Metal organic frameworks as efficient photosensitizer for TiO₂ nanoarray anode and application to water splitting in PEC cells

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Metal organic frameworks as eﬀicient photosensitizer for

TiO� nanoarray anode and application to water splitting

in PEC cells

Sheng Mu You

To cite this version:

Sheng Mu You. Metal organic frameworks as eﬀicient photosensitizer for TiO� nanoarray anode and application to water splitting in PEC cells. Catalysis. Université Paris-Saclay; National Chiao Tung University (Taiwan), 2020. English. �NNT : 2020UPASF015�. �tel-03092318�

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Metal organic frameworks as efficient

photosensitizer for TiO2 nanoarray anode and

application to water splitting in PEC cells

Thèse de doctorat de National Chiao Tung University

et de l'université Paris-Saclay

É cole doctorale n°571 Sciences chimiques : molécules, matériaux, instrumentation et biosystèmes (2MIB)

Unité de recherche : Université Paris-Saclay, CNRS, Institut de chimie moléculaire et des matériaux d'Orsay, 91405, Orsay, France

Spécialité de doctorat: Chimie

Thèse présentée et soutenue à Orsay, le 13 Oct. 2020, par

M. Sheng Mu YOU

Composition du Jury :

Mme Marie-Laurence Giorgi

Professeure (LGPM, CentraleSupélec, Université Paris-Saclay) Présidente M. Christian Beauger

Professeur (PERSEE, Mines ParisTech, Sophia Antipolis) Rapporteur & Examinateur

M. Kun-Yi Lin

Professeur (ECT, ENV, National Chung Hsing University) Rapporteur & Examinateur

M. Ruey-An Doong

Professeur (ECCL, IAES, National Tsing Hua University) Directeur de thèse M. Pierre Millet

Professeur (ERIEE, ICMMO, Université Paris-Saclay) Directeur de thèse Mme Sue-Min Chang

Professeure (EML, IEV, National Chiao Tung University) Co-Directrice de thèse

M. Ying-Jung Hsu

Professeur (NODL, MSE, National Chiao Tung University) Invité

Thè

se

de d

octorat

NNT : 2 0 2 0 UPA S F 0 1 5

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Titre : Photosensibilisation de nano-réseaux de TiO2 à l'aide de Metal Organic

Frameworks pour la dissociation photoelectrochimique de l'eau.

Mots clés : Photoélectrodes, nano-réseaux TiO2, Metal Organic Framework,

dissociation de l'eau, réaction de dégagement d'oxygène, catalyse électrochimique, photoélectrochimie.

Résumé : L’utilisation continue des réserves de combustibles fossiles depuis la

révolution industrielle conduit à leur raréfaction ; elle a aussi engendré de profonds déséquilibres climatiques, mesurables par l’amplitude des cycles de la température atmosphérique. Stocker l'énergie solaire incidente sous forme d'hydrogène produit par dissociation photoélectrochimique de l'eau pourrait contribuer à combattre le réchauffement climatique. Les matériaux de la famille des «Metal Organic Framework» (MOF) sont des photo-électrocatalyseurs de type semiconducteur, intéressants pour ce type d’application. Leur porosité extrêmement élevée et leur grande variabilité de composition chimique et de structure permettent d’ajuster leurs propriétés d'absorption du rayonnement solaire et de catalyse. En contrôlant la composition chimique et le dopage du linker utilisé dans le MOF, il est possible d'ajuster l'énergie de la bande interdite, de favoriser la fonctionnalisation sur des substrats très variés ou encore d'ajuster leur résistance à la corrosion dans divers environnements chimiques. Ce sont donc des matériaux d'un grand intérêt pour la catalyse, l'électrocatalyse ou la photo-électro-catalyse. De son côté, le TiO2 nano-structuré, par exemple sous forme de tapis

d’épaisseur micrométrique de nanotubes (TDNT) ou de nanofils (TDNR) formant des TiO2 nano-arrays (TNA), est couramment utilisé dans la fabrication de photoanodes

pour le dégagement d'oxygène en milieu aqueux. Au cours de notre thèse, nous avons fabriqué des matériaux composites constitués de MOF de métaux de transition (Ni, Co, Fe) déposés sur des TNA. Pour cela, nous avons utilisé une méthode électrochimique d'électrodéposition qui consiste à déposer des nanoparticules d’hydroxyde sur les TNA à potentiel fixe puis de les transformer par réaction chimique avec des ligands

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organiques (1,3,5- acide benzènetricarboxylique (BTC), 1,4 acide benzène dicarboxylique (BDC), et 2-méthylimidazole (2MZ)) par voie thermo-chimique. En fin, Nous sommes parvenus à fabriquer des photoélectrodes dont la couche photoactive est nanostructurée et constituée de nanoparticules de MOF déposées sur des tapis de nanofils ou de nanotubes de TiO2. Le niveau de performance photoélectrochimique de

ces photo électrodes vis-à-vis de la photo-oxydation de l'eau a été mesuré sous différentes conditions d'irradiation, dans un électrolyte aqueux de pH neutre (0.1 M Na2SO4). Les photoélectrodes à base de MOF présentent une photo-activité supérieure

à celle obtenue avec les tapis de TiO2 nus. Des photo-densités de courant de l’ordre du

mA cm−2_{ou de la dizaine de µA cm}−2_{ont été mesurées sous UV-Vis (100 mW/cm}2_{) et}

sous rayonnement visible (63 mW/cm2_{), respectivement. En détail, nous avons}

synthétisé des tapis ZIF-67@TDNR, ZIF-67 / TDNR, Ni-MOF / TDNR, Ni-MOF / c-TDNR et FeNi-MOF / TNTA en combinant des techniques de dépôt électrochimique et de transformation thermique. Nous avons mesuré des photocourants de 0,078 mA cm-2

(0,004 mA cm-2_{sous lumière visible), 0,88 mA cm}-2_{, 0,46 mA cm}-2_{(0,042 mA cm}-2

sous lumière visible), 4,48 mA cm-2_{et 1,91 mA cm}-2_{sous irradiation UV-Vis,}

respectivement. Ces photoélectrodes présentent à la fois un niveau élevé de performance électrochimique et une grande stabilité vis-à-vis de la photo-corrosion. Les mécanismes réactionnels ont été analysés par spectroscopie d’impédance photo-électrochimique. Les différentes étapes ont été identifiées et les valeurs des paramètres cinétiques ont été déterminées sur un domaine élargi de potentiel. Ce type de matériaux hybrides, utilisant des MOF comme photo-sensibilisateurs, vient enrichir la famille des matériaux photo-actifs déjà connus vis-à-vis de la photo-oxydation de l’eau. L’aute mot dire, ces matériaux composites ont été utilisés avec succès comme phase active de photo-anodes pour la réaction de dégagement d'oxygène moléculaire (OER). Ces photo-anodes possèdent une activité électrocatalytique significative et une excellente durabilité photoélectrochimique.

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Title: Metal organic frameworks as efficient photosensitizer for TiO2 nanoarray

anode and application to water splitting in PEC cells.

Keywords: Metal organic frameworks, TiO2 nanoarray, water splitting, oxygen

evolution reaction, electrochemical catalysis, photo-electrochemical catalysis.

Abstract: The continued use of fossil fuel reserves since the industrial revolution has

led to their scarcity; it has also created deep climatic imbalances, measurable by the amplitude of the atmospheric temperature cycles. Storing incident solar energy in the form of hydrogen produced by photoelectrochemical dissociation of water suggests the possibility of combating global warming. The materials of the "Metal Organic Framework" (MOF) family are semiconductor-type photoelectrocatalysts, which are advantageous for this type of application. Their extremely high porosity and their great variability in chemical composition and structure allow their properties of absorption of solar radiation and of catalysis to be adjusted. By controlling the chemical composition and the doping of the linker used in the MOF, it is possible to adjust the energy of the band gap, to promote the functionalization on a wide variety of substrates or to adjust their corrosion resistance in various chemical environments. They are therefore materials of great interest for catalysis, electrocatalysis or photo-electro-catalysis. For its part, nanostructured TiO2, for example in the form of a micrometric

thickness mat of nanotubes (TDNT) or nanorods (TDNR) forming TiO2 nano-arrays

(TNA), is commonly used in the manufacture of photoanodes for the evolution of gaseous oxygen (OER) in an aqueous medium. During our thesis, we fabricated composite materials consisting of MOF of transition metals (Ni, Co, Fe) deposited on TNAs. For this, we used an electrochemical method of electrodeposition. This allowed us to deposit metal hydroxides on TNAs at fixed potential and then transform them by

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chemical reaction with organic ligands (1,3,5 benzenetricarboxylic acid (BTC), 1,4 benzenedicarboxylic acid (BDC), and 2-Methylimidazole (2MZ)) thermochemically. Finally, we have succeeded in manufacturing photoelectrodes in which the photoactive layer is nanostructured and consists of MOF nanoparticles deposited on mats of nanowires or TiO2 nanotubes. The level of photoelectrochemical performance of these

photoelectrodes with respect to the photo-oxidation of water was measured under various irradiation conditions, in an aqueous electrolyte of neutral pH (0.1 M Na2SO4).

MOF-based photoelectrodes exhibit higher photoactivity than that obtained with bare TiO2 mats. Current photo-densities of the order of mA cm-2 or tens of µA cm-2 have

been measured under UV-Vis (100 mW/cm2_{) and visible (63 mW/cm}2_{) radiation,}

respectively. In detail, we synthesized ZIF-67@TDNR, ZIF-67 / TDNR, Ni-MOF / TDNR, Ni-MOF / c-TDNR and FeNi-MOF / TNTA mats by combining electrochemical deposition and thermal transformation techniques. We measured photocurrents of 0.078 mA cm-2_{(0.004 mA cm}-2_{under visible light), 0.88 mA cm}−2_{, 0.46 mA cm}−2_{(0.042 mA}

cm-2 _{under visible light), 4.48 mA cm}−2_{and 1.91 mA cm}−2_{under UV-Vis irradiation,}

respectively. These photoelectrodes exhibit both a high level of photoelectrochemical performance and high stability against photo-corrosion. The reaction mechanisms were analyzed by photoelectrochemical impedance spectroscopy. The different steps have been identified and the values of the kinetic parameters have been determined over a wide range of potential. This type of hybrid material, using MOFs as photosensitizers, enriches the family of photoactive materials already known with regard to the photo-oxidation of water. In addition, these composite materials have been used with success as the active phase of photoanodes for the OER. These photo-anodes have significant electrocatalytic activity and excellent photoelectrochemical durability.

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Declaration

I, ………Sheng-Mu YOU………

declare that this thesis and the work presented in it are my own and has been generated by me as the result of my own original research.

Title: Metal organic frameworks as efficient photosensitizer for TiO2 nanoarray anode

and application to water splitting in PEC cells.

I confirm that:

1. This work was done wholly or mainly while in candidature for a research degree at this University

2. Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated 3. Where I have consulted the published work of others, this is always clearly attributed 4. Where I have quoted from the work of others, the source is always given. With the

exception of such quotations, this thesis is entirely my own work 5. I have acknowledged all main sources of help

6. Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself

Signed: ……..……… Date: ……….………

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Acknowledgements

This Thesis was completed under the kind care and careful guidance of Prof. Ruey–an Doong and Prof. Pierre Millet. During my master and dual PhD degree in Taiwan and France, I have experienced many things no matter my family, study, and even my life. Finally, I achieved this limestone of my life and I will keep my positive attitude and brave to face everything in the future.

First, I have to thanks Prof. Ruey–an Doong owing to his serious scientific attitude, rigorous academic spirit, and work style of excellence have deeply infected and inspired me. I’m really appreciate that you are not only giving me meticulous guidance in my studies, but also gave me meticulous care in my thoughts and life. You encouraged me to study PhD abroad, and supported me as much as you can do. I would like to extend my sincere gratitude and high respect to you.

I would also like to thank Prof. Pierre Millet because of your help and support that I can overcome the difficulties and doubts one by one until the successful completion of this thesis and papers. When the thesis is about to be completed, my mood cannot be calm. Thanks for you patiently modified my papers and thesis, and also kindly helped me during this hard of period. I would like to express my deep thanks again.

Additionally, I would like to show my highly appreciation to Prof. Waleed M. A. El Rouby due to the support for studies and care. Although I’m not as smart as you, you still show your kind to teach me skills and technics for experiments. I can say without your help I cannot complete this thesis and papers. As well as I have to thanks Prof. Sue-min Chang, Prof. Sylvain Franger, Prof. Loic Assaud and for administrative support and life care. Your help and support allowed me to pay more concentration on my studies and works. Thank you very much.

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When I studied in National Chiao-Tung University, I was supported by many colleagues and friends. I would like to thank Prof. Ong, Dr. Vincent, Dr. Ruby, Dr. Ankan, Dr. Akhilesh, Dr. Kartick, Dr. Bihn, Sheila, Duncan, Jim, Jenny, Minor, Cindy, Alan Wang, Alan Yeh, Yi-Ting, Zoe, and all members in ECCL. Without your help and cooperation, I cannot finish everything alone. Thank you all again.

During the two years in University Paris Saclay, I’m appreciate with my dear friends and brothers Parviz, Arun, Juan, Remith, Ali, Alian, and Jesus. You are not stingy to help me for everything, and spread happiness to me. Especially, I won’t forget our promise with Parviz and Arun, we will meet one day in Taiwan. Also I want to see Arun singing on my marriage and enjoy everything with me in my hometown. I will invite you all to my country one day. Thanks a lot to you all.

Finally, I would also like to thank my parents who cultivated my hard work when I grew up. You never refuse my request and stubbornness, and even support me all the way to graduation. And also my brother and sister, thank you for tolerating my temper when I feel bad or under pressure. At last, I would like to say sorry to my passed maternal grandpa, because I’m late for the promise with you. Because of your take care since I was child, I would not be able show my talent for my PhD. Thank you again. Finally, I would like to express my heartfelt thanks again to all of my professors, friends, and brothers who cared about and helped me. I’m glad to share my happy and lucky to all of you. Thank you very much.

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Abbreviations

PEC

OER

HER

DSSC

RHE

SCE

PC

CA

CV

GDC

LSV

PEIS

MOF

TNA

TNTA

TDNR

NW

BTC

BDC

ZIF

FTO

TMO

TMD

MeOH

DMF

2MZ

RE

CE

EXAFS

BET

XAS

XRD

SEM

TEM

FTIR

Photoelectrochemistry Oxygen evolution reaction Hydrogen evolution reaction

Dye sensitized solar cells Reversible hydrogen electrode

Saturated calomel electrode Photocatalysis Chronoamperometry

Cyclic voltammetry Galvanostatic charge & discharge

Linear sweep voltammetry

Photoelectrochemical impedance spectroscopy Metal organic frameworks

TiO2-based nanoarray

TiO2 nanotubes array

TiO2 nanorods array

Nanowires

1, 3, 5-benzenetricarboxylic acid 1, 4-benzenedicarboxylic acid Zeolitic imidazolate framework

Fluorine doped tin oxide Transition metal oxide Transition metal dichalcogenide

Methanol

N, N-Dimethylformamide 2-Methylimidazole Reference electrode Counter electrode

Extended X-ray absorption fine structure Brunauer−Emmett−Teller

Hard X-ray absorption spectroscopy X-Ray diffraction

Scanning electron microscope Transmission electron microscope Fourier transform infrared spectroscopy

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Motivation

Orderly hybrid nanomaterials formed from titanium dioxide nano-arrays (TNA) and Metal Organic Frameworks (MOFs) have demonstrated high activity levels when used in the fields of photodegradation of organic pollutants, the dissociation of water and energy storage. Improvements are still possible. The theoretical surface area of TiO2

nanotubes produced by the microwave-assisted hydrothermal method is limited to only 246 m2_{/g. Another problem is that surface properties influence the rate of recombination}

of photo-induced charge carriers and limit the charge transfer kinetics to the electrolyte. In view of the recent work reported in the literature, higher specific surfaces can be obtained using mats of TiO2 nanotubes on titanium (Ti / TiO2 NT). However, TiO2 has

inherent limitations. The value of the band gap of pure TiO2 (Anatase: 3.2 eV, Rutile:

3.0 eV) allows only 30 to 50% of the total solar spectrum to be collected, and the rapid recombination of the photo-generated electron-hole pairs limits photo-electro-catalytic activity. These limitations disqualify pure nanostructured TiO2 for water

photo-dissociation. Despite the amount of defects present in these materials (parasitic recombination centers of photogenerated charge carriers), photocatalytic activity can be significantly enhanced by forming highly ordered 1D nanocomposites of TNA / MOF in which MOFs act as photosensitizers, more sensitive to visible light. These nano-hybrid materials can combine the (photo-) electrochemical performance of organic ligands and TNAs and can present higher photo-activity with regard to either the hydrogen (HER) or oxygen (OER) evolution reactions under visible light. Our thesis objective is therefore the fabrication of new photoanodes based on MOF / TNTA nanocomposites and their application to the photoelectrochemical dissociation of water.

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Objectives and methodology

The objective of this thesis work is to build ordered hybrid nanomaterials combining mats of TiO2 nano-arrays (TNA) and MOFs of transition metals, and to

measure their photoelectrochemical activity with respect to the dissociation of water into molecular hydrogen and oxygen. Optimization of these structures requires a detailed understanding of their properties (surface, light absorption, electrochemical activity and charge carrier transport). TiO2-based nanomaterials are frequently used as

photoelectrodes due to their unique electronic properties and limited cost. The addition of MOFs with transition metals (eg, Fe-MOF, Ni-MOF and ZIF-67) makes it possible to increase the specific surface area by introducing a large quantity of micropores into the structure (0.5 to 2.0 nm in diameter), which improves the photocatalytic properties. The nanostructure of MOFs and the MOF / TNA interface contribute to the increase in surface area. During our work, the morphology as well as the elemental composition of MOF, TNA, and MOF/TNA were characterized using different techniques (e.g. SEM, TEM, XPS, and EDX). A conventional three-electrode cell was used to perform the electrochemical experiments. Linear sweep voltammetry (LSV) was used to record voltammograms over the potential range extending from -0.6 to +1.2 V/Ag/AgCl, under dark and chopped illumination conditions. Chronopotentiometric measurements were made at a constant potential of +1V vs Ag/AgCl, under chopped and continued illumination. Photoelectrochemical impedance spectroscopy (PEIS) was used under Uv-vis, Vis light and dark conditions, at different potentials ranging from -0.6 to +1.2 V vs Ag/AgCl, over the 100 kHz–50 mHz frequency range, using an AC voltage amplitude of 10 mV. Mott-Schottky measurements were made at a constant frequency of 10 Hz over the +1.0 V vs Ag/AgCl, to -1.0 V vs Ag/AgCl, potential range.

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1. Introduction

1.1. Water splitting

World energy consumption has increased by a factor of at least two compared to the 1970s, due to strong demographic and economic growth over the period. This demand has been met from fossil (oil, coal and gas, etc.) and nuclear energy resources.1

However, the progressive depletion of fossil fuel reserves has forced the scientific community working in the energy field to seek unconventional sources of energy. The so-called renewable energy resulting from the interaction of solar radiation with the Earth makes it possible to imagine a sustainable energy organization, compatible with the continuous development of human societies. This requires the development of high performance energy converters using abundant and recyclable chemical elements, whose life cycle would have a limited environmental impact. The intermittent nature of these renewable energies has placed the problem of their storage at the heart of the concerns of the energy transition. The interconversion of the electricity vector into a chemical vector (for example via the dissociation of water into molecular hydrogen) allows the development of low-carbon infrastructures acclaimed by the public and the public authorities. It also contributes to the energy independence of nations and to a reduction in geopolitical tensions around fossil energy reserves. Fig. 1 summarizes the situation schematically and illustrates the challenges researchers face in transforming renewable electricity into hydrogen using Earth abundant chemicals.2

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Fig. 1. Diagram showing the challenges posed to research in materials science to be able to dissociate water into hydrogen and oxygen using transition metals.3

The electrochemical dissociation of water involves two synchronous redox half reactions, the hydrogen evolution reaction (HER) which occurs at a cathode and the oxygen evolution reaction (OER) which occurs at an anode. Under standard temperature and pressure conditions, the reaction requires the application of a voltage at least equal to 1.23 V corresponding to the Gibbs free energy change in free enthalpy of the reaction.4_{HER involves the formation (adsorption) and desorption of}

ad-hydrogen atoms on catalytic sites, on the surface of materials used as electrocatalysts (Fig. 2). The Gibbs free energy variation associated with the adsorption / reduction of protons at an electrocatalytic site (ΔGH+_{) also gives a good idea of the value of the}

desorption energy leading to the formation of molecular hydrogen.5_{A good HER}

catalyst is a material for which the variation in free energy associated with the formation of hydrogen ad-atoms is neither too strong nor too weak. Too low, it will require a significant overvoltage to reduce the protons and form the ad-atoms. Too strong, the desorption into molecular hydrogen will have low kinetics.6

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On the side of the OER, the reaction mechanisms are more complex. In fact, different reaction pathways exist depending on the type of reaction site and the type of electrolyte. However, there also exist reactional intermediates more or less adsorbed, such as HO-_{, O}-_{and HOO}-7_{. The rate-limiting step of OER is generally the}

transformation of the O-_{species into HOO}-_{. The formation of OOH* from O* is ascending}

for the OER at the equilibrium potential of 1.23 V (vs. reversible hydrogen electrode, RHE). The kinetics of OER being slow under the temperature and pressure conditions in which the electrolysis of water takes place, a large overvoltage must be applied to obtain a reasonable current density (e.g., 1 A/cm2_{). As a result, the operating potential of the}

anodes in an acid environment is generally close to 1.6-1.7 Volt instead of 1.23 Volt (thermodynamic requirement).

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The electrocatalysts required to maximize the kinetics of HER and OER play a key role in the energy efficiency of the water dissociation reaction. (Fig.3) They must be selected in a very rigorous way. It is now clearly established that noble metals such as Pt, Pd, Ir, Au, Ag and rare elements give the best results. However, other compounds based on non-noble metals, for example carbides and phosphides, are also known as effective cocatalysts of the water dissociation reaction by photocatalytic (PC) or photoelectrochemical (PEC) processes.9_{Several families of materials have a known}

activity with respect to the photodissociation of water: transition metal oxide (TMO)10-11_,

metal organic framework (MOF)12-13_{, transition metal dichalcogenide (TMD)}14-15_{, and}

non-metal graphitic carbon material(C3N4)16-17 which are the state-of-the-art cathode and anode

electrocatalysts with suitable band gap for hydrogen production. Commercial electrolyzers typically operate at a cell voltage of 1.8 – 2.0 V due to the large over potentials associated with the reaction kinetics.18_{Therefore, the search for active, stable, and cost-efficient}

electrocatalysts for oxygen evolution via water splitting could make a substantial impact on energy technologies that do not rely on fossil fuels.19

Fig. 3. (a) I-V curve for the overall water splitting reaction (b) Electrochemical mechanism of HER/OER in water spitting.20

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Photo-electrochemical water splitting (PEC-WS)

The production of hydrogen from water and sunlight is one way to overcome the intermittency of renewable energy production. Photoelectrochemical water splitting (PWS) takes place using a photoelectrochemical (PEC) cell powered directly by sunlight and operating at temperatures close to room temperature. The incident photons are captured using inexpensive semiconductor materials. The challenge is to increase the photon-to-hydrogen conversion yields.21_{The use of a PEC type cell has several}

advantages over thermochemical methods: the efficiency of solar-hydrogen conversion is relatively higher, and it is possible to size the cells in a flexible manner allowing for different small-scale applications. However, improving photon-to-hydrogen and energy conversion efficiencies faces enormous challenges. The efficiency losses are linked to the limited absorption of light photons, and to the rapid recombination of photo-generated charge carriers. In addition, there are problems of chemical stability and of degradation of the photoelectrodes in contact with the aqueous electrolyte by photo-corrosion. Several cell designs have been described in the scientific literature (Fig. 4), including that of single and dual compartment wireless cell or non-contact (separator embedded) cell. In addition, it is possible to adjust several other parameters (the intensity and composition of the light source, the composition of the electrolyte, its concentration and pH, the reagent flow rates and the composition of the gas.22

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To obtain high efficiency performances, the conception and the optimization of the design of the cell is not the only problem: the type and the quality of the photoelectrode (s) is also extremely important. The necessary conditions required for the selection of the semiconductors used in PEC water splitting cells are shown in the Fig..5. The semiconductor which allows the absorption of light radiations must have an appropriate band gap to convert the photons into electric charges and must be chemically stable in contact with the electrolytic solutions used, inexpensive (free of noble metals: more and more research is being carried out to find semiconductors using elements abundant on Earth such as Fe, Co, Ni, Cu, Ti, Zn, etc. 24_{) and must be able to be produced in a well}

crystallized form with a concentration of defects as low as possible. This is because the defects act as electron-hole recombination sites and thus reduce the energy conversion efficiency. Other properties (particularly absorption of UV, visible or sunlight) and charge transfer properties are also important for material selection. Titanium dioxide TiO2 (in various morphologies) is the most commonly used material for the

manufacture of photoelectrodes, due to its large gap energy value (≥ 3.0 eV) and its excellent chemical stability in aqueous media.

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d e f

Fig. 4. Schematic illustration of different strategies of effective PEC cell (a)-(c) are typically general cell21_{, and (d)-(f) are compact cell}23_.

Fig. 5. List of properties required for semiconductor materials used in water dissociation photoelectrochemical cells 24-25

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- 21 - 1.2. Metal organic framework

Metal–organic frameworks (MOFs) are compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. They form a subclass of coordination polymers with the particularity of having microporous architectures.26-27_{used in these materials for the formation of the}

crystal structure (e.g. BTC, BDC, imidazole or glycine) are sometimes referred to as " struts "28_{The structure of MOFs is characterized by an open framework that can be}

porous (microporous materials). MOFs can be used for gas storage (Fig. 6)29-30_,

purification31-32_{and separation}33-34_{, as well catalysis}35-36_{and sensing}37-38_{. To date,}

MOFs which have electrocatalytic properties have received increasing attention in recent years, especially for the oxidation of water. This is due to the great variety of synthesizable materials, their ultra-large surface area / volume ratio (an important property in catalysis) and the possibility of adjusting the pore diameters.39_Several

MOFs having an interesting electrochemical activity with respect to the reactions of hydrogen and oxygen evolution have already been identified.12, 40_.

Fig. 6. Schematic representation of important reported MOFs which are known to have high gas storage properties.28

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In addition to its proven electrocatalytic properties, any MOF capable of being used as an electrocatalyst for the dissociation of water must be chemically stable in contact with aqueous electrolyte solutions, either at acidic pH or at alkaline pH, over a range of expanded temperature (from ambient to boiling water temperature).41_In

addition, its resistance to photo-corrosion (combined effect of working potential and incident radiation) must also be verified. Duan et al. (Fig. 7) 42_{have developed a}

methodology for the in situ growth of 2D ultra-thin nanosheet arrays of NiFe-MOF electrodes. Electrodes containing MOF have a higher level of performance with respect to the oxygen evolution reaction: an overvoltage of 240 mV is required to circulate a current density of 10 mA / cm2_{for 20,000 without significant loss of activity. A turnover}

frequency of 3.8 s-1_{was measured on this type of electrode at an overvoltage of 400}

mV. This study and its encouraging results were the starting point for the use of MOFs deposited on various semiconductor substrates. It has also served as the starting point for a set of strategies for fabricating arrays of ultrafine 2D MOF nanosheets.

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Other nanomaterials derived from MOFs also give good performance in water electrolysis. Chaikittisilp and co‐workers43_{were the first to use a Co-based MOF}

(zeolite imidazolate framework-9, ZIF-9) as a precursor compound for the synthesis of a nanoporous CoxOy-C hybrid electrochemically active with respect to OER (Fig. 8-A).

The idea of using MOF to form hybrid metal oxide nanomaterials was intended to increase the amount of MOF and create high specific surface area, high porosity and high concentration of active sites for the electrooxidation of water. With the same idea, Cardenas-Morcoso et al. 44_{modified photoanodes of BiVO}

4 with an extremely porous

cobalt oxide cocatalyst, obtained from a MOF of cobalt-imidazolium (ZIF-67, Fig.

8-B). Due to the large specific surface area of the MOF precursor, the porous cocatalyst

obtained has a large amount of active sites for the oxidation of water, which allows a significant acceleration of the oxidation kinetics in PEC cells.

A

B

Fig. 8. Synthesis of (A) Nanoporous CoxOy‐C Hybrid43_{and (B) CoOx co-catalyst}

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- 24 - 1.3. Titanium dioxide nanoarray (TNAs)

In the 1970s, a TiO2-based catalyst was used for the first time in a commercial device

for controlling air pollution. This drew the attention of academic research to titanium dioxide which subsequently became the subject of many scientific studies. As is known, TiO2 exists in three diferent crystal forms: anatase, brookite and rutile. The crystal

structure of anatase is the most stable form at low temperature (<700 °C).45_Commercial

TiO2 nanomaterials, (e.g., Degussa-P25, consist of a mixture of 85% anatase and 15%

rutile). Due to their high specific surface and their high photoactivity, they have been widely used as catalyst in photocatalysis, for example for the dissociation of water and the production of hydrogen, or for the degradation of chemical compounds dangerous for the environment.46_{Efforts have been directed towards finding efficient but also}

sustainable catalysts based on metals abundant on Earth. Titanium oxide has many advantages such as low cost, high chemical and thermal stability, and low toxicity.47

Fig. 9. Schematic diagram of TiO2 nanostructures according to structural

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In recent years, research has sought to establish the relationship between different nano-dimensional morphologies of TiO2 and the level of performance obtained in

different applications (Fig. 9). Different morphologies of TiO2, for example, nanotube

arrays (TNTA), nanorod arrays (TDNR), nano-flowers, nanofibers and nanopaticules48, have been used to effect the photo-driven degradation of a wide variety of pollutants, as well as for energy conversion and storage (e.g., water dissociation, solar cells, lithium batteries, supercapacitors and DSSC type cells). Although there are many semiconductor materials having a band structure suitable for the photodissociation of water, TiO2 is particularly well suited (Fig. 10). The energy position of the conduction

(CB) and valence (VB) bands, appropriately frames the two energy levels of the two half-reactions of dissociation of water. The other most studied materials are ZnO49-50_,

CuxO51-52, CeO253-54, BiVO455, WO356.

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Regarding the level of photoelectrochemical performance, Zhifeng et al. 58_have

succeeded in fabricating a TiO2 / BiVO4 bilayer photoelectrode using a simple spin

coating method. They measured a high photocurrent density of 35 µA / cm2_{at 1.23 V}

relative to RHE under an illumination of 100 mW / cm2 in 0.5M Na2SO4 (Fig. 11).

For their part, Jeong, K et al.59_{presented an original study on the synthesis of aligned}

nanowires of ZnO-TiO2 with core-shell structure (NWs). This heterostructure enabled

them to measure a photocurrent density approximately 2.41 times (1.23 mA cm-2₎

greater than that measured on pristine ZnO nanowires. The use of a TiO2 support makes

it possible to considerably reduce the parasitic recombination of the electron-hole pairs appearing on the surface of the ZnO nanowires. The core-shell 1D sites are the most influential factors in the efficient separation and transport of charge carriers. This work not only proved that TiO2 is a good material for photo-sensitization, but also

demonstrated that the well-ordered development of a 1D structure with ideal semiconductors is essential to improve the performance levels of PEC cells for the dissociation of water.

Fig. 11. Schematic representation of synthesis process, TEM image, water splitting efficiency, and possible mechanisms for ZnO-TiO2 core–shell NWs heterostructures.59

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1.3.1 TiO2 nanotube arrays (TNTAs)

TNTAs (crystalline anatase form) have a large specific surface area, high photo activity. They can be manufactured simply by electrochemical anodization of Ti metal sheets. Fig. 12 recapitulates the operating conditions used in recent years for the synthesis of these TNTAs. There are three basic rules to be observed in order to obtain optimal morphologies and performance levels: (i) the synthesis leading to titanium membranes covered with a thick and uniform carpet of TiO2 nanotubes is carried out in

an ethylene glycol electrolyte; (ii) the optimum anodization voltage is between 30 and 60 V; (iii) the titanium membranes coated with these TiO2 nanotubes exhibit better

photocatalytic and photoelectrochemical performance compared to those coated with networks of nanotubes prepared on Ti sheet.60_{As shown in Fig.. 6, membrane thickness,}

tube diameter and surface morphology are key geometric parameters. They are adjusted by adjusting the electrochemical parameters, such as the composition of the electrolyte, the voltage, the anodization time and the temperature. Several relationships have been demonstrated between the experimental parameters of synthesis, the morphology of the nanotube membrane and the levels of performance.

Fig. 12. Strategies used optimizing the geometry (array surface, thickness and tube diameter) of the free-standing and flow-through membranes.61

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1.3.2 TiO2 nanorod arrays (TDNR)

TDNRs are of particular interest because there are fewer defects likely to block the transport of charge carriers and facilitate their recombination. The length of nanorods is a key factor influencing the efficiency of photon collection and charge carrier formation. Using the split-etching process, it is possible to increase their surface area, produce macro-pore and meso-pores, and build a stable and oriental structure. There are two important factors that control the architecture of TDNRs: the contact time for acid etching and the acid concentration (HCl is normally used as a mediator). The time of the etching process is also critical (Fig. 13). The etching treatment leads to the division of the TDNRs into secondary nanorods of reduced diameter, which considerably enlarges the internal surface of the assembly. The rutile structure of TiO2

is also interesting because it has a low band gap (3.0 eV). It is possible to synthesize nanowires rich in 1D sites, very stable at high temperature (> 500 o_C)62_{. In addition, it}

has been demonstrated in the field of DSSC that the efficiency is better with rutile nanowires than with anatase nanowires. 63

Fig. 13. Schematic illustration for splitting of single-crystalline rutile TiO2 nanorod

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- 29 - 1.4. MOF/TNTA nanocomposites

The control of the chemical composition and the structure of MOFs allows to adjust the conductivity and the transfer of electrons from metal centers such as Cu, Ni, Fe, Co. The TiO2-based compounds such as TNTs or TNTAs can be doped with anions such as

N and C or decorated with nanoparticles of Ag, Au, Ru or Zn to improve electrochemical performance. This has been demonstrated for lithium battery or supercapacitor type applications. The same materials can be used in photoelectrochemical cells (PEC) incorporating photoelectrodes. This allows higher photocatalytic and electrochemical activities to be obtained. In the PEC cell of Fig. 14, a photoanode and a conventional cathode are immersed in the electrolyte and connected via an external electrical circuit. Typically, the photoelectrode (photoanode) is made of a semiconductor material which absorbs sunlight, and the counter electrode (cathode) is generally made of a metal foil. Incident photons with energies above the bandgap of the semiconductor can be absorbed by the semiconductor, thus creating electron-hole pairs that are separated by the local electric field formed at the interface (in the charge region space) between the semiconductor and the electrolyte.

Fig. 14. Schematic representation of PEC system (A) Designed ideal reactor. (B) Overall hydrogen production system.64

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MOF-based active phases can be obtained with a great variety of morphologies and dimensional structures. They make it possible to significantly improve the level of electrochemical performance of the photoelectrodes used for the photo-dissociation of water into hydrogen and oxygen. Recently, various synthetic methods have been developed to manufacture MOFs with a morphology suitable for the dissociation of water. For example, Duan et al.65_{have developed a strategy for the}

in-situ growth of ultrathin nanosheet arrays of two dimensional MOFs on various supports (Fig..15). The electrodes incorporating the MOFs demonstrated superior performance levels with respect to OER, HER (for example a current density of 10 mA cm-2_was

measured at a voltage of 1.55 V). Zhan et al. have managed to advance the state of the art by synthesizing new MOF-based core-shell heterostructures combining ZnO nanorods and ZIF-8 for the same type of application. This example shows the interest of MOFs and that of heterostructures of semiconductor + MOF for many fields of application.

Fig. 15. Schematic illustration of ZnO@ZIF-8 Nanorods Synthesized via the Self-Template methodology.66

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As shown in Fig. 16, Cai et al.67_{have developed the same kind of MOF array.}

They tried to address the issue that the MOF particles are prone to aggregate during pyrolysis at high temperature, which rendered the resultant materials unfavorable for electronic conductivity as well as for the transport of reactant and products in electrocatalytic reaction. With their approach they were able to retain the advantage of the MOF superstructure while preventing particle agglomeration. This result also demonstrated that the fabrication of a one-dimensional MOF array improves performance.

Fig. 16. Schematic Illustration of the template-directed growth of well-aligned MOF hybrid arrays.67

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Recently, TiO2-based nanoarray have been used as a support of MOFs and have

shown interesting performance levels for photoelectrochemical dissociation of water. Yang et al.68_{successfully fabricated TiO}

2@Co-MOF nanorod array photoanodes (Fig.

17), which exhibit a photocurrent density of 2.93 mA/cm2_{at 1.23 V (vs. RHE): this is}

~ 2.7 times the photocurrent achieved with bare TiO2 nanorod array under irradiation

of an unfiltered 300 W Xe lamp with an output power density of 100 mW/cm2_{. As a}

result of the beneficial and attractive electrochemical characteristics of MOF@TNTA, it is possible to fabricate high performance hybrid (photo)electrocatalyst with high specific surface area, good capacity of electrons generation and superior (photo)electrocatalytic activity under unique voltage range. In these systems, the titanium dioxide nanotube array serves as the support, while MOF not only provide ordered heterostructure to enhance reduction activity but also can rapidly generate electrons.

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2. Methodology and materials

2.1. Chemicals and materials

All the chemicals were analytical grade and Milli-Q grade water (18.2 Ω) was used in all the experiments. Titanium plate (99.9%) was purchased form M&T Co., Ltd. (Taiwan), Titanium tetrachloride (98%), titanium(IV) n-butoxide (98%), Iron(III) nitrate hydrate (99%), Nickel (II) nitrate hexahydrate (98%), Cobalt (II) nitrate hexahydrate (99%), Cobalt chloride (II) hexahydrate (99%), Terephthalic acid (BDC) (98%), Trimethyl 1,3,5-benzenetricarboxylate (98%), Ammonium fluoride (98%), Ethylene glycol (99%) were purchased from Alfa Aesar. The electrolyte solution was prepared with Sodium sulfate (≥99.0%) which was bought from Sigma-Aldrich. HCl (36%), N, N-Dimethylformamide (anhydrous, 99.8%), Methanol (≥99.9%) and washing solvent (Ethanol, 2-propanol and acetone), Isopropanol, (99.9%), 1-Butanol were all purchased from VWR. 2-methylimidazole (99%), Copper(II) nitrate trihydrate (>99.9%), Sodium sulfate anhydrous, Boric acid (>99.9%), Glycine (>98%), Dimethyl sulfoxide (DMSO, 99%,) were all obtained from Sigma-Aldrich.

2.2. Synthesis of various MOFs

2.2.1 Cu-BTC-MOF

In a typical synthesis method, Copper nitrate trihydrate (0.2416g, 1mmole) and Benzene-1,3,5-tricarboxylic acid (1.414g, 6.7mmole) were dissolved in 50 mL 1-butanol/ethanol/water (1:3:3, v/v). The light blue flocs observed initially were formed by the binding of copper ions and organic ligands. The resulting viscous mixture was then transferred to a Teflon-coated stainless steel autoclave with continuous stirring for 1 h. The autoclave was heated at 150 °C under hydrothermal conditions for 12 h. The final products (precipitates of Cu-BTC-MOF) were then washed with ethanol and centrifuged three times after cooling to room temperature. The blue Cu-BTC crystals

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were dried in an oven at 60 °C, 100 °C, and the resulting powders were then calcined at 400 °C, 500 °C or 600 °C for 2 h under nitrogen atmosphere.

2.2.2 Fabrication of Cu-BTC-MOF-Ole and Boron doped Cu-BTC-MOF

The process used for the manufacture of Cu-MOF uses oil (oleic acid) and a dopant (boron). This makes it possible to obtain Cu-MOF particles of nanometric size and to increase the photoelectrochemical performances. For the synthesis of Cu-BTC-MOF-Ole, oleic acid and DMSO were added with constant stirring for 1 hour while the Cu / BTC flocs were formed. The precipitates were calcined at 400 °C. In addition, boron-doped Cu-BTC (B-Cu-MOF) was synthesized using different levels of boron (1%, 2%, 5% and 10%, p / p) then dissolved boric acid in 1.414 g BTC in 50 ml of ethanol / water (1: 1, v / v) solution, and mixed with 1 mmol Cu (NO3)2 under permanent sonication

for 30 minutes. The blue crystals of B-Cu-MOF were collected after centrifugation and calcined at 400°C for 2 h under a nitrogen atmosphere.

2.2.3 Synthesis of Cu-Gly-MOF

To make a nanoscale copper MOF, we used the simple organic ligand which has stable functional groups to bind to the metal ion. We followed the same synthetic process as for Cu-BTC-MOF and replaced BTC with glycine in an equal molar ratio. Copper nitrate trihydrate and glycine were mixed in a 1: 1 and 3: 1 ratio, then dissolved in 50 ml of ethanol / water (1: 1, v / v) which led to the formation of color flocs light blue. The light blue crystals of Cu-Gly-MOF were dried in an oven at 60 °C or 100 °C, and the resulting powders were calcined at 400 ° C for 2 h under a nitrogen atmosphere.

2.2.4 Synthesis of Ni-MOF

We used a procedure slightly different from that described in the work of M.K. Wu. Et al. 69_{(Fig. 18). For this, 0.1 M Ni (NO}

3)2 • 6H2O, and 0.4 M 1,4-benzenedicarboxylic

acid and 0.83 mL of methanol in 7.5 mL of N, N-dimethylformamide (DMF) were mixed at room temperature under ultrasound for 30 min (the mass ratio of methanol

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and DMF is set at 1: 9). Then the solution was transferred to a 30 ml Teflon-coated autoclave and the autoclave was kept at 150 ° C for 4 h or 5 h. After gradual cooling to room temperature, a light green precipitate was obtained. It was centrifuged and washed with anhydrous ethanol several times, then dried at 60 ° C for 12 h.

.

Fig. 18. Schematic representation of synthesis Ni-MOF catalysts

2.3. Synthesis of TiO2 nanoarray

2.3.1 Production of TNTAs/Ti electrodes

The synthesis of arrays of titanium oxide nanotubes by electrochemical anodization of metallic titanium foils has been extensively studied for over a decade. By playing on the experimental anodization parameters, it makes it possible to control the morphology of the nanotubes, to adjust their diameter to approximately 100 nm and their length between 100 nm and 1 mm. Prior to anodization, the titanium sheets (3.0 x 1.0 cm2_{) are ultrasonically cleaned in an equivolumic mixture of acetone and ethanol}

for 20 minutes. Then, the clean Ti sheets are anodized at room temperature in a conventional two-electrode cell, using a Pt plate as a counter electrode, in an ethylene glycol solution containing 1% by weight NH4F and 2% by weight. of H2O. First, a

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allows nucleation and growth of the nanotubes. The anodized Ti plate is then ultrasonicated in pure ethanol for at least 10 minutes to remove TiO2 residues. After

electrochemical anodization and ultra-sonication, the samples are rinsed with distilled water and air dried. The amorphous TNTA thus obtained is then annealed in air at various temperatures of 400 °C, 450 °C, 500 °C or 600 °C for 2 h with a heating rate and a cooling rate of 5°C min-1_{to ensure crystallographic transformation into Anatase.}

Practical details are summarized in Fig. 19.

Fig. 19. Schematic representation of titanium nanotube array anodization procedure

2.3.2 Production of TDNRs/FTO electrodes

Photoelectrodes are made from pieces of glass covered with a thin layer of fluorine-doped tin oxide (FTO) providing electronic conductivity. In a typical experiment, a 1.3 cm * 2.5 cm piece of glass (transparent electrode) from Solaronix Co. Ltd (Switzerland) is ultrasonically cleaned for 30 minutes in a mixed solution of isopropanol / acetone / deionized water (1: 1 : 1 by volume), then dried at room temperature. The hydrothermal solution used for the growth of the TiO2 nanorods was prepared as follows. First, 60 ml

of an aqueous solution of hydrochloric acid was obtained by equivolumic mixture of HCl (36%) and deionized water. Then, 1 ml of titanium (IV) n-butoxide was added and

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the whole was stirred for 10 minutes. The piece of transparent electrode was introduced into a teflon-coated stainless steel autoclave, turning the conductor side (FTO) down. The vessel was then filled with the stirred synthesis solution, sealed and heated at 150 °C for 4 h in an oven. After the reaction, the vessel was naturally cooled to room temperature in contact with ambient air, then the electrode was removed from the autoclave. The residue of synthesis solution on the surface of the electrode was removed by washing with deionized water. Then the electrode was dried at room temperature. Details are provided in Fig. 20.

Fig. 20. Schematic representation of the procedure used for the synthesis of TDNR using the hydrothermal method.

2.4. Synthesis of MOF/TNA composite

2.4.1 Formation of Ni-MOF on TDNRs/FTO

The nickel MOF (Ni-BDC MOF) was deposited on the surface of the TiO2 nanowire

mat by electrodeposition. Electroplating was performed in a conventional electrochemical cell with a three-electrode system, comprising an Ag / AgCl reference electrode (RE), and a glassy carbon plate as a counter electrode (CE). The photoelectrode (piece of glass covered with TDNR / FTO) was used as a working electrode. The synthesis procedure is summarized in Fig. 21. First, the as prepared

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TDNRs/FTO electrodes were immersed into 40 ml of freshly prepared Ni(NO3)2 • 6H2O

electrolyte solution of various concentrations (0.001M~0.1M). Second, the electroplating of nickel hydroxide was carried out by scanning cyclic voltammetry over the potential window between -0.2 V and -1.0 V, at the scanning speed of 10 mV / s. A different number of cycles (2c, 5c, 10c) were carried out to expose more or less large quantities of nickel. After electrodeposition, the Ni hydroxide deposited on TDRs / FTO was rinsed 3 times with deionized water and dried. The final step was to convert the nickel hydroxide to MOF by chemical reaction with the appropriate organic ligands. For this, we used a 0.03 M solution of terephthalic acid (BDC) dissolved in a mixed solution of methanol (MeOH) and DMF (1: 9 by volume) with constant stirring for 1 hour. The glass photoelectrodes were placed in the autoclave, with the coating of nickel hydroxide facing up. Nickel hydroxide was converted to MOF by reaction at 150°C for 4 h. At the end of the operation, the samples were washed with deionized water and then with absolute ethanol three times in a row and finally dried at 60 ° C, for 1 hour. The photoelectrodes were then cooled to room temperature before taking the photoelectrochemical measurements in the PEC cell.

FTO Electrodeposition Ni2+ FTO FTO Ni-MOF TDNR e-_e-_e-_e -e-_e-_e -h+_h+_h+_h+ h+_h+_h+ H2O/H+ H2 H2O/OH -._OH

Fig. 21. Schematic representation of the protocol used for synthesis of Ni-MOF on TDNR/FTO.

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2.4.2 Formation of ZIF-67 MOF coating on TDNR

Another MOF of ZIF-67 was also studied. It was deposited on the TiO2 nanowire

photoelectrode in one step. The method used is that described in the literature by Hu et al. 70_{. Nickel nitrate was a precursor. First, the cobalt nitrate was dissolved in 25 ml of}

ethanol (solution A, 0.04 M) and the 2-methylimidazole was dissolved in an additional 25 ml of methanol (solution B, 0.16 M). Then, a piece of glass-FTO-TDNRs substrate was placed in a glassware and solutions A and B were poured inside (Fig. 22). The amount of MOF deposited on TiO2 was adjusted by controlling the time of the chemical

reaction. Four different photoelectrodes were prepared using different coating durations (respectively 1, 2, 4 and 16 h). Then, the photoelectrodes were removed from the autoclave and washed 3 times with methanol to remove the unattached and unreacted chemicals. Finally, the photoelectrodes were dried at room temperature. In the following sections, the four photoelectrodes used for the experiments will be referred to as xh ZIF-67/TDNRs (with x = 1 h, 2 h, 4 h and 16 h).

2-methylimidazole

Cobalt nitrate

Ethanol

T D N R T D N R T D N R 1/2/4/16 hrs R.T. ZIF-67

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2.4.3 Formation of ZIF-67 MOF deposited on TDNR/FTO

The as-prepared TDNR was covered by electrodeposition of Co(OH)2 on top of

TDNR (Fig. 23). The electrolyte contained 0.892 g of CoCl2.6H2O in 50 mL H2O. A

glassy carbon plate was used as counter electrode and an SCE as reference electrode. Cobalt hydroxide was deposited by chronoamperometry, at a fixed potential of −1.0 V vs. saturated calomel electrode (SCE) for 5, 10, 20, 40, 60 and 80 seconds. After electrodeposition, the obtained Co(OH)2/TDNRs samples were rinsed with distilled

water three times and then dried in air. The electrodeposited Co(OH)2 was then

transformed to ZIF-76 MOF. This was done by immerging the photoelectrodes into 2-methylimidazole (2.463 g in 30 mL H2O) for 4 hours under continuous stirring. The

active area of 1 cm2_{was delimited using CAPTON film. During the reaction, the photo}

electrodes were placed against the wall of the beaker. After reaction, the samples were washed 3 times with water and then dried. Finally, all samples were calcinated at 350o_C

for 2h in muffle furnace to form Co3O4 nanoparticles over the TDNRs mats.

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2.4.4 Formation of FeNi-MOF on TNTA

A bi-metallic MOF has also been prepared and tested. First, the as-prepared TNTA (TiO2/Ti) electrode was immersed into 40 ml Ni(NO3)2 or Ni(NO3)2 and Fe(NO3)3

mixture electrolyte solution (using various molar ratio). Then it was polarized at negative potentials of fixed -1.0 V at 10 mV/s in 2, 5, and 10 cycles by cyclic voltammetry method. In detail, The sample of Ni/TNTA was made under the concentration of 0.005M to 0.05M which referred previous methodology and the FeNi/TNTA were also fabricated under same range of concentration with different ratio in 1:1, 2:1 and 3:2 (Fe:Ni).Then, the TNTA covered with Ni or Ni/Fe hydroxide deposits was obtained after dried at 60 o_{C overnight. The electrodeposition was carried out using}

a conventional three-electrode electrochemical cell, equipped with an Ag/AgCl reference electrode (RE) and a graphite counter electrode (CE). After electrodeposition, all the electrodes were rinsed three times with absolute ethanol and dried in air at room temperature. The final step was to transform the Fe and Ni species to MOF by chemical reaction with organic ligand precursors. 30 mL of a 0.03M BTC solution was prepared in a mixed solution of methanol (MeOH) and DMF (1:9 in volume) under continuous stirring for 1hr. The FeNi/TNTA layer was placed face-up at the bottom of Teflon tube. Finally, the FeNi-MOF was formed by heating at 150o_{C for 4 hr. After hydrothermal}

reaction, the as prepared samples were washed by DI water and absolute ethanol for three times before drying at 60o_{C for 1 hr. Before PEC measurement, the}

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- 42 - H+ H2 H2O O2 Solar light Fe Ni 1,3,5-benzenetricarboxylate

Anodization TiO2 nanotube array

N

F

B

F

Fig. 24. Schematic representation the synthesis and mechanism of FeNi-MOF grows on TNTA

2.5. Analytical techniques for Characteristics

2.5.1 X-Ray Diffraction (XRD)

X-ray diffraction measurements were performed to determine the crystal structure of the samples. These measurements were made using a Bruker D8-Advance, CuKa radiation (k = 1.5405 A˚), 40 kV and 40 mA. The diffraction patterns of the samples were compared to the JCPDS powder diffraction files. Mathematical Bragg peak analysis was used to calculate particle size using Scherrer's formula.

t = ¼ k λ/β cosθ (Eq.1) where t is the crystalline size, k a constant (k = 0.94, assuming the grains to be spherical), λ is the wavelength of the X-ray radiation (λ = 1.5406 A˚), β is the full width at half-maxima expressed in radians (obtained after correction for instrumental error) and θ is half of the diffraction angle.

2.5.2 Hard X-ray Absorption Spectroscopy

This beamline is a high resolution DCM X-ray beamline with both collimating and focusing mirrors, which will provide monochromatic photon beams with energy ranging from 6 keV to 33 keV for Extended X-ray Absorption Fine Structure (EXAFS),

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powder diffraction and the related experiments. At the sample position, the expected photon flux is 1*1011_{photon/sec /200mA with an average energy resolution (△E/E) of}

1.6*10-4_{and the focused beam size is about 0.9 mm*0.2 mm.}

2.5.3 X-ray photoelectron spectroscopy (XPS)

The XPS measurements were carried out using a K Alpha equipment (Thermo Fisher Scientific) equipped with a monochromatic aluminum source (Al Ka, 1486.68 eV). All measurements were carried out in an ultra-vacuum chamber (UHV) at a pressure below 10-9_{mbar. A beam of 400 micrometers corresponding to an irradiated}

area of about 1 mm2_{was used. The hemispherical analyzer was used at a take-off angle}

of 0o_{in the constant analyzer Energy mode (CAE), with a passage energy of 200 eV}

and an energy step of 1 eV for the acquisition of wide scans. Narrow scan spectra were recorded at a pass energy of 50 eV and an energy step of 0.1 eV. The charge compensation was performed by means of a "dual beam" flood gun, using low energy electrons (5 eV) and argon ions. The samples were attached to the holder using conductive adhesive tape. The recorded spectra were processed using Avantage software and a peak adjustment routine with a Shirley background and 70% - 30% symmetrical mixed Gaussian-Lorentzian peak shapes. Atomic ratios were evaluated after normalization of the peak areas with Scofield sensitivity factors.

2.5.4 Brunauer−Emmett−Teller specific surface area analyzer (BET)

Nitrogen adsorption-desorption isotherms were measured at 77 K using a Micrometrics ASAP 2020 analyzer. Prior to measurement of isotherms, samples were degassed under high vacuum at 120 ° C. The Brunauer - Emmett - Teller method was used to calculate the specific surfaces and pore texture of MOFs and TNAs.

2.5.5 Scanning Electron Microscope (SEM)

The morphology as well as the particle size of MOF and titanium oxide nano arrays were determined using a scanning electron microscope (JEOL) by high

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resolution thermal field emission JSM-7610F (HRFEG-SEM) at 15 kV. The silicon wafer was prepared from the fine powder samples which were taken from an alcoholic solution after sonication for 30 min. A few drops of samples were placed on a silicon wafer and dried well at 50°C before being placed in the microscope.

2.5.6 Transmission Electron Microscope (TEM)

TEM experiments were performed on a JEOL JEM 2100 Plus UHR microscope operating at 200 kV, interfaced to Oxford Instruments AZtec EDX system with an X-Max T large area (80mm2_{) SDD detector. The images were collected with a 4008*2672}

pixel CCD camera (Gatan Orius SC1000). Additionally, high-angle annular dark-field scanning TEM (HAADF-STEM) and EDX X-ray microanalysis were carried out to determine the chemical mapping.

2.5.7 Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared (FTIR) spectra were recorded with a HORIBA FT 720 spectrometer (Kyoto, Japan). All the organic ligands used in the MOFs were characterized by FTIR. This technique allowed us to verify the position of the specific peaks associated with the expected characteristic bonds.

2.5.8 UV-Vis spectroscopy

The optical properties of the samples (in reflection and in absorption) were measured using a UV-vis spectrophotometer (Cary 60 UV-Vis). It has also been used for the identification of organic and inorganic compounds.

2.5.9 Experiments for Electrochemical studies

A conventional three-electrode electrochemical cell was used to perform the electrochemical experiments, i.e., to measure cyclic voltammograms (CV), galvanostatic charge / discharge (GCD) and impedance diagrams by electrochemical impedance spectroscopy. (EIS). The measurements were made with an Autolab PGSTAT 302N potentiostat (Metrohm Autolab B.V.).