FIELD OF THE INVENTION

This invention relates to photosensitive and/or photoactive electrochemical compounds. More particularly, but not exclusively so, this invention relates to photosensitive electrochemical compounds which may be used as a photo electrocatalyst in an electrochemical process and/or as a semiconductor in a photovoltaic cell.

BACKGROUND TO THE INVENTION

The use of a photoactive material as an electrocatalyst is known in the art. These types of catalysts, which are often referred to as photo-electrocatalysts, have been used in, amongst others, electrochemical water splitting reactions and fuel cells for the production of electricity.

Metal oxides have been extensively employed in photovoltaic cells as semiconductors. Examples of such metal oxides include CU2O, ZnO and BiFe03, to name but a few.

These traditional photo-electrocatalysts, however, require continuous illumination with ultraviolet or visible light to render them active (induce a threshold electro-active state) for a specific reaction. It is a distinct disadvantage of these photo-electrocatalysts that once illumination is stopped, the photo-electrocatalyst’s activity is lowered significantly, within a very short period of time, or lost completely. In interpreting this specification, it should be understood that the phrase“increased electroactive state” means “a sustained photo-charging effect that results in an enhanced and, at least partially, sustained dark current (subsequent to the termination of illumination with a UV light), which is greater than the current obtained through applied electrical potential alone”.

OBJECT OF THE INVENTION

It is accordingly an object of the present invention to provide a photo-electrocatalyst and/or semiconductor which overcomes, at least partially, the abovementioned problems and/or which will be a useful alternative to existing photo-electrocatalysts and/or semiconductors.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a europium- tellurium-oxide compound which retains an increased electro-active state induced therein during a preceding illumination step.

The europium-tellurium-oxide compound may retain (in the dark), for a period of at least 40 minutes, an increased electro-active state induced therein during a preceding illumination step.

There is provided for the europium-tellurium-oxide compound to be selected from the group consisting of Eu2Te06, Eu3Te06, Eu2Te60is and Eu2Te40n. According to a second aspect of the present invention, there is provided for the use of a europium-tellurium-oxide compound, according to the first aspect of the present invention, as a photo-electrocatalyst.

It is envisaged that the europium-tellurium-oxide compound may be used as a photo electrocatalyst in an electrochemical reaction. The electrochemical reaction may be an oxygen evolution reaction in an electrolysis reaction for the production of hydrogen. The electrochemical reaction may, also, be an oxygen reduction reaction in fuel cells, which is used for the supply of electricity.

According to a third aspect of the present invention, there is provided for the use of a europium-tellurium-oxide compound, according to the first aspect of the present invention, as a semiconductor.

It is envisaged that the europium-tellurium-oxide compound, when used as a semiconductor, may be used as an active material in a photovoltaic cell.

According to a fourth aspect of the present invention, there is provided for the use of the europium-tellurium-oxide compound in a coating solution for a working electrode of an electrochemical reaction. There is provided for the coating solution to be prepared by a process including the steps of:

(i) dispersing 30 mg of a compound selected from the group consisting of Eu2Te06, Eu3Te06, Eu2Te60is and Eu2Te40n in 1 ml_ ethylene glycol under ultrasonication for 120 minutes; and

(ii) adding 0.4 ml_ of Nafion and 0.2 ml_ of 0.1 M NaOFI under ultrasonication for 30 minutes.

These and other objects, features and advantages of the invention will become apparent to those skilled in the art following the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described further, by way of example only, with reference to the accompanying drawings wherein:

Figure 1 (a) shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample ETO-400) in an O2 purged 0.1 M KOH solution under UV illumination and at a working electrode rotation of 0 rpm;

Figure 1 (b) shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample ETO-900) in an O2 purged 0.1 M KOFI solution under UV illumination and at a working electrode rotation of 0 rpm;

Figure 1 (c) shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample ETO-400) in an O2 purged 0.1 M KOFI solution under UV light illumination and at a working electrode rotation of 1600 rpm;

Figure 1 (d) shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample ETO-900) in an O2 purged 0.1 M KOFI solution under UV light illumination and at a working electrode rotation of 1600 rpm;

Figure 2 (a) shows the current density for a single europium-tellurium-oxide sample (sample ETO-400) at a fixed potential (1 .8 V, RFIE), at different illumination times, at a scan rate of 10 rn.V.s ¹ in O2 purged 0.1 M KOFI solution and at a working electrode rotation of 0 rpm;

Figure 2 (b) shows the current density for a single europium-tellurium-oxide sample (sample ETO-900) at a fixed potential (1 .8 V, RFIE), at different illumination times, at a scan rate of 10 m.V.s ^-1 in O2 purged 0.1 M KOFI solution and at a working electrode rotation of 0 rpm; Figure 3 (a) shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample ETO-400) in an O2 purged 0.1 M KOFI solution after termination of UV illumination and at a working electrode rotation of 0 rpm;

Figure 3 (b) shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample ETO-900) in an O2 purged 0.1 M KOFI solution after termination of UV illumination and at a working electrode rotation of 0 rpm;

Figure 3 (c) shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample ETO-400) in an O2 purged 0.1 M KOFI solution after termination of UV illumination and at a working electrode rotation of 1600 rpm;

Figure 3 (d) shows the photocurrent density-potential curve for a single europium-tellurium-oxide sample (sample ETO-900) in an O2 purged 0.1 M KOFI solution after termination of UV illumination and at a working electrode rotation of 1600 rpm;

Figure 4 (a) shows the current density over time for a single europium- tellurium-oxide sample (sample ETO-900) in an O2 purged 0.1 M KOFI solution at 1 .8 V vs RFIE and at a working electrode rotation of 1600 rpm; Figure 4 (b) shows the current density over time for a single europium- tellurium-oxide sample (sample ETO-900) in an O2 purged 0.1 M KOFI solution at 1 .8 V vs RFIE and at a working electrode rotation of 0 rpm;

Figure 5 shows the results of a sequential switching chronoamperometry experiment which was conducted on a single europium-tellurium- oxide sample (sample ETO-900) at 1 V (vs Pt) in a N2 purged 0.1

M Na2S04 solution;

Figure 6 shows the electrochemical impedance spectroscopy (EIS) spectra of a single europium-tellurium-oxide sample (sample ETO-900) in an O2 purged 0.1 M KOH electrolyte under different conditions and at a working electrode rotation of 1600 rpm; and

Figure 7 shows the variation in the valence band energy of a single europium-tellurium-oxide sample (sample ETO-900) before and after illumination with a UV light.

The presently disclosed subject matter will now be described in greater detail hereinafter with reference to the accompanying examples, in which representative embodiments are shown. The presently disclosed subject matter can, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art.

DETAILED DESCRIPTION OF THE DRAWINGS

Synthesis of europium-tellurium-oxide compounds

Europium-tellurium-oxide compounds were synthesised by employing the Pechini sol- gel process (Reference 1 ) whereby stoichiometric ratios of EUCI2.H2O and HQTQOQ were combined. This was done by first dissolving 0.1 M HQTQOQ in a 50 ml_ solution of ethanol and water (1 :1 volumetric ratio) at temperatures above 30°C.

This was followed by pouring 0.2 M EUCI2.H2O (which was also prepared in a water- ethanol mixture) into the HQTQOQ solution followed by an aqueous solution of 0.6 M citric acid. Lastly, 2.5 mL of ethylene glycol was added as a cross linking agent and heated to 70°C under constant stirring to get rid of the excess water. A transparent gel was obtained that was dried overnight at 120°C in an oven followed by heating in a furnace at 300°C for 2 hours. The resulting material was crushed and subjected to sintering at 400°C and 900°C for 4 hours and labelled as ETO-400 and ETO-900, respectively.

Finally, the different ETO samples were ground to a fine powder using an agate mortar, subjected to X-ray diffraction spectrometry to ascertain its composition (shown in Table 1 ) and stored for further use. Table 1 : Composition of the europium-tellurium-oxide compounds which were synthesised

Photo-electrochemical experimental setup and conditions

All synthesised samples were tested for their electrocatalytic and photo- electrocatalytic activity towards the oxygen evolution reaction (OER, reaction 1 ), chosen as a representative electrochemical reaction, in an alkaline medium (0.1 M KOH) at 25 °C at a scan rate of 10 rnV.s ¹ and two rotation rates (i.e. 0 rpm and 1600 rpm).

Reaction 1 : 40H -> O2 + 2H2O + 4e

A three-electrode double walled photo-electrochemical cell made from polypropylene, with illumination directly in front of the rotating disk working electrode that contains the photo-electrocatalyst under investigation (any one of the ETO samples), was utilised to investigate the electrochemical and photo-electrochemical activity of the compounds synthesised. Linear sweep voltammetry (LSV) was used to examine the oxygen-evolution-reaction activity of the various ETO compounds. A platinum wire (Pine Research Instrumentation) was used as the counter electrode while a Hg/HgO (Bio-Logic Science Instruments) electrode was used as the reference electrode. All experiments were carried out in an O2 purged 0.1 M KOH solution at 25°C employing a VSP double channel potentiostat (Bio-Logic Science Instruments). The temperature of the electrolyte was directly monitored and adjusted accordingly be means of a thermostatic control unit to ensure a constant operating temperature of 25 °C employing a Julabo F12-ED thermostatic control unit. All potentials are communicated relative to the reversible hydrogen electrode (RHE) having been corrected for iR-drop.

The rotating disk working electrode, a 5 mm diameter glassy carbon insert (inside a Teflon holder), was coated with the catalyst (any one of the ETO samples) that consisted of a solution that was prepared by: i) dispersing 30 mg of catalyst (any one of the ETO samples) in 1 mL of ethylene glycol under ultrasonication for 120 minutes;

ii) adding 0.4 mL of Nafion and 0.2 mL of 0.1 M NaOH under ultrasonication for 30 minutes; and

iii) pipetting 10 pL of the resultant catalyst solution onto the glassy carbon insert and drying it overnight in a vacuum oven at 70°C. Photo-electrochemical results for the europium-tellurium-oxide compounds

For the photo-electrocatalysis experiments, that involved the europium-tellurium-oxide compounds, the working electrode (having a catalyst loading of 0.95 mg. cm ² , the catalyst being a europium-tellurium-oxide compound) was illuminated using an ultraviolet (UV) lamp (Philips, 9W, TUB PL-S); this lamp (i.e. illumination device) was chosen due to the bandgap of the europium-tellurium-oxide compounds correlating with the ultraviolet region of the electromagnetic spectrum.

As shown in Figures 1 (a) to (d), the electrocatalytic (EC) as well as photo- electrocatalytic (PEC) activity of both ETO-400 (Figures 1 (a) and (c)) and ETO-900 (Figures 1 (b) and (d)), towards the alkaline OER, were probed by means of linear sweep voltammetry (LSV) of oxygen purged 0.1 M KOFI solutions (25 °C) at a scan rate of 10 mV.s ^-1 , and two rotation rates, i.e. 0 rpm and 1600 rpm.

An EC LSV, in the absence of any illumination, as well as PEC LSVs at increasing illumination times, were recorded for both samples at the two rotation rates. In all instances it is clear that the pure EC LSVs result in the lowest current densities.

The rotation rate, as expected, has an effect on both the EC and PEC activities of the samples with current densities being quite higher at 1600 rpm compared to no rotation. This is due to improved mass transfer to the electrode surface and the removal of oxygen gas bubbles from the electrode surface. Illumination of the samples resulted in increased activity, however, increasing illumination times do not correlate linearly with activity or current density. This is evident from Figures 2 (a) and (b), which shows the current densities at 1 .8 V (RHE) for the ETO-400 (Figure 2 (a)) and ETO-900 (Figure 2 (b)) samples (at 0 rpm), with the activity of ETO-400 going through a maximum at around 60 mins, subsequent to which activity deminishes, and that of ETO-900 plateauing out after 150 mins. At 0 rpm the EC activity is greater for the amorphous ETO-400, while the PEC activity (at maximum potentials) is greater for the polycrystalline ETO-900. It would therefore seem that Eu2Te06 (ETO-900) has a greater photonic energy capacity, resulting in increased PEC activity, while ETO-400 (as a result of its amorphous structure) has an increased EC activity. Enhanced PEC activity could be due to the longer lifetime of the photo-induced charge carriers, resulting from an improved separation efficiency of the photogenerated charge carriers, and the absorption of a higher density of photons.

Compared to other semiconductors, such as ZnO and EU2O3/B1VO4 , Eu2Te06 would seem to perform surprisingly well, specifically in the OER region. For example, nanocoral-ZnO and N-doped ZnO nanowire photo-electrocatalysts displayed photocurrent density values lower than 0.3 mA.cnr ² at 1 .8 V vs. RFIE, while EU2O3/B1VO4 photocatalyst displayed a photocurrent density of only 0.14 mA.cnr ² at 1 .8 V vs. RFIE, which is comparatively less than the values of 0.80 mA.cnr ² and 2.66 mA.cnr ² recorded respectively at 0 rpm and 1600 rpm for ETO-900 at 1 .8 V vs. RFIE.

In addition to EC and PEC probing, associated with increased illumination times, the EC activity was also probed subsequent to a total illumination time of 150 mins (in the dark) as shown in Figures 3 (a) to (d). This would reveal any capacity of the ETO samples to store charge, release stored charge, or to give effect to increased charge separation. Again, LSV scans were obtained for both ETO-400 (Figures 3 (a) and (c)) and ETO-900 (Figures 3 (b) and (d)) at 0 rpm and 1600 rpm.

As shown in Figure 3 (a), it is evident that, in the case of no rotation, the amorphous ETO-400 does not retain any PEC activity subsequent to illumination having been terminated, as all the LSVs are grouped together with the pure EC LSV (obtained in the absence of any light).

As shown in Figure 3 (b), ETO-900 exhibits excellent prolonged charge carrier separation, subsequent to illumination having been terminated, with all post illumination dark EC LSVs grouped together with the 150 mins PEC.

With the introduction of electrode rotation (at 1600 rpm) both the amorphous ETO-400 (Figure 3 (c) and the polycrystalline ETO-900 (Eu2Te06) (Figure 3 (d)) exhibit increased PEC activity. However, PEC activity in the dark decreases over time with the amorphous ETO-400 losing its activity quite fast compared to the polycrystalline

ETO-900.

A clearer picture is obtained on the prolonged charge storage capacity of ETO, after illumination has been terminated, when comparing the current densities, at increased illumination times, of ETO-900 at 1 .8 V for both the rotated (1600 rpm) and non-rotated (0 rpm) samples (Figures 4 (a) and (b)). Under rotation, a decrease in dark current is observed over time with the dark current, after 40 mins, still being greater than that of the non-illuminated EC current. Surprisingly, in the absence of rotation, the dark current remains constant (at an average value of 0.86 rmA.cnr ² ) for each time interval up to 40 mins, subsequent to which the dark current was not monitored. This stable dark current, still represents 95% of the PEC current, and is double that of the initial pure/dark EC current of 0.44 rmA.cnr ² .

The observed charge storage phenomenon was further validated by conducting a sequential switching experiment in the presence of a different alkali metal ion, i.e Na ⁺ . A chronoamperometry (CA) experiment was conducted at 1 V (vs. a Pt electrode) in I\l2-purged 0.1 M Na2S04 at 25 °C, with the termination of illumination for 20 sec after every 10 mins. The results obtained during the aforesaid experiment is shown in Figure 5. Upon the termination of illumination very little current was lost (only 2-3%) relative to the EC base-line, which point to enhanced charge separation. The indefinite termination of illumination after 150 mins revealed a slow decline in the dark current, which points to the effective photo-charging of the material. Subsequent to a 150 min dark period Eu2Te06 still retained 186% of its photo-charged current relative to the EC base.

An improved understanding of the EC and PEC processes at the electrode electrolyte interface is obtained, under dark and illuminated conditions, by employing an EIS technique for ETO-900 at 1600 rpm. The results obtained during the aforesaid experiment is shown in Figure 6. Similar semi-circular arcs, of different sizes, were observed in all instances, i.e. un-illuminated (EC), illuminated (PEC) and post- illuminated (40 min off, EC). It is clear that the EIS spectra are greatly affected by illumination as the light-affected samples revealed comparatively lower charge transfer resistance in comparison to the virgin EC scan. The lower charge transfer resistance values, in the presence of and subsequent to illumination, are attributed to the greater efficiency of charge transfer across the electrode/electrolyte interface, which is the result of (a) increased charge separation (for the illuminated PEC sample), and (ii) reduced charge recombination (for the post-illuminated 40 min off EC sample). The lower charge transfer resistance value for the post-illuminated 40 min off EC sample, compared to the virgin EC measurement, is indicative of the material’s capability to store charge in its crystal lattice and release the charge in the dark subsequent to the termination of illumination.

To further investigate the effect that illumination has on europium tellurium oxide (ETO), a sample was illuminated (employing a Hel - 21 .2 eV UV source) for different time periods (0 mins, 15 mins, 35 mins and 65 mins), subsequent to which XPS spectra were recorded. As the sample is illuminated, for prolonged time periods, the binding energy of the valence band electrons increase sequentially, as shown in Figure 7. The overall shift between the unilluminated and 65 mins illuminated sample is 0.35 eV towards higher binding energy. This is in line with an increased number of electrons that have been excited to the conduction band, which subsequently results in an increase of the energy by which the remaining electrons in the valence band is bound. As the XPS spectra have been obtained some time (~ 5 mins) after illumination has been terminated, it is clear that the lifetime of the excited electrons has increased in correlation with the prolonged illumination time. This is further proof that europium tellurium oxide (ETO) has the capacity of decreasing charge (positive hole and electron) recombination. Use of the ETO compounds described herein

The above described surprising effect can, ideally, be exploited by using any one of the described compounds (i.e. Eu2Te06, Eu3Te06, Eu2Te60is and Eu2Te40n) as an electrocatalyst and/or photo-electrocatalyst in an electrochemical reaction, an electrocatalyst and/or photo-electrocatalyst in an electrolysis cell and/or fuel cell, a photo-sensitive semiconductor or a photo-sensitive semiconductor in a photovoltaic cell.

It will be appreciated by those skilled in the art that the invention is not limited to the precise details as described herein and that many variations are possible without departing from the scope and spirit of the invention. For example, a combination of any of the photo-sensitive electrochemical compounds described herein may be used as a photo-electrocatalyst or a photo-sensitive semiconductor.

REFERENCES

1. J Llanos, R Castillo, D Barrionuevo, D Espinoza, S Conejeros,“The family of Ln2Te06 compounds (Ln = Y, La, Sm and Gd): Characterization and synthesis by the Pechini sol-gel process”, Journal of Alloys and Compounds (2009), 565- 568