Iron Cuboid Nanoparticles for Catalysis

Iron Cuboid Nanoparticles for Catalysis

Daniel A. Pohoryles BSc Chemistry - Year 3 Physical Chemistry Research Project Candidate number: 00508020 Supervisor: Dr Paul Wilde Date of Submission: March 26th, 2010

Daniel A. Pohoryles


Iron Cuboid Nanoparticles for Catalysis Daniel A. Pohoryles

ABSTRACT Iron cuboid nanoparticles are useful catalysts for electrochemical denitrification as result of small size effects in addition to surface structure effects of the (1 0 0) facets. Pure Iron nanoparticles supported on glassy carbon (denoted nm-Fe/GC) were prepared by two different electrochemical deposition methods. Cyclic Voltammetry (CV) generated cuboid nanoparticles solely, whereas potential step deposition allowed the synthesis of a variety of shapes. The active surface area of the nm-Fe/GC was measured and it was shown that the nanoparticles were grown dispersedly on the GC surface. The electrocatalytic activity of the nanoparticles towards denitrification was investigated. Enhanced activity of the nanoparticles compared to bulk Iron was observed, the onset potential of reduction being positively shifted and the steady reduction current density being more than doubled. The effect of time and re-use of nm-Fe/GC for nitrite reduction was also investigated and an exponential decay of activity with time was observed. The current-time transients of Iron nanoparticle electrodeposition were analysed to examine deposition mechanisms.

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Introduction In the past decades1, nanomaterials have attracted substantial and increasing attention in a wide range of fields, ranging from catalysis, optoelectronics, and magnetic data storage to bio-sensors2. In electrochemistry this interest is due to the unique advantages over bulk material arising from small size effects such as enhancement of mass transport and high catalytic activity. Iron nanoparticles present a particular interest as Fe is one of the most abundant elements on earth3. Iron is also a very important catalyst in nitrate and nitrite reduction4, which are very important processes as agriculture (fertilizers) and some industrial sectors cause severe nitrate contamination of ground-water. Nitrate, once absorbed by humans, is converted to toxic nitrite by bacteria. Developing effective nitrate and nitrite reduction catalysts is hence of great importance for humans and the envIronment. Electrochemical denitrification using Iron was shown to be more envIronmentally friendly, more selective and more cost and energy efficient than other technologies (such as biological denitrification or ion-exchange)5. Other metal electrodes, Cu modified gold6 or Pd/Sn/Au electrodes for instance7, have shown similar advantages in electroreduction of nitrate and nitrite. One advantage of Fe is, however, that it is very versatile and very effective for the detoxification of a wide range of other common envIronmental pollutants, such as chlorinated organic solvents or organochlorine pesticides4. It is, hence, of interest to generate Iron nanoparticles as they have an enhanced catalytic activity in denitrification due to nanoscale properties, i.e. large surface areas and high surface reactivity. The synthesis of Iron nanoparticles was the object of extensive research. However, most research to date has concentrated on the generation of Iron-oxide nanoparticles. For instance, Giri et al.8 prepared mixed α-Fe2O3, γ-Fe2O3 and Fe3O4 Iron oxide nanoparticles and oxide-coated Iron nanoparticles were prepared by Banerjee et al.9. The research concerning synthesis of pure Fe nanoparticles was less fruitful though due to the rapid and easy oxidation of Iron. Chen et al. however achieved preparation of pure Iron nanoparticles by electrodeposition onto a glassy carbon substrate28,36 (GC on which Fe nanoparticles were deposited was labelled as nm-Fe/GC thereafter). The success of this deposition is due to the low surface energy of GC which allows metal nanoparticles to grow as individual islands rather than monolayers (Volmer-Weber mode, see Fig. 5). This will be discussed in more detail later in this report. Two different methods, cyclic voltammetry which generated cuboid nanoparticles and a potential step technique which allowed shape control, were presented by Chen et al. in 2008 and 2009 respectively. In both cases it was shown that the nanoparticles were grown dispersedly on the GC surface and the catalytic activity was enhanced compared to bulk Iron. This enhanced activity is ascribed not only to a nanosize effect, but also to the relative surface structure effect of different facets of Iron nanocrystals28.

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The interest in controlling the shape of the nanoparticles is that different shapes exhibit different catalytic activities for nitrite reduction. The coordination number of the atoms at the facets of the crystals determines their surface energy and hence the catalytic activities of the nanoparticles for chemical reactions10. Closest-packed facets are made up by atoms with a high coordination number and hence have a low surface energy; in turn crystals with an open structure have atoms with low coordination number at the surface and therefore a higher surface energy11. For Fe the (111) plane is an open structure surface for the bcc lattice and hence leads to a stronger catalytic activity than other planes, (110) for instance, which has the lowest activity, being the closestpacking surface12,13 (see Fig.1) .

Fig.1 - The (110), (100) and (111) crystal planes that can constitute the facets of Iron nanocrystals

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Another important aspect is the durability of the catalytic properties of Iron nanoparticles in denitrification. This has not yet been studied and will be investigated in this project. To understand the effects on catalytic activities, also the mechanisms of nitrite reduction on nm-Fe/GC and the state of the particles will be examined in this report.

Aims and Objectives The aim of this project was to synthesise Fe nanoparticles for catalysis, more precisely for the catalytic reduction of nitrite, a main pollutant of water sources. The replication of the electrodeposition of Iron particles on Glassy Carbon substrate by two different techniques, cyclic voltammetry28 and potential step35, were attempted. The theoretical backgrounds to the depositions will be explained. It was aimed at depositing different shapes of Fe nanoparticles (cuboid and rhombic dodecahedral) and to describe their differences in catalytic activity. In order to assess the success of these attempts, the electroactive surface was investigated. The obtained data was used to compare catalytic activity experiments to the results obtained by Chen et al.28,35 and to bulk Fe. Two different assessments of catalytic activity were performed, one involving reduction over a potential range and the other stepping to three distinct set potentials. These experiments should ultimately show if nanoparticles (better catalytic activity) or bulk-Fe were grown on the GC surface.

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The relative catalytic activities and surface areas of the particles grown by the two different methods were compared. The effect of time and re-use of the Fe nanocrystals was not investigated by Chen et al.28 and was hence analysed in this project. This experiment should give information on the durability of the particles which can be used to complement the data obtained on their efficiency (i.e. catalytic activity). It was tried to gain a better understanding of the surface structure of the particles and the mechanisms of nitrite reduction on the nm-Fe/GC, as the depreciation of the catalytic activity is strongly dependent on these factors. This was attempted by analysing the current-time transients observed during particle deposition. This was also not investigated by Chen et al. and no real knowledge of the state of the particles was found in the literature.

Experimental 1. Reagents, solutions, materials and apparatus Reagents used: Ferrous sulfate (FeSO4.7H2O), sodium hydroxide (NaOH), sulfuric acid (H2SO4), sodium nitrite (NaNO3), sodium nitrite (NaNO2), Lithium Perchlorate, Ferricyanide and ethanol. These reagents were obtained commercially (Sigma-Aldrich) in the purest available grade. Ultrapure water of resistivity not less than 18 MΩ.cm was taken from a Millipore Direct Q water system (Nihon Millipore Ltd.). Nitrogen (Pure shield, BOC) was used for de-aeration of electrolyte solutions. Glassy carbon substrate: The glassy carbon electrode was 3.0 mm in diameter with a geometric area of 0.07cm2. It was polished mechanically using successively alumina powder of size 3 and 0.3µm. It was cleaned with a stream of ultrapure water directed at the GC surface. The nm-Fe/GC were stored in alcohol after being taken out of solution at reduction potential as advised by Chen et al.17 Preparation of deposition solutions: Solutions of 0.02M (or 0.002M) FeSO4 + 0.1 M NaNO3, 0.1 M NaNO3, 0.2M NaOH and 0.2M NaOH with 0.01M NaNO2 were prepared freshly for the relevant electrochemical experiments. The deposition solution was always used directly after preparation and the solution was kept at a pH of 3 to 3.5 by adding a few drops of H2SO4 in order to avoid the oxidation of Fe(II) to Fe(III) by pH effect. If this was not done, at pH=5 there would be a change in colour from a transparent solution to a pale yellow solution with a flocculent precipitate (see Fig. 15) within a few hours due to oxidation of Fe(II) to Fe(III). The solutions were prepared with Millipore water (18 MΩ cm) provided by a Millipore Direct Q water system.

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Electrochemical experiments: Electrochemical experiments were carried out at room temperature. The set-up consisted of the GC electrode (working electrode), a platinum rod as counter electrode and an SCE (saturated calomel electrode) as reference electrode. This reference electrode was placed in a separate beaker, filled with the relevant background electrolyte, and connected to the reaction cell by a bridge (also filled with the background electrolyte). All potentials were given with respect to SCE. Electrode potential was controlled by a BAS 100B/W electrochemical workstation. Before each electrochemical measurement, the nm-Fe/GC electrode was subjected to a cathodic polarization at −1.25V for 15 min in 0.2M NaOH solution and the solutions were de-aerated by bubbling highpurity N2. All the different electrochemical experiments are summarised in Table 1 at the end of the Experimental section. 2. A brief description of Cyclic Voltammetry Cyclic Voltammetry is a method to investigate the electrochemistry or electron transfer properties of a compound or a system. It is what could be called “electrochemical spectroscopy”: instead of scanning through different wavelengths in order to investigate absorption processes, it is possible to scan over a range of potentials. This allows analysing electron transfer processes, i.e. oxidations and reductions14. One important concept that makes theoretical treatment easier is that in cyclic voltammetry reactant transport is by diffusion only15, hence migration (motion as a result of the electric field) and convection (eg. Stirring) must be suppressed. In order to accommodate for this, no degassing or stirring is applied during the scans and a background electrolyte is used in a high concentration to suppress migration. This electrolyte is chosen so as not to have any electron transfer reactions in the potential range of interest. The set-up consists of a working electrode (in this case a Glassy Carbon or GC electrode) at which the reaction of interest takes place, a counter electrode (in this case a platinum electrode) which allows passing current and a reference electrode through which no current is passed. The reference electrode in this project was an SCE (saturated calomel electrode) which was placed in a separate beaker (filled with the relevant background electrolyte) and connected to the reaction cell by a bridge (also filled with the background electrolyte). All potentials were given with respect to this SCE. It is possible to vary the potential range, the number of cycles of potentials applied to the system, the scan rate and the direction (negative or positive) of the scan. The range of potentials used largely varies on the system analysed and the direction depends on the process studied. In this project the scans were all performed at negative potentials in the negative direction as reductions (of Iron, of nitrite) were analysed. The scan rate is chosen appropriately to the rate of the studied reaction. Some electron transfers might only appear if a sufficiently slow scan rate is used16. 6

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It is important to mention that the state of the surface of the electrode plays an important role, especially because the processes studied in this project all happened on particles adsorbed on the surface of the GC electrode. An alteration of surface conditions (e.g. rough or oxidised surface) can shift the voltammogram and this will alter the processes occurring at the set potentials. The preparation of a smooth GC surface is hence very important25,26. 3. Preparation of Fe nanoparticles by cyclic voltammetry The Fe cuboid nanoparticles were deposited electrochemically onto GC substrate under cyclic voltammetric (CV) conditions. Ten cycles of potential cycles between -900mV and -1250mV were applied in 0.02M FeSO4 solution at a scan rate of 50mV.s-1. An electrode onto which Iron nanoparticles were deposited was denoted thereafter as nm-Fe/GC. 4. Preparation of Fe nanoparticles by potential step deposition The electro-deposition was performed on a GC electrode in a solution of 0.002M FeSO4 with 0.1M NaNO3. Fig. 16 illustrates schematically the procedure of electrochemical deposition. The potential was first stepped from open current to a nucleation potential and then to a growth potential. The standard parameters for nucleation time and potential were 4s and -1200mV. These parameters were changed in a few experiments to attempt an investigation of the mechanisms of deposition. For the cuboid particles (e) the growth time was 300 sec and Egrowth was -1070mV, for the rhombic dodecahedral crystals the growth parameters were 900sec and -1030mV. 5. Investigation of active surface area This was recorded after electrochemical activation at -1250mV for 900s with N2 de-aerating. The cyclic voltammogram of the nm-Fe/GC electrode in 0.2M NaOH was recorded with potential cycling from -500mV to -1100mV in the negative direction at a scan rate of 25 mV.s-1. The integral of the current against time plot was then calculated to assess the charge which was used to obtain the active surface area. 6. Determinations of catalytic activity of nm-Fe/GC Before the investigation of catalytic activity investigation, the electrode was activated at a potential of -1250mV for 15minutes in 0.2M NaOH in order to remove any possible surface oxide from the Iron which might decrease or inhibit catalytic activity. In order to assess the catalytic activity of the Fe nanoparticles, the voltammogram of NO2- reduction on nm-Fe/GC2 was recorded in solutions of 0.2M NaOH with 0.01M NaNO2. The potential range was from -1000mV to -1350mV with a scan rate of 1mV.s-1. In order to demonstrate the differences in steady catalytic activity between nm-Fe/GC and bulk-Fe electrodes, j–t curves were recorded on nm-Fe/GC, at −1.2, −1.25 and −1.3V within a time window of 300 s.

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Table 1 - Summary of the different electrochemical methods used throughout the research project

Clean Electrode method

time

potential

solution

activation

900sec

-1250mV

0.2M NaOH Cyclic voltammetry

method

min

max

direction

scan rate

segments

solution

Fe deposition

-1250mV

-900mV

negative

50mV.s-1

20

0.02M FeSO4

Catalytic activity

-1350mV

-1000mV

negative

1mV.s-1

1

0.01M NaNO2 in 0.2M NaOH

Surface area

-1100mV

-500mV

negative

25mV.s-1

2

0.2M NaoH

Potential step method

1

3

5

7

a) Step deposition rhombic dodecahedra

0.265mV /1sec

-1200mV /4sec

-1030mV /900sec

x

e) Step deposition cuboid NC

0.265mV /1sec

-1200mV /4sec

-1070mV /300sec

x

Steady state catalytic activity

800mV /3sec

-1200mV /300sec

-1250mV /300sec

-1300mV /300sec

solution

0.002M FeSO4 in 0.1M NaNO3 (pH= 3-3.5)

0.01M NaNO2 in 0.2M NaOH

Results and Discussion Y-X Chen et al.17 describe a simple method for the electrochemical synthesis of pure Iron nanocrystals. The nanoparticles are prepared by reduction of dissolved Fe2+ onto a Glassy Carbon (referred to as GC) substrate by cyclic voltammetry (CV). Depositing pure Fe nanocrystals and maintaining them in their reduced form is generally a difficult process as Iron oxidises very easily18, which leads to the formation of Iron oxide nanoparticles19. The first task was to reproduce the literature method before trying any new investigations. Cleanliness of the experimental set-up and most significantly the state of the GC electrode surface can have a large influence on the outcome of the experiment and can change the electrochemical process20.

1. Determination of the surface state of the glassy carbon electrodes Before tackling Iron nanoparticle deposition, determining the state of the surface of the glassy carbon electrodes is crucial. Glassy carbon (GC) is a non-graphitizing allotrope of carbon that is very useful as an electrode material since it is a cheap and relatively inert material and a wide range of potentials can be applied to

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it21. An important drawback however is that it can oxidise, producing carbon-oxygen functionalities (such as phenolic-, quinonic-, carboxylic-, carbonylic and epoxidic-like groups) on the surface22. This oxidation and the adsorption of impurities can substantially affect its electrochemistry25. Surface treatment is hence a fundamental aspect that must be considered when working with GC electrodes. The surface structure is very dependent on the method of its preparation23, in order to get reproducible results, it is important to follow a strict protocol for the cleaning and activation of the electrode throughout a series of experiments. Most commonly, the GC surface is polished using micro-sized abrasives which exposes fresh new surface, as the old surface was gradually deactivated by exposure to the atmosphere. Clean conditions are absolutely necessary, Chen et al.28 for instance suggest wearing nitrile gloves during the polishing so that skin oils do not get onto the electrode. The polishing should be performed on larger abrasives first, going to smaller and smaller sizes. In this experiment the electrodes were polished using 3µm and then 0.3µm alumina. The polishing was performed with light pressure on a flat glass surface following an 8 pattern for 30 seconds. Ideal preparation would require the use of a larger range of different abrasives. It is important to note that the use of particles that are too large or agglomerated might scratch the surface which is also not desired. In order to remove carbon particles and remains of polishing material, the GC has to be rinsed with ultrapure water and sonication is suggested for an ideal removal of impurities. The ideal modes of activation are vacuum heat treatment (VHT) as suggested by Fagan et al.24 or polishing with alumina in cyclohexane (or other organic solvents)26. The interest of both these methods is the absence of oxygen-containing functionalities on the carbon surface, which, as discussed before, have a large effect on the electrochemistry. The state of the GC surface was hence investigated by recording a cyclic voltammogram in 0.1M Lithium perchlorate solution. Three GC electrodes (labelled GC 1, 2 and 3) were analysed after being polished with 0.3µm alumina powder by applying one potential cycle between -500 and 1000 mV at a scan rate of 50mV.s-1. The background electrolyte contains no redox species and so the only current that is passed is the charging current, which flows in order to change the potential of the electrode. When the electrode surface is larger, the current passed is larger. Ideally the response should show no peaks and a lower current suggests a lower area and hence a smoother surface. The results of this experiment are displayed in Fig. 2, indicating that GC2 seems to have the smoothest surface as the background current is the lowest for this electrode.

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Current/µA

-15

0

15 -600

-100

400 Potential/mV

900

Fig. 2 - Cyclic voltammograms of the glassy carbon substrates GC1 (-), GC2 (- - -) and GC3 (.....) in Lithium Perchlorate solution, scan rate 50mV.s-1, with potential scan range of -500 to 1000mV

In order to investigate the state of the surface in more depth, measuring the difference in peak potential for the ferri/Ferrocyanide redox couple is a useful test25. The CV of Potassium Ferricyanide (small amount, concentration not important) in 0.1M Lithium Perchlorate (i.e. at roughly neutral pH) was recorded from 800 to -400mV at two scan rates (25 and 50mV.s-1). For a clean and electrochemically active GC, this difference is expected to be close to 60mV26. This separation represents an ideal electron transfer process (which ferri/ferrocyanide should be since there are no bonds made or broken) with rapid kinetics. GC electrodes were polished consecutively with 3µm and then 0.3µ m alumina powder. The results are displayed in Table 2 and confirm the findings from the background scans, for GC2 the peak separation is the closest to 60mV, hence it has the smoothest and most active surface of the three electrodes. However it has to be said that the peak separation of 84mV does still not correspond to the ideal active surface. Consequently, the GC2 electrode was used for most of the following experiments.

Table 2 - Peak separations of the ferri/Ferrocyanide redox couple

GC1 GC2 GC3

V /mV.s-1 ∆peak/mV= ∆peak/mV= ∆peak/mV=

25 107 84 92

50 126 87 117

2. Fe deposition by cyclic voltammetry After assessing the state of the surface of the GC electrodes, the Fe-nanocrystal deposition was undertaken. Ten cycles of potential cycles between -900mV and -1250mV in 0.02M FeSO4 solution were

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applied at a scan rate of 50mV.s-1. The conditions of the experiment were utterly crucial; it took a few failed depositions till a similar cyclic voltammogram to the deposition graph in literature was obtained. The failure of the deposition was determined as no deposition was visible on the surface and no catalytic activity (described in section 3) for the nitrite reduction was obtained. It was noted that the polishing of the GC electrode had a large effect on the success of the deposition, after more vigorous polishing was applied, the deposition of nanoparticles was visible by inspection (and also from the electrochemical data). The cyclic voltammogram recorded during the electrodeposition of the Fe nanoparticles on GC is presented in Fig. 3. In the first cycle (represented by a single arrow) a large overpotential is needed for Iron reduction. This is shown by a low reduction current which is increasing slowly until -1220mV, where it then starts to increase very steeply. This is very similar to the data recorded by Chen et al.28 which can be seen in Fig. 4. In the second cycle, the current increases straight from the start of the cycle and the peak reduction current is larger than it was for the first cycle. The current is increasing continuously in the negative-going potential scan (NGPS), this is unlike the literature, where it increases to a plateau at -1000mV and then increases again at around -1100mV in literature. Also, the cross-over observed on Fig. 4 in the second cycle, cannot yet be seen in the obtained data, it occurs in the third cycle. This cross-over, corresponding to a potential range where the cathodic current on the reverse scan is higher than on the forward scan, is a typical feature of nucleation27 and can be seen very well in Fig. 4.

Fig. 3 - Cyclic voltammograms of the electrodeposition of Fe on the GC2 electrode, at a scan rate of 50mV.s-1 in 0.02M FeSO4 solution. The first three cycles are highlighted by arrows, the dashed line corresponds to the subsequent cycles.

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Fig. 4 - Cyclic voltammograms of the electrodeposition of Fe on GC electrode performed by Chen et al.28

On the NGPS for the first cycle, when the glassy carbon has no Iron particles on its surface, Iron particles hence need to nucleate onto the GC surface. Nucleation onto the carbon surface is however a difficult process as small centres are thermodynamically unstable and should hence redissolve29. For the nucleus to become stable on the substrate, it has to reach a specific size which can only be achieved if the atoms assemble rapidly. This is achieved by driving the electron transfer process by applying a sufficiently strong overpotential. Hence, on the first NGPS, deposition of Fe cannot begin until this overpotential has been reached. However, once stable nuclei have grown on the GC surface, the growth can continue readily. This is what can be seen on the reverse scan, Fe can deposit on top of Fe particles which is a much faster process and is thermodynamically more favourable. The growth of individual nanoparticles, unlike two-dimensional layer formation, is governed by a Volmer-Weber growth mode30 (illustrated in Fig. 5), leading to island formation. This island formation is achieved as the substrate has a smaller surface energy than the Fe deposits; the adatom cohesive force is hence stronger than that of the GC surface, thus the particles grow in islands (4a) rather than spreading evenly across the surface (4b). With an increasing number of cycles, Fe deposition becomes easier, because there are more Fe particles on the surface on which further deposition can take place. The subsequent cycles in Fig. 3 are quite similar in reduction current and shape to the second and third cycle. This is different to the literature, where the reduction current decreases with increasing number of cycles.

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Iron Cuboid Nanoparticles N for Catalysis

Fig. 5 – (a) Volmer-Weber Weber (island formation) compared to (b) Frank-van Frank der Merwe (layer-by by-layer) growth modes

The dissimilarities of the obtained data and the literature might originate from the state of the surface. As mentioned previously, the surface of the GC used in this experiment is not very smooth. The increased roughness of the surface could mean that Fe deposition is easier, which can ultimately lead to the formation of bulk Iron as islands of particles may merge together if too much Fe was deposited. The deposition of Iron particles onto the GC substrate, substrate as described by Chen et al. al 28, was hence somewhat reproducible. The deposits are visible on the GC electrode and Fig. 6 shows a GC electrode on which particles have been deposited: a greyish film covers the GC surface.

(a)

(b)

Fig. 6 – (a) GC2 on which Iron particles have been deposited (nm-Fe/GC2) (nm compared to (b) a GC with no deposition

3. Catalytic activity for nitrite reduction after the first successful Fe deposition As shown in the previous section the deposition of Iron particles seemed successful, the next n step was to test their ability to function as catalyst for the reduction of nitrite. This is of interest as nitrite is a major agricultural pollutant and its reduction is hence very favourable for the environment. env As we will see below, this his experiment showed that Iron was deposited on the GC surface as reduction current in the nitrite reduction was observed.

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In order to assess the catalytic activity of the Fe nanoparticles, the voltammogram of NO2 - reduction on nm-Fe/GC2 was recorded in solutions of 0.2M NaOH with 0.01M NaNO2. The potential range was from -1000mV to -1350mV with a scan rate of 1mV.s-1. Before the investigation of catalytic activity, the electrode was activated at a potential of -1250mV for 15minutes in 0.1M NaOH in order to remove any possible surface oxide from the Iron which might decrease or inhibit catalytic activity. An interesting observation was the evolution of gas bubbles during nitrite reduction on the nm-Fe/GC electrodes which was also pointed out by Chen et al. Under the conditions of this experiment, most probably this gas corresponded to NO and H2 31. This is due to the following reactions: -

Nitrite reduction to nitrous oxide NO2- + H2O + e-

-

NO + 2OH-

Hydrogen evolution 2H2O + 2e-

H2 + 2OH-

The first trials of the catalytic activity investigation were performed on the nm-Fe/GC2 for which the deposition is shown in Fig. 3. The catalytic activities of the freshly prepared particles and for the same particles after being stored for one day in ultra-pure water can be seen in Fig. 7. 0,00E+00

1,00E-04 I/A

background GC Fresh nmFe/GC2

2,00E-04

1 day old nm-Fe/GC2

3,00E-04 -1400

-1200 E/mV

-1000

Fig. 7 - Comparison of the catalytic activities of fresh and one day-old nanoparticles

Catalytic activity is observed as expected for the freshly deposited particles. The reduction current starts to increase at about -1080mV which is the same value obtained by Chen et al.17. Two current (shoulder) peaks (indicated by two vertical arrows in Fig. 7) were observed in the nitrite reduction, a less defined one at -1250mV and a more defined one at -1290mV. These peaks are similar to those obtained in literature (-1180 and -1280mV), however their relative intensities cannot be compared as the data above are presented as

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current whilst the literature values are presented as current density or current per unit area. A background scan of the GC substrate with no Iron was also performed, with next to no reduction current observable. This confirms that there is indeed Iron deposition on the GC electrode. It seems very important though, to investigate the electroactive surface area of the Iron in order to compare the data to the literature and to the response of bulk Iron. Without this data, it is not possible to confirm that nanoparticles were actually formed. The catalytic activity for the particles after one day of storage in ultrapure water is also shown in Fig. 7. It can clearly be seen that there are two effects of time on catalytic activity. Firstly the peak currents are decreasing and the peaks are much less defined. Secondly the increase in reduction current (indicating the onset of nitrite reduction) starts at much more negative potential (-1140mV compared to -1080mV for the fresh particles). These features indicate that nitrite reduction on the electrode coated with Iron becomes more difficult, so that a more negative potential is required before a measurable current is observed, and these features are similar to the ones observed by Chen et al. for bulk Iron. It is possible that the islands of nanoparticles obtained the previous day have grown together and formed clusters or that they have been passivated by an oxide layer (perhaps by atmospheric driven corrosion of the Fe particles). In order to keep the particles active, the nm-Fe/GC has to be taken out from solution at reduction potential (e.g. after the nitrite reduction) and then stored in absolute alcohol (ethanol was used in this project).

4. Determination of the electroactive surface-area of the nm-Fe/GC electrode It is of great importance to know the electroactive surface area of the nanoparticles so as to obtain results that can be compared to literature. Also in order to analyse the effect of time (or repetitive use) on the catalytic activity of the nanoparticles, determining the surface area would allow a greater understanding and a more quantitative comparison would be possible. As we will see below, results similar to literature and hence an active area of less than half of the total GC surface were obtained. This indicates that Iron was deposited dispersedly, giving another indication for the deposition of nanoparticles. In order to assess the electroactive surface area, the density of electric charge corresponding to the electrooxidation of Fe into Fe(II) and Fe (III) species32 was compared between the Iron nanoparticles and bulk Fe. The nm-Fe/GC electrode was placed in alkaline 0.2M NaOH solution, in which the nm-Fe undergoes oxidation and passivation processes under electrochemical conditions. Multilayers of Fe oxide will be formed on the surface and the electric charge density will depend on the number of Fe oxide layers formed33.

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Under the electrochemical conditions used in this experiment, Iron(III) oxide-hydroxide was determined, by different UV-vis spectroscopic techniques32, to be the main product of the electrooxidation of Fe in alkaline solution. The oxidation process can be described as follows34: Fe + 3OH-

FeOOH + H2O + 3e-

The cyclic voltammogram of the nm-Fe/GC electrode in 0.2M NaOH can be seen in Fig. 8. It was recorded with the potential cycling from -500mV to -1100mV in the negative direction at a scan rate of 25 mV.s-1 so as to obtain a cyclic voltammogram with one peak for reduction and one for oxidation. It is important to mention that the electrode has to be electrochemically activated at -1250mV for 15 minutes just as it was for the catalytic activity measurements. This activation at reduction current was performed in order to reduce any native oxide layer formed on Iron upon contact with air.

i /µA

-25

0

25 -1200

-1000

-800 E/V (SCE)

-600

-400

Fig. 8 - Cyclic voltammogram of nm-Fe/GC electrode, 0.2M NaOH solution, scan rate 25mV.s-1, potential range -500mV to -1100mV

The cyclic voltammogram features two peaks, one corresponding to the reduction process at -1025mV and one for the oxidation which is at -700mV. The data reported by Chen et al. is shown in Fig. 9. As it can be seen the cyclic voltammogram is similar to the one obtain in this research project, with a peak at -1030mV in the anodic scan and one at -690mV in the cathodic scan. One important difference however is that the current values obtained here are lower than those reported by literature. This can be explained easily by the fact that the surface area of the GC substrate used is lower than the one used by Chen et al. (0.07cm² compared to 0.28cm²). In order to compensate for this difference the ratio of electroactive area (i.e. of Fe particles) and total GC surface area was calculated for each surface area investigation in order to normalise the results.

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Fig. 9 - Cyclic voltammogram of nm-Fe/GC electrode, 0.2M NaOH solution, scan rate 25mV.s-1, potential range -500mV to -1100mV obtained by Chen et al.17

From this cyclic voltammogram, the anodic (qA) and cathodic (qC) electric charge (in mC) were measured by integration (see Fig. 10). The data obtained can be used to calibrate the electroactive surface area of the Iron nanoparticles using the electric charge density measured for bulk Fe for the same processes in this potential range. The data for bulk Fe were taken from the experiments performed by Chen et al.17. Fig. 10 also summarises the calculations performed by Chen et al. to obtain the charge denisities qA and qC for bulk Fe (a) and hence surface area of the Fe nanoparticles (b): The integration of the current-potential plots against time gives the charge as Q=I.t. Chen et al. calculated the average of the cathodic (qC) and anodic (qA) charges for bulk Fe (a) of known surface area. This allowed charge density to be obtained, which serves as conversion factor between charge and surface area. This conversion factor was found to be 5.14mC.cm-2. Again by integration the cathodic and anodic charges for the nanoparticles were obtained. By dividing the average charge by the conversion factor, the surface area of electroactive particles was obtained.

Fig. 10 - Cyclic voltammograms of bulk Fe (a) and cube nano-Fe (b) electrodes, 0.2 M NaOH solution, scan rate 25 mV s-1, with potential scan range between -1.1 and -0.5V. 35

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Iron Cuboid N Nanoparticles for Catalysis

The same calculations were applied in this project; however the current-time current time plot was used as it made the integration much easier to perform. The current current-time time plot was obtained from conversion of potential to time by dividing by the scan-rate rate (25mV.s-1). Fig. 11 shows the graph used for the surface area calculations. The area under the curve was calculated by multiplying the current current at each point in time by the difference in time between the points (dt = 0.04s). The cathodic and anodic charges were averaged and the average divided by the aforementioned conversion factor (5.14mC.cm-2). Finally the surface area was normalised by the total GC area for comparison son with the normalised literature active area. An An example for the calculations for this process is shown in Table 3. -20 20

-10 10

I/µA

0

10

20 0

20 time/s

40

Fig. 11 - Current-time time plot of the surface area investigation experiment for nm nm-Fe/GC, 0.2M NaOH solution, scan rate 25mV.s-1, potential range -500mV to -1100mV 1100mV

Table 3 shows one example out of many surface calculations. In general the ratio of particle area to GC area was similar to the one obtained by Chen et al.28 A general observation is that the cathodic charge is always larger than the anodic charge. This was also reported by Chen et al. and is related to Hydrogen evolution that can occur at the negative ative end of the range of the potential cycle. A more important observation is that the ratio of electroactive surface to total GC surface is lower than one, indicating that the Fe nanoparticles were deposited dispersedly on the GC surface and hence neithe neitherr a monolayer of Fe nor bulk Fe were formed. Table 3 - Example of the calculations performed to obtain the normalised surface areas

nm-Fe/GC2 literature

17

Qc

Qa

Qaverage

Electroactive

total GC

/mC

/mC

/mC

surface/cm2

surface/cm2

ratio

0.176

0.140

0.158

0.031

0.07

0.43

/

/

/

0.096

0.28

0.34

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Daniel A. Pohoryles


The investigation of surface area was the first proof that nanoparticles were formed in this project. The surface area experiment was performed after each deposition and before each catalytic activity experiment. Therefore current densities could be calculated and the catalytic activities could be compared to the data for bulk Fe and the nm-Fe/GC obtained by Chen et al.17.

5. Investigation of the effects of time on the catalytic activity of nm-Fe/GC In the preceding section, it was shown that the deposition of particles was successful, and now knowing the surface area of the particles, the catalytic activity can be analysed in more depth. Chen et al. analysed the activity of particles and compared it to bulk Iron and other shapes of particles, however they did not investigate the effects of time or repeated use of the particles. As we will see below, this was analysed over the course of two weeks in this project, and it was observed that catalytic activity deteriorates with time. The exact reasons for this are not known and further investigation would be required to explain this phenomenon. Iron nanocrystals were deposited again on the GC substrate and this time their surface area was calculated. The data shown in Table 3 corresponds to this experiment. The catalytic activity was then tested and the current divided by the value of surface area to get current density j in mA.cm-2 . Fig. 12 shows the nitrite reductions at the nm-Fe/GC obtained on the day of deposition, six, eleven and 14 days later.

0

j / mA.cm-2

2

4

6

8 -1400

-1300

-1200

-1100

-1000

E/ mV (SCE)

Fig. 12 - Effect of time on catalytic activity–nitrite reduction on nm-Fe/GC, CV in 0.2M NaOH with 0.01M NaNO2, 1mV.s-1, freshly deposited (-), five days old (- - -), ten days old (- · - ·) and 14 days old (···) particles

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Daniel A. Pohoryles


To begin with, as seen in Fig. 12, the catalytic activity of the freshly deposited particles has the features described by Chen et al., that were also obtained in the first nitrite reduction mentioned beforehand: The reduction current starts to increase at about -1080mV and two current peaks (indicated by arrows) were observed in the nitrite reduction, one less defined one at -1260mV (j=5.5 mA.cm-2) and a more defined one at -1295mV (j=6.8 mA.cm-2). Moreover, knowing the current densities, these can now be compared to the results obtained by Chen et al.28 for bulk Fe and the nanoparticles. The peak current densities obtained in this project are lower (by about 20%), however they are reasonably similar and double the current density for nitrite reduction on the bulk-Fe. It can be said with confidence that Iron nanoparticles were grown, not only because the surface area indicates that it was not a monolayer of Iron, but also because enhanced catalytic activity compared to bulk Iron was observed. Chen et al. report that they grew cuboid nanoparticles which have a very strong catalytic activity due to optimal surface effects36. It could be that a different shape of nanoparticles was grown here which would explain the differences in catalytic activity. Fig. 12 also allows a quantitative assessment of the effect of time by comparing the decrease in current density with time. The nm-Fe/GC was stored in alcohol after being taken out of solution at reduction potential as advised by Chen et al.17. The current densities at the potentials corresponding to the peaks in nitrite reduction for the freshly prepared particles, i.e. at -1260 and -1295 mV, were compared for the four sets of data. The graph obtained is shown in Fig. 13. 8 7

j/mA.cm-2

6 y = 6,6163e-0,059x R² = 0,9935

5

at -1260mV at -1295mV

4

Expon. (at -1260mV) 3

Expon. (at -1295mV) y = 5,5618e-0,08x R² = 0,9886

2 1 0 0

2

4

6

8

10

12

14

16

time/days

Fig. 13 - Evolution of current density at the current peaks at -1260 and -1295mV with time. The trend lines correspond to exponentials, their equations indicated next to the lines. The deviation from the experimental data is shown with error bars.

For the peak at -1260mV, there is a drop in current density of about 33% after six days, 60% after eleven days and 66% after two weeks. For the second peak the drops are of 35%, about 50% and about 57% respectively. The

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drop looks very much like an exponential decay and a model of the form y=A.exp(-xt) was fitted using Microsoft Excel. The coefficient of determination gives information on the goodness of fit of a model. An R2 of 1.0 indicates that the regression line fits the data perfectly37. For a good approximation though, this value should ideally be higher than 0.99. For the exponential models used in Fig. 13, the R2 values are 0.9886 for the peak at -1260mV and 0.9935 for the peak at -1295mV. This means that the peak current vs. time graph actually describes an exponential decay. It is very difficult to offer an explanation for this decay, as the actual mechanism for nitrite reduction on the nm-Fe/GC is not known and has also not been investigated by Chen et al.28. Also the state of the particles would have to be investigated further. It may be a slow irreversible oxidation of the Iron, or surface sites blocked by adsorptive impurities or perhaps some agglomeration of particles into larger clumps. SEM (Scanning electron microscopy) visualisation over time might be some help in unravelling this. The second observable trend in the evolution of catalytic activity with time Fig. 12is that the point at which reduction current starts to increase is shifted to more and more negative potentials the older the particles are. This indicates that nitrite reduction at the nm-Fe/GC electrodes requires a stronger overpotential; it is hence a more difficult process indicating that the catalytic activity of the particles was deteriorating with time (trend shown by a horizontal arrow in Fig. 12). 6. Deposition of Fe nanoparticles of different shape on GC substrate using a potential step technique After the successful deposition of Iron nanoparticles by cyclic voltammetry and good results were obtained for their electroactive surface area and catalytic activity in nitrite reduction, a second paper written by Chen et al. was discovered. Almost a year after the paper in Electrochimica Acta, a Communication in the J. Am. Chem. Soc. describes the electrodeposition of Fe nanocrystals of different shape controlled by potential step method stepping to a nucleation potential (ENUC) and then to a growth potential (EGROWTH)36. Depending on the growth time (tGROWTH) and potential used, different shapes of Fe nanocrystals were obtained and there shapes were confirmed by Scanning electron microscopy (SEM), X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), small angle electron diffraction (SAED) and transmission electron microscopy (TEM and HDTEM). This second way of growing Fe nanoparticles on GC was investigated and is reported below. In summary though, the deposition seemed successful as good surface areas were obtained and deposition was visible on the electrode, however the catalytic activities obtained for the nitrite reduction were about a hundred times too low. In order to understand why this was the case, the j-t transients of the nucleation and deposition were analysed. These were not examined by Chen et al.36 and might give crucial information on the mechanism of deposition as well as the state of the produced particles. However time did not allow in-depth analysis of these transients. 21

Daniel A. Pohoryles


Growing specific shapes of particles allows controlling their catalytic activity. Chen et al. synthesised crystals with surface structures varying from {110} (highest stability, lowest activity) to a mixture with different fractions of {110} and {100} to {100} (highest surface energy, highest activity). They showed that the higher the overpotential EGROWTH, the higher the fraction of {100} and this result can be explained by two-dimensional nucleation theory38. The rate of formation of the crystals hence depends on the work of formation of specific nuclei which changes depending on the overpotential applied36. Fig. 14 shows a selection of shapes obtained by this method. The shapes marked in red labelled a) (rhombic dodecahedral) and e) (cube) were the two extremes in catalytic activity, e) being composed of {100} facets solely has the best catalytic activity for nitrite reduction. Due to a restricted amount of time, it was decided to concentrate on the deposition of these particle shapes in this project. The table in Fig. 14 also shows the different values for EGROWTH and tGROWTH used to deposit the particles.

a)

e)

Fig. 14 - Fe Nanocrystals obtained by Chen et al.35 in 0.002 M FeSO4 + 0.1 M Na2SO4 solution, Eoc=0.265 V, toc=1.0 s, Enuc=-1.2 V, tnuc= 4.0 s, Egrowth and tgrowth are listed in the table. Images of the particles obtained from SEM.

The electro-deposition was performed on a GC electrode in a solution of 0.002M FeSO4 with 0.1M NaNO3. It is important to keep this solution at a pH of 3 to 3.5 by adding a few drops of H2SO4 to keep the solution stable, otherwise (at pH=5) the solution changes in colour to a pale yellow with a flocculent precipitate (see Fig. 15) within a few hours due to oxidation of Fe(II) to Fe(III).

22

Daniel A. Pohoryles

(a)


(b)

Fig. 15 – Change of colour after about 3h from (a) to (b) of 0.002M FeSO4 solution with pH=5

As discussed for the cyclic voltammetry deposition, the growth of Fe nanoparticles is driven by the very low surface energy of the GC substrate, which encourages individual particle growth through a Volmer-Weber mode39 (see Fig. 5). Furthermore this also favours shape-controlled deposition of the nanoparticles40. The deposition experiment consisted of two potential steps which can be seen in Fig. 16. First the potential was stepped from open current (EOC) to ENUC= -1200mV at which Fe nanoseeds were nucleated on the GC surface for tNUC=4 seconds and then the potential was stepped to more positive potential EGROWTH to allow the particles to grow slowly for a time tGROWTH. Variation of the growth times and potentials allowed precise shape-control of the synthesised particles. Chen et al. also assessed the change of concentration from 0.002M FeSO4 to 0.02M FeSO4 to synthesise different shapes of particles. This was not investigated in this projected due to a limited amount of time.

Fig. 16 - Typical experimental procedure for the shape-controlled deposition of Fe nanoparticles on GC substrate in 0.002M FeSO4+0.1M NaNO3. The standard parameters used by Chen et al were EOC=0.265V, tOC=1.0s and ENUC=-1.2V, tNUC=4.0s. For particles a) (rhombic dodecahedra): Egrowth=-1.03V, tgrowth=900s, for e) (cubes): Egrowth=-1.07V, tgrowth=300s.

23

Daniel A. Pohoryles


The depositions of the rhombic dodecahedral and cubic (a and e respectively) Fe nanocrystals were attempted using the parameters described in Fig. 14. For both experiments deposition on the GC substrate was visible, a greyish layer was covering the surface just like it was seen for the cyclic voltammetry deposition (see Fig. 6). After electrochemical activation of the nm-Fe/GC, the electroactive surface area was measured in the same way as described in a previous section. The obtained surface areas for all attempted potential step depositions were in the same order of magnitude as those calculated for the CV depositions. Fig. 17 is a comparison of the surface area investigation for the (a) and (e) potential step depositions with the CV deposition. The surface area for the CV deposition is slightly higher (0.03 cm²) than for the e (0.026) and a (0.024) deposition techniques. The observation that the active surface area is lower for (a) than for (e) is in good agreement with the discussion on different facets and their relative activities ({110} vs. {100}) which infers that the cubic nanocrystals have the higher electroactive surface area.

i/A

-2E-05

0

2E-05 0

10

20

30 time (s)

40

50

60

Fig. 17 - Current-time plot of the surface area investigation experiment for nm-Fe/GC, 0.2M NaOH solution, scan rate 25mV.s-1, potential range -500mV to -1100mV. The solid black line corresponds to the active area for nanoparticles (a), the solid grey line to (e), and the dashed grey line to the particles deposited by CV.

Next, the catalytic activities of the deposited particles was determined as described previously, by recording a voltammogram of NO2- reduction on nm-Fe/GC2 in solutions of 0.2M NaOH with 0.01M NaNO2, the potential varying from -1000mV to -1350mV with a scan rate of 1mV.s-1. Examples of the results can be seen in Fig. 18. Even though the electroactive surface area coincided with the expected results, the catalytic activities were much lower (between 50 and 100 times lower) than those reported by Chen et al.36.

24

Daniel A. Pohoryles


E/m V -1400

-1350

-1300

-1250

-1200

-1150

-1100

-1050

-1000

0.00E+00 2.00E-01 4.00E-01

j/ mA.Cm-2

6.00E-01 8.00E-01 us ing the a- step tec hnique us ing the e- step tec hnique 1.00E+00 1.20E+00 1.40E+00 1.60E+00 1.80E+00

Fig. 18 – Comparison of nitrite reduction on nm-Fe/GC deposited by methods (a) (blue) and (e) (red)

As no catalytic activities comparable to literature were obtained, the nucleation times and potentials were increased in separate experiments, of which some gave slightly higher catalytic activities. However at no point were catalytic activities matching the reduction current observed by Chen et al. obtained. The range of potential over which Chen et al grew different crystals is only 40mV (Fig. 14). Thus the careful control of the nucleation potential is critical. An alteration of surface conditions can shift the voltammogram and the onset of Iron reduction and hence this will alter the processes occurring at the set potentials. An interesting route of investigation that might give more information about the mechanisms of Fe nanoparticles deposition is the analysis of i-t transients from the step deposition. The shape of these transients gives essential information on the mechanism of deposition41. Different current-time transients at nucleation and growth potentials can be seen in Fig. 19. These different transients arise from different nucleation conditions: (a) corresponds to the nucleation conditions used by Chen et al.36 (-1200mV, 4s); (b) to a nucleation at a higher overpotential (-1250mV) for the same time (4s); (c) corresponds to an increase in time compared to (b) to 100s (at -1250mV); and (d) corresponds to a nucleation at strong overpotential for 40 s. The growth conditions were the same for the four nucleations. It would be expected that a larger overpotential generates more nucleation and hence more particle deposition. This would mean that for the nucleation transients, the graph corresponding to (d) should have the largest current passing. This is however not the case, the currents for the two nucleation for 4 seconds at -1200 and -1250mV (a and b respectively) are two orders of magnitude stronger than for (c) and (d).

25

Daniel A. Pohoryles


Fig. 19 - Current-time transients for nucleation and growth using the potential step technique. Different nucleation times and potentials were used, the growth time and potential was -1.03V and 900sec for each of the four experiments. The colour code indicated in graph (e) applies to the four nucleation i-t curves (a) to (d)

For nucleations (a) and (b), as reduction current of about 0.01 mA was passed, it can be assumed that centres are nucleating. This is also reflected by the growth curves: once centres have nucleated, a step is taken back to the growth potential (-1.03V in this case) and significant current is observed, inferring that significant deposition is taking place. It is worthwhile noticing that the current passed at growth potential is larger for (b), which makes sense as the nucleation was driven at a stronger overpotential. Another observation that can be made for the

26

Daniel A. Pohoryles


growth transients of (a) and (b), is that the current decreases at first and then increases again to reach a peak (labelled by arrows in Fig. 19 (e)). This effectively resembles a pseudo sinusoidal function, which however corresponds to layer-by-layer growth41. As discussed in section 2, we should not be getting layers under these conditions but particles, as the low surface energy of GC makes island-growth more favourable (see Fig. 5). The reason for this is not fully understood and would have to be investigated over a larger number of experiments. For the other two nucleations, (c) and (d), much lower (and oscillating) currents were observed even though higher overpotentials were applied. Also the growth transients show no current for a considerable time. This suggests that no or very few particles were grown at the GC surface. For experiment (d) with a nucleation potential of -1.3V, there is a sudden increase in current for the growth transient. The reason for this is again unknown as so little is known on the deposition of the particles and the state of the GC surface. One possible hypothesis is that the surface condition of the electrode degraded over time (or with extended use) and this led to a shift of the nucleation to more negative potentials. The choice of growth potential might have hence been in a region with insufficient overpotential to nucleate. If we take a look back at CV deposition of particles, it can be seen that nucleation occurred at about -1230mV (see Fig. 3). If this potential shifted to more negative potentials, no nucleation will be observed. This would also explain why the nucleation at -1300mV gave no significant current despite the application of a strong overpotential, but however after 250 seconds of constant application of growth potential, nucleation was forced to start and led to some deposition (labelled by an arrow on the red curve of Fig. 19 (e)). Even though the analysis of the transients could not deliver any clear results and further investigations would be necessary to understand the modes of deposition first, one important statement can be made: the state of the surface is absolutely crucial for the deposition and activity of nanoparticles. An alteration of surface conditions can shift the voltammogram and the onset of Iron reduction and hence this will alter the processes occurring at the set potentials, especially since variation over a small region of 40mV has such a dramatic effect on the properties of the particles produced. A method of generating a smooth and reproducible GC surface for all investigations would mean a significant improvement to this type of experiment.

7. Bulk-Fe vs. nanoparticles: the importance of the state of the GC surface As very low catalytic activity was produced by the potential step method, it was decided to re-investigate deposition by cyclic voltammetry. In order to see if the cyclic voltammetric nanoparticle deposition described at the beginning of this report would still give good catalytic activity, the deposition was reproduced at the end of the project. However, two major differences to the usual nanoparticle deposition were observed:

27

Daniel A. Pohoryles

-


Active area (see Fig. 20 (a)): the ratio of electroactive Fe surface to total GC surface was 1.78 and hence far too large to be dispersedly deposited nanoparticles. This data rather suggested that the whole surface was covered by a layer of bulk Iron.

-

Catalytic activity (see Fig. 20(b)): The obtained maximum current density in nitrite reduction was much lower than for the nm-Fe/GC produced earlier in this project.The values were very close to those reported for bulk-Fe by Chen et al.

Fig. 20 - Electroactive surface investigation and catalytic activity for the nanoparticles (- - -) and the mistakenly deposited layer of Fe (-)

This suggests that instead of individual nanocrystals, a layer of Iron covering the whole GC surface was obtained. The reason for this was found in the cyclic voltammograms from the deposition. The data obtained from the deposition CVs (see Fig. 21) shows that the current decreases at -1050mV (black arrow) in the first deposition cycle, while it should decrease at around -1230mV (blue arrow, and see section 1.) This suggests that it was easier to deposit Fe as less overpotential was needed for nucleation and hence more Fe was deposited. This led to the formation of clusters of Fe nanoparticles, which grew together to form a layer after ten deposition cycles. This might be due to the state of the surface of the GC electrode. If the surface was too rough or if it was partially oxidised and contained functional groups which might assist nucleation then nucleation on the surface would be easier. More thorough polishing was hence needed.

28

Daniel A. Pohoryles


Fig. 21 – First cycle of the cyclic voltammogram of Fe deposition on GC, at a scan rate of 50mV.s-1 in 0.02M FeSO4 solution.

After more thorough polishing, the deposition was repeated. This time it was possible to synthesise nanoparticles, the surface area was now half of the total GC surface and the catalytic activity was more than doubled compared to bulk Iron, which means that values similar to those reported by Chen et al. for nm-Fe/GC were obtained. This experiment proved how crucial the state of the surface is for Fe deposition. As a rougher surface made the nucleation easier, too much Iron deposited on the GC substrate, ultimately leading to the formation of bulk instead of individual particles.

8. Steady catalytic activities in nitrite reduction It was shown in an earlier section that the catalytic activity of the particles grown by the CV method was in good agreement with the findings of literature. Another way of assessing the catalytic activity is the measure of steady state catalytic activity of the nm-Fe/GC. This was done by stepping the potential from -0.8V to -1.2V, -1.25V and -1.3V where the potential was held constant for 300s. This experiment assesses the current with time at a constant potential compared to the previous examination of activity where the potential is scanned through a wider range (Fig. 12). The particles deposited by cyclic voltammetry were used in this experiment and it can be seen below that results close to the literature were obtained and an enhanced catalytic activity compared to bulk Iron was observed for the nanoparticles.

The j-t curves are broadly similar to those obtained by Chen et al. and are compared in Fig. 22 (it should be noted that the reduction current increases on the y axis). However their intensities were lower. A strong sharp peak was obtained at the beginning of the curve for -1300mV, and for the curves at -1250mV and -1200mV, the intensity at t=0 is much lower. It can also be observed that on both electrodes the steady current of nitrite

29

Daniel A. Pohoryles


reduction was achieved after 50 s and that a smaller and broader peak is observed at about t=100s for the curves at -1250mV and -1300mV in both graphs. The steady catalytic activity was also measured for the bulk-Fe that was mistakenly deposited and the steady current for this was about half of the one for nm-Fe/GC. This is in good agreement with Chen et al. who found a steady current for the nanoparticles to be two to three times larger than for bulk Iron36. This experiment was not further investigated as it was the last in this project. (b)

6 5

j/mA.cm-2

4 3 2 1 0 0

100

200

300

400

t/s

Fig. 22 - j–t Curves recorded on nm-Fe/GC (a) and bulk Fe (b) electrodes, 0.01mol L−1 NaNO2 + 0.2 mol L−1 NaOH solution, reduction potentials, respectively, at −1.20V(...), −1.25V (- - -), and −1.30V (—), as obtained by Chen et al. (a) and in this project (b).

Conclusions and Avenues of Future Research In this project the deposition of Iron particles on glassy Carbon by two different electrochemical techniques was achieved. The surface areas of the nanoparticles were in good agreement with the cited literature28 for both techniques of electrodeposition and showed that particles were grown dispersedly on the GC substrate. The catalytic activity of the nanoparticles for nitrite reduction was assessed in two different ways and for the particles deposited by cyclic voltammetry very strong catalytic activities, comparable to those reported by Chen et al., were obtained. Enhanced catalytic activity for denitrification was observed and this showed that nanoparticles were grown rather than a monolayer. These results are also in good agreement with Volmer-Weber (island formation) growth mode, which is favoured by the low surface energy of glassy carbon. The effect of time and re-use of the nanoparticles for catalysis was examined. It was shown that the peak currents obtained in nitrite reduction decreased with time and use and that the overpotential needed to initiate the reduction increased with time. The reasons for this decline are assumed to be related to the state of the particles. It may be a slow irreversible oxidation of the Iron, or surface sites blocked by adsorptive impurities

30

Daniel A. Pohoryles


or perhaps some clustering of particles into larger clumps. SEM (Scanning electron microscopy) visualisation over time might be some help in unravelling this. The particles synthesised by step deposition produced insignificant catalytic activity. The reason for this was investigated by analysis of the deposition transients. The state of the surface was shown to be absolutely crucial for all the performed experiments. An alteration of surface conditions can shift the voltammogram and this will alter the processes occurring at the set potentials. A method of generating a smooth and reproducible GC surface for all investigations would mean a significant improvement to this type of experiments. The analysis of the current-time transients was a very difficult process as little is known on the surface state of the particles and the mechanism of deposition. The data seemed interesting and further investigation might give a much better understanding of the deposition mechanism. X-ray photoelectron spectroscopy (XPS) and SEM would also be useful to understand the state of the particles better.

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2

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a)T. Teranishi, H. Hori and M. Mixake, J. Phys. Chem. B, 1997, 101, 5774. b) Y. Volokitin, J. Sinzig, L. de Jong, G. Schmid, M. N. Vargaftic and I. I. Moiseev, Nature, 1998, 384, 621. c) V. Palermo, M. Palma, P. Samori, Adv. Mater., 2006, 18,145. 3

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The Care and Feeding of Electrodes,


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Lin Wang, J. Am. Chem. Soc., 2009, 131, 10860–10862, Supplementary Material, Figure S7 36

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