WO_3 thin films have been deposited in a hierarchically structured core–shell morphology, with the cores consisting of an array of Si microwires and the shells consisting of a controlled morphology WO_3 layer. Porosity was introduced into the WO_3 outer shell by using a self-assembled microsphere colloidal crystal as a mask during the deposition of the WO_3 shell. Compared to conformal, unstructured WO_3 shells on Si microwires, the hierarchically structured core–shell photoanodes exhibited enhanced near-visible spectral response behavior, due to increased light absorption and reduced distances over which photogenerated carriers were collected. The use of structured substrates also improved the growth rate of microsphere-based colloidal crystals and suggests strategies for the use of colloidal materials in large-scale applications

Arpin, Kevin A.

Braun, Paul V.

Brunschwig, Bruce S.

Coridan, Robert H.

Lewis, Nathan S.

Caltech Authors - Main

Supporting Information 
 
Materials and methods 
 
10% buffered hydrofluoric acid (BHF, Transene Inc.), 30%(w/w) hydrogen peroxide 
(H2O2), concentrated hydrochloric acid (HCl), tungsten metal powder (W, Sigma 
Aldrich), platinum black (Pt-black, Sigma Aldrich), acetonitrile (CH3CN, 99.8%, Sigma 
Aldrich), tetrabutylammonium hydrogen sulfate (TBA-HSO4, 97%, Sigma Aldrich), 
tetrabutylammonium tetrafluoroborate (TBA-BF4, 99%, Sigma Aldrich), silver nitrate 
(AgNO3, >99%, Sigma Aldrich) and tetrahydrofuran (THF, Sigma Aldrich) were used as 
received.   18 MΩ-cm resistivity water was obtained from a Barnstead Nanopure 
filtration system. 
Si microwire growth 
Boron-doped (0.001-0.005 Ω-cm resistivity), single-side polished, (111)-oriented p+-Si 
wafers (WRS Materials, San Jose, CA), obtained with a 300 nm-thick thermal oxide 
layer, were used as substrates for the growth of Si microwires1.  Photolithography was 
used to produce a pattern of 3 µm diameter holes on a 7 µm-pitch square lattice in a 
polymeric photoresist, and the oxide exposed by the holes in the photoresist was etched 
by immersion in BHF.  500 nm of high-purity Cu (99.9999%,) was then deposited onto 
the patterned wafer by thermal evaporation.   The photoresist layer was then removed 
with acetone, leaving a patterned, oxide-confined, Cu catalyst layer on the wafer.   Si 
microwires were grown by the vapor-liquid-solid chemical-vapor deposition process at 
1000 °C at atmospheric pressure in flowing H2 (500 sccm) and SiCl4 (45 sccm), with 
small partial pressures of BCl3 (1.3 sccm) as a p-type dopant.  After microwire growth, 
the Cu catalyst was removed by immersing the substrates in an RCA2 etch (1:6:1 by 
volume HCl:H2O:H2O2) for 20 min at 70 °C.   This process produced patterned 1 µm 
diameter p-Si microwires on the degenerately doped p+-Si substrate. 
To prepare the core-shell microwire structures, the substrates containing Si microwire 
arrays were etched in BHF for 20 s to remove native oxide on the microwire surface.   A 
125 nm layer of tin-doped indium oxide (ITO) was then deposited over the entire 
substrate by RF sputtering (90% In2O3/10% SnO2 target, Plasmaterials, Inc.).  The ITO 
layer produced a low-resistivity ohmic contact between the p-Si core and the n-type WO3 
shell layer, and was sufficiently transparent to allow light of energies below the WO3 
band gap to be absorbed in the Si2.   
Porous core-shell Si/ITO/WO3 microwire synthesis 
Conformal layers of WO3 were then electrodeposited on the Si microwire/ITO substrates.   
To prepare the electrodeposition bath2, a solution of peroxytungstic acid was prepared by 
slowly dissolving 4.6 g of W powder in 50 mL of 30%(w/w) H2O2(aq).   After the powder 
had dissolved, the remaining H2O2 was decomposed with Pt black until the peroxide 
concentration was less than 3 mg mL-1, as measured with peroxide testing strips.  The 
solution was then filtered to remove the Pt black, and diluted with water and isopropanol 
until the final solution was 50 mM peroxytungstic acid in 65% water/35% isopropanol by 
volume.  
Porous core-shell Si/ITO/WO3 microwire arrays were prepared using electrodeposition of 
WO3 in conjunction with sidewall-microsphere lithography.  First, a Si microwire-array 
substrate with a sputtered ITO layer was connected to a wire by an alligator clip.  The 
back and sides of the substrate were then painted with nail polish, to prevent deposition 
on these surfaces.  A conformal base layer of WO3 was then cathodically electrodeposited 
on the substrate.   The electrodeposition was performed potentiostatically at -450 mV 
versus a Ag/AgCl reference electrode (CH Instruments), in a standard three-necked flask 
with a Pt wire as the counter electrode.   The deposited tungsten oxide was solidified by 
annealing in air for 20 min at 200 °C.  This base layer insulated the conductive ITO layer 
from making electrical contact with solution during photoelectrochemical 
characterization.  
Solutions of polystyrene microspheres of a chosen diameter were prepared in Teflon vials 
by diluting 2.6%(w/w) stock aqueous solutions of microspheres (PolySciences Inc.) to 
0.26% with water, to produce~ 4 mL of diluted microsphere solution for each deposition.  
The microwire substrates were placed along the inside of the vial, and were oriented 
vertically with the full deposition area submerged in the microsphere solution.  The vials 
were then placed in a temperature-controlled sand bath and covered to control the 
surrounding airflow.  The vials were submerged in the sand to the initial level of the 
microsphere solution and were held at ~ 55 °C.    
After the solvent had completely evaporated (~24 hr), the film of microspheres was 
observed to have infiltrated the microwire substrate.  Slight variations in the optical 
density were observed along the substrate in locations where temperature changes 
affected the evaporation rate of the solvent.  Hence, the temperature was controlled (55 ± 
3 °C) for all of the samples described herein.  The substrates were removed from the vials 
and excess polystyrene material was scraped from the sides and the back of the substrate.  
The infilling polystyrene colloidal assembly was stabilized by sintering for 1 h at 98 °C, 
slightly below the glass-transition temperature of polystyrene3 .   
To prepare the porous shell layer, the microsphere-patterned substrate was electrically 
contacted to a wire by an alligator clip.  The back and sides of the substrates were again 
painted with nail polish, to prevent deposition on these surfaces.  WO3 was 
potentiostatically electrodeposited, as described above, on the templated substrate until 
the desired charge density had been passed.  The substrates were then annealed at 200 °C 
in air for 20 min to solidify the electrodeposited film.   After cooling to room 
temperature, the substrate was submerged overnight in THF to dissolve the polystyrene 
template.  Finally, the substrates were annealed at 500 °C for 2 h in air (ramp rate = 1 °C 
min-1) to crystallize the WO3 in the monoclinic phase.  Plain core-shell Si/ITO/WO3 
microwire arrays were prepared using nominally the same procedure, except that the 
production of the microsphere template was omitted from the process steps.    
The total amount of electrodeposited WO3 was controlled by limiting the total projected-
areal charge passed during electrodepositon (-1.0 C cm-2 and -2.0 C cm-2), not accounting 
for any additional surface area due to the microwires.  For photoelectrochemical 
experiments, control electrodes of plain core-shell microwire arrays were prepared on 
substrates that had arrays of 15 µm-long p-Si microwires.  WO3 was electrodeposited 
onto these electrodes deposited in two steps, first with a base layer of -0.25 C cm-2, and 
then with a second layer deposited until the desired total charge density (-0.75 C cm-2 or -
1.75 C cm-2) was reached.  Scanning electron microscopy (SEM) images (Zeiss LEO 
1550VP field-emission SEM) indicated that the diameters for these core-shell microwire 
structures were 2.2(± 0.1) µm for -1.0 C cm-2 of total deposition charge density and 
3.8(±0.1) µm for -2.0 C cm-2 of total deposition charge density.  To study the effect of 
patterning the WO3 shell using sidewall microsphere lithography, core-shell microwire 
electrodes were prepared on 15 µm-long p-Si microwires with -1.0 C cm-2 of total charge 
density passed (-0.25 C cm-2 in the base layer, -0.75 C cm-2 in the porous layer), using 
microspheres having diameters of 350 nm, 500 nm, 750 nm, or 1000 nm.  To minimize 
growth-to-growth variations in the Si microwires, all of the electrodes described herein 
were prepared from a single growth of Si microwire arrays. 
Electrodes were prepared from microwire-array substrates by scratching Ga-In eutectic 
into the back of the substrate, and affixing the substrates to a tinned Cu wire with 
conductive Ag paint (SPI Supplies, Inc.).  After the paint had dried, the wires were 
threaded through a glass tube and the substrate was sealed with epoxy (Hysol 9460), 
leaving only the microwire array exposed.  The epoxy was cured overnight at room 
temperature and dried at 60 °C for 2 h.  The area of each electrode was determined by 
scanning the electrode surface with a high-resolution (600dpi) scanner, followed by 
measurement of the active area with the ImageJ software package.   
Photoelectrochemical characterization of core-shell Si/ITO/WO3 microwire electrodes 
Photoelectrochemical current density vs. potential (J-E) data were obtained in CH3CN 
solutions that had 100 mM TBA-HSO4 as the supporting electrolyte.  The 
electrochemical cell was a standard three-necked flask that had been modified to include 
a quartz window in the path of illumination.  Under these solution conditions, HSO4- 
anions are photoanodically oxidized to S2O82- at the WO3 surface4. A platinum mesh was 
the counter electrode, and a Ag/Ag+ reference electrode was prepared from a Ag wire in a 
solution of 10 mM AgNO3/10 mM TBA-BF4 in CH3CN.  The light source was a 150 W 
Xe arc lamp (Oriel), mounted with a collimator and an Air-Mass 1.5 G (AM1.5G) filter.   
Spectral response measurements were performed in the same electrolyte and cell 
conditions, but illumination was provided from a 150 W Xe arc lamp (Oriel) that was 
passed through a monochromator (Oriel Cornerstone 260, Newport Corp.) before striking 
the electrochemical cell.  The electrode was held at a fixed potential (1.0 V vs. a Pt wire 
poised at the Nernstian potential of the solution) and the photocurrent was measured with 
a Gamry Series G300 potentiostat.  The illumination was focused to an area smaller than 
the active area of the photoelectrode (<1 mm2) and was chopped at a frequency of 1.0 Hz.  
A lock-in amplifier (SR830, Stanford Research Systems) was used to discriminate the 
chopped photocurrent from the dark current.  The external quantum yield was calculated 
based on the wavelength-dependent photon flux as measured with a calibrated Si 
photodiode (UV-50, OSI Optoelectronics) at the sample position.   
Simulations of Si/WO3 core-shell absorption 
Simulations of the light-absorption profiles in representative porous and plain core-shell 
structures were performed by finite-difference time-domain (FDTD) methods5 using the 
freely available software package MEEP6.  Monochromatic simulations were performed 
by initially running the FDTD simulation until steady state was reached.  The time-
averaged absorption profile was then calculated from a single illumination period 
(1/frequency) of further simulation.  The core and shell structures were assigned 
wavelength-dependent complex index of refraction values for Si7 and WO38, respectively, 
which were interpolated from published, spectroscopically determined data.  Finite values 
for the extinction coefficient for wavelengths corresponding to energies near the band gap 
are not reported in the available data.  However, spectroscopic measurements indicted 
that photocurrent was produced at these wavelengths (c.f., Figure 4e).  To account for 
such behavior, the absorption values at these wavelengths were interpolated in the 
simulation between the last finite entry and a value of zero for wavelengths larger than 
450 nm.  The simulations were performed as a single core-shell microwire structure in a 7 
µm x 7 µm box (voxel size = 20 nm), with periodic boundary conditions in the x and y 
directions to represent an array of structures matching the geometry of the actual 
substrates used in the experiments.  Perfectly matched absorber-layer (PML) boundaries 
were defined on the ends of the simulation box along the z-direction.   
The core structure of the core-shell microwire was modeled as a 10 µm long Si microwire 
(radius = 0.5 µm) with a 0.1 µm thick WO3 base layer.  The shell structure was modeled 
as a cylinder with spherical voids of a single diameter to represent the pores generated by 
the templating microspheres.  Because SEM images indicated that the microsphere 
assemblies were close-packed and highly periodic, the spherical voids were arranged in a 
face-centered cubic lattice, in accord with the behavior of artificial opals grown by 
evaporative self- assembly9.  Spheres that overlapped with the core structure were 
omitted from the lattice, so the core structure was unchanged by the presence of voids.  
The radius of the cylinder that represented the shell structure was chosen so that the 
volume of the porous structure was equal to the volume of a plain core-shell microwire 
having a radius of 1.3 µm.  This radius was nearly 2 µm for all chosen diameters of the 
microspheres, except for the 750 nm porous core-shell microwire.  Compared to spheres 
of other diameters, the omission condition of the definition of the lattice of void spheres 
in the simulation resulted in a longer distance of closest approach to the microwire core 
for 750 nm diameter spheres.  This condition resulted in a larger volume of untemplated 
WO3 near the center of the 750 nm case compared to the other porous structures, and an 
overall smaller diameter for the cylinder representing the shell structure required to 
satisfy the equal volume constraint of the simulation geometry.  Table SI.1 contains a 
table of the chosen radii and volumes for each geometry. 
	    
 	  Template	  sphere	  diameter	   radius(um)	   volume(um3)	  None	  (equal	  volume	  case)	   1.3	   61.58	  None	  (equal	  radius	  case)	   2.065	   175.7	  350nm	   2.079	   61.88	  500nm	   2.063	   61.58	  750nm	   1.837	   61.13	  1000nm	   2.064	   61.63	  	  Table	  SI.1	  -­‐	  Chosen	  radius	  and	  calculated	  volume	  for	  plain	  and	  porous	  core-­‐shell	  structures	  used	  in	  FDTD	  simulations.	   	  
	  	  	  Figure	  SI.1	  -­‐	  SEM	  image	  of	  a	  Si	  microwire	  array	  patterned	  with	  a	  continuous	  assembly	  of	  polystyrene	  microspheres.	  	  The	  regularly	  ordered	  white	  dots	  in	  the	  image	  are	  the	  tips	  of	  the	  microwires	  	  near	  the	  top	  of	  the	  template.	  	  Microwire	  substrates	  can	  be	  patterned	  with	  this	  uniformity	  over	  all	  of	  the	  substrates	  sizes	  tested	  here,	  up	  to	  2cm2.	   	  
	  	  Figure	  SI.2	  -­‐	  An	  SEM	  image	  of	  microsphere	  templated	  porosity	  on	  a	  Si/ITO/WO3	  microwire	  array.	  	  This	  image	  represents	  the	  maximum	  field	  of	  view	  of	  the	  SEM,	  but	  patterning	  with	  this	  fidelity	  was	  observed	  throughout	  the	  entire	  sample	  (1cm2).
	  	  Figure	  SI.3	  -­‐	  Integrated	  depth	  profiles	  of	  fractional	  absorption	  in	  the	  WO3	  shell	  of	  a	  Si/WO3	  core-­‐shell	  microwire	  as	  calculated	  from	  FDTD	  simulations.	  	  For	  plain	  core-­‐shell	  equal	  volume	  and	  equal	  radius	  electrodes,	  a	  significant	  fraction	  of	  illumination	  is	  absorbed	  at	  depths	  much	  larger	  than	  the	  minority	  carrier	  diffusion	  length	  (Lp=500nm,	  dashed	  black	  line).	  	  In	  the	  porous	  core-­‐shell	  structures,	  almost	  all	  of	  the	  absorption	  occurs	  at	  depths	  much	  shallower	  than	  Lp	  due	  to	  the	  small	  feature	  size	  of	  the	  inverse	  opal	  structure.	  	  	  	   	  
0 0.5 1 1.5
0.05
0.1
0.15
0.2
0.25
absorption depth (um)
fra
cti
on
al 
ab
so
rp
tio
n
plain (equal volume)
 
 
320nm
330nm
350nm
370nm
380nm
390nm
400nm
410nm
420nm
430nm
440nm
0 0.5 1 1.5 2 2.5
0.05
0.1
0.15
0.2
0.25
absorption depth (um)
fra
cti
on
al 
ab
so
rp
tio
n
plain (equal radius)
 
 
320nm
330nm
350nm
370nm
380nm
390nm
400nm
410nm
420nm
430nm
440nm
0 0.2 0.4 0.6
0.2
0.4
0.6
0.8
1
absorption depth (um)
fra
cti
on
al 
ab
so
rp
tio
n
350nm pores
 
 
320nm
330nm
350nm
370nm
380nm
390nm
400nm
410nm
420nm
430nm
440nm
0 0.2 0.4 0.6
0.2
0.4
0.6
0.8
1
absorption depth (um)
fra
cti
on
al 
ab
so
rp
tio
n
500nm pores
 
 
320nm
330nm
350nm
370nm
380nm
390nm
400nm
410nm
420nm
430nm
440nm
0 0.2 0.4 0.6
0.2
0.4
0.6
0.8
1
absorption depth (um)
fra
cti
on
al 
ab
so
rp
tio
n
750nm pores
 
 
320nm
330nm
350nm
370nm
380nm
390nm
400nm
410nm
420nm
430nm
440nm
0 0.2 0.4 0.6
0.2
0.4
0.6
0.8
1
absorption depth (um)
fra
cti
on
al 
ab
so
rp
tio
n
1000nm pores
 
 
320nm
330nm
350nm
370nm
380nm
390nm
400nm
410nm
420nm
430nm
440nm
	  	  	  Figure	  SI.4	  -­‐	  External	  quantum	  yield	  measurements	  for	  independently	  prepared,	  p-­‐Si/ITO/WO3	  core-­‐shell	  electrodes	  for	  the	  (red)	  -­‐0.25/-­‐0.75	  C	  cm-­‐2,	  750nm	  porous	  core-­‐shell	  electrodes	  and	  (blue)	  -­‐0.25/-­‐1.75	  C	  cm-­‐2	  plain	  core-­‐shell	  electrodes.	  	  	  	   	  
340 360 380 400 420 440 460 4800
0.01
0.02
0.03
0.04
0.05
0.06
h (nm)
Ex
te
rn
al 
qu
an
tu
m
 yi
eld
 
 
750nm porous, #1
750nm porous, #2
plain (2X deposition), #1
plain (2X deposition), #2
References: 
 
1. Kelzenberg, M. D.; Boettcher, S. W.; Petykiewicz, J. A.; Turner-Evans, D. B.; Putnam, 
M. C.; Warren, E. L.; Spurgeon, J. M.; Briggs, R. M.; Lewis, N. S.; Atwater, H. A. Nat Mater 
2010, 9, (3), 239-244. 
2. Coridan, R. H.; Shaner, M.; Wiggenhorn, C.; Brunschwig, B. S.; Lewis, N. S. J. Phys. 
Chem. C 2013, 117, (14), 6949-6957. 
3. Wunderlich, B.; Bodily, D. M.; Kaplan, M. H. Journal of Applied Physics 1964, 35, (1), 
95-102. 
4. Mi, Q.; Coridan, R. H.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S. Energy Environ. 
Sci. 2013, 6, (9), 2646-2653. 
5. Taflove, A.; Hagness, S. C., Computational Electrodynamics: The Finite-Difference 
Time-Domain Method. 3rd ed.; Artech House: 2005. 
6. Oskooi, A. F.; Roundy, D.; Ibanescu, M.; Bermel, P.; Joannopoulos, J. D.; Johnson, S. G. 
Computer Physics Communications 2010, 181, (3), 687-702. 
7. Aspnes, D. E.; Studna, A. A. Phys. Rev. B 1983, 27, (2), 985-1009. 
8. von Rottkay, K.; Rubin, M.; Wen, S. J. Thin Solid Films 1997, 306, (1), 10-16. 
9. Míguez, H.; López, C.; Meseguer, F.; Blanco, A.; Vázquez, L.; Mayoral, R.; Ocaña, M.; 
Fornés, V.; Mifsud, A. Applied Physics Letters 1997, 71, (9). 	  


Photoelectrochemical Behavior of Hierarchically Structured Si/WO_3 Core–Shell Tandem Photoanodes

https://authors.library.caltech.edu/45238/7/nl404623t_si_001.pdf

Photoelectrochemical Behavior of Hierarchically Structured Si/WO_3 Core–Shell Tandem Photoanodes

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main