Solution-Processed Nb-Substituted BaBiO3 Double Perovskite Thin

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Solution-processed Nb-substituted BaBiO3 Double Perovskite Thin Films for Photoelectrochemical Water Reduction Jie Ge, Wan-Jian Yin, and Yanfa Yan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04880 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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Chemistry of Materials

Solution–processed Nb–substituted BaBiO3 Double Perovskite Thin Films for Photoelectrochemical Water Reduction †‡*

Jie Ge,

§

Wan-Jian Yin, and Yanfa Yan

†*

†

Department of Physics and Astronomy & Wright Center for Photovoltaics Innovation and Commercialization, the University of Toledo, Toledo, Ohio 43606, United States. ‡

SNU Materials Division for Educating Creative Global Leaders, Seoul National University, Seoul 08826, Republic of Korea.

§

Soochow Institute for Energy and Materials InnovationS (SIEMIS), College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215006, People’s Republic of China. * Dr Jie Ge, [email protected]; Prof Yanfa Yan, [email protected].

Abstract Photoelectrochemical (PEC) water reduction is a long–term strategical technology for hydrogen production. In this work, we synthesize a series of compact and nano/meso–porous Nb–substituted BaBiO3 [i.e. Ba2Bi(Bi1–xNbx)O6, 0 ≤ x ≤ 0.93, BBNO] thin films using cost–effective chemical solution methods. The synthesized BBNO alloy based thin films demonstrate tuneable bandgaps from 1.41 eV (x = 0) to 1.89 eV (x = 0.93) to efficiently absorb the solar spectrum and p–type conductivities suitable for hydrogen production. The photoelectrodes with a configuration fluorine−doped –2 SnO2 / BBNO (0 ≤ x ≤ 0.93) / Pt produce cathodic photocurrents of 0.05~1 mA·cm at 0 VRHE (volt versus reversible hydrogen electrode) measured –2 in a neutral (pH=7.2) phosphate buffer and under simulated AM 1.5G illumination (100 mW·cm ). The BaBiO3 without Nb alloying based electrode –2 delivers the best photocurrent of 1 mA·cm at 0 VRHE but is subjected to severe corrosions during the PEC related tests. Alloying Nb has an obvious influence on enhancing the material stability against corrosion. With Nb alloying, the screen–printed nanoporous BBNO (x = 0.6, bandgap = 1.62 −2 eV) based photoelectrode generates a better photocurrent of 0.2 mA·cm at 0 VRHE with a highly positive onset at 1.5 VRHE enabling unbiased water reduction.

Introduction Hydrogen is a clean and renewable chemical fuel, which can be used in hydrogen fuel cells to generate power using a chemical reaction, producing only water and heat as byproducts without carbon emissions. Hydrogen production from water using abundant solar energy is a promising technology pathway with low or no greenhouse gas emissions and is expected to become viable. Photoelectrochemical (PEC) water splitting is a process in which hydrogen is produced from water using sunlight and semiconductor materials that can utilize sunlight to directly dissociate water molecules into hydrogen and oxygen. The semiconductor materials used in the efficient PEC process include those used in solar photovoltaic 1-2 3-4 5-6 7-10 electricity generation, eg. p–type tetrahedrally–bonded chalcogenides such as CdTe, Cu(In, Ga)S(e)2, Cu2ZnSnS4, and Cu2BaSnS(e)4. Metal oxides are alternative PEC electrode materials and they can be prepared by low–cost chemical solution method such as spin coating and screen printing for small– and large–scale depositions, respectively. Most of oxides are n–type with conduction band edges not suitable for hydrogen production and have large bandgaps that cannot sufficiently absorb the main part of the solar spectrum. The p–type oxides with suitable bandgaps 11-13 14-16 17-19 20-22 to absorb solar spectrum are limited to some Cu based oxides like CuxO, CuFeO2, CuBi2O4, and CuAlO2. It is tremendously interesting to seek new cost–eﬀective p–type PEC oxides with suitable bandgaps of < 2.1 eV to efficiently absorb visible light, good charge transport properties, 23 and suitable conduction band positions for solar water hydrogen production. Recently, organic–inorganic hybrid lead halide based perovskites have attracted global interest due to its superb performance as solar absorbers. The superior properties of perovskite, including high hole mobility and defect tolerance, have been attributed to the lone–pair s orbital 24 (i.e. occupied s orbital), which can cause a strong antibonding with anion p orbital in high cubic Oh symmetry. However, lead halide based 25 perovskite solar absorbers may not be suitable for water splitting application because they will decompose immediately upon exposure to water. Inspired by the superior performance of s–p coupling in lead halide based perovskite solar absorbers, it is very meaningful to search for alternative 3+ 3+ 2+ 2+ 2+ p–type perovskite based PEC water splitting materials which are made of the elements with lone–pair s orbitals (e.g. Bi , Sb , Pb , Sn , Ge ) and 26 3+ 5+ 27-29 are able to demonstrate the very stability against water. Double perovskite Ba2Bi Bi O6 (i.e. BaBiO3, BBO), a p–type semiconductor, has been 30-32 demonstrated with the photocatalytic activity to generate H2. But its bandgap is still at issues: theoretical calculations predict a ~ 1 (or 1.45) eV 33-35 36 31 30, 32, 37 bandgap, while there is as well no consensus on the experimentally determined bandgap values (e.g. 1.7 eV, 1.66 eV, and 2.05 eV ). 3+ 5+ 3+ 5+ And its antimony and tantalum derivatives Ba2Bi (Bi1–xSbx) O6 (BBAO) and Ba2Bi (Bi1–xTax) O6 (BBTO) have been experimentally confirmed to be 29, 38 p–type as well. Additionally, recent theoretical calculation results of Yin et al. confirm that these compounds ought to be good p–type semiconductors with dominant shallow acceptors of Ba vacancy, Bi vacancy and cation antisites (e.g. Ba on Bi site and Bi on Ta site) and that all the 39 5+ 5+ 5+ 5+ donor levels are deep defects with very high formation energies. In addition to Ta and Sb , Nb has been also used to partially replace Bi in 3+ 5+ 40 BBO, leading to for instance Ba2Bi (Bi0.4Nb0.6) O6 (BBNO). For lattice structure, BBO exhibits a monoclinic space group I2/m symmetry while BBNO/BBAO/BBTO show a trigonal space group 3 symmetry at room temperature, all of which can be derived through tiny distortions and 41 octahedral tiltings from their parent cubic perovskite symmetry 3. For the electronic band structure, these double perovskites demonstrate 3+ 2 similar valence band maximum (VBM) components consisting of the antibonding between Bi 6s lone pair orbitals and O 2p orbitals, and the

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antibonding interaction between the Bi 6s and O 2p states pushes up the VBM energy position, favoring the p–type character with shallow 33, 38 5+ 5+ 5+ acceptors (i.e. mitigating p–type doping limits). Meantime, Sb 4d / Nb 4d / Ta 5d orbitals, contributing to the conduction band minimums (CBMs) of BBAO / BBNO / BBTO, are relatively high in energy position, and as a result, the CBMs of BBAO, BBNO, and BBTO are higher in energy 33, 38 than that of BBO. Thus, the high CBM and VBM will ensure the p–type characters for these derived double perovskites and also ensure more 3+ 2 suitable band positions for hydrogen production. Additionally, theoretical calculations suggest that Bi with occupied 6s orbitals have strong coupling with O 2p orbitals in the Oh symmetry, increasing the extent of the dispersion at VBM and effectively reducing the valence effective mass, 38, 42 finally leading to good hole transport properties. Thus, these compounds may be suitable for use as PEC absorber materials for hydrogen production. Recently, Weng et al. reported a method to prepare BBNO films via post annealing spin–coated precursor films, which uses a mixed solvent of acetic acid and ethylene glycol (1:1, vol. ratio) to make precursor solutions and thermal annealing of the precursor films in air at 550 °C –1 33 for 1 h with a ramping rate of 10 °C min . The high viscosity constant and a high boiling temperature (~200 °C) of the solvent may potentially have trouble to prepare uniform precursor films by spin–coating method. Additionally, the thermal annealing with a slow ramping rate of 10 °C –1 min may cause the element losses of Nb and Bi, which could lead to the formation of secondary phases and compositional gradient as reported 33 33 by Weng et al. It is noted that the BBNO films prepared by Weng et al. are n–type semiconductors, possibly due to the presence of numerous 43 hole–killer defects such as oxygen vacancies, which are more preferred for oxygen evolution reactions. In this work, we develop a new chemical recipe to prepare BBNO alloy films, which is based on the mixed solvents of acetic acid (HAc) and 2–methoxyethanol (2–MOE) (1:1 vol. ratio). The 2–MOE solvent has a very low viscosity constant and a low boiling temperature (~120 °C), which enables the synthesis of smooth, uniform, and pin–hole free BBNO alloy films using spin coating. This chemical solution can be further aged into a slabby slurry or paste after heating at low temperatures (< 100 °C), which can therefore be used for the cost–effective large–scale screen–printing of BBNO nanoporous samples. Additionally, we use rapid thermal annealing to crystallize the spin–coated/screen–printed precursor films (ramping –1 rate, ~100 °C min ), which effectively minimizes the element losses. With an oxygen gas flow introduced into the furnace chamber during the thermal annealing to suppress the formation of hole–killer defects, such as oxygen vacancies, the rapid thermal annealing process allows us to prepare nearly single–phase BBNO films with a p–type conductivity, holding promise for use as PEC photocathodes for solar water hydrogen production.

Results and discussion Material preparation: The procedure of making the precursor solutions is illustrated in Scheme 1. First, the functional solvents of HAc, 2–MOE, and 2,4–pentanedione are premixed under agitation, wherein the HAc dissolves barium acetate salts, the 2–MOE assists to dissolve bismuth acetate salts, and the 2,4– 44 pentanedione additive (1 mL) reduces the sensitivity of precursor solution to hydrolysis in the air. Then, we add barium acetate salts into the premixed solvents. The full dissolution of the barium acetate salt needs moderate heating and stirring for about one hour. Bismuth acetate salt was then added after the barium salt is completely dissolved, which can rapidly dissolve into this hot barium–containing solution within 5~10 min. Then, the solution is cooled down to room temperature and is transferred into a nitrogen–filled glove box. In the glove box, the liquid niobium ethoxide chemical (extremely moisture sensitive) is then added into the solution using a pipette. After adding the niobium ethoxide, the solution is stirred for about one hour in the glove box. Finally, the obtained clear solution is ready for spin–coating use in the air. Additionally, white slabby slurry / paste for large–scale screen printing of the BBNO alloy films is obtained by aging the clear spin–coating solution at 95 °C for overnight in the air. In this work we prepare three types of chemical solutions with the metal salt concentrations expected for BBO, Ba2BiNbO6 (BBNO–1), and Ba2Bi(Bi1– xNbx)O6, (0.4 < x < 0.6, BBNO–2), as shown in Scheme 1 and Table 1. The crystallization of the precursor films prepared by spin coating and screen coating in air (Scheme 2a and 2b) was carried out using a rapid –1 thermal annealing system (ramping rate ~100 °C min ). As the heating profile shown in Scheme 2c, the first temperature dwell at 250 °C for 10 min 44 is introduced to ensure the pyrolysis (or decomposition) of the metal–organic complex precursor. Afterwards, the temperature rapidly increases to 600 °C to promote the crystallization of the desired BBNO–related phases. The entire annealing process was carried out in the oxygen environment. For the spin–coated BBNO films, we repeated the spin–coating of precursors and thermal annealing for four times in order to obtain thick enough film layers. The final annealing temperature of 600 °C was determined by an optimization study, where we annealed spin–coated BBNO–2 films at different target temperatures varying from 400 °C to 600 °C. We find that BBNO phase starts to form at 550 °C where the film starts to become black; while, the annealing at temperatures below 550 °C produces no BBNO phases, leading to transparent films (Figure S1). X– ray diffraction (XRD) measurements indicate that the film annealed at 600 °C exhibits the best crystalline feature (Figure S1a). We, therefore, chose 600 °C as the target annealing temperature for all the samples studied here. The spin–coated BBNO–1 and BBO films exhibit a brown color after thermal annealing; while, the spin–coated BBNO–2 films exhibit a black color with a smooth and uniform appearance after annealing (Figure S1b and Figure S2). Material characterization: Panels (a–c) of Figure 1 show that the surface morphological and cross–sectional scanning electron microscope (SEM) images of spin–coated BBNO–1 film on FTO substrate. As seen, this film demonstrates the large–scale uniformity in morphology and grain size (~50 nm), and no significant secondary phases can be observed. The compositional analyses using the energy dispersive X–ray (EDX) spectroscopy (Table 1 and Figure S2) suggests that the spin–coated BBNO–1 film is deficient in Nb content with an atomic ratio of Nb/Bi = 0.87. The screen–printed BBNO–1 film on FTO


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substrate exhibits a nanoporous crystalline feature, but with numerous overgrown large grains (100–400 nm) segregating atop the film such as the circled area on Figure 1d. The EDX analyses suggest that these secondary phases are rich in Nb (Nb/Bi=1.26, Figure S2c), compared to the nanocrystalline BBNO grains (Nb/Bi=0.79, Figure S2b). Besides, these two spin–coated and screen–printed BBNO–1 films also show slightly Ba poor contents with the atomic ratios of Ba/(Nb+Bi)≈0.86–0.83 less than 1. Figure 2 shows the surface morphological and cross–sectional SEM images of the spin–coated and screen–printed BBNO–2 films on FTO substrates. As seen, the spin–coated BBNO–2 film is pinhole–free and compact without any secondary phases, with uniform grain sizes of ~50 nm, while the film prepared by screen printing is nanoporous and also secondary phase free, with grain sizes less than 40 nm. The composition results in Table 1 suggest that the Nb/Bi ratios in these BBNO–2 films are 0.43 and 0.53, respectively. Notably, all the Nb/Bi ratios in these synthesized BBNO–1 and BBNO–2 films are less than the ratios of Nb and Bi metal salt concentrations in their precursor solutions (Table 1), which suggests that potential element losses have occurred during the thermal processing. The BBNO–2 films are also slightly richer in Ba because of their Ba/(Nb+Bi) ratios of 1.03/1.04 greater than 1. The spin–coated compact BBO film exhibits distinctly larger grains (~200 nm) as compared to the spin–coated BBNO films, but with numerous micro–voids atop the film (Figure 3a and 3b). As seen from the cross–sectional SEM image of Figure 3c, these observed surface voids do not penetrate through the entire film thickness, while leading to a mesoporous microstructure. Besides, the BBO film shows a Ba rich composition with Ba/Bi=1.09 greater than 1, implying the formation of Bi vacancy. Figure 4a shows the X–ray diffraction (XRD) patterns of these synthesized BBNO alloy films with the reflections from standard phases of BaBiO3 (PDF 97–017–2757) and Ba2Bi1.4Nb0.6O6 (PDF 97–018–5580) being given in the figure top panel for comparison. As seen, all the films demonstrate multiple diffraction peaks which can be well assigned to BBO or BBNO phases. Notably, the BBNO–1 sample grown by screen printing additionally exhibits spurious diffraction peaks at 2θ≈28.8°, 30.4°, and 42.4° labelled by “#”, which may be assigned to (004), (103), and (105) reflections of BaNb7O9 (PDF 00–079–1353) respectively. This is consistent with the SEM observation in Figure 1d that large Nb rich secondary phases segregate atop the film surface. Besides, the BBNO–1 sample grown by the spin coating method also shows trace secondary phases, evidenced by the observation of two tiny XRD peaks at 2θ≈30.4° and 44.4° (labelled by “?”) which cannot be assigned to any known phases for sure due to lack of substantial diffraction intensity and peak number. The BBNO phase remains the dominant component in this film because the XRD peaks from this unknown secondary phase are very weak. Figure 4b presents the magnified XRD patterns at the range from 26° to 31° in order to highlight the dominant XRD peaks from BBNO and BBO phases. As shown, these synthesized films demonstrate very broad XRD peaks with full widths at half maxima of 0.3°~0.37°, and as a result, the two most intense XRD peaks, i.e. (–120) and (–114), for BBNO are not distinguishable. The average grain size can be then estimated to be 40~70 nm for BBNO and to be ~190 nm for pure BBO using the software of Jade 2010 based on the Debye– Scherrer formula, in line with those from the SEM images. We additionally find that the dominant peak summits shift to high angles with the increases of the composition Nb/Bi ratio, meaning that the lattice constant of BBNO alloy reduces as the Nb/Bi atomic ratio increases. This observation is in sharp contrast to that in Weng’s publication, where there is no XRD peak position shift observed between his BBNO and BBO 33 5+ 5+ 40 samples. The Shannon ionic radii for Bi and Nb are 0.76 Å and 0.64 Å, respectively. Thus, BBNO phases ought to have smaller lattice constants than BBO. This observed peak shift to higher diffraction angle confirms that BBNO exhibits smaller lattice size than BBO, suggesting that Nb has indeed incorporated into the BBO lattice and form the expected BBNO phases. Besides, we also place the dominant XRD peaks of Ba2BiTaO6 (PDF 97–015–4149) and Ba2BiSbO6 (PDF 97–016–2454) in the panels of Figure 4b for comparison. As seen, Ba2BiTaO6 and Ba2BiSbO6 as well exhibit 5+ 5+ 5+ smaller lattice constants than BBO, as the Shannon ionic radii of Ta (0.64 Å) and Sb (0.60 Å) are smaller than that of Bi (0.76 Å). Ba2BiNbO6 is 5+ 5+ anticipated with the identical lattice constants as Ba2BiTaO6 because Nb and Ta have the same Shannon ionic radius (0.64 Å). Among these synthesized BBNO alloy films, the spin–coated BBNO–1 film exhibits the smallest lattice constant, with the dominant XRD peak located at 29.300° which is at the higher angle position than the dominant (–114) reflection (29.351°) of Ba2Bi1.4Nb0.6O6 (PDF 97–018–5580) but at lower angle position than the dominant (–101) reflection (29.367°) of Ba2BiTaO6 (PDF 97–015–4149). This suggests that the Nb/Bi atomic ratio in this spin– coated BBNO–1 film is higher than that of Ba2Bi1.4Nb0.6O6 (PDF 97–018–5580, Nb/Bi=0.43) but still lower than Nb/Bi = 1 of Ba2BiTaO6, consistent with the EDX compositional result of Nb/Bi=0.87 in Table 1. Figure 5 shows the Raman scattering spectra of the synthesized single–phase BBNO alloy films on FTO substrates. Monoclinic BBO (space group I2/m) can totally generate 12 Raman active modes ( = 7 ⊕ 5 ), wherein the breathing mode of BiO6 octahedra of Ag symmetry (i.e. –1 45-46 –1 564.56 cm peak in Figure 5d) has the largest scattering intensity. The second most intense peak at 310.95 cm observed in Figure 5d can be –1 assigned to the asymmetrical breathing mode of BiO6 octahedra. The low frequency peak at 156.28 cm may be assigned to the Ba translation 46-47 45-46 mode. Trigonal BBNO (space group 3) can produce 8 Raman active modes ( = 4 ⊕ 4 ). Notably, the BBNO films demonstrate –1 similar Raman spectra with its parent BBO film, where the new peaks at higher frequencies of 630~639 cm (Figure 5 a–c) may possibly correspond 48 to the symmetric stretching mode of NbO6 octahedra in BBNO lattice. We then acquired the optical transmission spectra on these synthesized BBNO alloy films on FTO substrates. As shown in Figure 6a, the low transmittance at the IR spectral region (wavelength > 1500 nm) suggests that FTO substrates are suitably conductive even after high temperature annealing. Apparent transmittance drops at the spectral region from 500 nm to 1000 nm indicates the dominant optical absorption edges of the 4 –1 films. The derived optical absorption coefficients shown in Figure 6b indicate that BBNO alloys exhibit high absorption coefficients α > 10 cm at 5 –1 the visible–ultraviolet light region, wherein the pure BBO sample can deliver the highest absorption coefficient of > 10 cm . Recent theoretical calculation results suggest that BBNO alloys when 0 ≤ x < 1 have their lowest conduction bands primarily made of a lower–energy–lying unoccupied 5+ 5+ 5+ 6s orbital of Bi ; alloying Nb on the Bi sites in the BBO matrix will therefore dilute the joint density of states of the conduction bands, causing an obvious reduce of light absorption coefficient. Notably, the synthesized films do not have very sharp or straight absorption edges, indicating the prevalence of band tailing states in the films. To characterize the degree of band tailing, we use the Urbach band–tailing model of =

exp

to fit the absorption coefficient curve at the vicinity of the band gap value, where EU is the Urbach characteristic energy and Eg is



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band gap value estimated from the plots of Figure 6c. Our synthesized films show very large fitted Urbach energy Eu values of 0.33–0.52 eV 49 compared to the chalcogenides, suggesting the presence of large degrees of band–tailing states that may cause severe non–radiative recombination of the photo–generated carriers in bulk. The calculated band electronic structures suggest that BBNO when 0 ≤ x < 1 are nearly– 33-34 direct bandgap semiconductors. The bandgap plots using the linear part of a direct transition relation, along with the bandgap values 5+ increasing from 1.41 eV (x = 0) to 1.89 eV (x = 0.93) with the Nb/Bi ratio due to the reduced contribution from Bi 6s orbitals, are given in Figure 6c, wherein the estimated bandgap values are in line with the absorption onsets seen from the transmission spectra in Figure 6a. We additionally tried with an indirect relation to determine the bandgaps of our synthesized films (Figure S4); however, none of them can exhibit good linear regions for the bandgap fit, and the estimated values are ~0.5 eV smaller than those using the direct relation, very inconsistent with the absorption onsets observed from the transmission spectra. As seen in Figure 6d, the alloy bandgaps (y = Eg) increase with the composition x but not linearly, which instead tends to follow a parabola relation: Eg[Ba2Bi(Bi1–xNbx)O6] = (1–x)Eg[Ba2BiBiO6] + xEg[Ba2BiNbO6]+bx(1–x), where b is the bowing coefficient. Photoelectrochemical (PEC) Test: Prior to the PEC tests, it is important to know the conductivity type and flat band potential (EFB) of our synthesized BBNO alloy films. Mott–Schottky (MS) plot and illuminated open circuit potential (OCP) in a three–electrode electrochemical cell are two straightforward techniques to determine the conductivity and EFB. First, the bare BBNO alloy films were characterized by dark capacitance–voltage (CV) profiles measured in a neutral phosphate buffer solution (pH = 7.2). The p−type character is conﬁrmed by the nega_ve slopes of the measured Mo`–Schottky (MS) plots of CV profiles with selected test frequencies shown in Figure 7, because the negative slope indicates the negative doping density (i.e. electrons) at the space charge regions. The 13 15 –3 linear part of Mott–Schottky (MS) plot can yield flat band potentials EFB ≈ +0.17~+0.37 VRHE and carrier densities Na ≈ 10 ~10 cm of BBNO alloy films based on the expression of

!

"!

=

# $%& %' ()

* − ,- −

./ $

2

, where A=0.636 cm is the area of BBNO (BBO) electrode, e is element electron 50

charge, 01 is the vacuum dielectric constant, 02 ≈ 100 (≈ 31) is the relative dielectric constant of BBNO (BBO) alloys, and kT = 0.0259 eV at room temperature. The Fermi levels of BBNO films, EF ≈ 0.19~0.33 eV above the VBMs, can be estimated based on the expression 3- = −, + 5678 ∗ #= > ./ A ! @!

where the effective density of states in the valence band ;3 = 2

≈ 8 × 10GH IJ with hole effective mass @∗ = 0.451.

39, 51

(9

(:

,

With the

assumption of , = ,- , we further obtain the VBMs EVB ≈ +0.43~0.68 volts versus reversible hydrogen electrode (VRHE), and the CBMs are + – 0.78~1.36 VRHE above the hydrogen evolution level (2H + 2e →H2). Our MS tests suggest that all the synthesized BBNO alloy films exhibit conduction band positions and p–type conductivities well suited for hydrogen production. We additionally find that the increase of the film Nb/Bi content can reduce the carrier density and the conductivity, consistent with the trend found in the case of BBSO and BBTO alloys; however, some of the BBNO electrode films are too insulating that the MS measurements are not successful to determine the conductivity type and the carrier density [Panels (a~d) of Figure S6]. Notably, our MS plots show frequency dependence: 1. some process different slopes but converge to an identical intercept (see Figure 7a for spin–coated BBNO–1 sample and Figure 7c for spin–coated BBNO–2 sample); 2. some show the same slope but yield a different EFB for each frequency (see Figure 7b for screen–printed BBNO–1 sample); 3. The rest electrodes (see Figure 7d for screen– printed BBNO–2 sample and Figure 7e for spin–coated BBO samples) show moderately different slopes and intercepts for each frequency. We do observe the film color change more or less before and after MS tests (eg. spin–coated BBNO–2 thin film based electrode shown in Figure S7a) for these electrodes studied, in particular for the spin–coated BBO electrode which looked thinner and more transparent compared to the pristine area (Figure S7b), suggesting the dissolution loss of the film. As a result, sometimes the MS tests failed to show the p–type conductivity characteristic for some BBO electrodes (Figure S6e). We also note that the electrode samples made of high Nb/Bi usually deliver better stability during MS tests than the electrodes with low Nb/Bi. For the spin–coated BBNO–1 electrode with the biggest Nb/Bi, we did not observe any significant film color change after MS tests. But, all the BBNO alloy films instead demonstrate the very stability without any observed film color change when exposure to the neutral phosphate buffer electrolyte for couple hours. Thus, the observed corrosion during the MS test, particularly for BBO electrode, should be due to the applied bias potentials that may probably incur some unknown redox reactions with the electrolyte tested. Further, these corrosions may likely lead to the inaccuracy for the estimated Na and/or EFB to some degree. In parallel, the conductivity type of the synthesized BBNO electrodes can also be decided from the direction of the shift in the open circuit potential (OCP) upon illumination. For a p–type semiconductor, illuminating its electrode surface will shift the Fermi level of the bulk (i.e. measured potential) more anodic. Thus, p–type electrode materials will show a positive shift in OCP upon illumination; instead, n–type semiconductors will exhibit a 51-52 negative shift in illuminated OCP. Figure 8 shows the measured OCPs of our synthesized BBNO electrodes under chopped illumination with various light intensities, where the positive shift in OCP upon illumination indicates that our synthesized BBNO alloy films show the p–type conductivity. We tested about 80 pieces of BBNO electrodes with various compositions and all the OCP tests confirm that our synthesized BBNO based semiconductors process the p–type conductivity even for the samples for which MS plots fail to determine. ΔOCP is defined as the magnitude of the difference of the OCP in the dark versus under illumination. As seen from Figure 8, our BBNO photoelectrodes except BBO can –2 deliver several tens of millivolts of ΔOCP under AM 1.5G illumination (100 mW·cm ) and ΔOCP reduces under the illuminations with smaller intensities. Likewise, the corrosion of the photoelectrodes may also occur during the OPC measurements for BBO and BBNO–2 samples, because we observe remarkable color change for BBO electrode and very mild changes for BBNO–2 electrodes after a series of OCP tests under the manually chopped illuminations with various light intensities (Figure S7b and S7c). This photocorrosion directly results in slow and fast drifts of the measured potentials over the test course of about 60 s for BBNO–2 (Figure 8c and 8d) and BBO electrode (Figure 8e), respectively; meanwhile, it additionally accounts for the significant overall shifts in chopped OCPs in–between the test circles under different illumination intensities [see 52 panels (c–e) of Figure 8]. All of our BBO electrodes suffer from severe photocorrosions during the illuminated OCP tests (Figure S7b); as a result, some of them did not exhibit very obvious ΔOCP when turn the light on and off (Figure S6f). Besides, BBNO–1 samples do not exhibit significant


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photocorrosion during the illuminated OCP tests, because we observed neither considerable color film changes after OCP tests with various light intensities nor the drifts/shifts of the measured potentials over the entire test courses (Figure 8a and 8b). But, these two wide–bandgap BBNO–1 electrodes demonstrate very slow ΔOCP responses upon turning the light on and off (Figure 8a and 8b), compared to the two low–bandgap BBNO– 2 electrodes (Figure 8c and 8d), indicating that BBNO–2 electrodes with smaller bandgaps and better electrical properties possess better material 52 qualities for PEC applications. In principle, illuminated OCP technique can additionally be used to estimate the EFB as long as the above–bandgap illumination is sufficiently intense to completely flatten the band bending and that the materials do not experience severe Fermi lever pinning at 51 –2 the electrode–electrolyte interface. The reduce of the ΔOCP over illumination intensity suggests that the AM 1.5G with a 100 mW·cm irradiation intensity seems not sufficiently intense to saturate OCP. Thus, the measured illuminated OCPs under this condition may be more cathodic than the true EFB. Likewise, the photocorrosion during the OCP test may additionally cause more inaccuracies for EFB estimation. With the vast MS and illuminated OCP tests of about 80 pieces of BBNO alloy samples, we can fully confirm that all the BBNO alloy samples should 39 demonstrate very strong p–type conductivity, consistent with the theoretical predications of Yin et al, even though the corrosion concurrent with the MS and illuminated OCP tests may blur the estimation of EFB which therefore may cause the inaccuracy to estimate the band position versus hydrogen evolution level to some extent. At least, we can make sure that BBNO alloys should have their conduction band positions with energy higher than the hydrogen evolution level to ensure water reduction reaction owing to their p–type conductivity natures (see our early remark in the introduction part). The selection of the correct electrolyte type (acidic, neutral, or alkaline) is important for efficient PEC water splitting, because it is usually associated with the semiconductor material corrosion when immersed in the solution and the kinetics for driving hydrogen or oxygen evolution reaction. For driving the hydrogen evolution reaction, the kinetics is much higher at acidic solution than in respective neutral or alkaline media; while, the alkaline solution is usually used for the oxygen evolution reaction because of its lower driving kinetics. We also note that the p–type 22 oxide photoelectrodes can corrode when immersed in the acidic media; therefore, most of non–protective p–type Cu based oxides were usually 15-16, 18-21 evaluated their water reduction abilities in either neutral or alkaline media. In order to identify the optimum electrolyte type for our BBNO photoelectrodes, we carried out the linear sweep voltammetry (LSV) measurements in acidic (pH=1.9), neutral (pH=7.2), and alkaline (pH=14) –2 solutions under manually chopped AM 1.5G (100 mW·cm ) illumination using the spin–coated BBNO–2 film as a case study (Figure 9a–c). A fast –1 scan rate of 50 mV·s was used to minimize the film corrosion during the illuminated LSV scans. Besides, an O–ring seal set–up were employed as depicted by Figure 9d, which allows different small regions from the same electrode sample to be tested in the three individual pH electrolytes. Each small region was repeated three times for the LSV scan test to evaluate the stability & reliability of the photocurrent generated. As shown in Figure 9a–c, cathodic photocurrents were produced under chopped illumination, confirming that our BBNO alloy film is a p−type semiconductor; –2 and this electrode can deliver about 0.11~0.13 mA·cm photocurrents at 0 VRHE (volts versus hydrogen reversible electrode) with onset potentials at about 1.5 VRHE for the initial LSV scans, without any significant differences in photocurrent or its onset potential when tested in these three pH electrolytes. Notably, we did observe the photocurrent degradation over the test circles, suggesting that film corrosion has occurred during each of nd rd the LSV scan circles: for the 2 and 3 LSV scans in the acidic solution, the dark currents become significantly large when the bias goes cathodic of st 0 VRHE and the corresponding photocurrent onsets as well become more cathodic compared to those of the 1 LSV scan; for the test in the neutral nd rd st electrolyte, both photocurrent and its onset potential become smaller and more cathodic for the 2 and 3 LSV scan compared to those of the 1 LSV scan; for the test in the alkaline electrolyte, the onset potential of the photocurrent did not significantly degrade over the three LSV scan circles while the photocurrent at 0 VRHE did. A digital photo of the tested regions of the electrode after 3 LSV scans shown in Figure S7a indicates that the electrode film suffers from more–or–less corrosions in these three different pH electrolytes. The electrode film was subjected to severe corrosion in the acidic bath because the film region after test looks more transparent compared to those tested in alkaline and neutral baths. Moreover, it appears to exhibit the best stability in the neutral phosphate electrolyte, but it still has the corrosion issue with film color change even after one– time LSV scan (Figure S7c). It should be pointed out again that all our BBNO electrodes could demonstrate suitable stability for several hours when immersed in the neutral phosphate buffer without bias applied or illumination. Thus, the film corrosion observed after PEC and MS tests should be due to the light illumination and the applied bias. For efficient PEC water reduction, efficient extraction of the photo–carriers at the electrode surface is essential. Surface modification using Pt catalysis has been commonly employed to enhance the carrier extraction at the electrode surface. Herein, we platinized the synthesized BBNO electrode surfaces using photo–assisted electrodeposition techniques before the PEC tests. Figure 10 shows the LSV scans under chopped AM –2 1.5G illumination (100 mW·cm ) in the neutral phosphate buffer (pH=7.2) after platinizing the electrode surfaces. Likewise, we carried out the LSV scan for three consecutive circles to evaluate the stability of the photocurrent for each electrode. As shown, the multiple LSV scans confirm that all the BBNO alloy electrodes are indeed subject to corrosion to some extent, leading to the reduce of the saturated cathodic photocurrents and the cathodic shift of the onset potentials of the photocurrents. We also note that our BBO electrodes have the least stability during the LSV scan, as a result, which can only stand one–time LSV scan test from +1.2 VAg/AgCl (volts versus Ag/AgCl reference electrode) to – 0.8 VAg/AgCl (Figure 10e inset). At least two significant redox peaks can be observed from the initial LSV scan curves of the BBO electrodes, one at –0.1~+0.2 VAg/AgCl, another at ~–0.6 VAg/AgCl (Figure 10e), both contributing to the significant increase of the dark current. Nonetheless, the BBO –2 electrode still delivers the best photocurrent of ~ 1 mA·cm at ~0 VRHE compared to the other BBNO electrodes presented in panels (a)–(d) of Figure 10, likely due to its good light absorption and carrier transport properties. We additionally find that minimizing the applied bias range, such as the range from +0.55 VAg/AgCl to –0.35 VAg/AgCl shown in Figure 10e, may moderately alleviate the BBO corrosion during the LSV scan test, rendering the fresh BBO film capable of standing three consecutive LSV scan tests; after LSV tests, numerous large barium phosphate based educts appeared atop the film evidenced by the SEM image and EDX measurements in Figure S8, while the BBO film becomes very deficient in Ba content. For the other BBNO electrodes with Nb alloying, the BBNO–2 electrodes with lower Nb contents can deliver better photocurrents –2 –2 of 0.2 mA·cm (for the screen–printed sample) and 0.16 mA cm (for the spin–coated sample) at 0 VRHE. Comparing the photocurrents shown



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in Figure 9b and Figure 10c, we find that Pt surface catalyzation indeed enhanced the cathodic photocurrents of the electrode, while the onset of the photocurrent remains unchanged at a highly positive potential of about +1.5 VRHE. The highly positive onset potentials may imply that BBNO electrodes do not have significant kinetic overpotential loss for water reduction reaction, which stands out from the p–type Cu based 11, 13, 15-22 oxides. Figure 10f shows chronoamperometric test results of two pristine BBNO–2 electrodes measured at a mildly negative bias of – –2 0.15 VAg/AgCl with the AM 1.5G illumination (100 mW·cm ) on and off. As seen, the electrode can only sustain a relatively stable photocurrent output for about ~2 min under this operation condition; afterwards, the dark current significantly increased. After about 5 min chronoamperometric test, the electrode films became white and looked transparent, and SEM and EDX measurements shows that the remnant films have Ba deficient contents as well as with large Ba phosphate based grains and numerous cracks atop the surfaces (Figure S9 and S10). We additionally tested the compositional variances after three consecutive LSV scans for BBNO–2 electrodes. As shown in Figure S11 and S12, the films exhibit the Ba poor compositions and dissolved regions compared to the pristine surfaces, although Ba phosphate related phases did not appear after the LSV scans. The corrosion issue, primarily in terms of constituent Ba element leaching from the perovskite matrix, can greatly limit the application of BBNO alloy based electrodes for water splitting. But, we believe that applying n–type protective overlayers like 8, 11, 13 CdS/TiO2 onto the BBNO surface can overcome its corrosion drawback and further improve the photocurrent.

Conclusion We have explored the state–of–the–art low–cost Nb substituted BaBiO3 [i.e. Ba2Bi(Bi1–xNbx)O6, 0 ≤ x ≤ 0.93, BBNO] thin films for use as photoelectrode in PEC solar water reduction. A new and cost–effective chemical solution has been formulated in this work, which can work for both spin coating of compact thin–film samples and screen–printing of nanoporous thin–film samples. The synthesized BBNO thin films after the optimized heat annealing show uniform and nano–crystalline grains (~50 nm), while the spin–coated BaBiO3 (BBO) film demonstrate a mesoporous crystalline microstructure with larger grain sizes of ~200 nm. Our characterization results indicated that BBNO (0 < x ≤ 0.93) demonstrates smaller lattice constants and additional lattice vibrational modes compared to its BBO parent. The synthesized alloy films showed tunable bandgaps from 1.89 eV (x = 0.93) to 1.41 eV (x = 0). Our Mott–Schottky and illuminated open circuit potential analyses suggested that BBNO along with its parent BaBiO3 are all p–type semiconductors suitable for water reduction. However, the BBNO films with high Nb contents (x = 0.93 and 0.88) do not exhibit good light absorption or carrier transport properties well appropriate for efficient PEC water splitting. –2

The FTO /BBNO (0 ≤ x ≤ 0.93)/Pt photoelectrodes produced cathodic photocurrents of 0.05~1 mA·cm at 0 VRHE measured in a neutral (pH=7.2) –2 phosphate buffer and under simulated AM 1.5G illumination (100 mW·cm ). Among them, the spin–coated BBO mesoporous electrode delivers –2 the best photocurrent of 1 mA·cm at 0 VRHE but concurrent with severe corrosion during the PEC related tests primarily due to Ba constituent elements leaching from the perovskite matrix. BBNO alloys with Nb show less degrees of the corrosion issue but deliver smaller photocurrents of –2 0.05~0.2 mA·cm at 0 VRHE, wherein the screen–printed BBNO (x = 0.6) nanoporous film with a bandgap of 1.62 eV delivers a better photocurrent –2 of 0.2 mA·cm at 0 VRHE. Promisingly, the BBNO alloy based electrodes can generate their cathodic photocurrents at highly positive onset potentials of ~+1.5 VRHE enabling unbiased water reduction under AM 1.5G one–sun illumination. The corrosion issue may be the dominant factor limiting the photocurrents of our BBNO based photoelectrodes studied. Employing protective overlayers to modify the electrode surface is one legitimate method to overcome this corrosion issue and further improve the photocurrents in the future. Besides, we still need to improve the material quality of BBNO alloys to reduce the bulk recombination loss of photo carriers.

Acknowledgement This paper presents results from an NSF project in the U.S. (award number CBET−1433401) compe__vely−selected under the solicitation “NSF 14−15: NSF/DOE Partnership on Advanced Fron_ers in Renewable Hydrogen Fuel Produc_on via Solar Water Splidng Technologies”, which was co−sponsored by the Na_onal Science Founda_on, Division of Chemical, Bioengineering, Environmental, and Transport Systems (CBET), and the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. Jie Ge at Seoul National University acknowledges Brain Korea 21 Program for Leading Universities & Students (BK 21 PLUS, No. 21A20131912052). Wanjian Yin at Soochow University acknowledges the funding support from National Key R&D Program of China under grant No. 2016YFB0700700, National Natural Science Foundation of China (under Grant No. 51602211, No. 11674237), Natural Science Foundation of Jiangsu Province of China (under Grant No. BK20160299).

Associated contents Supporting Information (SI) available: [characterization method details; additional characterization results including energy dispersive X–ray spectroscopic profiles and X–ray diffraction patterns of BBNO films obtained from different annealing temperatures; additional MS plots of BBNO electrodes; Digital Photos of BBNO based photoelectrodes before and after PEC tests; SEM and EDX compositional analyses of BBNO based electrodes after PEC tests]

Figures and tables


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Scheme 1. Flow chart showing the steps of solution make–ups for spin–coated and screen–printed BBNO [Ba2Bi(Bi1–xNbx)O6, 0 ≤ x < 1] alloy films.

Scheme 2. Sketches showing the preparation processes of spin–coated (a) and screen–printed (b) BBNO [Ba2Bi(Bi1–xNbx)O6, 0 ≤ x < 1] precursors on FTO substrates, as well as the thermal annealing temperature profile and the rapid thermal annealing set–up for the crystallization of BBNO alloy films (c).



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Figure 1. Surface morphological and cross–sectional SEM images of spin–coated and screen–printed BBNO–1 films: (a) low (×25k) and (b) high (×100k) magnification SEM images showing the surface morphology of spin–coated BBNO–1 film, and (c) cross–sectional SEM image of spin–coated BBNO–1 film; (d) low (×30k) and (e) high (×100k) magnification SEM images showing the surface morphology of screen–printed BBNO–1 film, and (f) cross–sectional SEM image of screen–printed BBNO–1 film.

Figure 2. Surface morphological and cross–sectional SEM images of spin–coated (a–c) and screen–printed (d–f) BBNO–2 films: (a) low (×5k) and (b) high (×100k) magnification SEM images showing the surface morphology of spin–coated BBNO–2 film, and (c) cross–sectional SEM image of spin– coated BBNO–2 film; (d) low (×5k) and (e) high (×100k) magnification SEM images showing the surface morphology of screen–printed BBNO–2 film, and (f) cross–sectional SEM image of screen–printed BBNO–2 film.


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Figure 3. Surface morphological and cross–sectional SEM images of spin–coated BBO films: (a) low (×10k) and (b) high (×80k) magnification SEM images showing the film surface morphology, and (c) cross–sectional SEM image.

Table 1. Element atomic ratios of BBNO [Ba2Bi(Bi1–xNbx)O6, 0 ≤ x < 1] alloy thin films grown on FTO substrates determined by energy dispersive X– ray spectroscopy (EDX) and the metal salt concentrations of the precursor solutions.

Sample

BBNO– 1 BBNO– 2 BBO

Deposition method Spin coating Screen printing Spin coating Screen printing Spin coating

Element atomic percentage (%) of the annealed film

Element atomic ratio of the annealed film

Metal salt concentration (mM) of the precursor solution Barium Bismuth Nb Nb/Bi acetate acetate ethoxide 5 2.5 2.5 1.0

Ba

Bi

Nb

Ba/(Nb+Bi)

Nb/Bi

x=

46.31

28.72

24.97

0.86

0.87

0.93

45.22

30.58

24.20

0.83

0.79

0.88

5

2.5

2.5

1.0

50.70

32.17

17.13

1.03

0.53

0.69

5

3.2

1.8

0.56

51.06

34.22

14.72

1.04

0.43

0.60

5

3.4

1.6

0.47

52.18

47.82

0

1.09

0

0

5

5

0

0

Figure 4. X–ray diffraction (XRD) patterns of synthesized BBNO [Ba2Bi(Bi1–xNbx)O6, 0 ≤ x ≤ 0.93] alloy thin films on FTO substrates: (a) a wide range scan (10° ≤ 2θ ≤ 80°) showing the all the observed XRD reflections of the samples and (b) a small angle range (26° ≤ 2θ ≤ 31°) highlighting the peak position shifts of the dominant XRD reflections of the samples with compositional ratio Nb/Bi. Note: XRD peaks labelled by “*” at 2θ = 26.500°, 33.600°, 37.800°, 51.400°, 61.520°, and 65.540° correspond to the (110), (101), (200), (211), (310), and (301) reflections of the FTO substrate (SnO2, PDF 97–026–2768), respectively.



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Figure 5. Raman spectra of BBNO [Ba2Bi(Bi1–xNbx)O6, 0 ≤ x ≤ 0.93] alloy thin films grown on FTO substrates: (a) spin–coated BBNO–1 film; (b) spin– coated BBNO–2 film; (c) screen–printed BBNO–2 film; (d) spin–coated BBO film. Note: spectral profiles were fitted by Lorentzian curves; crystallographic structure of Ba2Bi1.4Nb0.6O6 (PDF 97–018–5580, space group 3) and representative displacement vectors of NbO6 and BiO6 internal modes in the trigonal/monoclinic structure of BBNO/BBO are given in the insets of panels (b), (c), and (d), respectively.


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Figure 6. Optical transmission spectra (a), absorption coefficient (b), direct bandgap plots (c), and compositional dependence of bandgaps Eg (d) of BBNO [Ba2Bi(Bi1–xNbx)O6, 0 ≤ x ≤ 0.93] alloy thin films grown on FTO substrates. Note: the transmittance of intact FTO substrate was included in panel (a) for comparison; the absorption coefficient α in panel (b) was derived from the optical transmittance (T) and film thickness (d) based on the relation α = 1 ln 1 .

d

T



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Figure 7. Mott–Schottky (MS) plots of frequency–dependent capacitance–voltage (CV) profiles of glass/FTO/BBNO alloys based electrodes measured in the dark: (a) spin–coated BBNO–1 film, (b) screen–printed BBNO–1 film, (c) spin–coated BBNO–2 film, (d) screen–printed BBNO–2 film, and (e) spin–coated BBO film. Note: all the measurements were carried out in a pH=7.2 phosphate buffer containing 0.1 M disodium hydrogen phosphate and 0.1 M sodium dihydrogen phosphate; capacitance used for MS plots corresponds to the imaginary part of the measured complex impedance; in figure insets, VRHE = volts versus the reversible hydrogen electrode (RHE), EVB (ECB) = energetic position of the valence (conduction) band, FB = the flat band;


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Figure 8. Open circuit potentials (OCPs) of glass/FTO/BBNO alloys based photoelectrodes in a pH=7.2 phosphate buffer with light off and on and –2 under AM 1.5G (100 mWcm ) illumination without and with various neutral optical filters: (a) spin–coated BBNO–1 film, (b) screen–printed BBNO– 1 film, (c) spin–coated BBNO–2 film, (d) screen–printed BBNO–2 film and (e) spin–coated BBO film. The positive shifts in OCPs upon illumination indicates all the BBNO films possess p–type conductivities. Note: phosphate buffer solution contains 0.1 M disodium hydrogen phosphate and 0.1 M sodium dihydrogen phosphate.



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Figure 9. Linear sweep voltammetry (LSV) scanning curves of glass/FTO/spin–coated BBNO–2 based photoelectrodes under manually chopped AM –2 1.5G illumination (100 mW·cm ): (a) tests in an acidic electrolyte (pH=1.9) containing 20 mL phosphate buffer solution plus 1 mL phosphoric acid (85% wt); (b) tests in a neutral electrolyte (pH=7.2) of 20 mL phosphate buffer; (c) tests in an alkaline electrolyte (pH=14) of 20 mL 1 M KOH 2 solution; (d) side–view of the BBNO photoelectrode in a three–electrode PEC cell with an O–ring seal. Note, illuminated cell area=0.636 cm ; LSV scans were repeated three times to show the stability of BBNO photoelectrodes in the electrolytes with different pH values; phosphate buffer solution contains 0.1 M disodium hydrogen phosphate and 0.1 M sodium dihydrogen phosphate.


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Figure 10. Linear sweep voltammetry (LSV) scanning curves of glass/FTO/BBNO alloys/Pt photoelectrodes under manually chopped AM 1.5G –2 illumination (100 mW·cm ): (a) spin–coated BBNO–1 film, (b) screen–printed BBNO–1 film, (c) spin–coated BBNO–2 film, (d) screen–printed BBNO–2 film , (e) spin–coated BBO film, and (f) chronoamperometric tests of BBNO–2 electrodes measured at –0.15 VAg/AgCl. Note, illuminated 2 cell area=0.636 cm ; all the measurements were carried out in a 20 mL neutral phosphate buffer solution (pH=7.2); prior to the LSV tests, all the electrodes were loaded with Pt catalysis through photo–assisted electrodeposition of a chloroplatinic solution which contains 0.2 mL chloroplatinic acid and 20 mL phosphate buffer; phosphate buffer solution contains 0.1 M disodium hydrogen phosphate and 0.1 M sodium dihydrogen phosphate.



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Solution-Processed Nb-Substituted BaBiO3 Double Perovskite Thin

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