Quantitative In Situ Monitoring of Cu-Atom Release by Cu2O Nanocatalysts under Photocatalytic CO2 Reduction Conditions: New Insights into the Photocorrosion Mechanism

Cu2O is among the most promising photocatalysts for CO2 reduction, however its photocorrosion remains a standalone challenge. Herein, we present an in situ study of the release of Cu ions from Cu2O nanocatalysts under photocatalytic conditions in the presence of HCO3 as a catalytic substrate in H2O. The Cu-oxide nanomaterials were produced by Flame Spray Pyrolysis (FSP) technology. Using Electron Paramagnetic Resonance (EPR) spectroscopy in tandem with analytical Anodic Stripping Voltammetry (ASV), we monitored in situ the Cu2+ atom release from the Cu2O nanoparticles in comparison with CuO nanoparticles under photocatalytic conditions. Our quantitative, kinetic data show that light has detrimental effect on the photocorrosion of Cu2O and ensuing Cu2+ ion release in the H2O solution, up to 15.7% of its mass. EPR reveals that HCO3 acts as a ligand of the Cu2+ ions, promoting the liberation of {HCO3-Cu} complexes in solution from Cu2O, up to 27% of its mass. HCO3 alone exerted a marginal effect. XRD data show that under prolonged irradiation, part of Cu2+ ions can reprecipitate on the Cu2O surface, creating a passivating CuO layer that stabilizes the Cu2O from further photocorrosion. Including isopropanol as a hole scavenger has a drastic effect on the photocorrosion of Cu2O nanoparticles and suppresses the release of Cu2+ ions to the solution. Methodwise, the present data exemplify that EPR and ASV can be useful tools to help quantitatively understand the solid–solution interface photocorrosion phenomena for Cu2O.


Introduction
The rapid development of human society has led to an increase in energy demands and ensuing environmental deterioration, making the use of new and renewable energy sources imperative. Photocatalysts have become a research hotspot over the last decades. The pioneer work of Fujishima and Honda in 1972 [1] paved the way for light-induced water dissociation by TiO 2 and has ignited numerous studies on photocatalysts, especially TiO 2 [2,3]. Since then, research interest has increased exponentially, combined with the discovery of numerous photocatalysts ranging from metal oxides (e.g., ZnO [4], WO 3 [5], and SnO 2 [6]), non-oxides (e.g., CdS [7], CuInS 2 [8] and ZnS [9]) and metal-free semiconductors (C 3 N 4 [10]). Among them, Cu 2 O stands out as particularly interesting [11][12][13] thanks to its highly reducing conduction band edge located at E CB = −1000 mV vs. NHE (pH = 0) [12].
Cu 2 O is a promising photocatalyst for CO 2 [14][15][16] reduction and H 2 production [17,18], i.e., it has a direct band-gap structure with a small energy gap of 2.0-2.2 eV [12], allowing it to absorb efficiently in the visible range of the solar spectrum, maximizing sunlight harvesting. Despite these advantages, photostability issues are among the well-documented drawbacks of Cu 2 O [13,19,20]. The so-called photocorrosion phenomenon encodes the key problem, i.e., the photogenerated holes (h + ) and electrons (e − ) can be adversely consumed to the self-decomposition of Cu 2 O itself [20]. At low degrees of photocorrosion, some Cu 1+

Flame Spray Pyrolysis (FSP) Synthesis of CuO and Cu 2 O Nanoparticles
A conventional FSP process was used for the synthesis of CuO, as described in detail in previous works [39][40][41]. A precursor solution of 0.25 M was prepared by dissolving Copper (II) Nitrate trihydrate (Cu(NO 3 ) 2 • 3H 2 O 99-104%, Sigma-Aldrich (Saint Louis, MO, USA)) in a 1:1 (by volume) mixture of acetonitrile (≥99.9%, Supelco (Bellefonte, Pennsylvania, USA)) and ethylene glycol (≥99%, Supelco (Bellefonte, PA, USA)). This precursor solution (P) was fed at a rate of P = 5 mL min −1 to our system and atomized to fine droplets using an oxygen dispersion flow of D = 5 L min −1 at a pressure drop of 1.5 bar. The spray was ignited and sustained by an oxygen/methane pilot flame of O 2 /CH 4 : 4/2 L min −1 . For the particle collection, an additional 10 L min −1 O 2 sheath was used, and the produced particles were deposited on a glass microfiber filter (Hahnemühle GF 6 257) with the assistance of a vacuum pump (BUSCH V40).
The synthesis of high-purity Cu 2 O nanoparticles required a more-demanding FSPsetup with control of the combustion-atmosphere surrounding the spray nozzle (see Figure 1a). We used a cylindrical metal chamber consisting of two concentric tubes, a sinter metal tube (outer tube) and a porous metal tube (inner tube) to isolate the flame compartment from the surrounding atmosphere The porous walls of the inner tube allow the radial inflow of an inert mixing gas, in our case, N 2 , to exclude O 2 . Moreover, to provide an additional O 2 -excluding source and aid the particle collection, a 10 L min −1 N 2 sheath was used. Once again, a 0.25 M precursor solution of Cu(NO 3 ) 2 • 3H 2 O dissolved in a 1:1 mixture of acetonitrile and ethylene glycol was sprayed into our system with a P/D ratio of 3/3. A series of N 2 radial inflows were tested in the range 0 to 30 L min −1 , resulting in progressively higher Cu 2 O-phase percentages. In all cases, in addition to the radial N 2 , a N 2 sheath gas was fixed at 10 L min −1 , except in the case of pristine CuO, where we used a 10 L min −1 O 2 sheath. The produced materials, listed in Table 1, are codenamed as Cu-xN, where x = the radial N 2 -inflow in L/min −1 . In Table 1, we list the most pertinent materials with the Cu-20N to contain the higher Cu 2 O fraction (>95%). Higher radial N 2 inflows resulted in the deterioration of particle crystallinity and are not discussed herein. To understand the role of photoinduced holes in the observed photocorrosion process.

Flame Spray Pyrolysis (FSP) Synthesis of CuO and Cu2O Nanoparticles
A conventional FSP process was used for the synthesis of CuO, as described in detail in previous works [39][40][41]. A precursor solution of 0.25 M was prepared by dissolving Copper (II) Nitrate trihydrate (Cu(NO3)2• 3H2O 99-104%, Sigma-Aldrich (Saint Louis, MO, USA)) in a 1:1 (by volume) mixture of acetonitrile (≥99.9%, Supelco (Bellefonte, Pennsylvania, USA)) and ethylene glycol (≥99%, Supelco (Bellefonte, PA, USA)). This precursor solution (P) was fed at a rate of P = 5 mL min −1 to our system and atomized to fine droplets using an oxygen dispersion flow of D = 5 L min −1 at a pressure drop of 1.5 bar. The spray was ignited and sustained by an oxygen/methane pilot flame of O2/CH4: 4/2 L min −1 . For the particle collection, an additional 10 L min −1 O2 sheath was used, and the produced particles were deposited on a glass microfiber filter (Hahnemühle GF 6 257) with the assistance of a vacuum pump (BUSCH V40).
The synthesis of high-purity Cu2O nanoparticles required a more-demanding FSPsetup with control of the combustion-atmosphere surrounding the spray nozzle (see Figure 1a). We used a cylindrical metal chamber consisting of two concentric tubes, a sinter metal tube (outer tube) and a porous metal tube (inner tube) to isolate the flame compartment from the surrounding atmosphere The porous walls of the inner tube allow the radial inflow of an inert mixing gas, in our case, N2, to exclude O2. Moreover, to provide an additional O2-excluding source and aid the particle collection, a 10 L min −1 N2 sheath was used. Once again, a 0.25 M precursor solution of Cu(NO3)2• 3H2O dissolved in a 1:1 mixture of acetonitrile and ethylene glycol was sprayed into our system with a P/D ratio of 3/3. A series of N2 radial inflows were tested in the range 0 to 30 L min −1 , resulting in progressively higher Cu2O-phase percentages. In all cases, in addition to the radial N2, a N2 sheath gas was fixed at 10 L min −1 , except in the case of pristine CuO, where we used a 10 L min −1 O2 sheath. The produced materials, listed in Table 1, are codenamed as Cu-xN, where x = the radial N2-inflow in L/min −1 . In Table 1, we list the most pertinent materials with the Cu-20N to contain the higher Cu2O fraction (>95%). Higher radial N2 inflows resulted in the deterioration of particle crystallinity and are not discussed herein.

Characterization of Materials
Powder X-Ray Diffraction (pXRD): The as-prepared nanomaterials were characterized using a powder X-ray diffractometer (Bruker D8 Advanced using CuKα radiation = 1.5405 Å) with a scanning step of 0.03 • at a rate of 2 s per step and 2-theta (θ) angle ranging from 10-80 • at current 40 mA and voltage 40 kV. The average crystal size was calculated by using the Scherrer Equation (1) [42]: where, d XRD is the crystallite size (nm), k is a shape constant (in this case 0.9), λ is the wavelength of CuKα radiation, β is the full width at half maximum and θ is the peakdiffraction angle. To determine the percentage of CuO/Cu 2 O crystal phase in each Cubased nanomaterial, we used Profex, which is a graphical user interface for Rietveld refinement [43].

Electron Paramagnetic Resonance Spectroscopy (EPR)
EPR spectra were recorded at 77 K using a Bruker ER200D spectrometer equipped with an Agilent 5310 A frequency counter operating at X-band (~9.6 GHz) with a modulation amplitude of 10 G peak to peak. The spectrometer is controlled with a custom-made software based on LabView. To obtain an adequate signal-to-noise ratio, each spectrum is an average of 5-10 scans. Theoretical analysis of the Cu 2+ EPR signals was performed using a spin Hamiltonian and can be simulated using EasySpin MATLAB toolbox [44] assuming a spin system with S = 1/2 and I = 3/2 for 63,65 Cu 2+ .

Analytical Cu 2+ Leaching Study by Anodic Stripping Voltammetry (ASV)
The concentration of Cu 2+ in aqueous solution was determined by Anodic Stripping Voltammetry (ASV) using a Metrohm 797 VA computrace stand equipped with a Metrohm multimode electrode (MME). More specifically, a conventional three-electrode arrangement was used comprising Hanging Mercury Drop Electrode (HMDE) as the working electrode, Platinum rod (Pt) as the auxiliary electrode and Ag/AgCl (3 mol L −1 KCl) as the reference electrode. Cu standard solutions used for the quantification of our unknown samples were prepared by dissolving Cu(NO 3 ) 2 • 3H 2 O in ultrapure triple-distilled (3d) water obtained from a Millipore-Q water purification system (USA) with a resistivity of >18 MΩ cm and diluting to obtain the desired Cu concentrations. The measurements were carried out at a volume of 10 mL of 0. 1 M KNO 3 and 3 d water of pH:4 adjusted with HNO 3 to ensure the maximum presence of Cu 2+ ions based on the theoretical copper speciation for hydroxo complexes in pure water [45]. The instrumental settings were as follows: mercury drop size 0.4 mm and scan rate 20 mV s −1 . Moreover, a deposition potential of −0.6 V versus Ag/AgCl (+0.2 V versus SHE at 25 • C) was used and the deposition time was carried out for 90 s. The reported data represent an average of three independent experimental repetitions. Figure 1a shows the FSP reactor set-up and photos of as-produced pure CuO and Cu 2 O powders on the FSP filter. The black color is typical for CuO, while the red-brown color of Cu 2 O originates from its band gap Eg = 2.0-2.2 eV [12]. Figure 1b shows the XRD patterns of Cu materials, also listed in Table 1 The XRD data in Figure 1 show that increasing N 2 inflow, promoted the formation of Cu 2 O at the expense of the originally predominating CuO phase. The XRD-estimated particle diameters values (d XRD ) of the CuO and Cu 2 O phases as well as their respective phase percentages are listed in Table 1. We see that Cu-20N is a Cu 2 O material with at least 95% and a minor fraction of CuO. Based on several trials, we conclude that a small percentage (2-5%) of CuO was formed upon exposure of the originally pure Cu 2 O to atmospheric O 2 during the particle handling. Once formed, this CuO did not further increase. Thus, the Cu 2 O/CuO phase compositions listed in Table 1 represent stable compositions of FSP-made nanomaterials.

Results
To underscore the Cu 2 O-formation process, we note that in FSP, the gas atmosphere where the particle formation takes place, is of key importance [34,46]. Under an oxygen-rich atmosphere, i.e., such as ambient air inflow with 20% O 2 , the produced materials are highly stable and fully oxidized ceramic powders [47]. In the present case of Cu oxide formation, this FSP protocol results in the formation of pristine CuO, see Figure 1. Decreasing the oxygen concentrations in the FSP reactor by the N 2 sheath and mostly by the radial N 2 inflow, see Figure 1a, resulted in the promotion of stable, reduced metal oxide (Cu 2 O) whose lattice is formed by Cu 1+ ions. In our case, the use of N 2 in our FSP reactor played a dual role: first, the exclusion of oxygen and second, the reduction of oxygen partial pressure inside the reactor, resulting in the progressive formation of Cu 2 O. We should note here that the formation of metallic Cu 0 was not observed, which led us to conclude that this modified FSP setup allows meticulous exploration of the formation of suboxides rather than metallic particles.

Cu 2+ Ion Release under CO 2 -Photoreduction Conditions
The Role of pH: First, we examined the chemical stability, without light, by monitoring the Cu ions' release from CuO and Cu 2 O in H 2 O under different pH values. Figure 2a presents results based on ASV determination of Cu 2+ ions in solution after 3 h of exposure. This time scale (3 h) is typical time span for photocatalytic experiments. As we see in Figure 2a, under acidic pH (pH:2), both CuO and Cu 2 O materials were 100% dissolved after 3 h. On the contrary, increasing the pH towards more alkaline values, Cu 2+ release decreased rapidly, with a threshold pH > 7, where the Cu 2+ release was <5% at 3 h. Notice that the CuO phase exhibited better chemical stability than Cu 2 O. Even at neutral pH, Cu 2 O was more unstable, having a dissolution of 7%, which is 3.5-fold higher vs. the corresponding leaching of CuO (better viewed at the zoomed Figure 2a inset).
The Role of Light-Photons: Based on these results, a series of Xenon-lamp illuminations were performed under a slightly alkaline pH (pH:8), often used in CO 2 photocatalysis in HCO 3 − /H 2 O systems [12,14], and both CuO and Cu 2 O are relatively stable, with Cu 2+ release of 0.6% and 3.5%, respectively ( Figure 2a). As seen in Figure 2b, under full-Xenon spectrum illumination, hv > 200 nm, CuO showed~1.5% light-induced Cu 2+ release, that is a +1% increase vs. no light. Elimination of UV photons by filtration hv > 340 nm resulted in a lower Cu 2+ release by CuO, i.e., by~1% ( Figure 2b). Overall, the data in Figure 2a show that the damage of light on the CuO nanoparticles was limited.
Overall, the data in Figure 2b,c reveal that [i] Cu2O is far more prone, about 10 fold, to Cu 2+ release in solution than CuO.
[ii] This is a direct manifestation of photocorrosion. That is to say, photocorrosion starts as an oxidation event inside the Cu2O crustal, as evidenced by many previous data [19,20], and, in the following, the present data show that photocorrosion persists until the physical detachment of the Cu ions from the particle matrix. As we show hereafter, photoinduced holes are the origin of the Cu 1+ to Cu 2+ oxidation. The effect of photon wavelength can be understood as follows: the band gap of Cu2O particles near 2.1 eV entails that photons with λ ≤ 580 nm, i.e., visible and UV photons, can photoexcite it, creating holes and electrons. This includes 200 nm photons, i.e., hv~6 eV, which excite highly energetic "deep" holes with energies well below the valence band top. Similarly, electrons well above the conduction-bend edge can be excited. The data in Figure 2c, with hv > 200 nm, indicate that the high energetic holes dramatically boost the Cu 2+ release. This results in a significant 15% of the Cu2O mass to literally deteriorate. In the same context, allowing hv > 340 nm contains photons with energy ≤ 3.4 eV that can also photoexcite "deep" holes, though with less energy than the 200 nm photons. Thus, the hv > 340 nm results in about half of the Cu 2+ release by the Cu2O particles. On the contrary, light photons exerted a severe effect on Cu 2+ leaching by the Cu 2 O nanophase (material Cu-20N) (Figure 2c). Full-Xenon illumination, hv > 200 nm, resulted in dissolution higher than >15% of the Cu 2 O matrices, releasing the Cu 2+ ions in the aqueous solution. Thus, hv > 200 nm photons enhanced the Cu release by 500%, i.e., from~3% in the dark to~15%. Filtering out the UV photons, hv > 340 nm, resulted in a significant drop of Cu 2+ ions release to 7% (Figure 2c), which is about 200% versus no light. Overall, the data in Figure 2b,c reveal that [i] Cu 2 O is far more prone, about 10 fold, to Cu 2+ release in solution than CuO.
[ii] This is a direct manifestation of photocorrosion. That is to say, photocorrosion starts as an oxidation event inside the Cu 2 O crustal, as evidenced by many previous data [19,20], and, in the following, the present data show that photocorrosion persists until the physical detachment of the Cu ions from the particle matrix. As we show hereafter, photoinduced holes are the origin of the Cu 1+ to Cu 2+ oxidation.
The effect of photon wavelength can be understood as follows: the band gap of Cu 2 O particles near 2.1 eV entails that photons with λ ≤ 580 nm, i.e., visible and UV photons, can photoexcite it, creating holes and electrons. This includes 200 nm photons, i.e., hv~6 eV, which excite highly energetic "deep" holes with energies well below the valence band top. Similarly, electrons well above the conduction-bend edge can be excited. The data in Figure 2c, with hv > 200 nm, indicate that the high energetic holes dramatically boost the Cu 2+ release. This results in a significant 15% of the Cu 2 O mass to literally deteriorate. In the same context, allowing hv > 340 nm contains photons with energy ≤ 3.4 eV that can also photoexcite "deep" holes, though with less energy than the 200 nm photons. Thus, the hv > 340 nm results in about half of the Cu 2+ release by the Cu 2 O particles.
The Role of HCO 3 − : As mentioned previously [48,49], Cu 2 O is identified as a promising CO 2 photocatalyst. In aqueous-phase photocatalytic processes, carbonate species are pertinent. Herein, we tested the role of HCO 3 − as a photocatalytic substrate that prevails in the pH range 6.5-10.5 in H 2 O systems [50]. We used 30 mM HCO 3 − , which is an average amount used in CO 2 -photocatalytic experiments [51,52]. Control data show that HCO 3 − with no illumination had an insignificant effect on Cu 2+ release (Figure 3a) from CuO. Similarly, the Cu 2+ release data in Figure 3a show that during underexposure of CuO in HCO 3 − plus light, Cu-atom release was extremely low, i.e., 0.75% without irradiation and 1% with hv > 200 nm. This confirms the stability of CuO under light and as well as light +HCO 3 − .
The Role of HCO3 − : As mentioned previously [48,49], Cu2O is identified as a promising CO2 photocatalyst. In aqueous-phase photocatalytic processes, carbonate species are pertinent. Herein, we tested the role of HCO3 − as a photocatalytic substrate that prevails in the pH range 6.5-10.5 in H2O systems [50]. We used 30 mM HCO3 − , which is an average amount used in CO2-photocatalytic experiments [51,52]. Control data show that HCO3 − with no illumination had an insignificant effect on Cu 2+ release (Figure 3a) from CuO. Similarly, the Cu 2+ release data in Figure 3a show that during underexposure of CuO in HCO3 − plus light, Cu-atom release was extremely low, i.e., 0.75% without irradiation and ~1% with hv > 200 nm. This confirms the stability of CuO under light and as well as light +HCO3 − .
In the case of Cu2O, the presence of HCO3 − alone with no light (Figure 3b) caused a Cu-atom release ~11%. This is higher than the Cu 2+ release by Cu2O with no HCO3 − , i.e., 3.5% (compare Figure 3b vs. Figure 2c). This reveals a direct chemical, not photochemical effect of HCO3 − on the Cu2O atoms. As we show hereafter by EPR data, HCO3 − extracts Cu 2+ ions from the Cu2O particles s via formation of Cu-HCO3 complexes. As seen in Figure 3b, under light photons, the HCO3 − severely intensifies the Cu 2+ release, which reached ~27% of its mass (Figure 3b) under hv > 200 nm. Filtering off UV photons (Figure 3b), hv > 340 nm, resulted in ~15% Cu 2+ release. These results clearly reveal that carbonate, i.e., HCO3 − exerts a deteriorating effect in two ways: [i] In the dark, HCO3 − is able to drive detachment of some Cu atoms from the Cu2O particles.
[ii] Under illumination, the photocorrosive Cu release is exacerbated by the presence of carbonates. Figure 4a shows X-band EPR spectra for Cu 2+ ions released by Cu2O particles under Xenon light irradiation, either in the presence or absence of HCO3. All spectra displayed in Figure 4a are typical for mononuclear Cu 2+ (electron spin S = 1/2, nuclear spin I = 3/2) [30,53]. The well-resolved hyperfine lines of Cu 2+ EPR spectra correspond to isolated Cu 2+ ions in solution. All EPR spectra can be simulated, assuming a spin system with S = 1/2, I = 3/2, i.e., for Cu 2+ , see dotted lines in Figure 4a with Cu 2+ Spin Hamiltonian parameters (tensors g and A), listed in Table 2. In Figure 4b, we represent a so-called Peisach-Blumberg plot [54] for Cu 2+ species using the g// and A// from Table 2. Peisach and Blumberg developed a method which correlates EPR parameters (g//, A//) with the number and type of ligand donor atoms in Cu 2+ complexes. Previously, we showed that this method may be used to precisely detect the coordination of Cu 2+ ions on metal oxides' surfaces and to distinguish the form of Cu atoms in solution [30,31]. In the case of Cu 2 O, the presence of HCO 3 − alone with no light (Figure 3b) caused a Cu-atom release~11%. This is higher than the Cu 2+ release by Cu 2 O with no HCO 3 − , i.e., 3.5% (compare Figure 3b vs. Figure 2c). This reveals a direct chemical, not photochemical effect of HCO 3 − on the Cu 2 O atoms. As we show hereafter by EPR data, HCO 3 − extracts Cu 2+ ions from the Cu 2 O particles s via formation of Cu-HCO 3 complexes.

EPR Spectroscopy
As seen in Figure 3b, under light photons, the HCO 3 − severely intensifies the Cu 2+ release, which reached~27% of its mass (Figure 3b) under hv > 200 nm. Filtering off UV photons (Figure 3b), hv > 340 nm, resulted in~15% Cu 2+ release. These results clearly reveal that carbonate, i.e., HCO 3 − exerts a deteriorating effect in two ways: [i] In the dark, HCO 3 − is able to drive detachment of some Cu atoms from the Cu 2 O particles.
[ii] Under illumination, the photocorrosive Cu release is exacerbated by the presence of carbonates. Figure 4a shows X-band EPR spectra for Cu 2+ ions released by Cu 2 O particles under Xenon light irradiation, either in the presence or absence of HCO 3 . All spectra displayed in Figure 4a are typical for mononuclear Cu 2+ (electron spin S = 1/2, nuclear spin I = 3/2) [30,53]. The well-resolved hyperfine lines of Cu 2+ EPR spectra correspond to isolated Cu 2+ ions in solution. All EPR spectra can be simulated, assuming a spin system with S = 1/2, I = 3/2, i.e., for Cu 2+ , see dotted lines in Figure 4a with Cu 2+ Spin Hamiltonian parameters (tensors g and A), listed in Table 2. In Figure 4b, we represent a so-called Peisach-Blumberg plot [54] for Cu 2+ species using the g // and A // from Table 2. Peisach and Blumberg developed a method which correlates EPR parameters (g // , A // ) with the number and type of ligand donor atoms in Cu 2+ complexes. Previously, we showed that this method may be used to precisely detect the coordination of Cu 2+ ions on metal oxides' surfaces and to distinguish the form of Cu atoms in solution [30,31].

Discussion
The present data show that in the presence of HCO3 − , the Cu2O photocorrosion is severely accentuated. Even in the dark, bicarbonate should be viewed as a highly active coordinating agent that can bind on the Cu2O surface and promote the release of Cu (HCO3 − )2 complexes in aqueous solution. Additionally, light photons can promote the formation of Cu 2+ via self-oxidation.
The Role of Hole Scavenger: The data in Figures 2 and 3 clearly exemplify the photocorrosion phenomena that prevail in Cu2O. As mentioned by Toe [19,20], photoinduced holes should be considered as the key reactive species that promote the Cu 2+ release from    Table 2).(b) The relation between g// and A// parameters for Cu 2+ ions in the presence and absence of HCO3 -.
The structural significance of the EPR spectral features can be understood by comparison of the g// and A// parameters with the literature data according to the method established by Peisach and Blumberg. These data indicate that: (a) In the absence of carbonates, the Cu 2+ ions are released from illuminated Cu2O as aqua-coordinated ions in solution. (b) In the presence of HCO3 − as a photocatalytic substrate, copper ions are released in the form of Cu(HCO3 − )2 complexes in the aqueous solution. In all cases, the Cu 2+ ions are coordinated by O atoms in an octahedral symmetry with the ground-state orbital of the Cu-unpaired electron to be 2 − 2 [55,56]. (Cu 2+ in zeolites) Cu-CHA hydrated gx = gy = g ̝ ⊥ = 2.07 gz = g// = 2.394 157 [57] (Cu 2+ in zeolites) Cu-MOR hydrated gx = gy = g ̝ ⊥ = 2.08 gz = g// = 2.4 154 [57]

Discussion
The present data show that in the presence of HCO3 − , the Cu2O photocorrosion is severely accentuated. Even in the dark, bicarbonate should be viewed as a highly active coordinating agent that can bind on the Cu2O surface and promote the release of Cu (HCO3 − )2 complexes in aqueous solution. Additionally, light photons can promote the formation of Cu 2+ via self-oxidation.
The Role of Hole Scavenger: The data in Figures 2 and 3 clearly exemplify the photocorrosion phenomena that prevail in Cu2O. As mentioned by Toe [19,20], photoinduced holes should be considered as the key reactive species that promote the Cu 2+ release from ⊥ = 2.08 g z = g // = 2.   Table 2).(b) The relation between g// and A// parameters for Cu 2+ ions in the presence and absence of HCO3 -.
The structural significance of the EPR spectral features can be understood by comparison of the g// and A// parameters with the literature data according to the method established by Peisach and Blumberg. These data indicate that: (a) In the absence of carbonates, the Cu 2+ ions are released from illuminated Cu2O as aqua-coordinated ions in solution. (b) In the presence of HCO3 − as a photocatalytic substrate, copper ions are released in the form of Cu(HCO3 − )2 complexes in the aqueous solution. In all cases, the Cu 2+ ions are coordinated by O atoms in an octahedral symmetry with the ground-state orbital of the Cu-unpaired electron to be 2 − 2 [55,56].  [29] (Cu 2+ in zeolites) Cu-CHA hydrated gx = gy = g ̝ ⊥ = 2.07 gz = g// = 2.394 157 [57] (Cu 2+ in zeolites) Cu-MOR hydrated gx = gy = g ̝ ⊥ = 2.08 gz = g// = 2.4 154 [57]

Discussion
The present data show that in the presence of HCO3 − , the Cu2O photocorrosion is severely accentuated. Even in the dark, bicarbonate should be viewed as a highly active coordinating agent that can bind on the Cu2O surface and promote the release of Cu (HCO3 − )2 complexes in aqueous solution. Additionally, light photons can promote the ⊥ = 2.07 g z = g // = 2.394 157 [57] (Cu 2+ in zeolites) Cu-MOR hydrated g x = g y = Nanomaterials 2023, 13, x FOR PEER REVIEW 8 of 12  Table 2).(b) The relation between g// and A// parameters for Cu 2+ ions in the presence and absence of HCO3 -.
The structural significance of the EPR spectral features can be understood by comparison of the g// and A// parameters with the literature data according to the method established by Peisach and Blumberg. These data indicate that: (a) In the absence of carbonates, the Cu 2+ ions are released from illuminated Cu2O as aqua-coordinated ions in solution. (b) In the presence of HCO3 − as a photocatalytic substrate, copper ions are released in the form of Cu(HCO3 − )2 complexes in the aqueous solution. In all cases, the Cu 2+ ions are coordinated by O atoms in an octahedral symmetry with the ground-state orbital of the Cu-unpaired electron to be 2 − 2 [55,56].  [29] (Cu 2+ in zeolites) Cu-CHA hydrated gx = gy = g ̝ ⊥ = 2.07 gz = g// = 2.394 157 [57] (Cu 2+ in zeolites) Cu-MOR hydrated gx = gy = g ̝ ⊥ = 2.08 gz = g// = 2.4 154 [57]

Discussion
The present data show that in the presence of HCO3 − , the Cu2O photocorrosion is severely accentuated. Even in the dark, bicarbonate should be viewed as a highly active ⊥ = 2.08 g z = g // = 2.4 154 [57]

Discussion
The present data show that in the presence of HCO 3 − , the Cu 2 O photocorrosion is severely accentuated. Even in the dark, bicarbonate should be viewed as a highly active coordinating agent that can bind on the Cu 2 O surface and promote the release of Cu (HCO 3 − ) 2 complexes in aqueous solution. Additionally, light photons can promote the formation of Cu 2+ via self-oxidation.
The Role of Hole Scavenger: The data in Figures 2 and 3 clearly exemplify the photocorrosion phenomena that prevail in Cu 2 O. As mentioned by Toe [19,20], photoinduced holes should be considered as the key reactive species that promote the Cu 2+ release from photo-cited Cu 2 O. In Figure 5, we examine the role of hole scavenger using isopropanol as a standard hole scavenger. Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 12 photo-cited Cu2O. In Figure 5, we examine the role of hole scavenger using isopropanol as a standard hole scavenger. In the presence of 2-propanol plus NaHCO3, a significant suppression of the photocorrosion is observed, as evidenced by the decrease from 27% to 3% of Cu 2+ -ion release (Figure 5a). This provides clear evidence that scavenging of the photoinduced holes, provides significant protection against photocorrosion of Cu2O under realistic CO2-photocatalytic conditions. This is a very encouraging result, showing a route to address the Cu2O photocorrosion problem.
To further understand the process, we examined by XRD the Cu2O particles after 3 h photocatalytic exposure (Figure 5b). As seen in Figure 5b, in the presence of NaHCO3, after 3 h of irradiation (Xenon, hv > 200 nm) the initial Cu2O-crystal phase composition is changed from >95% Cu2O (see Table 3) to 60% CuO. We underline that the particles collected after 3 h photocorrosion represent only the fraction that is not dissolved to Cu 2+ ions. Thus, the photocorrosion of Cu2O in the presence of NaHCO3 has two consequences: [i] Part of the Cu2O particle is dissolved towards Cu 2+ ions.
[ii] The remaining Cu-oxide particle phase is altered from Cu2O to CuO. Importantly, in the presence of 2-propanol, the Cu 2+ -release and XRD data show that [i] Practically minimal Cu 2+ -ions release occurs. That is the Cu-oxide particles remain mostly intact.
[ii] The crystal composition is modified, i.e., according to Table 3, the Cu-oxide particles consist of 25% CuO, i.e., the initial 95% Cu2O has been retained to 75%. We consider that the formed 25% CuO forms a protective layer around the Cu2O, and this inhibits the Cu 2+ -ion release.

Conclusions
Using EPR spectroscopy in tandem with ASV, the in situ study of the release of Cu ions from Cu2O nanocatalyst under photocatalytic conditions provides new insight into the role of HCO3 as a catalytic substrate. Light and HCO3 − have detrimental effects on the photocorrosion of Cu2O and the ensuing Cu 2+ -ion release in the H2O solution. EPR reveals that HCO3 − acts as ligand of the Cu 2+ ions, promoting the liberation of {HCO3-Cu} com- In the presence of 2-propanol plus NaHCO 3 , a significant suppression of the photocorrosion is observed, as evidenced by the decrease from 27% to 3% of Cu 2+ -ion release ( Figure 5a). This provides clear evidence that scavenging of the photoinduced holes, provides significant protection against photocorrosion of Cu 2 O under realistic CO 2photocatalytic conditions. This is a very encouraging result, showing a route to address the Cu 2 O photocorrosion problem.
To further understand the process, we examined by XRD the Cu 2 O particles after 3 h photocatalytic exposure (Figure 5b). As seen in Figure 5b, in the presence of NaHCO 3 , after 3 h of irradiation (Xenon, hv > 200 nm) the initial Cu 2 O-crystal phase composition is changed from >95% Cu 2 O (see Table 3) to 60% CuO. We underline that the particles collected after 3 h photocorrosion represent only the fraction that is not dissolved to Cu 2+ ions. Thus, the photocorrosion of Cu 2 O in the presence of NaHCO 3 has two consequences: [i] Part of the Cu 2 O particle is dissolved towards Cu 2+ ions.
[ii] The remaining Cu-oxide particle phase is altered from Cu 2 O to CuO. Importantly, in the presence of 2-propanol, the Cu 2+ -release and XRD data show that [i] Practically minimal Cu 2+ -ions release occurs. That is the Cu-oxide particles remain mostly intact.
[ii] The crystal composition is modified, i.e., according to Table 3, the Cu-oxide particles consist of 25% CuO, i.e., the initial 95% Cu 2 O has been retained to 75%. We consider that the formed 25% CuO forms a protective layer around the Cu 2 O, and this inhibits the Cu 2+ -ion release.

Conclusions
Using EPR spectroscopy in tandem with ASV, the in situ study of the release of Cu ions from Cu 2 O nanocatalyst under photocatalytic conditions provides new insight into the role of HCO 3 2 complexes in aqueous solution. On top of this, light photons can promote the formation of Cu 2+ via self-oxidation. XRD data show that under prolonged irradiation, part of Cu 2+ ions can re-precipitate on the Cu 2 O surface, creating a passivating CuO layer that stabilizes the CuO-Cu 2 O from further photocorrosion. Moreover, including isopropanol as a hole scavenger has a drastic impact on the photo-oxidation of Cu 2 O to CuO as well as suppresses the release of Cu 2+ ions. Method-wise, the present data exemplify that EPR and ASV can be useful tools to quantitatively understand the solid-solution interface photocorrosion phenomena for Cu 2 O.