Photocatalytic Hydrogen Evolution catalysts Editorial Photocatalytic Hydrogen Evolution Vignesh Kumaravel 1,* and Misook Kang 2,* 1 Department of Environmental Science, Institute of Technology Sligo, Ash Lane, Co., Sligo F91 YW50, Ireland 2 Department of Chemistry, College of Natural Sciences, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Korea * Correspondence: Kumaravel.Vignesh@itsligo.ie (V.K.); mskang@ynu.ac.kr (M.K.) Received: 21 April 2020; Accepted: 26 April 2020; Published: 1 May 2020 ���������� ������� Solar energy conversion is one of the sustainable technologies that tackles the global warming and energy crisis. Photocatalytic hydrogen (H2) production is a clean technology to produce eco-friendly fuel with the help of semiconductor nanoparticles and abundant sunlight irradiation. Titanium oxide (TiO2), graphitic-carbon nitride (g-C3N4) and cadmium sulfide (CdS) are the most widely explored photocatalysts in recent decades for water splitting. As the guest editors, we have comprehensively investigated the role of sacrificial agents on the H2 production efficiency of TiO2, g-C3N4 and CdS photocatalysts [1]. The activity of the catalysts was evaluated without any noble metal co-catalysts. The effects of the most widely reported sacrificial agents were evaluated in this work. The activity of the catalysts was influenced by the number of hydroxyl groups, alpha hydrogen and carbon chain length of the sacrificial agent. We found that glucose and glycerol are the most suitable sacrificial agents to produce H2 with minimum toxicity to the solution. The findings of this study would be highly favorable for the selection of a suitable sacrificial agent for photocatalytic H2 production. Hong et al. demonstrated the photoelectrochemical (PEC) efficiency of MoSe2/Si nanostructures for H2 production and carbon dioxide (CO2) reduction [2]. PEC deposition coupled with the rapid thermal annealing method was applied to fabricate the electrodes on the Si substrate. PEC H2 evolution and CO2 conversion efficiencies of the MoSe2/Si electrode were higher in visible light irradiation as compared to dark conditions. Kim et al. synthesised monodispersed spherical TiO2 particles with a disordered rutile surface for photocatalytic H2 production [3]. The photocatalyst was synthesised through sol-gel and a chemical reduction technique using Li/ethylenediamine (Li/EDA) solution. The samples were calcined at various temperatures to tune the anatase to the rutile phase ratio. The disordered rutile surface and mixed crystalline phase of TiO2 significantly increased the H2 production under solar light irradiation. Idrees et al. reported the photocatalytic activity of Nb2O5/g-C3N4 heterostructures for molecular H2 production under simulated solar light irradiation [4]. A hydrothermal technique was utilised to develop the three dimensional Nb2O5/g-C3N4 heterostructure with a high surface area. H2 production efficiency of Nb2O5/g-C3N4 (10 wt. %) was higher than that of pure Nb2O5 and g-C3N4. The photogenerated electron hole pairs were successfully separated through a direct Z-scheme mechanism at the heterojunction. Kim and Woodward described the band gap modulation of tantalum (V) perovskite by anion control [5]. Perovskites such as BaTaO2N, SrTaO2N, CaTaO2N, KTaO3, NaTaO3 and TaO2F were studied in this work. Pt-loaded CaTaO2N was utilised as a visible-light-driven photocatalyst for H2 production using CH3OH as the sacrificial agent. Son et al. reported the impact of sulfur defects on the H2 production efficiency of a CuS@CuGaS2 heterojunction under visible light irradiation [6]. The activity of the CuS@CuGaS2 heterojunction was higher as compared to pure CuS. This was ascribed to the introduction of structural defects to promote the photo-generated electron hole separation. Catalysts 2020, 10, 492; doi:10.3390/catal10050492 www.mdpi.com/journal/catalysts http://www.mdpi.com/journal/catalysts http://www.mdpi.com https://orcid.org/0000-0003-4755-189X https://orcid.org/0000-0002-7199-9881 http://www.mdpi.com/2073-4344/10/5/492?type=check_update&version=1 http://dx.doi.org/10.3390/catal10050492 http://www.mdpi.com/journal/catalysts Catalysts 2020, 10, 492 2 of 2 The recent accomplishments in the synthesis and application of various photocatalysts for H2 production are briefly reviewed by Zhang et al. [7] Tremendous efforts should be taken in the future to commercialise this photocatalytic technology at the industry level. The studies should also be performed with cheap materials, industrial wastewater and seawater for H2 production. Finally, we would like to convey our sincere thanks to all the authors for their significant contributions in this special issue. Author Contributions: Conceptualization, V.K. and M.K.; Review and editing, V.K and M.K. All authors have read and agreed to the published version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest References 1. Kumaravel, V.; Imam, M.D.; Badreldin, A.; Chava, R.K.; Do, J.Y.; Kang, M.; Abdel-Wahab, A. Photocatalytic hydrogen production: Role of sacrificial reagents on the activity of oxide, carbon, and sulfide catalysts. Catalysts 2019, 9, 276. [CrossRef] 2. Hong, S.; Rhee, C.K.; Sohn, Y. Photoelectrochemical Hydrogen Evolution and CO2 Reduction over MoS2/Si and MoSe2/Si Nanostructures by Combined Photoelectrochemical Deposition and Rapid-Thermal Annealing Process. Catalysts 2019, 9, 494. [CrossRef] 3. Kim, N.Y.; Lee, H.K.; Moon, J.T.; Joo, J.B. Synthesis of Spherical TiO2 Particles with Disordered Rutile Surface for Photocatalytic Hydrogen Production. Catalysts 2019, 9, 491. [CrossRef] 4. Idrees, F.; Dillert, R.; Bahnemann, D.; Butt, F.K.; Tahir, M. In-Situ Synthesis of Nb2O5/g-C3N4 Heterostructures as Highly Efficient Photocatalysts for Molecular H2 Evolution under Solar Illumination. Catalysts 2019, 9, 169. [CrossRef] 5. Kim, Y.-I.; Woodward, P.M. Band gap modulation of Tantalum (V) perovskite semiconductors by anion control. Catalysts 2019, 9, 161. [CrossRef] 6. Son, N.; Heo, J.N.; Youn, Y.-S.; Kim, Y.; Do, J.Y.; Kang, M. Enhancement of Hydrogen Productions by Accelerating Electron-Transfers of Sulfur Defects in the CuS@ CuGaS2 Heterojunction Photocatalysts. Catalysts 2019, 9, 41. [CrossRef] 7. Zhang, Y.; Heo, Y.-J.; Lee, J.-W.; Lee, J.-H.; Bajgai, J.; Lee, K.-J.; Park, S.-J. Photocatalytic hydrogen evolution via water splitting: A short review. Catalysts 2018, 8, 655. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://dx.doi.org/10.3390/catal9030276 http://dx.doi.org/10.3390/catal9060494 http://dx.doi.org/10.3390/catal9060491 http://dx.doi.org/10.3390/catal9020169 http://dx.doi.org/10.3390/catal9020161 http://dx.doi.org/10.3390/catal9010041 http://dx.doi.org/10.3390/catal8120655 http://creativecommons.org/ http://creativecommons.org/licenses/by/4.0/. References