SILICA – CARBON QUANTUM DOTS DECORATED TITANIUM DIOXIDE AS SUNLIGHT-DRIVEN PHOTOCATALYST TO DIMINISH ACETAMINOPHEN FROM AQUATIC ENVIRONMENT

Journal Pre-proof SILICA – CARBON QUANTUM DOTS DECORATED TITANIUM DIOXIDE AS SUNLIGHT-DRIVEN PHOTOCATALYST TO DIMINISH ACETAMINOPHEN FROM AQUATIC ENVIRONMENT Viona Wongso (Conceptualization) (Methodology) (Formal analysis) (Investigation) (Writing - original draft) (Writing - review and editing), Hui Khee Chung (Validation) (Investigation) (Writing - review and editing), Nonni Soraya Sambudi (Writing - review and editing) (Resources) (Visualization) (Supervision) (Funding acquisition), Suriati Sufian (Supervision) (Resources), Bawadi Abdullah (Supervision) (Resources), Muhammad Dzul Hakim Wirzal (Supervision) (Resources)

PII:

S1010-6030(19)31967-7

DOI:

https://doi.org/10.1016/j.jphotochem.2020.112436

Reference:

JPC 112436

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

19 November 2019

Revised Date:

20 January 2020

Accepted Date:

3 February 2020

Please cite this article as: Wongso V, Chung HK, Sambudi NS, Sufian S, Abdullah B, Dzul Hakim Wirzal M, SILICA – CARBON QUANTUM DOTS DECORATED TITANIUM DIOXIDE AS SUNLIGHT-DRIVEN PHOTOCATALYST TO DIMINISH ACETAMINOPHEN FROM AQUATIC ENVIRONMENT, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.112436

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SILICA – CARBON QUANTUM DOTS DECORATED TITANIUM DIOXIDE AS SUNLIGHT-DRIVEN PHOTOCATALYST TO DIMINISH ACETAMINOPHEN FROM AQUATIC ENVIRONMENT

Viona Wongso1, Hui Khee Chung1, Nonni Soraya Sambudi1,2†, Suriati Sufian1,3, Bawadi

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Abdullah1,4, Muhammad Dzul Hakim Wirzal1,5

Department of Chemical Engineering, Universiti Teknologi PETRONAS, Seri Iskandar, Perak

Center for Advanced Integrated Membrane System, Universiti Teknologi PETRONAS, Seri

Iskandar, Perak 32610, Malaysia.

Center of Innovative Nanostructure and Nanodevices (COINN), Universiti Teknologi

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32610, Malaysia.

PETRONAS, Seri Iskandar, Perak 32610, Malaysia. Center of Contaminant Control and Utilization (CenCoU), Universiti Teknologi PETRONAS,

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Seri Iskandar, Perak 32610, Malaysia. 5

Center of Research in Ionic Liquid (CORIL), Universiti Teknologi PETRONAS, Seri Iskandar,

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Perak 32610, Malaysia.

† Corresponding Author Email: [email protected] Tel: +6053687600; Fax: +6053654088

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Graphical Abstract

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Highlights

Transformation of TiO2 in different hydrothermal temperature is investigated.

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Removal of acetaminophen is performed under sunlight irradiation.

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The optimum amount of Si-CQDs in the composite is suggested.

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The Si-CQDs/TiO2 composite exhibits excellent performance and good stability.

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Abstract

The presence of pharmaceutical compound (i.e., acetaminophen) in aquatic environment has been declared as environmental issue since researchers found that it has potential risk to human health. Photocatalytic process as a promising method for waste degradation commonly employs

titanium dioxide, TiO2. However, TiO2 has narrow light absorption and rapid charge recombination resulting in ineffective photocatalytic activity. In this study, silica – carbon quantum dots (Si-CQDs) from rice husk are decorated into TiO2 matrix through facile mixing approach to minimize the limitations of TiO2. Preliminary studies regarding TiO2 transformation and Si-CQDs incorporation in various amount were systematically investigated. It is observed

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that 1 wt% is the optimum amount of Si-CQDs in composite in order to maximize the photocatalytic ability of TiO2. Under sunlight irradiation, 1 wt% Si-CQDs/TiO2 composite is

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able to completely degrade 5 mg/L of acetaminophen within 240 min (33.3% faster than pure TiO2). The excellent performance of the composite is attributed to synergistic effect of Si-CQDs

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addition on TiO2 surface, which acted as photo sensitizer and electron trapper. Si-CQDs extend

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light absorption of TiO2 by reducing band gap energy from 3.20 to 3.12 eV, as confirmed by UV-Vis Diffuse Reflectance Spectroscopy (DRS) spectra. Photoluminescence (PL) spectra and

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N2 sorption isotherm reveal that Si-CQDs addition prolongs the lifetime of charge separation and improves surface area (17% larger than TiO2), respectively. The composite of Si-CQDs/TiO2

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Keywords

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also demonstrates good stability which is beneficial for pharmaceutical waste removal in the

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Titanium dioxide, carbon quantum dots, photocatalyst, pharmaceutical, acetaminophen, sunlight.

1.

Introduction

Acetaminophen (C8H9NO2), widely known as paracetamol, is an analgesic and antipyretic drug which is used to relieve pain and/or reduce fever. Acetaminophen is distributed all around the

world and being consumed with prescription and without prescription (over the counter drugs). Hundreds to thousand tons of acetaminophen were sold in several countries, such as England (403 tons in 2000), Germany (622 tons in 2001) and Korea (1069 tons in 2003) [1]. In Malaysia, acetaminophen was top ten most consumed drugs which were sold up to 271 tons on 2014 and increased approximately 5% each year during 2011 – 2014 [2]. Recent studies show that some of pharmaceutical compounds (one of them is acetaminophen) have been found in aquatic

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environment. In USA, 10 μg/L of acetaminophen had been detected in natural waters, while more

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than 65 μg/L of acetaminophen was detected in Tyne River, UK [3]. In Malaysia, especially at Klang River, acetaminophen was among top two of detected pharmaceutical waste in the range

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of 0.12 – 1.45 μg/L [4]. The presence of pharmaceutical waste in environment can be caused by

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incomplete human excretion, since body does not fully absorb the active compound of medicine, and improper disposal of unused and expired medicines [5]. Besides, pharmaceuticals plants are

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also contributed to the presence of acetaminophen in environment, since waste water treatment facilities are generally only able to remove 80 – 86% of acetaminophen [1]. Pomati et al. [6]

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reported that the exposure of low concentration of pharmaceutical compound in long term may potentially risk the human health. Therefore, the presence of pharmaceutical waste in aquatic environment should be diminished to avoid its severe effects.

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Heterogeneous photocatalysis has been proven as the most effective method to degrade waste compound in water environment [7, 8]. TiO2 as one of common semiconductors has tremendous potential to be applied in photocatalytic waste treatment due to its relatively low-cost, high stability and low-toxicity [9]. During degradation, sufficient light energy is required to initiate charge (electron-hole) separation. The electron produces hydroxyl radical which is used to break the aromatic ring of acetaminophen and convert it into CO2 and H2O [7]. The production of

hydroxyl radical is influenced by light energy and charge recombination. Large band gap energy (~3.2 eV) becomes a barrier for TiO2 to be applicable in sunlight irradiation, since TiO2 requires UV light irradiation to activate the photocatalyst, while sunlight only contains 3% of UV spectrum [10]. Besides, the charge recombination also occurs very fast (approximately 10 ns) resulting in ineffective photocatalytic activity [11]. Considering these limitations, modification

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on TiO2 by adding metal or non-metal was performed to enhance the photocatalytic ability [8]. Studies regarding composite of carbonaceous materials and TiO2 on acetaminophen removal had

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been reported. de Luna et al. [12] synthesized carbon self-doped TiO2 from titanium butoxide

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using sol-gel method in various calcination temperatures. Different crystalline phases were found in this study, and it suggested that 1 g/L of photocatalyst in anatase phase is able to remove 94%

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of acetaminophen (15 ppm) within 540 min of 5 x 1 W blue LED light irradiation. Afterwards, Gómez-Avilés et al. [13] are successfully reducing the required time for acetaminophen removal

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by using carbon doped TiO2 nanotube under Xenon lamp irradiation. The TiO2 was prepared using sol-gel method and titanium isopropoxide as the precursor, while lignin was employed as

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the carbon precursor. These precursors were mixed, hydrothermally heated and calcined in air atmosphere. It was reported that the photocatalyst (0.25 g/L) completely degrades 5 ppm of acetaminophen in 60 min irradiation. The presence of carbon in the composite reduces the band

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gap energy of TiO2, thus it extends the light absorption of TiO2 into visible light irradiation. Unlike previous studies, Shaban and Fallata [14] utilized sunlight irradiation to activate the prepared photocatalyst for acetaminophen degradation. Shaban and Fallata [14] prepared carbon doped TiO2 and reported that 2 g/L of photocatalyst can degrade 3 mg/L of acetaminophen within 75 min of sunlight irradiation. Similar to Gómez-Avilés et al. [13], the enhanced acetaminophen removal over TiO2 is accounted for low band gap energy.

Recently, carbon quantum dots (CQDs) as the latest class of carbon nanomaterials have attracted much interest due to its promising properties, such as: low-toxicity [15, 16], good conductivity and up-converting photoluminescence [17]. The addition of CQDs into TiO2 enhances photocatalytic activity by extending light absorption to visible light and reducing charge recombination rate [18]. Therefore, the composite of CQDs/TiO2 has been utilized for

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photocatalytic applications, such as: air purification [18, 19] and dye degradation [8]. However, the performance of CQDs/TiO2 composite for pharmaceutical waste removal has been rarely

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reported, particularly for acetaminophen.

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Herein, this study investigated the performance of CQDs/TiO2 composite as sunlight-driven photocatalyst for acetaminophen removal. The CQDs were synthesized from natural carbon

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sources (i.e., rice husk). Natural carbon sources have received much interest nowadays than synthetic carbon sources due to its highly abundance, low-cost and environmental friendly

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characteristics [15]. Rice husk, as major waste from rice production (ca. 15 – 28 %) comprises of cellulose, hemicellulose and lignin can be utilized as carbon precursor [20]. Beside carbon, rice

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husk also consists of silica elements that can be employed in synthesis of CQDs to enhance the photoluminescence property [21, 22]. Utilization of rice husk as precursor of carbon and silica not only can convert low-value rice husk into high-value product, but also decrease presence of

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solid waste (rice husk) and liquid waste (pharmaceutical waste) at the same time. The transformation of TiO2 under various hydrothermal temperature, the amount of Si-CQDs added into TiO2 and the stability of composite were investigated to achieve excellent photocatalytic ability.

2.

Materials and Method

2. 1.

Materials

Titanium (IV) isopropoxide (TTIP, Ti(OCH(CH3)2)4, 97%, Sigma Aldrich, USA) was employed as TiO2 precursor, while rice husk from Jabatan Pertanian Negeri Perak, Malaysia was utilized as Si-CQDs precursor. Nitric acid (65%) and sodium hydroxide (NaOH pellets, CAS No. 1310-73-

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2) were purchased from Merck (Germany). Ethanol (95%), hydrochloric acid (~37%) and sulphuric acid (95-97%) were purchased from HmbG Chemicals (Germany), Fisher Scientific

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utilized as pharmaceutical compound for photocatalytic study.

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(UK) and Qrec (Asia) Sdn. Bhd, respectively. Acetaminophen (>99%, Sigma Aldrich, USA) was

Synthesis of TiO2 samples

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TiO2 samples were prepared via one-step hydrothermal method. First, TTIP (12 mL) as TiO2

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precursor was added into 40 mL of ethanol under constant stirring. HNO3 (1 M) was slowly dropped to the mixture until the pH reached 3.5. After being magnetically stirred for 1.5 h,

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NaOH solution (21 mL, 10 M) was added and the mixture was sonicated for another 30 min. The white suspension was transferred into Teflon-lined stainless steel autoclave and hydrothermally treated at specific temperature (100 – 200oC) for 36 h. Then, the autoclave was allowed to cool down to room temperature. After that, the precipitation was washed using HCl (0.1 M) and DI

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water until neutral pH was achieved. Finally, the obtained TiO2 were dried at 60oC, and calcined at 400oC for 2 h. The samples were denoted as Tx, which x represented the temperature of hydrothermal (100, 125, 150, 175 and 200oC).

2. 3.

Synthesis of Si-CQDs/TiO2 composites

Si-CQDs were synthesized from rice husk using hydrothermal method that had been published in previous study [22] with some modifications in hydrothermal condition. Briefly, the rice husk was heated in tube furnace at 700oC for 2 h under nitrogen atmosphere to obtain rice husk ash. Afterwards, 100 mg of rice husk ash was oxidized using 10 mL of H2SO4 and 3 mL of DI water in a water bath sonicator for 5 h. This was followed by immersion in 20 mL of HNO3 for 10 h in

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sonicator. The mixture was vacuum-filtered using 0.45 μm membrane and washed using DI

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water until the pH of supernatant became neutral. The obtained precipitate was then dispersed in 30 mL of DI water and NaOH solution was added to adjust the pH to 12. The mixture was

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transferred into autoclave and hydrothermally heated in oven at 200oC for 10 h. Si-CQDs

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solution was obtained by separating the supernatant from precipitate using 0.45 μm membrane and dialysis tubing with 10 kDa of molecular weight cut off for 72 h.

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The composites of Si-CQDs/TiO2 were prepared by mixing TiO2 samples to certain amount of Si-CQDs solution (1 to 3 wt% of TiO2). DI water was added to adjust total volume of the

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mixture to 60 mL. The mixture was then magnetically stirred for 2 h. The samples were kept in oven at 60oC overnight until Si-CQDs/TiO2 composites in powder form were obtained. The composites of Si-CQDs/TiO2 were labelled as y%C-Tx where x represented the temperature hydrothermal of TiO2 samples and y represented the weight percentage of Si-CQDs in TiO2

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samples. 2. 4.

Characterization methods

The morphology of TiO2 samples was investigated using Transmission Electron Microscope (TEM) Philips CM200. The crystallinity of samples was estimated using X-ray Diffractometer on X’Pert3 Powder & Empyrean PANalytical with Cu Kα irradiation (λ = 1.54) range of

diffraction angles (2θ) from 20 to 80

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with step size of 1 o/step and exposure time of 1 s/step.

Fourier Transform Infrared Spectroscopy (FTIR) was studied using Perkin Elmer Spectrum One. Light absorption of samples was obtained by using Agilent Technologies Cary Series UV-Vis Spectrophotometer in the range of 350 – 800 nm. The textural properties of the prepared samples specifically surface areas and pore size were investigated by using N2 sorption isotherm analysis

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(Micromeritics ASAP 2020 equipment). Photoluminescence (PL) spectra were recorded using Maya2000 Pro Spectrometer. Photocatalytic activity study

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2. 5.

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Photocatalytic activity of samples was evaluated in removal of acetaminophen as pharmaceutical waste compound. The photocatalyst with dose of 1 g/L was added in a test tube with certain

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amount of acetaminophen. The experiment was carried out under sunlight irradiation on June 2019 at Perak, Malaysia between 10.00 am to 04.00 pm. At certain time interval, 5 mL of

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solution was taken out and filtered to remove the photocatalyst. The concentration of acetaminophen was determined by measuring the intensity of the filtrate (λ = 243 nm) through

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UV-Vis spectroscopy (Shimadzu UV 1800).

Results and Discussion

3. 1.

The characteristics of TiO2 samples

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Figure 1 TEM images of TiO2 samples at different hydrothermal temperature (a) T100, (b) T125, (c) T150, (d) T175 and (e) T200, insets are its TEM images in higher magnification. The transformation of TiO2 morphology in varying hydrothermal temperature (100, 125, 150,

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175 and 200oC) is examined through TEM images. Figure 1 depicts the transformation of TiO2 morphology from nanotube to nanoribbon by the change of hydrothermal temperature. The average diameter is measured to be 6.0, 5.6, 5.0, 38.8 and 66.4 nm for T100, T125, T150, T175 and T200, respectively. When hydrothermal temperature increases from 100 to 150oC, tube-like morphology is formed. As the temperature increases from 100 to 150oC, the diameter of tube is decreasing and the length is increasing (length of T100 and T125 are 18.5 and 24.5 nm,

respectively). Further increase hydrothermal temperature to 200oC causes different type of morphology. The surface of TiO2 becomes rigid and it has larger diameter, which is typically called as nanoribbon. The transformation of TiO2 morphology is associated to Ostwald ripening mechanism [23]. During hydrothermal treatment, NaOH solution initiates the growth of TiO2 into nanotube structure. As the process goes further, the excess of NaOH dissolves the nanotube

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then recrystallizes into nanoribbon structure [24]. Functional groups on the surface of samples are studied using FTIR spectra (Figure 2). The

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broad peaks appearing within wavenumber in the range of 3368 to 3433 cm-1 assign to the

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stretching vibration of –OH groups [25], while bending vibration of –OH groups from absorbed water molecules is displayed by peak at 1632 cm-1 [26]. The presence of –OH groups on the

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surface of photocatalyst is beneficial for photocatalytic activity since it produces hydroxyl radical (•OH) after charge separation [27]. As shown in Figure 2, the intensity of –OH groups is

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decreasing as the hydrothermal temperature increases which might be caused by dehydroxylation of TiO2 [28]. The occurrence of Ti-O-Ti and Ti-O bending vibration are assigned by band

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appearing within range of 1000 to 400 cm-1 [14, 29].

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Figure 2 FTIR spectra of TiO2 samples at different hydrothermal temperature (a: 3368-3433, b:

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1632, c: 974, d: 819 and e: 786 cm-1).

Figure 3 represents XRD spectra for TiO2 samples after being calcined in air at 400oC for 2 h. The samples exhibit the combination diffraction peaks of anatase phase (JCPDS: 98-000-9853)

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and monoclinic phase which is commonly denoted as TiO2 (B) (JCPDS: 98-004-1056). In order to present the composition of anatase and TiO2 (B), quantitative study is performed using HighScope software with Rietveld method (Table 1). The proportion of anatase is increasing as the hydrothermal temperature increases, whereas the proportion of TiO2 (B) is decreasing and completely disappearing when hydrothermal temperature reaches 175oC. The crystallinity of produced TiO2 can be explained by the presence of HNO3 as peptizing agent in synthesis process

to control the crystalline phase of TiO2 [25]. In this study, utilization of HNO3 is used to adjust the pH of TTIP solution to 3.5, which can lead to completed transformation of produced TiO2 into anatase phase [30]. Since TiO2 (B) is considered as metastable polymorph of anatase [27], this study suggests that hydrothermal temperature also influences the crystalline phase of TiO2. Pure anatase phase of TiO2 is obtained from TTIP in presence of HNO3 solution when the

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temperature of hydrothermal is more than 175oC.

Figure 3 XRD spectra of TiO2 samples at different hydrothermal temperature. Table 1 The crystalline phase of TiO2 samples at different hydrothermal temperature. Sample

Anatase (%)

Monoclinic (%)

47.6

52.4

T125

84.9

15.1

T150

97.2

2.8

T175

100.0

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T200

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T100

temperature.

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Figure 4 (a) UV-Vis DRS spectra and (b) Tauc plot of TiO2 samples at different hydrothermal

The ability of samples in absorbing light is studied using UV-Vis DRS spectra (Figure 4a). The

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estimated band gap energy, as illustrated in Figure 4b, is calculated using Tauc plot [31, 32], that ploting (αhv)2 function against band gap energy (Eg = hv = hc/λ), where α is absorption coefficient, h is the Plank’s constant, and v is the frequency of radiation. The band gap energy is

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estimated by the value of x-axis after extrapolating the linear portion of graph to zero of y-axis. The value of band gap energy is determined by the position of valence and conduction band that influences the ability of photocatalyst to harvest light. As shown in Figure 4b, the samples exhibit band gap energy of 3.20, 3.15, 3.14, 2.95 and 2.87 eV for T100, T125, T150, T175 and T200, respectively. The band gap energy of TiO2 samples is controlled by its particle size.

Smaller particle size has stronger quantum confinement effect between electron and hole which

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leads to increase in band gap energy [33].

Figure 5 N2 sorption isotherms for TiO2 samples at different hydrothermal temperature.

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Surface area and pore size distribution of TiO2 samples at different hydrothermal temperature are estimated from N2 sorption isotherm curves (Figure 5). T100, T125 and T150 show type IV isotherm with H3 hysteresis loop indicating slit like pores [25]. According to IUPAC, T175 and T200 exhibit type III isotherm of macroporous nature, however, hysteresis loop is observed representing mesoporous materials [34]. The position of hysteresis loop in sorption isotherm curves at the highest relative pressure (P/P0 = 1) indicates the pore size of samples [25], where

high position of end hysteresis loop is related to large size of pores. As shown in Figure 5, the order position of samples is T200 < T175 < T100 < T125 < T150, suggests that T150 has the largest pore size while the smallest pore size is obtained in T200 samples. The surface area, pore size and total pore volume of TiO2 samples are summarized in Table 2. Table 2 The structural properties of TiO2 samples at different hydrothermal temperature. Surface Area Pore

Size Total

Pore

(nm)

Volume (cm3/g)

T100

126

16.51

0.520

T125

161

17.51

0.942

T150

268

23.38

1.175

T175

34

14.24

0.120

T200

29

10.31

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Sample

Photocatalytic ability of TiO2 samples

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The ability of TiO2 samples in photocatalyst application is carried out in 1 L of batch reactor, equipped by magnetic stirrer and 500 W Xenon lamp as simulated sunlight irradiation. In order

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to maintain the temperature of solution, the lamp is placed inside cylindrical quartz jacket with water circulation that has been set at 25oC. Prior to turning on the lamp, the mixture of photocatalyst and acetaminophen is stirred for 30 min in dark condition. The experiment is repeated for three times, so the removal efficiency (C/C0) of acetaminophen curves with error

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bars (vertical line) is presented (Figure 6). As the comparison, the photocatalytic activity in the absence of photocatalyst is also studied. It shows that acetaminophen is barely removed when only light is applied.

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Figure 6 Photocatalytic performances of TiO2 samples under 500 W Xenon lamp irradiation by

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removing 5 mg/L of acetaminophen.

Figure 6 reveals that TiO2 samples reduce the concentration of acetaminophen as the function of time. During the light off, the acetaminophen removal of all samples is merely 1 to 2 %, indicating insignificant adsorption process in this study. Acetaminophen is difficult to be

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adsorbed on TiO2 surface since it has stereochemical configuration [35]. Low adsorption ability suggests that photocatalytic performs dominant role in the removal of acetaminophen. Among TiO2 samples, T100 with mixed crystalline phase shows the highest photocatalytic activity, removing almost 100% of acetaminophen within 240 min under Xenon lamp irradiation.

In the early of photocatalyst investigation, researchers suggest that high surface area and low band gap energy are crucial parameters to enhance photocatalytic performance [36]. High surface area provides more active sites to contact with the pollutant, whereas low band gap energy is able to extend light absorption of TiO2 [37, 38]. However, these properties are less effective when the recombination within the semiconductor is still occurred very fast [39].

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Therefore, in order to investigate the charge separation within TiO2, PL study of the samples is performed. Figure 7 illustrates that PL intensity of TiO2 samples is increasing as hydrothermal

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temperature increases. PL emission is influenced by the properties of TiO2 samples, one of them is its surface defects [40]. When the temperature increases, re-arrangement of TiO2 bonding is

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occurred, then it creates Ti3+ defects on the surface as illustrated in PL spectra [41]. It can be

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noted that surface defects can be acted as recombination center, resulting decreased photocatalytic performance. Hence, this finding strongly suggests that T100 performs excellent

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photocatalytic performance due its effectiveness on charge separation.

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Figure 7 PL spectra of TiO2 samples at different hydrothermal temperature with excitation

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wavelength of 405 nm.

Photocatalytic ability of Si-CQDs/TiO2 composites

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Figure 8 Photocatalytic performance of (a) various photocatalyst in 5 mg/L of acetaminophen removal; (b) 1%C-T100 composite in removing of various acetaminophen concentration and (c) its kinetic data which is fitted by pseudo-first order model; and (d) life cycle study of 1%C-T100

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under sunlight irradiation.

The composites of Si-CQDs/TiO2 are prepared by varying Si-CQDs loading (1, 2 and 3 wt%) into TiO2 (T100). Figure 8a shows that sole Si-CQDs have no ability to remove acetaminophen, whereas, Si-CQDs/TiO2 composites perform different photocatalytic ability under sunlight irradiation. Among the variation of Si-CQDs loading into TiO2, 1%C-T100 is found to have the

highest photocatalytic ability, compared to 2%C-T100 and 3%C-T100 (Figure 8a). After 120 min of sunlight irradiation, removal efficiency of acetaminophen for 1%C-T100, 2%C-T100 and 3%C-T100 are 53%, 46% and 42%, respectively. Further irradiation to 240 min, 1%C-T100 composite completely removes the acetaminophen from water. The ability of T100 under direct sunlight irradiation is also tested and presented in Figure 8a. It shows that T100 requires 360 min

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of sunlight irradiation to remove 5 mg/L of acetaminophen. These results clearly suggest that the ability of TiO2 to remove acetaminophen could be enhanced by adding proper amount of Si-

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CQDs into TiO2. The optimum loading of Si-CQDs in the composite to maximize the removal efficiency of acetaminophen is 1 wt% of TiO2, which removes acetaminophen 33.3% faster than

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T100 (without addition of Si-CQDs).

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Further test for photocatalytic ability of 1%C-T100 composite is performed by increasing the initial concentration of acetaminophen from 5 mg/L to 10 and 20 mg/L. The initial concentration

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of acetaminophen influences the performance of photocatalyst, as shown in Figure 8b. When initial concentration increases from 5 to 20 mg/L, the ability of photocatalyst to remove

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acetaminophen is decreasing from 100% to 58%. This phenomenon could be caused by decreased ratio between photocatalyst and molecules of acetaminophen [14]. The efficiency of photocatalyst to remove acetaminophen is influenced by the amount of produced hydroxyl

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radical [7]; in this case, constant amount of photocatalyst produces same amount of hydroxyl radical. However, higher initial concentration of acetaminophen has more molecules to be removed. Consequently, the ability of photocatalyst to remove acetaminophen is reduced. Thus, it can be concluded that initial concentration and removal efficiency of acetaminophen are inversely related.

The removal efficiency of 1%C-T100 composite for various initial concentration of acetaminophen is fitted by pseudo-first order kinetic model, which represented by −ln(𝐶𝑡 /𝐶0 ) = 𝑘 𝑡 [14, 27, 29]. 𝐶𝑡 and 𝐶0 are concentration of acetaminophen (mg/L) at time 𝑡 of the reaction and at time 0 (beginning of the reaction), respectively; while 𝑘 is removal rate constant. The result indicates that pseudo-first order kinetic model fits well for 1%C-T100 composite with R2

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value in the range of 0.987 to 0.995 (Figure 8c). Compared to T100 sample, 1%C-T100 composite performs better performance in various initial

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concentration of acetaminophen. Figure S1 depicts that T100 is only able to remove 48% of 20

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mg/L acetaminophen within 360 min of sunlight irradiation, which is ~20% lower than 1%CT100.

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The stability of 1%C-T100 composite is assessed by recycling the photocatalyst. After each

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experiment, the composite is collected and repeatedly used without any treatment. As presented in Figure 8d, after three consecutive cycles of removal, the composite of 1%C-T100 is still able

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to completely remove 5 mg/L of acetaminophen within 240 min of sunlight irradiation. Even though the ability of 1%C-T100 composite is slightly decreasing (less than 2%) at 120 min of irradiation, the composite is considered to have good stability. 3. 4.

The characteristics of Si-CQDs/TiO2 composites

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The possible explanation regarding its enhancement on photocatalytic ability is studied by examining the characteristics and properties of the composites. XRD spectra of Si-CQDs/TiO2 composites with different amount of Si-CQDs (Figure 9) reveal that the crystalline phase of photocatalyst is changed from mixture of TiO2 (B) and anatase to pure anatase. This transformation is probably not caused by Si-CQDs addition because addition

of Si-CQDs in different amount exhibits identical XRD spectra. This argument is in good agreement to previous study [14, 29, 42] which reported that addition of carbon material in small amount does not change the crystallization of TiO2. The transformation in crystallinity could be explained due to heating treatment in water at 60 oC during synthesis process of Si-CQDs/TiO2

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composite, transforming TiO2 (B) into stable phase (i.e., anatase) [43].

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Figure 9 XRD spectra of Si-CQDs/TiO2 composites. FTIR spectra of Si-CQDs/TiO2 composites are shown in Figure 10, presenting that SiCQDs/TiO2 composites have similar spectra to TiO2 samples. However, additional peak appears in the range of 1383-1388 cm-1 which could be due to stretching vibration of O-Na bond [34]. The intensity of O-Na bond becomes higher with increasing amount of Si-CQDs addition in the

composite. This could be explained due to the presence of Na+ from NaOH in synthesis of Si-

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CQDs, which is carried and attached to the composite.

Figure 10 FTIR spectra of Si-CQDs/TiO2 composites (a: 3400, b: 1632, c: 1383-1388, d: 974, e: 786, f: 470 cm-1).

The presence of Si-CQDs in TiO2 is not clearly observed in low magnification of TEM images;

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therefore, further observation is done within white-square in Figure 11a. The lattice spacing of samples is observed and measured to be 0.220 nm and 0.147 nm. The lattice spacing of 0.220 nm is ascribed to (100) in-plane of graphite [44, 45], while lattice spacing of 0.147 nm is close to lattice spacing in T100 samples (as shown in Figure 11c). Figure 11b suggests that Si-CQDs decorate TiO2 on its surface.

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Figure 11 HRTEM images of 1%C-T100 in (a) low and (b) high magnification, and HRTEM

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images of (c) T100 in high magnification.

EDX mapping (Figure S2) shows the existence of carbon and silica elements, indicating that Si-

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CQDs are successfully decorating TiO2 surface. Besides, Figure S2 also reveals that silica and

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carbon elements are evenly distributed over TiO2 surface. Table 3 displays the composition of Si-CQDs/TiO2 composites from EDX analysis. As the loading of Si-CQDs increases, decreased

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weight percentage of carbon elements is observed within the composite. As illustrated in Figure S3, the CQDs are attached on silica elements. Since increased weight percentage of silica

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elements is detected by increasing loading of Si-CQDs, the silica probably covers the carbon (CQDs) during Si-CQDs/TiO2 preparation. Table 3 The weight percentage of elements within Si-CQDs/TiO2 composite. weight%

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Sample

Ti

O

C

Si

Na

1%C-T100

40.94

50.15

6.16

0.98

1.77

2%C-T100

37.42

52.84

5.46

1.96

2.32

3%C-T100

42.29

48.39

3.48

2.86

2.97

As shown in Figure 12, Ti 2p spectra displays two peaks at 458.6 and 464.2 eV, which can be ascribed to Ti 2p3/2 and Ti 2p1/2, respectively [46]. Two peaks are detected in C 1s spectra: 285.2

eV for C=C and 289.7 eV for O-C=O, while Si 2p spectra only display one peak at 102.9 eV ascribed to Si-O-Si [22]. Furthermore, O 1s spectra are deconvoluted into three peaks located at 529.9, 533.7 and 535.7 eV that assigned as Ti-O [47], C=O and adsorbed O [48], respectively. These spectra suggest that Si-CQDs are composited with TiO2 through Ti-O-C bonding [46, 47,

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49].

Figure 12 (a) Ti 2p, (b) O 1s, (c) C 1s and (d) Si 2p spectra of 1%C-T100 The enhanced photocatalytic ability of Si-CQDs/TiO2 could be attributed to its surface area. As revealed by Table 4, addition of 1 wt% Si-CQDs increases the surface area from 126 m2/g (T100) to 148 m2/g (1%C-T100). Large surface area has synergic effect to photocatalytic activity

since it provides more active sites to adsorb the acetaminophen [9]. However, Si-CQDs addition more than 1 wt% of TiO2 decreases the surface area of composite since excessive of Si-CQDs might blocks the pore, which is confirmed by Figure 13 and pore size of each composite as listed

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in Table 4.

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Figure 13 N2 sorption isotherms for Si-CQDs/TiO2 composites. Table 4 The structural properties of Si-CQDs/TiO2 composites.

Sample 1%C-T100

Surface

Pore Size

Total

Pore

Area (m2/g) (nm)

Volume (cm3/g)

148

0.631

17.47

2%C-T100

137

17.01

0.597

3%C-T100

113

15.41

0.435

Beside surface area, lower band gap energy is also accounted for enhanced photocatalytic ability (Figure 14). The band gap energy for Si-CQDs/TiO2 composite with Si-CQDs loading of 1, 2 and 3 wt% are determined to be 3.12, 3.14 and 3.20 eV, respectively. This suggests that the optimum loading of Si-CQDs in TiO2 (1 wt%) is able to utilize sunlight more effectively than

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pure TiO2, as confirmed by its ability to remove acetaminophen in Section 3.3.

Figure 14 (a) UV-Vis DRS spectra and (b) Tauc plot of Si-CQDs/TiO2 composites. Figure 15 depicts the PL spectra of pure TiO2 (T100) and Si-CQDs/TiO2 composite (1%C-T100) at 300 nm excitation. Both samples exhibit several peaks in the range of 380 – 480 nm

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representing hetero-structured nature of the photocatalyst [19]. Overall, it clearly shows that the PL intensity of the photocatalyst is significantly decreasing when Si-CQDs are added, indicating that Si-CQDs could be acted as electron trapper thus it prolongs the lifetime of charge separation within TiO2.

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Figure 15 PL spectra of the photocatalyst at excitation wavelength of 300 nm. Addition of Si-CQDs onto TiO2 significantly enhances the removal of acetaminophen from water under direct sunlight irradiation. The enhancement is mainly attributed to its properties, such as: enhanced surface area, extended light absorption and prolonged charge separation. In this study,

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Si-CQDs act as photo sensitizer and electron trapper. When the composite of Si-CQDs/TiO2 is irradiated by direct sunlight, TiO2 absorbs UV spectrum of the sunlight while visible spectrum is harvested by Si-CQDs. Following that, the separation of electron and hole is occurred within TiO2 and Si-CQDs. Acting as an electron trapper, the electron at conduction band of TiO2 is transferred to Si-CQDs to prolong the recombination between electron and hole within TiO2.

4.

Conclusion

The sunlight-driven photocatalyst of Si-CQDs/TiO2 to remove acetaminophen from water are successfully prepared in this study. The investigation regarding TiO2 transformation suggests that TiO2 which is synthesized through hydrothermal treatment at 100oC (T100) has better photocatalytic ability among the TiO2 samples. Besides, it also suggests that the photocatalytic

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performance is not only affected by high surface area and low band gap energy, but also affected by the charge recombination within TiO2 samples. Si-CQDs act as photo sensitizer and electron

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trapper in the composite that enhance the photocatalytic activity for acetaminophen removal under direct sunlight irradiation. It is found that 1 wt% is the optimum loading of Si-CQDs into

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TiO2 which is able to completely remove acetaminophen 33.3% faster than pure TiO2. The

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excellent photocatalytic activity is accounted for enhanced surface area, lower band gap energy

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Declaration of interests

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and prolonged charge separation as the result of Si-CQDs addition into TiO2.

The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

CRediT author statement Viona Wongso: Conceptualization, Methodology, Formal Analysis, Investigation, Writing Original Draft, Writing - Review & Editing. Khee Chung Hui: Validation, Investigation, Writing - Review & Editing. Nonni Soraya Sambudi: Writing - Review & Editing, Resources, Visualization, Supervision, Funding acquisition. Suriati Sufian: Supervision, Resources. Bawadi Abdullah: Supervision, Resources. Mohamad Dzul Hakim Wirzal: Supervision, Resources. Ang Wei Lun: Writing - Review & Editing.

5.

Acknowledgments

This work was supported by Fundamental Research Grant Scheme (FRGS) project with code

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FRGS/1/2018/TK02/UTP/03/3.

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