Cr-Gd co-doped TiO2 Nanoribbons as Photoanode in Making Dye Sensitized Solar Cell

Pure and (Cr-Gd) co-doped TiO2 nanoribbons (TiNRs) were synthesized by a hydrothermal method at 200°C with stirring at 400 rpm. The structures and morphology of the prepared product were characterized using X-ray diffraction, Fourier transform infrared, Raman spectroscopy, Transmission electron microscopy and Scanning electron microscope. The results showed that the Cr and Gd metals ions dopant were incorporated into interstitial position of the TiO2 lattice nanoribbons with diameter of (30-100) nm and length of few micrometers. The calculated optical band gap for undoped and co-doped of the TiNRs were 3.12 eV and 3.01, respectively. These modified properties of the TiNRs showed an important effect on the conversion efficiency of the assembled dye sensitized solar cells, where the efficiency of the undoped and codoped TiNRs were 1.71 and 1.99, respectively.


Introduction
Titanium dioxide (TiO 2 ) technologically is an important material owing to its notable chemical stability, combination of properties, and environment-friendly [1]. Therefore, great attention has paid to the research dealing with photocatalytic and catalytic properties in wide field of applications, ranging from the energy applications to environmental [2].
Nowadays, the researches show that the reaction between TiO 2 powder and concentrated NaOH solution under hydrothermal conditions is effective in forming onedimensional 1-D structures of TiO 2 [3]. As nanostructures is a transition step bulk solid materials and crystal unit cells, these 1-D TiO 2 -based structures, such as fibers, nanotubes, and ribbons, have an important interests, where it could exhibit novel physicochemical properties and reaction behaviors [4].
The doping of transition metals (TMs) in the TIO 2 help in forming new energy levels between conduction band (CB) bottom and valence band (VB), involving a shift of light absorption towards the region of the visible light. Usually, the activity of Photocatalytic is based on the nature and the amount of doping elements [5]. The doping of TiO 2 with rare-earth elements (REEs) has attracted significant attention regarding its unique 4f electronic configuration, catalytic properties and special luminescent. Multiple electronic configurations can be easily generated and the oxide is polymorphism exhibiting strong adsorption selectivity and good thermal stability. Especially, after doping there is remarkable shift to the visible range [6].
A solar cell directly converts solar energy into electrical power using a semiconductor. Recently, dye-sensitized solar cells (DSSCs) have considered as an attractive alternative for third solar cells generation due to their convenient of manufacturing, high efficiency of power conversion and low cost. Comparing with other organic cells, the dye-sensitized solar cells (DSSCs) is an effective method owing to their high conversion efficiency. The DSSCs consist of five main components transparent conducting glass, semiconductor materials, dyes as a sensitizer, redox-couple electrolyte and counter electrode [7].
In this study the undoped TiO 2 and Cr/Gd co-doped TiO 2 nanoribbons via hydrothermal method have prepared from TiO 2 nanoparticles. In addition, the effect of Cr/Gd dopant on the fabrication and characterization of the dye sensitized solar cell (DSSCs) has been evaluated.

Synthesis TiO 2 Nanoparticles (TiNPs)
TiNPs were synthesized by sol-gel [8] utilizing titanium (IV) isopropoxide (TTIP), distilled water, ethyl alcohol, and HCl as the starting materials. Concentrations the volume ratio of [TTIP: EtOH: H 2 O: HCl] were chosen as (1:7:0.35:0.07). (7 ml) of TTIP was dissolved in 30 ml of anhydrous ethanol to prepare solution A. Meanwhile, 0.5 ml of concentrated HCl was mixed with 20 ml of anhydrous ethanol and 2.5 ml of water to prepare solution B. Solution A was subsequently added dropwise to solution B under vigorous stirring within 30 min. A homogeneous solution was obtained after stirring vigorously for 2h, and a sol was formed. After 24h, the sol was transformed into gel and in order to obtain nanoparticals and the product was separated by centrifugations. Then the product has dried at 80°C for 12h. The dried gel was annealed at 400°C for 4h, to acquire the desired TiO 2 NPs.

Synthesis of Undoped TiO 2 Nanoribbons (TiNRs)
The details of the synthesis steps for the TiNRs were reported in hydrothermal method [9]. Briefly, 0.5 g of Titanium(IV) oxide was mixed with 50 ml of 10 M NaOH alkaline solution and the suspension system had stirred for 1h at RT. Then, the resultant mixture had moved into a teflonlined autoclave, at 200°C and constant magnetic stirring at 400 rpm for 72h. The obtained precipitates from centrifuging process had washed with deionized water. Afterward, the products underwent an ultrasonic treatment with 0.1 M HCl and then washed by distilled water equal to pH=7 of washing solution was obtained. Finally, the product powder of TiNRs was placed into a ceramic crucible for annealing at 400°C for 4h.

Synthesis of Cr-Gd co-doped TiNPs
[Cr 3+ -Gd 3+ ] co-doped TiNRs were prepared as follows: The synthesized anatase TiO 2 powder nanoparticles, (0.5 g), was stirred with (50 ml) of 10 mol/l NaOH aqueous solution containing 0.5 mol% each of Chromium(III) nitrate nonahydrate Cr(NO 3 ) 3 .9H 2 O and Gadolinium(III) nitrate hexahydrate Gd(NO 3 ) 3 .6H 2 O for 1 h at room temperature to form a suspension. Afterward, the mixed solution was transferred into a Teflon-lined autoclave. The final precipitate was treated in the same manner to obtain the TiNRs.

Fabrication of Photoelectrodes and DSSCs
The FTO glass was used as substrate after careful cleaning. Fabrication of photo anodes using TiO 2 NPs and TiNRs which mixed at ratios (100% TiNPs, 75% TiNPs: 25% TiNRs) with total weight of 1 g and ground in a mortar. Ethanol (20 ml), deionized water (2 ml), and CH 3 COOH (0.5 ml) were added dropwise to diffuse the TiO 2 NPs and TiNRs under ultrasonic probe. The TiO 2 composite moved with an excess of ethanol (50 ml) into beaker and stirred at 300 rpm for 3h. Then the composite had concentrated by evaporating. The paste was coated on FTO coated glass by tape casting method. Then the films that had formed were sintered at 450°C for 15 min and at 500°C for 15 min in air. After cooling to 80°C, the TiO 2 electrodes were immersed into (0.5 mM) N3-535 dye solution and kept at room temperature for (20-24 h) to insure complete sensitizer uptake. The platinum-coated FTO is used as the counter. A drop of electrolyte solution was injected into the photoelectrode and then the counter was clamped onto the photoelectrode; the electrolyte solution prepared by dissolving (0.5M) of KI and (0.05M) of I in ethylene glycol. Solar cell assembly; the cell assembly was performed by adding a few drops of (I / I ) electrolyte on the photoanode and fix both electrodes (anode and counter) electrodes facing each other using a clips binder. Finally all the assembled DSSCs are tested by two electrodes potentiostat using (100mW/cm 2 ) deuterium light source.

Scanning Electron Microscopic (SEM)
The SEM micrographs, morphology and particles size distribution of the precursor TiO 2 nanoparticles obtained by sol-gel methods ( Figure 1). The morphology of the sample refers to uniform nanostructures with average size 30-50 nm.  The images show the presence long and straight of ribbonlike with typical lengths is ranging from several micrometers to tens of micrometers. These samples are confirmed the well-defined 1D TiNRs without any sign of an impurity cluster. In contrast, These surface morphologies of TiNRs are not affected by the Chromium and Gadolinium doping, indicating that (Cr 3+ -Gd 3+ ) are successfully incorporated into the TiO 2 lattice crystal, thus eliminating any impurity and confirming that (Cr 3+ -Gd 3+ ) metal clusters are forming from the morphological point of view. In hydrothermal reaction, TiNRs were obtained during the reaction of high concentration of NaOH aqueous solution and pure TiO 2 powder in Teflon line stainless autoclave at suitable hydrothermal temperature and time. The reaction is defined by sequence reaction step below: a) High concentrated alkaline reaction At the first reaction step, the high concentrated of NaOH (alkaline solution) will reacted with TiO 2 powder at temperature of 200°C for 72h. The sodium titanate (Na 2 Ti 2 O 5 .H 2 O) was the main product. (1)

b) Replacement of Na+ with H+
The hydrogenotitanate was formed during the ion exchange between Na + and H + occurs during the acid wash treatment. The reason for this sodium titanate was thermodynamically unstable. The general chemical reaction can be described as follows:

c) Dehydration process
The dehydration process is performed by undergone drying process at 80°C overnight and followed by calcination process at high temperature for two hours. The reaction consists of two steps as follows:

X-ray Diffraction and ICP
X-ray diffraction (XRD) pattern of starting TiO 2 nanoparticles (TiNPs) prepared by sol gel method is shown in Figure 4  The XRD results of the hydrothermally prepared undoped TiO 2 and (Cr 3+ -Gd 3+ ) co-doped TiO 2 using 10 M NaOH and constant magnetic stirring at 400 rpm at 200°C for 72 h are shown in Figure 4 (b and c). XRD patterns illustrate that all of prepared materials are a mixed phase combination of both TiO 2 anatase and TiO 2 -(B). It can be seen that two XRD peak positions of anatase phase are detected at around 2θ = 24.12° and 48.20° which corresponds to (101) 2015) showed that the transformation from TiO 2 -(B) to anatase was completed at 650°C [2]. The result was a mix of two phases (anatase and TiO 2 -(B)) because we have used in our calcination process 400°C. Police et al (2010) confirmed that no peaks of dopant were observed in XRD pattern due to low concentration of dopant loading into TiNRs [12]. The XRD cannot identify the dopant peaks because of their detection limit. The result of (Cr 3+ -Gd 3+ ) in the doped TiO 2 NRs measured by the Inductively Coupled Plasma-Optical Emission spectroscopy (ICP-OES). Greene et al. (2006) confirms the doping process is complex in aqueous system because the doping species would form metal-aquo complexes easily and not incorporate into the crystal lattice [13]. The final doping level is approximately (0.43 and 0.39)% which is lower than the (Cr/Gd) concentration in the precursor solution. The difference between the theoretical and the measured concentration of (Cr/Gd) in the Ti 1-x M x O 2 samples is due to the incomplete degree of hydrothermal reaction of substrates during the synthesis of Ti 1-x M x O 2 . The colored solution after hydrothermal treatment at washing process, confirms that not all the (Cr/Gd) in solution was consumed during the hydrothermal synthesis.

Fourier Transform Infrared Spectroscopy
The studied samples were characterized by FT-IR analysis in the range between (4000-400) cm -1 , where the FT-IR results for the TiNPs are shown in Figure 6 [18]. The peak at about 486 cm -1 might be a sign to bending modes of TiO 6 [19,20]. The broad band (from about 800 till 400 cm -1 ) was assigned to Ti−O and Ti−O−Ti skeletal frequency region. The sodium ions present in the samples are not only physically adsorbed but also incorporate within the lattice, and the composition is other than titanium dioxide [21].

UV-Visible Spectroscopy
The optical absorbance coefficient of a semiconductor close to the band edge can be expressed by the following equation: Where a α is the absorption coefficient, E g is the absorption band gap, A is constant, n depends on the nature of the transitions and may have values (½, 2, 3 / 2 and 3) depending on the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions respectively [22]. The absorption spectra of TiO 2 nanoparticles are shown in Figure 7A (a). The absorption band edges were estimated to be about 381 nm.
The band gap energy can be determined by extrapolation to the zero coefficients, which is calculated from the above equation. As shown in Figure 7B, the intercept of the tangent to the plot (αhυ) 2 versus hυ gives a good estimation of the band gap for this semiconductor of direct band gap. The band gap value of 3.21 eV is for sol-gel derived TiO 2 nanoparticles. Light absorption characteristics of the hydrothermally synthesized 1D TiO 2 NRs and the precursor TiO 2 are shown in Figure 7A (a) (b). The as-synthesized sample exhibit a red shift of the absorption edge and considerable absorption in the visible violet region. As a result of doping process, the band gap is slightly decreased (Figure 7 (c)). According to the TiO 2 energy band structure, the CB bottom and VB top correspond mainly to O 2p and Ti 3d states, respectively [23]. The optical absorption around 378 nm for undoped TiO 2 is only due to the band-to-band (O 2p → Ti 3d) transition, while the slight red shift in [(Cr 3+ -Gd 3+ )] co-doped TiO 2 NRs can be explained as being mainly due to (sp-d) exchange interactions between the band electrons and the localized (d-f) electrons of the ions substituting cations. The s-(d-f) and p-(d-f) exchange interactions give rise to downward shifting of the CB edge and an upward shifting of the VB edge, causing a band gap narrowing [24]. The indirect band gap was determined by the linear fit to the linear portion of the (αhυ) 2 versus photon energy (hυ) plot ( Figure 7B (6) and (7) respectively and are listed in Table 1.

DSSCs Based on Composite TiO 2 NPs/TNRs
% / % +, * * -, 01#234 * $$ * 100 (7) The best energy conversion efficiency 2.04 with full factor of 0.58 was achieved with using TiO 2 -NPs. The co-doping with these two metals are applied to further increase energy conversion efficiency, where each element can separately enhance device properties [25]. In addition, one element can reinforce the effect of the other [26], or may counteract some of the detrimental effects caused by the other element [27]. Cr 3+ doping of TiO 2 led to a clear red shift in the UV/Vis absorption spectrum, evidencing a decrease in band gap and VB shift, which resulted in a slightly lower value of V oc [28], while the worst was on using TiO 2 -NPs/TiO 2 -TNRs, where η% was at minimum 1.71 and the FF was 0.59. This can be attributed to less surface area which reduces the adsorption of the dye [29], and consequently the TiNRs collected electron number is reduced.

Conclusions
The aim of this study is to synthesize the undoped and codoped TiO 2 nanoribbons from the TiO 2 nanoparticles prepared by sol gel method using alkaline hydrothermal method. Both TiO 2 nanoribbons and TiO 2 nanoparticles are used as photoanode in making the dye sensitized solar cell. Then we evaluate the effect of doping process of TiO 2 nanoribbons on the DSSCs efficiency. Shape and size of the TiNRs could be controlled by the reaction temperature and the reaction time. Yet, it is long time method with complicated alkaline washing process. The conversion efficiency of the co-doping TiNRs is better than undoped TiNRs, while the conversion efficiency of TiO 2 NPs of DSSC was the best.