Rhodium nanoparticles impregnated on TiO2: Strong morphological effects on hydrogen production

Article abstract of DOI:10.1039/d0nj02419h

The effect of the shape of rhodium nanoparticles impregnated on TiO2 on photocatalytic hydrogen generation using methanol as a sacrificial reagent has been investigated by using a simple home-made UV-LED light strip reactor to activate the catalyst surface. For rhodium nanocubes, the normalised rate of H2 production was 22.4 mmol h-1 m-2 at 365 nm and 7.5 mmol h-1 m-2 at 400 nm. The normalised activity/surface area of the studied Rh nanoparticles showed a trend that led to establishing a relationship between the exposed [100] facets and photocatalytic activity. The Rh shape-dependent H2 production follows a trend of cubic > spherical > octahedral, which is associated with a higher surface energy of the Rh nanoparticles. This journal is

Full text of DOI:10.1039/d0nj02419h

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March 2018
Pages 3963-4776
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Rhodium nanoparticles impregnated on TiO
2
: Strong morphologi
D
c
O
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I:
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D0NJ02419H
t
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on hydrogen production
a
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Brunno L. Albuquerque,* Gustavo Chacón, Michael Nazarkovsky and Jairton Dupont
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a. LAMOCA – Laboratory of Molecular Catalysis, Institute of Chemistry – Universidade
Federal do Rio Grande do Sul. Av. Bento Gonçalves, 9500 Porto Alegre 91501-970 RS
(Brazil).
b. Pontifícia Universidade Católica de Rio de Janeiro, Rua Marquês de São Vicente, 225,
Rio de Janeiro 22451-900 RJ (Brazil).
(*) Corresponding author: E-mail address: brunno.albuquerque@ufrgs.br
Abstract
The effect of the shape of rhodium nanoparticles impregnated on TiO on photocatalytic
2
hydrogen generation using methanol as sacrificial reagent has been investigated by using a simple
home-made UV-LED light strips reactor to activate the catalyst surface. For rhodium nanocubes, the
-
1
-2
-1
-2
normalised rate of H production was 22.4 mmol h m at 365 nm and 7.5 mmol h m at 400 nm.
2
Normalised activity/surface area of the studied Rh nanoparticles showed a trend that led to establishing
a relationship between the exposed [100] facets and photocatalytic activity. The Rh shape-dependent
H production follows a trend of cubic > spherical > octahedral, which is associated with a higher
2
surface energy of the Rh nanoparticles.
Introduction
The process of photocatalytic hydrogen production using TiO as a catalyst and Pt as co-catalyst
2
1
, 2
under UV irradiation was reported almost half a century ago. Since then, huge efforts have been
dedicated to the development of new materials capable of being activated with the visible region of the
3-5
6-8
solar spectrum for a diverse set of applications such as hydrogen production, CO reduction, and
2
9-11
decomposition of organic dyes, amongst others. The emission of the solar spectrum in the UV region
1

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DOI: 10.1039/D0NJ02419H
accounts for only about 3-4% of all the light that reaches the Earth’s surface. However, the advent of
newer technologies of light-emitting diodes (LED) allows for the use of common photovoltaic cells and
hence, UV absorbing semiconductors can be used without modification to harvest photons in the visible
1
3, 14
region indirectly.
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Although there have been several investigations regarding the composition of the co-catalyst in
1
5-17
H
2
production,
only a few specific studies about crystal morphology and nanoparticle shape vs
1
8-23
activity has been reported thus far.
With the recent development of shape-controlled synthesis of
2
4-27
28,
nanocrystals,
a new field for catalysis has been opened for a plethora of specific transformations.
2
9
The effect of octahedral and cubic Pd nanoparticles impregnated on TiO
2
has been investigated on
water photolysis using various alcohols as sacrificial agents and showed that exposed Pd [111] facets
30
increased the heterojunction between metal and semiconductor, thus enhancing hydrogen production.
However, an overall higher trend for H
2
production was demonstrated to be directly related to the [100]
For Rh nanoparticles deposited over graphitic carbon (g-C ), there
is no size dependence for hydrogen production in aqueous methanol. It was hypothesized that metallic
3
1-33
facets of cubic nanoparticles.
3
N
4
3
4
Rh was essential for the removal of photogenerated electrons at the surface. Additionally, studies on
the effect of the size of Rh nanoparticles on GaN:ZnO in a sulfuric acid solution for photocatalytic
water splitting, indicated that the activity of Rh@Cr
2
O core-shell nanoparticles (NPs) is dependent
3
3
5
upon the size of the Rh core. It is known that upon variation of the size of the nanocrystal, the ratio of
atoms in the edges, corners, faces and vertices is not constant; hence the normalisation of the surface
36
atoms by using metallic nanoparticles should be corrected.
In this study, we report the development of Rh NPs with different shapes as co-catalysts in the
photocatalytic reforming of methanol for the production of hydrogen in aqueous media. Three different
Rh NPs shapes were studied (cuboctahedra RhNP, cubic RhNC and octahedral RhOh) to evaluate the
3
6
surface composition activity by using the magic numbers approach (Figure 1). The catalysts were
characterized by standard techniques such as, electron microscopy, textural and electronic properties.
Also, it was developed the LED reactor lamp which permitted this study.
2

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Figure 1. Illustration of Rh nanoparticles shapes used in this work showing the lattice planes of
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the exposed facets.
1
. Experimental
1
.1. Materials and methods
Rhodium chloride (RhCl
3
) was purchased from Stream Chemicals, 2-bromoethanol was
aeroxide (P25) was obtained from Evonik Industries.
1-methyl imidazole was obtained from Spectrum Chemical and distilled under reduced pressure prior
to use. Rhodium acetate dimer (Rh (OAc) ) and the other reagents were purchased from Sigma-Aldrich
Merck) and used without further purification. Methanol was HPLC grade and the deionized water (18.2
MW cm) were used throughout this study.
purchased from Oakwood Chemical China, TiO
2
2
4
(
1
.2. Reactor lamp design and reaction Schlenk vessel.
Two reactors were built by using 300-LED strips with emission at 365 ± 5 nm and 400 ± 10
nm. These strips were glued to the inner side of a cylinder (Æ = 15 cm, 15 cm height) were the top part
was fitted with three small fans (60 mm) for forced ventilation (See Figure S1a, b for illustration). The
power output of each strip is 20 W and 24 W, 365 nm and 400 nm, respectively. Both of the lamps were
powered by the same 12 V, 20 amps power supply. The reaction vessel consisted on a Schlenk tube
adapted with a GC septum for gas sampling as shown in Figure S1c.
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.2.1. Determination of UV-LED photon flux.
The photon flux of the UV-LED reactor assembly was measured using the ferrioxalate
3
8
actinometry adapted from the literature. Three stock solutions were prepared using distilled water. The
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stock solution A was prepared by dissolution of 0.967 g of Fe(NO
water. Oxalate stock solution B was prepared by dissolution of 2.0 g of K
of water. Stock solution C was prepared by dissolution of 10 mg of 1,10-phenantroline (5.5 mmol L )
3
)
3
.9H
2
O (0.4 mol L ) in 10 mL of
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C
2
O (1.2 mol L ) in 10 mL
4
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1
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and 2.245 g sodium bicarbonate (1.65 mol L ) in 10 mL of H
2
O. Stock solutions A and C were protected
from light.
The reaction solution was prepared by a mixture of 1.0 mL of solution A and B, diluted to 20
mL in darkness. Then, 10 mL of the reaction solution was transferred to the Schlenk tube used in the
photocatalytic experiments equipped with a magnetic stirring bar and then irradiated for 5 min. After
this time, 0.5 mL was taken from the reaction solution, mixed with 0.5 mL of stock solution C and
diluted to 5 mL with H O. The spectrum was measured before and after the irradiation and the
2
2
+
absorption at 510 nm used for quantification of Fe formed. The photon flux was calculated by the
formula:
ꢂ ꢂ ∆ꢃ
ꢂ ꢃ
ꢀ
ꢀꢁꢀꢁ
=
(1)
ꢂ ꢄꢅ
ꢀ
Were, V
reaction solution (0.5 mL) and V
maximum absorbance measured at 510 nm before and after the irradiation, l is the optical path length
1
is the volume of the irradiated solution (10 mL), V
2
is the volume taken from the
3
is the volume measured in the UV-vis (5 mL). DA is the variation of
2
+
-1
-1
of the cuvette (l = 1.0 cm) and e is the molar extinction coefficient for [Fe(phen)] (11100 L mol
-1
cm at 510 nm). The number of incident photons is calculated by the formula:
ꢆꢄꢅꢆꢇꢆꢈꢉ
ꢇꢈꢆ
ꢆꢊꢋꢀꢁ
=
(2)
∅ꢌꢉꢀ
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Were the quantum yield of Fe production (F365 = 1.270 and F400 = 1.188) , t is the irradiation
time (5 min) and F is the mean fraction of the absorbed light (F = 1).
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.3. Synthetic procedures
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1.3.1. Synthesis of BMIm.OAc.
Previously, BMIm.Cl was prepared by reacting butyl chloride and 1-methylimidazole at 75-80
ºC for 48 hours. After recrystallization in acetone/methanol mixture, BMIm.Cl was obtained as white
1
crystals (yield: 80%). H NMR (400 MHz, D
2
O) δ ppm 0.89 (t, J = 7.4 Hz, 3H); 1.22–1.35 (m, 2H);
1
.82 (qp, J = 7.4 Hz, 2H); 3.86 (s, 3H); 4.17 (t, J = 7.2 Hz, 2H); 7.40 (d, J = 2.0 Hz, 1H); 7.45 (d, J =
2
.0, 1H). In a second step, the corresponding BMIm.OAc, was obtained by anion exchange using
-
Amberlyst X resin saturated with OH ions. After the anion exchange, glacial acetic acid was added and
the produced water removed by vacuum to obtain BMIm.OAc as a pale-yellow oil. Yield: 80%. NMR
1
analysis. H NMR (400 MHz, CDCl
3
) δ ppm 0.97 (t, J = 7.4 Hz, 3H); 1.31–1.41 (m, 2H); 1.83-1.89 (m,
2
H); 1.97 (s, 3H); 4.05 (s, 3H); 4.23 (t, J = 7.4 Hz, 2H); 7.24 (s, 1H); 7.30 (s, 1H); 11.08 (s, 1H).
Residual water contents determined by Karl-Fischer titration (3%).
1
.3.2. Synthesis of HEMIm.Br.
In a typical procedure, in a round bottomed flask equipped with a magnetic stirrer 10 mL of 1-
methylimidazole (0.125 mol) and 10 mL of 2-bromoethanol (0.141 mol) were mixed at room
temperature. After 48h the reaction flask was cooled until induce the precipitation of the HEMIm.Br.
Then, resulting solid was washed with diethyl ether and dried over vacuum. Yield: 83 %, NMR analysis.
1
H NMR (400 MHz, D O) δ ppm; 3.80 (s, 1H); 3.84 (m, 2H); 4.64 (s, 2H); 7.35 (t, J = 2.0 Hz, 1H); 7.40
2
(
t, J = 2.0, 1H).
1.3.3. Preparation of spherical Rhodium-impregnated TiO (RhNP).
2
The RhNP were prepared by chemical reduction with NaBH
4
. Typically, 8.0 mg of Rh
2
(OAc)
4
(
0.036 mmol in Rh) were dissolved in a solution containing 75.0 mg of BMIm.OAc (0.38 mmol) in 40
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mL of deionized water. After 30 minutes under vigorous stirring, 10 mL of a freshly prepared ice-cold
NaBH
4
solution in deionized water (7.0 mg of NaBH , 0.185 mmol) was added to the solution. After
4
few minutes, the initial blue colour turns to dark brown, characterizing the formation of the
Rh@BMIm.OAc.
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The impregnation of Rh nanoparticles in TiO
2
procedure was adapted from literature. In a
typical experiment, 1000 mg of TiO (Evonik, P25) freshly macerated in an Agatha mortar were added
2
to a 50 mL of the Rh@BMIm.OAc colloidal solution under vigorous stirring during 48 hours. After this
time, RhNPs were isolated by centrifugation, washed with deionized water, and dried under vacuum at
60 ºC for 12 hours. (Isolated yield: 917 mg.)
1
.3.4 Preparation of cubic Rhodium-impregnated TiO (RhNC).
2
The rhodium nanocubes were synthesized as follow: 42.0 mg of RhCl (0.2 mmol), 444.5 mg
3
of PVP (55 Da, 4.0 mmol in monomer units) and 207.1 mg of HEMIm.Br (1.0 mmol) were placed in a
Schlenk tube and dissolved in ethylene glycol. This mixture was heated to 80 °C for 20 minutes under
vigorous stirring. After this time, the resulting solution was placed in a silicone bath at 185 °C and the
reaction conducted for 1.5 h. After this, acetone was added and the solution centrifuged to obtain a
precipitated that was re-dissolved with few drops of ethanol and acetone in order to remove remaining
PVP. RhNC were recovered by centrifugation and dried by vacuum (Isolated yield: 20.0 mg).
For impregnation of rhodium nanocubes in TiO
2
, 5.0 mg of previously isolated powder were
in 25 mL of water. After
dissolved in 25 mL of water and added dropwise to a solution of 1000 mg TiO
2
addition, the resulting solution was stirred for 48 h at room temperature, and RhNC isolated by
centrifugation. The RhNC were washed with deionized water, and dried under vacuum at 60 ºC for 12
hours. (Isolated yield: 800 mg).
1
.3.5 Preparation of octahedral Rhodium-impregnated TiO (RhOh).
2
In a typical synthesis, PVP (55 Da, 33.3 mg, 0.29 mmol in monomer units), ascorbic acid (480
mg, 2.72 mmol) and citric acid (60 mg, 0.31 mmol) were dissolved in 9.0 mL of ethylene glycol under
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magnetic stirring at 145 C. In another flask, RhCl (6.2 mg, 0.03 mmol) was totally dissolved in 2.0
3
mL of ethylene glycol and then added to the PVP, ascorbic acid and citric acid solution mixture. The
o
reaction was carried out at 145 C for 2 hours. After cooling to room temperature, the solution was
centrifugated and washed with ethanol to remove the excess of PVP. Finally, the octahedral rhodium
nanoparticles were isolated by centrifugation at 12000 RPM. (yield: 5 mg.) Same RhNC impregnation
procedure was used for RhOh (Isolated yield: 740 mg)
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1.4 Rh nanoparticles characterization.
The TEM samples were prepared by drop-casting the colloidal nanoparticles and the supported
catalyst onto Cu grids (holey carbon – EMS), 300 mesh. The TEM images were acquired in a JEOL
JEM 1011 electron microscope operating at 100 kV using at least 400k magnification. The HRTEM
and HAADF-STEM images were acquired using a JEOL JEM 2100F electron microscope operated at
N
2
00 kV equipped with an Oxford Instruments X-Max 80 LTE EDS detector, which was used for
elemental mapping with STEM imaging. For elemental mapping, the sample holder was tilted 17.0º
towards the detector for better signal-to-noise ratio and the EDS maps were acquired in the range of 0
to 40 keV. The Ka emission line was chosen for the composition of the colour map of the elements,
1
Rh at 20.217 keV and Ti at 4.508 keV. The as-prepared colloidal samples were isolated by
centrifugation, dried under vacuum and analysed by X-ray photoelectron spectroscopy (XPS) using Al-
Kα radiation and a Scientia Omicron hemispherical analyser. Adventitious C 1s (284.8 eV) was used
as a reference. Textural characterization was performed from low-temperature N
isotherms (Tristar Kr 3020, Micromeritics). Prior to the measurements, the samples were degassed at
20 ºC under vacuum for 12 h. The specific surface area, SBET was determined by BET (Brunauer,
2
adsorption-desorption
1
4
1
Emmett and Teller) multipoint technique. The total pore volume, V
adsorption point (p/p = 0.98 – 0.99). The pore size distribution by volume (PSDV) was computed
through the self-consisted regularization under non-negative conditions of the pore size distribution
function, f(R )>0 and at the regularization parameter α = 0.01. The method allows us to estimate the
contributions of different pore types based on their size: micro- (Vmicro at R < 1 nm), meso- (Vmeso at 1
p
, was calculated at the maximal
0
p
p
nm < R
p
< 25 nm) and macropores (Vmacro, R
p
> 25 nm). The mathematical aspects are discussed
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particularly in the papers.
method involving the isotherm's adsorption branch within p/p
calculated from the plot corresponding to the equation ln(V/V ) = (D
V and V are the adsorbed volume of N , the saturation volume of adsorbed N
are the equilibrium and saturation pressure of nitrogen adsorbed, respectively.
The fractality was determined from the Frenkel-Halsey-Hill (FHH)
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< 0.8. The fractal dimension D
– 3)·ln[ln(p /p)] + const, where
, respectively; p and p
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The electronic properties of Rh impregnated on TiO
reflectance spectroscopy using a Shimadzu UV2100 equipped with an integration sphere accessory.
Powder samples were packed into the sample holder and measured against BaSO as standard.
2
were measured by UV-Vis diffuse
4
1
.5 Photocatalytic H production experiments
2
In a reactor vessel, 10 mg of the corresponding catalyst were dissolved in 9 mL of deionized
water and 1 mL of methanol. The mixture was sonicated for 30 s at 1000 W in order to disperse the
catalyst and the solution was deaerated by bubbling argon with a stainless-steel needle during 20
minutes. After this time, the reactor was placed in the UV-LED reactor and periodic samples of the
headspace analysed (H
equipped with TCD detector. The production of CO and CH
of CO is about 1/3 of the production of H , as a sub-product of methanol oxidation.
2
, CO, CH
4
and CO
2
content) with the aid of a gastight syringe (100 μL) in a GC
4
is negligible, but the overall production
2
2
4
6
The relative photonic efficiency (z
r
) was estimated by using the Formula 3 below, which
-
correlates the amount of H
2
produced, considering a 2e reaction, with the photon flux measured by the
ferrioxalate actinometry.
ꢎꢏꢐꢑ ꢐꢒꢍꢓꢔꢕꢖꢗꢔꢐꢘꢙꢚꢓꢛꢐꢚꢜꢏ"#
ꢝ
!
ꢀꢁ (%) =ꢁ
ꢂꢁ100 (3)
ꢍ
ꢏꢐꢒꢞꢓꢟꢓꢏꢠꢐ(ꢚꢜꢏ"#
)
2
. Results and discussion
2
.1 UV-LED Lamp reactor and actinometry
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In this work it was used a home-made UV-LED reactor as a light source which is low cost and
has low energy consumption in comparison to other types of light sources and high intense
4
7, 48
monochromatic emission.
Ferrioxalate actinometry was used to probe the photon flux of the UV-
LED reactor and in Figure 2 is shown the UVVis spectra after/before 5 minutes irradiation in both
wavelengths studied here. Using the absorbance value at 510 nm it was estimated the photon flux of
each UV-LED strip, as shown in Table 1.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. UV-Vis spectra of ferrioxalate actinometry, before (black line) and after irradiation
(
red line), using 365 nm UV-LED and (blue line) using 400 nm UV-LED.
Table 1. Measured photon flux of each UV-LED assembly.
-
1
-6
Lamp Reactor
65 nm
400 nm
DA
Photon flux (photons min x10 )
3
1.56
1.25
2.22
2.06
2
.2 Rh-TiO characterization
2
The average size distribution and the morphology of the Rh nanoparticles was probed by
conventional TEM (Figure 3). As expected, the chemical reduction of Rh by NaBH afforded to very
4
small spherical nanoparticles with a narrow size distribution. The cubic and octahedral nanoparticles
-
were prepared by polyol solvolysis at high temperature using Br ions and citrate as shape inducers. The
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2
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2
2
3
3
3
3
3
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3
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mean size and distribution of the nanoparticles are summarized in the Table 2. RhNC were measured
diagonally meanwhile RhOh were measured by the longest length of the nanoparticles.
As the statistical analysis of the Rh nanoparticles size distribution has shown (Figure S2), all
three samples are characterized by a strict tendency in the average diameter growth together with the
standard deviation in the RhNP – RhNC – RhOh series. This, in turn, speaks for a less dispersed
character of the spherical nanoparticles – the standard deviations (SDs) for RhNC and RhOh are as
higher as ca. 2 and 4 in comparison to SD for RhNP. The enlargement of the interquartile range (0.4875,
0.9563, 2.47 for RhNP, RhNC and RhOh, respectively) and normal character for non-spherical particles
may favour a view that polyol affected synthesis procedures for RhNC and RhOh occur stochastically,
i.e., in both cases, the formation of larger nanoparticles required more time to achieve the equilibrium.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
The reduction in the presence of NaBH for RhNP could be controlled kinetically resulting in enhanced
4
selectivity with the set nanoparticles size. Taking a closer look at the distribution’s profiles, RhNC and
RhOh demonstrate normality validated by the D’Agostino-Pearson test, revealing pD-P > 0.05 (0.275
and 0.068, respectively). Contrastingly, RhNP has not passed the normality test at pD-P << 0.05 (Figure
S2).
After impregnation on TiO
2
, the catalysts were characterized by HRTEM as shown in Figure
, Figure 3e, f,
3. For RhNC and RhOh it was possible to measure the lattice spacing of the Rh and TiO
2
and the measured distances by FFT are in accordance to fcc structure of Rh. The lattice planes of RhNC
and RhOh shows contributions of (200) and (111) planes at 0.20 nm and 0.23 nm, respectively, which
49
are close to the previously reported data. Since the RhNP were on top of TiO , the FFT results could
2
not be assigned. Also, the catalysts were analysed by STEM-EDS and it was confirmed the elemental
composition of the nanoparticles on the surface of TiO (Figure 4). The heterojunction of Rh and TiO
was investigated by UV-vis DRS (Tauc plots in Figure S3). The influence of ionic liquids on TiO was
investigated by impregnation of [BMIm.OAc] on TiO . It was observed that the presence of IL shifted
down the bandgap value. The calculated direct band gaps of impregnated Rh materials showed to have
increased values in comparison with pristine TiO (Table 2). Upon increasing the metal content
2
2
2
2
2
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impregnated on TiO2, the bandgap shifted to higher values. This behaviour is probably due to the overlap
in the absorption spectra of the Rh colloids, ionic liquids, and TiO .
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. TEM image of colloidal a) RhNP, b) RhNC, c) RhOh and HRTEM images of d)
RhNP, e) RhNC showing the (200) atomic planes and corresponding FFT for Rh (green box) and TiO
red box) and f) RhOh a single RhOh nanoparticle in TiO showing the (111) atomic planes of Rh and
corresponding FFT (inset).
2
(
2
Table 2. Physico-chemical characterisation of the catalysts.
+
x
a
b
System
TiO
TEM Size (nm)
-
Rh amount FAAS (%) Ratio Rhº/Rh (%) Bandgap (eV)
2
-
-
3.20
3.58
3.40
3.47
RhNP
RhNC
1.6 ± 0.4
5.7 ± 0.7
8.4 ± 1.7
0.24
0.09
0.11
38,8
63,2
78,1
b
RhOh
a
b
Determined by XPS analysis of unsupported nanoparticles. Direct bandgap determined by linear extrapolation
c
of Tauc plots. Increasing the metal content to 0.56 % (w/w) shifted the direct bandgap to 3.76 eV.
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1
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1
1
1
1
1
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2
2
2
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2
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2
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2
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3
3
3
3
3
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1
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4
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8
9
0
1
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5
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7
8
9
0
1
2
3
4
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. HAADF-STEM/EDS of a) RhNP, b) RhNC and c) RhOh, the inset presents the EDS
mapping and individual contribution of Ti and Rh (colorized images).
The electronic state of Rh NPs synthesized were measured by XPS and the Rh 3d region are
shown in Figure 5 and in Table 2 (the survey spectrum can be found in the ESI, Figure S4). The data
were acquired on the as-prepared non-supported nanoparticles, after isolation by centrifugation and
washing with DI H O, due to the low Rh loading on the final catalyst. As the nanoparticles were in
2
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different sizes, the Rh 3d region shows an increase in the Rh contribution in the smaller particles as
34, 50, 51
+x
already observed by others.
Two contributions from Rhº and Rh were used for the
deconvolution of the data with peaks in 307.0 and 309.2 eV, respectively.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5. XPS of Rh 3d region of the nanoparticles synthesized in our work, highlighting the
+
x
presence of both Rhº and Rh contributions centred in 307.0 and 309.2 eV, respectively.
The N
showed similar behaviour for all catalysts Figure 6a. Specific surface area, SBET was determined from
the standard BET method and the total pore volume, V was calculated at p/p = 0.98-0.99. The pore
2
adsorption/desorption was evaluated for the catalysts synthesized in this work and
p
0
radius analysis, Figure 6b, indicated a decreased porosity that is reflected in lowered total pore
volume and augmented average pore radius in RhNC and RhOh, as compared to RhNP. Hierarchically,
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the key alteration in the morphology occurs with aggregates and agglomerates of aggregates – one can
see this by elevated values of the fractal dimensions – from 2.44 (RhNP) up to 2.51 and 2.52 (RhNC
and RhOh, respectively, shown in Figure S5). The rearrangement of the aggregated structure is
especially visible comparing RhNC and RhOh, while at lower V
maintained, as in case of RhOh, i.e. the contributions of meso- and macropores are almost equal,
whereas the size of TiO primary particles did not undergo any alteration. Notice, that RhNC and RhOh
p
(RhNC) the mesoporosity is
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
feature in slightly enlarged rhodium nanoparticles giving an additional fraction despite their lower
concentration in both hybrid catalysts in comparison to RhNP. Hence, a complex introduction of
modificators (1) and magnified Rh nanoparticles (2) leads to the decrease of empty volume (Vem) related
to the true (ρ
0
) and bulk (ρ
b
) density of the powders: Vem = 1/ρ
b
−1/ρ
0
, where the value of ρ undergoes
b
increase.
Figure 6. a) N
2
adsorption/desorption isotherms and b) the pore size distribution by volume of
the catalysts synthesized in this work.
Table 3. Textural characteristics of RhOh, RhNC and RhNP materials.
2
3
Sample
RhOh
RhNC
RhNP
S
BET (m /g)
V
p
(cm /g)
V
micro/V
p
(%) Vmeso/V
p
(%)
V
macro/V
p
(%)
R
p
, (nm)
D
S
50
50
54
0.355
0.317
0.371
1.3
1.5
1.0
52.1
46.6
44.3
35.0
53
49
34
2.52
2.51
2.44
54.3
64.0
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2
.3 Photocatalytic H
2
production experiments.
The H production reactions were carried out at room temperature using a 10% methanol
2
-
1
aqueous solution (10 mL) and 1.0 g L of catalyst concentration during a period of five hours (The
kinetics of H production of all catalysts is shown in Figure 7). In the absence of MeOH the H
production was negligible; this is due to the methanol is scavenging holes decreasing the recombination,
but a part of H is coming from photoreforming (coming from photooxidation) of methanol. A 25-fold
increase in H formation by the spherical nanoparticles in comparison with pristine TiO and the
colloidal nanoparticles was observed (up to 23.0 mmol g of H after 5 hours of reaction). Since RhNP
was shown to have the smallest size among the Rh nanoparticles applied in this study, the number of
particles on the surface of TiO presents a higher number of active sites for the generation of H
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2
2
2
-
1
2
2
2
.
Figure 7. Kinetic profiles of H production over gram of catalyst per time, (a) at 365 nm
2
irradiation and (b) at 400 nm irradiation.
As shown in Figure 8, the wavelength influence in the H
upon the comparison of the H production per gram of catalyst. The production per gram of catalyst of
RhNP at 365 nm irradiation figure among the best results so far in these conditions (using commercial
2
production can be also assessed
2
5
2
TiO
2
and methanol as sacrificial reagent). In comparison with other Rh catalysts under irradiation at
the same region, our result is around 5 times better than previous reported, when normalized by catalyst
5
3
loading. A comparison of other Rh-based catalysts can be found on Table S1, but the direct
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2
2
2
2
2
2
2
2
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3
3
3
3
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3
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comparison is difficult since it was used different conditions, such as choice of support, sacrificial
reagent and irradiation wavelength/source.
Pristine TiO showed the same production after five hours of reaction independent of the
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
incident wavelength. However, by using the colloidal RhNP, it is clear that activation of the metallic
surface upon irradiation at shorter wavelengths occurs. For non-plasmonic metals, interband transitions
5
4
play a significant role in the light-activated catalysis. Excitation of d-band electrons can be absorbed
5
5
by the support and contribute to the generation of hot electrons and holes. The excitation of d-band
electrons can also interact with the LUMO of the adsorbed species at the surface of the Rh nanoparticles,
therefore, promoting the reaction.
Figure 8. Normalised H production per gram of catalyst after 5 hours of reaction.
2
5
6
Although Rh nanocubes can exhibit localized surface plasmon resonance bands (LSPR), the
5
7
position of the plasmonic band depends on size whereas the size of RhNC is much smaller than the
Rh nanocubes which exhibit LSPR in the emission wavelengths of LED used in this work. The position
of the interband transitions of the Rh nanoparticles studied here overlaps with the absorption of UV
light by TiO
2
. Therefore, a dual-excitation pathway can lead to a better efficiency in the generation of
58
electron-hole pairs on TiO
2
,
culminating with a strong synergetic effect in the catalysts. Upon
preparing a catalyst with a higher amount of RhNC, the activity did not increase linearly (See Figure
S6). This fact can be explained by an RhNC blocking out effect over TiO light absorption that leads to
lower activities. The irradiation of a 300 W Xe lamp without an UV bandpass filter, commonly
2
5
9
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employed in solar simulation photocatalysis, didn’t affect the catalyst performance as tested using
RhNP (the comparison of kinetics over 4 hour reaction is shown in Figure S7). Therefore, we estimated
that using visible light (> 400 nm) would not increase the H production, as observed by comparison of
2
365 nm and 400 nm LED lamps.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The specific rate of H
2
production was calculated after measuring the average size of the
production and, the amount of Rh on TiO determined by
nanoparticles by TEM analysis, the rate of H
FAAS, assuming that the surface in contact with TiO
SI for details). This approximation is similar to already estimated for colloidal nanoparticles in
2
2
6
0
2
was proportional for all samples, (Table 4 and
61
-1
-2
heterogeneous reactions . The specific rate of H
2
production was 3.0 µmol h m for pristine TiO
2
and
.95 µmol h m for colloidal Rh at 365 nm, and 3.2 µmol h m and 0.19 µmol h m at 400 nm,
respectively. Under these conditions, there is no dependency upon the incident wavelength of the UV-
LED lamp using only TiO as a catalyst. However, when Rh is impregnated on TiO the H production
-
1
-2
-1
-2
-1
-2
0
2
2
2
is highly dependent on the incident wavelength. For RhNP, the activity at 365 nm had a two-fold
enhancement in comparison with the 400 nm irradiation. For RhNC activity was about a three-fold
increased, meanwhile RhOh, showed to be not very dependent upon the incident wavelength, which
constitutes evidence of a better activity in the presence of [100] faceted NPs. Spherical (cuboctahedra)
37
nanoparticles have both [111] and [100] facets on the surface, and therefore showed an increase in
37
activity for H production. RhNC, which is composed mainly of [100] exposed facets, the activity was
2
even higher at shorter wavelengths. Finally, for RhOh, which surface is mainly composed of [111]
facets, activity remained almost unaltered. The higher activity of the RhNP, among the catalysts tested,
could be associated with two mainly factors: 1) the higher concentration of Rh impregnated on TiO ,
2
and 2) the higher surface-to-volume ratio in comparison with the other Rh nanoparticles synthesised in
this study. The stability of the catalysts before the reaction was assessed by conventional TEM, using
both TiO -supported and unsupported RhNP (spheres) as shown in Figure S8. Although is difficult to
2
have a good statistical counting on supported nanoparticles, is possible to observe that the size remains
unaltered after the catalytic cycle, which is also true when using unsupported nanoparticles.
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Table 4. Specific rate of H
2
production and relative photonic efficiency, z
r
, of the catalysts upon
different incident wavelengths.
-
1
-2 a
System
Specific Rate (mmol h m )
zr (%)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
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8
9
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9
0
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0
365 nm
400 nm
365 nm
2.7
400 nm
2.7
-
3(b)
-3(b)
TiO
2
3.0x10
3.2x10
RhNP
RhNC
RhOh
13.5
22.4
5.6
73.8
26.1
9.2
30.6
8.7
7.5
7.9
5.2
6.2
c
-4
-4
RhNP
9.5x10
1.9x10
0.08
0.02
a
Values were determined by the linear fit of the slope of H production kinetics in a
2
5-hour reaction at room temperature, the reported data is an average of at least three
b
independent experiments. Specific Rate calculated using BET surface area value
2
-1
c
(
56 m g ). Colloidal nanoparticles.
Upon normalising the average activity by surface area for the amount of Rh in each sample,
it is possible to observe a higher activity of RhNC. The observed activity trend of RhNC > RhNP >
RhOh is similar with earlier reports on Pd nanocubes and octahedra with different sizes for H
2
6
2
production in aqueous methanol. The higher activity of the RhNC might be due to the higher surface
energy of the Rh nanoparticles, in comparison with other metals such as Au, Pt, and Pd, regardless their
6
3
shape. The higher activity observed for the RhNC and RhNP could be an effect of the presence of
imidazolium ionic liquids in its preparation. It is well known that the interaction of imidazolium cations
6
4-66
with metal nanoparticle surface could produce Metal-N-heterocyclic carbene species (M-NHC)
consequently different catalytic activities. To confirm the influence of IL in water photolysis, the RhOh
catalyst was treated with [BMIm.OAc] and TGA analysis showed a 2 w.t % in remained on TiO (Figure
S9). The small increase in activity after impregnation with the IL (Figure S10), can be explained by two
and
2
6
5
different mechanisms: 1) the formation of Rh-NHC carbene species at the surface and, 2) the
6
7
interaction of the ionic pair with the TiO
for the IL-doped RhOh also decreased from 3.47 to 3.44 eV, a difference of 0.03 eV, which is similar
to the 0.05 eV difference when doping the TiO with the IL. This result is evidence of the IL cation
participation on the photocatalytic effect as observed before.
2
surface which synergically shifts the bandgap. The bandgap
2
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7
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The z was determined and since the estimation was carried out using the same catalyst loading
r
and the LED lamps in our study, an apparent quantum efficiency (Fapp) was not used in this study. The
results are compiled in Table 4. It can be noted that all Rh-impregnated catalysts showed better
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
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2
2
3
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3
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3
3
3
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4
4
4
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5
5
5
5
5
5
5
5
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0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
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7
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9
0
1
2
3
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0
efficiency than pristine TiO
2
. The high z
r
values obtained in this work are also observable in the high
H
2
production of all catalysts, in special the RhNP, which converted approximately ~74 % of photons
68
to H
2
. Similar z
r
values for equivalent H production has been observed before, but referred as
2
apparent quantum efficiency, Fapp. The discussion of using Fapp to heterogeneous photocatalysis can be
4
6, 69
found elsewhere.
directions minimizing the scattering effects of the powdered sample. The higher efficiency of the
systems at lower wavelength is probably due to the better photon absorption by TiO and, therefore a
better charge transfer to the co-catalyst. TiO exhibited the same z for both wavelengths, due to its
absorption spectra in the UV region. In the case of RhOh, the z difference between the wavelengths is
less evident, 9.2 % versus 6.2 % for 365 nm and 400 nm, respectively. This effect for Rh photocatalysis
Also, the reactor design may play a major role since the sample is irradiated in all
2
2
r
r
7
0
was also observed in the photodegradation of 4-chlorophenol by UV and Visible light.
3. Conclusions
The results presented in this paper show that the photocatalytic activity is directly related to the
exposed facets of the Rh-cocatalyst and that can be tailored by the shape of MNPs. The H production
is among the best reported so far using UV LED lamps and commercial TiO . Despite using Rh as a
cocatalyst, it was observed that a very low amount of impregnation (i.e. RhNP at 0.24 %) is enough to
2
2
-
1
achieve a Hydrogen production of about 4.68 mmol h per gram of catalyst. A triple synergic effect of
TiO , Rh nanoparticle and the ionic liquid was observed. The incident photon energy was also an
important observation in this work. By only a few nanometres of difference in the UV-LED emission,
an increase of about 30% in the H production activity and z using spherical and cubic nanoparticles.
The application of magic numbers approach to unveil the surface activity of the catalysts was essential
to rationalize the morphological effects. All of these observations reveal a trend in the H production of
the exposed facets [100] > [111] on studied conditions. The hydrogen generation presented in this work
2
2
r
2
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figure among the best reported so far for rhodium co-catalysts impregnated in TiO . Also, common
2
LED strips can be used as a photon source for the production of hydrogen in aqueous methanolic
solutions. This can be applied to different alcohols to seek a more environmentally friendly H
2
production. The electric energy demand for LED light sources shows good potential for sunlight
powered systems with the aid of common photovoltaic cells.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are thankful for financial support from CAPES (158804/2017-01 and 001),
FAPERGS (16/2552-0000 and 18/2551-0000561-4), CNPq (406260/2018-4, 406750/2016-5 and
4
65454/2014-3). We are grateful for the National Laboratory of Nanotechnology (LNNano-Campinas)
under proposal TEM-24375 for the HRTEM analysis, the Central Laboratory of Electron Microscopy
LCME) at UFSC for the TEM analysis, the Laboratory of Atomic Spectrometry at IQ-UFRGS for the
(
FAAS analysis and Prof. Claudio Radtke (UFRGS) for XPS experiments.
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Rhodium nanoparticles with different morphology were synthesized to assess the influence of the
exposed facet towards the hydrogen production in aqueous methanolic solution.