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Published by kik_saowarose, 2021-08-29 23:18:00

Influence of calcination temperature on structure, morphology, and electrochemical properties of Zn2P2O7 nanostructures

Surfaces and Interfaces 23 (2021) 100961

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Influence of calcination temperature on structural, morphological, and
electrochemical properties of Zn P O nanostructure
2 2 7
Attaphol Karaphun a,d , S. Sawadsitang a,d , T. Duangchuen a,d , P. Chirawatkul , T. Putjuso ,
Pisist Kumnorkaew , S. Maensiri , E. Swatsitang a,d,*
Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), NANOTEC -KKU RNN on Nanomaterials Research and Innovation for Energy, Khon Kaen
University, Khon Kaen, 40002, Thailand
Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang, Nakhon Ratchasima 30000, Thailand
c School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand
Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, 40002, Thailand
School of General Science, Faculty of Liberal Arts, Rajamangala University of Technology Rattanakosin, Wang Klai Kangwon Campus, Hua Hin, Prachaup Khiri khan
77110, Thailand
National Nanotechnology Center (NANOTEC), National Science and Technology, Development Agency (NSTDA), Pathumthani, 12120, Thailand
Keywords: Ammonium zinc phosphate hydrates (NH 4 ZnPO 4 : NZPO) precursor powder was synthesized by a facile hydro-
Zinc pyrophosphate nanostructure (Zn 2 P 2 O 7 : thermal method at 200 C for 8 h. Effect of calcination temperature on structure, morphology and electro-

ZPO Nstr)
chemical properties of zinc pyrophosphate nanostructure (Zn 2 P 2 O 7 : ZPO Nstr) was studied by calcination NZPO
Hydrothermal method

precursor powder at 500 (ZPO5), 600 (ZPO6), 700 (ZPO7) and 800 (ZPO8) C under argon flow for 1 h.
Influence of calcination temperature
Calcination temperature significantly affect structure of samples, since the X-ray diffraction (XRD) analysis
Structure and morphology
confirmed a monoclinic phase with space group P2 1 /n (Z = 4) in NZPO precursor powder, an orthorhombic
Electrochemical properties
phase of γ-Zn 2 P 2 O 7 with space group Pbcm in ZPO5 Nstr, and a monoclinic phase of space group I2/c (Z = 12) in
ZPO6, ZPO7 and ZPO8 Nstr. Moreover, the crystallite size of calcined ZPO Nstr increased with increasing
calcination temperature. The micrographs of all ZPO Nstr revealed by scanning electron microscope (SEM)
indicated the dependence of morphology on calcination temperature i.e. a dense agglomeration of perfect
rectangular bars of smooth surface with irregular size intercalated by small incomplete grown crystals with small
cracked fractures spread on their surfaces was observed in ZPO5 Nstr. For ZPO6 and ZPO7 Nstr, similar
morphology was observed with a less dense agglomeration. Selected area electron diffraction (SAED) patterns of
all samples obtained by transmission electron microscope (TEM) confirmed their crystal structures. The analyzed
data of X-ray absorption near edge spectroscopy (XANES) indicated the presence of Zn 2+ cations in all ZPO Nstr
samples. Electrochemical properties of all active ZPO Nstr working electrodes studied by the cyclic voltammetry
(CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectrum (EIS) using a three-
–electrode system in a 3 M KOH electrolyte displayed the pseudo-capacitor performance correlated with Faradaic
redox reaction. In addition, the specific capacitance (C s ) was found to decrease at higher calcination temperature.
The highest C s of 102.9 F g 1 at 1 A g 1 with 81.58% retention of cycling stability after 1000 cycle test was
achieved in ZPO5 Nstr electrode.
1. Introduction benefit and disadvantage. Among these, supercapacitor (SC) is the
interesting one that also attracts researchers attention due to the simply
From past to present, various types of energy storage systems (ESSs) structural arrangement of either electrochemical double-layer capaci-
such as conventional capacitors, batteries, supercapacitors and fuel cells tive (EDLC) or pseudocapacitive (PDC) type with a chemical redox re-
have been continuously developed for more high performance to meet a action of a Faradaic process functioned by the charge transfer between
rapid growth of global energy demand [1–4]. Each of them has different electrolyte and electrodes that can provide a high capacitance value,

* Corresponding author.
E-mail address: [email protected] (E. Swatsitang).
Received 12 November 2020; Received in revised form 17 January 2021; Accepted 19 January 2021
Available online 23 January 2021
2468-0230/© 2021 Elsevier B.V. All rights reserved.

Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.

Fig. 1. TG and DTG curves of NH 4 ZnPO 4 precursor powder performed at a heating rate of 10 C/min in N 2 atmosphere.
high specific power and fast charging property accompanied by a high one-dimensional Co 2 P 2 O 7 nanorods and employed to be an excellent
energy density and rate capability [4–9]. In fact, one important Co 2 P 2 O 7 electrode for a high-performance pseudo-capacitor in 3 M KOH
parameter that significantly influences the electrochemical perfor- electrolyte, which exhibited the specific capacitance of 483 F g 1 at 1 A
mances of the device is the electrode materials. Accordingly, various g . Besides, Zhang et al., 2017 [29] reported the successful synthesis of
materials have been attempted for this purpose and one of these is the microsphere nickel phenyl phosphonates with the hierarchical
transition metal pyrophosphates such as Co 2 P 2 O 7 [5,10,11], Mn 2 P 2 O 7 flower-like structure self-assembled by the layer-structured nanosheets
[12], Fe 2 P 2 O 7 [13], Cu 2 P 2 O 7 [14–16], Ni 2 P 2 O 7 [8,9,17–20] and for electrode material that displayed the pseudocapacitive performance
Zn 2 P 2 O 7 [21,22] owing to their potent for wide applications, and pres- with specific capacitance of 500.8 F g 1 and retained the value of 305.5
ently used as a promising one for pseudocapacitive type [23–27]. In F g 1 after 1000 cycle test. Furthermore, according to our previous
particular, Pang et al., 2012 [28] synthesized Mn 2 P 2 O 7 report by Karaphun et al., 2019 [18], the electrochemical properties in 3
micro/nano-structures and reported the electrochemical capacitance of M KOH electrolyte of mesoporous Ni 2 P 2 O 7 microplates obtained by

30 F g 1 at a scan rate of 5 mV s 1 in an electrolyte of 1 M Na 2 SO 4 so- calcination of NH 4 NiPO 4 •H 2 O at 600 C for 1 h could exhibit a good
lution. Moreover, the synthesized Co 2 P 2 O 7 nano/micro-structures by cycling stability 90.80% retention after 1000 cycle test with the

calcination of NH 4 CoPO 4 •H 2 O precursor at 500 C could yield the spe- maximum specific capacitance of 905.687 F g 1 at 1 A g . For other
cific capacitance of 367.2 at a current density 0.625 A g 1 , as reported report of Karaphun et al., 2018 [16], the electrochemical results of
by Pang et al., 2013 [11]. In addition, Hou et al., 2013 [24] had Cu 2 P 2 O 7 nanocrystals obtained by calcination of the NH 4 CuPO 4 •H 2 O

developed the efficient one-step template-free strategy to produce the powder at 550 C for 1 h could display a pseudocapacitive behavior

Fig. 2. XRD patterns of (a) NZPO precursor powder, (b) ZPO5, (c) ZPO6, (d) ZPO7 and (e) ZPO8 Nstr.


Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.

Fig. 3. Refinement analysis of (a) ZPO5, (b) ZPO6, (c) ZPO7 and (d) ZPO8 Nstr with their corresponding Williamson-Hall plots in (e), (f), (g) and (h), respectively.
associated with Faradaic redox reactions and achieved the highest spe- with unit cell parameters a = 4.950 Å, b = 13.335 Å and c = 16.482 Å
cific capacitance of 297.521 F g 1 at 1 A g 1 with a good cycling stability [31,33,34]. With the diversity of phase, Zn 2 P 2 O 7 is considered to be a
after 1000 cycle test of 94.04%. In addition to these mentioned pyro- fascinating material that can provide the uniqueness of electrochemical
phosphates, another one that gains much attraction to researcher is properties. From literature, Zn 2 P 2 O 7 nanoparticles could provide the
Zn 2 P 2 O 7 . Usually, at room temperature, Zn 2 P 2 O 7 has a monoclinic space high surface area and yield enough redox-active sites for pseudocapa-
group of I 2/c with the unit cell parameters a = 20.068 Å, b = 8.259 Å, c citive type, leading to the enhancement of specific capacitance [21].
= 9.099 Å and β = 106.35 [30–32]. Furthermore, above transition According to the report by Qian et al., 2018 [22], the electrochemical

temperature of 130 C, Zn 2 P 2 O 7 can transform from α to β- Zn 2 P 2 O 7 properties of Zn 2 P 2 O 7 nanoparticles with micro-/mesoporosity exhibi-
phase with a monoclinic space group of C 2/m , unit cell parameters a = ted the pseudocapacitive active site with the capacitances of 263.2 F g 1
6.61 Å, b = 8.29 Å, c = 4.51 Å and β = 105.4 [31,32]. Moreover, at 5 A g 1 and 132.0 F g 1 at 10 A g . This was suggested to be based on
Zn 2 P 2 O 7 can be found in the γ- Zn 2 P 2 O 7 phase of orthorhombic structure the surface adsorption of electrolyte ions between K + cations on

Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.
Table 1 precursor powder, a chemical reaction can be displayed as in the
Lattice parameters (a, b and c), β angle, cell volume (V), refinement parameters followings,
(R ex , R p , R wp GOF), average crystallite sizes obtained by Williamson-Hall plots
● ●
(D xrd ) with micro-strain (ε) of all ZPO Nstr. 2Zn (NO 3 ) 2 6H 2 O (aq) + 6NH 4 OH (aq) + 2H 3 PO 4 (aq) → 2ZnNH 4 PO 4 H 2 O (aq)
+ 4NH 4 NO 3(aq) + 11H 2 O (aq) (1)
Parameters NZPO ZPO Nstr
Lattice a 8.794 4.950(4) 20.050 20.080 20.081

parameter (4) (1) (1) (9) ZnNH 4 PO 4 H 2 O (aq) → NH 4 ZnPO 4 (s) + H 2 O (g) (2)
(Å) b 5.452 13.299 8.259(5) 8.270(5) 8.286(4)
(1) (5) To obtain Zn 2 P 2 O 7 Nstr powders, NH 4 ZnPO 4 precursor powders were

c 8.963 16.469 9.107(5) 9.097(5) 9.110(4) further calcined in a quartz tube at 500, 600, 700 and 800 C for 1 h
(2) (9) under argon flow and the final products were designated as ZPO5, ZPO6,
β ( ) 90 90 106.27 106.30 106.31
(9) (1) (1) ZPO7 and ZPO8, respectively.
Cell volume (V/10 429.73 1084.16 1447.66 1450.25 1454.81
pm ) 2.2. Characterization
R ex % 6.442 4.817 5.775 5.559 5.578
R p % 5.860 2.970 3.184 3.151 3.161
R wp % 4.476 4.147 4.623 4.620 4.593 To explore a proper calcination temperature for phase formation of
GOF 3.313 2.822 2.906 2.903 2.868 Zn 2 P 2 O 7 Nstr, the obtained NZPO precursor powder was pyrolyzed

Slope - 1.415 0.941 1.119 1.190 under nitrogen flow from RT to 800 C using thermogravimetric (TG)
Intercept - 0.0066 0.0061 0.0046 0.0028 with differential thermogravimetric (DTG) (TG/DTG, Pyris diamond
R 2 - 0.874 0.855 0.856 0.895
Perkin Elmer, USA). X-ray diffraction (XRD, EMPYREAN, PANalytical
D xrd (nm) - 21.08 22.73 30.14 49.51
ε - 0.707 0.470 0.559 0.595 diffractometer with Cu K α radiation, λ= 0.15406 nm) was employed for
structure investigation of all the samples. Morphology of all ZPO Nstr
samples was studied using scanning electron microscopy (SEM, SNE-
Zn 2 P 2 O 7 nanoparticles and OH anions transported in the materials. 4500 M SEM, South Korea). Moreover, their structure was further
Likewise, the porous surface of Zn 2 P 2 O 7 nanoparticles could yield the investigated and confirmed by transmission electron microscopy (TEM,
diffusion paths for electrolyte, that was beneficial to the reversibility of JEOL2010 Japan). X-ray absorption near edge spectroscopy (XANES,
the electrodes, resulting in the high-rate performance and excellent Beam line 1.1W: Multiple X-ray techniques [KKU] at SLRI, Thailand)
cycling stability [35]. was performed in the transmission mode to verify the oxidation states of
However, modification of zinc pyrophosphate (Zn 2 P 2 O 7 ) for the Zn ions in all the ZPO Nstr products. Cyclic voltammetry (CV), galva-
electrochemical capacitance has never been reported. Thus, the main nostatic charge–discharge (GCD) and electrochemical impedance spec-
purpose of this work is to study the effect of calcination temperature on troscopy (EIS) performed in 3 M KOH aqueous electrolyte was carried
phase formation, crystal structure, morphology and electrochemical out by three electrodes arrangement using a potentiostat/galvanostat
properties of Zn 2 P 2 O 7 nanostructure (ZPO Nstr) obtained by calcination station (CS350 electrochemical workstation; Corrtest, Hubei, China).
the hydrothermally obtained NH 4 ZnPO 4 (NZPO) precursor powder at

500, 600, 700 and 800 C for 1h in argon atmosphere. Many techniques
2.3. ZPO electrode fabrication
were employed to characterize the products such as thermogravimetric
(TG) with differential thermogravimetric (DTG) analysis, X-ray diffrac-
To prepare the paste for each ZPO Nstr sample, 80 wt % of each
tion (XRD), scanning electron microscopy (SEM), transmission electron
product, polyvinylidene difluoride (PVDF) binder (10 wt %), activated
microscopy (TEM), X-ray absorption near edge structure (XANES). carbon (AC) black (10 wt %), and N-methyl-2 pyrrolidone (0.5 ml) were
Moreover, the electrochemical performance of active ZPO Nstr working 2
thoroughly mixed using ball milling. After that, a 1.25 × 1.25 cm nickel
electrodes were studied using cyclic voltammetry (CV) and galvano- foam current collector plate was coated with the prepared paste in an
static charge–discharge (GCD) measurements with electrochemical 2

area of ~ 1 cm and oven dried at 80 C for 5 h. Then, these ZPO Nstr-
impedance spectroscopy (EIS) analysis. Interestingly, the maximum coated plates were uniaxially pressed at1.5 ton for further assembly as
specific capacitance in 3 M KOH electrolyte of ZPO Nstr working elec- working electrodes.

trode obtained by calcination NZPO at 500 C for 1 h was 102.9 F g 1 at a
current density of 1 A g 1 and the retention after 1000 cycle test dis-
2.4. Electrochemical properties study
played good cycling stability of 81.58%.
To investigate the electrochemical properties of all the ZNO Nstr-
2. Experimentals
assembled cells, the CV test was performed at scan rate of 5, 10, 20,
50 and 100 mV s 1 in a potential window range of 0 - 0.5 V. Meanwhile,
2.1. Material synthesis
the GCD test employed for the specific capacitance study was performed
at current density of 1, 1.5, 2, 3, 4, 6, 8 and 10 A g 1 in a potential
The hydrothermal method was employed for the preparation of
window range 0 - 0.45 V. The obtained GCD data after 1000 cycle test at
NH 4 ZnPO 4 precursor powder. Firstly, 2 mmol of each ammonium hy- 1
droxide solution (NH 4 OH) and 70% orthophosphoric acid (H 3 PO 4 ) were 10 A g was analyzed to evaluate the stability of all electrodes. Besides,
the EIS measurements were performed at 10 mV DC voltage in a fre-
dissolved in 50 ml deionized (DI) water with a control pH in a moderate
range of 4 - 6 and mechanically stirred for 20 min at room temperature quency range of 0.01 Hz - 100 kHz.
(RT). Secondly, stoichiometric amount of zinc (II) nitrate hexahydrate

(Zn(NO 3 ) 2 6H 2 O, 2 mmol), ethylene glycol (HOCH 2 CH 2 OH, 10 ml) and 3. Results and discussion
ethyl alcohol (CH 3 CH 2 OH, 5 ml) were subsequently added in the
mixture and further stirred at RT for 25 h. Next, the well mixed solution 3.1. Phase formation investigation

was hydrothermally treated at 200 C for 10 h in a Teflon-lined stainless
steel autoclave. Finally, the fine sediment of NH 4 ZnPO 4 precursor Fig. 1 shows the TG-DTG curves of NH 4 ZnPO 4 precursor powder. As

seen in Fig. 1, the TG curve gradually decreases from RT to about 300 C,
powder was rinsed with DI water to obtain a neutral pH and dried in an

◦ corresponding to the exothermic peak of DTG curve at 280.41 C with
electric oven at 85 C for 8 h. According to the synthesis of NH 4 ZnPO 4
the weight loss of approximately 2.2 % owing to the evaporation of

Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.

Fig. 4. SEM images of (a) ZPO5, (b) ZPO6, (c) ZPO7 and (d) ZPO8 Nstr.

water and the loss of some inorganic matter with the formation of zinc
ammonium phosphate hydrate group (NH 4 ZnPO 4 ). From ~350 to 440 2NH 4 ZnPO 4 (s) → Zn 2 P 2 O 7(s) + 2NH 3(g) + H 2 O (g) (3)
◦ C, the TG curve drops rapidly, corresponding to the exothermic DTG

peak at 405.86 C with the major weight loss of approximately 12.4 % Therefore, according to the TG and DTA results, an appropriate range
due to the decomposition of inorganic matter in NH 4 ZnPO 4 and NH 3 in of temperature to be conducted for the influence study of calcination
phase structure. After that, the observed decrease of TG curve from 500 temperature on structure, morphology and electrochemical properties of

to 535 C, correlating to the estimated weight loss of about 1.1 % and the ZPO Nstr is suggested to be in a range of 500 - 800 C. Thus, NH 4 ZnPO 4

appeared exothermic DTG peak at 516.92 C, is attributed to the residual precursor powder was subjected to calcine at different temperatures of

of zinc nitrate and phosphates group [33], and the transformation of 500, 600, 700 and 800 C for 1h as described in the experimental part.
γ-Zn 2 P 2 O 7 phase to α-Zn 2 P 2 O 7 phase is suggested to occur around this
temperature [30,31]. Finally, a clear plateau of TG curve is observed 3.2. Structure examination

from 560 to 800 C, correspond to a constant DTG line, suggesting the
formation of α-Zn 2 P 2 O 7 crystalline phase as provided by the following The XRD patterns of as-prepared NH 4 ZnPO 4 precursor powder and
reaction [31–34,36]. all ZPO Nstr are displayed in Fig. 2(a)–(e). Fig. 2(a) depicts the
diffraction peaks at 2θ angle of 14.20, 19.80, 25.69, 27.94, 32.92 and

Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.

Fig. 5. TEM images with SEAD patterns of (a) ZPO5, (b) ZPO6, (c) ZPO7 and (d) ZPO8 Nstr.

Fig. 6. Normalized Zn K-edge XANES spectra of standards Zn foil and ZnO (Zn ) with those of ZPO5, ZPO6, ZPO7, and ZPO8 Nstr. Inset displays these spectra from
9650 to 9680 eV.
38.69 , corresponding to (101), (110), (012), (112), (020) and (220) diffraction angle 15.80, 17.48, 20.32, 21.29, 23.74, 29.51, 29.75, 31.40,
planes of a monoclinic phase of NZPO with P2 1 /n (Z = 4) space group, 32.96, 35.21, 37.51, 39.82, 41.33, 43.23, 45.15, 47.12, 48.58, 52.14,
and matching to the classified JCPDS no. 00-022-0025 of standard 53.51, 57.40 and 58.11 degree, respectively, matching to those of α -
NH 4 ZnPO 4 [30,32]. For ZPO5 Nstr, the XRD pattern in Fig. 2(b) shows Zn 2 P 2 O 7 phase (JCPDS no. 01-072-1702) [31–34,36,37]. The crystalline
the γ - Zn 2 P 2 O 7 phase with all detectable peaks correspond to an parameters such as lattice constant (a, b, and c) with β angle, cell vol-
orthorhombic phase of γ - Zn 2 P 2 O 7 with space group Pbcm (JCPDS no. ume (V) and other related parameters obtained by the Rietveld refine-
00-049-1240) [30]. However, the XRD results of ZPO6, ZPO7 and ZPO8 ment (i.e. weighted profile residual (R wp ), profile residual (R p ), expected
Nstr, as shown in Fig 2(c)–(e), reveal a monoclinic phase of I2/c (Z = 12) residual (R ex ) and goodness of fit (GOF)), as shown in Fig. 3(a)–(d) for all
space group. In addition, there are more diffraction peaks appeared in a calcined samples, were listed in Table 1. As shown in Table 1, it is
2θ range of 15 - 60 degree i.e. (211), (310), (002), (411), (121), (602), obvious that the value of calculated lattice parameters correspond with
(811), (022), (213), (222), (620), (123), (004), (332), (622), (024), γ-Zn 2 P 2 O 7 standard data for ZPO5 Nstr and a monoclinic phase of
α-Zn 2 P 2 O 7 standard data for ZPO6, ZPO7 and ZPO8 Nstr. Moreover, the
(624), (334), (534), (215) and (814) planes, consistent with the 2θ


Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.

Fig. 7. CV curves at different scan rates 5, 10, 20, 50, and 100 mV s 1 of (a) ZPO5, (b) ZPO6, (c) ZPO7 and (d) ZPO8 Nstr-working electrodes.

average crystallite size (D xrd ) and micro-strain (ε) of all calcined samles in Fig. 5 (a)–(d). As seen in all of these figures, a shape of rectangular bar
were determined by Williamson-Hall equation (shown in Fig. 3(e)–(h)) cannot be seen (as shown by SEM micrographs) due to the sample
and all the values were listed in Table 1. It is clearly seen in Table 1 that preparation process before TEM observation i.e. all samples were soni-
increasing calcination temperature result in the increase of D xrd , cated in ethanol. However, the average particle size of these ZPO Nstr
whereas the ε value decreases from 0.707 for ZPO5 Nstr to 0.470, 0.559 samples was evaluated and found to increase from 0.233± 0.074 to
and 0.595 for ZPO6, ZPO7 and ZPO8 Nstr, respectively, owing to the 0.654 ± 0.183 μm with increasing the temperature of calcination.
growth of Zn 2 P 2 O 7 nanocrystals and the increase of their cell volumes Additionally, the SAED patterns of ZPO5, ZPO6, ZPO7 and ZPO8 Nstr
(V) [33,34]. are depicted in the inset of Fig. 5(a)–(d), respectively. As depicted in the
inset of Fig. 5(a), the halo-like rings with bright spots on their circum-
ferences are observed due to the passing of electron beam through a
3.3. Morphology study
small and thin piece of ZPO5 Nstr, revealing a polycrystalline nature of a
sample. In case of ZPO6 sample, the SAED pattern reveals a more clear
As displayed in Fig. 4(a)–(d) are SEM images with magnification
ring with a more regular pattern of bright spots correspond with its SEM
view depicted on their right hand side, revealing the morphology of all image shown in Fig. 4(b) i.e. a better growth of nanocrystal with a more
ZPO Nstr. As seen in Fig. 4(a), ZPO5 Nstr calcined at low temperature
perfect structure and a larger in crystalline size. For ZPO7 sample, the
reveals the agglomeration of smooth surface - rectangular bars corre- observed SAED pattern is similar to that of ZPO6, however, with a more
sponding to an orthorhombic structure with thickness of about 188 nm
clearer pattern of bright spots correspond with its SEM image (shown in
measured for one selected bar. These bars of irregular size (length, width Fig. 4(c)), revealing a more clearer and larger bars of particles. For ZPO8
and thickness) are densely agglomerated with the intercalation of small
sample, a regular pattern of bright spots with some of the scattered
incomplete grown crystals and the spread of their cracked fractures. For points is detected, corresponding with an irregularly rectangular bars

samples calcined at higher temperature of 600 and 700 C (ZPO6 and
observed by SEM image shown in Fig. 4(d). To verify the crystalline
ZPO7 Nstr) as shown Fig. 4(b) and (c), SEM images display their
structure of all the samples, their SAED patterns in the insets of Fig. 5(a)–
morphology similar to that of ZPO5 Nstr, however, their particle sizes
(d) were indexed and the obtain results reveal the certain crystalline
increase and less dense agglomeration of the particles is observed with
planes correspond to those of the XRD results in Fig. 2(b)–(e). Finally,
more space between them. For ZPO8 Nstr, SEM image in Fig. 4(d) re-
according to the XRD, SEM and TEM results, it can be concluded that
veals a piece of incomplete separation of grown bar with small fractures
calcination temperature can significantly affect the phase formation,
distributed on the surface. Thus, it is difficult to measure the particle size structure, morphology and the quality of the Zn 2 P 2 O 7 nanocrystal
of ZPO Nstr. However, the average length of ZPO6, ZPO7 and ZPO8 Nstr
growth [34].
were measured employing Image J program and found to be about 0.475
± 0.133, 0.644 ± 0.148 and 1.305 ± 0.255 μm, respectively. As a result,
it is obvious that the particle size increases with increasing calcination 3.4. Oxidation state analysis
temperature. Furthermore, the structure and morphology of all ZPO Nstr
were observed and investigated by TEM, and their images are displayed Fig. 6 shows the normalized Zn K-edge XANES spectra of all ZPO Nstr

Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.

Fig. 8. GCD curves at different current densities 1, 1.5, 2, 3, 4, 6, 8 and 10 A g 1 of (a) ZPO5, (b) ZPO6, (c) ZPO7 and (d) ZPO8 Nstr-working electrodes.

Fig. 9. Specific capacitance plots of ZPO5, ZPO6, ZPO7 and ZPO8 Nstr-working electrodes as a function of current density.

and those of standard Zn foil and ZnO (Zn 2+ ) measured in a transmission oxidation state of 2+ for Zn ion in all ZPO Nstr, correspond with the
mode at RT. As depicted in Fig. 6, the energy edge positions of Zn foil report by Gupta et al., 2017 [38]. Usually, the Zn 2 P 2 O 7 crystal structure
and ZnO appear at approximately 9659.00 and 9661.97 eV, respec- consists of the layers of zeolite type that connected ZnO (Zn 2+ ) in the
tively. In addition, the energy edge positions of ZPO5, ZPO6, ZPO7, and P–O network at PO 4 polyhedral sites, which might possibly affect their
ZPO8 Nstr occur at about 9662.49, 9662.59, 9662.72 and 9662.64 eV, electrochemical properties, correlating with the reversible Faradaic
respectively. These values are near to that of ZnO, implying the redox reactions caused by the active Zn 2+ ions and OH electrolyte [11].


Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.
Table 2 peak are observed, correlating to a quasi-reversible electron transfer at
Specific capacitance (C s ) at different current densities from 1 to 10 A g , ca- the electrode/electrolyte interface induced by the Faradaic redox re-
pacity retention after 1000 cycles test and fitting values of the equivalent circuit actions, resulting in the electrochemical oxidation and reduction pro-
elements of all ZPO Nstr-working electrodes. cesses due to the switching of OH ions in KOH solution as reported by
Parameter ZPO Nstr-working electrodes Hou et al., 2013 [24]. The electrochemical mechanism of redox energy
ZPO5 ZPO6 ZPO7 ZPO8 storage of Zn 2 P 2 O 7 Nstr in KOH electrolyte should be attributed to the
C s (F g ) 1 A g ¡1 102.9 93.7 81.2 70.2 diffusion of charge carrier 2OH anion into the surface of Zn 2 P 2 O 7 Nstr
1.5 A g 97.9 89.7 76.0 66.6 [22,24,40–42], according to the following reaction.
2 A g ¡1 94.28 85.2 71.8 64.2
3 A g ¡1 86.8 77.1 65.2 58.1 Zn 2 P 2 O 7 + 2OH ↔ Zn 2 P 2 O 7 (OH) 2 + 2e (4)
4 A g ¡1 80.2 70.7 60.6 52.0
6 A g ¡1 71.0 62.8 53.6 45.2 Moreover, the area enclosed by CV curve of ZPO5 Nstr electrode is
8 A g ¡1 62.0 54.5 45.8 39.2 greater than those of ZPO6, ZPO7 and ZPO8 Nstr-working electrodes,
10 A g ¡1 58.2 50.4 40.7 31.7 suggesting the superior electrochemical capacitance of ZPO5 Nstr elec-
Capacity retention after 1000 cycles test (%) 81.58 80.16 79.25 84.51
trode over those of other electrodes.
R s (Ω) 0.946 0.935 0.939 0.707
R ct (Ω) 0.901 0.697 0.655 0.327 Fig. 8(a)–(d) present the GCD results of all ZPO Nstr-working elec-
CPE CPE_T (x 10 ¡4 ) 0.004 0.012 0.050 0.004 trodes obtained in a voltage range from 0 to 0.45 V at 1, 1.5, 2, 3, 4, 6, 8
CPE_P 0.600 0.774 0.694 0.883 and 10 A g 1 current density. All the plots display a non-symmetric
W W R 3.140 2.476 4.770 5.870
curve, representing the dominant pseudo-capacitor characteristic [5,
16,18,24,39]. Moreover, the analysis of discharge curves of all ZPO
3.5. Electrochemical properties analysis Nstr-working electrodes display an internal resistance (R i ), suggesting
the low conductivity of electrode material in good agreement with the
The CV measurements for all ZPO Nstr-working electrodes were report of Khan et al., [25]. Furthermore, the specific capacitance (C s )
performed at 5, 10, 20, 50 and 100 mV s 1 scan rates of which the results value of each ZPO Nstr-working electrode was determined from the
are shown in Fig. 7(a)–(d). It is seen in Fig. 7(a)–(d) that increasing scan discharge curve at 1, 1.5, 2, 3, 4, 6, 8 and 10 A g 1 current density, using
rate can result in the gradual increase of intensity and area of redox the following equation.
peaks, indicating the reversible redox reaction of a pseudo-capacitance IΔt
type and occurring at the interface of electrode material [5,16,18,22, C s = (5)
24,39]. As a result, a broaden main anodic peak and another of cathodic

Fig. 10. Specific capacitance retention of (a) ZPO5, (b) ZPO6, (c) ZPO7 and (d) ZPO8 Nstr-working electrodes after 1000 cycle of charge-discharge test at 10 A g 1
with inset showing the charge-discharge curve at 10 A g 1 in the 2 nd and 1000 th cycle.


Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.

Fig. 11. (a) Nyquist impedance plots of ZPO5, ZPO6, ZPO7 and ZPO8 Nstr-working electrodes with inset showing the enlargement of plots near the origin and (b) the
corresponding equivalent circuit.

Where I, Δt, m, and ΔV are the constant discharge current (A), the decrease of R s results in the increasing conductivity of electrolyte [24].
discharge time (s), the mass of active materials in the electrode (g) and Next region demonstrates a semicircle arc, belonging to R ct at electro-
the potential window (V), respectively [18]. The plots of C s values as a de/electrolyte interface, as a result of the Faradaic redox process. As
function of current density of these ZPO Nstr working electrodes are seen in Table 2, the R ct values decrease with increasing calcination
shown in Fig. 9 with the obtained values listed in Table 2. In Fig. 9, it can temperature, implying the greater diffusion of electrolytic ions [24,39,
be seen that as the current density increases, the C s values decrease, 41,42]. Finally, an abrupt increase of Z’’ is observed as a straight line in
suggesting to be initiated by the incomplete redox reaction at high a region of low frequency, of which the slope represents the Warburg
current density due to the imperfection of ZPO Nstr-working electrodes resistance (W R ) that describes the ion diffusion process of redox mate-
[24,39–41]. Moreover, it is obvious that ZPO5 Nstr electrode demon- rial. As seen in Table 2, W R values of ZPO5 and ZPO6 Nstr-working
strates a superior C s value to those of other electrodes, correspond with electrodes are lower than those of other ZPO Nstr-working electrodes.
the CV results. Fig. 10(a)–(d) shows the specific capacitance retention The lower value of W R implies the faster ion transferability from the
plots of all ZPO Nstr-working electrodes after 1000 cycle charge/di- electrolyte to the electrodes surface and a greater C s value can be ob-
scharge tests at 10 A g 1 . The specific capacitance retention of ZPO5, tained [24,41,42].
ZPO6, ZPO7 and ZPO8 Nstr-working electrodes were evaluated to be
81.58, 80.16, 79.25 and 84.51 %, respectively. These results indicate a 4. Conclusion
good cycling stability of ZPO Nstr-working electrodes with the achieved

highest value in a sample calcined at high temperature of 800 C (ZPO8 NH 4 ZnPO 4 (NZPO) precursor powder was successfully prepared by a
Nstr), which might be due to the solidity of crystalline structure of facile hydrothermal method. Zn 2 P 2 O 7 (ZPO) Nstr were obtained by

monoclinic α- Zn 2 P 2 O 7 [16]. calcination NZPO powder at 500, 600, 700 and 800 C under argon flow
Fig. 11(a) shows the Nyquist impedance plots of all ZPO Nstr for 1 h. XRD analysis revealed a monoclinic phase of NZPO powder of

working-electrodes in a frequency range of 0.01 Hz - 10 MHz at 10 mV P2 1 /n (Z = 4) space group, whereas the powder calcined at 500 C
DC voltage amplitude. As generally seen, the plot typically consists of a (ZPO5) Nstr showed an orthorhombic γ-Zn 2 P 2 O 7 structure with space
semicircle in a high frequency region and a decline line in a low fre- group Pbcm, and those of samples calcined at higher temperature (ZPO6,
quency region. As depicted in Fig. 11(b) is the Randle’s equivalent cir- ZPO7 and ZPO8 Nstr) were identified to be a monoclinic phase of I2/c (Z
cuit, employed to fit the plots, of which the main elements are solution = 12) space group. The average crystalline size of ZPO Nstr increased
resistance (R s ), charge transfer resistance (R ct ), constant phase element with increasing calcination temperature, corresponding with the
(CPE) and Warburg element (W) [5,18,24,39,41,42]. According to the decrease of ε value due to the growth of Zn 2 P 2 O 7 nanocrystals. As
fitting, the values of these parameters are obtained as listed in Table 2. In revealed by SEM, the morphology of calcined ZPO Nstr was changed
addition, from the magnification of plots near the origin as depicted in from irregularly small size-rectangular bars that densely and randomly
the inset of Fig. 11(a), it is obvious that the obtained results can be agglomerated with the intercalation of small fractures and incomplete
identified into three distinct regions, depending on frequency. In a high grown nanocrystals to larger rectangular bars with less intercalation of
frequency region, the intercept of plot on Z’ axis (near the origin) rep- small fractures and incomplete grown nanocrystals as calcination tem-
resents R s at electrolyte/ZPO Nstr-working electrodes interface. The perature increased. The CV, GCD and EIS results of all ZPO Nstr-working


Surfaces and Interfaces 23 (2021) 100961
A. Karaphun et al.
electrodes exhibited a pseudo-capacitance characteristic, relating to the positive electrode for stable solid-state hybrid supercapacitor, Electrochim. Acta
Faradaic redox reactions. According to these results, it is emphasized 319 (2019) 435–443.
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