Financial support was provided by IIT Gandhinagar and Government of India. We would also like to thank the extramural funding provided by Science and Engineering Research Board (SERB) (File no. EMR/2015/002462).

1. Synthesis of the photosensitizer-catalyst hybrid
<ol>
	<li>Synthesis of catalyst precursor Co(dmg)2Cl2 complex 
	NOTE: This complex was synthesized following the modified version of the reported procedure29.
	<ol>
		<li>Dissolve 232 mg (1 mmol) of dimethylglyoxime (dmg) ligand (two equivalents in this reaction) in 27 mL of acetone.</li>
		<li>Dissolve 118 mg (0.5 mmol) of CoCl2&#8729;6H2O (one equivalent in this reaction) in 3 mL of deionized water separately that produces a pink color solution.</li>
		<li>Add the aqueous CoCl2 solution drop wise to the acetone solution containing dmg with continuous stirring at room temperature.</li>
		<li>Closely monitor the change in the solution color, which will sequentially turn to bluish green color following the metal addition.</li>
		<li>Continue the reaction for 2 h.</li>
		<li>Filter the reaction mixture through a Grade 40 filter paper and keep the filtrate at 4 &#176;C overnight.</li>
		<li>The next day, obtain the green colored precipitate of Co(dmg)2Cl2 complex (cobaloxime) from the solution and filter it through grade 40 filter paper.</li>
		<li>Dry the sample under air.</li>
	</ol>
	</li>
	<li>Synthesis of the photosensitizer (PS)-cobaloxime hybrid 
	NOTE: The stilbene based photosensitizer (PS) was synthesized as per the reported method28. The following steps were followed for the PS-catalyst hybrid complex synthesis.
	<ol>
		<li>Add 100 mg (0.277 mmol) of cobaloxime (one equivalent) (synthesized in Step 1) in 5 mL of methanol. It will form a green suspension.</li>
		<li>Add 38 &#181;L (0.277 mmol) of triethylamine (TEA) base (one equivalent) to the green suspension with continuous stirring. The solution will turn transparent brown after within 1 min.</li>
		<li>Add 65 mg (0.277 mmol) of solid stilbene dye (one equivalent) to the previously mentioned TEA added cobalt solution in methanol.</li>
		<li>Continue the stirring for 3 h. Closely monitor the change in the solution, which will sequentially produce the reddish-brown precipitate of the PS-cobaloxime hybrid.</li>
		<li>Filter the reddish-brown precipitate with Grade 40 filter paper and wash it with copious amount of cold methanol (20 mL).</li>
		<li>Dissolve the precipitate in chloroform (10 mL) and collect the reddish-brown filtrate.</li>
		<li>Evaporate the filtrate under reduced pressure using a rotavapor at room temperature.</li>
		<li>Collect the solid reddish-brown product [Observed yield: 76 mg (65%)].</li>
		<li>Recrystallize the product from chloroform solution at room temperature, where the chloroform evaporates slowly to produce reddish-brown crystals of the complex.</li>
	</ol>
	</li>
</ol>
2. Characterization of the photosensitizer-cobaloxime hybrid
<ol>
	<li>NMR characterization
	<ol>
		<li>Dissolve 5.0 mg of the purified PS-Cobaloxime hybrid complex in 650 &#956;L of d6-DMSO.</li>
		<li>Record the 1H NMR in NMR spectrometer at room temperature. 
		NOTE: 1H NMR signals, in &#948; (ppm) units with the corresponding number of protons, their identity, and the splitting pattern in parentheses (s = singlet, d = doublet, m = multiplet), are as following: 1H NMR: 2.34 (12H, -dmg-CH3, s), 2.97 (6H, -dye-N-(CH3)2, s), 6.74 (2H, dye-aromatic, d), 6.84 (1H, allylic-H, d), 7.48 (5H, four dye-aromatic, one allylic-H, m), 7.82(2H, dye-aromatic, d), 18.47(2H,dmg-NOH, s).</li>
	</ol>
	</li>
	<li>UV-Vis spectroscopy
	<ol>
		<li>Prepare a 1.0 mM solution of the PS-cobaloxime complex in N,N&#8242;-dimethylformamide (DMF) by adding the appropriately weighed amount of the complex in the solvent.</li>
		<li>Dilute the solution 10 times with blank DMF to generate 0.1 mM solution of the hybrid complex in DMF.</li>
		<li>Further dilute it 5 times with blank DMF to generate 20 &#956;M solution of the hybrid complex in DMF.</li>
		<li>Record the optical spectra of the 20 &#956;M PS-cobaloxime complex solution using a spectrophotometer. 
		NOTE: UV-Vis peaks (&#955;/nm), with the corresponding molar extinction coefficient (&#949; /M-1cm-1) in parentheses, are as follows: 266 (13400) and 425 (14600).</li>
	</ol>
	</li>
	<li>Single crystal structure determination
	<ol>
		<li>Prepare a concentrated 0.2 M sample of the PS-catalyst hybrid complex in 5 mL of chloroform. Grow reddish-brown (cubic) crystals of the complex from that chloroform solution over 3 days.</li>
		<li>Select a suitable crystal of the complex and mount on a cryo-loop using cryoprotectant (e.g., Paratone oil).</li>
		<li>Collect the single crystal diffraction data for the hybrid complex at 298 K on the diffractometer.</li>
		<li>Apply the empirical absorption correction to the data by employing the multi-scan method in SADABS programming30.</li>
		<li>Resolve the structure by direct methods with SHELXS-97 and refine by the full matrix least square methods on F2 using the SHELXL-201431.</li>
	</ol>
	</li>
	<li>Electrochemical studies
	<ol>
		<li>Sample preparation
		<ol>
			<li>Prepare a 1 mM solution of the PS-catalyst hybrid complex in HPLC grade DMF containing 0.1 M tetra-N-butyl ammonium fluoride (n-Bu4N+F&#8722;/TBAF).</li>
			<li>Place 2 mL of the sample solution prepared in step 1 in the electrochemical cell (volume 5 mL).</li>
			<li>Purge N2 gas through the solution for 30 min to remove oxygen.</li>
		</ol>
		</li>
		<li>Electrode preparation
		<ol>
			<li>Polish the 1 mm diameter glassy carbon-disc working electrode with 0.25 &#956;m alumina paste prepared in water on a polishing pad.</li>
			<li>Rinse the polished electrode thoroughly with a copious amount of deionized water.</li>
			<li>Place the clean working electrode in the electrochemical cell.</li>
			<li>Place the Ag/AgCl (in 1.0 M AgNO3) reference electrode and the platinum (Pt)-wire counter electrode in the electrochemical cell.</li>
			<li>Connect all the electrodes accordingly to the potentiostat.</li>
		</ol>
		</li>
		<li>Collecting data
		<ol>
			<li>Stop the N2 gas purging before the electrochemical experiment.</li>
			<li>Keep a continuous flow of N2 above the sample solution in the electrochemical cell.</li>
			<li>Record cyclic voltammograms (CV) of the sample starting from the anodic direction to cathodic direction with appropriate scan rate (0.1 V/s scan rate was used in this experiment).</li>
			<li>Repeat the above experiment by adding appropriate amounts of water (30% water in DMF) and trifluoroacetic acid (TFA) (8 &#956;L of 10x diluted neat TFA), respectively.</li>
			<li>Add ferrocene to the sample solution and record the corresponding CV. Adjust the potential scale with the ferrocene couple (FeCp2+/0 = 0V vs. Ferrocene) for all the collected data. Thus, all the potential values mentioned in this work was internally referenced against Ferrocene couple.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
3. Catalytic H2 production by the photosensitizer-catalyst hybrid in sunlight
<ol>
	<li>Photocatalytic H2 production by the PS-catalyst hybrid complex
	<ol>
		<li>Prepare 0.2 mM PS-catalyst hybrid complex in 10 mL of 70:30 DMF water (pH 7, 0.1 MES buffer) in a two-neck test tube.</li>
		<li>Add 1 mL of triethanolamine (TEOA) as the sacrificial electron donor to the sample solution.</li>
		<li>Close the two openings of the test tube with the air-tight septum.</li>
		<li>Connect this setup with the H2 detector with appropriate tubing connections. 
		NOTE: The H2 detector has two tube connections. One of them acts as the input that goes through an in-built detector to measure the amount of H2 (in ppm units) present in the sample. The measured gas sample then connects back to the reaction vessel by the output tubing.</li>
		<li>Place the set up under sunlight for 30 min and monitor the H2 production rate via the detector.</li>
	</ol>
	</li>
	<li>Monitoring the solar-driven H2 production via gas chromatography (GC)
	<ol>
		<li>Collect 1 mL of headspace gas via gas-tight syringe.</li>
		<li>Inject the collected gas in the gas chromatography (GC) instrument.</li>
		<li>Monitor the resulted gas chromatograph.</li>
		<li>Inject 1 mL of headspace gas collected from a control sample placed under dark.</li>
		<li>Inject 1 mL of gas from a calibrated standard gas mixture containing 1% H2.</li>
	</ol>
	</li>
</ol>

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<li>Das, A., Han, Z., Haghighi, M. G., Eisenberg, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photogeneration+of+hydrogen+from+water+using+CdSe+nanocrystals+demonstrating+the+importance+of+surface+exchange%5BTitle%5D)+AND+%22Proceedings+of+the+National+Academy+of+Sciences+of+the+United+States+of+America%22%5BJournal%5D)">Photogeneration of hydrogen from water using CdSe nanocrystals demonstrating the importance of surface exchange.</a> Proceedings of the National Academy of Sciences of the United States of America. 110 (42), 16716-16723 (2013).</li>
<li>Das, A., Han, Z., Brennessel, W. W., Holland, P. L., Eisenberg, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Nickel+Complexes+for+Robust+Light-Driven+and+Electrocatalytic+Hydrogen+Production+from+Water%5BTitle%5D)+AND+%22ACS+Catalysis%22%5BJournal%5D)">Nickel Complexes for Robust Light-Driven and Electrocatalytic Hydrogen Production from Water.</a> ACS Catalysis. 5 (3), 1397-1406 (2015).</li>
<li>Eckenhoff, W. T., Eisenberg, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Molecular+systems+for+light+driven+hydrogen+production%5BTitle%5D)+AND+%22Dalton+Transactions%22%5BJournal%5D)">Molecular systems for light driven hydrogen production.</a> Dalton Transactions. 41 (42), 13004-13021 (2012).</li>
<li>Dutta, A., Appel, A. M., Shaw, W. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Designing+electrochemically+reversible+H+2+oxidation+and+production+catalysts%5BTitle%5D)+AND+%22Nature+Reviews+Chemistry%22%5BJournal%5D)">Designing electrochemically reversible H 2 oxidation and production catalysts.</a> Nature Reviews Chemistry. 2 (9), 244(2018).</li>
<li>Savéant, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Proton+Relays+in+Molecular+Catalysis+of+Electrochemical+Reactions%3A+Origin+and+Limitations+of+the+Boosting+Effect%5BTitle%5D)+AND+%22Angewandte+Chemie+International+Edition%22%5BJournal%5D)">Proton Relays in Molecular Catalysis of Electrochemical Reactions: Origin and Limitations of the Boosting Effect.</a> Angewandte Chemie International Edition. 58 (7), 2125-2128 (2019).</li>
<li>Khandelwal, S., Zamader, A., Nagayach, V., Dolui, D., Mir, A. Q., Dutta, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Inclusion+of+Peripheral+Basic+Groups+Activates+Dormant+Cobalt-Based+Molecular+Complexes+for+Catalytic+H2+Evolution+in+Water%5BTitle%5D)+AND+%22ACS+Catalysis%22%5BJournal%5D)">Inclusion of Peripheral Basic Groups Activates Dormant Cobalt-Based Molecular Complexes for Catalytic H2 Evolution in Water.</a> ACS Catalysis. , 2334-2344 (2019).</li>
<li>Staffell, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+role+of+hydrogen+and+fuel+cells+in+the+global+energy+system%5BTitle%5D)+AND+%22Energy+%26+Environmental+Science%22%5BJournal%5D)">The role of hydrogen and fuel cells in the global energy system.</a> Energy & Environmental Science. 12 (2), 463-491 (2019).</li>
</ol>

The authors have nothing to disclose.

The organic photosensitizer stilbene moiety was successfully incorporated into the cobaloxime core via the axial pyridine linkage (Figure 1). This strategy allowed us to devise a photosensitizer-cobaloxime hybrid complex C1. The presence of both the oxime and organic dye in the same molecular framework was evident from the single crystal structure of the C1 (Figure 4). The phenyl and pyridine functionalities of the stilbene moti...

In the modern world, fossil fuels such as coal, oil, and natural gas supply a majority share of the energy. However, they produce copious amount of CO2 during the energy harvesting to negatively impact the global climate1. In coming years, a steep rise in energy demand is predicted worldwide following the continuous growth of population and constant improvement in human lifestyle. Thus, there is an active search for a suitable alternative energy resource to match the global energy requirement. Renewable energy resources like solar, wind, and tidal power have emerged as one of the best solutions due to their environment-friendly zero carbon energy transduction process2. However, the intermittent nature of these energy resources has so far limited their extensive application. A possible solution of this problem can be found in biology; solar energy is efficiently transformed into chemical energy during photosynthesis3. Following this clue, researchers have developed artificial photosynthetic strategies for storing solar energy into chemical bonds following a number of small molecule activation reactions4,5. The H2 molecule has been considered one of the most appealing chemical vectors due to their high energy density and simplicity of their chemical transformation6,7.
The presence of a photosensitizer and a H2 production catalyst are essential for an active solar-driven H2 production setup. Here in this work, we will focus on the cobalt-based molecular complex cobaloxime for the catalytic segment. Typically, a hexa-coordinated cobalt center is bound in a square planar N4 geometry, derived from the dimethylglyoxime (dmg) ligands, in cobaloximes. The complementary Cl- ions, solvent molecules (such as water or acetonitrile) or pyridine derivatives ligate in the residual axial positions8. Cobaloximes are long known for active H2 production electrocatalysis and their reactivity can be tuned by appending variable functionalities on the axial pyridine9,10,11,12. The relatively uncomplicated syntheses, oxygen tolerance under catalytic conditions, and moderate catalytic response of cobaloximes have prompted researchers to explore their photocatalytic H2 production reactivity. The Hawecker group was the pioneer in demonstrating the light-driven H2 production activity of cobaloximes by utilizing Ru(polypyridyl)-based photosensitizers13. Eisenberg and his coworkers utilized platinum (Pt)-based inorganic photosensitizers to induce photocatalytic H2 production in tandem with cobaloxime catalysts14,15. Later, the Che group utilized organo-gold photosensitizer to replicate similar activity16. Fontecave and Artero expanded the range of photosensitizers by applying iridium (Ir)-based molecules17. The practical applications of these photocatalytic systems were heading towards a roadblock due to the use of expensive metal-based photosensitizers. The Eisenberg and Sun research groups have countered that by independently devising organic dye-based photo-driven H2 production systems18,19. Despite the successful photo-driven H2 production by all these systems, it was observed that the overall catalytic turnovers were relatively slow20. In all these cases, the photosensitizer and cobaloxime molecules were added as separate moieties in the solution, and the lack of direct communication between them might have hindered the overall efficiency of the system. A number of photosensitizer-cobaloxime dyads were developed to rectify this issue, where a variety of photosensitizers were directly linked with the cobaloxime core via the axial pyridine ligand21,22,23,24,25,26. Sun and co-workers were even successful in developing a noble-metal free device by introducing a Zn-porphyrin motif as a photosensitizer24. Recently, Ott and coworkers have successfully incorporated the cobaloxime catalyst within an metal organic framework (MOF) that displayed photocatalytic H2 production in the presence of organic dye27. However, the inclusion of the high molecular weight photosensitizers into the cobaloxime framework reduced the water solubility while affecting the long-term stability of the dyads under catalytic conditions. The stability of the active dyads under aqueous conditions during the catalysis is crucial as the omnipresent water is an attractive source of protons during the catalysis. Thus, there is a serious need for developing an aqueous soluble, air-stable photosensitizer-cobaloxime dyad system to establish an efficient and economical photo-driven H2 production setup.
Here in this work, we have anchored a stilbene-based organic dye28 as photosensitizer to the cobaloxime core via the axial pyridine linker (Figure 1). The light molecular weight of the dye ensured improved water solubility of the dyad. This stilbene-cobaloxime hybrid molecule was characterized in detail via optical and 1H NMR spectroscopy along with its single crystal structure elucidation. The electrochemical data revealed the active electrocatalytic H2 production by the cobaloxime motif even with the appended organic dye. This hybrid complex exhibited significant photo-driven H2 production when exposed to direct sunlight in the presence of an appropriate sacrificial electron donor in a 30:70 water/DMF (N,N&#8242;-dimethylformamide) solution without any degradation of the hybrid structure as complemented by optical spectroscopy studies. A simple photocatalytic device, consisting of a H2 detector, was employed during the photocatalysis of the hybrid complex that demonstrated continuous production of H2 gas under aqueous aerobic condition without any preliminary lag period. Thus, this hybrid complex has the potential to become the base for developing the next generation of solar-driven H2 production catalysts for efficient renewable energy utilization.

<table><tbody><tr><td>1 mm diameter glassy carbon disc electrode</td><td>ALS Co., Limited, Japan</td><td>2412</td><td>1</td></tr><tr><td>Acetone</td><td>SD fine chemicals</td><td>25214L10</td><td>27 mL</td></tr><tr><td>Ag/AgCl reference electrode</td><td>ALS Co., Limited, Japan</td><td>12171</td><td>1</td></tr><tr><td>Co(dmg)&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;</td><td>Lab synthesised</td><td>NA</td><td>100 mg</td></tr><tr><td>CoCl&lt;sub&gt;2&lt;/sub&gt;.6H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma Aldrich</td><td>C2644</td><td>118 mg</td></tr><tr><td>d&lt;sup&gt;6&lt;/sup&gt; dmso</td><td>Leonid Chemicals</td><td>D034EAS</td><td>650 &amp;micro;L</td></tr><tr><td>Deionized water from water purification system</td><td>NA</td><td>NA</td><td>500 mL</td></tr><tr><td>Dimethyl formamide</td><td>SRL Chemicals</td><td>93186</td><td>5 mL</td></tr><tr><td>Dimethyl glyoxime</td><td>Sigma Aldrich</td><td>40390</td><td>232 mg</td></tr><tr><td>Gas-tight syringe</td><td>SGE syringe Leur lock</td><td>21964</td><td>1</td></tr><tr><td>MES Buffer</td><td>Sigma</td><td>M8250</td><td>195 mg</td></tr><tr><td>Methanol</td><td>Finar</td><td>67-56-1</td><td>15 mL</td></tr><tr><td>Platinum counter electrode</td><td>ALS Co., Limited, Japan</td><td>2222</td><td>1</td></tr><tr><td>Stilbene Dye</td><td>Lab synthesised</td><td>NA</td><td>65 mg</td></tr><tr><td>TBAF(Tetra-n-butylammonium fluoride)</td><td>TCI Chemicals</td><td>T1338</td><td>20 mg</td></tr><tr><td>Triethanolamine</td><td>Finar</td><td>102-71-6</td><td>1 mL</td></tr><tr><td>Triethylamine</td><td>Sigma Aldrich</td><td>T0886</td><td>38 &amp;micro;L</td></tr><tr><td>Trifluoroacetic acid</td><td>Finar</td><td>76-05-1</td><td>10 &amp;micro;L</td></tr><tr><td>Whatman filter paper</td><td>GE Healthcare</td><td>1001125</td><td>2</td></tr></tbody></table>

developing photosensitizer-cobaloxime hybrids for solar-driven h2 production in aqueous aerobic conditions

Developing photocatalytic H2 production devices is the one of the key steps for constructing a global H2-based renewable energy infrastructure. A number of photoactive assemblies have emerged where a photosensitizer and cobaloxime-based H2 production catalysts work in tandem to convert light energy into the H-H chemical bonds. However, the long-term instability of these assemblies and the need for hazardous proton sources have limited their usage. Here, in this work, we have integrated a stilbene-based organic dye into the periphery of a cobaloxime core via a distinct axial pyridine linkage. This strategy allowed us to develop a photosensitizer-catalyst hybrid structure with the same molecular framework. In this article, we have explained the detailed procedure of the synthesis of this hybrid molecule in addition to its comprehensive chemical characterization. The structural and optical studies have exhibited an intense electronic interaction between the cobaloxime core and the organic photosensitizer. The cobaloxime was active for H2 production even in the presence of water as the proton source. Here, we have developed a simple airtight system connected with an online H2 detector for the investigation of the photocatalytic activity by this hybrid complex. This photosensitizer-catalyst dyad present in the experimental setup continuously produced H2 once it was exposed in the natural sunlight. This photocatalytic H2 production by the hybrid complex was observed in aqueous/organic mixture media in the presence of a sacrificial electron donor under complete aerobic conditions. Thus, this photocatalysis measurement system along with the photosensitizer-catalyst dyad provide valuable insight for the development of next generation photocatalytic H2 production devices.

In this work, a stilbene photosensitizer-cobaloxime hybrid complex (C1) was synthesized successfully by anchoring the organic dye (L1) derived pyridine motif as the axial ligand to the cobalt core. The 1H NMR data of the hybrid complex clearly demonstrated the presence of both the cobaloxime and organic dye protons in the same complex. As shown in Figure 2, the up-fielded aliphatic region highlighted the presence of both oxime-bound methyl and sti...

Watch this Scientific Journal Video about Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions at JoVE.com

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

We have directly incorporated a stilbene-based organic dye into a cobaloxime core to generate a photosensitizer-catalyst dyad for photocatalytic H2 production. We have also developed a simple experimental setup for evaluating the light-driven H2 production by photocatalytic assemblies.

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

Developing photocatalytic H2 production devices is the one of the key steps for constructing a global H2-based renewable energy infrastructure. A number ...

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Research

JoVE Journal

Chemistry

8.3K Views.  Indian Institute of Technology Gandhinagar. We have directly incorporated a stilbene-based organic dye into a cobaloxime core to generate a photosensitizer-catalyst dyad for photocatalytic H2 production. We have also developed a simple experimental setup for evaluating the light-driven H2 production by photocatalytic assemblies.

Video: Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Engineering

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 &#181;m were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min&#160;of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Procedures are outlined to prepare segmented and coaxial nanowires via templated electrodeposition in nanopores. As examples, segmented nanowires consisting of Ag and ZnO segments, and coaxial nanowires consisting of a TiO2 shell and a Ag core were made. The nanowires were used in photocatalytic hydrogen formation experiments.

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and ...

Light-driven Enzymatic Decarboxylation

Oxidoreductases belong to the most-applied industrial enzymes. Nevertheless, they need external electrons whose supply is often costly and challenging. Recycling of the electron donors NADH or NADPH requires the use of additional enzymes and sacrificial substrates. Interestingly, several oxidoreductases accept hydrogen peroxide as electron donor. While being inexpensive, this reagent often reduces the stability of enzymes. A solution to this problem is the in situ generation of the cofactor. The continuous supply of the cofactor at low concentration drives the reaction without impairing enzyme stability. This paper demonstrates a method for the light-catalyzed in situ generation of hydrogen peroxide with the example of the heme-dependent fatty acid decarboxylase OleTJE. The fatty acid decarboxylase OleTJE was discovered due to its unique ability to produce long-chain 1-alkenes from fatty acids, a hitherto unknown enzymatic reaction. 1-alkenes are widely used additives for plasticizers and lubricants. OleTJE has been shown to accept electrons from hydrogen peroxide for the oxidative decarboxylation. While addition of hydrogen peroxide damages the enzyme and results in low yields, in situ generation of the cofactor circumvents this problem. The photobiocatalytic system shows clear advantages regarding enzyme activity and yield, resulting in a simple and efficient system for fatty acid decarboxylation.

We describe a protocol for the light-catalyzed generation of hydrogen peroxide &#8212; a cofactor for oxidative transformations.

Oxidoreductases belong to the most-applied industrial enzymes. Nevertheless, they need external electrons whose supply is often costly and challenging. ...

Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide (CdSe@CdS) nanorod photocatalysts. Seeded rods are grown using a colloidal hot-injection method and then moved to an aqueous medium by ligand exchange. CdSe@CdS nanorods, an iridium precursor and other salts are mixed and illuminated. The deposition process is initiated by absorption of photons by the semiconductor particle, which results with formation of charge carriers that are used to promote redox reactions. To insure photochemical oxidative growth we used an electron scavenger. The photogenerated holes oxidize the iridium precursor, apparently in a mediated oxidative pathway. This results in the growth of high quality crystalline iridium oxide particles, ranging from 0.5 nm to about 3 nm, along the surface of the rod. Iridium oxide grown on CdSe@CdS heterostructures was studied by a variety of characterization methods, in order to evaluate its characteristics and quality. We explored means for control over particle size, crystallinity, deposition location on the CdS rod, and composition. Illumination time and excitation wavelength were found to be key parameters for such control. The influence of different growth conditions and the characterization of these heterostructures are described alongside a detailed description of their synthesis. Of significance is the fact that the addition of iridium oxide afforded the rods astounding photochemical stability under prolonged illumination in pure water (alleviating the requirement for hole scavengers).

A protocol for the photochemical oxidative growth of small crystalline iridium oxide nanoparticles on the surface of CdSe@CdS seeded rod nanoparticles is presented.

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide ...

A Simple, Low-cost, and Robust System to Measure the Volume of Hydrogen Evolved by Chemical Reactions with Aqueous Solutions

There is a growing research interest in the development of portable systems which can deliver hydrogen on-demand to proton exchange membrane (PEM) hydrogen fuel cells. Researchers seeking to develop such systems require a method of measuring the generated hydrogen. Herein, we describe a simple, low-cost, and robust method to measure the hydrogen generated from the reaction of solids with aqueous solutions. The reactions are conducted in a conventional one-necked round-bottomed flask placed in a temperature controlled water bath. The hydrogen generated from the reaction in the flask is channeled through tubing into a water-filled inverted measuring cylinder. The water displaced from the measuring cylinder by the incoming gas is diverted into a beaker on a balance. The balance is connected to a computer, and the change in the mass reading of the balance over time is recorded using data collection and spreadsheet software programs. The data can then be approximately corrected for water vapor using the method described herein, and parameters such as the total hydrogen yield, the hydrogen generation rate, and the induction period can also be deduced. The size of the measuring cylinder and the resolution of the balance can be changed to adapt the setup to different hydrogen volumes and flow rates.

The study of methods to generate on-demand hydrogen for fuel cells continues to grow in importance. However, systems to measure hydrogen evolution from the reaction of chemicals with water can be complicated and expensive. This article details a simple, low-cost, and robust method to measure the evolution of hydrogen gas.

There is a growing research interest in the development of portable systems which can deliver hydrogen on-demand to proton exchange membrane (PEM) ...

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic polymers, with the aim of photo-activating oxygen-evolving manganese oxide catalysts, which are important components in artificial photosynthesis. The cross-linking technique was applied to obtain a self-standing membrane with a high PW12O403- content. Incorporation and structure retention of PW12O403- within the polymer matrix were confirmed by FT-IR and micro-Raman spectroscopy, and optical characteristics were investigated by UV-Vis spectroscopy, which revealed successful construction of the metal-to-metal charge transfer (MMCT) unit. After deposition of MnOx oxygen evolving catalysts, photocurrent measurements under visible light irradiation verified the sequential charge transfer, Mn &#8594; MMCT unit &#8594; electrode, and&#160;the photocurrent intensity was consistent with the redox potential of the donor metal (Ce or Co). This method provides a new strategy for preparing integrated systems involving catalysts and photon-absorption parts for use with photo-functional materials.

Here, we present a protocol to prepare charge transfer chromophores based on a polyoxometalate/polymer composite membrane.

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic ...

A Complete Method for Evaluating the Performance of Photocatalysts for the Degradation of Antibiotics in Environmental Remediation

Various antibiotics such as tetracycline, aureomycin, amoxicillin, and levofloxacin are found in large quantities in groundwater and soil systems, potentially leading to the development of resistant and multi-drug resistant bacteria, posing a threat to humans, animals, and environmental systems. Photocatalytic technology has attracted keen interest due to its rapid and stable treatment and direct use of solar energy. However, most studies evaluating the performance of semiconductor catalysts for the photocatalytic degradation of organic pollutants in water are currently incomplete. In this paper, a complete experimental protocol is designed to comprehensively evaluate the photocatalytic performance of semiconductor catalysts. Herein, rhombic dodecahedral silver phosphate was prepared by a simple solvent phase synthesis method at room temperature and atmospheric pressure. BrSubphthalocyanine/Ag3PO4 heterojunction materials were prepared by the solvothermal method. The catalytic performance of as-prepared materials for the degradation of tetracycline was evaluated by studying different influencing factors such as catalyst dosage, temperature, pH, and anions at atmospheric pressure using a 300 W xenon lamp as a simulated solar light source and a light intensity of 350 mW/cm2. Compared with the first cycle, the constructed BrSubphthalocyanine/Ag3PO4 maintained 82.0% of the original photocatalytic activity after five photocatalytic cycles, while the pristine Ag3PO4 maintained only 28.6%. The stability of silver phosphate samples was further tested by a five-cycle experiment. This paper provides a complete process for evaluating the catalytic performance of semiconductor catalysts in the laboratory for the development of semiconductor catalysts with potential for practical applications.

Presented here is a protocol to explore a universal set of experimental procedures for comprehensive laboratory evaluation of photocatalysts in the field of environmental purification, using the example of photocatalytic removal of antibiotic organic pollutant molecules from water by phthalocyanine sensitized silver phosphate composites.

Various antibiotics such as tetracycline, aureomycin, amoxicillin, and levofloxacin are found in large quantities in groundwater and soil systems, ...

Environment

A Facile Synthetic Method to Obtain Bismuth Oxyiodide Microspheres Highly Functional for the Photocatalytic Processes of Water Depuration

Bismuth oxyhalide (BiOI) is a promising material for sunlight-driven-environmental photocatalysis. Given that the physical structure of this kind of materials is highly related to its photocatalytic performance, it is necessary to standardize the synthetic methods in order to obtain the most functional architectures and, thus, the highest photocatalytic efficiency. Here, we report a reliable route to obtain BiOI microspheres via the solvothermal process, using Bi(NO3)3 and potassium iodide (KI) as precursors, and ethylene glycol as a template. The synthesis is standardized in a 150 mL autoclave, at 126 &#176;C for 18 h. This results in 2-3 &#181;m-sized mesoporous microspheres, with a relevant specific surface area (61.3 m2/g). Shortening the reaction times in the synthesis results in amorphous structures, while higher temperatures lead to a slight increase in the porosity of the microspheres, with no effect in the photocatalytic performance. The materials are photo-active under UV-A/visible light irradiation for the degradation of the antibiotic ciprofloxacin in water. This method has demonstrated to be effective in interlaboratory tests, obtaining similar BiOI microspheres in Mexican and Chilean research groups.

This article describes a synthetic method to obtain bismuth oxyiodide microspheres, which are highly functional to perform the photocatalytic removal of organic pollutants, such as ciprofloxacin, in water under UV-A/visible light irradiation.

Bismuth oxyhalide (BiOI) is a promising material for sunlight-driven-environmental photocatalysis. Given that the physical structure of this kind of ...

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the Earth's atmosphere. So-called 'solar fuel' devices, currently developed for terrestrial applications in the quest for realizing a sustainable energy economy on Earth, provide promising alternative systems to existing air-revitalization units employed on the International Space Station (ISS) through photoelectrochemical water-splitting and hydrogen production. One obstacle for water (photo-) electrolysis in reduced gravity environments is the absence of buoyancy and the consequential, hindered gas bubble release from the electrode surface. This causes the formation of gas bubble froth layers in proximity to the electrode surface, leading to an increase in ohmic resistance and cell-efficiency loss due to reduced mass transfer of substrates and products to and from the electrode. Recently, we have demonstrated efficient solar hydrogen production in microgravity environment, using an integrated semiconductor-electrocatalyst system with p-type indium phosphide as the light-absorber and a rhodium electrocatalyst. By nanostructuring the electrocatalyst using shadow nanosphere lithography and thereby creating catalytic 'hot spots' on the photoelectrode surface, we could overcome gas bubble coalescence and mass transfer limitations and demonstrated efficient hydrogen production at high current densities in reduced gravitation. Here, the experimental details are described for the preparations of these nanostructured devices and further on, the procedure for their testing in microgravity environment, realized at the Bremen Drop Tower during 9.3 s of free fall.

Efficient solar-hydrogen production has recently been realized on functionalized semiconductor-electrocatalyst systems in a photoelectrochemical half-cell in microgravity environment at the Bremen Drop Tower. Here, we report the experimental procedures for manufacturing the semiconductor-electrocatalyst device, details of the experimental set-up in the drop capsule and the experimental sequence during free fall.

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the ...

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

The interest on nitrogen-containing heterocycles has expanded rapidly in the synthetic community since they are important motifs for new drugs. Traditionally, they were synthesized through thermal cycloaddition reactions, whereas today, photocatalysis is preferred due to the mild and efficient conditions. With this focus, a new photocatalytic method for the synthesis of nitrogen-containing heterocycles is highly desired. Here, we report a protocol for the biosynthesis of cercosporin, which could function as a metal-free photocatalyst. We then illustrate cercosporin-photocatalyzed protocols for the synthesis of nitrogen-containing heterocycles 1,2,3-thiadiazoles through annulation of azoalkenes with KSCN, and synthesis of 1,4,5,6-tetrahydropyridazines [4+2] through cyclodimerization of azoalkenes under mild conditions, respectively. As a result, there is a new bridge between the microbial fermentation method and organic synthesis in a mild, cost-effective, environmentally friendly and sustainable manner.

New routes for the synthesis of nitrogen-containing heterocycles utilizing cercosporin as a metal-free photocatalyst were developed.

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H₂ Production in Aqueous Aerobic Conditions

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