Study of Sn doped NiO microwires with waveguiding behaviour grown by a vapor-solid process

Article abstract of DOI:10.1016/j.jallcom.2021.160654

Sn-doped NiO microwires with tailored structure of defects, optical properties and waveguiding behaviour were fabricated by a one-step vapor-solid method. The presence of metallic Sn or SnO₂ in the precursor mixture favours the formation of elongated microstructures with singular morphologies. This geometry, not explored so far, provides an increased surface to volume ratio and a characteristic morphology, which is of potential interest in a wide range of surface properties and optoelectronic functionalities. In this work, the structural, compositional, electronic and optical properties of the microwires have been analyzed as a function of Sn precursor. Depending on the employed Sn precursor, the optical properties such as their luminescence or waveguiding behaviour can be tailored. In some of the analysed microwires a red laser can be guided along the microstructure. This behaviour has not been previously reported for NiO so far, hence increasing the functionality and applicability of this material.

Full text of DOI:10.1016/j.jallcom.2021.160654

Journal of Alloys and Compounds 881 (2021) 160654
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Study of Sn doped NiO microwires with waveguiding behaviour grown by
a vapor-solid process
M. Taeño⁎, D. Maestre, A. Cremades
Departamento de Física de Materiales, Facultad de CC. Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Sn-doped NiO microwires with tailored structure of defects, optical properties and waveguiding behaviour
were fabricated by a one-step vapor-solid method. The presence of metallic Sn or SnO2 in the precursor
mixture favours the formation of elongated microstructures with singular morphologies. This geometry, not
explored so far, provides an increased surface to volume ratio and a characteristic morphology, which is of
potential interest in a wide range of surface properties and optoelectronic functionalities. In this work, the
structural, compositional, electronic and optical properties of the microwires have been analyzed as a
function of Sn precursor. Depending on the employed Sn precursor, the optical properties such as their
luminescence or waveguiding behaviour can be tailored. In some of the analysed microwires a red laser can
be guided along the microstructure. This behaviour has not been previously reported for NiO so far, hence
increasing the functionality and applicability of this material.
Received 16 March 2021
Received in revised form 24 May 2021
Accepted 29 May 2021
Available online 2 June 2021
Keywords:
Nickel oxide
Microwires
Waveguiding behaviour
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
demonstrated improvement of several applications such as catalytic
activity or gas detection. This improvement is usually attributed to
Nickel oxide (NiO) is one of the scarce p-type semiconducting
oxides, which is gaining increasing interest in the last years based on
their interesting properties and growing applications in diverse
fields such as gas sensors, electrochromic and supercapacitors,
among others [1–5]. However, the integration of NiO micro and
nanostructures in novel devices requires specific dimensions, mor-
phology and properties which can be tailored by controlled synth-
esis methods. Diverse chemical and physical methods have been
employed so far for the synthesis of NiO in forms of ceramics, thin
films, or nanoparticles [2,6]. Actually, morphology engineering of
NiO can lead to optimize most of the applications by reducing the
size and selecting proper dimensions. In particular, some authors
reported the growth of some NiO elongated structures in form of
nanowires, rods and tubes [7–9] by using diverse synthesis methods.
Alternatively, doping is an additional parameter, which can induce
property improvements and originate new morphologies, and
therefore new applications in the field of optical and optoelectronic
devices. Nb5+, Sn4+, Li+ or Al3+ [10] have been widely investigated as
dopants in NiO since they can lead to changes in the NiO stoichio-
metry and its physicochemical properties. Among these dopants,
Sn4+ has become in one of the most promising candidates due to its
the formation of Ni3+ in order to maintain the charge neutrality due
to Sn4+ incorporation, which produces an enhanced conductivity in
NiO. Moreover, Sn4+ and Ni2+ exhibit very similar ionic radii (0.69 Å),
favouring the substitution of Ni2+ by Sn4+. As has been recently re-
ported, Sn incorporation can also modify the morphology of NiO or
decrease the size of NiO [11], which contributes to the increase of the
surface area. Diverse authors have reported the growth of Sn doped
NiO, most of them in form of micro and nanostructures such as
nanospheres [12], nanoparticles [13] or microcubes [14]. However,
the fabrication of elongated structures similar to those described in
this work has not been extensively explored so far. The particular
geometry, dimensions and composition of these microwires could
extend their use not only in optical devices but also in novel fields of
research, not explored yet for NiO.
In a previous work, a preliminary study of Sn based NiO, mainly
focused on bulk materials, were reported [15]. However, in the
present work, special attention has been paid to individual micro-
structures in order to gain a deeper insight in their structural,
compositional, electronic and optical properties which can broaden
their applicability as building blocks in optoelectronic devices.
In this work, Sn doped NiO microwires have been synthesized
and characterized by x-ray diffraction (XRD), scanning electron mi-
croscopy (SEM), Raman spectroscopy, energy dispersive spectro-
scopy (EDS), electron back scattered diffraction (EBSD) in a SEM,
x-ray photoelectron spectroscopy (XPS) in a synchrotron facility, and
⁎
Corresponding author.
E-mail address: m.taeno@ucm.es (M. Taeño).
https://doi.org/10.1016/j.jallcom.2021.160654
0925-8388/© 2021 Elsevier B.V. All rights reserved.

M. Taeño, D. Maestre and A. Cremades
Journal of Alloys and Compounds 881 (2021) 160654
photoluminescence (PL). A confocal microscope has been used to
evaluate the waveguiding behaviour of the microwires.
940 °C following a wet chemical method. TEM analysis showed the
presence of conical tips at the ends of the nanowires, which ex-
hibited lengths ranging from 150 to 250 nm as a function of the
annealing temperature. HRTEM analysis was also performed, con-
firming that the preferred growth direction on the nanowires was
the [111] direction. On the other hand, Wei et al. [20] reported a
novel vapor-based metal-etching oxidation method to synthesize
high-quality NiO nanowire arrays using Ni foil as substrate. In this
case the diameter of the nanowires was 130 nm and the HRTEM
analysis suggested that the NiO nanowires were single crystalline,
and the growth direction was along [200], although in a few nano-
wires the growth direction was along [220]. In this work, it is only
when a low amount of SnO2 or metallic Sn is used in the precursor
mixture that the growth of large amount of elongated micro-
structures is drastically promoted at the surface of the treated pel-
lets, therefore the present work is focused on the Sn doped NiO
elongated structures, topic no widely reported in the literature,
contrary to undoped NiO. Fig. 1a shows a SEM image from the
sample containing SnO2 in the precursor mixture (NiO:SnO2). These
elongated microstructures exhibit lengths ranging from 5 to 50 µm
with an average length of 18.8 µm and widths of a few microns or
even submicrometric. Occasionally, microwires with some branches
are also observed, as has been previously reported after Sn in-
corporation [21]. Most of the microwires show smooth lateral sur-
faces and regular sections, although in some cases a terraced
appearance is promoted at their base, as shown in the inset of Fig. 1a.
The incorporation of metallic Sn in the precursor mixture leads to
a higher concentration of elongated microstructures (NiO:Sn), as
shown in Fig. 1b. These microwires exhibit lengths ranging from 10
to 30 µm with square or rectangular sections and an average length
of 16.3 µm. In this case, most of the microstructures show a terraced
appearance contrary to the smooth lateral surfaces observed in the
samples with SnO2 in the precursor mixture. Some of these struc-
tures present a decrease in the diameter from the base to the tip,
known as tapering effect [22]. Some authors proposed a screw-dis-
location assisted mechanisms during the formation of NiO whiskers
or microchannels by metallic Ni oxidation at temperatures over
1000 °C and low oxygen pressure [23,24]. In this case, similar pro-
cesses can be considered for the growth of NiO microwires, although
the presence of Sn should also induce variable diffusion processes
and oxidation kinetics, probably leading to enhanced nucleation
sites for the growth of microwires. These results confirm that Sn or
SnO2 incorporation, not only favours the growth of a high number of
elongated structures, but also a decrease in the average lengths as
compared to undoped NiO. At 1400 °C Ni bulk diffusion is promoted
instead of grain boundary diffusion [23,24], which should be in-
volved in the promotion of the microwires growth. Furthermore,
1400 °C also demonstrated to be the optimum temperature for the
growth of SnO2 microstructures by a similar vapor-solid process
[25,26], which could also explain why it is only at 1400 °C that the
growth of NiO:Sn microstructures is promoted, as observed in
this work.
2. Materials and methods
In the vapor-solid process used in this work, a controlled mix of
metallic Ni (Sigma Aldrich 99.9%) with either metallic Sn (Sigma
Aldrich 99.9%) or SnO2 (Sigma Aldrich 99.9%) is used as precursor
material. The initial powders were mixed in a weight ratio 9:1 (Ni:
Sn) and milled in a centrifugal mill during 5 h and subsequently
pressed into pellets. Thermal treatments were performed at 1400 °C
during 10 h under a controlled Ar flow (1.6 l/min). Following this
method, a large concentration of elongated structures grows on the
surface of the treated pellet in a single step. In this work, the sam-
ples will be named as NiO:SnO2 and NiO:Sn, depending on the se-
lected Sn precursor. Treatments without Sn in the precursor mixture,
named as NiO, were also carried out in order to study the influence
of Sn incorporation.
Structural characterization of the samples was performed by
X-ray diffraction (XRD) in a Philips X′Pert Pro diffractometer using
Cu Kα radiation (λ = 1.54158 Å) in Bragg-Brentano configuration. The
morphological characterization was carried out using a Leica 440
Stereoscan and a FEI-Inspect S scanning electron microscopy (SEM).
For the compositional study by means of energy dispersive x-ray
spectroscopy (EDS), a Bruker AXS 4010 detector mounted on a Leica
440 SEM was used. Raman spectroscopy and Photoluminescence
(PL) analysis were performed at room temperature with a Horiba
Jobin-Yvon LabRam Hr800 confocal microscope, using a He-Cd laser
(λ = 325 nm) as excitation source. A Bruker e-Flash electron back-
scattered diffraction (EBSD) system was employed for the structural
analysis of the microwires. X-ray photoelectron spectroscopy (XPS)
analysis was carried out at the ESCA microscopy beamline at the
Elettra synchrotron facility in Trieste (Italy), using a 640 eV photon
energy with a resolution of 200 meV and spatial resolution down to
250 nm. The elongated structures were carefully detached from the
pellet where they grow and placed onto a Si (100) substrate in order
to avoid the overlapping of the microstructures and the contribution
to the signal from the surface of the pellets during their analysis, as
well as to assure the correct assessment of their orientation.
3. Results and discussion
3.1. Morphological and structural characterization
It is only when thermal treatments are carried out at 1400 °C
using a controlled Ar flow that the growth of elongated structures
with different morphologies is favoured. The presence of the Ar flow
induces a supersaturated atmosphere and low oxygen pressure
conditions during the growth process, favouring the formation of
elongated structures (Fig. 1) that grow following a vapor-solid
method [16,17]. When the thermal treatments are carried out
without Sn in the precursors, the growth of microcrystals with
variable dimensions and terraced appearance is mainly promoted, as
has been previously reported [18], being this the main morphology
of the sample. However, a small amount of NiO microwires with
lengths ranging from 20 to 40 µm also grow mainly at the edge of the
treated pellets as shown in Fig. S1. NiO one-dimensional structures
are usually fabricated following techniques which require several
steps. For example, Wang et al. [8] synthesized single crystal cubic
NiO nanorods with lengths of up to tens of micrometres by thermal
decomposition method using NiCl2.6H2O as precursor. TEM analysis
from these structures confirmed a rod-like shape and no spherical
droplets, suggesting that the nanorods may not grow by vapor-li-
quid-solid mechanism. Raman and XRD analysis confirmed the
crystallinity and purity of the synthesized nanorods. Xu et al. [19]
reported the growth of NiO nanowires after annealing at 890 °C and
Fig. 1c shows XRD patterns from the treated pellets (including
the sample fabricated using only metallic Ni as a reference) where all
the peaks can be assigned to NiO with rock-salt structure. The in-
tensity and well-defined peaks indicate high crystallinity and pre-
ferential (200) texture, more evident in the case of NiO:SnO2 sample.
Different authors have studied that the preferential orientation in
NiO thin films is mainly governed by a minimization of surface en-
ergy. Ryu et al. reported that preferential (111) orientation reduces
the surface energy when NiO thin films grow in an oxygen-rich at-
mosphere, while (200) orientation is favoured in oxygen-deficient
environments [27]. In our case, the presence of an Ar atmosphere
during the growth could favour the preferential (200) texture ob-
served in all cases. Peaks from the precursors of Sn-O based com-
pounds were not detected in the XRD patterns, however the
2

M. Taeño, D. Maestre and A. Cremades
Journal of Alloys and Compounds 881 (2021) 160654
Fig. 1. (a) SEM image from microwires grown on the pellet with SnO2 in the precursor mixture. (b) SEM image from microwires grown on the pellet with metallic Sn in the
precursor mixture. (c) XRD patterns from the samples NiO, NiO:SnO2 and NiO:Sn showing cubic structure in all the cases.
presence of SnO2 or metallic Sn in the precursor promotes a shift to
higher diffraction angles in the XRD peaks, as compared with the
reference NiO. This shift cannot be directly related to a lattice con-
traction due to the substitutional incorporation of Sn4+, since Ni2+
and Sn4+ cations have a similar ionic radius (0.69 Å and 0.71 Å, re-
spectively). Actually, some works based on Sn doped NiO reported
either lattice contraction or expansion due to substitutional Sn4+
[13,28]. In order to further analyse the elongated microstructures
observed in this work and avoid possible contributions from the
pellet where they grow, some of the Sn doped NiO microwires were
detached from the pellet and placed onto Si substrates.
droplet is usually observed in elongated structures grown following
a vapor-liquid-solid method [16]. The gradual decrease of Ni from
the base to the tip could be related to an inhomogeneous Ni oxi-
dation through the microwire and/or the incorporation of a small
amount of Sn, not detectable by this technique. This analysis has
been also performed for the microwires from the NiO:Sn sample,
showing only the presence of Ni and O in the compositional map-
pings (Fig. S2). Neither in the NiO:SnO2 nor in NiO:Sn samples Sn
was detected by EDS, taking into account the resolution limit of the
technique, which indicates that, if present, the amount of Sn in the
microwires should be low.
Fig. 2a shows the SEM image and compositional maps acquired
with the O or Ni signals, from the corresponding EDS spectra shown
in Fig. 2b, acquired on one NiO:SnO2 microwire. All the analysed
microwires are formed only by Ni and O. Peaks from Sn were not
detected by EDS, within the detection limit of the technique.
Quantification from the EDS signal indicates a composition around
Ni: 54.6 2.7 at%; O: 45.3 2.3 at% at the base of the microwire
(point 1), however the amount of Ni decreases when approaching
the apex of the microwire (point 3), where a droplet can be appre-
ciated in some cases, as marked in the SEM image from Fig. 2a. This
Raman signal from the microwires was also analysed (Fig. 3a). In
this case, all the Raman modes can be assigned with rock-salt NiO
[29,30] showing a high crystallinity, while peaks from the precursors
or other Sn-O compounds were not observed, in agreement with
XRD and EDS. A weak peak from the Si substrate where the micro-
wires were placed can be also appreciated at ~520 cm−1. Vibrational
modes at 438 cm−1, 594 cm−1, 728 cm−1, 902 cm−1 and 1154 cm−1
correspond to TO (transverse optical mode), LO (longitudinal optical
mode), 2TO, TO+LO and 2LO modes, respectively. In this case the 2 M
mode from NiO at 1490 cm−1 is suppressed when using a UV laser as
3

M. Taeño, D. Maestre and A. Cremades
Journal of Alloys and Compounds 881 (2021) 160654
Fig. 2. (a) SEM image and compositional EDS mappings from a NiO:SnO2 microwire and (b) the corresponding EDS spectra acquired in different regions marked in (a).
excitation source due to resonant excitation effects, as reported by
Aytan et al. [31].
such as NiO, there is no crystalline habit that explains the observed
anisotropic growth of these microstructures under study. Another
process and mechanisms, such as the minimization of surface energy
should be considered in the growth of NiO microwires. According to
Wulff model [32] the faces thermodynamically favourable will be
developed. Some authors attribute the growth of elongated struc-
tures of other semiconductor oxides, such as ZnO [33], or In2O3 [34],
by thermal treatments and vapour-solid methods, to the presence of
extensive defects such as dislocations or voids that extend along the
growth axis. In particular, a screw-dislocation driven mechanism
was reported for the growth on microrods from In2O3, also with a
cubic bixbyite crystalline system [34]. Another example of this type
of growth can be found in SnO2, where the most favourable growth
directions are the [101] and [001] [35]. In our work, in addition to a
possible growth based on dislocations, some other different kinetic
and thermodynamic aspects promoted by the presence of Sn or SnO2
in the precursor mixture, should be considered. It should be noted
that the formation of the elongated structures similar to those
shown in this work, has not been previously reported in the case of
NiO. However, different works based on the growth of one-dimen-
sional structures such as NiO nanowires can be found. For example,
C. Xu et al. [19] reported the formation of NiO nanowires synthesized
Raman signal from NiO:Sn microwires shows an increase in the
relative intensity of the LO mode in comparison with NiO:SnO2
microwires. Mironova et al. [30] reported an increase in the first LO
mode due to disorder-induced defects in nanosized NiO, which could
indicate more defective NiO:Sn microwires. Some of these micro-
wires deposited on Si substrates were also individually analysed by
EBSD. As EBSD analysis requires a well-controlled orientation of the
probed surface with respect to the detector, the terraced and vari-
able diameter of the NiO:Sn microstructures hinder their study.
Hence, EBSD measurements have been performed on NiO:SnO2 mi-
crowires which exhibit more regular morphology and smooth lateral
faces. Fig. 3b shows an EBSD pattern of the NiO:SnO2 microwire
shown in the inset, where the Kikuchi bands from NiO can be ob-
served. The corresponding polar figure is depicted in Fig. 3c. Ac-
cording to the EBSD results from different microwires, the probed
surface from the microwires usually corresponds to the (100), in
agreement with XRD measurements, while occasionally the signal
from the (111) can be also appreciated for some wires. These results
suggest that the preferential directions for the growth of elongated
structures in NiO are the [100] and [111], however, in cubic systems
Fig. 3. (a) Raman spectra from NiO:Sn and NiO:SnO2 microwires acquired with a He-Cd laser (λ = 325 nm) (b) Kikuchi lines and (c) polar figure from the region marked in the SEM
image shown in the inset in (b) from a microwire of the NiO:SnO2 sample.
4

M. Taeño, D. Maestre and A. Cremades
Journal of Alloys and Compounds 881 (2021) 160654
Fig. 4. (a) XPS image acquired with the Ni3p signal from NiO:SnO2 microwire, showing the analyzed regions and the corresponding XPS spectra from (b) Ni 3p, (c) Sn 3d, (d) O 1s
and (e) Valence band region.
by a wet chemical method, showing that these structures grow along
[111] direction. Wei et al. [20] used Ni foil substrates to grow NiO
nanowires, confirming by HRTEM that the structures grow along
[200] direction. Both results suggest that the preferential growth
directions for NiO are [100] and [111], similar to those proposed
in this work, in which [100] growth direction is more commonly
observed.
low and variable amount of Ni3+ can be also observed. The Ni3+/Ni2+
ratio estimated from the XPS spectra, indicates values around 0.17 at
the Point A and 0.23 at the Point B. Some authors have previously
reported a decrease in the resistivity as a function of the Ni3+ in-
crease, responsible for the electrical conductivity in NiO [38,39]. The
presence of Ni vacancies, the most commonly defect in NiO [38,40],
implies that some Ni2+ ions can be oxidized to Ni3+ to maintain the
charge neutrality [38]. Nickel vacancies can be described by the
following reaction:
3.2. Electronic characterization
1
2
2Ni2+
+
O2
2Ni3+ + O2 + VNi
Surface sensitive XPS analysis was performed at the Elettra
synchrotron facility, from individual microwires placed on Si sub-
strates. Fig. 4a shows a XPS image acquired with the Ni 3p signal
from a NiO:SnO2 microwire. XPS spectra from Ni 3p, O 1s, Sn 3d and
the valence band region were acquired at different points of the
microwire, marked in Fig. 4a. It should be noted that the point B is
related to the droplet at the tip of some microwires previously ob-
served in the SEM image from Fig. 2a, while point A is quite re-
presentative from the behaviour from the rest of the microwire. The
XPS spectra were calibrated using the C1s signal from adventitious
carbon (285 eV) and deconvolutions were performed using Voigt
functions after appropriate Shirley background correction. Ni 3p core
levels shown in Fig. 4b indicates the presence of four bands at 68 eV
and 70 eV, due to the Ni3p3/2 and Ni3p1/2 from Ni2+ in NiO, and 72 eV
and 74 eV associated with Ni3p3/2 and Ni3p1/2 related to Ni3+ [36,37].
The possible presence of satellite peaks in the analysed energy re-
gion has not been considered in the XPS analysis for the shake of
clarity. Ni2+ signal dominates all the Ni3p XPS spectra, although a
where two Ni2+ ions react with oxygen to produce a nickel vacancy
(doubly ionized) and two Ni3+ ion. The presence of Ni3+ is also di-
rectly related to non-stoichiometry in NiO [39,40].
Sn 3d core levels shown in Fig. 4c confirm the presence of a low
amount of Sn at the surface of the probed NiO:SnO2 microwire, not
detected by EDS. The high surface-sensitivity of the XPS technique
allows to detect Sn at the surface of the microwires, thus confirming
Sn doping. Two main contributions at around 486 eV and 487 eV can
be appreciated in the corresponding spectrum at the Point A, which
can be associated to Sn2+ and Sn4+ oxidation states, respectively [36].
However, at the Point B, Sn is mainly incorporated as Sn4+, which can
be associated with the presence of SnO2-rich regions at the apex of
the microwire. A possible incorporation of Sn in the NiO lattice and
Sn-O rich environments can be also considered in the analysis of
these contributions. Nevertheless, this incorporation of Sn is not
homogeneous along the wire. Substitutional Sn4+ is commonly
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M. Taeño, D. Maestre and A. Cremades
Journal of Alloys and Compounds 881 (2021) 160654
reported in Sn doped NiO, as both Sn4+ and Ni2+ exhibit similar ionic
radii in octahedral configuration, 0.71 Å and 0.69 Å respectively. The
substitution of Ni2+ by Sn4+, leads to a charge imbalance that is
usually compensated by the formation of Ni3+. However, when Ni2+
is replaced by isovalent Sn2+ the amount of Ni vacancies, and
therefore the amount of Ni3+, should not be altered. In our case, the
low amount of Ni3+ observed for the NiO:SnO2 microwire, at the
Point A is directly related to a low amount of Sn4+, being the sub-
stitution of Ni2+ by Sn2+, the predominant mechanism in the mi-
crowires under study.
XPS spectra from O 1s core levels are shown in Fig. 4d, with two
main contributions at 532 eV, associated with O2- in NiO [36], and
534 eV due to adsorbed oxygen species and hydroxyl groups [41],
being the latter the one with higher relative intensity at both points.
The valence band region, shown in Fig. 4e, is dominated by con-
tributions at 2 eV and 4 eV related to Ni 3d and O 2p hybridized
states in NiO, respectively. It should be noted that the valence band
from undoped NiO is dominated by the contribution at 4 eV [42],
while Sn incorporation leads to an increase in the relative intensity
of the contribution at 2 eV. A similar contribution has been attrib-
uted to Sn 5s states in reduced SnO2 surfaces, with features similar
to SnO, as previously reported by Herrera et al. [43]. The lower in-
tensity of the contribution at 2 eV at the Point B, as compared with
Point A, could be related to a higher concentration of Sn4+. The es-
timation of the difference between the Fermi level and the max-
imum of the valence band confirms the p-type character of the
doped NiO microwires.
shown in Fig. 5b indicates the presence of mainly Ni2+ and a low
contribution of Ni3+. In this case, the estimated ratio Ni3+/Ni2+ is
around 0.05, lower than for NiO:SnO2 microwires. The low amount
of Ni3+ in this sample can be directly related to the high con-
centration of Sn2+, as observed in Fig. 5c, since the substitution of
Ni2+ for Sn2+ does not lead to a charge imbalance. In addition, the
higher concentration of Sn2+ observed in this sample, could be also
related to the oxidation state of the metallic Sn used as precursor, as
a difference with the NiO:SnO2 sample where the main Sn oxidation
state corresponds to Sn4+. Fig. 5d shows the O1s core level, where
also two contributions are observed, similar to those previously
described for NiO:SnO2. Finally, in this case the valence band region
(Fig. 5e) is dominated for the contribution at 2 eV, related to Ni 3d
hybridized states, showing a higher intensity probably associated
with the presence of Sn2+
.
Due to the lower concentration of Sn onto the surface of the
microwires, and therefore the low signal of Sn 3d, which hinders the
proper quantitative analysis, only a qualitative study has been per-
formed. However, the Ni3+/Ni2+ ratio and the difference between the
Fermi level and the maximum of the valence band have been esti-
mated as shown in Table 1. This study confirms that the higher
presence of Sn4+ (Point B from NiO:SnO2), leads to a higher p-type
Table 1
Ni3+/Ni2+ ratios and EF-EVBM from NiO:SnO2 and NiO:Sn.
NiO:SnO2
A
NiO:Sn
Point
B
NiO:Sn microwires were also analyzed by XPS following a similar
procedure, as shown in Fig. 5. XPS image from a probed microwire,
acquired with Ni 3p signal, is shown in Fig. 5a. Ni 3p core level
EF-EVBM
1.1
0.17
0.81
0.23
1.28
0.05
Ni3+/Ni2+ ratio
Fig. 5. (a) XPS image acquired with the Ni3p signal from NiO:Sn microwire, showing the analyzed region and the corresponding XPS spectra from (b) Ni 3p, (c) Sn 3d, (d) O 1s and
(e) Valence band region.
6

M. Taeño, D. Maestre and A. Cremades
Journal of Alloys and Compounds 881 (2021) 160654
character since the difference between the Fermi level and the
maximum of the valence band is lower. Moreover, the point which
contains higher Sn4+ (Point B from NiO:SnO2), also contains higher
Ni3+, confirming that the imbalance charge due to the Sn4+ in-
corporation is compensated by the formation of Ni3+. Taking this into
account, it would be expected that the electric conductivity of
NiO:SnO2 microwires should be higher than the conductivity of
NiO:Sn, since the higher presence of Ni3+ is directly related to a
higher conductivity as has been previously discussed. In addition,
and based on our previous works [44] related to the growth of Sn
doped NiO nanostructures, the incorporation of Sn usually leads to a
higher p-type character as compared with undoped NiO.
symmetry. According to the previously described Raman analysis,
NiO:Sn microwires are more defective than NiO:SnO2 microwires,
which could be associated with the increased luminescent signal.
Therefore, the optical properties of Sn doped NiO elongated struc-
tures can be tailored as a function of the employed Sn-based pre-
cursor. Previous luminescence studies based on ceramic pellets also
pointed out to variations in the luminescence signal due to Sn
doping. In particular, in Fig. 6b, PL signals from the NiO:Sn ceramic
pellet and the microwires are shown for comparison, showing
changes in the relative intensity of the different emissions as a
function of the sample. PL signal from NiO:Sn microwires is domi-
nated by emissions centered at 2.37 and 2.45 eV as has been pre-
viously mentioned, while for NiO:Sn ceramic pellet, an increase in
the relative intensity of the emission centered at 2.8 eV can be ob-
served. These results confirm that the morphology and dimensions
of the analyzed samples can promote different luminescent signals.
Moreover, and based on our previous works [18,44], NiO can
exhibit an emission around 4.5 eV attributed to excitonic re-
combination. However, this emission has been only observed by
cathodoluminescence measurements, as for the generation of that
UV emission by PL a laser with wavelength λ < 275 nm is required,
lower than the one used in this work. It should be noted that PL
spectra shown in the present work, were acquired with and UV laser
(λ = 325 nm), therefore emissions at energy higher than 3.8 eV
cannot be observed. Emissions around 3.2 eV, has been also observed
for this oxide, however, in our case, the visible emission is the
dominant signal, which is related to Ni vacancies and Ni2+ defect-
states, promoted by Sn incorporation.
Finally, waveguiding behaviour was also observed only for the
Ni:SnO2 microwires under illumination with a He-Ne red laser
(λ = 633 nm). The waveguiding behaviour was not observed for
NiO:Sn microwires, probably due to the different structure of defects
and their terraced and more irregular appearance. It should be noted
that to observe a waveguiding behaviour, several requirements are
necessary such as specific dimensions, morphology and high crys-
tallinity to avoid energy losses. Taking this into account, the mor-
phology of NiO:Sn microwires is irregular, and usually exhibits
terraced appearance on the surface, which lead to light scattering
and hinder the waveguiding behaviour. On the contrary, NiO:SnO2
microwires show smooth surfaces and more regular sections and
morphology which is an advantage for the light confinement and
waveguiding. Fig. 7 shows bright field (BF) and the corresponding
dark field (DF) images from two NiO:SnO2 microwires placed on Si
substrates. The microstructures were illuminated by one end and a
bright signal corresponding to the guided light from the laser is
appreciated at the opposite end, thus confirming waveguiding be-
haviour. In most of the cases, scattered light due to optical losses
cannot be appreciated which can be related to the good crystallinity
of the structures. Waveguiding behaviour has been also reported for
Based on the presence of a droplet with higher amount of Sn4+ at
the final tip of some of the microwires, together with the regular
sections observed in other cases, in this work a combination of a
vapor-solid process and an autocatalytic vapor-solid process is pro-
posed as growth mechanisms to obtain Sn doped NiO elongated
structures. The presence of extended defects can also play a key role
during the growth. In order to depict the proposed growth me-
chanisms, a scheme is shown in Fig. S3.
3.3. Optical properties
In order to study the influence of the incorporation of Sn on the
optical properties of the microwires as a function of the selected Sn-
based precursor, PL spectra were acquired at individual micro-
structures from both samples, as well as from NiO as reference, as
shown in Fig. 6. PL spectra from NiO shows a wide emission in the
visible region, similar to those observed in NiO:Sn sample. However,
the luminescent intensity of this signal is enhanced with Sn in-
corporation. PL spectrum from NiO:Sn microwires exhibits a wide
and complex emission in the VIS region. Emissions from NiO in the
visible range, as observed in this work, are commonly related to
defect-induced states associated with Ni vacancies and Ni2+ defect-
states, as reported by diverse authors [45,46]. In addition, a weak
emission around 2.8 eV can be also observed in the spectrum from
NiO:Sn microwires. This emission has been previously attributed to
Ni interstitials or 1T2g(D)-3A2g transitions as referred by Gangwar
et al. [47]. Nevertheless, the intensity of the PL signal drastically
decreases for the NiO:SnO2 microwires, as shown in Fig. 6. In that
case, emission from the Si substrate is mainly observed in the cor-
responding PL spectrum. Variations in the presence of defects in NiO
and local distortions during the growth and doping of the samples
can lead to changes in the optical properties. As an example, Usha
et al. [48] reported variations in various optical constants from NiO
thin films, including the refractive index, as a function of the para-
meters used during the growth process by radio-magnetron sput-
tering. In particular, an improvement in the NiO luminescence has
been associated with surface induced defects that lower the crystal
Fig. 6. PL spectra acquired with an UV laser from a) NiO, NiO:Sn, NiO:SnO2 microwires and silicon substrate as reference and b) NiO:Sn microwire and pellet.
7

M. Taeño, D. Maestre and A. Cremades
Journal of Alloys and Compounds 881 (2021) 160654
Fig. 7. (a) and (c) Bright field and (b) and (d) the corresponding dark field images from two NiO:SnO2 microwires showing waveguiding behaviour.
elongated microstructures from other oxides such as SnO2 [49],
β- Ga2O3 [50], or In2O3 [51] among others. In some of these ex-
amples, the light excited by the laser in the material is guided along
the microstructure [50], while in some others [49,51], including the
NiO microwires described in this work, the light of the laser itself is
guided instead. This waveguiding behaviour has not been previously
reported for NiO, hence increasing the functionality and applicability
of this versatile oxide in the optical field, in which still lags in
performance.
used in the precursor mixture, leading to elongated NiO micro-
structures which have not been explored so far, to the best of our
knowledge, with proper morphology and dimensions for their im-
plementation in optoelectronic devices and other fields of research.
The microwires, up to tens of microns length, exhibit different
morphology as a function of the employed Sn-based precursor. Using
SnO2 in the precursors leads to microwires with smoother surfaces
and regular appearance, while terraced and more irregular micro-
wires were achieved when using Sn in the precursor mixture. Raman
signal indicates more defective microwires when using Sn as pre-
cursor, which also leads to an increase in the luminescence signal, as
confirmed by PL analysis. Most of the microwires grow along the
[100] direction, as confirmed by EBSD. XPS results confirm the
low presence of Sn as a dopant, mainly as Sn2+, as well as a non-
homogeneous incorporation of Sn along the probed microwires. This
4. Conclusions
Sn doped NiO microwires have been fabricated by a single step
process following a vapor-solid method. The growth of these mi-
crostructures is drastically promoted when Sn-based compounds are
8

M. Taeño, D. Maestre and A. Cremades
Journal of Alloys and Compounds 881 (2021) 160654
non-homogeneous incorporation of Sn can favour the growth of
elongated structures following a vapor-solid and/or autocatalytic
vapor-solid process.
Finally, the waveguiding behaviour not previously reported for
NiO was confirmed only in those microwires fabricated using SnO2
in the precursors, hence increasing the functionality and applic-
ability of this material.
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CRediT authorship contribution statement
M. Taeño: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Resources, Writing - original draft, Writing -
review
& editing, Visualization. D. Maestre: Conceptualization,
Methodology, Validation, Formal analysis, Investigation, Resources,
Writing - original draft, Writing - review & editing, Visualization,
Supervision. A. Cremades: Conceptualization, Methodology,
Validation, Formal analysis, Investigation, Resources, Writing - ori-
ginal draft, Writing - review & editing, Visualization, Supervision,
Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by MINECO/FEDER/M-ERA.Net Cofund
projects: MAT2015-65274-R, RTI2018-097195-B-I00 and PCIN-2017-
106. Authors thank the ESCA-microscopy staff at the Elettra
Synchrotron for their help during XPS measurements.
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Appendix A. Supporting information
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2021.160654.
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