Showing papers by "Wonyong Choi published in 2021"

Mechanistic analysis of multiple processes controlling solar-driven H2O2 synthesis using engineered polymeric carbon nitride.

Abstract: Solar-driven hydrogen peroxide (H2O2) production presents unique merits of sustainability and environmental friendliness. Herein, efficient solar-driven H2O2 production through dioxygen reduction is achieved by employing polymeric carbon nitride framework with sodium cyanaminate moiety, affording a H2O2 production rate of 18.7 μmol h −1 mg−1 and an apparent quantum yield of 27.6% at 380 nm. The overall photocatalytic transformation process is systematically analyzed, and some previously unknown structural features and interactions are substantiated via experimental and theoretical methods. The structural features of cyanamino group and pyridinic nitrogen-coordinated soidum in the framework promote photon absorption, alter the energy landscape of the framework and improve charge separation efficiency, enhance surface adsorption of dioxygen, and create selective 2e− oxygen reduction reaction surface-active sites. Particularly, an electronic coupling interaction between O2 and surface, which boosts the population and prolongs the lifetime of the active shallow-trapped electrons, is experimentally substantiated. Solar-driven H2O2 production presents a renewable approach to chemical synthesis. Here, authors perform a mechanistic analysis on the contribution of the sodium cyanaminate moiety to the 2-electron oxygen reduction reaction performance of polymeric carbon nitride frameworks.

Photocatalytic activity enhancement of PDI supermolecular via π-π action and energy level adjusting with graphene quantum dots

Abstract: Herein, perylene bisimide supramolecular (PDI) and graphene quantum dots (GQDs) were successfully layer-by-layer assembled via electrostatic interaction. The apparent rate constant of photocatalytic reaction for phenol degradation of GQDs/PDI-14 % (0.018 min−1) was 4.73 times as high as nano PDI under visible light. Besides, the rate of H2 production of GQDs/PDI-14 % (1.6 mmol g−1· h−1) was 1.88 times over that of pure PDI. GQDs is helpful for the electrons delocalization via π-π action with PDI. Moreover, the quantum confinement effect of GQDs promotes electron transfer from GQDs to PDI, which helps the conduction band of PDI shift to more negative position and further enhance the reduction ability of PDI. The work may afford some interesting ideas for designing efficient quantum dots-modified supramolecular organic photocatalysts.

Complexes of Fe(III)-organic pollutants that directly activate Fenton-like processes under visible light

Abstract: The major challenge of Fenton and Fenton-like technologies is promoting the effective transformation of Fe3+ to Fe2+. Photoinduced ligand-to-metal charge transfer (LMCT) enables charge to transfer effectively from the complex ligand to metal ions for the subsequent redox reactions. This study shows that photoactivated LMCTs relying on internal charge transfers occurred from the pollutant complex to the Fe3+ center and followed the in situ transformation of Fe3+ to Fe2+ without the addition of other ligands or agents. Using the antibiotic pollutant sulfamethoxazole (SMX), a direct Fe-SMX complex is formed and enables visible light to be used to activate peroxydisulfate (PDS) by Fe3+ for the rapid degradation of SMX at a rate 6.5-times higher than that observed by the conventional Fe2+/PDS system. This study outlines a new and cost-effective LMCT activation approach and broadens our knowledge of the ability of Fe3+ to be applied in Fenton-like reactions for environmental remediation.

Photocatalytic air purification mimicking the self-cleaning process of the atmosphere.

Abstract: Photocatalytic air purification is a promising technology that mimics nature’s photochemical process, but its practical applications are still limited despite considerable research efforts in recent decades. Here, we briefly discuss the progress and challenges associated with this technology.

Crystal phase-dependent generation of mobile OH radicals on TiO2: Revisiting the photocatalytic oxidation mechanism of anatase and rutile

Abstract: Titanium dioxide has been the most popular environmental photocatalyst of which role critically depends on the generation of OH radicals. In particular, the mobile free OH racial ( OHf) generation and the subsequent diffusion from the surface are critical in achieving the mineralization of non-adsorbing substrates by extending the reaction zone from the surface to the solution bulk. Here the origin of the crystalline phase-dependent generation of OHf was investigated using tetramethylammonium (TMA) cation as a main probe compound for OHf in a UV/TiO2 photocatalytic system. We found a clear evidence that the mobile free OH radical is generated through a reductive conversion of dissolved O2 on anatase only (O2 → H2O2 → OHf). The surface trapped holes are not involved in OHf formation, but lead to the generation of surface-bound OH radical ( OHs) on both anatase and rutile. The generation of OHf is favorable on anatase because more H2O2 are evolved (via dioxygen reduction) and adsorbed on the anatase surface. Rutile showed little sign of OHf formation. The generation of 18O-labelled p-hydroxybenzoic acid on anatase only (not rutile) from benzoic acid oxidation under 18O2-saturated condition provides a solid evidence that the OHf generation mechanism on anatase involves the reductive pathway. Better understanding of OHf production pathway in photocatalysis will provide a new insight leading to an engineering solution for how the production of OHf can be maximized, which is critically important in achieving the efficient photocatalytic oxidation of various pollutants.

Fouling of TiO2 induced by natural organic matters during photocatalytic water treatment: Mechanisms and regeneration strategy

Abstract: Photocatalysis has been intensively investigated for the removal of pollutants but little attention was paid to the deactivation of photocatalysts during the long-term operation. Herein we used TiO2 as the model photocatalyst to investigate its durability in the presence of humic acid (HA), a ubiquitous constituent of real water. No sign of TiO2 deactivation was observed without HA, whereas significant deactivation occurred with HA. Interestingly, the adsorption of intact HA had only a minor contribution to the deactivation, and the major contributing factor was the adsorption of in-situ oxidized HA, which formed surface complexes with TiO2, blocked the active sites, and decreased the efficiency of photogenerated charge transfer. The regeneration of the deactivated TiO2 was also systematically investigated. The present study provides basic information that is required to understand and hinder the complex fouling phenomenon that can be serious in photocatalytic treatment of water containing natural organic matters.

Self-wetting triphase photocatalysis for effective and selective removal of hydrophilic volatile organic compounds in air.

Abstract: Photocatalytic air purification is widely regarded as a promising technology, but it calls for more efficient photocatalytic materials and systems. Here we report a strategy to introduce an in-situ water (self-wetting) layer on WO3 by coating hygroscopic periodic acid (PA) to dramatically enhance the photocatalytic removal of hydrophilic volatile organic compounds (VOCs) in air. In ambient air, water vapor is condensed on WO3 to make a unique tri-phasic (air/water/WO3) system. The in-situ formed water layer selectively concentrates hydrophilic VOCs. PA plays the multiple roles as a water-layer inducer, a surface-complexing ligand enhancing visible light absorption, and a strong electron acceptor. Under visible light, the photogenerated electrons are rapidly scavenged by periodate to produce more •OH. PA/WO3 exhibits excellent photocatalytic activity for acetaldehyde degradation with an apparent quantum efficiency of 64.3% at 460 nm, which is the highest value ever reported. Other hydrophilic VOCs like formaldehyde that are readily dissolved into the in-situ water layer on WO3 are also rapidly degraded, whereas hydrophobic VOCs remain intact during photocatalysis due to the “water barrier effect”. PA/WO3 successfully demonstrated an excellent capacity for degrading hydrophilic VOCs selectively in wide-range concentrations (0.5−700 ppmv). Photocatalytic air purification is promising but it calls for more efficient photocatalytic materials and systems. Here, the authors report a strategy to introduce an in-situ water layer on WO3 by coating hygroscopic periodic acid that effectively remove hydrophilic volatile organic compounds.

Solar photoelectrochemical synthesis of electrolyte-free H2O2 aqueous solution without needing electrical bias and H2

Abstract: The conventional synthesis of hydrogen peroxide (H2O2) such as heterogeneous catalytic and electrochemical processes requires H2 and O2 as reagents, costly noble metals, and organic solvents, which are energy/waste-intensive and hazardous. An alternative method of photoelectrochemical (PEC) synthesis that needs only water and sunlight is environment-friendly but its practical application is limited due to the energy-demanding method for the separation of the synthesized H2O2 from the electrolytes. Herein, we demonstrated the direct synthesis of an electrolyte-free aqueous solution of pure H2O2 by developing a PEC system with solid polymer electrolyte (SPE) and engineered electrodes. Ruthenium catalyst-decorated TiO2 nanorods (RuOx/TNR: photoanode) and anthraquinone-anchored graphite rods (AQ/G: cathode) are placed in an anode compartment and a cathode compartment, respectively, while a middle compartment containing SPE is located between these compartments. Upon solar simulating irradiation (AM 1.5G, 100 mW cm−2), the photoanode generates H+ ions via water oxidation reaction (WOR) and the cathode generates HO2− ions via two-electron oxygen reduction reaction (ORR), while the SPE selectively transports H+ and HO2− into the middle compartment to form pure H2O2 solution. The combined system enabled continuous H2O2 synthesis over 100 h even under bias-free (0.0 V of cell voltage) conditions with the production of ∼80 mM H2O2 (electrolyte-free) and a faradaic efficiency of ∼90%, which is the highest concentration of pure H2O2 obtained using PEC systems. This study successfully demonstrates the proof-of-concept that might enable the production of a concentrated pure (electrolyte-free) aqueous solution of H2O2 using sunlight, water, and dioxygen only.

Bifunctional Carbon Nitride Exhibiting both Enhanced Photoactivity and Residual Catalytic Activity in the Post-Irradiation Dark Period

Abstract: Graphitic carbon nitride (CN) as an environmental photocatalyst works under continuous illumination and stops under the dark condition. Here, we develop a bifunctional CN (4-phenoxyphenol-functiona...

Photocatalytic production of H2O2 from water and dioxygen only under visible light using organic polymers: Systematic study of the effects of heteroatoms

Abstract: The photocatalytic production of H2O2 that needs dioxygen, water, and light only has been proposed and investigated. However, most of the studies employed organic electron donors. In this work, organic photocatalysts that can produce H2O2 without organic electron donors are designed with a bottom-up approach. Although the heptazine-based g-C3N4 cannot induce water oxidation, a different kind of organic polymer (CN) obtained from the condensation of s-triazine and pyrimidine can produce H2O2 via dioxygen reduction coupled with water oxidation. The incorporation of O and P elements in CN structure (CNO, CNP, CNOP) increases the visible light absorption and hinders the photodecomposition of H2O2, respectively. The C-O and P-O bonds act as the charge trapping sites and also the preferred adsorption sites of a key intermediate, •OOH. As a result, the CNOP exhibits the highest production of H2O2 using dioxygen, water, and visible light only.

Photoconversion of Cyanide to Dinitrogen Using the Durable Electrode of a TaON Overlayer-Deposited WO3 Film and Visible Light

Abstract: Chemical treatments of toxic cyanide (CN–) typically involve its conversion to cyanate (OCN–), which is less toxic. An ideal treatment should be its conversion to N2 and CO2. This study proposed an...

Solar denitrification coupled with in situ water splitting

Abstract: Utilizing solar energy as a sustainable means of controlling the nitrogen pollutant is proposed. The photochemical conversion of nitrate (NO3−) to dinitrogen (N2) without using chemical reductants is an ideal solution but difficult to be realized. Here we demonstrate a successful case of solar denitrification (with 0.1–10 mM nitrate) coupled with in situ water splitting (without chemical reductants) by developing a ternary composite photocatalyst composed of TiO2, Cu–Pd bimetals, and reduced graphene oxide (rGO) (Cu–Pd/rGO/TiO2). Direct transformation of NO3− to N2 occurs on Cu–Pd/rGO/TiO2via using in situ H2 generated from water splitting in a broader pH range with achieving near 100% conversion and selectivity to N2 while it is not possible at all with rGO/TiO2 and Cu–Pd/TiO2. The unique activity is ascribed to the synergic action of Cu as a co-catalyst for nitrate-to-nitrite conversion, Pd for nitrite-to-dinitrogen conversion, and rGO for the enhanced charge separation/transfer and H2 production. The combined roles of Cu–Pd and rGO in retarding the charge recombination and accelerating the electron transfer from TiO2 to NO3− are confirmed by monitoring the time-resolved photoluminescence and slurry-type photocurrent generation, respectively. The in situ water splitting on Cu–Pd/rGO/TiO2 was confirmed by the concurrent H2 and O2 evolution and the in situ generated H2 was immediately consumed in the presence of nitrate. The introduction of rGO enabled the denitrification even under visible light (up to 450 nm) and the apparent quantum yield (AQY) of N2 production reached a maximum of 4.9% at 320 nm. The proposed composite photocatalytic system realizes the selective solar conversion for chemical reductant-free denitrification (nitrate to N2) by coupling nitrate reduction and water oxidation.

Synergistic effect of Sn doping and hydrogenation on hematite electrodes for photoelectrochemical water oxidation

Abstract: Hematite (α-Fe2O3), which is abundant, chemically stable, and environmentally benign, is a promising photoanode material that can oxidize water in photoelectrochemical (PEC) water splitting. However, the poor electrical properties of hematite limit its intrinsic activity with regard to PEC water splitting. Herein, we report an innovative dual modification strategy that combines metal (Sn) doping and H2 treatment to prepare an efficient hematite photoanode for PEC water splitting. Sn doping and the subsequent H2 treatment generate different electron donors, Sn4+ and oxygen vacancies (Vo), in hematite. Electrochemical impedance measurements revealed that dual-modified hematite with a synchronous presence of Sn4+ and Vo exhibited significantly improved electrical properties with 111 times higher donor density and 10 times smaller charge transfer resistance than those of bare hematite. As a result, the photooxidation current was 4.61 mA cm−2 (at 1.6 VRHE), which is 55 times higher than that of bare hematite (83 μA cm−2) and substantially higher than the sum of the photooxidation current exhibited by the two single-modified hematites (298 μA cm−2 for Sn doping and 681 μA cm−2 for H2 treatment). A stable photocurrent was maintained under prolonged illumination over 12 h without showing any sign of deactivation. Additional cobalt treatment further increased the PEC water oxidation performance of the dual-modified hematite, achieving a superior Faraday efficiency (ca. 99%) and stability. Compared to the oxygen-deficient heat treatment (under Ar) of Sn-doped hematite, the combination of Sn doping and H2 treatment induces outstanding synergistic improvement of PEC activity (photocurrent twice that of Sn-doped and Ar-treated hematite).

Confronting Racism in Chemistry Journals

Abstract: The following joint Editorial was originally published in ACS Applied Materials & Interfaces (DOI: 10.1021/acsami.0c10979). We confront the terrible reality that systemic racism and discrimination impacts the daily personal and professional lives of many members of the scientific community and broader society. In the U.S., the brutal killing of George Floyd while in police custody is one of the most recent examples of the centuries of systemic violence suffered by Black Americans. This moment and its aftermath lay bare the legacies of racism and its exclusionary practices. Let us be clear: we, the Editors, Staff, and Governance Members of ACS Publications condemn the tragic deaths of Black people and stand in solidarity with Black members of the science and engineering community. Moreover, ACS condemns racism, discrimination, and harassment in all forms. We will not tolerate practices and viewpoints that exclude or demean any member of our community. Despite these good intentions, we recognize that our community has not done enough to provide an environment for Black chemists to thrive. Rep. Eddie Bernice Johnson, Chairwoman of the U.S. House Committee on Science, Space, and Technology said, “So far, we have gotten by with a STEM workforce that does not come close to representing the diversity of our nation. However, if we continue to leave behind so much of our nation’s brainpower, we cannot succeed.”(1) Indeed, the U.S. National Science Foundation notes that Blacks and other under-represented minority groups continue to be under-represented in science and engineering education and employment.(2) What is abundantly clear in this moment is that this lack of representation is a symptom of systemic racism across all levels of education and professional life. We know that supportive words are not enough. We must develop and implement a concrete plan for changing our trajectory. Publications and citations are academic currency, and while we like to think publishing a manuscript is “just about the science”, we know that is not true for everyone. We have seen the biases (largely through the lens of gender and in Western countries because of the limitations in bibliometric analyses) and applaud our colleagues at the RSC for their massive study that explored these gender barriers in the publishing pipeline(3) and their recent Inclusion and Diversity Framework.(4) At the present time, unfortunately, less is known about the effects of race and ethnicity on publishing success. A study published in PeerJ, however, found that unprofessional reviewer comments had a disproportionate effect on authors from under-represented groups.(5) As the world’s leading society publisher, we have a responsibility to aggressively combat bias in all aspects of the publishing process, including systemic under-representation of Blacks in this endeavor (no ACS journal is currently led by a Black Editor-in-Chief). Within ACS Publications, we actively track gender and geographic diversity of editors, advisors, authors, and reviewers, and we anecdotally report on race of editors. Diversity encompasses many more dimensions than these, and we acknowledge that we can do much more than we have. We affirm that diversity and inclusion strengthen the research community and its impact, and we are committed to developing, implementing, tracking, and reporting on our progress to ensure that our editors, advisors, reviewers, and authors are more diverse and that all authors receive the same fair treatment and opportunity to publish in our journals. We acknowledge that we do not have all the answers now, but we seek to hear from and listen to our community on how we can improve our journals to be more diverse and inclusive. As first steps, we commit to the taking the following actions: Gathering and making public our baseline statistics on diversity within our journals, encompassing our editors, advisors, reviewers, and authors; annually reporting on progress Training new and existing editors to recognize and interrupt bias in peer review Including diversity of journal contributors as an explicit measurement of Editor-in-Chief performance Appointing an ombudsperson to serve as a liaison between Editors and our Community Developing an actionable diversity plan for each ACS journal These are only initial plans and the start of a conversation: other ideas are beginning to germinate, and we commit to sharing them with you regularly. We invite you contribute your ideas on how we can do better via our Axial website. We are listening carefully. We encourage you to take immediate action in your own circles. In a recent editorial, JACS Associate Editor Melanie Sanford(6) offered practical steps to take now. Take a moment to find out more about these actions and how to bring them into your work and your life. We all have a responsibility to eradicate racism and discrimination in the science and engineering community; indeed, to make a real difference, we need to be antiracist. The tragic events we have seen in the Black community provide great urgency to this goal. The work will be difficult and will force us to confront hard realities about our beliefs and actions. We fully expect that you, and everyone in the community, will hold us accountable.

Confronting Racism in Chemistry Journals

Abstract: The following joint Editorial was originally published in ACS Applied Materials & Interfaces (DOI: 10.1021/acsami.0c10979). We confront the terrible reality that systemic racism and discrimination impacts the daily personal and professional lives of many members of the scientific community and broader society. In the U.S., the brutal killing of George Floyd while in police custody is one of the most recent examples of the centuries of systemic violence suffered by Black Americans. This moment and its aftermath lay bare the legacies of racism and its exclusionary practices. Let us be clear: we, the Editors, Staff, and Governance Members of ACS Publications condemn the tragic deaths of Black people and stand in solidarity with Black members of the science and engineering community. Moreover, ACS condemns racism, discrimination, and harassment in all forms. We will not tolerate practices and viewpoints that exclude or demean any member of our community. Despite these good intentions, we recognize that our community has not done enough to provide an environment for Black chemists to thrive. Rep. Eddie Bernice Johnson, Chairwoman of the U.S. House Committee on Science, Space, and Technology said, “So far, we have gotten by with a STEM workforce that does not come close to representing the diversity of our nation. However, if we continue to leave behind so much of our nation’s brainpower, we cannot succeed.”(1) Indeed, the U.S. National Science Foundation notes that Blacks and other under-represented minority groups continue to be under-represented in science and engineering education and employment.(2) What is abundantly clear in this moment is that this lack of representation is a symptom of systemic racism across all levels of education and professional life. We know that supportive words are not enough. We must develop and implement a concrete plan for changing our trajectory. Publications and citations are academic currency, and while we like to think publishing a manuscript is “just about the science”, we know that is not true for everyone. We have seen the biases (largely through the lens of gender and in Western countries because of the limitations in bibliometric analyses) and applaud our colleagues at the RSC for their massive study that explored these gender barriers in the publishing pipeline(3) and their recent Inclusion and Diversity Framework.(4) At the present time, unfortunately, less is known about the effects of race and ethnicity on publishing success. A study published in PeerJ, however, found that unprofessional reviewer comments had a disproportionate effect on authors from under-represented groups.(5) As the world’s leading society publisher, we have a responsibility to aggressively combat bias in all aspects of the publishing process, including systemic under-representation of Blacks in this endeavor (no ACS journal is currently led by a Black Editor-in-Chief). Within ACS Publications, we actively track gender and geographic diversity of editors, advisors, authors, and reviewers, and we anecdotally report on race of editors. Diversity encompasses many more dimensions than these, and we acknowledge that we can do much more than we have. We affirm that diversity and inclusion strengthen the research community and its impact, and we are committed to developing, implementing, tracking, and reporting on our progress to ensure that our editors, advisors, reviewers, and authors are more diverse and that all authors receive the same fair treatment and opportunity to publish in our journals. We acknowledge that we do not have all the answers now, but we seek to hear from and listen to our community on how we can improve our journals to be more diverse and inclusive. As first steps, we commit to the taking the following actions: Gathering and making public our baseline statistics on diversity within our journals, encompassing our editors, advisors, reviewers, and authors; annually reporting on progress Training new and existing editors to recognize and interrupt bias in peer review Including diversity of journal contributors as an explicit measurement of Editor-in-Chief performance Appointing an ombudsperson to serve as a liaison between Editors and our Community Developing an actionable diversity plan for each ACS journal These are only initial plans and the start of a conversation: other ideas are beginning to germinate, and we commit to sharing them with you regularly. We invite you contribute your ideas on how we can do better via our Axial website. We are listening carefully. We encourage you to take immediate action in your own circles. In a recent editorial, JACS Associate Editor Melanie Sanford(6) offered practical steps to take now. Take a moment to find out more about these actions and how to bring them into your work and your life. We all have a responsibility to eradicate racism and discrimination in the science and engineering community; indeed, to make a real difference, we need to be antiracist. The tragic events we have seen in the Black community provide great urgency to this goal. The work will be difficult and will force us to confront hard realities about our beliefs and actions. We fully expect that you, and everyone in the community, will hold us accountable.