A novel approach for enhancing hydrogen production

Science of the Total Environment 627 (2018) 1464–1472

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

A novel approach for enhancing hydrogen production from bio-glycerol photoreforming by improving colloidal dispersion stability Xiaoyan Cai a, Chao Wang a,⁎, Ying Chen a, Zhengdong Cheng a,b, Riyang Shu a, Jingtao Zhang a, Enqi Bu a, Mingzheng Liao a, Qingbin Song c a b c

Guangdong Provincial Key Laboratory on Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USA Macau Environmental Research Institute, Macau University of Science and Technology, Macau, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Photocatalytic activities were studied for anatase TiO2 nanoparticles with different shapes. • Colloidal dispersion stability and H2 production were improved by binary TiO2 system. • Improved H2 production from bioglycerol/water photoreforming was achieved over TiO2.

a r t i c l e

i n f o

Article history: Received 5 January 2018 Received in revised form 30 January 2018 Accepted 1 February 2018 Available online xxxx Keywords: Hydrogen production Bio-glycerol photoreforming Colloidal dispersion stability TiO2 nanoparticle

a b s t r a c t In photocatalytic systems, TiO2 based particles in suspensions tend to aggregate spontaneously, resulting in low efficiency for light utilization and photocatalytic activity. In this study, various TiO2 nanoparticles with different shapes were synthesized and characterized via various analysis. In atmosphere and aqueous environment, the nanoparticles demonstrated different properties in the nature of the agglomerations. Photocatalytic performances of the as-prepared samples were confirmed to be strongly influenced by the dispersion stabilities with TiO2 nanotube exhibited higher colloidal dispersion stability and photocatalytic activity. Mixing binary TiO2 photocatalysts with different shapes as a simple approach was firstly proposed for enhancing photoreforming hydrogen production from bio-glycerol aqueous solution. At a specific mixing ratio, the mixed suspension of TiO2 nanosphere and nanosheet retained excellent colloidal dispersion stability, and photocatalytic hydrogen production was significantly promoted with its maximum H2 production amount as 2.1–2.9 times high as that of TNP and TNS, respectively. This strategy of enhancing colloidal dispersion stability may provide new ideas for the design of efficient and energy-saving photocatalytic system. © 2018 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. E-mail address: [email protected] (C. Wang).

https://doi.org/10.1016/j.scitotenv.2018.02.009 0048-9697/© 2018 Elsevier B.V. All rights reserved.

Nowadays, the major resources for hydrogen production are fossil fuels. The usage of those fossil fuels brings about the emission of greenhouse gases and pollutions (Dou et al., 2018; Wang et al., 2015) and

X. Cai et al. / Science of the Total Environment 627 (2018) 1464–1472

requires large energy consumptions with much energy losses caused by high temperature, multiple chemical reactions and complex auxiliary processes (C. Wang et al., 2012). Photo-reforming H2 production using solar energy has received great attention due to its integration for both solar energy and renewable sources utilization. This method combines reduction of water and oxidation of organic compounds (such as bio-alcohols) into a single process. The kinetics and reaction efficiency of H2 evolution could be significantly enhanced because the organic substrate (like biomass) reacts rapidly and irreversibly with photogenerated holes of semiconductor photocatalyst thereby suppressing recombination reactions. The overall efficiencies could be also increased because such photo-reforming reactions utilize both the energy generated by the incident photons and the chemical potential stored in the organic substrate (Nomikos et al., 2014). Many biomassderived compounds as renewable sources have been proposed for this photoreforming hydrogen production (Chen et al., 2015; Daskalaki et al., 2011; Gallo et al., 2012). Among those, either crude glycerol or refined glycerol shows great potential for photocatalytic hydrogen production (Fangrui et al., 1999; Gombac et al., 2016). As reported, the general hydrogen production reaction from glycerol aqueous solution could be described as below (Fujita et al., 2016): C3 H8 O3 þ H2 O

hv

→

photocatalyst

CO2 þ H2

ðR1Þ

It is generally recognized that TiO2 has been proven to be the most suitable and promising semiconductor catalyst in heterogeneous photocatalysis due to its physical and chemical stability, nontoxicity, effectivity, oxidizing property, and a wide variety of chemicals, as well as its low cost (AkbarAshkarran et al., 2014; Chen et al., 2012). Recent studies have shown that the shapes of TiO2 nanoparticles have attracted a great deal of research interests, they reported that the shapes may have some effects on the photocatalytic activity due to their different structural, surface and morphological features (Liu et al., 2010; Mor et al., 2005; Wahi et al., 2005; Wu et al., 2010; Zhao et al., 2014). For example, it is reported that TiO2 nanotubes could exhibit higher photocatalytic activity than that of commercial P25-TiO2 nanoparticles due to their better abilities for delocalization of excited e−/h+ pairs. This was mainly because those TiO2 nanotubes have well developed space charge regions which could reduce the recombination time of photo generated electrons (Z. Wang et al., 2012; Zhao et al., 2014). In the case of TiO2 nanobelts, one considered that they could possess high photocatalytic activities because of that the nanobelts had higher charge carrier mobilities and provided the pathways for the transport of charge carriers throughout the longitudinal direction, in which charge separation was expected to be facilitated (Wu et al., 2010). Like many other nanoparticles, when suspended in aqueous-like solutions, TiO2 nanoparticles have large specific surface areas with high surface energy, and this high surface energy could lead to particle aggregation resulting in the decrease of surface areas. In an aquatic system, the photocatalytic activities of metal oxide nanoparticles have been found to decrease of generating reactive oxygen species caused by the aggregation. Jassby et al. reported that the photocatalytic activity of TiO2 in generating free hydroxyl radicals varied with its aggregate structure and size (Jassby et al., 2012). The study presented that the reactivity of TiO2 could be reduced as aggregates become larger and denser. Lakshminarasimhan et al. proposed that the agglomeration of TiO2 nanoparticles was critical for photocatalytic hydrogen production in methanol aqueous solution (Lakshminarasimhan et al., 2008). The authors emphasized that hydrogen production and colloid agglomeration should be directly related. And the photoinduced electron transfer could be increased as a result of the colloid agglomeration based on antenna mechanism (Franco et al., 2006). The energy balance of suspension system dispersed by stirring was evaluated by Feng et al. They claimed that the electric energy input to drive the light source, pump and stirrer is tens of thousands of even hundreds of thousands of times more than hydrogen energy

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output. Therefore, an alternative approach with less external energy input while maintaining photocatalytic activity is required rather than a conventional magnetic stirring system (Feng et al., 2008). Li et al. found that reduction of aggregation could be achieved by controlling the pH (Li et al., 2010). Using this method, they observed the deaggregation of TiO2 nanoparticles could improve the adsorption of reactants and the electrostatic attraction accelerated the photoreaction rate. This discrepancy of the above discussions may be due to the different scopes of the aggregation involved. Usually, TiO2 nanoparticles are largely agglomerated with the trend of precipitation in an aqueous system in which alcohols act as sacrificial agents for hydrogen production experiments. In summary, there is a demand for the study on the aggregation and colloidal dispersion stability of TiO2 nanoparticles for photocatalytic hydrogen production. Many studies have focused on the enhancement of dispersion stability for TiO2 particles in water (French et al., 2009; Li et al., 2010). Mechanical dispersion, surfactant addition and surface modification were studied for improving the colloidal dispersion stability of TiO2 nanoparticle suspension (S. Kim et al., 2012; Othman et al., 2012). However, the disadvantages of these methods are obvious. Although adding surfactants or regulating pH value seemed to be effective for enhancing dispersion stability (Chen et al., 1998), it is difficult to remove the added cation or anion in the liquid, and it also increases the complexity and uncertainty for controlling the reaction system. And it is obvious to consume much energy when employing conventional magnetic stirring method in hydrogen production reaction. There is a fact that TiO2 tend to flocculate in a low dielectric solvent because the Hamaker constant between the TiO2 nanoparticles is high in such solvent. J. Kim et al. (2012) reported that surface fluorination treated TiO2 particles could maintain dispersion stability in organic solvents and have a better photocatalytic activity. But the surface catalytic active sites of TiO2 may be sacrificed using such surface treatment. Mixing nanoparticles with different shapes is a new method to improve colloidal dispersion stability without conducting any surface modifications on TiO2 nanoparticles. Liu et al. (2015) stabilized the TiO2 nanoparticle suspensions through the use of ultrathin ZrP nanoplatelets. Shao et al. reported that stability of nanoparticle suspensions mixture with different shapes of TiO2 is beneficial to the dispersion stability of TiO2 nanoparticles in water (Shao et al., 2015). However, this effective method for enhance dispersion stability has never been studied for photoreforming hydrogen production. The aim of this study was to investigate the effect of TiO2 with different morphologies on their the colloidal dispersion stability; and the correlation mechanism between colloidal dispersion stability and photocatalytic activity of photoreforming hydrogen production. For such specific analysis, various TiO2 nanoparticles were synthesized with different shapes. The dispersion stability of the TiO2 aqueous suspensions were characterized using particle size analyzer, zeta potential measurements, transmission electron microscope (TEM), and Turbiscan Stability method. For further analysis, binary TiO2 nanoparticle systems were introduced by dispersing certain amount of two types of TiO2 with different shapes. In order to demonstrate that the relationship between dispersion stability of TiO2 nanoparticle suspensions and hydrogen production from photoreforming of bio-glycerol aqueous solution was carried out, and the experimental results were discussed to clarify the relationship.

2. Experimental 2.1. Synthesis of various shapes of TiO2 nanoparticles 2.1.1. Synthesis of TiO2 nanosphere The TiO2 nanosphere was purchased from Aladdin Industrial Corporation and used as received (denoted as TNP), which was also served as a precursor for the synthesis of TiO2 nanobelt, nanotube and nanosheet.

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2.1.2. Synthesis of TiO2 nanobelt TiO2 nanobelt was prepared according to the method reported in previous literatures (Zhao et al., 2015; Zhou et al., 2010). There were three main steps in the formation of TiO2 nanobelt according to the following three equations: 3TiO2 þ 2NaOH→Na2 Ti3 O7 þ H2 O

ðR2Þ

Na2 Ti3 O7 þ 2HCl→H2 Ti3 O7 þ 2NaCl

ðR3Þ

H2 Ti3 O7 →3TiO2 þ H2 O

ðR4Þ

In a typical preparation procedure, 0.1 g commercial anatase TiO2 nanoparticle was firstly dispersed into a 20 ml NaOH aqueous solution with a concentration of 10 M and then hydrothermally treated at 180 °C for 24 h. The treated powder was washed thoroughly with deionized water followed by a filtration and drying process. The sodium titanate nanobelt was then obtained. Afterwards, the obtained nanobelt was immersed in 0.1 M HCl aqueous solution for 24 h and then washed thoroughly. The obtained sample was isolated from the solution by centrifugation and sequential washed for several times. Finally, the sample was dried at 70 °C for 10 h and then calcined at 550 °C for 3 h. The obtained TiO2 nanobelt was denoted as TNB. 2.1.3. Synthesis of TiO2 nanotube TiO2 nanotube was synthesized by the hydrothermal process in concentrated NaOH aqueous solution (Mogilevsky et al., 2008; Zhao et al., 2014). A commercial anatase TiO2 powder was used as the precursor, and a typical process could be described as follows: 4 g anatase TiO2 powder was mixed with 400 ml of 10 M NaOH aqueous solution under continuously stirring for 30 min at room temperature, followed by hydrothermal treatment at 140 °C for 72 h. Subsequently, a solid sample was obtained by filtration and washing with distilled water until the rinsing solution reached a pH at about 7. Then the sample was dried at 70 °C for 10 h. The obtained sample was denoted as TNT. 2.1.4. Synthesis of TiO2 nanosheet TiO2 nanosheet was synthesized by the hydrothermal process (Leng et al., 2014). Firstly, 0.1 g anatase TiO2 powder and 8 g TBA-OH were added to the 10 ml NaOH (10 M) aqueous solution under stirring. After 30 min, the mixture was transferred into a 100 ml Teflon container, then sealed in a stainless autoclave and treated in an air-flow electric oven at 130 °C for 24 h. After cooling down naturally, the precipitate is collected using a centrifuge then washed with water and ethanol. Finally, the sample is dried in an 80 °C oven. The obtained sample is denoted as TNS. 2.2. Characterization of catalysts The specific areas of the catalysts were determined by the BrunauerEmmett-Teller (BET) method using a N2 adsorption and desorption isotherms at 77 K measured with Micrometric Acusorb 2100E apparatus. The sample was degassed prior to the measurement at 100 °C for 1 h and then at 120 °C for 2 h. The structural of the samples were investigated using powder X-ray diffraction (XRD, Shimadzu XRD-6000). The detailed morphology of the photocatalysts was investigated by a highresolution transmission electron microscopy (HRTEM) using JEOL2100. For the HRTEM analysis, the samples were ground and dispersed with ethanol, and deposited on a Cu grid covered with a perforated carbon membrane. The particle sizes of the catalyst samples were measured at 25 °C using dynamic light scattering (DLS) at a scattering angle of 173° with a Zeta sizer Nano ZS particle size analyzer (Malvern Instruments Ltd., England). UV–vis absorption spectra of the samples were obtained by a UV–vis spectrophotometer (UV2550, Shimadzu, Japan).

2.3. Colloidal dispersion stability measurements Zeta potential profiles of the as-prepared particles were measured using a zeta-potential measurement device (Delsa Nano C/SS). 15 mg of TiO2 sample was suspended with 15 mL solvents and sonicated for 1 h and a typical procedure was used to prepare the suspension. The dispersion stabilities were measured by Turbiscan Lab Expert Apparatus (Formulation, France). The light transmission and backscattering of the nanoparticle suspensions were recorded. The dispersion stabilities can be represented by the variation of light transmission and backscattering. This method of using TSI to evaluate suspension stability has been introduced in detail in previous studies (Buron et al., 2004; Fang et al., 2012; Kang et al., 2012; Wiśniewska, 2010). And TSI can be used to evaluate the dispersion stability quantitatively. 2.4. Photocatalytic activity measurements The photocatalytic H2 production experiments were conducted in a duplex Pyrex flask at ambient temperature, and openings of the flask were sealed with a silicone rubber seals and glass lid. The crylindrical shaped reactor was used carrying fittings with two gas outlets. A 300 W Xe arc lamp (320–780 nm, Beijing Philae Technology Co., Ltd., China), employed as light sources, was vertical fixed at 10 cm away from the photocatalytic reactor. The intensity of radiation at the center of the reactor measured by a radiometer as approximately 350 W·m− 2 , and the light receiving area was 0.28 cm−2 calculated by the spot diameter of xenon lamp. In each photocatalytic experiments, total amount of 100 mg catalyst was suspended in 100 mL refined bio-glycerol aqueous solution (containing 10 vol% of glycerol). Prior to irradiation, the entire experimental system was evacuated to completely remove the dissolve oxygen or air and the system pressure was maintained at − 0.1 MPa to ensure that the system was in an anaerobic condition. We made every effort to ensure that the colloidal dispersion stability was not subject to any external interference during the experiment. The produced hydrogen was analyzed by gas chromatographer (GC-2014c AT, Shimadzu, Japan, TCD, and nitrogen as a carrier gas and 5 Å molecular sieve column). 3. Results and discussion 3.1. Characterization crystal structure and morphology The crystal structures of the samples were confirmed by the X-ray diffraction (XRD) patterns (Fig. 1). In the synthesis process, the general formation of the tube geometry is based on the exfoliation of TiO2 crystal planes in the alkali environment and stabilized as Ti-O-Na+. Therefore, the crystallinity of the as-prepared nanotube was poor, which was in accordance with other work, for such alkali treatment and low temperature preparation method (Grover et al., 2013). It was found that all the crystallographic planes of as-prepared TNS and TNB were corresponding with anatase phase (JCPDS 28-1152). There were no apparent peaks corresponding to other crystal structures. These findings confirmed that all the as-prepared TiO2 nanoparticles with different shapes were apparently in pure anatase crystal structure. And such crystal structure would be benefit to photocatalytic hydrogen production efficiency compared with those containing rutile structures (Gao et al., 2014; Luís et al., 2011; Toyoda, 2004). HRTEM was used to study the morphologies of the TiO2 nanoparticles as illustrated in Fig. 2(a)–(d) with the average geometric sizes analyzed by Nano Measurer 1.2.0 software. In Fig. 2(a), open ended straw like hollow nanotubes, which had the diameters of 5.2–8.1 nm and lengths of 82.0–145.0 nm. The wall thickness of the tubes and the internal diameter of TNTs were measured as 1.0–1.2 nm and 3.1–5.2 nm, respectively. Similar types of TNT with a much higher diameter ca. 70.0–100.0 nm and wall thickness of 14.0–50.0 nm was also obtained by Hoyer (1996). The HRTEM image in Fig. 2(b) revealed the formation

(c)

(b)

(a)

40

50

60

70

80

2θ (degree) Fig. 1. XRD patterns of (a) TNT, (b) TNB, (c) TNP and (d) TNS samples.

35

b $YJ QP

30 25 20

100 nm

15 10 5 0 80

Number Frequency (%)

100 nm

a


of nanobelts with various dimensions. There were some long and shortrice like morphologies of TNB particles with the width of 5.0–11.1 nm and length of 39.0–169.0 nm in narrow rectangular outlines. The average length of 93.5 nm and width of 7.7 nm were determined by randomly selected 20 individual particles. The micrographic view of TiO2 nanospheres (Fig. 2(c)) showed that the average particle size was 21.5 nm and the nanospheres appeared to be agglomerated but in relatively uniform particle size. From the HRTEM image of nanosheets in Fig. 2(d), it could be observed that the TNS particles were substantially in rectangular shape with the edge length in the range of 30.0–60.0 nm. Those observed as thick black lines were supposed to be the vertical arrangements of the nanosheets. The insets in Fig. 2 are typical image of SEM. Fig. 3 displayed the average particle size distributions measured in aqueous phase by dynamic light scattering (DLS) technique. From the

15 10 5 30

60

90

120 150 180

Length (nm) Avg.=7.67nm

20 15 10 5 0

4

5

6

7

8

9

10 11

Width (nm)

d Number Frequency(%)

20

25

c Avg.=21.50nm

40

40 Avg.=43.31nm

30 20 10 0 10 15 20 25 30 35

30 20 10 0 30 35 40 45 50 55 60 Size(nm)

Size (nm) 100 nm

Avg.=102.42nm

25

0

100 120 140 Length (nm)

35 Avg.=6.4nm 30 25 20 15 10 5 0 5.0 5.5 6.0 6.5 7.0 7.5 8.0 Outer Diameter (nm)

50

30


30

results, the obtained average particle sizes using DLS technique for all samples were increased in values compared with the particle sizes analyzed by HRTEM. This may be caused by the different environments in which the solid nanoparticles were observed. For HRTEM, individual particles were observed in gas phase environment, and those nanoparticles may aggregate when being suspended in water for DLS analysis. The distribution of the sizes peaked with relatively large tails toward either larger or smaller values shown in Fig. 3 indicating both nanoparticles and irregular sized clusters coexist in all samples. The average particle size differences of the HRTEM and DLS results could reflect the dispersion stabilities of the synthesized nanoparticles in the aqueous suspension. After simple calculations, the average particle size differences of TNT, TNB, TNP and TNS were around 17.46%, 36.94%, 83.09%, 76.11%. Among the results, TNT showed a relatively small average size differences between nanoparticles in gas phase and aqueous phase which may have a positive impact for deaggregation in aqueous system. On the other hand, better dispersion stability in aqueous suspension could exhibit more advantages to surface contact between catalyst particle and solution for the creation of active radicals. Fig. 4 showed the nitrogen adsorption-desorption isotherms and the corresponding pore-size distribution curves of the samples. The nitrogen adsorption-desorption isotherms of all samples could be categorized as type IV isotherms with a H3 hysteresis loop (IUPAC classification) at a high relative pressure range of 0.8–1.0, indicating the presence of slit like pores due to the stacking of TiO2 particles (Meng et al., 2016). Further observation showed that isotherms of TNS shifted up comparing with other samples at a high P/P0 range (approaching 1.0), this suggested that TNS sample had bigger pore volumes. The pore size distribution curves (see inset of Fig. 4) were derived from desorption branch of the nitrogen isotherms by BarrettJoyner-Halenda (BJH) method. From the calculations, the pore diameters for TNP, TNS and TNB samples were about 20.0 nm, and ca. 10.0 nm for TNT sample. Table 1 depicts the physical properties of the prepared TiO2 nanoparticles. The BET surface area (SBET) of asprepared TNT, TNB, TNP and TNS particles were 85.8, 52.3, 65.9 and 98.3 m2 g−1, respectively. With the similar preparation process, the BET values could reflect their structural morphologic properties. The


20

1467


(215)

(116) (220)

(204)

(d)

(105) (211)

(200)

Anatase TiO2 (004)

Intersity(a.u.)

(101)


100 nm

Fig. 2. HRTEM and typical SEM (insets) images of the as-prepared nanoparticles: (a) TNT, (b) TNB, (c) TNP and (d) TNS.


Number Frequncy (%)

16

Avg.=137.55nm Std.Dev.=51.49nm

14

a

12 10 8 6 4 2 0

Number Frequncy (%)

1468

80 100 120 140 160 180 200 220

14 Avg.=162.41nm 12 Std.Dev.=85.55nm 10 8 6 4 2 0 50

100

Size (nm)


12

150

200

250

300

350

Size (nm) 7

c

10 8 6 4 2 0

Number Frequncy (%)

Number Frequncy (%)

14

b

d


6 5 4 3 2 1 0

60

80 100 120 140 160 180 200 220

100

Size (nm)

200

300

400

500

Size (nm)

Fig. 3. Particle size distributions of TiO2 nanoparticles suspended in aqueous solution: (a) TNT, (b) TNB, (c) TNP and (d) TNS.

400 300 200

0.020

dV/dW (cm3/(gnm))

Volume adsorbed (cm3/g,STP)

particles of TNB and TNP could be considered as zero-dimensional and one-dimensional (1-D) nanostructures, the BET value of TNT and TNS were the higher for their unique 3-D and 2-D nanostructures. It has been known in our previous study that mixing nanotubes and nanosheets could enhance the stability of the suspension (Shao et al., 2015). To further study the photocatalytic effect of mixing two types of nanoparticles with different shapes, new binary nanoparticle systems were introduced. The ratios of nanoparticles with different shapes for each binary nanoparticle system were selected based on our previous studies about nanofluid considering the collaborative effects of both particle mixing and particle interaction (Mo et al., 2016; Shao et al., 2015). The typical UV–vis absorption spectra of the TiO2 nanosuspensions were illustrated in Fig. 5. The absorption onsets were determined by linear extrapolation from the inflection point of the curve to

TNP TNB TNT TNS

0.016 0.012 0.008

the base-line. Light absorption was mainly determined by the band structure and the dipole matrix elements (Wu et al., 2010). Since all those samples were consisted by anatase TiO2, the peaks of the UV–vis absorption spectra curves located at the same wavelength range. And the optical properties of TiO2 nanoparticles are extremely sensitive to their surface morphology (Djaoued et al., 2010), the peak intensities had differences of the samples due to their different shapes. It was seemed that the peaks had hypochromic shift when adding another nanoparticles to the system. As reported, such hypochromic effect could be observed when the dye molecules were degraded by the oxygen-containing radical species (Li and Cai, 2016). Thus, it could be deduced that most of those binary systems could promote the generation of oxygen-containing radical species. And the most likely oxygencontaining radical species should be hydroxyl radicals which were the intermediate products of photoreforming hydrogen production reaction. For the samples with TNT, mixing particles with another morphology to TNT system had no positive effect on hydrogen production regardless of time effect on reaction kinetics. 3.2. Colloidal dispersion stability analysis As stated in Section 3.1, poor dispersion stabilities and certain degrees of aggregations were confirmed by HRTEM and DLS analysis for

0.004 0.000

100

0

20

40

60

80

100

120

Pore diameter(nm)

Table 1 Physical property of various TiO2 nanoparticles.

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure ( P/P0) Fig. 4. Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves (inset) of TNT, TNB, TNP and TNS.

Photocatalyst sample notation

SBET (m2 g−1)

Pore volume (cm3 g−1)

Average pore size (nm)

Crystallite size (nm)

TNT TNB TNP TNS

85.82 52.34 65.86 98.33

0.16 0.27 0.36 0.50

7.25 20.50 21.88 20.51

– 22.47 23.62 17.63


Intensity (a.u.)

TNT TNB TNP TNS TNT+TNP TNT+TNS TNT+TNB TNP+TNB TNP+TNS TNS+TNB

300

400

500

600

700

800

900

Wavelength (nm) Fig. 5. Absorption spectra of various nanoparticle systems suspended in aqueous solutions.

all nanoparticle samples. In order to further investigate the dispersion stability properties, the zeta potentials of those TiO2 based nanoparticle suspensions were measured (shown in Fig. 7). The zeta potential value could act as an indicator of the intensity of repulsive force among particles in the suspension. It is also an aid in predicting long-term stability (Othman et al., 2012). Actually, the zeta potential value has a strong correlation with environmental pH value, the point of zero charge (PZC) value represents a certain pH value at which a solid submerged in an electrolyte exhibiting zero net electrical charge on the surface (Schematic diagram shown in Fig. 6), and pHPZC value was reported to be around 5.8 for P25-TiO2 (Karunakaran et al., 2011; Suttiponparnit et al., 2011). According to the DLVO theory (Majidian et al., 2011), the higher surface charge (as proved by the higher of the negative value of zeta potential) serves the higher repulsive potential between the particles, leading to a more stable suspension can be obtained. As shown, the observed zeta potential values were −54.42 mV for TNT, − 21.48 mV for TNB, −13.27 mV for TNP and −10.41 mV for TNS photocatalysts, respectively. When taking 5.8 as a reference value, the pHsolution values of TNT, TNB, and TNS suspensions were all larger than

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that of pHPZC, indicating that those aqueous solutions were stable because the electrostatic repulsions between the particles were with the same charge property. When TiO2 particles was dispersed in water, the particle surface became anionic in nature and increase in surface area that would render more coverage of hydroxyl groups (\\OH) from H2O (Sahu and Biswas, 2011). This led to the generation of less hydrogen ions in solution and hence increased the pH value. The pHsolution value of TNT was much more than that of TNB and TNS, meaning its zeta potential value was more negative. Therefore, the result of the zeta potential indicated that TNT sample possessed the best dispersion stability, followed by TNB, TNP and TNS. Zeta potential values of the different binary nanoparticle systems were measured and the results were also shown in Fig. 7. For TNB, TNP and TNS containing systems, the nanoparticles in binary nanoparticle suspensions were less aggregated due to the greater electronic repulsion interaction, so the stabilities of binary nanoparticle systems were better than those of the single component suspensions. However, the situation was different for TNT containing samples. This may be due to its unique 3-D hollow structure and high specific surface area promoting its zeta potential value. The transmission and backscattering curves of TNT, TNB, TNS and TNP were monitored and analyzed by Turbiscan Lab. Fig. 8 showed the typical transmission and backscattering curves of the suspension systems standing for 5 h. The x-axis denoted the height of the sample cell from bottom to top. It was noteworthy that the high transmission values near the bottom and top of the sample cell were caused by the shape of the meniscus of the solid-liquid and air-liquid interfaces. Among the obtained transmission curves, TNT displayed lower light transmissions than other samples. This confirmed that TNT had better dispersion stability than those of the other three samples under the same conditions. Such performance may be due to the homogenously dispersed TNT particles possessing 3-D material properties which could block more light from transmitting through water than those with only 2-D and 1-D material properties (Tang et al., 2012). The Turbiscan Stability Index (TSI) is a parameter used for estimating the suspension stability. This index is a statistical factor with its value being obtained as the sum of all processes taking place in the studied system. A smaller TSI value represents better dispersion stability for

pHPZC < pHsolution

Solid particles

pHPZC (point of zero charge)

Dispersion

Zeta Potential (mV)

0

Stable

pH Negative Fig. 6. Conceptual representation of the effect of pH on zeta potential value.

Higher

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could be related to depletion attraction (Shao et al., 2015). Nanoparticles with different shapes could introduce depletion interaction when being suspended in aqueous phase (Zhang et al., 2013). TNT suspension stabilities were seriously deteriorated because strong depletion attractions were introduced when sufficient concentration of other types particles was added. And the TNT agglomerates would precipitate due to the gravitational force after aggregating to large clusters, which could also accelerate the sedimentations of other containing nanoparticles in the binary systems.

-50 -40 -30 -20 -10 0

3.3. Photocatalytic activity analysis (a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

Sample Name Fig. 7. Zeta potential value: (a) TNT, (b) TNB, (c) TNP and (d) TNS, (e) 0.06 wt% TNT + 0.14 wt% TNP, (f) 0.06 wt% TNT + 0.04 wt% TNS, (g) 0.06 wt% TNT + 0.04 wt% TNB, (h) 0.06 wt% TNP + 0.04 wt% TNB, (i) 0.08 wt% TNP + 0.1 wt% TNS and (j) 0.06 wt% TNS + 0.04 wt% TNB.

the suspension. The TSI values were calculated with the special computer program using the following equation (Wiśniewska, 2010): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 i¼1 ðXi −XBS Þ TSI ¼ n−1

ðR5Þ

where Xi means the average backscattering for each minute of measurements, XBS means average Xi, and n represents the number of scans. As observed in Fig. 9, the stabilities of the binary nanoparticle systems with suitable concentration ratios were better than those of single component systems in the same total amounts of TiO2 except the TNT containing systems. The unstable state of nanoparticles in solutions

T(%)

0.5h 2.5h 4.5h

1.0h 3.0h 5.0h

a

1.5h 3.5h

48

T (%)

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The photoreforming hydrogen production activities of single and binary nanoparticle systems were investigated in a bio-glycerol aqueous solution under a 300 W xenon lamp irradiation. Fig. 10(a) presented the photocatalytic H2 evolution amounts of the samples. Obviously, the TiO2 nanosphere showed a lower photocatalytic H2 production activity due to its poor colloidal dispersion stability and small specific surface area. Among the four single component nanoparticle systems, TNT displayed a higher H2 production amount with its maximum evolution rate of 12 μmol g−1 h−1 in the 5 h test. Compared with the other single component nanoparticle systems, the better photocatalytic activity of TNT could be ascribed to: (i) a hollow interior structure and a high specific surface area, that would more favor of catalytic surface adsorption; (ii) a better delocalization of charge carriers along its longitudinal dimension, which could decrease electron-hole recombination and guide electron transfer; and (iii) the presence of hydrated water in its crystal structure, which provokes more number of \\OH formation, thus increasing the photocatalytic activity (J. Kim et al., 2012; Toledo Antonio et al., 2010; Wahi et al., 2005). Compared with TNS, the crystallinity of the as-prepared TNT was poor, which may be have the poor photoactivity (Toyoda, 2004). In addition, TNS possessed the highest surface area, the biggest pore volume and the better crystallinity, but

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the photocatalytic activity of TNS was not better than that of TNT or TNB. When observing the results of zeta potential, transmission curves and TSI value, the dispersion stability of TNS was poor. That is to say, the effect of dispersion stability to the photocatalytic activity was not less than that of specific surface area and pore volume or crystallinity. It can be seen from Fig. 10 that most of the photocatalytic activities were enhanced by mixing certain shape of TiO2 with the other one of a different shape. For TNT containing systems, mixing other types of TiO2 nanoparticles was not helping it improving the photocatalytic activity which was consistent with colloidal dispersion stability analysis. Those results confirmed that the dispersion stability had a great influence on photocatalytic H2 production. Among the photocatalytic activity of all the binary components, TNP mixed with TNS showed the

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highest photocatalytic activity. The maximum H2 evolution were 33.62 μmol·g−1 for TNP and 46.56 μmol·g−1 for TNS while the maximum H2 evolution of TNP mixed with TNS was 97.02 μmol·g−1, which was much greater than that of single TNP or TNS. Various factors may be responsible for this result, such as optimized reaction geometry for charge transfer, UV diffuse reflection characteristics, and solution diffusion effects. To be noted, TNS with 2-D structure characteristics may have a higher efficiency of receiving light under the same experimental condition, also have a larger adsorption area because of its highest specific surface area. However, the dispersion stability and photocatalytic H2 production activity of the TNS was the least desirable among the single component systems. This further indicated that dispersion stability was a key factor for photocatalytic hydrogen production. The TiO2 colloid absorbed less light because of more incident light should be scatted out in the agglomerated state. During photoreforming process, the aggregation of TiO2 particles could decrease photocatalytic activities by reducing or alter the generation of reactive oxygen species (ROS) (Lin et al., 2006; Siedl et al., 2009), therefore suppress hydrogen production. The poor dispersion stability and photocatalytic activity of TNS could be remarkably enhanced after mixing TNT, TNS and TNB, respectively. Meanwhile, Fig. 10 showed that the low photocatalytic H2 production activity of TNB could also be enhanced after mixing other particles. 4. Conclusions In summary, a series of TiO2 nanoparticles with various shapes were synthesized by hydrothermal synthesis processes and all the samples showed certain photocatalytic activity for hydrogen production. Among those, TNT nanoparticles showed better colloidal dispersion stability in the suspensions. The dispersion stabilities of binary nanoparticle systems were better than that of single component ones at the same total concentration, with the exception of the TNT. This was possibly related to the electrostatic repulsion and depletion interaction between the particles with different shapes and the special case of TNT suggested that the influence of TiO2 nano-dimension on the colloidal dispersion stability and its photocatalytic activity requires further study. Novel binary nanoparticle systems were successfully introduced to enhance dispersion stability of the nanoparticles, thereby improving hydrogen production amount, reaction rate and efficiencies. Binary nanoparticle system consisted of TNS and TNP displayed a higher catalytic activity. The results emphasized the importance of colloidal dispersion stability and aggregation when discussing the fate of photocatalytic activity in hydrogen production. Mixing particles of different shapes could help to conduct a more energy-efficient photocatalytic hydrogen production system. Notes The authors declare no competing financial interest. Acknowledgement The authors gratefully acknowledge the financial support provided by Natural Science Foundation of China (No. 51606046) and Guangdong Provincial Science and Technology Plan (No. 2015A010106013).

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Irradiation Time (h) Fig. 10. Hydrogen evolution versus time by photoreforming over 0.1 g the as-prepared photocatalysts is in 10 vol% aqueous solutions of glycerol: H2 production amount (a) and rate (b) (Note: the proportions of the binary components are as the same as Fig. 7).

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