Synthesis of Micron-sized Hexagonal and Flower-like Nanostructures of Lead Oxide (PbO2) by Anodic Oxidation of Lead

Micron sized hexagon- and flower-like nanostructures of lead oxide (α-PbO2) have been synthesized by very simple and cost effective route of anodic oxidation of lead sheet. These structures were easily obtained by the simple variation of applied voltage from 2–6 V between the electrodes. Lead sheet was used as an anode and platinum sheet served as a cathode. Anodic oxidation at 2 V resulted in the variable edge sized (1–2 μm) hexagon-like structures in the electrolyte. When the applied potential was increased to 4 V a structure of distorted hexagons consisting of some flower-like structures were obtained. Further increment of potential up to 6 V resulted in flower like structures of α-PbO2 having six petals. The diameter of the flower-like structures was ∼200–500 nm and the size of a petal was ∼100–200 nm.


Introduction
The fabrication of micrometer and submicrometersized materials are of great scientific and technological importance in applications such as photonic materials, microchip reactor, miniaturized sensor, separation technologies, and non-linear optical apparatus [1][2][3][4][5][6]. Lead monoxide nanostructures and its composites [7] have been synthesized by various methods such as spray pyrolysis [8], porous alumina template based method [9], sonochemical method [10] and sol gel method, [11] etc. PbO 2 electrodes are applied in the industrial processes such as energy conversion, synthesis, process recycling and environmental treatment [12][13][14][15][16][17]. It is well known that PbO 2 exhibits excellent chemical stability in an acid medium and nanostructured lead dioxide are re-ported as a novel stationary phase for solid phase microextraction [18]. PbO 2 can be obtained easily as anodic deposits from solutions of the low-valence lead ions [19]. The properties of lead dioxide are highly dependent on its method of synthesis, which affects the structure, morphology and phase composition. PbO 2 sub micrometer-sized hollow spheres and microtubes [20], lead oxide nanotubes [21], porous PbO 2 electrodes [22], lead oxide nanobelts [23], and lead dioxide films [24] have been synthesized by applying different synthesis routes. Liang Shi et al. reported lead oxide nanosheets, scrolled nanotubes, and nanorods in a controlled way [25].
PbO 2 nano-powders were synthesized by the ultrasonic irradiation of an aqueous suspension of dispersed. β-PbO, as a precursor, in the presence of ammonium peroxydisulfate as an oxidant [26]. Recently a sonochemical method was used for the synthesis of pure lead (II) oxide nanoparticles or nanobelts by utilizing mixed nano lead (II) two-dimensional coordination polymer as precursor [27][28][29].
Moreover, the electrocatalytic properties of PbO 2 may be enhanced by the incorporation of ions such as F − . In this context, fluorine-doped PbO 2 has been synthesized in the presence of some additives of fluorine-containing compounds (F − , potassium salt of nonafluoro-1-butanesulfonic acid C 4 F 9 O 3 SK and Nafion) [30]. Due to the limited studies on PbO 2 morphology and their subsequent application in nanoscale and microscale electronic devices, investigation on the synthesis and size dependent properties of lead oxide are significantly delayed. Micron sized hexagons and flower-like structures of α-PbO 2 have been synthesized by a very simple route of anodic oxidation of lead sheet. These structures can be easily obtained by the simple variation of voltage from 2 to 6 V between the platinum and lead electrodes. Synthesis of these structures is very simple and no costly chemicals, catalysts and surfactants are required.

Experimental Method
The experiment was performed in an electrochemical bath of Perspex with two electrodes set up. High purity lead sheet (alfa ascar, 99.9%) was utilized as a working electrode (the anode). A platinum sheet served as the counter electrode (the cathode) for anodization. The distance between the electrodes was kept 1 cm. A constant voltage of 2 V, 4 V or 6 V was applied between the electrodes using a potentiostat for a time span of one hour in every case. The volume of the electrochemical bath was 4 × 3 × 3 cm 3 and the surface area of each electrode was 2 × 1 cm 2 . 5 ml distilled water (with very low ionic conductivity ∼ 6-10 μS/m and pH=6.5) was used as the electrolyte. After every electrolysis run the synthesized material (settled down on the bottom of the electrolytic cell) was dried on the formvar coated copper grid and further characterized by transmission electron microscopy (TEM, Tecnai 20 G 2 ), and x-ray diffraction technique (XRD, X Pert Pro Panalytical).    Figure 2(b) is the magnified TEM image of such a distorted hexagon. Interestingly, it can be seen that the hexagons still have maintained its boundary; whereas the region inside the boundary of the hexagons have changed into small particle-like structures. The further magnified TEM micrograph as shown in Fig. 2(c) reveals that these particles are in the form of small circular and flower-like structures. Circular and flower-like particles have been indicated by white arrow in the Fig. 2(c). Figure 2(d) is the SAED pattern from these structures.

Results and discussion
As we increased the applied potential to 6 V, the hexagon-like structures disappeared and only flowerlike structures (which were also previously observed inside the hexagons) were observed in the sample as can be seen from Fig. 3(a) and 3(b). Figure 3 clearly six petals obtained at higher applied potentials. The diameter of the flower-like structures is 200-500 nm and each petal has the size 100-200 nm. Figure 3(d) is the SAED pattern from a single flower-like structure.
The white product obtained after several electrolysis runs was collected and further characterized by XRD for structural analysis. Figure 4 is the XRD pattern from the materials obtained after several electrolysis run at 6 V for 1 h. The diffraction peaks from these flower-like structures were indexed to the orthorhombic system of α-PbO 2 with lattice parameter a=4.971Å, b=5.956Å, c=5.438Å, confirming that the synthesized materials were α-PbO 2 . In addition to the delaminated nanostructures in the electrolyte, investigations of the lead anode, which were subjected to electrolysis runs, revealed the presence of large numbers of PbO 2 micron- Although the exact mechanism for the formation of lead oxide nanostructures is not quite understood, a plausible explanation for the formation of lead oxide nanostructures by electrolysis can be provided. The reactions for the formation of α-PbO 2 structures from Pb electrodes during electrolysis can be described by the mechanism suggested by Lee et al. [31]. In our case the electrochemical situation is [Pt/water(mildly acidic)/Pb], with applied voltage greater than ∼1.23 V, which corresponds to electrolysis with moderate to vigorous oxygen and hydrogen evolution. As we apply the potential greater than 2 V, the water gets electrolyzed. Under this condition the hydrogen is liberated at the platinum electrode whereas a continuous layer of surface adsorbed oxygen molecules is expected at the lead electrode. Simultaneously the lead electrode was oxidized into lead ions, which after reaction with the surface-adsorbed oxygen molecules, resulted in the lead oxide nanostructures. The formation of PbO 2 nanostructures, as is evident from Fig. (1, 2 and 3), is dependent dominantly on the applied potential. At mild applied potential of 2 to 4 V, oxygen evolution is expected to be mild and the migration of fresh Pb 2+ ions from the interior will be slow [32]. The formation of lead ions and surface-adsorbed oxygen and hence the reaction kinetics depends on the applied potential, which governs the formation of different lead oxide nanostructures. The reactions for the formation of α-PbO 2 structures from Pb electrodes after electrolysis can be described by the following equations [31,33]. Figure 5 presents a schematic on the formation of different structures of PbO 2 at various applied potential of 2 V, 4 V and 6 V. Left-side images are schematic presentation of the structures and the right-side images are corresponding to the nanomaterials as obtained at various applied potentials. It is a very simple and cost-effective method for the synthesis of the desired micron-sized hexagon-like structures or nano-sized sixpetal flower-like structure.  There are several effective parameters that can affect the structure, morphology and yield of the product, such as the distance between the electrodes, temperature, time of oxidation, and applied potential. Among all of these parameters, the applied potential between the electrodes was the most influential for the synthesized nanostructures. That is why the effect of the applied potential has been studied in detail, keeping in mind that the rest of the parameters could also influence on the yield of the products. As we increase the time of oxidation at a particular applied potential, we obtain a higher yield of the corresponding nanomaterials.

Conclusions
Submicron sized hexagons and six-petal flower-like structures of α-PbO 2 have been synthesized by a very simple route of anodic oxidation of lead sheet. Anodic oxidation at 2 V results in the formation of hexagon-like structures, with a size ranging from 1 to 2 μm in the electrolyte. Applied potential of 6 V results in six-petal flower-like structures of PbO 2 .