Photoinduced Electrochemiluminescence at Silicon Electrodes in

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Photo-Induced Electrochemiluminescence at Silicon Electrodes in Water Yiran Zhao, Jing Yu, Guobao Xu, Neso Sojic, and Gabriel Loget J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06743 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 3, 2019

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Journal of the American Chemical Society

Photo-Induced Electrochemiluminescence at Silicon Electrodes in Water Yiran Zhao,†,# Jing Yu,‡,# Guobao Xu,§ Neso Sojic,*,‡ Gabriel Loget*,† Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes)-UMR 6226, F-35000 Rennes, France. University of Bordeaux, Bordeaux INP, ISM, UMR CNRS 5255, 33607 Pessac, France. § State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, P.R. of China. †

‡

ABSTRACT:

We introduce the photo-induced electrochemiluminescence (P-ECL) of the model ECL system involving the simultaneous oxidation of [Ru(bpy)3]2+ and trin-propylamine (TPrA). This system classically requires highly anodic potentials, >+1 V vs SCE for ECL generation. In the reported approach, the ECL emission is triggered by holes (h+) photogenerated in an n-type semiconductor (SC) electrode, which is normally highly challenging due to competing photocorrosion occurring on SC electrodes in aqueous electrolytes. We employ here Si-based tunnel electrodes protected by a few nm-thick SiOx and Ni stabilizing thin films and demonstrate that this construct allows generating P-ECL in water. This system is based on an upconversion process where light absorption at 810 nm induces ECL emission (635 nm) at a record low electrochemical potential of 0.5 V vs SCE. Neither this excitation wavelength nor this low applied potential is able to stimulate ECL light if applied separately. But their synergetic actions lead to a stable and intense ECL emission in water. This P-ECL strategy can be extended to other luminophores and is promising for ultrasensitive detection, light-addressable and imaging devices.

to ECL in water-based electrolyte, probably because most SCs suffer from corrosive degradation mechanisms. These effects are even more severe under anodic bias in aqueous media.7 Among available SCs, silicon (Si) is attracting particular attention as a photoelectrode material because of its abundance, its low toxicity, and its tunable electronic properties.8 Besides, Si is also employed as a substrate to manufacture a broad diversity of microfluidic systems,9 including diagnosis microchips,10-12 which makes it a relevant candidate as an ECL electrode. However, this material is generally highly unstable when used as a photoanode in water.13 In this communication, we report on ECL-active and stable Si-based electrodes allowing us to combine, for the first time, Si photoelectrochemistry with [Ru(bpy)3]2+/TPrA ECL, thus creating a photo-electrochemiluminescent (P-ECL) system producing light from photogenerated h+. This system emits at a wavelength that is below its absorption, being reminiscent of photon upconversion and allows triggering [Ru(bpy)3]2+/TPrA ECL at a record low potential of 0.5 V vs SCE and to photo-address locally the ECL emission.

Electrochemiluminescence (ECL) is a light-emitting process that has become a powerful tool in analytical chemistry, especially for immunoassays and clinical diagnosis. This phenomenon is induced by the excited state of a luminophore, which is generated by an electrochemical reaction at an electrode surface.1 ECL systems, like the model one involving tris(bipyridine)ruthenium(II) ([Ru(bpy)3]2+) as a luminophore and tri-n-propylamine (TPrA) as a sacrificial co-reactant, typically involve the production of high-energy intermediates, generated usually at high potentials. For example, ECL emission of [Ru(bpy)3]2+ requires imposing a potential above +1 V vs SCE on usual electrode materials.2 On the other hand, photoelectrochemistry on illuminated semiconductors (SCs) is a process where an electrode absorbs light, as opposed to ECL, in which the electrode can be considered as a light emitter. This concept relies on employing photogenerated minority carriers within a depleted SC (e.g. holes (h+) in the case of an n-type SC photoanode), in order to decrease the potential required to trigger an electrochemical reaction.3,4 This concept has been mostly exploited for energy applications (e.g. photoelectrochemical water splitting or CO2 reduction). Although a few pioneer examples have been reported on SCs (p-Si, InP and GaAs) in annihilation mode in dried organic solvents,5,6 SC photoelectrochemistry has never been applied

Figure 1. a) Scheme showing a n-Si/SiOx/Ni MIS photoanode during P-ECL (the side‐product of the TPrA radicals formed during the ECL process is denoted by p). b) Scheme showing the Si/SiOx/Ni electrodes employed in this work with ECL emission (λECL). c) IPCE spectra recorded on n-Si/SiOx (green curve) and n-Si/SiOx/Ni (1.5 nm) (purple curve) at 0.7 V vs SCE. d) Normalized absorption spectrum of the electrolyte (black curve) and P-ECL emission spectra recorded on nSi/SiOx (green curve) and n-Si/SiOx/Ni (1.5 nm) (purple curve) at 0.6 V vs SCE under illumination at λLED = 810 nm. These measurements were performed in PBS (pH = 7.3) with 5 mM [Ru(bpy)3]2+ and 100 mM TPrA.

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Due to the notorious instability of Si in aqueous media,13 we coated it by a stabilizing nickel (Ni) thin film, which was deposited by magnetron sputtering on a freshlydecontaminated and oxidized Si (100) surface to yield Si/SiOx/Ni (# nm, where # denotes the thickness of the Ni layer) metal-insulator-SC (MIS) junctions. The SiOx tunnel layer was, in all cases, 1.5 nm-thick, as measured by ellipsometry (Figure S1) and several thicknesses of Ni thin film were investigated in this work. Note that Ni was in all case covered by a native NiO/Ni(OH)2 layer,14 as confirmed by X-ray photoelectron spectrometry (XPS, Figure S2). The choice of the Ni protection layer originates from recent reports in the field of photoelectrochemical water splitting that have highlighted the outstanding properties of several Si-Ni-based photoanodes in harsh media.15,16 In order to evaluate the electrical conductivity and the stability of our MIS junctions, the first experiments were performed on nonphotoactive model electrodes that were prepared with degenerated p+-Si surfaces modified with a 5 nm-thick Ni thin film to yield non-photoactive p+-Si/SiOx/Ni (5 nm). Dark electrochemical analyses of these electrodes in the ECL electrolyte (pH = 7.3) revealed that ECL could occur on such surfaces at high potentials, as shown by cyclic voltammetry (CV, Figure S3) and chronoamperometry (CA, Figure S4). In the latter case, current densities (j) in the range of 1 mA cm-2 were recorded at 1 V vs SCE, associated with ECL that could be observed by naked eyes and recorded by a digital camera for more than 2 h (Figure S4). This is in strong contrast with the behavior observed on Ni-free hydrogenated Si (Si-H) and Si/SiOx electrodes which exhibited a quick decay in j and ECL, which disappeared after a few minutes (Figures S3 and S4). These experiments demonstrate the importance of the Ni layer for stabilizing the anode and show that stable ECL can occur at Si/SiOx/Ni MIS electrodes, allowing the study of photoactive n-Si/SiOx/Ni electrodes, which is described in the following. The P-ECL experiments were performed in a home-made cell that comprised a quartz window in front of which was localized the photoelectrode (Figure S5). We first recorded the incident photon-to-current efficiency (IPCE) spectrum of n-Si/SiOx/Ni (1.5 nm) (Figure 1c, purple curve), which indicated an effective photoconversion at wavelengths >550 nm, with a maximum conversion efficiency of ~10% around 700 nm. The cut-off below 550 nm is explained by the absorption of [Ru(bpy)3]2+ dissolved in the electrolyte, as confirmed by the absorption spectrum of the electrolyte (black curve of Figure 1d). The IPCE spectrum recorded on nSi/SiOx in the same conditions (Figure 1c, green curve), presents no photoconversion. This behavior indicates that passivation of the latter electrode occurred in the timeframe of this experiment, and further confirms the importance of the Ni thin film for promoting a stable charge transfer at the solid/liquid interface. To record real-time ECL on illuminated n-Si/SiOx/Ni surfaces, we employed a 810 nm incident wavelength (the LED spectrum is shown in Figure S6), which corresponds to a reasonable 9% IPCE (Figure 1c) and that is far enough from the ECL emission peak (λECL = 635 nm, Figure 1d). The LED and the optical fiber coupled to a spectrometer were placed in front of the quartz window (Figure S5) for illuminating the surface and simultaneously acquiring ECL emission spectra every 500 ms, respectively. This setup allows to separate the incident excitation LED light (λLED = 810 nm) from the ECL signal (i.e. λECL). It

enables to record the ECL intensity, which is integrated from 550 nm to 650 nm on the ECL spectra, as a function of the potential during CV cycles. Since Si and Ni are not usual electrode materials for ECL generation,2 we compared the ECL spectra of the [Ru(bpy)3]2+/TPrA solution obtained on n-Si/SiOx, nSi/SiOx/Ni (Figure 1d) with a classic glassy carbon electrode. ECL spectra were identical indicating that the same metal-toligand charge transfer (MLCT) excited state [Ru(bpy)3]2+* was reached on the different electrode surfaces.

Figure 2. a) CVs recorded on n-Si/SiOx/Ni (1.5 nm) under LED illumination at 810 nm (6 consecutive scans, purple curves) and in the dark (black curve). b) Corresponding ECL intensity vs E curves. c) CP curves (lines) recorded at 1 mA cm-2 on n-Si/SiOx/Ni (1.5 nm) (purple curve) and n-Si/SiOx (green curve) under illumination at 810 nm, the ECL intensity evolution on n-Si/SiOx/Ni (1.5 nm) is shown by purple circles. d) CVs recorded in the dark on p+-Si/SiOx/Ni (1.5 nm) (red curve) and under illumination on n-Si/SiOx/Ni (1.5 nm) (purple curve) and n-Si/SiOx/Ni (25 nm) (blue curve). e) Corresponding ECL intensity vs E curves. These measurements were performed in PBS (pH = 7.3) with 5 mM [Ru(bpy)3]2+ and 100 mM TPrA at 20 mV s-1. The red curves in 2d and 2e were divided by 3 for clarity. Figures 2a and 2b show the CV and the corresponding ECL intensity, respectively, recorded on n-Si/SiOx/Ni (1.5 nm). It is interesting to note the absence of current and ECL signals in the dark (black curves), showing the rectifying behavior of the junction in that potential range. In contrast, when the electrode was illuminated, a photocurrent density in the range of 1 mA cm-2 coupled with ECL appeared at potentials >0.45 V (Figure 2a,b purple curves) with no decay during 6 consecutive cycles. The stability was further confirmed by


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Journal of the American Chemical Society chronopotentiometry (CP) under illumination where a constant current density of 1 mA cm-2 was applied for 15 min, and, during which only negligible ECL variations were recorded (Figure 2c, purple plots). In contrast, n-Si/SiOx exhibited a fast deactivation during CV cycling (Figure S7) and CP measurements (Figure 2c, green curve). The comparison of the ECL intensity vs E curves obtained on illuminated photoactive n-Si/SiOx/Ni (1.5 nm) with the ones recorded on non-photoactive p+-Si/SiOx/Ni (1.5 nm) (Figure 2e) allows estimating a considerable photovoltage (Voc) of 410 mV. In addition, preliminary Mott-Schottky results (Figure S8) indicate that the n-Si/SiOx/Ni (1.5 nm) is in depletion with a flat band potential very cathodic with respect to the ECL onset potential. These results demonstrate for the first time the occurrence of P-ECL in water and show that this concept allows triggering [Ru(bpy)3]2+)/TPrA ECL at a potential close to 0.5 V, which, to the best of our knowledge, has never been reported before. We then investigated the effect of the Ni layer by employing a n-Si/SiOx/Ni (25 nm) electrode. This system was also active for P-ECL, although presenting a lower photocurrent density (as reported in the case of water splitting electrodes),15 and ECL intensity. In addition, the thickening of the Ni film was also associated with a considerable decrease in photovoltage (Voc = 150 mV), in good agreement with recent reports showing that ultrathin metal films on n-Si-based Schottky junctions photoelectrodes considerably increases the junction barrier height.15,17 We finally investigated the effect of the illumination power on the P-ECL, as shown in Figures 3a and 3b. In these experiments, we recorded CVs and corresponding ECL intensity profiles at different values of illumination power. These results show that both signals are dependent on illumination power (plots are shown in Figure S10).

in various fields such as ECL. Indeed, the concept of P-ECL can be applied to other types of reactions (including cathodic ones) on a variety of absorbing n- and p-type SCs, thus a wide range of P-ECL systems with different absorption/emission characteristics should be readily accessible. Besides being beneficial in terms of electrochemical potential gain, this concept may find application for the design of new detection strategies, lightaddressable systems or infrared imaging devices.

ASSOCIATED CONTENT Supporting Information Experimental section and additional electrochemical and optical data are provided in Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION

Corresponding Authors [email protected] [email protected] ORCID Gabriel Loget: 0000-0003-4809-5013 Neso Sojic: 0000-0001-5144-1015

Author Contributions # These authors contributed equally.

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by Univ Rennes and CAS President’s International Fellowship Initiative (PIFI). Y. Z. acknowledge the program “Défis Scientifiques” from Univ Rennes for financing her internship. J. Y. acknowledges the China Scholarship Council for her PhD fellowship. Dr. O. de Sagazan (IETR, Rennes), Dr. Yoan Léger (INSA, Rennes), Dr. B. Goudeau (ISM, Bordeaux), Dr. C. Mériadec (IPR, Rennes), Dr. S. Ababou-Girard (IPR, Rennes), Dr C. Ayela and Dr. L. Hirsch (IMS, Bordeaux) are acknowledged for their help. The authors wish also to acknowledge the support from the SinoFrench international research network “New nanostructured materials and biomaterials for renewable electrical energy sources” for providing facilities.

Figure 3. a) CVs recorded on n-Si/SiOx/Ni (1.5 nm) at different LED power density. b) Corresponding ECL intensity vs E curves. These measurements were performed in PBS solution (pH = 7.3) containing 5 mM [Ru(bpy)3]2+ and 100 mM TPrA with a 810 nm incident light. Scan rate: 20 mV s-1. To conclude, we reported for the first time the photoelectrochemiluminescence (P-ECL) of the aqueous model system [Ru(bpy)3]2+/TPrA, which was demonstrated at MIS n-Si/SiOx/Ni photoanodes. The upconversion process, which combines photoactivation at SC and electrochemistry gives a remarkably stable and intense ECL emission in water. Our findings show that the recent developments on photoelectrode protection7,18 can be beneficial for applications outside energy, opening exciting opportunities

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