It is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collision from the balls. This process was developed by Benjamin and his coworkers at the International Nickel Company in the late of 1960. It was found that this method, termed mechanical alloying, could successfully produce fine, uniform dispersions of oxide particles (Al2O3, Y2O3, ThO2) in nickel-base superalloys that could not be made by more conventional powder metallurgy methods. Their innovation has changed the traditional method in which production of materials is carried out by high temperature synthesis. Besides materials synthesis, high-energy ball milling is a way of modifying the conditions in which chemical reactions usually take place either by changing the reactivity of as-milled solids (mechanical activation increasing reaction rates, lowering reaction temperature of the ground powders)or by inducing chemical reactions during milling (mechanochemistry). It is, furthermore, a way of inducing phase transformations in starting powders whose particles have all the same chemical composition: amorphization or polymorphic transformations of compounds, disordering of ordered alloys, etc.
The alloying process can be carried out using different apparatus, namely, attritor, planetary mill or a horizontal ball mill. However, the principles of these operations are same for all the techniques. Since the powders are cold welded and fractured during mechanical alloying, it is critical to establish a balance between the two processes in order to alloy successfully. Planetary ball mill is a most frequently used system for mechanical alloying since only a very small amount of powder is required. Therefore, the system is particularly suitable for research purpose in the laboratory. The ball mill system consists of one turn disc (turn table) and two or four bowls. The turn disc rotates in one direction while the bowls rotate in the opposite direction. The centrifugal forces, created by the rotation of the bowl around its own axis together with the rotation of the turn disc, are applied to the powder mixture and milling balls in the bowl. The powder mixture is fractured and cold welded under high energy impact.
The figure below shows the motions of the balls and the powder. Since the rotation directions of the bowl and turn disc are opposite, the centrifugal forces are alternately synchronized. Thus friction resulted from the hardened milling balls and the powder mixture being ground alternately rolling on the inner wall of the bowl and striking the opposite wall. The impact energy of the milling balls in the normal direction attains a value of up to 40 times higher than that due to gravitational acceleration. Hence, the planetary ball mill can be used for high-speed milling.
During the high-energy ball milling process, the powder particles are subjected to high energetic impact. Microstructurally, the mechanical alloying process can be divided into four stages: (a) initial stage, (b) intermediate stage, (c) final stage, and (d) completion stage.
(a) At the initial stage of ball milling, the powder particles are flattened by the compressive forces due to the collision of the balls. Micro-forging leads to changes in the shapes of individual particles, or cluster of particles being impacted repeatedly by the milling balls with high kinetic energy. However, such deformation of the powders shows no net change in mass.
(b) At the intermediate stage of the mechanical alloying process, significant changes occur in comparison with those in the initial stage. Cold welding is now significant. The intimate mixture of the powder constituents decreases the diffusion distance to the micrometer range. Fracturing and cold welding are the dominant milling processes at this stage. Although some dissolution may take place, the chemical composition of the alloyed powder is still not homogeneous.
(c) At the final stage of the mechanical alloying process, considerable refinement and reduction in particle size is evident. The microstructure of the particle also appears to be more homogenous in microscopic scale than those at the initial and intermediate stages. True alloys may have already been formed.
(d) At the completion stage of the mechanical alloying process, the powder particles possess an extremely deformed metastable structure. At this stage, the lamellae are no longer resolvable by optical microscopy. Further mechanical alloying beyond this stage cannot physically improve the dispersoid distribution. Real alloy with composition similar to the starting constituents is thus formed.
Theoretical considerations and explorations of planetary milling process have been broadly studied in order to better understand and inteprate the concept. Joisels work is the first report to study the shock kinematics of a satellite milling machine. This work focused on the determination of the milling parameters that were optimized for shock energy. The various parameters were determined geometrically and the theoretical predictions were examined experimentally using a specifically designed planetary mill. Schilz et al. reported that from a macroscopical point of view, the geometry of the mill and the ratio of angular velocities of the planetary to the system wheel played crucial roles in the milling performance. For a particular ductile-brittle MgSi system, the milling efficiency of the planetary ball was found to be heavily influenced by the ratio of the angular velocity of the planetary wheel to that of the system wheel as well as the amount of sample load. Mio et al. studied the effect of rotational direction and rotation-to-revolution speed ratio in planetary ball milling. Some more theoretical issues and kinematic modeling of the planetary ball mill were reported later in related references. Because mechanical alloying of materials are complex processes which depend on many factors, for instance on physical and chemical parameters such as the precise dynamical conditions, temperature, nature of the grinding atmosphere, chemical composition of the powder mixtures, chemical nature of the grinding tools, etc., some theoretical problems, like predicting nonequilibrium phase transitions under milling, are still in debate.
For all nanocrystalline materials prepared by high-energy ball milling synthesis route, surface and interface contamination is a major concern. In particular, mechanical attributed contamination by the milling tools (Fe or WC) as well as ambient gas (trace impurities such as O2, N2 in rare gases) can be problems for high-energy ball milling. However, using optimized milling speed and milling time may effectively reduce the contamination. Moreover, ductile materials can form a thin coating layer on the milling tools that reduces contamination tremendously. Atmospheric contamination can be minimized or eliminated by sealing the vial with a flexible O ring after the powder has been loaded in an inert gas glove box. Small experimental ball mills can also be completely enclosed in an inert gas glove box. As a consequence, the contamination with Fe-based wear debris can be reduced to less than 12 at.% and oxygen and nitrogen contamination to less than 300 ppm. Besides the contamination, long processing time, no control on particle morphology, agglomerates, and residual strain in the crystallized phase are the other disadvantages of high-energy ball milling process.
Notwithstanding the drawbacks, high-energy ball milling process has attracted much attention and inspired numerous research interests because of its promising results, various applications and potential scientific values. The synthesis of nanostructured metal oxides for gas detection is one of the most promising applications of high-energy ball milling. Some significant works have been reported in recent years. Jiang et al. prepared metastable a-Fe2O3MO2 (M: Ti and Sn) solid solutions by high-energy milling for C2H5OH detection. The 85 mol% a-Fe2O3SnO2 sample milled for 110 hours showed the highest sensitivity among all the samples studied. The best sensitivity to 1000 ppm C2H5OH in air at an operating temperature of 250 C was about 20. Zhang et al. synthesized FeSbO4 for LPG detection. They found that there were two-step solid-state reactions occurring in the raw powders during the ball milling:
The response and recovery times of their sensor were less than one second. The sensitivity to 1000 ppm C2H5OH at an operating temperature of 375 C was about 45. Diguez et al. employed precipitation method to prepare nanocrystalline SnO2 and planetary milling to grind the obtained powder for NO2 detection. They found that the grinding procedure of the precursor and/or of the oxide had critical effect on the resistance in air. As a result, the gas sensing properties to NO2 had been considerably improved. Cukrov et al. and Kersen et al. synthesized SnO2 powders by mechanochemical processing for O2 and H2S sensing applications, respectively. The O2 sensor exhibited stable, repeatable and reproducible electrical response to O2. More recently, Yamazoes group reported the sensing properties of SnO2Co3O4 composites to CO and H2. A series of SnO2Co3O4 thick films containing 0100% Co3O4 in mass were prepared from the component oxides through mixing by ball-milling for 24 h, screen-printing and sintering at 700 C for 3 h. The composite films were found to exhibit n- or p-type response to CO and H2 depending on the Co3O4 contents in the composites. The n-type response was exhibited at 200 C or above by SnO2-rich composites (Co3O4 content up to 5 mass%). The sensor response to both CO and H2 was significantly enhanced by the addition of small amounts of Co3O4 to SnO2, and the response at 250 C achieved a sharp maximum at 1 mass% Co3O4. The p-type response was obtained at 200 C or below by the composites containing 25100 mass% Co3O4. The sensitivity as well as selectivity to CO over H2 could thus be increased by the addition of SnO2 to Co3O4.
Besides the above mentioned researches, significant efforts on the synthesis of nanostructured metal oxide with high-energy ball milling method for gas sensing have been actively pursued by the authors of this chapter. In our research, we use the high-energy ball milling technique to synthesize various nanometer powders with an average particle size down to several nm, including nano-sized a-Fe2O3 based solid solutions mixed with varied mole percentages of SnO2, ZrO2 and TiO2 separately for ethanol gas sensing application, stabilized ZrO2 based and TiO2 based solid solutions mixed with different mole percentages of a-Fe2O3 and synthesized SrTiO3 for oxygen gas sensing. The synthesized powders were characterized with XRD, TEM, SEM, XPS, and DTA. Their sensing properties were systematically investigated and sensing mechanisms were explored and discussed as well.
Ball milling is a size reduction technique that uses media in a rotating cylindrical chamber to mill materials to a fine powder. As the chamber rotates, the media is lifted up on the rising side and then cascades down from near the top of the chamber. With this motion, the particles in between the media and chamber walls are reduced in size by both impact and abrasion. In ball milling, the desired particle size is achieved by controlling the time, applied energy, and the size and density of the grinding media. The optimal milling occurs at a critical speed. Ball mills can operate in either a wet or dry state. While milling without any added liquid is commonplace, adding water or other liquids can produce the finest particles and provide a ready-to-use dispersion at the same time.
Grinding media comes in many shapes and types with each having its own specific properties and advantages. Key properties of grinding media include composition, hardness, size and density. Some common types include alumina, stainless steel, yttria stabilized zirconia and sand. Ball milling will result in a ball curve particle size distribution with one or more peaks. Screening may be required to remove over or undersized materials.
CdS/TiO2 composite photocatalysts were made by the method of secondary ball milling at different ball milling speeds, milling time, and material ratios. After the secondary ball milling process, parts of the samples were calcined at high temperatures. X-ray diffraction (XRD) and UV-Vis diffuse reflectance spectroscopy (DRS) were used to observe the powder particle size, structural defect, bandgap, and absorption spectrum of the samples. Combined with the observation results, the effects of ball milling speed, time, material ratio, and high-temperature calcination on the photocatalytic performance of CdS/TiO2 composite samples were analyzed. Furthermore, the methyl orange (MO) was chosen to simulate pollutants, and the photocatalytic degradation rate of CdS/TiO2 composite photocatalysts for MO was evaluated under sunlight and UV irradiation conditions. The photocatalytic degradation efficiency of CdS/TiO2 photocatalyst under UV irradiation is much higher than that under sunlight irradiation. The experimental results reveal that secondary ball milling can effectively promote the formation of CdS/TiO2 composite nanostructure and the high-temperature calcination can reduce the bandgap width, which makes the samples easier to be excited. When the ball milling speed, time, and material ratio were respectively 400rpm, 10h, 25:75, and then calcined at high temperature, after 2h of irradiation under UV light, CdS/TiO2 composite photocatalysts exhibited maximum photocatalytic degradation efficiency of 57.84%.
Nowadays, with the speedy development of the economy and increasing awareness of environmental protection, people pay more and more attention to the utilization of solar energy and photocatalytic treatment of organic pollutants by semiconductors . Photocatalysis technology has been considered as one of the most efficient solutions to address air pollutants due to its preferable properties and complete degradation [2,3,4]. Among ZnO , CeO2, TiO2, etc, TiO2 has been the preferred photocatalyst on account of its excellent photocatalytic performance, environmental protection, stable chemical properties [6,7], inexpensiveness, nontoxicity, and long service life [8,9]. In particular, when compared with the traditional methods, it is of great significance to the degradation of pollutants in wastes and can even completely degrade pollutants in sewage [10,11]. Therefore, TiO2 has been used in extensive variety of applications, such as optics, self-cleaning, water splitting, catalysts, electrical porcelain [12,13], reducing environmental pollution , and other aspects [15,16,17], which has prompted many people to use various methods, such as microwave-assisted solvent heat method and ion exchange method to prepare TiO2 [18,19]. For example, Maruthapandi et al.  presented polyaniline composite formed with TiO2; SiO2 is an effective adsorbent for the degradation of the organic contaminant in water, and the adsorbed contaminant can be desorbed to achieve reuse. Zangeneh et al.  used l-histidine with TiO2/CdS photocatalytic nanocomposite materials to prepare a polyethersulfone membrane. The results showed that due to the addition of them, the surface roughness of the membrane had a good change and its porosity and hydrophilic energy were also improved, thus improving the antifouling performance, which is of great significance to the protection of water resources. In addition, activated carbon nanocomposites synthesized with TiO2 and other substances have been used to remove airborne pollutants such as xylene [22,23]. In 2020, Rambabu et al.  proposed a tricomponent photocatalyst consisting of TiO2 multileg nanotubes, CdS nanoparticles, and reduced graphene oxide, which has good light absorption performance and can efficiently generate easily separated carriers under light irradiation. Furthermore, TiO2 was modified with Cu(OH)(2) as a catalyst to make composite catalyst and used it for photocatalytic hydrogen production, which shows a great hydrogen yield , and Qin et al.  reached a similar conclusion through experiments. However, since TiO2 with a wide bandgap width of 3.2eV, especially the crystalline phase of anatase can merely be excited under UV light, resulting in about 50% visible light cannot be utilized. At the same time, some of the electron-hole pairs induced by UV are easy to recombine, which greatly limits its photocatalytic performance [27,28].
In recent years, more in-depth studies have been conducted at home and abroad, to conquer these defects of TiO2 mentioned earlier and improve its photocatalytic activity. A large number of studies have shown that modification methods such as ion doping, semiconductor compounding, noble metal deposition, and photosensitization can extend the optical photoabsorption wavelength of pure titanium dioxide to the visible light range and enhanced its catalytic activity to different degrees [29,30]. Trejo-Tzab et al.  proposed that the photocatalytic activity of TiO2 can get improvement by nitrogen (N2) doping and deposition of Cu nanoparticles, the degradation rate of MB solution was used to evaluate the photocatalytic activity of this improved TiO2. The experimental phenomenon indicated that under the condition of low-intensity gas plasma, incorporating Cu into TiO2 P25 to obtain nitrogen-doped TiO2/Cu had higher photocatalytic activity than pure anatase TiO2. Also, Tokmakci et al.  proved that the photocatalytic performance of titanium dioxide photocatalysts mixed with both boron (B) and zirconium (Zr) is higher than that of single element mixed TiO2 and pure anatase TiO2. This kind of phenomenon can attribute to the reduction in the size of the photocatalyst and the successful weaving of B and Zr into the crystal structure by the mechanical ball milling method. Similarly, the sulfur (S)-doped TiO2 photocatalysts newly designed by Jalalah et al.  extended their absorption edge to the visible light range via incorporating sulfur into the lattice structure of TiO2, revealing an excellent photocatalytic activity of the new photocatalysts to MB in the visible region. Petrovic et al.  synthesized TiO2/CeO2 photocatalyst by high energy ball milling method. According to the degradation efficiency for MO solution, one could see that TiO2/CeO2 composite material had higher photocatalytic activity than anatase TiO2, resulting from the effective separation of electron/hole pairs on TiO2 because of the addition of CeO2. Aysin et al.  prepared silver-loaded TiO2 photocatalyst, the number of photogenerated charge carriers involved in the degradation process greatly increased because of the introduction of Ag and gave rise to the improvement of photocatalytic activity. Even with very little Ag loading, the photocatalytic degradation efficiency for MO of the silver-loaded samples was 50% higher than that of the unloaded TiO2 after 1h of irradiation under UV light. Chen et al.  obtained TiN/TiO2 composite nanoscale photocatalyst by ball milling of TiO2 in TiN-doped aqueous solution, and when compared with pure anatase TiO2, TiN/TiO2 composite nanoscale photocatalyst has better photocatalytic performance under both sunlight and ultraviolet light irradiation. The enhancement in the photocatalytic performance of the TiN/TiO2 may well owe to the extension of photoabsorption wavelength of the photocatalyst and the raise of the Ti3+ reaction center on the surface. Moreover, Balakrishnan et al.  used methylene blue to simulate pollutants to investigate the photocatalytic performance of TiO2ZnO nanostructures. The result proves that TiO2ZnO nanostructures had good photocatalytic performance in both visible and ultraviolet light. Habib et al.  also conducted an in-depth study on the photocatalysis of TiO2ZnO nanocomposites.
It can be seen from the studies listed above that in recent years, although some effects have been achieved through the modification of TiO2, most modification methods are difficult to be applied and industrialized due to high preparation cost, complicated process, and limited performance improvement. In 2015, Zhou et al.  prepared CdS/TiO2 composite photocatalytic material by a simple mechanistic method to boost the photocatalytic activity of TiO2 by sensitization and surface hybridization of CdS. In the paper, CdS/TiO2 samples were made by similar mechanochemical secondary ball milling process (dry ball milling and wet ball milling), to explore the optimal ball milling process and the effect of high-temperature calcination on the photocatalytic performance of CdS/TiO2 composite photocatalyst produced by secondary ball milling process; methyl orange (MO) was used to simulate pollutants, and the photocatalytic activity of CdS/TiO2 composite photocatalyst was analyzed by UV-Vis diffuse reflectance spectroscopy (DRS) and X-ray diffraction (XRD). CdS is a kind of commonly used photosensitive resistor, which has a strong photoelectric effect on visible light. The bandgap width of it is relatively narrow, and the spectral response range is close to that of visible light. However, electron holes have strong redox ability and can oxidize S2 on the surface under light irradiation, resulting in severe photocorrosion in the use of CdS alone. After the combination of TiO2 and CdS with a large difference in bandgap width, electrons can transition within the visible region, leading to the high-efficiency separation of electron holes. Therefore, TiO2 can be excited in the visible range, and the photogenic holes of CdS can be neutralized, which can inhibit the photocorrosion phenomenon and enhance the photocatalytic performance of CdS/TiO2, thus achieving more efficient degradation of pollutants in water. Although there have been many solutions for organic pollutants in water, for example, Yi et al.  studied the adsorption performance of silica gel to organic pollutants in water but only at low concentrations of pollutants. The production method is complex and has strict requirements on precision and time, which is not suitable for large-scale production or industrial utilization.
The materials used in this experiment are nanoscale pure anatase titanium dioxide (TiO2, if used as an industrial application, Wang et al.  proposed large-scale synthesis of high purity TiO2 by ion-exchange method, cadmium sulfide (CdS) and MO, all of which are analytical pure drugs produced by China National Pharmaceutical Corporation.
CdS/TiO2 composite photocatalysts were made by the secondary balling milling method, which was carried out in two steps. The first step was to fully compound the raw materials evenly mixed by dry ball milling, and the second step was to refine the particle size of powders by wet ball milling. The detailed preparation processes of CdS/TiO2 composite photocatalysts are as follows: (1) according to the designed material ratios (i.e., the mass ratio of CdS to TiO2), the TiO2 and CdS powders were accurately weighed and added into the corundum ball mill tanks in turn. Corundum balls with diameters of 10, 5, and 2.5mm and a mass ratio of 1:3:6 with a total mass of 200g were selected for grinding ball beads. The mass ratio of corundum balls and materials was 20:1. Place the ball mill tanks in the ball mill after the ingredients are finished. (2) On the basis of ball milling speed and ball milling time designed in the experiment, the TiO2 and CdS powders were ground by dry ball milling. (3) After the completion of dry ball milling, anhydrous ethanol was added to make the mixed materials show sticky shape and then put it into the ball mill again with the same milling parameters for wet ball milling for 2h. (4) After the ball milling was finished, the products were collected immediately, then dried and put into the resistance furnace to calcine at 400C for 2h. (5) The materials prepared by the above processes were fully ground for 30min to make the samples.
The properties and structures of the samples were characterized by UV-Vis DRS and XRD (Cu K, scanning rate was 4/min, and the transport current and acceleration voltage. Respectively. were 40mA and 45kV). Based on the above analyses, the XRD patterns of the CdS/TiO2 composite photocatalysts and the average particle sizes were obtained. Moreover, the absorbance spectrum of the photocatalysts was acquired by utilizing the UV-Vis spectrophotometer (type LAMBDA 650), and the band gaps as well as the light absorption capacity were further analyzed.
MO is a common organic dye, which has no volatilization and is difficult to decompose and oxidize under light and can be used to simulate experimental contaminants. In this experiment, MO simulating pollutants were utilized to study the photocatalytic properties of CdS/TiO2 composite photocatalysts, and the degradation rate was calculated by UV-2600 UV-Vis spectrophotometer. MO is a state of sodium sulfonate dyes, with the extension of photocatalysis time; MO was adsorbed on the surface of CdS/TiO2 composite photocatalysts and eventually degraded to H2O CO2 and other inorganic substances. During the degradation, new substances appeared and the concentration of new substances was constantly increasing. However, due to the complexity of the products, limited test time, and instrument conditions, the intermediate products could not be completely separated.
UV-high-pressure mercury lamp and long-arc xenon lamp were used to simulate the environment of UV light and sunlight irradiation conditions. The experimental procedures are as follows: (1) 10mg/L mixture solution of MO was made by weighing a certain amount of MO and deionized water with a balance and a measuring tube, respectively. (2) Weighing 25mg of photocatalyst with a balance and weighing 250mL of MO solution with a measuring tube, both were placed in a container together, and 4mL solution was removed by a pipette and put into the centrifuge tube in the opaque collection box. (3) The photocatalyst and MO mixed solution was put into the dark box and stirred with a magnetic stirrer for 30min, then 4mL solution was removed with a pipette and placed in the centrifuge tube in the opaque collection box. (4) Turn on the lamp needed for the experiment to preheat for 10min. (5) The photocatalyst and MO mixture was irradiated with simulated UV light or sunlight for 2h, while the cooling cycle was turned on. Place the solution directly under a light source so that it is fully illuminated and then stirred with a magnetic stirrer. Every 30min, 4mL of the solution was removed by a pipette and placed it in a centrifuge tube in the opaque collection box. (6) The samples of the solution collected in steps (3) and (5) were centrifuged in a centrifuge, and the supernatant liquor was removed with a pipette. The supernatant liquor was then centrifuged until the centrifuged solution did not precipitate. (7) Turn on the UV-2600 spectrophotometer and set its parameters. The centrifugal fluid collected from step (6) was poured into the colorimetric dishes, respectively, and the absorbance was measured (the absorbance of the centrifugal fluid was recorded as A t according to the illumination time). After each measurement, the colorimetric dish was cleaned with deionized water. (8) In this experiment, the photocatalytic degradation efficiency (D) can be obtained by the following formula:
The CdS/TiO2 composite photocatalyst prepared by the ball milling method has an effective composite structure. During the formation of the composite structure, CdS has a larger negative conductive potential and a smaller bandgap width than TiO2, which results to the migration of the electrons (e) of CdS to the conductive band of TiO2, achieving the purpose of e and holes (h+) separation resulting in the enhancement of the photocatalytic capacity of CdS/TiO2 samples.
When the composite photocatalyst is irradiated with light with energy greater than the bandgap energy of it, electrons are excited and enter the conduction band across the forbidden band, producing negatively charged highly active electrons (e) in the conduction band, leaving positively charged holes (h+) in the valence band, thus producing highly active electronhole pairs (h+e) on the surface of the photocatalyst. Under the action of an electric field, electrons and holes separate and migrate to the particles surface. Holes on the surface of the composite photocatalyst can oxidize hydroxyl (OH) and water (H2O) adsorbed on its surface to hydroxyl radicals (HO), which is a kind of nonselective oxidant with strong oxidation capacity and can completely oxidize MO to CO2, H2O, and other inorganic substances.
The material ratio is the ratio of raw materials needed to prepare unit products. This experiment prepared seven groups of CdS/TiO2 samples with a material ratio of 10:90, 15:85, 20:80, 25:75, 30:70, 35:65, and 40:60, respectively, to explore the influence of different material ratios on the photocatalytic performance of CdS/TiO2 composite photocatalysts. Under the conditions of ball milling speed of 400rpm and ball milling time of 10h, the photocatalytic performance of samples prepared with different material ratios was evaluated.
Figure 1 shows the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts that of different material ratios after 2h of irradiation under UV light or sunlight. As can be seen from Figure 1 whose horizontal axis and the vertical axis, respectively, represent the material ratio and photocatalytic degradation efficiency of the samples, when the material ratio of CdS/TiO2 composite photocatalysts is 25:75, the photocatalytic degradation efficiency is the highest, which is 8.04% and 54.32%, respectively, under sunlight and UV light. It follows that the photocatalytic performance of CdS/TiO2 composite photocatalysts synthesized at a material ratio of 25:75 is higher than that of pure TiO2, with photocatalytic degradation efficiency of 3.75% and 49.04%, respectively, under the irradiation of sunlight and UV light. Figure 2 exhibits the UV-Vis diffuse reflectance spectra of the TiO2 and CdS/TiO2 composite photocatalysts. One can see that from the figure, when compared with pure TiO2 (whose light absorption edge is at 390nm in the ultraviolet light range), the absorption edge of CdS/TiO2 composite photocatalyst after ball milling extends to the vicinity of 525nm in the visible region, resulting in greatly enhancement of photocatalytic activity. Also, Figure 2 shows that the CdS/TiO2 composite photocatalyst with a material ratio of 25:75 has the largest absorption spectral area, that is, the best photocatalytic performance, which is consistent with the data measured in Figure 1. The improvement of photocatalytic degradation efficiency can be put down to the effective composite structure of CdS/TiO2 composite photocatalyst formed by ball milling under this material ratio. In addition, under the same conditions, the photocatalytic degradation efficiency of UV irradiation is much higher than that of sunlight irradiation, since the bandgap width reduces the utilization rate of sunlight. According to the XRD pattern shown in Figure 3, the characteristic diffraction peak value of TiO2 and CdS still exist in the crystal phase of the CdS/TiO2 composite photocatalysts after ball milling, and the decrease in crystallinity of the samples made by ball milling processes may be caused by the strain effect of lattice and the defects of lattice structure due to the action of mechanical force.
The ball milling speed is the speed of the stirring shaft of the ball mill. Under the condition of low speed, medium speed, and high speed, the ball milling process can mix, compound, and crush the sample powders, respectively. As the speed of ball milling increases gradually, the ball milling energy will gradually increase and the composite will be more sufficient, but the damage to the material structure and the polymerization of the powders will also be more violent. To avoid the effect of the powder polymerization on the photocatalytic performance, dry ball milling was first used in this experiment, and then wet ball milling was carried out to refine the powder particle size. The photocatalytic properties of CdS/TiO2 composite photocatalysts were evaluated at different ball milling speeds under the conditions of the preset material ratio of 25:75 and ball milling time of 10h.
Figure 4 shows the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts exposed to UV light and sunlight for 2h at different ball milling speeds. It is evident from Figure 4 that the speed of ball milling has an obvious effect on the photocatalytic performance of the samples, especially under UV irradiation. When the ball milling speed is no more than 400rpm, the photocatalytic degradation efficiency of the CdS/TiO2 composite photocatalysts exhibits a positive linear relationship with the ball milling speed, which may be due to the fact that the powders are not fully compounded, resulting in the low separation efficiency between holes and electrons; hence, the photocatalytic activity of the composite does not reach the maximum value. When the ball milling speed is 400rpm, the CdS/TiO2 composite photocatalyst has the highest photocatalytic degradation efficiency of 55.39%, indicating that the powders are most fully compounded and the composite structure has fewer defects under this ball milling speed. When the ball milling speed is higher than 400rpm, a lot of adverse defects occur in the composite structure, which is not conducive to carrier separation and transfer resulting in the decrease of photocatalytic activity of the samples.
Moreover, DebyeScherrer formula was employed to calculate the average particle sizes of CdS/TiO2 composite photocatalysts prepared at different ball milling speeds, to explore the relationship between powder particle size and photocatalytic performance. Table 1 shows the detailed particle size of CdS/TiO2 composite photocatalysts at different ball milling speeds. As the table indicates, with the gradual increasing ball milling speed, the average particle size of CdS/TiO2 composite photocatalyst gradually decreases from 42.3nm and finally stabilizes at about 7.4nm. When the ball milling speed increases from 200 to 400rpm, the average particle size of the sample decreases significantly by 32.1nm, 75.9% compared with the average particle size at 200rpm. However, when the ball milling speed increases from 400r/min to 600r/min, the particle size is reduced by only 2.8 mm. The photocatalytic degradation efficiency of CdS/TiO2 composite photocatalyst is the highest at the ball milling speed of 400rpm, corresponding to the nano-effect of nanoparticles and the trend of particle size reduction as shown in Figure 5.
The time of ball milling directly affects the particle size and purity of the product. During ball milling, due to the severe collision and friction of ball milling beads, part of the product will fall off. The longer of ball milling time, the more serious the pollution of the product will be. On the other hand, as mentioned above, with an increase in the ball milling time, the particle size of CdS/TiO2 composite photocatalysts will gradually decrease to a certain value and tend to be stable, even if the ball milling time still increases. Under such a condition, the increase in the ball milling time will only lead to the contamination of the CdS/TiO2 composite photocatalysts, but no longer improve the photocatalytic performance. Hence, one can see that finding the optimal ball milling time is quite crucial.
Under the conditions of the preset material ratio of 25:75 and ball milling speed of 400rpm, Figure 6 shows the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts prepared at different milling time after 2h of UV or sunlight irradiation. As is shown in the diagram, the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalyst with a ball milling time of 10h is better than that of other ball milling time under both UV light and sunlight irradiation, so it can be considered that 10h is the best ball milling time for preparing the CdS/TiO2 composite photocatalysts. Similarly, DebyeScherrer formula was also made use of calculating the average particle size of CdS/TiO2 composite photocatalysts prepared under certain ball milling time, which is revealed in Table 2. The particle size of the sample significantly decreases when the ball milling time is no more than 10h and gradually becomes stable after more than 10h, which is consistent with the changing trend of the photocatalytic performance of the composite photocatalysts, which one can see from the comparison between Figures 6 and 7.
To explore the influence of high-temperature calcination on the photocatalytic activity of CdS/TiO2 photocatalysts (ball milling time of 10h, i.e., optimal ball milling time) with different material ratios, the samples were put into a resistance furnace and calcined at 400C for 2h. Subsequently, the photocatalytic test was carried out under UV or sunlight irradiation for 2h, and the calculated photocatalytic degradation efficiency was compared with that of the uncalcined samples, as shown in Figure 8. On the whole, the variation trend of photocatalytic degradation efficiency of CdS/TiO2 photocatalysts with material ratio after calcination is approximately the same as that of uncalcined samples, but the photocatalytic degradation efficiency of calcined samples increases slightly, which may be owing to the reduction of adverse structural defects of CdS/TiO2 composite photocatalysts. In addition, the bandgap width of the samples prepared and calcined under the optimal ball milling parameters and that of TiO2 was calculated. As shown in Figure 9(a) and (b), the bandgap width of pure anatase TiO2 is 3.05eV, which is consistent with the research showing that the bandgap width of TiO2 is 3.2eV, while the bandgap width of CdS/TiO2 composite photocatalyst is 1.88eV. The wider the bandgap, the greater the energy required to excite the materials. In this experiment, the CdS/TiO2 composite photocatalysts obtained by the secondary ball milling and calcining method have smaller bandgap width, easier excitation, and stronger photocatalytic performance.
In this paper, CdS/TiO2 composite photocatalysts are made by the method of secondary ball milling, and the effects of material ratio, ball milling time, ball milling speed, and high-temperature calcination on the photocatalytic performance are investigated. The following conclusions can be drawn from the experimental results:
The mechanochemical action of secondary ball milling can promote the dispersion of CdS on the surface of TiO2 and the interaction between these two, forming an effective composite nanostructure with extended light absorption edge and small bandgap width, resulting in a significant improvement on the photocatalytic degradation rate of CdS/TiO2 composite photocatalysts.
High-temperature calcination of CdS/TiO2 composite photocatalysts with different material ratios did not change the variation trend of their photocatalytic degradation efficiency, but the photocatalytic degradation efficiency increased slightly after calcination in general, which may be because of the reduction in adverse structural faults of CdS/TiO2 composite photocatalyst after calcination.
The material ratio and ball milling process had an obvious influence on the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts when compared with high-temperature calcination. When the ball milling speed, time, and material ratio were 400rpm, 10h, and 25:75, respectively, the CdS/TiO2 composite photocatalysts obtained by calcining after ball milling had the strongest photocatalytic performance on MO.
The irradiation conditions also had a significant influence on the photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts. The photocatalytic degradation efficiency of CdS/TiO2 composite photocatalysts under UV irradiation was much higher than that under sunlight irradiation.
As for the application, it is suggested that the CdS/TiO2 composite photocatalysts can be prepared under the optimum conditions of the ball milling process if it is used in the industry. Since high-temperature calcination has a relatively small effect on its photocatalytic performance, for the sake of economy and operability, this step is not performed unless it is particularly necessary.
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1. As the name suggests, the ball milling method consists of balls and a mill chamber. Therefore over all a ball mill contains a stainless steel container and many small iron, hardened steel, silicon carbide, or tungsten carbide balls are made to rotate inside a mill (drum). Continue reading Ball Milling method for synthesis of nanomaterials Share and Like article, please: