Controlling the morphological and redox properties of the CuTCNQ catalyst through solvent engineering
The solution-based synthesis of the coordination polymer, CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane), which exists in two phases, has predominately used acetonitrile (MeCN) as the solvent. However, our knowledge on the growth and properties of CuTCNQ in other solvents remains limited. In this work, the synthesis of CuTCNQ on Cu foil in two protic (MeOH, EtOH) as well as four aprotic (MeCN, DMSO, DMF, THF) solvents has allowed us to obtain new insights into the important role of the reaction medium in the spontaneous crystallization of CuTCNQ in discrete morphologies and phases. A new electrochemical method for phase identification also has been developed to support this study. Findings reveal that (i) the solvents with higher dielectric constants favor CuTCNQ crystallization; (ii) irrespective of the solvent, use of high temperature (60 °C vs. 25 °C in conventional synthesis) promotes CuTCNQ crystallization and facilitate conversion of phase I to phase II; (iii) phase I CuTCNQ possess enhanced redox catalysis (ferricyanide reduction by thiosulfate) performance over phase II CuTCNQ; and (iv) the amount of catalyst is not necessarily the most important factor for driving catalytic reactions, and other factors, such as, morphology, redox characteristics and solvent in which the CuTCNQ is synthesized may dictate the overall catalytic performance. These findings emphasize the importance of understanding the influence of parameters, such as, solvent and temperature in CuTCNQ synthesis as a means of providing materials with improved catalytic activity.
KeywordsCatalysis Metal-organic semiconductor CuTCNQ Charge-transfer complex Morphological control
The ambipolarity, superconductivity, and a myriad of interesting properties of charge transfer (CT) complexes based on 7,7,8,8-tetracyanoquinodimethane (TCNQ) have seen significant interest in exploring these materials for new applications [1, 2, 3]. As such, a number of strategies based on electrochemical [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14], vapor deposition [6, 14, 15, 16, 17, 18, 19, 20], and wet chemical routes [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31] have been employed to fabricate these materials. A large body of work in this area has focused on MTCNQ [9, 25], as the strong electron withdrawing ability of neutral TCNQ (TCNQ0) from a metal (M0) allows a wide breadth of MTCNQ (M+TCNQ−) materials to be fabricated with relative ease [8, 9, 25]. The unique electronic properties of these metal-organic CT complexes have allowed them to be used in field emission and molecular electronics [9, 32]. Our group and others have been able to recently expand the applicability of these materials beyond the field of electronics [7, 16, 17, 21, 22, 23, 24, 26, 27, 28, 29, 30, 31, 33, 34, 35, 36, 37, 38]. Some examples include, nanoarrays of CuTCNQ, AgTCNQ, CuTCNQF4 (2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane), AgTCNQF4, CuTCNQ/AgTCNQ hybrids, CuTCNQ/noble metal hybrids, and AgTCNQ/noble metal hybrids as excellent photocatalysts [21, 24, 25, 26, 27, 28, 31, 39]; CuTCNQF4/Ag hybrids as redox catalysts ; heterojunctions of CuTCNQ/ZnO as a humidity sensor ; alkali-TCNQ nanoarrays as gas sensors [16, 17]; AgTCNQ nanowires as an antibacterial material ; Fe(TCNQ)2, Mn(TCNQ)2, and Co(TCNQ)2 as electrocatalysts for oxygen and hydrogen evolution reactions [33, 34, 38, 41], and CuO/TCNQ and Co(OH)2/TCNQ nanoarrays on copper foams as electrocatalysts [42, 43, 44]. These wide-ranging possibilities using MTCNQ materials have seen renewed interest to improve their fabrication strategies.
Since the initial report on MTCNQ materials by Melby et al. , much of the effort in the synthesis of these materials has focused on solution-based or vapor deposition strategies. Of these, the former is preferred as it only requires simple immersion of a metal foil in an acetonitrile (MeCN) solution containing TCNQ0 [9, 25]. The ease of reduction (strong electron withdrawing ability) of TCNQ facilitates its spontaneous reaction with many metals, resulting in the formation of MTCNQ on the surface of metal foil [9, 25]. Although this strategy is simple and robust, it poses certain limitations in controlling the aspect ratio, the overall morphology and the phase of the resulting MTCNQ structures. Since these factors can significantly influence the properties of MTCNQ, there is a strong motivation to explore alternative fabrication strategies. Specifically, in the context of CuTCNQ, our group recently developed two independent approaches to control the aspect ratios of the resulting CuTCNQ microrods. In the first strategy, we employed a bi-solvent approach where the addition of small quantities of water to MeCN allowed dynamic control over the reaction kinetics in favor of CuTCNQ crystallization, leading to microrods of ≈ 100 μm in length, instead of 5–10 μm achieved through conventional synthesis . In another approach, we could overcome the equilibrium barrier using a recycling strategy, where the replenishment of the reaction solution with fresh TCNQ0(MeCN) over multiple cycles favored CuTCNQ crystallization, allowing the formation of ≈ 70 μm long microrods . In most studies, a polar aprotic solvent, typically MeCN, has remained the preferred reaction medium for CuTCNQ growth. Notably, CuTCNQ is known to exist in two distinct phases (phase I and phase II) [9, 45, 46]. During its synthesis in MeCN, phase I is formed under ambient conditions, while phase II is typically observed at elevated temperatures . Given that the choice of the solvent and the synthesis temperature can have a large influence on the reaction kinetics and reaction equilibrium [47, 48], it is important to understand the influence of these parameters on the morphology and the phase of CuTCNQ.
In line with the above considerations, this work is focused on the investigation of the crystallization and growth of CuTCNQ in a range of polar solvents at 25 and 60 °C. To obtain a detailed understanding of the role of solvent and temperature on CuTCNQ crystallization, we also synthesized phase-pure CuTCNQ using well-established methods . These materials served as reference standards for the characterization of CuTCNQ formed in this study (Fig. S1, Supporting Information). The study of CuTCNQ crystallization in a range of solvents provides an in-depth understanding of factors involved in modulating the crystallization kinetics. The outcomes reveal that the CuTCNQ crystallization kinetics, their morphological characteristics and phases are significantly influenced by (i) the nature of the solvent (protic vs. aprotic, dielectric constant and dipole moment) and (ii) the reaction temperature. Importantly, it was discovered that the morphology of CuTCNQ has a major influence on the catalytic activity. However, the catalytic efficiency of morphologically distinct CuTCNQ materials was independent of the amount of CuTCNQ crystallized.
2 Materials and methods
Copper foil (99% pure), potassium ferricyanide, copper (I) iodide (Cu2I2), DMSO, DMF, ethanol, methanol, THF, sodium thiosulfate, and copper sulfate were purchased from Sigma Aldrich, Australia. 7,7,8,8-tetracyanoquinodimethane (TCNQ) was purchased from Chem Supply, Australia. MeCN was purchased from Ajax Fine Chem, Australia. All chemicals were used as received. Deionized water was dispensed from a Millipore Milli-Q Ultrapure Water system fitted with Organex-Q Cartridge filters. To remove surface oxides, before the growth of CuTCNQ, the copper foil was treated with dilute nitric acid, washed with deionized water, and dried with a flow of nitrogen immediately prior to use.
Synthesis of CuTCNQ in different solvents
A 1.0 × 1.0 × 0.50 cm3 piece of Cu foil was placed in contact with 5 mM TCNQ dissolved in the solvent of interest. The reaction between Cu foil and TCNQ in each solvent was allowed to proceed for 24 h at 25 °C or 60 °C. Subsequently, the Cu foils were washed with the relevant solvent to remove any unreacted TCNQ from the surface. These substrates were used for materials characterization and catalysis.
Synthesis of pristine phase-pure CuTCNQ reference standards
Pure phase I and II CuTCNQ were synthesized using a literature method . Phase I CuTCNQ was formed by a redox reaction between TCNQ (hot solution dissolved in MeCN) with Cu2I2 . This reaction resulted in the formation of phase I CuTCNQ, which was precipitated, then washed with MeCN. Phase II CuTCNQ was obtained by prolonged stirring of a suspension containing the phase I CuTCNQ in MeCN at room temperature .
The Cu foils containing CuTCNQ grown in different solvents were characterized by a range of methods. Instrumental techniques used for this characterization are as follows: a FEI Verios 460L FE-SEM instrument operated at an accelerating voltage of 10 kV; EDX analysis with a FEI Verios 460L FE-SEM instrument coupled with EDX Oxford X-Max Silicon Drift Detector; FTIR spectroscopy with a Perkin–Elmer D100 spectrophotometer in attenuated total reflectance mode with a resolution of 4 cm−1; Raman spectroscopy with a Perkin–Elmer Raman Station 200F at an excitation wavelength of 785 nm and 100 μm spot size.
Electrochemical solid-solid state interconversions for phase identification
A BASi Epsilon-EC electrochemical workstation was used for voltammetric experiments. A glassy carbon (GC) working electrode was first prepared by polishing it with 0.3 μm alumina (Buehler, Lake Bluff, IL, USA) on a clean polishing cloth (Buehler, USA), followed by washing several times with deionized water under sonication for 5 min. The GC electrode was finally washed with acetone and dried under a N2 stream before modification with the CuTCNQ sample. The CuTCNQ samples grown on Cu foil were collected by scraping from the Cu foil surface using a spatula. These CuTCNQ samples, as well as phase I and II reference standards were loaded onto the GC (0.00786 cm2) macro-disc electrode. Pt wire was used as a counter electrode and Ag/AgCl (3.0 M KCl) was used as a reference electrode in a 3-electrode cell. An aqueous 0.10 M CuSO4.5H2O solution was used as a solvent and supporting electrolyte medium. The presence of Cu2+ in the aqueous electrolyte minimizes the dissolution of the solid from the CuTCNQ-modified electrode upon repetitive cycling of the potential . Given that the Cu2+ is reduced at potentials more negative than 0.200 V, the electrochemistry of CuTCNQ adhered to a GC electrode in contact with 0.10 M CuSO4 was studied over the potential range of 0.250 to 0.600 V. Samples for electrochemical studies were prepared freshly as prolonged immersion of the CuTCNQ-loaded GC electrode in aqueous Cu2+ electrolyte solution facilitated the conversion of phase I to phase II. All experiments were performed in triplicate.
The ferricyanide reduction reaction was used as a model pseudo-first order reaction to test the catalytic efficiency of CuTCNQ samples grown in different solvents and temperatures. A 30 mL solution was used for the catalytic reaction in the presence of visible light. The reaction contained 1.0 mM ferricyanide, 0.10 M thiosulfate, and CuTCNQ catalysts grown on Cu foil (1.0 × 1.0 × 0.50 cm3). Care was taken to maintain the temperature of the reaction at 25 ± 1 °C under stirring at 1200 rpm using a magnetic stirring bar. The catalytic conversation of ferricyanide to ferrocyanide was monitored as a function of time using UV-vis absorbance spectroscopy (Cary 50 Bio spectrophotometer).
3 Results and discussions
Properties of the polar solvents used in this study at 20 °C
Energy dispersive X-ray (EDX) spectral analyses of the CuTCNQ crystals formed in different solvents show characteristic energy lines corresponding to C Kα (0.277 eV) and N Kα (0.392 keV) [25, 26, 30, 31] that are assigned to the presence of TCNQ− in CuTCNQ (Fig. S10, supporting information). In addition, the expected energy lines corresponding to Cu Kα and Cu Kβ are observed [25, 26, 30, 31]. The absence of an O Kα signal confirms minimal oxidation of Cu in all cases. An elemental EDX map establishes that the growth of CuTCNQ remains uniform under all experimental conditions (Fig. S11, supporting information).
The phase of the CuTCNQ cannot be distinguished by Raman spectroscopy alone, as evident from the Raman spectra of phase-pure CuTCNQ in Fig. S1f (supporting information). Thus, the phase was assessed using FTIR spectroscopy (Fig. 3b and c and Fig. S1g and h, supporting information). All FTIR spectra including the phase-pure CuTCNQ (Fig. S1g) showed the δ(C–H) bending vibrations at ca. 823 cm−1, as expected from TCNQ− in CuTCNQ, negating the possibility of TCNQ0 or TCNQ2− (Fig. 3b) [22, 26, 27, 28, 29, 30, 31]. However, the band at ca. 2200–2215 cm−1 can be used to identify the phase, such that phases I and II show bands at ca. 2200 and ca. 2215 cm−1, respectively (Fig. S1h, supporting information) [39, 45]. An additional band at ca. 2170 cm−1 is noted in all the samples; however, it is more prominent in phase II. FTIR spectral analysis reveals that irrespective of the solvent, the CuTCNQ grown at 25 °C shows characteristics of a phase I material (Fig. 3c). However, if grown at 60 °C, predominantly phase II material with a minor residual phase I is produced (Fig. 3c), consistent with the morphological characteristics of these materials described earlier (Fig. 1). This implies that during the synthesis of CuTCNQ at 60 °C, CuTCNQ first crystalizes as phase I and converts to the thermodynamically stable phase II.
Under voltammetric conditions, the kinetically-favored phase I CuTCNQ is first formed electrochemically in the initial cycles of potential but is then transformed to the thermodynamically-stable phase II on extensive, repetitive cycling of the potential. Therefore, in this voltammetric method of phase identification, if the material under investigation is phase I CuTCNQ, making the initial scan direction positive, starting from 0.400 V vs. Ag/AgCl, followed by repetitive cycling of the potential, over the range of 0.600 to 0.250 V, leads to conversion to phase II, in aqueous solution containing Cu2+(aq). Alternatively, if a phase II material is adhered to a GC electrode, insignificant changes in its voltammetry on repetitive cycling is expected. If both phases co-exist, then the voltammetric response is expected to contain features of both phases.
Catalytic performance of CuTCNQ catalysts under designated conditions
Induction time [min]
Time taken for 95% reaction completion [min]
In summary, this work provides new insights into the important role of the reaction medium in the spontaneous crystallization of CuTCNQ on the surface of Cu foil. Changing the solvent can have important implications on the morphological characteristics of CuTCNQ. While polar aprotic solvents typically have the ability to crystalize CuTCNQ, we note that solvents with higher dielectric constant can favorably drive CuTCNQ crystallization. Further, irrespective of the solvent, the elevated temperatures not only promote CuTCNQ crystallization, the conversion of phase I to phase II is simultaneously favored. To extend the phase identification resolution capabilities available with FTIR, we have also introduced a robust and more sensitive technique based on electrochemical solid-solid state interconversion. While the amount of catalyst used in a reaction is typically considered important for driving a reaction, we observe that the catalyst with the lowest amount of CuTCNQ (DMSO/25 °C) outperformed all other materials with least induction time. These results outline the importance of understanding the influence of simple synthesis parameters such as solvent and temperature that can induce morphological and phase changes during CuTCNQ crystallization. Combining the outstanding redox capabilities of the new CuTCNQ materials reported here with optically-tunable two-dimensional (2D) materials [53, 54, 55, 56, 57, 58, 59, 60, 61] will offer opportunities for developing new photo-redox catalysts.
V.B. thanks the Australian Research Council (ARC) for a Future Fellowship (FT140101285). V.B., R.R., L.L.M., and A.M.B. acknowledge the ARC for funding this project though an ARC Discovery (DP170103477) Grant. The authors are appreciative of the generous support of the Ian Potter Foundation towards establishing the Sir Ian Potter NanoBioSensing Facility at RMIT University. R.R. acknowledges RMIT University for a Vice Chancellor Fellowship. Authors acknowledge the support from the RMIT Microscopy and Microanalysis Facility (RMMF) for technical assistance and providing access to characterization facilities.
V.B.: Australian Research Council (ARC), Future Fellowship (FT140101285).
V.B., R.R., L.L.M., A.M.B.: ARC, Discovery Grant (DP170103477).
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Compliance with ethical standards
Conflict of interest statement
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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