Nano Res

1

Reduced graphene oxide/silicon nanowire

heterostructures with enhanced photoactivity and

superior photoelectrochemical stability

Xing Zhong1,†, Gongming Wang1,†, Benjamin Papandrea1, Mufan Li1, Yuxi Xu1, Yu Chen2, Chih-Yen Chen1,

Hailong Zhou1, Teng Xue2, Yongjia Li2, Dehui Li1 ,Yu Huang2,3 and Xiangfeng Duan1,3 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0790-2

http://www.thenanoresearch.com on April 17, 2015

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-015-0790-2

1

TABLE OF CONTENTS (TOC)

Reduced Graphene Oxide-Silicon Nanowire

Heterostructures with Enhanced

Photoactivity and Superior

Photoelectrochemical Stability

Xing Zhong1,†, Gongming Wang1,†, Benjamin

Papandrea1, Mufan Li1, Yuxi Xu1, Yu Chen2,

Chih-Yen Chen1, Hailong Zhou1, Teng Xue2,

Yongjia Li2, Dehui Li1, Yu Huang2,3 and

Xiangfeng Duan1,3*

1 Department of Chemistry and Biochemistry,

University of California, Los Angeles, Los Angeles,

California, 90095, USA. 2 Department of Materials Science and Engineering,

University of California, Los Angeles, California,

90095, USA. 3 California Nanosystems Institute, University of

California, Los Angeles, California, 90095, USA.

† These authors contributed equally to this work.

Page Numbers.

We report a hybrid photocatalyst by enwrapping silicon nanowires with

reduced graphene oxide sheets to achieve greatly enhanced photoactivity and

superior photoelectrochemical stability.

Provide the authors’ website if possible.

Prof. Xiangfeng Duan, http://xduan.chem.ucla.edu/

2

Reduced Graphene Oxide/Silicon Nanowire Heterostructures

with Enhanced Photoactivity and Superior

Photoelectrochemical Stability

Xing Zhong1,†, Gongming Wang1,†, Benjamin Papandrea1, Mufan Li1, Yuxi Xu1, Yu Chen2, Chih-Yen Chen1, Hailong Zhou1, Teng Xue2, Yongjia Li2, Dehui Li1 ,Yu Huang2,3 and

Xiangfeng Duan1,3 ()

1 Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California, 90095, USA. 2 Department of Materials Science and Engineering, University of California, Los Angeles, California, 90095, USA. 3 California Nanosystems Institute, University of California, Los Angeles, California, 90095, USA.

† These authors contributed equally to this work.

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT Silicon nanowires (SiNWs) have been widely explored as light harvesting antenna in photocatalysts due to

their ability to absorb broad solar spectrum, but are typically limited by poor photoelectrochemical stability.

Here we report the synthesis of reduced graphene oxide-SiNW (rGO-SiNW) heterostructures to achieve

greatly improved photocatalytic activity and stability. The SiNWs were synthesized through a metal-assisted

electroless etching process, and functionalized with reduced graphene oxide (rGO) flakes through a chemical

absorption process. Here the rGO can not only function as a physical protection layer to isolate the SiNWs

from the harsh electrochemical environment, but also serve as a charge mediator to facilitate the charge

separation and transport processes. Furthermore, the rGO may also function as the redox catalysts to ensure

efficient utilization of photo-carriers for the desired chemical reactions. Photocatalytic dye degradation

studies show that the photoactivity of the heterostructures can be significantly enhanced with an initial

activation process and maintained without apparent decay over repeated reaction cycles. Electrochemical

and photochemical studies indicate that the enhanced photoactivity and photostability can be attributed to

the more efficient separation of photoexcited charge carriers in SiNWs, and the reduced self-oxidation on

SiNWs surface during the photocatalytic dye degradation process. The ability to significantly improve the

photocatalytic activity and stability in rGO-SiNW heterostructures can not only open up more opportunities

in the application of silicon based photocatalysts/photoelectrodes for solar energy harvesting, but also

provide new insights in the stabilization of other unstable photocatalytic systems.

KEYWORDS Graphene, Silicon nanowire, Photocatalyst, Photoactivity, Stability

3

1. Introduction

Silicon based materials have garnered considerable

interest due to their unique properties and potential

applications in broad areas including electronics

[1-6], thermoelectrics [7], solar energy harvesting

[8-11], and biotechnology [12, 13]. With a room

temperature band gap of 1.12 eV, silicon promises

efficient solar energy harvesting across the entire

solar spectrum from UV to near infrared (IR) range.

To this end, silicon based photoelectrodes and

photocatalysts have been widely explored for solar

energy harvesting and conversion.

For photoelectrochemical or photocatalytic

processes, nanostructured materials typically

exhibit much better activity than their bulk

counterparts, due to their large surface areas and

short charge carrier diffusion distances [10, 14-17].

Therefore, nanoscale morphology engineering

represents one of the most common strategies to

develop highly efficient photocatalysts. We have

recently developed a metal-assisted electroless wet

chemical etching approach for the synthesis of

single crystal silicon nanowires (SiNWs) in large

quantity and explored their potential for efficient

photocatalysts [18-21]. With excellent optical

absorption, charge transport properties and large

surface area, these SiNWs have been shown to

exhibit excellent photocatalytic activity in the

visible range [18, 19, 22]. However, they are severely

limited by their poor photoelectrochemical stability

and rapid decay of the photoactivity under light

illumination. To this end, there have been

considerable efforts in developing effective

approaches to stabilize photoactivity of SiNWs,

typically using different surface protecting layers [4,

19, 23-26]. For example, Chen et al. have reported

the use of atomic layer deposition method to

deposit a thin TiO2 protection layer (~2 nm) on the

surface of SiNW photoelectrodes to enable excellent

photostability [24]. Qu et al. have designed an

integrated photocatalyst with a SiO2 protecting shell

to insulate SiNWs from direct photoelectrochemical

reactions and ensure excellent photoelectrochemical

stability [26]. These approaches, however, usually

involve complicated processes that are difficult to

scale up for large quantity production [18, 26].

Additionally, the use of an oxide-protecting layer

can usually result in a negative impact on both

charge separation and charge transport properties,

leading to a partially compromised photoactivity.

Therefore, there is a considerable interest in

developing alternative low cost and scalable

strategies for highly stable and active photocatalysts

based on SiNWs.

Graphene, a single atomic layer of honeycomb

lattice of carbon atoms, has recently become the

central focus of material research for both

fundamental studies and its potential applications

in diverse areas [27-34]. The covalently bonded

carbon lattice can exhibit excellent chemical stability

and function as a natural protecting barrier. For

example, it has been shown that graphene can

function as an effective passivation layer to protect

metal surfaces from oxidation [35, 36]. Additionally,

graphene can exhibit excellent electrical transport

properties and facilitate charge separation and

transport in semiconductors and their interfaces

[37-39]. Lastly, single or few layer graphene sheets

exhibit high optical transparency in visible

wavelengths [40], which will not affect the light

absorption of underlying materials. Together, these

combined attributes make graphene an excellent

candidate as a novel protection material and charge

mediating layer for SiNW photocatalysts.

Here, we report a facile and simple solution

phase method to enwrap SiNWs with reduced

graphene oxide (rGO) sheets. We show the

as-prepared rGO-SiNW heterostructures can exhibit

both enhanced photoactivity and remarkable

photostability. The improved photoactivity and

photostability is mainly attributed to the surface

protection of SiNWs by rGO and the more efficient

charge separation induced by the Schottky

junctions at the rGO-SiNW interface.

4

2. Experimental

2.1 Material preparation. The SiNWs in this study

were synthesized from a p-type silicon wafer (10-30

∙cm) through a wet chemical etching method.

Briefly, the silicon pieces were first sonicated in

acetone and isopropyl alcohol and then immersed

into a buffered oxide etchant (BOE) to remove the

native oxide layer. The H-terminated silicon pieces

were coated with Ag using an electroless deposition

process in a solution containing 0.005M AgNO3 and

4.8 M HF for 1 minute at room temperature. After Ag

deposition, the color of the silicon wafer surface turn

from dark to colorful. The resulting silver coated

silicon wafer was further rinsed with de-ionized

water to remove extra silver ions and then

immediately immersed into an etching solution

containing 4.8M HF and 0.3M H2O2 for 40 minutes.

Finally, the Ag nanoparticles were removed from the

nanowires by immersing in concentrated nitric acid

for one hour. The SiNWs were then scratched off the

substrate using a razor blade and collected for

further functionalization.

Graphene oxide sheets were prepared using an

improved Hummers’ method followed by strong

sonication [41]. The as-prepared graphene oxide

sheets were further reduced by a hydrazine solution

to produce the rGO solution for the subsequent

studies. To prepare the rGO-SiNW heterostructures,

1 mg of SiNW powder were dispersed in 3 ml of 2%

(3-Aminopropyl)triethoxysilane (APTES) ethanol

solution, stirred for 2 hrs and then centrifuged and

washed out with ethanol for three times. The APTES

functionalized SiNWs were re-dispersed in 5 ml of

water and mixed with various amount of rGO for 2

hrs under vigorous stirring. The product was finally

centrifuged and washed with water for three times.

2.2 Material characterization. The resulted rGO,

SiNWs and rGO-SiNWs were studied with a

scanning electron microscope (SEM) (JEOL 6700)

with 10 kV of electron acceleration voltage, or a

transmission electron microscope (TEM), high

resolution TEM (HRTEM), energy dispersive X-ray

spectroscopy (EDS) (Phillips CM120 with a 120 kV

operation voltage and FEI Titan with a 300 kV

operation voltage) and Raman spectroscopy with

488 nm laser wavelength (Renishaw 1000).

2.3 Photoactivity, electrochemical and

photoelectrochemical studies. Photocatalytic dye

degradation reactions were studied with SiNWs or

rGO-SiNWs in 100 mM of Indigo carmine (IC)

aqueous solution under a 300 W xenon lamp. The

rGO and SiNWs weight ratio in this experiment is

1:4. The IC degradation was monitored by a

Beckman DU-800 UV-vis spectrophotometer. All

photocatalytic reactions were carried out under

ambient conditions. Electrochemical and

photoelectrochemical studies were carried out using

a Princeton Applied Research electrochemical

workstation.

3. Results and discussion

3.1 Structure of rGO-SiNWs. Figure 1a shows the

cross sectional SEM image of the etched SiNWs on a

p-type silicon wafer. It can be clearly seen that

SiNWs with a length of around 20 µm are produced

perpendicular to the substrate. The resulting SiNWs

can be removed from the substrate with a razor

blade and dispersed in proper solution for further

characterization, functionalization and

photocatalytic studies. Transmission electron

microscopy (TEM) studies show that the

as-prepared SiNWs typically exhibit diameters on

the order of 100 nm (Fig. 1b).

Before conjugating with rGO, the surface of

SiNWs was functionalized with APTES to render a

positive surface charge. The positively charged

SiNWs can then be coupled with negatively

charged rGO through the electrostatic interactions.

The TEM images of an rGO functionalized SiNW

clearly show that numerous pieces of rGO sheets

are closely wrapped on the surface of SiNWs (Fig.

1c, d), forming a physical diffusion barrier between

the silicon surface and the electrochemical

5

Figure 1 (a) Cross sectional SEM image of p-type SiNWs. The scar bar is 5 µm. (b) TEM image of bare SiNWs, the scale bar is 50

nm. (c) TEM image (d) HRTEM image of SiNWs functionalized with rGO sheets. The scar bars are 100 nm and 30 nm.

environment. Raman spectra collected from

rGO-SiNWs samples also show well-defined D and

G bands for rGO, indicating the formation of

rGO-SiNW conjugate and there is no obvious

change of Raman characteristics for rGO after

coupling with SiNW (Fig. S-1 in Electronic

Supplementary Material (ESM)).

3.2 Photocatalytic activity and stability. To verify

our hypothesis that rGO could improve both the

photocatalytic activity and stability of SiNWs, we

have conducted photocatalytic dye degradation

studies using bare SiNWs and rGO-SiNWs as the

photocatalysts and compared their activity and

stability. The IC degradation process was monitored

by a UV-vis spectrophotometer as a function of

reaction time. Figure 2a displays the first three

photocatalytic cycles of bare SiNWs and

rGO-SiNWs. Overall, the bare SiNWs show rapid

degradation behavior in the first 45 minutes of the

first cycle; however, the degradation rate

substantially slows down after the first 45 minutes.

With the increasing test cycles, the photocatalytic

performance is continuously degraded, suggesting

the decreased photoactivity is not recoverable in

new testing cycles. These studies are consistent with

poor photocatalytic stability of bare SiNWs

previously reported. On the other hand, the rGO

-SiNW heterostructures show a slightly lower dye

degradation photoactivity at the beginning of each

cycle (i.e. with an initial activation stage, which will

be discussed further later), but with an overall

superior photoactivity and photostability in

repeated IC dye degradation cycles. As a control,

rGO by itself has also been tested for dye

degradation study and shows quite poor activity

(Fig. S-2 in ESM). We have also compared the

overall photocatalytic activity over multiple cycles

of dye degradation reactions (Figure 2b). It is

evident that the rGO-SiNWs exhibit significantly

improved photostability when compared with bare

SiNWs. For bare SiNWs, only less than 20% of

initial photoactivity is maintained after 5 cycles; in

contrast, rGO-SiNWs do not show any obvious

6

Figure 2 (a) The comparison of the IC degradation catalyzed by the SiNWs (black square) and rGO-SiNWs (red dot) for the first

three cycles. (b) The photocatalytic stability of the SiNWs (black square) and rGO-SiNWs (red dot). The photoactivity of the both

photocatalysts is represented by the percentage of IC degraded at the reaction time of 105 minutes for each cycle.

Figure 3 (a). IC degradation catalyzed by functionalized rGO-SiNWs (red dot), physical mixture of rGO and SiNWs (blue square). (b)

Cyclic voltammogram curves of bare SiNWs (black), rGO (green), rGO-SiNWs (red) and physical mixture of rGO and SiNWs (blue)

in 0.5M Na2SO4 aqueous solution at a scan rate of 100 mV/s. (c) Continuous IC degradation catalyzed by the rGO-SiNWs.

Calculated amount of IC solution was added every 10 minutes after a total initial reaction time of 90 minutes.

decay in photocatalytic activity even after 10 cycles,

indicating rGO functionalization is an effective

method to stabilize the photoactivity of SiNWs.

To further understand the role of rGO in the

heterostructure photocatalysts, we have prepared a

physical mixture of rGO and SiNWs as a control

sample (Fig. S-3 in ESM) and investigated its

photocatalytic properties. Figure 3a shows that the

functionalized rGO-SiNWs and mechanically mixed

rGO/SiNWs (with the same SiNWs/rGO ratio)

exhibit very similar activation process and

photoactivity at the beginning; however, the IC dye

degradation rate of the mechanically mixed

rGO-SiNWs slows down shortly after the activation

process, which is different from the functionalized

rGO-SiNWs (without apparent decay). This decay

of photocatalytic activity in the physical mixture of

rGO and SiNWs may be attributed to the weak

interaction between rGO and SiNWs and the poor

protection of the SiNWs by the rGO.

It is noted that both mechanically mixed and

functionalized rGO-SiNWs show a similar

activation process, indicating that rGO is the key

factor responsible for the initial activation process.

Considering the large surface area of rGO, we

hypothesize the activation process is due to the

light driven internal charging effect [42]. Under

light irradiation, a large fraction of photogenerated

electron-hole pairs in SiNWs at the beginning are

used to charge rGO sheet, instead of the

photoelectrochemical process in solution. After the

initial charging process and when rGO is fully

charged, the photocatalytic activity shows an

apparent acceleration, as all the photogenerated

7

charges are used in the photocatalytic dye

degradation process from this point on. As a result,

the activation time in the photocatalytic process can

be understood by recognizing that the photoexcited

electron/hole pairs are not used for IC dye

degradation at the beginning but internal charging,

until the rGO based electrochemical capacitors are

fully charged.

To further understand the charging process, we

have conducted cyclic voltammogram (CV) of the

same amount of bare SiNWs, rGO, functionalized

rGO-SiNWs and physical mixture of rGO and

SiNWs in 0.5M Na2SO4 aqueous solution with a

scan rate of 100 mV/s (Fig. 3b). The significantly

increased rectangular CV area in the sample with

rGO indicates that the rGO-SiNWs has a more

pronounced charge storage capability compared to

bare SiNWs, resulting in a longer activation time. To

minimize the impact of the activation process on the

overall photocatalytic performance, we have further

performed the cycling experiment by repeatedly

adding a fixed amount of IC dye into the reaction

system at the end of each cycle, without

centrifuging out the photocatalysts. Significantly, no

activation stage is observed in the subsequent

reaction cycles since the electrochemical capacitor in

rGO-SiNW heterostructures has been already fully

charged during the first cycle (Fig. 3c). More

importantly, with the exclusion of the initial

charging process, the absolute photocatalytic

activity of rGO-SiNWs is at least one order of

magnitude higher than that of bare SiNWs (>90%

degradation in 10 minutes for rGO-SiNWs and

<70% degradation in 100 minutes for bare SiNWs).

The significantly enhanced photocatalytic

activity in rGO-SiNWs may be attributed to the

improved charge separation and transport at

rGO-SiNW interface, or the improved charge

injection efficiency from rGO to the solution (e.g.

the catalytic effect of rGO). To probe these

possibilities, we conducted systematic

electrochemical and photoelectrochemical studies.

Figure 4 (a) Cyclic voltammogram of bare SiNWs and rGO-SiNWs in 0.5 M Na2SO4 aqueous solution under N2 and O2 condition.

(b) Linear sweep of SiNWs and rGO-SiNWs in 0.5M Na2SO4 aqueous solution with IC dye under dark condition and light

illumination. (c) Photocurrent response of bare SiNWs (black curve) and rGO-SiNWs (red curve) at the potential of -1.5V vs.

Ag/AgCl under chopped light illumination. (d) Photocurrent response of bare SiNWs (black curve) and rGO-SiNWs (red curve) at a

fixed potential of 1V vs. Ag/AgCl under chopped light illumination.

8

As one of the photocatalytic reactions involves

oxygen reduction that generates reactive oxygen

species (ROS), we first studied the role of rGO for

oxygen reduction reaction. Figure 4a shows the

cyclic voltammogram of same amount of bare

SiNWs and rGO-SiNWs in 0.5M Na2SO4 solution

with/without oxygen. These plots clearly show that

rGO-SiNWs exhibit more pronounced oxygen

reduction activity than bare SiNWs, suggesting rGO

could facilitate oxygen reduction during the

photocatalytic process and thus improve the

photoactivity of the SiNWs.

By photoelectrochemical method, we have also

studied the separation of photoexcited charges at

the interface between rGO and SiNWs. Figure 4b

shows the linear sweep of bare SiNWs and

rGO-SiNWs in 0.5M Na2SO4 solution with IC dye at

the scan rate of 50 mV/s, under dark condition and

light illumination. As expected, both SiNWs and

rGO-SiNWs show cathodic photocurrents and the

rGO modification could enhance the photocurrent

density of SiNWs in the negative potential region.

Figure 4c shows the chopped cathodic

photocurrents of SiNWs and rGO-SiNWs at the

fixed potential of -1.5 V vs. Ag/AgCl. It also clearly

shows the photoresponse of rGO-SiNWs is much

higher than that of bare SiNWs, which is consistent

with the linear sweeps in Figure 4b. The increased

photocurrent is attributed to the more efficient

electron injection from silicon to rGO. A longer time

test with cathodic photocurrent of rGO-SiNWs

(Figure S-4 in ESM) further demonstrates the

photoelectrochemical stability of rGO-SiNWs. On

the other hand, in order to study the

hole-separation in rGO-SiNW interface, we applied

a reversible (positive) bias on silicon

photoelectrodes to tune the p-type property to

n-type behavior. As we can see in Figure 4d, both

bare SiNWs and rGO-SiNWs show anodic

photocurrent response with applying a positive

potential of 1.0 V vs. Ag/AgCl. Interestingly, the

rGO-SiNWs sample also shows higher photocurrent

density than that of bare SiNWs, suggesting rGO

can also facilitate the hole transport at the

rGO-SiNW interface.

Together, our studies have demonstrated

that rGO cannot only facilitate the electron injection,

but also hole injection process, which might be

attributed to the incomplete reduction and

heterogeneous nature of rGO (conjugated graphene

domains with oxygenic defects in between). These

different domains on rGO may form different types

of Schottky junctions with SiNWs to either facilitate

the electron or hole injection processes. For the

region with high degree of reduction, rGO would

be more like graphene and have a work function

around 4.5 eV [43], which can form a p-n junction

with p-type SiNWs with rGO facilitating the

electron separation and transport to solution. On

the other hand, it has been previously suggested the

less reduced region of rGO with oxygenic defects

could exhibit work function of 4.9 eV, more like

graphene oxide [44, 45], to align well with the

valence band of SiNWs. These parts of rGO could

facilitate the hole transport process. In this way, the

conjugation of rGO can not only function as an

effective protection layer to isolate the SiNW from

direction electrochemical process to ensure

excellent photostability, but also enhance the

electron hole separation and transport process to

greatly improve the photochemical activity.

Lastly, we have conducted TEM study on bare

SiNWs and rGO-SiNWs after photocatalytic testing.

Figure 5a,b show that a 5-10 nm amorphous SiOx

shell can be clearly seen on the SiNW surface after

photocatalytic testing. EDS studies (Fig. 5c) further

confirm that the apparent amorphous shell around

the SiNW consists of SiOx, suggesting the SiNW has

been severely oxidized during photocatalytic

process. In contrast, the SiNWs are well protected in

rGO-SiNW heterostructure by the enwrapping rGO

sheets. The rGO-SiNW interface remains clean

9

Figure 5 (a,b) TEM and selected area HRTEM images of the bare SiNW after photocatalytic reaction. The arrows indicate the

formation of silicon oxide shell. The scar bars are 100 nm and 5 nm, respectively. (c) Normalized energy dispersive X-ray spectra of

point A and B shown in (a), indicating the apparently amorphous shell primarily consists of SiOx. (d,e) TEM and selected area

HRTEM images of the SiNW functionalized with rGO after photocatalytic reactions. The arrows indicate the rGO on SiNW surface.

The scar bars are 100 nm and 5 nm respectively. (f) Normalized energy dispersive X-ray spectra of point A and B shown in (d),

indicating the primary presence of carbon outsize the crystalline silicon.

without obvious SiOx formation after the

photocatalytic process (Fig. 5d,e). The EDS studies

(Fig. 5f) further confirm that the amorphous layer

around the SiNW is not SiOx but the rGO

enwrapping the SiNW.

4. Conclusion

In summary, we have developed a simple and

effective strategy to protect SiNWs from oxidation

by functionalizing them with thin layer rGO sheets

and used them as highly efficient and stable

photocatalysts for dye degradation. Photocatalytic

studies demonstrate that the rGO enwrapped

SiNWs exhibit superior stability and photoactivity,

compared with the unprotected SiNWs.

Electrochemical and photoelectrochemical studies

suggest that the enhanced photoactivity and

photostability can be attributed to the physical

protection of SiNWs from direct photochemical

processes and the more efficient separation and

transport of photo-excited charges at rGO-SiNW

interface. The preparation of stable and efficient

rGO-SiNW photocatalysts could have enormous

impacts on the design and application of silicon

based materials for visible-light photocatalysis and

photoelectrochemical fuel generation.

Acknowledgements

We acknowledge the support from the U.S.

Department of Energy, Office of Basic Energy

Sciences, Division of Materials Science and

Engineering through Award DE-SC0008055. We

acknowledge Electron Imaging Center for

Nanomachines (EICN) at UCLA for the support of

TEM, supported with funding from NIH-NCRR

shared resources Grant (CJX1-443835-WS-29646)

and NSF Major Research Instrumentation Grant

(CHE-0722519).

Electronic Supplementary Material:

Supplementary material (Raman spectroscopy

10

measurement, TEM image of mechanical mixing

rGO/SiNWs, IC degradation catalyzed by rGO, and

long-time photocurrent stability test) is available in

the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher).

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12

Electronic Supplementary Material

Reduced Graphene Oxide/Silicon Nanowire Heterostructures

with Enhanced Photoactivity and Superior

Photoelectrochemical Stability

Xing Zhong1,†, Gongming Wang1,†, Benjamin Papandrea1, Mufan Li1, Yuxi Xu1, Yu Chen2, Chih-Yen Chen1, Hailong Zhou1, Teng Xue2, Yongjia Li2, Dehui Li1 ,Yu Huang2,3 and

Xiangfeng Duan1,3 ()

1 Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California, 90095, USA. 2 Department of Materials Science and Engineering, University of California, Los Angeles, California, 90095, USA. 3 California Nanosystems Institute, University of California, Los Angeles, California, 90095, USA.

† These authors contributed equally to this work.

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

Figure S-1 Raman spectra of rGO and rGO-SiNWs. The broad peak located at around 970nm is originated from silicon, due to

2TO-photon overtone scattering from the critical point L. D and G bands are from graphene.

Figure S-2 IC degradation catalyzed by rGO itself.

13

Figure S-3 TEM image of mechanical mixture of rGO and SiNWs, showing poor adhesion between rGO and SiNWs. The scale bar is

500 nm.

Figure S-4 Photocurrent stability test of rGO-SiNWs at the potential of -1.5V vs. Ag/AgCl under chopped light illumination for 1

hour.