Tuning inter-crystalline void-like defects in nanowire clusters to TiO2

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Tuning Intercrystalline Void-like Defects in Nanowire Clusters to TiO2 Quantum Wires with Enhanced Photocatalytic Performance Ji-Hyeon Song,†,⊥ Sanjiv Sonkaria,‡,⊥ Byeongil Lee,§ Young Gyu Kim,§ Sung-Hoon Ahn,†,‡ Caroline S. Lee,∥ and Varsha Khare*,‡ †

Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanak-Ro 1, Gwanak-Gu, Seoul 08226, Korea Institute of Advanced Machinery and Design, Seoul National University, Gwanak-Ro 1, Gwanak-Gu, Seoul 08226, Korea § Department of Chemical and Biological Engineering, Seoul National University, Gwanak-Ro 1, Gwanak-Gu, Seoul 08226, Korea ∥ Department of Materials Engineering, Hanyang University, Ansan 15588, Korea

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‡

S Supporting Information *

ABSTRACT: Enthalpically driven dopant induced defects generated from the interaction of bulk titanium oxide and structure directed agents composed of charge separated ions have important consequences in directing interfacial energies in organometallic semiconductors. Such external factors imposed by the chemical environment at contrasting surfaces generated at bulk catalytic interfaces are capable of introducing lattice defects at the structural and morphological level. Here, we demonstrate that post-modification of primary defect sites of TiO2 nanowires from the bulk state in a group reactive ionic liquid (IL) environment has the potential to structurally redirect defect states altering both the order of dimensionality and electronic properties. This is demonstrated by the fabrication of surface modified quantum wires (SMoQWs) from zero-dimensional nanoclustered nanowires (NCNWs) formed under moderate temperature annealing and ambient pressure. This approach demonstrates that structural distortions that exist within crystal lattices of the NCNWs can generate new functionalities at non-equilibrium sites by modulating the mixed Ti3+/Ti4+ valence signature. The concentration of displaced oxygen molecules and their consumption have a direct impact on vacancy growth patterns, increasing the Ti3+/Ti4+ ratio at crystal sites when mediated by nitrogen rich species. Evidence shows that the growth confinement of TiO2 is locked in a one-dimensional geometrical configuration formed by a complex of caged Tiporphyrin clusters bridged by polyphenylquinoxiline linkers formed via O−Ti−N bonding. The importance of charge carrier separation and charge mobility was demonstrated by spatial reordering of functionalized TiO2 quantum wires via dye adsorption (N719). SMoQWs demonstrate superior photocatalytic degradation properties to the NCNWs, enhancing their utility in DSSC device applications. KEYWORDS: TiO2 quantum dots, quantum nanowires, nanoclustered nanowires, intercrystalline void, ionic liquids

1. INTRODUCTION Structural imperfections of metal oxides1 at the semiconductor surface account for much of the surface instabilities generated through surface modification processes. The defect imprint that is intrinsic to the material surface and subsurface can often be influenced or altered by fabrication processes locally or more extensively beyond regional areas that originate as point or area defects. The unpredictability of these irregularities defined by edges, cracks, steps, dislocations, and substitutional and interstitial defects2 and other surface contributions via temperature and pressure effects can be diminished by a better understanding of preferential lattice relaxations and motilities around deformed sites and surrounding vicinities. Such issues are of immense importance to electron-transfer dynamics of semiconductor materials of TiO2,3 which not only are sensitive to surface imperfections but also are affected by intrinsically © XXXX American Chemical Society

driven catalytic properties of TiO2 at heterogeneous interfacial boundaries introduced through dopants4 to increase visible light photocatalytic performance.5 The need for stabilization of high energy surface states often lead to drastic modifications in shape6 from bulk to the nanoscale characterized by precursor decomposition, dehydrogenation, functional group chemistries via impurities,7 polymerization, adsorption/desorption, and water effects with interesting shape driven geometrical, morphological, and size effects on TiO2. A better understanding of the chemical complexities associated with these changes will have important implications to accelerate the “discovery path” to controlling material properties desirably. Received: April 23, 2019 Accepted: July 3, 2019 Published: July 3, 2019 A

DOI: 10.1021/acsaem.9b00803 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

low-dimensional pillar-like nanocrystals of TiO 2 form aggregated clusters. This provided unhindered access to tune thermally induced defect sites trapped at boundary regions to engineer superior photocatalytic materials. In this context, the morphological transition of NCNWs to quantum wires (SMoQWs) was achieved through nitrogen doping of intercrystalline void spaces assisted by surface functionalization of the NCNWs in a nitrogen rich reaction medium comprising [EMIM][DCA]. While the Ti4+ atomic configuration of the NCNWs is largely preserved after IL surface modification, a 29% reduction in Ti4+ vacancy state is accompanied (offset) by a substantially sharp increase of 63% in T2O3 (Ti3+) oxygen vacancy sites of SMoQWs providing the necessary nonequilibrium sites to drive low dimensionality to the order of 2− 3 nm. This approach demonstrates that structural distortions that exist within crystal lattices can generate new functionalities at nonequilibrium sites by modulating the mixed valence boundary of Ti3+/Ti4+ signature pattern favoring oxygen vacancy evolution with narrowing of nanowires.

Accessibility to structural variants of TiO2 that are morphologically tuned to participate in charge transfer events is a key goal in dye sensitized solar cell (DSSC) fabrication. There is growing awareness that the dynamics of chemically steering the shape and size of TiO2 crystals relies on heterogeneous precursor chemistry which can be enthalpically driven and predictably controlled through specific interactions. We propose that the “ground-state” reaction environment should permit a “gateway” to such assemblies that advocates a strong dependency on a rich supply of nonassociating cationic and anionic one-dimensional structures to direct catalytically desorbed molecules to reassociate under spatial control and space confinement. Sequential ordering of chemically nucleating species targeted around binding sites on the semiconductor surface. This also provides the potential for multicompartmentalization from bulk to low-dimensional scales, imposing growth restriction around metal sites. Ordered confinement at material interfaces provides an architectural route to crystalline porous materials with selective pore size and shape which can be suitably adapted for charge transfer processes via a templating scaffold. Reaction environments that have the potential to impose intrinsic and extrinsic behavioral growth patterns affecting shape and size variability, surface energetics, site and group specific coordination, bond associations to enhance photocatalytic chemistry enabling semiconductor defect engineering8,9 are offering new alternatives. Such states of matter that are introduced from pathways that drive the larger bulk material to a nanoscale environment considerably affect interfacial properties particularly in contrasting materials. Weaker ionic associations at the interfacial boundary are enthalpically tunable at the precursor level and serve as building blocks to higher hierarchical structures. Hence, the utilization of functional group chemistry combined with the structural and physical characteristics of a nonrigid anionic− cationic template, self-assembly processes can be exploited to generate architectural features as “guest” structures in fabricated materials to effectively improve DSSC performance. Altering band-gap structure at grain boundary defect sites by chemical intervention offers the potential to considerably redirect charge transfer to more efficient routes.10 Imidiazolium type ionic liquids (ILs) that reportedly adhere to charge separation chemistry and ordinarily conform to anionic−cationic layering structurally sustained by short-range electrostatic bonding show multifunctional intrinsic and extrinsic behavior on supports as absorbed and chargebalanced bistable materials.11 Coordination strength of absorbed bonds at metal surfaces occurs as a function of metal specific complexation distinguished by the magnitude of N 1s binding energies.12 Nitrogen doped heterogeneity at defect sites within in the TiO2 lattice provides parametrical guidance13 to deliver heterogeneous to homogeneous mechanisms using nitrogen rich ionic liquid interactions. This has the potential to inspire and drive the assembly of more optically active materials assisted by surface catalytic events of Ti during C−N bond formation favoring lower energy defect structures such as pyrroles.14 In the case of TiO2, the structural organization of the IL,1ethyl-3-methylimidazolium dicyanamide [EMIM][DCA], at the IL−semiconductor interface has proved to be useful for tuning surface properties both molecularly and elementally impacting shape and size. We show that by applying a moderate temperature annealing phase at ambient pressure,

2. EXPERIMENTAL DETAILS 2.1. Synthesis and Characterization Methods of SMoQWs and NCNWs. In the present study nanowire were grown on FTO substrate (Fine Chemicals Industry Co., Ltd.) of 25 mm × 25 mm × 2.2 mm dimension and 10 Ω cm. The cleaning of the substrate was performed with acetone (Daejung Chemicals & Metals Co. Ltd.), isopropyl alcohol (Sigma-Aldrich), and deionized (DI) water. Titanium isopropoxide (Sigma-Aldrich) was used as the bulk titanium precursor for the synthesis of the NCNWs, and 1-ethyl-3methylimidazolium dicyanamide ([EMIM][DCA]) (synthesized in our lab) was used for the surface functionalization of NCNWs to generate surface modification of nanowires (SMoQWs). 2.2. Synthesis of SMoQWs and NCNWs. The NC nanowire synthesis was conducted by hydrothermal method at ambient pressure in a thermal furnace. However, a similar method was used by Akshay Kumar et al.15 where the reaction was pressure driven in an autoclave. In a typical reaction solution mixture was prepared by 1:1 mixing of 10 mL of DI water and 10 mL of concentrated hydrochloric acid (35%) followed by dropwise addition of titanium precursor resulting in a clear transparent solution. Furthermore, the substrate was dipped in the mixed solution and then placed in a thermal furnace at 120 and for a 2 h period. Prior to placement in a solution to initiate the reaction, the FTO substrate was ultrasonically cleaned sequentially in acetone, isopropyl alcohol, and DI water for 3 min each followed by drying under N2 flow. Further for the synthesis of NCNWs, well grown nanowire crystals were surface functionalized in [EMIM][DCA] ionic liquid. The functionalization was performed by a posttreatment of the NCNWs by dropwise addition of [EMIM][DCA] on well grown wires. Nanowire clusters were exposed to heat treatment at 90 °C for 30 min in a convection oven to obtain well functionalized SMoQWs. Additional [EMIM][DCA] used for functionalization was synthesized by the modified Jean’ne M. Shreeve’s method (details in Supporting Information). 2.3. Characterization of NCNWs and SMoQWs. All samples were equilibrated in ethanol. Gross structural and microstructural investigations of NCNWs and SMoQWs were performed with wideangle X-ray scattering using D8 Advance DAVINCI, Bruker diffractometer. The 2θ range was chosen from 3° to 50° with a 0.02° step. The detailed structural and microstructural characterization was done by use of an energy-filtering transmission electron microscope (TEM, LIBRA 120, Carl Zeiss) and high-resolution TEM (HRTEM, JEM-3000F, JEOL). Surface morphological features of the two samples were investigated by field-emission scanning electron microscope (SEM, SUPRA 55VP, Carl Zeiss). The chemical distribution in bulk was investigated by Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Scientific) in the wavenumber range of 4000−400 cm−1. X-ray photoelectron specB


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Figure 1. Comparative deconvoluted X-ray diffraction patterns of (a) NCNWs showing rutile TiO2 indicated by R as the major phase accompanied by Ti2O3 impurities at regions marked by T. (b) A reversal in surface properties shows Ti2O3 as the major phase in SMoQWs with traces of rutile TiO2 (Table S1). (c) Raman scattering of NCNWs and SMoQWs reveals major peak assignments residing at peak positions 150, 437, 605, and 616 cm−1 for SMoQWs and 159, 439, and 616 cm−1 for NCNWs. The Raman shifts reported here (red arrows indicate NCNWs, and blue arrows indicate SMoQWs) are numerically close to the Raman spectrum for rutile (indicated by the black arrows) where R signifies the rutile phase configuration from its precursor state. troscopy (XPS, AXIS-His, KRATOS) was used to investigate the surface chemical changes due to functionalization of nanowires. A Nd:YAG laser was used to obtain photoluminescence (PL) emission data with excitation beam set at 355 nm. The emission was collected using a i-ccd and pmt spectrometer (laser external trigger signal at a shutter exposure time of 25 ms). The transient PL was used to measure the decay time of these samples. Samples were cleaned in ethanol, and photocatalytic activity measurements were performed under UV−vis conditions. Samples were dissolved in 0.5 mM N719 dye solution and incubated for 2 h in the dark to achieve the equilibrium of dye absorption.

3. RESULTS AND DISCUSSION 3.1. One-Dimensional TiO2 Nanowires Are Chemically Tunable to a Quantum Wire Architecture from the Bulk State Geometry. Differential chemical selectivity from bulk to nucleation growth of titanium oxide in heteroaromatic and nonheteroaromatic reaction environments was pursued by driving the bulk titanium precursor mediated synthesis of NCNWs and SMoQWs, respectively. Phase transformation of the bulk was observed by examining the gross structure of the NCNWs and SMoQWs through deconvolution of the X-ray diffraction (XRD) patterns exposing the presence of a broad peak shown in Figure 1. The respective XRD scans of NCNWs and SMoQWs in Figure 1a and Figure 1b reveal that the intrinsic structure in NCNWs is associated with the TiO2 rutile phase, However, in the Ti2O3 phase, Ti3+ vacancies preferentially occupy lattice sites and dominate SMoQW type morphology. This dominance of Ti3+ vacancies results from the surface interaction of the NCNWs with [EMIM][DCA] by increasing the activation barrier for the formation of Ti3+ in SMoQWs (Figure 1 a, Table S1). Ionic liquid induced reversal of the dominant defect state observed here challenges the intrinsic nature of TiO2 in preferentially forming oxygen vacancy defects toward the more native configuration.16 Drastic changes to the surface properties of TiO2 are indicative of an altered electronic configuration that is chemically stabilized through the hybridization of nitrogen ligands that overlap with vacant metallic orbitals. The surface morphological trends in the microstructure between IL functionalized and nonfunctionalized nanowires were assessed using SEM imaging. Figure 2 shows the overall and detailed SEM image of NCNWs and SMoQWs. In Figure 2a−c, NCNWs are seen to form a porous network of nanopillars of clustered nanowire(s) with

Figure 2. SEM images showing the growth of nanopillars in NCNWs and SMoQWs. (a) Porous network of nanopillars in NCNWs. (b) Cauliflower-like individual units of pillars leading to a clustered growth of nanowires indicated in (c). Growth patterns show (d) multidirectional growth from a nucleus. (e) SEM image shows the occurrence of nanopillars grown from a bed of nanowires via surface modification resulting from chemical interaction proceeding to the necking of nanopillars into SMoQWs and evolving into (f) units of romanesco cauliflower-like morphologies. The inset (g) outlines the loss of wires and multidirectional growth patterns.

cauliflower-like morphology. These nanopillar-like morphologies that form unit structures of the cauliflower structure that C


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Figure 3. TEM images of nanowires equilibrated in ethanol solution. (a) Bright field micrograph of NCNWs indicates the clustering effect. (b) High resolution TEM images of nanowires of 4−5 nm thickness. (c) HRTEM image of NCNWs from separated regions showing the elongated nanoparticles (shown within white enclosures) of mainly rutile phase as correlated by FFTs data insets. (d) Inset shows selected regions for the bright field image analysis of elongated nanoparticles which are absent in SMoQWs. (e) HRTEM of SMoQWs shows nanowires of 2.5 nm thickness and a change in the interface. (f) HRTEM of free-standing single NCNWs of thickness of ∼7.42 and 2.94 nm in (g). (h) Visualization of a schematic representation of the possible mechanism of formation of nanopillar assemblies showing the evolution of NCNWs to SMoQWs transitioning from a to b, respectively.

growth characteristics of separated and narrowed architectures (Figure S1b and Figure S1d) approaching or within the quantum dimensional scale. Further, analysis by high resolution TEM (HRTEM) indicates the presence of well oriented nanowires shown in Figure 3. The bright field image of clustered nanowires of the NCNWs shows that the interface between the wires is clearly visible, indicated by the black arrows outlined in red in Figure 3b. It can be observed that the NCNWs are surrounded by spherical (4−5 nm) and elongated structures in the inset (Figure 3a). The HRTEM image from Figure 3a is shown in Figure 3b outlining visible streaks of individual wires highlighted by yellow dotted lines. The nanowires are about 5−7 nm thick with 5−10 nm rod-like nanoparticles (along axis) (Figure 3d). FFT analysis signifies a mixed rutile and anatase TiO2 random growth phase evidenced by analysis of selected regions encircled within white borders in Figure 3c. These nanoparticles however are not visible in SMoQWs (Figure 3e), suggesting their disappearance might be attributed to newly formed bond associations with the ionic liquid (IL) environment. Growth restriction and surface alteration of SMoQWs result in narrowing of nanowires in IL, and the FFT taken at the interface (FFT 2 of Figure 3e) reveals a degree of overlapping of wires. Double diffraction patterns from the existence of two interfaces are clearly established by FFT 2 in Figure 3e in contrast to FFT 1 (Figure 3e), which correlates to lattice fringes of singular wires showing a sharp diffraction pattern of the rutile phase of TiO2. It is noteworthy here that the thickness of nanowires is within the quantum confinement range of TiO2 and the growth restriction is a result of IL interactions in the reaction medium. The proposed mechanism nanopillar growth and transformation of NCNWs to SMoQWs is shown in Figure 3g formulated on the basis of detailed analysis by SEM and TEM.

possibly arise from the nucleation and multidirectional growth and exhibit well separated shapes that hide embedded nanowires are also visible under HRTEM (Figure 2d). In the presence of IL, structural modifications are marked by the loss of material from the surface and subsequent coalescence of the cauliflower assemblies through interfacial interactions between the metal oxide surface and the cationic−anionic charged IL surface. This variation in microstructure due to surface interface interactions in SMoQWs leads to an “interconnected” nanopillar-like network of assemblies via the removal of the nanowire film (Figure 2e) grown on FTO. Surface alterations are accompanied by sharp and spike contours observed on the surface of SMoQWs inducing shape change characteristics from cauliflower-like to a romanesco cauliflower architecture. Microstructural features shown in (g) indicate similar type of coalescence behavior of the romanesco cauliflower indicated by the disintegration of nanowires on the surface of interconnecting nanopillar films. The structural organization here provides some insight into the repositioning of nanowires on edges and cracks as evidence of drastic alliterations to the surface morphology. The possibility of unmasking more subtle effects of nitrogen doping on nanopillar growth was pursued with transmission electron microscopy (TEM). In contrast to SEM, the effects of [EMIM][DCA] mediated SMoQWs formation from NCNWs are more clearly visible by TEM showing in part the disassembly of aggregates as nanoparticles (detached regions) that may re-form into well faceted nanopillars. TEM images detailing structural and microstructural differences between NCNW and SMoQWs nanopillars (Figure S1) show aggregates of strongly bonded NCNWs with well faceted features (Figure S1a) and are of darker contrast to the network of clusters with indistinct facets in SMoQWs (Figure S1c). Surface functionalized structures of SMoQWs reveal different D


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ACS Applied Energy Materials Further, free-standing NCNWs and SMoQWs shown in Figure 3f and Figure 3h confirm the nanosize and quantum size range of individual wires. The mechanism shows that the growth of nanopillars is a simple two-step process combining the nucleation and growth of TiO2 NCNWs that further mature to well grown NCNWS. The morphological evolution of NCNWs to SMoQWS is the result of considerable surface modification of NCNWs at the ionic liquid interface. The HRTEM imaging in Figure 4 shows that cauliflower NCNWs

Figure 5. Schematic representation of surface modified quantum wires (SMoQWs) from nanoclustered nanowires (NCNWs). (a) Centers of nucleation form and (b) assemble into larger particles by the process of coalescence driven by Ostwald ripening and undergo (c) multidirectional growth into NCNWs and (d) removal of high energy weakly bonded particles to form (e) nanopillars of SMoQWs.

surface modified nanowires (SMoNWs) (Figure 5e) embedded in the cationic layer of the ionic liquid environment adopt crystal structures that are reminiscent of Romanesco cauliflower type architectures (Figure 5e) to their cauliflower counterparts formed in the absence of nitrogen precursor complexes or their dissociated constituents under thermal control. Single elongated 3D structures of rutile TiO2 have been reported to occur in highly acidic environments from precursor molecules,18 but the dimensional widths of TiO2 exceed, between 7 and 13, the order of TiO2 geometries reported here signifying considerable differences between the single crystal interaction of TiO2 with chloride and nitrogen anions at the nonmetal−metal interface. Elongation along the 110 plane is consistent with computed “short-lived” high energy growing surfaces19 that show higher dissipation rates compared to the more increasingly dominant slower contracting planes aligned to 002 observed here supporting a narrowing architecture. 3.2. Nanowire TiO2 Defects Are Precursor Sites for Complex Hieratical Assembly via Carbonization and Ti−N Bond Passivation. In this approach, we explored the extent to which intercrystalline voids introduced through moderate temperature annealing might contribute to the metastable content of nanocrystalline TiO2 and the role of nitrogen based impurities in atomically compensating metastable surfaces by lowering the free energy of potentially transient states through passivation of intrinsic defects entrapped in a crystalline phase. In this context, the change in the chemical shift pattern of binding energies was shown to be dominated by asymmetric Ti 2p3/2 shape peaks (Figure 6a and Table S2) corresponding to NCNWs and SMoQWs. The broadness of the peak Ti 2p1/2 (Figure 6a,b and Table S2) of the NCNWs compared to that observed for Ti 2p3/2 in SMoQWs is indicative of a change in the surface chemical state that correlates well with the full width at half-maximum (fwhm). NCNWs and SMoQWs reveal almost identical binding energy positions of the spin orbit doublets 2p3/2 and 2p1/2 around 458 and 463 eV (Figure 6a,b and Table S2) but are marked by differences in the peak areas corresponding to NCNWs and SMoQWs. The difference in separation energy of the doublet of Ti 2p spectra was determined to be 0.171 and 0.028 eV with respect to Ti 2p3/2 and Ti 2p1/2 of the Ti3+ atomic states. The effective increase in the surface population of Ti3+ defect states in SMoQWs was correlated to the reactivity of oxygen vacancy sites. This is expected to occur because of the rise in adsorbates and their chemical stabilization via TiO2 catalytic chemistry. Here, we sought

Figure 4. Colored HRTEM images depicting the growth mechanism of NCNWs. (a) Overall multidirectional growth of self-assembly of particles bound in neckless shape. (b) A closer look suggests that the individual particles grow via a nucleation and growth phenomenon that later follows. (c) Ostwald ripening process via diffusion at the weak interfaces of self-assembled particles in combination with the island growth at the nondiffusional surface.

units adhere to multidirectional necklace type self -assemblies that unfold from a growth epicenter of nuclei indicated by the red arrows in Figure 4b. In view of morphological growth characteristics of HRTEM shown in Figure 4b and Figure 4c (red arrows and white outlines), the genesis of necklace type of self-assemblies might be rationalized through a growth combination of individual particles via nucleation and Ostwald ripening. The Ostwald ripening process in the formation of self-assembled neckless growth likely proceeds via adaption to macro/nanoscale interfacial diffusion at weak interfaces between defect TiO2 crystals supported by Volmer−Weber growth regimes localized to those defect sites away from diffused interfaces favoring island growth (Figure 4c). High regions of instability result in defragmentation of modified surfaces that assemble as separated low dimensional SMoQW morphologies. This possibility extends the idea of spontaneous transformation of spherical assemblies to nanowire growth by removal of surfactant stabilizers at the semiconductor edge in alcoholic solvents17 which has broader implications in altering the surface chemistry of self-assembled linkers particularly at the water−solvent−metal interface. This is discussed further in subsequent sections. On the basis of this analysis, the assembly can be described by a mechanism that begins with centers of nucleation in which crystalline islands (Figure 5a) exhibit coalescence between neighboring crystal sites such that Ostwald ripening dominates generating larger radial particles (Figure 5b) that align along a multidirectional axis (Figure 5c) forming aggregates of nanoclustered nanowires (NCNWs) assisted by the surface chemistry between TiO2 and [EMIM][DCA]. The surface defragmentation of energetically unstable particles that surround the NCNWs forms a subnanometer layer of quantum confined morphologies that reassemble into a dispersed population of nanowired structures stabilized by complex heterocyclic pyridinic and pyrrolic ligand chemistry (discussed below). The morphologically driven structures of E


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(Figure 6d and Table S5) and Ti−N−O lattice bond arrangements, but diffraction patterns of nitrogen doped TiO2 have directly implicated stronger electron clouds represented by N−TiO2,20 suggesting a Ti centered metalloporphyrin-like arrangement. This type of metalloporphyrin arrangement is supported by N 1s core level photoelectron spectra specific to two different Ti−N complexed environments at the central porphyrin core at BEs of 398.097 and 400.06 eV (Figure 6c and Table S3) that affect the electronic configuration from a number of possibilities including substituents affecting binding.21 Further, FTIR bands at 3393 and 3464 signify N−H stretching vibration bands of pyrrolic structures22 (Figure 7) in which cyclization is catalytically assisted by the Ti surface via C−N bond formation in a nitrogen rich environment.14

Figure 6. Comparative deconvoluted core level XPS pattern of (a) Ti 2p of NCNWs, (b) Ti 2p of SMoQWs, (c) N 1s of SMoQWs, and (d)) N 1s of NCNWs. The peak patterns are numbered sequentially in (a) to (d), indicating Ti4+ and Ti3+ states in (a) and (b), respectively. The nature of the titanium−nitrogen bonded configurations in SMoQWs and NCNWs is revealed by the binding energies (BEs) in (c) and (d), respectively.

evidence for the surface passivation of Ti3+ defect sites using XPS and FTIR after functionalization with [EMIM][DCA]. The binding energy distributions of N 1s (Figure 6c, Figure S2, and Table S3) for SMoQWs and NCNWs show N substitution of O lattice positions and is evidenced to occur at crystalline nanocluster sites of NCNWs close to BEs of 396−399 eV (Figure 6c and Table S3). This is due to the strong anionic nature of nitrogen with the ability to induce paramagnetic shifts and core level changes at the nitrogen−titanium oxide interface which affect energy gaps. This declares that oxygen diffusion from bulk surfaces and those that migrate at crystal void sites both being nitrogen directed events can generate Ti3+ sites from localized growth escalating to more abundant regions across the lattice. However, growth mechanisms and the nature of interactions largely depend on the precursor structure. XPS of SMoQWs shows that binding energies around O 1s 530 and 532 eV (Figure 6d and Table S4) are exclusive to NCNWs−[EMIM][DCA] interactions and originate from nonstoichiometric interfacial atomic oxygen sites. These are characteristic of oxygen deficient Ti3+ point vacancies associated with the high occupancy of N atoms or their complexes from the rich nitrogen environment and unsaturated coordination from the substitutional loss of oxygen at TiO2. By their nature, these vacancy sites equally function as an index of charge imbalance at the lattice surface and operate as strong indicators of reactive O22−, CO32− and OH− species that can react with small hydrocarbon building blocks at titanium oxide catalytic sites. Selective surface modification at the crystalline surface of the NCNWs was dominant preferentially at adsorption sites specific to the formation of porphyrin 5,10,15,20-tetraphenylporphyrin (C44H30N4) assemblies around BEs of 398.097 and 400.06 eV of the N 1s spectra (Figure 6c and Table S3). The relative high carbon content of C44H30N4 is reflected by the C 1s core spectra located at 284.5 eV (Table S4). This inference is also compatible with O 1s BEs positioned at 531.4 eV

Figure 7. Comparative Fourier transform infrared (FTIR) spectroscopy of zero-dimensional nanoclustered nanowires (NCNWs) and surface modified quantum wires (SMoQWs).

This behavioral property has been explicitly attributed to the anionic characteristic of the dicyanamide structure in previous works from the onset of nucleation23 and has been discussed conceptually9 in which new material functionalities can be acquired by driving the dimensionality to the quantum scale. Here, the restricted growth of nanowires to 2−3 nm in SMoQWs at void positions suggests a general growth phenomenon of dicyanamide chemistry at the TiO2 bulk precursor surface favoring the catalytic assembly of nanocage porphyrin structures from self-assembled pyrrolic units. The central core of the metalloporphyrin architecture is defined by the metal−nitrogen porphyrin plane and conforms to a significant Ti−N bonding stretch at 396.9 eV (Figure 6c and Table S3) forming at the substitutional Ti3+ with the metal at the center of the porphyrin ring. This energy peak is inferred to be a key route to improve photocatalysis via N-substituted atomic states.24 Porphyrin hybrids of this has type have been very rarely reported in comparison to other metal−porphyrin hybrids. The work demonstrably shows here that the passivation of voids through populating of Ti3+ sites with densely rich N− complexes from the reaction medium can occur in the absence of a primary nucleation phase at low temperatures at the nanoscale, driving quantum scale confinement of TiO2 into a pre-existing crystalline assembly. The dissociation of secondary crystals originating from localized regions of the parent structure adopts a mechanism of assembly assisted by the more stable cationic imidazolium ring clusters in intercalated arrangements at the solid−liquid interface of organometallics.25 This conclusion is supported by large C−H stretching frequencies assigned to the imidazolium F


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Figure 8. Differences in surface states and charge transfer properties of NCNWs and SMoQWs were investigated by (a) photoluminescence emission and (b) photoluminescent transient decay measurements. Metalloporphyrin SMoQWs show distinguishable surface characteristics to NCNWs that demonstrate a strong dependency on Ti3+/Ti4+ defect states. The low Ti3+/Ti4+ ratio of NCNW relative to SMoQW is a dominant feature in driving rapid charge transfer in the absence of dye N719.

activity, the preferential location of nitrogen adsorbed onto the TiO2 surface impairs current generation in a concentration dependent manner.28 The interplay between thermal processes and structural composition of the nitrogen source and the choice of methods to drive the interfacial chemistry become critically important determinants in a host of complex phase driven reactions. An aspect of this scenario might explain the effect of the polyphenylquinoxiline linker coupled to TiO2 via O−Ti−N bridging through which the charge density on nitrogen is depleted resulting from the electronegative pull of oxygen at the anchor sites and the subsequent shift in the charge balance and alignment properties around the central titanium atom in the porphyrin center. Improvement in the surface exciton dynamics of transient states and the diffusion properties of electrons could be improved through dye adsorption. The emission intensity profiles of SMoQWs and the NCNWs shown in Figure 8a depict the photoluminescence characteristics of the respective quantum wires and nanowires and establish that their excitation dependency occurs in the visible region. The adequate supply of electrons ensured that valence−conduction band transition exceeded the energy difference of the band-gap of both samples through the excitation of photons at 355 nm (3.4925 eV) above the threshold wavelength of TiO2 (>3.2 eV). It can be clearly inferred that the surface states of both materials are sensitive to contrasting degrees in behavior, and the result confirms that surface interaction at the NCNW−nitrogen interface alters its intrinsic nature to a secondary crystallization phase as supported by TEM and XRD. The considerable red shift to unmodified TiO2 is consistent with two emission peaks at 424 and 507 nm in SMoQWs (Figure 8a) that span the wavelength region from 375 to 759 nm. This differs considerably from single emission broad peak at 463 nm of the NCNWs (Figure 8a). The contrasting difference in PL performance advocates that the charge transfer properties of TiO2 quantum wires in the absence of dye are exceptionally hindered as a direct consequence of the structural features introduced during surface modification with [EMIM][DCA]. The relative high PL intensity in SMoQWs is indicative of a high state of recombination in the absence of dye after photon absorption and generates a loss in charge transfer properties. However, improving the orientation of porphyrin molecules at the TiO2 surface can impart a shielding effect offering better resistance to recombination effects.29 The substantially suppressed

C5N ring configuration or associated alkyl moieties at 1088 cm−1 26 and 1323 cm−1 (Figure 7) associated with sizable C− N asymmetric vibrations that extend from 1066 to 1130 cm−1 (Figure 7). The low temperature binding functionality of the imidazolium ring in SMoQWs is better resolved at the interfacial surfaces as judged from the FTIR spectral bond vibration between the positively charged imidazolium moieties and the carboxylate groups at the TiO2 surface at 1171 cm−1 (Figure 7), revealing a sharp spectral band between surface carboxylates (COO̅ ) and protonated N+ of the EMIM ring clusters. Although the vibration bands around 1084 and 1088 cm−1 (Figure 7) are observed in the NCNWs which are likely due their metallic origin, they are substantially enhanced in the presence of nitrogen complexes or chemically catalyzed pyrrolic products or other low temperature induced pyrolytic yields. Further, peak splittings between 3010 and 3184 cm−1 at 3035, 3112, and 3156 cm−1 (Figure 7) are all synonymic with C−H vibrational peaks and the last band in addition to the C− H stretch of ringed configuration also shows additional compatibility with the deprotonated state of the imidazolium group above 3040 cm−1 (Figure 7).27 See Supporting Information for a detailed discussion. 3.3. Photoluminescence (PL) Shows Enhancement in Charge Carrier Properties of SMoQWs Tunable by Surface Modification of NCNWs. The photoluminescence decay times for as-synthesized NCNWs and SMoQWs are shown in Figure 8b. The radiative decay in electron transfer for SMoQWs occurs between 200 and 500 ns (τSMoQWs = 300 ns) on an exponential scale, while for the NCNWs, it is undectable and instantaneous quenching may be attributed to very rapid decay patterns for much shorter durations possibly due to fast diffusion properties on the picosecond time scale. This observation highlights the importance of the surface structural order between the assembly of TiO2 centered porphyrin and the effective relaxation of Ti3+ sites mediated by N 2p−Ti formed through hydrocarbon complexes. In SMoQWs, the transfer of electrons across the cationic organic layer and metalloporphyrin interface dominates the exterior surface of TiO2 perturbing electron transfer across the interface. The transient PL (Figure 8b) semiconductor properties show consistency with PL emission data Figure 8a (discussed below), and this interpretation is supported by the low energy of the excited state correlating to slower charge transfer rates with the red-shift behavior of the hybrid crystal states. While nitrogen improves the visible photocatalytic G


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hypothesis. We extend this argument to the electron−hole proximity of excited states located close to the conduction band. The architectural feature of porphyrin present in our self-assembled structure likely enables localized electron tunneling via frontier orbitals. Also, the chemically formed polyphenylquinoxiline ligand formed through self-assembly facilities this process via the spatial organization of porphyrin bound TiO2 molecules. This mechanism drives the fast separation of electron−hole pairs via the tunneling effect at the porphyrin−semiconductor interface aided by the nanowire morphology which results in the prolonged PL decay time observed for SMoQWs. 3.4. Hieratical Self-Assembly Supports Improved Spatial Ordering and Charge Transfer Dynamics in SMoQWs. In order to investigate the catalyst dye interaction, we applied direct methods (total organic content − chemical oxygen demand (TOC)) to estimate the total organic content in the solution. However, the presence of organic constituents prevented an accurate estimation. For this reason, indirect measurements were used to provide some insight toward understanding interaction between the dye and catalyst and associated absorption events. UV−vis spectroscopy was used to monitor the adequate binding of dye molecules to the metal oxide surface by observing the shift in the absorption bands to the visible light region and to establish the role of the IL functionalized surface of SMoQWs in enhancing charge transfer processes between porphyrin−TiO2 and the ligand environment during visible photoexcitation. To this effect, the photocatalytic activities of the NCNWs (Figure 9a) and

radiative recombination rate observed for the NCNWs represents almost a 7-fold reduction (Figure 8a) to SMoQWs at the peak maxima favoring a more conducive role for Ti3+ defect states in nanoclusters to the nitrogen passivated defect states of quantum wires. However, we have shown that this impediment to structure−function performance has been observed previously in our lab and is attributed to the poor absorptive state of porphyrin that influences the surface properties of SMoQWs in the Soret region of 380−500 nm. This finding is consistent with a conclusive study that predicts that a Ti4+ centered metalloporphyrin configuration can markedly favor the use of axial ligands by influencing the HOMO−LUMO energy gap levels via the modulation of frontal orbital interaction and their associated shape characteristics.30 This has significant implications for self-assembled ligand structures that actively participate in donor−acceptor bond complexation reducing barriers to charge transfer events. X-ray photoelectron spectroscopy revealed the cosynthesis of polyphenylquinoxiline cojointly with porphyrin contributed by the N 1s binding energies at 399.23 and 401.01 eV (Figure 6c and Table S3) reflecting the respective polymer and pyridinic characteristics. Polyphenylquinoxiline has been shown to exhibit self-polymerizable properties from quinoxiline monomers31 and has been long recognized as an electron transporting conjugated polymer coupled with electroluminescence properties.32 The chemical heterogeneity of nitrogen as an elemental dopant shows significant binding energy contributions in the range of 396−402 eV,13 which is highly evident in this investigation. While the larger part of the influence of nitrogen occurs in the region assigned to porphyrin which chemically adheres to the TiO2 surface at substitutional sites across the N 1s binding energy contour from 398 to 400 eV, the notable increase in the BE from 399 to 402 eV is correlated to O−Ti−N (Figure 6c and Table S3) and preferred sites are of an interstitial nature.33 However, Ti− O−N and Ti−O−N−O linkages are also generated by oxygen substitution reflecting the heterogeneity of N-doping events at low temperatures.34 The charge flexibility around the polyphenylquinoxiline structure modulated by the nature of the N-dopant anion affecting the cationic/neutral charge ratio35 and the interstitial space allows the oxidation state dynamics to occur along the quantum wire. In view of the opinion that the design features of porphyrin conjugates likely govern their chemical and physical organization at the metal surface that strongly impact recombination processes, we proceeded to examine whether the structural arrangements between self-assembled porphyrin ring and polyphenylquinoxiline ligand were structurally flexible and sufficiently compatible with the industry standard dye product N719 (cisbis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) bistetrabutylammonium) in driving photoinduced charge separation in SMoQWs relative to NCNWs. Better insight of the transfer dynamics was obtained from the deconvolution of the two broad spectral PL data peaks for SMoQWs into eight Gaussian bands (Table S6). The electronic states identified for SMoQWs are clearly absent in NCNWs. Differences in photoexcitation behavior and hence PL intensity stem from interactions at the TiO2−porphyrin interface and the associated surface and deep trapped states. The difference in the configuration of states between SMoQWs and NCNWs drives e−−h+ separation very differently from each other, The structural basis of this argument is supported by the work of Andrade et al.36 explained via a tunneling

Figure 9. Photocatalytic efficiency degradation of (a) NCNWs and (b) SMoQWs in the presence of ruthenium N179 based dye monitored by UV−vis spectroscopy. The role reversal in charge transfer properties of ruthenium based dye (N179) is shown in (c). (d) Comparative kinetic plot of the residual photocatalytic efficiency monitored by the degradation of dye N719 with NCNWs and SMoQWs. Inset in (d) shows the comparative absorbance with time plot.

SMoQWs (Figure 9b) were monitored by degradation of a ruthenium based dye (N179) in the presence of NCNWs and SMoQWs under solar light exposure at time intervals ranging between 0 and 15 min. The dye-led UV−vis degradation profile follows an absorption spectroscopic pattern typical of bis(4,4′-dicarboxy-2,2′-bipyridine)ruthenium(II) (abbreviated H


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Figure 10. Comparative Fourier transform infrared (FTIR) spectroscopy of zero-dimensional nanoclustered nanowires (NCNWs) and surface modified quantum wires (SMoQWs) between 1000 and 4000 cm−1 following dye degradation over a 15 min exposure period. Black and red lines correspond to NCNW and SMoQW, respectively.

Figure 11. Bright field TEM images show the overall morphology of (a) NCNW and (b) SMoQW after dye absorption. The inset shows the mechanism of dye absorption on the two catalyst assemblies. HRTEM images of (c) nanoparticle clusters attached to NCNWs in TiO2−dye complex in NCNW reveal TiO2 metal oxide sites and (d) single SMoQWs with increased thickness possibly due to absorption of dye on the surface of SMoQWs. Although the catalyst retains the overall flower-like morphology, NCNWs are surrounded by nanoparticle clouds while in SMoQWS fragmentation of petals at the surface is visible. TEM attacked EDS elemental mapping of ruthenium (Ru) in SMoQWs after solar radiation exposure (e) for 5 min indicating presence of equally distributed Ru within the area shown in inset I1 (f) for 15 min indicating drastic reduction in Ru in the area shown in inset I3. The elemental composition of Ru shown in the insets indicates (I2) high Ru content after 5 min (I4) and drastically reduced Ru content after 15 min. HRTEM images showing evolution of size reduction with time (g−i) with insets showing details of single wire (j) analysis of size reduction with exposure time. All scale bars are 5 nm.

as Ru(dcbpy)2(NCS)2 or N71937 characterized by maxima absorption peaks at 521 and 383 nm which correspond to metal-to-ligand charge transfer while 309 nm signifies ligandto-ligand charge transfer (Figure 9c).38 The analysis shows that the reduction of the peak absorption maxima after the adsorption of dye in SMoQWs occurs spontaneously on exposure to UV light such that the initial kinetic contribution to dye degradation is substantial in permitting only 15.8% detection of photocatalytic activity relative to the NCNWs. In the presence of dye, we note a role reversal in the performance of the NCNWs and SMoQWs reflecting the dynamic spectral properties from nanosized to quantum-sized morphologies during dye degradation of the ruthenium based dye, N719. This trend is also observed under “UV−visible” conditions (Figure 9c) demonstrating that SMoQWs are associated with enhanced activity in the absence and presence of sunlight. Further, a red shift in the peak maxima indicates charge

transfer initiated at lower energies in NCNWs within the presence of dye. The larger shift however is only weakly active for the degradation of dye compared to SMoQWs. The kinetic plot shown in Figure 9d allows the estimation of the lower limit of dye degradation by SMoQWs to a value 8 times faster than NCNWs. Further, we used FT-IR analysis to investigate the intermediate components of photocatalytic activity by establishing the binding mode of the Ru dye to the TiO2 surface due to the existence of the resonance forms of the thiocyanate ion of the dye shown in Figure 10. The FT-IR spectrum in Figure 10 shows two possible binding orientations of the dye as either (1) metal−C−N or (2) metal−N−S binding coordination. The vibrational stretching frequencies for both dye-treated SMoQWs and NCNWs occur at 2116 cm−1 which establishes that metal binding is coordinated via the C−N bond.39 However, the absorption band previously I


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ACS Applied Energy Materials identified around 1998 cm−1 but observed here as the broader shoulder of peak 2116 cm−1 (Figure 10) is associated with N− S binding for SMoQWs and NCNWs.40 From the peak spectra we conclude that N179 dye is predominately bound via the C−N orientation in both SMoQWs and NCNWs although notably more for SMoQWs showing a distinctive absorption peak at 1995 cm−1 (Figure 10). Information related to TiO2 surface stability, dye interaction, and sensitization of NCNWs and SMoQWs with the ruthenium complex of N719 can be affected by dye product accumulation during charge transfer reactions. Identification of the products of dye degradation by TOC − COD proved difficult due to interference by organic matter. However, the work by Grünwald and Tributsch40 shows that Ru(bcy)(CN)2, where bcy is 2,2′-bipyridine, is a major fragment lost following dye absorption that may have some significance with the merged smaller peaks with the metal−C−N (2116 cm−1) emerging at 2013 and 2104 cm−1 (Figure 10). There is also evidence that more complex structures, such as [Ru(bcy)(CN)2]Ru(bpca)2]2 where bcpa is 2,2-bipyridine-4,4-dicarboxylate, are also liberated around 2095 cm−1. Degradation is accompanied by CO/CO2 adsorption at the TiO2 surface at 2166 and 2178 cm−1 (Figure 10) with respect to NCNWs and SMoQWs during the detachment of pyridinc rings of Ru(bcy)(CN)2 via COO− linkage to the TiO2 surface. Among the numerous weaker vibrations, FTIR shows a broad asymmetric band around 2010−2158 cm−1 (Figure 10). The asymmetric nature and broadness of the peak can result from overlapping chemistry of SCN and NCS vibrational bands which arise between 2175−2140 and 1990−2140 cm−1 (Figure 10). This is expected due to the binding of ligands at the Ru center. Further, smaller peak depth in SMoQWs indicates enhanced activity to dissociate the thiocynate ligand which is weakest in N719 dye. Figure 11a−d shows the overall morphological pattern of catalytic surfaces by HRTEM following dye adsorption. The images reveal the microstructural differences in the cauliflower type NCNW (Figure 11a) and SMoQW (Figure 11b) after 15 min of solar light irradiation. The surface modification of cauliflower NCNWs transitions to compact clustered shapes forming quantum wires (SMoQWs) that self-assemble into dispersed petal-like structures. The disaggregation of the “cloud”-like appearance of NCNW clusters under prolonged exposure of solar energy uncovers the active metal regions of TiO2 (Figure 11c). The inset in Figure 11b captures the evolution of SMoQWs into thin wires (inset of Figure 11b) emerging from the petal-like morphologies in which the HRTEM lattice fringes correspond to TiO2.. This suggests that the progression to quantum wires under simulated sunlight likely reveals a kinetically driven codependency between (1) the unmasking of TiO2 sites, (2) the supply of nitrogen and carbon-rich structures by the surrounding IL environment and their disassembly to a pool of reactive monomers as the building blocks to hierarchical structures via (3) metal oxide (TiO2) catalytic activation of C− H, C−C, and Ti−N bonds at interfacial nucleation sites. This provides an ordered route to directing particle attachment and caging effects that exert growth restriction around metal oxide nucleation sites. However, the fast kinetic growth of SMoQWs did not permit an informed investigation of intermediate products, but we are able to explore microstructural and elemental changes due to UV exposure. Ruthenium elemental mapping of the SMoQWs after 5 and 15 min of UV exposure is shown in Figure 11e and Figure 11f. Under UV exposure, the concentration of Ru reduces by a third, indicating efficient dye

degradation in contrast to the interaction at the dye−NCNW interface. This difference might signify (a) an organometallic hierarchical assembly that favors superior charge transfer properties and (b) spatial ordering at quantum scale dimensions initiated by the templating properties of the IL surroundings. It is also notable that under UV exposure (≤15 min), the diameter of TiO2 wires is comparably smaller than their original size after dye absorption undergoing size reduction by a factor of 150% to ∼1 nm. The striking difference in the charge transfer properties between NCNW and SMoQW incubated in excess with dye is shown in Figure 12 along with the control dye dissolved in ethanol alone. The

Figure 12. UV absorbance profile comparing the degradation properties of dye adsorbed NCNW (unmodified) and SMoQW in ethanol. Incubation of the dye with the respective TiO2 complexes shows that the associated dye peaks at 400 and 540 nm are substantially reduced by SMoQWs but not in the presence of the NCNW complex. The small peak shift in dye adsorbed NCNWs compared to the control sample (N719 in ethanol) indicates that the interaction at the organometallic−dye interface does not result in dye degradation.

quantum confinement of TiO2 within the porphyrin cavity generates an efficient geometrically aligned “passageway” for the mobility of charge carriers potentially improving charge separation properties. 3.5. Insight into the Mechanism of Self-Assembly of Porphyrin Mediated Charge Transfer in Caged TiO2. Previous9 and current41 work suggests that the complexity of hierarchical structural assembly brings into interplay key events that have central importance to low-dimensional growth including (1) nitrogen passivation of the TiO2 surface via Ti−N bonds at oxygen vacancy sites and (2) carbon−carbon and carbon−hydrogen bond activation at the IL−metal oxide interface catalytically driven at TiO2 complementary sites leading to (3) monomer assembly directed by space confinement around newly emerging nucleation sites. The results of this approach suggest that passivation of unoccupied regions at the TiO2 sites in a nitrogen rich environment can effectively impose growth restriction of secondary nucleating sites on preassembled TiO2 nanowires that alters size, morphological shape, and functional properties of an existing structure. In this study, evidence favors multigrowth coordinated cooperativity which is a typical hallmark of synergistic bonding networks in self-assembly to effectively steer to build compartmentalized carbon−hydrogen shells and interlinking bridges to stabilize J


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cosensitization of the SMoQW assembly demonstrating that nitrogen doping of NC wires localized in void spaces can provide accessibility to defects to tune the Ti3+/Ti4+ ratio around self-assembled π-conjugated structures with growth restricting properties. Experimental observations have shown that solvation by ethanol at the rutile TiO2 surface can possibly exist as a chemically modulating factor influencing the performance of SMoNWs. The photooxidation of ethanol to its congruent ethoxide (CH3CH2O) form assisted by the TiO2 catalytic surface can enable its participation as a hole (h+) scavenger which is more effective at nondefect sites of the TiO2 lattice structure,44 and the highest performance was notable at TiO2(110) in their investigation supported by other independent studies.45 The findings here bring forth the question on the feasibility of engineering of void defects present in pre-existing crystals particularly in driving lowdimensional quantum material growth with enhanced photocatalytic performance. The opinion that structured sites within rutile TiO2 lattice offer superior surface morphologies to defect/void regions in driving efficient and more active photocatalytic reactions in ethanol is decidedly true for unmodified surfaces44 but does not necessarily hold true for mature TiO2 crystals in which void sites can be engineered to generate better performing materials. We show here that careful selection of modifying agents and their interaction with defect sites at the primary crystal interface can be used to stabilize secondary structures derived from the parent crystal improving shape and size characteristics inducing novel ligand structures that exhibit better overall performance.

supramolecular growth. Such an arrangement might be visualized to form a periodic assembly shown in Figure 12 in which TiO2 centered metalloporphyrins are separated by bridging linkers as evidenced by XPS. Photodegradation data demonstrate that the change in the molecular environment during the morphological transition from NCNW to SMoQW leads to increased photoactivation in electron transfer between the excitable porphyrin shell and TiO2 in the dye complex. Enhancement in the charge transfer properties arises from probable effects of quantum confinement of charge carriers in SMoQWs that is hindered in the absence of dye molecules. The confinement of TiO2 quantum wires self-assembled within porphyrin molecules is consistent with the dispersed wire-like morphologies observed in TEM that assemble into parallel arrangements sporadically in the ionic liquid environment. The nonaggregated collection of the complexed organometallic quantum wires shown in Figure 12 presents a possible crystal arrangement that makes use of the binding capabilities of benzoheterocyclic structures such as phenylquinoxaline type derivatives with Ti metallic centers42 that can form extended moieties of porphyrin via alkyne triple bonds.29 This hypothesis is further evidenced by FTIR assignments of the broad vibrational band at 3407 cm−1 (Figure 8) that is a functional characteristic of CH−H bonding supporting a geometrical configuration between the porphyrin shell and the bridging linker comprising phenylquinoxaline units. The possible mechanistic implications in extending the role play of phenylquinoxaline in a donor−π− acceptor configuration and the need for better spatial architecture for electron charge transfer processes, one that would explain the superior photodegradation properties of SMoQWs, rest on the probable role of the cationic [EMIM][DCA] molecule. Sensitizing properties of organic ligands can considerably compensate the low retention of dye N719 on TiO2 rutile structures of low surface areas43 or highly congested porphyrin quantum wires. The interfacial alignment of the TiO2−porphyrin complex might contribute to a better spatial divide formed from a polymerized assembly involving the complexation of imidazolium with phenylquinoxaline vibrational bands at 3150, 3104, and 2900 cm−1 associated with imidazolium moieties positioned at [C4, 5-H], [C2-H], and the alkyl [(−CH3 and CH2CH3)]27 that are considerably red-shifted to 3155, 3116, and 2975 cm−1 that suggests increased stabilization of the imidazolium ring to a lower energy structure marked by significant changes to the C−H environment around the ring structure. Enhancement in the delocalization across the imidazolium−phenylquinoxaline complex is consistent with this observation and the strong presence of CN at 1572 cm−1 that is completely absent in NCNWs. The presence of the CN stretch significantly changes the electronic distribution of the imidazole ring in the complex. The anchoring of these groups at low temperature caters for a structural arrangement that results in a visible shift of the complex to lower energies shown by the PL emission profile (Figure 9a). The reversal in the enhancement of charge properties between the NCNWs and SMoQWs shows a strong dependency on the surface chemistry and better photovoltaic efficiency in the presence of the dye. This is likely explained by structural compatibility in the positioning of the interfacial alignment between the band-gap energies of the porphyrin− TiO2 complex, bridging linkers, and N719 dye molecules. The residual time−absorbance optical plot shown in (Figure 8b) shows rapid degradation of the dye molecules with increased

4. CONCLUDING COMMENTS The work demonstrates the importance of heterocyclic chemistry of nitrogen rich ionic liquids and the incorporation of nitrogen at the IL-TiO2 liquid−solid interface and the interaction of self-assembled structures in quantum confinement of TiO2. These findings provide alternative routes to advancing photocatalytic performance at the photoanode in DSSC by decoration of the photoanode structure via surface modification enabling better nanostructuring and exposure to the effects of low dimensionality, defect geometries, hierarchical layering resulting from shape and size interactions generating new heterocoordination sites and dependencies. The use of ionic liquid as a soft matter material has key advantages in shaping the topography of bulk to nano nucleated crystalline semiconductor solids as demonstrated by lateral growth of nanoclusters to quantum nanowires. The reactivity of heterocyclic compounds is of special interest within nanostructured surfaces due to associations that result in the reduction and coordination number leading to increase in photocatalytic activity that originate from changes associated with different shapes and dimensionality.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00803. Detailed information on materials and methods, supplementary figures and tables originating from structural information (XPS and TEM), additional references, and further mechanistic interpretation (PDF) K


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(12) Caporali, S.; Pedio, M.; Chiappe, C.; Pomelli, C. S.; Acres, R. G.; Bardi, U. Surface study of metal-containing ionic liquids by means of photoemission and absorption spectroscopies. Surf. Sci. 2016, 648, 360−365. (13) Viswanathan, B.; Krishanmurthy, K. R. Nitrogen Incorporation in TiO2: Does It Make a Visible Light Photo-Active Material? Int. J. Photoenergy 2012, 2012, 1−10. (14) Gilbert, Z. W.; Hue, R. J.; Tonks, I. A. Catalytic formal [2+2+1] synthesis of pyrroles from alkynes and diazenes via TiII/TiIV redox catalysis. Nat. Chem. 2016, 8, 63. (15) Kumar, A.; Madaria, A. R.; Zhou, C. Growth of Aligned SingleCrystalline Rutile TiO2 Nanowires on Arbitrary Substrates and Their Application in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2010, 114 (17), 7787−7792. (16) Abdelouahed, S.; McKenna, K. P. Relevance of non-equilibrium defect generation processes to resistive switching in TiO2. J. Appl. Phys. 2015, 118 (13), 134103. (17) Tang, Z.; Kotov, N. A.; Giersig, M. Spontaneous Organization of Single CdTe Nanoparticles into Luminescent Nanowires. Science 2002, 297 (5579), 237−240. (18) Lin, J.; Heo, Y.-U.; Nattestad, A.; Sun, Z.; Wang, L.; Kim, J. H.; Dou, S. X. 3D Hierarchical Rutile TiO2 and Metal-free Organic Sensitizer Producing Dye-sensitized Solar Cells 8.6% Conversion Efficiency. Sci. Rep. 2015, 4, 5769. (19) Morgan, B. J.; Scanlon, D. O.; Watson, G. W. Small polarons in Nb- and Ta-doped rutile and anatase TiO2. J. Mater. Chem. 2009, 19 (29), 5175−5178. (20) Agyeman, D. A.; Song, K.; Kang, S. H.; Jo, M. R.; Cho, E.; Kang, Y.-M. An improved catalytic effect of nitrogen-doped TiO2 nanofibers for rechargeable Li-O2 batteries; the role of oxidation states and vacancies on the surface. J. Mater. Chem. A 2015, 3 (45), 22557−22563. (21) Gassman, P. G.; Ghosh, A.; Almlof, J. Electronic effects of peripheral substituents in porphyrins: x-ray photoelectron spectroscopy and ab initio self-consistent field calculations. J. Am. Chem. Soc. 1992, 114 (25), 9990−10000. (22) Kawamata, K.; Chowdhury, P. K.; Ito, F.; Sugawara, K.-i.; Nakanaga, T. Investigation of the N-H Stretching Vibrations of the Aniline-Pyrrole Binary Complex and Its Cation by Infrared Depletion Spectroscopy. J. Phys. Chem. A 1998, 102 (25), 4788−4793. (23) Sonkaria, S.; Kang, S.; Kim, S.-Y.; Choi, D.; Lee, C. S.; Ahn, S.H.; Khare, V. Quantum multi-compartmentalisation of non-photoactivated TiO2 via an extended nanoporphyrin metal-organic ring assembly. In preparation, 2016. (24) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293 (5528), 269−271. (25) Sonkaria, S.; Kim, H.-T.; Kim, S.-Y.; Kumari, N.; Kim, Y. G.; Khare, V.; Ahn, S.-H. Ionic liquid-induced synthesis of a graphene intercalated ferrocene nanocatalyst and its environmental application. Appl. Catal., B 2016, 182, 326−335. (26) Guleria, A.; Singh, A. K.; Adhikari, S. Optical properties of irradiated imidazolium based room temperature ionic liquids: new microscopic insights into the radiation induced mutations. Phys. Chem. Chem. Phys. 2015, 17 (16), 11053−11061. (27) Zhang, Q.-G.; Wang, N.-N.; Yu, Z.-W. The Hydrogen Bonding Interactions between the Ionic Liquid 1-Ethyl-3-Methylimidazolium Ethyl Sulfate and Water. J. Phys. Chem. B 2010, 114 (14), 4747−4754. (28) Wang, J.; Tapio, K.; Habert, A.; Sorgues, S.; Colbeau-Justin, C.; Ratier, B.; Scarisoreanu, M.; Toppari, J.; Herlin-Boime, N.; Bouclé, J. Influence of Nitrogen Doping on Device Operation for TiO2-Based Solid-State Dye-Sensitized Solar Cells: Photo-Physics from Materials to Devices. Nanomaterials 2016, 6 (3), 35. (29) Si, L.; He, H.; Zhu, K. 8-Hydroxylquinoline-conjugated porphyrins as broadband light absorbers for dye-sensitized solar cells. New J. Chem. 2014, 38 (4), 1565−1572. (30) Ornso, K. B.; Pedersen, C. S.; Garcia-Lastra, J. M.; Thygesen, K. S. Optimizing porphyrins for dye sensitized solar cells using large-

AUTHOR INFORMATION

Corresponding Author

*Phone: +82-2-880-1697. Fax: +82-2-888-9073. E-mail: [email protected]. ORCID

Sung-Hoon Ahn: 0000-0002-1548-2394 Varsha Khare: 0000-0003-1602-1255 Author Contributions ⊥

J.-H.S. and S.S. share first authorship for equal contribution to the work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by Korea Government (MEST), Grants NRF-2012R1A1A2008196, NRF 2012R1A2A2A01047189, NRF 2017R1A2B4008801, NRF 2018R1A2A1A13078704, and NRF 2018R1A4A1059976, and NRF Basic Research Programme in Science and Engineering by the Ministry of Education (Grant 2017R1D1A1B03036226). V.K. gratefully acknowledges financial support from the BK21 program of the Government of Korea.

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