Paramagnetic Resonance Absorption Research Articles

Photochemical transformations of exo-2,3-norbornene oxide radical cations in the CF3CCl3 matrix at 77 K

The structure and nature of the underlying photochemical transformations of radical cations formed upon irradiation of exo-2,3-norbornene oxide solutions in a CF3CCl3 matrix at 77 K have been established using electron paramagnetic resonance and UV-visible absorption spectroscopy as well as quantum chemical calculations. For photochemical reactions proceeding in this system, a mechanism has been proposed, which consists in the transformation of the initially formed ring-open radical cation into a ring-closed C-centered radical cation, followed by photoelimination of H2.

Pre-Planarized Triphenylamine-Based Linear Mixed-Valence Charge-Transfer Systems.

Three linear dimers with two redox‐active planarized triphenylamines were synthesized and their structures verified by X‐ray crystallography. Their radical cations, which exhibit electron self‐exchange between the two redox centers, are of great interest. This process was thoroughly investigated by means of electron paramagnetic resonance spectroscopy, absorption spectroscopy, and (time‐dependent) density functional theory calculations. A comparison of the key parameters of electron transfer with non‐planarized nitrogen‐centered building blocks emphasizes the impact of using redox centers with low internal reorganization energies. However, the distance‐dependence attenuation factor of the super‐exchange mechanisms remains similar.

Effects of octahedral tilting on the site of substitution of manganese in CaTiO3

Electron paramagnetic resonance (EPR) and X-ray absorption fine structure measurements were combined with first principles calculations to investigate the substitutional behavior of Mn ions in perovskite CaTiO3 ceramics. While transition-metal dopants in perovskite-structured oxides often act as aliovalent defects, Mn in CaTiO3 is amphoteric and concurrently occupies both Ca and Ti sites as Mn2+ and Mn4+, respectively. Contrary to the behavior of Mn in SrTiO3 and BaTiO3, which exhibit larger geometric perovskite tolerance factors, in CaTiO3, it is determined that Mn2+ prefers A-site substitution. Density functional theory (DFT) calculations provide insight to the unique defect chemistry of Mn-doped CaTiO3 compared to SrTiO3 and BaTiO3, highlighting the role of octahedral rotations which accommodate ionic size mismatch between the larger host and smaller dopant cations on the cuboctahedral sites without significant dopant-ion displacements. Superposition models of the EPR zero-field splitting parameters for multiple types of Mn defect centers were considered based on the structural parameters of DFT calculations, and these results, combined with the EPR, DFT, and X-ray absorption analysis, were used to determine the point defect substitution mechanisms of Mn-CaTiO3.

Preparation and spectroscopic characterization of lyophilized Mo nitrogenase

Mo nitrogenase is the primary source of biologically fixed nitrogen, making this system highly interesting for developing new, energy efficient ways of ammonia production. Although heavily investigated, studies of the active site of this enzyme have generally been limited to spectroscopic methods that are compatible with the presence of water and relatively low protein concentrations. One method of overcoming this limitation is through lyophilization, which allows for measurements to be performed on solvent free, high concentration samples. This method also has the potential for allowing efficient protein storage and solvent exchange. To investigate the viability of this preparatory method with Mo nitrogenase, we employ a combination of electron paramagnetic resonance, Mo and Fe K-edge X-ray absorption spectroscopy, and acetylene reduction assays. Our results show that while some small distortions in the metallocofactors occur, oxidation and spin states are maintained through the lyophilization process and that reconstitution of either lyophilized protein component into buffer restores acetylene reducing activity.Graphic abstract

EPR Theories for Selection Rules to Observe the Spin Gap

The direct electron paramagnetic resonance (EPR) absorption between the singlet ground state and the triplet excited states of spin gap systems is investigated. Such an absorption, which is forbidden by the conservation of the total spin quantum number in isotropic Hamiltonians, is allowed by some anisotropies; the staggered g-tensor and the Dzyaloshinskii–Moriya interaction. Selection rules in the presence of these interactions were presented in our previous works. We review the selection rules of the EPR transition of the spin gap with some numerical demonstrations and discuss some applications.

Power of Aerogel Platforms to Explore Mesoscale Transport in Catalysis.

We describe the opportunity to deploy aerogels-an ultraporous nanoarchitecture with co-continuous networks of meso/macropores and covalently bonded nanoparticulates-as a platform to address the nature of the electronic, ionic, and mass transport that underlies catalytic activity. As a test case, we fabricated Au||TiO2 junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO2 network (Au "in" TiO2, AuIN-TiO2 aerogel) or deposited onto preformed TiO2 aerogel (Au "on" TiO2, AuON/TiO2 aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but AuIN-TiO2 aerogels impose a greater degree of Au contact with TiO2 particles than does the AuON/TiO2 form. Both aerogel variants enable transport of electrons over micrometer-scale distances across the TiO2 network to Au||TiO2 junctions, as evidenced by electron paramagnetic resonance (EPR) and ultrafast visible pump-IR probe time-resolved absorption spectroscopy. The siting of gold nanoparticles in the TiO2 network more effectively disperses trapped electrons. Density functional theory (DFT) calculations find that increased physical contact between Au and TiO2, induced by oxygen vacancies, produces increased hybridization of midgap states and quenches unpaired trapped electrons. We assign the apparent differences in electron-transport capabilities to a combination of the relatively better-wired Au||TiO2 junctions in AuIN-TiO2 aerogels, which have a greater capacity to dilute accumulated charge over a larger interfacial surface area, with an enhanced ability to discharge the accumulated electrons via catalytic reduction of adsorbed O2 to O2- at the interface. Solid-state 1H nuclear magnetic resonance experiments show that proton spin-lattice relaxation times and possibly proton diffusion are strongly coupled to Au||TiO2 interfacial design, likely through spin coupling of protons to unpaired electrons trapped at the TiO2 network. Taken together, our results show that Au||TiO2 interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in catalytic aerogel architectures.

FeFe]-hydrogenase maturation: H-cluster assembly intermediates tracked by electron paramagnetic resonance, infrared, and X-ray absorption spectroscopy

[FeFe]-hydrogenase enzymes employ a unique organometallic cofactor for efficient and reversible hydrogen conversion. This so-called H-cluster consists of a [4Fe–4S] cubane cysteine linked to a diiron complex coordinated by carbon monoxide and cyanide ligands and an azadithiolate ligand (adt = NH(CH2S)2)·[FeFe]-hydrogenase apo-protein binding only the [4Fe–4S] sub-complex can be fully activated in vitro by the addition of a synthetic diiron site precursor complex ([2Fe]adt). Elucidation of the mechanism of cofactor assembly will aid in the design of improved hydrogen processing synthetic catalysts. We combined electron paramagnetic resonance, Fourier-transform infrared, and X-ray absorption spectroscopy to characterize intermediates of H-cluster assembly as initiated by mixing of the apo-protein (HydA1) from the green alga Chlamydomonas reinhardtii with [2Fe]adt. The three methods consistently show rapid formation of a complete H-cluster in the oxidized, CO-inhibited state (Hox-CO) already within seconds after the mixing. Moreover, FTIR spectroscopy support a model in which Hox-CO formation is preceded by a short-lived Hred′-CO-like intermediate. Accumulation of Hox-CO was followed by CO release resulting in the slower conversion to the catalytically active state (Hox) as well as formation of reduced states of the H-cluster.Graphic abstract

Electron paramagnetic resonance and optical properties of Cu+2 in PbO-Li2O-As2O3-B2O3 glasses

The room temperature electron paramagnetic resonance (EPR) and optical absorption studies of PbO-Li2O-As2O3-B2O3 glasses have been done by introducing a transition metal ion Cu+2 ion as a spin probe with fixed mole% in the given glass composition. Glasses containing transition metal (TM) ion give the information of site symmetry around the Cu+2 ion of the host glass. The EPR spectra of Cu+2 in all glass samples recorded in the X-band frequency (9.1–9.7 GHz) have similar features. The spin-Hamiltonian parameters have been calculated; the observed values are g|| > g⊥ > ge and A|| > A⊥. It suggests that the site symmetry around Cu+2 ion is tetragonal distorted octahedral and the ground state of Cu+2 is dx2-y2 (2B1g) orbital. The Cu+2 ion is coordinated with six ligand atoms in a distorted octahedron site symmetry. The optical absorption spectra exhibit a broadband corresponding to d-d transition bonds of Cu+2 ion. Calculated bonding parameters suggest the covalent nature of in-plane σ-bonding (α2) and pure ionic nature for out-of-plane (β2) and in-plane π-bonding (β12). This data is correlated with the theoretical optical basicity (Λth) values, and it is observed that the Γσ value increases whereas Γπ values decrease with increasing Λth.

Unveiling Catalytic Sites in a Typical Hydrogen Photogeneration System Consisting of Semiconductor Quantum Dots and 3d-Metal Ions.

Semiconductor quantum dots (QDs) in conjunction with non-noble 3d-metal ions (e.g., Fe3+, Co2+, and Ni2+) have emerged as an extremely efficient, facile, and cost-effective means of solar-driven hydrogen (H2) evolution. However, the exact structural change of the active sites under realistic conditions remains elusive, and the mechanism of H2 evolution behind the remarkable activity is poorly understood. Here, we successfully track the structural variation of the catalytic sites in the typical H2 photogeneration system consisting of CdSe/CdS QDs and 3d-metal ions (i.e., Ni2+ used here). That is, the nickel precursor of Ni(OAc)2 changes to Ni(H2O)62+ in neutral H2O and eventually transforms to Ni(OH)2 nanosheets in alkaline media. Furthermore, the in operando spectroscopic techniques of electron paramagnetic resonance and X-ray absorption spectroscopy reveal the photoinduced transformation of Ni(OH)2 to a defective structure [Nix0/Ni1-x(OH)2], which acts as the real catalytic species of H2 photogeneration. Density functional theory (DFT) calculations further indicate that the surface Ni-vacancies (VNi) on the Ni(OH)2 nanosheets enhance the adsorption and dissociation of H2O molecules to enhance the local proton concentration, while the Ni0 clusters behave as H2-evolution sites, thereby synergistically promoting the activity of H2 photogeneration in alkaline media.

Correlation between the C-C Cross-Coupling Activity and C-to-Ni Charge Transfer Transition of High-Valent Ni Complexes.

High-valent Ni complexes have proven to be good platforms for diverse cross-coupling reactions that are otherwise difficult to be achieved with conventional low-valent catalysts. However, their reductive elimination (RE) activities are still significantly variable by up to 5 orders of magnitude, depending on the supporting ligand and oxidation state of the Ni center. To elucidate frontier molecular orbitals (FMOs) that determine the RE activity of the Ni center, the electronic structures of cycloneophyl (CH2C(CH3)2-o-C6H4) NiIII and NiIV complexes have been characterized by utilizing various transition metal-based spectroscopic techniques such as electronic absorption, magnetic circular dichroism, electron paramagnetic resonance, resonance Raman, and X-ray absorption spectroscopies. In combination with density functional theory computations, the spectroscopic analyses have shown that the energies of the C-to-Ni charge-transfer (CT) electronic transitions are strongly correlated to the rates of C-C bond-forming RE reaction. This correlation suggests that the kinetic barrier of the RE reaction is determined by energy cost for internal CT (ICT) from the coordinated carbon moiety to the Ni center, and that FMOs involved in the RE reaction and the C-to-Ni CT electronic transitions are essentially identical. This FMO determination has led us to discover that photoexcitation to the C-to-Ni CT excited states accelerates the C-C cross-coupling reaction by up to 105 times, as the CT electronic transition can substitute for the rate-determining ICT step of the RE reaction at the ground electronic state.

Effect of Structural Inhomogeneities on the Spin-Wave and Magnetic Properties of Monocrystalline Spinel Ferrite Films in the Microwave Range

We have studied magnetic parameters of monocrystalline manganese spinel ferrite films differing in composition and structural perfection and assessed the effect of dislocation distribution on the frequency dependence of the ferromagnetic resonance (FMR) linewidth in the microwave range. The results demonstrate that, if an external magnetic field is perpendicular to the film surface, electron paramagnetic resonance (EPR) absorption lines correspond to ferromagnetic and spin wave resonances (SWRs). SWR is due to nanostructural surface inhomogeneities. The EPR spectrum of a tangently magnetized film represents a single dipole exchange oscillation spectrum. Parameters of dipole and exchange modes have been determined.

Spectroscopic properties of the V-doped borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra in borate glasses of Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V compositions with added 0.5 and 1.0 mol.% V2O5, were investigated. The EPR spectroscopy clearly shows that the V impurity is incorporated into the network of studied glasses, mainly, as isolated vanadyl (VO2+) molecular complex centres. Spin Hamiltonian parameters of axial symmetry (g||, g⊥, A||, A⊥), dipolar hyperfine coupling (P) and Fermi contact interaction (K) constants of the VO2+ centres in octahedral sites with a tetragonal compression (C4v local symmetry) for all investigated glasses were determined from their experimental EPR spectra. Optical absorption bands, observed in the investigated glasses have been identified and interpreted. Crystal field parameters (Dq, Ds, Dt) and molecular orbitals (bonding) coefficients (β⁎2, επ⁎2) for VO2+ centres in the studied glasses were calculated and compared with referenced data for other V-doped borate glasses with similar compositions.

Unifying Activity, Structure, and Spectroscopy of [NiFe] Hydrogenases: Combining Techniques To Clarify Mechanistic Understanding.

Achieving a unified understanding of the mechanism of a multicenter redox enzyme such as [NiFe] hydrogenase is complicated by difficulties in reconciling information obtained by using different techniques and on samples in different physical forms. Measurements of the activity of the enzyme, and of factors which perturb activity, are generally carried out using biochemical assays in solution or with electrode-immobilized enzymes using protein film electrochemistry (PFE). Conversely, spectroscopy aimed at reporting on features of the metalloclusters in the enzyme, such as electron paramagnetic resonance (EPR) or X-ray absorption spectroscopy (XAS), is often conducted on frozen samples and is thus difficult to relate to catalytically relevant states as information about turnover and activity has been lost. To complicate matters further, most of our knowledge of the atomic-level structure of metalloenzymes comes from X-ray diffraction studies in the solid, crystalline state, which are again difficult to link to turnover conditions. Taking [NiFe] hydrogenases as our case study, we show here how it is possible to apply infrared (IR) spectroscopic sampling approaches to unite direct spectroscopic study with catalytic turnover. Using a method we have named protein film IR electrochemistry (PFIRE), we reveal the steady-state distribution of intermediates during catalysis and identify catalytic "bottlenecks" introduced by site-directed mutagenesis. We also show that it is possible to study dynamic transitions between active site states of enzymes in single crystals, uniting solid state and solution spectroscopic information. In all of these cases, the spectroscopic data complement and enhance interpretation of purely activity-based measurements by providing direct chemical insight that is otherwise hidden. The [NiFe] hydrogenases possess a bimetallic [NiFe] active site, coordinated by CO and CN- ligands, linked to the protein via bridging and terminal cysteine sulfur ligands, as well as an electron relay chain of iron sulfur clusters. Infrared spectroscopy is ideal for probing hydrogenases because the CO and CN- ligands are strong IR absorbers, but the suite of IR-based approaches we describe here will be equally valuable in studying substrate- or intermediate-bound states of other metalloenzymes where key mechanistic questions remain open, such as nitrogenase, formate dehydrogenase, or carbon monoxide dehydrogenase. We therefore hope that this Account will encourage future studies which unify information from different techniques across bioinorganic chemistry.

Incorporation of Ni2+, Co2+, and Selenocysteine into the Auxiliary Fe-S Cluster of the Radical SAM Enzyme HydG.

The radical SAM enzyme HydG generates CO- and CN--containing Fe complexes that are involved in the bioassembly of the [FeFe] hydrogenase active cofactor, the H-cluster. HydG contains a unique 5Fe-4S cluster in which the fifth "dangler" Fe and the coordinating cysteine molecule have both been shown to be essential for its function. Here, we demonstrate that this dangler Fe can be replaced with Ni2+ or Co2+ and that the cysteine can be replaced with selenocysteine. The resulting HydG variants were characterized by electron paramagnetic resonance and X-ray absorption spectroscopy, as well as subjected to a Tyr cleavage assay. Both Ni2+ and Co2+ are shown to be exchange-coupled to the 4Fe-4S cluster, and selenocysteine substitution does not alter the electronic structure significantly. XAS data provide details of the coordination environments near the Ni, Co, and Se atoms and support a close interaction of the dangler metal with the FeS cluster via an asymmetric SeCys bridge. Finally, while we were unable to observe the formation of novel organometallic species for the Ni2+ and Co2+ variants, the selenocysteine variant retains the activity of wild type HydG in forming [Fe(CO)x(CN)y] species. Our results provide more insights into the unique auxiliary cluster in HydG and expand the scope of artificially generated Fe-S clusters with heteroatoms.

(Invited) Atomistic Insights into Cathode Dissolution and Ion Transport in Li-Ion Batteries

Fundamental understanding of inter-connected electrochemical phenomena, including chemical reactions, charge/ion transport, defect evolution, and solvation dynamics) is crucial to accelerate development of low-cost, safe, and high-performance Li-ion battery architectures. In particular, molecular mechanisms underlying (a) dissolution of cathode into liquid electrolyte; and (b) fast Li-ion transport in novel solid-electrolytes, need to be clearly identified to design Li-ion batteries with exceptional capacity, rate-capability, and cycle life; however, such knowledge remains in its infancy. In this talk, we will first address the key mechanisms governing dissolution of a commercially promising LiMn2O4 (LMO) cathode. Using a combination of ab initio molecular dynamics simulations (AIMD) and quantum-chemical calculations, we demonstrate that dissolved Mn ions exist in 2+ oxidation state in carbonate based electrolytes. This finding resolves the debate raised by recent electron paramagnetic resonance and X-ray absorption near edge structure experiments suggesting that Mn could preferentially exist in 3+ state within solution; and provides evidence supporting the long-standing belief that Hunter’s disproportionation mechanism (2 Mn3+ --> Mn4+ + Mn2+) precedes Mn dissolution. Furthermore, we employed AIMD and density functional theory (DFT) calculations to probe formation of Mn2+ on stable LMO (511) surfaces via surface Hunter charge disproportionation mechanisms, as well as chemical mechanisms involving hydrofluoric acid (HF) electrolyte impurities and oxidation of ethylene carbonate (EC) electrolyte molecules. These studies would guide future strategies to suppress Mn dissolution, and in turn, combat capacity fading in LiBs based on LMO cathodes. Next, we will demonstrate that strongly-correlated rare-earth perovskite nickelates exhibit fast Li-ion conduction with simultaneous suppression of electronic transport via Mott transition. The pronounced Li-ion conduction is facilitated by electron injection into Ni orbitals, and concomitant lattice expansion when the Li ions are inserted from a reservoir. These results will be discussed in the context of developing next-generation lithium ion batteries with exceptional performance.

Photocatalytic Oxygenation Reactions with a Cobalt Porphyrin Complex Using Water as an Oxygen Source and Dioxygen as an Oxidant.

Photocatalytic oxygenation of hexamethylbenzene occurs under visible-light irradiation of an O2-saturated acetonitrile solution containing a cobalt porphyrin complex CoII(TPP) (TPP2- = tetraphenylporphyrin dianion), water, and triflic acid (HOTf) via a one-photon-two-electron process, affording pentamethylbenzyl alcohol and hydrogen peroxide as products with a turnover number of >6000; in this reaction, H2O and O2 were used as an oxygen source and a two-electron oxidant, respectively. The photocatalytic mechanism was clarified by means of electron paramagnetic resonance, time-resolved fluorescence, and transient absorption measurements as well as 18O-labeling experiments with H218O and 18O2. To the best of our knowledge, we report the first example of efficient photocatalytic oxygenation of an organic substrate by a metal complex using H2O as an oxygen source and O2 as a two-electron oxidant.

A MnII MnIII -Peroxide Complex Capable of Aldehyde Deformylation.

Ribonucleotide reductases (RNRs) are essential enzymes required for DNA synthesis. In class Ib Mn2 RNRs superoxide (O2.- ) was postulated to react with the MnII2 core to yield a MnII MnIII -peroxide moiety. The reactivity of complex 1 ([MnII2 (O2 CCH3 )2 (BPMP)](ClO4 ), where HBPMP=2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol) towards O2.- was investigated at -90 °C, generating a metastable species, 2. The electronic absorption spectrum of 2 displayed features (λmax =440, 590 nm) characteristic of a MnII MnIII -peroxide species, representing just the second example of such. Electron paramagnetic resonance and X-ray absorption spectroscopies, and mass spectrometry supported the formulation of 2 as a MnII MnIII -peroxide complex. Unlike all other previously reported Mn2 -peroxides, which were unreactive, 2 proved to be a capable oxidant in aldehyde deformylation. Our studies provide insight into the mechanism of O2 -activation in Class Ib Mn2 RNRs, and the highly reactive intermediates in their catalytic cycle.

Structural isomers of the S2 state in photosystem II: do they exist at room temperature and are they important for function?

In nature, an oxo-bridged Mn4 CaO5 cluster embedded in photosystem II (PSII), a membrane-bound multi-subunit pigment protein complex, catalyzes the water oxidation reaction that is driven by light-induced charge separations in the reaction center of PSII. The Mn4 CaO5 cluster accumulates four oxidizing equivalents to enable the four-electron four-proton catalysis of two water molecules to one dioxygen molecule and cycles through five intermediate S-states, S0 -S4 in the Kok cycle. One important question related to the catalytic mechanism of the oxygen-evolving complex (OEC) that remains is, whether structural isomers are present in some of the intermediate S-states and if such equilibria are essential for the mechanism of the O-O bond formation. Here we compare results from electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) obtained at cryogenic temperatures for the S2 state of PSII with structural data collected of the S1 , S2 and S3 states by serial crystallography at neutral pH (∼6.5) using an X-ray free electron laser at room temperature. While the cryogenic data show the presence of at least two structural forms of the S2 state, the room temperature crystallography data can be well-described by just one S2 structure. We discuss the deviating results and outline experimental strategies for clarifying this mechanistically important question.

Electric field modulation of magnetic exchange in molecular helices.

The possibility to operate on magnetic materials through the application of electric rather than magnetic fields-promising faster, more compact and energy efficient circuits-continues to spur the investigation of magnetoelectric effects. Symmetry considerations, in particular the lack of an inversion centre, characterize the magnetoelectric effect. In addition, spin-orbit coupling is generally considered necessary to make a spin system sensitive to a charge distribution. However, a magnetoelectric effect not relying on spin-orbit coupling is appealing for spin-based quantum technologies. Here, we report the detection of a magnetoelectric effect that we attribute to an electric field modulation of the magnetic exchange interaction without atomic displacement. The effect is visible in electron paramagnetic resonance absorption of molecular helices under electric field modulation and confirmed by specific symmetry properties and spectral simulation.

Enhanced photocatalytic hydrogen evolution activity of carbon and nitrogen self-doped TiO2 hollow sphere with the creation of oxygen vacancy and Ti3+

Introducing oxygen vacancies and Ti3+ into TiO2 is thought to be an effective method of increasing the utilization of sunlight and promoting the efficiency of electron transfer. In this work, novel carbon and nitrogen self-doped TiO2 hollow spheres (CNTH) with oxygen vacancies and Ti3+ were synthesized by a one-pot hydrothermal strategy in H2OC2H5OHHFH2O2 mixed solution with TiCN as a precursor of anatase TiO2 and self-doping source of N and C. The presence of oxygen vacancies and Ti3+ in the resulting CNTH photocatalyst was supported by electron paramagnetic resonance (EPR) and X-ray absorption near-edge structure spectroscopy (XANES). X-ray photoelectron spectroscopy (XPS) analysis showed that the C and N atoms were successfully self-doped into TiO2 lattice, which resulted in the CNTH materials exhibiting the enhanced absorption in the UV–visible region with a clear red shift at the absorption edges. The electronic structure of CNTH was examined by first-principle density functional theory (DFT) calculations, which further confirmed the optical properties of CNTH for the enhanced visible-light absorption properties and the red-shift phenomenon of the absorption edges. The resulting CNTH photocatalyst showed enhanced visible-light photocatalytic activity for H2 production. The enhanced activity of CNTH was due to the introduction of C and N self-doping and the existence of oxygen vacancies and Ti3+. The former can promote the photocatalytic reactions of the photogeneration electrons and holes on CNTH under visible-light irradiation, while the latter create more active sites and prolong the life time of electrons and holes by their donor states.