On-chip assembly of 3D graphene-based aerogels for chemiresistive gas sensing [original]

450 |Chem. Commun., 2020, 56, 450--453 This journal is ©The Royal Society of Chemistry 2020

Cite this: Chem. Commun., 2020,

56,450

On-chip assembly of 3D graphene-based aerogels

for chemiresistive gas sensing†

Gaofeng Shao, * Oleksandr Ovsianytskyi, Maged F. Bekheet and

Aleksander Gurlo

Integration of the material preparation step into the device fabrication

process is of prime importance for the development of high perfor-

mance devices. This study presents an innovative strategy for the in situ

assembly of graphene-based aerogels on a chip by polymerization–

reduction and annealing processes, which are applied as chemiresistive

gas sensors for the detection of NO

Graphene aerogels represent an emerging category of porous

conductive materials with hierarchical pore structures and high

specific surface area, which have received enormous interest in

energy storage and conversion,

environmental remediation,

catalysis

and gas sensors.

The adsorption of gas molecules on

graphene’s surface leads to changes in its electrical conductivity

that can be attributed to the change in the local charge carrier

density induced by the surface adsorbates which act as electron

donors or acceptors.

Besides the adsorption onto low-energy bind-

ing sites (sp

-bonded carbon), that onto higher-energy binding sites

such as vacancies, defects, and functional groups on the surface of

graphene leads to an improved change in the conductance of

graphene-based sensors.

Furthermore, assembly of 2D graphene

sheetsinto3Dhydrogel/aerogelstructuresisapromisingwayfor

enhancing sensing performance by maintaining a high surface

area in an accessible porous network.

Up to now, most eﬀorts have

been ongoing to focus on chemical modifications

7c,8

and functiona-

lization of graphene-based aerogels, such as those decorated with

metal oxides and transition metal sulphides.

7b,9

However, up to now the fabrication of sensors based on 3D

graphene aerogels was achieved in two steps, in that the materials’

synthesis was followed by the deposition step.

7a,b,9,10

Current

strategies involving drop-casting or coating a suspension with

graphene aerogel powders onto the platform fail to control their

deposition, leading to variable morphology, uncontrolled coverage,

loss of surface area from aggregation, and poor electrical contact

with the sensing electrodes.

8a,11

Direct integration of 3D graphene-

based aerogels on a chip represents a significant challenge that

requires integration of the materials’ preparation/synthesis step

into the fabrication of sensor devices. These strategies for in situ

growth greatly simplify and strengthen the integration of the

sensing material with the microheater platform, maximizing

sensing performance, avoiding also issues of nanomaterial aggre-

gation and poor electrical contact that conventional methods of

materials’ integration face.

Here we report on the fabrication of chemiresistive gas

sensors through the localized on-chip assembly of 3D inter-

connected porous polypyrrole (PPy) coupled graphene/W

nanowire aerogels. This was achieved with the redox reaction

between graphene oxide (GO) and pyrrole (Py), which resulted

in reduction of GO and polymerization of Py spontaneously

without the addition of any other oxidizing or reducing agents.

To further enhance the sensitivity of aerogel sensors to oxidiz-

ing gases such as NO

nanowires were integrated into

reduced graphene oxide (rGO) aerogels during the assembly

process. As demonstrated previously, W

nanowires pos-

sess good sensing response to low concentrations of NO

These graphene based ternary aerogels facilitate gas adsorp-

tion by functioning as active gas adsorption sites, resulting

in enhanced gas sensitivity, indicating their high potential

for gas sensors. As a proof of concept, high-performance gas

sensors based on graphene aerogels were fabricated and

studied.

nanowires are synthesized via a mild solvothermal

method. Fig. 1a shows the XRD pattern of the as-prepared

nanowires. The narrow (010) and (020) reflections indi-

cate that the crystal growth direction of W

nanowires is

[010].

The high-resolution W4f spectrum can be deconvoluted

into two doublets, which correspond to W

and W

oxidation

states (Fig. 1b). The dominant peaks at 530.7 eV, 531.5 eV and

533.2 eV in the O 1s XPS spectra (Fig. 1c) represent the oxygen

bond of W–O–W, oxygen vacancies and the adsorbed oxygen

species,

respectively.

Fachgebiet Keramische Werkstoﬀe/Chair of Advanced Ceramic Materials,

Technische Universita

¨t Berlin, Hardenbergstr. 40, 10623, Berlin, Germany.

E-mail: Gaofeng.[email protected], [email protected].cn

†Electronic supplementary information (ESI) available. See DOI: 10.1039/

c9cc09092d

Received 22nd November 2019,

Accepted 29th November 2019

DOI: 10.1039/c9cc09092d

rsc.li/chemcomm

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As shown in Fig. 1d and e, the as-grown W

is comprised

of entangled nanowires with an average length of several micro-

meters and width of 10–20 nm. The wires are single-crystalline in

nature, confirmed by HRTEM (Fig. 1f). Clear lattice fringes with a

d-spacing of 0.38 nm can be discerned, corresponding to the

(010) planes of monoclinic W

This implies preferential

growth in the [010] direction, consistent with XRD observations.

Fig. 2 illustrates an in situ fabrication process of PPy coupled

graphene/W

nanowire aerogels (PGWAs) on a chip (a Al

substrate integrated with a Pt electrode on the positive side and a

Pt heater on the negative side). First, the substrate was hydroxyl

functionalized by Piranha solution in order to increase the bond-

ing strength between GO based solution and the substrate with

hydrogen bonding. GO aqueous suspension and pre-synthesized

nanowires were mixed together by ultrasonication, and

then a reducing agent, a conductive polymer monomer, Py, was

introduced (Fig. S2, ESI†). After that, W

nanowire containing

Py/GO suspension was dropped on the chip by a syringe with a

fine needle, followed by a polymerization–reduction process to

functionalize it and initiate sheet assembly into a 3D network.

After freeze-drying, the solvent was removed from the hydrogel to

produce polypyrrole coupled reduced graphene oxide/W

nanowire aerogels (PrGOWAs). After the further annealing process,

PGWAs were successfully integrated on a chip.

The direct on chip deposition is explained as follows. The GO

sheet is hydrophilic mainly due to the ionizable –COOH groups

at edges, while its basal plane is essentially hydrophobic due to a

network of polyaromatic islands of unoxidized benzene rings.

Therefore, the GO sheet can be considered as a surfactant with a

largely hydrophobic basal plane and hydrophilic edge, and

as a result, it would have the capability to disperse other nano-

materials in water homogeneously.

In this case, the GO sheets

support and prevent insoluble W

nanowires from precipita-

tion, forming stable suspensions. The Py monomer has a typical

conjugated structure with an electron-rich N atom, which may

easily attach on the surfaces and galleries of GO sheets through

hydrogen bonding, p–pstacking and electrostatic interaction,

coupling W

nanowires on graphene sheets simultaneously.

Therefore, the existence of Py will effectively prevent the self-

stacked behavior of GO during the reducing process, and

accordingly increase the available GO sheets for forming a

large volume of 3D graphene aerogels. A further annealing

process was utilized for further reduction of GO and partial

degradation of PPy, improving electrical contact and providing

more chemisorption sites to enhance the response.

Fig. 3(a–d) show the SEM images of the PGWA on the Al

substrate interdigitated Pt electrodes. They present the features

of tight adhesion between the PGWA and the device (Fig. 3(a))

and a highly porous structure with interconnected pores ranging

from tens to hundreds of micrometers (Fig. 3(b)), which endows

the sensing materials with low electrical resistance (Fig. S3, ESI†)

and a high specific surface area of 396 m

1

(Fig. S4, ESI†). In

contrast to the smooth surface of graphene sheets, the W

nanowires are randomly decorated on the micrometer sized

graphene sheet walls of the network (Fig. 3(c–e)), which are

coupled by a PPy thin layer in the PGWA. Furthermore, the

structure could be vividly described as a leaf tissue-like structure:

PPy is the epi-dermis, W

nanowires are the leaf veins and

graphene sheets are the mesophyll. For bare N-doped graphene

aerogels, even when the annealing temperature reaches 900 1C,

the aerogels still show robust adhesion on the device with typical

3D interconnected porous structures (Fig. S5, ESI†). It is evident

that this method is suitable for in situ assembly heteroatom-doped/

defective graphene aerogels and nanomaterial functionalized

graphene aerogels, which need post-treatment processes, such

as high temperature annealing or plasma treatment.

Fig. 1 (a) XRD pattern of W

nanowires, (b and c) W4f and O1s XPS

spectra, (d) SEM and (e) TEM images of W

nanowires, and (f) HRTEM

image of a single W

nanowire.

Fig. 2 Schematic illustration of the in situ fabrication process of 3D

graphene based aerogels on a chip.

Fig. 3 (a–d) SEM images of the PGWA on a chip at different magnifications

and (e) TEM image of the PGWA.

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452 |Chem. Commun., 2020, 56, 450--453 This journal is ©The Royal Society of Chemistry 2020

To confirm the interaction between PPy and graphene after

the polymerization–reduction process, the ATR-FTIR spectra of

GO and graphene based aerogels are shown in Fig. S6 and S7

(ESI†). It can be found that the bonds related to the CQO,

C–OH and C–O groups of GO almost disappear, indicating the

efficient reduction of GO. The strong band around 1564 cm

1

attributed to typical ring vibrations of PPy,

andthatat1196cm

1

is due to the C–N stretching.

The band at 1041 cm

1

corre-

sponds to the C–H band in plane vibrations, suggesting

that PPy was indeed loaded onto the surface of graphene

throughinteractionssuchashydrogenbondingandp–pstacking

between them.

19,21

After the introduction of W

nanowires,

peaks at 787 and 715 cm

1

are observed, which are assigned to

O–W–O stretching modes. The bands centered at 655 and

585 cm

1

are attributed to the W–O–W stretching modes.

The

partial characteristic peaks assigned to C–H in-plane and out-of-

plane vibrations at 1041 cm

1

and 916 cm

1

of the PPy diminished

after the annealing treatment, and no new characteristic peaks were

observed, showing the degradation of the polymer. Furthermore, the

chemical states of PGWAs are investigated by XPS (Fig. S8, ESI†). The

initial GO is well reduced to graphene by polymerization–

reduction and annealing processes, confirmed by significantly

improving the intensity of sp

C–C bonds of graphene at 284.6 eV

and decreasing the oxygen containing carbon (epoxy C–O at

286.6 eV, carbonyl CQO at 287.6 eV, and carboxyl O–CQOat

288.9 eV), as analyzed from the corresponding deconvolution of

the C1s spectra of the as-prepared aerogels and GO (Fig. S9 and

S10, ESI†). It should be noticed that the PGWA exhibits a nitrogen

contentof4.26at%afterannealing(TableS1,ESI†). Besides, the

high resolution N1s spectrum of the PGWA could be fitted well

with three different signals corresponding to pyridinic N, pyrrolic

N, and graphitic N, respectively (Fig. S11, ESI†), which further

confirms PPy functionalization on the surfaces of graphene and N

atoms doped in graphene. Further investigation of the phase

evolution of GO is characterized by XRD. As shown in Fig. S12(a)

(ESI†), the (001) crystal plane of GO was evident, with an interlayer

spacingof8.33Å.Afterthepolymerization–reduction process, the

PGA showed a broad reflection at 2y=261(Fig. S12(b), ESI†),

which is typical of reduced GO materials. The nanowires still

exhibit the feature of a monoclinic crystal phase of W

after

the further annealing process.

Based on all these findings, we can conclude that our fabrica-

tion procedure is suitable for the incorporation of 3D based

graphene aerogels into electronic devices. As a proof-of-concept,

the as-fabricated 3D graphene based aerogels are directly used as

a gas sensor for the detection of low concentrations of NO

gas.

Fig. 4a shows the time dependent response of the graphene

based aerogel sensors to various NO

concentrations, from 200

to 975 ppb, with an operating temperature of 140 1C (Fig. S13,

ESI†), which represents an optimum for the sensor signal and

response/recovery time. At low temperature, the adsorption of

is substantial and continues during gas exposure, while its

desorption is negligible due to the strong bonding between NO

and the PGA as well as the PGWA. The NO

desorption is

enhanced at higher temperature, which speeds up the time to

reach a balance between adsorption and desorption both during

gas exposure and during recovery. Although the response and

recovery times might be even faster at higher temperature, 140 1C

is considered an eﬀective temperature when also considering the

sensor signal.

Upon exposure to NO

at 140 1C, the PGWA sensor presents

higher response (DR/R

= 29.78%) towards 0.975 ppm of NO

gas

compared to the PGA sensor (DR/R

= 18.04%) demonstrating the

remarkably improved response achieved by functionalization

with W

nanowires. Furthermore, to investigate the repeat-

ability and stability of aerogel based sensors, the PGA and PGWA

sensors were exposed to 0.975 ppm of NO

for seven successive

cycles. An average response (DR/R

= 25.4%) of the PGWA sensor

with a small standard deviation of 1.9% is measured (Fig. 4b),

verifying the reliable repeatability of the sensors. Moreover,

compared with the gas sensing performance of graphene-based

aerogels by the traditional deposition method (Fig. S14, ESI†),

the PGWA assembled on a chip not only shows higher response,

but also presents low noise of the sensor signal originating from

its low electrical resistance (Fig. S15, ESI†).

Fig. 4(c) displays sensor response to NO

above 200 ppb,

displaying a nonlinear relationship between the sensor

response (DR/R

) and gas concentration, which is typical for

Fig. 4 (a) Transient sensor response at 140 1C towards diﬀerent NO

con-

centrations. (b) Reproducibility of sensor response exposed to 0.975 ppm NO

at 140 1C. (c) Comparison with reported graphene based aerogels/hydrogels

at optimized temperatures for NO

sensors (a,

11a

10c

10b

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chemiresistive sensors.

Assuming that the gas adsorption is

dominated by high-energy defect sites, at low NO

concentra-

tions when most of the sites are available, the charge transfer is

directly proportional to the gas concentration. At higher NO

concentrations, all sites may eventually be occupied during the

gas exposure, and thus the response reaches a saturation point.

To highlight the high sensing performance of the PGWA sensor

fabricated in this work, the response of reported graphene based

aerogels/hydrogels and two-dimensional metal sulfide aerogels are

summarized in Fig. 4(c), which demonstrated the outstanding

response towards low concentrations of NO

gas.

The sensing performance of the in situ assembled 3D graphene

based aerogels results from: (a) high conductivity of 3D intercon-

nected graphene aerogels necessary for eﬃcient operation of the

sensors, (b) a highly porous structure with a high surface area

that provides a large number of surface active sites necessary for

the reliable detection of low NO

concentrations, and (c) hetero-

junctions between n-type W

nanowires and p-type graphene

as well as p-type PPy which is beneficial for enhancing gas

sensitivity in chemiresistors.

In conclusion, we have successfully in situ assembled 3D

interconnected porous PPy coupled graphene/W

nanowire

aerogels on a chip by a polymerization–reduction route followed

by an annealing process. Py initiates and mediates the assembly

of GO sheets and W

nanowires into a 3D network in a facile,

environment-friendly and scalable route. As a proof of concept,

the as-fabricated 3D graphene based aerogels are applied as gas

sensors for the detection of low concentrations of NO

gas. The

strategy developed here allows for direct integration of graphene

based aerogels into functional devices and opens up new per-

spectives for the fabrication of functional devices.

Conflicts of interest

There are no conflicts to declare.

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