itr on buk Kor Bis (2,20-bipyridine) nitratocopper(II) ed ope he a bo ned and a lowest unoccupied molecular orbital (LUMO) levels of �4.692 and �4.071 eV, respectively. The photoelectric efficiency in DSSCs was approximately 0.032% with the nanometer-sized TiO in the condi- s) hav advan up, lig onven An important element of DSSC materials is the dye compound, consisting of conjugated p-electrons, which should have the following four properties. (1) The transfer of electrons from HOMO to LUMO (d? p electron transition) in the dye molecules after light absorption resulted to the injection of e-LUMO into the TiO2 electrode. (2) It should absorb all light in the visible area. (3) The chemical combination of solid oxide (TiO2) and dye molecule methylene)-1,3-dithiole-4,5-dicarboxylate) and cis-[Ru(H2dpydt)2 (NCS)2] (3, H2dpydt = 2-(di(2-pyridyl)methylene)-1,3-dithiole-4,5- dicarboxylate), and two types of ruthenium bipyridyl dyes incorpo- rating sulfur-donor bidentate ligands with general formula [Ru (R-bpy)2C2N2S2] and [Ru(R-bpy)2(S2COEt)] [NO3] for application into DSSCs, respectively. On the other hand, Tian and co-workers [12] reported that the synthesis of Iridium(III) bis[2-phenylpyridi- nato-N,C20]-4,40-(dicarboxylicacid)-2,20-bipyridine facilitated a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 85%, a Jsc of 9.59 mA/cm2, a Voc of 0.552 V, and a FF of 0.54, corresponding to an overall conversion efficiency of ⇑ Corresponding author. Tel.: +82 53 810 2363; fax: +82 53 815 5412. Inorganica Chimica Acta 365 (2011) 400–407 Contents lists availab Ch w.e E-mail address:
[email protected] (M. Kang). vices [1,2]. When a dye molecule absorbs light, it leads to the exci- tation of electrons on the highest occupied molecular orbital (HOMO) orbital to an electronically excited state, the lowest unoc- cupied molecular orbital (LUMO) orbital. The excited dye molecule injects an electron into the conducting band of the TiO2 film. The oxidized dye is restored by electron donation from the reducing ions in the electrolyte, usually an organic solvent containing a re- dox system. The donated electron is in turn regenerated by the reduction of conjugated ions in electrolytes. The circuit is com- peted by electron migration through an external load [3–5]. semiconductor electrode (TiO2). Funaki et al. [9] has synthesized a new type of ruthenium(II) complex containing a 2 quinolinecarb- oxylate ligand as a sensitizer for DSSCs. They attained an overall conversion efficiency of 8.2% under standard air mass 1.5 irradia- tion (100 mW/cm2) with a short-circuit photocurrent density (Jsc) of 18.2 mA/cm2, an open-circuit photovoltage (Voc) of 0.63 V and a fill factor (FF) of 0.72. Zuo and co-workers [10] and Robertson and co-workers [11] have also studied new ruthenium(II) com- plexes containing coupled di(2-pyridyl) and 1,3-dithiole units, cis-[Ru(Medpydt) 2(NCS)2] (2,Medpydt = dimethyl 2-(di(2-pyridyl) nitrate Hydrothermal method Five-coordination Cyclic voltammetry Dye-sensitized solar cells Photoelectric efficiency 1. Introduction Dye-sensitized solar cells (DSSC sively on account of their attractive less toxic manufacturing, easy scale- use of flexible panels, compared to c 0020-1693/$ - see front matter � 2010 Elsevier B.V. A doi:10.1016/j.ica.2010.09.041 2 tion of an open-circuit voltage (Voc) of 0.346 V, a short-circuit current density (Jsc) of 0.166 mA/cm 2 at an incident light intensity of 100 mW/cm2. � 2010 Elsevier B.V. All rights reserved. e been studied exten- tages, such as low cost, ht weight and potential tional p–n junction de- should be strong. (4) It should be stable to light and heat. Particu- larly, the commercial dyes possessing all of the above properties for application to DSSCs use the Ru complex as a typical example [6–8]. The high efficiency of the Ru complex arises because elec- trons of both the non-thermalized singlet excited state and the ex- cited triplet state can be injected into the conduction band of the Keywords: fer at around 350–600 nm, and d–d transfer at around �650 nm. Cyclic voltammetry in acetonitrile revealed a reversible Cu(I)? Cu(II) oxidation process at a highest occupied molecular orbital (HOMO) Rapid synthesis of bis (2,20-bipyridine) n a hydrothermal method and its applicati Youngmi Kim a, Jong Hwa Jeong b, Misook Kang a,⇑ aDepartment of Chemistry, College of Science, Yeungnam University, Gyeongsan, Gyeong bDepartment of Chemistry, Kyungpook National University, Daegu 702-701, Republic of a r t i c l e i n f o Article history: Received 16 November 2009 Received in revised form 16 August 2010 Accepted 27 September 2010 Available online 8 October 2010 a b s t r a c t In this study we synthesiz crystal structure, optical pr ysis results revealed that t bipyridine and the oxygen absorptions that were assig Inorganica journal homepage: ww ll rights reserved. atocopper(II) nitrate using to dye-sensitized solar cells 712-749, Republic of Korea ea bis (2,20-bipyridine) nitratocopper(II) nitrate in order to examine its the rty and application to dye-sensitized solar cells (DSSCs). Single X-ray anal- cquired complex exhibited five-coordination with four nitrogen atoms of nd of the NO3 � ion. The reflectance UV–Vis absorptions showed three to ligand-to-ligand at around 230–350 nm, metal-to-ligand charge trans- le at ScienceDirect imica Acta l sevier .com/locate / ica tricarbonyl complexes with bipyridinyl ligands attached to a sul- because bipym can act as a doubly bidentate bridging unit for couples observed were referenced to that. 0.5) cm , respectively. imica the preparation of bimetallic. Bipyridine shows a delocalized p- bonding combining the chelating properties of molecules such as 2,2-bipyridine and 1,4-diazines, respectively [19]. Aromatic nitro- gen heterocycles such as 2,2-bipyridine (bpy), 1,1-phenanthroline (phen) and di-2-pyridylamine (dpa) are classical [20–22]. Despite all these studies, only a few researches [23,24] have focused on the use of the Cu-complex in the solar case. We therefore attempted to synthesize the copper(II) complex easily and stably by forming bipyridine chelate ligands to the metal center. We used a new synthesis approach consisting of a hydro- thermal method under N2 condition in an autoclave, which differs from the reflux method that is conducted under room temperature and atmosphere pressure, in order to achieve perfect crystallinity and rapid synthesis time. The crystal structure and optical proper- ties of the synthesized complex were analyzed by single crystal X-ray, UV–Vis, and photoluminescence spectra. Finally, the application of this complex to DSSC is discussed. 2. Experimental 2.1. Synthesis and recrystallization purification of bis (2,2’-bipyridine) nitratocopper(II) nitrate The following chemicals were purchased and used without fur- ther purification. The Cu(II) complex was prepared in an autoclave under N2 gas atmosphere at 403 K for 2 h. As shown in Fig. 1, Cu (NO3)2�6H2O (Aldrich Co, 99%) and 2,20-bipyridine (Wako Co, 99.0%) were added to a two-necked 250 mL flask at a molar ratio of approximately 1:3 with 120 mL of distilled water, and the mix- ture was stirred for 1 h. The final solution was transferred into the autoclave and thermally treated at 403 K for 24 h after N2 gas purg- ing. The solid copper complex was separated after the reaction. A blue bis (2,2’-bipyridine) nitratocopper(II) nitrate was attained after drying at 353 K. The complex was recrystallized by slow dif- fusion for 3 days in saturated acetonitrile solution to afford blue cubic crystals. 2.2. Characterization fur-rich core, and osmium sensitizers containing 2,2-bipyridine- 4,4-bisphosphonic acid ligand, respectively. As shown in these examples, these dyes were composed of no- vel high-cost metals, such as Ru, Re, Ir and Os. In contrast, the use of low-cost dyes will hasten the commercial availability in DSSCs. Therefore, in this study, we have to tried to obtain stable transition metal complexes at low cost and apply them to the dye in DSSCs. First, we investigate the starting copper complex. Over the last few years, considerable efforts have been focused on the design and synthesis of the copper complex, which has attracted exten- sive interest in many fields such as molecular magnetism [15], metalloproteins, enzymes [16], and blue/green emitting materials [17]. For catalysis, the unique characteristic of the CuII/CuI redox couple renders many of their complexes suited for various catalysis reactions [18], especially for the green catalysis process since cop- per is a cheap and low toxicity metal. Recently, the synthesis, solid state structure and the photo physical properties of mono- and di-nuclear (d–f) transition metal complexes based on the 2,2-bipy- rimidine (bipym) ligand have received increasing attention 2.86% under AM 1.5 sunlight. Additionally, Zuo and co-workers [13] and Bignozzi and co-workers [14] have researched rhenium(I) Y. Kim et al. / Inorganica Ch Single crystals suitable for X-ray crystallographic analyses were obtained by recrystallization in acetonitrile, for bis (2,20-bipyridine) nitratocopper(II) nitrate. The data for the 3. Results and discussion The ORTEP view of the compound is shown in Fig. 2. The struc- ture refinement and selected bond angles and distances are given in Tables A1 and A2. The monoanionic bis (2,20-bipyridine) nitrato- copper(II) nitrate is analyzed in the monoclinic system, space group P21/n. It was reported [28] that the Cu atom of the title com- plex, (I), has a distorted octahedral coordination and is linked by the four bipyridine N atoms and a chelating NO3 group, for which one of the O-atom donors lies further from the Cu atom due to Jahn–Teller distortions. However, the prepared complex was ex- pected to the five-coordinated Cu(II)-complex with non-chelated NO3, because we did not determine the Cu–O(2) bond length in this study. The Cu atom has a stereochemistry with a pseudo-C2 sym- metry bisecting the NO3 ligand and passing between the bipyridine ligands. The atoms of the vectors Cu–N(1) and Cu–N(2) lie 1.974(7) and 2.026(8) Å apart, respectively, and are designated as forming the equatorial plane, with a little elongation of the Cu–N(3) and 2.3. Manufacturing dye-sensitized solar cell (DSSCs) The TiO2 thin film was prepared by solvothermal method as de- scribed in a previous paper [27]. Briefly slurry was produced by mixing 5.0 g nanometer-sized TiO2 powder with 10 mL alcohol after sonication for 24 h at 1200W/cm2. The TiO2 was fabricated by twice coating onto a fluorine-doped, SnO2 conducting glass plate (Hartford FTO, �30X/cm2, 80% transmittance in the visible region) using a squeeze printing technique to give an approximate thickness of 10.0 lm. The film was treated by heating at 723 K for 30 min to remove the alcoholic solvent. The TiO2 thin film elec- trode was immersed in a 3.0 � 10�4 M N719 dye solution at room temperature for 24 h, rinsed with anhydrous ethanol and dried. A Pt-coated FTO electrode was placed over the dye-adsorbed TiO2 electrode, and the edges of the cell were sealed with a sealing sheet (PECHM-1, Mitsui-Dupont Poly Chemical). The redox electrolyte consisted of 0.50 mol KI, 0.05 mol I2, and 0.5 mol 4-tert-butylpyri- dine as a solvent. The photocurrent–voltage (I–V) curves were used to calculate the Jsc, Voc, FF, and overall conversion efficiency of the DSSCs. The I–V curves were measured under white light irradiation from a xenon lamp (maximum 150W, Newport). The incident light intensity and active cell area were 100 mW/cm2 and 0.40 (0.8 � 2 compoundwere collected on an Enraf–Nonius CAD4 diffractometer at 293 K using x/2h scan mode. The usual corrections were ap- plied. A semi empirical absorption correction was carried out based onW scans. The structures were elucidated by direct methods and refined by full matrix least-squares methods with the SHELXS97 and SHELXL97 program [25]. Molecular graphics were prepared using OR- TEP III [26]. The crystal data, structure and refinement are summa- rized in Tables A1 and A2. The bis (2,2’-bipyridine) nitratocopper(II) nitrate underwent fluorescence (FL) spectroscopy (JASCO, Model FP-777) to examine the number of photo-excited electron hole pairs, with a target wavelength of 325 nm, and 0.5 mM Cu-complex was diluted in acetonitrile. Cyclic voltammetry data were obtained using a BAS 100B. The measurements were performed at room temperature in 0.1 M KCl (0.2 mM Cu-complex) as the supporting electrolyte, with a scan rate of 100 mV/s using platinum wire, working and counter electrodes and a Ag/AgCl reference electrode. Ferrocene was used as the internal reference and the potentials of the redox Acta 365 (2011) 400–407 401 Cu–N(4) distance to 1.988 and 2.1123 Å (designated as the axial atoms), respectively. The corresponding angles are also distorted from the ideal octahedral value of 180, with N(1)–Cu– Stirring for 1 h 1.0 mol Cu(NO3)2·6H2O 3.0 mol 2,2’-bipyridine Cupper aqueous solution H2O solvent + Autoclave 403K, 2 h Washing, filtering, and drying 1.0 mol Cu(NO3)2 3.0 mol 2,2’-bipyridine Open the clave Thermal treatment at 403K for 2 h Move to Autoclave Stirring for 1 h at room temperature Separation of phase liquid phase Solid phase H2O solvent Fig. 1. Preparation of bis (2,20-bipyridine) nitratocopper(II) nitrate using a hydrothermal method. Fig. 2. ORTEP view of the structure of bis (2,20-bipyridine) nitratocopper(II) nitrate. Wavelength (nm A bs or ba nc e (a. u. ) Coppe 200 300 400 500 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 330nm 350 nm 280 nm 320nm 265nm 230nm Bis(2,2’-245 nm Fig. 3. Reflectance UV–Vis absorption spectrum of 402 Y. Kim et al. / Inorganica Chimica Acta 365 (2011) 400–407 N(3) = 176.33, N(2)–Cu–O1 = 149.87. The distortions in the coordi- nation geometry agree with following observations reported [29] for pseudo-Jahn–Teller structures: as one Cu–O bond lengthens, the other shortens, and the Cu–N trans-bond to each O atom lengthens or shortens, respectively, while the second Cu–N bond within the same bipyridine ligand also lengthens or shortens cor- respondingly, but to a lesser extent. The Cu–N(1 and 3) bond lengths to the N atoms in the equatorial plane lay in the range 1.974–1.988 Å, with the elongated axial Cu–N(2 and 4) bond length being 2.026–2.112 Å (see Table A2), and the equatorial Cu–O(1) bond length being 2.114 Å. The axial NO3 O atom that lies at a dis- tance of about 2.520 Å from the N atom, but the other O atom com- bined with Cu atom did not appear from the Cu atom which constitutes the major distortion from the regular octahedral coor- dination. There are no unusual bond dimensions within either the bipyridine ligand or the chelating NO3 ligand, where the N–O bond lengths lie within the range 119.8–1.257 (7) Å. Within the nitrate anion, bond angles O(3)–N(5)–O(1) and O(2)–N(5)–O(1) lie within the typical range for this group of 121.7–116.8 Å. On the other ) 2,2’-bipyridine r(II) nitrate Cu NO3 bipyridine 600 700 800 650 nm bipyridine)nitratocopper(II) nitrate bis (2,20-bipyridine) nitratocopper(II) nitrate. hand, coordination by the second O atom in Cu(I) at 2.520 Å is tigh- ter than in the reported solvated [Cu(bipy)2(NO3)]+ complexes, but looser than in the [Cu(bipy)2(NO2)]+ complexes at room tempera- ture. However, the angles about the Cu atom in (I) more closely resemble those in the NO2-ligated structures than the NO3 � ligated structures, leading to a geometry closer to those in the un-solvated Fig. 3. Three types of absorption band, which were assigned to L–L (p? p or p? p*) transitions localized on the bipyridine li- gand, were seen at 230, 265, and 320 nm in the case of bipyridine compound. The Cu(II) nitrate, a Cu(II) starting material, was broadly absorbed at around 330 and 650 nm. This electronic spec- trum in [Cu(bipy)2(NO3)]+ shows two bands in the UV region at 245 and 280 nm, which is attributed to L–L (p? p or p? p*) transi- tions localized on the bipyridine ligand, a series of four regularly spaced bands with decreasing intensity and distinct maxima at 350 nm, which is attributed to Cu? bipy metal-to-ligand charge transfer (MLCT) transitions, and a single broad feature at 600 nm, which is attributed to a spin-forbidden singlet–triplet MLCT [30]. The relative order of the dxz, dxy, and dyz orbitals is dictated by the C2 symmetry of the complex and by the mixing of these orbi- tals with the p* orbital of the bipy ligand and the ligand group orbi- tal of NO3, of symmetry A2, B1, and B2. The two broad bands observed in the wavelength interval 350–600 nm can therefore be assigned to the transitions MLCT1 and MLCT2, as shown in Fig. 3. This last transition at above 650 nm, of remarkable intensity, is allowed by the large spin–orbit coupling constant of the d–d transfer of copper [26], and the band observed in the visible region can be rationalized in terms of symmetry arguments. The FL curve suggests that the electrons in the HOMO were transferred to the LUMO, after which the excited electrons were stabilized by photoemission, as shown in Fig. 4. In general, the FL intensity increases with increasing number of emitted elec- trons resulting from recombination between excited electrons and holes, and consequently, a decrease in photo activity. There- fore, there is a strong relationship between the FL intensity and 300 400 500 600 700 -100 0 100 200 300 400 Wavelength (nm) In te ns ity 335 nm 650 nm Violet Yellow Fig. 4. FL curve for bis (2,20-bipyridine) nitratocopper(II) nitrate. Y. Kim et al. / Inorganica Chimica Acta 365 (2011) 400–407 403 structures. From these results, we confirmed that the geometry of the [Cu(bipy)2(NO3)]+ complex expresses five-coordination with the NO3 � ligated structure. The reflectance UV–Vis absorption spectrum in the wavelength interval 200–800 nm of the [Cu(bipy)2(NO3)]+ species is shown in 0.0001 Epa=190mV i Onset=312mV Cu rre nt (A ) 1500 1000 500 0 -500 -1 1500 1000 500 0 -500 -1 -0.0005 -0.0004 -0.0003 -0.0002 -0.0001 0.0000 Epc=-120mV Reducible reaction Cu2+ + e- Cu+ c E (mV) vs. Ag/AgCl -0.00014 -0.00012 -0.00010 -0.00008 -0.00006 -0.00004 -0.00002 0.00000 0.00002 0.00004 0.00006 Irreducible reaction Epa=113mV ic Cu rre nt (A ) Onset=-309mV Onset=307mV Fig. 5. Cyclic voltammetry of bis (2,20-bipyridine) nitratocopper(II) nitrate, in distilled w and 0.1 M KCl as the supporting electrolyte. photo activity. In particular, the FL intensity decreases greatly when a metal can capture excited electrons or exhibit conductiv- ity, which is known as the relaxation process. The FL curves of the two-shaped, Cu-complex sample showed blue/green emission at 335 and 650 nm. The band broadening was attributed to the * 0.2mM complex solution * Glassy carbon as the working electrode * 0.1M KCl as the supporting electrolyte * Ag/AgCl, E0/V=+0.197, * Epc=reduction, Epa=oxidation * E0 (=E1/2) =Eox+Ered/2 (a) Synthesized Cu-complex (b) Copper nitrate 000 -1500 000 -1500 ater solutions of the 0.2 mM complex using glassy carbon as the working electrode overlapped emission from the higher and lower excited states to the ground states, corresponding to the ligand-to-metal charge transfer (LMCT) and MLCT transitions, respectively, of Fig. 3. A theoretical reason has been proposed to understand the transition from the free ligand affects the complex emission energy [31]. The HOMO of the complex is a p-orbital, localized on the Table 1 Electronic parameters for cyclic voltammetry. Materials Ered vs. Ag/Ag+ (V) Eox vs. Ag/Ag+ (V) E0 vs. Ag/Ag+ (V) Ered-onset vs. Ag/Ag+ (V) Eox-onset vs. Ag/Ag+ (V) HOMO (eV) LUMO (eV) Eg (eV) Synthesized Cu- complex �0.120 +0.190 0.155 �0.309 +0.312 �4.692 �4.071 0.621 Copper nitrate Non-detected +0.113 – – +0.307 – �4.687 – *E0ferrocene = +0.42 V vs. Ag/Ag +, *EHOMO(LUMO) = �4.8 eV � [Eonset � E1/2ferrocene], *Eg (=band gap) = HOMO � LUMO. 4500 4000 3500 3000 2500 2000 1500 1000 500 0 a) b) c) Ti-O C=CN=O Wavenumber(cm-1) In te ns ity (a . u .) (A) FT-IR spectrum of Bis(2,2’-bipyridine)nitrato copper(II) nitrate (B) A adsorbed model between the surface of TiO2 and Cu(II) complex CO2 N N N N Cu O N O O Ti O O Fig. 6. FT-IR spectroscopy of bis (2,20-bipyridine) nitratocopper(II) nitrate and an adsorbed model between the surface of TiO2 and the Cu(II) complex: (a) bis (2,20-bipyridine) nitratocopper(II) nitrate, (b) TiO2, and (c) TiO2 absorbed by bis (2,20-bipyridine) nitratocopper(II) nitrate. A) 120 140 160 180 e- -2 404 Y. Kim et al. / Inorganica Chimica Acta 365 (2011) 400–407 Cu rre nt (u 60 80 100 Voc(V) Jsc(mA) FF Efficiency (%) 0.346 0.166 0.55 0.032 Voltage (V) (A) Photocurrent-voltage (I-V) curves assembled by Cu comp 0 20 40 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 - - Fig. 7. Photocurrent–voltage (I–V) curve of a solar cell assembled by TiO2 adsorbed with b TiO2 e- -3 eV lex (B) Energy diagram between TiO2 and Cu complex Dye e- e- h+4.692 -4.25 I3-/I- electrolyte -5 -44.071 Difficulty to transfer is (2,20-bipyridine) nitratocopper(II) nitrate, and the energy diagram between them. and ampere meter (Model 2000, Keithley) with a variable load. A 150W illuminant Xenon lamp was employed as a radiation source sured using a power analyzer and thermal smart-sensor. The FF and solar energy conversion efficiency (g) are calculated using imica non-coordinating nitrogen atom and the carbon atom in the bipyridyl ring, owing to the low symmetry. The energy of the HOMO level of the complex is much lower than that of the free ligand. The LUMO of the complex is a p* orbital consisting of mostly atomic orbits from one of the bipyridyl rings. There are few contributions from the bipyridyl ring in the LUMOs of the complexes. The role of the metal ion is to increase the co- planarity and conformational rigidity in the molecule structure and decrease the p–p* energy gap. The results lead to a red shift in the transition energy from ligand to metal center. The utility of cyclic voltammetry is highly dependent on the analytic condition being studied, and it has to be redox active within the experimental potential window. It is desirable to dis- play a reversible wave, which gives some information as the fol- lows; the reversible reactions display a hysteresis between absolute potential between the reduction (Epc) and oxidation peak (Epa). Conveniently in an ideal system the relationships re- duces to, DE(=Epc � Epa) = 57 mV/n, for an n-electron process [32]. Reversible reactions will show a ratio of the peak currents passed at reduction (ipc) and oxidation (ipa) that is near unity (1 = ipa/ipc). When such reversible peaks are observed thermody- namic information in the form of half cell potential E01=2 (Epc + Epa/2) also can be determined. Especially when waves are semi- reversible such as when ipa/ipc is less than or greater than 1, it can be possible to determine even more information especially kinetic processes. In this study, the oxidation potentials are mea- sured by means of cyclic voltammogram in distilled water solu- tions of the 0.2 mM complex using glassy carbon as the working electrode and 0.1 M KCl as the supporting electrolyte, as shown in Fig. 5, and the electronic parameters for cyclic voltammetry are listed in Table 1. In the synthesized [Cu(bipy)2(NO3)]+, the Cu(I)? Cu(II) redox process looks like to a reversible reaction, thus it deviates from the theoretical value (57 mV) [32] in ideal revers- ible system to giving E1/2 values of 155 mV for the [Cu(bipy)2 (NO3)]+ species. The absolute potentials between the reduction (Epc) and oxidation peak (Epa) in [Cu(bipy)2(NO3)]+ complex are seen at �120 and +190 mV, respectively. Otherwise only oxidation peak shows in copper nitrate, supposing to a non-reversible, and it is impossible to determine what their thermodynamic E01=2 is with cyclic voltammetry. The compounds could be irreversible because of a following chemical process of a common example, for transi- tion metals is a shift in the geometry of the coordination sphere. If this is the case, then higher scan rates may show a reversible wave. It is also possible that the wave is irreversible due to a phys- ical process most commonly some form of precipitation. Recently, some researchers have reported a useful equation, which can determine the energy levels of HOMO and LUMO using cyclic vol- tammetry method [33–35]. First, the ferrocene (E1/2 versus Ag/ Ag+ = +0.42 eV) potential as a standard should be measured in the electrolyte solution using the same reference electrode, and it be fixed �4.8 eV as a energy level at the vacuum set. Finally the en- ergy levels of HOMO or LUMO can be calculated using the follow- ing formula HOMOðor LUMOÞ ðeVÞ ¼ �4:8� ðEonset � E1=2ðFerroceneÞÞ Here, the Eonset is a starting point of the redox potential, and this is more used than peak potential values in most papers [32–35]. In Table 1, the onset potentials for oxidation and reduction with the Ag/AgCl reference electrode are �0.309 and 0.312 V, in [Cu(bipy)2 (NO3)]+ complex case, respectively. Therefore, the HOMO and LUMO energy levels can be calculated to, respectively, �4.692 and �4.071 eV. Therefore, the expected band gap was 0.621 eV. Meanwhile, the LUMO of the Cu(II) nitrate compound is not Y. Kim et al. / Inorganica Ch determined. This result indicated that the Cu ion in the prepared [Cu(bipy)2(NO3)]+ species was reduced more easily than in the complex without a pyridine ring. Additionally, the progressive shift Eqs. (1) and (2), respectively [37,38] FF ¼ Imax � Vmax=Isc � Voc ð1Þ gð%Þ ¼ Pout=Pin � 100 ¼ Imax � Vmax=Pin � 100 ¼ Isc � Voc � FF ð2Þ Fig. 7A shows the I–V curves of TiO2 adsorbed with [Cu(bipy)2- (NO3)]+. The FF, Voc, Jsc, and overall energy efficiency were determined using the equations described above. A TiO2 DSSC assembled with [Cu(bipy)2(NO3)]+ had a Voc of 0.346 V and a Jsc of 0.166 mA/cm2 at an incident light intensity of 100 mW/cm2. The power conversion efficiency of the TiO2/[Cu(bipy)2(NO3)]+ DSSC was 0.032%. In an unusual result, the prepared [Cu(bipy)2 (NO3)]+ dye was well adsorbed on the Ti in the TiO2 film, despite the absence of the COO- group in the bipyridine ring, due to the attachment of two O atoms in NO3 to the Ti metal, as shown in Fig. 6. Naturally, the efficiency was smaller than that of the Ru- complex (about 3.0–5.0%, N719), a commonly used dye in DSSC. The most important fact is that the LUMO energy level of the [Cu(bipy)2(NO3)]+ dye complex is lower than the conduction level of TiO2 particles as shown in figure B), expected a model for energy diagram from the cyclic voltammetry data. Typically to indicate the better optical conversion efficiency, compared to the LUMO energy levels in dye-complex, the conduction band in TiO2 should be low energy. However, the electrons excited from [Cu(bipy)2(NO3)]+ dye could not easy transfer to the TiO2 surface, and consequently, the photoelectric efficiency of TiO2/[Cu(bipy)2 (NO3)]+ DSSC is eventually forced to lower. However, we are expecting the photovoltaic efficiency to be greatly enhanced in at an AM-1.5 radiation angle. The intensities of light were mea- in [Cu(bipy)2(NO3)]+ at lower potential values upon deprotonation of the NO3 groups is in agreement with the destabilization effect of the negative NO3 charge on the bipy p* orbital, which is therefore less effective in accepting back-donation from the metal. Fig. 6 shows the interfacial biding energy between the dye mol- ecule, [Cu(bipy)2(NO3)]+, and the surface of the modified TiO2, which was examined by FT-IR spectroscopy. Generally, the effi- ciency of the charge injection process is strongly dependent on the bonding structure of the dye molecules adsorbed on the TiO2 film. In addition, electron transfer in a DSSC is strongly affected by electrostatic and chemical interactions between the TiO2 sur- face and the adsorbed dye molecules. Regarding the specific adsorption for FT-IR [36], the IR spectra showed absorption at 1600–1500 and 1390–1300 cm�1, which were assigned to the C@C and N@O(–NO2) stretching modes, respectively. After dye- adsorption, the N@O band decreased because it was transferred to the N–O stretching mode due to bidentate coordination of the [Cu(bipy)2(NO3)]+ dye on the surface of the TiO2 films. The bond between NOO� and the surface of the TiO2 film was assumed to be strong due to the perfect bidentate linkage. Furthermore, the IR spectrum of dye-TiO2 showed a broad band at around 500 cm�1, which was assigned to metal–O, due to a new Ti–O bond between the O of NOO� and the Ti atom. Therefore, we suggest an adsorbed model between the surface of TiO2 and the [Cu(bipy)2 (NO3)]+ dye, as shown in Fig. 6B. The photoelectric properties were measured using a voltmeter Acta 365 (2011) 400–407 405 future research if the bipyridine group is modified with electron donating or withdrawing ligands so that it can be combined stably with the transition metals. 4. Conclusion A simple and rapid synthetic procedure, using a conventional hydrothermal method, was used to prepare the [Cu(bipy)2(NO3)]+ complex covering the whole UV-region and part of the visible region, via a perfectly reversible electrochemical process. Single X-ray analysis revealed the presence of five-coordination in the acquired complex with four nitrogen atoms of bipyridine and the oxygen bond of the NO3 � ion. The FL spectra of the complex displayed blue/green luminescence in acetonitrile solvent. The reflectance UV–Vis absorptions in the complex showed three absorptions that were assigned to L–L, MLCT, and d–d transitions. The IR spectra suggested that the two O atoms of NO3 � were attached to the surface of TiO2. The HOMO and LUMO energies of [Cu(bipy)2(NO3)]+ presented to �4.692 and �4.071 eV, respec- tively, and the LUMO energy level of the [Cu(bipy)2(NO3)]+ dye complex is lower than the conduction level of TiO2 particles, and consequently, the photoelectric efficiency of TiO2/[Cu(bipy)2 (NO3)]+ DSSC is to be lower. Therefore, to further enhance the photovoltaic properties in DSSC devices, we intend to fabricate a new transition metal-complex composed of a bipyridyl ring with electron-withdrawing or -donating groups. Acknowledgements This research was supported by the Basic Science Program through the National Research Foundation of Korea (NRF) funded by Ministry of Education, Science and Technology (2009- Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2010.09.041. References [1] C.S. Chou, R.Y. Yang, C.K. Yeh, Y.J. Lin, Powder Technol. 194 (2009) 95. [2] H. An, B. Xue, D. Li, H. Li, Q. Meng, L. Guo, L. 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Empirical formula C H CuN O N(2)–Cu–O(1) 149.87(8) N(4)–Cu–O(1) 100.99(8) C(1)–N(1)–C(5) 119.1(2) C(1)–N(1)–Cu 125.15(19) C(5)–N(1)–Cu 115.31(17) N(1)–C(1)–C(2) 121.8(3) C(3)–C(2)–C(1) 118.8(3) C(2)–C(3)–C(4) 120.0(3) C(3)–C(4)–C(5) 119.0(3) N(1)–C(5)–C(4) 121.3(2) N(1)–C(5)–C(6) 114.3(2) C(4)–C(5)–C(6) 124.4(2) N(2)–C(6)–C(7) 121.5(3) N(2)–C(6)–C(5) 114.5(2) C(7)–C(6)–C(5) 123.9(2) C(6)–C(7)–C(8) 118.7(3) C(9)–C(8)–C(7) 119.6(3) C(10)–C(9)–C(8) 118.9(3) N(2)–C(10)–C(9) 122.4(3) C(10)–N(2)–C(6) 118.9(2) C(10)–N(2)–Cu 127.41(18) C(6)–N(2)–Cu 113.53(16) C(15)–N(3)–C(11) 119.6(2) C(15)–N(3)–Cu 115.91(17) C(11)–N(3)–Cu 123.66(18) N(3)–C(11)–C(12) 121.7(3) C(13)–C(12)–C(11) 118.9(3) C(14)–C(13)–C(12) 119.9(3) C(13)–C(14)–C(15) 119.1(3) N(3)–C(15)–C(14) 120.7(2) N(3)–C(15)–C(16) 115.5(2) C(14)–C(15)–C(16) 123.8(2) N(4)–C(16)–C(17) 121.6(3) N(4)–C(16)–C(15) 115.0(2) C(17)–C(16)–C(15) 123.4(3 C(16)–C(17)–C(18) 118.5(3) C(19)–C(18)–C(17) 119.7(3) C(20)–C(19)–C(18) 119.1(3) N(4)–C(20)–C(19) 121.8(3) C(20)–N(4)–C(16) 119.1(2) C(20)–N(4)–Cu 128.2(2) C(16)–N(4)–Cu 112.19(16) N(5)–O(1)–Cu 104.78(18) O(3)–N(5)–O(2) 121.5(3) O(3)–N(5)–O(1) 121.7(3) O(2)–N(5)–O(1) 116.8(3) 406 Y. Kim et al. / Inorganica Chimica 20 16 6 6 Formula weight 499.93 Wavelength 0.71073 Å Crystal system Monoclinic Space group P21/n Unit cell dimensions a (Å) 11.3342(7) b (Å) 12.2826(8) c (Å) 15.0806(7) b (�) 98.365(5) V (Å3) 2077.1(2) Z 4 Dcalc 1.599 Mg/m3 Absorption coefficient 1.104 mm�1 F(0 0 0) 1020 Crystal size 0.35 � 0.40 � 0.40 mm Theta range for data collection 2.11–25.47� Index ranges 0 < h < 13, 0 < k < 14, �18 < l < 18 Reflections collected 4208 Independent reflections 3857[R(int) = 0.0077] Reflections observed (>2r) 2765 Data completeness, 1.000 Refinement method Full-matrix least-squares on F2 Data/restraints/parameters 3857/0/297 Goodness-of-fit on F2 1.032 Final R indices [I > 2r(I)] R1 = 0.034, wR2 = 0.097 R indices (all data) R1 = 0.059, wR2 = 0.103 Table A1 Summary of the crystal data, structure and refinement for bis (2,20-bipyridine) nitratocopper(II) nitrate. Crystal data and structure refinement 0064865), for which the authors are very grateful. Appendix Largest difference in peak and hole (e Å�3) 0.558 and �0.397 Table A2 Selected bond angles and distances for the structure of bis (2,20-bipyridine) nitratocopper(II) nitrate. Bond lengths (Å) Cu–N(1) 1.974(2) Cu–N(3) 1.988(2) Cu–N(2) 2.026(2) Cu–N(4) 2.112(2) Cu–O(1) 2.114(2) N(1)–C(1) 1.339(4) N(1)–C(5), 1.343(3), C(1)–C(2), 1.381(4) C(2)–C(3), 1.361(5), C(3)–C(4), 1.369(5) C(4)–C(5) 1.382(4) C(5)–C(6) 1.481(4) C(6)–N(2) 1.353(3) C(6)–C(7) 1.371(4) C(7)–C(8) 1.390(4) C(8)–C(9) 1.373(4) C(9)–C(10) 1.364(4) C(10)–N(2) 1.339(3) N(3)–C(15) 1.338(3) N(3)–C(11) 1.341(3) C(11)–C(12) 1.376(4) C(12)–C(13) 1.367(4) C(13)–C(14) 1.367(4) C(14)–C(15) 1.393(3) C(15)–C(16) 1.475(4) C(16)–N(4) 1.345(3) C(16)–C(17) 1.373(4) C(17)–C(18) 1.380(5) C(18)–C(19) 1.368(5) C(19)–C(20) 1.366(5) C(20)–N(4) 1.340(3) O(1)–N(5) 1.256(3) Angles (�) N(1)–Cu–N(3) 176.33(9) N(1)–Cu–N(2) 81.16(8) N(3)–Cu–N(2) 99.60(8) N(1)–Cu–N(4) 103.29(9) N(3)–Cu–N(4) 79.90(8) N(2)–Cu–N(4) 109.07(9) N(1)–Cu–O(1) 90.12(8) N(3)–Cu–O(1) 87.48(8) Acta 365 (2011) 400–407 Derek, N. Robertson, J. Photochem. Photobiol. A 202 (2009) 196. [12] Z. Ning, Q. Zhang, W. Wu, H. Tian, J. Organomet. Chem. 694 (2009) 2705. 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