Solutionâprocessed Nbâsubstituted BaBiO3 Double Perovskite Thin

Article Cite This: Chem. Mater. 2018, 30, 1017−1031

pubs.acs.org/cm

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 S Supporting Information *

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/mesoporous 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 tunable 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 fluorinedoped SnO2/BBNO (0 ≤ x ≤ 0.93)/Pt produce cathodic photocurrents of 0.05−1 mA·cm−2 at 0 VRHE (volt versus reversible hydrogen electrode) measured in a neutral (pH = 7.2) phosphate buffer and under simulated AM 1.5G illumination (100 mW· cm−2). The BaBiO3 without Nb alloying based electrode delivers the best photocurrent of 1 mA·cm−2 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 eV) based photoelectrode generates a better photocurrent of 0.2 mA·cm−2 at 0 VRHE with a highly positive onset at 1.5 VRHE enabling unbiased water reduction.

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CuBi2O4,17−19 and CuAlO2.20−22 It is tremendously interesting to seek new cost-effective p-type PEC oxides with suitable bandgaps of 105 cm−1. Recent theoretical calculation results suggest that BBNO alloys when 0 ≤ x < 1 have their lowest conduction bands primarily made of a lowerenergy-lying unoccupied 6s orbital of Bi5+; alloying Nb5+ on the Bi5+ 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

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 (ΓRaman = 7Ag ⊕ 5Eg), wherein the breathing mode of BiO6 octahedra of Ag symmetry (i.e., 564.56 cm−1 peak in Figure 5d) has the largest scattering intensity.45,46 The second most intense peak at 310.95 cm−1 observed in Figure 5d can be assigned to the asymmetrical breathing mode of BiO6 octahedra. The low frequency peak at 156.28 cm−1 may be assigned to the Ba translation mode.46,47 Trigonal BBNO (space group R3̅) can produce 8 Raman active modes (ΓRaman = 4Ag ⊕ 4Eg).45,46 Notably, the BBNO films demonstrate similar Raman spectra with its parent BBO film, where the new peaks at higher frequencies of 630−639 cm−1 (Figure 5 a−c) may possibly correspond to the symmetric stretching mode of NbO6 octahedra in BBNO lattice.48 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 to 1000 nm indicates the dominant optical absorption edges of the films. The derived optical absorption coefficients shown in

E − Eg

( ) to fit the

band-tailing model of α(E) = α(Eg ) exp

EU

absorption coefficient curve at the vicinity of the band gap value, where EU is the Urbach characteristic energy and Eg is 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 compared to the chalcogenides,49 suggesting the presence of large degrees of band-tailing states that may cause severe non-radiative recombination of the photogenerated carriers in bulk. The calculated band electronic structures suggest that BBNO when 0 ≤ x < 1 are nearly−direct bandgap semiconductors.33,34 The bandgap plots using the linear part of a direct transition relation, along with the bandgap values 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 Bi5+ 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 1022

DOI: 10.1021/acs.chemmater.7b04880 Chem. Mater. 2018, 30, 1017−1031

Article

Chemistry of Materials

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 1 1 the relation α = d ln T .

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: E g [Ba 2 Bi(Bi 1−x Nb x )O 6 ] = (1 − x)E g [Ba 2 BiBiO 6 ] + 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 negative slopes of the measured Mott− 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 linear part of Mott−Schottky (MS) plot can yield flat band potentials EFB ≈ +0.17 ∼ +0.37 VRHE and carrier densities Na ≈ ∼1013−1015 cm−3 of BBNO alloy

films based on the expression of

A2 C2

=

2 eεr ε0Na

(V − EFB − kTe ),

where A = 0.636 cm2 is the area of BBNO (BBO) electrode, e is element electron charge, ε0 is the vacuum dielectric constant, εr ≈ 100 (≈ 31) is the relative dielectric constant of BBNO (BBO) alloys,50 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 N EVB = −EF + kT ln NA , where the effective density of states V

(

in the valence band NV = 2

2πmh*kT 2

h

3/2

)

≈ 8 × 1018 cm−3

with hole effective mass m*h = 0.45m0.39,51 With the assumption of EF = EFB, we further obtain the VBMs EVB ≈ +0.43−0.68 V 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 1023


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Figure 7. Mott−Schottky (MS) plots of frequency-dependent capacitance−voltage (CV) profiles of glass/FTO/BBNO alloy 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 the figure insets, VRHE = volts versus the reversible hydrogen electrode (RHE), EVB (ECB) = energetic position of the valence (conduction) band, and FB = the flat band.

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

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 of the 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 (e.g., 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 1024


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Figure 8. Open circuit potentials (OCPs) of glass/FTO/BBNO alloy based photoelectrodes in a pH = 7.2 phosphate buffer with light off and on and under AM 1.5G (100 mW·cm−2) 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 indicate 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.

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 ptype 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 negative shift in illuminated OCP.51,52 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 ptype 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 deliver several tens of millivolts of ΔOCP under AM 1.5G illumination (100 mW·cm−2) 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 1025


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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 1.5G illumination (100 mW·cm−2): (a) tests in an acidic electrolyte (pH = 1.9) containing 20 mL of phosphate buffer solution plus 1 mL of phosphoric acid (85 wt %); (b) tests in a neutral electrolyte (pH = 7.2) of 20 mL of phosphate buffer; (c) tests in an alkaline electrolyte (pH = 14) of 20 mL of 1 M KOH 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 cm2; 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.

illumination is sufficiently intense to completely flatten the band bending and that the materials do not experience severe Fermi lever pinning at the electrode−electrolyte interface.51 The reduce of the ΔOCP over illumination intensity suggests that the AM 1.5G with a 100 mW·cm−2 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 demonstrate very strong p-type conductivity, consistent with the theoretical predications of Yin et al.,39 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). The selection of the correct electrolyte type (acidic, neutral, or alkaline) is important for efficient PEC water splitting,

and very mild changes for BBNO-2 electrodes after a series of OCP tests under the manually chopped illuminations with various light intensities (Figures S7b,c). 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,d) and BBO electrode (Figure 8e), respectively; meanwhile, it additionally accounts for the significant overall shifts in chopped OCPs between the test circles under different illumination intensities (see panels c−e of Figure 8).52 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 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,b). But, these two wide-bandgap BBNO-1 electrodes demonstrate very slow ΔOCP responses upon turning the light on and off (Figure 8a,b), compared to the two low-bandgap BBNO-2 electrodes (Figure 8c,d), indicating that BBNO-2 electrodes with smaller bandgaps and better electrical properties possess better material qualities for PEC applications.52 In principle, illuminated OCP technique can additionally be used to estimate the EFB as long as the above-bandgap 1026


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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 illumination (100 mW·cm−2): (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 cell area = 0.636 cm2; all the measurements were carried out in a 20 mL of 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 of chloroplatinic acid and 20 mL of phosphate buffer; phosphate buffer solution contains 0.1 M disodium hydrogen phosphate and 0.1 M sodium dihydrogen phosphate.

note that the p-type oxide photoelectrodes can corrode when immersed in the acidic media;22 therefore, most of nonprotective p-type Cu based oxides were usually evaluated their water reduction abilities in either neutral or alkaline media.15,16,18−21 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),

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 1027


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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 to ∼+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 electrode still delivers the best photocurrent of ∼1 mA·cm−2 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 of 0.2 mA·cm−2 (for the screen-printed sample) and 0.16 mA·cm−2 (for the spin-coated sample) at 0 VRHE. Comparing the photocurrents shown in Figure 9b and 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 oxides.11,13,15−22 Figure 10f shows chronoamperometric test results of two pristine BBNO-2 electrodes measured at a mildly negative bias of −0.15 VAg/AgCl with the AM 1.5G illumination (100 mW·cm−2) on and off. As seen, the electrode can only sustain a relatively stable photocurrent output for about ∼2 min under this operation condition; afterward, the dark current significantly increased. After about 5 min chronoamperometric test, the electrode films became white and looked transparent, and SEM and EDX measurements show that the remnant films have Ba deficient contents as well as with large Ba phosphate based grains and numerous cracks atop the surfaces (Figures S9 and S10). We additionally tested the compositional variances after three consecutive LSV scans for BBNO-2 electrodes. As shown in Figures 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 CdS/TiO2 onto the BBNO surface can overcome its corrosion drawback and further improve the photocurrent.8,11,13

neutral (pH = 7.2), and alkaline (pH = 14) solutions under manually chopped AM 1.5G (100 mW·cm−2) illumination using the spin-coated BBNO-2 film as a case study (Figure 9a− c). A fast scan rate of 50 mV·s−1 was used to minimize the film corrosion during the illuminated LSV scans. Besides, an O-ring seal setup was 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 and 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; this electrode can deliver about 0.11− 0.13 mA·cm−2 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 the LSV scan circles: for the second and third LSV scans in the acidic solution, the dark currents become significantly large when the bias goes cathodic of 0 VRHE and the corresponding photocurrent onsets as well become more cathodic compared to those of the first LSV scan; for the test in the neutral electrolyte, both photocurrent and its onset potential become smaller and more cathodic for the second and third LSV scan compared to those of the first 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 photocarriers 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 1.5G illumination (100 mW·cm−2) 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

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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 nanocrystalline grains (∼50 nm), while the spin-coated BaBiO3 (BBO) film demonstrates a 1028


Chemistry of Materials 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 ptype 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 appropriate for efficient PEC water splitting. The FTO/BBNO (0 ≤ x ≤ 0.93)/Pt photoelectrodes produced cathodic photocurrents of 0.05−1 mA·cm−2 at 0 VRHE measured in a neutral (pH = 7.2) phosphate buffer and under simulated AM 1.5G illumination (100 mW·cm−2). Among them, the spin-coated BBO mesoporous electrode delivers the best photocurrent of 1 mA·cm−2 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 0.05−0.2 mA·cm−2 at 0 VRHE, wherein the screen-printed BBNO (x = 0.6) nanoporous film with a bandgap of 1.62 eV delivers a better photocurrent of 0.2 mA·cm−2 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 photocarriers.

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ACKNOWLEDGMENTS

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REFERENCES

This paper presents results from an NSF project in the U.S. (Award No. CBET-1433401) competitively selected under the solicitation “NSF 14-15: NSF/DOE Partnership on Advanced Frontiers in Renewable Hydrogen Fuel Production via Solar Water Splitting Technologies”, which was co-sponsored by the National Science Foundation, 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. J.G. at Seoul National University acknowledges Brain Korea 21 Program for Leading Universities & Students (BK 21 PLUS, No. 21A20131912052). W.-J.Y. 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), and Natural Science Foundation of Jiangsu Province of China (under Grant No. BK20160299).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04880. 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; and SEM and EDX compositional analyses of BBNO based electrodes after PEC tests (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*Jie Ge. E-mail: [email protected]. *Yanfa Yan. E-mail: [email protected]. ORCID

Jie Ge: 0000-0001-5481-4059 Wan-Jian Yin: 0000-0003-0932-2789 Yanfa Yan: 0000-0003-3977-5789 Notes

The authors declare no competing financial interest. 1029


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