Oxidation of copper nanoparticles in water monitored in situ by localized surface plasmon resonance spectroscopy

Abstract

Disc-shaped copper nanoparticles (Cu NPs) were fabricated by hole–mask colloidal lithography on bare and thin titania film covered fused silica substrates. The dynamics of Cu oxide formation around the NPs were studied in water by localized surface plasmon resonance (LSPR) spectroscopy. We found that the oxidation rate is strongly enhanced under UV irradiation when the NPs are on the surface of the titania film, in comparison to NPs deposited on an inert fused silica substrate. The reason is sought in the ability of TiO₂ to create hydroxyl radicals with strong oxidative potential in water under UV irradiation and the charge transfer at the interface between the Cu NPs and the TiO₂. The post irradiation analysis of the samples with X-ray photoelectron spectroscopy provides complementary evidence for Cu oxide formation. In addition, size and shape changes of the NPs, as measured by atomic force microscopy and scanning electron microscopy, indicate (photo) corrosion of the NPs. Our results demonstrate the potential of using LSPR spectroscopy to monitor the oxidation of Cu NPs in situ and in different environments.

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Introduction

Copper nanoparticles (Cu NPs) play a key role as co-catalysts in the photocatalytic reduction of carbon dioxide (CO₂) to hydrocarbons in humid or aqueous environments.¹ The combination of Cu NPs with titanium dioxide (TiO₂) as photocatalysts is a widely proposed scheme in this endeavour.^2–6 However, the CO₂ photoreduction technology based on TiO₂ still faces major challenges such as: enhancement of the reaction rate, efficient utilization of the solar spectrum and control of the selectivity of the products. Cu films and NPs, empowered by several oxidation states, play a key role as co-catalysts for TiO₂. They provide active sites for electron trapping, suppress electron–hole recombination and enhance multi-electron reactions on their own^7–9 or in combination with other metal NPs.^6,10 Most importantly, Cu NPs are found to be capable of fixing CO₂,^11,12 oxidizing hydrocarbons selectively⁴ and enhancing the CO₂ photoreduction in water.^8,13 This motivates our interest in the system consisting of photocatalytically active TiO₂ and Cu NPs.

Cu NP oxidation has been studied in different environments.^14–24 Here, we intend to contribute to the understanding of Cu oxidation in water by studying the process on the nanoscale. For that purpose, we fabricate samples of Cu NPs in a highly controlled way, and employ a spectroscopic technique, based on the NPs’ localized surface plasmon resonance (LSPR), that gives the possibility to follow the chemical transformations in water, as a possible medium for CO₂ photoreduction. Basically, the changes of the LSPR peak position and amplitude can be assigned to (i) a shrinking of the nanoparticle's metallic core, (ii) the formation and growth of an oxide layer (or corrosion layer in general) and (iii) the change of the refractive index of the nanoparticle's surrounding environment.^16,42 We have used bare Cu NPs on top of fused silica, with and without a TiO₂ layer, in which Cu NPs function as a plasmonic sensor. Our aim is to distinguish the TiO₂ layer photocatalytic effect on the oxidation of Cu in water under UV illumination. We contribute to establishing nanoplasmonic sensing as a remote and in situ optical technique to characterize qualitatively the oxidation state of metal NPs in photocatalytic reactions.

Experimental

Metallic Cu NPs were fabricated by hole–mask colloidal lithography²⁵ directly on fused silica, an inert substrate, or with an interlayer of titanium dioxide (TiO₂) to render our model system photoactive. The metallic deposition was done by e-beam evaporation (AVAC HVC600) at a rate of 0.5 to 1 Å s⁻¹ under vacuum (P ∼ 10⁻⁵ mbar). The fabricated Cu NPs’ nominal height and diameter are 20 and 80 nm, respectively. Fused silica (University Wafer, thickness 0.5 mm) was cleaned in an ultrasonic bath with acetone and isopropyl alcohol separately, dried by nitrogen gas and subjected to oxygen plasma treatment (Plasmatherm batchtop m/95; 250 mTorr, 10 SCCM, 60 seconds). The TiO₂ layers were deposited without intentional doping by DC reactive magnetron sputtering (FHR MS150). The deposited TiO₂ layers were subjected to a rapid thermal annealing at 500 °C for 10 min in argon (JIPLEC Jet First 200) and the annealing process yielded polycrystalline anatase TiO₂ films.²⁶ The typical thickness of the layers used in this study was ∼50 nm. For structural characterization, Cu NPs with the same characteristics with and without a TiO₂ interlayer were prepared on silicon wafers.

The Cu NPs’ structural characterization was performed with scanning electron microscopy (SEM, Zeiss Supra 60 VP) and atomic force microscopy (AFM, Bruker Dimension 3100). For optical measurements, spectrophotometry (Cary 5000 Varian) was done in transmission mode. For surface analysis, X-ray photoelectron spectroscopy (XPS, PHI 5500) was done with monochromatic Al Kα as the X-ray source (hν = 1486.6 eV). The charging effects and associated binding energy shifts were corrected to the C1s peak at 285 eV. The surface composition calculations were done by following Briggs and Seah²⁷ and Physical Electronics Inc. procedures.²⁸

The in situ LSPR spectroscopy setup consisted of a fibre optic spectrometer (AvaSpec 2048), light source calibrated for the visible range (350–1095 nm, integrated output P = 35 mW) and a UV LED (Hamamatsu, λ = 365–367 nm, P = 250 mW). Note the use of two different sources of light. The UV LED was used for bandgap excitation of TiO₂ and the visible lamp was used to independently excite and monitor the Cu NPs LSPR. The sample was placed in a quartz cell containing deionised (DI) water (Milli-Q water: 18.2 MΩ cm) and irradiated with visible light normal to the surface and with UV light at an angle of ∼45°. The solution was continuously bubbled with nitrogen to create stable working conditions at the sample surface. The spectroscopic data were each recorded for 10 s in the wavelength range between 250 and 900 nm using the Avasoft software. The data treatment was done according to Dahlin et al.²⁹ and in our analysis, we have verified particularly that the maximum peak and the centroid peak show the same relative wavelength position and extinction along with the data treatment.

Results and discussion

Fig. 1 shows SEM images of the Cu NPs fabricated on top of the TiO₂ layer and directly on silicon and AFM images directly on fused silica and with the TiO₂ interlayer. The shape and distribution are similar when the Cu NPs are deposited directly on fused silica or silicon substrates. The size and height of the NPs are measured by AFM (see Fig. 1e and f). The measured AFM mean height corresponds to (20.5 ± 0.9) nm, which is close the nominal deposition height. The mean Cu NPs diameter and distance between the nanoparticles are 79 ± 9 and 180 ± 40 nm, respectively. These values correspond well to the choice of the size of the spherical colloids used in the hole–mask colloidal lithography under the conditions of the fabrication process (see ref. 25 for further details).

Fig. 1 SEM images of Cu NPs on top of (a) TiO₂ and (b) silicon. AFM images (color scale: 50 nm) for Cu NPs on top of (c) TiO₂ and (d) directly on fused silica. Height profile across the two lines in the previous images for Cu NPs on top of (e) TiO₂ and (f) directly on fused silica.

In Fig. 2, the optical extinction spectra of the Cu NPs deposited on bare and TiO₂ covered fused silica are presented. The Cu NPs deposited on fused silica show their LSPR at a wavelength of ∼630 nm, in agreement with LSPR measurements and simulations for Cu NPs of a similar size range.^15,17,19 The LSPR resonance wavelength, given by λ_LSPR = λ₀(1 + 2ε_d)^1/2, will red-shift with an increase in the dielectric constant (ε_d) of the NPs’ environment, compared to the vacuum LSPR wavelength (λ₀).^16,30 Note, the dielectric constant of a material is a complex magnitude and is dependent on the wavelength and related to the refractive index.³¹ Hereafter, we will discuss using the real part of the refractive index (n) as a reference for the NPs’ environment, and evaluate the qualitative effects of the λ_LSPR shifts (Δλ_LSPR) and the variations of the maximum extinction value (M_EXT) at the λ_LSPR position.

Fig. 2 Optical extinction spectra for the samples with and without (w/o) TiO₂. Inset: X-ray photoelectron (XPS) spectra detail from the binding energy region for Cu 2p^1/2 and 2p^3/2 peaks. Note that the lines correspond to the fitting of deconvoluted peaks: XPS raw measurements (hollow squares and black line), the base line (BL) taken as background (grey line) for the deconvolution of Cu (dots) and CuO (slashed-dotted line) contribution to simulation (Sim.) of the measured data (blue line).

As measured in Fig. 2, the λ_LSPR of the Cu NPs deposited on TiO₂ is red-shifted in comparison to those on bare fused silica. The shift is around 100 nm, which is due to a change in the effective refractive index of the NPs' environment: it increases for TiO₂ covered samples (n ∼ 2.5, for anatase)^32,33 compared to only fused silica (n = 1.45).^34,35 This phenomenon and the magnitude of the shift were observed for all the samples prepared for this study. The observed extinction increment in the UV range (λ < 400 nm) corresponds to the absorption edge of the anatase phase of TiO₂, with a bandgap energy, E_g = 3.2 eV (i.e. λ = 388 nm).²⁶ The inset of Fig. 2 shows the measured and simulated (Sim.) XPS spectra of the surface composition in the energy window for Cu and CuO, taking into account a standard baseline (BL). Taking into account the vacuum deposition of Cu and the observed narrow XPS peak width at ∼932 eV for Cu 2p^3/2, we confirm that there is almost no evidence of Cu₂O in these nanoparticles. In addition, the clear presence of the satellite structures at ∼955 eV and ∼965 eV are characteristic for the CuO 2p^1/2 and 2p^3/2 lines. Our observations and spectral line identification are in agreement with XPS measurements of similar Cu NPs and film structures.^{8,11–13,24,36–37} The peak deconvolution indicates the relative concentration of Cu and CuO to be (60 ± 2)% and (40 ± 2)%, respectively. The Cu NPs deposited on the different substrates did not show any specific composition variations for the CuO and Cu peaks.

The oxidation tests in DI water were performed by immersing the samples (with and without TiO₂) for 70 min. Fig. 3 shows the optical extinction spectra for the samples at the initial moment and at the end of the experiment (t = 70 min). The observations for λ_LSPR and M_EXT are summarized in Table 1. At the initial moment in DI water, both samples show red shifted Cu NP plasmon resonances compared to the measurements in air, because DI water has a higher refractive index (n = 1.3–1.4)³⁸ compared to that of air (n = 1). The magnitude of the plasmon resonances shift, comparing the air and DI water environment measurements for both samples, is in agreement with predicted values for environment sensing by Larsson et al.³⁹ At the end of the experiment, both Cu NP plasmon resonances are blue shifted, which points out the modification of the metallic size of the NPs. The extinction change for the Cu NPs on fused silica is negligible or within experimental accuracy.

Fig. 3 Optical extinction spectra for the samples with and without (w/o) TiO₂ at the initial and final (t = 70 min) time of their immersion in DI water after the UV illumination.

Table 1 Cu NPs λ_LSPR and M_EXT for samples with and w/o TiO₂ in air and in water before (t = 0) and after UV illumination (t = 70 min). Note that the measurements in the air were taken by a Cary 5000 while those in water were measured by a fibre spectroscopy setup. The experimental errors are: ±1 nm for λ_LSPR and ±2% for M_EXT

Medium	Cu NPs/SiO2		Cu NPs/TiO2/SiO2
	λLSPR (nm)	MEXT (%)	λLSPR (nm)	MEXT (%)
Air	630	33	737	44
Water (initial)	650	28	784	32
Water (after UV)	624	26	703	25

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Immediately after the immersion in DI water, the optical spectra were recorded every 10 s for 70 minutes. Fig. 4 shows the time evolution of the Cu NPs’ Δλ_LSPR and the M_EXT variation (ΔM_EXT), referenced to the initial position and value presented in Table 1. Note that two identical samples with Cu NPs on top of a TiO₂ layer were immersed in DI water, of which one sample was irradiated with UV light while the other one was in the dark (used as a reference). Right after the immersion in DI water, the λ_LSPR blue shifts for both samples, with and without TiO₂, and remains almost constant after 15 min. In dark conditions, the Cu NPs on top of TiO₂ show an almost constant λ_LSPR for 70 min. Under UV illumination (t = 30 min in Fig. 4) λ_LSPR clearly blue shifts for the Cu NPs on top of TiO₂, while λ_LSPR remains almost constant for the NPs on fused silica. The Cu NPs’ M_EXT is also affected, showing a small decrease for the samples. As noted in Fig. 4, returning to dark conditions, the λ_LSPR and the extinction for the Cu NPs with TiO₂ remains constant. In this sense, the shift of λ_LSPR and the extinction are coupled to the UV illumination only for the Cu NPs with the TiO₂ layer. Furthermore, the dependence on UV illumination was evaluated using different photon fluxes. In fact, we measured an increment of the Δλ_LSPR for increased UV light illumination. The increased shifts are from 0.009 nm min⁻¹ to 0.015 nm min⁻¹ and 0.022 nm min⁻¹ for 30, 60 and 100% of the maximum power (250 mW cm⁻²), respectively. Note that the thickness of the TiO₂ layer was constant for all the samples and could be optimized to obtain a more significant effect on the Cu NPs’ LSPR with the UV illumination power.

Fig. 4 Variation in time of Cu NPs’ λ_LSPR and M_EXT of samples with and without (w/o) TiO₂, immersed in DI water, in the dark and under UV illumination. Note, one of the samples with Cu NPs on top of a TiO₂ layer (diamond symbol) was immersed in DI water but not irradiated with UV light as a reference.

After completing the measurements in water, we performed XPS surface analysis of the samples and measured the optical spectra with the spectrophotometer again (see Fig. 5). From the XPS, the Cu NPs on fused silica show a similar amount of oxidation as measured before the immersion in DI water, with a relative composition of (59 ± 2)% and (41 ± 2)% for the Cu and CuO peak, respectively. In contrast, the Cu NPs with TiO₂ after immersion in DI water and irradiation with UV light show a substantial surface oxidation, where the relative composition for the Cu and CuO XPS peaks is (42 ± 5)% and (58 ± 5)%, respectively. In the same manner, the TiO₂ sample exhibits a very low Cu NP λ_LSPR, almost covered by the background absorption of the TiO₂, while the M_EXT was just reduced for the bare Cu NPs. Furthermore, the AFM images still show an even distribution of NPs on the surface only for the Cu NPs deposited on top of fused silica (similar to Fig. 1d), compared to a high disorder and different size aspects for the Cu NPs with the TiO₂ layer (see Fig. 5). The mean height and diameter of the Cu NPs on top of TiO₂ after the immersion in DI water and irradiated with UV light are (12 ± 4) nm and (100 ± 30) nm, respectively. In comparison, the Cu NPs deposited directly on top of fused silica showed a (19.2 ± 0.7) nm height and a (77 ± 7) nm diameter, which are close to the initial values measured before the immersion in DI water. Similarly, the Cu NPs deposited on top of TiO₂ immersed in DI water but not irradiated with UV light show a (19.5 ± 0.8) nm height and a (81 ± 9) nm diameter. In summary, only the Cu NPs deposited on TiO₂ immersed in DI water after the UV irradiation showed chemical and structural modifications.

Fig. 5 (Left) SEM image of Cu NPs on TiO₂ after UV irradiation in water. Insets: AFM image (color scale: 50 nm) and the height profile across the three lines in the image. (Right) Optical extinction spectra for the Cu NPs on TiO₂ after UV irradiation. Inset: XPS spectra of the Cu NPs. The full scale for the XPS intensity is the same for both samples.

Under dark conditions in water, the Cu NPs’ λ_LSPR with and without TiO₂ show a clear blue shift and small ΔM_EXT. The trend is similar to the one observed when glacial acetic acid is used to remove the oxide.^15,17 Cu oxides are highly hydrophilic and the patina (natural oxide in Cu film) is able to absorb significant amounts of water, due to its porosity.⁴⁰ Hence, we assume a partial removal and/or a composition rearrangement of the CuO in the NPs’ surface in contact with the water. This will reduce the effective mean dielectric constant surrounding the inner metallic Cu NPs. At the same time, we should take into account the decrease of the dielectric constant in water for thin TiO₂ film,⁴¹ in order to evaluate the initial Δλ_LSPR as a general environment adaptation/sensing of the inner Cu NPs.

The slight increment and subsequent decrease of M_EXT, in DI water and dark conditions, are similar to the ones observed in the oxidation of pure Cu NPs in different solvents (toluene and benzene)²⁰ and water.²² The ΔM_EXT rate is 7 × 10⁻⁴ s⁻¹, in agreement with similar immersions of Cu NP films in water,²² and slightly higher compared to humid air exposure (3 × 10⁻⁴ s⁻¹).⁴² In general, the transfer of electrons between the Cu NPs and the DI water will be responsible for the constant decrease of M_EXT as pointed out already for the interaction of gold NPs with solvents and colloids.⁴³

Under UV illumination, the photogenerated electrons and holes in titania film are an efficient source to react with the water, the NPs, and eventually impurities in the system.³ To assess the exact outcome of this process is complicated, since the CuO layer may affect the interface charge transfer between TiO₂ and the copper NPs. In any case, the Cu NPs on the TiO₂ layer show a strong blue shift of the λ_LSPR of more than ca. 40 nm. Such a Δλ_LSPR is coherent with shrinkage of the inner core of the metallic NP, as shown by theoretical calculations of cubic NPs using the discrete dipole approximation.⁴⁴ The reduced extinction may be interpreted as a result of the reduced diameter of the metallic NPs.

However, to explain the induced decrease of the effective size of the NPs, we should take into account not only the surface in contact with the water, where hydroxyl radicals are created under UV illumination, but also the interface with the TiO₂, where an oxide compound might grow, since the charge transfer from the TiO₂ may influence the Cu/CuO reaction rates. In this sense, the higher Δλ_LSPR ratios, measured in relation to the increment of illumination flux, i.e. higher rate of electron–hole pairs generation in TiO₂, clearly indicate the acceleration of the reaction at the interface with Cu NPs. Therefore, the Δλ_LSPR of the Cu NPs may be able to monitor oxide growth at the interface of the NPs and the semiconductor layer, triggered by the UV illumination. Furthermore, the influence of the Cu oxidation state of the outer NPs layer should be studied separately for CuO and Cu₂O in photocatalytic reactions.

Finally, under immersion in water, Cu and the natural Cu oxides are considered to be insoluble in water.⁴⁵ Only yearlong experiments reveal how the protective nature of the Cu₂O on Cu films is degraded by the presence of water layers on its surface.^40,46 Looking closely at Fig. 4, the Cu NPs deposited on fused silica experience a very slow red shift of the λ_LSPR, regardless of the UV illumination. The LSPR peak shifts linearly with the oxide thickness and corresponds to a ca. 0.1 nm surface oxide growth, in agreement with other Cu NP surface oxide growth and corrosion measurements.^15,17,19,42

Conclusions

Disk-shape Cu NPs with a diameter of 80 nm and height of 20 nm were fabricated on top of a fused silica substrate with and without a TiO₂ photocatalytic active layer beneath. The dynamics of Cu oxide formation around the NPs was monitored in situ in water by means of LSPR spectroscopy. The λ_LSPR shifts were followed in the dark and under UV light irradiation. Under dark conditions, both Cu NPs fabricated on fused silica with and without a TiO₂ layer show a ca. 20–30 nm λ_LSPR blue shift when immersed in DI water. The observation was interpreted as evidence for partial dissolvation and/or composition rearrangement of the native Cu oxide in water, but the Cu NPs remained stable. In contrast, the Cu NPs deposited on TiO₂ underwent rapid oxidation under UV illumination in water. We found a clear coupling between the λ_LSPR blue shift and the duration and intensity of UV irradiation. The λ_LSPR blue shift measured ca. 40 nm after 1 hour irradiation in water and was assigned to the formation of Cu oxide, the shrinkage of the metallic core and partial photocorrosion of NPs. The Cu oxidation triggered by the UV illumination can proceed both on top of the Cu NPs and at the TiO₂ interface. Comparing the findings made by in situ monitoring of the Cu NPs oxidation in water with the post-irradiation characterization analysis, we concluded that visible light spectroscopy of metallic LSPR enables us to monitor the oxide growth on the Cu NPs.

Acknowledgements

Financial support is from the Nordic Initiative for Solar Fuel Development (Project NISFD-52). We thank Lars Ilver for technical support with the X-ray spectroscopy equipment. F.G-P. thanks Prof. I. Zorič for helpful explanations and sharing references from M. Schwind et al., Dr H. Kannisto for technical assistance with X-ray spectroscopy measurements, and Dr A. Hellman, Dr C. Langhamer, C. Wadell, Dr B. Wickman, and Dr V. Zhdanov for fruitful discussions.

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Footnotes

† Present address: Institut d'Electronique et Systemes, UMR 5214, University of Montpellier 2 – CNRS, 34095 Montpellier, France.

‡ Present address: School of Electronics Engineering, VIT University, Vellore-632014, India.

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