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Sensors and Actuators B 185 (2013) 1– 9

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locat e/snb

oom temperature impedance spectroscopy-based sensing of formaldehyde withorous TiO2 under UV illumination

ipeng Liua, Xiaogan Lia,∗, Prabir K. Duttab, Jing Wanga

School of Electronic Science and Technology, Institute for Sensing Technologies, Key Laboratory of Liaoning for Integrated Circuits Technology, Dalian University of Technology,alian 116024, ChinaDepartment of Chemistry, The Ohio State University, 120 W 18th Avenue, Columbus, OH 43210, United States

a r t i c l e i n f o

rticle history:eceived 28 November 2012eceived in revised form 14 April 2013ccepted 19 April 2013vailable online 28 April 2013

eywords:

a b s t r a c t

The room temperature sensing of formaldehyde with macroporous TiO2 based semiconductor-type sen-sor in the presence of humidity and UV irradiation was investigated using impedance spectroscopy. Thenormalized change in impedance in the presence of the formaldehyde was used to calculate the sen-sor response. A linear increase in sensor response to concentrations of formaldehyde in the range from10 ppm to 50 ppm was observed. Significant enhancement of the sensor response was observed in thepresence of both UV illumination and low relative humidity (∼6%). It is proposed that the role of UV

acroporous TiO2

V illuminationormaldehydeas sensors

illumination was to generate hydroxyl groups from the adsorbed water on the titania, and these activehydroxyl groups reacted with formaldehyde to enhance the sensor response. The a.c. impedance-typesignals with formaldehyde were more stable than the conventional multimeter-based d.c. resistancechanges. A possible explanation for the more stable response under a.c. conditions is that the poten-tial barrier due to the build-up of the dissociated protons from water adsorbed on the oxide surface is

der t

effectively suppressed un

. Introduction

Formaldehyde (HCHO) is a colorless, pungent-smelling vapornd a possible carcinogen. It can cause central nervous systemamage, immune system disorders, as well as blindness and respi-atory diseases [1–3]. Thus, there is a need for portable, inexpensiveas detectors at home and work place for real-time monitoringf formaldehyde, so that precautions can be taken in the pres-nce of this harmful gas. Metal oxide semiconductor gas sensorso detect toxic gas species are attractive due to their simple work-ng principle, high sensitivity, portability and low cost [3–9]. Theseensors exhibit sensitivity of ppm or sub-ppm levels to formalde-yde and usually operate around 200–400 ◦C, thus necessitating

heat source [3–9]. Recent efforts have focused on modifyinghe sensing materials and detection strategies to lower operationemperatures of the sensors. These strategies have included utiliza-ion of nanosized particles or porous nanostructural architecturesnd/or resorting to UV–vis illumination during the sensor operation10–38].

UV illumination with wavelengths shorter than the bandgapnergy of oxides leads to formation of electron–hole pairs, whichan activate adsorbed oxygen enhancing their reactivity with

∗ Corresponding author. Tel.: +86 41184706710.E-mail addresses: lixg@dlut.edu.cn (X. Li), dutta.1@osu.edu (P.K. Dutta).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.04.090

he a.c. condition.© 2013 Elsevier B.V. All rights reserved.

analyte gas [19–38]. Prades et al. demonstrated the equiva-lence between thermal and room-temperature UV photoassistedresponses of single SnO2 nanowire sensors [34,35]. Lu et al. inves-tigated room temperature NO2 sensing properties of the compositeoxide based sensors and proposed that better charge separation inthe ZnO/SnO2 heterojunction under UV illumination contributed tohigher response of the sensor [37]. Peng et al. investigated the roomtemperature formaldehyde sensing using ZnO nanorods operatedunder UV illumination [28,29]. Besides, UV illumination has alsobeen investigated for improving selectivity of metal oxide gas sen-sors [19,30].

For chemical gas sensors, mesoporous metal oxides (pore size:2–50 nm) have potentially high sensitivity to targeted gases due totheir larger specific surface area [39–44]. However, sintering of themesoporous oxides during operation in the temperature range of200–400 ◦C is an issue. Macroporous materials (pore size: >50 nm)synthesized using template methods have a larger pore structure(∼100 nm), and better thermal stability is expected, which shouldlead to long term stability [39,44].

Porous TiO2 has been studied for low-temperature chemicalgas sensing [13–15,18,40–42,45–51]. Synthesized by the modi-fied sol–gel method, mesoporous TiO2 showed higher sensitivity

to hydrogen and CO relative to TiO2 powders [40–42]. Orderedmacroporous TiO2 nanotube arrays have been explored for sensingvarious gases and humidity [13–15,18,40–42,45–51]. This type ofordered macroporous TiO2 exhibited high response to hydrogen

2 nd Actuators B 185 (2013) 1– 9

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L. Liu et al. / Sensors a

ven at room temperatures [46]. In presence of humidity, formalde-yde sensing with macroporous TiO2 nanotube array at lowemperatures has been reported [18].

In this paper, we focus on room temperature sensing oformaldehyde with macroporous TiO2 based thick-film type sen-ors in humid air under UV illumination using a.c. impedancepectroscopy. Changes in the impedance spectra, which serve ashe sensor response, were correlated with the interactions amonghe gas analyte, water vapor and surface of the TiO2 nanoparticlesnder UV illumination.

. Experimental

.1. Materials preparation and microstructure characterization

The two-step process for synthesis of the nanotube-basediO2was reported previously [51]. In brief, the highly orderedniform TiO2 nanotube array was grown by conventional electro-hemical anodization of a Ti metal sheet. The calcined nanotuberrays were then ground manually to obtain TiO2 nanoparticlesith residual nanotube fragments.

The crystal phases of the fabricated samples were analyzedsing a powder X-ray diffractometer (XRD: D8 Advance, Bruker-XS, Germany). The XRD patterns were collected using Ni-filteredu K� radiation at 40 kV and 25 mA between 2� of 15◦ and 65◦ at

scanning speed of 12◦/min. The surface morphology of the nano-ubes was examined using a scanning electron microscope (SEM:-3000 N, Hitachi, Japan) on gold-coated specimens. Close exam-nation of the microstructure of the samples was further carriedut by transmission electron microscopy (TEM: JEOL 2100, Japan).igh-resolution X-ray photoelectron spectroscopy (XPS) studiesere performed with a Kratos AXIS Ultra X-ray photoelectron spec-

rometer. The specific surface area was also measured by the BETethod with a TriStar 3000 gas adsorption analyser using N2 as the

dsorbent.

.2. Sensor fabrication and electrical measurements

The macroporous TiO2 nanopowders were mixed with alpha-erpineol (Aldrich, Shanghai) forming a printable paste. The pasteas then deposited on commercial interdigitated gold electroderinted substrates (Shenyang Instruments, Shenyang, China) [51].

The sensing apparatus was a sealed chamber with a total volumef 50 L, as illustrated in Fig. 1. During the sensing measurements,he corresponding liquid solutions for obtaining the different con-entrations of the corresponding gases were injected into the smallurnace at the corner of the chamber as illustrated in Fig. 1. Theure liquid methanol and acetone were vaporized to prepare theorresponding gases in the chamber. For the formaldehyde andmmonia gas vapor preparations, concentrated formaldehyde andmmonia aqueous solutions with concentrations of 40 wt% and5 wt%, respectively, were used. Table 1 specifies the compositionf the aqueous formaldehyde solutions used for preparation of theormaldehyde vapors, for example, 50 ppm formaldehyde vapor

as 122.9 ppm water vapor and 50 ppm ammonia has 87.7 ppmater vapor. The influence of the accompanying water with the

aporized formaldehyde/ammonia on the sensor impedance wasxamined. For recovery to the background, the front window of the

able 1omposition of the aqueous formaldehyde solutions used for preparation of the formalde

Targeted CHCHO (ppm) 10

Total volume of formaldehyde solution used (�L) 1.65

Volume of the accompanying H2O (�L) 0.99

Concentrations of the accompanying H2O in the chamber (ppm) 24.7

Fig. 1. Illustration of the testing apparatus.

chamber was opened and the gas vapors released. The fluctuation ofthe relative humidity is ±0.5% before and after the measurementsaccording to the readings from the digital hydrometer. The dry mix-ture of the formaldehyde with 21% oxygen was made by mixingdry oxygen (purity 99.995%, Guangming, Dalian, China), pure drynitrogen (purity 99.999%, Guangming, Dalian, China) and the par-ent 50 ppm formaldehyde gas tank balanced by nitrogen (purity99.999%, Guangming, Dalian, China). The dry 30 ppm formaldehydein 21% O2/N2 was admitted into a sealed 500 mL glass container forthe electrical measurements.

The a.c. impedance spectra of the macroporous TiO2 basedsensor in gaseous formaldehyde, methanol, ethanol and ammo-nia in the background of humid ambient air (relative humidity(RH) ∼ 5–62%) or in dry condition at room temperature (19 ± 3 ◦C)were measured by an LCR meter with digital data acquisitioncapability (Agilent E4980A, USA). The scanning frequency rangewas from 40 Hz to 2 MHz. The impedance spectra were fitted bycommercial ZView software (Scribner, Inc., USA). The absoluteimpedance of the sensor at a fixed frequency (1 kHz) was used as thesensor signal. The response (Rs) of the sensor is defined as the rela-tive change of the absolute impedance in the ambient backgroundand that in the various analytes: Rs = (|Z|a − |Z|g)/|Z|a × 100, where|Z|a and |Z|g are the impedances of the sensor in ambient air andthe analytes at a fix frequency of 1 kHz, respectively. The reportedresponse Rs to formaldehyde/ammonia was corrected by deductingthe response to the accompanying water. The sensor response Rs towater, as explained below is <5 and negligible as compared to theresponse from the analytes (>40 at the lowest concentrations exam-ined). The d.c. resistance of the sensors was measured by using thedigital electromultimeter (Agilent 34410A, USA). The UV irradiationwas introduced by placing two LED light sources as close as possibleto the top surface of the sensor. The total power density of the LEDswas ∼40 mW unless otherwise specified (Shengda Optoelectronics,

Zhuhai, China) and the wavelength range was 360–365 nm. Duringeach measurement, the sensors were exposed to UV for about 1 h to

hyde vapors.

20 30 40 50

3.3 4.94 6.6 8.231.98 2.96 3.96 4.94

49.3 73.7 98.6 122.9

L. Liu et al. / Sensors and Actuators B 185 (2013) 1– 9 3

20 30 40 50 60 70 80

(215)

(200)

(116)

(213)

(211)

(105)(200)

(112)

(004)

(103)

Inte

nsity (

a.u

.)

(101)

Two Theta (deg.)

Anatase TiO2

Fig. 2. XRD pattern of the prepared macroporous TiO2.

Table 2BET specific surface area of the various TiO2 samples.

Samples Degussa P25 Nanotube array As-grounded

st

3

3

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TsrFpnsb

i(fcs

3

3

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454 45 6 458 46 0 46 2 46 4 46 6 46 8 470

CP

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Binding En ergy (eV )

Ti2p458.8

464.5

(a)

528 53 0 532 534 53 6

CP

S

Binding En ergy (e V)

529.9

531 .4

O1s (b)

BET Area (m2/g) 35–45 27.9 44.5

tabilize the impedance of the sensors and UV remained on duringhe entire measurement period.

. Results

.1. Microstructure of the prepared macroporous TiO2

Fig. 2 shows the powder XRD pattern of the synthesized TiO2 andeveals that the product is anatase. Fig. 3(a) shows the XPS in theegion of Ti2p. The major peak centered at 458.8 eV indicates theresence of Ti4+. Fig. 3(b) shows XPS data in O1s region. Two peakst 529.9 eV (relative area: 73.2%) and 531.4 (relative area: 26.8%)orrespond to the lattice oxygen and surface OH group, respectively52,53].

The SEM of the as-prepared porous TiO2 is shown in Fig. 4(a).he sample consists of a mixture of larger rod-like particles andmall aggregates. TEM in Fig. 4(b) shows that the larger porousod-like TiO2 particles are the residual nanotube array fragments.ig. 4(c) shows that the small aggregates consisted of two types ofarticles: the residual broken nanotubes (∼100–400 nm) and someearly spherical nanoparticles (∼10 nm). These nanoparticles pos-ibly come from the broken wall of the nanotubes and are looselyonded together.

The porosity of the samples, as measured by BET is shownn Table 2. The ground sample has a larger specific surface area45 m2/g) compared to the corresponding nanotube arrays. Thiseature is desirable for metal oxide based semiconductor-typehemical gas sensors where surface redox reactions determine theensor signals [4–6].

.2. Impedance measurements

.2.1. Impedance spectra of the sensor under UVFig. 5 shows the impedance spectra of TiO2 in formaldehyde

10–50 ppm) in a background of ambient (RH ∼ 52%) under UV illu-ination. The frequency range for all measurements was 40 Hz

Fig. 3. XPS spectra of the prepared macroporous TiO2 in the regions of (a) Ti2p and(b) O1s.

to 2 MHz. The impedance spectra in Fig. 5(a) includes two parts:the depressed semicircle which arises from bulk conductivity,the increasing tail at low frequency due to polarization at theelectrode–electrolyte interface, arising from the effect of blockingelectrodes [54,55]. The semicircle can be simulated by an equiva-lent electric circuit illustrated in Fig. 6 consisting of a resistor (R1)in parallel with a capacitive component (CPE1: constant phase ele-ment) representing the electrochemical cell. The impedance Z isexpressed as [54,55]

Z = Z ′ + iZ ′′ = R1

(1 + ZCPE1 )= R1

(1 + iω�)n (1)

where Z′ is the real part and Z′′ the imaginary part of the impedance,ZCPE1 represents the impedance contributed from the capacitivecomponent of the equivalent circuit, ω is the angular frequency, �(=R1CPE1) is the relaxation time and n is a fractional index whosemagnitude is related to the suppression of the semicircle above thereal axis. The peak angular frequency of the semicircle (ωp) alsosatisfies the following equation:

ωp.R1 · CPE1 = 1 (2)

The best fitting parameters to the impedance curves in air and

various concentrations of formaldehyde are shown in Table 3. Thebulk resistances (R1) of the sensor decreased while the capacitivecomponent (CPE1) of the bulk increased upon exposure of the sen-sor to the different concentrations of formaldehyde in humid air

4 L. Liu et al. / Sensors and Actuators B 185 (2013) 1– 9

Fig. 4. (a) SEM images of the prepared macroporous TiO2 and TEM images

Table 3Fitted parameters of the impedance spectra of the macroporous TiO2 based sensorin different concentrations of formaldehyde and in air (RH ∼ 52%, T ∼ 19 ◦C).

Bulk Interface (W1)

CHCHO R1(×105 �) CPE1(pF) n Ri(×105 �) CPEi (F) ni

Air 2.3 15.6 0.91 7.06 0.045 0.4110 ppm 1.51 23.8 0.89 5.32 0.068 0.37

ua

kdttoi

3

s

◦

30 ppm 0.69 39.3 0.87 1.05 0.051 0.3550 ppm 0.37 92.8 0.82 0.76 0.058 0.32

nder UV. The fraction index n is around 0.8–0.9 (n < 1), indicating diffusion process under the applied electrical field.

The corresponding frequency-dependent impedance plots (alsonown as Bode plot) are shown in Fig. 5(b). In the long range ioniffusion limited region, the plateau corresponds to the d.c. resis-ance. The decrease in the impedance of the sensor as a function ofhe applied frequency at higher frequency follows the relationshipf |Z|(ω) ∝ ω−0.74, which is associated with the hopping of electronsn crystalline TiO2 with extrinsic defects [54,55].

.2.2. Sensor response under UV at the fixed frequencyThe frequency at 1 kHz was selected for the time-dependent

ensing measurements.

of (b) the larger particles and (c) the smaller aggregates shown in (a).

The influence of the accompanying water vapor present withthe formaldehyde or ammonia gas vapors was examined. Fig. 7(a)shows the impedance changes only with 41.1–204.8 ppm watervapor, these concentrations being chosen based on the water thatis introduced along with the analyte vapor (Table 1). The insertof Fig. 7(a) shows the sensor response Rs to the varying levels ofwater vapor and is <5, even for the highest concentrations. Fig. 7(b)shows the impedance change with 50 ppm formaldehyde vapor,and the impedance change after 204.8 ppm water was injectedinto the sample chamber. There is minimal effect on the sensorimpedance with the added water. However, since we measuredthe sensor response to pure water vapor (Fig. 7(a)), the reportedresponses of the sensor to formaldehyde/ammonia were correctedby deducting the response to the accompanying water vapor. Forexample, if the response of the sensor to the mixture of 50 ppmformaldehyde and the accompanying 122.9 ppm H2O is 87 in thebackground with RH∼52% and the response to the 122.9 ppm watervapor alone is 4, the response to 50 ppm formaldehyde is then 83.

Fig. 8 shows the changes of the impedance of the sensorto different concentrations of formaldehyde from 10 to 50 ppm,

with ambient relative humidity of ∼52% and temperature ∼19 C.Consistent with the Nyquist plots, the impedance of the sensordecreased when the sensor was exposed to various concentrationsof formaldehyde vapors as shown in Fig. 8(a). The response and

L. Liu et al. / Sensors and Actuators B 185 (2013) 1– 9 5

0 100000 2000 00 3000 00 400000 500000

0

100000

200000

300000

400000

500000-Z

'' (O

hm

)

Z' (Ohm )

air

10 ppm

30 ppm

50 ppm

(a)

10 10 0 100 0 1000 0 10000 0 100000 0

10000

100000

|Z| (O

hm

)

f (Hz)

Air

10 pp m

30 pp m

50 pp m

(b)

Fig. 5. (a) Impedance spectra and (b) the corresponding frequency-dependentimpedance plots of the sensor in different formaldehyde concentrations (frequencyrange 40 Hz to 2 MHz, T ∼ 19 ◦C, RH ∼ 52%).

Ft

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|Z| (M

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Time (s)

41.1 ppm H2O

82.2 ppm H2O

204.8 ppm H2O

Inje cted H2O

Open the cha mbe r

(a)

0 200 400 600 800 10 00 12000

1

2

3

4

Air

|Z| (

MO

hm

)

Time (s )

Injec ted 204.8 ppm H2O

Injected 8.23 uL aqueous HCHO

solut ion to obta in 50 ppm HC HO vaporAir

T~23oC,

RH~ 20%

(b)

Fig. 7. (a) The impedance changes of the sensor with macroporous TiO2 under UV

ig. 6. Proposed equivalent electrical circuits for fitting the impedance spectra ofhe porous TiO2 based sensors shown in Fig. 4(a).

he recovery time (defined as the time taken for the sensor signalo reach 90% of the steady-state response and decrease down to0% above the impedance in the humid air background), is ∼3 minnd ∼2.5 min, respectively. The sensor response Rs to formaldehydeas calculated from Fig. 8(a) and plotted as a calibration curve in

ig. 8(b). The sensor responses increased linearly with the increasef the gas concentrations and the sensitivity, defined as the slope ofhe calibration curve is 1.2 ppm–1 below 40 ppm. Above 40 ppm, theensor response becomes saturated. It is noted here that withoutV, the response of the sensor to formaldehyde in humid condition

s relatively small (Rs = 2.5) [51].Fig. 9(a) shows the repeatability of the impedance changes of the

ensor to 40 ppm formaldehyde at the fixed frequency of 1 kHz, andompares it with the d.c. resistance changes. The sensor indicatedetter stability of a.c. impedance changes in both the air backgroundnd 40 ppm formaldehyde compared to the d.c. resistance changes

Fig. 9(b)).

Fig. 10 shows the responses of the sensor to possible inter-erents, 50 ppm of methanol, ammonia and acetone at room

light to different amounts of water vapor (insert is a plot of the sensor response Rs);(b) The influence of added water vapor to formaldehyde on the impedance of thesensor (T ∼ 23 ◦C, RH ∼ 20% and frequency = 1 kHz).

temperature under UV illumination. The response to 50 ppmformaldehyde is also illustrated in the same figure for comparison.It is observed that the responses of the sensor to the interferentsare smaller compared to that for formaldehyde. It also noticed thatthe sensor response to acetone is in the opposite direction, indicat-ing an increase in impedance, the mechanism of which is unclearat the present time.

The resistance changes and the corresponding Rs of the threesensors are shown in Fig. 11(a) and (b), respectively. The absoluteresistances of the sensor 2 and 3 in different gas environments arealmost identical while that of the sensor 1 is slightly lower. This isnot unexpected since all the three sensors were made manually.However, the responses of the three sensors are close to each otheras shown in Fig. 11(b) indicating good reproducibility.

4. Discussion

Under UV illumination of 360–365 nm, electrons and holes aregenerated in the titania (band gap ∼3.2 eV) [38,56–58]:

h�TiO2−→h+ + e− (3)

With the titania exposed to the dry air, the positive holes are

proposed to combine with the adsorbed oxygen ions according toreaction [38,56–58]:

h+ + O2(ad)− → O2

•(g) (4)

6 L. Liu et al. / Sensors and Actuators B 185 (2013) 1– 9

0 20 0 40 0 600 80 00

150000

300000

450000

600000

|Z| (O

hm

)

Time (s )

10 ppm

20 ppm

30 ppm

40 ppm

50 ppm

f=1 kHz

RH~56 %

10 20 30 40 50

40

50

60

70

80

90

Rs

Rs

HCHO Co nc. (pp m)

(b)

(a)

Fig. 8. (a) Impedance traces of the macroporous TiO2 based sensor with differentcb1

e

e‘

O

wt

bc

H

o

O

t

0 1000 2000 3000 4000 5000 6000 7000

0

200

400

600

800

|Z| (k

Ohm

)

Time (s)

40 pp mf=1 kH z

(a)

0 10 00 200 0 300 0 400 0

0

2

4

6

8

10

12

14

40 pp m

R (

MO

hm

)

Time (s )

d.c .

(b)

Fig. 9. Repeatability and stability of responses of the macroporous TiO2 based sensorto 40 ppm formaldehyde using (a) a.c. method at a frequency of 1 kHz (T ∼ 19 ◦C,RH ∼ 19%) and (b) d.c. method. (T ∼ 20 ◦C, RH ∼ 54%).

1 2 3 4

-20

0

20

40

60

80

100

Rs

HCHO

CH3OH

NH3

CH3COCH

3

oncentrations of formaldehyde (10–50 ppm) and (b) the corresponding correlationetween the responses Rs and formaldehyde concentrations (frequency: 1 kHz, T∼9 ◦C and RH ∼ 52%).

The desorption of O2• is considered to be facile and the “trapped”

lectrons lead to a decrease in resistance.Fan et al. [38] proposed that a fraction of the photogenerated

lectrons (e−(h�)) can react with the adsorbed oxygen formingrefreshed’ oxygen ions (O2

−(h�)) according to:

2 + e−(h�) → O2−(h�) (5)

It should be noted that the oxygen removal via reaction (4)ould overwhelm oxygen re-adsorption (5) and the overall resis-

ance of the oxide will decrease under UV illumination.These “refreshed” photo-activated oxygen ions on the oxides are

elieved to be more energetic [36–38] and can speed up the photo-atalytic reaction of formaldehyde at room temperatures [57,58]:

CHO + O2−(h�) → CO2 + H2O + e− (6)

Under humid conditions, besides reaction (4), the neutralizationf the hydroxyls by the photo-generated holes is possible:

H− + h+(h�) → OH•(h�) (7)

These OH•(h�) are proposed to be the major active oxidant forhe formaldehyde decomposition under UV in the presence of water

Fig. 10. Comparison of the response Rs of the macroporous TiO2 based sensor tovarious volatile gases (gas concentration: 50 ppm, T ∼ 19 ◦C, RH ∼ 52%; frequency:1 kHz).

vapor [57,58].Thus, redox reaction (8) can occur in addition to reac-tion (6):

HCHO + 2OH•(h�) → CO2 + H2O + 2H+ + 2e− (8)

The electrons released via reactions (6) and (8) will lead to thedecrease of the sensor impedance, as observed in Figs. 5, 7 and 8.Under humid conditions, the OH · (h�) based oxidative reactions

L. Liu et al. / Sensors and Actuators B 185 (2013) 1– 9 7

0 200 40 0 600 800 1000 1200 140 00

3

6

9

12

15

R (

MO

hm

)

Time (s)

Sensor 1

Sensor 2

Sensor 3

10

20

30

40 50

(a)

Unit: ppm

10 20 30 40 50

70

80

90

100

Rs

HCHO Conc. (pp m)

Sen sor 1

Sensor 2

Sensor 3

T~20oC, RH~50 %

DC si gnal s

(b)

Fa

wticgd

sFtbqiafssTeaoonira

1000 2000 30 00

0

200 0

4000

600 0

800 0

HCHO on

HCHO off

|Z| (O

hm

)

Time (s)

30 ppm HC HO in dry N2 un der UV

T~ 25oC, f=1 kH z

HCHO on

0 700 1400 2100 28 00 35 00

0

3

6

9

12

15N

2

30 ppm HCHO off

|Z| (M

oh

m)

Time (s )

21% O2 flow i n

pure N2 flow in

Dry 21%O2/N

2

30 ppm HCHO on

(b)

(a)

Fig. 13 shows the comparison of the sensor response Rs to30 ppm formaldehyde in various backgrounds. In the dry conditionunder UV, the response was lower in dry N2 than that in 21% O2.In the dry N2 background, the formaldehyde could react with the

dry N2 dry 21%O2 6%RH Air 15%RH Air 50%RH Air

0

20

40

60

80

100

Rs

Variou s Back ground

30 ppm HCHO

f=1 kH z

N2 O2

ig. 11. Reproducibility of the three macroporous TiO2 based sensors (a) resistancend, (b) sensor response Rs (T ∼ 20 ◦C, RH ∼ 50%, d.c. method).

ill enhance the sensitivity of the sensors. The better stability ofhe a.c. signals of the sensor relative to the d.c. signals as shownn Fig. 8(b) can be attributed to the suppression of the electri-al polarization due to the H+ building-up at the interface of theold electrode and titania. This makes the a.c. testing method moreesirable than the d.c. method during the sensor measurements.

In support of the sensing mechanism proposed above, the sen-or was exposed to dry nitrogen and 21% O2 balanced with N2.ig. 12(a) and (b) shows three cycles of the impedance changes ofhe sensor to the 30 ppm formaldehyde in dry N2 and 21% oxygenackground, respectively, (under UV illumination, T ∼ 25 ◦C and fre-uency at 1 kHz).The sensor showed higher background impedance

n the dry 21% oxygen than that in dry N2 which can be explained asrising from the extraction of the electrons by the adsorbed oxygenrom the bulk oxide (reaction 5). In both environments, the sen-or exhibited similar impedance changes to formaldehyde: a sharppike in impedance followed by a slow decrease in the impedance.he sensor also showed the largest response during the first-timexposure to formaldehyde and the response was not fully recover-ble. The initial sensor response can be attributed to the reactionf the formaldehyde with the residual hydroxyls or the ionizedxygen adsorbed strongly on the surface of the TiO2 which wereot removed during the treatment with flowing dry nitrogen. Dur-

ng the first-time exposure, the formaldehyde would consume theesidual oxygen and make the subsequent responses more repeat-ble and recoverable.

Fig. 12. Impedance traces of the macroporous TiO2 based sensor to 30 ppmformaldehyde in (a) dry nitrogen and (b) dry 21% O2 balanced with nitrogen(T ∼ 25 ◦C, frequency: 1 kHz).

Fig. 13. Comparison of the sensor response Rs of the macroporous TiO2 based sen-sor to 30 ppm formaldehyde in various background gases. (T = 22 ± 3 ◦C, frequency:1 kHz).

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L. Liu et al. / Sensors a

angling lattice oxygen at the surface of the porous TiO2 nanocrys-als releasing electrons into the bulk. The sluggish sensor responseelative to that shown by the sensor in the oxygen environmentFig. 12(b)) supports this idea, though more work is needed tolucidate the exact mechanism. When the sensor was exposed to0 ppm formaldehyde in dry 21% oxygen, the response almost dou-led compared to that in dry N2. The increase in the sensor responsean be attributed to reaction (6).

With humidity of RH ∼ 6% in the background of air (21% oxygen),he response was significantly enhanced and almost triple thatf the dry 21% oxygen. The reaction with the activated hydroxylsccording to eq. (8) explains this observation.

However, as also seen in Fig. 13, the sensor responses decreaseds the relative humidity increased beyond ∼6%. The electronsrapped by the chemisorbed oxygen ions can be released to the bulkpon exposure to water vapor, by displacement of oxygen ions byydroxyls or adsorbed water, leading to decrease in the resistance59–61]. This is supported by evidence that the impedance of theensor decreased as the humidity increased from 11% to 90% (dataot shown). Consequently, there are less chemisorbed oxygen ionsarticipating in the sensing process with increase of humidity, andnly with the optimal level of humidity in a fixed oxygen back-round (RH ∼ 6%, Fig. 13), the highest response is observed. Similarbservations are reported in the literature [62,63].

. Conclusions

The macroporous TiO2 based impedance-type sensors showedood sensitivity and selectivity to formaldehyde in the concen-ration range of 10–50 ppm in humid air at room temperaturender UV illumination. With the presence of the water vapor in theackground (RH ∼ 6%), the response of the sensor was significantlynhanced and proposed to be due to hydroxyl-promoted reactionsith formaldehyde. However, with further increase in humidity,

he sensor response decreased due to the possible displacement ofhemisorbed oxygen ions by the dissociated water at the surfacef the oxide. A.c. impedance-type signals were more stable thanhe d.c. resistance signals. We propose that polarization potentialarrier with the build-up of the protons (H+) dissociated from theater molecules at the interface of the porous oxide sensing mate-

ials and the gold electrodes is suppressed with the a.c. method.

cknowledgements

The financial supports from the Fundamental Research Fundsor the Central Universities and the National Natural Science Foun-ation of China (Grant No.: 61001054, 61131004, 61176068) arecknowledged.

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Biographies

Lipeng Liu is a master student in the School of Electronic Science and Tech-nology, Dalian University of Technology, China. He is currently investigating thelow-temperature gas sensing of the porous TiO2 based semiconductor type sensors.

Xiaogan Li is an associate professor in the School of Electronic Science and Tech-nology in Dalian University of Technology (DUT), Dalian, Liaoning, P. R. China andserves as the director of the Institute for Sensing Technologies in DUT. He receivedhis Ph.D. in materials science and engineering from University of Leeds, U.K. andconducted two-year postdoctoral research in chemical gas sensors at the Ohio StateUniversity in USA. His current research interests are in chemical gas sensors.

Prabir K. Dutta received his Ph.D. degree in chemistry in 1978 from PrincetonUniversity. After 4 years of industrial research at Exxon Research and EngineeringCompany, he joined The Ohio State University, where currently he is professor ofchemistry. His research interests are in the area of microporous materials and ceram-ics, including their synthesis, structural analysis, toxicity and as hosts for sensors,chemical and photochemical reactions.

Jing Wang is a professor in the School of Electronic Science and Technology, DalianUniversity of Technology, China. She received her master degree from the Depart-ment of Electronic Engineering, Jilin University, China in 1981. Her current scientificinterests are gas and humidity sensors.