Issue

Vol. 18 (2026)

Textured and Hierarchically Porous Hematite Photoanode for Efficient Hydrogen Production via Photoelectrochemical Hydrazine Oxidation

https://doi.org/10.1007/s40820-025-02045-z
Runfa Tan
Department of Materials Science & Engineering, Ajou University, Suwon 16499, Republic of Korea
Yoo Jae Jeong
Department of Materials Science & Engineering, Ajou University, Suwon 16499, Republic of Korea
Hyun Soo Han
Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA
Samadhan Kapse
Department of Materials Science & Engineering, Ajou University, Suwon 16499, Republic of Korea
Seong Sik Shin
SKKU Advanced Institute of Nanotechnology (SAINT) and Department of Nanoengineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
Xiaolin Zheng
Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA
In Sun Cho
Department of Materials Science & Engineering, Ajou University, Suwon 16499, Republic of Korea

Corresponding Author: In Sun Cho

insuncho@ajou.ac.kr

Nano-Micro Letters, Vol. 18 (2026), Article Number: 201

Abstract

The performance of hematite (α-Fe2O3) photoanodes for photoelectrochemical (PEC) water splitting has been limited to around 2–5 mA cm−2 under standard conditions due to their short hole diffusion length and sluggish oxygen evolution reaction kinetics. This work overcomes those challenges through a synergistic strategy that co-designs the hematite architecture and the surface reaction pathway. We introduce a textured and hierarchically porous Ti-doped Fe2O3 (tp-Fe2O3) photoanode, synthesized via multi-cycle growth and flame annealing method. This unique architecture features a high texture (110), enlarged surface area, and hierarchically porous structure, which enable significantly enhanced bulk charge transport and interfacial charge transfer compared to typical nanorod Ti-doped Fe2O3 (nr-Fe2O3). As a result, the tp-Fe2O3 photoanode achieves a photocurrent density of 3.1 mA cm−2 at 1.23 V vs. RHE with exceptional stability over 105 h, notably without any co-catalyst. By replacing the OER with the hydrazine oxidation reaction, the photocurrent further reaches a record-high level of 7.1 mA cm−2 at 1.23 VRHE. Finally, when we integrate the tp-Fe2O3 with a commercial Si solar cell, it achieves a solar-to-hydrogen efficiency of 8.7%—the highest reported value for any Fe2O3-based PV-tandem system. This work provides critical insights into rational Fe2O3 photoanode design and highlights the potential of hydrazine as an efficient alternative anodic reaction, enabling waste valorization.


Highights:
1 A multi-cycle growth and flame annealing strategy was developed to construct textured and hierarchically porous Ti-doped hematite (tp-Fe2O3) photoanodes with enhanced charge transport and surface kinetics.
2 The hydrazine oxidation reaction was introduced as a fast and thermodynamically favorable alternative to the oxygen evolution reaction, enabling the simultaneous production of hydrogen and the remediation of toxic hydrazine.
3 The tp-Fe2O3-based bias-free photovoltaic-photoelectrochemical tandem device achieved a record solar-to-hydrogen efficiency of 8.7%, demonstrating excellent stability and scalability for sustainable solar fuel generation.

Keywords

Hematite Hierarchically porous Texture Hydrazine oxidation reaction Solar-to-hydrogen
Tan, Runfa, Yoo Jae Jeong, Hyun Soo Han, Samadhan Kapse, Seong Sik Shin, Xiaolin Zheng, and In Sun Cho. 2026. “Textured and Hierarchically Porous Hematite Photoanode for Efficient Hydrogen Production via Photoelectrochemical Hydrazine Oxidation”. Nano-Micro Letters 18 (January):201. https://doi.org/10.1007/s40820-025-02045-z.
Download Citation
Endnote/Zotero/Mendeley (RIS)
BibTeX
References
  1. K. He, Z. Huang, C. Chen, C. Qiu, Y.L. Zhong et al., Exploring the roles of single atom in hydrogen peroxide photosynthesis. Nano-Micro Lett. 16, 23 (2024). https://doi.org/10.1007/s40820-023-01231-1
  2. M. Abbas, S. Chen, Z. Li, M. Ishaq, Z. Zheng et al., Highest solar-to-hydrogen conversion efficiency in Cu2ZnSnS4 photocathodes and its directly unbiased solar seawater splitting. Nano-Micro Lett. 17, 257 (2025). https://doi.org/10.1007/s40820-025-01755-8
  3. Y. Guo, X. Tong, N. Yang, Photocatalytic and electrocatalytic generation of hydrogen peroxide: principles, catalyst design and performance. Nano-Micro Lett. 15, 77 (2023). https://doi.org/10.1007/s40820-023-01052-2
  4. C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Photoelectrochemical devices for solar water splitting–materials and challenges. Chem. Soc. Rev. 46(15), 4645–4660 (2017). https://doi.org/10.1039/c6cs00306k
  5. B. He, Y. Cao, K. Lin, Y. Wang, Z. Li et al., Strong interactions between Au nanops and BiVO4 photoanode boosts hole extraction for photoelectrochemical water splitting. Angew. Chem. Int. Ed. 63(23), e202402435 (2024). https://doi.org/10.1002/anie.202402435
  6. K.-J. Lin, B. He, Z.-H. Xiao, L.-Y. Li, Z.-Y. Qiao et al., Electron-rich Mn: NiFe-LDHs onto BiVO4 photoanode for improved photoelectrochemical water splitting. Rare Met. 44(10), 7476–7485 (2025). https://doi.org/10.1007/s12598-025-03494-6
  7. Y. Cao, Y. Tian, B. He, Z. Qiao, L. Li et al., Modulating hole extraction and water oxidation kinetics in CoPi/Au/BiVO4 photoanode via strong metal-support interactions. J. Energy Chem. 109, 315–324 (2025). https://doi.org/10.1016/j.jechem.2025.05.048
  8. G. Gao, Z. Sun, X. Chen, Z. Guang, B. Sun et al., Recent advances in hydrogen production coupled with alternative oxidation reactions. Coord. Chem. Rev. 509, 215777 (2024). https://doi.org/10.1016/j.ccr.2024.215777
  9. Y. Miao, M. Shao, Photoelectrocatalysis for high-value-added chemicals production. Chin. J. Catal. 43(3), 595–610 (2022). https://doi.org/10.1016/S1872-2067(21)63923-2
  10. R.-Y. Fan, X.-J. Zhai, W.-Z. Qiao, Y.-S. Zhang, N. Yu et al., Optimized electronic modification of S-doped CuO induced by oxidative reconstruction for coupling glycerol electrooxidation with hydrogen evolution. Nano-Micro Lett. 15, 190 (2023). https://doi.org/10.1007/s40820-023-01159-6
  11. S. Li, D. Liu, G. Wang, P. Ma, X. Wang et al., Vertical 3D nanostructures boost efficient hydrogen production coupled with glycerol oxidation under alkaline conditions. Nano-Micro Lett. 15, 189 (2023). https://doi.org/10.1007/s40820-023-01150-1
  12. S. Hong, J. Kim, J. Park, S. Im, M.R. Hoffmann et al., Scalable Ir-doped NiFe2O4/TiO2 heterojunction anode for decentralized saline wastewater treatment and H2 production. Nano-Micro Lett. 17, 51 (2025). https://doi.org/10.1007/s40820-024-01542-x
  13. C. Yang, H. Pang, X. Li, X. Zheng, T. Wei et al., Scalable electrocatalytic urea wastewater treatment coupled with hydrogen production by regulating adsorption behavior of urea molecule. Nano-Micro Lett. 17(1), 159 (2025). https://doi.org/10.1007/s40820-024-01585-0
  14. X. Gao, Y. Chen, Y. Wang, L. Zhao, X. Zhao et al., Next-generation green hydrogen: progress and perspective from electricity, catalyst to electrolyte in electrocatalytic water splitting. Nano-Micro Lett. 16, 237 (2024). https://doi.org/10.1007/s40820-024-01424-2
  15. Z. Chen, R. Zheng, T. Bao, T. Ma, W. Wei et al., Dual-doped nickel sulfide for electro-upgrading polyethylene terephthalate into valuable chemicals and hydrogen fuel. Nano-Micro Lett. 15, 210 (2023). https://doi.org/10.1007/s40820-023-01181-8
  16. S. Shen, Y. Li, L. Ouyang, L. Zhang, M. Zhu et al., V-Ti-based solid solution alloys for solid-state hydrogen storage. Nano-Micro Lett. 17, 175 (2025). https://doi.org/10.1007/s40820-025-01672-w
  17. M. Wang, X. Wang, E. Sun, Z. Kang, F. Gong et al., Advancements and innovations in low-temperature hydrogen electrochemical conversion devices driven by 3D printing technology. Nano-Micro Lett. 18(1), 61 (2025). https://doi.org/10.1007/s40820-025-01907-w
  18. J. Li, N. Wu, J. Zhang, H.-H. Wu, K. Pan et al., Machine learning-assisted low-dimensional electrocatalysts design for hydrogen evolution reaction. Nano-Micro Lett. 15(1), 227 (2023). https://doi.org/10.1007/s40820-023-01192-5
  19. F. Wang, L. Xie, N. Sun, T. Zhi, M. Zhang et al., Deformable catalytic material derived from mechanical flexibility for hydrogen evolution reaction. Nano-Micro Lett. 16(1), 32 (2023). https://doi.org/10.1007/s40820-023-01251-x
  20. B.K. Jha, S. Chaule, J.-H. Jang, Enhancing photocatalytic efficiency with hematite photoanodes: principles, properties, and strategies for surface, bulk, and interface charge transfer improvement. Mater. Chem. Front. 8(10), 2197–2226 (2024). https://doi.org/10.1039/D3QM01100C
  21. D. Zhou, K. Fan, Recent strategies to enhance the efficiency of hematite photoanodes in photoelectrochemical water splitting. Chin. J. Catal. 42(6), 904–919 (2021). https://doi.org/10.1016/S1872-2067(20)63712-3
  22. J. Kang, B.G. Ghule, S.G. Gyeong, S.-J. Ha, J.-H. Jang, Alleviating charge recombination caused by unfavorable interaction of P and Sn in hematite for photoelectrochemical water oxidation. ACS Catal. 14(13), 10355–10364 (2024). https://doi.org/10.1021/acscatal.4c01150
  23. H. Niu, L. Gao, M. Liu, Y. Zou, J. Wang et al., Rapid charge extraction via hole transfer layer and interfacial coordination bonds on hematite photoanode for efficient photoelectrochemical water oxidation. Appl. Catal. B Environ. Energy 358, 124369 (2024). https://doi.org/10.1016/j.apcatb.2024.124369
  24. J. Park, K.-Y. Yoon, B.G. Ghule, H. Kim, J.-H. Jang, Morphology-engineered hematite photoanode for photoelectrochemical water splitting. ACS Energy Lett. 9(6), 3169–3176 (2024). https://doi.org/10.1021/acsenergylett.4c01347
  25. K. Kang, C. Tang, J.H. Kim, W.J. Byun, J.H. Lee et al., In situ construction of ta: Fe2O3@CaFe2O4 Core-Shell nanorod p–t–n heterojunction photoanodes for efficient and robust solar water oxidation. ACS Catal. 13(10), 7002–7012 (2023). https://doi.org/10.1021/acscatal.3c00932
  26. H. Wang, C. Xu, M. Ye, K. Liang, Y. Zhang et al., Inducing multiporous hematite nanorod photoanode by zirconium oxide overlayer and niobium oxide underlayer for efficient solar water splitting. Small 21(23), 2502503 (2025). https://doi.org/10.1002/smll.202502503
  27. C. Xu, H. Wang, H. Guo, K. Liang, Y. Zhang et al., Parallel multi-stacked photoanodes of Sb-doped p-n homojunction hematite with near-theoretical solar conversion efficiency. Nat. Commun. 15(1), 9712 (2024). https://doi.org/10.1038/s41467-024-53967-y
  28. L. He, Q. Zhang, C. Gong, H. Liu, F. Hu et al., The dual-function of hematite-based photoelectrochemical sensor for solar-to-electricity conversion and self-powered glucose detection. Sens. Actuat. B Chem. 310, 127842 (2020). https://doi.org/10.1016/j.snb.2020.127842
  29. J. Yuan, Y. Yuan, J. Zhang, H. Xu, Z. Mao et al., Mechanistic insights into selective acetaldehyde formation from ethanol oxidation on hematite photoanodes by operando spectroelectrochemistry. Chemsuschem 15(5), e202102313 (2022). https://doi.org/10.1002/cssc.202102313
  30. X. Li, J. Wang, M. Sun, X. Qian, Y. Zhao, Ti–Fe2O3/Ni(OH) as an efficient and durable photoanode for the photoelectrochemical catalysis of PET plastic to formic acid. J. Energy Chem. 78, 487–496 (2023). https://doi.org/10.1016/j.jechem.2022.12.031
  31. C.A. Mesa, A. Kafizas, L. Francàs, S.R. Pendlebury, E. Pastor et al., Kinetics of photoelectrochemical oxidation of methanol on hematite photoanodes. J. Am. Chem. Soc. 139(33), 11537–11543 (2017). https://doi.org/10.1021/jacs.7b05184
  32. L. Zhu, Z. Li, Y. Cheng, X. Zhang, H. Du et al., A highly efficient hematite photoelectrochemical fuel cell for solar-driven hydrogen production. Int. J. Hydrog. Energy 48(84), 32699–32707 (2023). https://doi.org/10.1016/j.ijhydene.2023.05.042
  33. X. Sun, X. Jiang, Z. Wang, Y. Li, J. Ren et al., Fluorescent probe for imaging N2H4 in plants, food, and living cells and for quantitative detection of N2H4 in soil and water using a smartphone. J. Hazard. Mater. 479, 135701 (2024). https://doi.org/10.1016/j.jhazmat.2024.135701
  34. R. Tan, Y.J. Jeong, Q. Li, M. Kang, I.S. Cho, Defect-rich spinel ferrites with improved charge collection properties for efficient solar water splitting. J. Adv. Ceram. 12(3), 612–624 (2023). https://doi.org/10.26599/jac.2023.9220709
  35. Y.J. Jeong, R. Tan, S. Nam, J.H. Lee, S.K. Kim et al., Rapid surface reconstruction of In2S3 photoanode via flame treatment for enhanced photoelectrochemical performance. Adv. Mater. 37(26), 2403164 (2025). https://doi.org/10.1002/adma.202403164
  36. I.S. Cho, H.S. Han, M. Logar, J. Park, X. Zheng, Enhancing low-bias performance of hematite photoanodes for solar water splitting by simultaneous reduction of bulk, interface, and surface recombination pathways. Adv. Energy Mater. 6(4), 1501840 (2016). https://doi.org/10.1002/aenm.201501840
  37. I.S. Cho, M. Logar, C.H. Lee, L. Cai, F.B. Prinz et al., Rapid and controllable flame reduction of TiO2 nanowires for enhanced solar water-splitting. Nano Lett. 14(1), 24–31 (2014). https://doi.org/10.1021/nl4026902
  38. I.S. Cho, C.H. Lee, Y. Feng, M. Logar, P.M. Rao et al., Codoping titanium dioxide nanowires with tungsten and carbon for enhanced photoelectrochemical performance. Nat. Commun. 4, 1723 (2013). https://doi.org/10.1038/ncomms2729
  39. R. Tan, S.Y. Hong, Y.J. Jeong, S.S. Shin, I.S. Cho, Interfacial engineering and rapid thermal crystallization of Sb2S3 photoanodes for enhanced photoelectrochemical performances. J. Energy Chem. 108, 417–426 (2025). https://doi.org/10.1016/j.jechem.2025.04.044
  40. J. Zou, C.-Z. Li, C.-Y. Chang, H.-L. Yip, A.K.Y. Jen, Interfacial engineering of ultrathin metal film transparent electrode for flexible organic photovoltaic cells. Adv. Mater. 26(22), 3618–3623 (2014). https://doi.org/10.1002/adma.201306212
  41. T. Butburee, Y. Bai, H. Wang, H. Chen, Z. Wang et al., 2D porous TiO2 single-crystalline nanostructure demonstrating high photo-electrochemical water splitting performance. Adv. Mater. 30(21), 1705666 (2018). https://doi.org/10.1002/adma.201705666
  42. R. Tan, S.W. Hwang, A. Sivanantham, I.S. Cho, Solution synthesis and activation of spinel CuAl2O4 film for solar water-splitting. J. Catal. 400, 218–227 (2021). https://doi.org/10.1016/j.jcat.2021.06.004
  43. Z. Zhang, H. Nagashima, T. Tachikawa, Ultra-narrow depletion layers in a hematite mesocrystal-based photoanode for boosting multihole water oxidation. Angew. Chem. Int. Ed. 59(23), 9047–9054 (2020). https://doi.org/10.1002/anie.202001919
  44. S. Kment, P. Schmuki, Z. Hubicka, L. Machala, R. Kirchgeorg et al., Photoanodes with fully controllable texture: the enhanced water splitting efficiency of thin hematite films exhibiting solely (110) crystal orientation. ACS Nano 9(7), 7113–7123 (2015). https://doi.org/10.1021/acsnano.5b01740
  45. H. Zhang, Y. He, X. Bao, Z. Wang, W. Jiang et al., Fabrication of hematite photoanode consisting of (110)-oriented single crystals. Chemsuschem 16(19), e202300666 (2023). https://doi.org/10.1002/cssc.202300666
  46. Q. Liu, C. Chen, G. Yuan, X. Huang, X. Lü et al., Morphology-controlled α-Fe2O3 nanostructures on FTO substrates for photoelectrochemical water oxidation. J. Alloys Compd. 715, 230–236 (2017). https://doi.org/10.1016/j.jallcom.2017.04.213
  47. G. Park, Y.-I. Kim, Y.H. Kim, M. Park, K.Y. Jang et al., Preparation and phase transition of FeOOH nanorods: strain effects on catalytic water oxidation. Nanoscale 9(14), 4751–4758 (2017). https://doi.org/10.1039/C6NR09790A
  48. H. Huang, J. Wang, Y. Liu, M. Zhao, N. Zhang et al., Stacking textured films on lattice-mismatched transparent conducting oxides via matched Voronoi cell of oxygen sublattice. Nat. Mater. 23(3), 383–390 (2024). https://doi.org/10.1038/s41563-023-01746-3
  49. Gurudayal, S.Y. Chiam, M.H. Kumar, P.S. Bassi, H.L. Seng et al., Improving the efficiency of hematite nanorods for photoelectrochemical water splitting by doping with manganese. ACS Appl. Mater. Interfaces 6(8), 5852–5859 (2014). https://doi.org/10.1021/am500643y
  50. A.M. Schultz, Y. Zhu, S.A. Bojarski, G.S. Rohrer, P.A. Salvador, Eutaxial growth of hematite Fe2O3 films on perovskite SrTiO3 polycrystalline substrates. Thin Solid Films 548, 220–224 (2013). https://doi.org/10.1016/j.tsf.2013.09.073
  51. G. Liu, N. Li, Y. Zhao, M. Wang, R. Yao et al., Porous versus compact hematite nanorod photoanode for high-performance photoelectrochemical water oxidation. ACS Sustain. Chem. Eng. 7(13), 11377–11385 (2019). https://doi.org/10.1021/acssuschemeng.9b01045
  52. K.-Y. Yoon, J. Park, M. Jung, S.-G. Ji, H. Lee et al., NiFeOx decorated Ge-hematite/perovskite for an efficient water splitting system. Nat. Commun. 12(1), 4309 (2021). https://doi.org/10.1038/s41467-021-24428-7
  53. J. López-Sánchez, A. del Campo, S. Román-Sánchez, Ó. Rodríguez de la Fuente, N. Carmona et al., Large two-magnon Raman hysteresis observed in a magnetically uncompensated hematite coating across the morin transition. Coatings 12(4), 540 (2022). https://doi.org/10.3390/coatings12040540
  54. Y. Liu, R.D.L. Smith, Identifying protons trapped in hematite photoanodes through structure-property analysis. Chem. Sci. 11(4), 1085–1096 (2019). https://doi.org/10.1039/c9sc04853g
  55. X. Meng, C. Zhu, X. Wang, Z. Liu, M. Zhu et al., Hierarchical triphase diffusion photoelectrodes for photoelectrochemical gas/liquid flow conversion. Nat. Commun. 14(1), 2643 (2023). https://doi.org/10.1038/s41467-023-38138-9
  56. Y.P. Hou, S.L. Feng, L.M. Dai, Y.M. Zheng, Droplet manipulation on wettable gradient surfaces with micro-/nano-hierarchical structure. Chem. Mater. 28(11), 3625–3629 (2016). https://doi.org/10.1021/acs.chemmater.6b01544
  57. M. Distaso, O. Zhuromskyy, B. Seemann, L. Pflug, M. Mačković et al., Interaction of light with hematite hierarchical structures: experiments and simulations. J. Quant. Spectrosc. Radiat. Transf. 189, 369–382 (2017). https://doi.org/10.1016/j.jqsrt.2016.12.028
  58. S. Tang, W. Qiu, S. Xiao, Y. Tong, S. Yang, Harnessing hierarchical architectures to trap light for efficient photoelectrochemical cells. Energy Environ. Sci. 13(3), 660–684 (2020). https://doi.org/10.1039/c9ee02986a
  59. L. Lyu, X. Hu, S. Lee, W. Fan, G. Kim et al., Oxygen reduction kinetics of Fe–N–C single atom catalysts boosted by pyridinic N vacancy for temperature-adaptive Zn–air batteries. J. Am. Chem. Soc. 146(7), 4803–4813 (2024). https://doi.org/10.1021/jacs.3c13111
  60. L. Lyu, X. Hu, W. Fan, B. Shao, Q. Wang et al., Operando hydroxylation of the Sn–co diatomic catalyst boosting ultrastable Zn–air batteries. Nano Lett. 25(28), 11164–11173 (2025). https://doi.org/10.1021/acs.nanolett.5c02744
  61. I. Kwon Jeong, M.A. Mahadik, S. Kim, H.M. Pathan, W.S. Chae et al., Transparent zirconium-doped hematite nanocoral photoanode via in situ diluted hydrothermal approach for efficient solar water splitting. Chem. Eng. J. 390, 124504 (2020). https://doi.org/10.1016/j.cej.2020.124504
  62. J. Kang, K.-Y. Yoon, J.-E. Lee, J. Park, S. Chaule et al., Meso-pore generating P doping for efficient photoelectrochemical water splitting. Nano Energy 107, 108090 (2023). https://doi.org/10.1016/j.nanoen.2022.108090
  63. R. Chen, L. Meng, W. Xu, L. Li, Cocatalysts-photoanode interface in photoelectrochemical water splitting: understanding and insights. Small 20, 2304807 (2024). https://doi.org/10.1002/smll.202304807
  64. Z. Zhang, X. Chen, R. Hao, Q. Feng, E. Xie, Van der Waals heterojunction modulated charge collection for H2O2 production photocathode. Adv. Funct. Mater. 33(34), 2303391 (2023). https://doi.org/10.1002/adfm.202303391
  65. Y.J. Jeong, D.H. Seo, J.H. Baek, M.J. Kang, B.N. Kim et al., Crystal reconstruction of Mo: BiVO4: improved charge transport for efficient solar water splitting. Adv. Funct. Mater. 32(52), 2208196 (2022). https://doi.org/10.1002/adfm.202208196
  66. H. Wu, L. Zhang, S. Qu, A. Du, J. Tang et al., Polaron-mediated transport in BiVO4 photoanodes for solar water oxidation. ACS Energy Lett. 8(5), 2177–2184 (2023). https://doi.org/10.1021/acsenergylett.3c00465
  67. G. Meng, Z. Chang, L. Zhu, C. Chen, Y. Chen et al., Adsorption site regulations of [W–O]-doped CoP boosting the hydrazine oxidation-coupled hydrogen evolution at elevated current density. Nano-Micro Lett. 15, 212 (2023). https://doi.org/10.1007/s40820-023-01185-4
  68. L. Zhu, J. Huang, G. Meng, T. Wu, C. Chen et al., Active site recovery and N-N bond breakage during hydrazine oxidation boosting the electrochemical hydrogen production. Nat. Commun. 14(1), 1997 (2023). https://doi.org/10.1038/s41467-023-37618-2
  69. S. Li, Y. Hou, L. Jiang, G. Feng, Y. Ge et al., Progress in hydrazine oxidation-assisted hydrogen production. Energy Rev. 4(1), 100105 (2025). https://doi.org/10.1016/j.enrev.2024.100105
Read More
Author biographies are not available.
Download this PDF file
PDF SUPPLEMENTARY FILE

Most read articles by the same author(s)