Surface-Modified Ta3N5 Nanocrystals with Boron for Enhanced

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Surface-Modified TaN Nanocrystals with Boron for Enhanced Visible-Light -Driven Photoelectrochemical Water Splitting Young Woon Kim, SeungHwan Cha, In Hye Kwak, Ik Seon Kwon, Kidong Park, Chan Su Jung, Eun Hee Cha, and Jeunghee Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09040 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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ACS Applied Materials & Interfaces

Surface–Modified Ta3N5 Nanocrystals with Boron for Enhanced VisibleLight -Driven Photoelectrochemical Water Splitting Young Woon Kim,† Seunghwan Cha,† Inhye Kwak,‡ Ik Seon Kwon,‡ Kidong Park,‡ Chan Su Jung,‡ Eun Hee Cha,*,† and Jeunghee Park*,‡ †

Graduate School of Green Energy Engineering, Hoseo University, Asan 336-795, Korea ‡

Department of Chemistry, Korea University, Jochiwon 339-700, Korea

ABSTRACT Photocatalysts for water splitting are the core of renewable energy technologies such as hydrogen fuel cells. The development of photoelectrode materials with high efficiency and low corrosivity has great challenges. In this study, we report new strategy to improve performance of tantalum nitride (Ta3N5) nanocrystals as promising photoanode materials for visible-light-driven photoelectrochemical (PEC) water splitting cells. The surface of Ta3N5 nanocrystals was modified with boron whose content was controlled, with up to 30% substitution of Ta. X-ray photoelectron spectroscopy revealed that boron was mainly incorporated into the surface oxide layers of the Ta3N5 nanocrystals. The surface modification with boron increases significantly the solar energy conversion efficiency of the water-splitting PEC cells by shifting the onset potential cathodically and increasing the photocurrents. It reduces the interfacial charge-transfer resistance and increases the electrical conductivity, which could cause the higher photocurrents at lower potential. The onset potential shift of the PEC cell with the boron incorporation can be attributed to the negative shift of the flat band potential. We suggest that the boron-modified surface acts as a protection layer for the Ta3N5 nanocrystals, by catalyzing effectively the water splitting reaction. KEYWORDS: Ta3N5, boron, photoelectrochemical cells, Faraday efficiency, water splitting.

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INTRODUCTION Solar-driven photoelectrochemical (PEC) water splitting is an attractive way to generate hydrogen fuel that does not cause environmental issues associated with the combustion of fossil fuels.1–4 Since Fujishima and Honda first succeeded in splitting water with TiO2 under UV illumination in 1972, TiO2 has been one of the most studied semiconductor materials owing to its high stability in aqueous electrolytes.1,5,6 However, the large band gap of TiO2 (3.0 eV for rutile and 3.2 eV for anatase) has limited the performance of PEC water-splitting devices. So far, tremendous efforts have focused on searching for photostable semiconductors responsive to visible light. Tantalum nitride (Ta3N5) is an n-type semiconductor that has received increasing attention recently as a photoanode material for solar water splitting.7–13 With a band gap of about 2.1 eV and suitable energy levels, Ta3N5 can utilize a large portion of the solar spectrum (420 nm) illumination at 100 mW cm–2. The applied potential vs. Ag/AgCl was converted to the value vs. RHE, as described in the experimental section, unless otherwise specified. The 8


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measurements were performed by anodically sweeping the potential from 0.2 to 1.4 V at a scan rate of 10 mV s–1 in a stirred Ar-saturated 0.1 M Na2SO4 and 5 M NaOH electrolyte adjusted to pH 13. The currents, generated from “on/off” light cycles (with a time interval of 2 s), show a quick increase (with some spikes) under light irradiation. The smooth photocurrent curves were generated by connecting the “on” signals. TN exhibits the onset potential (= open-circuit voltage, VOC) of about 0.45 V. The current density is about 0.36 mA cm-2 at the water oxidation potential (E0 = 1.23V), corresponding to the short-circuit current, (JSC). With the B substitution, the onset potential was decreased and the photocurrent enhanced. The lowest onset potential and highest photocurrent were observed for 10% B (BTN-1). The onset potential of BTN-1 was about 0.25 V, which is negatively shifted by 0.20 V relative to that of TN, whereas the photocurrent density at E0 was increased to 0.95 mA cm–2. However, higher B contents produced lower photocurrents, and less negative shift of the onset potentials (see Table 1). The measurements were also performed by cathodically sweeping the potential from 0 to 1.4 V (vs. RHE) at a scan rate of 20 mV s–1, showing consistently the lowest onset potential and highest photocurrent for BTN1 (see Supporting Information, Figure S6). In Figure 3b, the solar energy conversion efficiency (η) of the TN and BTN photoanodes is plotted as a function of applied potential, using the current–potential curves shown in Figure 3a. The η value as a function of the applied potential was calculated using the following equation: η =

,

where FF is the fill factor determined using the photocurrent density at each applied potential and Iph is the incident photon density (= 100 mW cm–2). The maximum η values achieved for TN, BTN-0, BTN-1, 2, and -3, were 0.032% (at 0.93 V), 0.094% (at 0.65 V), 0.35% (at 0.64 V), 0.22% (at 0.67V), and 0.17% (at 0.70 V), respectively (see Table 1). The cell parameters are summarized in Supporting Information, Table S1. The solar energy conversion efficiencies determined for various B contents showed that (i) the

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B substitution at surface increases the PEC cell efficiency, (ii) as the B content increases, the efficiency increases significantly (for up to 10%) and then decreases. To increase the photocurrents, the TN and BTN-1 photoanodes were modified with a CoPi co-catalyst that is known to be effective for preventing self-oxidation of Ta3N5 by catalyzing water splitting.15,16,45,46 The current–potential curves of the modified photoanodes are shown in Figure 3c. Modification increased the solar energy conversion efficiency for TN and BTN-1: 0.088% (at 0.97 V) and 0.54% (at 0.64 V), respectively, as shown in Figure 3d (see the cell parameters in Supporting Information, Table S1). The CoPi functions as a water oxidation catalyst, which increases the photocurrents of both TN and BTN-1. This result shows that B-incorporation with the CoPi co-catalyst is an effective strategy for increasing the efficiency of water-splitting PEC cells. We performed IPCE measurements to study their photoresponse as a function of the wavelength of the incident light, confirming that the photocurrents are originated from the Ta3N5 NCs (Supporting Information, Figure S7). The IPCE spectrum shows a significant enhancement of IPCE for BTN-1, which is well linked with that of the photocurrents. The use of ITO substrates instead Ti foil showed consistently the larger photoconversion efficiency of BTN-1 relative to that of TN (Supporting Information, Figure S8 and Table S2). In order to confirm that the photocurrents observed in the PEC cell was really due to water splitting, we measured simultaneously the photocurrent and the evolution (for 5 h) of hydrogen (H2) and oxygen (O2) in a closed PEC cell with an in-line gas chromatography at an applied potential of 0.9 V vs. RHE (See Figure S9 and Table S3 in Supporting Information). As the B incorporates, the stoichiometric ratio of H2:O2 becomes closer to 2. For BTN-1, an exact 2:1 stoichiometric confirmed excellent water-splitting photocatalytic activity with a negligible self-oxidation. The Faradic efficiency was ca. 73%, 83%, 83%, and 80%, respectively, for TN, BTN-1, BTN-2, and BTN-3. The XPS data of the electrodes before and after the PEC measurement (with the SEM images showing the morphology of the electrode) confirmed that the B content remain nearly the constant for 5 h (see Supporting Information, Figure S10). The XRD pattern also showed the maintenance of Ta3N5 phase. 10


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To explore the effect of B-substituted surface on the photogenerated carriers, electrochemical impedance spectroscopy (EIS) measurements were carried out. A frequency range of 105 to 0.1 Hz and an amplitude of 10 mV were used under dark and visible light irradiation (>420 nm; 100 mW cm–2) for the EIS measurements (Figure 4). The charge-transfer resistance between the electrode and electrolyte, Rct, is a key parameter for characterizing the semiconductor–electrolyte charge transfer process. A semicircle in the Nyquist plot at high frequencies represents the charge transfer process, with the diameter of the semicircle reflecting the charge-transfer resistance. The x- and y-axes are the real part (Z′) and negative imaginary part (–Z′′) of the impedance, respectively. The simulation of EIS spectra using an equivalent circuit model (inset) yielded the Rct values, as shown in Table S4 (Supporting Information). The fitting parameter Re represents the internal resistance of the electrolyte, and CPE represents the constant-phase element. The B incorporation decreases the charge-transfer resistance, most significantly with 10% (see Table 1). The value of Rct (in dark) is 12 and 8.5 kΩ, respectively, for TN and BTN-1. Under visible light irradiation, the Rct values decreases to 2.3 and 1.5 kΩ, respectively. The Re value is 2-3 Ω. The sequence is the same for both dark and light irradiation conditions. It has been reported that the modification of TiO2 and WO3 with boron oxide enhances the photocatalytic activity for water splitting.47,48 The improved PEC properties were attributed to the increased surface hydroxylation favoring water photo-oxidation reaction.48 We suggest that the Bsubstituted surface oxide layers adsorb more effectively H2O and hydroxyl ions in the electrolyte, and can assist the water oxidation reaction of Ta3N5, which decrease the charge-transfer resistance. The efficient visible-light-driven PEC water splitting could decrease significantly the charge-transfer resistance by reducing the recombination of photogenerated carriers (electron–hole pairs) at the interface with the electrolyte. The maximum B-O bonding at 10% B substitution decreases the charge-transfer resistance most significantly. As the B content increases from 10% to 30%, the charge-transfer resistance increases, probably owing to the increased B-N bonding.

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We also measured the electrical conductivity of the samples using four-point probe method, as shown in Table S5 (Supporting Information). The electrical conductivity is 26, 29, 165, 90, and 56 S cm-1 for TN, BTN-0, BTN-1, BTN-2, and BTN-3, respectively (see Table 1). The sequence is consistent with that of Rct. The higher conductivity of the BTN series could be correlated with the increase of the B-O bonding that eliminate the surface defects as we suggested above. The B-substituted oxide layers could bridge the Ta3N5 in a plausible route for electron transportation. The increased electrical conductivity allowed the electrons to be collected more efficiently at the back contact. The higher electron concentration contributes to the decrease of charge-transfer resistance and the improvement in photocurrents of the PEC cells The flat band potentials (Efb) were investigated using Mott–Schottky (MS) plots. Figure 5 shows the MS plots, i.e., the reciprocal of capacitance as a function of the applied potential at 0.5, 1, and 2 kHz. These MS plots were measured by anodically sweeping the potential with an AC amplitude of 10 mV. The lines show the fitting of the linear regions of the MS plots. The Efb values are obtained from the intercepts of the extrapolated lines. The Efb values of TN and BTN-1–3 were determined as –0.25, –0.41, –0.40, and –0.38 V (vs. RHE), respectively (see Table 1). In the case of BTN-0, Efb = -0.32 V (not plot here). This result means that the Efb position in BTN is negatively shifted by 0.07, 0.16, 0.15, and 0.13 V relative to that of TN, for 5%, 10%, 20%, and 30% B. This shift is similar to the negative shift of the onset potentials in the PEC cell; 0.07, 0.20, 0.19, and 0.10 V, respectively. Therefore the shift of the onset potential is attributed to that of the Efb position. The discrepancy between the Efb and the photocurrent onset is likely due to the water oxidation overpotential losses and voltage drop in the circuit. The negative shift in the Efb could be correlated with the higher electron concentration, based on the charge-transfer resistance and electric conductivity measured above. The elevation of the Ef position could provide more energetic photogenerated electrons for water reduction, which would result in the cathodic shift of the photocurrent onset potentials.

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The photocatalytic water splitting process has been suggested as follows. As soon as Ta3N5 absorbs light, photocarriers are generated within Ta3N5 (presumably an electron in the VB jumps into the CB). The B-substituted surface oxide layers facilitate the water oxidation reaction of Ta3N5 (and also CoPi), which reduces the self-oxidation. The water oxidation reaction captures the holes from Ta3N5 to continue the photoexcitation process. The elevation of the Ef position facilitates the H2-evolution water reduction reaction. Therefore, the efficient water splitting consequently enhances the performance of the PEC cell. Oxide layers or oxygen-doping of Ta3N5 NCs have been reported previously by other groups, suggesting that the oxide layers play an important role in increasing or decreasing the PEC efficiencies.49,50 Herein, the B-substituted surface oxide layers enhance photocatalytic water splitting, so they can act as a protection layer for the Ta3N5 NCs. CONCULSIONS We developed a sol–gel method and a subsequent nitration (using NH3 at 900 °C) to synthesize pure Ta2N5 (TN) and B-containing Ta3N5 (BTN-0-3) NCs using B2O3. The B cations substituted the Ta cations with the content of 5, 10, 20, and 30%, which was confirmed by EDX and XPS. The NCs exhibited a unique nanorod morphology. An average diameter was 20 nm. The electronic structures of these samples were thoroughly investigated using XPS, and this analysis provided robust evidence for the B–O bonding in amorphous surface oxide layers. The B-modified surface shifted cathodically the onset potential and increased the photocurrents of PEC water splitting cells. The Faradic efficiency of H2 and O2 evolution reaches 83% (at 0.9 V vs. RHE). The highest solar energy conversion efficiency under visible light irradiation was achieved with 10% B content and CoPi co-catalyst: 0.54% at 0.64 V vs. RHE. The impedance measurements showed that B substitution of surface oxide reduced the charge transfer resistance, consistently with the increased electrical conductivity. The MS plots showed that the B substitution shifted negatively the Efb position, which is responsible for the cathodic shift of the onset potential. The B-O bonding at surface can catalyze effectively the water oxidation, and elevate the Ef 13



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position that facilitates the water reduction. Therefore, the B-modified surface oxide layers play as a protection layers for the Ta3N5 NCs, which improves the water-splitting PEC activity.

ASSOCIATED CONTENT Supporting Information. Tables S1-S5, and Figures S1-S10: This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

E-mail: [email protected] (J.P); [email protected] (E.C)

ACKNOWLEDGMENTS This study was supported by 2017R1D1A3B03034523 and 2016K000295 (funded by the Ministry of Science). The HVEM measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH. It was also supported by the Korea Basic Science Institute under the R&D program (Project No. D37700) supervised by the Ministry of Science, ICT and Future Planning. REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (2) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141-145. (3) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338-344. (4) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light Photocatalysis in Nitrogen14


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(35) Zhong, M.; Hisatomi, T.; Sasaki, Y.; Suzuki, S.; Teshima, K.; Nakabayashi, M.; Shibata, N.; Nishiyama, H.; Katayama, M.; Yamada, T., et al. Highly Active GaN-Stabilized Ta3N5 Thin-Film Photoanode for Solar Water Oxidation. Angew. Chem. Int. Ed. 2017, 56, 4739-4743. (36) Zhao, W.; Ma, W.; Chen, C.; Zhao, J.; Shuai, Z. Efficient Degradation of Toxic Organic Pollutants with Ni2O3/TiO2-xBx under Visible Irradiation. J. Am. Chem. Soc. 2004, 126, 4782-4783. (37) In, S.; Orlov, A.; Berg, R.; Garcia, F.; Pedrosa-Jimenez, S.; Tikhov, M. S.; Wright, D. S.; Lambert, R. M. Effective Visible Light-Activated B-Doped and B,N-Codoped TiO2 Photocatalysts. J. Am. Chem. Soc. 2007, 129, 13790-13791. (38) Liu, G.; Zhao, Y.; Sun, C.; Li, F.; Lu, G. Q.; Cheng, H. M. Synergistic Effects of B/N Doping on the Visible-Light Photocatalytic Activity of Mesoporous TiO2. Angew. Chem. Int. Ed. 2008, 47, 45164520. (39) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114, 9919-9986. (40) Morikawa, T.; Saeki, S.; Suzuki, T.; Kajino, T.; Motohiro, T. Dual functional modification by N doping of Ta2O5: p-type Conduction in Visible-Light-Activated N-Doped Ta2O5. Appl. Phys. Lett. 2010, 96, 142111. (41) Jang, D. M.; Kwak, I. H.; Kwon, E. L.; Jung, C. S.; Im, H, S.; Park, K.; Park, J. Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution. J. Phys. Chem. C 2015, 119, 1921-1927. (42) Powell, C. J.; Jablonski, A. Evaluation of Calculated and Measured Electron Inelastic Mean Free Paths Near Solid Surfaces. J. Phys. Chem. Ref. Data 1999, 28, 19-62. (43) Bao, J.; Zhang, X.; Fan, B.; Zhang, J.; Zhou, M.; Yang, W.; Hu, X.; Wang, H.; Pan, B.; Xie, Y. Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation. Angew. Chem. 2015, 127, 7507-7512. (44) Zhu, Y.; Zhou, W.; Yu, J.; Chen, Y.; Liu, M.; Shao, Z. Enhancing Electrocatalytic Activity of Perovskite Oxides by Tuning Cation Deficiency for Oxygen Reduction and Evolution Reactions. Chem. Mater. 2016, 28, 1691-1697. (45) Kanan, M. W.; Nocera, D. G. In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072–1075. (46) Surendranath, Y.; Dincă, M.; Nocera, D. G. Electrolyte-Dependent Electrosynthesis and Activity of Cobalt-Based Water Oxidation Catalysts. J. Am. Chem. Soc. 2009, 131, 2615–2620. (47) Moon, S. C.; Hiroaki Mametsuka, H.; Tabata, S.; Suzuki, E. Photocatalytic Production of Hydrogen from Water Using TiO2 and B/TiO2. Catal. Today 2000, 58, 125-132. (48) Barczuka, P. J.; Krolikowska, A.; Lewera, A.; Miecznikowski, K.; Solarska, R.; Augustynski, J. Structural and Photoelectrochemical Investigation of Boron-Modified Nanostructured Tungsten Trioxide Films. Electrochm. Acta 2013, 58, 125-132. (49) Nurlaela, E.; Ould-Chikh, S.; Harb, M.; Gobbo, S. D.; Aouine, M.; Puzenat, E.; Sautet, P.; Domen, K.; Basset, J. -M.; Takanabe, K. Critical Role of the Semiconductor-Electrolyte Interface in 17



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Photocatalytic Performance for Water-Splitting Reactions Using Ta3N5 Particles. Chem. Mater. 2014, 26, 4812-4825. (50) Wang, Z.; Qi, Y.; Ding, C.; Fan, D.; Liu, G.; Zhao, Y.; Li, C. Insight into the Charge Transfer in Particulate Ta3N5 Photoanode with High Photoelectrochemical Performance. Chem. Sci. 2016, 7, 4391-4399.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Summary of the XPS, PEC, and Electrochemical Data Studied in the Present Works.

B-O/ B-Na

XPS Ta-O (and B-O)/O-Ob

VOC (V)

PECc JSC (mA cm-2)

Rct (kΩ)d

ρ (S cm-1)e

Efb (V)f

2.3

26

-0.25 (0)g

2.2

29

-0.32 (0.07)

0.35 (0.54)h

1.5

165

-0.41 (0.16)

0.74

0.22

1.8

90

-0.40 (0.15)

0.60

0.17

2.0

56

-0.38 (0.13)

Sample

B (%)

TN

0

N/A

0.5

0.45 (0)g

0.36

BTN-0

5

0.7

0.7

0.38 (0.07)

0.38

η (%) 0.032 (0.088)h 0.094

BTN-1

10

7

0.8

0.25 (0.20)

0.95

BTN-2

20

7

0.6

0.26 (0.19)

BTN-3

20

1.3

0.6

0.33 (0.12)

a

The ratio of B–O band to B-N band (Figure 2c); b The ratio of Ta-O/B–O band to B-N band (Figure 2d);

c

PEC cell parameters (Figure 3), VOC = onset potential or open-circuit voltage, JSC = short-circuit current,

η = solar energy conversion efficiency;

d

Charge-transfer resistance (under visible light irradiation)

determined by Nyquist plots (Figure 4b); e Electrical conductivity measured by four-point probe method; f The flat band potentials determined by MS plots (Figure 5); g The values in parenthesis correspond to the negative shift relative to that of TN; h The values in parenthesis correspond to η obtained using CoPi cocatalyst.

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Figure 1. (a) HRTEM and corresponding FFT images of Ta3N5 NCs. The distance between the adjacent (023) planes was close to that of the corresponding bulk phase (2.8 Å). (b) EDX spectra of TN (0% B), BTN-1 (10% B), and BTN-3 (30% B) reveal Ta:N ratios of 3:5, 2.7:5, and 2:5, respectively. (c) EDX mapping (with HAADF STEM images) shows the homogeneous distribution of 30% B over the NCs.

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(a) Ta 4f

4f7/2

4f5/2 Ta-N Ta-O

Ta

0

(b) N 1s

4f5/2 4f7/2

Ta-N

BTN-3

N

(c) B 1s

0

Ta-N

Ta-O

B-O

B B-N

0

(d) O 1s

0

O

N-O O-O Ta-O/B-O

B-N

BTN-2

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BTN-1

BTN-0

TN 28

26

24

Ta-N

22 399 398 397 396 395

O-O Ta-O/B-O N-O

194

192

190

188 536 534 532 530 528

Binding Energy (eV) Figure 2. Fine-scan XPS peaks of (a) Ta 4f, (b) N 1s, (c) B 1s, and (d) O 1s for Ta3N5 and B-Ta3N5. The

data (open circles) are fitted by Voigt functions, and the sum of the resolved bands is represented by black lines. The positions of the neutral peaks (Ta0, N0, B0, and O0) are marked by dotted lines to delineate the shift.

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0.4

(a)

0

TN (0% B) BTN-0 (5% B) BTN-1 (10% B) BTN-2 (20% B) BTN-3 (30% B)

1.0 0.8

Solar Energy Conversion Efficiency (%)

-2

1.2

E

0.6 0.4 0.2 0.0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

E

2.0

TN (with CoPi) BTN-1 (with CoPi)

1.5 1.0 0.5 0.0 0.2

0.4

0.6

0.8

1.0

1.2

Potential (V vs. RHE)

TN BTN-0 BTN-1 BTN-2 BTN-3

0.3

0.2

0.1

0.0 0.2

0.4

0.6

0.8

1.0

1.2

Potential (V vs. RHE) Solar Energy Conversion Efficiency (%)

-2

(c)

Page 22 of 25

(b)

Potential (V vs. RHE) Current Density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Current Density (mA cm )


1.4

0.7

(d)

0.6

BTN-1 (with CoPi)

0.5 0.4 0.3 0.2

TN (with CoPi)

0.1 0.0 0.2

0.4

0.6

0.8

1.0

1.2


Figure 3. (a) Current density vs. potential for Ta3N5 (TN) and B-Ta3N5 (BTN-0–3) photoanodes measured in a 0.1 M Na2SO4 and 5 M NaOH electrolyte under visible light (>420 nm) irradiation at 100 mW cm–2. (b) Solar energy conversion efficiency of the photoanodes calculated from the photocurrent– potential curves. (c) Current density vs. potential for Ta3N5 (TN) and 10% B-Ta3N5 (BTN-1) photoanodes modified by the CoPi co-catalyst, and (d) corresponding the solar energy conversion efficiency curves.

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8

2.0

(a)

TN BTN-1 BTN-3

6

-Z'' (kΩ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BTN-0 BTN-2

(b)

TN BTN-1 BTN-3

1.5

BTN-0 BTN-2

1.0

4 CPE

0.5

2 Rs Rct

0 0

2

4

6

8

10

0.0 0.0

0.5

1.0

1.5

2.0

2.5

Z' (kΩ)

Z' (kΩ)

Figure 4. Nyquist plots of Ta3N5 (TN) and B-Ta3N5 (BTN-0–3) for EIS experiments in the range from 100 kHz to 0.1 Hz at a representative potential of 1.23 V (vs. RHE) under (a) dark and (b) visible light irradiation (>420 nm; 100 mW cm–2). The equivalent circuit is shown in (a).


23


(b) BTN-1 (10% B)

(a) TN (0% B)

0.5 kHz 1 kHz 2 kHz

20 15

9

4

C /10 cm F

-2

25

10

-2

-0.25

-2

(d) BTN-3 (30% B)

(c) BTN-2 (20% B)

20

0.5 kHz 1 kHz 2 kHz

15

9

4

-0.41

5 0 25

C /10 cm F

10

-0.40

-0.38

-2

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Page 24 of 25

5 0 -0.6

-0.4

-0.2

0.0

0.2


0.4

-0.6 -0.4 -0.2

0.0

0.2

0.4


Figure 5. Mott–Schottky plots for (a) TN, (b) BTN-1, (c) BTN-2, and (d) BTN-3 by impedance measurements. The flat band potentials are obtained from the intercepts of the extrapolated lines.


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Table of Contents Graphic

We report synthesis of Ta3N5 nanocrystals, and the remarkable finding that the boron-modified surface enhances greatly visible-light-driven photoelectrochemical water splitting.


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Surface-Modified Ta3N5 Nanocrystals with Boron for Enhanced

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