Graphene Electrocatalyst in Dye-Sensitized

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Efficient NiSe-Ni3Se2/Graphene Electrocatalyst in DyeSensitized Solar Cells: the Role of Hollow Hybrid Nanostructure Xiao Zhang, Mengmeng Zhen, Jinwu Bai, Shaowei Jin, and Lu Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02350 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 22, 2016

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ACS Applied Materials & Interfaces

Efficient

NiSe-Ni3Se2/Graphene

Electrocatalyst

in

Dye-Sensitized Solar Cells: the Role of Hollow Hybrid Nanostructure Xiao Zhang,† Mengmeng Zhen,† Jinwu Bai,† Shaowei Jin,*, ‡ Lu Liu*, †

†

College of Environmental Science and Engineering/Ministry of Education Key

Laboratory of Pollution Processes and Environmental Criteria. Nankai University, Tianjin 300071, P. R. China. ‡

School of Physics and Materials Science, Anhui University, Hefei, 230601, P. R.

China.

Author information * Corresponding author. E-mail: [email protected]; [email protected].

ACS Paragon Plus Environment


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ABSTRACT Hollow and hybrid nanomaterials are excellent electrocatalysts on account of their novel electrocatalytic properties compared with homogeneous solid nanostructures. In this report, NiSe-Ni3Se2 hybrid nanostructure with morphology of hollow hexagonal nanodisk was synthesized in situ on graphene. A series of NiSe-Ni3Se2/RGO with different phase constitutions and nanostructures were obtained by controlling the durations of solvothermal treatment. Owing to their unique hollow and hybrid structure, NiSe-Ni3Se2/RGO hollow nanodisks exhibited higher electrocatalytic performance than NiSe/RGO and solid NiSe-Ni3Se2/RGO nanostructure for reducing I3− as counter cell (CE) of dye-sensitized solar cells (DSSCs). Additionally, NiSe-Ni3Se2/RGO hollow nanodisks achieved much lower charge transfer resistance (Rct = 0.68 Ω) and higher power conversion efficiency (PCE) (7.87 %) than those of Pt (Rct =1.41 Ω, PCE = 7.28 %). KEYWORDS: NiSe-Ni3Se2; hollow hybrid nanostructure; mesoporous hexagon; graphene; electrocatalyst.

1. INTRODUCTION Energy conversion fields based on the electrocatalysis frequently used nanostructured materials to enhance the electrocatalytic performance. Such studies have mainly focused on the architecture−function relationships of nanoscale inorganic materials in electrocatalytic processes.1-5 Researches indicated that adjusting the chemical composition and crystal structure of nanoscale inorganic materials contributed significantly to not only obtain unique electrocatalytic property, but also tune their charge transfer and catalytic properties as desired.6-10 Very recently, hollow and hybrid nanostructures of electrocatalysts are widely studied because of their novel electrocatalytic properties in comparison with homogeneous solid nanostructures.11-14 On one hand, the hollow structures increase the surface-to-bulk ratio as well as the contact area between active material and electrolyte.11 Moreover, it can promote


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efficient ion transportation, which are favorable for the adsorption and desorption of reactant.15 On the other hand, the design and architecture of homo/heterojunction nanomaterials can accelerate charge separation and transfer, and optimize band gap position then finally enhance activity of electrocatalysis.16 For example, Chou and coworkers synthesized unique Pt–Ni nanostructure, which boosted the bulk heterojunction polymer solar cells efficiency to 8.48%.17 Lou and coworkers reported that the designed synthesis Co3O4/NiCo2O4 hollow hybrid structures revealed much better electrocatalytic activity in the oxygen evolution reaction than Co3O4.18 As one of the most important functional materials of transition metal chalcogenides, nickel selenides own small difference in electronegativity between Ni (χ = 1.92) and Se (χ = 2.55), which is beneficial to form the narrow band gap of nickel selenides (about 2.0 eV).19-20 Therefore during heterogeneous electrocatalysis reaction, conduction band and valence band of nickel selenides both generate overlaps with the energy levels of electrolyte redox in space charge region. In addition, the great extent overlaps among energy levels can contribute greatly to exchange current between nickel selenides and electrolyte redox. With the high conductivity and good electrocatalytic activity, nickel selenides have shown extraordinary performance in electrocatalysis.21-23 Furthermore, the composite nanojunction of different nickel selenides phases provide easier electrons transfer and more catalytic active sites due to their unique valence electronic configuration and unsaturated selenium atoms.24-25 Nonetheless, most researches about nickel selenides were limited to pure phase and non-hollow nanostructures, and the research about hollow and hybrid nanostructures of nickel selenides has not been reported yet.19, 26-28 It is generally known that graphene, composed of one-atom-thick planar sheets of sp2-bonded carbon atoms with a two-dimensional honeycomb structure, possesses a high surface area for the dispersion of nanostructures and good electrical conductivity for electron transfer.29-31 Herein, we presented a facile one-step solvothermal synthesis of hollow NiSe-Ni3Se2 hybrid nanostructure in situ on reduced graphene oxide (NiSe-Ni3Se2/RGO-HD). To investigate the formation mechanism of NiSe-Ni3Se2/RGO-HD and the effect of the hollow hybrid structure on the



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electrocatalytic performance, NiSe-Ni3Se2/RGO series with different compositions and

nanostructures,

including

NiSe/RGO

nanoparticles,

NiSe-Ni3Se2/RGO

nanoparticles and solid NiSe-Ni3Se2/RGO nanodisks, were synthesized by adjusting the reaction time. The unique hollow and hybrid structure improved the availability of catalytic active sites and facilitate the diffusion of electrons and reactants, eventually endowed the NiSe-Ni3Se2/RGO hollow nanodisks with much better electrocatalytic performance as CEs of DSSCs than NiSe/RGO nanoparticles, NiSe-Ni3Se2/RGO nanoparticles and solid NiSe-Ni3Se2/RGO nanodisks. Owning to the unique hollow hybrid nanostructure of NiSe-Ni3Se2/RGO-HD and fast charge transfer channels of RGO, the fabricated device with NiSe-Ni3Se2/RGO-HD achieved a relatively high PCE of 7.87 %, which is far superior to that of the Pt-based device (7.28 %).

2. EXPERIMENTAL SECTION 2.1 Synthesis of NiSe-Ni3Se2/RGO-HD and NiSe-Ni3Se2/RGO Series. For the preparation of NiSe-Ni3Se2/RGO-HD in situ on graphene, graphite oxide (GO) was firstly prepared by a modified Hummer’s method,32 and the concentration of GO aqueous solution was about 10 mg mL-1. NiSe-Ni3Se2/RGO-HD were prepared by a typical solvothermal method with NiCl2·6H2O and SeO2 as nickel source and selenium source. In a typical synthesis, 3 mmol NiCl2·6H2O was dissolved in 20 mL distilled water, then, 5 mL GO aqueous solution and 1g sodium citrate was added under stirring. The mixed solution was agitated for 1 hour to enable good GO dispersion in the suspension. After this, 3 mmol SeO2 was placed into the mixture. Subsequently, for reducing SeO2 and GO, 10 mL hydrazine hydrate was added dropwise. The resulting solution was put into Teflon-lined autoclave of 50 ml capacity and heated at 180 °C for 15 h. Then, the autoclave was allowed to cool to room temperature naturally. The product was washed with water and absolute ethanol to remove impurities, and then dried at 60 °C. Similarly, NiSe-Ni3Se2/RGO series were synthesized by the same method, except that the reaction times were adjusted to 1h, 2h, 6h and 10h.


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2.2 Characterization of Obtained Samples. The crystallinity and composition of the samples were characterized by X-ray diffraction (XRD, D/max-2500, JAPAN SCIENCE) with Cu Kα radiation (λ = 1.54056 Å). The purity of the samples was performed

by

the

X-ray

photoelectron

spectroscopy

(XPS)

analysis

(PHI5000VersaProbe). The morphology of samples was studied by field-emission scanning electron micro-scopy (FE-SEM, Nanosem 430, FEI). More detailed insight into the microstructure of the sample was given by high-resolution transmission electron microscopy and energy dispersive spectrometer (EDS) linescan (TEM, Tecnai G2 F20, operating at 200kV, FEI). The nitrogen adsorption-desorption measurement was analyzed using a Tristar 3000 nitrogen adsorption apparatus.

3. RESULTS AND DISCUSSION X-ray diffraction (XRD) was used to verify the phase structures of the final products. The diffraction peaks of NiSe-Ni3Se2/RGO-HD in XRD pattern (Figure 1) can be unambiguously indexed and assigned to NiSe (JCPDS Card No. 02-0892) and Ni3Se2 (JCPDS Card No. 19-0841). Moreover, there are no other peaks from crystalline by-products, such as Ni3Se4 and NiO, confirming the successful synthesis of NiSe-Ni3Se2 hybrid. On the other hand, the strongest peak of NiSe belongs to 32.78° (101), accordingly, the strongest peak of Ni3Se2 occurs at 29.55 (110). It can be estimated from the XRD patterns that NiSe (~89 wt%) occupies the major part of NiSe-Ni3Se2 hybrid with a small amount of Ni3Se2 (~11 wt%). XPS spectra were employed to further evaluate the quality and surface composition of the as-synthesized NiSe-Ni3Se2/RGO-HD products. As shown in Figure S1, besides the O KLL, C KLL, and Ni LMM peaks, Ni 2p, Se 3d, C 1s and O 1s peaks can also be observed. No other elemental peaks were observed, indicating that the samples were pure. Furthermore, the C 1s and O 1s peaks demonstrate the existence of graphene in samples. As shown in Figure S1B, the Se 3d features of NiSe-Ni3Se2/RGO-HD have a binding energy of 54.7 eV, which is blue-shifted compared with that of previously reported NiSe.33 Similar phenomena were observed



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for that of Ni 2p (856.5 and 875.0 eV) (Figure S1C). This demonstrates the oxidation state of the Ni and Se changes, arising from the interaction between NiSe and Ni3Se2.34 The morphology of the products was investigated by SEM and TEM measurements. Low magnification SEM image (Figure 2A) shows that NiSe-Ni3Se2/RGO-HD nanodisks with homogeneous size uniformly sprinkle on graphene nanosheets. Furthermore, some nanodisks dispersed on graphene, and some were warpped up by the semitransparent ultrathin graphene nanosheets. High magnification TEM was carried out to further characterize the hexagonal disk shape and hollow structure. The inset in Figure 2B shows that the NiSe-Ni3Se2 presents the morphology of hexagonal disks, which is hard to synthesize since both size and shape need to be uniform. Furthermore, the TEM images in Figure 2C and D show that the center of the individual hexagonal disk is brighter than the edge, which reveals that the synthesized NiSe-Ni3Se2/RGO-HD

owns

hollow

structure.35

The

hollow

structure

of

NiSe-Ni3Se2/RGO-HD was further demonstrated by the EDS linescan (Figure S3). In addition, the average diameter of NiSe-Ni3Se2/RGO-HD is about 100 nm. On the other hand, the shadow in Figure 2B also proves the existence of graphene in NiSe-Ni3Se2/RGO-HD samples. The SEM and TEM measurements demonstrate the hollow hexagonal nanodisks of NiSe-Ni3Se2 in situ on graphene. Nitrogen adsorption-desorption measurement (Figure 3 and inset) was performed to understand

the

porous

structure

of

NiSe-Ni3Se2/RGO-HD.

The

Barrett−Joyner−Halenda (BJH) pore size distribution calculated from adsorption branches demonstrates the presence of mesopores (2−50 nm): 35.8 nm. Nitrogen sorption isotherm of NiSe-Ni3Se2/RGO-HD shows a typical adsorption hysteresis that belongs to type IV curve, indicating that the composites had a mesoporous structure. TEM and the pore size strongly indicate that the as-prepared NiSe-Ni3Se2 hexagonal disks have a mesoporous hollow structure. Such hollow mesoporous structure can facilitate good contact of the internal active materials with I−/I3− electrolyte, and would be beneficial for the electrocatalytic performance.36


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Upon different durations of hydrothermal treatment, we observed an apparenttly time-dependent phase constitution and morphology transformation (Figure 4). Figure 4A shows the evolution of the XRD patterns from NiSe/RGO nanoparticles to NiSe-Ni3Se2/RGO-HD. In the initial stage of reaction, the dissolved SeO2 in our system was reduced to Se by the N2H4·H2O and the produced Se will be further converted to Se2 ̄, whereas the Ni2+ cannot be reduced to Ni atoms at this stage.37-38 Ni2+ reacted with Se2- and produced NiSe, then many primary NiSe monomers aggregate to nanoparticles without a preferential growth orientation, resulting in an irregular shape under hydrothermal conditions.39 Therefore the reactions in the hydrothermal process can be described as follows: 3SeO2 + N2H4·H2O → 3Se + N2 + 3H2O

(1)

2

3Se + 6 OH ̄ → 2Se2 ̄ + SeO3 ̄ + 3H2O

(2)

Ni2+ + Se2 ̄ → NiSe

(3)

Thus, when the reaction time was 1 h, the product was NiSe with the morphology of nanoparticles (NiSe/RGO-NP). With the extension of the reaction time, the excess Ni2+ will be reduced to elemental Ni with the presence of hydrazine hydrate.19 This kind Ni was highly reactive and would react with NiSe to produce Ni3Se2.19 Then the hybrid phases of NiSe-Ni3Se2/RGO were formed at 2h, and the main product was NiSe with a small quantity of Ni3Se2 (NiSe-Ni3Se2/RGO-NP). The corresponding reaction can be described as follows: 2Ni2+ + N2H4·H2O + 4OH ̄ → 2Ni +N2 + 4H2O

(4)

2NiSe + Ni → Ni3Se2

(5)

As shown in the XRD and TEM of obtained samples at different reaction times, as the reaction time further prolonging, hollow hexagonal nanodisks formed gradually with the amount of Ni3Se2 in the obtained samples decreasing.39,

40

After further

increasing the reduction time to 10h (NiSe-Ni3Se2@RGO-HD-10h), the hollow structures of NiSe-Ni3Se2 hexagonal nanodisks could be observed. However the formation of hollow structures of NiSe-Ni3Se2@RGO-HD-10h was incomplete and there were also some nanoparticles and solid nanodisks. When the reaction time was



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15h, the amount of Ni3Se2 and the hollow nanostructure of the NiSe-Ni3Se2 hybrid phases became stable. To elucidate the electrochemical characteristics of Pt and NiSe-Ni3Se2/RGO series electrodes, the Nyquist plots of EIS for the CEs were carried out on dummy cells with a symmetric sandwich-like structure–counter electrode/electrolyte/counter electrode (Figure 5). The corresponding parameters are listed in Table 1. High frequency (corresponding to low Z') intercept on the real axis (Z' axis) represents the series resistance (Rs).41 The semicircle in the high-frequency range results from charge-transfer resistance (Rct) and the corresponding constant phase-angle element (CPE) at the electrolyte–counter electrode interface.42 The semicircle in the low-frequency range (corresponding to a high Z') arises from Nernst diffusion impedance of the I3−/I− couple of the electrolyte.43 Before the formation of NiSe-Ni3Se2/RGO-HD,

the

Rct

values

of

NiSe/RGO-NP

(2.49

Ω),

NiSe-Ni3Se2/RGO-NP (1.73 Ω) and NiSe-Ni3Se2/RGO-SD (2.27 Ω) CEs were higher than Pt CE (1.43 Ω). It is noteworthy to mention that after forming the hollow hexagonal nanodisks Rct values of NiSe-Ni3Se2/RGO-HD-10h (1.09 Ω) and NiSe-Ni3Se2/RGO-HD (0.68 Ω) were much smaller than Pt CE. The lower Rct values of NiSe-Ni3Se2/RGO hollow hybrid structures indicate their superior electrocatalytic activity and better charge transfer ability in comparison with NiSe/RGO and NiSe-Ni3Se2/RGO solid structure.44 Table 1. EIS, Tafel polarization and CV parameters of NiSe-Ni3Se2/RGO series and Pt CEs CEs

Rs/

Jlim/

Rct/ b

J0 /

Epp/

a

Ohm

Ohm

log (mA cm )

log (mA cm )

mVe

NiSe-Ni3Se2/RGO-HD

13.51

0.68

1.85

0.51

281

NiSe/RGO-NP

12.47

2.49

1.57

0.33

414

NiSe-Ni3Se2/RGO-NP

13.18

1.73

1.71

0.45

341

NiSe-Ni3Se2/RGO-SD

13.38

2.27

1.63

0.35

367

NiSe-Ni3Se2/RGO-HD-10h

13.10

1.09

1.78

0.49

290

-2 c


-2 d

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Pt a c

12.99

1.43

1.64

0.47

429

Rs: series resistance; bRct: charge-transfer resistance between the CE and electrolyte;

Jlim: limiting diffusion current density;

d

J0: exchange current density;

e

Epp:

peak-to-peak separation. Figure 6 shows the Tafel Polarization curves of different CEs, which were measured by the dummy cells with Pt and NiSe-Ni3Se2/RGO series CEs. The information regarding the exchange current density (J0) and the limiting diffusion current density (Jlim) related to the electrocatalytic activity of the CEs can be obtained from the curves (Figure S5), and corresponding parameters are listed in Table 1. The intersection of the cathodic branch with the Y-axis can be considered as the limiting diffusion current density (Jlim), which is determined by the diffusion properties of the redox couple and the CE materials. The values of calculated Jlim are in the increasing order of NiSe-Ni3Se2/RGO-HD (1.85 log (mA cm-2)) > NiSe-Ni3Se2/RGO-HD-10h (1.78 log (mA cm-2)) > NiSe-Ni3Se2/RGO-NP (1.71 log (mA cm-2)) > NiSe-Ni3Se2/RGO-SD (1.63 log (mA cm-2)) > NiSe/RGO-NP (1.57 log (mA cm-2)). On the other hand, the large slope for anodic or cathodic branch indicates a high J0, which stands for the good catalytic activity toward triiodide reduction of CEs.45 Likewise, the J0 values of NiSe-Ni3Se2 CEs series also follow an order of NiSe-Ni3Se2/RGO-HD > NiSe-Ni3Se2/RGO-HD-10h > NiSe-Ni3Se2/RGO-NP > NiSe-Ni3Se2/RGO-SD > NiSe/RGO-NP. Both of J0 and Jlim match well with the results of EIS experiments, and further demonstrate the excellent diffusion property and high catalytic activity for reducing I3− of NiSe-Ni3Se2/RGO hollow hexagonal nanodisks CEs. To understand the catalytic activity of the resultant film on the I−/I3− redox reaction, the electrochemical behavior of NiSe-Ni3Se2 series and Pt electrodes has been studied by cyclic voltammetry (CV). CV profiles of NiSe-Ni3Se2 series and Pt at a scan rate of 25 mV s-1 are presented in Figure 7. Two pairs of redox peaks corresponding to I− ↔ I3− interconversion [Red−1 (I3− + 2e = 3I−)/Ox−1 (3I− − 2e = I3−); Red−2 (3I2 + 2e = 2 I3−)/Ox−2 (2I3− − 2e = 3I2)] are detected in each CV curve. From left to right, the



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two couples of the redox current peaks in the CV curves correspond to the I3−/I− and I3−/I2 redox reactions.46 Catalytic activities of different CEs can be analyzed by the peak-to-peak separation (Epp) between Red−1 and Ox−1 and the peak current density for Red1 reaction in lower potential.47 The Epp values of NiSe/RGO-NP (414 mV), NiSe-Ni3Se2/RGO-NP

(341

mV),

NiSe-Ni3Se2/RGO-SD

(367

mV),

NiSe-Ni3Se2/RGO-HD-10h (290 mV), NiSe-Ni3Se2/RGO-HD (281mV) and Pt (429 mV) indicate the better intrinsic electrocatalytic activity of the I−/I3− redox reaction of NiSe-Ni3Se2/RGO-HD-10h and NiSe-Ni3Se2/RGO-HD than other CEs.48 Remarkably, as shown in Figure 7, NiSe-Ni3Se2/RGO-HD-10h and NiSe-Ni3Se2/RGO-HD exhibit higher peak current density in comparison with other CEs, revealing the better electrocatalytic activity for reducing I3− of these two CEs materials.49 Results of CV experiments also agree well with the EIS experiments, and manifest the superior electrocatalytic performance of NiSe-Ni3Se2/RGO hollow hexagonal nanodisks.

Table 2. Photovoltaic parameters of DSSCs with NiSe-Ni3Se2/RGO and Pt CEs Voc (V)a

CEs

Jsc (mA cm-2)b

FFc

PCE (%)d

NiSe-Ni3Se2/RGO-HD

0.75

16.31

0.64

7.83

NiSe/RGO-NP

0.75

13.80

0.60

6.21

NiSe-Ni3Se2/RGO-NP

0.75

14.41

0.65

7.02

NiSe-Ni3Se2/RGO-SD

0.75

14.03

0.64

6.73


0.76

15.48

0.63

7.41

Pt

0.76

15.25

0.63

7.28

a

Jsc: short-circuit current density; bVoc: open-circuit voltage; cFF: fill factor; dPCE:

power conversion efficiency. The photovoltaic property of DSSCs were measured using a conventional N719 dye attached TiO2 nanoparticle layer and an organic solution containing conventional I−/I3− redox species as the photoelectrode and electrolyte, respectively. The current density−voltage (J−V) characteristics of the DSSCs under 1 sun illumination (AM


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1.5G, 100 mW cm−2) are shown in Figure 8. The corresponding values of open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), power conversion efficiency (PCE) are summarized in Table 2. It can be found that phase constitution and structure of NiSe-Ni2Se3 series affect the performance of DSSCs. The DSSC based on NiSe-Ni3Se2/RGO-HD CE has a Jsc of 16.40 mA cm-2, a Voc of 0.75 V, an FF of 0.64, and yields a total PCE of 7.83 %, which are higher than those of Pt CE (Jsc = 15.25 mA cm-2, PCE = 7.28 %). While the cell devices with NiSe/RGO-NP, NiSe-Ni3Se2/RGO-NP, NiSe-Ni3Se2/RGO-SD, NiSe-Ni3Se2/RGO-HD-10h obtained PCE values of 6.21 %, 7.02 %, 6.73 % and 7.41%, respectively. It is a remarkable fact that the Jsc values of the various electrodes are found in the same sequence with PCE values, which can represent the charge transfer ability and catalytic reaction velocity at the CEs materials/electrolyte interface.50 Simultaneously, with much higher Jsc values than other CEs, NiSe-Ni3Se2/RGO-HD (FF = 0.64) and NiSe-Ni3Se2/RGO -HD-10h (FF = 0.63) still exhibited similar even better FF values, revealing their great utilization efficiency of photogenerated electron. Better charge transfer ability and higher catalytic activity spontaneously produced more efficient electrocatalysis and superior PCE of NiSe-Ni3Se2/RGO-HD and NiSe-Ni3Se2/RGO-HD-10h than NiSe/RGO-NP, NiSe-Ni3Se2/RGO-NP and NiSe-Ni3Se2/RGO-SD. The inferior electrocatalytic

performance

of


than

NiSe-Ni3Se2/RGO-HD resulted from that the incomplete hollow nanostructure contributed less to the electrocatalytic performance compared with hollow nanodisks. The above results demonstrate the excellent electrocatalytic performance of NiSe-Ni3Se2 hollow hexagonal nanodisks in situ on graphene in terms of charge transfer ability, diffusion properties and catalytic activity. The excellent performance of NiSe-Ni3Se2/graphene-HD may be attributed to several reasons. First and most importantly, NiSe-Ni3Se2/graphene-HD possesses a hollow mesoporous structure. Such structure is conducive to increase interior reaction space, and improve contact area between NiSe-Ni3Se2/graphene-HD active material and I−/I3− electrolyte. And then, diffusion lengths for both charge and ion transport are shortened, and the availability of the catalytic active sites is improved.36,


51

Secondly, the hybrid


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nanostructures of NiSe-Ni3Se2/graphene-HD partly contribute to exchange current between NiSe-Ni3Se2/graphene-HD and I−/I3− electrolyte and accelerate charge transfer

at

the

NiSe-Ni3Se2/graphene-HD/electrolyte

interface,

which

was

demonstrated by EIS and Tafel Polarization measurements.52 Thirdly, the excellent charge transfer ability of graphene improves the charge transfer among NiSe-Ni3Se2/graphene-HD, in addition, the large specific surface area of graphene greatly promotes the absorption of electrolyte molecules onto the surface of NiSe-Ni3Se2/graphene-HD CEs.53-54

4. CONCLUSION In summary, through a facile one-step solvothermal method we successfully synthesized NiSe-Ni3Se2 hollow hexagonal nanodisks on graphene with excellent electrocatalytic performance in energy conversion field. The formation mechanism of hollow hybrid nanostructure of NiSe-Ni3Se2 was studied by adjusting the durations of solvothermal treatment. The better charge transfer ability and catalytic activity of NiSe-Ni3Se2/RGO

than

NiSe/RGO

and

NiSe-Ni3Se2/RGO

solid

structure

demonstrated the effect of hollow and hybrid nanostructure on electrocatalytic performance. Furthermore, benefitting from the well defined structural features of hollow and hybrid nanostructure, NiSe-Ni3Se2/RGO hollow hybrid nanodisks showed more remarkable electrocatalytic property as CE of DSSC than standard Pt CE. The high quality of NiSe-Ni3Se2/RGO hollow hybrid nanostructure could be extended to obtain advanced electrocatalytic materials by tuning charge transfer and catalytic activity.

ASSOCIATED CONTENT Supporting Information. Fabrication of DSSCs, characterization of CEs and DSSCs, XPS spectra, HRTEM image and EDS linescan profile of NiSe-Ni3Se2@RGO-HD, TEM images of graphene, and annotations of getting J0 and Jlim from the Tafel curves, XRD,

SEM

images

and

photocurrent

density−voltage


characteristics

of

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NiSe-Ni3Se2-24h. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L. L); [email protected] (S. W. J).

ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. 21271108), the Ministry of Science and Technology (Grant 2014CB932001), National Natural Science Foundation of China (No. 21425729), and China–U.S. Center for Environmental Remediation and Sustainable Development.

REFERENCES (1)Sneed, B. T.; Young, A. P.; Jalalpoor, D.; Golden, M. C.; Mao, S.; Jiang, Y.; Wang, Y.; Tsung, C. K. Shaped Pd-Ni-Pt Core-Sandwich-Shell Nanoparticles: Influence of Ni Sandwich Layers on Catalytic Electrooxidations. ACS Nano 2014, 8, 7239–7250. (2) Sun, Y.; Gao, S.; Lei, F.; Xie, Y. Atomically-Thin Two-Dimensional Sheets for Understanding Active Sites in Catalysis. Chem. Soc. Rev. 2015, 44, 623–636. (3) Guo, Z.; Percival, S. J.; Zhang, B. Chemically Resolved Transient Collision Events of Single Electrocatalytic Nanoparticles. J. Am. Chem. Soc. 2014, 136, 8879–8882. (4) Xia, B. Y.; Wu, H. B.; Yan, Y.; Lou, X. W.; Wang, X. Ultrathin and Ultralong Single-Crystal Platinum Nanowire Assemblies with Highly Stable Electrocatalytic Activity. J. Am. Chem. Soc. 2013, 135, 9480–9485. (5) Tang, J.; Gomez, A. Control of the Mesoporous Structure of Dye-Sensitized Solar Cells with Electrospray Deposition. J. Mater. Chem. A 2015, 3, 7830–7839. (6) Biacchi, A. J.; Vaughn, D. D., 2nd; Schaak, R. E. Synthesis and Crystallographic Analysis of Shape-Controlled SnS Nanocrystal Photocatalysts: Evidence for a Pseudotetragonal Structural Modification. J. Am. Chem. Soc. 2013, 135, 11634–11644. (7) Choi, S. I.; Choi, R.; Han, S. W.; Park, J. T. Shape-Controlled Synthesis of Pt3Co



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nanocrystals with High Electrocatalytic Activity toward Oxygen Reduction. Chem. Eur.–J. 2011, 17, 12280–12284. (8) Kim, Y.; Lee, Y. W.; Kim, M.; Han, S. W. One-Pot Synthesis and Electrocatalytic Properties of Pd@Pt Core-Shell Nanocrystals with Tailored Morphologies. Chem. Eur. –J. 2014, 20, 7901–7905. (9) Liyanage, W. P. R.; Wilson, J. S.; Kinzel, E. C.; Durant, B. K.; Nath, M. Fabrication of CdTe Nanorod Arrays over Large Area Through Patterned Electrodeposition for Efficient Solar Energy Conversion. Sol. Energy Mater. Sol. Cells 2015, 133, 260–267. (10)

Mishra, S.; Nath, M. Growth of Vertically Aligned CdTe Nanorod Arrays

through Patterned Electrodeposition. Nano Energy 2013, 2, 1207–1213. (11)

Sun, B.; Feng, X.; Yao, Y.; Su, Q.; Ji, W.; Au, C. T. Substantial Pretreatment

Effect on Co Oxidation over Controllably Synthesized Au/FeOχ Hollow Nanostructures via Hybrid Au/Β-FeOOH@SiO2. ACS Catal. 2013, 3, 3099–3105. (12)

Hong, J. W.; Kang, S. W.; Choi, B. S.; Kim, D.; Lee, S. B.; Han, S. W.

Controlled Synthesis of Pd-Pt Alloy Hollow Nanostructures with Enhanced Catalytic Activities for Oxygen Reduction. ACS Nano 2012, 6, 2410–2419. (13)

Nath, M.; Rao, C. N. R. MoSe2 and WSe2 Nanotubes and Related Structures.

Chem. Commun. 2001, 2236–2237. (14)

Mishra, S.; Song, K.; Ghosh, K. C.; Nath, M. Enhancement of

Superconducting Tc (33 K) by Entrapment of FeSe in Carbon Coated Au–Pd17Se15 Nanoparticles. ACS Nano 2014, 8, 2077–2086. (15)

Pu, J.; Cui, F.; Chu, S.; Wang, T.; Sheng, E.; Wang, Z. Preparation and

Electrochemical Characterization of Hollow Hexagonal NiCo2S4 Nanoplates as Pseudocapacitor Materials. ACS Sustainable Chem. Eng. 2014, 2, 809–815. (16)

Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. Engineering

Heterogeneous Semiconductors for Solar Water Splitting. J. Mater. Chem. A 2015, 3, 2485–2534. (17)

Chou, S. W.; Chen, H. C.; Zhang, Z. Y.; Tseng, W. H.; Wu, C. I.; Yang, Y. Y.;

Lin, C. Y.; Chou, P. T. Strategic Design of Three-Dimensional (3D) Urchin-Like Pt-Ni


Page 14 of 23

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanoalloys: How This Unique Nanostructure Boosts the Bulk Heterojunction Polymer Solar Cells Efficiency to 8.48%. Chem. Mater. 2014, 26, 7029–7038. (18)

Hu, H.; Guan, B. Y.; Xia, B. Y.; Lou, X. W. Designed Formation of

Co3O4/NiCo2O4 Double-Shelled Nanocages with Enhanced Pseudocapacitive and Electrocatalytic Properties. J. Am. Chem. Soc. 2015, 137, 5590–5595. (19)

Zhuang, Z.; Peng, Q.; Zhuang, J.; Wang, X.; Li, Y. Controlled Hydrothermal

Synthesis and Structural Characterization of a Nickel Selenide Series. Chem. Eur. –J. 2005, 12, 211–217. (20)

Xue, M. Z.; Fu, Z. W. Lithium Electrochemistry of NiSe2: A New Kind of

Storage Energy Material. Electrochem. Commun. 2006, 8, 1855–1862. (21) Gao, M. R.; Xu, Y. F.; Jiang, J.; Yu, S. H. Nanostructured Metal Chalcogenides: Synthesis, Modification, and Applications in Energy Conversion and Storage Devices. Chem. Soc. Rev. 2013, 42, 2986–3017. (22)

Swesi, A. T.; Masud, J.; Nath, M. Nickel Selenide as a High-Efficiency

Catalyst for Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9, 1771–1782. (23)

Shi, J. L.; Hu, J. M.; Luo, Y. L.; Sun, X. P.; Asiri, A. M. Ni3Se2 Film as a

Non-precious Metal Bifunctional Electrocatalyst for Efficient Water Splitting. Catal. Sci. Technol. 2015, 5, 4954–4958. (24)

Gong, F.; Wang, H.; Xu, X.; Zhou, G.; Wang, Z. S. In Situ Growth of

Co0.85Se and Ni0.85Se on Conductive Substrates as High-Performance Counter Electrodes for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2012, 134, 10953–10958. (25)

Gao, M. R.; Lin, Z. Y.; Zhuang, T. T.; Jiang, J.; Xu, Y. F.; Zheng, Y. R.; Yu, S.

H. Mixed-Solution Synthesis of Sea Urchin-Like NiSe Nanofiber Assemblies as Economical Pt-Free Catalysts for Electrochemical H2 Production. J. Mater. Chem. 2012, 22, 13662–13668. (26)

Reyes-Retana, J. A.; Naumis, G. G.; Cervantes-Sodi, F. Centered

Honeycomb NiSe2 nanoribbons: Structure and Electronic Properties. J. Phys. Chem. C 2014, 118, 3295–3304. (27)

Lee, C. T.; Peng, J. D.; Li, C. T.; Tsai, Y. L.; Vittal, R.; Ho, K. C. Ni3Se4



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Hollow Architectures as Catalytic Materials for the Counter Electrodes of Dye-Sensitized Solar Cells. Nano Energy 2014, 10, 201–211. (28)

Zhang, X.; Yang, Y. X.; Guo, S. Q.; Hu, F. Z.; Liu, L. Mesoporous Ni0.85Se

Nanospheres Grown in Situ on Graphene with High Performance in Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 8457–8464. (29)

Sun, L.; Bai, Y.; Zhang, N.; Sun, K. The Facile Preparation of a Cobalt

Disulfide-Reduced Graphene Oxide Composite Film as an Efficient Counter Electrode for Dye-Sensitized Solar Cells. Chem. Commun. 2015, 51, 1846–1849. (30)

Janani, M.; Srikrishnarka, P.; Nair, S. V.; Nair, A. S. An in-Depth Review on

the Role of Carbon Nanostructures in Dye-Sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 17914–17938. (31)

Liu, H. W.; Liang, S. P.; Wu, T. J.; Chang, H. M.; Kao, P. K.; Hsu, C. C.;

Chen, J. Z.; Chou, P. T.; Cheng, I. C. Rapid Atmospheric Pressure Plasma Jet Processed Reduced Graphene Oxide Counter Electrodes for Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 15105–15112. (32)

Su, C. Y.; Xu, Y. P.; Zhang, W. J.; Zhao, J. W.; Tang, X. H.; Tsai, C. H.; Li, L.

J. Electrical and Spectroscopic Characterizations of Ultra-Large Reduced Graphene Oxide Monolayers. Chem. Mater. 2009, 21, 5674–5680. (33)

Yu, X.; Sun, W.; Chu, Y. One-Pot Solvothermal Synthesis and Properties of

1d NiSe and NiSe–Ni3S2 Alloyed Compound Nanorod Arrays. New J. Chem. 2014, 38, 70–76. (34)

Liao, Y.; Pan, K.; Pan, Q.; Wang, G.; Zhou, W.; Fu, H. In situ Synthesis of a

NiS/Ni3S2 Nanorod Composite Array on Ni Foil as a FTO-Free Counter Electrode for Dye-Sensitized Solar Cells. Nanoscale 2015, 7, 1623–1626. (35)

Wang, X.; Yu, L. J.; Hu, P.; Yuan, F. L. Synthesis of Single-Crystalline

Hollow Octahedral NiO. Cryst. Growth Des. 2007, 7, 2415–2418. (36)

Xia, Y.; Xiao, Z.; Dou, X.; Huang, H.; Lu, X.; Yan, R.; Gan, Y.; Zhu, W.; Tu,

J.; Zhang, W.; Tao, X. Green and Facile Fabrication of Hollow Porous MnO/C Microspheres from Microalgaes for Lithium-Ion Batteries. ACS Nano 2013, 7, 7083–7092.


Page 16 of 23

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37)

Shi, W. D.; Zhang, X.; Che, G. B. Hydrothermal Synthesis and

Electrochemical Hydrogen Storage Performance of Porous Hollow NiSe Nanospheres. Int. J. Hydrogen Energy 2013, 38, 7037–7045. (38)

Fan, H.; Zhang, M. F.; Zhang, X. W.; Qian, Y. T. Hydrothermal Growth of

NiSe2 Tubular Microcrystals Assisted by PVA. J. Crys. Growth 2009, 311, 4530–4534. (39)

Chen, W. M.; Liu, W.; Chen, C. P.; Wang, R. M.; Feng, Q. R. Single-crystal

MgB2 Hexagonal Microprisms via Hybrid Physical-chemical Vapor Deposition. CrystEngComm, 2011, 13, 3959–3961. (40)

Wang, W. S.; Dahl, M.; Yin, Y. D. Hollow Nanocrystals through the

Nanoscale Kirkendall Effect. Chem. Mater. 2013, 25, 1179−1189. (41)

Liu, F.; Zhu, J.; Hu, L.; Zhang, B.; Yao, J.; Nazeeruddin, M. K.; Grätzel, M.;

Dai, S. Low-Temperature, Solution-Deposited Metal Chalcogenide Films as Highly Efficient Counter Electrodes for Sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 6315–6323. (42)

Li, S. S.; Luo, Y. H.; Lv, W.; Yu, W. J.; Wu, S. D.; Hou, P. X.; Yang, Q. H.;

Meng, Q. B.; Liu, C.; Cheng, H. M. Vertically Aligned Carbon Nanotubes Grown on Graphene Paper as Electrodes in Lithium-Ion Batteries and Dye-Sensitized Solar Cells. Adv. Energy Mater. 2011, 1, 486–490. (43)

Hao, F.; Dong, P.; Zhang, J.; Zhang, Y. C.; Loya, P. E.; Hauge, R. H.; Li, J. B.;

Lou, J.; Lin, H. High Electrocatalytic Activity of Vertically Aligned Single-Walled Carbon Nanotubes Towards Sulfide Redox Shuttles. Sci. Rep. 2012, 2, 2045–2322. (44)

He, Q.; Huang, S.; Zai, J.; Tang, N.; Li, B.; Qiao, Q.; Qian, X. Efficient

Counter Electrode Manufactured from Ag2S Nanocrystal Ink for Dye-Sensitized Solar Cells. Chem. Eur. –J. 2015, 21, 15153–15157. (45)

Patil, S. A.; Shinde, D. V.; Lim, I.; Cho, K.; Bhande, S. S.; Mane, R. S.;

Shrestha, N. K.; Lee, J. K.; Yoon, T. H.; Han, S. H. An Ion Exchange Mediated Shape-Preserving Strategy for Constructing 1-D Arrays of Porous CoS1.0365 nanorods for Electrocatalytic Reduction of Triiodide. J. Mater. Chem. A 2015, 3, 7900–7909. (46)

Yoo, K.; Kim, J. Y.; Lee, J. A.; Kim, J. S.; Lee, D. K.; Kim, K.; Kim, J. Y.;



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Kim, B.; Kim, H.; Kim, W. M.; Kim, J. H.; Ko, M. J. Completely Transparent Conducting Oxide-Free and Flexible Dye-Sensitized Solar Cells Fabricated on Plastic Substrates. ACS Nano 2015, 9, 3760–3771. (47)

Tan, Z.; Liu, P.; Zhang, H.; Wang, Y.; Al-Mamun, M.; Yang, H. G.; Wang, D.;

Tang, Z.; Zhao, H. An in Situ Vapour Phase Hydrothermal Surface Doping Approach for Fabrication of High Performance Co3O4 Electrocatalysts with an Exceptionally High S-Doped Active Surface. Chem. Commun. 2015, 51, 5695–5697. (48)

Jiang, S.; Yin, X.; Zhang, J. T.; Zhu, X. Y.; Li, J. Y.; He, M. Vertical Ultrathin

MoS2 Nanosheets on a Flexible Substrate as an Efficient Counter Electrode for Dye-Sensitized Solar Cells. Nanoscale 2015, 7, 10459–10464. (49)

Liu, J.; Tang, Q.; He, B.; Yu, L. Cost-Effective, Transparent Iron Selenide

Nanoporous Alloy Counter Electrode for Bifacial Dye-Sensitized Solar Cell. J. Power Sources 2015, 282, 79–86. (50)

Zhang, X.; Yang, Y.; Guo, S.; Hu, F.; Liu, L. Mesoporous Ni0.85Se

Nanospheres Grown in Situ on Graphene with High Performance in Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 8457–8464. (51)

Huang, G.; Zhang, L.; Zhang, F.; Wang, L. Metal-Organic Framework

Derived Fe2O3@NiCo2O4 Porous Nanocages as Anode Materials for Li-Ion Batteries. Nanoscale 2014, 6, 5509–5515. (52)

Somorjai, G. A. Introduction to Surface Chemistry and Catalysis, 1st ed;

John Wiley & Sons, Inc: 1994. (53)

Zhen, M.; Zhu, X.; Zhang, X.; Zhou, Z.; Liu, L. Reduced Graphene

Oxide-Supported TiO2 Fiber Bundles with Mesostructures as Anode Materials for Lithium-Ion Batteries. Chem. Eur.–J. 2015, 21, 14454–14459. (54)

Wang, H.; Hu, Y. H. Graphene as a Counter Electrode Material for

Dye-Sensitized Solar Cells. Energ. Environ. Sci. 2012, 5, 8182–8188.


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Figure Captions:

Figure 1. XRD patterns of the obtained NiSe-Ni3Se2/RGO-HD products, the standard NiSe (JCPDS 02-0892) and Ni3Se2 (JCPDS 19-0841).

Figure 2. Typical (A) SEM and (B, C and D) TEM images of NiSe-Ni3Se2/RGO-HD.



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Figure

3.

Barrett−Joyner−Halenda

pore

size

distribution

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plot

and

N2

adsorption/desorption isotherm (inset) of NiSe-Ni3Se2/RGO-HD.

Figure 4. Evolution of the XRD patterns (A) and TEM images with reaction times of 1h (B), 2h (C), 6h (D) and 10h (E).


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Figure 5. Nyquist plots for symmetric cells fabricated with NiSe-Ni3Se2/RGO series and Pt CE.

Figure 6. Cyclic voltammograms curves of iodide/triiodide redox species for NiSe-Ni3Se2/RGO series and Pt CE at a scan rate of 25 mV s−1.



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Figure 7. Tafel polarization curves of I−/I3− symmetrical cells of NiSe-Ni3Se2/RGO series and Pt CE.

Figure

8.

Photocurrent

density−voltage

characteristics

of

DSSCs

with

NiSe-Ni3Se2/RGO series and Pt CE, measured at AM1.5G illumination (100 mW cm−2).


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GRAPHICAL ABSTRACT


Graphene Electrocatalyst in Dye-Sensitized

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