Effectiveness of Nd doping and graphene oxide modification on electrochemical performance of CdSe nanorod material

Accepted Manuscript Title: Effectiveness of Nd doping and graphene oxide modification on electrochemical performance of CdSe nanorod material Authors: Nazanin Hamnabard, Younes Hanifehpour, Sang Woo Joo PII: DOI: Reference:

S1226-086X(17)30028-X http://dx.doi.org/doi:10.1016/j.jiec.2017.01.012 JIEC 3256

To appear in: Received date: Revised date: Accepted date:

28-8-2016 30-12-2016 6-1-2017

Please cite this article as: Nazanin Hamnabard, Younes Hanifehpour, Sang Woo Joo, Effectiveness of Nd doping and graphene oxide modification on electrochemical performance of CdSe nanorod material, Journal of Industrial and Engineering Chemistry http://dx.doi.org/10.1016/j.jiec.2017.01.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effectiveness of Nd doping and graphene oxide modification on electrochemical performance of CdSe nanorod material Nazanin Hamnabard, Younes Hanifehpour*, Sang Woo Joo * School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea E-mail: [email protected]; [email protected]; Tel.: +82-53-810-3239; Fax: +8253-810-2062 (SWJ)

1

Graphical abstract

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Abstract Bare CdSe, neodymium (Nd)-doped CdSe, and a graphene-oxide (GO)/CdSe nanohybrid were prepared using a hydrothermal method. These materials were characterized by XRD, SEM, TEM, Raman spectoscopy, XPS, and the Brunauer-Emmett-Teller (BET) method. The phase stability during cycling and charge-transfer behavior were greatly improved by the formation of the GO-based composite and Nd doping. The GO/CdSe electrode had better electrochemical performance (1042.8 mF.g-1 at 10 mV.g-1 and 3454.5 mF.g-1 at a current density of 9 mA.g-1). Among different levels of Nd doping, the CdSe nanorods doped with 0.015 mmol of Nd showed the highest specific capacitance (1025 mF.g-1 at 10 mV.g-1 and 2389.3 at current density of 9 mA.g-1). Keywords:

Cadmium selenide; Neodymium doping; Graphene oxide composite;

Electrochemical capacitor

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1. Introduction The ever-growing demand for energy motivates researchers to study clean energy and improve energy storage devices. Supercapacitors are being considered for their high power density, charge–discharge characteristics, and environmental friendliness[1-3]. Key concepts in supercapacitor research are the design and simple synthesis of new nanomaterials with low cost and high performance. Two types of supercapacitors are electrical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs have nonfaradic charge separation at the electrode/electrolyte interface, and pseudocapacitors are based on faradic reaction of the electrode material with the electrolyte [1,4,5]. Nanostructured transition metal chalcogenides (TMD) are promising candidates for electronic and optoelectronic devices because they have metallic, semimetallic, semiconducting, and charge density wave behavior, depending on the chemistry and the type of compound [6]. One-dimensional (1D) and two-dimensional (2D) inorganic materials have attracted much attention as a pseudocapacitor material, especially transition metal selenides such as GeSe2, SnSe2, CoSe, MoSe2, and WSe2. These materials are notable for their physicochemical, electrical, optical, and magnetic properties [7-9]. Cadmium selenide (CdSe) is one of the most important II–IV group semiconductors and a type of

cadmium

chalcogenide

that

has

interesting

optoelectronic,

photovoltaic,

electroluminescence, and catalyst properties [5,10,11]. However, to the best of our knowledge, little attention has been paid to CdSe electrode materials for supercapacitors. This could be due to limitations such as poor electrical conduction, a lack of electrochemical cycling stability, and low capacitance. An approach is needed to gain desired properties by improving the synthesis strategies. 4

Graphene has a flexible porous structure, high electrical conductivity, superior mechanical properties, good electrochemical stability, and high surface area. It also creates a more efficient conducting network compared to widely used carbon black and even carbon nanotubes [4,5,1214]. Graphene has higher capacitance than reduced graphene due to the oxygencontaining functional groups of graphene oxide (GO) on its basal planes, which play a significant role in improving pseudocapacitive effects [5,7,12-16]. For some practical applications, doping with nanomaterials such as transition or lanthanide ions is frequently done to improve the material properties, such as the electrical, electrochemical, and optical properties. This results from the nature of the electronic states and the propensity to occupy the crystal structure [1,17-21]. Doping can also be an attractive way to enhance the intrinsic conductivity and capacitive performance of CdSe electrodes [1,22]. There is great interest in the study of nanohybrids called van der Waals heterostructures, which are typically composed of graphene and other nanomaterial. This kind of hybrid could have an excellent electrocatalytic activity due to the presence of conductive graphene sheets and strong synergistic effects between graphene and norganic nanomaterial [23]. We

propose a

hydrothermal approach for fabricating 2D nanostructures of CdSe electrodes, lanthanide metal neodymium (Nd) doped CdSe nanorods, and a GO/CdSe nanohybrid to enhance the conductivity and charge transfer process. CdSe nanorods were formed by a green and facile hydrothermal method, followed by Nd doping and combination with GO. We compared the pseudo-capacitance of the materials and studied the physicochemical properties of the samples using a combination of physical and electrochemical characterization techniques.

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2. Experimental methods 2.1. Materials Graphite powder was purchased from Kanto Chemical Co. Inc., Japan. Carbon black (Vulcan XC-72) was supplied by the Gad Hub Company. Cadmium acetate dihydrate (Cd (CH3COO)2. 2H2O, 99% purity) was obtained from DAEJUNG Pure Chemicals Co. Ltd., Korea. Potassium chloride (KCl, 99% purity), N,N-dimethylformamide (DMF, 99.5% purity), sulfuric acid (H2SO4, 95% purity), hydrogen peroxide (H2O2, 30% purity), nitric acid (HNO3, 61% purity), ethyl alcohol (C2H5OH, 94% purity), and ammonia water (NH4OH, 25-30% purity) were supplied by Duskan Pure Chemicals Co. Ltd., Korea. Potassium permanganate (KMnO4, 99% purity), hydrazine hydrate (N2H4 H2O, 50-60% purity), sodium selenite (Na2SeO3, 99% purity), polyvinylidene fluoride (PVDF), neodymium (III) acetate hydrate (Nd (CH3COO)3.H2O, 99% purity), and polyvinylpyrrolidone (PVP) were obtained from Sigma Aldrich.

2.2. Synthesis of Nd-doped and undoped CdSe nanorods TNd-doped CdSe nanorods were prepared by a one-step hydrothermal method with variable Nd content (0, 0.015, 0.03, 0.05, and 0.075 mmol) using hydrazine hydrate (N2H4-H2O) as a reducing agent [24]. In a typical procedure, an appropriate amount of Cd(CH3COO)2. 2H2O was added to 30 ml of distilled water. After strongly stirring for about 15 min at room temperature, 2.5 ml of ammonia was added to the solution, followed by PVP (50 mg) and a corresponding amount of Nd(CH3COO)3. The solution was left for 1 hour, and then Na2SeO3 (1 mmol) and 4 ml of N2H4 H2O were added to the mixture under constant stirring. The transparent solution was transferred to a 50-ml hydrothermal reactor and maintained at 170oC for 24 h in a furnace (Lindberg/Blue M, Thermo Scientific, Box Furnace). The solution was then cooled to room temperature and centrifuged, washed with DI water and ethanol several 6

times, and dried at 323 K for 18 h in a vacuum oven (DZF-6030A). For comparison, CdSe was also prepared using the same method but without Nd salt. 2.3 Synthesis of GO GO was prepared using natural graphite powder and a simple room-temperature method [25]. In brief, graphite powder (3 g) was added to a mixture of concentrated sulfuric acid (320 mL) and phosphoric acid (80 mL) under constant stirring. Then, 12 g of potassium permanganate was gradually added to the mixture at room temperature. The color of the mixture changed from dark purplish green to dark brown during oxidation. After 5 days of aging with continuous stirring, H2O2 (30 ml) was added drop by drop to quench the solution. The color changed to bright yellow, indicating a high oxidation level. The GO was collected and washed with an aqueous solution of 1 M HCl and DI water several times to remove impurities from the target compound via centrifugation. The product was then dried in a vacuum oven at 333 K for 24 h. 2.4. Preparation of GO/CdSe The GO/CdSe hybrid was prepared via a hydrothermal method [26]. First, 10 mg of GO was dispersed in 20 ml of water and sonicated for 2 h (JAC ultrasonic 2010, KODO Technical Research Co., Ltd) to obtain a stable and homogenous GO solution. Then, 10 mg of CdSe nanorods was dispersed in 20 ml of DI water and sonicated for 2 h, followed by addition of the GO suspension. The mixture was then for 3 h at room temperature to obtain a homogeneous suspension. Next, it was transferred to a Teflon-lined stainless steel autoclave and heated at 150oC for 3 h. After cooling to room temperature, the resulting solid product was centrifuged, washed several times with absolute ethanol and distilled water, and dried at 323 K for 24 h under vacuum. Fig. 1 shows a schematic diagram of the syntheses.

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2.5. Characterization All samples were characterized using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). The phase and crystalline structures were analyzed by XRD (D8 Advance, Bruker Germany) with Cu K radiation (=1.5406 Å) at an accelerating voltage of 40 kV and cathode current of 30 mA. The morphologies of the samples were characterized via field-emission SEM (S-4200, Hitachi, Japan) and high-resolution TEM (HRTEM, G2 F20, Tecnai, Japan). XPS data were obtained using a K-Alpha XPS system (Thermo Fisher Scientific, UK) with a monochromatic Al K source, a spot size of 400 mm, and pass energy of 30 eV. The Gaussian (70%) and Lorentzian (30%) GL3 functions in Avantage software were used for fitting and analyzing the obtained binding energy peaks. The XPS analysis was calibrated using the C1s peak at a binding energy of 284.5 eV for the GO/CdSe sample. Raman spectroscopy measurements were performed at room temperature with 532-nm laser excitation (HORIBA Scientific, Xplora plus, France). The Brunauer-Emmett-Teller (BET) surface area was obtained from N2 adsorption–desorption isotherms using a Quantachrome instrument (AS1, USA). 2.6. Electrochemical measurements All electrochemical experiments were carried out with a CHI660C electrochemical workstation (CH Instruments Inc., USA) using the synthesized samples as a working electrode, Ag/AgCl (3 M KCl) as the reference electrode, and a platinum plate as the counter electrode. The test electrode was prepared by loading a slurry consisting of the active samples (80%), carbon black (15%), and PVDF binder (5%) in DMF on a Ti foil electrode (99.7% purity, 0.1-mm thickness), which was dried at 50oC for 12 h [27,28]. Before deposition, the mixture was sonicated for 2 h, and the rectangular Ti foil substrate (11 cm) was cleaned by soaking in acetone and drying. 8

The weight of the electrodes was determined by a high-precision balance (AR2140, Adventurer, Ohaus, USA). Each experiment was repeated three times with a new pair of sample working electrodes, and the electrolyte solution was KCl (1 M). The cells were studied by cyclic voltammetry (CV) over a voltage range of -0.6 to 0.2 V at different scan rates (10-500 mV/S), as well as galvanostatic charge/discharge at varying current densities, and electrochemical impedance spectroscopy (EIS) in the frequency range of 0.1 Hz to 100 kHz. The specific capacitance (C F/g) was calculated by integrating the area under the CV curves and galvanostatic charge–discharge curve using C =

(

)

∫

( )

and C = ∆

×∆ ×

,

respectively [7,12], where  (Vs-1) is the scan rate, V2 -V1 is the potential window, m (g) is the mass of the active material, I (A) is the discharge current, t (s) is the discharge time, and V (V) is the potential change during discharge.

3. Results and discussion 3.1 XRD analysis Fig. 1 shows the XRD patterns. All the diffraction peaks of the samples can be readily indexed to the those typical of pure well-crystallized CdSe nanorods and are in good agreement with the values in the literature (JCPDS No. 08-0549) [24, 29]. No impurity peaks were observed in the XRD data of the Nd-doped CdSe nanorods, indicating that the hydrothermal method was successful and that Nd ions were dispersed uniformly in the CdSe lattice. No obvious peak was observed for GO in the GO/CdSe sample due to the lower percentage of the crystalline phase of the carbon material, which led to a very weak XRD signal for the graphitic carbon that could not be distinguished from the background noise. However, the XPS measurements demonstrate the presence of carbon in the GO/CdSe sample since amorphous and crystalline phases of material appeared in the XPS data. GO was also detected in the GO/CdSe sample by SEM, 9

TEM (Figs. 3 and 4), and Raman analysis (Fig. 6). Compared to the pure CdSe, the slight shift of diffraction peaks to lower angles in all samples indicated the presence of either Nd+3 ions or GO [4,20,30]. The blue-shift observed in the doped materials can be explained by the larger radius of Nd+3(112 nm) compared to Cd+2 ions (109 nm). This confirms the incorporation of Nd ions into the CdSe host lattice and their substitution at the Cd sites. The intensity of diffraction peaks decreased when increasing the doping concentration up to 0.075 mmol, which may be attributed to the strain that resulted from the incorporation of Nd [31]. The (002) reflection showed the strongest intensity peak, indicating [002]-oriented growth of the CdSe nanorods. 3.2 SEM and TEM studies Fig. 3 shows typical images of the samples. The non-doped CdSe (Fig. 3a) has a unique nanorod-like morphology with a smooth and uniform surface. The diameter of the nanorods is around 10-50 nm with length of 20-30 nm. Fig. 3b shows that the addition of 0.015 mmol of Nd had little effect on the morphology of the CdSe nanorods. When increasing the amount of dopant, the density of the nanorod arrays gradually decreased, which could impact the electrochemical properties (Fig. S1). The GO had a flaky structure with stacks of several sheets (Fig. 3c). The GO/CdSe composite (Fig. 3d) was composed of irregular CdSe nanorods attached to the GO layers, which might hinder aggregation. The CdSe nanorods were relatively uniform in size and stacked randomly in the GO layers. Fig. 4 shows the TEM images of the samples. The pure CdSe nanocrystals are mostly faceted with rod morphology, and the sizes are between 10 and 50 nm with diameters of 20-22 nm (Fig. 4a). The A typical HRTEM image of a single nanorod is shown in Fig. 4b, and the distinct lattice fringe confirms that the nanorod is a well-crystallized single crystal. Lattice planes with a spacing distance of about 0.35 nm can be seen along the growth direction, which corresponds 10

to the {111} lattice planes of the individual CdSe [31]. To observe the morphology of the Nd-doped CdSe in more detail, TEM and HRTEM images of nano-arrays with rod-like morphology are shown in Figs. 4c and d. The fringe patterns indicate that the Nd-doped CdSe nanorods are single crystalline in nature. As shown in Fig. 4d, the HRTEM images rule out the presence of any unwanted impurity phase in the sample and confirm the complete substitution of Nd ions in the CdSe lattice, which is in good agreement with the Rietveld analysis and SEM results. Moreover, there is a noticeable increase in the interlayer distance from 0.35 to 0.37 nm due to the incorporation of Nd ions in the CdSe nanorods, which can shift the XRD peaks toward lower diffraction angles (see section 3.1) [ 32]. Fig. 4e clearly shows that the CdSe nanorods are well distributed and wrapped in the transparent GO layers in the GO/CdSe composite. The interplanar spacing determined from the HRTEM image in Fig. 4f is about 0.36 nm, which is consistent with the (111) planes of CdSe nanorods. The clear interface between one CdSe nanorod and one graphene layer can be seen in Fig. 4f, which shows good adhesion between the two components in the composite. The uniform CdSe/GO heterostructure shown in Figs. 4e and f can be ascribed to the facile physical method. Initially, the ultrasonication and continuous stirring facilitate uniform nanorod distribution in the graphene matrix, and the temperature in the hydrothermal method leads to bonding between the GO and CdSe nanorods. 3.3. XPS study XPS can provide detailed information on the chemical composition and bonding of samples. Fig. S(2) shows evidence of Cd, Se, Nd, O, and C. The intensity of the oxygen peak around 500 eV increases from the pure CdSe to the Nd-doped CdSe and the GO/CdSe hybrid due to the oxygen in the GO layers [32]. The binding energies of different elemental core-level spectra 11

and the corresponding elemental compositions are given in Table 1 and Table 2, respectively. Fig. 5 shows the narrow scan spectra of the Cd 3d, Se 3d, C 1s, and O1s regions. Double peaks of spin-orbit components (3d

3/2

and 3d

5/2)

of the Cd+2 are observed at 412.1 ± 0.1 eV and

405.4 ± 0.1 eV for pure CdSe, at 411.46 ± 0.1 eV and 404.76 ± 0.1 eV for Nd-doped CdSe, and at 412.3 ± 0.1 eV and 405.6 ± 0.1 eV for GO/CdSe. The difference between the binding energies of Cd 3d5/2 and Cd 3d3/2 is 6.7 eV, which corresponds to the presence of the Cd+2 oxidation state at the surface for all samples [34]. The Se 3d5/2 peak at a binding energy of 54.26 ± 0.1 eV and the Se 3d3/2 peak at 55.10 ± 0.1 eV were assigned to Se bonded to Cd for the bare CdSe and GO/CdSe composite. Also, the XPS spectra of Nd-doped CdSe nanorods exhibited peaks at 54.46 and 53.59 eV originating from Se, as shown in Fig. 5b [35]. There are no extra decoupled components in the fitted Se curve, which allows us to exclude the possibility of impurities such as SeO2 or Se. Fig. S(3) shows the O 1s core level spectra of the Nd-doped CdSe. The peak around 532.68 eV can be attributed to the surface hydroxyl groups of chemisorbed water molecules on the CdSe nanorods [4]. Furthermore, there is no peak around 529-530 eV, indicating there are no O2- ions due to Cd or Nd oxides [4, 36]. The of XPS results in Fig. 5(c) show coupled peaks around 1004.74 and 981.8 eV [20], which indicate the presence of Nd 3d within the sample. This suggests that the Nd dopants can be incorporated into the CdSe lattice as Nd 3d ions in place of the Cd 3d ions. Moreover, the peak positions for Nd-doped CdSe shifted, which confirms the substitution of Nd ions at Cd sites [36]. The signals from carbon (285.15 eV (C-C), 286.22 eV (C-O), 287.40 eV (C=O), 288.86 eV (COOH)) and oxygen (532.07(C=O), 532.96 (C-O), 533.37 (OH))

4,5,25

in the GO are assigned to the incorporation of GO in the GO/CdSe

composite ( Figs. 5d and e) The C/O intensity ratio of GO/CdSe is 2.35, where C refers to the sum of C-C/C=C bonds, and 12

O refers to all combinations of carbon and oxygen bonds, including C-O, C=O, and O=C-OH [10]. It is apparent in Fig. 5e that the areas of oxygen-containing functional group peaks are high, indicating that no reduction occurred for the GO/CdSe composite [5]. The Se/Cd ratio was consistent among all samples, as shown in Table 2. The ratio clearly increases with the Nd doping, which can indicate the presence of Nd into the CdSe nanorods [38]. Notably, there is not much change in the Se/Cd ratio of the GO/CdSe sample. 3.4. Raman spectroscopy Fig. 6 shows the Raman spectra of the pure nanorods, the 0.015 mmol Nd-doped CdSe, and the composite. As shown in Figure. 6a, the peaks of the CdSe nanorods correspond to first-order longitudinal optical (LO) and second-order (2LO) phonons, which are close to νLO = 211 cm−1 and ν2LO = 415 cm−1, respectively [41]. The blue shift for LO and 2LO compared to the bulk CdSe can be attributed to material strain during deposition or quantum confinement of the phonon mode due to the size effect [39,40]. Zhang et al. reported that lattice contraction on the surface due to nano-crystallite growth is the main reason for the peak shift [40,41]. The shift in wave number towards a lower value could be due to the increased effective mass of the Cd(Nd)

Se bond because of the larger atomic weight of Nd than Cd42. As shown in

Fig. 6b, the intensity of the LO and 2LO peaks of Nd-doped CdSe is different from that of the pure CdSe nanorods. This can be explained by the Nd+3 affecting the lattice vibration modes and its distribution in the structure [43]. In Fig. 6c, there are two peaks around 204 cm−1 and 412 cm−1 corresponding to the LO and 2LO modes of the CdSe nanorods assembled on the GO layers. They shift slightly to a low frequency compared to pure CdSe nanorods, which may be due to the small size effect [44]. This pattern shows two extra peaks at about 1359 cm−1 and 1597 cm−1 corresponding to disordered (D) and graphitic (G) carbon structures, respectively [13]. The D/G intensity ratio (ID/IG) is 0.90 for bare GO (Fig. S4), but it is higher for the 13

GO/CdSe composites (ID/IG= 1.00). These results indicate that the GO/CdSe sample is a disordered material with dispersed and broken layers GO [45], which is consistent with the XPS and TEM results. 3.5. Formation mechanism We can describe the chemical reaction as follows: +4

→

(

)

+4

(1)

According to Eq. 1, the Cd+2 ions released from Cd(COOCH3)2 in the solution are complexed with NH3 - H2O to form Cd(NH3)4+2, which makes the solution more homogenous and avoids the formation of Cd(OH)2 and Cd(SeO3). + +

→ →

+ +

+ +3

+2

+

(2)

(3)

N2H4 serves as a reducing agent and can convert Na2SeO3 (the water-soluble Se source) into Se-2 and Se2O3-2, as in Eqs. 2 and 3. The oxide product Se2O3-2 could be reduced by N2H4 according to Eq. 2. +

→

(4)

Finally, the Cd+2 ions bond with Se-2 to form CdSe or SeO3-2 and produce CdSeO3. The CdSeO3 can appear as a peak around 58.9 eV in the Se XPS spectrum. However, there is no obvious peak (Fig. 5b), indicating CdSeO3 is not formed or that the amount is too low for detection by XPS. The PVP is a non-ionic molecule that contains a strongly hydrophilic component (the pyrrolidone moiety) and a considerably hydrophobic group. PVP has been successfully used in recent years as a stabilizing agent and a structure-directing agent for the preparation of gold and silver nanorods and nanowires [46]. In the present work, 14

PVP plays a vital role in the nanorod growth due to the formation micelles, which can decrease the aggregation of nanoparticles. With its lone pair of electrons on the nitrogen and oxygen atoms, the PVP adsorbs on the freshly nucleated nanoparticles and forms a shell around them, which prevents grain growth as a consequence of its steric effect [46-48]. The -system and oxygen-containing groups in the GO can also react with the Cd atoms on the CdSe nanorods [49]. The GO also provides highly reactive functional groups on its surface that may serve as immobilizers to hold the CdSe [50]. 3.6. Electrochemical analysis To evaluate the materials for use in the electrodes of supercapacitors, several electrochemical tests were carried out with a three-electrode configuration. Fig. 7a shows the CV curves of the different electrodes at a scan rate of 10 mV S-1 in a 1 M KCl aqueous solution with an optimized potential range of -0.6 to +0.2 V. The CV curve has a quasi-rectangular shape that indicates capacitive behavior, which arises from the charge and discharge of the electric double layer. GO/CdSe has the largest specific capacitance (1042 mF.g-1), while the bare CdSe and the samples doped with Nd contents of 0.015, 0.03, 0.05, and 0.075 mmol show capacitance values of 1011, 1025, 911, 889, and 849 mF.g-1 at the same scan rate, respectively. Fig. 7b shows the specific capacitance versus the scan rate from 0.01 to 0.5 V s−1. The specific capacitance of GO/CdSe is 1042 mF.g-1 at 10 mV s−1, and it decreases to 285 mF.g-1 as the scan rate increases to 500 mV s−1. This also occurred for the other samples due to an ion exchange mechanism [4]. The capacitive behavior was also studied using galvanostatic charge/discharge curves at current densities of 0.008 to 0.06 A.g-1 (8 to 60 mA.g-1) in a potential window of -0.2 to 0.5 V.s-1. As shown in Fig. 7c, the charge/discharge curves are perfectly linear without any IR drop, which indicates prominent capacitive behavior of the electrodes [51]. Compared with the other samples, the GO/CdSe showed a longer charge/discharge time in the V–t curves, indicating 15

larger capacitance. This was consistent with the CV results. Fig. 7d shows the typical comparative charge–discharge profiles of the sample at different current densities. The capacitance of the GO/CdSe, Nd-doped CdSe (0.015, 0.03, 0.05, and 0.075 mmol), and bare CdSe was calculated to be 3454, 2389, 647, 623, 433, and 1320 mF.g1

, respectively. These values were obtained at a current density of 0.008 A.g-1 by applying the

equation C = It/mV. The GO/CdSe nanorods had much higher specific capacitance than the Nd-doped CdSe and bare CdSe. The specific capacitance is improved by the presence of GO layers and the formation of graphene conductive networks. In Figs. 7b and d, the CdSe nanorods doped with 0.075 mmol of Nd shows the lowest capacitance, which increases among the other samples in the following order: 0.075 mmol < 0.05 mmol < 0.03 mmol < bare CdSe < 0.015 mmol < GO/CdSe. The higher capacitance of the GO/CdSe can be attributed to the synergistic effect between the CdSe and GO. Anchored CdSe on the GO sheets act as spacers that can effectively prevent aggregation and restacking of the GO sheets, leading to higher capacitance [5,13]. In order to study the quantifiable influence of Nd doping, the specific capacitances of samples were calculated from the CV and charge/discharge curves at 0.01 V.s-1 and 0.008 A.g-1, as shown in in Figs. 8a and b, respectively. The results show that low Nd doping (0.015 mmol) led to the greatest increase in specific capacitance, and the sample with 0.075 mmol of Nd showed the lowest increase. At a scan rate of 0.01 V.s-1, doping with 0.015 mmol of Nd led to a specific capacitance of 1025 mF.g-1, which is higher than the other values of 1011 mF.g-1 (bare CdSe), 910.7 mF.g-1(0.03 Nd), 889.7 mF.g-1 (0.05 Nd), and 849.2 mF.g-1 (0.075 Nd). This result indicates that Nd doping significantly improves the pseudo-capacitance of the CdSe nanorods. Incorporating Nd also prevents the aggregation of CdSe nanorods, which increases the total 16

surface area, electrical conductivity, and specific capacitance [52]. However, Fig. 8 shows a stress reduction of the specific capacitance with more Nd doping. This may be attributed to the lower electroactivity with high Nd doping relative to pure CdSe in terms of pseudocapacitance. At high doping, this result could be due to the presence of Nd nanoparticles on the CdSe nanorods, which prevent reaction between the CdSe and the electrolyte [53]. Ragone plot relative to the energy and power densities was calculated from the galvanostatic capacitance in a voltage window of -0.2 to 0.6 V with current densities of 0.009 to 0.06 A .g-1, as shown in Fig. 8c. The plots were obtained according to the following equation: E = 0.5C∆



(5)

P = ⁄ (6) where P, C, V, t, and E represent the power density (W kg-1), specific capacitance based on the volume of the electroactive material (F.g-1), operating voltage (V), discharge time (s), and energy density (W.h.kg-1), respectively. The maximum energy density of 0.31 Wh.kg-1 was obtained for GO/CdSe at a power density of 3.63 W.kg-1, which is much higher than those of the bare CdSe nanorods (0.12 Wh.kg-1 at 3.53 W.kg-1) and the samples with 0.015 mmol (0.21 Wh.kg-1 at 3.43 W.kg-1), 0.03 mmol (0.058 Wh.kg-1 at 2.28 W.kg-1), 0.05 mmol (0.055 Wh.kg1

at 3.53 W.kg-1), and 0.075 mmol of Nd (0.038 Wh.kg-1 at 3.43 W.kg-1). The high energy

density of the GO/CdSe can be partly credited for the presence of graphene layers as a carbon source [7]. Based on these findings, the CdSe/GO, 0.015 mmol Nd-doped CdSe, and CdSe nanorod electrodes were chosen for subsequent studies. Impedance spectroscopy analysis was conducted, and the corresponding Nyquist plot as shown in Fig. 9. These plots are well fitted using the electrical equivalent circuit shown in the inset, where Rs, Rct, and Rc represent the 17

solution resistance, charge-transfer resistance, contact resistance, and interface resistance among the active material and current collector, respectively [54]. Cdl is related to the double layer capacitance, and Zw represents the Warburg impedance. The Nyquist plots indicate compressed semicircles in the high frequency range of each spectrum, which describe the charge transfer resistance (Rct) of these electrodes. The intersection point of the semicircle on the real axis at high frequencies represents the equivalent series resistance (Rs), and the straight lines in the low frequency range are related to ion diffusion into the active materials [5,7-9,52]. The diffusion coefficients (D) of the potassium ions diffusing into the electrode materials are calculated using Eq. (7) [55]:

D=

(7)

where R is the gas constant, T is the absolute temperature, F is Faraday's constant, A is the area of the electrode surface, W is the Warburg impedance coefficient, and C is the molar concentration of K+ ions. σ can be obtained from the linear fitting of Zre vs. ω−1/2 in the medium–low frequency range. The main impedance parameters of the materials are shown in Table (3). Consistent with the other data, GO/CdSe material has the lowest ohmic resistance (0.56 Ω), the smallest chargetransfer resistance (2.87 Ω), and the best electrolyte diffusion (1.1310-18cm2. s-1), suggesting better electrochemical performance than the other samples. Regardless of the impedance data, the 0.015 mmol Nd-doped CdSe nanorods have higher ohmic resistance, charge transfer resistance, and electrolyte diffusion compared to bare CdSe nanorod. According to the characterization data, this could be attributed to the presence of graphene oxide layers in the GO/CdSe electrode. This phenomenon indicates that the synergetic effect between GO and CdSe could accelerate the charge transfer process and increase the reaction rate owing to the 18

high conductivity of the GO/CdSe and the increased contact area of the sample. In addition, the high conductivity of the GO/CdSe could lead to higher energy of the electrons transported to the electrode, resulting in lower charge transfer resistance and high ion diffusion [56]. This can be attributed to the increased surface area (Fig. S6) and improved charge transfer from CdSe nanorods to the substrate via graphene . Long cycle life is a vital parameter for the practical application of supercapacitors [51]. Fig. 10 shows the electrochemical stability of the GO/CdSe electrode as a function of cycle number. These results were obtained at a current density of 0.015 A.g-1 in a voltage window of -0.2 to 0.6 V for up to 1000 cycles. The capacitance decreases by 24% in the first 100 cycles and then remains constant throughout the cycling period. After 1000 cycles, the capacitor retains a high capacitance of 700 mF.g-1, which is about 63% of the initial capacitance (1122 mF.g-1). Conclusion We have used a hydrothermal process for the synthesis of GO/CdSe, Nd-doped CdSe, and pristine CdSe nanorods. The electrochemical performance of the samples was compared. The GO/CdSe nanorod composite showed a maximum specific capacitance of 1042.8 mF.g-1 at 10 mV.s-1 and a value of 3454.5 mF.g-1 at a current density of 9 mA.g-1 in an electrolyte solution of 1 M KCl. It also had a good specific capacitance retention of 63% after 1000 continuous charge/discharge cycles. The higher capacitance of the GO/CdSe nanorods was attributed to the high surface area and conductivity caused by the incorporation of GO. The doped CdSe nanorods with 0.015 mmol of Nd had a great influence on the electrochemical performance compared with the other samples.

19

Acknowledgment This work is funded by the Grant NRF-2015002423 of the National Research Foundation of Korea.

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List of Figures: Fig 1. Schematic diagram of synthesis procedure of (I) CdSe nanorods, (II) Nd-doped CdSe nanorod and (III) GO/CdSe using hydrothermal method Fig 2. XRD patterns of bare CdSe nanorods, Nd-doped CdSe, and Go/CdSe nanorods Fig 3. SEM images of bare (a) CdSe nanorod, (b) 0.015Nd doped CdSe, (c) bare graphene oxide, and (d) GO/CdSe nanorod. Fig 4. TEM and HRTEM images of (a, b) CdSe nanorods, (c, d) 0.075 Nd-doped CdSe nanorods, (e, f) graphene oxide and GO/CdSe nanorods. Insets show the corresponding SAED patterns. Fig 5. High-resolution XPS spectra of (a) Cd 3d, (b) Se 3d, and (c) Nd 4f, (d) C 1s, and (e) O 1s. Fig 6. Raman spectra of (a) bare CdSe nanorods, (b) 0.015 mmol Nd-doped CdSe nanorods, 24

and (c) GO/CdSe nanorods. Fig 7. (a) CV curve of bare CdSe, Nd-doped CdSe, and GO/CdSe samples at scan rate of 10 mV.s-1 in 1M KCl. (b) Specific capacitance of prepared samples at different scan rates (10-500 mV.s-1). (c) Galvanostatic charge–discharge voltage profile of the bare CdSe, Nd-doped CdSe, and GO/CdSe samples at a current density of 9 mA.g-1. (d) Specific capacitance of prepared samples at different current densities. Fig 8. Specific capacitance of Nd-doped CdSe nanorods (0, 0.015, 0.03, 0.05, and 0.075 mmol) from (a) CV curves at 10 mV.s-1 and (b) galvanostatic charge–discharge voltage profile at current density of 9 mA.g-1. (c) Ragone plot related to energy and power densities of prepared samples from galvanostatic discharge. Fig 9. (a) Nyquist plots of the electrochemical impedance spectra for (a) GO/CdSe, (b) 0.015 mmol Nd-doped CdSe, and (c) bare CdSe nanorods. b) Cycle life of GO/CdSe nanorods at 9 mA.g-1 over 1000 cycles. List of Tables: Table 1: XPS peak positions of fitted high-resolution spectra of Cd 3d, Se 3d, Nd 3d, C 1s, and O 1s for the CdSe, Nd doped CdSe, and GO/CdSe

Table 2: XPS analysis results of the synthesized samples, atomic weight percent, and Cd/Se and C/O ratios Table 3: Impedance parameters of the GO/CdSe, 0.015 mmol Nd-doped CdSe and bare CdSe nanorod5

25

26

Figure 1

Figure 2 27

Figure 3

28

Figure 4

29

Figure 5

30

Figure 6

31

Figure 7

32

Figure 8

33

Figure 9

34

Table 1: XPS peak positions of fitted high-resolution spectra of Cd 3d, Se 3d, Nd 3d, C 1s, O 1s and for the CdSe, Nd doped CdSe and GO/CdSe Binding energy (eV) for corresponding peak positions Sample

Cd 3d3/2

Cd 3d 5/2

Se 3d3/2

Se 3d 5/2

Nd 3d3/2

Nd 3d5/2

C-C

C-O

C=O

COOH

C-O

C=O

OH

Pure CdSe

412.13

405.41

55.12

54.26

-

-

-

-

-

-

Nd doped CdSe

411.46

404.76

54.46

53.59

1004.74

981.8

-

-

-

-

-

-

-

CdSe/GO

412.38

405.66

55.10

54.27

-

-

285.15

286.22

287.4

288.86

532.96

532.07

533.37

Tables for Our reference: JIEC 3256

Table 2: XPS analysis results of the synthesized samples and atomic weight percent and Cd/Se and C/O ratios Elemental Elemental percentage ratio Sample

Cd

Se

Nd

C

O

Se/Cd

C/O

Pure CdSe

46.2

53.8

-

-

-

1.16

-

Nd doped CdSe

42.72

51.64

4.64

CdSe/GO

10.05

11.45

-

-

-

1.21

55.07

23.43

1.14

2.35

Table 3: The impedance parameters of the GO/CdSe, 0.015 Nd doped CdSe and Bare CdSe nanorod Sample Pure CdSe 0.015 Nd doped CdSe CdSe/GO

Rs(

Rct(

D( cm2s-1)

0.56

2.87

1.13E-18

2.22

7.95

7.41E-20

3.57

10.71

3.51E-20

35