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Reports: UR552145-UR5: Synthesis of Ruthenium Dioxide Nanoparticles and Clusters with Tailored Size-and Surface-Dependent Properties for Enhanced Catalytic Applications

Vladimir Kitaev, Wilfrid Laurier University

The emphasis of the project has shifted from oxides of ruthenium and iridium to manganese and iron oxides. The shift was primarily driven by feasibility of applications: cost limitations are significant for applications of platinum and iridium, making only highly selective ones feasible. At the same time, manganese (III/IV) oxides and Fe(III) oxides to a significant extent feature comparable electro- and photochemical activity. The low cost of manganese and iron makes cost-competitive applications, such as water splitting and photoelectrochemical environmental degradations, more practical and realistic.

In the last year, we have concentrated our efforts primarily onto two systems: birnessite-like layered manganese oxides (MnO2-x) and hematite (a-Fe2O3). We have explored stable colloidal systems where we systematically studied different doping to enhance electrical (film conductivity) and optical (UV-vis absorption) properties of these materials, as well as elucidating the effect of doping on morphology of these nanostructures.

In one direction, we have explored noble metals (largely Au(0)) to both enhance conductivity and optical absorption by plasmonic resonance (while keeping cost factors in mind). In another broader direction we have explored several oxides: chromium, cobalt, copper, and tungsten, which are known to either be photoelectrochemically active themselves or serving as beneficial dopants.

Figure 1 shows morphological and optical effect of doping in MnO2-x system. Non-doped samples are shown in Figure 3B. Metallic gold (1) has the most prominent doping effect both in morphology (dendritic structures of gold) and in strong enhancement of absorption (broad plasmonic absorption of dendritic structures). At the same time, the photocurrents (Table 1) of these samples are relative low, likely due to the large size of the particles preventing good electrical contact with the conducting surface. Au-doped manganites will be explored for more specialized plasmonic- based sensing applications.

Figure 1. Manganese oxide doping. A) – morphological changes evidenced by TEM images and B) – optical properties evidenced by UV-vis of imaged samples of MnO2-x  (1.6 mM total Mn concentration) doped with 1) 10% Au(0), 2) 80% Fe(III), 3) 20% W(VI), 4) 80% Co(II/III). 

Fe(III) (2)and Co(II/III) (4) doping decreased the size of MnO2-x  platelets with comparable values of photocurrents for Fe(III) doping and significant enhancement for Co(II/III) doping (Table 1). Doping with W(IV) (3) lead to significantly smaller nanoparticle size which may become an instructive tuning parameter in this system. Cr(III) doping was also promising for the photoelectrochemical activity (see Table 1).

Table 1. Photocurrent densities in water splitting tests measured as a difference between dark and illuminated anodes. Each anode was prepared on FTO and tested at 0.7 V bias at 100 mW/cm2 light intensity in 0.15 M NaOH and platinum cathode.

Anode Composition

 Photocurrent Density (µA/cm2)
MnO2-x

70
MnO2-x + 10% Au(0)

 26
MnO2-x + 5% Fe(III)

 62
MnO2-x + 20% W(VI)

 42
MnO2-x + 10% Co(II/III)

 100
MnO2-x + 80 % Cr(III)

 114
α-Fe2O3

 10
α-Fe2O3 + 10% Au(0)

 12
α-Fe2O3 + 10% W(VI)

 12
α-Fe2O3 + 5% Cr(III)

 24
α-Fe2O3 + 10% Mn(II/III)

 20

Similar to Figure 1, Figure 2 presents several examples of changes in morphological and optical properties upon doping in iron(III) oxide system that consisted predominantly of hematite with some amounts of akaganéite (β-FeO(OH)).). Since the latter does not display any significant photocurrents, the emphasis was on hematite formation (monitored by its characteristic optical properties, XRD and photoelectrochemical activity). Compared to manganese oxides, the photocurrents of iron oxide were lower (Table 1).

Figure 2. Iron oxide doping. A) – morphological changes evidenced by TEM images and B) – optical properties evidenced by UV-vis of imaged samples (5.0 mM total iron concentration, except 5) doped with 1) 20% Au(0), 2) 5% W(VI), 3) 40% Cr(III), 4)  5% Mn(III), 5) 1.0 mM, no doping, 6) control.

Figure 3. UV-vis spectra (A) and TEM images (B) of MnO2-x nanoplates prepared by two-stage seeding for size control. 1) one-stage control, 2) 1/4 seeding, 3) 1/8 seeding, 4) 1/16 seeding, 5) 1/32 seeding, 6) 1/3 seeding. Each sample had the same total Mn concentration (0.16 mM); fraction listed is the relative amount of the first portion in two-stage addition.

Doping with Cr(III) was most promising for electrophotochemical properties (Table 1). Doping with W(VI) allowed for the production of smaller hematite particles that are advantageous given that the effective electron-hole separation is crucial for hematite photoelectrochemical efficiency. We continue to investigate this system further.

In the MnO2-x system, we have also realized synthetic control over the size of the platelets (strips) by a two-stage addition process, for which we were able to experimentally find optimal parameters. Figure 3 summarizes the results with the average size varied from ~ 50 nm (1/4 seeding) to 80 nm (1-stage). Optical absorption was found to be the strongest for samples with ¼ seeding (1) in Figure 3).

In the current stage, we continue with several most promising doped systems of manganese and iron oxides for the applications in water splitting, as well as photochemical and photoelectrochemical degradation of persistent pollutants.

In another development, we have continued with the synthesis of size-disperse latex, concentrating on negatively charged particles with the sizes ranging from 200 to 350 nm to approach controllable porosity of the anode materials. Figure 4 summarizes results in latex size control achieved with very good reproducibility (Figure 4A). Negatively charged particles were found to interact strongly with positively charged iron oxide and are beneficial for the porous composite materials for anodes. Both positively charged and negatively charged spheres can now be produced on a scale of kilograms when required for applications.

Figure 4. Size control in the synthesis of negatively charged polystyrene latex by varying the ratio of styrene to sodium salt of 4-vinyl benzene sulfonic acid (ST(-)). A) – dependence of the latex size on the molar ratio of styrene to ST(-) in the system. B) and C) – representative SEM images of two latex samples. (350 nm for B) and 220 nm for C)).

Overall, the project progressed significantly in controlling morphology and electrophotochemical properties of several oxide materials that will be further explored in applications.