Concentration-Dependent Photoredox Conversion of As(III)/As(V) on Illuminated Titanium Dioxide Electrodes

Monllor-Satoca, Damiá,n,Gó,mez, Roberto,Choi, Wonyong American Chemical Society 2012 Environmental science & technology Vol.46 No.10

<P>The photoconversion of As(III) (arsenite) and As(V) (arsenate) over a mesoporous TiO<SUB>2</SUB> electrode was investigated in a photoelectrochemical (PEC) cell for a wide range of concentrations (μM–mM), under nonbiased (open-circuit potential measurements) and biased (short-circuit current measurements) conditions. Not only As(III) can be oxidized, but also As(V) can be reduced in the anoxic condition under UV irradiation. However, the reversible nature of As(III)/As(V) photoconversion was not observed in the normal air-equilibrated condition because the dissolved O<SUB>2</SUB> is far more efficient as an electron acceptor than As(V). Although As(III) should be oxidized by holes, its presence did not increase the photooxidation current in a monotonous way: the photocurrent was reduced by the presence of As(III) in the micromolar range but enhanced in the millimolar range. This abnormal concentration-dependent behavior is related with the fate of the intermediate As(IV) species which can be either oxidized or reduced depending on the experimental conditions, combined with surface deactivation for the water photooxidation process. The lowering of the photooxidation current in the presence of micromolar As(III) is ascribed to the role of As(IV) as a charge recombination center. Being an electron acceptor, the addition of As(V) consistently lowers the photocurrent in the entire concentration range. A global concentration-dependent mechanism is proposed accounting for all the PEC results and its relation with the photocatalytic oxidation mechanism is discussed.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/esthag/2012/esthag.2012.46.issue-10/es203922g/production/images/medium/es-2011-03922g_0009.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/es203922g'>ACS Electronic Supporting Info</A></P>

Band energy levels and compositions of CdS-based solid solution and their relation with photocatalytic activities

Zhang, G.,Monllor-Satoca, D.,Choi, W. Royal Society of Chemistry 2013 Catalysis science & technology Vol.3 No.7

Photooxidation of Arsenite under 254 nm Irradiation with a Quantum Yield Higher than Unity

Ryu, Jungho,Monllor-Satoca, Damiá,n,Kim, Dong-hyo,Yeo, Jiman,Choi, Wonyong American Chemical Society 2013 Environmental science & technology Vol.47 No.16

<P>Arsenite (As(III)) in water was demonstrated to be efficiently oxidized to arsenate (As(V)) under 254 nm UV irradiation without needing any chemical reagents. Although the molar absorption coefficient of As(III) at 254 nm is very low (2.49 ± 0.1 M<SUP>–1</SUP>cm<SUP>–1</SUP>), the photooxidation proceeded with a quantum yield over 1.0, which implies a chain of propagating oxidation cycles. The rate of As(III) photooxidation was highly enhanced in the presence of dissolved oxygen, which can be ascribed to its dual role as an electron acceptor of photoexcited As(III) and a precursor of oxidizing radicals. The in situ production of H<SUB>2</SUB>O<SUB>2</SUB> was observed during the photooxidation of As(III) and its subsequent photolysis under UV irradiation produced OH radicals. The addition of <I>tert</I>-butyl alcohol as OH radical scavenger significantly reduced (but not completely inhibited) the oxidation rate, which indicates that OH radicals as well as superoxide serve as an oxidant of As(III). Superoxide, H<SUB>2</SUB>O<SUB>2</SUB>, and OH radicals were all in situ generated from the irradiated solution of As(III) in the presence of dissolved O<SUB>2</SUB> and their subsequent reactions with As(III) induce the regeneration of some oxidants, which makes the overall quantum yield higher than 1. The homogeneous photolysis of arsenite under 254 nm irradiation can be also proposed as a new method of generating OH radicals.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/esthag/2013/esthag.2013.47.issue-16/es402011g/production/images/medium/es-2013-02011g_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/es402011g'>ACS Electronic Supporting Info</A></P>

Simultaneous production of hydrogen with the degradation of organic pollutants using TiO₂ photocatalyst modified with dual surface components

Kim, Jungwon,Monllor-Satoca, Damiá,n,Choi, Wonyong The Royal Society of Chemistry 2012 Energy & environmental science Vol.5 No.6

<P>The simultaneous production of hydrogen and degradation of organic pollutants (4-chlorophenol, urea, and urine) was successfully achieved using titania photocatalysts which were modified with both anion adsorbates (fluoride or phosphate) and (noble) metals (Pt, Pd, Au, Ag, Cu, or Ni). The dual-function photocatalysis worked only when both components coexisted on the surface of TiO<SUB>2</SUB>, whereas TiO<SUB>2</SUB> modified with a single surface component (F–TiO<SUB>2</SUB>, P–TiO<SUB>2</SUB>, or Pt/TiO<SUB>2</SUB>) was inactive under the same experimental condition. Two main surface-modified photocatalysts, F–TiO<SUB>2</SUB>/Pt (surface fluorinated and platinized) and P–TiO<SUB>2</SUB>/Pt (surface phosphated and platinized), were similarly active for dual-function photocatalysis in the anoxic suspension under UV irradiation. With these catalysts employed, the degradation of 4-chlorophenol (or urea) was accompanied by the concurrent production of H<SUB>2</SUB>. The synergistic effect greatly depended on the kind of metal and pH. The activity of F–TiO<SUB>2</SUB>/Pt gradually decreased with increasing pH, which makes the application of F–TiO<SUB>2</SUB>/Pt limited to the acidic pH region. On the other hand, P–TiO<SUB>2</SUB>/Pt exhibited a consistent activity over a wide range of pH, which makes P–TiO<SUB>2</SUB>/Pt a more practical dual-function photocatalyst. The synergistic effect of anions and metal deposits on the surface of TiO<SUB>2</SUB> enhanced the interfacial electron transfer and reduced the charge recombination which resulted in a maximum of 20-fold increase of H<SUB>2</SUB> production compared to metal deposited TiO<SUB>2</SUB> in the presence of 4-chlorophenol.</P> <P>Graphic Abstract</P><P>The simultaneous production of hydrogen and degradation of organic pollutants (4-chlorophenol, urea, and urine) was successfully achieved using titania photocatalysts which were modified with both anion adsorbates (fluoride or phosphate) and (noble) metal deposits. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2ee21310a'> </P>

Visible light photocatalysis of fullerol-complexed TiO₂ enhanced by Nb doping

Lim, J.,Monllor-Satoca, D.,Jang, J.S.,Lee, S.,Choi, W. Elsevier 2014 Applied Catalysis B Vol.152 No.-

Visible light photocatalysis by TiO<SUB>2</SUB> nanoparticles modified with both fullerol complexation and Nb-doping (fullerol/Nb-TiO<SUB>2</SUB>) demonstrated an enhanced performance. Nb-doped TiO<SUB>2</SUB> (Nb-TiO<SUB>2</SUB>) was firstly prepared by a conventional sol-gel method, and subsequently fullerol was adsorbed on the surface of Nb-TiO<SUB>2</SUB>. The physicochemical and optical properties of as-prepared fullerol/Nb-TiO<SUB>2</SUB> were analyzed by various spectroscopic methods (TEM, EELS, XPS, and DRS). The adsorption of fullerol on Nb-TiO<SUB>2</SUB> surface increased the visible light absorption through a surface-complex charge-transfer (SCCT) mechanism. Nb-doping enhanced the charge transport and induced the Ti cation vacancies that retarded the recombination of photo-generated charge pairs by trapping the electrons injected from the HOMO level of fullerol. Due to the advantage of simultaneous modification of fullerol and Nb-doping, the visible light photoactivity of fullerol/Nb-TiO<SUB>2</SUB> was more enhanced than either Nb-TiO<SUB>2</SUB> or fullerol/TiO<SUB>2</SUB>. The photocatalytic activities of fullerol/Nb-TiO<SUB>2</SUB> for the reduction of chromate (Cr<SUP>VI</SUP>), the oxidation of iodide, and the degradation of 4-chlorophenol were all higher than bare TiO<SUB>2</SUB> and singly modified TiO<SUB>2</SUB> (i.e., Nb-TiO<SUB>2</SUB> and fullerol/TiO<SUB>2</SUB>) under visible light (λ>420nm). A similar result was also confirmed for their photoelectrochemical behavior: the electrode made of fullerol/Nb-TiO<SUB>2</SUB> exhibited an enhanced photocurrent under visible light. On the other hand, the decay of open-circuit potential of the fullerol/Nb-TiO<SUB>2</SUB> electrode after turning off the visible light was markedly slower than either that of Nb-TiO<SUB>2</SUB> or fullerol/TiO<SUB>2</SUB>, which implies the retarded recombination of photo-generated charge pairs on fullerol/Nb-TiO<SUB>2</SUB>. In addition, the electrochemical impedance spectroscopic (EIS) data supported that the charge transfer resistance is lower with the fullerol/Nb-TiO<SUB>2</SUB> than either Nb-TiO<SUB>2</SUB> or fullerol/TiO<SUB>2</SUB>. This specific combination of the bulk (Nb-doping) and surface (fullerol complexation) modifications of titanium dioxide might be extended to other cases of bulk+surface combined modifications.

Enhanced photoelectrochemical and hydrogen production activity of aligned CdS nanowire with anisotropic transport properties

Kim, Wooyul,Monllor-Satoca, Damiá,n,Chae, Weon-Sik,Mahadik, Mahadeo A.,Jang, Jum Suk Elsevier 2019 APPLIED SURFACE SCIENCE - Vol.463 No.-

<P><B>Abstract</B></P> <P>Various solar conversion materials with 1D nanostructure have been developed and are being widely investigated for various solar fuel generation applications. In this study, aligned and non-aligned CdS nanowires (NWs) were synthesized on Cd foil or in solution via solvothermal processes. In the case of aligned CdS NWs, the relative intensity of the (0 0 2) diffraction peak was higher than that of the non-aligned CdS NWs, which indicated that the NWs grew preferentially in the (0 0 1) direction. The systematic comparison between the photoelectrochemical properties of both electrodes revealed that the aligned CdS NW electrode displayed markedly enhanced photocurrent (by a factor of 7), photoelectrochemical hydrogen production (by a factor of 10), and photostability in comparison with those of the non-aligned NWs electrode fabricated on FTO glass. Resistance (R<SUB>µ</SUB>) through the inner part of the aligned CdS NWs was very small due to a low grain-boundary resistance (by a factor of 130). This low resistance induced efficient charge transfer, reducing the charge recombination loss and assisting the charge transport along the axial direction of the aligned NWs. Time-resolved photoluminescence spectroscopy confirmed that the charge separation in the aligned CdS NWs is longer than that in the non-aligned CdS NWs (by a factor of 1.6).</P> <P><B>Highlights</B></P> <P> <UL> <LI> Aligned CdS nanowire was hydrothermally synthesized on Cd foil. </LI> <LI> The superiority of aligned CdS nanowire is explained for PEC and hydrogen production. </LI> <LI> The aligned CdSnanowire photoanode exhibits the anisotropic transport properties. </LI> <LI> The charge transport mechanism of aligned and non-aligned CdS nanowire is proposed. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

N-doped TiO₂ nanotubes coated with a thin TaO_<i>x</i>N_<i>y</i> layer for photoelectrochemical water splitting: dual bulk and surface modification of photoanodes

Kim, Hyoung-il,Monllor-Satoca, Damiá,n,Kim, Wooyul,Choi, Wonyong The Royal Society of Chemistry 2015 ENERGY AND ENVIRONMENTAL SCIENCE Vol.8 No.1

<P>TaON is a good photoanode material with a suitable band structure for water splitting as well as coupling with TiO<SUB>2</SUB> for efficient charge separation. However, the synthesis of TaON that requires high temperature nitridation (850 °C) limits the combination with other materials. In this work, we deposited a thin amorphous TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> layer on N-doped TiO<SUB>2</SUB> nanotubes (N-TNTs) through low temperature nitridation (500 °C) and demonstrated its successful performance as an efficient photoanode for water-splitting. Since the preparation temperature is low, TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> on N-TNTs has a unique amorphous structure with a smooth thin layer (5 nm). It is proposed that the thin amorphous TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> layer plays dual roles: (i) surface sensitization and/or charge rectification at the heterojunction between the TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> layer and N-TNTs, and (ii) passivation of N-TNT surface trap states to retard the charge recombination. TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> layer-decorated N-TNTs as dual modified TNTs (N-doping in the bulk and TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> overlayer deposition on the surface) have significantly improved both visible (<I>ca.</I> 3.6 times) and UV (<I>ca.</I> 1.8 times) activities for PEC water-splitting as well as the faradaic efficiency (<I>ca.</I> 1.4 times, <I>η</I> = 98%) for H<SUB>2</SUB> production. Making the amorphous TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> layer crystalline at higher temperatures reduced the PEC activity of the hybrid photoanode, in contrast, which indicates that the amorphous TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> layer deposition on N-TNTs through low temperature nitridation (500 °C) is optimized for the PEC activity. A range of spectroscopic and electrochemical techniques were systematically employed to investigate the properties of the PEC process.</P> <P>Graphic Abstract</P><P>Thin amorphous TaO<SUB><I>x</I></SUB>N<SUB><I>y</I></SUB> layer-coated <I>N</I>-doped TiO<SUB>2</SUB> nanotubes successfully serve as a water splitting photoanode. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4ee02169j'> </P>

Promoting water photooxidation on transparent WO₃ thin films using an alumina overlayer

Kim, Wooyul,Tachikawa, Takashi,Monllor-Satoca, Damiá,n,Kim, Hyoung-il,Majima, Tetsuro,Choi, Wonyong The Royal Society of Chemistry 2013 ENERGY AND ENVIRONMENTAL SCIENCE Vol.6 No.12

<P>Tungsten trioxide (WO<SUB>3</SUB>) is being investigated as one of the most promising materials for water oxidation using solar light. Its inherent surface-related drawbacks (<I>e.g.</I>, fast charge recombination caused by surface defect sites, the formation of surface peroxo-species, <I>etc.</I>) are nowadays being progressively overcome by different methods, such as surface passivation and the deposition of co-catalysts. Among them, the role of surface passivation is still poorly understood. Herein, transparent WO<SUB>3</SUB> (electrodeposited) and Al<SUB>2</SUB>O<SUB>3</SUB>/WO<SUB>3</SUB> (prepared by atomic layer deposition, ALD) thin film electrodes were employed to investigate the role of an alumina overlayer by using both photoelectrochemical and laser flash photolysis measurements. Films with a 5 nm-alumina overlayer (30 ALD cycles) showed an optimum photoelectrochemical performance, portraying a 3-fold photocurrent and Faradaic efficiency enhancement under voltage biases. Moreover, IPCE measurements revealed that alumina effect was only significant with an applied potential <I>ca.</I> 1 V (<I>vs.</I> Ag/AgCl), matching the thermodynamic potential for water oxidation at pH 1 (0.97 V <I>vs.</I> Ag/AgCl). According to the investigation of electron accumulation through optical absorption measurements, the alumina overlayer dominantly decreased the number of electron trapping sites on the WO<SUB>3</SUB> surface, eventually facilitating photoelectron transfer to the external circuit in the presence of a positive bias. In addition, the laser flash photolysis measurements of WO<SUB>3</SUB> and Al<SUB>2</SUB>O<SUB>3</SUB>/WO<SUB>3</SUB> thin films clearly showed that the electron trapping decreased in the presence of the alumina overlayer whereas the hole trapping relatively increased with alumina, facilitating water photooxidation and rendering a more sluggish recombination process. These results provide a physical insight into the passivation process that could be used as a guideline for further development of efficient photoanodes in artificial photosynthesis.</P> <P>Graphic Abstract</P><P>In the water photooxidation process for the artificial photosynthesis, the alumina overlayer on a WO<SUB>3</SUB> photoanode markedly decreases the number of electron trapping sites, eventually facilitating the hole transfer to water. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3ee42151a'> </P>

Development of Aligned Nanostructures Grown on Cd Foil by Hydrothermal Synthesis for Solar Hydrogen Production

장점석,김우열,( Damian Monllor-satoca ),최선희,최원용 한국공업화학회 2013 한국공업화학회 연구논문 초록집 Vol.2013 No.1

Solar Photoconversion Using Graphene/TiO₂ Composites: Nanographene Shell on TiO₂ Core versus TiO₂ Nanoparticles on Graphene Sheet

Kim, Hyoung-il,Moon, Gun-hee,Monllor-Satoca, Damiá,n,Park, Yiseul,Choi, Wonyong American Chemical Society 2012 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.116 No.1

<P>Size controlled nanographene oxides (NGOs; <50 nm) were prepared by a two-step oxidation process and NGOs were self-assembled with TiO<SUB>2</SUB> nanoparticles to form the core/shell structure. Nanosized GO-coated TiO<SUB>2</SUB> nanoparticles (NGOTs) were then reduced by a photocatalytic process under UV irradiation to obtain graphene-coated TiO<SUB>2</SUB>. This is clearly different from the typical graphene/TiO<SUB>2</SUB> composite with the particles-on-a-sheet geometry and is the first study on the core/shell structure of its kind. The physicochemical properties of NGOs and the reduced NGOTs (r-NGOTs) were characterized by various analytical and spectroscopic methods (AFM, FT-IR, XPS, TEM, EELS, etc.). The photocatalytic and photoelectrochemical activities of r-NGOT were compared with a composite of r-GO/TiO<SUB>2</SUB> that has TiO<SUB>2</SUB> nanoparticles loaded on a larger graphene sheet (r-LGOT). The photocatalytic production of hydrogen was measured in the aqueous suspension of the composite photocatalyst under UV irradiation (λ > 320 nm), and the photoelectrochemical behaviors were characterized using the electrode coated with the composite photocatalyst. The rates of H<SUB>2</SUB> production and photocurrent generation were higher with r-NGOT than r-LGOT, which indicates that the presence of r-GO shell on the surface of TiO<SUB>2</SUB> facilitates the interfacial electron transfer. The direct contact between r-NGO and TiO<SUB>2</SUB> is maximized in r-NGOT by retarding the charge recombination and accelerating the electron transfer. The geometry of the core/shell structure should be effective in the design of a graphene/TiO<SUB>2</SUB> composite for solar conversion applications.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2012/jpccck.2012.116.issue-1/jp209035e/production/images/medium/jp-2011-09035e_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp209035e'>ACS Electronic Supporting Info</A></P>