1
In situ Raman spectroscopic evidence for oxygen reduction 1
reaction intermediates at platinum single crystal surfaces 2
Jin-Chao Dong
1
, Xia-Guang Zhang
1
, Valentín Briega-Martos
2
, Xi Jin
1
, Ji Yang
1
, Shu Chen
3
, 3
Zhi-Lin Yang
3
, De-Yin Wu
1
, Juan Miguel Feliu
2,
*, Christopher T. Williams
4
, Zhong-Qun 4
Tian
1
, Jian-Feng Li
1,3,5,
* 5
1
MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of 6
Physical Chemistry of Solid Surfaces, iChEM, and College of Chemistry and Chemical 7
Engineering, Xiamen University, Xiamen 361005, China 8
2
Instituto de Electroquímica, Universidad de Alicante, Apt. 99, Alicante, E-03080, Spain 9
3
Department of Physics, Research Institute for Biomimetics and Soft Matter, Xiamen University, 10
Xiamen 361005, China 11
4
Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 12
29208, USA 13
5
Shenzhen Research Institute of Xiamen University, Shenzhen 518000, China 14
Email: Li@xmu.edu.cn and juan.feliu@ua.es 15
16
Abstract 17
Developing an understanding of structure-activity relationships and reaction mechanisms of 18
catalytic processes is critical to the successful design of highly efficient catalysts. As a 19
This is a previous version of the article published in Nature Energy. 2019, 4: 60-67. doi:10.1038/s41560-018-0292-z
2
fundamental reaction in fuel cells, elucidation of the oxygen reduction reaction (ORR) mechanism 20
at Pt(hkl) surfaces has remained a significant challenge for researchers. Here, we employ in situ 21
electrochemical surface-enhanced Raman spectroscopy (SERS) and density functional theory 22
(DFT) calculation techniques to examine the ORR process at Pt(hkl) surfaces. Direct 23
spectroscopic evidences for ORR intermediates indicates that under acid conditions, the pathway 24
of ORR at Pt(111) occurs through the formation of HO
2
*, while at Pt(110) and Pt(100) it occurs 25
via the generation of OH*. However, we propose that the pathway of ORR under alkaline 26
conditions at Pt(hkl) surfaces mainly occurs through the formation of O
2
-
. Significantly, these 27
results demonstrate that the SERS technique offers an effective and reliable way for real-time 28
investigation of catalytic processes at atomically flat surfaces not normally amenable to Raman 29
study. 30
31
In recent energy researches, significant focus has been placed on understanding the mechanism of 32
catalytic reactions at the atomic level. The direct operando monitoring of surface catalytic 33
reactions has always been a "holy grail" in electrochemistry and heterogeneous catalysis, and will 34
aid significantly in the design and development of more highly efficient catalysts.
1,2
As a classical 35
catalytic reaction, the process and mechanism of the oxygen reduction reaction (ORR) at platinum 36
surfaces have been a focus of attention in the literature for a long time.
3,4
Though lots of research 37
groups have carried out experimental and theoretical studies to reveal the ORR mechanism, the 38
detailed surface process is still not clear. 39
Generally, the mechanism of ORR process at platinum electrodes in acidic condition is 40
3
considered to occur by two main pathways: one involves oxygen being reduced directly via a 41
four-electron pathway into H
2
O; the other first reacts oxygen via a two-electron pathway to 42
hydrogen peroxide, followed by a two electron transfer reduction of the latter to water; hydrogen 43
peroxide also can directly diffuse into the solution as a final product, which then quickly 44
decomposes. However, some essential questions and uncertainties remain about ORR processes, 45
including slow kinetics, the origin of observed high overpotentials, and the rate determining 46
step.
5-11
The main reason is that as a multi-electron reaction, there are varieties of intermediates 47
(e.g., OH*, O
2
2-
, O
2
-
, HO
2
*, etc.) that are generated during ORR process, and most of the 48
intermediates have a short life-time, low coverage and are also influenced by other co-adsorbed 49
species. Thus, the key factor to unravel the ORR mechanism is to develop an in situ method to 50
identify the various reaction intermediates and their adsorbed configurations at platinum surfaces 51
during the ORR process. With their well-defined surface structures, optical and electric field 52
properties, and ability to be modeled at the atomic level, single crystal surfaces play a key role in 53
probing catalytic reaction mechanisms in surface science.
12
However, most of the current 54
spectroscopic methods are not suitable for the single crystal studies in aqueous solution, especially 55
for the ORR reaction at Pt(hkl) electrode surfaces.
13-19
56
Surface-enhanced Raman scattering (SERS) is a powerful fingerprint spectroscopy that can be 57
used for in situ investigation of trace chemical species and identification with single-molecule 58
sensitivity.
20-22
However, its applications are generally restricted to ‘free-electron-like’ metals such 59
as Au, Ag and Cu that have non-smooth surfaces. To overcome the long-term limitation of SERS 60
on morphology and material generality, previously we developed a surface vibrational 61
spectroscopic method that was named Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy 62
4
(SHINERS).
23
In SHINERS, an ultrathin and uniform silica shell coated onto a gold nanoparticle 63
can efficiently enhance the Raman signal of molecules that are in located near the nanoparticle 64
surface without any interference. It is possible to obtain Raman signals from any substrate and any 65
material surface. A unique advantage of SHINERS is its particular applicability to explore the 66
adsorption configuration and catalytic processes of probe molecules at single crystal surfaces.
24-32
67
Here, we employ in situ electrochemical (EC)-SHINERS coupled with density functional theory 68
(DFT) calculations to study the ORR process at Pt(hkl) electrode surfaces. We obtain direct 69
spectral evidence that allows the ORR mechanism at these surfaces to be elucidated at a molecular 70
and atomic level. 71
72
SHINERS enhancement at Pt(hkl) surfaces 73
For a clear understanding of the relationship between the shell-isolated nanoparticles (SHINs) 74
enhancement and the electric field distribution, a 2 × 2 Au@SiO
2
nanoparticles (NPs) array was 75
modeled on a perfectly smooth platinum substrate surface and simulated using a 76
3D-Finite-Difference Time-Domain (3D-FDTD) theoretical system. Fig. 1a shows the schematic 77
diagram of in situ EC-SHINERS at low index Pt(hkl) surfaces. The SHINs used in this experiment 78
had a gold nanoparticle core (~55 nm) with SiO
2
shell (~2 nm) (Fig. 1b and Supplementary Fig. 1), 79
with the coverage of SHINs at the Pt(hkl) electrode surface at around 30% (Fig. 1c). The 80
3D-FDTD technique has been employed to model the SHINERS system effectively.
33-36
The hot 81
spots are mainly located around the particle-surface junctions under 638 nm excitation (Fig. 1d), 82
and the average SERS enhancement factor of this configuration is about 1.0×10
5
on the Pt(hkl) 83
5
surface.
24
84
85
86
Figure 1 | Schematic illustration of SHINERS study of ORR process and correlated characterization and 87
3D-FDTD results at Pt(hkl) surfaces. (a) The model of shell-isolated nanoparticles (Au@SiO
2
NPs, SHINs) at 88
Pt(111) surface, and the mechanism of ORR process revealed by EC-SHINERS method. The silver-white, red, and 89
white spheres represent Pt, O, and H atoms, respectively. The large golden spheres with transparent shells 90
represent SHINs. The SHINs, when being excited by a laser, can generate strong electromagnetic fields to enhance 91
the Raman signals of molecules adsorbed at the Pt(hkl) single crystal surface; (b) The transmission electron 92
microscope (TEM) image of Au@SiO
2
nanoparticle; (c) Scanning electron microscope (SEM) image of Pt(111) 93
single crystal electrode surface modified with SHINs; (d) 3D-FDTD simulations of four SHINs NPs with a model 94
of 2 × 2 array on a Pt substrate. 95
96
ORR processes at Pt(hkl) surfaces in acidic condition 97
