What Limits the Performance of Ta3N5 for Solar Water Splitting? Yumin He, James E. Thorne, Cheng Hao Wu, Peiyan Ma, Chun Du, Qi Dong, Jinghua Guo, Dunwei Wang  Chem  Volume 1, Issue 4, Pages 640-655 (October 2016) DOI: 10.1016/j.chempr.2016.09.006 Copyright © 2016 Elsevier Inc. Terms and Conditions

Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 1 Schematics of the Main Issues that Limit the Performance of Photoactive Materials for Solar Water Splitting The representative materials corresponding to each issue are shown in the center. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 2 Base Performance and TEM Characterization of Ta3N5 (A) The current-voltage relationship of Ta3N5 in 1 M NaOH (pH 13.6). Illumination condition = 100 mW/cm2, AM 1.5; scan rate = 20 mV/s. (B–D) TEM image of the Ta3N5 surface as prepared (B), after one cycle of CV scan (C), and after 3 hr of photoelectrolysis at 1.23 V (D). Scale bars represent 1 nm. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 3 X-Ray Spectra of Ta3N5 (A and B) Total electron yield of O 1s and N 1s X-ray absorption spectra. The dotted vertical line in (B) serves as a visual guide. (C–E) Binding energies of O 1s (C), N 1s (D), and Ta 4f (E) electrons as measured by XPS. Samples compared in this figure include as-prepared Ta3N5 (fresh), Ta3N5 after one cycle of CV (1 CV), and Ta3N5 after 3 hr of photoelectrolysis (3 hr). The dotted vertical line in (E) serves as a visual guide. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 4 PEC Performance in the Presence of Hole Scavengers (A) The stability of bare Ta3N5 was much better when H2O2 (0.1 M; 1 M NaOH [pH 13]) was added and the best when K4Fe(CN)6 (0.1 M; potassium phosphate buffer [pH 10] with 0.1 mM K3Fe(CN)6) was present. Applied potential (Vapp) = 1.23 V. (B) The initial current-voltage relationship of Ta3N5 in the solutions as identified above. Scan rate = 20 mV/s. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 5 Light Open-Circuit Potentials of Three Groups of Ta3N5 Electrodes as Measured in Phosphate Buffer with 0.1 M K4Fe(CN)6 and 0.1 mM K3Fe(CN)6 The samples in (A) were tested in 1 M NaOH for one cycle of CV and 3 hr of photoelectrolysis. Similarly, the samples in (B) were tested in 1 M NaOH with 0.1 M H2O2. The samples in (C) were tested in phosphate buffer (pH 10) with 0.1 M K4Fe(CN)6 and 0.1 mM K3Fe(CN)6. Vapp for all tests = 1.23 V. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 6 Ar 2p XPS Spectra of 100 mTorr Ar Gas Phase at the Near Surface Region of Ta3N5 The testing samples included fresh Ta3N5, Ta3N5 after one cycle of CV under illumination, and Ta3N5 after 3 hr of photoelectrolysis at 1.23 V. The test electrolyte was 1 M NaOH, and the light intensity was 100 mW/cm2 (AM 1.5). The dotted vertical line serves as a visual guide. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 7 The Evolution of Ta3N5 Surface Energetics Stage I: fresh Ta3N5 free of H2O. Stage II: Ta3N5 with partial H2O adsorption due to exposure to ambient air. Stage III: Ta3N5 immersed in H2O. Stage IV: Ta3N5 with surface oxides. The horizontal lines correspond to the surface Fermi-level position of Ta3N5 in stages I–IV. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 8 Recovery of the Initial Performance Degradation by Water-Oxidation Catalysts (A–C) The current-voltage relationships for bare Ta3N5 with different cycles of water oxidation tests (A, 1st scan; B, 10th scan; C, 25th scan) and the performance with re-deposited NiFeOOH. (D) The J-V curve of freshly prepared Ta3N5 with NiFeOOH or CoFeOOH as the catalyst. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 9 Performance of Ta3N5 with an MgO Protection Layer and Co(OH)x Catalyst The J-V curves (A) and the stability of Ta3N5 (B) with and without MgO as a protection layer and Co(OH)x as the catalyst in 1 M NaOH. For the stability test, the potential was fixed at 1.23 V. Chem 2016 1, 640-655DOI: (10.1016/j.chempr.2016.09.006) Copyright © 2016 Elsevier Inc. Terms and Conditions