photocatalytic water splitting

File:Water splitting with a photocatalyst.png

Two mole of {{H2O}} is split into 1 mole {{chem|O|2}} and 2 mole {{chem|H|2}} using light in the process shown below.

: $\backslash begin\{matrix\}\{\backslash scriptstyle\; h\backslash nu\backslash \; >\backslash \; 1.23~\backslash ce\{eV\}\}\backslash \backslash$

\text{Light absorption: } \ce{SC + h\nu -> e-_{cb} + h+_{vb}}\\

\text{Oxidation reaction: } \ce{2H2O + 4h+_{vb} -> O2 + 4H+}\\

\text{Reduction reaction: } \ce{2H+ + 2e-_{cb} -> H2}\\

\text{Overall water splitting: } \ce{2H2O -> 2H2 + O2}\\

{\scriptstyle (h\nu \text{: photon energy, SC: semiconductor,} e-_{cb}\text{: electron in conduction band,} h+_{vb}\text{: hole in valence band})}

\end{matrix}

A photon with an energy greater than 1.23 eV is needed to generate an electron–hole pairs, which react with water on the surface of the photocatalyst. The photocatalyst must have a bandgap large enough to split water; in practice, losses from material internal resistance and the overpotential of the water splitting reaction increase the required bandgap energy to 1.6–2.4 eV to drive water splitting.

The process of water-splitting is a highly endothermic process (ΔH > 0). Water splitting occurs naturally in photosynthesis when the energy of four photons is absorbed and converted into chemical energy through a complex biochemical pathway (Dolai's or Kok's S-state diagrams).{{Cite journal |last1=Yano |first1=Junko |last2=Yachandra |first2=Vittal |date=2014-04-23 |title=Mn 4 Ca Cluster in Photosynthesis: Where and How Water is Oxidized to Dioxygen |journal=Chemical Reviews |language=en |volume=114 |issue=8 |pages=4175–4205 |doi=10.1021/cr4004874 |issn=0009-2665 |pmc=4002066 |pmid=24684576}}

O–H bond homolysis in water requires energy of 6.5 - 6.9 eV (UV photon).{{Cite journal |last1=Gligorovski |first1=Sasho |last2=Strekowski |first2=Rafal |last3=Barbati |first3=Stephane |last4=Vione |first4=Davide |date=2015-12-23 |title=Environmental Implications of Hydroxyl Radicals ( • OH) |url=https://pubs.acs.org/doi/10.1021/cr500310b |journal=Chemical Reviews |language=en |volume=115 |issue=24 |pages=13051–13092 |doi=10.1021/cr500310b |pmid=26630000 |issn=0009-2665}}{{cite journal|last1=Fujishima|first1=Akira|title=Electrochemical Photolysis of Water at a Semiconductor Electrode|journal=Nature|date=13 September 1971|volume=238|issue=5358|pages=37–38|doi=10.1038/238037a0|pmid=12635268|bibcode = 1972Natur.238...37F |s2cid=4251015}} Infrared light has sufficient energy to mediate water splitting because it technically has enough energy for the net reaction. However, it does not have enough energy to mediate the elementary reactions leading to the various intermediates involved in water splitting (this is why there is still water on Earth). Nature overcomes this challenge by absorbing four visible photons. In the laboratory, this challenge is typically overcome by coupling the hydrogen production reaction with a sacrificial reductant other than water.{{Cite journal |last1=Zhou |first1=Dantong |last2=Li |first2=Dongxiang |last3=Yuan |first3=Shengpeng |last4=Chen |first4=Zhi |date=2022-08-25 |title=Recent Advances in Biomass-Based Photocatalytic H 2 Production and Efficient Photocatalysts: A Review |url=https://pubs.acs.org/doi/10.1021/acs.energyfuels.2c01904 |journal=Energy & Fuels |volume=36 |issue=18 |language=en |pages=10721–10731 |doi=10.1021/acs.energyfuels.2c01904 |s2cid=251852086 |issn=0887-0624}}

Materials used in photocatalytic water splitting fulfill the band requirements and typically have dopants and/or co-catalysts added to optimize their performance. A sample semiconductor with the proper band structure is titanium dioxide ({{chem|TiO|2}}) and is typically used with a co-catalyst such as platinum (Pt) to increase the rate of {{chem|H|2}} production.{{cite journal | last1 = Kudo | first1 = A. | last2 = Miseki | first2 = Y. | year = 2009 | title = Heterogeneous photocatalyst materials for water splitting | journal = Chem. Soc. Rev. | volume = 38 | issue = 1| pages = 253–278 | doi = 10.1039/b800489g | pmid = 19088977 }} A major problem in photocatalytic water splitting is photocatalyst decomposition and corrosion.

Photocatalysts must conform to several key principles in order to be considered effective at water splitting. A key principle is that {{chem|H|2}} and {{chem|O|2}} evolution should occur in a stoichiometric 2:1 ratio; significant deviation could be due to a flaw in the experimental setup and/or a side reaction, neither of which indicate a reliable photocatalyst for water splitting. The prime measure of photocatalyst effectiveness is quantum yield (QY), which is:

: QY (%) = (Photochemical reaction rate) / (Photon absorption rate) × 100%

To assist in comparison, the rate of gas evolution can also be used. A photocatalyst that has a high quantum yield and gives a high rate of gas evolution is a better catalyst.

The other important factor for a photocatalyst is the range of light that is effective for operation. For example, a photocatalyst is more desirable to use visible photons than UV photons.

The efficiency of solar-to-hydrogen (STH) of photocatalytic water splitting, however, has remained very low.

A STH efficiency of 9.2% indium.{{cite journal |last1=Zhou |first1=Peng |last2=Navid |first2=Ishtiaque Ahmed |last3=Ma |first3=Yongjin |last4=Xiao |first4=Yixin |last5=Wang |first5=Ping |last6=Ye |first6=Zhengwei |last7=Zhou |first7=Baowen |last8=Sun |first8=Kai |last9=Mi |first9=Zetian |title=Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting |journal=Nature |date=January 2023 |volume=613 |issue=7942 |pages=66–70 |doi=10.1038/s41586-022-05399-1|pmid=36600066 |bibcode=2023Natur.613...66Z }}

{{chem|NaTaO|3}}:La yielded the highest water splitting rate of photocatalysts without using sacrificial reagents. This ultraviolet-based photocatalyst was reported to show water splitting rates of 9.7 mmol/h and a quantum yield of 56%. The nanostep structure of the material promotes water splitting as edges functioned as {{chem|H|2}} production sites and the grooves functioned as {{chem|O|2}} production sites. Addition of NiO particles as co-catalysts assisted in {{chem|H|2}} production; this step used an impregnation method with an aqueous solution of {{chem|Ni(NO|3|)|2}}•6{{chem|H|2|O}} and evaporated the solution in the presence of the photocatalyst. {{chem|NaTaO|3}} has a conduction band higher than that of NiO, so photo-generated electrons are more easily transferred to the conduction band of NiO for {{chem|H|2}} evolution.{{cite journal | last1 = Kato | first1 = H. | last2 = Asakura | first2 = K. | last3 = Kudo | first3 = A. | year = 2003 | title = Highly Efficient Water Splitting into H and O over Lanthanum-Doped NaTaO Photocatalysts with High Crystallinity and Surface Nanostructure | journal = J. Am. Chem. Soc. | volume = 125 | issue = 10| pages = 3082–3089 | doi=10.1021/ja027751g| pmid = 12617675 | bibcode = 2003JAChS.125.3082K }}

{{chem|K|3|Ta|3|B|2|O|12}} is another catalyst solely activated by UV and above light. It does not have the performance or quantum yield of {{chem|NaTaO|3}}:La. However, it can split water without the assistance of co-catalysts and gives a quantum yield of 6.5%, along with a water splitting rate of 1.21 mmol/h. This ability is due to the pillared structure of the photocatalyst, which involves {{chem|TaO|6}} pillars connected by {{chem|BO|3}} triangle units. Loading with NiO did not assist the photocatalyst due to the highly active {{chem|H|2}} evolution sites.T. Kurihara, H. Okutomi, Y. Miseki, H. Kato, A. Kudo, "Highly Efficient Water Splitting over {{chem|K|3|Ta|3|B|2|O|12}}Photocatalyst without Loading Cocatalyst" Chem. Lett., 35, 274 (2006).

({{chem|Ga|.82|Zn|.18}})({{chem|N|.82|O|.18}}) had the highest quantum yield in visible light for visible light-based photocatalysts that do not utilize sacrificial reagents as of October 2008. The photocatalyst featured a quantum yield of 5.9% and a water splitting rate of 0.4 mmol/h. Tuning the catalyst was done by increasing calcination temperatures for the final step in synthesizing the catalyst. Temperatures up to 600 °C helped to reduce the number of defects, while temperatures above 700 °C destroyed the local structure around zinc atoms and were thus undesirable. The treatment ultimately reduced the amount of surface Zn and O defects, which normally function as recombination sites, thus limiting photocatalytic activity. The catalyst was then loaded with {{chem|Rh|2-y|Cr|y|O|3}} at a rate of 2.5 wt% Rh and 2 wt% Cr for better performance.K. Maeda, K. Teramura, K. Domen,

"Effect of post-calcination on photocatalytic activity of ({{chem|Ga|1-x|Zn|x}})({{chem|N|1-x|O|x}}) solid solution for overall water splitting under visible light" J. Catal., 254, 198 (2008).

Proton reduction catalysts based on earth-abundant elements{{Cite journal|last1=McKone|first1=James R.|last2=Marinescu|first2=Smaranda C.|last3=Brunschwig|first3=Bruce S.|last4=Winkler|first4=Jay R.|last5=Gray|first5=Harry B.|date=2014|title=Earth-abundant hydrogen evolution electrocatalysts|url=http://xlink.rsc.org/?DOI=C3SC51711J|journal=Chem. Sci.|language=en|volume=5|issue=3|pages=865–878|doi=10.1039/C3SC51711J|issn=2041-6520}}{{Cite journal|last1=Sutra|first1=P.|last2=Igau|first2=A.|date=April 2018|title=Emerging Earth-abundant (Fe, Co, Ni, Cu) molecular complexes for solar fuel catalysis|url=https://linkinghub.elsevier.com/retrieve/pii/S2452223617300998|journal=Current Opinion in Green and Sustainable Chemistry|language=en|volume=10|pages=60–67|doi=10.1016/j.cogsc.2018.03.004|bibcode=2018COGSC..10...60S }} carry out one side of the water-splitting half-reaction.

A mole of octahedral nickel(II) complex, [Ni(bztpen)]²⁺ (bztpen = N-benzyl-N,N’,N’-tris(pyridine-2-ylmethyl)ethylenediamine) produced 308,000 moles of hydrogen over 60 hours of electrolysis with an applied potential of -1.25 V vs. standard hydrogen electrode.{{Cite journal|last1=Zhang|first1=Peili|last2=Wang|first2=Mei|last3=Yang|first3=Yong|last4=Zheng|first4=Dehua|last5=Han|first5=Kai|last6=Sun|first6=Licheng|date=2014|title=Highly efficient molecular nickel catalysts for electrochemical hydrogen production from neutral water|url=http://xlink.rsc.org/?DOI=C4CC05511J|journal=Chem. Commun.|language=en|volume=50|issue=91|pages=14153–14156|doi=10.1039/C4CC05511J|pmid=25277377 |issn=1359-7345}}

Ru(II) with three 2,2'-bipyridine ligands is a common compound for photosensitization used for photocatalytic oxidative transformations like water splitting. However, the bipyridine degrades due to the strongly oxidative conditions which causes the concentration of Ru(bpy)32+ to diminish. Measurements of the degradation is difficult with UV-Vis spectroscopy but MALDI MS can be used instead.{{Cite journal |last1=Bergman |first1=Nina |last2=Thapper |first2=Anders |last3=Styring |first3=Stenbjörn |last4=Bergquist |first4=Jonas |last5=Shevchenko |first5=Denys |date=2014 |title=Quantitative determination of the Ru(bpy) 3 2+ cation in photochemical reactions by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry |url=http://xlink.rsc.org/?DOI=C4AY01379D |journal=Anal. Methods |language=en |volume=6 |issue=21 |pages=8513–8518 |doi=10.1039/C4AY01379D |issn=1759-9660}}

Cobalt-based photocatalysts have been reported,{{Cite journal | doi = 10.1002/anie.201007987| pmid = 21748828| title = Splitting Water with Cobalt| year = 2011| last1 = Artero | first1 = V. | last2 = Chavarot-Kerlidou | first2 = M. | last3 = Fontecave | first3 = M. | journal = Angewandte Chemie International Edition| volume = 50| issue = 32| pages = 7238–7266}} including tris(bipyridine) cobalt(II), compounds of cobalt ligated to certain cyclic polyamines, and some cobaloximes.

In 2014 researchers announced an approach that connected a chromophore to part of a larger organic ring that surrounded a cobalt atom. The process is less efficient than a platinum catalyst although cobalt is less expensive, potentially reducing costs. The process uses one of two supramolecular assemblies based on Co(II)-templated coordination of {{chem2|Ru(bpy)32+}} (bpy = 2,2′-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle. The Co(II) centers of both assemblies are high spin, in contrast to most previously described cobaloximes. Transient absorption optical spectroscopies indicate that charge recombination occurs through multiple ligand states within the photosensitizer modules.{{cite journal|url=http://www.kurzweilai.net/a-less-expensive-way-to-duplicate-the-complicated-steps-of-photosynthesis-in-making-fuel |title=A less-expensive way to duplicate the complicated steps of photosynthesis in making fuel |journal=Physical Chemistry Chemical Physics |volume=15 |issue=48 |pages=21070–6 |doi=10.1039/C3CP54420F |pmid=24220293 |access-date=2014-01-23|bibcode=2013PCCP...1521070M |last1=Mukherjee |first1=Anusree |last2=Kokhan |first2=Oleksandr |last3=Huang |first3=Jier |last4=Niklas |first4=Jens |last5=Chen |first5=Lin X. |last6=Tiede |first6=David M. |last7=Mulfort |first7=Karen L. |year=2013 }}{{Cite journal | last1 = Mukherjee | first1 = A. | last2 = Kokhan | first2 = O. | last3 = Huang | first3 = J. | last4 = Niklas | first4 = J. | last5 = Chen | first5 = L. X. | last6 = Tiede | first6 = D. M. | last7 = Mulfort | first7 = K. L. | doi = 10.1039/C3CP54420F | title = Detection of a charge-separated catalyst precursor state in a linked photosensitizer-catalyst assembly | journal = Physical Chemistry Chemical Physics| volume = 15 | issue = 48 | pages = 21070–21076 | year = 2013 | pmid = 24220293| bibcode = 2013PCCP...1521070M | url = https://zenodo.org/record/1230024 }}

Bismuth vanadate is a visible-light-driven photocatalyst with a bandgap of 2.4 eV.{{Cite journal|last1=Shafiq|first1=Iqrash|last2=Hussain|first2=Murid|last3=Shehzad|first3=Nasir|last4=Maafa|first4=Ibrahim M.|last5=Akhter|first5=Parveen|last6=Amjad|first6=Um-e-salma|last7=Shafique|first7=Sumeer|last8=Razzaq|first8=Abdul|last9=Yang|first9=Wenshu|last10=Tahir|first10=Muhammad|last11=Russo|first11=Nunzio|date=August 2019|title=The effect of crystal facets and induced porosity on the performance of monoclinic BiVO4 for the enhanced visible-light driven photocatalytic abatement of methylene blue|url=http://dx.doi.org/10.1016/j.jece.2019.103265|journal=Journal of Environmental Chemical Engineering|volume=7|issue=4|pages=103265|doi=10.1016/j.jece.2019.103265|s2cid=198742844 |issn=2213-3437}}Shafiq, I. (2018). Mesoporous monoclinic BiVO4 for efficient visible light driven photocatalytic degradation of dyes (Doctoral dissertation, COMSATS University Islamabad, Lahore Campus.). BV have demonstrated efficiencies of 5.2% for flat thin films{{cite journal|last=Abdi|first=Fatwa F|author2=Lihao Han |author3=Arno H. M. Smets |author4=Miro Zeman |author5=Bernard Dam |author6=Roel van de Krol |title=Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode|journal=Nature Communications|date=29 July 2013|doi=10.1038/ncomms3195|bibcode = 2013NatCo...4.2195A |volume=4 |pmid=23893238 |page=2195|doi-access=free }}{{cite journal | doi = 10.1002/cssc.201402901 | volume=7 | title=Inside Cover: Efficient Water-Splitting Device Based on a Bismuth Vanadate Photoanode and Thin-Film Silicon Solar Cells (ChemSusChem 10/2014) | year=2014 | journal=ChemSusChem | page=2758 | last1 = Han | first1 = Lihao | last2 = Abdi | first2 = Fatwa F. | last3 = van de Krol | first3 = Roel | last4 = Liu | first4 = Rui | last5 = Huang | first5 = Zhuangqun | last6 = Lewerenz | first6 = Hans-Joachim | last7 = Dam | first7 = Bernard | last8 = Zeman | first8 = Miro | last9 = Smets | first9 = Arno H. M.| issue=10 | doi-access = free | bibcode=2014ChSCh...7.2758H }} and 8.2% for core-shell WO₃@BiVO₄ nanorods with thin absorbers.{{Cite journal|last1=Pihosh|first1=Yuriy|last2=Turkevych|first2=Ivan|last3=Mawatari|first3=Kazuma|last4=Uemura|first4=Jin|last5=Kazoe|first5=Yutaka|last6=Kosar|first6=Sonya|last7=Makita|first7=Kikuo|last8=Sugaya|first8=Takeyoshi|last9=Matsui|first9=Takuya|last10=Fujita|first10=Daisuke|last11=Tosa|first11=Masahiro|date=2015-06-08|title=Photocatalytic generation of hydrogen by core-shell WO 3 /BiVO 4 nanorods with ultimate water splitting efficiency|journal=Scientific Reports|language=en|volume=5|issue=1|pages=11141|doi=10.1038/srep11141|issn=2045-2322|pmc=4459147|pmid=26053164|bibcode=2015NatSR...511141P }}{{Cite journal|last1=Kosar|first1=Sonya|last2=Pihosh|first2=Yuriy|last3=Turkevych|first3=Ivan|last4=Mawatari|first4=Kazuma|last5=Uemura|first5=Jin|last6=Kazoe|first6=Yutaka|last7=Makita|first7=Kikuo|last8=Sugaya|first8=Takeyoshi|last9=Matsui|first9=Takuya|last10=Fujita|first10=Daisuke|last11=Tosa|first11=Masahiro|date=2016-02-25|title=Tandem photovoltaic–photoelectrochemical GaAs/InGaAsP–WO3/BiVO4device for solar hydrogen generation|journal=Japanese Journal of Applied Physics|volume=55|issue=4S|pages=04ES01|doi=10.7567/jjap.55.04es01|bibcode=2016JaJAP..55dES01K |s2cid=125395272 |issn=0021-4922}}{{Cite journal|last1=Kosar|first1=Sonya|last2=Pihosh|first2=Yuriy|last3=Bekarevich|first3=Raman|last4=Mitsuishi|first4=Kazutaka|last5=Mawatari|first5=Kazuma|last6=Kazoe|first6=Yutaka|last7=Kitamori|first7=Takehiko|last8=Tosa|first8=Masahiro|last9=Tarasov|first9=Alexey B.|last10=Goodilin|first10=Eugene A.|last11=Struk|first11=Yaroslav M.|date=2019-07-01|title=Highly efficient photocatalytic conversion of solar energy to hydrogen by WO3/BiVO4 core–shell heterojunction nanorods|journal=Applied Nanoscience|language=en|volume=9|issue=5|pages=1017–1024|doi=10.1007/s13204-018-0759-z|bibcode=2019ApNan...9.1017K |s2cid=139703154|issn=2190-5517}}

Bismuth oxides are characterized by visible light absorption properties, just like vanadates.{{Cite journal |last1=Ropero-Vega |first1=J.L. |last2=Pedraza-Avella |first2=J.A. |last3=Niño-Gómez |first3=M.E. |date=September 2015 |title=Hydrogen production by photoelectrolysis of aqueous solutions of phenol using mixed oxide semiconductor films of Bi–Nb–M–O (M=Al, Fe, Ga, In) as photoanodes |url=https://linkinghub.elsevier.com/retrieve/pii/S0920586114007512 |journal=Catalysis Today |language=en |volume=252 |pages=150–156 |doi=10.1016/j.cattod.2014.11.007}}{{Cite journal |last1=Ropero-Vega |first1=J. L. |last2=Meléndez |first2=A. M. |last3=Pedraza-Avella |first3=J. A. |last4=Candal |first4=Roberto J. |last5=Niño-Gómez |first5=M. E. |date=July 2014 |title=Mixed oxide semiconductors based on bismuth for photoelectrochemical applications |url=http://link.springer.com/10.1007/s10008-014-2420-4 |journal=Journal of Solid State Electrochemistry |language=en |volume=18 |issue=7 |pages=1963–1971 |doi=10.1007/s10008-014-2420-4 |s2cid=95775856 |issn=1432-8488|hdl=11336/31744 |hdl-access=free }}

Tungsten diselenide has photocatalytic properties that might be a key to more efficient electrolysis.{{cite web |url=http://www.nbcnews.com/science/science-news/discovery-brightens-solars-future-energy-costs-be-cut-n385841 |title=Discovery Brightens Solar's Future, Energy Costs to Be Cut |author= |date=July 2, 2015 |website=NBC News |publisher=NBC News from Reuters |access-date=July 2, 2015 }}

Systems based on III-V semiconductors, such as InGaP, enable solar-to-hydrogen efficiencies of up to 14%.{{cite journal|last=May|first=Matthias M|author2=Hans-Joachim Lewerenz |author3=David Lackner |author4=Frank Dimroth |author5=Thomas Hannappel |title=Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure|journal=Nature Communications|date=15 September 2015|doi=10.1038/ncomms9286|volume=6 |pages=8286|arxiv = 1508.01666 |bibcode = 2015NatCo...6.8286M |pmid=26369620 |pmc=4579846}} Challenges include long-term stability and cost.

2-dimensional semiconductors such as Molybdenum disulfide are actively researched as potential photocatalysts.{{cite journal|last1=Luo|first1=Bin|last2=Liu|first2=Gang|last3=Wang|first3=Lianzhou|author-link3=Lianzhou Wang|year=2016|title=Recent advances in 2D materials for photocatalysis|journal=Nanoscale|volume=8|issue=13|pages=6904–6920|doi=10.1039/C6NR00546B|issn=2040-3364|pmid=26961514|bibcode=2016Nanos...8.6904L }}{{cite journal|last1=Li|first1=Yunguo|last2=Li|first2=Yan-Ling|last3=Sa|first3=Baisheng|last4=Ahuja|first4=Rajeev|title=Review of two-dimensional materials for photocatalytic water splitting from a theoretical perspective|journal=Catalysis Science & Technology|volume=7|issue=3|year=2017|pages=545–559|issn=2044-4753|doi=10.1039/C6CY02178F|url=https://discovery.ucl.ac.uk/id/eprint/1555518/}}

An aluminum‐based metal-organic framework made from 2‐aminoterephthalate can be modified by incorporating Ni²⁺ cations into the pores through coordination with the amino groups.{{cite journal |title=Ni(2) Coordination to an Al-Based Metal–Organic Framework Made from 2-Aminoterephthalate for Photocatalytic Overall Water Splitting |author= |journal=Angewandte Chemie International Edition |date=February 7, 2017 |volume=56 |issue=11 |pages=3036–3040 |doi=10.1002/anie.201612423 |pmid=28170148 }}Molybdenum disulfide

Organic semiconductor photocatalysts, in particular porous organic polymers (POPs), attracted attention due to their low cost, low toxicity, and tunable light absorption vs inorganic counterparts.{{Cite journal|last1=Kalsin|first1=A. M.|last2=Fialkowski|first2=M.|last3=Paszewski|first3=M.|last4=Smoukov|first4=S. K.|last5=Bishop|first5=K. J. M.|last6=Grzybowski|first6=B. A.|date=2006-04-21|title=Electrostatic Self-Assembly of Binary Nanoparticle Crystals with a Diamond-Like Lattice|journal=Science|language=en|volume=312|issue=5772|pages=420–424|doi=10.1126/science.1125124|pmid=16497885|bibcode=2006Sci...312..420K |issn=0036-8075|doi-access=free}}{{Cite journal|last1=Martin|first1=David James|last2=Reardon|first2=Philip James Thomas|last3=Moniz|first3=Savio J. A.|last4=Tang|first4=Junwang|date=2014-09-10|title=Visible Light-Driven Pure Water Splitting by a Nature-Inspired Organic Semiconductor-Based System|journal=Journal of the American Chemical Society|volume=136|issue=36|pages=12568–12571|doi=10.1021/ja506386e|pmid=25136991|issn=0002-7863|doi-access=free|bibcode=2014JAChS.13612568M }}{{Cite journal|last1=Weingarten|first1=Adam S.|last2=Kazantsev|first2=Roman V.|last3=Palmer|first3=Liam C.|last4=Fairfield|first4=Daniel J.|last5=Koltonow|first5=Andrew R.|last6=Stupp|first6=Samuel I.|date=2015-12-09|title=Supramolecular Packing Controls H2 Photocatalysis in Chromophore Amphiphile Hydrogels|journal=Journal of the American Chemical Society|volume=137|issue=48|pages=15241–15246|doi=10.1021/jacs.5b10027|issn=0002-7863|pmc=4676032|pmid=26593389|bibcode=2015JAChS.13715241W }} They display high porosity, low density, diverse composition, facile functionalization, high chemical/thermal stability, as well as high surface areas.{{Cite journal|last1=Zhang|first1=Ting|last2=Xing|first2=Guolong|last3=Chen|first3=Weiben|last4=Chen|first4=Long|date=2020-02-07|title=Porous organic polymers: a promising platform for efficient photocatalysis|journal=Materials Chemistry Frontiers|language=en|volume=4|issue=2|pages=332–353|doi=10.1039/C9QM00633H|issn=2052-1537|doi-access=free}} Efficient conversion of hydrophobic polymers into hydrophilic polymer nano-dots (Pdots) increased polymer-water interfacial contact, which significantly improved performance.{{Cite journal|last1=Wang|first1=Lei|last2=Fernández-Terán|first2=Ricardo|last3=Zhang|first3=Lei|last4=Fernandes|first4=Daniel L. A.|last5=Tian|first5=Lei|last6=Chen|first6=Hong|last7=Tian|first7=Haining|date=2016|title=Organic Polymer Dots as Photocatalysts for Visible Light-Driven Hydrogen Generation|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201607018|journal=Angewandte Chemie International Edition|language=en|volume=55|issue=40|pages=12306–12310|doi=10.1002/anie.201607018|pmid=27604393|issn=1521-3773}}{{Cite journal|last1=Pati|first1=Palas Baran|last2=Damas|first2=Giane|last3=Tian|first3=Lei|last4=Fernandes|first4=Daniel L. A.|last5=Zhang|first5=Lei|last6=Pehlivan|first6=Ilknur Bayrak|last7=Edvinsson|first7=Tomas|last8=Araujo|first8=C. Moyses|last9=Tian|first9=Haining|date=2017-06-14|title=An experimental and theoretical study of an efficient polymer nano-photocatalyst for hydrogen evolution|journal=Energy & Environmental Science|language=en|volume=10|issue=6|pages=1372–1376|doi=10.1039/C7EE00751E|issn=1754-5706|doi-access=free|bibcode=2017EnEnS..10.1372P }}{{Cite journal|last1=Rahman|first1=Mohammad|last2=Tian|first2=Haining|last3=Edvinsson|first3=Tomas|date=2020|title=Revisiting the Limiting Factors for Overall Water-Splitting on Organic Photocatalysts|journal=Angewandte Chemie International Edition|language=en|volume=59|issue=38|pages=16278–16293|doi=10.1002/anie.202002561|issn=1521-3773|pmc=7540687|pmid=32329950}}

Beweries, et al., developed a light-driven "closed cycle of water splitting using ansa-titanocene(III/IV) triflate complexes".{{Cite journal |last1=Godemann |first1=Christian |last2=Hollmann |first2=Dirk |last3=Kessler |first3=Monty |last4=Jiao |first4=Haijun |last5=Spannenberg |first5=Anke |last6=Brückner |first6=Angelika |last7=Beweries |first7=Torsten |date=2015-12-30 |title=A Model of a Closed Cycle of Water Splitting Using ansa -Titanocene(III/IV) Triflate Complexes |url=https://pubs.acs.org/doi/10.1021/jacs.5b11365 |journal=Journal of the American Chemical Society |language=en |volume=137 |issue=51 |pages=16187–16195 |doi=10.1021/jacs.5b11365 |pmid=26641723 |bibcode=2015JAChS.13716187G |issn=0002-7863}}

An Indium gallium nitride (InxGa1-xN) photocatalyst achieved a solar-to-hydrogen efficiency of 9.2% from pure water and concentrated sunlight. The effiency is due to the synergistic effects of promoting hydrogen–oxygen evolution and inhibiting recombination by operating at an optimal reaction temperature (~70 degrees C), powered by harvesting previously wasted infrared light. An STH efficiency of about 7% was realized from tap water and seawater and efficiency of 6.2% in a larger-scale system with a solar light capacity of 257 watts.{{Cite journal |last1=Zhou |first1=Peng |last2=Navid |first2=Ishtiaque Ahmed |last3=Ma |first3=Yongjin |last4=Xiao |first4=Yixin |last5=Wang |first5=Ping |last6=Ye |first6=Zhengwei |last7=Zhou |first7=Baowen |last8=Sun |first8=Kai |last9=Mi |first9=Zetian |date=January 2023 |title=Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting |url=https://www.nature.com/articles/s41586-022-05399-1 |journal=Nature |language=en |volume=613 |issue=7942 |pages=66–70 |doi=10.1038/s41586-022-05399-1 |pmid=36600066 |bibcode=2023Natur.613...66Z |s2cid=255474993 |issn=1476-4687}}

Solid solutions {{chem|Cd|1-x|Zn|x|S}} with different Zn concentration (0.2 < x < 0.35) have been investigated in the production of hydrogen from aqueous solutions containing $\backslash ce\{SO3^2-\}/\backslash ce\{S^2-\}$ as sacrificial reagents under visible light.{{cite journal |last1=del Valle |first1=F. |last2=Ishikawa |first2=A. |last3=Domen |first3=K. |last4=Villoria De La Mano |first4=J.A. |last5=Sánchez-Sánchez |first5=M.C. |last6=González |first6=I.D. |last7=Herreras |first7=S. |last8=Mota |first8=N. |last9=Rivas |first9=M.E. |date=May 2009 |title=Influence of Zn concentration in the activity of Cd1-xZnxS solid solutions for water splitting under visible light |journal=Catalysis Today |volume=143 |issue=1–2 |pages=51–59 |doi=10.1016/j.cattod.2008.09.024}} Textural, structural and surface catalyst properties were determined by {{chem|N|2}} adsorption isotherms, UV–vis spectroscopy, SEM and XRD and related to the activity results in hydrogen production from water splitting under visible light. It was reported that the crystallinity and energy band structure of the {{chem|Cd|1-x|Zn|x|S}} solid solutions depend on their Zn atomic concentration. The hydrogen production rate increased gradually as Zn concentration on photocatalysts increased from 0.2 to 0.3. The subsequent increase in the Zn fraction up to 0.35 reduced production. Variation in photoactivity was analyzed for changes in crystallinity, level of the conduction band and light absorption ability of {{chem|Cd|1-x|Zn|x|S}} solid solutions derived from their Zn atomic concentration.

• {{cite journal|last1= Chu |first1= Sheng |last2= Li |first2= Wei |last3= Hamann |first3= Thomas |last4= Shih |first4= Ishiang |last5= Wang |first5= Dunwei |last6= Mi |first6= Zetian |year= 2017 |title= Roadmap on solar water splitting: current status and future prospects |journal= Nano Futures |volume= 1 |issue= 2|pages= 022001 |doi= 10.1088/2399-1984/aa88a1 |bibcode=2017NanoF...1b2001C|s2cid= 3903962 }}
• {{Cite journal |last1=Takata |first1=Tsuyoshi |last2=Jiang |first2=Junzhe |last3=Sakata |first3=Yoshihisa |last4=Nakabayashi |first4=Mamiko |last5=Shibata |first5=Naoya |last6=Nandal |first6=Vikas |last7=Seki |first7=Kazuhiko |last8=Hisatomi |first8=Takashi |last9=Domen |first9=Kazunari |date=2020-05-28 |title=Photocatalytic water splitting with a quantum efficiency of almost unity |url=http://www.nature.com/articles/s41586-020-2278-9 |journal=Nature |language=en |volume=581 |issue=7809 |pages=411–414 |doi=10.1038/s41586-020-2278-9 |pmid=32461647 |bibcode=2020Natur.581..411T |s2cid=218912943 |issn=0028-0836}}
• {{cite journal |last1=del Valle |first1=F. |last2=Álvarez Galván |first2=M. Consuelo |last3=Del Valle |first3=F. |last4=Villoria De La Mano |first4=José A. |last5=Fierro |first5=José L. G. |display-authors=etal |date=Jun 2009 |title=Water Splitting on Semiconductor Catalysts under Visible-Light Irradiation |journal=ChemSusChem |volume=2 |issue=6 |pages=471–485 |doi=10.1002/cssc.200900018 |pmid=19536754|bibcode=2009ChSCh...2..471N }}
• {{cite book |last1=del Valle |first1=F. |title=Photocatalytic water splitting under visible Light: concept and materials requirements |last2=Del Valle |first2=F. |last3=Villoria De La Mano |first3=J.A. |last4=Álvarez-Galván |first4=M.C. |last5=Fierro |first5=J.L.G. |chapter=Advances in Chemical Engineering |year=2009 |isbn=9780123747631 |volume=36 |pages=111–143 |doi=10.1016/S0065-2377(09)00404-9 |ref=CONACYT Mexico |display-authors=etal}}

• Artificial photosynthesis
• High-temperature electrolysis
• Photochemical reduction of carbon dioxide
• Electrolysis of water
• Photoelectrolysis of water

{{reflist|30em}}

Category:Environmental chemistry

Category:Hydrogen production

Category:Photochemistry