Chapter 3 Experiment Methods
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
level of the electrode.[72] This ET rates are given in terms of the density of
available states, the electron-resonance width and the strength of the coupling to the
phonon bath.[73,74] The activation energy for the reaction decreases with increasing
electronic interaction width. The structure of the NTs, as well as their local density
of states, might be responsible for the increase of the electronic-energy interaction
width. In our case, arrayed CNx NT microelectrode prepared with N2 flow rate of 40
sccm has higher local-density of states (including surface defects induced states
from XPS results), and promotes the enhanced ET kinetics.
The unique structures thus created in the in-situ N doping process can be verified
by selective EC deposition technique, which is a direct and quantitative measure of
deposition of metal at the chemical reactive sites such as the defect sites on CNx
NTs. Figures 6 shows the comparative TEM images of the CNx NTs prepared with
various N2 flow rate during growth and followed by 0.5M H2SO4 & 0.0025M
H2PtCl6 mixture solutions. In Figure 4.6(a) most of the Pt particles aggregate on the
top of the electrode to form large agglomerates, indicating lack of nucleation sites on
the side surface of CNx NT prepared with no N2 flow during growth. Figure 4.6(b)
shows more uniform and higher density of Pt NPs nucleating on the side surface of
CNxNTs prepared with 40 sccm N2 flow rate during growth. However, excessive N2
flow rate may cause adverse effect. Figure 4.6(c) shows Pt agglomerates of tens of
nanometers scattered throughout the CNx NTs, which can be attributed to low
nucleation density at this growth condition (120 sccm N2 flow rate). Figure 4.6(b)
shows clear indication of the nearly monodisperse Pt NPs of 2-5 nm on the side
surface of CNx NTs having optimal defect density created under the 40 sccm N2
flow rate during growth. Furthermore, a typical CV curve measured in 1 M
CH3OH/1 M H2SO4 solution for the arrayed Pt NP-CNx NTs hybrid nanocomposites
was shown in Figure 4.7. Remarkably, a significantly enhanced electrocatalytic
activity is yielded for 40 sccm nominal N2 flow rate mainly due to keep higher
accessible surface area of discrete Pt NPs finely dispersed on CNx NT. In sharp
the pretty good electrocatalytic activity can be obtained from Pt NP/CNx NT
nanocomposites with the N2 flow rate of 40 sccm dopant, indicating that this arrayed
cable-like nanocomposites can be the most promising candidate for useful catalytic
applications in the μFCDs.
4.4 Summary
Vertically aligned CNx NTs have been synthesized using microwave plasma
enhanced chemical vapor deposition with different nitrogen flow rate. The
microstructure, bonding and electrochemistry properties have been investigated
utilizing Raman, XPS, CV and AC impedance measurements to establish
correlations among the N content, electronic structure, microstructure and EC
activities. The results show that nitrogen incorporation provides a simple pathway to
engineering/modifying the microstructure and electronic bonding structure, which is
crucial to the ET kinetics of the CNx NT electrode. In-situ N doping of the CNTs at
an optimal 3.5 at. % N is effective in converting the nanotubes into bamboo-type
morphology and promoting substitutional graphite-like defect structure, which is
favorable for fast ET in electrochemistry. The surface defects thus created further
enhance anchoring of Pt atom in the subsequent selective EC deposition to produce
ultrahigh density Pt NPs to form nanocomposite, which is ideal for many
highly desirable in many EC based applications.
Figure 4.1 Cross-sectional SEM images of the vertically aligned CNTs synthesized
at different flow rate of N2: (a) 0, (b) 80, and (c) 120 sccm.
( ( a a ) )
(c ( c) )
(b ( b) )
Figure 4.2 (a) Comparison of the peak intensities and the full width at half
maximum (FWHM) of the first-order Raman spectra for the vertically aligned CNx
NTs prepared with different N2 flow rate during growth. (b) D-band position as a
function of N2 flow rate. (c) ID/IG as a function of N2 flow rate.
Figure 4.3 (a) The C1s XPS spectra of the vertically aligned CNTs prepared with
various N2 flow rate during growth. The inset is the FWHM of the C-N bonding
component as a function of N2 flow rate. (b) The N1s XPS spectra of the vertically
aligned CNTs prepared with different N2 flow rate. (c) The IP and IG as a function of
the N2 flow rate for the N1s peak. (d) The N-doping concentration as a function of
Figure 4.4 (a) Cyclic voltammetry of the vertically well-aligned CNTs modified with
different N-doping level in 1 M KCl and 5 mM K4Fe(CN)6. (b) The ferricyanide
peak current versus the scan rate (v)1/2 plot for CNTs using various flow rate of N2
for both anode and cathode. (★: 0 sccm, ■: 40 sccm, ●: 80 sccm, ◆: 120 sccm, ▲:
Figure 4.5 AC impedance analysis of the vertically aligned CNTs with different
N-doping level in 1 M KCl and 5 mM K4Fe(CN)6. The inset shows the internal
resistance as a function of the N2 flow rate.
Figure 4.6 TEM images of the Pt NP-CNx NT hybrid nanostructures synthesized at
different flow rate of N2: a) 0, b) 40, and c) 120 sccm.
( ( a a ) ) ( ( b b ) )
( ( c c ) )
10 nm 5 nm
5 nm
0.0 0.2 0.4 0.6 0.8 1.0 0
10 20 30
Cur ren t D e ns ity ( mA /c m
2)
Potential(V vs. Ag/AgCl)
0 sccm 40 sccm 120 sccm
Figure 4.7 Typical CV curve of the arryed Pt NP-CNx NT nanocomposites with
different N2 flow rate at a scan rate of 50 mV/s in 1 M CH3OH + 1 M H2SO4
solution.
Chapter 5
Nanostructured ZnO Nanorod@Cu Nanoparticle as Catalyst for Microreformers
5.1 Introduction
The use of hydrogen for energy generation has attracted significant attention in the
recent years as a clean, sustainable and transportable fuel alternative, and has
consequently sparked a rapid global development of hydrogen fuel cells for electric
power generation.[1] Catalytic reformation of hydrocarbons, with careful attention
to avoid storage and safety issues,[75,76] is currently the predominant process for
hydrogen generation. One of the leading and most promising techniques for
hydrogen generation is catalytic reformation of methanol.[77,78] Cu/ZnO-based
catalysts are, therefore, of great importance for industrial scale catalytic production
of reformate hydrogen.[13] Owing to their wide commercial relevance,
Cu/ZnO-based catalysts, prepared via several preparation routes, are being
extensively investigated; and substantial improvements in their efficiency of
catalytic activity brought about by addition of suitable promoter/support,
hetero-nanostructures as reforming catalysts is still lacking to date. This inspired us
to design the core-shell nanostructured catalyst, consisting of a ZnO nanorod (NR)
core and an outer shell of Cu nanoparticles (NPs), i.e. NR@NPs, for achieving high
efficiency of catalytic conversion.
In addition, the idea of using microreformers is highly attractive for several
applications such as on-board hydrogen sources for small vehicles and portable fuel
cells (FCs).[79,80] However, two key issues have hindered the realization of
microreformers for catalysis, viz. poor adhesion between the catalyst layer and the
microchannels; and poor utilization of catalyst layer deposited in the form of thick
film.[81] Notwithstanding the several approaches investigated to overcome these
issues, catalyst immobilization and its efficient utilization inside the microchannel
remains a challenge.[82,83] Most of these approaches involve a two-step process,
wherein active catalysts are prepared in the first step, followed by its immobilization
on the surface of the microchannels in the second step. In this communication, we
report a simple and reliable method for integrating in-situ synthesis of catalyst and
its immobilization for microreformer applications.
5.2 Structural Characterization of Cu Nanoparticle/ZnO Nanorod
Nanocomposites
ZnO NR arrays at low temperature subsequently resulted in spontaneous formation
of the cable-like nanostructures. Since the ZnO NR@Cu NP nanocomposites were
in-situ synthesized directly on microreactor, the arrayed ZnO@Cu nanocomposites
were strongly anchored onto the microchannel. This was evidenced by observation
of no material loss after sonication in the water for several hours, which highlighted
the strong mechanical anchorage of nanostructured catalysts on the surface of
microchannel. The interaction between Cu NP and ZnO NR was studied by several
analytical techniques, including electron microscopy (EM), X-ray diffraction (XRD),
X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and
temperature-programmed reduction (TPR). The structure of the microreformer
design based on the ZnO NR@Cu NP nanocomposite is illustrated in Figure 5.1,
which is evidenced by the photographs comparing the microchannels before and
after the deposition of the ZnO NR@Cu NP nanocomposite (see Figure 5.2).
One of the most significant advantages of the core-shell nanocomposites, which
are clearly distinct from the traditional catalysts, is the large surface area they offer
for effective surface contact between the reactants and catalysts. Figure 5.3a
illustrates the cross-sectional SEM image of vertically aligned ZnO NRs grown on
the inner surface of the microchannel. The size of the NRs range from 35 to 50 nm