Indeed, the Au nanocoral electrode has 35-fold higher Au content than Au/C. Furthermore, among the nanostructures obtained by the electrodeposition (see Figure 3), Au nanocorals exhibit the best electrocatalytic activity that is 1.54-and 2.2fold higher than AuNPs and the sputtered Au film electrodes, respectively [44]. This has been ascribed to its structure, especially the presence of Au(1 1 0) facets [43,44].
To better evaluate the ability of these electrodes, some direct glucose fuel cell (DGFC) tests have been performed. Figure 10A and B displays the DGFC performances using the electrochemically grown Au nanostructures on carbon paper as anode materials [44]. The open-circuit voltage (OCV) is 0.45, 0.61, and 0.64 V for sputtered film, nanoparticles, and nanocorals, respectively. The achieved maximum output power density (P max ) is 0.12, 0.34, and 0.85 mW cm −2 for the same sequence. This tendency, in agreement with the CV data, can be explained by the high surface area of Au nanocorals electrode and definitely, its structure that may better facilitate the diffusion of the species. This is supported by AuNPs electrode where the ohmic drop (IR) is more significant than its Au nanocorals counterpart with similar OCV. Among the three electrodes, the sputtered film is certainly the densest, which explained the inaccessibility to some actives sites and hard mass transport phenomenon. Figure 10C presents the DGFC performances at different concentrations of glucose using Au/C synthesized from the BAE method as anode material [51]. The OCV is quite similar, 0.90V (±20 mV); P max = 0.86 (0.  [74]. In addition, it outperforms the previous results from the electrodeposition method [44]. Importantly, a record OCV of 1.1 V has been reached in 0.5 M NaOH using Au/C from BAE as anode [51]. In conclusion, the BAE method foreshadows good prospects for the development of efficient anode materials.

Spectroelectrochemical investigations on AuNPs
Coupling electrochemistry to spectroscopy enables better understanding of the challenging anodic reaction. Mostly, in situ spectroelectrochemical experiments consist of coupling either cyclic voltammetry to FTIRS (CV-FTIRS or SPAIRS) or chronoamperometry to FTIRS (CA-FTIRS) [47]. For the mechanistic purpose, the in situ FTIRS in its Single Potential Alteration Infrared Reflectance (SPAIR) variant was undertaken to study the glucose electrooxidation on gold nanoparticles in alkaline medium [3,47,75]. This study enables the identification of adsorbed reaction intermediates and/or the final products. Figure 11 shows the SPAIR spectra obtained for spherical gold catalysts during the glucose electrooxidation reaction. From the SPAIRS spectra, the presence of gluconate is revealed by the three down-going vibration bands at 1350, 1416, and ca. 1575 cm −1 [3] corresponding, respectively, to CH 2 deformation, O─C─O symmetric, and asymmetric stretchings of this compound. The band at 1732 cm −1 together with the small band at ca. 1385 cm −1 is assigned to the C═O stretching and CH 2 deformation δ(CH 2 ) vibration modes of δ-gluconolactone [61,76]. In addition, the formation of gluconate occurs on the full range of potential, that is, from ca. 0.2 to 1.6 V vs. RHE as well as during the backward potential scan. Furthermore, the presence of the adsorbed reaction intermediate (δ-gluconolactone) on the electrode surface was confirmed by a slight downshift of the ν(C═O) band to lower wave numbers. δ-Gluconolactone is the primary reaction product of the glucose oxidation. After it desorption from the electrode surface, the lactone diffuses into solution and its cyclic structure becomes carboxylate anion after its electrolysis. The main product after this process is the gluconate. Finally, the feature vibration band of oxalate was identified through a sharp band at 1308 cm −1 which indicated a C─C bond cleavage of gluconate, as already reported [77]. Therefore, AuNSs electrode materials prepared herein are the best to convert glucose into gluconate via its corresponding δ-lactone in alkaline medium; the further dissociative adsorption of this main product at the electrode surface results in a C─C bond cleavage of the initial six-carbon molecule skeleton.

(Electro)Analytical investigations
Organic chemistry that consists of breaking and coupling various chemical bonds, for example, C─C, C─H, C─N, C─O, etc., enables the total synthesis of wide range of organic molecules and indirectly involves electron-transfer-driven reactions. Thus, electrochemical methods could serve as straightforward and powerful routes that could inspire the development of numerous elegant approaches to produce chemicals from C─C coupling reactions, functional-group interconversion, and installation of heteroatom moieties [78,79]. Otherwise, organic electrosynthesis replaces toxic or hazardous reagents, avoids large quantities of stoichiometric oxidants and reductive reagents, and can be used for the in situ production of unstable and hazardous reagents [80]. Consequently, the waste originating from the reagents used is almost negligible since only an unconsumed and recyclable electrode material and electrical current serve as reagents. Thus, electrochemistry complies with all the criteria of "green chemistry" [79,80]. One of the keys of the major gate leading to such breakthroughs is the electrode material that must exhibit high selectivity. Metal nanomaterials from BAE method have been used as electrodes for the electrochemical conversion of carbohydrates using glucose, galactose, and lactose as models [51].
The high-performance liquid ionic chromatographic (HPLIC) analysis of the sample after electrolysis at Au/C (BAE method) is depicted in Figure 12A and highlights the presence of two unresolved peaks at t R = 5.5 and 6.1 min, assigned to gluconate. Importantly, the combination of in situ FTIRS and HPLIC unambiguously state that C─C bond cleavage does not occur, which highlights the high selectivity of the electrode material [51]. The inset shows the time-dependent experimental number of electrons (n exp ) and subsequently the experimental Faradaic yield (τ F ). After 7.5 h of electrolysis, n exp = 2.05 and τ F = 102%. It was demonstrated that the catalyst efficiency was nearly 90% [51], which illustrates the excellent capability of Au/ C to overcome the deactivation phenomena and oxidize selectively most of the carbohydrates at the C1-position in two-electron process. To validate previous results, analytical determination of the reaction products by liquid chromatography coupled with mass spectrometry (LC-MS) was performed. Figure 12B shows the spectrum of the sample from glucose electrolysis and after resin exchange (H + -exchange) neutralization followed by lyophilization to generate the acid form of the reaction products. It displays a major peak at m/z = 195 that belongs to the pseudo-molecular ions (M−H) − of gluconic acid, whereas those at m/z = 129 and 391 are attributed to the fragmentation and simple dimerization processes. The 13 C NMR spectrum of the sample is displayed in Figure 12C. The 13 C chemical shifts of 95.9 and 92 ppm are assigned to the carbon at C1-position of the β-glucose and α-glucose (unreacted) and those at 176.9, 175.8, and 173.6 ppm as attributed to C1-position of gluconic acid, γ-gluconolactone and δ-gluconolactone [51]. More importantly, the spectrum of the lyophilized gluconic acid solution (commercial) and that of gluconate (commercial) after H +exchange and lyophilization are identical to the electrolysis sample. Consequently, the electrooxidation of carbohydrates at this Au/C concerns exclusively the carbon at C1-position and involves two electrons, thus underpinning the conclusion that the electrocatalysis can be used to oxidize selectively carbohydrates. This fundamental understanding of glucose oxidation taken as model can enable an efficient design of high-output-power co-generation devices for the energy and chemicals production from selective oxidation of the anomeric carbon of glucose and its related carbohydrates without any restrictive function protection.

Concluding remarks
Wet chemical methods are most versatile and highly flexible to synthesize size and shape controlled gold nanoparticles. From a fundamental point of view, the growth kinetic of nanomaterials constitutes a very important field of study in the understanding and designing of catalysts. This chapter explained nanocrystals growth. It also shows the main role of such studies in the development of new synthesis routes. The dependence of diameter and the form of the size distribution in the reaction-limited regime were well demonstrated. It is observed that the growth of nanocrystals can be controlled either by diffusion or by the reaction at the surface.
Such size control processes enable the synthesis of nanoparticles with good size distribution and well-defined surface structure allowing electrocatalytic reactions with good reactivity and selectivity. Transmission electron microscopy images and the electrochemical characterization through the sensitive method of UPD of lead allowed to analyze extensively the surface orientation of different gold nanomaterials. It was found that the as-prepared gold nanomaterials are structurally composed of (1 1 1), (1 1 0), and (1 0 0) index facets.
The electrocatalytic activity of different AuNPs was elucidated. It was shown that AuNPs exhibited high electrochemical activity toward glucose oxidation in alkaline medium. The correlation of the physical/electrochemical characterizations leads to show that the glucose electrooxidation is a size, shape, and crystallographic structure sensitive reaction. Small spherical gold with mean diameter of 4.2 nm shows higher performance to this reaction than counterparts. The small particles induced an increase in surface electronic effect, which enhanced the catalytic activity. In contrast, (1 0 0) facet appeared as the more active facet by improving the catalytic activity of the materials.