Chapter 2 Literature Review
2.3 Photoelectrochemical water splitting
2.3.1 PEC hydrogen generation based on nanomaterial photoelectrodes
The use of nanomaterial-based PEC for water splitting dates back to 1997.
Fitzmaurice and coworkers reported the studies of charge separation in a
nanostructured TiO2 membrane sensitized with Ru complexes.[44] The long-lived
charge separation observed was an important finding that suggests that water
splitting is practically feasible on nanostructured TiO2 material. In the following few
years, Khan and Akikusa reported the photoresponse of nanocrystalline n-TiO2,
photoanodes are stable for water splitting. Significantly, the nanocrystalline n-Fe2O3
films showed higher photoresponse compared to those prepared by compression of
n-Fe2O3 powder or by thermal oxidation of metallic Fe sheets, indicating that
high-quality nanostructured materials could improve the overall efficiency of the
water splitting reaction. Since then, the PEC performance of different
nanocrystalline metal oxide films has been studied.[26]
Besides 0D nanostructures, 1D nanostructures, such as nanotubes, nanowires, and
nanorods, are expected to exhibit much improved transport properties than
nanoparticles. The first demonstration of PEC water splitting using 1D nanostructure
as photoelectrodes was reported by Lindquist and coworkers in 2000.[47,48] They
reported the photoelectrochemistry of hematite nanorod arrays for the photoanode. It
is generally accepted that recombination of electrons and holes, trapping of electrons
by oxygen deficiency sites, and low mobility of the holes cause the low
photoresponse for hematite films. In comparison to nanoparticles, nanorods improve
the transportation of carriers and thus reduce the recombination losses at grain
boundaries, as illustrated in Figure 2.11a. Moreover, the small diameter of the
nanorods minimizes the distance for holes to diffuse to the SC/electrolyte interface,
as shown in Figure 2.11b. Ideally, when the nanorod radius is smaller than the hole
well beyond the efficiencies reported for polycrystalline and nanocrystalline films of
hematite. This work demonstrated that nanorod arrays could potentially address
some of the fundamental PEC issues and increase the photon-to-current yield of
hematite.
Figure 2.1 Schematic operating principles of DMFC.[27]
Figure 2.2 Breakdown of anode, cathode, and electrolyte-related performance
losses in a DMFC.[29]
Figure 2.3 Schematic illustration of the synthesis procedure of the composite.[30]
Figure 2.4 TEM images of Pt/CNT (inset: enlarged image).[30]
Figure 2.5 TEM images of 20 wt% PtRu/C nanocatalyst prepared with different
molar ratios of Pt:Ru (a) LRTEM of 1:1, (b) HRTEM of 1:1.[31]
Figure 2.6 Typical TEM image of the CNT/PyPBI/Pt. Pt nanoparticles are loaded
Table 2.1 Reactor temperature ranges for initial processing of different fuels and
CO content of different process streams after the initial reaction.[11]
Figure 2.7 Schematic diagram of the integrated methanol steam reformer
system.[11,38]
Figure 2.8 Schematic of a typical PEC device and its basic operation mechanism
for hydrogen generation from water splitting.[40]
Figure 2.9 Energy diagram of a PEC cell consisting of an n-type SC photoanode
and a metal cathode for water splitting.[40]
Figure 2.10 Type of photoelectrode for PEC water splitting (SC-semiconductor;
M-metal).[21]
Figure 2.11 (a) Schematic representation of the electron transport through
spherical particles and nanorods. (b) A schematic of a Fe2O3 photoanode for water
splitting. The small dismeter of the nanowires ensures a short hole diffusion
length.[40,48]
1D & 3D nanoarchitectures
Carbon nanotube ZnO nanorod ZnO-CuO
inverse opal
Electrochemicalmeasurement
1D & 3D nanoarchitectures
Carbon nanotube ZnO nanorod ZnO-CuO
inverse opal
Electrochemicalmeasurement
Chapter 3
Experiment Methods
3.1 Flowchart of experiment process
3.2 Preparation of nanomaterials
3.2.1 Synthesis of CN
xNTs and Pt NPs deposition
For the synthesis of the CNx NTs, an iron catalyst layer was deposited on Ti/Si
substrates by ion beam sputtering prior to the NT growth step. Then CNx NTs were
grown on the precoated substrates by MPECVD method (Figure 3.1), which has
been reported in our previous paper.[7] For Pt NPs deposition, a potential of -0.1 V
vs Ag/AgCl in 0.5M H2SO4 & 0.0025M H2PtCl6 mixture solutions was performed
for 15 sec as shown in Figure 3.2. Other details for the electrochemical deposition
process are reported by Quinn and co-workers.[65]
3.2.2 Fabrication of ZnO NRs and metallic Cu NPs deposition
For ZnO array preparation, a thin film of ZnO was first deposited on the inner
surface of microchannels prior to the NR growth step by solution method, which
acted as a seed layer. This was followed by growth of aligned ZnO NRs on the
precoated substrates by chemical bath deposition (CBD) method. The CBD growth
was performed using equimolecular mixtures of zinc nitrate hexahydrate (99.5 %,
Aldrich) and hexamethylenetetramine (99 %, Aldrich) as source precursors and a
reaction temperature of 90 oC for 7 h (Figure 3.3). Other details of the NR array
growth process were same as those described in the procedure proposed by
precursor salts (copper nitrate trihydrate, 99.5 %, Aldrich) that were subsequently
reduced (Figure 3.4). This reducing step is the boiling under reflux, which leads to
Cu/Cu+ colloids. The synthesis of size-selected Cu NPs immobilized on ZnO NRs
was carried out in ethylene glycol (EG; as reducing agent) solutions containing
different amounts of sodium hydroxide (0.5 M NaOH in EG) with pH value between
9 and 11.[85] The decoration concentrations were controlled between 0.5 and 3 mM.
The solutions were stirred for 30 min at room temperature, subsequently heated
under reflux to 190 ℃ for 2 h, and then cooled in air. Dark brown solutions
containing Cu NPs were formed in this manner, referred to as colloidal solutions in
this work.
3.2.3 Synthesis of microwave-activated CuO NT/ZnO NR nanocomposites
For synthesis of ZnO nanorods inside the microchannels, chemical bath deposition
growth was performed using the equimolecular mixtures of zinc nitrate hexahydrate
(99.5 %, Aldrich) and hexamethylenetetramine (99 %, Aldrich) as source precursors,
at a reaction temperature of 90 oC for 7 h. Other details for the NR array growth
process were similar to the procedure proposed by Vayssieres.[84] Typically, the
one-step direct impregnation method involved impregnation of ZnO
mixing Cu(NO3)2‧3H2O (99.5 %, Aldrich) in deionized water (18.2 MΩ).
Cu(NH3)42+ complex cations were prepared by adding a concentrated ammonia
solution (28-30 wt %) dropwise into the above aqueous solution until the pH value
reached 11 under vigorous stirring. The resulting homogeneous solutions were then
gently heated in an oven at 90 oC for over 2 h. Finally, CuO nanotip/ZnO nanorod
nanoarchitectures on the inner surface of microchannels were formed.
The as-derived CuO nanotip/ZnO nanorod catalyst precursors were processed in a
microwave oven (2.45 GHz; TMO-17MA; TATUNG CO., Ltd.; Taiwan) for 10 min
to obtain the final microwave-irradiated CuO nanotip/ZnO nanorod catalyst
precursors. The microwave chamber was 452 mm in length, 262 mm in width, and
325 mm in height. During the microwave irradiation, the substrate temperature,
environmental atmosphere and microwave power were maintained at room
temperature, air and 600 W, respectively. For comparison, the as-derived CuO
nanotip/ZnO nanorod catalyst precursors were also treated in a conventional
annealing furnace at 450 oC for 1 h to obtain the conventional thermal-treated CuO
nanotip/ZnO nanorod catalyst precursors
3.2.4 Preparation of O
2plasma-activated CuO-ZnO inverse opals
For synthesis of CuO-ZnO inverse opals, 10 wt % of polystyrene (PS) colloidalwell-defined structures were formed inside the microchannels. These PS
particle-packed microchannels were heated at 90 ℃ for 3-6 h in order to improve
the connectivity between the neighboring particles. The copper-zinc precursor
solution containing 0.5 M Cu(NO3)2‧3H2O (99.5%, Aldrich) and 0.5 M Zn(NO3)2‧
6H2O (98%, Aldrich) in ethanol was then applied dropwise over the surface of the
PS layer. These infiltrated samples were then placed in air at room temperature for
2-4 h. Finally, the resulting CuZnO/PS composites were heated in air to 300 ℃ to
obtain the CuO-ZnO inverse opals (Figure 3.6).
These pristine CuO-ZnO inverse opals were then exposed to O2 plasma for a short
duration of 3-15 min to obtain O2-plasma treated CuO-ZnO inverse opals. The
process of plasma-chemical surface modification was performed in a parallel-plate
reactor with a DC flowing discharge (PCD-150; ALL REAL TECHNOLOGY CO.,
Ltd.; Taiwan). The distance between ground electrode and powered electrode was
about 6 cm. The plasma chamber was 250 mm in diameter and 140 mm in height.
The electrode was in the form of disk with around 6 inch in size. For the O2 plasma
treatment, the substrate temperature, total gas pressure and DC power were
maintained at room temperature, 200 mTorr and 50 W, respectively.
3.2.5 Preparation of carbon-modified ZnO inverse opals
After room temperature evaporation of water from the suspension, PS opals with
well-defined structures were formed on the ITO substrate. These PS particle-packed
templates were heated at 90 ℃ for 3-6 h in order to improve the connectivity
between the neighboring particles. The zinc precursor solution containing 0.5 M
Zn(NO3)2‧6H2O (98%, Aldrich) in ethanol was then applied dropwise over the
surface of the PS layer. These infiltrated samples were then placed in air at room
temperature for 2-4 h. Finally, the resulting ZnO/PS composites were heated in air to
300 ℃ to obtain the carbon-modified ZnO inverse opals. For comparison, the ZnO
noninverse opals could be prepared by the same fabrication procedure only without
using PS particle-packed templates.
3.3 Experimental measurements and characteristics analysis
For methanol reforming reaction, the Al-alloy (6061) chip of the microreactor was
made by ourselves through a laser machining method. Ten microchannels per
Al-alloy chip were separated by 800 μm fins. The width, depth and length of the
microchannels were 500μm, 200μm and 4.3 cm, respectively. For the calculation of
the catalyst weight for the steam reforming reactions, we measure the total weight
by precision electronic balance (± 0.1 mg; Sartorius). After reduction of catalysts in
a H2/N2 (5/95) at a flow rate of 50 mL min-1 at 200 ℃ for 1 hr, premixed water,
gas streams were analyzed online with a thermal-conductivity-detector gas
chromatograph (TCD-GC; China Chromatography CO., LTD.) and CO detector
(Gastech CO., Ltd.; GTF200).
For photoelectrochemical measurement, a water-splitting photoelectrode was used
as the working electrode with surface area of 0.5-1 cm2, a platinum plate as counter
electrode, and Ag/AgCl as reference electrode. All PEC studies were operated in a
1M Na2SO4 (pH7.0) solution as supporting electrolyte medium by using
Electrochemical Multichannel Solartron Analytical System. The water-splitting
photoelectrode was illuminated with a xenon lamp equipped with filters to simulate
the AM1.5 spectrum.
For material characterization, XRD analyses were performed on a Bruker D8
Advance diffractometer with Cu (40 kV, 40 mA) radiation. SEM measurements were
made on a JEOL 6700 filed-emission SEM. XPS spectra were obtained using a
Microlab 350 system. For obtaining TEM images, the products on the substrate were
scratched and dispersed on a carbon-coated Cu grid, and analyzed using a JEOL
JEM-2100 TEM system. Micro-Raman analyses were performed on a Jobin Yivon
Labram HR800 spectrometer. XAS analyses were performed on a beamline BL17C1
and BL20A1 at the National Synchrotron Radiation Research Center (NSRRC),
Cooling lines
Dummy Load
A-5000 Microwave power source
Mechanical Pump
Figure 3.1 Schematic diagram of microwave-plasma enhanced chemical-vapor
deposition (MPECVD) facility.
Figure 3.2 Schematic diagram of electrochemical deposition facility.
Zn2+
HMT Heating by oven
Arrayed
Arrayed ZnOZnONRsNRs
9Concentration
Arrayed ZnOZnONRsNRs
9Concentration
Cu NPs/ZnO ZnO NRs NRs
9pH Value
Cu NPs/ZnO ZnO NRs NRs
9pH Value
9Temp. Ramp Rate Figure 3.3 ZnO array preparation.
Figure 3.4 Cu NP/ZnO NR nanocomposites preparation.
Cu2+
Figure 3.5 Cu NT/ZnO NR nanocomposites preparation.
Figure 3.6 Procedure for the preparation of CuO-ZnO inverse opals using
polystyrene colloidal crystal templates.
Chapter 4
Effects of Nitrogen-Doping on the Microstructure, Bonding and Electrochemical Activity of Carbon Nanotubes
4.1 Introduction
Carbon nanoscience and nanotechnology have been developed very rapidly over
the past decade since the discovery of carbon nanotubes (CNTs). Miniaturization of
electronic and electrochemical (EC) devices using the single-walled CNTs has been
demonstrated.[49-51] Meanwhile, the remarkable structure of CNTs offers attractive
scaffolds for further anchoring of nanoparticles (NPs) and biomolecules, which is
highly desirable for energy conversion/storage and molecular sensing
applications.[6,52-62] For these applications, surface modification of the CNTs or
attaching functional groups on the sidewall become a key issue. In particular,
surface modification offers an opportunity to improve the EC reactivity of CNTs
through facilitating an efficient route for their electron-transfer (ET) kinetics with
ambient species or specific biomolecules. Therefore, understanding the ET behavior
between the CNT surface structures and the active entities are essential.
effectively functionalize CNTs surface.[63,64] Despite that many solutions to
modify CNTs are available, simple and reliable process to achieve such goal is still
lacking. In the past we have introduced heteroatom dopant such as nitrogen (N)
in-situ during the CNT growth and found it effective not only to change the atomic
structure of the CNTs into bamboo like,[7] but also to improve their electrochemical
(EC) performance down the road.[6] Although many reports on nitrogen doped
carbon nanotubes (CNx NTs) are available in the literature,[7-10] the role of
N-doping in carbon nanotube and its resultant functionality is still not clearly
understood.
In this paper, systematic studies on the effect of N incorporation in CNTs on the
morphology, microstructures, electronic states, and electrochemical properties have
been carried out. CNx NTs with different nitrogen content have been produced using
a simple in-situ nitrogen doping in a microwave plasma enhanced chemical vapor
deposition (MPECVD) reactor.[7] Comparative studies to correlate the nitrogen
content, microstructure, electronic structure, and EC performance of the CNx NTs
have been carried out. Subsequent loading of Pt NPs on the CNx NTs to study the
correlation of N dpoing with surface defect density and distribution has been carried
out.
Figure 4.1 shows the cross-sectional scanning electron microscopy (SEM) images
of the vertically aligned CNTs synthesized using different flow rate of N2 gas but
otherwise identical growth parameters. The diameters of the CNTs thus produced are
in the range of 20-50 nm with an approximate length of 2-4 μm. Bamboo-like
structure of the CNx NTs is observed, as reported in our previous paper.[7] However,
as the N2 gas flow is higher than 80 sccm the average diameter of the CNTs goes
beyond 100 nm with a reduced length of 1-2 μm. This may be attributed to the
higher growth temperature at higher N2 flow rate, which is deviated from the
nominal growth condition.
The Raman spectra of the CNx NTs prepared under different nitrogen flow rate are
presented in Figure 4.2. The 1350 cm-1 peak (D band) in Figure 4.2(a) corresponds
to the disorder-induced feature due to the finite particle size effect or lattice
distortion, while the 1580 cm-1 peak (G band) corresponds to the in-plane stretching
vibration mode E2g of single crystal graphite.[66] A D′ band around 1620 cm-1 at the
shoulder of the G band is attributed to the symmetry breaking duo to the
microscopic sp2 crystallite size.[67] The D-band position and the ratio of the D- and
G-band integrated intensities as a function of N2 flow rate are depicted in Figure
4.2(b) and 4.2(c), respectively. As the N2 flow rate increases from 0 to 40 sccm the
dependence on the N2 flow rate. Interestingly, the intensity ratio I(D)/I(G) increases
strongly as the N2 flow rate is increased to 40 sccm and also decreases above the 40
sccm optimal flow rate. In principle, the I(D) increases rapidly as a result of the
enhanced defect density. Therefore, the up-shift of the D band and the increase in the
ratio of the integrated intensities can be attributed to the increase of defect density in
the graphitic structure and/or the enhancement of the edge plane by N-induced
deformation in CNx NTs. However, further increase of N2 flow rate over 40 sccm
leads to a higher growth temperature and results in enhanced graphitization degree.
The growth temperature determined by pyroelectric thermal detector is 420, 450,
and 750 oC at the N2 flow rate of 0, 40, 160 sccm, respectively.
In order to obtain better understanding on the electronic structures of the CNx NTs,
X-ray photoemission spectroscopy (XPS) is applied to samples grown using
different N2 flow rates. Figure 4.3(a) shows the C 1s spectra of the CNx NTs after
background subtraction using Shirley’s method.[68] The C 1s spectra thus obtained
can be decomposed into three Gaussian peaks including C1 peak at 284.4±0.1 eV
representing the delocalized sp2-hybridized carbon or graphite-like C-C bonding,[69]
C2 peak at 285.1±0.1 eV reflecting defect-containing sp2-hybridized carbon
associated with the trigonal phase with a sp2 bonding,[69,70] and the C3 peak at
peak position shows a slight up-shift from 0 to 40 sccm optimal N2 flow rate
followed by a down-shift at higher flow rate. This result is in agreement with the
variation in the degree of structural disorder, which is associated with the electron
density of the defect-containing sp2-hybridized carbon and reflected in the FWHM
of the C2 peak as illustrated in the inset of Figure 4.3(a). The C2 FWHM reaches a
maximum value at 40 sccm manifesting that the most pronounced disruption in the
sp2 carbon framework due to the incorporation of N atoms into the graphene lattice
occurs at a N2 flow rate of 40 sccm.
Likewise, the N 1s XPS spectra of the CNx NTs with increasing N2 flow rate are
fitted by two Gaussian lines as shown in Figure 4.3(b). The peaks at 398.1±0.2 eV
(denoted by IP) and 400.8±0.2 eV (IG) are assigned to tetrahedral nitrogen bonded to
a sp3-hybridized carbon (so-called substitutional pyridine-like dopant structure) and
trigonal nitrogen bonded to a sp2-coordinated carbon (so-called substitutional
graphite-like dopant structure), respectively.[69] Figure 4.3(c) displays the fraction
of pyridine-like and graphite-like defects as a function of the N2 flow rate.
Increasing N2 flow rate is shown to give rise to an increase of the peak intensity
ratios of the N-sp3 C bonding (IP), which is strongly related to graphene sheets.
However, increasing N2 flow rate, keeping the rest of the process parameters the
graphene planes through the sp3-coordinated carbons. Meanwhile, the peak intensity
ratios of the N-sp2 C bonding decreases slightly over 40 sccm nominal N2 flow rate.
In general, N-doping is shown to lower the energy of pentagon defects in the
graphite-like dopant structures. Thus, through the N-sp2 C bonding, N atoms are
easily incorporated in the graphene sheets. The total N-doping levels for the peak
ratio of N/(N + C) are plotted against the respective N2 flow rate in Figure 4.3(d).
The CNx NTs with N2 flow rate of 40 sccm contain maximum N atomic ratio.
4.3 Nitrogen-Doping Effect on Electrochemical Activity
The ET behavior of CNx NTs are explored using a potassium ferrocyanide redox
probe (5 mM K4Fe(CN)6 in 1M KCl). A typical CV curve of CNx NTs
microelectrode in this redox couple system is shown in Figure 4.4(a). The
well-defined peaks obtained in the forward and reverse scans are due to the
Fe3+/Fe2+ redox couple. The reversible redox reaction of the CNx NT
microelectrodes is further evidenced by the linear Ipa and Ipc vs υ1/2 plots shown in
Figure 4.4(b), where Ipa、Ipc and υ are the corresponding peak current densities of the
cathodic and anodic reactions and the scan rate, respectively. These data indicate
that the whole reactions are limited by semi-infinite linear diffusion of the reactants
to the electrode surface. Moreover, the effective surface area of CNx NT arrays can
CNx NTs surface to the geometrical electrode surface area, as a function of N2 flow
rate is depicted in the inset of Figure 4.4(b). It is noted that the roughness factor of
CNx NT microelectrode prepared with N2 flow rate of 40 sccm is ~33, showing
significant enhancement in the effective surface area. Clearly, the N-doping induced
electrochemically active sites on the surface of CNx NT microelectrodes prepared
with N2 flow rate of 40 sccm are optimized.
The peak-to-peak separation ( Ep△ ) of potassium ferrocyanide redox probe is
strongly dependent on the ET rate, namely, the reactivity of electrode materials to
the electrolyte. In Figure 4.4(c), Ep△ of CNx NTs with N2 flow rate of 40 sccm is
around 59 mV, reflecting excellent ET reactions. This is also related to the reduced
internal resistance of the CNx NT structures, which was determined by EC
impedance (EIS) in 1M KCl solution containing 5 mM K4Fe(CN)6 at an AC
frequency varying from 0.1 to 100 kHz as shown in Figure 4.5. From the point
intersecting with the real axis in the range of high frequency, the internal resistance
of the electrode is obtained. As shown in the inset of Figure 4.5, the arrayed CNx
NTs microelectrode prepared with N2 flow rate of 40 sccm shows the lowest
resistance of all, which is in good agreement with the above mentioned
measurements. Moreover, the Nyquist complex plane plot of the CNx NT
capability with selective N dopant.
The enhanced ET kinetics observed at CNx NTs surface may in part be attributed
to higher electronegativity of the CNx NTs surface. The attractive interaction
between the C-N dipoles present at the surface may attract the negatively charged
members of the Fe(CN)63-/4- and accelerates the redox reactions. In fact, the rate
constant for ET from the reactant to the electrode can be expressed as
kox = ∫ dє (1 - f(є,T))wox(є),
where wox is the rate of ET from an occupied level of the reactant to an empty
where wox is the rate of ET from an occupied level of the reactant to an empty