SO2 and h2s sensing properties of hydrothermally synthesized cuo nanoplates

We recently successfully synthesized different p-type semiconductors for gas sensors, such as Co3O4 nanorods,24 NiO nanosheets,25 and CuO nano-wires.26 The advantages of p-type semicondu

SO 2 and H 2 S Sensing Properties of Hydrothermally

Synthesized CuO Nanoplates

PHAM VAN TONG,1,2NGUYEN DUC HOA ,1,3,4HA THI NHA,1 NGUYEN VAN DUY,1CHU MANH HUNG,1and NGUYEN VAN HIEU1

1.—International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No 1, Dai Co Viet Str., Hanoi, Vietnam 2.—Department of Physics, Faculty of Mechanical Engineering, National University of Civil Engineering (NUCE), No 55, Giai Phong Str., Hanoi, Vietnam 3.—e-mail: ndhoa@itims.edu.vn 4.—e-mail:

hoa.nguyenduc@hust.edu.vn

CuO nanoplates were synthesized by a facile hydrothermal method for a SO2

gas-sensing application The synthesized materials were characterized by field-emission scanning electron microscopy (FE-SEM), powder x-ray diffrac-tion (XRD), Raman spectroscopy, and photoluminescence spectroscopy Gas-sensing characteristics were measured at various concentrations of SO2and

H2S at 200–350°C The results showed that rectangular CuO nanoplates with

an average size of approximately 700 9 500 9 30 nm3were synthesized FE-SEM and XRD analyses also depicted that the nanoplates were polycrystalline with an average crystal size of 12.85 nm Gas-sensing measurements demonstrated that the synthesized CuO nanoplates exhibited p-type semi-conducting behavior, where the sensor resistance increased upon exposure to

H2S and decreased when exposed to SO2 The sensor showed a considerably higher response to SO2 than to H2S in the measured concentrations ranging from 1 ppm to 10 ppm, suggesting that the CuO nanoplates are suitable for high-sensitivity SO2sensing We also clarified the sensing mechanism of the CuO nanoplate-based SO2 sensors

Key words: CuO nanoplates, hydrothermal, SO2sensing, sensing

mechanism

INTRODUCTION

In recent years, the rapid growth in the economy

and the industrialization of Vietnam have required

a significant increase in energy supply; however,

fossil fuels are limited, and the utilization of

household biogas is a possible solution.1 Especially

in Vietnam, the potential for biogas energy sources

is very high because many pig farms and

agricul-tural waste products are available.2Biogas is

exten-sively and readily produced and has been used in

rural Vietnam in recent years3thanks to the policy

of the government to encourage the use of biogas for

electric generation However, the biogas contains

some toxic gases, such as sulfur dioxide (SO2), hydrogen disulfide (H2S), carbon monoxide (CO), and carbon dioxide (CO2).4 Furthermore, in big cities such as Hanoi, SO2is also present in vehicle emissions as a result of fuel combustion in diesel buses and trucks.5 SO2 is a colorless gas with a strong odor and is considered an extremely toxic gas because, when combined with water, it becomes highly corrosive sulfuric acid which can damage the environment and constructions.6 The inhalation of low-concentration SO2 gas can cause chemical burns, as well as irritation of the nose, throat, and airways The occupational safety and health admin-istration (OSHA, USA) designated permissible exposure limits (PEL) for SO2 to be only 5 ppm Along with SO2, H2S is also an extremely toxic, explosive, and hazardous gas with an odor similar to that of bad eggs.7 It occurs naturally in

(Received June 27, 2018; accepted September 1, 2018)

Ó2018 The Minerals, Metals & Materials Society

crude petroleum and natural gas, and can be

pro-duced by the breakdown of organic matter and

human/animal wastes in livestock biogas.8

Espe-cially, in rural Vietnam, people use biogas for

cooking purposes without desulfurization Biogas

contains toxic H2S gas with a concentration of up to

0.5%,9 but most plants in Vietnam use it without

any monitoring or desulfurization The PEL for H2S

set by OSHA is 20 ppm; thus, its detection in the

environment and monitoring are essential.10 Thus,

monitoring of H2S11 and SO212 at low

concentra-tions (ppm level) is very important and is the key

issue in the safe use of biogas and industrial

processes.13 Different materials and/or structures

have been used for SO2 and H2S sensors For

instance, an integrated microchip with Ru/Al2O3/

ZnO as the sensing material has been developed for

SO2 sensors with a limit detection of 5 ppm.14 An

electrochemical sensor based on diamond-like

car-bon-modified polytetrafluoroethylene membranes

has been fabricated for the detection of SO2.15

Fe-doped metal oxides have also been prepared for

conventional solid-state SO2and CO2gas sensors.16

Stabilized zirconia-based mixed potential-type

sen-sors utilizing MnNb2O6-sensing electrodes have

been prepared for the detection of low-concentration

SO2, in which the sensor has a low detection limit of

50 ppb at an operating temperature of 700°C.17

Most of the developed SO2 sensors are solid

elec-trolyte types, but such devices are limited by their

short lifetime,18 bulky size,19 and high operation

temperature.17 In contrast to the conventional

electrochemical sensor, metal oxide-based resistive

devices have some advantages, such as small size,

simple operation, easily integration in circuits, good

reproducibility, and good reversibility.20 Both

n-type and p-n-type metal oxide semiconductors have

been extensively studied for gas sensor

applica-tions.21The n-type SnO2dodecahedron was used for

the monitoring of SO2, where the maximum

response to 10 ppm SO2 at 183°C was only 1.92.22

Transition metal (Ni, Fe, and Co)-doped MoS2

nanoflowers have also been synthesized for

room-temperature SO2 gas sensors,23 where the

responses are strongly dependent on the doped

metal and reach the maximum value of

approxi-mately 20% to 4000 ppm SO2

We recently successfully synthesized different

p-type semiconductors for gas sensors, such as Co3O4

nanorods,24 NiO nanosheets,25 and CuO

nano-wires.26 The advantages of p-type

semiconduc-tors27,28 over n-types include low-humidity

dependence, high catalytic properties,29and a high

signal-to-noise ratio.30Among others, CuO, a p-type

semiconductor with a narrow band gap ( 1.2 eV)

has been the priority choice due to its robustness

and abundance.31 CuO has been used as a sensing

material in various sensors, such as VOCs,32 H2,33

H2S,34 CO,35 and NO2.36 Nanostructured CuO

materials, such as core–shell nanoparticles,37

tad-poles, spindles, leaves/spheres and fusiform,38

nanotubes and nanocubes,39 nanowires,40 nanor-ods,41nano-thin films,42polyhedrons,43urchin- and fiber-like,44and nanoplates,45have been extensively studied for gas-sensing applications utilizing the large specific surface area and effective adsorption sites for surface reactions.46Different methods have been used to synthesize CuO nanostructures in which the sputtering or physical methods require a high vacuum and expensive equipment.37,42 The inexpensive wet chemical method is very advanta-geous in the synthesis and control of the morphology

of CuO materials for gas sensor applications.38 However, the SO2-sensing characteristics of CuO nanoplates have not yet been studied In addition, sensing mechanisms of CuO-based sensors to sul-fur-containing gases, such as H2S and SO2, remain unclear

Here, CuO nanoplates were synthesized by a facile hydrothermal method and then spin-coated onto a thermally oxidized silicon substrate equipped with a pair of comb-type interdigitated Pt electrodes (Pt IDEs) for gas-sensing characterization Dynamic measurement of the change in resistance of sensors

on exposure to different concentrations of SO2and

H2S gases was performed at various temperatures ranging from 200°C to 350°C The results demon-strated that the synthesized CuO nanoplates are excellent for monitoring low concentrations of SO2

gas The SO2 gas-sensing mechanism of the CuO nanoplates has also been discussed

EXPERIMENTAL The materials used in this study were analytical copper(II) chloride (CuCl2), potassium hydroxide (KOH), and deionized (DI) water CuO nanoplates were synthesized by a facile hydrothermal method without any surfactant or post-thermal calcination Processes for the synthesis of CuO nanoplates are summarized in Scheme1 In a typical synthesis, 1.2 g of CuCl2 and 1.7 g of KOH were dissolved in

DI water under magnetic stirring at a room tem-perature of 27°C The clear blue solution was transferred to a Teflon-lined autoclave (100 mL in volume) A hydrothermal process was carried out in

an electric oven at 220°C for 4 h After the reaction was completed, the autoclave was cooled naturally

to room temperature The precipitated powders were collected and washed five times with DI water and subsequently two times with ethanol to remove unreacted ions by centrifuging at 4000 rpm for

15 min Finally, the precipitate powders were dried

at 45°C overnight before characterization The synthesized materials were characterized by pow-der x-ray diffraction (XRD; Advance D8, Bruker) and field-emission scanning electron microscopy (FE-SEM; JEOL 7600F), respectively Photolumi-nescence (PL) was measured at room temperature using an excited laser at 328 nm Raman spec-troscopy was measured using the Renishaw Invia Confocal micro-Raman System

For gas-sensing characterization, the

as-synthe-sized materials were dispersed in N-vinylpyrrolidone

to form a colloidal solution, which was then

spin-coated onto a thermally oxidized Si substrate

equipped with Pt IDEs to form the sensing devices.47

The Pt IDEs contain 37 digits with a length of

approximately 780 lm, while the width and the

space between two digits are 20 lm (Fig.1a) For

gas-sensing measurement, the fabricated sensor was

placed on a heating plate to control the working

temperatures Prior to the gas-sensing

measure-ment, the sensor was pre-heated at 400°C for 1 h to

stabilize the resistance and increase the contact

between the sensing materials and the Pt IDEs The

resistance of the sensor was continuously measured

using a Keithley instrument (Model 2602) interfaced

with a computer, while the dried air and analytic

gases were switched on/off in each cycle Here, the

analytic H2S and SO2, with a concentration of

100 ppm diluted in nitrogen were used as tested

gases To obtain lower concentrations (1–10 ppm),

the analytic gas was further diluted with dry air

using a mixing system Details of the mixing system

can be found elsewhere.48 The response time (sres)

was estimated by fitting the sensor resistance versus

time from the introduction of the SO2 gas until the

resistance reached saturation, using Eq.1

R tð Þ ¼ R0expð t=sresÞ ð1Þ The recovery time (srec) was also estimated by fitting

the sensor resistance versus time after stopping the

introduction of SO2gas until the resistance returns

to the initial value, using Eq.2

R tð Þ ¼ RSexp t=sð recÞ ð2Þ where R0and RSare the resistances of the sensor in

air and the saturation value of the resistance in

SO , respectively

RESULTS AND DISCUSSION After hydrothermal synthesis, the obtained prod-ucts exhibited a black color, as shown in the inset of Fig.1a The morphologies and microstructure anal-ysis of the obtained products were investigated by FE-SEM (Fig.1a–c) Obviously, the low-magnifica-tion FE-SEM image (Fig.1a) demonstrates that the as-prepared products are composed of homogenous nanoplates, which were well separated but not aggregated together because none of the surfactants was used during material synthesis The nanoplates exhibited an irregular rectangular shape with a size

of approximately 700 9 500 nm2and a thickness of approximately 30 nm (Fig.1 and c) The nanosheets were composed of nanocrystals of an average size of less than 20 nm (Fig 1d) In the study of Li et al.,31 the CuO nanoplates were synthesized by a hydrothermal method using an ionic liquid precursor, benzyltrimethylammonium hydroxide, and the nanoplates have the length of approximately 262 nm, depending on the synthesis conditions Here, we did not use an ionic liquid surfactant or structure-directing agent but still obtained a nanoplate structure During hydrother-mal synthesis, the CuCl2 reacts with KOH to form Cu(OH)2 Subsequently, the Cu(OH)2dehydrates to form CuO nanocrystals at high temperature, according to the following equations:

CuCl2þ KOH ¼ Cu OHð Þ2 ð3Þ

Cu OHð Þ2DT!CuOþ H2O ð4Þ The CuO crystals thus serve as seeds for the growth

of nanoplates through the Ostwald ripening mech-anism.49Note that the dehydration of Cu(OH)2can also occur at room temperature but at a very slow rate, and the produced CuO material has a flake morphology, but the nanoplates do not During hydrothermal synthesis, the CuO nanocrystals (seeds) grew along two [100] and [010] preferential growth directions to form nanoplates according to the Ostwald-ripening mechanism.50During growth, the CuO seeds aggregated together to form the nanoplates through van der Walls forces However, the mis-orientation of the nanocrystals during aggregation forms the polycrystalline nanoplates but not the single crystal structure

The powder XRD pattern of the synthesized CuO nanoplates is shown in Fig.2a, where all the diffraction peaks were readily indexed to the mon-oclinic structure of CuO (JCPDS, No 80-1917) The diffraction peaks were broad as a result of the nanocrystalline nature of the synthesized materi-als.51 The average crystal size of the CuO nano-plates calculated using the Scherer equation was approximately 12.85 nm This value is considerably smaller than the size of the CuO nanoplates esti-mated by the SEM images, again confirming the poly-crystallinity of the nanoplates

Scheme 1 Processes for the hydrothermal synthesis of CuO

na-noplates.

The Raman spectrum of the CuO nanosheets

shown in Fig 2b displays two peaks, at 286.5 cm 1

and 337.7 cm 1, which were assigned to the active

modes Ag and B1g, respectively.52 The Raman

spectrum of the sheet-like CuO mesocrystals has

three modes at around 281 cm 1, 345 cm 1, and

630 cm 1belonging to the Agand Bgmodes of CuO,

respectively.53Note that, in the CuO Raman modes,

the copper atoms remain stationary, and only the

oxygen atoms take part in the motion because

oxygen atoms are considerably lighter than the

copper atoms The oxygen motion for the Agmode is

parallel to, and that for the Bg mode is

perpendic-ular to, the monoclinic axis.54Here, the wavelength

numbers of those modes in the CuO nanoplates are

shifted from the values reported in the bulk

liter-ature (298 cm 1, 345 cm 1, and 632 cm 1) due to

the size effects.53,55 Here, the active mode B2g at

623 cm 1 did not appear, possibly due to the

anisotropic structure of the CuO nanoplates.56

The PL spectrum of the synthesized CuO

nano-plates shown in Fig 2c exhibits broad emission

peaks centered at approximately 498 nm (2.48 eV)

The band gap of bulk CuO ranged from 1.9 eV to

2.1 eV.57 The higher near-band-edge emission of

nanocrystalline CuO was explained due to the

Burstein–Moss effect.58 The broad emission at

498 nm (2.48 eV) is assigned to the energy levels

of defect sites, such as VCu (copper vacancies), Cui

(copper interstitial), and Vo (oxygen vacancy) in CuO nanoplates or the quantum confinement effect.59 This result is consistent with other CuO nanostructures reported to have an optical band gap

of approximately 2.48 eV.60,61 The high defect level

is expected to show a high gas-sensing performance The transient resistance versus time upon expo-sure to different concentrations of SO2measured at temperatures ranging from 200°C to 350°C is shown

in Fig.3a The base resistances of the sensor in air were 24.76 kX, 4.53 kX, 1.65 kX, and 0.80 kX for temperatures of 200°C, 250°C, 300°C, and 350°C, respectively The resistance of the CuO nanoplates decreases with increasing temperature and exhibits

an obvious negative temperature coefficient of resistance in the measured range This result is consistent with other reports on CuO thin film.62 Prior to the introduction of the analytic gas, the resistance of the sensor is very stable and hardly changed with time However, upon exposure to SO2, the sensor resistance abruptly decreased and reacted to the saturation values within a minute, depending on the working temperatures and gas concentrations The sensor also shows good recovery characteristics where the resistances returned to the initial values when the flow of analytic gas was stopped Figure3a also reveals that the response

Fig 1 (a, b) Low- and (c, d) high-magnification FE-SEM images of the synthesized CuO nanoplates-based sensor Inset in (a) is a photo of the hydrothermal product.

and recovery speed increases with increasing

work-ing temperature At all measured temperatures, the

sensor shows reversible response characteristics

Reversible adsorption of analytic gas molecules on

the surface of the sensing material is very

impor-tant in the practical application and reusability of

gas sensors The sensor shows a remarkable

response to low SO2 concentrations down to

1 ppm, which is very effective because CuO

nanosheets did not sense 40 ppm SO2 at room

temperature.63 The sensing mechanism of SO2

based on metal oxides is complex because SO2

molecules can directly adsorb on the surface of

CuO and/or sulfidate CuO into CuS In the study

reported by Ma et al.,22 the resistance of n-type SnO2nanocrystal-based sensors decreases with the introduction of SO2gas because SO2 molecules can react with the chemisorbed oxygen species, and the trapped electrons are released back to the conduc-tion band of SnO2, according to Eq.5

SO2 gasð Þþ O ¼ SO3 ads ð Þþ e ð5Þ According to this, given that CuO is a p-type semiconductor, upon exposure to SO2, the sensor resistance should increase and not decrease as observed in our study The result is opposite to that

of n-type WO3, where the sensor resistance increased upon exposure to 10 ppm SO2 at 220°C.64 A decrease in the resistance of p-type semiconductor-based sensors upon exposure to SO2

gas was also found in MoS2materials.23We believe that SO2gas can act as both an oxidizing agent and

a reducing agent because of the multiple valences of sulfur Here, the SO2exhibits characteristics of an oxidizing gas by direct adsorption The SO2 mole-cules act asan oxidizing agent, capturing electrons

Fig 2 (a) XRD pattern, (b) Raman spectrum, and (c) PL spectrum

of CuO nanoplates: average crystal size of 12.85 nm Inset in (b)

shows an optical microscopic image of CuO nanoplate powder.

Fig 3 SO2-sensing characteristics of CuO nanoplates measured at different temperatures: (a) transient resistance versus time upon exposure to different SO2concentrations, (b) sensor response as a function of SO2concentrations.

from the sensing material and adsorbing on the

surface in the form of SO2, as follows:

CuOþ SO2 gas ð Þþ e ¼ CuO
SO2 adsð Þ ð6Þ

In addition, upon SO2 exposure, the SO2molecules

can react with CuO to form Cu2SO3, as shown

below.65

2CuOþ SO2 gasð Þþ 2e ¼ Cu2SO3þ 1=2O2 ð7Þ

The adsorption of SO2 molecules capture electrons

and generate hole carriers, thereby decreasing the

resistance of p-type CuO nanoplate-based gas

sensors

The sensor response (R0/R), as a function of SO2

concentrations measured at different temperatures,

is shown in Fig 3b At all measured temperatures,

the sensor response increased to the SO2

concen-trations The response value increased from 1.5 to

2.8 when the SO2 concentration increased from

1 ppm to 10 ppm at a measured temperature of

200°C The response value is considerably higher

than that of the n-type SnO2 dodecahedron22 or

SnO2thin film loaded with metal oxide catalysts.66

At a given concentration, the sensor response

decreases with increasing working temperatures

The sensor response can be improved by decreasing

the working temperature to below 200°C However,

decreasing the working temperature increased the

response and recovery time For practical

applica-tion, the response and recovery time should be

limited; thus, we did not check the sensor response

at temperatures lower than 200°C

Response and recovery times are important

fac-tors which determine the performance of gas

sen-sors By using the exponential decay and growth

functions, we could estimate the response and

recovery times of the sensors, which were measured

at different temperatures to various SO2

concentra-tions, as shown in Fig 4 The response time of the

sensor decreased significantly from approximately

40 s to about 5 s with the increase of working

temperature from 200°C to 350°C (Fig.4a) A higher

SO2concentration requires a shorter response time

The fast response time at high working

tempera-tures can be explained by the acceleration of

thermal energy for the gas adsorption The sensor

requires a longer recovery time of approximately

100–200 s at 200°C, depending on Sthe O2

concen-trations, indicating the strong adsorption on the

surface of the CuO However, those values

decreased exponentially to around 15 s with the

increase of working temperature from 200°C to

350°C (Fig.4b) The decrease of the response and

recovery times is consistent with the Langmuir

isotherm model, where the adsorption and

desorp-tion are exponentially dependent on the

temperature.67

H2S is a reducing gas, which is mainly present in

biogas, has high reactivity to CuO Thus, its sensing

characteristics were also measured at different

temperatures, and the data are shown in Fig.5 Figure5a shows the transient resistance versus time of the sensor measured at different tempera-tures upon exposure to various H2S concentrations

As demonstrated, the sensor resistance increased with the exposure to H2S gas, again confirming the p-type characteristics of CuO This response trend was opposite to those of other reports, where the resistance decreases (conductance increases) upon exposure to H2S by the formation of CuS percolation paths.68 However, the sensor exhibited a relatively poor recovery characteristic, whereas the sensor resistance could not recover to the initial values after the H2S gas flow was stopped H2S gas is highly reactive to CuO69; thus, it can react with CuO surfaces to form CuSO4 or CuS depending on the analytic gas concentration and working temper-atures.70 The working principle of the sensor was believed to be based on the phase transition of semiconducting p-type CuO to strongly degenerated p-type CuS with metallic conductivity,71because, in the form of CuS,65 CuxS exhibits semi-metallic or semiconducting properties72 with a band gap of approximately 1.6 eV.73 Indeed, the sensing mech-anism of H2S sensors based on CuO material relying

on the sulfidation of CuO into CuS was reported in

Fig 4 (a) Response and (b) recovery times as functions of the working temperatures measured to different SO2concentrations.

Ref 68 However, in the study of Ramgir et al.,74

they found that, at intermediate concentrations

(500 ppb to 50 ppm), the response curve of CuO

thin film at room temperature is governed by both

H2S oxidation and CuS formation mechanisms

Here, the sensor resistance increased upon

expo-sure to H2S gas Thus, we believe that the H2S

response is mainly based on the following reaction:

2H2SðgasÞþ 3O2 adsð Þ¼ 2H2OðgasÞþ 2SO2 gas ð Þþ 3e

ð8Þ The released electrons will neutralize free holes,

and thus reduce the main carrier density,

increas-ing sensor resistance

Sensor response (R0/R), as a function of H2S

concentrations measured at different temperatures

(Fig.5b), reveals that the sensor has the highest

response value at the low operating temperature of

250°C In addition, the sensor response increased

from approximately 1.5 to 2.8 with an increase of

H2S concentration from 1 ppm to 10 ppm These

values are relatively high when the response to

1 ppm H2S of CuO thin film at room temperature is

very low at approximately 1.29.74 Decreasing the

temperature can increase the response value, but

the recovery characteristic is very poor and not

effective for practical application

The response and recovery times of the sensor when measured at different temperatures to various

H2S concentrations are shown in Fig.6 The response and recovery times of the sensors decrease with increasing working temperature, such as that

of the SO2 response The response time of approx-imately 60 s at 250°C decreased to approxapprox-imately

20 s when the temperature increased to 350°C The recovery time was longer (100–250 s) at 250°C, depending on the H2S concentration However, the recovery time decreased to approximately 24 s at 350°C for all H2S concentrations Such fast response and recovery times at a relative high temperature of approximately 350°C are effective for practical applications in the monitoring of H2S in environ-mental pollution

A comparative result on the response of the sensor to various concentrations of SO2 and H2S measured at their optimal temperatures is plotted

in Fig.7 The response values to SO2 are approxi-mately three times higher than those to the H2S gas despite its working temperature being lower In addition, hydrogen-sensing characteristics of the sensor were also tested at 200°C to different con-centrations ranging from 50 ppm to 1000 ppm (Fig-ure S1, Supplementary) The sensor showed very

Fig 5 H2S sensing characteristics of CuO nanoplates measured at

different temperatures: (a) transient resistance versus time upon

exposure to different H2S concentrations, (b) sensor response as a

function of H2S concentrations.

Fig 6 (a) Response and (b) recovery times as functions of working temperatures measured for different H2S concentrations.

low response values of 1.15–1.36 to the H2

concen-tration of 50–1000 ppm, respectively Such results

suggest that the CuO nanoplates are effective for

monitoring SO2and/or H2S gases

CONCLUSION

We introduced a facile and saleable hydrothermal

synthesis of CuO nanoplates for effective SO2 and

H2S gas-sensing applications The CuO nanoplates

are highly crystalline and allow reversible

monitor-ing of low concentrations (1–10 ppm) of SO2 and

H2S at moderate temperatures (250–350°C) The

maximum response value to 1 ppm SO2at 200°C is

2.74 with the response time of less than 40 s,

sufficient for practical applications The SO2

-sens-ing mechanism mainly relied on the direct surface

adsorption/desorption and not on the sulfidation

process The developed sensor is suitable for

mon-itoring toxic SO2and H2S gases in biogas

ACKNOWLEDGEMENT

This study was supported by the Hanoi

Univer-sity of Science and Technology (Grant No

T2017-PC-171)

ELECTRONIC SUPPLEMENTARY

MATERIAL The online version of this article (https://doi.org/

10.1007/s11664-018-6648-0) contains

supplemen-tary material, which is available to authorized

users

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