Preparation of p-type AgCrO2 nanocrystals through low-temperature hydrothermal method and the potential application in p-type dye-sensitized solar cell

Article abstract of DOI:10.1016/j.jallcom.2015.04.072

Abstract The synthesis of nano-sized ternary delafossite oxides with pure crystal phases is of great challenge. We present a novel hydrothermal method for the synthesis of AgCrO₂ nanocrystals with ultrafine size of 10-20 nm at relatively low temperature range (190-230 °C). It is the first time to report that AgCrO₂ nanocrystals can be hydrothermally synthesized at such a low temperature (190°C) and applied as photocathode in dye sensitized solar cells (DSSCs). The as-synthesized AgCrO₂ nanoproducts, including their crystal phases, morphologies, element compositions, valence state information, thermal stability, electrical and optical properties, have been systematically studied. This facile method employed metal nitrates (AgNO₃ and Cr(NO₃)₃) as the starting materials and NaOH as the mineralizer, where Cr(NO₃)₃ undertook the dual functions of Cr³⁺ source material and weak reducing reagent. The in-situ oxidation-reduction reaction between Cr³⁺ and Ag⁺/Cu²⁺ during the hydrothermal crystal growth is the noteworthy feature of this general method. The crystal formation mechanism disclosed in the synthesis of chromium based delafossite oxides will certainly be benefit for the preparation of other delafossite oxides.

Full text of DOI:10.1016/j.jallcom.2015.04.072

Journal of Alloys and Compounds 642 (2015) 104–110
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Preparation of p-type AgCrO2 nanocrystals through low-temperature
hydrothermal method and the potential application in p-type
dye-sensitized solar cell
Dehua Xiong a,b, Huan Wang a, Wenjun Zhang a, Xianwei Zeng a, Haimei Chang b, Xiujian Zhao b,
Wei Chen a, , Yi-Bing Cheng a,c
⇑
a Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
b State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, People’s Republic of China
c Department of Materials Engineering, Monash University, Melbourne, Victoria 3800, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of nano-sized ternary delafossite oxides with pure crystal phases is of great challenge. We
present a novel hydrothermal method for the synthesis of AgCrO2 nanocrystals with ultrafine size of
10–20 nm at relatively low temperature range (190–230 °C). It is the first time to report that AgCrO2
nanocrystals can be hydrothermally synthesized at such a low temperature (190 °C) and applied as pho-
tocathode in dye sensitized solar cells (DSSCs). The as-synthesized AgCrO2 nanoproducts, including their
crystal phases, morphologies, element compositions, valence state information, thermal stability, electri-
cal and optical properties, have been systematically studied. This facile method employed metal nitrates
(AgNO3 and Cr(NO3)3) as the starting materials and NaOH as the mineralizer, where Cr(NO3)3 undertook
the dual functions of Cr3+ source material and weak reducing reagent. The in-situ oxidation–reduction
reaction between Cr3+ and Ag+/Cu2+ during the hydrothermal crystal growth is the noteworthy feature
of this general method. The crystal formation mechanism disclosed in the synthesis of chromium based
delafossite oxides will certainly be benefit for the preparation of other delafossite oxides.
Ó 2015 Elsevier B.V. All rights reserved.
Received 10 February 2015
Received in revised form 9 April 2015
Accepted 11 April 2015
Available online 20 April 2015
Keywords:
Hydrothermal synthesis
p-type semiconductor
Delafossite oxide
Photocathode
Dye-sensitized solar cell
1. Introduction
replacement of conventional NiO, and several successful examples
include CuAlO2 [5,6], CuGaO2 [7–10] and CuCrO2 [11–14]. As
The crystal structure of delafossite oxides, deriving their name
from the mineral CuFeO2, was first confirmed by Pabst in 1946
[1]. The delafossite structure is constructed from alternate layers
of two-dimensional close-packed copper cations with linear
O–Cu+–O bonds and slightly distorted edge shared Fe3+O6 octahe-
dras [2]. To date, numerous delafossite oxides (AMO2, A = Ag or
Cu, M = B, Al, Ga, In, Fe, Cr, Sc, Y, etc.) have been reported to serve
important roles in diverse applications, such as photovoltaics,
transparent photodiode, catalysts, batteries, ferroelectrics and so
on [2,3]. The most attractive features of delafossite oxides should
rest with their high p-type conductivity and optical transparency,
which have been firstly discovered in CuAlO2 by Hisono et al.
and well explained by the ‘‘chemical modulation of valance band’’
theory [4]. Recently, several groups including ours started the
applications of delafossite oxides nanocrystalline as the photocath-
ode materials in p-type dye-sensitized solar cells (DSSCs), in
reflected in all of the previous works [10–14], size control of the
delafossite nanocrystals are critical for the high performance of
p-type DSSCs [15,16].
Generally, delafossite oxides powders could be prepared
through high temperature solid-state reactions [17–19], cation
exchange reactions [19–23], and hydrothermal synthesis [7–14].
However, it is very difficult to synthesize nano-sized crystals of
delafossite oxides, which largely restricts their application fields.
To the best of our knowledge, only a few literatures focused on
the synthesis of nano-sized delafossite oxides; the successful
examples are not exceeding CuAlO2 [24], CuGaO2 [7–10,25] and
CuCrO2 [11–14,26]. Though, more copper based delafossite oxides
(CuAlO2, CuCrO2, CuFeO2, CuScO2 and so on) could be synthesized
readily via high temperature solid-state reactions under N2 or Ar
atmosphere at 800–1200 °C [2], avoiding the valence for the mono-
valent copper (Cu+) being oxidized into bivalent Cu2+, the higher
temperature usually led to excessive growth of big delafossite crys-
tals. Compared with the copper based delafossites, the synthesis of
silver based delafossites is even more difficult. The related reports
⇑
Corresponding author. Tel./fax: +86 27 8779 3867.
E-mail address: wnlochenwei@mail.hust.edu.cn (W. Chen).
http://dx.doi.org/10.1016/j.jallcom.2015.04.072
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

D. Xiong et al. / Journal of Alloys and Compounds 642 (2015) 104–110
105
for the synthesis of silver based delafossite oxides are much fewer.
The inherent cause is suggested to derive from the fact that, the
simple binary oxide of silver (Ag2O) is easy to decompose at a tem-
perature of ꢀ300 °C, which limits the preparation of silver based
delafossites through solid-state reactions. Therefore, it becomes a
reasonable choice to synthesize silver based delafossite oxides at
low-temperature and/or closed reaction systems [2,27]. For exam-
After reaching a homogeneous state, the solution was loaded into
a 100 ml Teflon-lined autoclave, which was sealed and maintained
at 190–230 °C for 60 h. After the reaction finished, the obtained
yellow green precipitate was washed with diluted nitric acid,
deionized water and absolute alcohol in sequence for several
times, and then stored in absolute alcohol solution. Finally, about
2.0 g nano-sized AgCrO2 product could be gained from each
reaction.
ple, AgCrO2 crystals with an average size of around 1 lm have been
synthesized through cation exchange reactions, by heating LiCrO2,
KNO3 and AgNO3 at 350 °C in an evacuated silica tube for 4 days
[19,21]. Recently, AgCrO2 crystals with an average size of 2 lm
Fig. 1 shows the XRD patterns and morphologies of AgCrO2
nanocrystals prepared at different reaction temperature (190 °C,
210 °C and 230 °C). Fig. 1a shows that all of the diffraction peaks
can be indexed to delafossite AgCrO2 (JCPDS File Card No. 70-
1703) with the hexagonal R3m crystal structure, and no impurity
phase can be detected. The full width at half maximum of the
XRD patterns becomes wider as the reaction temperature
decreases, which reflects the crystal size decreases. By applying
the Scherrer equation to the broadened diffraction peaks, the aver-
age crystal sizes at different synthesis temperatures of 190 °C,
210 °C and 230 °C were calculated to be 18.4 nm, 15.9 nm and
14.1 nm, respectively. This result is consistent with their corre-
sponding SEM and TEM images in Fig. 1b–d. From TEM images,
the nanocrystals are with the hexagonal nanoplates morphology.
The observed diameters of the nanoplates obtained at 230 °C,
210 °C and 190 °C are 15–50 nm, 15–30 nm and 10–30 nm, respec-
tively, while their thicknesses are much thinner. From these, it is
known that the decreased reaction temperature leads to slower
crystal growth rate, and therefore smaller crystal size of the
nano-products.
have been prepared under a critical hydrothermal condition
(400 °C, 40 MPa) [27]. Note that, those reported processes are com-
plicated and include tough conditions; to synthesize nano-sized
silver based delafossite oxides still remains as great challenge.
In this work, delafossite AgCrO2 nanocrystals with ultrafine
sizes of 10–20 nm have been synthesized for the first time via a
low temperature hydrothermal method, and its potential as photo-
cathode in p-type DSSC has been primarily examined. The synthe-
sis parameters effecting on the crystal phases and morphologies
have been studied. The element composition and valence state
information, thermal stability, electrical and optical properties of
AgCrO2 nanocrystals have been systematically investigated. At last,
a general crystal formation mechanism for the hydrothermal syn-
thesis of chromium based delafossite oxides (AgCrO2 and CuCrO2
nanocrystals) based on the in-situ oxidation–reduction reactions
have been proposed.
Furthermore, the SEM–EDS mapping results of AgCrO2
nanocrystals are shown in Fig. 2, to reduce the measurement errors
caused by absorption of water molecules on the sample surface,
the AgCrO2 was analyzed after annealing at 120 °C for 2 h in vac-
uum. It can be observed that all of the Ag, Cr and O elements are
homogeneously distributed (Fig. 2a–c), the elementary percent-
ages of Ag (25.75 at.%), Cr (26.88 at.%), O (47.37 at.%) are nearly
consistent with their source materials’ concentrations in the
hydrothermal precursor and close to the stoichiometric proportion
of AgCrO2 (Ag: Cr: O = 1: 1.04: 1.84, see Fig. 2e). In addition, the
elemental chemical states of the AgCrO2 crystals have been inves-
tigated by XPS. The corresponding results are shown in Fig. 3. The
peaks located at near 368.5 eV (Ag 3d 5/2) and 373.9 eV (Ag 3d 3/2)
shown in Fig. 3a confirm the monovalent state of silver element
(Ag+) in the samples [20]. The peaks located at near 576.3 eV and
585.8 eV (Fig. 3b) are corresponding to the binding energies of Cr
2p 3/2 and Cr 2p 1/2, which confirm the trivalent state of chro-
mium element (Cr3+) in the samples [12,26].
2. Experimental section
All of the chemicals in these experiments without special notification were pur-
chased from Sigma Aldrich with analytical grade and used without further purifica-
tion. In
a typical hydrothermal synthesis, certain amounts of reactants were
dissolved in deionized water, and the obtained solution was transferred into a
Teflon-lined autoclave. The sealed autoclave was maintained at 190–230 °C for
reaction. After the reaction finished, the autoclaves were naturally cooled to room
temperature. Finally, the obtained precipitate was washed for several times in a
centrifugal cleaning machine and was finally stored in absolute alcohol solution
for further use.
Powder X-ray diffraction patterns were collected at room temperature by using
a Panalytical X’pert Pro diffractometer (XRD, Cu K
a radiation). A field emission
scanning electron microscope (FESEM) system (Hitachi-S-4800) coupled with
energy dispersive X-ray spectroscopy (EDX) and a transmission electron micro-
scope (FETEM, Tecnai G2 F30) were used to observe the microstructure and deter-
mine the composition of the as-synthesized nanocrystals. The thermal stability of
nanocrystals was investigated by a differential scanning calorimeters-thermo gravi-
metric analyzer (DSC-TG, Diamond TG/DTA, Perkin–Elmer Instruments), these sam-
ples were heated in air from room temperature to 800 or 1000 °C at a heating rate of
10 °C minꢁ1. The ultraviolet–visible–near infrared (UV–vis–NIR) spectra of films
were recorded on
a Perkin–Elmer UV/Vis spectrophotometer (UV–vis, Model
Lambda 950) in the wavelength range of 300–800 nm. Hall effect measurements
were done on a Hall effect analysis system (Accent HL 5500 PC), of which the sam-
ples were prepared by the powder pellet method, using Ag coating at four contact
points to decrease the contact resistance. X-ray photoelectron spectroscopy mea-
surements (XPS) were performed with a physical electronics surface analysis equip-
ment (Model PHI 5600), and the C (1s) line (at 285.0 eV) corresponding to the
surface adventitious carbon (C–C line bond) has been used as the reference binding
energy. By using a mask with a size of 4.5 ꢂ 4.5 mm2 to prevent the scattering of
light, the solar cells were tested using a solar simulator with an AM 1.5 G filter
(Oriel, model 91192) at a light intensity of 100 mW cmꢁ2, and calibrated using a
standard silicon reference cell.
3.2. Thermal stability of AgCrO2 nanocrystals
From the thermogravimetric (TG) curve shown in Fig. 4a, the
initial weight losses of AgCrO2 samples are suggested to be due
to the evaporation of chemically combined water of crystallization
and the variation of oxygen vacancy in the sample. The sharp mass
decrease from the temperature of above 400 °C should be due to
AgCrO2 decomposition in air at the temperature. Fig. 4b shows
the diffraction patterns of AgCrO2 samples sintered in air at
350 °C, 400 °C, 500 °C, and 550 °C for 1 h, respectively. The diffrac-
tion peaks of the AgCrO2 powder sintered at 350 °C should be iden-
tified as pure AgCrO2 crystal phase; whilst for the AgCrO2 powder
sintered at 550 °C, the diffraction peaks owning to the newly gen-
erated by-products of Ag (JCPDS File Card No. 65-8428) and Cr2O3
(⁄, JCPDS File Card No. 38-1479) could be clearly identified. The
appearance of Cr2O3 inside the AgCrO2 powders sintered at the
temperatures of >400 °C suggests that the decomposition of
AgCrO2 occurs. This phenomenon is consistent well with the TG
analysis result. It is suggested that the following chemical reaction
3. Results and discussion
3.1. Hydrothermal synthesis of AgCrO2 nanocrystals
AgCrO2 nanocrystals were prepared similarly to our previously
hydrothermal procedure for CuCr1ꢁxMgxO2 (x = 0, 0.05, 0.10.)
[11,12]. At first, 15 mMol Cr(NO3)3ꢃ9H2O and 15 mMol AgNO3 were
dissolved in 70 ml deionized water at room temperature, 2.40 g
NaOH was added to the above solution and stirred for 10 min.

106
D. Xiong et al. / Journal of Alloys and Compounds 642 (2015) 104–110
Fig. 1. XRD patterns (a) of products freshly obtained AgCrO2 nanocrystals from the hydrothermal synthesis at 190–230 °C. Also shown are SEM and TEM images of freshly
obtained AgCrO2 nanocrystals (b, 230 °C; c, 210 °C; d, 190 °C).
Fig. 2. (a–c) EDS elemental mapping, (d) SEM image, and (e) elemental analysis report of the AgCrO2 nanocrystals.
should be involved during the high temperature (>400 °C)
sintering:
CuCrO2 [11]. The above mentioned thermal stability of CuCrO2/
AgCrO2 are similar to our previous report on thermal stability of
CuAlO2/AgAlO2 [3]. It is common that Cu based delafossite oxides
tend to be oxidized by O2 in the air at higher sintering temperature,
whilst Ag based delafossite oxides tend to decompose to be metal-
lic Ag at higher sintering temperature. If one compares the thermal
stability of Al based delafossite oxides and Cr based delafossite
4AgCrO2 ¼ 2Cr2O3 þ 4Ag þ O2
ð1Þ
On the contrary, according to our previous work [11], the TG
curve of CuCrO2 shows that mass increases sharply from the tem-
perature of above 450 °C, which should be due to the oxidation of

D. Xiong et al. / Journal of Alloys and Compounds 642 (2015) 104–110
107
Fig. 3. Typical XPS spectra of AgCrO2 nanocrystals: (a) Ag 3d and (b) Cr 2p.
were confirmed with p-type conductivity. The conductivity of the
delafossite oxides are closely to that of NiO [27], but the carrier
densities of Cu based delafossite oxides are several orders of mag-
nitude higher. Moreover, the conductivity and carrier density of
AgCrO2 are 2.22 ꢂ 10ꢁ6 S cmꢁ1 and 4.33 ꢂ 1011 cmꢁ3, respectively,
which are much lower than that of CuCrO2. This low conductivity
problem of AgCrO2 is consistent with Poeppelmeier’s report [30].
They demonstrated that the conductivities of silver delafossites
are much lower than that of copper delafossites with the same
M-site cations, and suggested that the mixed valence state from
O-2p and Ag-4d limits the hole mobility, resulting in the low con-
ductivity of silver delafossites [19,30].
3.4. Optical properties of AgCrO2 nanocrystals
In order to study the optical properties of p-type AgCrO2
nanocrystals, uniformity films were deposited on the glass slides
through spray deposition method. After heated in air at 300 °C
for 1 h, the deposited films were examined by the UV–Vis spec-
troscopy. The optical bandgaps are estimated by the following
equation: [3,31,32]
1=n
ða
hv
Þ
¼ Aðh
v
ꢁ EgÞ
ð2Þ
where
a
, h, , A, and Eg are the absorption coefficient, Planck
m
constant, the frequency of light, a constant, and the band gap,
respectively. Moreover, the exponent n depends on the type of tran-
sition, for direct-allowed transition, n = 1/2; for indirect-allowed
transition, n = 2, for direct-forbidden transition, n = 3/2, and for
direct-forbidden transition, n = 3 [3].
Fig. 5 shows the optical transmittance spectra within the wave-
length range of 300–800 nm and the calculated bandgaps of
AgCrO2 films. In detail, the average optical transmittance of green-
Fig. 4. Thermogravimetric (TG) curves of (a) AgCrO2 at a heating rate of 10 °C minꢁ1
in air. Also shown are the corresponding XRD patterns for (b) AgCrO2 powder after
sintering in air at different temperatures.
ish-yellow AgCrO2 film (about 2.0 lm) is around 60–75% in visible
range (Fig. 5a), and the direct bandgap of AgCrO2 is around 3.32 eV
(Fig. 5b). The calculated bandgap value of AgCrO2 is much bigger
than that of CuCrO2 (3.25 eV) [32], this phenomena is similar to
our previous report on that of CuAlO2/AgAlO2 [3]. And the larger
optical bandgap of AgCrO2 is suggested to be owing to the shift
oxdies, the former samples are with much higher decomposition
temperature of ꢀ800 °C, about 400 °C higher than that of the latter
ones. From these, it can be deduced that the thermal stability of the
AMO2s are closely related to the M site elements. Since AlO6 octa-
hedras are more stable and with stronger interaction toward A site
Cu+ or Ag+ than CrO6 octahedras, therefore Al based delafossite
oxides with better phase stability than that of Cr based delafossite
oxides are understandable.
Table 1
The test results of Hall Effect measurements for CuCrO2 and AgCrO2 Nanocrystals.
Materials Type Conductivity
(S cmꢁ1
Carrier density
(cmꢁ3
Hall coefficient
(cm/C)
)
)
3.3. Hall effect measurements on AgCrO2 nanocrystals
NiO [24]
CuCrO2
AgCrO2
p
p
p
8.33Eꢁ05
4.60Eꢁ05
2.22Eꢁ06
3.91E+11
4.86E+13
4.33E+11
–
1.29E+05
1.44E+01
The Hall Effect measurement results of AgCrO2 and CuCrO2
delafossite oxides are shown in Table 1. All these delafossite oxides

108
D. Xiong et al. / Journal of Alloys and Compounds 642 (2015) 104–110
In this work, during the hydrothermal synthesis of AgCrO2, the
original reaction products include Ag, AgCrO2, Na2CrO4 and NaNO3
from the XRD patterns in Fig. 6a. It is suggested that part of silver
cations (Ag+) were reduced to elemental silver (Ag), accompanied
with part of trivalent chromium cations (Cr3+) were oxidized to
hexavalent chromium cations (Cr6+). Finally, we can get the pure
phase AgCrO2 nanocrystals after the corresponding washing proce-
dure, by which the other byproducts were dissolved and washed
off as ions. Moreover, the similar circumstance was present in
the preparation of CuCrO2 [11,12]. From the XRD patterns of
Fig. 6b, it can be seen that CuO, CuCrO2, Na2CrO4 and NaNO3 were
detected in the origin reaction products for CuCrO2; it is suggested
that cupric cations (Cu2+) were reduced to cuprous cations (Cu+),
accompanied with part of trivalent chromium cations (Cr3+) were
oxidized to hexavalent chromium cations (Cr6+). After these reac-
tion products were washed with diluted hydrochloric acid and
absolute alcohol, pure phase CuCrO2 nanocrystals could be
obtained.
In summary, we prepared both AgCrO2 and CuCrO2 nanocrystals
similarly from metal nitrates (Cu(NO3)2 or AgNO3 and Cr(NO3)3)
and sodium hydroxide (NaOH) [11,12]. It can be concluded that
the in-situ oxidation–reduction reactions during the hydrothermal
process play the critical role for the crystal formation of desired
delafossite oxides. The chemical reactions can be written as the fol-
lowing equations:
3Agþ þ Cr3þ ¼ 3Ag þ Cr6þ
ð2-aÞ
Fig. 5. The optical transmittance (a) and the calculated bandgap (b) of AgCrO2
films; the inset in (a) is corresponding optical image.
of valence band states toward to lower energy, associated with the
replacement of copper 3d states with silver 4d states [3,30].
3.5. The proposed synthesis mechanisms for AgCrO2
High temperature solid-state reaction and hydrothermal reac-
tion are the most common methods used for preparing the copper
based delafossite oxides (such as CuAlO2, CuCrO2 and CuFeO2)
[30,34]. However, until now, there has been very few literatures
except the following ones focusing on the hydrothermal synthesis
mechanisms [3,26,27]. For example, Kenneth R. Poeppelmeier and
coworkers reported the synthesis of broad families of delafossite
oxides for CuMO2 (M = Al, Sc, Cr, Mn, Fe, Co, Ga, and Rh), and
AgMO2 (M = Al, Sc, Fe, Co, Ni, Ga, Rh, In, and Tl) [2].
Unfortunately, their report did not involve AgCrO2. Xiaodong
Fang and coworkers reported the hydrothermal synthesis of
CuCrO2 at 220 °C by using Cu2O, Cr(NO3)3 and NaOH as the reac-
tants [26]. They demonstrated that aqueous soluble Cu(OH)2ꢁ
and Cr(OH)4ꢁ species result in eruptible nucleation of CuCrO2. W.
C. Sheets and coworkers have tried the hydrothermal synthesis of
AgCrO2 at 210 °C by using Ag2O and Cr(OH)3 dissolved in 2.5 M
NaOH solution, but no AgCrO2 crystal has been obtained; only sil-
ver metal and highly soluble CrO24ꢁ species were stabilized [2].
Recently, Sanjay Kumar and coworkers reported that AgCrO2 poly-
crystalline (with average size about 2 lm, much bigger than our
AgCrO2 nanocrystals) were synthesized by one-step reaction
between Ag2O and Cr(OH)3 in supercritical water conditions
(400 °C, 40 MPa), using K2Cr2O7 as an oxidizing agent through
hydrothermal method [27]. Just like the statements from
Xiaodong Fang, Sanjay Kumar also declared that only at a certain
temperature, pressure, reaction time and K2Cr2O7 as the oxidizing
agent, the stability of Ag(OH)2ꢁ and Cr(OH)4ꢁ species could be
guaranteed, and finally the supersaturation state was reached
and then AgCrO2 was obtained.
Fig. 6. XRD patterns of reaction products for the preparation of (a) AgCrO2 and (b)
CuCrO2.

D. Xiong et al. / Journal of Alloys and Compounds 642 (2015) 104–110
109
Fig. 7. (a) J–V characteristic curve based on AgCrO2 photocathode, (b) digital photo of P1 dye sensitized AgCrO2 photocathode, (c) SEM image of mesoporous AgCrO2 film and
(d) IPCE spectra of the solar cells based on bare AgCrO2 and P1 dye sensitized AgCrO2 photocathode.
Agþ þ Cr3þ þ 4OHꢁ ¼ AgCrO þ 2H2O
ð2-bÞ
ð2-cÞ
ð2-dÞ
to be further elucidated and resolved in the following work. At the
moment, the low conductivity of AgCrO2 and/or improper band
alignment between P1 dye and AgCrO2 should be firstly suspected.
2
3Cu2þ þ Cr3þ ¼ 3Cuþ þ Cr6þ
Cuþ þ Cr3þ þ 4OHꢁ ¼ CuCrO2 þ 2H2O
4. Conclusions
As mentioned above, the synthesis mechanism of chromium
based delafossite oxides (AgCrO2 and CuCrO2 nanocrystals) based
on an oxidation–reduction reaction was pointed out for the first
time. This novel synthesis mechanism based on an oxidation–re-
duction reaction between the A site and M site elements in
AMO2, which is totally different from the combination reaction
taking place during the high-temperature solid-state method. On
the basis of this mechanism, we may propose a novel and general
approach for the preparation of large families of delafossite oxides,
especially for the M-site element of AMO2 can change their valence
states during the hydrothermal conditions.
In summary, we report a novel, controllable and one-step syn-
thesis of p-type delafossite oxides AgCrO2 nanocrystals by a low-
temperature (190–230 °C) hydrothermal method and, the AgCrO2
nanocrystals as photocathode material in p-type DSSC for the first
time. The obtained nano-sized AgCrO2 crystals are around
10–20 nm, with p-type conductivity, wide bandgap (>3.10 eV)
and high optical transparency (60–75%). The synthesis tempera-
tures for AgCrO2 nanocrystals has been expanded to as low as
190 °C, which is much lower than ever reported. According to the
analysis of reaction products generated during the hydrothermal
process, we have proposed the general synthesis mechanism for
these chromium based delafossites (AgCrO2 and CuCrO2 nanocrys-
tals). Only metal nitrates (AgNO3 or Cu(NO3)2 and Cr(NO3)3) and
sodium hydroxide (NaOH) were selected as starting materials,
and therefore formed CuCrO2 and AgCrO2 through an in-situ
oxidation–reduction reaction. Namely, the oxidation–reduction
reactions occurred between Cr3+ and Ag+/Cu2+ under facile
hydrothermal conditions, and then formed these chromium based
delafossites. The proposed synthesis mechanism for this low-
temperature hydrothermal method is different from that in tradi-
tional high-temperature solid-state method, and our finding may
open a new avenue for the preparation of nanocrystals with
delafossite structure.
3.6. Application in dye-sensitized solar cells
In our previous reports [11,12], both CuCrO2 (ꢀ15 nm) and
CuCr0.90Mg0.10O2 (ꢀ10 nm) nanocrystals were successfully
employed to fabricate photocathodes for p-type DSSCs. In this
work, we first applied AgCrO2 nanocrystals as photocathodes in
p-type DSSCs, using P1 dye as the sensitizer. The detailed solar cell
fabrication and measurement are according to our previous works
[11,12]. Fig. 7a presents the photocurrent–voltage (J–V) curve of
the P1 dye sensitized AgCrO2 based p-type DSSC under 1 Sun AM
1.5G illumination. An open-circuit voltage (Voc) of 246 mV, short-
circuit current density (Jsc) of 0.149 mA cmꢁ2, fill factor of 0.409,
and overall photoconversion efficiency of 0.0145% could be
achieved. From Fig. 7b, the P1 dye sensitized AgCrO2 film presents
cardinal red color, which proves abundant P1 dye molecules
absorbing on the mesoporous film. Moreover, the highly porous
morphology of AgCrO2 film shown in Fig. 7c can explain its good
dye absorbing capability. Though, the Voc of AgCrO2 based p-type
DSSC is very high in comparison to most of previously reported
p-type DSSCs based on P1 dye, the Jsc is very low, only
0.149 mA cmꢁ2. These result in the DSSC device with an inefficient
efficiency of around 0.0145%. The low IPCE shown in Fig. 7d can
explain the very low Jsc of the solar cell. The IPCE spectral differ-
ence before and after P1 dye sensitization on the AgCrO2 photo-
cathodes can prove the presence of the sensitization effect in the
p-type DSSC. It is known that IPCE of p-type DSSC is determined
by synergistic effect of (1) light harvesting of dye sensitized photo-
cathode, (2) hole injection from P1 dye to valance band of AgCrO2
and (3) the hole collection through the AgCrO2 network. Which
effect plays the decisive role on the poor device performance needs
Acknowledgements
The authors would like to express sincere thanks for the finan-
cial support by the National Natural Science Foundation of China
(Nos. 21103058, 51402223), 973 Program of China (No.
2011CBA00703) and Basic Scientific Research Funds for Central
Colleges (HUST: 2012YQ027, 2013TS040). D. Xiong also thanks
the financially supported by China Postdoctoral Science
Foundation (No. 2014M552098) and the Fundamental Research
Funds for the Central Universities (WUT: 2014-IV-091). We also
thank Analytical and Testing Center of Huazhong University
Science & Technology for the sample measurements.
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