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A 1D conical nanotubular TiO2/CdS heterostructure with superior photon-to-electron
conversion†
R. Zazpe, a,b H. Sopha, a,b J. Prikryl,
L. Hromadko a and J. M. Macak *a,b
a
M. Krbal,
a
J. Mistrik,
a
F. Dvorak,
a
Herein, a new strategy to efficiently harvest photons in solar cells is presented. A solar cell heterostructure
is put forward, based on a 1D conical TiO2 nanotubular scaffold of high aspect ratio, homogenously
coated with a thin few nm layer of CdS light absorber using atomic layer deposition (ALD). For the first
time, a large variety of conical nanotube layers with a huge span of aspect ratios was utilized and ALD was
used for the preparation of a uniform CdS coating within the entire high surface area of the TiO2 nanotubes.
The resulting 1D conical CdS/TiO2 tubular heterostructure acts as a sink for photons. Due to the multiple
Received 23rd March 2018,
Accepted 23rd May 2018
light scattering and absorption events within this nanotubular sink, a large portion of photons (nearly 80%)
DOI: 10.1039/c8nr02418a
is converted into electrons. It is the combination of the scaffold architecture and the light absorber present
on the high surface area as a very thin layer, the optimized charge transport and multiple optical effects that
rsc.li/nanoscale
make this heterostructure very promising for the next generation of highly performing solar cells.
Introduction
The depletion of fossil resources and all the associated
harmful effects on the environment have encouraged the development of sustainable energy technologies. Among all alternative energy sources (solar, hydro, wind, biomass, geothermal,
etc.), photovoltaic technologies are very important renewable
energy suppliers. Over the past decades, there have been considerable ongoing efforts into the development of highly
efficient solar cells on a large scale. The original crystal silicon
solar cells,1 even though their production costs have been
gradually decreasing and their efficiency gradually increasing,
have shown limitations: (i) a narrow range of solar conditions for good performance (direct sunlight required) and
(ii) specific installation requirements (i.e. a suitable angle against
the sun). Much effort has also been put into second generation
solar cells based on thin films of amorphous silicon and
chalcogenides.2–5 The further inclusion of inorganic nanostructure architectures within solar cell development, has
enabled the realization of the third generation of solar cells
a
Center of Materials and Nanotechnologies, Faculty of Chemical Technology,
University of Pardubice, Nam. Cs. Legii 565, 530 02 Pardubice, Czech Republic.
E-mail: jan.macak@upce.cz
b
Central European Institute of Technology, Brno University of Technology,
Purkynova 123, Brno, Czech Republic
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nr02418a
This journal is © The Royal Society of Chemistry 2018
(heterojunction and multijunction) utilizing organic materials
as light absorbers. Well-known examples are solar cells based
on mesoporous TiO2 sensitized with different chromophores
(organic dyes and perovskites).6–8 To achieve efficient solar
cells of this kind, the interplay between optical absorption,
high interfacial surface area and good electrical contact
between TiO2 and the chromophores has to be optimized. As
well as organic dyes and perovskites, significant promise lies
also in the utilization of quantum dots or thin layers of chalcogenides such as CdS,9 CdSe,10 CdTe,11 CIGS,12 kesterites,13 and
Sb2S3 14 among others due to their optimal light absorption,
and high stability under UV light and ambient conditions.
A promising further step in the design of these solar cells
lies in the utilization of anodic TiO2 nanotubular structures
that provide a high surface area for anchoring the light absorber, unique directionality for the charge separation, and a
straight pathway for the electron transport along the axis of
the nanotube, resulting in highly effective charge collection.15,16 All these properties, together with the simple and low
cost fabrication process, make self-organized TiO2 nanotube
layer architectures very promising for next generation photovoltaics cells.17,18 However, the integration of light absorbers
within TiO2 nanotube layers of various aspect ratios, in a conformal and reproducible fashion, is not trivial. Conventional
thin film deposition methods such as chemical vapour deposition (CVD), physical vapour deposition (PVD) or sputtering
are not suitable, as the deposited material cannot reach
the deepest part of the TiO2 nanotube layers, and effective
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utilization of the tube interiors is disabled due to tube mouth
clogging. Other attempts, using spin-coating,19,20 electrodeposition,21,22 and chemical bath deposition,23,24 were
reported to infiltrate secondary materials in the inner surface
of the nanotubes, however complete infiltration of secondary
materials was unfortunately constrained to a reduced number
of materials.
Ideally, the most efficient solar cells based on TiO2 nanotube layers should have a continuous and homogeneous layer
of a suitable light absorber on the nanotube walls to lead to a
good interfacial contact. To date, the atomic layer deposition
(ALD) technique is the only deposition method that meets this
requirement.25 ALD is capable of depositing continuous and
conformal layers of secondary materials into mesoporous and
high aspect ratio nanostructures.26–29 ALD is a vapour-phase
deposition technique based on sequential self-limiting reactions between gaseous molecules of the precursor and the
solid surface. The self-limiting nature of the ALD reactions is
the key difference between ALD and CVD, and leads to its
unparalleled sub-nanometer thickness control and conformal
deposition.
With regard to TiO2 nanotube layers, ALD has already been
demonstrated to be the ideal choice to synthesize nanotubular
heterostructures by depositing different secondary materials
such as Al2O3,30–33 ZnO,34–36 TiO2,37 In2O3,38 and Co3O4 within
nanotubes.39 The resulting nanotubular heterostructures exhibited interesting synergic effects reflected either in the significant
improvement of their photoelectrochemical and photocatalytic
performance (essentially due to an enhanced charge separation
induced by coatings of secondary materials) or in the optical
properties and mechanical and chemical stability.
Surprisingly, the use of ALD for the deposition of a suitable
chalcogenide sensitizer within high aspect ratio TiO2 nanotube
layers towards photovoltaic applications is still unexplored,
except for our recent paper, which utilized very thin and low
aspect ratio nanotube layers.40 CdS is a wide-bandgap semiconductor (Eg ≈ 2.4 eV) and one of its most prominent applications is related to photovoltaics devices as a light absorber.
In addition, the suitable band-edge position of the CdS/TiO2
heterojunction allows the effective transfer of photogenerated
electrons. The deposition of CdS into TiO2 nanotubes has
been reported previously by a range of techniques: successive
ionic layer adsorption and reaction (SILAR),23 a close space
sublimation technique,41 chemical bath deposition,42–45 a
solvothermal technique,46 electrochemical deposition,9,47–50
spray pyrolysis deposition,51 successive electrochemical deposition (SECD),52 and pulsed laser deposition.53 However, these
techniques led to limited interfacial contact between the sensitizer and the TiO2 nanotubes, unlike the complete interfacial
contact obtained by means of ALD.40
Thus, here we take all the benefits of ALD to fabricate a new
type of heterostructured photochemical half-cell by depositing
continuous and uniform chalcogenide (CdS) coatings into 1D
conical TiO2 nanotubular scaffolds of different aspect ratios,
including extremely high aspect ratios (≈900). We utilize for
this purpose highly ordered anodic TiO2 nanotube layers of
16602 | Nanoscale, 2018, 10, 16601–16612
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low (1 μm) thickness up to ultra-high (130 μm) thickness (and
correspondingly increasing aspect ratio and surface area),
coated via ALD with uniform thin CdS coatings (2.5, 5 and
10 nm thick). We compare the performance of coated nanotube layers with the performance of blank (uncoated) layers.
The chemical composition and crystalline structure of the CdS
thin coating was analysed by means of X-ray photoelectron
spectroscopy (XPS) and X-ray diffractometry (XRD). The photoelectrochemical properties were characterized by means of
photocurrent measurements, yielding the incident photon-tocurrent conversion efficiencies (IPCEs). Optical properties were
investigated using diffuse reflectance and ellipsometric
measurements. The 1D conical nanotubular CdS/TiO2 heterostructures show a superior photoresponse in both the UV and
visible regions compared to their blank TiO2 counterparts confirming the synergetic effect between the nanotubular scaffold
and optimal light propagation within the coated nanotube
layer. We discuss in detail the various features leading to these
excellent results.
Experimental
Self-organized TiO2 nanotube layers with different thicknesses
and nanotube diameters were fabricated via anodization of Ti
foils. Prior to anodization, the Ti foils (Sigma-Aldrich
0.127 mm thick, 99.7% purity) were degreased by sonication in
isopropanol and acetone, then rinsed with isopropanol and
dried in air. The electrochemical setup consisted of a 2-electrode configuration using a Pt foil as the counter electrode,
while Ti foils (working electrodes) were pressed against the
O-ring of the electrochemical cell, leaving 1 cm2 open to the
electrolyte. The electrochemical experiments were carried out
at 20 °C by a high-voltage potentiostat (PGU-200 V, IPS
Elektroniklabor GmbH). The electrolyte used for the preparation of 1 μm thick TiO2 nanotube layers was glycerol–deionized water (50 : 50 vol%) containing 270 mM NH4F.
Ethylene glycol containing 10 vol% deionized water and
150 mM NH4F was used to synthesize the 5 μm thick TiO2
nanotube layers, whereas ethylene glycol containing 1.5 vol%
deionized water and 176 mM NH4F was used for the anodization of the rest of the TiO2 nanotube layers (20, 60, 90 and
130 μm thick). For details on the anodization potentials/currents and durations we refer to Table 1. All electrolytes were
prepared from reagent grade chemicals (Sigma-Aldrich).
Ethylene glycol based electrolytes were aged before the first
use for 15 h by anodization of blank Ti foils at 60 V under the
same conditions for the anodization experiments; the reasons
for aging are described in the literature.54 After anodization,
the Ti foils were rinsed and sonicated in isopropanol and
dried. The shorter TiO2 nanotube layers, i.e., 1,55 5,22 and
20 μm thicknesses,32 were synthesized potentiostatically,
whereas the thicker, i.e. 60, 90 and 130 μm thicknesses were
fabricated galvanostatically.
Upon the anodization process, the transformation of the
amorphous as-anodized TiO2 nanotube layers into an anatase
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Table 1 Average thickness, inner diameters and aspect ratios of the different TiO2 nanotube layers (TNTs) together with the corresponding anodization process conditions: electrolyte, potential or current applied, and anodization process time
Average inner Average
Average
aspect Electrolyte
thickness diameter
composition
ratio
(nm)
(μm)
Average
Total surface area
Anodization Anodization Anodization number of
of TNTs on 1 cm2
TNTs (cm−2) of substrate (cm2)a
potential (V) current (mA) time (min)
1
80
13
20
—
100
66 × 108
16.5
5
230
22
100
—
240
12 × 108
44
20
60
90
130
130
130
130
145
154
461
692
897
—
7.5
10
15
240
180
180
180
46 × 108
45 × 108
48 × 108
37 × 108
377
1080
1776
2183
Glycerol + H2O
(50 : 50 vol%), 270 mM NH4F
Ethylene glycol,
10 vol% H2O, 150 mM NH4F
60
—
Ethylene glycol,
1.5 vol% H2O, 176 mM NH4F —
—
a
The total surface area of the nanotubes was estimated from the average values of the inner diameter and the number of nanotubes calculated
statistically from the SEM images. The anodized area of the Ti substrate was 1 cm2.
crystalline structure proceeded by an annealing process in a
muffle oven in air. Annealing of 60, 90 and 130 μm thick TiO2
nanotube layers was carried out at 320 °C for 1 h using a
heating rate of 0.5 °C per minute. These annealing conditions
were required to avoid lift-off and cracking of the TiO2 nanotube layers. The annealing conditions of the remaining TiO2
nanotube layers were 400 °C for 1 h.
The deposition of CdS within the different TiO2 nanotube
layers was carried out via ALD (Beneq TFS-200). Dimethyl
cadmium (Strem, elec. gr. 99.995 + % – Cd) and hydrogen sulphide (99.5%) were used as the cadmium and sulphur precursors, respectively. Unlike planar substrates, the deposition of
secondary materials into (ultra)high aspect ratio nanotubular
structures involves a rather large area to coat and long
diffusion distances that the precursors/by-products must travel
into/off the TiO2 nanotubes.27 Therefore, to achieve uniform
and homogeneous coatings the ALD process demands relatively high precursor doses and long precursor diffusion
times.32 In order to meet all the mentioned requisites, instead
of the conventional ALD continuous flow mode, we used the
ALD exposure mode (also called quasi-static mode).27 Therein
an additional step called exposure time was included between
the pulsing of the precursor and the chamber purge step. The
exposure time aids: (i) diffusion of precursors towards the
deepest part of the nanotube, and (ii) diffusion of the resultant
by-products and unreacted precursors out of the nanotube
upon ALD reaction. Under these deposition conditions, one
growth ALD cycle was defined by the following sequence:
dimethyl cadmium pulse (3 s) – dimethyl cadmium exposure
(45 s) – N2 purge (85 s) – H2S pulse (3 s) – H2S exposure (45 s) –
N2 purge (85 s). All processes were carried out at a temperature
of 150 °C, and using N2 (99.9999%) as a carrier gas at a flow
rate of 400 standard cubic centimetres per minute (sccm). CdS
deposition was carried out running 17, 33, and 66 ALD cycles,
leading to 2.5, 5, and 10 nm of nominal coating thicknesses.
The number of cycles was estimated from the growth per cycle
value of the CdS process at 150 °C (≈1.5 Å per cycle). The ALD
heating rate was slow enough (≈0.5 °C min−1) to ensure the
absence of cracking and lift-off of the thicker TiO2 nanotube
layers (60, 90 and 130 μm thicknesses).
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The structure and morphology of the TiO2 nanotube layers
with and without CdS coatings were characterized by fieldemission SEM (FE-SEM JEOL JSM 7500F) and a high-resolution
transmission electron microscope (FEI Titan Themis 60, operated at 300 keV) equipped with a high angle annular dark field
scanning transmission detector (HAADF-STEM). The cross-sectional views were taken from mechanically bent samples. The
bending process led to the rupture of the nanotube layers,
enabling visualization of the coatings within the nanotubes in
various directions and nanotube layer depths. Dimensions of
the nanotubes along with the CdS coating were measured and
statistically evaluated using proprietary Nanomeasure software.
Average values and standard deviations were calculated from at
least three different locations with a high number of measurements (n > 50). The thickness of the CdS coating was in
addition to SEM measured also by variable angle spectroscopic
ellipsometry (VASE® ellipsometer, J.A. Woollam, Co., Inc.)
using CdS-coated Si wafers deposited within identical ALD
runs as for the nanotube layers. Recorded ellipsometry spectra
were fitted against a model sample consisting of a crystalline
Si substrate, SiO2 native oxide (the thickness of which was
determined to be 2.6 nm) and a CdS overlayer. The optical constants of Si and SiO2 were taken from the literature,56 whereas
those of CdS were parameterized by summation of three
Gaussian oscillators together with one oscillator based on the
Herzinger–Johs function.57
Diffraction analyses of the blank and CdS/TiO2 nanotube
layers were carried out using an X-ray diffractometer (XRD, D8
Advance, Bruker AXE) using Cu Kα radiation with a secondary
graphite monochromator and an NaI(Tl) scintillation detector.
The chemical composition of the CdS coating was determined using X-ray photoelectron spectroscopy (XPS, Phoibos
150, SPECS) using a monochromatized Al Kα X-ray source (hν =
1486.7 eV) and a PHOIBOS 150 hemispherical analyser with a
1D DLD detector. The binding energy scale was referenced to
the adventitious carbon (EB = 284.8 eV). After the initial
measurement, the sample was sputtered with Ar+ ions with a
kinetic energy of 500 eV and a current density of 1.5 µA cm−2
for 20 min to remove surface contaminants and explore the
deeper surface region. In parallel, energy dispersive X-ray spec-
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troscopy (EDX, fitted to HR-TEM) was used in addition to
reveal the chemical composition.
The photocurrent measurements were carried out in
aqueous electrolytes containing either 0.1 M Na2SO4 or 0.1 M
Na2S employing a photoelectric spectrophotometer (Instytut
Fotonowy) connected with the modular electrochemical system
AUTOLAB (PGSTAT 204, Metrohm Autolab B.V.) operated with
Nova 1.10 software. A three-electrode cell with a flat quartz
window was employed with an Ag/AgCl reference electrode, a
Pt wire counter electrode and the anodized and annealed Ti
substrate as the working electrode, pressed against the O-ring
of the electrochemical cell leading to an irradiated sample
area of 0.28 cm2. Monochromatic light was provided by a 150
W Xe lamp with an areal intensity (λ = 500 nm) of ≈1.50 mW
cm−2. The photocurrents generated by the TiO2 nanotubes
were measured at 0.4 V vs. Ag/AgCl (3 M KCl) in the spectral
range from 300 up to 800 nm with 10 nm steps. The photocurrent transients were measured for 10 s (light-on period).
Diffuse reflectance measurements were carried out on all
types of nanotubes using a UV/VIS/NIR spectrophotometer
Jasco V-570 equipped with an integrating sphere (Jasco
ISN-470).
Results and discussion
Fig. 1 SEM top (a, c, and e) and bottom (b, d, and f ) view images of the
CdS coated (5 nm) TiO2 nanotube layers of different thicknesses: 1 μm (a
and b) 60 μm (c and d) and 130 μm (e and f ).
1D conical TiO2 nanotube layers with thicknesses spanning
from 1 μm to 130 μm were prepared by anodization of the Ti
substrates. The conditions used are described in the
Experimental section and listed in Table 1, which also provides
additional important information about the aspect ratio and
the total surface area of the TiO2 nanotube layers. Subsequent
annealing treatments yielded crystalline anatase TiO2 nanotube layers with suitable electron transport properties.15
Afterwards, the TiO2 nanotube layers were coated with 5 nm
thick CdS layers by atomic layer deposition (ALD) at 150 °C –
details of all these procedures are given in the Experimental
section.
Fig. 1 shows representative SEM images of 1D CdS/TiO2
nanotube layers of different thicknesses: 1 μm (Fig. 1a and b),
60 μm (Fig. 1c and d), and 130 μm (Fig. 1e and f ). The layers
consist of close-packed 1D nanotubes with strictly conical
shapes. This shape is apparent from significantly larger inner
tube diameters at the top of the layer compared to the bottom.
In contrast to the single wall 1 μm thick TiO2 nanotube
layers, all the other TiO2 nanotube layers exhibited a double
wall structure that is characteristic for the ethylene glycol
based electrolytes used for the anodization process.58
However, uniform CdS coating all along the nanotubes was
verified through SEM images from the top (Fig. 1a, c and e)
and bottom (deepest) parts of the different TiO2 nanotube
layers (Fig. 1b, d and f ), demonstrating the capability of ALD
to uniformly coat thick TiO2 nanotube layers with secondary
materials. Further representative SEM images from the top
parts of the CdS/TiO2 nanotube layers, along with the corresponding cross-sectional SEM images that verified the thick-
nesses of the nanotube layers, are shown in Fig. S1.† The
thickness of the CdS coating measured from the SEM images
(≈ 5.1 ± 0.7 nm, see Experimental section) was in excellent
agreement with the thickness value (≈ 5.9 ± 0.2 nm) obtained
by ellipsometry measurements of a CdS thin film deposited on
Si wafers within the same ALD run.
STEM and EDX analyses were carried out to verify the uniformity of the CdS coatings of the TiO2 nanotube layers as it was
achieved in our previous work using other materials.32,33,36,38
Fig. 2a shows a high angle annular dark field (HAADF)
scanning transmission electron microscopy (STEM) image of a
fragment of a 20 μm thick TiO2 nanotube layer coated with
10 nm of CdS. Therein, one can clearly see the uniform and
continuous CdS coating at the interior as well as the exterior
part of the TiO2 nanotube. Fig. 2b shows an overall STEM/EDX
elemental map. EDX maps of individual elements (namely Cd,
S, Ti and O) acquired on fragments of the TiO2 nanotube layer,
coated either with 5 nm or with 10 nm of CdS are illustrated in
Fig. S2 and S3,† respectively. Finally, Fig. 2c shows the compositional profile of these elements recorded across the line scan
(the green line in Fig. 2a and b). In addition, STEM imaging
and element mapping were used to verify the coating on a
larger scale. Fig. S4 and S5† show the STEM image and EDX
maps of a bundle of multiple TiO2 nanotubes coated with
5 nm and 10 nm of CdS, respectively. Overall, analyses in
various TEM modes possess a solid verification of the CdS
coatings along the tube walls and its corresponding
composition.
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Fig. 3 (a) XPS survey spectra of a 20 nm thick CdS film on a Si (100)
wafer before and after sputtering. High resolution XPS spectra of (b) Cd
and (c) S. (d) Cd MNN Auger spectra.
Fig. 2 (a) High angle annular dark field (HAADF) scanning transmission
electron microscopy (STEM) image and (b) the overall STEM/EDX
elemental map of a fragment of a 20 μm thick TiO2 nanotube layer
coated with 10 nm of CdS. The green arrow indicates the EDX line scan
across the nanotube fragment. (c) The EDX compositional profile corresponding to the line scan.
X-ray photoelectron spectroscopy (XPS) analyses were conducted to determine the chemical composition of the CdS
coatings. Fig. 3a illustrates a survey XPS spectra of the CdS
coating (20 nm thick) on a Si (100) witness sample, measured
in the as deposited state and after a mild sputtering. C and O
species were only detected on the surface before sputtering.
These impurities were therefore related to sample handling in
air following ALD. After sputtering by Ar+ ions, only Cd and S
species were visible in the coating. The high-resolution XPS
spectrum of Cd and S (Fig. 3b and c) shows the position of the
Cd 3d and S 2p peaks at 405.3 eV and 161.5 eV, respectively,
which correspond to typical CdS binding energies.59 However,
the small shifts of the binding energies of the Cd metal and
the Cd oxides and sulphides hinder their differentiation from
each other. In order to determine the chemical status of the
Cd more precisely, the Auger spectra of the MNN energy levels
were acquired (Fig. 3d) and confirmed that the peak at 381.1
eV corresponded to the Cd–S bond.59 Further XPS analyses
(shown and discussed later in the text) were carried out to
determine the valence band offset between TiO2 and CdS to
gain valuable information for a deeper understanding of the
photoelectrical behaviour of the coated nanotube layers.
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The crystalline structure of blank and CdS/TiO2 nanotube
layers was characterized using X-ray diffractometry (XRD).
Fig. 4a and b show the corresponding XRD patterns of 1 μm
and 130 μm thick TiO2 nanotube layers (Fig. S6† summarizes
the corresponding XRD patterns obtained for all types of TiO2
nanotube layers). The XRD patterns of the blank TiO2 nanotube layers (Fig. 4a and S6†) exhibited the peaks of the metallic
Ti substrate, associated with the planes (100), (002), (101) and
(102), as well the peaks corresponding to the TiO2 anatase
phase planes (101), (004), (200) and (105) of the TiO2 nanotube
layers. The XRD patterns obtained for the CdS/TiO2 nanotube
layers, illustrated in Fig. 4b and S6,† showed the patterns
described for their blank counterparts plus the peaks associated with hexagonal (wurtzite) crystalline CdS planes (002),
(101) and (110) with a dominant orientation (002).60 Fig. 4c
shows a detail of the wurtzite peaks, corresponding to planes
(002) and (101), where the intensity of the CdS peaks clearly
increased along with the thickness of the TiO2 nanotube
layers, indicating a higher CdS loading. Moreover, broad CdS
peaks revealed a polycrystalline CdS coating with a small grain
size.
Blank and CdS/TiO2 1D conical nanotube layers were
further investigated for their photoresponse. Fig. 5 shows the
incident photon-to-current efficiency (ICPE) spectra of the
blank TiO2 nanotube layers with different thicknesses (Fig. 5a)
and their counterpart nanotube layers after CdS coating
(Fig. 5b). Blank TiO2 nanotube layers showed no photocurrent
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Fig. 4 XRD pattern of the (a) blank and (b) CdS coated TiO2 nanotube
layers 1 and 130 μm thick. (c) Details of the anatase peak corresponding
to the plane (101) and CdS peaks associated to planes (002) and (101)
obtained from all the CdS/TiO2 nanotube layers.
Fig. 5 (a and b) Incident photon-to-current efficiency and (c and d)
transient photocurrent at 360 nm and at 470 nm, respectively, recorded
for blank and CdS/TiO2 nanotube layers of different thicknesses. Arrows
indicate light ON (↑) and light OFF (↓). (e) Cycling performance of photocurrents transients, measured at 470 nm for 60 μm thick CdS coated
TiO2 nanotube layers (21 cycles).
in the visible spectral region due to the band gap energy of
anatase TiO2 Eg ∼ 3.2 eV (≈ 387 nm), while in the UV region
the IPCE values were decreasing with increasing thickness of
the TiO2 nanotube layers, in line with the literature and as discussed later.61
On the contrary, the CdS/TiO2 nanotube layers (Fig. 5b)
showed a superior photoresponse in both the UV and visible
spectral range. More specifically, their IPCE values were about
3 times higher for the CdS/TiO2 nanotube layers in the UV
region (reaching 60–80% IPCE at 360 nm) as compared with
the blank TiO2 nanotube layers. The enhancement of the IPCE
values in the UV spectral region can be ascribed to the presence of sulphur (S) within the TiO2 nanotube layers which
removed TiO2 surface defects and thus inhibited significant
charge carrier recombination.19,20,40,62,63 The surface incorpor-
ation of S atoms occurred during the very early ALD cycles as
the bare TiO2 surface and H2S came into close contact for a
significant time (the exposure time was 45 s). The photoresponse of the CdS/TiO2 nanotube layers in the visible spectral region was outstanding as compared to their blank
counterparts. The CdS coatings within the nanotube layers
caused superior visible light sensitization represented by high
IPCE values over a wide range of wavelengths (up to 675 nm).
In addition, the IPCE values shown in Fig. 5b were superior to
those reported in previous work on TiO2 nanotubes sensitized
with CdS by other techniques, regardless of the variety of
experimental conditions used for IPCE measurements.23,42–46
We believe this phenomenon stems from the fact that conventional deposition methods lead to decoration of the TiO2 nanotubes by individual particles or particle bundles and hence
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only affords partial coverage of the TiO2 walls. In contrast, ALD
CdS leads to a uniform coating that is in close contact with the
entire available surface of the TiO2 nanotubes, and participates
then in the light harvesting. The potential of the surface and
1D-architecture of the TiO2 nanotubes is therefore fully
exploited, unlike TiO2 nanotubes with particle decoration.
Therefore, lower performance of TiO2 nanotubes with particle
decoration is understandable.
After all the preliminary tests, a selection of blank
(uncoated) nanotube layers as an appropriate reference to their
CdS-coated counterparts turned out to be the most suitable for
at least three reasons: (i) it is not reasonable to compare the
performance of nanotube layers (especially the thick ones, e.g.
130 μm) with mesoporous TiO2 layers of such a thickness due
to difficulties for electrons to percolate such a thick porous
structure, (ii) the pores of such mesoporous layers are clogged
by ALD at the very top of the layer with CdS (yielding low
photocurrents, and iii) such comparison was already provided
for low aspect ratio nanotube layers by Baker et al.23 and
yielded nanotube layers superior to mesoporous layers.
The transient photocurrents of the layers were measured at
360 and 470 nm (Fig. 5c and d), respectively, to obtain
additional information about the charge separation within
blank and CdS/TiO2 nanotube layers. These wavelengths were
selected as they represented (on average) the highest photocurrents achieved for all the layers. Fig. 5c shows that in the UV
spectral range, the photocurrent values of the TiO2 nanotube
layers increased for 1, 5 and 20 μm thick blank layers, where
they reached saturation. In addition, the photoresponse was
rather immediate with the light on and off. This is in line with
the outcomes of previous reports that showed that nanotube
layers with thicknesses of 5–20 μm are the most favourable for
efficient performance in the UV spectra range.61,64,65 As
further seen in Fig. 5d for thicker layers (60, 90 and 130 μm),
the photocurrents decreased with increasing TiO2 nanotube
thickness. Moreover, these layers showed very slow responses
with the light on and off in contrast to their thinner counterparts (1, 5 and 20 μm). The delayed photocurrent response
with the light on and off is clearly due to numerous traps in
the nanotube walls, mainly due to surface defects (oxygen
vacancies and under-coordinated Ti4+ sites). This phenomenon
was already demonstrated in previous literature but never for
such a huge span of nanotube layer thicknesses.37,66,67 These
features clearly illustrate the strong effect of the TiO2 nanotube
layer thickness on the electron transport efficiency. In order to
address the stability of the achieved photocurrents, we performed photocurrent transients in numerous cycles with the
light “on” and “off”. Fig. 5e shows an example of the photocurrent transients obtained for a 60 μm thick CdS coated TiO2
nanotube layer measured at 470 nm during 21 illumination
cycles. As one can see, a minor decay of photocurrent is
observed, most likely due to a partial dissolution of CdS in the
used electrolyte. However, in a complete solid state cell, such
dissolution has no reason to take place, so we consider the
stability of the presented CdS coatings and obtained photocurrents as very sufficient.
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In order to give a physical description of these results, it is
necessary to consider the retrieval length (Lr), defined as the
length resulting from adding the electron diffusion length (Ln)
and the penetration length (Lλ) of the light at a given wavelength (Lr = Ln + Lλ).61 It is important to note that Lλ is inversely proportional to the absorption coefficient α(λ), which is
higher for short wavelengths. The short wavelengths are
absorbed at the upper parts of the TiO2 nanotube layers
leading to high IPCE values, but it also implies a reduction of
the light penetration length Lλ, or eventually shortening Lr.
Thus, it could be expected that those TiO2 nanotube layers are
thicker than certain Lr values (in this case ≈20 μm) and must
experience a detrimental worsening of the electron transport
and the photocurrent response. That is, as the electron
diffusion distance requires a longer time than the photoelectron lifetime, the electron–hole (e+–h−) recombination
probability significantly increases and the photocurrent
declines, which is clearly apparent in the case of the 60, 90
and 130 μm layers in Fig. 5a. These results match with the
work of Lynch et al.61 who calculated ≈24 μm as the electron
diffusion distance within TiO2 nanotube layers synthesized in
ethylene glycol.
In contrast, Fig. 5d shows how all the differences in electron
diffusion distance were attenuated for the CdS/TiO2 nanotube
layers, which is seen from the prompt response with the light
on and off and the remarkably enhanced photocurrents for all
thicknesses. Such a great photoresponse in the visible spectral
region stems from three effects: (i) absorption of the visible
light photons in CdS given the bandgap energy of CdS (Eg ≈
2.4 eV), (ii) the suitable positions of the conduction bands of
CdS and TiO2 in the energy diagram allowing for efficient electron transfer of electron,68,69 and (iii) removal of surface
defects of the TiO2 nanotubes by CdS leading to a very efficient
charge carrier separation. Interestingly, at a wavelength of
470 nm (Fig. 5d), the photocurrents were also highest for the
20 μm thick CdS/TiO2 nanotube layers. This suggests that for
this particular thickness, the light absorption and the electron
transport within the CdS/TiO2 nanotube layer were optimal
(even though all the other layers showed an excellent response
too). This means that even the deepest parts of the thickest
CdS-coated TiO2 nanotube layers were still involved in efficient
electron transport and acted as efficient electron highways.
Another highly significant feature that can be observed for
the CdS/TiO2 nanotube layers is the red shift of the IPCE
values with increasing thickness of the CdS/TiO2 nanotube
layers, as shown in Fig. 5b. For a physical elucidation of the
observed red shift, different factors were explored. The first
considered factor was the aforementioned content of S within
the TiO2 nanotube layers. The S 3p states of the S atoms occupying the substitutional sites are delocalized and can contribute to the valence band (VB) together with the O 2p and Ti 3d
states, resulting in an increase of the VB width, and as a consequence a narrowing of the band gap.62,63 Thicker TiO2 nanotube layers exhibited a more prominent red shift that could
stem from the higher S content and the more pronounced narrowing of the band gap. Thus, the effect of the incorporated S
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atoms would be dual: (i) contributing to passivation of the
surface defects of the TiO2 nanotube layers ( principally the
oxygen vacancies), and (ii) narrowing of the optical band gap
spreading the absorption to a wider spectral range (i.e. toward
longer wavelengths). In line with the content of S, the formation of TiS2 on the TiO2 nanotube surface was also considered. The substitution of S for O was found in the literature
to cause a significant shift in the absorption edge to a lower
energy and might contribute to the red shift observed.62
However, XPS measurements shown in Fig. S7† did not indicate the presence of TiS2 as only Ti–O bonds were found
within the Ti 2p XPS spectra.
The second considered factor was the possible sub bandgap
tail of the CdS thin coating within the TiO2 nanotubes. Optical
constants of the bulk and thin films of CdS were studied by
different authors and most of the works evidenced the existence of a sub bandgap tail (the so called Urbach tail).56,70,71
This tail is responsible for a weak light absorption even for
photons with energies lower than the CdS bandgap which is
usually believed to be 2.4 eV (≈ 516 nm). The span of the
Urbach tail to a lower energy region is characterized by the
Urbach energy that can reach values as high as a few hundreds
of meV, depending on the method used for the CdS deposition.72 Several suggestions were formulated to assign the
presence of the Urbach tail to structural disorder, bulk defects
or other as yet undefined inhomogenities in CdS.71–73
To support the discussion of our IPCE results in terms of
the possible sub bandgap tail of CdS, we have carried out
complementary ellipsometry measurements on planar CdS
thin films with a determined thickness of (≈ 5.9 ± 0.2 nm, see
Experimental section). These films were deposited onto Si substrates during the same ALD run as was used for the CdS
coating of TiO2 nanotubes to keep the deposition conditions
of CdS in both cases as close as possible. The obtained spectral dependence of the extinction coefficient of the CdS film is
presented in Fig. 6a, where the sub bandgap tail is evident. It
is worth mentioning here that for the flat thin CdS film one
should expect only a negligible signature of the sub bandgap
tail in either the optical reflectance or the transmittance
spectra. This is due to relatively low values of absorption coefficients in the CdS tail spectral region and the small thickness
of the films (a few nm). However, a significantly different situation could be expected within the 1D conical nanotubular
layers and an extremely high surface area of CdS coating on
them. In the case of the 1D conical CdS/TiO2 nanotube layers,
one can quantify the rather complex light scattering phenomena by the multiple light reflections, absorptions and transmissions in between the tube walls coated by CdS, as schematically depicted in Fig. 6b, whereby the light harvesting ability
is enhanced. Similar reflection, scattering and transmittance
features were described already for porous alumina.74 The
penetration depth of light in the CdS sub bandgap tail spectral
region is rather large (about 1 μm) and the TiO2 nanotubes are
optically transparent for these wavelengths, therefore it is easy
to imagine a huge number of successive transmissions of light
through the CdS/TiO2 nanotube walls before the photons
16608 | Nanoscale, 2018, 10, 16601–16612
Nanoscale
Fig. 6 (a) Spectral dependence of the extinction coefficient of CdS,
where the solid arrow indicates the sub-band gap tail and the dash dot
line the CdS energy band gap (2.4 eV or 516 nm); (b) a simplified sketch
illustrating the different interactions of light into the CdS/TiO2 nanotube
layers: incident light (long dash line), multi-scattering at different incident light penetration depths (solid line), absorption (red square dot
line), transmission (dash dot line) and reflected light (round dot line with
white squares); (c) a simplified energy diagram of the valence and conduction energy bands, showing alignment of the CdS/TiO2 junction
according to the valence band offsets determined by XPS. The CdS sub
bandgap tail allows the photoelectron promotion at a lower energy (E)
than the nominal CdS bandgap energy (Eg).
escape from the TiO2 nanotube layer as diffuse reflected light
or become absorbed inside the CdS/TiO2 nanotube layers. In
fact, there are mainly these multiple transmissions, each
coupled with small, but not negligible light absorption in the
CdS Urbach tail, which results in an effective band gap lowering of the CdS/TiO2 nanotube layer (Fig. 5b). The multiple
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light scattering with the associated transmission and absorption processes along the nanotubular structure become more
pronounced with the increasing thickness of the nanotube
layers, as observed in Fig. 5b. In order to support the physical
interpretation of this observation, further XPS analyses were
carried out to determine the relative positions of the TiO2 and
CdS valence and conduction bands at the TiO2/CdS heterojunction. The exact calculations can be found in the ESI
(Fig. S8†). The obtained valence band offset value was 1.3 ± 0.2
eV and that implies a conduction band offset value of 0.5 ± 0.2
eV (assuming the band gap energy values of anatase TiO2 and
CdS to be 3.2 eV and 2.4 eV, respectively). Therefore, it is
evident that the CdS sub bandgap tail (schematically depicted
in Fig. 6c) enables the injection of photo-induced electrons
into the TiO2 conduction band and this physical scenario fully
correlates with our interpretation of an effective band gap lowering based on the optical and IPCE measurements.
The demonstrated effective band gap lowering has been
previously observed and discussed also in the case of CdS/ZnO
microplatelets,72 CdS/ZnO nanoporous films,73 and CdS/TiO2
nanotubes with a red shift reported from 516 nm to about
600 nm.52 TiO2 nanorods or nanotubes, regardless of the
aspect-ratio, sensitized by CdS in the form of quantum dots,
do not show a comparably prominent shift,75–77 probably due
to the only partial coverage of the TiO2 walls by the CdS
material.
Thus, the most plausible description of the red shift
observed in Fig. 5b is a multiple scattering effect of light confined within the 1D conical CdS/TiO2 nanotube layer originating from multiple transmission and absorption processes,
without excluding the potential effect of the S content within
the TiO2 nanotube layers on the narrowing of the band gap, as
previously described. The scattering effect and its impact on
the red shift must be directly related to the aspect ratio and
the total surface area of the TiO2 nanotube layers, shown in
Table 1. Thus, a higher surface and aspect ratio (thicker TNTs)
drives to a stronger scattering effect and a noteworthy enhancement of the absorption coefficients in the CdS tail spectral
region, triggering a larger red shift of the IPCE values.
The light scattering effect (as an effective means to increase
the light harvesting) has been reported for dye sensitized solar
cells. Different double layered hierarchical TiO2 structures
have been developed containing nanospindles,78 flower
shaped structures,79,80 or aggregated TiO2 structures.81,82
Despite the observed significant performance enhancement,
they are unfortunately based on complex and tedious fabrication processes. In contrast, herein we present an open conical
shaped nanotubular structure ( produced using a comparably
easier protocol), wherein multiple light scattering effects
remarkably enhance the light harvesting. Even though the
conical shape of the nanotube layers was previously reported
by So et al.83 as highly advantageous for dye sensitized solar
cells, that study was limited to 13 μm-thick nanotube layers,
the study did not have continuous coatings and did not
discuss the multiple scattering phenomena, which are
described in this work.
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Finally, we studied the photoresponse of the 20 μm thick
TiO2 nanotube layers with different CdS coating thicknesses
since for this particular thickness, the light absorption and
the electron transport within the CdS/TiO2 nanotube layer
were found to be optimal as discussed above. The optimal CdS
thickness should be such a thickness allowing maximal light
absorption within nanotubes while retarding electron–hole
recombination. In this way, taking advantage of ALD to precisely control the coating thickness, additional samples of
20 μm thick TiO2 nanotube layers were coated with 17 and 66
ALD cycles to obtain CdS coatings with a nominal thickness of
Fig. 7 (a) Photocurrent density and (b) incident photon-to-current
efficiency recorded for 20 μm thick TiO2 nanotube layers sensitized with
different CdS coating thicknesses (2.5, 5 and 10 nm). (c) Diffuse reflectance spectra of TiO2 nanotube layers with a CdS coating of different
thickness: 2.5, 5 and 10 nm.
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2.5 and 10 nm, respectively (shown in Fig. S9†), in addition to
5 nm. As shown in Fig. 7a and b, a clear increase of the photocurrent and IPCE values was obtained after increasing the
thickness from 2.5 to 5 nm. However, almost identical results
were obtained for CdS coating with 5 and 10 nm, despite the
diffuse reflectance measurements that showed stronger light
absorption for 10 nm thick CdS coatings, as shown in Fig. 7c.
These results clearly show that thicker CdS coatings can quantitatively absorb more photons, but they do not produce quantitatively corresponding amounts of electrons. In other words,
the lifetime of the photogenerated electrons at the outermost
part of the CdS coating is not long enough to successfully
reach the TiO2 nanotubes. From this point of view, using a
5 nm CdS coating in this work as the main coating thickness
is fully justified and represents the best choice for the optimal
photoelectrochemical performance of the CdS/TiO2 nanotube
layers.
All in all, from the results presented herein it becomes clear
that 1D conical TiO2 nanotubes act as effective sinks for
photons. The conical nanotube shape has a strong influence
on the light absorption profile within the CdS/TiO2 heterostructure and influences the sub bandgap tail of CdS within
the TiO2 nanotubes. The results of this work clearly show how
effective a strategy it is to deposit suitable secondary materials
within the TiO2 nanotube layers by means of ALD to tailor
their surface properties and expand the application portfolio
of these unique 1D layers. This strategy can be exploited for a
whole range of secondary materials with specific functional
properties. A whole range of novel solid-state photovoltaic
devices (and electrochemical devices in general), which were
not feasible before, can be realized using this approach.
Conclusions
In conclusion, we present the superior performance of a 1D
conical heterostructured photochemical half-cell based on
TiO2 nanotubular scaffolds ALD coated with a thin layer of
CdS. The excellent photon-to-electron conversion is a direct
consequence of the synergistic effect between the 1D conical
nanotubular structure that acts as an electron sink, and the
uniform and conformal ALD coating of the CdS light absorber.
The resulting high interfacial contact surface area between the
scaffold and the light absorber (not achievable otherwise), and
the multiple light scattering effects yielded very effective
photon absorption and conversion to electrons. We showed
that a whole range of nanotube layer thicknesses can be used
for this purpose and that the optimal thickness of the absorber coating is approximately 5 nm. This work is a proof of
concept for the efficient use of ALD and nanotubular structures for photovoltaic applications. For this purpose, a binary
system such as CdS/TiO2 should be considered as a proof of
principle tandem. In order to prepare a large scale functional
device, promising for future solar cells and other applications,
the presented results open up a new pathway to coat TiO2
nanotubes e.g. by kesterites and other functional materials.
16610 | Nanoscale, 2018, 10, 16601–16612
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The door is open to produce a whole range of functional
devices, based on the findings presented in this work, including highly efficient solar cells, photocatalytic membranes and
catalytic materials.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge support from the
European Research Council (ERC, project no. 638857) and the
Ministry of Education, Youth and Sports of the Czech Republic
(no. LM2015082, LQ1601). The authors acknowledge the
CERIC-ERIC Consortium for access to experimental facilities
and for financial support under proposal no. 20172101.
Dr Katerina Veltruska, Dr Bretislav Smid, Dr Peter Matvija, and
Dr Tomas Duchon are acknowledged for their support during
XPS measurements. We thank Dr Veronika Podzemna, Mr Jan
Michalička and Assoc. Prof. Ludvík Beneš for SEM, HR-TEM
and XRD analyses, respectively.
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