Disentangling the Role of Ag‐Based Nanocorals as Efficient Cocatalyst over CuBi2O4 Photocathodes Toward Hydrogen Evolution Reaction

CuBi2O4 is one of the most studied potential candidates for photoelectrocatalytic solar fuel generation, from H2 production, to CO2 reduction, or even N2 fixation. Hence, understanding its performance and catalytic behavior is key to use this material under real working conditions. Herein, Ag nanocorals are successfully deposited over CuBi2O4 photocathodes for enhancing its performance as a promising candidate for photoelectrocatalytic reduction reactions. An in‐depth study of this novel structure through a combination of several materials’ characterization techniques, confirming the tetragonal structure and the stoichiometric proportion of the elemental components, is presented. In addition, the different charge transfer processes and catalytic mechanisms behind the performance of Ag‐decorated CuBi2O4 photocathodes are unveiled through a combination of electrochemical impedance spectroscopy and transient absorption spectroscopy. The combination of these advanced spectroscopies reveals that Ag is acts as a true catalyst, enhancing the charge extraction and decreasing the charge accumulation and the recombination at the CuBi2O4 surface, thus boosting the photocathode performance.


Introduction
The development of sustainable and efficient methods for solar fuel production has garnered significant attention in recent years as a promising solution to address the global energy and environmental challenges. [1,2]mong the extensive variety of different materials investigated for photo(electro)catalytic applications, CuBi 2 O 4 has emerged as a highly attractive semiconductor due to its unique structural and electronic properties. [3]CuBi 2 O 4 , a p-type ternary oxide compound composed of copper (Cu), bismuth (Bi), and oxygen (O), exhibits excellent visible light absorption, favorable band-edge positions, and efficient charge carrier separation, making it a promising candidate for photocathodic applications. [4]otably, CuBi 2 O 4 photocathodes have demonstrated remarkable performance in the hydrogen evolution reaction (HER), enabling the efficient conversion of solar energy into hydrogen fuel. [5,6]In addition, some works have also reported the employment of a CuBi 2 O 4 photocathode in a tandem photoelectrochemical cell coupled with a BiVO 4 photoanode. [7]However, CuBi 2 O 4 also suffers from three main drawbacks: 1) low chemical stability under aqueous conditions, as a consequence of photocorrosion, as it happens in other Cu-based photocathodes, [8] 2) low visible light absorption, [9] and 3) poor charge separation and transport properties, [10,11] leading to low photocurrent densities.Trying to overcome these limitations, several studies have explored different strategies to enhance the photoelectrocatalytic activity of CuBi 2 O 4 , including postsynthetic treatments, [12] surface modification, [13] surface protection, [3] heterostructure formation, [14,15] the introduction of ferroelectric nanodomains, [16] self-doping by the introduction of Cu vacancies, [17] external doping, [18] and cocatalyst deposition, [19] resulting in an improved photoelectrochemical performance.Specifically, MoS 2 has been reported as an effective cocatalyst toward photoelectrocatalytic H 2 production in alkaline media. [19]owever, examples of the use of cocatalysts to improve the performance of CuBi 2 O 4 photocathodes toward the HER under mild pH conditions remain scarce.Besides, many reported studies are just focused in enhancing the performance of CuBi 2 O 4 photocathodes but with a lack of understanding of the reasons behind this improvement.
The present study tries to shed light on the role of Ag as a cocatalyst in enhancing the photoelectrocatalytic performance of CuBi 2 O 4 , making it a promising candidate for HER at mild pH conditions, among other photoelectroreductive solar fuels reactions.A combination of a wide range of characterization techniques has been employed to achieve this ambitious purpose.Finally, through a combination of electrochemical impedance spectroscopy (EIS) and transient absorption spectroscopy (TAS), the role of Ag was found as a true catalyst, being able to enhance charge extraction and to decrease the charge accumulation and the recombination at CuBi 2 O 4 surface, hence, boosting its photoelectrochemical activity.

Results and Discussion
The CuBi 2 O 4 samples reported in the present work were fabricated following modified electrodeposition methods. [5,20]igure S1, Supporting Information, shows the employed chronoamperometry for the electrodeposition of CuBi 2 O 4 thin films.
The inset shows the aspect of the electrodeposited films over soda lime fluorine-doped tin oxide (FTO)-coated substrates.The number of electrodeposited layers was optimized to 20 by measuring the J-V response of the fabricated samples in 0.1 M KPi buffer (pH = 7) under 1 sun chopped illumination, as shown in Figure S2, Supporting Information.The morphology of the fabricated CuBi 2 O 4 electrode was analyzed by field emission-scanning electron microscopy (FE-SEM).Figure 1A shows the cross-section image of the fabricated CuBi 2 O 4 photocathode where the different layers can be observed: soda lime glass ≈2 mm, FTO ≈300 nm, and CuBi 2 O 4 ≈500 nm. Figure S3A-C, Supporting Information shows the FESEM images acquired in the CuBi 2 O 4 sample at different magnifications.Figure S3A, Supporting Information, shows the frontier region between the electrodeposited semiconducting layer of CuBi 2 O 4 sample and the fluorine-doped SnO 2 (FTO) transparent conductive substrate.Figure S3B,C, Supporting Information, shows the porous nature of the electrodeposited samples, with grain sizes in the order of several tens of nm.On the other hand, the chemical composition of the fabricated photocathode was analyzed by energy dispersive X-ray spectroscopy (EDS) (Figure 1B-E), confirming the presence of Cu, Bi, and O in the stoichiometric proportions (Cu ≈ 12 at%, Bi ≈ 21 at%, and O ≈ 40 at%, Figure S4, Supporting Information).Only Sn and F were also detected from the FTO substrate.The presence of any other elements as contaminants was discarded in the fabricated samples.
The crystallinity of the CuBi 2 O 4 electrodes was analyzed by X-ray diffraction (XRD) (Figure 1F), where the presence of peaks associated with the tetragonal phase of CuBi 2 O 4 (P4/ncc) was confirmed. [21]Only two peaks associated with the rutile F: SnO 2 from the FTO were detected. [22]The presence of any other elements, other crystalline phases, or other compounds was also discarded by this technique, confirming the successful electrodeposition of stoichiometric CuBi 2 O 4 layers.Raman spectroscopy was employed to confirm the crystalline structure of the fabricated films.Specifically, the acquired Raman spectrum in the synthesized photocathodes is shown in Figure 1G, showing the characteristic vibrational modes located at 130 cm À1 (2A 1g þ B 2g ), 186 cm À1 (E g ), 257 cm À1 (A 1g ), 396 cm À1 (A 1g ), and 578 cm À1 (A 1g þ E g ), as previously reported, [23] confirming the P4/ncc tetragonal unit cell of CuBi 2 O 4 .The inset in Figure 1G corresponds to an atomic force microscopy (AFM) image showing the high porosity and roughness of the electrodeposited CuBi 2 O 4 samples.
Figure 2 shows the X-ray absorption near-edge structure (XANES) of samples and references.The spectra of Cu 2 O and CuO are similar to those reported in previous works. [24,25]For the Cu 2 O reference, the edge position, taken as the maximum of the first derivative spectrum, is around 8089.8 eV.The absence of any pre-edge peak indicates the presence of Cu 1þ species because no 1s-3d transitions are allowed for Cu 1þ ions due to the d10 electronic configuration.Instead, for the CuO reference, the edge position is shifted to 8983.9 eV, similar to what was observed for other Cu 2þ compounds. [26]In this case, a clear pre-edge peak is observed around 8978.2 eV, confirming the presence of oxidized Cu 2þ ions.For the CuBi 2 O 4 sample, the spectrum resembles the CuO reference, indicating the presence of Cu 2þ ions.Similar results were also reported by other authors through the analysis of X-ray absorption spectroscopy (XAS) spectra at Cu L 3 edge. [27]The edge position of CuBi 2 O 4 is around 8984.0 eV, practically the same as for the CuO.Moreover, the pre-edge peak is at the same energy for the CuO as well as the white line intensity that peaks around 8997 eV.However, there are also differences compared to the CuO, such as the shoulder around 8981 eV that is associated with shake-down transition in CuO that is not observed in the CuBi 2 O 4 sample.
The extended X-ray absorption fine structure (EXAFS) was analyzed in k-range 3-11.5 Å À1 and the fit was performed in the R-range 1-2.2 Å in order to obtain information about the first coordination sphere, see Figure 2B.The results of the fit are summarized in Table 1.The structure of Cu 2 O is made of two Cu─O bonds at a distance of 1.85(1) Å, instead for CuO, the Cu 2þ ions are four-coordinated and the bond distance is 1.94(1) Å.The EXAFS signal of the CuBi 2 O 4 sample is very similar to that of CuO, indicating a quite similar local structure of Cu 2þ in this compound.The first coordination sphere is made of four Cu─O bonds at a distance of 1.94(1) Å.The intensity of the first shell for the CuBi 2 O 4 is higher than for the CuO reference, likely related to a lower structural disorder for the former, as indicated by the lower value of the σ 2 parameter (Table 1).
The XANES spectrum at Bi L 3 -edge of the CuBi 2 O 4 sample is shown in Figure 2C.The edge position is around 13 425 eV, similar to what was observed for Bi 2 O 3 , [28] thus suggesting the presence of Bi 3þ ions.The white line peaks at 13 442 eV and a quite broad XANES resonance appear around 13 496 eV.The analysis of the EXAFS signal is shown in Figure 2D and indicates that Bi 3þ ions are four coordinated with an average Bi─O bond distance of 2.17 Å, see Table 1.
Figure 3A shows the open-circuit potential (OCP) under 1 Sun chopped illumination.Under illumination, the CuBi 2 O 4 samples show an increase in the OCP values, associated with the positive  splitting of the pseudo-Fermi levels, characteristic of a p-type semiconductor, confirming the photoactive and semiconducting nature of this material and its possible use as a photocathode. [29]pecifically, the samples presented herein show a photovoltage of 0.35AE0.05V. Figure 3B shows the J-V under front (blue trace) and back (brown trace) chopped illumination at the CuBi 2 O 4 photocathode.As shown in this figure, the sample shows around twice photocurrent densities under back illumination, due to a slow hole transport in the film.Figure S5, Supporting Information, shows a detailed scheme of the charge generation and extraction processes under back and front illumination.Under back illumination, the photocarriers are generated close to the FTO/CuBi 2 O 4 interface, and then, the photogenerated holes need to travel a shorter distance than the photogenerated electrons, being the limiting factor in terms of transport. [30]Thus, from now on, all the photoelectrochemical results shown in the present manuscript were measured under back illumination.Figure 3C shows a cyclic voltammetry (including three cycles) of the CuBi 2 O 4 sample under dark and illumination conditions.No differences are observed among the consecutive cycles, ensuring the photoelectrochemical (PEC) stability of the fabricated photocathode under these conditions.Additionally, no hysteresis was observed, discarding the charge accumulation of the material along the J-V curve.The dark current was detected at around À0.1 V versus Ag/AgCl.In addition, Raman spectroscopy was also employed as an indirect measurement of the stability of the CuBi 2 O 4 photocathodes (Figure S6, Supporting Information), showing the appearance of the same vibrational modes before and after photoelectrochemical measurements and hence, demonstrating a certain stability of the synthesized photocathodes under working conditions.
Aiming to further investigate the maximum photoelectrochemical response of the fabricated CuBi 2 O 4 films, the activity of the materials was analyzed in the presence of H 2 O 2 (adding 1 mL to the electrochemical cell) as electron scavengers, favoring the reduction of this specie into H 2 compared with the water present in the 0.1 M KPi buffer. [31]In this direction, Figure 3D shows the comparison between the photoelectrochemical response of the CuBi 2 O 4 in 0.1 M KPi and 0.1 showing around 3 times of higher performance in the latter case and demonstrating the maximum potential of the electrodeposited CuBi 2 O 4 films as photocathodes.
With the intention of improving the PEC performance of the fabricated CuBi 2 O 4 photocathodes, the deposition of an Ag cocatalyst was performed.For this purpose, a metallic silver target was evaporated over the photocathodes for 30 s (check Experimental Section for more details).
Figure S7, Supporting Information, shows the XRD pattern acquired in an Ag-decorated CuBi 2 O 4 photocathode.The main diffraction peaks associated with the tetragonal P4/ncc CuBi 2 O 4 as well as two peaks associated with the rutile FTO substrate were detected.Moreover, one additional peak associated with Ag 2 O (32.8°; (111) plane) was also detected, confirming the successful deposition of the catalyst with its concomitant partial oxidation in air.As can be observed in the EDS spectrum, the presence of silver was confirmed in a concentration ≈1.8 at%.Additionally, the EDS mapping shows the presence of Ag islands homogeneously distributed along the whole analyzed surface.Consequently, the PEC performance of the Ag-decorated samples was evaluated, showing increased catalytic properties.Figure 4C shows the PEC performance of the bare and the Ag-decorated CuBi 2 O 4 samples under 1 Sun illumination in 0.1 M KPi buffer.As observed, the sample containing Ag as cocatalyst shows higher photocurrent densities along the whole cathodic potential window; even in both cases a high surface recombination was detected, as can be clearly observed in the spikes under chopped illumination conditions. [32]The presence of surface recombination in the Ag-decorated photocathode is attributed to the islandlike aspect of the Ag-based nanocorals, leaving exposed areas of CuBi 2 O 4 to the electrolyte.On the one hand, Figure 4D shows the performance of the Ag-CuBi 2 O 4 photocathode comparing 0.1 M KPi buffer and 0.1 M KPi þ H 2 O 2 electrolyte, to analyze the maximum performance of this sample if no limiting factors as diffusion limitations were present. [33]A nearly 7-fold increase in photocurrent density was observed in the presence of H 2 O 2 as electron scavenger (Figure 4D), showing that the Ag-CuBi 2 O 4 photocathode is a promising candidate for photoelectroreductive reactions, as HER, CO 2 Reduction Reaction (CO 2 RR), or N 2 fixation, among others.In addition, Figure S10, Supporting Information, shows a chronoamperometry test performed at 0.5 V versus reference hydrogen electrode (RHE) (point of maximum photoactivity before reaching the dark current regime), showing a reasonable PEC stability up to 30 min.
To gain further insights in the electrical properties of the fabricated photocathodes, EIS measurements were carried out. [34]igure 5A shows the obtained Nyquist, under dark and 1 Sun conditions, plots acquired at 0.45 V versus RHE.As observed, a single arc was observed in the bare CuBi 2 O 4 samples, while two arcs were observed in the Ag-decorated photocathodes. Figure S11, Supporting Information, shows the measured Bode plot at 0.45 V versus RHE, where the bare CuBi 2 O 4 shows only one plateau in the capacitance, while two plateaus can be observed in the Ag-decorated CuBi 2 O 4 sample.Due to these reasons, a Randles' circuit was employed to fit the EIS raw data of the CuBi 2 O 4 , [35] while a circuit contacting two R-C couples in parallel (inset Figure 5B) was employed to fit the data of the Ag-decorated samples due to the porous nature of this photocathode, considering the possible CuBi 2 O 4 /electrolyte and Ag/ electrolyte interfaces (Figure 5A-C), as previously reported by Klahr et al. in Fe 2 O 3 /Co-Pi photoanodes. [36]On the other hand, Figure 5B shows the series resistances, R s , accounting for the wiring and the FTO substrate, showing a flat behavior with the applied potential with values between 40 and 70 Ω cm 2 .The calculated charge transfer resistances, R ct , of the analyzed samples versus the applied potential are plotted in Figure 5C.Here, two different groups of resistances can be observed: first, the resistance associated with the CuBi 2 O 4 /electrolyte interface present in both samples and second, the resistance associated with the Ag cocatalyst.R ct associated with the CuBi 2 O 4 active layer shows a decrease in with the illumination and with the application of the external bias, as expected in a photoelectrode, as a consequence of a higher extraction of the photogenerated electrons as these potentials. [37,38]It is also noticeable that the R ct of the Ag-decorated sample shows lower values, in the dark and under illumination conditions, than the R ct of the bare CuBi 2 O 4 , in good agreement with the higher photocurrent densities observed in the former one (Figure 4D) and with very recent studies in Ag-modified CuBi 2 O 4 photocathodes for CO 2 reduction reaction. [39]On the other hand, the observed R ct associated with the Ag/electrolyte interface shows a flat behavior with the applied potential and no critical changes under illumination, due to the catalytic activity of Ag even in the dark. [40,41]In the extracted capacitances associated with the CuBi 2 O 4 /electrolyte interface, a decrease in the capacitive values was observed at higher applied potentials (0.4 < V < 0.8) under illumination, [42] which could be attributed to a lower accumulation of the photogenerated electrons at the interface, especially noticeable in the Ag-decorated sample due to an enhanced charge extraction under these favorable conditions.Moreover, the extracted Ag/electrolyte capacitance is one order of magnitude lower than the CuBi 2 O 4 /electrolyte one along the whole potential window, due to a fast charge extraction at this interface, as expected in a cocatalyst. [43]o further understand the role of the Ag nanocorals on the charge separation and transfer processes on CuBi 2 O 4 photocathodes, UV-visible spectroscopy, steady-state and time-resolved photoluminescence (PL and TRPL, respectively), and TAS experiments were performed.
After laser excitation at 355 nm, Ag-CuBi 2 O 4 showed the formation of a broad band covering the range 400-550 nm attributed to the surface plasmon resonance of Ag (Figure S12, Supporting Information). [44]This behavior is clearly noticeable by darkening of the sample due to the formation of Ag particles (Figure S12, Supporting Information inset). [45][48] In contrast, the intensity of the PL emission for Ag-CuBi clearly decreased, accompanied by an increase in its lifetime (1.22 and 9.77 ns, with τ PL average of 3 ns), implying an increase in the electron-hole (e À /h þ ) time-life (Figure 6A,B green traces). [46]This depletion of the photoluminescent response (radiative recombination pathway) with the employment of cocatalysts or heterojunctions has already been reported in other photoelectrocatalytic systems. [49]In our case, the inhibition of e À /h þ recombination could be directly ascribed to the influence of Ag particles to trap charge, as it has been previously reported for HER, [50,51] enhancing the charge extraction and the catalytic activity, in good correlation with the photocurrent measurements and the EIS results (Figure 4 and 5).
On the other hand, TA spectrum for CuBi 2 O 4 exhibited a positive broad band from 400 nm (Figure 6C).This TA exhibited a biexponential lifetime of 150 ns and a second contribution in the μs scale (Figure 6D with detection at 600 nm).The reported fs-TAS studies on CuBi 2 O 4 have reported the generation of a TA broad spectrum from 400 nm, exhibiting two small peaks located at ≈550 and ≈650 nm, which decrease over time combining into one broad peak.These peaks were attributed to the ligand-to-metal charge transition between O-to-Bi and O-to-Cu, respectively. [27,52]In our case, the broad signal observed in nanosecond scale should contain these two contributions but it was not possible to clearly distinguish as two small peaks.
Interestingly, the Ag-decorated samples exhibited a negative band at ≈430 nm, attributed to the ground-state depletion of Ag nanoparticles (Figure S12, Supporting Information) and a markedly increase of threefold of ΔOD absorption from 500 nm during the first 10 μs after pulse (Figure 6C,D, green traces and Figure S13, Supporting Information).Additionally, the remarkable enhancement of absorption resulted in a large transient lifetime measured up to 700 μs after pulse (Figure S14, Supporting Information), indicative of the accumulation of long life (e À /h þ ) in the semiconductor upon electron transfer to Ag.We have previously reported this increased lifetime of the photogenerated electrons in both Au and Ag-decorating TiO 2 systems for CO 2 photoreduction due to the charge trapping ability of these metals, leading to a reduced surface recombination. [53,54]It is also noteworthy that after the laser pulses (by means of TRPL as well as TAS), a darkening of the sample due to Ag photochromism after the charge trapping is clearly noticeable.
This result, combined with those obtained from TRPL and EIS, confirms an improved charge separation, a slow-down of the e-/hþ pair recombination process, and therefore, a higher density of photogenerated electrons in the Ag-CuBi 2 O 4 system.In this manner, this improved system is able to engage in the photoelectrocatalytic production of solar fuels, demonstrating the function of Ag as a genuine catalyst rather than a passivator of recombination center. [18,46,55]Additionally, these results are in good agreement with a recent study from G. Liu et al.where the authors report an enhanced carrier dynamics with an efficient inhibition of carrier recombination in CuBi 2 O 4 photocathodes through its modification with Ag plasmonic nanoparticles for CO 2 reduction. [39]Scheme 1 represents the photogeneration of electron-hole pairs and the possible suggested pathways for charge extraction: 1) the photogenerated holes are extracted to the back contact, 2) the majority of the photogenerated electrons at the semiconductor are transferred to the Ag cocatalysts and then extracted to the electrolyte to drive the desired photoelectrochemical reduction reaction, and 3) a small portion of this photogenerated holes can be directly transferred to the solution and drive also the reaction.

Conclusion
This article reports the successful evaporation of Ag nanocorals on electrodeposited CuBi 2 O 4 photocathodes with applications toward photoelectrochemical reduction reactions.The synthesized p-type CuBi 2 O 4 has been extensively characterized with a wide range of advanced techniques, confirming its tetragonal structure and composition.The deposition of Ag nanocorals over CuBi 2 O 4 photocathodes has demonstrated its boosted photoelectrocatalytic performance toward the HER compared with bare one.From EIS measurements, a decrease in the charge transfer resistance together with a low accumulation of the photogenerated electrons was observed in the Ag-decorated photocathodes.On the other hand, a decrease in the PL indicates a low radiative recombination when the Ag cocatalyst was present.Additionally, a higher density of the photogenerated electrons and an extended lifetime of those carriers were detected by TAS on the Ag-CuBi 2 O 4 sample.The combination of these mechanistic studies confirms that Ag acts as an effective cocatalyst reducing charge recombination and improving charge generation and extraction and thus, improving the overall kinetics toward the HER.

Figure 1 .
Figure 1.A) Cross-section SEM image of the photocathode device showing the different layers: glass substrate, FTO electrical contact, and CuBi 2 O 4 absorbing layer.B-E) EDS mapping results acquired in the CuBi 2 O 4 sample.F) XRD pattern of the CuBi 2 O 4 photocathode.G) Raman spectrum of the sample.The inset shows an AFM image acquired at the surface.

Figure 2 .
Figure 2. A) Normalized XANES spectra at Cu K-edge of samples and references.B) Fourier-transform moduli of the Cu K-edge EXAFS of data (sphere) and fit (black line) in the phase-shift-uncorrected scale.C) Normalized XANES spectrum at Bi L 3 -edge of CuBi 2 O 4 .D) Fourier-transform modulus of EXAFS data (sphere) and fit (black line) in the phase-shift-uncorrected scale.

Figure 3 .
Figure 3. A) OCP under light and in the dark.J-V response under 1 Sun B) chopped and C) constant illumination in 0.1 M KPi buffer.The inset in (B) shows the photogenerated carriers explaining the higher performance under back illumination.D) J-V response in the presence of H 2 O 2 as electron scavenger under 1 Sun chopped illumination.
Figure 4A,B shows the aspect of the Ag-decorated CuBi 2 O 4 photocathodes, showing a nanocoral-like aspect (due to the

Figure 4 .
Figure 4. A,B) SEM images of the Ag-decorated CuBi 2 O 4 photocathode.J-V response under 1 Sun chopped illumination of the CuBi 2 O 4 and Ag-decorated CuBi 2 O 4 samples under C) 0.1 M KPi electrolyte and D) 0.1 M KPi þ H 2 O 2 electrolyte.
presence of the Ag/Ag 2 O domains), typical from evaporated metal grain sizes in the range of few tens of nm.Other FESEM images of the morphology of the evaporated cocatalyst are shown in FigureS8A-D, Supporting Information, where the formation of continuous Ag/Ag 2 O domains of few tens of nm is observed.FigureS9, Supporting Information, shows the EDS measurements acquired in an Ag-decorated CuBi 2 O 4 .

Figure 5 .
Figure 5. EIS results.A) Nyquist plots acquired at 0.45 V versus RHE.Extracted parameters from the fitting of the EIS raw data along the applied potential window.B) Series resistance (R s ), C) charge transfer resistance (R ct ), and D) capacitances, (C).

Table 1 .
Results of the EXAFS fit.N is the coordination number of the Cu-O or bi-o shell, R is the bond distance and σ 2 is the Debye-Waller factor.