Effects of Carbon Nanotube and Carbon Sphere Templates in TiO2

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Kinetics, Catalysis, and Reaction Engineering

Effects of carbon nanotube and carbon sphere templates in TiO2 composites for photocatalytic hydrogen production Mahmoodreza Karimiestahbanati, Mehrzad Feilizadeh, Marziehossadat Shokrollahi Yancheshmeh, and Maria C. Iliuta Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05815 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Industrial & Engineering Chemistry Research

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Effects of carbon nanotube and carbon sphere

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templates in TiO2 composites for photocatalytic

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hydrogen production

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M.R. Karimi Estahbanati1, Mehrzad Feilizadeh2, Marziehossadat Shokrollahi Yancheshmeh1, Maria C.

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Iliuta1*

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1

Department of Chemical Engineering, 1065 Av. De la Médecine, Université Laval, Québec, Québec G1V 0A6,

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Canada.

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2 School

of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran.

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KEYWORDS: Carbon sphere; Carbon nanotube; Photocatalysis; Hydrogen production; TiO2; sol-gel;

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hydrothermal.

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Abstract. Incorporation of carbonaceous templates (CT) into TiO2 composites is a promising alternative

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to increase the photocatalytic activity of TiO2. In this work, the effects of carbon sphere (CS) and carbon

*

Corresponding author ([email protected]). 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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nanotube (CNT) incorporation (as CT) in TiO2 composites were thoroughly investigated and the roles of

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these CTs as template, cocatalyst, and adsorbent were studied. To this end, three different methods

18

were utilized to form a layer of TiO2 on the CT: alcoholic phase sol-gel, aqueous phase sol-gel, and

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hydrothermal. The role of CT as template was examined through morphology analysis of the prepared

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composites. The cocatalyst and adsorbent roles of CT were investigated based on photocatalytic

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hydrogen production from glycerol. Interestingly, it was found that the incorporation of CNT into TiO2

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composite can approximately double the rate of hydrogen production (i) in the absence of Pt or (ii) at

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low glycerol concentration. Accordingly, it was concluded that in addition to being a template, the CNT

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can play two important roles as cocatalyst and adsorbent.

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1.

INTRODUCTION

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Glycerol is a by-product of biodiesel production from biomass1. Due to glycerol overproduction, its

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price in the market has decreased significantly in the past years, setting it up as a promising renewable

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feedstock2-3. Photocatalytic valorization of bio-based glycerol to hydrogen is an alternative to today’s

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critical energy, environmental, and sustainability issues4-5. Photocatalysis is currently being considered

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as a promising technique with many applications, such as renewable energy production and treatment

31

of environmental pollution. Although TiO2 is the most prominent photocatalyst and has attracted the

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most attention in the photocatalysis research, it still suffers from low efficiency6-7. Utilization of

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carbonaceous template (CT) incorporated composite to enhance the photocatalytic activity of TiO2 has

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been widely investigated in the recent years8-9. Incorporation of bio-based carbonaceous templates to

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prepare TiO2@CT composites is an effective and sustainable approach10-11.

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The discovery of fullerenes in 1985 drew tremendous attention to the application of carbon

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(nano)materials in contemporary research12. For instance, since the discovery of carbon nanotube (CNT)

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in 199113, its applications in different fields including catalysis have been extensively studied. As the 2 ACS Paragon Plus Environment

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most well-known one-dimensional carbonaceous material with unique structural, chemical, thermal,

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and electrical properties14, CNT can be used in many applications such as preparation of TiO2

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composites15-16. Carbon sphere (CS) is another carbonaceous material that has a long history, but has

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especially gained a lot of attention in the past decade. CS benefits from the advantages of both carbon

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materials and spherical colloids that provide it with remarkable characteristics such as uniform

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geometry, good liquidity, controllable porosity, surface functionality, adjustable particle size

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distribution, and excellent chemical and thermal stability17-18. These characteristics suggest CS as a

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promising CT for photocatalytic applications.

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To prepare a TiO2@CT composite, simple mechanical mixing of pre-synthesized semiconductors and

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CT may not enhance the photocatalytic activity19. Thus, to have an intimate contact between

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semiconductor and CT, the semiconductor should be synthesized in the presence of CT20. Sol-gel and

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hydrothermal methods are the most common approaches for the preparation of carbonaceous TiO2

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composites8, 15. In the synthesis, the thermal treatment is important because it affects the crystallinity,

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particle sintering, and electronic interphase interaction between TiO2 and CT21.

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Several studies have been performed on the photocatalytic activity of TiO2 and CT composites for

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hydrogen generation22-25. Although many of them indicated the effectiveness of CNT and CS in

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photocatalytic activity, some showed that they may not be beneficial in specific cases8, 26. Leary et al.8

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conducted a comprehensive literature review on the effect of carbonaceous materials on the

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enhancement of TiO2 photocatalytic activity and concluded that the first challenge in further

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exploitation of synergistic effects of carbonaceous materials is a better understanding of the

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fundamentals of enhancement of photocatalytic activity using carbonaceous materials as well as

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controlling the synthesis method conditions.

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CTs can play different roles, such as support, cocatalyst, and adsorbent27. First, as a support, CTs are

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promising materials available in different 0D, 1D, 2D, and 3D allotropes28. This vast variety provides the 3 ACS Paragon Plus Environment


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opportunity of TiO2 formation in different shapes such as nanotube, sphere, sheet, and nanodiamond.

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Second, CTs can also play the role of cocatalyst as they possess conjugated π electrons that can

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accelerate the transfer of photogenerated electrons from conduction band to H2 production active

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sites29. Considering the new approach of using low-cost earth-abundant metals or metal free materials30,

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CTs are promising cheap, abundant, stable, and non-toxic cocatalysts. Third, the high adsorption

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capacity of CTs reveals the effectiveness of CTs incorporation as adsorbent in TiO2 composites, as CTs

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not only increase the concentration of the substrate on the surface of photocatalyst, but even enhance

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the product selectivity28. To the best of our knowledge, no study focussed on the individual effects and

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roles of CTs in the photocatalytic hydrogen production.

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In this context, the aim of the present work is to perform a thorough comparative study on

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carbonaceous TiO2 composites to explore the individual roles of CTs as template, cocatalyst, and

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adsorbent. The analysis was performed using different (i) carbonaceous material (CS and CNT), (ii)

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synthesis methods (hydrothermal, alcohol phase sol-gel, and aqueous phase sol-gel), and (iii) calcination

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temperatures (300-800 ºC). The role of CT as template was examined through the morphology analysis

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of the prepared composites. The synergistic functions of CT and Pt during photocatalytic hydrogen

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production reactions were evaluated to investigate the effect of CT as cocatalyst. Finally, the adsorbent

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role of CT was assessed by comparing the amount of hydrogen produced in the presence or absence of

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CT.

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2. EXPERIMENTAL SECTION

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2.1.

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Materials

Graphistrength® C100 multi-walled CNT (≥90%,10-15 nm diameter, 5-10 walls, 1-10 μm length31) was kindly provided by ARKEMA and functionalized as described below. Titanium (IV) isopropoxide (TTIP, 4 ACS Paragon Plus Environment

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≥98%) and titanium (IV) n-butoxide (TBOT, ≥99%) were obtained from Acros Organics. Glycerol (≥99.7%),

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acetic acid (≥99.7%), ethyl alcohol (99.99%), and 2-propanol (≥99.9%) were purchased from VWR

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Analytical, Caledon, Commercial Alcohols, and Fisher Scientific, respectively. Nitric acid (68.0-70.0%),

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sulfuric acid (95.0-98.0%), and hydrochloric acid (36.5-38.0%) were supplied from Anachemia. Benzyl

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alcohol (≥99%) and D-(+)-Glucose anhydrous (≥99%) were bought from Alfa Aesar. Hexachloroplatinic

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acid (IV) (H2PtCl6·6H2O, ≥37.50% Pt) was purchased from Sigma-Aldrich. Potassium bromide (≥99%) of

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Oakwood Chemicals brand was used to make pellets for FT-IR analysis.

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Table 1. List of the prepared samples. Nomenclature

Template

Synthesis method

Calcination temperature (ºC)

TiO2@CS_HT_CN

CS

Hydrothermal

Non-calcined

TiO2@CS_HT_C300

CS

Hydrothermal

300

TiO2@CS_HT_C400

CS

Hydrothermal

400

TiO2@CS_HT_C500

CS

Hydrothermal

500

TiO2@CS_HT_C600

CS

Hydrothermal

600

TiO2@CS_HT_C800

CS

Hydrothermal

800

TiO2@CNT_HT_CN

CNT

Hydrothermal

Non-calcined

TiO2@CNT_HT_C300

CNT

Hydrothermal

300

TiO2@CNT_HT_C400

CNT

Hydrothermal

400

TiO2@CNT_HT_C500

CNT

Hydrothermal

500

TiO2@CNT_HT_C600

CNT

Hydrothermal

600

TiO2@CNT_HT_C800

CNT

Hydrothermal

800

TiO2_HT_C400

Templateless

Hydrothermal

400

TiO2@CNT_ALSG_C400

CNT

Alcohol phase sol-gel

400

TiO2@CS_ALSG_C400

CS


400

TiO2_ALSG_C400

Templateless


400

TiO2@CNT_AQSG_C400

CNT

Aqueous phase sol-gel

400

TiO2_AQSG_C400

Templateless

Aqueous phase sol-gel

400

94 95

2.2.

Sample preparation

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2.2.1. Nomenclature

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The prepared samples were coded as a string with three elements: TiO2@A_B_CX (Table 1). A

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represents the type of template employed (CS or CNT). When no template was used during the

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synthesis, only the type of semiconductor (TiO2) is mentioned. B represents the synthesis method


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denoted by HT for hydrothermal, ALSG for alcoholic phase sol-gel, and AQSG for aqueous phase sol-gel.

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CX represents the calcination temperature, X=300-800 ºC (CN denotes a non-calcined sample).

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2.2.2. CS preparation

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The CS template was prepared using glucose as a sustainable carbon source based on the method

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described in other work32. Briefly, 36 g of glucose were dissolved in 200 ml deionized water and kept at

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room temperature under 30 min stirring. Then, the solution was transferred into a 460-ml autoclave and

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kept at 180 ºC for 18 h. The formed brownish carbon sphere particles were filtered and washed with

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distilled water 3 times using the centrifuge. Finally, the sample was dried at 80 ºC for 24 h. 5% CS33 was

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used to prepare TiO2@CS composite through all the synthesis methods.

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2.2.3. Functionalization of CNT

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To attach TiO2 particles on the surface of CT, oxygen-containing groups such as hydroxyl, carboxyl,

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carbonyl, and epoxy are required on the surface to provide nucleation sites and play the role of anchor

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towards the reagents34. Because of the absence of these groups in the case of pristine CNT, the

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functionalization is typically essential. An acid treatment was therefore performed to functionalize the

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CNT and remove the catalytic metal residues and amorphous carbon35. The pristine CNT was first

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washed with 32 wt. % HCl at 70 ºC for 12 h in a round bottom flask equipped with a condenser36. The

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sample was then washed with deionized water and kept at boiling temperature in 10 M nitric acid

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solution for 3 h21. It is worth mentioning that the functionalized CNT using a mixture of H2SO4:HNO3 =

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3:1 (volume ratio) was not found effective due to extensive damage to the CNT structure and low

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recovery. After cooling, the sample was filtered under vacuum and rinsed several times by deionized

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water to reach the neutral pH of filtrate. The prepared sample was dried in the oven at 80 ºC for 16 h.

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Approximately 9% weight loss was observed during the acid treatment process. All the TiO2@CNT



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composites were prepared with 20% CNT8, 37; however, samples containing 1% CNT were also prepared

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and used only to analyze the adsorbent role of CNT.

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2.2.4. Hydrothermal synthesis

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To prepare composites by hydrothermal synthesis, the required amount of TTIP was added to

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propanol (solvent) and BA (linking agent) with the volume ratio of TTIP:BA:PrOH equal to 10:17.5:150.

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After 30 min of continuous stirring, CT (CS or HCl treated CNT which were prepared as described in

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sections 2.2.2 and 2.2.3, respectively) was added to the solution and sonicated in an ice bath for 1 h,

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while covered to prevent alcohol evaporation and moisture absorption. The as-obtained suspension was

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transferred to a 460-ml Teflon-lined stainless-steel autoclave and heated in an oven at 180 ºC for 48 h.

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After formation of titanium on the CT, the brownish/black product was harvested by centrifugation. A

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washing process involving three cycles of centrifugation, washing, and re-dispersion in ethanol was then

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performed. The resulting composite was dried at 90 ºC overnight and calcined in air atmosphere for 2 h.

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For comparison purpose, the above-mentioned procedure was repeated without the addition of CT.

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2.2.5. Alcoholic phase sol-gel synthesis

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The required amount of CT (CS or HCl treated CNT) was added to ethanol and subjected to

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ultrasonication for 1 h. Then, BA and water were added to the suspension and mixed for 30 min

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(solution A). Separately, TBOT was dissolved in ethanol by mixing for 30 min (solution B). Solution B was

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added dropwise into solution A with the rate of 30 ml/h using a syringe pump to obtain a

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TBOT:BA:H2O:EtOH molar ratio of 1:5:5:100. After ultrasonication, the mixture kept under stirring for 1 h

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to completely hydrolyze the titanium precursor. The formed suspension was subjected to 3 cycles of

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centrifugation and washing and subsequently dried at 90 ºC overnight. The obtained powder was

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calcined at 400 ºC for 2 h (heating ramp of 5 ºC min-1). Templateless TiO2 was also prepared according to

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the procedure mentioned above without addition of CT for comparison purposes. 8 ACS Paragon Plus Environment

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2.2.6. Aqueous phase sol-gel synthesis

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The aqueous phase sol-gel synthesis was done according to our previous research38-39. Briefly, a

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beaker containing glacial acetic acid was placed in an ice bath to freeze. TTIP was added and mixed

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under magnetic stirring for 30 min. The beaker was covered by a layer of parafilm to avoid moisture

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absorption or acid evaporation. After injecting slowly deionized water using a syringe pump (pumping

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rate of 30 ml/h), the nitric acid treated CNT was added to the solution and ultrasonicated in the ice bath

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under magnetic mixing for 1 h. The obtained suspension was subsequently kept under stirring for 2 h to

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let TTIP completely hydrolyze. The mixture was sonicated again in the ice bath for 15 min and aged at

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room temperature in the absence of light for 24 h. Then, the suspension was gelated at 75 ºC overnight

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and dried at 120 ºC for 2 h. The dried gel was ground into a fine powder and calcined in air at 400 ºC

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(heat ramp of 5 ºC/min). Templateless TiO2 was also prepared according to the same procedure, but

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without employing CT.

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2.3.

Characterization techniques

The structure and morphology of the prepared samples were analyzed by scanning electron

160

microscopy (SEM) using JEOL JSM-840A device. Transmission electron microscopy (TEM) of the samples

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was captured using a JOEL JEM 1230 operated with 120 kV accelerating voltage. For SEM and TEM, 25

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ppm suspension of the samples in ethanol was sonicated to detach the agglomerated particles and the

163

analyses were performed after drying. Particle size analyses were performed using multiple SEM and

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TEM images. X-ray diffraction (XRD) was collected for 2=10-80º on a Bruker SMART APEXII X-ray

165

diffractometer equipped with Cu K radiation source (= 1.5418 Å). The percentage of rutile phase in

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the samples was calculated based on Eq. (1)40.



% Rutile 

1  100 1  0.8(I A / IB )

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(1)

167

where IA and IB respectively represent the intensities of anatase (101) and rutile (110) reflections.

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Thermogravimetry and differential thermal analysis (TG/DTA) of the prepared samples were carried

169

out on a TGA Q5000IR from 50 to 700 ºC with a heating ramp of 10 ºC/min and air flow of 25 ml/min.

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For BET measurements using Micrometrics TRISTAR 3000, the samples were dried for 4h at 120 ºC to

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remove the adsorbed water. FT-IR spectrum was taken using an FTS 45 infrared spectrophotometer with

172

the KBr pellet technique. ATR-FTIR absorption spectra were measured by Thermo Nicolet Magna-850

173

Spectrometer device and a Golden Gate® ATR accessory.

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2.4.

Photocatalytic hydrogen production

175

For each experiment, an aqueous solution containing 10% glycerol (which is the average glycerol

176

content in biodiesel production wastes) and 1 g/l catalyst was introduced in 10 ml vials. To investigate

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the effect of glycerol concentration, some tests were also performed for 1% and 50%. The required

178

amount of platinum (Pt) precursor solution in deionized water was added to form 1 wt% Pt as cocatalyst

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on the surface of the photocatalyst. The final volume of suspension was adjusted to 5 ml using deionized

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water. Before starting the experiments, each of the following steps was performed for 15 min: (a)

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ultrasonication by a Hielscher UP400S Ultrasonic Processor to detach the agglomerated catalyst, (b)

182

purging with 20 ml/min flow of nitrogen to remove oxygen, and (c) mixing the suspension to assure the

183

adsorption of glycerol on the surface of the catalyst. The photocatalytic hydrogen production

184

experiments were done at ambient temperature by starting irradiation using four Black-Ray® 20 W

185

mercury tubes (365 nm). Thermo-ScientificTM CimarecTM 15-Position magnetic stirrer was utilized to have

186

a uniform 500 rpm mixing during photocatalytic experiments. Pt was deposited on the photocatalyst at

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the beginning of the photocatalytic reaction based on in-situ photodeposition method41. The amount of 10 ACS Paragon Plus Environment

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hydrogen produced by each of the prepared samples was measured by analyzing 250 µl of the product

189

gas headspace after 4 h of irradiation, using Agilent Technologies 7820A gas chromatograph (GC). The

190

GC equipped with the HP-Molesieve column (Agilent) and TCD detector used nitrogen as carrier gas. The

191

experiments were performed in triplicate and a mean value was reported. Some experiments were done

192

in the absence of light and photocatalyst to be certain of the photocatalytic nature of the reaction.

193

Some tests were also performed using a pure template and no photocatalytic activity was observed.

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3. RESULTS AND DISCUSSION

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3.1.

197 198

Morphology analysis

The structure and morphology of TiO2 nanoparticles formed on the CT were investigated based on SEM and TEM analyses (Figure 1 and Figure 2).

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200

Figure 1. SEM images of (a) as-prepared CS, (b) TiO2@CS_HT_CN, (c) TiO2@CS_HT_C300, (d)

201

TiO2@CS_HT_C400, (e) TiO2@CS_HT_C500, (f) TiO2@CS_HT_C600, (g) TiO2@CS_HT_C800, (h)

202

TiO2@CS_ALSG_C400, and (i) TiO2@CNT_AQSG_C400.

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Figure 2. TEM images of (a) as-prepared CS, (b) TiO2@CS_HT_CN, (c) pristine CNT, (d) HCl treated CNT,

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(e) HCl and HNO3 treated CNT, (f) TiO2@CNT_HT_CN, (g) TiO2@CNT_HT_300, (h) TiO2@CNT_HT_400, (i)

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TiO2@CNT_HT_500, (j) TiO2@CNT_HT_600, (k) TiO2@CNT_HT_800, (l) TiO2_AQSG_C400, (m)

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TiO2@CNT_AQSG_C400.

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As can be seen from the SEM image of as-prepared CS (Figure 1a), the spheres have almost uniform

209

spherical morphology and no broken CS is observed. Based on the TEM image of as-prepared CS (Figure

210

2a), the spheres are not uniformly formed through the entire bulk. This tendency to accretion is

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common in the CS with less than 1000 nm diameter42. Indeed, the size distribution histogram of CS

212

depicted in the inset of Figure 1a reveals diameters between 400-600 nm with an average value of 497

213

nm.

214

Figure 1b-g depicts the SEM of TiO2 samples prepared on CS by hydrothermal method and calcined at

215

different temperatures. A spherical morphology with a particle diameter of around 3 µm can be seen for

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all samples. The surface of the spheres is rough, suggesting that they may be formed by nanocrystalline

217

flakes. This assumption was proved by the TEM image of spherical TiO2 surface (Figure 2b). Although the 13 ACS Paragon Plus Environment


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samples calcined at different temperatures show a similar spherical shape, the effect of increasing

219

calcination temperature on the sintering can be seen on the surface. For instance, the surface roughness

220

of the spheres calcined at 800 ºC is evidently reduced compared to those calcined at the lower

221

temperature (Figure 1c-g), in agreement with the obtained BET result (Table 2). Nevertheless, almost no

222

noticeable change in the sphere diameter is observed, which is consistent with the work done by

223

Bakardjieva et al.43. This observation suggests that the modification of the structure of the samples

224

prepared on CS (through carbon sphere removal by burning, transformation of amorphous phase to

225

anatase and anatase to rutile, and sintering) did not affect the sphere diameter but predominantly

226

influenced the morphology of carbon sphere surface.

227

Figure 1h depicts the SEM image and the spherical size distribution of the TiO2 samples grown on the

228

CS using the alcoholic phase sol-gel method. By considering the shape of CS (Figure 1a), it can be seen in

229

Figure 1h that the TiO2 particles formed uniformly on the surface of CS and retained its shape. The

230

diameter of particles is distributed between 800 and 1400 nm and the average diameter is 1129 nm.

231

Thus, the average diameter is almost doubled in comparison to CS.

232

The TEM images of the pristine, HCl treated, and HCl and HNO3 treated CNT samples are shown in

233

Figure 2c-e. As can be seen, the pristine CNT is a network of curved entanglements and intertwined

234

strands, with some intense black agglomerates that can be organic and metallic impurities. HCl

235

treatment had no significant effect on the morphology of the CNT; however, the treatment by HNO3

236

caused opening of the CNT bundles and shortening of the length of strands that facilitates the CNT

237

access by the TiO2 particles44. Moreover, in comparison to the pristine and acid treated CNT, acid

238

treatment successfully eliminated the impurities (as confirmed by TGA results).

239

TEM analysis of TiO2 samples formed on the CNT by hydrothermal method (Figure 2f-k) illustrates the

240

formation of TiO2 nanoparticle clusters with an intimate bound along the side walls as well as the ends

241

of CNT. Similar results have been previously obtained on TiO2@CNT composites45-48. As seen in Figure 2f14 ACS Paragon Plus Environment

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h, the TiO2 nanoparticles distributed between the bundles and the CNT are able to hinder the

243

agglomeration of TiO2 particles. FT-IR analysis of CNT also confirmed the formation of functional groups

244

on the surface of CNT as a result of the functionalization process. These functional groups provided

245

nucleation sites and played the role of anchor during formation of TiO2 nanoparticles on the CNT

246

surface. The diameter of CNT bundles is around 10-15 nm, confirming that the functionalization and

247

sample preparation had no effect on CNT diameter. The CNT is burned out in the samples calcined at

248

500 ºC or higher, in agreement with the TGA results. Even after burning the CNT templates, the TiO2

249

particles have almost retained their interconnected structure (Figure 2i-j). From TEM images, the

250

average particle size of the samples prepared by hydrothermal method on CNT and calcined at 300, 400,

251

500, 600, and 800 ºC was calculated to be around 18, 19, 20, 22, and 58 nm, respectively. The increase

252

of particle size can be attributed to the agglomeration of particles as well as the conversion of anatase

253

to rutile. The particle size of the samples calcined at 300-600 ºC is very close to the crystallite size

254

calculated from XRD results (Table 2). However, the particle size of the sample calcined at 800 ºC is

255

higher than its crystalline size, suggesting that each particle is composed of more than one crystal.

256

The TEM image of the templateless TiO2 sample produced by aqueous phase sol-gel method shows the

257

agglomeration of nanoscale particles with the average particle size of 9 nm (Figure 2l). However, in the

258

presence of CNT, the TiO2 particles are successfully distributed between CNT strands (Figure 2m). Due to

259

the use of acetic acid in the synthesis method, the CNT template agglomerates due to shifting CNT zeta

260

potential towards zero at low pH49. The agglomeration of CNT leads to an accumulation of TiO2 particles

261

which can also be seen in the corresponding SEM image (Figure 1i). It is worth mentioning that the

262

stability of the prepared functionalized CNT template in water was evaluated in different pH and only at

263

basic pH did it remain dispersed after several days. This observation confirms that the functionalized

264

CNT agglomerated during synthesis with aqueous phase sol-gel method.

265 15 ACS Paragon Plus Environment


266

Page 16 of 42

Table 2. Physicochemical properties and the rate of hydrogen production of different samples. Sample

BET surface area (m2/g)

Polymorph (%)

Crystallite size (nm)

H2 production (μmol g-1 hr-1)

anatase

rutile

anatase

rutile

5.7

-

-

-

-

-

CNT HCl treated

251.2

-

-

-

-

-

CNT HCl and HNO3 treated

296.4

-

-

-

-

-

TiO2@CS_HT_CN

66.1

-

-

-

-

-

TiO2@CS_HT_C300

77.3

100

0

14

-

795

TiO2@CS_HT_C400

45.8

100

0

16

-

3210

TiO2@CS_HT_C500

41.6

100

0

18

-

3000

TiO2@CS_HT_C600

11.8

100

0

25

-

1210

TiO2@CS_HT_C800

1.3

0

100

-

45

965

TiO2@CNT_HT_CN

103.5

-

-

-

-

-

TiO2@CNT_HT_C300

104.4

100

0

13

-

50

TiO2@CNT_HT_C400

90.2

100

0

14

-

350

TiO2@CNT_HT_C500

51.6

100

0

17

-

830

TiO2@CNT_HT_C600

42.1

100

0

21

-

265

TiO2@CNT_HT_C800

6.4

25

75

36

44

45

TiO2_HT_C400

66.0

100

0

13

-

1145

TiO2@CNT_ALSG_C400

100.8

100

0

14

-

580

TiO2@CS_ALSG_C400

2.7

100

0

19

-

545

TiO2_ALSG_C400

7.3

100

0

27

-

670

TiO2@CNT_AQSG_C400

194.9

100

0

9

-

4360

TiO2_AQSG_C400

133.1

100

0

9

-

4790

CS

267


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268


3.2.

XRD patterns

269

The XRD patterns of the samples prepared by hydrothermal and sol-gel methods are presented in

270

Figure 3. Figure 3a and b show, respectively, the XRD pattern of the samples prepared on the CS and

271

CNT by hydrothermal method and calcined at different temperatures. No crystalline impurities are

272

detected in the prepared samples. The crystallinity is enhanced by increasing the calcination

273

temperature. The samples calcined at 300-600 ºC demonstrate the characteristic XRD pattern of anatase

274

phase and all the corresponding characteristic planes (JCPDS no.: 01-084-1286) are clearly exhibited at

275

2 = 25.3 (101), 36.9 (103), 37.8 (004), 38.6 (112), 48.0 (200), 53.9 (105), 55.0 (211), 62.1 (213), 62.7

276

(204), 68.8 (116), 70.25 (220), 74.1 (107), 75.0 (215), and 76.0 (301). The high intensity of characteristic

277

diffraction peaks exhibits the high degree of crystallinity and low resistance of electron transport 34.

278

The phase transformation of the samples prepared on the CNT by the hydrothermal method from

279

anatase to rutile was observed by calcination at 800 ºC, according to results reported in the literature 50.

280

The TiO2@CNT_HT_C800 sample is composed of a mixture of anatase and rutile phases, while the

281

anatase phase was completely transformed to rutile phase in the case of the TiO2@CS_HT_C800. The

282

characteristic planes of the rutile phase at 2 = 27.5 (110), 36.1 (101), 39.2 (200), 41.2 (111), 44.0 (210),

283

54.3 (211), 56.6 (220), 62.6 (002), 64.0 (310), 69.0 (301), 69.7 (112), 72.5 (311), 76.5 (202), and 79.8

284

(212) are in good agreement with the standard rutile spectrum (JCPDS no.: 01-088-1175).

285

The XRD pattern of the samples prepared by sol-gel methods is depicted in Figure 3c. As expected, all

286

samples are composed of pure anatase. The CT incorporation increased the crystallinity of the sample

287

prepared by the aqueous sol-gel method but decreased it in the case of the alcoholic sol-gel method.

288

The values of anatase crystallite size, rutile crystallite size, anatase%, and rutile% are presented in

289

Table 2. The crystallite size of the prepared samples was estimated by Scherrer equation 40 and

290

anatase:rutile ratio of the samples containing both phases was predicted according to the peak heights

291

of anatase (101) and rutile (110) reflections 51. Accordingly, TiO2@CS_HT_C800 is composed of pure 17 ACS Paragon Plus Environment


292

rutile, TiO2@CNT_HT_C800 is made of 25% anatase and 75% rutile , and all the other samples are pure

293

anatase. Figure 4 illustrates the effect of the calcination temperature on the crystallite size of the

294

samples prepared on CT using the hydrothermal method. As can be seen, the crystallite size increases by

295

increasing the calcination temperature. For the case of TiO2@CNT sample, increasing the calcination

296

temperature from 300 to 800 ºC led to 2.8 times increase of the anatase crystallite size. It can also be

297

inferred from Figure 4 that the calcination temperature has almost the same effect on the crystallite size

298

of the samples formed on the CS or CNT templates. However, the CS assisted the transformation of

299

anatase to rutile at 800 ºC, as in contrast with the sample formed on the CNT, no anatase phase was

300

observed for the TiO2@CS_HT_C800 sample. For the case of TiO2@CNT_HT_C800 sample, the rutile

301

crystallite size is 20% larger than the anatase crystallite size.

302

According to Table 2, the incorporation of CT has no effect on the crystallite size of the sample

303

prepared by the aqueous phase sol-gel method. A comparison of the crystallite size of the samples

304

prepared by the hydrothermal method on the CT and calcined at 400 ºC with the templateless TiO2

305

sample synthesized by the same method shows that the incorporation of CT slightly increases the

306

crystallite size. Only for the case of alcoholic phase sol-gel method does employing the CT decrease the

307

crystallite size. The crystallite size of the TiO2@CS_ALSG_C400 and TiO2@CNT_ALSG_C400 samples is

308

respectively 19 and 14 nm, which are approximately 1.5 and 2 times lower than the crystallite size of the

309

TiO2_ALSG_C400 sample. Among all the samples prepared on the CT, TiO2@CNT_AQSG_C400 has the

310

lowest crystallite size of 9 nm. This value is the same as the particle size obtained based on the TEM

311

image (Figure 2g-k) and shows that each particle is formed by one crystal.


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312

Figure 3. XRD patterns of the composites synthesized on (a) CS template by hydrothermal method, (b)

313

CNT template by hydrothermal method, and (c) CS and CNT templates by sol-gel method.

Figure 4. The effect of calcination temperature on the crystallite size of samples prepared on CS and CNT using hydrothermal method. 314 315

3.3.

BET analysis

316

The surface area of the prepared samples is presented in Table 2 and Figure 5. Compared to CS (5.7

317

m2/g), the HCl treated CNT sample has a high surface area of 251.2 m2/g and it increases to 296.4 m2/g

318

by further treatment with HNO3. Moreover, the calcination of TiO2@CS_HT_CN at 300 ºC increased it by

319

around 17%. This increase in surface area may be attributed to the burning of organic impurities (as

320

their presence is confirmed by the TGA results) that block the pores, their presence being proved by TGA 19 ACS Paragon Plus Environment


321

(Figure 6b). However, further increase in the calcination temperature decreases the surface area

322

progressively due to the sintering of TiO2 particles (see Figure 5)50. According to Table 2, the BET surface

323

area of TiO2@CS_HT_C800 and TiO2@CNT_HT_C800 decreased to 1.3 and 6.4 m2/g, respectively. This

324

behavior suggests that the materials are drastically agglomerated and the active sites are located in the

325

interagglomerate pores43. For TiO2@CNT_HT_CN, the calcination at 300 ºC has no significant effect on

326

the surface area; however, the presence of CNT reduces the sintering effect in comparison with the

327

sample prepared on CS template. The decrease of surface area of the TiO2@CS_HT material from 500 ºC

328

(41.6 m2/g) to 400 ºC (45.8 m2/g) is not significant. This observation may be attributed to the combined

329

effect of the increase of porosity due to burning of the carbonaceous templates between 400 and 500

330

ºC (as confirmed by the TGA results) and the sintering at higher temperatures. Similar behavior was also

331

observed in the case of TiO2@CNT_HT between 500 and 600 ºC, corresponding to the CNT burning

332

temperature.

333

The surface area of TiO2 (templateless) as well as TiO2@CS and TiO2@CNT (CT based) samples

334

prepared in the same conditions (hydrothermal method and calcination at 400 ºC) was found to be 66.0,

335

45.8, and 90.2 m2/g, respectively. This could be explained by the analysis of these materials after

336

performing TGA. Figure 6d and i reveal that TiO2@CS_HT_C400 corresponds to pure TiO2, but 60% of the

337

initial CNT in TiO2@CNT_HT_C400 still remained unburned (TiO2@CNT_HT_C400 contains 12% CNT). By

338

considering the surface area of CTs (see Table 2) and their amount in the above-mentioned materials, it

339

can be concluded that the utilization of CS and CNT (as CT) in TiO2 composites respectively reduced the

340

surface area of TiO2 around 30% and increased its surface area about 17%.

341

Comparing the samples synthesized by the alcoholic phase sol-gel method reveals that the surface

342

area of TiO2_ALSG_C400, TiO2@CS_ALSG_C400, and TiO2@CNT_ALSG_C400 is respectively 7.3, 2.7, and

343

100.8 m2/g. Thus, for the hydrothermal method, the incorporation of the CS template leads to a

344

decrease of surface area. However, as TiO2@CNT_ALSG_C400 contains 20% CNT (with a surface area of 20 ACS Paragon Plus Environment

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345

296.4 m2/g), it can be concluded that using this template enhanced the surface area of TiO2 in the

346

sample 5.7 fold.

347

By comparing the materials prepared using different methods, the sample prepared by the aqueous

348

phase sol-gel on CNT method exhibits the highest surface area. The surface area of TiO2_AQSG_C400

349

and TiO2@CNT_AQSG_C400 is, respectively, 133.1 and 194.9 m2/g. As this increase corresponds to the

350

surface area of the CNT contained in the sample (20%), it seems that the utilization of the CNT as CT in

351

the aqueous phase sol-gel method has almost no effect on the surface area of TiO2 in the composite.

352 353

Figure 5. BET surface area of samples prepared without and with carbonaceous template (CS or CNT). 354 355

3.4.

TGA analysis

356

TGA was performed on CT and the samples containing template to analyze the required temperature

357

to burn out the template and evaluate the template content in the samples. For the CS template (Figure

358

6a), less than 2% weight loss is observed at 200 ºC, most probably due to the adsorbed water52. The

359

broad peaks ranging from 250 to 550 ºC corresponds to the gradual burning of CS template. The peak at 21 ACS Paragon Plus Environment


360

around 300 ºC may be attributed to the presence of amorphous carbon. It is well known that CT can

361

have different burning temperatures due to the burning of amorphous carbon or the presence of

362

different diameters or sites53. TGA analysis demonstrates the high purity of CS.

363

The TGA results of TiO2 coated CS samples, non-calcined or calcined at 300 or 400 ºC, are given in

364

Figure 6b-d. TiO2@CS_HT_NC demonstrated a broad peak which began at around 250 ºC, while

365

TiO2@CS_HT_C300 started to burn at 300 ºC, as the less stable compounds have already burned during

366

calcination at 300 ºC. According to Figure 6a-c, the template burned faster in the samples containing

367

TiO2, as it can catalyze the reaction54. For TiO2@CS_HT_NC the presence of TiO2 did not shift the

368

shoulder related to amorphous carbon (around 300 ºC) to lower temperatures, indicating that the

369

amorphous carbon may not be in contact with TiO2 and is located inside CS. For TiO2@CS_HT_C300,

370

5.6% weight loss related to CS burning is close to 5% CS content, confirming the complete hydrolysis of

371

the titanium precursor during the hydrothermal synthesis. All the template burned out by calcination at

372

400 ºC, where less than 2% weight loss is related to the adsorbed water (Figure 6d).

373

According to Figure 6e, the HCl treated CNT template illustrates 96% weight loss in the range of 450-

374

650 ºC, which is related to the combustion of CNT template. The sample weight is stable after 650 ºC,

375

indicating that all the template is burned out. In the case of HCl and HNO3 treated CNT template, around

376

5% gradual weight loss is observed between 100 and 400 ºC. Moreover, CNT starts to burn at lower

377

temperatures, indicating that the HNO3 treatment has partly affected the CNT structure and produced

378

some defects (Figure 6f). At the end of TGA analysis of the pristine CNT (not shown), HCl treated CNT

379

(Figure 6e), and HCl and HNO3 treated CNT (Figure 6f), 7.3, 3.0, and 1.0% of the initial sample remained

380

unburned, respectively. This behavior shows that both HCl and HNO3 treatments help to remove

381

impurities. By considering this 7.3% initial sample remaining from TGA and taking into account the

382

weight loss of pristine CNT during acid treatment (9%), it can be concluded that around 1.7% of the


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383

organic compounds in the pristine CNT removed during acid treatment that can be attributed to the

384

amorphous carbon.

385

The TGA analysis of the TiO2@CNT_HT_CN sample (Figure 6g) reveals that the amorphous carbon and

386

the compounds formed during the hydrothermal treatment are removed between 300 and 430 ºC,

387

followed by removing of the CNT template in the range of 430-660 ºC. In addition, a weight loss of 19%

388

is observed for the peak corresponding to CNT burning, which is very close to the 20% CNT content in

389

the sample. Moreover, the TiO2 catalyzes the CNT burning and decreases the burning temperature up to

390

430 ºC. Both TiO2@CNT_HT_C300 and TiO2@CNT_HT_C400 (Figure 6h and i) show that the burning of

391

CNT occurs between 350 and 655 ºC; the weight loss being 19.5% and 12%, respectively. By considering

392

20% CNT content of the samples, this observation demonstrates that the sample calcined at 400 ºC lost

393

around 40% of the containing CNT template during calcination. As expected, no burning of organic

394

compounds is observed for TiO2@CNT_HT_C500, indicating that no template remained after the

395

calcination at 500 ºC (Figure 6j).

396

The TGA analysis of the TiO2@CS_ALSG_C400 sample (Figure 6k) shows around 3% weight loss before

397

400 ºC, which can be attributed to the removal of the adsorbed water, followed by CS burning between

398

400 and 550 ºC. The 6.3% weight loss corresponding to the CS burning is close to the 5% CS content. The

399

TGA analysis of TiO2@CNT_ALSG_C400 is almost similar to TiO2@CNT_HT_C400 and both samples lost

400

around 5% of their CNT during calcination at 400 ºC due to the catalyzing effect of TiO2. Nevertheless,

401

19% weight loss of TiO2@CNT_AQSG_C400 in the temperature range of 400-700 ºC shows that the

402

material has not lost the containing CNT during the calcination at 400 ºC. In all the TGA graphs of Figure

403

6, the appearance of the exothermic peaks shows that the oxidative decomposition is the cause of

404

weight loss.

405



406

Figure 6. Thermogravimetric analysis curves of (a) as-prepared CS, (b) TiO2@CS_HT_CN, (c)

407

TiO2@CS_HT_C300, (d) TiO2@CS_HT_C400, (e) HCl treated CNT, (f) HCl and HNO3 treated CNT, (g)

408

TiO2@CNT_HT_CN, (h) TiO2@CNT_HT_C300, (i) TiO2@CNT_HT_C400, (j) TiO2@CNT_HT_C500, (k)

409

TiO2@CS_ALSG_C400, (l) TiO2@CNT_ALSG_C400, (m) TiO2@CNT_AQSG_C400.


Page 24 of 42

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410 411


3.5.

FT-IR analysis

FT-IR technique was used to investigate the chemical structure of the prepared samples. The infrared

412

ATR spectrum of the as-synthesized CS is shown in Figure 7a. The bands at 799 and 2980 can be

413

respectively assigned to the aromatic CH out-of-plane bending vibrations55 and stretching vibrations56,

414

illustrating the occurrence of aromatization during the hydrothermal treatment. The broad and

415

interconnected peaks from 1000 to 1400 can be attributed to the CO vibration in highly conjugated,

416

aromatic or other chemical structures56. The band at 1680 corresponds to the stretching of carbon-

417

carbon double bonds26. The IR-absorption bands at 1704 can be attributed to the main stretching bands

418

of C=O group26, suggesting the formation of carboxylic acid groups on the surface of CS. The very intense

419

band between 2300-2400 belongs the O=C=O asymmetric stretching vibration of the adsorbed carbon

420

dioxide56. The broad band which appears in the 3000-3700 range can be attributed to the stretching

421

vibrations of OH group as well as the physisorbed water20. As it can be seen, CO is the predominant

422

oxygen containing functional group on the CS surface.

423

To examine the success of CNT functionalization, the FT-IR spectra of functionalized and pristine CNT

424

samples are compared in Figure 7b. The band appearing at 651 may be attributed to the remained

425

metallic impurities, whose presence was proved by TGA (Figure 6f). The broad and intense band at 1158

426

belongs to the CCO ring and CCC asymmetric groups57. The band peaking at 1560 can be assigned

427

to the C=C bond vibration in the aromatic ring21. Moreover, the stretching vibration band of the carbonyl

428

(C=O) and carboxyl (C(=O)OH) groups can be seen at 1652 and 1699, respectively. By comparing the FT-

429

IR spectra of functionalized and pristine CNT, it can be inferred that the CNT sample was successfully

430

functionalized, in order to assist the formation of TiO2 on the CNT surface.



Figure 7. (a) Infrared ATR spectra of as-synthesized CS, (b) FT-IR spectra of pristine and functionalized CNT. 431 432

3.6.

Photocatalytic hydrogen production

The photocatalytic activity of the prepared samples was examined based on the rate of hydrogen

433

production and the results are presented in Table 2. Accordingly, the samples prepared by aqueous sol-

434

gel method (TiO2@CNT_AQSG_C400 and TiO2_AQSG_C400) illustrate the highest photocatalytic activity.

435

On the other hand, TiO2@CNT_HT samples calcined at 300 and 800 ºC illustrate the lowest

436

photocatalytic activity. By associating the rate of hydrogen production with the sample characterization

437

results, it can be seen that the samples which present simultaneously high surface area (Table 2) and

438

high crystallinity (Figure 3) demonstrate high photocatalytic activities. Higher crystallinity results in a

439

decrease of crystal defects which leads to a lower recombination rate of photoinduced species58.

440

Although TiO2_ALSG_C400 and TiO2@CS_ALSG_C400 samples were crystalized considerably (Figure 3c),

441

the hydrogen production was not significant due to their low surface area (respectively 7.3 and 2.7 m2/g,

442

according to Table 2). On the other hand, despite the relatively high surface area of the

443

TiO2@CNT_ALSG_C400 sample (100.8), the rate of hydrogen production was low as a result of a low

444

crystallinity (Figure 3c).


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445


To investigate thoroughly the effect of CT incorporation during the catalyst preparation on the rate of

446

hydrogen production by each of the prepared catalysts, the results were compared based on CT

447

effectiveness ratio and depicted in Figure 8. The CT effectiveness ratio was defined as follows:

CT effectiveness ratio  448 449 450

hydrogen produced by sample synthesized on CT (mol) hydrogen produced by templateless sample (mol)

(2)

The highest amount of hydrogen produced by the templateless sample which was prepared by the same method was considered the denominator for these calculations. The results of hydrogen produced by the samples synthesized on CS and CNT templates are illustrated

451

in Figure 8. The red line corresponds to a CT effectiveness ratio of 1. Larger values (than 1) denote the

452

positive effect of CT incorporation on the rate of hydrogen production. Accordingly, no significant

453

enhancement was observed for the composites prepared on CT (CS and CNT) by sol-gel methods

454

(aqueous phase and alcoholic phase).

455

The catalyst synthesized on the CS by hydrothermal method followed by calcination at 400 ºC shows

456

the best results, leading to an increase of the rate of hydrogen production 2.8 times in comparison to

457

the sample prepared in the same conditions but without CS. For the case of CNT incorporation by

458

hydrothermal synthesis, the highest effectiveness ratio corresponds to the calcination temperature of

459

500 ºC. According to BET (Figure 5) and XRD (Figure 3a-b) data, both surface area and crystallinity are

460

appropriate to the calcination temperatures corresponding to optimum hydrogen production. At the

461

lower and higher calcination temperatures corresponding to optimum hydrogen production,

462

respectively the crystallinity and surface area considerably decrease. Based on TGA results (Figure 6),

463

the samples where no template remained in the samples after calcination (TiO2@CS_HT_C400 and

464

TiO2@CNT_HT_C500) lead to the highest hydrogen production.



Page 28 of 42

465 466

Figure 8. The effect of carbonaceous templates incorporation on the enhancement of hydrogen

467

production. CT effectiveness ratio is defined by Eq. (2) and the red line is corresponding to the material

468

without CT incorporation. Experimental condition: 10% glycerol, 1% Pt, 1 g/l catalyst.

469 470

Figure 8 illustrates that all the samples prepared on the CNT have an effectiveness ratio less than 1. To

471

explore the reason for this behavior, some supplementary experiments were performed using the

472

templateless TiO2 and TiO2@CNT materials prepared by aqueous phase sol-gel method, whose

473

application led to the production of the highest amount of hydrogen (see Table 2). These experiments

474

were performed in the presence and absence of Pt as cocatalyst. For this investigation, the Pt

475

effectiveness ratio is defined as follows:

Pt effectiveness ratio  476 477

hydrogen produced by platinum deposited sample (mol) hydrogen produced by sample without platinum (mol)

Figure 9 illustrates the Pt effectiveness ratio for the templateless TiO2, TiO2@CNT, and TiO2@CS samples prepared by the sol-gel method. According to this figure, Pt deposition on the surface of 28 ACS Paragon Plus Environment

(3)

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478

samples led to a Pt effectiveness ratio of around 13 for the TiO2 and TiO2@CS samples, while for the

479

TiO2@CNT sample this value is almost halved. Additionally, the CT effectiveness ratio for the samples

480

prepared on CS and CNT in the presence and absence of Pt are depicted in Figure 10. This figure shows

481

that only in the case of the TiO2@CNT sample and in the absence of Pt could CT incorporation double

482

the rate of hydrogen production. In other words, no positive effect was observed in the case of the

483

TiO2@CS sample, which can be justified by the fact that all CS was burned during calcination at 400 ºC

484

(Figure 6d). It can be inferred from Figure 9 and Figure 10 that both CNT and Pt have a positive effect on

485

the rate of hydrogen production; however, the effect of Pt is around 6.5 times higher. The addition of

486

CNT can improve the rate of hydrogen production, but this enhancement is not synergistic with the

487

positive effect of Pt. This behavior can be explained by considering the electron transfer mechanism

488

during the photocatalytic process. The CNT can scavenge the photo-generated electrons since its work

489

function (4.3-5.1 eV59) is larger than the conduction band energy level of TiO2 (4.21 eV60). On the other

490

hand, the work function of Pt is in the range of 5.6-5.9 eV61 and larger than the work function of CNT,

491

indicating the facile electron transfer from CNT to Pt. Thus, in the presence of both of CNT and Pt, the

492

reduction of protons to hydrogen does not occur on CNT and only Pt plays the role of cocatalyst. As the

493

results of hydrogen production demonstrated that the positive effect of Pt and CNT are not synergistic,

494

it could be concluded that a role of CNT is to act as the cocatalyst. The conduction band edge of CNT is

495

lower compared to TiO262; thus, the photogenerated electrons can be transferred to CNT as a cocatalyst.

496

On the other hand, CNT can rapidly transfer the electrons to the reaction sites as it possesses an

497

excellent electrical conductivity and electron storage capacity 63. The high electron affinity of CNT

498

decreases the chance of electron and hole recombination, thus enhancing the rate of hydrogen

499

production 64.

500 501

As mentioned above, the former experiments were performed using a glycerol concentration of 10% (typical content in the biodiesel wastes). To check the effect of CNT as adsorbent, additional 29 ACS Paragon Plus Environment


502

photocatalytic experiments were also performed at very high (50%) and low (1%) glycerol concentration.

503

For these experiments, the aqueous phase sol-gel synthesized TiO2 and TiO2@CNT samples were used.

504

As can be seen in Figure 11, the CNT effectiveness ratio is around 1 at higher glycerol concentrations (10

505

and 50%). The CNT effectiveness ratio is doubled at the low glycerol concentration of 1%. These results

506

suggest that the amount of adsorbed glycerol on the surface of the templateless sample was not

507

sufficient at low glycerol concentrations and this shortage controls the rate of reaction (as the

508

adsorption step can be the predominant in the process for this condition). In this case, the adsorbent

509

role of CNT is influential. The CNT increases the glycerol concentration on the surface of catalyst by the

510

adsorption of glycerol molecules. It should be pointed out that additional experiments performed at 1%

511

glycerol concentration and lower CNT content (1%) led to similar results (not shown). We can conclude

512

that even 1% of CNT is sufficient to play the role of adsorbent. It is worth mentioning that the trend

513

illustrated in Figure 11 was not observed for the catalyst formed on CS. This can be justified by the fact

514

that CS is not present in the sample because it was burnt out (Figure 6d).

515

Figure 9. The effect of Pt deposition on the

Figure 10. The effect of CNT incorporation on the

enhancement of hydrogen production rate in the

enhancement of the hydrogen production rate,

presence of templateless TiO2, TiO2@CNT, and

with and without Pt deposition. CT effectiveness 30

ACS Paragon Plus Environment

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TiO2@CS. Pt effectiveness ratio is defined in Eq.

ratio is defined in Eq. (2) and the red line

(3) and the red line corresponds to the material

corresponds to the material without CT

without Pt incorporation (Pt effectiveness ratio =

incorporation (CT effectiveness ratio = 1).

1). Experimental condition: 10% glycerol, 1% Pt, 1

Experimental condition: 10% glycerol, 1% Pt, 1 g/l

g/l catalyst.

catalyst.

516 517

Figure 11. The effect of glycerol concentration on the enhancement of the hydrogen production rate

518

using an aqueous phase sol-gel prepared TiO2@CNT sample. CT effectiveness ratio is defined in Eq. (2)

519

and the red line corresponds to the material without CNT incorporation (CT effectiveness ratio = 1).

520

Experimental condition: 1% Pt, 1 g/l catalyst.

521 522 523

4. CONCLUSION In this work, the effects of incorporating CS and CNT as CT for the preparation of TiO2 composites

524

were investigated, and the roles of CTs as support, cocatalyst, and adsorbent were examined. To

525

evaluate the effect of incorporating CT and preparation methods thoroughly, hydrothermal and sol-gel

526

(alcoholic phase and aqueous phase) methods were utilized to form a layer of TiO2 on both CTs, as well



527

as to synthesize TiO2 samples without CT. The synthesized samples were characterized by SEM, TEM,

528

XRD, BET, TGA, and FT-IR. The analysis of the results revealed that:

529



TiO2 nanoparticles were uniformly distributed on the surface of the carbon sphere (average

530

diameter of CS around 1.1 µm for the alcoholic phase sol-gel method and 3 µm for the

531

hydrothermal method).

532



533 534

of TiO2 particles. 

535 536

539 540



Concerning the effects of incorporating CTs on the photocatalytic activity of the prepared samples, the following conclusions can be drawn: 



The incorporation of CS during the hydrothermal preparation of TiO2 followed by calcination at 400 ºC had the highest enhancement (2.8 times) compared to the other samples.



545 546

The highest amount of hydrogen production corresponded to the synthesis conditions that simultaneously lead to high crystallinity and high surface area.

543 544

Increasing the calcination temperature had almost similar effects on the enlargement of the crystallite size of TiO2 particles formed on both CS and CNT.

541 542

Incorporation of CNT could significantly increase the composite surface area; however, no positive effect was observed regarding CS.

537 538

TiO2 formation on the CNT was not uniform; however, CNTs could hinder the agglomeration

Using Pt and CNT individually enhanced photocatalytic activity thirteen- and twofold, respectively.



CNT has proved to have the role of cocatalyst in the TiO2 composite, but the incorporation of

547

CNT was not found to have a positive effect in the presence of Pt as cocatalyst (their effects

548

were not synergistic).

549 550



The adsorbent role of CNT was revealed to be considerable only at low substrate concentrations. 32 ACS Paragon Plus Environment

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551

Accordingly, this work contributes to the existing knowledge regarding the roles of CTs since (i) the

552

morphology analysis of the prepared composites showed that CT can play the role of template, (ii) the

553

analysis of synergistic effects of CT and Pt during the photocatalytic reaction revealed the role of CT as

554

cocatalyst, and (iii) comparing photocatalytic experiments in the presence and absence of CT indicated

555

the effective role of CT as adsorbent.

556 557

AUTHOR INFORMATION

558

Corresponding Author

559

*Tel.: 1-418-656-2204. Fax: 1-418-656-5993. E-mail: [email protected]. Address: 1065

560

Avenue de la Médecine, Université Laval, Québec City, Québec, Canada, G1V 0A6.

561 562

ACKNOWLEDGEMENT

563

The authors appreciate the generous financial support of this work and awarding Ph.D. scholarships

564

for M.R. Karimi Estahbanati, by the Ministry of Education and Higher Education of Quebec, The Fonds de

565

recherche du Québec Nature et technologies (FRQNT), The Natural Sciences and Engineering Research

566

Council of Canada (NSERC), TELUS, and Laval University. They are also very grateful to ARKEMA for

567

kindly providing Graphistrength® multiwall carbon nanotubes. Dr. Andrei Iliuta’s assistance (University of

568

Toronto) in the linguistic revision of the manuscript is appreciated as well.

569 570 571 572

REFERENCES 1.

Iliuta, I.; Iliuta, M. C. Biosyngas production in an integrated aqueous-phase glycerol

reforming/chemical looping combustion process. Ind. Eng. Chem. Res. 2013, 52 (46), 16142-16161.



573

2.

Tan, J. P.; Tee, Z. K.; Roslam Wan Isahak, W. N.; Kim, B. H.; Asis, A. J.; Jahim, J. M. Improved

574

fermentability of pretreated glycerol enhanced bioconversion of 1, 3-propanediol. Ind. Eng. Chem. Res.

575

2018, 57 (37), 12565-12573.

576 577 578

3.

Xu, C.; Xu, Q. A Novel Design for Simultaneous Production of Biodiesel and Glycerol Carbonate

from Soybean Oil. Ind. Eng. Chem. Res. 2018. 4.

Shokrollahi Yancheshmeh, M.; Radfarnia, H. R.; Iliuta, M. C. Sustainable Production of High-

579

Purity Hydrogen by Sorption Enhanced Steam Reforming of Glycerol over CeO2-Promoted Ca9Al6O18–

580

CaO/NiO Bifunctional Material. ACS Sustainable Chem. Eng. 2017, 5 (11), 9774-9786.

581

5.

Reddy, N. L.; Emin, S.; Kumari, V. D.; Muthukonda Venkatakrishnan, S. CuO Quantum Dots

582

Decorated TiO2 Nanocomposite Photocatalyst for Stable Hydrogen Generation. Ind. Eng. Chem. Res.

583

2018, 57 (2), 568-577.

584 585 586

6.

Gholipour, M. R.; Dinh, C.-T.; Béland, F.; Do, T.-O. Nanocomposite heterojunctions as sunlight-

driven photocatalysts for hydrogen production from water splitting. Nanoscale 2015, 7 (18), 8187-8208. 7.

Duan, B.; Zhou, Y.; Huang, C.; Huang, Q.; Chen, Y.; Xu, H.; Shen, S. Impact of Zr-Doped TiO2

587

Photocatalyst on Formaldehyde Degradation by Na Addition. Ind. Eng. Chem. Res. 2018, 57 (42), 14044-

588

14051.

589

8.

590 591

Leary, R.; Westwood, A. Carbonaceous nanomaterials for the enhancement of TiO2

photocatalysis. Carbon 2011, 49 (3), 741-772. 9.

Srisasiwimon, N.; Chuangchote, S.; Laosiripojana, N.; Sagawa, T. TiO2/Lignin-based Carbon

592

Composited Photocatalysts for Enhanced Photocatalytic Conversion of Lignin to High Value Chemicals.

593

ACS Sustainable Chem. Eng. 2018.


Page 34 of 42

Page 35 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

594 595 596


10.

Chung, W.-C.; Chang, M.-B. Simultaneous generation of syngas and multiwalled carbon

nanotube via CH4/CO2 reforming with spark discharge. ACS Sustainable Chem. Eng. 2016, 5 (1), 206-212. 11.

Quan, F.; Zhang, J.; Li, D.; Zhu, Y.; Wang, Y.; Bu, Y.; Qin, Y.; Xia, Y.; Komarneni, S.; Yang, D.

597

Biomass as Template Leads to CdS@Carbon Aerogels for Efficient Photocatalytic Hydrogen Evolution and

598

Stable Photoelectrochemical Cell. ACS Sustainable Chem. Eng. 2018.

599 600

12.

Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene.

Nature 1985, 318, 162.

601

13.

Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354 (6348), 56.

602

14.

Baughman, R. H.; Zakhidov, A. A.; De Heer, W. A. Carbon nanotubes--the route toward

603 604 605 606 607 608 609 610 611 612 613

applications. Science 2002, 297 (5582), 787-792. 15.

Cao, Q.; Yu, Q.; Connell, D. W.; Yu, G. Titania/carbon nanotube composite (TiO2/CNT) and its

application for removal of organic pollutants. Clean Technol. Environ. Policy 2013, 15 (6), 871-880. 16.

Woan, K.; Pyrgiotakis, G.; Sigmund, W. Photocatalytic Carbon-Nanotube-TiO2 Composites. Adv.

Mater. 2009, 21 (21), 2233-2239. 17.

Liu, J.; Wickramaratne, N. P.; Qiao, S. Z.; Jaroniec, M. Molecular-based design and emerging

applications of nanoporous carbon spheres. Nat. Mater. 2015, 14 (8), 763. 18.

Li, S.; Pasc, A.; Fierro, V.; Celzard, A. Hollow carbon spheres, synthesis and applications–a

review. J. Mater. Chem. A 2016, 4 (33), 12686-12713. 19.

Huang, B.-S.; Chang, F.-Y.; Wey, M.-Y. An efficient composite growing N-doped TiO2 on multi-

walled carbon nanotubes through sol–gel process. J. Nanopart. Res. 2010, 12 (7), 2503-2510.



614

20.

Dai, K.; Peng, T.; Ke, D.; Wei, B. Photocatalytic hydrogen generation using a nanocomposite of

615

multi-walled carbon nanotubes and TiO2 nanoparticles under visible light irradiation. Nanotechnology

616

2009, 20 (12), 125603.

617

21.

Silva, C. G.; Sampaio, M. J.; Marques, R. R. N.; Ferreira, L. A.; Tavares, P. B.; Silva, A. M. T.; Faria,

618

J. L. Photocatalytic production of hydrogen from methanol and saccharides using carbon nanotube-TiO2

619

catalysts. Appl. Catal. B:Environ. 2015, 178, 82-90.

620

22.

Moya, A.; Cherevan, A.; Marchesan, S.; Gebhardt, P.; Prato, M.; Eder, D.; Vilatela, J. J. Oxygen

621

vacancies and interfaces enhancing photocatalytic hydrogen production in mesoporous CNT/TiO2

622

hybrids. Appl. Catal. B:Environ. 2015, 179, 574-582.

623

23.

Haldorai, Y.; Rengaraj, A.; Lee, J.-B.; Huh, Y. S.; Han, Y.-K. Highly efficient hydrogen production

624

via water splitting using Pt@MWNT/TiO2 ternary hybrid composite as a catalyst under UV–visible light.

625

Synthetic Metals 2015, 199, 345-352.

626

24.

Veluru, J. B.; Manippady, K. K.; Rajendiren, M.; Mya, K. M.; Rayavarapu, P. R.; Appukuttan, S. N.;

627

Seeram, R. Photocatalytic hydrogen generation by splitting of water from electrospun hybrid

628

nanostructures. Int. J. Hydrogen Energy 2013, 38 (11), 4324-4333.

629

25.

Chen, Y.; Li, J.; Hong, Z.; Shen, B.; Lin, B.; Gao, B. Origin of the enhanced visible-light

630

photocatalytic activity of CNT modified gC3N4 for H2 production. Phys. Chem. Chem. Phys. 2014, 16 (17),

631

8106-8113.

632

26.

Justh, N.; Bakos, L. P.; Hernádi, K.; Kiss, G.; Réti, B.; Erdélyi, Z.; Parditka, B.; Szilágyi, I. M.

633

Photocatalytic hollow TiO2 and ZnO nanospheres prepared by atomic layer depositiondd. Sci. Rep. 2017,

634

7 (1), 4337.


Page 36 of 42

Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

635 636 637


27.

Cao, S.; Yu, J. Carbon-based H2-production photocatalytic materials. J. Photochem. Photobiol., C

2016, 27, 72-99. 28.

Georgakilas, V.; Perman, J. A.; Tucek, J.; Zboril, R. Broad family of carbon nanoallotropes:

638

classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene,

639

nanodiamonds, and combined superstructures. Chem. Rev. 2015, 115 (11), 4744-4822.

640

29.

Liu, X.; Zeng, P.; Peng, T.; Zhang, X.; Deng, K. Preparation of multiwalled carbon

641

nanotubes/Cd0.8Zn0.2S nanocomposite and its photocatalytic hydrogen production under visible-light. Int.

642

J. Hydrogen Energy 2012, 37 (2), 1375-1384.

643 644 645 646 647 648 649

30.

Ran, J.; Zhang, J.; Yu, J.; Jaroniec, M.; Qiao, S. Z. Earth-abundant cocatalysts for semiconductor-

based photocatalytic water splitting. Chem. Soc. Rev. 2014, 43 (22), 7787-7812. 31.

McAndrew, T. P.; Laurent, P.; Havel, M.; Roger, C.; Ave, F.; Prussia, K. In Arkema graphistrength®

multi-walled carbon nanotubes, Technical Proceedings of NSTI 2008; 2008; pp 47-50. 32.

Wang, Y.; Zhang, W.; Li, R.; Duan, W.; Liu, B. Design of stable cage-like CaO/CaZrO3 hollow

spheres for CO2 capture. Energy Fuels 2016, 30 (2), 1248-1255. 33.

Jiang, Q.; Li, L.; Bi, J.; Liang, S.; Liu, M. Design and Synthesis of TiO2 Hollow Spheres with Spatially

650

Separated Dual Cocatalysts for Efficient Photocatalytic Hydrogen Production. Nanomaterials 2017, 7 (2),

651

24.

652 653

34.

Di, J.; Li, S.; Zhao, Z.; Huang, Y.; Jia, Y. A.; Zheng, H. Biomimetic CNT@TiO2 composite with

enhanced photocatalytic properties. Chem. Eng. J. 2015, 281, 60-68.



654

35.

Gao, B.; Chen, G. Z.; Puma, G. L. Carbon nanotubes/titanium dioxide (CNTs/TiO2)

655

nanocomposites prepared by conventional and novel surfactant wrapping sol–gel methods exhibiting

656

enhanced photocatalytic activity. Appl. Catal. B:Environ. 2009, 89 (3-4), 503-509.

657

36.

Osorio, A. G.; Silveira, I. C. L.; Bueno, V. L.; Bergmann, C. P. H2SO4/HNO3/HCl—functionalization

658

and its effect on dispersion of carbon nanotubes in aqueous media. Appl. Surf. Sci. 2008, 255 (5), 2485-

659

2489.

660

37.

Wang, W.; Serp, P.; Kalck, P.; Faria, J. L. Photocatalytic degradation of phenol on MWNT and

661

titania composite catalysts prepared by a modified sol–gel method. Appl. Catal. B:Environ. 2005, 56 (4),

662

305-312.

663

38.

Moradi, S.; Vossoughi, M.; Feilizadeh, M.; Zakeri, S. M. E.; Mohammadi, M. M.; Rashtchian, D.;

664

Booshehri, A. Y. Photocatalytic degradation of dibenzothiophene using La/PEG-modified TiO2 under

665

visible light irradiation. Res. Chem.Intermed. 2015, 41 (7), 4151-4167.

666

39.

Feilizadeh, M.; Vossoughi, M.; Zakeri, S. M. E.; Rahimi, M. Enhancement of efficient Ag–S/TiO2

667

nanophotocatalyst for photocatalytic degradation under visible light. Ind. Eng. Chem. Res. 2014, 53 (23),

668

9578-9586.

669

40.

Ding, X. Z.; Liu, X. H.; He, Y. Z. Grain size dependence of anatase-to-rutile structural

670

transformation in gel-derived nanocrystalline titania powders. J. Mater. Sci. Lett. 1996, 15 (20), 1789-

671

1791.

672

41.

Jiang, X.; Fu, X.; Zhang, L.; Meng, S.; Chen, S. Photocatalytic reforming of glycerol for H 2

673

evolution on Pt/TiO2: fundamental understanding the effect of co-catalyst Pt and the Pt deposition

674

route. J. Mater. Chem. A 2015, 3 (5), 2271-2282.


Page 38 of 42

Page 39 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

675 676 677


42.

Deshmukh, A. A.; Mhlanga, S. D.; Coville, N. J. Carbon spheres. Mater. Sci. Eng. R Rep. 2010, 70

(1-2), 1-28. 43.

Bakardjieva, S.; Šubrt, J.; Štengl, V.; Dianez, M. J.; Sayagues, M. J. Photoactivity of anatase–rutile

678

TiO2 nanocrystalline mixtures obtained by heat treatment of homogeneously precipitated anatase. Appl.

679

Catal. B:Environ. 2005, 58 (3), 193-202.

680

44.

Cargnello, M.; Grzelczak, M.; Rodrı́guez-González, B.; Syrgiannis, Z.; Bakhmutsky, K.; La Parola,

681

V.; Liz-Marzán, L. M.; Gorte, R. J.; Prato, M.; Fornasiero, P. Multiwalled carbon nanotubes drive the

682

activity of metal@oxide core–shell catalysts in modular nanocomposites. J. Am. Chem. Soc. 2012, 134

683

(28), 11760-11766.

684

45.

Mohamed, M. M.; Osman, G.; Khairou, K. S. Fabrication of Ag nanoparticles modified TiO2–CNT

685

heterostructures for enhanced visible light photocatalytic degradation of organic pollutants and

686

bacteria. J. Environ. Chem. Eng. 2015, 3 (3), 1847-1859.

687 688 689

46.

Bao, J.; Dai, Y.; Liu, H.; Yang, L. Photocatalytic removal of SO2 over Mn doped titanium dioxide

supported by multi-walled carbon nanotubes. Int. J. Hydrogen Energy 2016, 41 (35), 15688-15695. 47.

Xu, Y.-J.; Zhuang, Y.; Fu, X. New insight for enhanced photocatalytic activity of TiO2 by doping

690

carbon nanotubes: a case study on degradation of benzene and methyl orange. Phys. Chem. C. 2010,

691

114 (6), 2669-2676.

692

48.

MamathaKumari, M.; Kumar, D. P.; Haridoss, P.; DurgaKumari, V.; Shankar, M. V. Nanohybrid of

693

titania/carbon nanotubes–nanohorns: a promising photocatalyst for enhanced hydrogen production

694

under solar irradiation. Int. J. Hydrogen Energy 2015, 40 (4), 1665-1674.



695

49.

Heister, E.; Lamprecht, C.; Neves, V.; Tîlmaciu, C.; Datas, L.; Flahaut, E.; Soula, B.; Hinterdorfer,

696

P.; Coley, H. M.; Silva, S. R. P. Higher dispersion efficacy of functionalized carbon nanotubes in chemical

697

and biological environments. ACS Nano 2010, 4 (5), 2615-2626.

698 699

50.

Hanaor, D. A. H.; Sorrell, C. C. Review of the anatase to rutile phase transformation. J. Mater. Sci.

2011, 46 (4), 855-874.

700

51.

Niemantsverdriet, J. W. Spectroscopy in catalysis. John Wiley & Sons: (2007).

701

52.

Subagio, D. P.; Srinivasan, M.; Lim, M.; Lim, T.-T. Photocatalytic degradation of bisphenol-A by

702

nitrogen-doped TiO2 hollow sphere in a vis-LED photoreactor. Appl. Catal. B:Environ. 2010, 95 (3-4), 414-

703

422.

704 705 706 707 708 709 710

53.

Ajayan, P. M.; Ichihashi, T.; Iijima, S. Distribution of pentagons and shapes in carbon nano-tubes

and nano-particles. Chem. Phys. Lett. 1993, 202 (5), 384-388. 54.

Zhang, D.; Fu, H.; Shi, L.; Fang, J.; Li, Q. Carbon nanotube assisted synthesis of CeO2 nanotubes. J.

Solid State Chem. 2007, 180 (2), 654-660. 55.

Li, M.; Li, W.; Liu, S. Hydrothermal synthesis, characterization, and KOH activation of carbon

spheres from glucose. Carbohydr. Res. 2011, 346 (8), 999-1004. 56.

Réti, B.; Kiss, G. I.; Gyulavári, T.; Baan, K.; Magyari, K.; Hernadi, K. Carbon sphere templates for

711

TiO2 hollow structures: Preparation, characterization and photocatalytic activity. Catal. Today 2017, 284,

712

160-168.

713 714

57.

Liu, Y.; Gao, L. A study of the electrical properties of carbon nanotube-NiFe2O4 composites:

effect of the surface treatment of the carbon nanotubes. Carbon 2005, 43 (1), 47-52.


Page 40 of 42

Page 41 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

715 716 717


58.

Tian, G.; Fu, H.; Jing, L.; Tian, C. Synthesis and photocatalytic activity of stable nanocrystalline

TiO2 with high crystallinity and large surface area. J. Hazard. Mater. 2009, 161 (2-3), 1122-1130. 59.

Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. Work

718

functions and surface functional groups of multiwall carbon nanotubes. Phys. Chem. B. 1999, 103 (38),

719

8116-8121.

720 721

60.

Kim, Y. K.; Park, H. Light-harvesting multi-walled carbon nanotubes and CdS hybrids: application

to photocatalytic hydrogen production from water. Energy Environ. Sci. 2011, 4 (3), 685-694.

722

61.

Haynes, W. M. CRC handbook of chemistry and physics. CRC press: 2014.

723

62.

Mehmood, U.; Hussein, I. A.; Harrabi, K.; Mekki, M. B.; Ahmed, S.; Tabet, N. Hybrid TiO2–

724

multiwall carbon nanotube (MWCNTs) photoanodes for efficient dye sensitized solar cells (DSSCs). Solar

725

Energy Materials and Solar Cells 2015, 140, 174-179.

726 727 728

63.

Kongkanand, A.; Kamat, P. V. Electron storage in single wall carbon nanotubes. Fermi level

equilibration in semiconductor–SWCNT suspensions. ACS nano 2007, 1 (1), 13-21. 64.

Du, P.; Song, L.; Xiong, J.; Li, N.; Wang, L.; Xi, Z.; Wang, N.; Gao, L.; Zhu, H. Dye-sensitized solar

729

cells based on anatase TiO2/multi-walled carbon nanotubes composite nanofibers photoanode.

730

Electrochimica Acta 2013, 87, 651-656.

731



732

TOC Graphic

733

734 735


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Effects of Carbon Nanotube and Carbon Sphere Templates in TiO2

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