(Aryan Azad)
12
(Sun Jae Kim)
1*
Copyright © 2022 The Korean Institute of Metals and Materials
Key words(Korean)
photoelectrochemical water-splitting, semiconductors, hematite (α-Fe2O3), Ti doping, hydrothermal method, doping method
1. INTRODUCTION
The reduction in fossil fuel resources has shifted the conventional energy system
in the direction of renewable energy sources such as water, wind, solar, and hydrogen
resources. Solar energy provides a promising option for hydrogen production from renewable
energy resources [1]. For solar hydrogen generation, photoelectrochemical (PEC) water-splitting has received
intense attention during the past decade due to its versatile properties [1]. Hematite (α-Fe2O3) is an n-type semiconductor candidate for solar water splitting that is earth-abundant
and stable and possesses an appropriate band-gap of 2.2 eV that ranges in the visible
region [2-4]. It was stable against photocorrosion in alkaline solutions. However, the performance
of hematite for solar water splitting is limited by factors such as poor conductivity,
oxygen evolution reactions (OER), a short hole diffusion length between 2 and 4 nm,
a short lifetime of the excitedstate carrier between 10 and 12 s, and improper band
positions [2-9]. TiO2 is another important transition metal oxide that displays a favorable band edge position,
strong optical absorption, and an inexpensive cost [1,10-12]. It can be used as a photoanode for water-splitting, batteries, dye removal, supercapacitors,
and other applications [13]. A number of factors such as the phase structure, surface area, crystallite size,
and number of surface hydroxyl groups directly influence the photocatalytic performance
of TiO2 [14,15]. In contrast, one of the key impediments to improving photocatalytic efficiency
is the restriction of the recombination of electrons and holes [14,16]. Hence, developing approaches that can promote charge separation in TiO2 is highly desired [14-17]. Various methods such as synthesizing branched structures [18,19], doping with metal or non-metal elements [20-24], post-growth hydrogen annealing, sensitizing with other small bandgap semiconductor
materials, and controlling the crystallite size and structure have been implemented
to improve the photocatalytic properties of TiO2 nanomaterials [25]. The different crystal structures of TiO2 (rutile and anatase) are the most significant factors that influence photocatalytic
performance [10,17]. For improving the performance of hematite, elemental doping has been also studied
using [26,27] Si and Sn, and they can be prepared by different methods to increase the photocurrent
density at low bias by reducing electron-hole recombination and enhancing donor density
beyond a few picoseconds [26-31]. This study aims to design a photoanode material while also discussing the key parameters
that affect the photocatalytic behavior of Fe2O3/TiO2/FTO. Surprisingly, the design of a new photoanode demonstrates excellent photocatalytic
activity by decreasing the bandgap energy and reducing electron-hole recombination.
2. EXPERIMENTS
2.1 α-Fe2O3/FTO hydrothermal (F HT/FTO)
For the hydrothermal coating of α-Fe2O3/FTO, fluorine-doped SnO2 glass (FTO, 14 Ω sq-1, Asahi Glass Co., Japan) was used as the substrate. The FTO glass was cleaned using
acetone, ethanol, and deionized water. A mixture of 0.85 g of NaNO3 and 0.4 g of FeCl3.6H2O at pH 1.5 (adjusted with HCl) was then prepared. A piece of clean FTO glass (3 cm
× 3 cm × 2 mm) was placed within the vial and heated at 100 °C for 6 h in an autoclave.
A thin layer of yellow FeOOH was formed on the FTO substrate. When the samples were
cooled to room temperature, the coated FeOOH on the substrates was cleaned with deionized
water and dried at 80 °C. The phase transition to α-Fe2O3 from β-FeOOH was achieved by annealing for 2 h at 550 °C. The samples were annealed
again at 750 °C for 10 min. This additional heat treatment did not affect morphology
formation; however, it prompted the formation of the target composition (from the
oxyhydroxide phase to the designed hematite phase).
2.2 Ti- Fe2O3/FTO hydrothermal (T-F HT/FTO)
To achieve Ti-doped Fe2O3/FTO, a piece of cleaned FTO glass (3 cm × 3 cm × 2 mm) was placed into a vial containing
0.85 g of NaNO3, 0.4 g of FeCl3.6H2O at pH 1.5 (adjusted by HCl), and 0.5 ml titanium carbonitride (TiCN, Sigma-Aldrich).
An aqueous solution was added as a precursor (10 mg/ml) to the solution. The vial
was then transferred to an autoclave and heated for 6 h at 100 °C. Consequently, TiCN
was created as a thin black layer on the FTO glass. The extra black powder was removed
from the synthesized samples using deionized water prior to sintering in air. The
samples were then annealed at 550 °C for 2 h. and then heat-treated at 750 °C for
an additional 10 min as described above.
2.3 TiO2/FTO Layer by Layer (T LBL/FTO)
In the layer-by-layer self-assembly method (LBL), the cleaning process for FTO glass
was different. In this method, the FTO surface must be fresh and ready to accept ions
with different charges. Therefore, piranha solution (7:3 = 70 % conc. H2SO4:30 % H2O2) was used to treat the surface of the FTO glass. Each piece of FTO glass was etched
for 20 min by dipping into 0.2 M polyethyleneimine (PEI, Aldrich Co.) at room temperature
to create a positive charge on the FTO glass. Subsequently, ions with negative charges
were fabricated by immersing the FTO glass for 20 min in 10 gL-1 of aqueous hydrogen titanate (H-TiNT) (powder obtained by a hydrothermal technique
[32]) dispersed with 0.2 M tetra butyl ammonium hydroxide (TBAOH, Aldrich Co.). The same
method was used to obtain a layer of positively charged ions. The FTO-treated films
were immersed in 0.2 M poly diallyl dimethyl ammonium chloride (PDDA, Aldrich Co.).
To clean the FTO glasses of all surfactants such as PEI, TBAOH, and PDDA and to obtain
H-TiNT/FTO, UV-Vis light irradiation (Hg-Xe 200 W lamp, Super-cure, SAN-EI Electric)
was used. Finally, the TiO2/FTO (T LBL/FTO) thin film samples were heated inside a box furnace at a heating rate
of 500 °C h-1 and then annealed for 10 min.
2.4 Fe2O3/TiO2/FTO photoanodes prepared by layer-by-layer (LBL) and hydrothermal (HT) methods (F
HT/T LBL/FTO)
To prepare the Fe2O3/TiO2/FTO photoanode, the TiO2/FTO (LBL) sample from section 2.3 was placed into a vial (stainless steel shell and
Teflon liner) containing a 0.85 g of NaNO3 and 0.4 g of FeCl3.6H2O at pH 1.5 (adjusted with HCl) solution. It was then placed in an autoclave and heated
to 100 °C for 6 h. The sample was washed with deionized water after cooling to room
temperature and then dried at 80 °C. To obtain the α-Fe2O3 phase, β-FeOOH samples were annealed at 550 °C for 2 h. The samples were annealed
at 750 °C for an additional 10 min to facilitate the formation of the target phase.
3. RESULTS AND DISCUSSION
Manipulating particle size is a general strategy to improve the quality of a photocatalyst.
Reducing the particle size of a photocatalyst enhances the density of the catalytic
site surface and the photocatalytic activity due to the shortened diffusion length
of photogenerated electron-hole pairs [33]. The surface morphology and particle sizes of F HT/FTO, T LBL/FTO, T-F HT/FTO, and
F HT/ T LBL/FTO sintered at different temperatures (500, 550, and 750 °C) are presented
in the FESEM images in Figure 1 that illustrate different nanostructures. Figures 1 (a) and (b) present the uniform distribution of the nanorods of F-HT nanostructures
on the FTO substrates. The growth of TiO2 by the LBL method on the FTO glass substrates exhibited short nanorod-like structures
as indicated in Figures 1 (c) and (d). The T-F HT/ FTO sample presented in Figures 1 (e) and (f) exhibits short urchin-like structures. Figure 1 (g, h) presents sample F HT/ T LBL/ FTO that was synthesized by both LBL and HT methods
in sequence and exhibits a short urchin-like structure. Therefore, samples exhibiting
urchin-like nanostructures possess smaller feature sizes that may increase the aspect
ratio of the effective interface of hematite and could be suitable for photoelectrochemical
splitting. Furthermore, the smaller feature size causes the photogeneration of holes
that are closer to the semiconductor liquid junction (SCLJ) [34]. Figure 2 indicates the aspect ratios of two samples with urchin-like structures.
X-ray diffraction (XRD, Rint-2000, Rigaku) was performed with Cu Kα radiation (λ=1.54056
Å) at 40 kV and 100 mA at a scanning speed of 2 ° min-1. Figure 3 presents the XRD data for samples (a) F HT/ T LBL, (b) T-F HT, and (c) F HT and for
(d) bare FTO glass. T-F HT sample (b) exhibits enhanced crystallinity of hematite
compared to that of sample (c) as observed from the increased intensity of the (110)
and (104) peaks (the intensity of the [110] peak in T-F HT is stronger than is that
of F HT). A general comparison of the intensity of the (110) XRD peak revealed that
the sample F HT/ T LBL that was sintered at 550 °C for 2 h. with an additional heat
treatment for 10 min at 750 °C yielded the best crystallinity. As described, the use
of both the HT and LBL methods reduces the feature size, and this ultimately leads
to improved crystallinity.
The measured photoelectrochemical performance of a 1 M aqueous solution of NaOH electrolyte
is presented in Figure 4. The counter and reference electrodes were platinum (Pt) wire and saturated calomel
electrode (SCE) Hg/Hg2Cl2, respectively, as illustrated in Figure 4 (1). Different substratebased photoelectrochemical cells that were sintered at 500,
550, and 750 °C were used as working electrodes in the dark and under light illumination
(100 mW cm-2 UV-Vis). The range of linear sweep voltammetry was between 0.0~+1.8 (V versus RHE).
The photocurrent at 1.23 VRHE was increased for samples (c), (b), and (a) as illustrated in Figure 4 (3), and the highest value of 2.04 mA cm-2 was observed for sample (a). Sample (c) T LBL at 1.23 V vs RHE exhibited a low photocurrent
density of 1.02 mA/cm2 as presented in Figure 4 (3). The photocurrent density of F-HT (sample [b]) at 1.23 V vs. RHE was observed
at 1.84 mA/cm2.
The addition of TiCN as a precursor to the HT method resulted in the formation of
new hematite nanostructures possessing an urchin-like morphology. In this structure,
the electron-hole recombination is reduced and a high photocurrent density at 1.23
V vs. RHE is obtained. The achieved current densities for samples (a)-(c) are compared
in Figure 4 (4). The photocurrent density of sample F HT/ T LBL at 1.23 V vs. RHE was remarkably
increased as shown in Figure 4 (2) due to the use of a TiO2 underlayer. TiO2 acts as a barrier layer and prevents electrons from recombining in F HT/ T LBL photoanodes
[35]. Thus, the TiO2 interlayer plays a significant role in the efficient collection and conversion of
photon energy [36].
A schematic of the process for charge transfer and separation in sample F HT/ T LBL
/FTO is presented in Figure 5. The photogenerated electrons of α-Fe2O3 under visible irradiation can be excited from the valence band (VB) to different
energy levels of the conduction band (CB) (including the high-energy and low-energy
regions). At a low energy level, the photogenerated electrons quickly relax to the
VB bottom of α-Fe2O3 to recombine with the holes. However, high-energy electrons are thermodynamically
transferred to the CB of TiO2, thus leading to the promoted separation of visible-excited charges [35-38].
The optical band gap energy presented in Figure 6 reveals the effect of the LBL coating and doping layer arrangement on all four samples.
Using both the HT and LBL methods and according to the highest obtained photocurrent
density in Figure 4, the bandgap energy is decreased in sample F HT/ T LBL / FTO. Typically, electrons
that gain energy that is equal to or greater than the bandgap energy in the valence
state are excited to the conduction band. In this study, TiO2 nanocrystals were the center for recombining electrons and holes. By decreasing the
bandgap energy and increasing the photocurrent density, the smaller urchin-like structure
of the F HT/ T LBL / FTO sample acted as a photoanode and enhanced PEC water splitting.
4. CONCLUSION
This research study presents an enhanced water-splitting efficiency technique using
a novel photoanode designed by utilizing LBL and HT methods. The morphology, particle
size, and surface area all play important roles in the improvement of water-splitting
performance, and thus, due to these factors and the effect of TiO2 as a compact layer between FTO and the α-Fe2O3 photoanode, the properties and water-splitting process are enhanced. A newly designed
Fe2O3/TiO2/FTO (F HT/ T LBL /FTO) photoanode (sintered at 550 °C for 2 h. with an additional
heat treatment for 10 min at 750 °C) demonstrated a current density of 7.68 mA/cm2. By applying both LBL and HT methods simultaneously, an innovative method is proposed
that leads to the formation of urchin-like structures possessing smaller feature particle
sizes. Eventually, decreasing the bandgap energy reduces the electron-hole recombination,
and this leads to a much higher photoelectrochemical water-splitting efficiency.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded
by the Ministry of Science, ICT, and Future planning [NRF-2018K1A4A3A01064272] and
[NRF-2020R1A6A1A03038540].
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Figures
Fig. 1.
SEM micrographs at 10,000× and 50,000× magnification of 4 different photoanodes: (a,
b) F HT/FTO sintered at 550 °C for 2 h with additional heat treatment at 750 °C for
10 min, (c, d) T LBL/FTO sintered at 500 °C for 10 min, (e, f) T-F HT/FTO sintered
at 550 °C for 2 h. with additional heat treatment at 750 °C for 10 min, and (g, h)
F HT/ T LBL/ FTO sintered at 550 °C for 2 h with additional heat treatment at 750
°C for 10 min.
Fig. 2.
Aspect ratio of samples (a) T-F HT sintered at 550 °C for 2 h with additional heat
treatment at 750 °C for 10 min and (b) F HT/ TLBL sintered at 550 °C for 2 h.
Fig. 3.
XRD spectra for (a) F HT/ T LBL, (b) T-F HT, (c) F HT, and (d) FTO glass.
Fig. 4.
(1) Schematic of photoelectrochemical performance of sample F HT/ T LBL as a working
electrode that was sintered at 550 °C for 2 h with additional heat treatment for 10
min at 750 °C in a 1 M aqueous solution of NaOH electrolyte. (2) I-V characteristics of F HT/ T LBL sintered at 550 °C for 2 h with additional heat treatment
for 10 min at 750 °C, (3) (a) T-F HT sintered at 550 °C 2 h with additional heat treatment
for 10 min at 750 °C, (b) F HT sintered at 550 °C for 2 h with additional heat treatment
for 10 min at 750 °C, and (c) T LBL sintered at 500 °C for 10 min. For all the measurements,
the simulated solar illumination was 100 mW cm2, and 1.0 M NaOH was used as an aqueous
electrolyte. (4) Comparison of photocurrent densities for samples (a)-(c).
Fig. 5.
Schematic of the mechanism for enhancing the photocatalytic properties of F HT/ T
LBL/ FTO.
Fig. 6.
Optical absorbances of the (a) F HT/ T LBL sintered at 550 °C for 2 h with additional
heat treatment for 10 min at 750 °C, (b) T-F HT sintered at 550 °C 2 h with additional
heat treatment for 10 min at 750 °C, (c) F HT sintered at 550 °C for 2 h with additional
heat treatment for 10 min at 750 °C, and (d) T LBL sintered at 500 °C for 10 min.