Influence of Meso-Substitution of the Porphyrin Ring on Enhanced

Subscriber access provided by - Access paid by the | UCSB Libraries

Article

The Influence of Meso-substitution of the Porphyrin Ring on Enhanced Hydrogen Evolution in a Photochemical System. Ekaterina Koposova, Xiao Liu, Andrey Anatolyevich Pendin, Bjoern Thiele, Galina Shumilova, Yury Ermolenko, Andreas Offenhäusser, and Yulia Mourzina J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01467 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

The Influence of Meso-substitution of the Porphyrin Ring on Enhanced Hydrogen Evolution in a Photochemical System Ekaterina Koposovaa,b,*, Xiao Liua,*, Andrey Pendinb, Björn Thielec, Galina Shumilovab, Yury Ermolenkob, Andreas Offenhäussera, Yulia Mourzinaa† a

Peter-Grünberg Institute-8, Forschungszentrum Jülich GmbH, 52428, Jülich and Jülich-Aachen

Research Alliance - Fundamentals of Future Information Technology (JARA-FIT) b

Institute of Chemistry, St. Petersburg State University, Universitetskaya nab. 7/9, 199034 St.

Petersburg, Russia c

Institute of Bio- and Geosciences-2, Forschungszentrum Jülich GmbH, 52428, Jülich

* - these authors contributed equally to the work. † - corresponding author

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 72

Abstract: This study establishes the relationships between the structure of a series of mesosubstituted tin(IV) porphyrins and their efficiency as photosensitizers for hydrogen generation in the Sn(IV)P/Pt - TiO2 nanocomposite system. The electrochemical properties of a series of SnPs, the catalytic performance of Pt nanomodifications, and the morphology of the Pt/TiO2 nanocomposites were characterized by electrochemical and electron microscopy methods. The dependence of photocatalytic performance on the structure for a series of Sn(IV) mesosubstituted phenyl porphyrins was studied and possible mechanisms are discussed employing the results of the electrochemical studies. It was found that the time course and type of the photochemically reduced species of Sn(IV)Ps, which are essential intermediates, are important factors and depend on

the electronegativity of the metal center, the character of meso-

substituents of porphyrin ring, and pH, and are correlated with the redox potential sequence of the

respective

Sn(IV)Ps:

SnTMPyP>SnTPyP>SnTPPS>SnTPPC.

Optimization

of

the

experimental parameters was performed with regard to the SnPs with different functional groups, pH values, concentrations of Pt/TiO2, light intensity, and Pt nanoparticles with different surface stabilizers. Finally, the maximum hydrogen yield under visible light was obtained from the system of Sn (IV) meso-tetra(4-pyridyl)porphyrin dichloride (SnTPyP) sensitized TiO2/Pt prepared by the citrate method/EDTA at pH 9.0. This demonstrates that the photochemically reduced species of SnTPyP are relatively long-lived, so they have enough time to complete electron transfer to TiO2 and/or Pt. The adsorption of SnTPyP on the TiO2/Pt surface is therefore not essential for hydrogen generation. Moreover, this study demonstrates for the first time the synergic effect of the excitation of TiO2 and mostly Q-bands of Sn(IV)P (wavelength range 390650 nm), which enhances the efficiency of photocatalytic hydrogen generation in the system. The Soret band of Sn(IV)TPyP was found to produce a minor (about 23 %) contribution to the


2

Page 3 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


photocatalytic activity of the porphyrin sensitizer in this system. Possible processes involved are discussed and mechanisms are proposed explaining different aspects of a series of photocatalytic systems with SnPs and Pt catalysts for hydrogen production under visible light. These structurefunction relationships are essential to effectively harness solar energy for hydrogen production as well as for a wide range of energy and environmentally related problems.

1. Introduction In recent decades, the emerging global energy crisis and the growing need to replace fossil fuels with clean and renewable energy resources are among the biggest challenges confronting chemists. The concept of harnessing and storing solar energy in chemical bonds as in photosynthetic systems

1,2

and the need for environmentally friendly carbon-neutral energy

2

make photocatalytic hydrogen production from water by the effective conversion of solar energy an attractive solution

2–8

. A photoelectrochemical system for H2 generation is typically designed

with a light-absorbing and charge-separation apparatus, including a photosensitizer and/or semiconductor, a catalyst for the reduction of protons to H2 and a sacrificial electron donor 9–11. Of the many metal oxide semiconductors that have been investigated

12–16

, TiO2 is the most

realistic due to its high thermal and chemical stability, environmental compatibility, and low cost 17

. However, one drawback of the application of TiO2 in photoelectrochemical systems is the

wide band gap (3.0 eV for rutile and 3.2 eV for anatase phase). TiO2 can only absorb UV light, which merely accounts for about 4% of the sun’s spectrum at the earth’s surface

18,19

. Therefore,

extending the absorption wavelength range to the visible light region is a significant topic from a practical point of view. Another disadvantage of TiO2 is the low quantum efficiency because of the rapid recombination of photoexcited charge carriers

20–22

. For example, the recombination of

photogenerated electrons and holes leads to the deactivation of the electronic excited states of


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TiO2. Therefore, modification of TiO2 with inorganic especially dyes

27–29

23,24

and organic

Page 4 of 72

25,26

compounds,

, is attracting increasing interest due to the improvement of the charge-pair

separation and extension of the absorption wavelength range from UV light to the visible light of the solar spectrum. Porphyrins are significant photosensitizer compounds for effective visible light energy conversion in the fields of solar cells, photo-hydrogen production, removal of aqueous pollutants, and medicine

30–34

. They are the basis of some natural pigments (e.g., chlorophyll,

pheophytin) and dramatically increase the sensitivity of the systems to the action of light. The best known natural light-absorbing molecules are chlorophylls contained in green plants in ensembles of the pigments P680 of PSI and P700 of PSII, which initiate the process of transforming sunlight for chemical energy production in photosynthesis. The quantum efficiency of electron transfer from the excited P680 and P700 to their respective acceptors pheophytin and iron-sulfur clusters is nearly 100%

35,36

. This concept of storing solar energy in chemical bonds

has generated interest in designing artificial photosynthetic systems. Because of the difficulty in reproducing the entire reaction (e.g., the oxidation of H2O and the reduction of either CO2 or protons) researchers divide the process into half-reactions. For a photo-driven system, where solar energy is stored as H2, it is necessary to use effective and stable porphyrin because chlorophylls extracted from natural systems are not stable

37

. A series of works have been

devoted to the characterization of the structural and photochemical properties of organized systems of porphyrins in inorganic micro- and nanostructured host materials for photochemical reactions, the intermediate structures of metal porphyrins in their photocatalytic reactions as well as Sb(V) and Sn(IV) porphyrins having strong oxidizing power as photosensitizers in photochemical systems 29,38–40.


4

Page 5 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The optical and electrochemical properties of porphyrin compounds are determined by the electronic state of the porphyrin ring, which depends on the type of central metal ion, substituents in the porphyrin ring, and axial ligands. Due to the favorable combination of optical and electrochemical properties tin(IV) porphyrins, SnPs have been considered for the transformation of the energy of visible light into biological and bioelectrochemical systems. SnPs are characterized by high molar attenuation coefficients of the order of 105 dm3 mol-1 cm-1 41

in the visible light region due to the long conjugated electron system of a porphyrin. Redox potentials of different couples in natural electron transport chains are organized in a

sequence, where at each step a part of the available free energy is used to drive the electron transfer along the chain

42

. Therefore, the redox potential of a porphyrin is also an important

criterion for using its excited state for photocatalytic processes. The energy of LUMO, which can be estimated from the energy corresponding to the first reduction potential of the porphyrin

43,44

should be higher than the energy of the proton reduction on a surface of a particular catalyst. However, electrochemical studies of water-soluble porphyrins are rare. Its highly charged Sn(IV) ion makes the porphyrin ring highly electrophilic and to be easily reduced that is unlike the popular Zn(II)-porphyrin 45. Reduction potentials of some Sn(IV)Ps in aqueous solutions (E1/2 red ˂ -0.3 V, NHE, pH=5.0) 41) indicate the possibility of using the energy of their excited state for proton reduction to hydrogen. The one-electron oxidation potentials, E1/2ox, of some mesosubstituted Sn(IV)TPPs in aqueous solution are >1.5 V (NHE) 41, which is higher than the water oxidation potential. Therefore, although in natural systems the main functions of porphyrincontaining molecules, like enzymes and photosynthetic chlorophylls, proceed via π-radical cation intermediate products

42,46,47

, the cationic species of some Sn(IV)Ps might lead to the oxidative

decomposition of water. In natural photosynthesis, the oxidation of excited P680 and P700


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 72

pigments gives rise to delocalized π-radical cations of chlorophylls in P680.+ and P700.+ (reduction potentials of 1.25 and 0.45 V, respectively) in the hydrophobic membrane which prevent side reactions 42,48,49. In contrast, it was found that the reduced species metalloporphyrin π-radical anions are important intermediates in the photochemical reduction of protons to hydrogen on the surface of a catalyst

45,50–52

. Additionally, the unsuitability of oxidative

quenching mechanisms for the excited state of Sn(IV)TPPC as a photosensitizer was discussed in 49

. Another aspect is that even though the two-electron reactions and products are often favored

for porphyrin compounds, a metal center with a strong electronegativity (e.g., Sb(V) or Sn(IV)) pulls the electron density to the porphyrin core and stabilizes the product of the one-electron reduction π-radical-anion by retarding porphyrin ring protonation, making π-radical-anions of Sb(V) or Sn(IV) porphyrins stable (for minutes at neutral solutions and up to hours in alkaline solutions in the absence of electron acceptor oxygen 53). The stability decreases in the order SbV, SnIV, InIII, GeIV, GaIII, AlIII, ZnII

45,53,54

, which was found to correspond to the order of the

porphyrin ring reduction potential. Thus, the reduction process in aqueous systems with Sn(IV)P is more productive although its π-radical anions react in several ways and may yield undesirable products, e.g., chlorins

20,49,51

. In the study on Sn(IV)TMPyP, it was discussed that upon

excitation with visible light, the excited state of Sn(IV)P, which is an oxidizing agent, favors the formation of the Sn(IV)P π-radical anion in the presence of the electron donor

20,50

, which is

stable and long-lived in the case of Sn(IV)Ps 20,45 in the absence of oxygen. The long-lived photochemically reduced species of Sn(IV)Ps are favorable for the flexible design of photocatalytic systems

20,49,51,52,55

because adsorption or linking the photosensitizer to

further components of the charge separation apparatus is not necessary, which contrasts with commonly used ruthenium complex-sensitized systems (in contrast to Sn(IV)P, free excited Ru


6

Page 7 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dyes rapidly decay to a ground state, ˂ 10 ns, having molar attenuation coefficients of the order of 104 M-1 cm-1) 20,56–58. Additionally, less expensive, and more abundant in contrast to Ru-based sensitizers, Sn(IV)Ps are soluble over a wide pH range. These properties make Sn(IV)Ps very interesting and promising candidates for designing systems converting solar energy into energy in the form of fuel and electricity

34,58–60

. Studies on the intermediates and final reduction and

oxidation products and redox pathways of porphyrins and metalloporphyrins depending on central metal ions, peripheral groups of porphyrin ring, and axial ligands have been performed by A. Harriman, P. Neta, P. Hambright, K.M. Kadish

43,45,53,54,61,62

. As in natural systems, precise

tuning of the redox potentials of the different constituents of the electron transport chains is achieved by controlling the coordination sphere and environment of different constituent redox centers

42,63–65

, it is important to study the dependence of the properties of SnP photosensitizers

in photocatalytic systems on their structure. Understanding these relationships will help to understand important factors and processes in order to design photosensitizers for photocatalytic systems. The Sn(IV)P-sensitized system has already been presented by a number of groups

20,29,49,50,66

because of the advantages mentioned above. However, details and structure-function relationships are still not fully understood and are crucial for maximizing their efficiency. Here, we studied the dependence between the periphery structure of a series of meso-substituted watersoluble SnPs shown in Figure 1 as photosensitizers and their photocatalytic action in the Sn(IV)P/Pt-TiO2 system in photochemical hydrogen evolution under visible light. For the first time, we compared Sn(IV) meso-tetra (4-sulfonatophenyl) porphin dichloride (SnTPPS), Sn(IV) meso-tetra(4-carboxyphenyl) porphin dichloride (SnTPPC), Sn(IV) meso-tetra (4-pyridyl) porphin dichloride (SnTPyP), 5,10,15,20-tetrakis(N-methyl-4-pyridyl) porphyrin-Sn(IV)(OH)2


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 72

tetrachloride (SnTMPyP(OH)2), 5,10,15,20-tetrakis(N-methyl-4-pyridyl) porphyrin-Sn(IV)(Cl)2 tetrachloride (SnTMPyP(Cl)2) over a wide pH range with adsorption of SnTPPS and SnTPPC on TiO2. The best peripheral group and optimal conditions for effective hydrogen generation were found for the series considered. The possible process, factors, and mechanisms involved are proposed and discussed in order to explain the structure-function relationships for the series. Two methods of synthesis of Pt material were also compared: citrate reduction and photoreduction methods with variation of the Pt concentration.

Figure 1. The structures of water-soluble SnPs. 2. Experimental section 2.1. Materials H2PtCl6ˑ6H2O, TiO2 (P25), sodium citrate, EDTA, and ITO electrode were purchased from Sigma-Aldrich Chemicals, Amberlite MB-3 ion exchangers were bought from VWR International GmbH and used as received. SnTPPS, SnTPPC, SnTPyP were purchased from Scientific Frontier, SnTMPyP(OH)2 and SnTMPyP(Cl)2 from Porphyrin System and used as received. A series of NMR analyses of SnTPPC and SnTPyP (Figure S1) showed that impurity in porphyrin samples may account for 10 - 15 %. Solutions for measurements the absorption spectra of SnPs with different peripheral groups at pH=5.2 were prepared by first dissolving appropriate amounts of SnPs in 20 mL distilled water to get 8 µM concentrations separately (the solution was acidified with HCl in case of SnTPyP and in case of SnTPPC NaOH was added in


8

Page 9 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


distilled water to promote dissolution). Then the 0.1 M sodium phosphate buffer solution was prepared, and finally, 20 mL of buffer solution was mixed with 20 mL SnPs solution to obtain the solution with pH 5.2 and 4 µM concentrations, respectively. Distilled water was used throughout all the experiments and other chemicals were reagent grade. 2.2. Apparatus Electrochemical experiments were performed in a three-electrode setup controlled by a potentiostat (AUTOLAB, The Netherlands). Three-electrode systems were composed of the ITO working electrodes modified with Pt nanoparticles, a platinum coil counter electrode, and a Ag/AgCl reference electrode without chloride leakage (achieved by KONBOa) in the experiment (3 M KCl, Ef = 0.210 V vs NHE, DRIREF-450, WPI). The three-electrode setup for cyclic voltammetry of SnPs consisted of a glassy carbon electrode (BASi), a platinum coil as a counter electrode and a Ag/AgCl double-junction reference electrode (3 M KCl, Ef = 0.210 V vs NHE, Metrohm, Switzerland). The solution in the bridge of the reference electrode was changed after each measurement to avoid contaminations of the electrochemical cell. The values of potentials were recorded against a Ag/AgCl double-junction reference electrode. The potential of the Ag/AgCl double-junction reference electrode was measured against a SCE and transferred to the NHE and energy scales using 0.24 V and 4.68 V for SCE, respectively

44

. The potential values

are given in the NHE scale. 1mM SnP solutions for measurements were prepared in a 0.1 M phosphate buffer for pH=7.0 and in 0.5 M KCl for pH=3.0 and pH=9.0. The pH was adjusted using hydrochloric acid and sodium hydroxide solutions. All solutions were purged with argon for 20 min prior to recording and kept under argon atmosphere throughout the measurements. SEM of the nanostructures was carried out on a Zeiss Gemini 1550 device. HRTEM was performed using an FEI TITAN 80-300 microscope with a Cs-image corrector operated at


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 72

300kV. A solar light simulator device (Oriel Instruments) with a 150 W xenon arc lamp (Oriel Instruments, Model No. 6255) with an air mass filter AM 1.5 (Oriel Instruments), 390-650 nm visible light filter, 435 nm long-pass filter (Thorlabs) and 315-445 & 715-1095 nm band-pass color filter (Thorlabs) were used for the photoelectrochemical reaction. The device was operated at different light intensities given by 150 W and 95 W lamp operation power, where the light intensity at 95 W corresponded to 1000 W m-2 as measured by a photodetector (CAS140CT-154 Kompakt-Array-Spektrometer Modell UV-VIS-NIR, Instrument Systems). The UV/Vis spectra of porphyrin solutions were recorded on a Perkin Elmer Lambda 900 spectrometer in 1 cm quartz cuvettes. 2.3. Synthesis of Pt catalysts by two different methods 2.3.1. Citrate reduction method 30 mg of H2PtCl6ˑ6H2O was dissolved in 75 mL of distilled water and the solution was stirred for 30 min without heating. 25 mL of a 1% sodium citrate solution was added to the mixture and refluxed for several hours until the color changed from yellow to brown. After cooling, the solution was stirred with Amberlite MB-3 ion exchangers to remove excess citrate until the conductivity decreased below 5 μs/cm. Finally, the solution was filtered through a 0.45 μm Millipore syringe filter unit and a clear liquid was obtained 67. 2.3.2. Photoreduction method The Pt NPs were produced by mixing a 38 mL aqueous solution of 12 mg H2PtCl6ˑ6H2O and 2 mL methanol. The mixture was purged with nitrogen gas for half an hour to drive away the residual air, and exposed to the 150 W xenon arc lamp (Oriel Instruments, Model No. 6255) without light filters for 4 hours while being stirred until the color of the mixture changed from yellowish to brown. The reaction was carried out in the fume hood with a PMMA window.


10

Page 11 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3.3. Preparation of TiO2/Pt nanocomposites 10 mg of TiO2 (P25) was added to 10 mL of the Pt catalyst solution synthesized by the citrate reduction method and exposed to ultrasound in a water bath for 8 min 67. 2.3.4. Photocatalytic process for hydrogen production TiO2/Pt composite solution was mixed with SnP and EDTA (electron donor) yielding a 50 μM concentration of a photosensitizer and a 20 mM concentration of the donor (400 time concentration excess of the EDTA donor compared to the SnP sensitizer). All solutions were prepared directly before the experiments. To achieve the adsorption of SnTPPS or SnTPPC on TiO2 these solutions were left in the dark for 17 h without being disturbed. The 30 mL reaction mixture was purged with nitrogen for 30 min to remove the dissolved oxygen and exposed to lamp illumination with AM 1.5 and various combinations of filters for 4.5 h under continuous stirring. The diameter of the reactor was 0.025 m and the cross-section of the reactor exposed to the lamp illumination was 12.5x10-4 m2. Some experiments were performed without waiting for the adsorption of porphyrin and the reaction mixture was used directly for the irradiation procedure. The amount of hydrogen was detected by the displacement of water from the inverted vessel immersed in the container with water. The absence of impurity gases from air and analysis of reaction gases was performed on a gas chromatograph equipped with a GC-TCD system consisting of a Trace GC Ultra gas chromatograph, two thermal conductivity detectors (Thermo Scientific, Waltham, MA, USA) and two valves (VICI Valco Instruments, Houston, TX, USA). 3. Results and discussion 3.1. Characterization 3.1.1. Characterization of Pt/TiO2 nanocomposite morphology


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 72

Platinum remains the best and most versatile catalyst in various important reactions, especially in the photocatalytic hydrogen evolution field owing to its large work function and low overpotential for hydrogen production

68,69

. Pt nanoparticles not only serve as electron sinks to

suppress the recombination process of photoexcited charge carriers in TiO2 semiconductor, but also provide active sites for considerably facilitated proton reduction reaction 21. Therefore, from the viewpoint of both electronic and catalytic properties, Pt is frequently considered the most suitable hydrogen generation catalyst. Figure 2 shows representative SEM and TEM images of Pt NPs synthesized by the citrate method on the surface of TiO2. Figure S2 shows a SEM image of the Pt NPs synthesized by the citrate method without TiO2. As shown in Figure 2 (A) and (B), numerous Pt nanoparticles were supported on the TiO2 (P25) with average dimensions of about 3 nm. The corresponding highresolution TEM image shows the monocrystalline Pt NPs and TiO2 formed by densely packed grains in Figure 2 (C). The enlarged high-resolution TEM image of individual grain displays lattice fringes of Pt NPs and TiO2 in Figure 2 (D) with an interplanar distance of about 0.227 nm for Pt NPs and 0.352 nm for TiO2, consistent with the interplanar spacing of Pt (111) planes and TiO2 (210) planes, respectively.


12

Page 13 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. SEM (A), TEM (B) and HRTEM (C) images of the Pt NPs synthesized by the citrate method on the surface of TiO2. Enlarged HRTEM image of individual grain showing lattice fringes (D). 3.1.2. Electrochemical characterization of the Pt NP-modified ITO electrode. To characterize the electrocatalytic activity of Pt NPs for the hydrogen evolution reaction, the Pt NPs synthesized by the citrate method were deposited on the ITO thin film substrate by the drop-casting method and immersed in 0.1 M HClO4 solution in a typical three-electrode cell setup. As can be seen in Figure 3 (A), the cyclic voltammetric curves illustrate hydrogen adsorption and desorption peaks in the potential range of 0 - 0.3 V recorded against NHE on the surface of Pt NPs, and the electrochemically active area of Pt can be estimated by integrating the cathodic


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

current for the hydrogen adsorption reaction

Page 14 of 72

70

. We applied the charge density of a clean

polycrystalline Pt electrode of 210 μC cm-2, which was reported in the previous work, by assuming the adsorption of one H atom per Pt atom

71,72

. The active area of the Pt NPs thus

obtained was 2.47 cm2. The typical polarization curve, which is the relationship between the active surface area normalized current density and potential is presented in Figure 3 (B). As is shown in Figure 3 (B), Pt NPs can initiate a large catalytic current density at low overpotential, indicating the high activity of the Pt NPs in the hydrogen reduction reaction. Experimentally measured Tafel slopes are often applied to characterize the intrinsic activity of an electrocatalyst toward a specific redox process. Thus, the corresponding Tafel plot of the Pt NPs is shown as the inset of Figure 3 (B). The Pt catalysts of the hydrogen production reaction usually display small Tafel slopes

73–77

. In our experiments, the Pt NPs synthesized by the citrate method exhibit a

Tafel slope of 62.7 mV/dec, and the Pt NPs prepared by the photoreduction method show a Tafel slope of 66.5 mV/dec, which was recently published elsewhere by our group

78

, showing

relatively fast kinetics to drive the hydrogen evolution reaction and the high catalytic performance.

Figure 3. Cyclic voltammogram (A) and polarization curve (B) of Pt NPs on the ITO thin film substrate in 0.1 M HClO4 solution, inset of (B): corresponding Tafel plot of Pt NPs.


14

Page 15 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2. UV-visible spectroscopy of SnPs Absorption spectra of SnTPyP, SnTPPS, SnTMPyP, and SnTPPC are shown in Figure 4. According to the four-orbital model applied by Gouterman

79

, the QI-absorption bands

correspond to the HOMO-LUMO gap. For SnTPyP, SnTPPS, SnTPPC, SnTMPyP(OH)2 and SnTMPyP(Cl)2, the QI-absorption occurs in the wavelength interval 589 to 594 nm. For these wavelengths, the electronic transition energy is 2.11 eV and 2.09 eV, respectively. This is consistent with previous work 80. The value of the HOMO-LUMO gap is very important for the characterization of the electron transfer in a system used for light-induced hydrogen production.

Figure 4. Absorption spectra of SnPs with different peripheral groups at pH=5.2, concentrations of SnPs was 4 µM (see section 2.1). 3.3. Electrochemical properties of SnPs From the regularities found in the redox behavior of the porphyrin ring of metalloporphyrins, it may be possible to predict the chemical reactivity and the reduction products of the macrocycle 62

. For the metalloporphyrins as the photosensitizers, the redox properties in aqueous media have

been investigated from the 1980s onwards 81. However, the analysis of electron transfer reactions of porphyrin in aqueous solution is limited by their tendency to aggregate and precipitate

82

and

detailed electrochemical studies of water-soluble porphyrins remain rare 41.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 72

We determined the electrochemical properties of SnPs by cyclic voltammetry at different pH within the potential range -1.0 V ˂ E ˂ +1.6 V (Figure 5, Table 1). During the measurements, problems were experienced with the solubility of water-soluble porphyrins because their solubility is limited by dissociation of peripheral groups. Thus, it was not possible to record cyclic voltammograms of each porphyrin from a series in the whole pH range. SnTPyP is only dissolved in acidic media. Basic media are more appropriate for SnTPPC. For SnTPPS, it was possible to record cyclic voltammograms at pH ≥ 3.5. SnTMPyP(OH)2 and SnTMPyP(Cl)2 are dissolved throughout the entire pH range employed (pH from 3.5 to 9.0). The latter two porphyrins have similar electrochemical behavior, so the data presented here correspond to one of them, namely SnTMPyP(Cl)2. 0.5 M KCl was a background solution in acidic (pH=3.0) and basic media (pH= 9.0), where the pH was adjusted with HCl or NaOH, respectively. Naphosphate buffer was used to make neutral media. Cyclic voltammograms of the background solutions demonstrate that Cl2 evolution starts above +1.4 V (Figure S3)

83

. However, this was

not found for the Na-phosphate buffer at pH=7.0. Therefore, further measurements at pH=3.0 and pH=9.0 were carried out in the narrower potential range to detect the first oxidation and reduction potentials to determine the electrochemical energy level of the HOMO-LUMO gap. The position of LUMO levels were estimated using the first reduction potential of the corresponding SnP, although one- and two-electron transfer processes may be not always resolved at all conditions.


16

Page 17 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fi gure 5. Cyclic voltammograms of 1mM SnPs: SnTPPC (A, inset: 0.2 V ˂ E ˂ +1.6 V potential range), SnTPPS (B, inset: 0.2 V ˂ E ˂ +1.6 V), SnTPyP (C), SnTMPyP (D) in 0.5M KCl of pH=3.5 (black) and pH=9.0 (blue) and in 0.1 M Na-phosphate buffer of pH=7.0 (red). Our results (Figure 5) prove that SnPs undergo oxidation reduction of the macrocyclic ring in several stages, which is in accordance with previous studies

53,80,81

. The electrochemistry of

SnTPPC was measured at pH=7.0 and pH=9.0 (Figure 5 (A)). Here, we found the first oxidation peaks at +1.20 V and +1.19 V for pH=7.0 and pH=9.0, respectively (Figure 5 (A), insert) and two reduction peaks. Cyclic voltammograms of SnTPPS (Figure 5 (B), insert) show the first oxidation peaks at pH=3.5 and pH=7.0 and also two reduction peaks at different pH like in the case of SnTPPC, Table 1. SnTPyP has several reduction peaks in acidic media (Figure 5 (C)) as


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 72

well as SnTMPyP(Cl)2 (Figure 5 (D)). But for the last porphyrin the number of reduction products was smaller in neutral media compared with acidic and basic media. It was not possible to detect oxidation potentials for the latter porphyrins since they are oxidized at higher potentials, which is in agreement with

41

. Additionally, possible impurities in the porphyrin samples and

dimerization of pyridinium groups (SnTMPyP)

84

might be responsible for some features on

complex CVs of Figure 5. Table 1. First half-wave reduction potentials (E1/2 red) and peak oxidation potentials (Ep

ox

) of

SnPs at different pH in aqueous solution. E1/2 red, Ep ox, V vs. NHE

SnP

pH=3.5

pH=7.0

pH=9.0

-0.69

-0.74

+1.40**

+1.39**

-0.57

-0.67

-0.71

+1.51**

+1.47**

>1.40

-

-

-0.23

-0.34

-0.35

>1.40

>1.40

>1.40

SnTPPC

-

SnTPPS

-0.27

SnTPyP*

>1.40

SnTMPyP(Cl)2* * at pH=5.0, E 1/2 41 ;

red

(vs. NHE) of SnTPyP -0.4 V and E 1/2

red

(vs. NHE) of SnTMPyP -0.3 V

** peak potentials are given in cases, where products were irreversible at experimental conditions (50mV/s, -1.0 V ˂ E ˂ +1.6 V (NHE)). Positions of the redox peaks are determined by the presence of electron withdrawing and electron donating groups at the porphyrin periphery and the π-ring system

43

and are sensitive to

pH as it is known that the reduction of metalloporphyrins is accompanied by the addition of protons to the macrocycle

45

, Scheme 1.


18

Page 19 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


All kinds of SnPs display different reduction behavior, where the central metal and substituents in the meso-positions of a porphyrin ring, namely their inductive and resonance effects, influence the course of the reaction and the site of protonation. The periphery used in our investigation is electron-withdrawing with different abilities, where the N-methyl-4-pyridyl group is the strongest electron-withdrawing group. Here, the important parameter is the first reduction potential, because there is a correlation between the first reduction potential and a net ring charge of the porphyrin 62. The values of the first half-wave reduction and peak oxidation potentials are shown in Table 1. Using this correlation we can estimate the sequence of the net electronegativity of the porphyrin ring from the first half-wave reduction potentials and oxidation potentials: SnTMPyP>SnTPyP>SnTPPS>SnTPPC [this study and

41

], which agree with the

sequence of the Hammet functions and electron-withdrawing properties of the corresponding substituents of the phenyl ring, p-MPy>p-S≥p-C

85,86

. The net electronegativity of the porphyrin

ring in SnTPyP with an electron-deficient nature of the pyridine substituents is similar to that of SnTMPyP in this series in acidic media. However, protonation of pyridyl groups of the mesosubstituents of SnTPyP producing pyridinium cations may partly be responsible for its similarity to SnTMPyP in acidic solutions. Unfortunately, attempts to estimate the reduction potentials of SnTPyP in water were restricted by pure solubility in this study and in

41

. Data at pH=5.0

41

confirmed the sequence SnTMPyP>SnTPyP>SnTPPS, with SnTMPyP and SnTPyP having a relatively more positive ring and being easier to reduce

45,62

. According to our data,

SnTMPyP(Cl)2, SnTMPyP(OH)2, and SnTPyP do not differ from each other and have more positive values of the reduction potential than SnTPPS at pH=3.5. In neutral media, again SnTMPyPs starts to reduce earlier in the potential range than SnTPPS and SnTPPC, while SnTPPS has a more positive value of the reduction potential than SnTPPC. For basic media, we


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 72

found a similar tendency for SnTMPyP, SnTPPS, and SnTPPC, which is in accordance with the sequence above. There are several spectrophotoelectrochemical studies describing products formed during the electrochemical reactions of metalloporphyrins in aqueous and organic solutions

53,81,82

. Most of

these peaks are distorted in aqueous media. In most cases, it is possible to identify some reversible one- and two-electron processes. Depending on the conditions, the reduction of metalloporphyrin can be started with the addition of 1-e to form a π-radical anion and finished with the formation of porphyrinogen via different intermediate products, e.g., Scheme 1

87

and

Scheme 2. Anions and the anion radical are more or less strong bases and are protonated in the presence of proton donors.

Scheme 1. Scheme of the reduction process for porphyrins with PhOH (r=100) and AcOH (r=5÷27) as proton donors (r-molar ratio), e – a 1-e electron transfer process, and p – a single protonation process.


20

Page 21 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2. Schematic representation of possible reactions of macrocycle of meso-substituted Sn(IV)Ps during photochemical hydrogen reduction in system Sn(IV)P sensitized TiO2/Pt with EDTA electron donor 49,52,55,88–91. Adapted from 49,52,55,88. The number of accepted electrons can lead to an assumption about the intermediate forms appearing during electroreduction. The identity of numerous reduction products strongly depends on conditions such as pH of the solution and periphery substituents of the porphyrin ring and can be produced according to different schemes, Scheme 1 and phlorin,

chlorin,

and

porphodimethene,

PH4

as

87

. Thus, PH2 can be considered as

porphomethene,

isobacteriochlorin, and bacteriochlorin, and PH6 as porphyrinogen (Scheme 1)

chlorin-phlorin, 87

. In our study,

the first reduction peaks might be related to the possible formation of π-radical anions (Figure 5, peaks 1), and π-dianions with possible subsequent formation of SnPH2 (Figure 5, peaks 2). Cyclic voltammograms of SnTMPyP and SnTPyP at pH=3.5 are very similar, thus reduction products may have the same nature (Figure 5 (C),(D), black curves). Their reduction mechanism can include adsorption process and protonation of pyridyl group

92

. Further studies would be

necessary to make detailed assignment of electron transfer processes. Cyclic voltammograms of SnTMPyP at pH=7.0 (Figure 5 (D), red curve) gave three reduction peaks and at pH=9.0 five products (Figure 5 (D), blue curve). The oxidation processes of porphyrins are known and consist of the formation of a π-radical cation, a π-dication and a form with an addition of a hydroxide ion – isoporphyrin 93, Scheme 3. In general, oxidation products of SnPs have very high formation potentials, so they could not be detected except for SnTPPC at pH=7.0 and 9.0 and SnTPPS at pH=3.5 and pH=7.0 . During the electrochemical process in the specified potential range, SnTPPC and SnTPPS yielded only πradical cations (Figure 5 (A), (B)-insets, oxidation peaks 1).


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 72

Scheme 3. Scheme of the oxidation process of tin-porphyrin. Spectroelectrochemistry would give more accurate information about the intermediate products and their lifetime, but these studies were beyond the scope of the present work. Nevertheless, important information is obtained from the electrochemical experiments and the first reduction-oxidation potentials. 3.4. Optimization of the system for photocatalytic hydrogen generation. 3.4.1. Effect of photosensitizers on hydrogen generation It was shown that the π-radical anion is an intermediate product of the photocatalytic process of hydrogen generation with Sn(IV)Ps 20,51,52. The long lifetime of these reduced species in the case of a electronegative central metal ion like Sn(IV)

45,50,53,54

favors an efficient electron transfer to

the semiconductor even without strong adsorption of the photosensitizer on the semiconductor surface. It was previously found that in outgassed aqueous solutions SnPs can be photochemically reduced to give a π-radical anion (product of 1-e reduction), a phlorin or a chlorin (products of 2-electron reduction and protonation). As illustrated by the chemical structures of phlorin and chlorin in Scheme 2, the protons attach to the meso-position in phlorin and to the pyrrole position in chlorin

53

. It was also assumed that the phlorins (phlorin

and a 4-electron reduction product chlorinphlorin) might act in like manner of hydridegenerating molecules49,55. The type of product depends on the porphyrin substituents, central metal ion, and pH of the media. The π-radical anion and phlorin, unlike chlorin, can be oxidized back to the original porphyrin, reducing water to hydrogen in the presence of the platinum catalysts 51. The π-radical anion may also disproportionate producing initial porphyrin and a 2-electron reduced phlorin 51.


22

Page 23 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


While chlorin, which is a product of irreversible

50

porphyrin reduction with protons attached to

the pyrrole positions, is not oxidized back to the porphyrin, which means the elimination of a porphyrin sensitizer from a reaction and a loss of hydrogen-evolving capacity of the system, as it was stated in the studies of A. Harriman and co-workers

51

. In the works of J.-H. Fuhrhop and

later on P. Kurz and co-workers it was stated that chlorin, isobacteriochlorin, and chlorinphlorin species were also involved in the photocatalytic hydrogen formation by Sn(IV)Ps and that these species might be produced via an intermediate step of phlorin formation, Scheme 2 49,52,55,88–90. In summary, the previous works indicate catalytic routes involving Sn(IV) phlorin, chlorin, and four-electron reduced species of Sn(IV) isobacteriochlorin or chlorinphlorin, Scheme 2. However, it was also supposed that catalytic route involving Sn(IV) chlorin might be less efficient than the one for Sn(IV) porphyrin 49. A. Harriman discussed that the formation of stable anion radical and phlorin is favorable for hydrogen production

51

. Analogously, in the case of

Sb(V)TPyP it was supposed that active water reductant is the phlorin anion, while photoreduction in the presence of Pt at pH7 lead to increased amount of chlorin so that, overall, the system was not useful for H2 generation 45. The tendency of the stability of the π-radical anion, which is high for the electronegative metal centers, like Sn(IV), as discussed above, was found to be in parallel with the formation of phlorin (not chlorin), with the longer-lived radical producing more phlorin: TMPyP>TPyP> TPP 45. This stability decreases in the order of the more negative reduction potential of the porphyrin ring and was found to be due to the inhibition of porphyrin ring protonation. Thus, metalloporphyrins with more positive reduction potentials, like SnTMPyP and SnTPyP, which is a demonstration of a more positive net ring charge 62 as discussed above, should produce long-lived π-radical anions and favor phlorin formation

45

. The photoelectrochemical hydrogen yield over photosensitizers


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 72

with different functional groups was evaluated under visible light irradiation at fixed concentrations of SnP, TiO2, Pt nanoparticles synthesized by the citrate method, and EDTA disodium salt. As shown in Figure 6, the maximum hydrogen yield was 232 µmol obtained from SnTPyP at pH=9.0 over 4.5 h irradiation periods with 1000 W m-2 light intensity. We assume the photochemical hydrogen generation of SnTMPyP, similar to SnTPyP, is also mediated through the formation of a π-radical anion and further reduction products (preferably phlorin as discussed above) that further transfer electrons to the conduction band of TiO2. However, under the same conditions, a higher volume of hydrogen was obtained with SnTPyP compared with SnTMPyPs, which has both chloride and hydroxide axial ligands (Figure 6 (m), (p), (q)). This can be explained by the ease with which the porphyrin 4-pyridyl group approaches the TiO2 due to the interaction between incompletely coordinated titanium atoms on the surface of TiO2 57,94 and the lone electron pair of the nitrogen atom, favoring electron transfer from a photosensitizer to the TiO2 surface. On the other hand, the steric effect of the methyl (-CH3) group

39

prevents the

SnTMPyP from approaching the surface of TiO2 or Pt, thus inhibiting the electron transfer. Additionally, it can be seen (see also Figure 11) that the energy of the electron in the π-radical anion of SnTMPyP (estimated by the first reduction potential of the respective porphyrin) and its further reduction products is the lowest among the porphyrins studied and approaches the energy required for the proton reduction, which could make this porphyrin less efficient, if any overpotential is present.


24

Page 25 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Hydrogen yields at different pH values and SnP complexes with different functional groups, pH=3.0: (a) SnTPyP/TiO2/Pt/EDTA, (b) SnTPPS/TiO2/Pt/EDTA, (c) SnTPPS adsorption on TiO2 overnight/Pt/EDTA, (d) SnTPPC/TiO2/Pt/EDTA, (e) SnTPPC adsorption on TiO2

overnight/Pt/EDTA,

SnTMPyP(Cl)2/TiO2/Pt/EDTA;

(f)

SnTMPyP(OH)2/TiO2/Pt/EDTA,

pH=7.4:

(h)

SnTPyP/TiO2/Pt/EDTA,

(g) (i)

SnTPPS/TiO2/Pt/EDTA, (j) SnTPPC/TiO2/Pt/EDTA, (k) SnTMPyP(OH)2/TiO2/Pt/EDTA, (l) SnTMPyP(Cl)2/TiO2/Pt/EDTA;

pH=9.0:

(m)

SnTPyP/TiO2/Pt/EDTA,

(n)

SnTPPS/TiO2/Pt/EDTA, (o) SnTPPC/TiO2/Pt/EDTA, (p) SnTMPyP(OH)2/TiO2/Pt/EDTA, (q) SnTMPyP(Cl)2/TiO2/Pt/EDTA;

pH=10.5:

(r)

SnTPyP/TiO2/Pt/EDTA;

pH=13.0:

(s)

SnTPyP/TiO2/Pt/EDTA. Furthermore, we also studied the photoelectrochemical properties of SnTPPS and SnTPPC. These porphyrins showed lower hydrogen generation in Figure 6 (b), (d), (i), (j), (n) and (o). The observed tendency is in agreement with the lower electron-withdrawing properties of 4sulfonato- and 4-carboxyphenyl meso-substituents of the porphyrin ring (less electronegative), which is also confirmed by the more negative reduction potentials of the corresponding SnTPPS and SnTPPC, respectively, as found in the electrochemical experiments.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 72

Therefore, the factors, which may influence the reactivity and efficiency tendency in this porphyrin series might be summarized as follows: in agreement with the redox potential sequence of the Sn(IV)Ps series, SnTMPyP >SnTPyP>SnTPPS> SnTPPC, the substances, which have a higher net electronegativity of the porphyrin ring and a more electropositive redox potentials and therefore are easier to reduce in the ground state should produce more stable πradical anion (which is also more stable in alkaline conditions and may disproportionate producing phlorin and initial porphyrin). For these substances, the 2-e reduction products of the π-radical anion include phlorin rather than chlorin (phlorin is however is unstable in the presence of oxygen). In turn, as it was discussed above and is illustrated below in section 3.6, formation of stable anion radical and phlorin is favorable for hydrogen production. Secondly, pyridinesubstituted SnPyPs with more electropositive redox potentials than SnTPPC and SnTPPS are easier to reduce in the ground state, making them better oxidizing agents in the excited state 95,96. Consequently, there is an especially large driving force for the reduction of SnP* by EDTA for these compounds, which also explains the reactivity trend in accordance with the redox potential sequence. Additional factor contributing to the photochemical reaction mechanism and efficiency of the porphyrin series might be the formation of associates between the porphyrin molecules and EDTA either before or during excitation, as the studies on the association of porphyrin molecules by hydrogen bonds or electrostatic interactions between reactants suggest

97–101

. A closer contact

and association of the electron donor EDTA and Sn(IV)P might then favor the formation and/or stabilization of the π-radical anion during irradiation. In this case, e.g., in alkaline and neutral media (pH 7.4 and 9.0), the interaction of SnTPyP (pKa(Py) = 5.2) with EDTA as HY3- (four carboxyl groups and one amine are deprotonated, while one amine is protonated,


26

Page 27 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pK1(CO2H)=0.0,

pK2(CO2H)=1.5,

pK3(CO2H)=2.0,

pK4(CO2H)=2.7,

pK5(NH+)=6.1,

pK6(NH+)=10.4) would be more favorable than with the negatively charged SnTPPC or SnTPPS (pKa(benzenesulfonic acid)=2.6, pKa(benzenecarboxylic acid)=4.2). Steric hindrance of methyl groups in TMPyP could also be a problem in this case diminishing the effectiveness. However, there are no data on the association of the excited SnPs with EDTA specifically and further studies would be necessary to prove this hypothesis. In order to obtain efficient electron injection, the excited orbitals of SnTPPS and SnTPPC should be strongly bound with Ti-3d orbitals through the chemical bond

20

. Thus, we mixed

SnTPPS (or SnTPPC) and TiO2 at the beginning of the experiment and left the mixture overnight (17 h) without light, and after that performed the same photocatalytic process as for the other samples. Adsorption of porphyrins on TiO2 was proved by the appearance of the transparent supernatant solutions after centrifugation of the mixture of TiO2 and dye solution after this time. Compared with Figure 6 (b) and (d), the higher hydrogen yields in Figure 6 (c) and (e) demonstrate that after overnight adsorption, SnTPPS (or SnTPPC) anchoring onto the surface of TiO2 enables greater enhancement of the electron transfer efficiency. The rate of hydrogen formation with time was investigated, Figure 7. The slope of the curve decreases with time showing a slight decrease of the hydrogen production rate. After 4.5 h the curve didn´t reach a plateau, which indicates higher resource of the system.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 72

Figure 7. Time profile of the hydrogen production in the SnTPyP/TiO2/Pt/EDTA system at pH=9.0, TON(H/SnP) – turnover number. 3.4.2. Effect of TiO2 on hydrogen generation To investigate the role of TiO2 in the hydrogen yield, we compared the photoelectrochemical properties of SnTPyP/TiO2/Pt/EDTA and SnTPyP/Pt/EDTA systems at different pH. As shown in Table 2, SnTPyP/TiO2/Pt/EDTA displays an enhancement of the hydrogen yield at both pH=7.4 and pH=9.0 with 150 W of lamp power compared to the hydrogen volume of the SnTPyP/Pt/EDTA system. In the section above, we mentioned the formation of the π-radical anion after reductive quenching of the excited state of SnTPyP by electron donors. Although the π-radical anion has a relatively long lifetime in case of SnTPyP, it is unstable with respect to disproportionation. The reversibly protonated π-dianion phlorin can be formed after disproportionation of the π-radical anion

50

. The phlorin can be reoxidized back to porphyrin

while the chlorin cannot. We assume that the presence of the nanoparticulate TiO2 with a large surface area made an essential contribution to the electron transfer from the π-radical anion and/or further reduction products to the Pt catalyst via TiO2. Moreover, the role of photo-excited TiO2 in hydrogen production was also studied and the synergic effect between the porphyrin and TiO2 excitation was proved by our observations. A recent study suggests that hot plasmonic


28

Page 29 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrons of gold nanoparticles are responsible for the enhanced photocatalytic activity that can be magnified when TiO2 starts to be simultaneously excited 102. In our investigation, two kinds of light filters with different wavelength ranges were applied in the experiments. Irradiation above 435 nm can excite the Q-bands of porphyrin, but cannot excite TiO2 or TiO2 defect/impurity states for H2 generation. However, light with a wavelength range between 390-650 nm can both create some charge carriers in TiO2 and produce excited porphyrin states, leading to enhanced photocatalytic activity for H2 generation. Comparison of sample 1 with sample 2 as well as sample 3 with sample 4 in Table 3 shows that the hydrogen yield decreased when the 390nm650nm light was replaced by light with λ> 435 nm (all conditions for the respective samples were the same except for the irradiation wavelength ranges). Table 2. Comparison of hydrogen generation performances in photocatalytic systems with and without TiO2 and different lamp powers, TON(H/SnP) – turnover number.

No.

Reactant

pH

Lamp power

Hydrogen yield

TON (H/SnP)

1

SnTPyP/Pt/EDTA

7.4

150W

67 µmol

89

2

SnTPyP/TiO2/Pt/EDTA

7.4

150W

121 µmol

161

3

SnTPyP/Pt/EDTA

9.0

150W

170 µmol

226

4

SnTPyP/TiO2/Pt/EDTA

9.0

150W

232 µmol

310

5

SnTPyP/TiO2/Pt/EDTA

9.0

95W

188 µmol

250

The photo-excited TiO2 shows conductivity because charge carriers are generated by the excitation. When light in the wavelength range between 390 nm and 650 nm is used for irradiation, both porphyrin and in part the TiO2 matrix are excited. The electrons from the photosensitizer are able to rapidly pass through the contacted TiO2 crystal to the nearby Pt NPs


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 72

for H2 production due to the higher conductivity of the excited TiO2. Whereas, if light in the 435 nm-650 nm range is applied, which can only excite the Q-bands of the porphyrin, the electrons can hardly be transported through the TiO2 nanocrystal to reach the nearby Pt NPs due to the low conductivity of non-excited TiO2, which is a possible reason for the lower H2 production in samples 1 and 3 in Table 3. Thus, the photo-excited state of TiO2 plays an important role in the electron transfer process for hydrogen generation. Table 3. Comparison of hydrogen generation performances in photocatalytic systems with different wavelength ranges and different reactants.

No.

Wavelength range

Reactant

pH

Hydrogen yield

1

435nm-650nm

SnTPyP/TiO2/EDTA/Pt (citrate method)

9.0

94 µmol

2

390nm-650nm

SnTPyP/TiO2/EDTA/Pt (citrate method)

9.0

232 µmol

3

435nm-650nm

SnTPyP/TiO2/EDTA/Pt (photoreduction)

9.0

85 µmol

4

390nm-650nm

SnTPyP/TiO2/EDTA/Pt (photoreduction)

9.0

223 µmol

5

390nm-650nm

TiO2/EDTA/Pt (citrate method)

9.0

9 µmol

6

390nm-650nm

TiO2/EDTA/Pt (photoreduction)

9.0

13 µmol

7

435nm-650nm

SnTPyP/EDTA/Pt (citrate method)

9.0

143 µmol

Additionally, comparison of sample 2 with sample 5 as well as sample 4 with sample 6 in Table 3 shows that the hydrogen yield decreased a lot when SnTPyP was absent (all conditions for the respective samples were the same except for the absence of SnTPyP). This indicates that TiO2 makes only about 5% contribution to the hydrogen generation process without the presence of SnTPyP under the wavelength range between 390 nm to 650 nm, and the sensitization function of SnTPyP for visible light absorption and charge separation are confirmed. Comparison of sample 1 with sample 7 reveals that the presence of TiO2 is not beneficial for the


30

Page 31 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hydrogen generation process, when it cannot be excited under the wavelength range between 435 nm to 650 nm, because the excited electrons in SnTPyP are transported to TiO2 and consumed by the defects of TiO2 in the sample 1, while the excited electrons can be directly transferred from SnTPyP to the nearby Pt NPs for proton reduction into H2 when TiO2 is absent in the sample 7. Therefore, the positive and negative impacts of TiO2 in the hydrogen production process depend on different experimental conditions. 3.4.3. Effect of pH on hydrogen generation The pH value of the medium is an important factor influencing hydrogen generation during the course of photolysis. We chose pH values of 3.0, 7.4, 9.0, 10.5, and 13.0 to investigate the effect of acidic, neutral, and alkaline medium on this photoelectrochemical system. Figure 6 illustrates the maximum yield of hydrogen for different SnP complexes at different pH values. The hydrogen yields in the SnTPPS/TiO2/Pt/EDTA and SnTPPC/TiO2/Pt/EDTA systems reached much higher values in the acidic region, because the adsorption of SnTPPS and SnTPPC on TiO2 is favored in the acidic medium. Therefore, overnight adsorption experiments for SnTPPS and SnTPPC in acidic medium were performed. However, even with the adsorbed SnTPPS (Figure 6(c)) or SnTPPC (Figure 6(e)), the hydrogen yield was not essentially higher than for SnTPyP/TiO2/Pt/EDTA (Figure 6(a)) and SnTMPyP/TiO2/Pt/EDTA (Figure 6(g)) systems at the same pH 3.0 with non-adsorbed porphyrins. In contrast, the SnTPyP/TiO2/Pt/EDTA and SnTMPyP/TiO2/Pt/EDTA systems exhibited relatively low hydrogen production in acidic solution in spite of the fact that hydrogen reduction is thermodynamically more favorable in acidic media. There are several reasons for a higher hydrogen yield in weakly alkaline media. The stability of a π-radical anion (due to the inhibition of protonation) and the tendency to form phlorin rather than chlorin decreases with lower pH (in


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

acidic media)

45,54

Page 32 of 72

. The preferable product of the photochemical transformation of both SnTPyP

and SnTMPyP in acidic solution is the respective chlorin, which is the irreversible reduction product of the porphyrins, and cannot be reoxidized to porphyrin, resulting in a loss of the initial porphyin sensitizer and in this sense hydrogen-evolving capacity of the system. This aspect was discussed in section 3.4.1 with respect to “phlorin” and “chlorin” pathways for hydrogen generation in Scheme 2. In alkaline medium, the lifetime of a π-radical anion is longer and the favorable transformation products of both SnTPyP and SnTMPyP are phlorins, which are reversible for hydrogen generation, thus much higher hydrogen yields for SnTPyP and SnTMPyP are obtained in weakly alkaline pH region. This aspect will be further discussed in section 3.6. An additional factor favoring alkaline media is a protonation of a free electron pair of the amine (electron source) in the EDTA donor in acidic media

49

. However, in strongly

alkaline media, like pH=10.5 and pH=13.0, the hydrogen generation decreases due to the lower concentration of hydrogen ions and water molecules which can be reduced to produce hydrogen (Figure 6 (r), (s)). Additionally, the energy, which is necessary for the hydrogen reduction in strongly alkaline media further increases (hydrogen is reduced at more negative potentials), while the redox potentials of porphyrins and their reduction products are not shifted with the change of pH significantly on the potential scale. This diminishes the driving force for hydrogen reduction in strongly alkaline media. Considering the effect of the axial ligand in SnTMPyPs, we found that a higher amount of hydrogen was produced with SnTMPyP(Cl)2 photosensitizer than with SnTMPyP(OH)2 at pH=9.0. This phenomenon can be explained by the formation of a dimer of SnTMPyP(OH)2 in the alkaline media, which occurs owing to the metal-axial ligand coordination 104

103

. According to

, the dimerization process results in a decrease of molecular absorption. Additionally,


32

Page 33 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


aggregation significantly decreases the excited lifetime of dyes

39

, which can also result in a

lower amount of hydrogen produced with SnTMPyP(OH)2. On the other hand, axial Cl- ligand could be exchanged to OH- in water solutions. However, according to the NMR experiment carried out by A.-M. Manke and coworkers for SnTPPC(Cl)2, no full exchange of the two Claxial ligands for OH- occurs in aqueous solution even at high pH 49. Except for chemical shift of the

119

Sn-NMR signal for SnTPPC(Cl)2 in D2O/2M NaOD, the authors found a signal for the

mixed complex SnTPPC(Cl)(OH) and a very small signal for SnTPPC(OH)2. Additionally, the spectrum had no further changes after 3 days. Thus, chloride ligand can hardly be replaced completely because pKa of HCl is lower than pKa of water 105. Therefore, based on these data we assume that in the timeframe of our experiments the exchange process of axial Cl - for axial OHligand doesn’t proceed significantly and therefore a higher amount of hydrogen was produced with SnTMPyP(Cl)2 photosensitizer than with SnTMPyP(OH)2 at pH=9.0. Although, according to

45,51

as well as our data, phlorin is the more favorable protonated π-

dianion for hydrogen production compared to chlorin, recently particular tin (IV) chlorin species were used as photocatalysts for the accumulation and transfer of hydrogen equivalents in combination with a suitable hydride-transfer mediator 106. 3.4.4. Effect of light intensity on hydrogen generation Table 2 shows the effect of light intensity on hydrogen yields under the same conditions. Almost 45 µmol hydrogen yield was lost when the lamp power was decreased from 150 W to 95 W, indicating the limiting parameter of this photoelectrochemical reaction is the number of photons absorbed into the system. Thus, a higher light intensity leads to a higher hydrogen yield. 3.4.5. Effect of Pt catalysts on hydrogen generation


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 72

Two kinds of Pt catalysts synthesized by the citrate method and the photoreduction method were applied in the photocatalytic system. The formation mechanism of the Pt catalysts by the citrate method is based on the reduction of Pt ions by sodium citrate to form small Pt nanoparticles of about 3 nm in diameter surrounded by citrate ions. During the photoirradiation process in the photoreduction method, the Pt ions are reduced to Pt (0) by receiving electrons from the electron donor methanol, and then Pt (0) is condensed to form nuclei. Once stable nuclei are formed, they grow larger and further produce Pt nanoparticles of about 15 nm in diameter, which have an active but less stable surface due to the absence of the stabilizing agent (except Cl- ions) and are prone to aggregation during storage 78. When the Pt catalysts synthesized by the photoreduction method were used in the photocatalytic system without TiO2, the volume of hydrogen production was very low and the photocatalytic reaction seemed to proceed for only 2 hours, indicating that the catalysts did not work well. The transformations of Pt catalysts in the presence of SnTPyP molecules were studied by SEM after the H2 production reaction. As shown in Figure S4, many large nanospheres with an average diameter of about 200 nm are obtained and the surface of the nanospheres is decorated with the aggregated Pt nanoparticles (clearly seen in the enlarged Figure S4 (B)), due to the coordination reaction between the amine ligand of SnTPyP and Pt catalysts

78,107,108

.

However, the nanospheres were not found in the product after the photoelectrochemical reaction between SnTPyP and Pt catalysts synthesized by the citrate method and the hydrogen yield was much higher than in the system with the photoreduced Pt catalyst. This indicates that the citrate ions which stabilized the Pt catalysts inhibited the coordination reaction between the Pt catalyst and the amine ligand of SnTPyP, which is more favorable for the hydrogen reduction reaction.


34

Page 35 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Therefore, the Pt catalysts synthesized by the citrate method were applied in our further experiments to avoid catalyst poisoning. Additionally, the effect of different concentrations of Pt catalysts on hydrogen generation was investigated. In a previous report in 1984, Harriman and coworkers found that at high concentrations of Pt catalysts an essential product of the photochemical transformation of SnTPyP is chlorin, which is the irreversible transformation product and is not favorable for the hydrogen reduction reaction 50. After studying the influence of different Pt concentrations on the hydrogen yields in our system, we found that low concentrations of Pt catalysts could decrease the hydrogen generation capacity of the system. Therefore, 0.2 mM of Pt catalysts was chosen. 3.5. Electron transfer process The process of electron transfer for hydrogen evolution has several steps 20,49,51 and its kinetics were considered in detail by Kim and coworkers

20

. The quantum of light transfers SnP into the

excited state (Figure 8). SnP* is reductively quenched by EDTA producing a long-lived π-radical anion (stable for minutes in neutral solutions and up to several hours in alkaline solutions 53) and the excited electron is either directly transferred to a platinum catalyst or injected into the conduction band of TiO2 and further used for the reduction of protons until hydrogen molecule formation via platinum. Hydrogen evolution depends on the quantum yield of excitation, the lifetime of the exited state, and the energy of an electron in the excited state of the photosensitizer

11,20

. SnPs adequately fulfill the requirements because they have a high molar

attenuation coefficient, appropriate fluorescence life time – 1.3 ns 109 for accepting electron from EDTA with an electron transfer rate of reductive quenching ˂ ns

20

, and suitable reduction

potential, namely LUMO energy level location relative to the TiO2 conduction band and hydrogen reduction reaction, which is considered below. The successful combination of a central


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 72

metal and the porphyrin ring substituents leads to a more efficient process. On the one hand, 4sulfonatophenyl and 4-carboxyphenyl groups can be anchored strongly on the TiO2 surface, which is favorable for direct electron transfer. However, they have less electron-withdrawing ability, the respective porphyrins have more negative reduction potential and a less long-lived πradical anion with a less tendency to form phlorin rather than chlorin 45. Moreover, according to the redox potential sequence they have a lower oxidizing strength in the excited state for reductive quenching with EDTA as discussed in section 3.4.1 95,96 . On the other hand, N-methyl4-pyridyl is the most electron-withdrawing substituent with a longer lifetime of its π-radical anion and a higher tendency to form phlorin rather than chlorin. However, it cannot be adsorbed on the semiconductor due to the steric effect 39 of the methyl group hindering its approach to the TiO2 or to the catalyst surfaces. The 4-pyridyl group has desirable characteristics, namely electron-withdrawing ability and no steric hindrance preventing it from approaching the TiO2 surface. Furthermore, as it was discussed in section 3.4.1. pyridine-substituted SnPyPs with more electropositive redox potentials than SnTPPC and SnTPPS are easier to reduce in the ground state, making them better oxidizing agents in the excited state thus facilitating the reduction of SnP* by EDTA in comparison with SnTPPC and SnTTPS.

Figure 8. Schematic of photoexcited electron transfer and hydrogen evolution under visible light irradiation.


36

Page 37 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Although the excitation of the porphyrin ring occurs in the 400-650 nm region, Q-bands (500650nm) are still mostly responsible for the photocatalytic activity of SnP. This fact was reported in 20 and our results showed that most hydrogen is produced due to the excitation of the Q-bands, although a higher energy Soret (419 nm) band makes a contribution of about 23 % to the process, as was found for SnTPyP. This is probably due to the fast S2  S1 transition with further participation of the produced S1 state in the photocatalytic process. The observed data are in agreement with the fact that the S2 excited state, corresponding to the absorption of the Soret band, has a very short lifetime, τ1/2 ~ 10-12 s and transfers to a longer-lived S1 state, τ1/2 ~ 10-9 s with the release of energy (heat), where S1 may be involved in the subsequent photochemical process and corresponds to the absorption of the Q-bands. The apparent photonic efficiency in the geometry of the experiment was 11 % (150 W) and 8% (150 W) and 20% (95 W) and 16% (95 W) for the Q- and Soret bands, respectively. However, this evaluation represents a lower limit, because it is uncorrected for the optical density and photon losses due to scattering and reflection

20,110

. Moreover, usage of a thin layer reactor would increase the apparent photonic

efficiency. 3.6. Photochemistry of SnP There are different pathways for H2 production following a first-step formation of the π-radical anion (SnP.-)

20,51

. Usually porphyrin is consumed as many sensitizers. Nevertheless, several

reactions return it to the original porphyrin, Scheme 4. Scheme 4 demonstrates that the formation of a stable π-radical anion or phlorin represents a way of reducing water to hydrogen returning to the original porphyrin sensitizer (not consumed in ideal case) and is in agreement with the reversibility of the addition of hydrogen to methine bridges 89, whereas the formation of chlorin, Scheme 2 and 4, results in an irreversible loss of the porphyrin sensitizer


50,51

, Scheme 2. The

37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 72

course of the reactions of phlorin-chlorin formation can be influenced. At higher pH, the stability of the π-radical anion and the formation of phlorin increases, Fig. 9, which leads to a higher H2 yield.

Scheme 4. Schematics of different pathways for hydrogen production. Further photochemical processes with participation of chlorin are shown in Scheme 2. Absorption spectra of the solution containing SnTPyP/Pt/EDTA were measured after visible light irradiation at pH=7.4 and pH=9.0 (Figure 9). The solution was continuously stirred and deaerated by blowing with nitrogen. Then the system was exposed to light for 1 min, 20 min, and left in dark for 20 minutes. After that the system was exposed to air. The color of the solution immediately changed from pink to orange after 1 minute of irradiation and the spectra were changed accordingly for both pH. Appearance of a new Q-band at about 620 - 621 nm indicates formation of chlorin

49,52,55,106

. Q-band of slightly shorter wavelength of about 605 - 607 nm

corresponds to the isobacteriochlorin formation

52,55

. Additionally, bacteriochlorin has the Q-

band absorption at about 760 nm 111,112, although isobacteriochlorin should be preferably formed instead of bacteriochlorin and should be stable to oxygenation

100

. A new peak at 446 nm and a

broad band of absorbance between 700 and 900 nm (with possible maxima at about 660, 720, 760, 820) were observed, especially at pH 9 (Fig. 9B). These spectral features were previously attributed to π-radical anion and/or phlorin by different authors or overlapping of the spectral features of these species

20,41,45,51–54,84,89,113,114

. Moreover, both π-radical anion and phlorin are

easily oxidized by oxygenation resulting in disappearing of their spectral features. Therefore,


38

Page 39 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


currently, we have no reliable data to attribute these spectral features specifically either to πradical anion or phlorin. However, formation of more stable π-radical anion is consistent with the favorable formation of phlorin as it was discussed above. As one can see from Fig. 9, after introducing air, the peak at 446 nm and a broad band of absorbance between 700 and 900 nm practically disappeared. The spectral features of π-radical anion and/or phorin were essentially less pronounced at pH 7.4 (Fig. 9B). At the same time, the intensities of Q-bands of chlorin and isobacteriochlorin were essentially higher at pH 7.4 than those at pH 9.0. This proves that πradical anion and/or phlorin formation notably occurs in basic media as favorable intermediate products in hydrogen production, Scheme 4, which is in agreement with our experiment on hydrogen evolution, in which the hydrogen volume was higher at pH=9 than at pH=7.4 (Figure 6 (h), (m)). Although hydrogen evolution from water requires less energy in acidic pH

115

, the

properties of the SnPs photosensitizer make photocatalytic hydrogen generation in this system more effective in basic media. Furthermore, the course of the photochemical reaction was compared in similar experiments for SnTPPC and SnTPyP at one pH, pH 9, Figure 10. Absorption spectra of the solutions containing SnTPyP/Pt/EDTA or SnTPPC/Pt/EDTA were measured after visible light irradiation at pH=9.0 (Figure 10). The solution was continuously stirred and deaerated by blowing with nitrogen. Then the system was exposed to light for 3 min and left in dark. The spectra were further measured in a wavelength interval of 650 - 920 nm at defined time intervals. The system was kept in dark between the successive measurements. We don’t expect the system to be in equilibrium during these measurements, therefore only qualitative information can be obtained from these data. After that the system was exposed to air. In case of SnTPyP, a broad band of absorbance between 700 and 900 nm (Figure 10A) was developed, which can be attributed to π-


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 72

radical anion and/or phlorin as discussed above, while the spectra of the SnTPPC/Pt/EDTA system were essentially featureless in this wavelength interval (Figure 10B). This again prove that π-radical anion and/or phlorin formation notably occurs in the system with SnTPyP sensitizer, which are thus favorable intermediate products in hydrogen production, and this fact is also in agreement with the experiments on hydrogen evolution, in which the hydrogen volume was higher for the SnTPyP sensitizer (Figure 6 (m), (o)).

Figure 9. Changes in the absorption spectra of SnTPyP solution in the presence of Pt NPs and EDTA at pH=7.4 (A) and pH=9.0 (B) under continuous N2 blowing and irradiation with visible light (λ > 390 nm).


40

Page 41 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Changes in the absorption spectra of SnTPyP/Pt/EDTA (A) and SnTPPC/Pt/EDTA (B) solution in the presence of Pt NPs and EDTA at pH=9.0 under continuous N2 blowing and irradiation with visible light (λ > 390 nm) for 3 min.

3.7. Energy levels in H2 production Taking into account half-wave potentials from the cyclic voltammograms, we arranged energy levels of porphyrin molecules relative to the bands of TiO2 and hydrogen reduction potentials at different pH (Figure 11). Periphery substituents of SnPs as well as pH affect the position of the levels. The positions of LUMO were clearly detectable from the CVs

43

but the positions of

HOMO were not so clearly distinguishable for all SnPs and overlapped with extraneous processes, as discussed in section 3.3. All kinds of porphyrins are capable of transferring electrons of the reductively quenched excited state to the conduction band of TiO2 or directly to Pt for the hydrogen reduction reaction. However, from the measured reduction potential values of SnTMPyP at pH=7.0 and pH=9.0, it seems likely that π-anion radicals of SnTMPyP can not participate in hydrogen reduction at these pH, but its further reduction products may participate in this process. SnTMPyP and SnTPyP have lower-lying LUMOs compared with SnTPPC. However, we obtained higher hydrogen volumes with the first two porphyrins not covalently linked to TiO2 or Pt, which have the most electron-withdrawing groups contributing to the stability of the π-radical anion and/or phlorin formation as well as stronger oxidizing properties of the excited states of the respective porphyrins (see sections 3.4.1 and 3.6). It can be assumed that a phlorin pathway is an essential pathway of H2 production in this case.


41


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 72

Figure 11. Schematic representation of the HOMO-LUMO gaps of SnPs, semiconductor bandgaps of TiO2, and hydrogen reduction potentials at different pH, where SnS – SnTPPS, SnPy – SnTPyP, SnMPy – SnTMPyP, SnC – SnTPPC. The positions of HOMO were not so clearly distinguishable for all SnPs (see text for detail) and are therefore marked with a dashed line on a schematic. The HOMO positions of SnTPPC and SnTPPS were defined from peaks of the oxidation potentials and are shown as pink bars. Ep of EDTA oxidation at GCE are +1.31 V (pH 7) and +1.25 V (pH 9), respectively. At pH 3.5 oxidation starts at about 1.25 V (NHE) at the experimental conditions (see Figure S5). Further studies may further clarify whether the singlet or the longer-lived triplet excited state of porphyrin is reductively quenched with EDTA to produce a π-radical anion 20,49. According to our experimental data and

41

, the energy of the triplet excited state (which is about 0.3 - 0.4 V

below the energy of the singlet excited state, ES-T ~0.3 - 0.4 V 11,116, would be essentially lower


42

Page 43 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


than the energy necessary for proton reduction, in particular for SnTPyP and SnTMPyP, as determined from the electrochemical characterization of the SnPs series, see Table 1 and Figure 11. Therefore, this allows us to assume that a singlet excited state of the Sn(IV) porphyrins is initially reductively quenched by EDTA, which is followed by the sequence of the charge transfer processes as described above. Literature data on the dynamics of the events suggest that participation of the singlet excited state in photocatalytic hydrogen generation in this system is possible, because the lifetime of the singlet excited state of Sn(IV) porphyrins is more than 1 ns 52,109

, while reductive quenching of the excited porphyrin dye with the electron donor proceeds

in the timescale below the ns range 20, thus making the reductive quenching of the singlet excited state of Sn(IV) porphyrins feasible within the lifetime of the singlet excited state 52. 4. Conclusions Under visible light, hydrogen was successfully generated in a SnP/TiO2/Pt/EDTA system by combining the advantages of both activation effects of Pt nanoparticles for hydrogen reduction and sensitization with SnPs for visible light absorption and charge transfer. The electrochemical properties of a number of meso-substituted SnPs and the catalytic properties of Pt catalysts were characterized by cyclic voltammetry and Tafel plots, and the morphology of the Pt/TiO 2 composites was illustrated by SEM and TEM. Several factors that affect the process of hydrogen generation have been described. Significantly, the hydrogen generation activities of a series of SnPs with different functional groups and Pt nanoparticles with different surface stabilizers were evaluated. As well as the effects of pH values, concentrations of Pt/TiO2, and light intensity were optimized for the photocatalytic reaction. The dependence of photocatalytic performance on the structure of a series of Sn(IV) meso-substituted phenyl porphyrins was studied and possible mechanisms discussed employing the results of the electrochemical studies. The time course and


43


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 72

type of the photochemically reduced species of Sn(IV)Ps, which are essential intermediates, are important factors and depend on the electronegativity of the metal center, the character of mesosubstituents of porphyrin ring, and pH, and are correlated with the redox potential sequence of the respective Sn(IV)Ps: SnTMPyP>SnTPyP>SnTPPS>SnTPPC. This is in good agreement with the electron-withdrawing properties of the phenyl substituents. Pre-adsorption of SnTPPC and SnTPPS on TiO2 is a favorable factor for efficient hydrogen generation in acidic media. After optimization of the experimental parameters, the maximum hydrogen yield was obtained for SnTPyP/TiO2/Pt

(citrate

method)/EDTA

system

at

pH=9.0,

demonstrating that

the

photochemically reduced species of SnTPyP are relatively long-lived in order to complete electron transfer, which means that the adsorption of SnTPyP on the TiO2 surface is not essential for hydrogen generation. The synergic effect of excitation of TiO2 and mostly Q-bands of Sn(IV)P enhances the efficiency of photocatalytic hydrogen generation in the visible wavelength range in the system (390 – 650 nm). The Soret band of Sn(IV)TPyP was found to produce a minor (about 23 %) contribution to the photocatalytic activity of the porphyrin sensitizer in this system. Furthermore, the expected working mechanism of the SnP/TiO2/Pt/EDTA system for hydrogen formation under irradiation with visible light was described. This system provides an effective way to harness solar energy for hydrogen energy generation and may be useful for finding further pathways in solving energy and environmental problems. Further studies regarding the electron injection and electron transfer processes and the variations of the intermediate conversion routes based on photoexcited SnPs are currently under way. ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org.


44

Page 45 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Description of the gas chromatography-thermal conductivity detection (GC-TCD) analysis, NMR analyses of SnTPPC and SnTPyP (Figure S1), a SEM image of the Pt nanoparticles synthesized by the citrate method (Figure S2), Cyclic voltammograms of the background solutions (Figure S3), SEM images of the nanospheres produced during the photocatalytic hydrogen reduction reaction (Figure S4), Cyclic voltammograms of Na2EDTA (Figure S5).

AUTHOR INFORMATION Corresponding Author *Dr. Y. Mourzina, E-mail: [email protected], Tel: +49 2461612364, Peter Grünberg Institute-8, Research Centre Jülich, 52428 Jülich Germany. Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) Funding Sources China Scholarship Council (File No. 201206890019). Russian Foundation for Basic Research (RFBR)

grant

14-03-01079.

Saint-Petersburg

State

University,

research

grant

No.

12.38.218.2015.


45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 72

Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT Xiao Liu would like to thank the China Scholarship Council (File No. 201206890019) for financial support. Financial support from the Russian Foundation for Basic Research (RFBR) grant 14-03-01079 for the electrochemical studies, and St. Petersburg State University grant 12.38.218.2015 for the photochemical studies are gratefully acknowledged. The authors would like to thank Dr. M. Heggen and M. Gocyla for TEM and HRTEM studies, Dr. S. Willbold for the NMR analysis, J. Klomfaß and C. Zahren for the light intensity measurements and support. ABBREVIATIONS SnP, Sn-porphyrin; NHE, normal hydrogen electrode; SnTPPS, Sn(IV) meso-tetra (4sulfonatophenyl) porphin dichloride; SnTPPC, Sn(IV) meso-tetra(4-carboxyphenyl) porphin dichloride; SnTPyP, Sn(IV) meso-tetra (4-pyridyl) porphin dichloride; SnTMPyP(OH)2, 5,10,15,20-tetrakis(N-methyl-4-pyridyl) porphyrin-Sn(IV)(OH)2 tetrachloride; SnTMPyP(Cl)2, 5,10,15,20-tetrakis(N-methyl-4-pyridyl)

porphyrin-Sn(IV)(Cl)2

tetrachloride;

EDTA,

ethylenediaminetetraacetic acid disodium salt; ITO, indium tin oxide; SCE, saturated calomel electrode; SEM, scanning electron microscopy; HRTEM, high-resolution transmission electron microscopy; Pt NPs, Pt-nanoparticles, GC-TCD, thermal conductivity detector, p-MPy, paramethylpyridyl substituent; p-S, para-sulfonate substituent; p-C, para-carboxyl substituent.

REFERENCES


46

Page 47 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(1)

Berardi, S.; Drouet, S.; Francàs, L.; Gimbert-Suriñach, C.; Guttentag, M.; Richmond, C.; Stoll, T. & Llobet, A. Molecular artificial photosynthesis. Chem. Soc. Rev. 2014, 43, 7501–7519.

(2)

Kärkäs, M. D.; Verho, O.; Johnston, E. V & Åkermark, B. Artificial photosynthesis: molecular systems for catalytic water oxidation. Chem. Rev. 2014, 114, 11863–2001.

(3)

Crabtree, R. Energy production and storage. Inorganic chemical strategies for a warming world; John Wiley & Sons Inc, 2010. 426.

(4)

Fujishima, A. & Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37–38.

(5)

Lingampalli, S. R.; Gautam, U. K. & Rao, C. N. R. Highly efficient photocatalytic hydrogen generation by solution-processed ZnO/Pt/CdS, ZnO/Pt/Cd1−xZnxS and ZnO/Pt/CdS1−xSex hybrid nanostructures. Energy Environ. Sci. 2013, 6, 3589.

(6)

Faber, M. S. & Jin, S. Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications. Energy Environ. Sci. 2014, 7, 3519–3542.

(7)

Wen, F. & Li, C. a N. Hybrid artificial photosynthetic systems comprising semiconductors as light harvesters and biomimetic complexes as molecular cocatalysts. Acc. Chem. Res. 2013, 46, 2355–64.

(8)

Collings, A. F. & Christa, C. in Artificial Photosynthesis - From Basic Biology to Industrial Application (eds. Collings, A. F. & Christa, C.) Wiley-VCH, 2006. 339.

(9)

Wang, Z.-Y.; Rao, H.; Deng, M.-F.; Fan, Y.-T. & Hou, H.-W. Photocatalytic H2 generation based on noble-metal-free binuclear cobalt complexes using visible-light. Phys. Chem. Chem. Phys. 2013, 15, 16665–71.

(10)

Amouyal, E. Photochemical production of hydrogen and oxygen from water: a review and state of the art. Sol. Energy Mater. Sol. Cells 1995, 38, 249–276.

(11)

Darwent, J. R.; Douglas, P.; Harriman, A.; Porter, G. & Richoux, M.-C. Metal phthalocyanines and porphyrins as photosensitizers for reduction of water to hydrogen. Coord. Chem. Rev. 1982, 44, 83–126.

(12)

Qiu, Y.; Yan, K.; Deng, H. & Yang, S. Secondary branching and nitrogen doping of ZnO nanotetrapods: Building a highly active network for photoelectrochemical water splitting. Nano Lett. 2012, 12, 407–413.

(13)

Yang, X.; Wolcott, A.; Wang, G.; Sobo, A.; Fitzmorris, R. C.; Qian, F.; Zhang, J. Z. & Li, Y. Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2009, 9, 2331–2336.


47


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 72

(14)

McDonald, K. J. & Choi, K.-S. Synthesis and photoelectrochemical properties of Fe2O3/ZnFe2O4 composite photoanodes for use in solar water oxidation. Chem. Mater. 2011, 23, 4863–4869.

(15)

Lytle, J. C.; Denny, N. R.; Turgeon, R. T. & Stein, A. Pseudomorphic transformation of inverse opal tungsten oxide to tungsten carbide. Adv. Mater. 2007, 19, 3682–3686.

(16)

Zhong, J. H.; Li, G. R.; Wang, Z. L.; Ou, Y. N. & Tong, Y. X. Facile electrochemical synthesis of hexagonal Cu2O nanotube arrays and their application. Inorg. Chem. 2011, 50, 757–763.

(17)

Pu, Y. C.; Wang, G.; Chang, K. Der; Ling, Y.; Lin, Y. K.; Fitzmorris, B. C.; Liu, C. M.; Lu, X.; Tong, Y.; Zhang, J. Z.; Hsu, Y. J. & Li, Y. Au nanostructure-decorated TiO2 nanowires exhibiting photoactivity across entire UV-visible region for photoelectrochemical water splitting. Nano Lett. 2013, 13, 3817–3823.

(18)

Tanaka, A.; Sakaguchi, S.; Hashimoto, K. & Kominami, H. Preparation of Au/TiO2 with metal cocatalysts exhibiting strong surface plasmon resonance effective for photoinduced hydrogen formation under irradiation of visible light. ACS Catal. 2013, 3, 79–85.

(19)

Tran, P. D.; Xi, L.; Batabyal, S. K.; Wong, L. H.; Barber, J. & Chye Loo, J. S. Enhancing the photocatalytic efficiency of TiO2 nanopowders for H2 production by using non-noble transition metal co-catalysts. Phys. Chem. Chem. Phys. 2012, 14, 11596.

(20)

Kim, W.; Tachikawa, T.; Majima, T.; Li, C.; Kim, H.-J. & Choi, W. Tin-porphyrin sensitized TiO2 for the production of H2 under visible light. Energy Environ. Sci. 2010, 3, 1789.

(21)

Zhang, Z.; Wang, Z.; Cao, S. & Xue, C. Au/Pt nanoparticle-decorated TiO2 nanofibers with plasmon-enhanced photocatalytic activities for solar-to-fuel conversion. 2013, 2,

(22)

Gerischer, H. Photocatalysis in aqueous solution with small TiO2 particles and the dependence of the quantum yield on particle size and light intensity. Electrochim. Acta 1995, 40, 1277–1281.

(23)

Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K. & Taga, Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269–271.

(24)

Kosowska, B.; Mozia, S.; Morawski, A. W.; Grzmil, B.; Janus, M. & Kałucki, K. The preparation of TiO2-nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis. Sol. Energy Mater. Sol. Cells 2005, 88, 269– 280.

(25)

Cho, Y. & Choi, W. Visible light-induced reactions of humic acids on TiO2. J. Photochem. Photobiol. A Chem. 2002, 148, 129–135.


48

Page 49 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26)

Zaleska, A.; Górska, P.; Sobczak, J. W. & Hupka, J. Thioacetamide and thiourea impact on visible light activity of TiO2. Appl. Catal. B Environ. 2007, 76, 1–8.

(27)

Ventura, B.; Flamigni, L.; Beyler, M.; Heitz, V. & Sauvage, J. P. Unusual photoinduced electron transfer from a zinc porphyrin to a tetrapyridyl free-base porphyrin in a noncovalent multiporphyrin array. Chem. - A Eur. J. 2010, 16, 8748–8756.

(28)

Yang, J.; Yoo, H.; Aratani, N.; Osuka, A. & Kim, D. Determination of the superradiance coherence length of directly linked linear porphyrin arrays at the single-molecule level. Angew. Chemie - Int. Ed. 2009, 48, 4323–4327.

(29)

Kurimoto, K.; Yamazaki, T.; Suzuri, Y.; Nabetani, Y.; Onuki, S.; Takagi, S.; Shimada, T.; Tachibana, H. & Inoue, H. Hydrogen evolution coupled with the photochemical oxygenation of cyclohexene with water sensitized by tin(IV) porphyrins by visible light. Photochem. Photobiol. Sci. 2014, 13, 154–6.

(30)

Campbell, W. M.; Burrell, A. K.; Officer, D. L. & Jolley, K. W. Porphyrins as light harvesters in the dye-sensitised TiO2 solar cell. Coord. Chem. Rev. 2004, 248, 1363–1379.

(31)

Malinka, E. A.; Kamalov, G. L.; Vodzinskii, S. V.; Melnik, V. I. & Zhilina, Z. I. Hydrogen production from water by visible light using zinc porphyrin-sensitized platinized titanium dioxide. J. Photochem. Photobiol. A Chem. 1995, 90, 153–158.

(32)

Mele, G.; Del Sole, R.; Vasapollo, G.; García-López, E.; Palmisano, L. & Schiavello, M. Photocatalytic degradation of 4-nitrophenol in aqueous suspension by using polycrystalline TiO2 impregnated with functionalized Cu(II) -porphyrin or Cu(II)phthalocyanine. J. Catal. 2003, 217, 334–342.

(33)

Lucky, S. S.; Soo, K. C. & Zhang, Y. Nanoparticles in photodynamic therapy. Chem. Rev. 2015, 115, 1990–2042.

(34)

Urbani, M. & Nazeeruddin, M. K. Meso-substituted porphyrins for dye-sensitized solar cells. Chem. Rev. 2014, 114, 12330–12396.

(35)

Balzani, V.; Credi, A. & Venturi, M. Photochemical conversion of solar energy. ChemSusChem 2008, 1, 26–58.

(36)

McCree, K. J. The action spectrum, absoptance and quantum yield of photosynthesis in crop plants. Agric. Meteorol. 9, 191–216.

(37)

Itoh, T.; Yano, K.; Inada, Y. & Fukushima, Y. Photostabilized chlorophyll a in mesoporous silica: Adsorption properties and photoreduction activity of chlorophyll a. J. Am. Chem. Soc. 2002, 124, 13437–13441.


49


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 72

(38)

Eguchi, M.; Tachibana, H.; Takagi, S. & Inoue, H. Microscopic structures of adsorbed cationic porphyrins on clay surfaces: molecular alignment in artificial light-harvesting systems. Res. Chem. Intermed. 2007, 33, 191–200.

(39)

Takagi, S.; Eguchi, M.; Shimada, T.; Hamatani, S. & Inoue, H. Energy transfer reaction of cationic porphyrin complexes on the clay surface: effect of sample preparation method. Res. Chem. Intermed. 2007, 33, 177–189.

(40)

Takagi, S.; Eguchi, M.; Tryk, D. A. & Inoue, H. Porphyrin photochemistry in inorganic/organic hybrid materials : clays, layered semiconductors, nanotubes, and mesoporous materials. J. Photochem. Photobiol. C Photochem. Rev. 2006, 7, 104–126.

(41)

Harriman, A. & Richoux, M. C. Redox chemistry of metalloporphyrlns in aqueous solution. J. Phys. Chem. 1983, 87, 4957–4965.

(42)

Bartlett, P. N. Bioelectrochemistry : Fundamentals, Experimental Techniques and Applications; John Wiley & Sons Inc, 2008. 494.

(43)

Kadish, K. & Caemelbecke, E. Van. Electrochemistry of porphyrins and related macrocycles. J. Solid State Electrochem. 2003, 7, 254–258.

(44)

Trasatti, S. The Absolute electrode potential: an explanatory note. Pure Appl. Chem. 1986, 58, 955–966.

(45)

Richoux, M.C., Neta P., Harriman, A., Baral, S., Hambright, P. One- and two-electron reduction of metalloporphyrins. Radiation chemical, photochemical, and electrochemical studies. Kinetics of the decay of pi-radical anions. J. Phys. Chem. 1986, 90, 2462–2468.

(46)

Dolphin, D. in Primary Photo-Processes in Biology and Medicine (eds. Bensasson, R. V., Jori, G., Land, E. J. & Truscott, T. G.) Springer US, 1985. 171–180. doi:10.1073/pnas.0703993104

(47)

Koposova, E.; Shumilova, G.; Ermolenko, Y.; Kisner, A.; Offenhäusser, A. & Mourzina, Y. Direct electrochemistry of cyt c and hydrogen peroxide biosensing on oleylamine- and citrate-stabilized gold nanostructures. Sensors Actuators B Chem. 2015, 207, 1045–1052.

(48)

Ulas, G. & Brudvig, G. W. in Energy production and storage. Inorganic chemical strategies for a warming world (ed. Crabtree, R. H.) John Wiley & Sons Inc, 2010. 426.

(49)

Manke, A.-M.; Geisel, K.; Fetzer, A. & Kurz, P. A water-soluble tin(iv) porphyrin as a bioinspired photosensitiser for light-driven proton-reduction. Phys. Chem. Chem. Phys. 2014, 16, 12029–42.

(50)

Handman, J.; Harriman, A. & Porter, G. Photochemical dehydrogenation of ethanol in dilute aqueous solution. Nature 1984, 307, 534–535.


50

Page 51 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51)

Harriman, A. Metalloporphyrin-photosensitized formation of hydrogen from organic and inorganic substrates. J. Photochem. 1985, 29, 139–150.

(52)

Mintrop, L.; Windisch, J.; Gotzmann, C.; Alberto, R.; Probst, B. & Kurz, P. SnIV Metalloporphyrin/Co(III) Complex: An all-abundant-element system for the photocatalytic production of H2 in aqueous solution. J. Phys. Chem. B 2015, 119, 13698– 13706.

(53)

Baral, S.; Hambright, P. & Neta, P. One-and two-electron reduction of aluminum and tin pyridylporphyrins. A kinetic spectrophotometric study. J. Phys. Chem. 1984, 88, 1595– 1600.

(54)

Sutter, T. P. G.; Rahimi, R.; Hambright, P.; Bommer, J. C.; Kumar, M. & Neta, P. Steric and inductive effects on the basicity of porphyrins and on the site of protonation of porphyrin dianions: radiolytic reduction of porphyrins and metalloporphyrins to chlorins or phlorins. J. Chem. Soc. Faraday Trans. 1993, 89, 495–502.

(55)

Krüger, W. & Fuhrhop, J.-H. Evolution of hydrogen from photochemically reduced watersoluble tin(IV)-porphyrin. Angew. Chem. Int. Ed. Engl. 1982, 21, 131.

(56)

Tachibana, Y.; Moser, J. E.; Gratzel, M.; Klug, D. R. & Durrant, J. R. Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films. J. Phys. Chem. 1996, 100, 20056–20062.

(57)

Nazeeruddin, M. K.; Kay, A.; Miiller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M.; Lausanne, C.- & April, R. Conversion of light to electricity by cis-X2bis(2,2’-bipyridyl4,4'-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 1993, 115, 6382–6390.

(58)

Grätzel, M. Conversion of sunlight to electric power by nanocrystalline dye-sensitized solar cells. J. Photochem. Photobiol. A Chem. 2004, 164, 3–14.

(59)

Walter, M. G.; Rudine, A. B. & Wamser, C. C. Porphyrins and phthalocyanines in solar photovoltaic cells. J. Porphyr. Phthalocyanines 2010, 14, 759–792.

(60)

Woolerton, T. W.; Sheard, S.; Chaudhary, Y. S. & Armstrong, F. a. Enzymes and bioinspired electrocatalysts in solar fuel devices. Energy Environ. Sci. 2012, 5, 7470.

(61)

Ou, Z.; Wenbo, E.; Zhu, W.; Thordarson, P.; Sintic, P. J.; Crossley, M. J. & Kadish, K. M. Effect of axial ligands and macrocyclic structure on redox potentials and electron-transfer mechanisms of Sn(IV) porphyrins. Inorg. Chem. 2007, 46, 10840–10849.

(62)

Fuhrhop, J. H.; Kadish, K. M. & Davis, D. G. The redox behavior of metallo octaethylporphyrins. J. Am. Chem. Soc. 1973, 95, 5140–5147.


51


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 72

(63)

Mashiko, T.; Marchon, J.-C.; Musser, D. T.; Reed, C. a.; Kastner, M. E. & Scheidt, W. R. Cytochrome c Models. J. Am. Chem. Soc. 1979, 101, 3653–3655.

(64)

Kassner, R. J. Theoretical model for the effects of local nonpolar heme environments on the redox potentials in cytochromes. J. Am. Chem. Soc. 1973, 95, 2674–2677.

(65)

Moore, G. R. & Williams, R. J. Structural basis for the variation in redox potential of cytochromes. FEBS Lett. 1977, 79, 229–32.

(66)

Ajayi, F. F.; Chae, Kyoung-Yeol Kim, K. J.; Choi, M. J. & Kim, I. S. Photocurrent and photoelectrochemical hydrogen production with tin porphyrin and platinum nanowires immobilized with nafion on glassy carbon electrode. Int. J. Hydrogen Energy 2009, 34, 110–114.

(67)

Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M. & Grätzel, M. Photochemical cleavage of water by photocatalysis. Nature 1981, 289, 158–160.

(68)

Montes-Navajas, P.; Serra, M. & Garcia, H. Influence of the irradiation wavelength on the photocatalytic activity of Au-Pt nanoalloys supported on TiO2 for hydrogen generation from water. Catal. Sci. Technol. 2013, 3, 2252–2258.

(69)

Wang, J.; Zhang, X.-B.; Wang, Z.-L.; Wang, L.-M.; Xing, W. & Liu, X. One-step and rapid synthesis of “clean” and monodisperse dendritic Pt nanoparticles and their high performance toward methanol oxidation and p-nitrophenol reduction. Nanoscale 2012, 4, 1549–1552.

(70)

Formo, E.; Peng, Z.; Lee, E.; Lu, X.; Yang, H. & Xia, Y. Direct oxidation of methanol on Pt nanostructures supported on electrospun nanofibers of anatase. J. Phys. Chem. C 2008, 112, 9970–9975.

(71)

Wang, J. & Swain, G. M. Fabrication and evaluation of platinum/diamond composite electrodes for electrocatalysis. J. Electrochem. Soc. 2003, 150, E24–E32.

(72)

Neyerlin, K. C.; Gu, W.; Jorne, J. & Gasteiger, H. A. Study of the exchange current density for the hydrogen oxidation and evolution reactions. J. Electrochem. Soc. 2007, 154, B631–B635.

(73)

Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S. & Stimming, U. Trends in the exchange current for hydrogen evolution. J. Electrochem. Soc. 2005, 152, J23–J26.

(74)

Bockris, J. O. M.; Ammar, I. A. & Huq, A. K. M. S. The mechanism of the hydrogen evolution reaction on platinum, silver and tungsten surfaces in acid solutions. J. Phys. Chem. 1957, 61, 879–886.


52

Page 53 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(75)

McKone, J. R.; Warren, E. L.; Bierman, M. J.; Boettcher, S. W.; Brunschwig, B. S.; Lewis, N. S. & Gray, H. B. Evaluation of Pt, Ni, and Ni–Mo electrocatalysts for hydrogen evolution on crystalline Si electrodes. Energy Environ. Sci. 2011, 4, 3573–3583.

(76)

Mann, R. F. & Thurgood, C. P. Evaluation of Tafel–Volmer kinetic parameters for the hydrogen oxidation reaction on Pt(110) electrodes. J. Power Sources 2011, 196, 4705– 4713.

(77)

Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. a & Lewis, N. S. Solar water splitting cells. Chem. Rev. 2010, 110, 6446–6473.

(78)

Liu, X.; Koposova, E.; Offenhäusser, A. & Mourzina, Y. Self-assembly of platinum nanoparticles and coordination-driven assembly with porphyrin. RSC Adv. 2015, 5, 86934–86940.

(79)

Bajema, L.; Gouterman, M. & Meyer, B. Spectra of porphyrins. J. Mol. Spectrosc. 1961, 27, 225–235.

(80)

Berezin, D. B. Makrociklicheskiy effect i strukturnaya himiya porfirinov; Krasand, 2010. 418.

(81)

Szulbinski, W., Zak, J., Strojek, J., W. Electrochemical and photocatalytic properties of water-soluble tin(IV) meso-tetraanilinporphyrin. J. Electroanal. Chem. 1987, 226, 157– 170.

(82)

Wilson, G. S. & Neri, B. P. Cyclic voltammetry of porphyrins and metalloporphyrins. Ann. New York Acad. Sci. 1973, 206, 568–578.

(83)

Ellis, C. D.; Gilbert, J. A.; Murphy, W. R. & Meyer, T. J. Electrocatalytic oxidation of chloride to chlorine based on polypyridine complexes of ruthenium. J. Am. Chem. Soc. 1983, 105, 4842–4843.

(84)

Olijve, L. L. C.; How, E. N. W.; Bhadbhade, M.; Prasad, S.; Colbran, S. B.; Zhao, C. & Thordarson, P. Structural, electrochemical and photochemical investigation of the watersoluble tin(IV) tetrakis(2-N-hydroxyethyl-4-pyridinium)porphyrin photocatalyst. J. Porphyr. Phthalocyanines 2011, 15, 1345–1353.

(85)

McDaniel, D. H. & Brown, H. C. An extended table of Hammett substitutent constants based on the ionization of substituted benzoic acids. J. Org. Chem. 1958, 23, 420–427.

(86)

Kruk, M. M. & Braslavsky, S. E. Structural volume changes upon triplet formation of water-soluble porphyrins depend on the resonant effect of the substituents. Photochem. Photobiol. Sci. 2012, 11, 972–8.

(87)

Askarov, K. A.; Berezin, B. D. & Bystrickaya, E. V. Porfiriny: spektroskopiya, elektrohimiya, primenenie; Nauka, 1987. 382.


53


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 72

(88)

Fuhrhop, J.-H.; Krüger, W. & David, H. H. Hydrogen evolution from viologen radicals and from photochemically reduced tin(IV) porphyrins in the presence of colloidal platinum. Liebigs Ann. Chem. 1983, 204–210.

(89)

Fuhrhop, J.-H. The reactivity of the porphyrin ligand. Angew. Chemie - Int. Ed. 1974, 13, 321–335.

(90)

Fuhrhop, J.-H. & Lumbantobing, T. The redox properties of tin(IV) - and germanium(IV) - octaethylporphynato - dihydroxides as compared to other matallo - octaethylporphins. Tetrahedron Lett. 1970, 2815–2818.

(91)

Inhoffen, H. H.; Jäger, P.; Mählhop, R. & Mengler, C.-D. Elektrochemische reduktionen an porphyrinen und chlorinen, IV. Liebigs Ann. Chem. 1967, 704, 188–207.

(92)

Karakostas, N.; Schaming, D.; Sorgues, S.; Lobstein, S.; Gisselbrecht, J. P.; Giraudeau, a.; Lampre, I. & Ruhlmann, L. Photophysical, electro- and spectroelectro-chemical properties of the nonplanar porphyrin [ZnOEP(Py)4 4+,4Cl-] in aqueous media. J. Photochem. Photobiol. A Chem. 2010, 213, 52–60.

(93)

Harriman, A. & Walters, P. Photooxidation of tin (IV) porphyrins in aqueous solution. Inorganica Chim. Acta 1984, 83, 151–154.

(94)

Yin, X.; Tan, W.; Zhang, J.; Weng, Y.; Xiao, X.; Zhou, X.; Li, X. & Lin, Y. The effect mechanism of 4-ethoxy-2-methylpyridine as an electrolyte additive on the performance of dye-sensitized solar cell. Colloids Surfaces A Physicochem. Eng. Asp. 2008, 326, 42–47.

(95)

Ceroni, P. & Balzani, V. in The Exploration of Supramolecular Systems and Nanostructures by Photochemical Techniques (ed. Ceroni, P.) Springer, 2012. 21–39. doi:10.1007/978-94-007-2042-8

(96)

Leonhardt, H. & Weller, A. Elektronenübertragungsreaktionen des angeregten perylens. Berichte der Bunsen-Gesellschaft 1963,

(97)

Langley, R.; Hambright, P. & Williams, R. F. X. Kinetics of reduction of Cobalt(III) porphyrins by Fe(II)-EDTA2-. Inorg. Chem. 1985, 24, 3716–3718.

(98)

Wang, Z.; Li, Z.; Medforth, C. J. & Shelnutt, J. A. Self-assembly and self-metallization of porphyrin nanosheets. J. Am. Chem. Soc. 2007, 129, 2440–2441.

(99)

Rehm, D. & Weller, A. Kinetik und mechanismus der elektronübertragung bei der flureszenzlöschung in acetonitril. Berichte der Bunsen Geselschaft 1969, 834–869.

(100) Harel, Y. & Manassen, J. Photoreduction of tetraphenylporphyrins by amines in the visible. Photochemical syntheses of reduced tetraphenylporphyrins and the mechanism of photoreduction. J. Am. Chem. Soc. 1978, 100, 6228–6234.


54

Page 55 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(101) Qian, D.; Wenk, S.; Nakamura, C.; Wakayama, T.; Zorin, N. & Miyake, J. Photoinduced hydrogen evolution by use of porphyrin, EDTA, viologens and hydrogenase in solutions and Langmuir – Blodgett films. Int. J. Hydrogen Energy 2002, 27, 1481–1487. (102) Zhang, Z. Y.; Li, A. R.; Cao, S. W.; Bosman, M.; Li, S. Z. & Xue, C. Direct evidence of plasmon enhancement on photocatalytic hydrogen generation over Au/Pt-decorated TiO2 nanofibers. Nanoscale 2014, 6, 5217–5222. (103) Fleischer, E. B.; Palmer, J. M.; Srivastava, T. S. & Chatterjee, a. Thermodynamic and kinetic properties of an iron-porphyrin system. J. Am. Chem. Soc. 1971, 93, 3162–3167. (104) Pasternack, R. F.; Huber, P. R.; P, B.; Engasser, G.; Francesconi, L.; Gibbs, E.; Fasella, P. & Cerio Venturo, G. On the aggregation of meso-substituted water-soluble porphyrins. J. Am. Chem. Soc. 1971, 669, 4511–4517. (105) Arnold, D. P. & Blok, J. The coordination chemistry of tin porphyrin complexes. Coord. Chem. Rev. 2004, 248, 299–319. (106) Oppelt, K. T.; Wöß, E.; Stiftinger, M.; Schöfberger, W.; Buchberger, W. & Knör, G. Photocatalytic reduction of artificial and natural nucleotide co-factors with a chlorophylllike tin-dihydroporphyrin sensitizer. Inorg. Chem. 2013, 52, 11910–11922. (107) Wang, Z.; Lybarger, L. E.; Wang, W.; Medforth, C. J.; Miller, J. E. & Shelnutt, J. a. Monodisperse porphyrin nanospheres synthesized by coordination polymerization. Nanotechnology 2008, 19, 395604. (108) Sun, X.; Dong, S. & Wang, E. Coordination-induced formation of submicrometer-scale, monodisperse, spherical colloids of organic-inorganic hybrid materials at room temperature. J. Am. Chem. Soc. 2005, 127, 13102–13103. (109) Delmarre, D.; Veret-Lemarinier, A. V. & Bied-Charreton, C. Spectroscopic properties of Sn(IV) tetrapyridyl and tetramethylpyridinium porphyrins in solution and in sol-gel matrices. J. Lumin. 1999, 82, 57–67. (110) Serpone, N. & Salinaro, A. Terminology, relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. Part I: Suggested protocol. Pure Appl. Chem. 1999, 71, 303–320. (111) Hoebeke, M.; Damoiseau, X.; Schuitmaker, H. J. & Van de Vorst, A. Fluorescence, absorption and electron spin resonance study of bacteriochlorin a incorporation into membrane models. Biochim. Biophys. Acta 1999, 1420, 73–85. (112) Kobayashi, M.; Akiyama, M.; Kano, H. & Kise, H. in Chlorophylls and Bacteriochlorophylls. Biochemistry, Biophysics, Functions and Applications (eds. Grimm, B., Porra, R. J., Rüdiger, W. & Scheer, H.) Springer Netherlands, 2006. 79–94.


55


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 56 of 72

(113) Segawa, H.; Shimidzu, T. & Honda, K. Control of pi-radical anion state of porphyrin with a polymer matrix. Polym. J. 1988, 20, 441–446. (114) Closs, G. L. & Closs, L. E. Negative ions of porphyn metal complexes. J. Am. Chem. Soc. 1963, 85, 818–819. (115) Bard, A. J.; Stratmann, M. & Fujishima, A. in Encyclopedia of Electrochemistry, Volume 6, Semiconductor Electrodes and Photoelectrochemistry (eds. Bard, A. J., Stratmann, M. & Licht, S.) Wiley-VCH, 2002. 608. (116) Whitten, D. G. Photochemistry of porphyrins and their metal complexes in solution and organized media. Rev. Chem. Intermed. 1978, 2, 107–138.


56

Page 57 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. The structures of water-soluble SnPs. 39x9mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. SEM (A), TEM (B) and HRTEM (C) images of the Pt NPs synthesized by the citrate method on the surface of TiO2. Enlarged HRTEM image of individual grain showing lattice fringes (D). 80x67mm (600 x 600 DPI)


Page 58 of 72

Page 59 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Cyclic voltammogram (a) and polarization curve (b) of Pt NPs on the ITO thin film substrate in 0.1 M HClO4 solution, inset of (b): corresponding Tafel plot of Pt NPs. 59x21mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Absorption spectra of SnPs with different peripheral groups at pH=5.2, concentrations of SnPs was 4 µM (see section 2.1). 288x201mm (300 x 300 DPI)


Page 60 of 72

Page 61 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Cyclic voltammograms of 1mM SnPs: SnTPPC (A, inset: 0.2 V ˂ E ˂ +1.6 V potential range), SnTPPS (B, inset: 0.2 V ˂ E ˂ +1.6), SnTPyP (C), SnTMPyP (D) in 0.5M KCl of pH=3.5 (black) and pH=9.0 (blue) and in 0.1 M Na-phosphate buffer of pH=7.0 (red). 160x122mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Hydrogen yields at different pH values and SnP complexes with different functional groups, pH=3.0: (a) SnTPyP/TiO2/Pt/EDTA, (b) SnTPPS/TiO2/Pt/EDTA, (c) SnTPPS adsorption on TiO2 overnight/Pt/EDTA, (d) SnTPPC/TiO2/Pt/EDTA, (e) SnTPPC adsorption on TiO2 overnight/Pt/EDTA, (f) SnTMPyP(OH)2/TiO2/Pt/EDTA, (g) SnTMPyP(Cl)2/TiO2/Pt/EDTA; pH=7.4: (h) SnTPyP/TiO2/Pt/EDTA, (i) SnTPPS/TiO2/Pt/EDTA, (j) SnTPPC/TiO2/Pt/EDTA, (k) SnTMPyP(OH)2/TiO2/Pt/EDTA, (l) SnTMPyP(Cl)2/TiO2/Pt/EDTA; pH=9.0: (m) SnTPyP/TiO2/Pt/EDTA, (n) SnTPPS/TiO2/Pt/EDTA, (o) SnTPPC/TiO2/Pt/EDTA, (p) SnTMPyP(OH)2/TiO2/Pt/EDTA, (q) SnTMPyP(Cl)2/TiO2/Pt/EDTA; pH=10.5: (r) SnTPyP/TiO2/Pt/EDTA; pH=13.0: (s) SnTPyP/TiO2/Pt/EDTA. 58x43mm (600 x 600 DPI)


Page 62 of 72

Page 63 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Time profile of the hydrogen production in the SnTPyP/TiO2/Pt/EDTA system at pH=9.0, TON(H/SnP) – turnover number. 55x38mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. Schematic of photoexcited electron transfer and hydrogen evolution under visible light irradiation. 42x22mm (300 x 300 DPI)


Page 64 of 72

Page 65 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Changes in the absorption spectra of SnTPyP solution in the presence of Pt NPs and EDTA at pH=7.4 (A) and pH=9.0 (B) under continuous N2 blowing and irradiation with visible light (λ > 390 nm). 66x25mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10. Changes in the absorption spectra of SnTPPC (A) and SnTPyP/Pt/EDTA (B) solution in the presence of Pt NPs and EDTA at pH=9.0 under continuous N2 blowing and irradiation with visible light (λ > 390 nm) for 3 min. 62x23mm (300 x 300 DPI)


Page 66 of 72

Page 67 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11. Schematic representation of the HOMO-LUMO gaps of SnPs, semiconductor bandgaps of TiO2, and hydrogen reduction potentials at different pH, where SnS – SnTPPS, SnPy – SnTPyP, SnMPy – SnTMPyP, SnC – SnTPPC. The positions of HOMO were not so clearly distinguishable for all SnPs (see text for detail) and are therefore marked with a dashed line on a schematic. The HOMO positions of SnTPPC and SnTPPS were defined from peaks of the oxidation potentials and are shown as pink bars. Ep of EDTA oxidation on GCE are +1.31 V (pH 7) and +1.25 V (pH 9), respectively. At pH 3.5 oxidation starts at about 1.25 V (NHE) at the experimental conditions (see Figure S5). 160x105mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. Scheme of the reduction process for porphyrins with PhOH (r=100) and AcOH (r=5÷27) as proton donors (r-molar ratio), e – a 1-e electron transfer process, and p – a single protonation process. 12x1mm (600 x 600 DPI)


Page 68 of 72

Page 69 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2. Schematic representation of possible reactions of macrocycle of meso-substituted Sn(IV)Ps during photochemical hydrogen reduction in system Sn(IV)P sensitized TiO2/Pt with EDTA electron donor 49,52,55,88–91. Adapted from 49,52,55,88. 73x33mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 3. Scheme of the oxidation process of tin-porphyrin. 11x0mm (600 x 600 DPI)


Page 70 of 72

Page 71 of 72

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 4. Schematics of different pathways for hydrogen production. Further photochemical processes with participation of chlorin are shown in Scheme 2. 28x5mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The TOC graphic. 47x32mm (300 x 300 DPI)


Page 72 of 72

Influence of Meso-Substitution of the Porphyrin Ring on Enhanced

Recommend Documents